Introduction

Metazoans detect innocuous and/or noxious environmental cues and appropriately generate relevant behavioral responses. There is a large diversity in types of nervous systems from relatively simple nerve nets to highly complex centralized organs dedicated for processing and executing behavioral commands. The ability of an organism to sense and respond to environmental cues is based on the underlying neural architecture and its connectivity to muscle groups for generating behavioral responses. Understanding the neural substrates underlying the execution of how behavioral commands are generated in the nervous system, spanning from the sensory neuron input to motor neuron output, constitutes one of the key areas of research in contemporary neuroscience.

Drosophila melanogaster is among the premier model organisms for studying molecular, cellular and circuit bases of behaviors in modern neuroscience driven by advances in electron microscopy (EM) connectomics, ability to perform high-throughput behavioral screens, characterization of stereotyped behavioral phenotypes, the ability to perform cell-type specific manipulations and genetic accessibility (for review see (Eschbach & Zlatic, 2020). Another critical benefit of using Drosophila to study neural connectivity at the synaptic level is its relatively small, but complex CNS, where there are only ∼10,000 neurons in the Drosophila larval CNS and ∼100,000 neurons in adult Drosophila melanogaster brain compared to 86 billion neurons in the human brain (Herculano-Houzel, 2009; Scheffer et al., 2020). Highly detailed serial section transmission electron microscopy (ssTEM) whole brain volumes with synaptic level resolution have been obtained for both Drosophila larval (Ohyama et al., 2015) and adult (Zheng et al., 2018) brains, where neurons were reconstructed using a collaborative web-based software, CATMAID (Saalfeld et al., 2009; Schneider-Mizell et al., 2016). Concerted efforts from many laboratories have made tremendous advances in reconstructing the Drosophila larval connectome at synaptic level resolution and there has been great progress in mapping out select connectomes at the EM level (Clark et al., 2018; Eschbach & Zlatic, 2020; Kohsaka et al., 2017). Connectomes have been further validated using behavioral and functional imaging studies for olfaction in combination with learning and memory (Berck et al., 2016; Eichler et al., 2017; Eschbach et al., 2020; Saumweber et al., 2018), feeding (Miroschnikow et al., 2018; Schlegel et al., 2016), visual processing (Larderet et al., 2017), locomotion (Carreira-Rosario et al., 2018; Fushiki et al., 2016; Heckscher et al., 2015; Hiramoto et al., 2021; Kohsaka et al., 2019; Zarin et al., 2019; Zwart et al., 2016), chemotaxis (Tastekin et al., 2018), thermosensation (Hernandez-Nunez et al., 2021), mechanosensation (Jovanic et al., 2016; Jovanic et al., 2019; Masson et al., 2020), nociceptive modalities (Burgos et al., 2018; Gerhard et al., 2017; Hu et al., 2017; Imambocus et al., 2022; Kaneko et al., 2017; Ohyama et al., 2015; Takagi et al., 2017), among others (Andrade et al., 2019; Huckesfeld et al., 2021; Imura et al., 2020; Mark et al., 2021; Valdes-Aleman et al., 2021; Winding et al., 2022).

We are particularly interested in how larval somatosensation functions through peripheral sensory neurons located along the body wall just below the larval cuticle. Larval somatosensory neurons are comprised of type I, mono-ciliated dendrites, (external sensory (es) and chordotonal (Ch) neurons) and type II, bipolar dendritic (td and bd) and highly branched multidendritic (md) neurons (classes I-IV, referred to as CI-IV md). Functional and behavioral roles of these sensory neurons include proprioception (CI md & Ch) (Caldwell et al., 2003; He et al., 2018; Vaadia et al., 2018), heat thermoreception (CIV md) (Babcock et al., 2009; Im et al., 2015; Ohyama et al., 2015; Tracey et al., 2003), cold thermoreception (Ch, CII md, & CIII md) (Turner et al., 2016; Turner et al., 2018), chemoreception (CIV md) (Himmel et al., 2019; Lopez-Bellido et al., 2019) and mechanosensation (Ch, CII md, CIII md, & CIV md) (Hu et al., 2017; Hwang et al., 2007; Jovanic et al., 2019; Masson et al., 2020; Ohyama et al., 2015; Scholz et al., 2015). Recent studies have unraveled circuit bases underlying thermo-(heat), chemo-and mechanosensation, however, circuit bases of noxious cold thermosensory evoked behaviors are yet to be described (Burgos et al., 2018; Hu et al., 2017; Hu et al., 2020; Imambocus et al., 2022; Jovanic et al., 2016; Jovanic et al., 2019; Hernandez-Nunez et al., 2021; Kaneko et al., 2017; Lopez-Bellido et al., 2019; Masson et al., 2020; Ohyama et al., 2015; Yoshino et al., 2017).

Neural circuitry downstream of select somatosensory neurons has been elucidated both physiologically and behaviorally in the context of nociceptive stimuli that evoke characteristic rolling behavior in Drosophila larvae. Specifically, Ch and CIV md neurons signal through multisensory integrator neurons (Basins) via ascending pathways to command neurons for mechanical and nociceptive stimulation that synergistically impact larval rolling escape responses (Ohyama et al., 2015). CIV md neuron mediated nociceptive escape behaviors also function through premotor neurons such as Down-and-Back (DnB) and medial clusters of CIV md interneurons (mCSI) (Burgos et al., 2018; Yoshino et al., 2017). Additionally, A08n projection neurons receive input from CIV md neuron and the peptidergic neuron DP-ilp7 to integrate multisensory inputs from CII, CIII and CIV md neurons (Imambocus et al., 2022). A08n and DP-ilp7 neurons are specifically required for nociceptive mechanical and chemical sensing but not for nociceptive thermosensation (Hu et al., 2017; Imambocus et al., 2022; Kaneko et al., 2017; Tenedini et al., 2019; Vogelstein et al., 2014). Lastly, Ch and CIII md neuronal connectivity, through ascending projection neurons A09e and TePns as well as premotor neuron Chair-1, are required for anemotaxis and innocuous mechanical sensing (Jovanic et al., 2019; Masson et al., 2020). While CIII md neuron connectivity to select second order neurons has been published (Masson et al., 2020), the functional and behavioral roles of these circuit components in the context of cold nociception remain unexplored. We hypothesized that the noxious cold sensitive somatosensory nociceptive circuit functions through shared circuitry amongst other somatosensory modalities.

Analyses of cold sensitive circuitry downstream of CIII md neurons elucidated dual pathways for signal transduction: (1) CIII md neurons directly signal through premotor neurons that modulate motor activity and (2) CIII md neurons also function via multisensory integrators and projections neurons to convey somatosensory information to higher order brain regions. Somatosensory neurons (Ch and CIII md) are required for cold-evoked responses but not CIV md neurons (Turner et al., 2016; Turner et al., 2018). Multisensory integration neurons, Basins 1-4, are required for cold nociception, facilitate CIII md evoked larval contraction (CT) responses, and select Basins exhibit cold evoked increases in calcium. Premotor neurons, DnB and mCSI, are required and facilitate cold-evoked behavioral responses, whereas Chair-1 premotor neurons are required for cold nociception but do not facilitate CIII md neuron mediated responses. Lastly, nociceptive projection neurons A09e and TePns also integrate cold stimuli but only A09e neurons enhance CIII md neuron mediated behavioral responses.

Results

Peripheral sensory neurons share synaptic partners

Recent work on competitive interactions and behavioral transitions in Drosophila larvae has reported CIII md and Ch connectomes in the context of larval mechanosensation (Masson et al., 2020). We performed a comparative circuit analysis of somatosensory (Ch, CIII md and CIV md) neurons using an online repository hosted by LMB Cambridge, which contains a ssTEM volume of Drosophila melanogaster first instar larval central nervous system and a repository of neural reconstructions (https://neurophyla.mrc-lmb.cam.ac.uk/). Here, we report and contextualize relevant CIII md neuron downstream neurons, whose functional roles in cold nociception were evaluated. The meta-analysis of previously published literature on Drosophila larval somatosensory connectomes revealed several common post-synaptic partners that are shared amongst Ch, CIII md and CIV md neurons (Figure 1A). CIII md neurons are upstream of multisensory integration neurons, pre-motor neurons and projection neurons. Class III md neurons are synaptically connected to multisensory integration neurons (Basin-2, -3 & -4). The downstream pathway from Basins includes connectivity to A00c neurons and a polysynaptic pathway to the rolling command neuron (Goro) via A05q neurons. CIII md neurons are also connected to various pre-motor neurons, which are ventral nerve cord localized neurons providing synaptic input to motor neurons, including DnB, A02n and Chair-1, and projection neurons including A05q, A09e, TePn05 and A08n. The CIII md neuron connectome analyses reveal broad second order connectivity and complex interconnectivity amongst second order neurons (Figure 1A, Figure 1-figure supplement 1). Functional and behavioral roles of CIII md neuron second order interneurons in cold nociception remain unexplored and below we address this knowledge gap.

Figure 1:

Functional analysis of somatosensory neurons in cold nociception

Due to shared post-synaptic neural connectivity of Ch, CIII md and CIV md neurons (Figure 1A, Figure 1-figure supplement 1), we first assessed functional roles of these sensory neurons in cold nociception by investigating: (1) stimulus-evoked calcium responses in the Drosophila larval ventral nerve cord; (2) cold-evoked calcium responses of these sensory neurons; (3) necessity and sufficiency of these sensory neurons in cold-evoked behaviors; and (4) examining whether co-activating multiple sensory neurons facilitates CIII-mediated behavioral response in Drosophila larvae.

1. CaMPARI analysis reveals distinct neuropil activation patterns by sensory stimuli

Drosophila larvae have distinct behavioral responses to various sensory stimuli including touch, heat or cold. Exposure to noxious cold temperatures (≤10°C) evokes highly stereotyped head and tail withdrawal towards the center of the animal, termed here as contraction (CT) response (Patel et al., 2022; Turner et al., 2016). The cold-evoked CT response is defined as at least 10% reduction in larval surface area (Figure 1-figure supplement 4). Innocuous mechanical stimuli evokes a suite of behaviors including pausing, turning, head withdrawal, and/or reverse locomotion. Noxious heat exposure leads to a corkscrew body roll escape response (Babcock et al., 2011; Im et al., 2015; Ohyama et al., 2015; Tracey et al., 2003). Previous work on stimulus-evoked changes in neural ensembles in larval zebrafish (Danio rerio) used the genetically encoded calcium integrator CaMPARI to reveal distinct CNS neural activation patterns in response noxious heat or cold exposure (Fosque et al., 2015). CaMPARI affords great spatial resolution as stimulus-evoked neural responses are captured from a freely moving animal.

With CaMPARI and pan-neural imaging of ventral nerve cord neurons, we visualized central representations of somatosensory stimuli across sensory modalities including innocuous mechanical, noxious cold or noxious heat, which are primarily detected via Ch, CIII md, & CIV md neurons, respectively. We pan-neuronally expressed CaMPARI using R57C10^GAL4 (Jenett et al., 2012; Pfeiffer et al., 2008) and Drosophila larvae were simultaneously exposed to diverse sensory stimuli and photoconverting light. Post-hoc imaging of intact Drosophila larvae ventral nerve cord revealed relatively little neural activity when larvae are not presented with any sensory stimulus (Figure 1-figure supplement 2). However, upon innocuous mechanical (gentle touch) stimulation, there is a marked increase in neural activation as reported by F_redLUT (Figure 1-figure supplement 2). Drosophila larvae exposed to noxious heat experience a large, spatially broad increase in neural activity. Incidentally, similar neuropil regions appear to be activated by both innocuous touch and noxious heat, albeit at different levels (Figure 1-figure supplement 2). Lastly, noxious cold exposure leads to robust neural activation medially, in the neuropil, however, cell bodies seem to have lower activation levels compared to touch or heat stimulations (Figure 1-figure supplement 2). These experiments measuring pan-neuronal ventral nerve cord activity in response to sensory stimuli would be ideal for comparative neural activation analyses, were the spatial resolution of the optical microscope sufficient. Acknowledging the limitations of the approach, we heretofore focus instead on single-cell type specific genetic driver lines for identified neurons that can be assigned to known neurons in the connectome,

2. CIII md somatosensory neurons exhibit robust Ca²⁺ responses to cold

Somatosensory neurons function as primary sensors of external stimuli. Ch and CIII md neurons have previously been reported to present cold evoked increases in Ca²⁺ (Turner et al., 2016; Turner et al., 2018). Previous reports indicate that CIV md neurons are weakly sensitive to cold stimuli (Turner et al., 2016). Cold sensitivity of sensory neurons (Ch, CIII md, & CIV md) was assessed by selectively expressing the CaMPARI2 Ca²⁺ integrator in sensory neurons using cell-type specific driver lines (Figure 1-figure supplement 3A). CaMPARI2 signal, as assessed by F_red/F_green ratios at the cell body, reveal all three neuron subtypes (Ch, CIII md and CIV md) have significantly higher response upon cold exposure compared to no stimulus controls (Figure 1B, Figure 1-figure supplement 3B). Unsurprisingly, as high-threshold nociceptors, CIII md or CIV md neurons present relatively low responses upon photoconverting light exposure sans cold (Figure 1B, Figure 1-figure supplement 3B). However, mechanosensitive Ch neurons exhibit relatively high responses upon exposure to only photoconverting light but no stimulus indicative of high baseline neuronal activity processing mechanosensory information (Figure 1B, Figure 1-figure supplement 3B).

Similar to the analysis at cell bodies, in the dendrites of Ch neurons we observe marked increases in CaMPARI2 response to photoconverting light (control), however, upon cold exposure (experimental condition) there is a further increase in CaMPARI2 response (Figure 1C). Sholl intensity analysis (see methods) likewise reveals that CIII md neurons present relatively low CaMPARI2 response in control conditions, however, there is a robust cold evoked increase in CaMPARI2 response throughout the dendrites (Figure 1C). Interestingly, CIV md neurons also exhibited significant increases in CaMPARI2 response upon cold exposure in the cell body (Figure 1B). we measure a higher cold-evoked CIV md neuron CaMPARI2 response closer to the soma (Figure 1C). However, there is no change in distal dendritic CIV md neuron CaMPARI2 response between cold and no stimulus conditions (Figure 1C). CIII md have robust cold evoked Ca²⁺ responses compared to Ch and CIV md neurons, which have relatively low cold evoked Ca²⁺ increases (Figure 1B,C). Therefore, we identified CIII md neurons as the most sensitive to noxious cold temperatures, and thus we focused our study on the neural circuits postsynaptic to CIII md neurons.

3. CIII md somatosensory neurons are necessary and sufficient for the cold response

The analysis of neuronal activity responses to cold exposure revealed distinct patterns for both central and somatosensory neurons, with most robust responses to stimuli evoked in CIII md neurons.

We assessed the necessity of somatosensory neurons in noxious cold-evoked behavioral responses (Figure 1-figure supplement 4) by expressing tetanus toxin light chain, which inhibits neurotransmitter release (Sweeney et al., 1995). Based on Ca²⁺ imaging, we expected both Ch and CIII md neurons may be necessary for cold evoked CT responses. Neural silencing of Ch and CIII md neurons led to significant reductions in cold evoked CT responses compared to either w1118 (parental line) or Empty^GAL4 controls (a GAL4 construct lacking the promotor sequence; Figure 1D, E). Instantaneous behavioral response curves also indicate lower cold sensitivity when either Ch or CIII md neurons are silenced. However, inhibiting neurotransmitter release in CIV md neurons alone did not result in significant reductions in cold evoked CT responses (Figure 1D, E). Among the three somatosensory neuron types tested, neural silencing of CIII md neurons resulted in the strongest impairment in cold evoked behavioral response.

Next, we evaluated whether neural activation of sensory neurons via optogenetics would be sufficient to elicit the CT behavioral response (Figure 1-figure supplement 5A). We expressed the engineered channelrhodopsin ChETA in individual sensory neuron subtypes and assessed evoked behavioral responses (Gunaydin et al., 2010). When assessing evoked responses, we first analyzed CT responses as measured by changes in surface area (Figure 1-figure supplement 5B). Only neural activation of CIII md neurons led to CT responses in Drosophila larvae (Figure 1F, G) consistent with our previously published work (Turner et al., 2016). Additionally, we analyzed larval mobility, which refers to changes in larval postures as measured by changes in occupied space (Figure 1-figure supplement 5C). Upon neural activation of CIII md neurons, there is a large increase in Drosophila larval immobility compared to controls (Figure 1-figure supplement 6A-C). However, there was no difference in immobility when either Ch or CIV md neurons were optogenetically activated compared to control (Figure 1-figure supplement 6A-C). Of the somatosensory neurons tested, only CIII md neurons are sufficient to elicit the CT response.

4. Effect of co-activating CIII md somatosensory neurons plus additional somatosensory neuron classes

Both Ch and CIV md neurons share common first order post-synaptic partners with CIII md neurons and all three somatosensory present cold-evoked increases in calcium levels. However, neither Ch nor CIV are sufficient to elicit a CT response. To further clarify the roles of Ch and CIV in cold evoked behaviors, we simultaneously activated CIII md neurons plus either Ch or CIV md neurons. We expected that co-activation of multiple sensory neuron subtypes would facilitate optogenetically-evoked CT responses. Optogenetic activation of CIII md neurons using two cell type specific driver lines (19-12^GAL4 and R83B04^GAL4) led to sustained increases in instantaneous CT responses compared to control, where only one driver line (19-12^GAL4) was used to activate CIII md neurons (Figure 1H, I). The activation of CIII md neurons using two driver lines (expressed in the same cell type, CIII md) led to a significant increase in immobility compared to single GAL4 driver-mediated activation of these neurons (Figure 1-figure supplement 6D-F), suggesting a single GAL4 line does not exhaust the dynamic range of the CT response. Co-activation of CIII md and Ch neurons, which are cold sensitive and required for cold evoked CT responses, led to a subtle initial increase in instantaneous CT response compared to CIII activation alone, however, the initial increase in evoked CT response was quickly reduced to well below control (Figure 1H, I). Therefore the co-activation of two somatosensory neuron types, Ch and CIII md, elicited a CT response that varied along the temporal axis relative to the activation of CIII md alone. Drosophila larval immobility was reduced during co-activation of Ch and CIII md neurons compared to CIII md neuron activation alone suggesting that Ch neurons do not facilitate CIII md neuron mediated CT responses (Figure 1-figure supplement 6D-F). CIV md and CIII md neurons share a large proportion of common second order interneuron connectivity including multisensory integration neurons, premotor and ascending neurons. We predicted that CIV md and CIII md neuron co-activation might potentiate CT responses. Interestingly, simultaneous activation of CIII and CIV md neurons led to small but insignificant reductions in instantaneous and peak CT responses compared to CIII md neuron activation alone (Figure 1H, I). Similarly, co-activation of CIII md and CIV md neurons did not alter larval immobility compared to only CIII md neuron activation (Figure 1-figure supplement 6D-F). Ch neurons are cold sensitive and required for cold nociceptive behaviors but do not facilitate CIII md neuron evoked behavioral responses. Meanwhile, CIV md neurons are modestly cold sensitive but are not required for cold nociception and do not facilitate CIII md neuron evoked CT responses. Collectively, CIII md neurons have the strongest cold-evoked calcium response, are required for cold-evoked behavioral response and sufficient for the CT response.

Multisensory integrators are required and facilitate cold nociception

Basin interneurons function as multisensory integrators receiving convergent inputs from mechano-, chemo- and thermo-sensitive peripheral sensory neurons (Figure 2A, Figure 2-figure supplement 1A). Previous behavioral and functional studies have revealed that Basin interneurons are required for nociceptive escape responses mediated by Ch and CIV md neurons (Ohyama et al., 2015). Both Ch and CIV md neurons play roles in CIII md mediated behavioral responses either in cold nociception and/or can detect cold stimuli. We hypothesized that Basin interneuron function is required downstream of cold nociceptive somatosensory neurons. We evaluated whether Basin interneurons are necessary and sufficient for cold nociception, exhibit cold evoked increases in calcium response and function downstream of CIII md neurons.

Figure 2:

Silencing Basin interneurons reduces cold-evoked CT

Basin interneurons receive somatosensory cues from CIII md neurons, thus we predicted that inhibiting neurotransmitter release from Basin neurons will result in impaired cold evoked behaviors (Figure 2A). We assessed the requirement of all Basin neurons using “pan-” Basin driver lines and also assessed roles of individual Basin neurons using subtype specific driver lines. Neural silencing of all Basin (1-4) neurons, using two independent driver lines (R72F11^GAL4 and R57F07^GAL4), led to significant reductions in cold-evoked CT responses compared to genetic background (w1118) and Empty^GAL4 controls (Figure 2B). Drosophila larvae with impaired Basin (1-4) neuronal function had significant reductions in CT duration, magnitude and cumulative %CT response compared to controls (Figure 2C-E). CIII neurons have differential connectivity to individual Basin subtypes (Figure 2A). Therefore, we assessed requirement of Basin-1, -2 or -4 neurons, for which there are previously validated independent driver lines (Jovanic et al., 2016; Ohyama et al., 2015). Tetanus toxin mediated neural silencing of Basin-2 or -4 led to significant reductions in cold evoked CT response, as measured by cumulative %CT response, CT duration and CT magnitude, compared to controls (Figure 2B-E). CIII md neurons do not synapse onto Basin-1 neurons according to the EM-mapped connectome (Figure 2A) (Masson et al., 2020), however, Basin-1 & -2 neurons share synaptic connectivity to both feedback and feedforward GABAergic interneurons, both of which are downstream of sensory neurons (Jovanic et al., 2016). Furthermore, Basin-1 neural activation leads to depolarizations in Basin-2 through GABAergic disinhibitory pathway (Jovanic et al., 2016). Therefore, we expected that inhibiting neurotransmitter release in Basin-1 neurons would result in reduced cold-evoked responses. Neural silencing of Basin-1 neurons resulted in modest, but significant reductions in cold evoked responses (Figure 2B-E). Impaired Basin neuron signaling results in at least 25% reduction in cold evoked responses with the strongest reductions for all-Basin, Basin-2 or Basin-4 driver lines (Figure 2J).

Co-activation of Basin interneurons and CIII md somatosensory neurons enhances CT

Next, we evaluated whether neural activation of Basin neurons would impair or elicit a CT response. Optogenetic activation of Basin neurons, either using all Basin or individual Basin driver lines, did not elicit a CT response (Figure 2-figure supplement 2). But the simultaneous co-activation of CIII md and Basin neurons led to sustained increases in CT responses compared to controls, where only CIII md neurons were activated, across multiple behavioral metrics (Figure 2F-I). Coactivation did not result in a change in larval immobility (Figure 2-figure supplement 1B-D), whereas activation of all Basins led to significantly greater immobility (Figure 2-figure supplement 1B-D). These results indicate that Basin neurons are not sufficient for the CT response, but that the combined activation of CIII md and Basins not only suffices but also elicits an even stronger CT response than activating CIII md neurons alone.

To parse the contribution of individual Basin neuronal subtypes, we next assessed the role of Basin-1, -2 or -4, and all four Basin together, in either facilitating or suppressing CT responses using our co-activation paradigm (Figure 2F-I). Co-activation of Basin-1 or Basin-2 with CIII md neurons led to an enhanced CT response compared to controls across all measures of behavioral response including instantaneous %CT response, peak %CT response, CT duration and CT magnitude (Figure 2F-I), with a subsequent significant increase in larval immobility (Figure 2-figure supplement 1B-D). Basin-2 plus CIII md neuron co-activation led to strong facilitation of CT responses, but surprisingly resulted in significantly reduced immobility (Figure 2-figure supplement 1B-D). Unlike for all other Basin neurons tested, Basin-4 and CIII md neuron co-activation led to a suppression of CT response, where peak instantaneous %CT response was similar to controls, however, there was a rapid reduction in instantaneous %CT response compared to controls (Figure 2F-I). Both CT duration and CT magnitude were significantly impaired for Basin-4 plus CIII md neuron co-activation compared to controls (Figure 2H, I), which also showed significantly lower immobility (Figure 2-figure supplement 1B-D). Interestingly, dual activation of CIII md and all Basin neurons led to weaker CT enhancement compared to co-activation of either Basin-1 or -2 coupled with CIII md neurons (Figure 2J), consistent with the finding that co-activation with Basin-4 reduced the CT response. Collectively, second-order Basin neurons are required for cold evoked responses and specifically Basin-1 and Basin-2 are able to enhance CIII md neuron evoked behavioral responses.

CaMPARI reveals Basin-2- and Basin-4 are activated in CT responses

To further explore how Basin interneurons function in cold nociception, we sought to investigate cold-evoked Ca²⁺ responses of Basin neurons. Since somatosensory neurons are cholinergic (Salvaterra & Kitamoto, 2001), we expected that Basin neurons postsynaptic to CIII md neurons will exhibit cold-evoked increases in Ca²⁺. Post-hoc imaging of evoked CaMPARI2 fluorescence revealed that Basin neurons have significantly higher F_red/F_green ratios compared to their respective controls, as assessed by two independent all-Basin driver lines (Figure 3A, B). We further investigated Ca²⁺ responses in greater detail using individual driver lines for Basin-1, -2 or -4. Basin-1 neurons are weakly required for cold-evoked CT responses (Figure 2J), and coherently do not exhibit cold-evoked increases in Ca²⁺ response (Figure 3C). In contrast, Basin-2 and -4 neurons both exhibit significant increases in Ca²⁺ responses compared to their respective controls (Figure 3D-E). Collectively, Basin-2 and -4 neuron subtypes that are required for cold-evoked CT responses (Figure 2J), also exhibit cold-evoked increases in Ca²⁺.

Figure 3:

CIII md neurons and Basin-2 and -4 neurons are functionally connected

Basin-2 and -4 are postsynaptic to CIII md neurons, are required for cold nociception and have cold-evoked increases in Ca²⁺. Next, we assessed whether CIII md neurons and Basin-2 or - 4 neurons are functionally connected. From the EM-reconstructed connectome we predicted that the activation of CIII md neuron will result in increased Ca²⁺ levels of Basin-2 or -4. As expected, optogenetic activation of CIII md neurons led to significant increases in Ca²⁺ levels in Basin-2 neurons, and repeated stimulation of CIII md neurons did not lead to sensitization of Basin-2 Ca²⁺ responses (Figure 3F, G). Previously, it was shown that Ch and CIII md neurons could elicit Ca²⁺ response in Basin-4 neurons (Kaneko et al., 2017), however, in this previous study the authors were unable to determine which of the two sensory neuron cell types led to increases in the Basin-4 Ca²⁺ response (Kaneko et al., 2017). Here, we show that upon specifically activating CIII md neurons, Basin-4 neurons have large, rapid increases in cytosolic Ca²⁺ followed by quick return to baseline levels. In contrast to Basin-2, Basin-4 neurons showed reduced Ca²⁺ responses upon repetitive activations of CIII md neurons (Figure 3H, I). Therefore, CIII md neuron activation is differentially encoded by Basin-2 and -4 neurons, where Basin-4 neurons have much larger CIII md neuron evoked increase in Ca²⁺ levels but exhibit sensitization compared to Basin-2 neurons, which do not show any sensitization upon repetitive stimulations. Collectively, our data demonstrate that Basin neurons are required for cold-evoked behaviors and Basin-2 and -4 neurons functionally operate downstream of CIII md neurons (Figure 3-figure supplement 1).

Multisensory integrators function independently of Goro pathway for cold nociception

Basin neurons innervate a set of projection neurons (A05q and A00c), which are upstream of a command neuron (Goro) that is responsible for initiating CIV md neuron-mediated nociceptive escape behaviors (Figure 4A, Figure 4-figure supplement 1A) (Ohyama et al., 2015). We set out to test whether cold-evoked behavioral responses mediated by CIII md and Basin neurons function through A00c, A05q, and/or Goro neurons. We first assessed whether these neurons are required for cold-evoked behavioral responses (Figure 4B-E). Neural silencing of A05q neurons via tetanus toxin led to mild, yet significant reductions in cold evoked responses, when compared to w1118 genetic control (Figure 4B-E). Neural silencing of A00c neurons resulted in significantly lower cold-evoked cumulative CT response compared to w1118 (Figure 4B, C). There were also significant reductions in CT duration and CT magnitude when A00c neurons were silenced compared to both controls (Figure 4D, E). In contrast, the Goro command neuron for nociceptive rolling behavior is not required for cold-evoked CT responses (Figure 4B-E). Next, we assessed whether neural activation of these neurons led to evoked CT responses. Like Basin neurons, single cell-type activation of A00c, A05q, or Goro neurons did not lead to CT behavior (Figure 2-figure supplement 2). Similarly, optogenetic co-activation of CIII md neurons and A05q or Goro neurons did not lead to significant facilitation of the CT response (Figure 4F-I). However, simultaneously co-activating A00c and CIII md neurons led to significant increases in CT duration and CT magnitude (Figure 4H, I). There were no changes in Drosophila larval immobility upon co-activation of CIII md plus A00c, A05q or Goro neurons (Figure 4-figure supplement 1B-D). Only co-activation of A00c neurons leads to notable enhancement of CIII md mediated CT response, whereas neural silencing of A00c or A05q neurons led to greater than 25% impairment in cold evoked behavioral responses (Figure 4J). In conclusion, the Basin to Goro polysynaptic pathway does not significantly contribute to the cold-evoked CT response.

Figure 4:

Premotor neurons function downstream of CIII md neurons to mediate cold nociceptive responses

Select Drosophila larval premotor neurons were previously implicated in CIV-mediated nociceptive escape responses. Specifically, DnB premotor neurons are involved in noxious thermal stimulus-evoked c-bending and rolling behavior (Burgos et al., 2018; Lopez-Bellido et al., 2019). Drosophila larvae also roll in response to activation of mCSI premotor neurons, which are synaptically connected to CIV md neurons and predicted to be A02m/n neurons from EM connectomes (Lopez-Bellido et al., 2019; Yoshino et al., 2017). Additionally, Chair-1 (A10a) premotor neurons have been implicated in anemotaxis (Jovanic et al., 2019). Collectively, these premotor neurons primarily receive inputs from both primary sensory neurons (CIII & CIV md) and multisensory integrators (Basin-2 & -4) (Figure 5A, Figure 5-figure supplement 1A). We predicted that premotor neurons are required for cold-evoked responses and function downstream of CIII md neurons in a stimulus-specific manner.

Figure 5:

Inhibition of neural transmission via cell type specific expression of tetanus toxin in individual premotor neurons led to reduced cold evoked responses in Drosophila larvae (Figure 5B-E). Silencing Chair-1 neurons resulted in the strongest reduction of cold-evoked CT responses, where instantaneous %CT was the lowest of all premotor neuron subtypes tested (Figure 5B). CT duration, magnitude and cumulative percent response were all significantly reduced compared to controls (Figure 5B-E). Impairment in Chair-1 neuronal signaling leads to 75% reduction from controls in cold-evoked behaviors (Figure 5J). We silenced DnB neurons using two independent cell-type specific driver lines (DnB’ (IT4051^GAL4) & DnB’’(IT412^GAL4)), where both resulted in reduced instantaneous %CT response, along with significant reductions in CT duration and magnitude compared to controls (Figure 5B-E). Silencing mCSI (R94B10^GAL4) neurons also resulted in significantly reduced cold-evoked CT responses compared to controls (Figure 5B-E). DnB or mCSI inhibition of neurotransmitter release leads to approximately 50% reduction in cold-evoked CT responses (Figure 5J). Collectively, these premotor neurons are required for cold-evoked behavioral responses.

Based on EM connectivity and neural silencing experiments, we predicted that activation of these premotor neurons would be sufficient for Drosophila larval CT response. First, we found that activation of premotor neurons alone did not evoke CT response (Figure 2-figure supplement 2). Next, we performed optogenetic co-activation of CIII md and premotor neuron subtypes. Chair-1 neuron co-activation with CIII md neuron did not have an effect on CT responses (Figure 5F-I), however, there was a reduction in larval immobility, where average mobility was increased and duration of immobility was reduced compared to when only CIII md neurons are activated (Figure 5-figure supplement 1B-D). Simultaneous activation of CIII md neurons and DnB or mCSI both led to significant increases in CT responses, as measured by peak CT response, CT duration or magnitude, compared to CIII md neuron activation alone (Figure 5F-I). DnB and CIII md neuron co-activation led to short lived, subtle but insignificant increases in larval immobility (Figure 5-figure supplement 1B-D). Co-activation of mCSI and CIII md neurons resulted in mild reductions in larval immobility (Figure 5-figure supplement 1B-D). These data reveal DnB and mCSI neuronal activity enhances CIII md neuron mediated CT responses.

Since these premotor neurons are postsynaptic to CIII md neurons and their activity is required for proper cold evoked behaviors, we predicted that activation of CIII md neurons would elicit cold-evoked increases in Ca²⁺. To test this prediction, we selectively expressed the Ca²⁺ integrator CaMPARI2 in premotor neurons. Unexpectedly, there was no change in cold evoked Ca²⁺ levels in mCSI or Chair-1 neurons (Figure 6C-D). Noxious cold exposure did however lead to significant Ca²⁺ increases in DnB neurons (Figure 6A-B). To further assess how CIII md neuronal activity affects DnB function, we optogenetically activated CIII md neurons and assessed evoked Ca²⁺ levels of DnB using GCaMP6. Interestingly, Drosophila larvae raised without all trans-retinal, a requisite light sensitive cofactor for optogenetic experiments, also had a mild light evoked increase in Ca²⁺ (Figure 6E, F). CIII md neuron optogenetic activation led to significant increases in DnB Ca²⁺ levels that slowly returned to baseline levels (Figure 6E, F). Upon repeated CIII md neuron activations, DnB neurons exhibit a blunted Ca²⁺ response relative to initial stimulation (Figure 6E, F). Taken together we find that premotor neuronal function is required for cold nociception and premotor neuron activity can facilitate CIII md neuron mediated CT responses (Figure 6-figure supplement 1).

Figure 6:

Ascending interneurons are required for cold nociceptive responses

Sensory, second order multisensory integration neurons (Basins) and a premotor neuron (DnB) have further direct synaptic connectivity to a set of projection neurons including A09e, A08n, and R61A01^GAL4 labeled neurons (labels: A10j, A09o, TePn04, TePn05) that have previously been implicated in anemotaxis, mechanosensory or chemosensory evoked behavioral responses (Hu et al., 2017; Jovanic et al., 2019; Kaneko et al., 2017; Masson et al., 2020; Vogelstein et al., 2014). Briefly, A09e receives synaptic inputs from CIII md, CIV md, A08n, DnB, and TePn05 (Figure 7A, Figure 7-figure supplement 1A). A09e and CIII md neurons are both required for Drosophila larval anemotaxis (Jovanic et al., 2019; Masson et al., 2020). A08n primarily receives inputs from CIV md neurons and very few inputs from CIII md neurons (Figure 7A, Figure 7-figure supplement 1A). However, Drosophila third instar larval synaptic connectivity visualization using GFP reconstituted across synaptic partners (GRASP) revealed that A08n are not synaptic partners of CIII md neurons (Kaneko et al., 2017). A08n neurons function downstream of CIV md neurons for noxious chemical and mechanical nociception (Hu et al., 2017; Kaneko et al., 2017) and neural activation of Ch and CIII md neurons does not lead to activation of A08n neurons (Tenedini et al., 2019). R61A01^GAL4 labeled neurons receive inputs from Ch, CIII md, CIV md, Basins, DnB, and A09e (Figure 7A, Figure 7-figure supplement 1A). Neurons labeled by R61A01^GAL4 have been implicated in mechanosensation (Masson et al., 2020). We assessed whether the following projection neurons A09e, A08n, and neurons in the expression pattern of the GAL4 line R61A01 (A10j, A09o, TePn04, TePn05) are required for cold evoked behaviors and function in conjunction with CIII md neurons for generating CT behavioral response.

Figure 7:

Neural silencing of projection neurons using cell specific expression of tetanus toxin led to impairments in cold evoked behaviors. Neurotransmission inhibition in projection neurons (A09e and R61A01) that receive strong connectivity from CIII md neurons led to significant reductions in all the CT behavioral metrics that we analyzed (Figure 7B-E). As expected, A08n neurons, which primarily receive inputs form CIV md neurons, are not required for cold-evoked CT response (Figure 7B-E). Whereas neural silencing of A09e or R61A01 neurons led to greater than 50% reduction in cold evoked responses from controls (Figure 7J).

Next, we assessed whether these projection neurons were able to elicit a CT response upon neural stimulation. Optogenetic stimulation of projection neurons alone did not lead to any CT response (Figure 2-figure supplement 2). Simultaneous co-activation of CIII md and R61A01^GAL4or A08n neurons led to mild but statistically insignificant increases in CT responses (Figure 7F-I, J). Co-activation of A09e and CIII md neurons led to facilitation of CT behavior, where compared to controls there were significant increases across all behavioral metrics that we analyzed (Figure 7F, G-I). There were mild, statistically insignificant, reductions in larval immobility when these projection neurons were co-activated with CIII md neurons, however, they were significant for A08n (Figure 7-figure supplement 1B-D). A09e, which has receives substantial inputs from CIII md neuron, was sufficient to facilitate CIII md neuron evoked behavioral responses (Figure 7J).

Our neural silencing analyses suggest that A09e and R61A01^GAL4(A10j, A09o, TePn04, TePn05) neurons are required for cold evoked responses. We hypothesized that these neurons are cold sensitive and function downstream of CIII md neurons. We predicted that these neurons will have increases in Ca²⁺ upon cold stimulation or optogenetic activation of CIII md neurons. A09e neurons present indeed significant increases in Ca²⁺ levels upon cold stimulation, as measured by cell type-specific expression of CaMPARI2 (Figure 8A). Optogenetic activation of CIII md neurons led to significant increases in evoked Ca²⁺ levels of A09e neurons (Figure 8B, C). Multiple stimulations of CIII md neurons did lead to slightly lower levels of evoked Ca²⁺ but overall Ca²⁺ response was largely similar between stimulations (Figure 8B, C). For R61A01^GAL4 Ca²⁺ imaging experiments, we restricted our analyses to TePn04 and TePn05 neurons, which were reliably identifiable in an intact Drosophila larval preparation. There were significant increases in CaMPARI2 response of TePn04/05 neurons upon cold stimulation (Figure 8E). Optogenetic stimulation of CIII md neurons led to strong increases in Ca²⁺ levels of TePn04/05 neurons, however, Ca²⁺ levels rapidly returned to baseline levels (Figure 8F, G). Repeated stimulations of CIII md neurons led to a blunted Ca²⁺ response in TePn04/05 neurons following the initial stimulation (Figure 8F, G). Combined, these data indicate A09e and R61A01^GAL4 neurons function downstream of CIII md neurons for cold nociception (Figure 8-figure supplement 1), indicating that ascending neurons relay cold somatosensation to the brain.

Figure 8:

Comparative analyses of cold sensitive neurons

Thus far, data were presented in logical groups based on their previously known cell-types and functions, however, these neurons function in an interconnected network and behavioral and functional imaging data must be assessed collectively to study circuit function. As discussed in previous sections, of all the cell types we tested only optogenetic activation of CIII md neurons alone is sufficient to elicit CT responses (Figure 2-figure supplement 2). Neural co-activation of CIII md neurons plus additional cell types resulted in marked increases in CT responses compared to only CIII md neuron activation (Figure 9A). Neural silencing of cell types downstream of CIII md neurons led to significant reductions in cold-evoked CT (Figure 9B). To understand how these behavioral phenotypes are interrelated, we performed t-distributed stochastic neighbor embedding (t-SNE) analysis on both neural silencing and co-activation behavioral datasets. We identified five different clusters that exhibit varying impacts on behavioral phenotypes upon either neural co-activation or silencing (Figure 9C, D). The first group clusters together with the control genotype and includes CIV, A05q, Goro, and A08n neurons. This group on average had less than 25% change in behavioral phenotypes in either neural co-activation or silencing experiments (Figure 9C, D). Basin-1 and DnB neurons clustered together, where on average they had 49% enhancement of CT response upon neural co-activation and 45% reduction in cold evoked CT response upon neural silencing (Figure 9C, D). R61A01 and CIII md neurons formed a cluster, where neural silencing resulted in nearly 60% reduction cold evoked behavior and 20% enhancement of CIII evoked behaviors upon neural co-activation (Figure 9C, D). A group of neurons including Chair-1, Basin-4 and Ch were required for cold evoked behavioral responses, however, these neurons did not facilitate CIII evoked CT responses (Figure 9C, D). In the last group, neural silencing of all Basins, Basin-2, mCSI, A00c or A09e led to an overall 69% reduction in cold evoked behaviors and 34% enhancement in CIII evoked CT response (Figure 9C, D).

Figure 9:

Analyses of Drosophila larval behavioral phenotypes revealed distinct roles for downstream neurons in cold nociceptive circuitry. Mapping synaptic connectivity of a particular circuit is only the first step in understanding how individual behaviors arise. Here, we draw attention to select CIII md neuron first-order neurons (Basins (−2 & -4), DnB, TePns and A09e) that showed robust requirements for cold nociceptive behaviors and had functional connectivity to CIII md neurons (Figure 9E). TePns synapse onto Basin-4 and form reciprocal connections to Basin-2, DnB, and A09e (Figure 9E). DnB further synapses onto Basin-4 and A09e. Within this circuit motif one might predict that A09e neurons function as the master integrators, where they might be computing synaptic information from various sources (Figure 9E). Consistent with EM connectivity, A09e neurons have the highest CIII md neuron evoked Ca²⁺ responses of the tested cell types (Figure 9F, G). Compared to Basin-4 neurons, Basin-2 neurons receive greater synaptic input from CIII md neurons (Figure 9E). However, CIII md neuron activation leads to nearly twice as much Ca²⁺ response in Basin-4 neurons than Basin-2 (Figure 9F, G). DnB neurons receive the second highest synaptic input from CIII md neuron and have the second highest CIII md neuron evoked Ca²⁺ response (Figure 9F, G). TePns receive significantly lower synaptic input compared to Basin-2 neurons, however, they both exhibit similar levels of CIII evoked Ca²⁺ response (Figure 9F, G). Comparative analyses reveal that synaptic connectivity is informative about functional neural activity, however, additional molecular and functional studies are required to fully understand roles of these neurons in cold nociception. Collectively, our findings on neural substrates of cold nociception indicate that second order multisensory integration by Basin neurons, select pre-motor neurons and projection neurons are preferentially activated in a stimulus specific manner to elicit appropriate behavioral responses.

Discussion

Environmental stimuli are detected by peripheral sensory neurons, which transduce relevant cues to downstream central circuitry responsible for processing multisensory input and generating stimulus-relevant behavioral responses. The study of the diverse molecular and cellular mechanisms involved in sensory discrimination have yielded key insights into how animals interact with their environment (Arnadóttir et al., 2011; Bandell & Patapoutian, 2009; Cho & Oh, 2013; Corfas & Vosshall, 2015; Coste et al., 2010; Derby et al., 2016; Dietrich et al., 2022; Fowler & Montell, 2013; Freeman & Dahanukar, 2015; Himmel et al., 2017; Leung & Montell, 2017; Montell, 2021; Wu et al., 2018; Xiao & Xu, 2021). However, how animals distinguish between innocuous and noxious stimuli at molecular, cellular and circuit level remains an important question in modern neuroscience (Bushnell et al., 1985; Dannhauser et al., 2020; Himmel et al., 2023; Imambocus et al., 2022; Moehring et al., 2018; Patel et al., 2022; Turner et al., 2016; Ward et al., 1996).

Here, we assessed the downstream circuitry of a multimodal sensory (CIII md) neuron that detects both innocuous mechanical and noxious cold temperatures. In CIII md neurons, detection of innocuous cues occurs via low threshold activation and noxious cues are detected via high threshold activation leading to stimulus-relevant behaviors (Turner et al., 2016). Transduction of nociceptive cold stimulus is predominantly mediated by multimodal (innocuous and noxious) sensitive Ch and CIII md neurons. However, nociceptive peripheral sensory neurons, CIII md and CIV md neurons, function through shared downstream neural circuitry. We describe the behavioral and functional requirements of multisensory integration (Basins) neurons, premotor (DnB and mCSI) neurons and projection (A09e and TePns) neurons in noxious cold evoked behavioral responses. We identified circuit components that play a role in amplifying noxious cues and differentiating between opposing noxious (heat versus cold) stimuli-evoked behaviors at various levels of sensory processing. Our findings provide key insights into how environmental cues are processed by multiplexed networks for multisensory integration and decision making.

Basin-1 neurons function across sensory modalities as gain modulators for noxious stimuli-evoked escape responses

Accurately responding to potentially harmful stimuli by appropriately executing energetically expensive escape behaviors is critical for animal survival. Incorrectly performing escape responses in absence of dangerous stimuli can have long-term detrimental consequences. Therefore the need for neural mechanisms responsible for integrating various noxious and innocuous sensory stimuli for accurately executing stimulus-appropriate escape responses. Here, we focus on the role of Basin neurons, known to mediate various escape responses (Jovanic et al., 2016; Masson et al., 2020; Ohyama et al., 2015). EM connectome analyses revealed that somatosensory (Ch, CIII md and CIV md) neurons all synapse onto multisensory integrating Basin neurons, and that amongst the Basins subtypes, Basin-1 neurons are presynaptic to Basin-2,-3 and -4 neurons (Masson et al., 2020; Ohyama et al., 2015). On the basis of the known connectome, we analyzed the function of Basin neurons in cold-evoked escape behaviors. Our findings indicate that optogenetic activation of Basin-1 is not sufficient to evoke noxious cold evoked behavioral response (Figure 2 – Supplement 2) and it was known that thermogenetic activation of Basin-1 is also not sufficient for evoked rolling escape (Ohyama et al., 2015). Therefore, Basin-1 neuron activity alone is not sufficient to elicit a noxious stimuli-evoked behavioral response. However, due to intricate connectivity amongst the Basins, the total integrated output of multiple Basin neurons may lead to threshold activation of noxious stimulus evoked escape responses. And indeed, thermogenetic co-activation assays found that Basin-1 neural activity could facilitate a Basin-4-mediated rolling response (Ohyama et al., 2015), and also the activation of Basin-1 neurons leads to the activation of Basin-2 neurons through lateral disinhibition (Jovanic et al., 2016).. Here, we found that co-activation of Basin-1 plus CIII md neurons led to strong evoked CT responses (Figure 2). Taken together, these studies concluded that Basin-1 facilitates the activation of other Basin neurons, and our results also provide additional support for the role of Basin-1 neurons in fine-tuning behavioral outcomes and enhancing cumulative output of multisensory integration Basin neurons leading stimulus specific escape responses.

Multisensory integration neurons drive behavioral selection downstream of sensory neurons

In order to evaluate the functional and behavioral roles of neurons within a circuit, a comprehensive analysis of both upstream and downstream synaptic connectivity is necessary. Basin-2 and -4 neurons integrate different amounts of synaptic input from somatosensory neurons, other Basin neurons, feedforward inhibitory local neurons, and projection neurons. Basin-4 neurons receive greater excitatory synaptic input compared to Basin-2 neurons, which have greater connectivity with local inhibitory neurons (Jovanic et al., 2016; Masson et al., 2020; Ohyama et al., 2015). Larger inhibitory connectivity of Basin-2 neurons likely arises from much broader downstream connectivity including several projection neurons, premotor neurons and feedback inhibitory local neurons, whereas Basin-4 downstream connectivity is restricted to limited set of neurons. These synaptic level differences in multisensory integration neurons underlie noxious stimuli-specific evoked responses, and suggest that CIII md neuron mediated responses primarily function through Basin-2 neurons and CIV md neuron mediated environmental cues are processed via Basin-4 neurons.

Chemical nociception in Drosophila larvae primarily functions via CIV md. Among the md neurons, CIII md neurons are the least sensitive to noxious chemical stimulus (Lopez-Bellido et al., 2019). Analyses of Basin neuron requirement in chemical nociception revealed that all Basins are necessary for chemical-evoked rolling, where comparative analyses of Basin subtypes revealed that chemical nociception is primarily mediated through Basin-4 with weaker behavioral phenotypes of Basin-1 or -2 (Lopez-Bellido et al., 2019). Our work shows that inhibition of neurotransmission in Basin-2 or Basin-4 neurons leads to significant reductions in cold nociceptive responses, where across the analyzed behavioral metrics Basin-2 had stronger deficits in cold evoked responses (Figure 2). Furthermore, neural co-activation analyses revealed that Basin-2 plus CIII md led to enhanced CT response, whereas Basin-4 plus CIII md co-activation did not facilitate but rather suppressed the CT response (Figure 2). Basin-2 neural activity likely has cascading effects on functions of downstream neurons, which we also found to be required for cold nociception. Behavioral assessment of multisensory integration neurons supports the notion that CIII md neuron sensory input is processed primarily by Basin-2 and CIV md neuron sensory cues are processed via Basin-4 neurons.

Despite the accumulating evidence that cold-evoked responses are primarily mediated by Basin-2, our Ca²⁺ analyses revealed that compared to Basin-2, Basin-4 neurons have much greater cold-or CIII md neuron-evoked Ca²⁺ responses. There are a few possible explanations for greater Basin-4 activity levels. Firstly, greater Basin-4 Ca⁺² activity could be due to greater synaptic input from projection neurons (TePn04/05) and premotor neurons (DnB), which are both required for cold nociception and exhibit cold-or CIII md neuron-evoked Ca²⁺ responses (Figure 7, Figure 8 – Supplement 1) (Masson et al., 2020). Secondly, reduced Basin-2 Ca²⁺ responses could be due to greater connectivity with feedback/feedforward inhibitory local neurons (Jovanic et al., 2016; Masson et al., 2020), therefore suggesting that Basin-2 could be mediating an onset response rather than a sustained response. Additional comparative Ca²⁺ imaging or electrophysiological analyses are required to further determine how Basin-2 and -4 neurons integrate sensory information across somatosensory modalities leading to distinct behavioral responses. However, to date, synaptic connectivity and behavioral analyses collectively indicate an initial sensory discrimination node amongst the Basin neurons, where downstream processing of CIII mediated sensory input is conveyed through Basin-2 and its downstream connectivity, meanwhile CIV somatosensory information is transduced through Basin-4 neuronal pathway.

Differential roles of pre-motor neurons lead to behavioral selection

Animal locomotive behavioral responses are implemented through neurosecretory systems and muscle contractions evoked by motor neuron activity. Motor neurons integrate a conglomerate of neural impulses from premotor neurons, leading to appropriate muscle activation patterns to implement specific behavioral responses (Bellardita & Kiehn, 2015; Berkowitz et al., 2010; Carreira-Rosario et al., 2018; Green & Soffe, 1996; Huang & Zarin, 2022; Liao & Fetcho, 2008; Talpalar et al., 2013; Zarin et al., 2019). Recent work in premotor and motor neuron connectomics has provided evidence for both labeled line and combinatorial connectivity between promotor and motor neurons that give rise to co-active motor neurons states leading to selective muscle group activation (Huang & Zarin, 2022; Zarin et al., 2019). Various Drosophila larval premotor neuron subtypes and/or motor pools have been implicated in locomotion, nociception, and innocuous mechanosensation (Burgos et al., 2018; Huang & Zarin, 2022; Jovanic et al., 2019; Kohsaka et al., 2017; Kohsaka et al., 2014; Kohsaka et al., 2019; Yoshino et al., 2017; Zarin et al., 2019). Neural reconstruction efforts in Drosophila larvae have revealed that somatosensory (CIII md and CIV md) neurons are not directly connected to motor neurons, but rather feed into polysynaptic pathways leading to motor neurons via premotor neurons (such as the DnB, mCSI, & Chair-1 neurons studied here) (Jovanic et al., 2019; Ohyama et al., 2015; Winding et al., 2022). Premotor neurons (DnB, mCSI, and Chair-1) that receive synaptic input from CIII md neurons are required for cold-evoked responses. However, only premotor neurons previously implicated in nociceptive escape responses, DnB and mCSI, can facilitate CIII evoked CT responses (Burgos et al., 2018; Lopez-Bellido et al., 2019; Yoshino et al., 2017). Meanwhile, Chair-1 neurons, which are required for innocuous mechanosensation, do not facilitate CIII md neuron mediated CT responses (Jovanic et al., 2019). We showed, with Ca²⁺ imaging, that Chair-1 neurons are not activated upon cold exposure. It remains unclear whether Chair-1 neuronal function is sufficient to drive specific sensory evoked behaviors or requires a combinatorial pre-motor neuronal activity for generating evoked behavioral responses. Given the data, we speculate that Chair-1 neurons likely gate innocuous-to-noxious stimulus evoked behaviors; however, additional functional studies are required to tease out how Chair-1 neurons function in behavioral selection.

Heterogeneity in thermosensory processing reflects variety of thermal stimuli

Animals detect changes in temperature through a variety of sensors that occupy and function in different spatiotemporal timescales. Thermosensory systems function to detect changes at the epidermal, brain and visceral levels, and these broad thermosensory inputs must be integrated to generate appropriate behavioral responses (Nakamura, 2018). The underlying neural circuitry for thermosensation differs based on body plan and type of thermosensation. In mice, cool and warm sensory signals, which are detected by TRP channels, are transmitted to the spinal dorsal horn, where distinct cool/warm pathways relay epidermal changes in temperature to thalamocortical regions for perception and discrimination (Nakamura, 2018). In Drosophila larvae and adults, thermotaxis occurs through temperature detection via dorsal organ ganglion (DOG) and antennal thermoreceptors, respectively (Frank et al., 2015; Gallio et al., 2011; Klein et al., 2015; Liu et al., 2003). Investigations of circuit bases of thermotaxis have revealed different strategies for larval and adult systems. Adult antennal thermoreceptors have distinct hot or cold sensors, which project to the proximal antennal lobe (PAL), where hot and cold information is segregated into discrete regions (Gallio et al., 2011; Macpherson et al., 2015). Thermal coding properties of thermosensory projections neurons (tPNs), whose dendrites are located in PAL, include both slow/fast adapting tPNs and broadly/narrowly tuned tPNs (Frank et al., 2017). Drosophila larval thermosensory DOG contains both warm- and cool-sensing cells that are connected to individual cool or warm projection neurons (Hernandez-Nunez et al., 2021). DOG thermosensory information is further integrated by integration projection neurons (iPNs), which receive inputs from both warm and cool projection neurons (Hernandez-Nunez et al., 2021). The Drosophila larval noxious thermosensory system has distinct hot (CIV md) or cold (CIII md) sensing neurons (Babcock et al., 2009; Tracey et al., 2003; Turner et al., 2016). Larval somatosensory thermal inputs mediated by CIII md neurons are heavily integrated by second order neurons (Basins, DnB, A09e, etc.), where population coding leads to cold-evoked behavioral response. Noxious cold stimulus is encoded downstream of CIII md neurons by ascending neurons (A09e and TePns) that allow for systems-level integration of multiple thermal inputs including those from cool/warm sensitive larval DOG neural circuits and cold/heat information from CIII/CIV md neurons, respectively. Thermal information from the larval DOG and somatosensory neurons converges onto the mushroom body, where cool or warm projection neurons downstream of DOG and feed-forward projection neurons downstream of ascending neurons (A09e) synapse onto the same mushroom body output neuron (Eichler et al., 2017; Hernandez-Nunez et al., 2021; Winding et al., 2023). Thus far, ascending pathways into the brain have been characterized for larval cold nociception, however, descending output neurons responsible for cold-evoked behaviors remains unidentified. Diversity in processing thermosensory stimuli reflects heterogeneity at molecular, cellular, and circuit levels that collectively function to maintain proper thermal homeostasis and ultimately behavioral action selection. The Drosophila larval thermosensory system is well-suited to dissect mechanisms underlying central nervous system integration of varying spatial and functional thermosensory information by utilizing various molecules, sensory systems, and interconnected circuits for appropriate behavioral responses.

Methods

Fly Strains

All Drosophila melanogaster strains used in this study are listed in (Table 1). All Drosophila reagents were maintained on standard cornmeal-molasses-agar diet in 12:12 hour light-dark cycle at ∼22°C. All experimental crosses were raised in 12:12 hour light-dark cycle at 29°C, unless otherwise stated. The following strains were a gift from Marta Zlatic: Basin-2 (SS00739^splitGAL4), Basin-4 (SS00740^splitGAL4), A09e (SS00878^splitGAL4), and Chair-1 (SS00911^splitGAL4). We used two separate CIII md neuron driver lines in this study:19-12^GAL4 and R83B04^lexA.

Table 1:

Molecular cloning and transgenic generation of CIII^lexA

GMR83B04^GAL4 was characterized as CIII md neuron driver using both optogenetics and visualization using membrane markers (Himmel et al., 2023; Patel et al., 2022). The R83B04 enhancer containing entry vector was a gift from FlyLight team at Janelia Research Campus, Ashburn, VA. We performed Gateway cloning to insert R83B04 enhancer upstream of lexA. LR reaction was performed using R83B04 containing entry vector and lexA containing (pBPLexA::p65Uw) destination vector (Addgene: 26231). Transgenic fly generation was conducted by GenetiVision. R83B04^lexAwas inserted in VK20 docking site using PhiC31 integrase mediated transformation.

EM connectomics

All data for synaptic connectivity, circuit diagrams and wire frame projections of neural cell types were extracted from Neurophyla LMB Cambridge (https://neurophyla.mrc-lmb.cam.ac.uk/). Relevant primary literature sources are cited within the main text. Sensory neurons from abdominal segments 1-4 were analyzed and for the remaining cell types, all reconstructed neurons were included (date accessed: March 04, 2022).

Cold Plate Assay

We assessed requirements of neurons downstream from peripheral sensory neurons in cold evoked responses using the cold plate assay (Patel & Cox, 2017; Patel et al., 2022; Turner et al., 2016). GAL4 drivers for select neuronal subtypes were used to express the light chain of tetanus toxin (UAS-TNT). We used two sets of controls: w1118, genetic background control, and Empty^GAL4, which contains a GAL4 construct but no regulatory promoter (GAL4 only control). Genetic crosses were raised on standard cornmeal-molasses-agar diet and at 29°C. To assess cold evoked behavioral responses of age matched Drosophila third instar larvae, we exposed the ventral surface to noxious cold (10°C) temperatures. Briefly, using a brush we remove third instar larvae from food and place them on wet Kimwipe. Food debris is removed passively by allowing the larvae to freely locomote on wet Kimwipe. We place 6-8 larvae on a thin black metal plate that is subsequently placed on a pre-chilled (10°C) Peltier plate TE technologies Peltier plate (CP-031, TC-48-20, RS-100-12). Larval responses are recorded from above using Nikon DSLR (D5300). Changes in larval surface area were extracted using FIJI and Noldus Ethovision XT (https://github.com/CoxLabGSU/CaMPARI-intensity-and-cold-plate-assay-analysis/branches) (Patel et al., 2022). Next, we isolated individual larva from each video, removed background and used Ethovision to measure larval surface area. Using custom built r scripts, we compiled data from each larva and each genotype. Utilizing r, we calculated percent change in larval surface area (Area change= (Area_N – Average_Area_baseline)/ Average_Area_baseline*100). From the percent change in area dataset, we report three behavioral metrics: Average change in area, which is average percent change in area for the stimulus duration. We defined cold evoked CT response as change in area of -10% or less for at least 0.5 consecutive seconds. CT duration, time spent at or below -10% change in area. Lastly, percent CT response, which is cumulative percent of animals that CT for at least 0.5 consecutive seconds.

Statistical analysis: We preformed following statistical tests for all cold plate assay data analysis. %CT response: Fisher’s exact with Benjamini-Hochberg for multiple comparison. We used r for performing comparisons of percent behavior response between genotypes, we used Benjamini-Hochberg multiple comparison correction. CT duration: Kruskal-Wallis with Benjamini, Krieger and Yekutieli for multiple comparisons. CT duration data are not normal. CT magnitude: One-way ANOVA with Holm-Šídák’s for multiple comparisons.

Neural activation via optogenetics

We performed two types of neural activations to assess sufficiency for CT response: single downstream neural activation and co-activation, where we simultaneously activated CIII md neurons and individual downstream neurons. For optogenetic experiments in Drosophila, a light sensitive co-factor all trans-retinal (ATR) is required. For all conditions, all adult animals in the genetic cross were placed in ATR (1500µM) supplemented food and subsequently F1 progeny were also raised in food containing ATR and raised in dark. For control condition, we used an Empty^GAL4 containing GAL4 construct but no regulatory promoter. Optogenetic experiments were conducted using a similar setup as previously described (Patel et al., 2022). Briefly, we used principles of dark field microscopy to enhance signal to noise ratio and capture high resolution larval videos. We created a custom dark field stage, where a Canon DSLR T3i camera captures video from above. Neural activation is performed by two blue led lights that are controlled remotely using the Noldus control box (Thorlabs: DC4100, DC4100-hub, and two M470L3-C4 led light. Noldus: mini-IO box). Larval behaviors are directly captured using Noldus Ethovision XT software, which also controls blue led activation. All optogenetic experiments were conducted in a dimly lit room. Third instar Drosophila larva were removed from food plug and placed onto wet Kimwipe, where larval locomotion allowed for passive removal of food debris. We lightly sprayed water onto a thin glass plate, then placed a single larva for optogenetic stimulation. The glass plate was manually moved on XY-axis to keep larva in the field of view. The following stimulus paradigm was used: 5 seconds of baseline (light off) and 5 seconds of neural activation (blue led lights on).

We performed video processing and behavioral analysis using FIJI and data compilation and analysis using r (https://github.com/CoxLabGSU/Drosophila_larval_optogenetic_analysis-area_and_mobility). In order to analyze Drosophila larval behavioral responses, we first automatically stabilized (XY axis) and then measured changes in larval surface area and mobility (described below).

Raw videos from Noldus Ethovision XT were uncompressed using video-to-video convertor (https://www.videotovideo.org/). The following steps were scripted in FIJI macro language for automatic video processing and data acquisition. For increasing processing speed, the uncompressed videos were automatically cropped to dimensions to contain all of the larva’s movement. Next, using a pre-determined threshold, we created a mask followed by background removal using erode, remove outlier and dilate functions. We then used the ‘Analyze Particles’ function to obtain XY coordinates of the larva in each frame. Larval movements were stabilized using XY coordinates and the ‘Translate’ function was used to create a highly stabilized video. Next, we measured larval surface area using automatic thresholding ‘Huang method’ and ‘Analyze Particles’ to obtain area. We define Drosophila larval mobility as changes in occupied pixels between two frames. We used our stabilized larval video to measure larval mobility, where larval peristaltic movements (XY displacement) are not captured. However, changes in occupied pixels resulting from turning and head sweeps are captured. Specifically, larval mobility was measured by subtracting thresholded larva in each frame from the previous frame (Raw mobility = Thresholded larva_{Frame N} - Thresholded larva_{Frame N-1}).

Data compilation and analysis was performed in r using custom scripts. Optogenetically evoked changes in behavior were analyzed independently for mobility and changes in area. For each larva, we calculated percent change in area (Area change= (Area_N – Average_Area_baseline)/ Average_Area_baseline*100). We measured CT duration as the amount of time the larva has -10% or lower change in area. We also measure CT magnitude by analyzing average change area for stimulus duration. We report percent instantaneous CT over time as percent of animals that are at or below -10% change in area. Additionally, we report peak %CT from percent instantaneous CT dataset for each genotype. For analysis of mobility, we calculated percent change in mobility (Mobility change= (Mobility_N – Average_ Mobility_baseline)/ Average_ Mobility_baseline*100). We report average percent change in mobility during stimulus for each genotype. We also plot percent immobility, which is calculated by the percent of animals with -25% or more reduction in mobility. Immobility duration is calculated based on amount of time individual animals spend at or below -25% mobility.

Statistical analysis: We preformed following statistical tests for all behavioral optogenetic data analysis. %CT peak response: Fisher’s exact with Benjamini-Hochberg for multiple comparison. We used r for performing comparisons of percent behavior response between genotypes, we used Benjamini-Hochberg multiple comparison correction. CT/immobility duration: Kruskal-Wallis with Benjamini, Krieger and Yekutieli for multiple comparisons. CT duration data are not normal. CT/immobility magnitude: One-way ANOVA with Holm-Šídák’s for multiple comparisons.

CaMPARI Imaging

Post-synaptic neuron CaMPARI2 imaging: For assessing cold-evoked Ca²⁺ responses of sensory neurons and CIII md neuron downstream neurons, we utilized CaMPARI2, which upon photoconverting light and high intracellular Ca²⁺ stably photoconverts fluorescence from green to red. We performed the cold plate assay as described above. For stimulus condition, the Peltier plate was set to noxious cold (6°C) temperature and for control condition, the Peltier plate was turned off (room temperature). We placed individual third instar larvae onto the Peltier plate and simultaneously exposed the animal to photoconverting light for 20 seconds. CaMPARI2 fluorescence was imaged in live, intact larvae via confocal microscopy. Zeiss LSM 780 Axio examiner microscope, Plan-Apochromat 20x objective, and excitation wavelengths of 561nm and 488nm were used to image larval ventral nerve cord or peripheral sensory neurons. We mounted live intact larva onto microscope slide and immobilized the larva by placing a coverslip, as previously described (Im et al., 2018; Patel & Cox, 2017; Patel et al., 2022; Turner et al., 2016). Three dimensional CaMPARI2 fluorescence in ventral nerve cord localized downstream neurons was imaged at 607.28µm x 607.28µm (XY resolution) and 2µm z-slices. Regions of interests were identified and area normalized fluorescence intensity for red & green signals were obtained via Imaris 9.5 software. CaMPARI responses are reported as ratio of F_red/F_green. Statistical comparisons: Parametric data – Welch’s t-test and non-parametric data – Mann-Whitney test.

PNS CAMPARI2 imaging: Sensory neuron CaMPARI2 responses were analyzed using custom FIJI macros that automatically detected cell bodies and sholl intensity analyses were performed using semi-automated custom FIJI macros (https://github.com/CoxLabGSU/CaMPARI-intensity-and-cold-plate-assay-analysis/branches) (Patel et al., 2022).

Cell body analysis – We created a set of three sequential macros that draw ROIs around the cell body, user verification of the ROIs and lastly quantification of fluorescence intensities. The first custom FIJI script generates maximum intensity projections of z-stacks, image masks were created by thresholding (Moments method) GFP signal, and next, background and dendritic branches removed using erode and dilate functions. At the end of background clearing only cell bodies remain, where Analyze particle function is used to draw ROIs around soma. The second FIJI script is used for manual verification of each ROI and manually redrawing any incorrect ROIs. Lastly, upon ROI verification, area normalized F_red and F_green intensities are quantified. As previously described (Fosque et al., 2015; Im et al., 2018; Patel & Cox, 2017; Patel et al., 2022; Turner et al., 2016), we report evoked photoconverted CaMPARI signal as F_red/F_green ratio. Statistical comparisons: Non-parametric data – Mann-Whitney test.

Sholl intensity analysis – Sholl intensity analysis was performed using a set of two custom FIJI scripts as previously described (Patel et al., 2022). Briefly, we first perform background clearing by manually thresholding the GFP signal and select all branches and soma of neuron of interest using the “Wand Tool” in FIJI and then a mask of neuron of interest is created. The second FIJI script draws five-pixel wide radial ROIs at a single pixel interval, here only the dendrites and soma from the neuron of interest are selected at each radial interval. After Sholl ROIs are drawn, area and area normalized F_red and F_green fluorescence intensities are extracted for each radial step away from the soma. Similar to CaMPARI cell body analysis, we CaMPARI2 signal as F_red/F_green ratios away from the soma.

Ventral nerve cord CaMPARI imaging: We utilized Pan-neural (R57C10^GAL4>CaMPARI) driver to visualize ventral nerve cord Ca²⁺ responses to various stimuli including innocuous touch, noxious heat (45°C) and noxious cold (6°C). Stimulus and photoconverting light were delivered for 20 seconds. Whole ventral nerve cord was imaged at 607.28µm x 607.28µm (XY resolution) and 2µm z-slices. The rest of the stimulus delivery and imaging was similar to previously described CaMPARI experiments. No statistical analyses were performed.

CIII activation and second order neuron GCaMP imaging

CIII md neuron-evoked responses in downstream neurons were evaluated by using optogenetics and GCaMP. We expressed lexAop-CsCrimson in CIII md neurons using R83B04^lexA and used downstream neuron specific GAL4 to drive expression of GCaMP6m. For experimental condition, all adult animals in the genetic cross were reared in ATR (1500µM) supplemented food and subsequently F1 progeny were also raised in food containing ATR. For control condition, adult flies and F1 progeny were raised in standard cornmeal-molasses-agar diet. Both control and experimental crosses were reared in 24hr dark. We mounted live intact third instar larva in between microscope slide and a coverslip. Larval ventral nerve cord and cells of interest were located using epifluorescence on Zeiss LSM 780 confocal microscope. Larval GCaMP6 responses were allowed to return to baseline for at least 2 minutes. Time-lapse acquisition was imaged at 250.06µm x 250.06µm x 307.2ms using 488nm laser wavelength. CIII md neural activation was performed using two oblique 617nm leds (Thorlabs M617F2 and M79L01) that were manually operated using Thorlabs led controller (Thorlabs DC4100 and DC4100-hub). We performed three sequential neural activations using the following paradigm was used: Baseline light off (30 seconds)-> neural activation (617nm for 15 seconds)-> light off (30 seconds)-> neural activation (617nm for 15 seconds)-> light off (30 seconds)-> neural activation (617nm for 15 seconds)-> light off (30 seconds). Time-lapse videos were stabilized using Stack reg – Rigid transformation in FIJI (Linkert et al., 2010; Schindelin et al., 2012; Thevenaz et al., 1998). Regions of interest were manually drawn and area normalized GCaMP fluorescence over time was exported. We report changes in GCaMP6m fluorescence as ΔF/F =(F-F_prestimulus)/ F_prestimulus*100 and max ΔF/F for each of the three neural activation epochs. Statistical comparisons: Parametric data – Welch’s t-test and non-parametric data – Mann-Whitney test.

Statistics and data visualization

Statistical analyses were performed using r (Fisher’s exact test) and GraphPad Prism. All graphical visualization of the data were created using Prism GraphPad. Details on specific statistical tests are listed in the respective methods section.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Author Contributions

Conceptualization: AAP and DNC. Methodology: AAP, and DNC. Cold-plate assays: AAP; Optogenetics, AAP; Calcium imaging: AAP. Statistics and other formal analyses: AAP. EM Connectome analysis: AAP and AC. Writing – Original Draft: AAP and DNC; Writing – Review & Editing: AAP, AC and DNC. Visualization: AAP. Supervision: DNC. Funding acquisition: DNC.

Funding

This work was supported by NIH R01 NS115209 (to DNC). AAP was supported by a Brains & Behavior Fellowship, a 2CI Neurogenomics Fellowship, and a Kenneth W. and Georganne F. Honeycutt Fellowship from Georgia State University. AC thanks the Wellcome Trust (award 205038/Z/16/Z and 205038/A/16/Z) and the HHMI Janelia Research Campus for funding.

Acknowledgements

We thank members of Cox Lab and Michael J. Galko (MD Anderson Cancer Center) for critical comments on the manuscript. We thank the Janelia Visiting Scientist program hosted by HHMI Janelia Research Campus for providing critical training in EM connectomics. We acknowledge the Imaging Core Facility of Georgia State University for training and instrument support associated with this work.