Genetically defined nucleus incertus neurons differ in connectivity and function

Reviewer #1 (Public Review):

Spikol et al. investigate the roles of two distinct populations of neurons in the nucleus incertus (NI). The authors established two new transgenic lines that label gsc2- and rln3a-expressing neurons. They show that the gsc2+ and rln3a+ NI neurons show divergent projection patterns and project to different parts of the interpeduncular nucleus (IPN), which receive inputs from habenula (Hb). Furthermore, calcium imaging shows that gsc2 neurons are activated by the optogenetic activation of the dorsal Hb-IPN and respond to aversive electric shock stimuli, while rln3a neurons are highly spontaneously active. The ablation of rln3a neurons, but not gsc2 neurons, alters locomotor activity of zebrafish larvae.

The strength of the paper is their genetic approach that enabled the authors to characterize many different features of the two genetically targeted populations in the NI. These two neuronal populations are anatomically closely apposed and would have been indistinguishable without their genetic tools. Their analyses provide valuable information on the diverse anatomical, physiological and behavioral functions of the different NI subtypes. On the other hand, these pieces of evidence are only loosely linked with each other to reach a mechanistic understanding of how the NI works in a circuit. For example, the anatomical study revealed the connections from the NI to the IPN, while the optogenetic mapping experiments investigate the other way around, i.e. the connection from the IPN to the NI.

Reviewer #2 (Public Review):

Summary:

Spikol et al performed a technical tour de force by combining numerous novel tools and approaches to investigate for the first time the connectivity and motor functions of nucleus incertus subset of neurons genetically defined by the expression of specific markers in the larval zebrafish brain.

Strengths:

By using expression of the specific markers relaxin 3 and gsc2, the authors generated novel knock-in transgenic lines enabling them to investigate the connectivity, recruitment and roles of these neurons in locomotion. Their work should enable numerous subsequent studies in zebrafish & inspire new paths of investigations in other animal models.

Weaknesses:

More precision is required for the anatomical data and further analysis is needed to describe the recruitment and role in spontaneous exploration of the rln3- and gsc2- expressing neurons.

Reviewer #3 (Public Review):

This study uses a range of methods to characterize heterogeneous neural populations within the nucleus incertus (NI). The authors focus on two major populations, expressing gsc2 and rln3a, and present solid evidence that these cells have different patterns of efferent and afferent connectivity, calcium activity and function in control of behavior. Although the study does not go as far as clarifying the role of NI in any specific neural computation or aspect of behavioral control, the findings will be valuable in support of future endeavors to do so. In particular, the authors have made two beautiful knock-in lines that recapitulate endogenous expression pattern of gsc2 and rln3a which will be a powerful tool to study the roles of the relevant NI cells. Experiments are well done and data are high quality and most claims are well supported. However, there are a few issues, detailed below, where I believe additional analysis could strengthen the paper.

• The data very clearly show different patterns of neurites for gsc2 and rln3a neurons in the IPN and the authors interpret these are being axonal arbors. However, can they rule out that some arbors might be dendritic in nature? Notably, they cite the recent Portugues lab study that confirmed that, as in other species, tegmental neurons in zebrafish extend spatially segregated axonal and dendritic arbors into IPN, and the authors speculate that these GABAergic cells might in fact be part of NI.

Author Response

We thank the reviewers for their rigorous and insightful comments, as well as their positive feedback on the manuscript. We agree with reviewer #1 that substantial additional work is needed for a complete mechanistic understanding of how NI circuitry works and we expect that the transgenic tools we generated will be valuable for such experiments. It is noteworthy that specific driver lines do not currently exist for IPN neurons, which limited our ability to perform optogenetic experiments activating the IPN to NI pathway. Reviewer 2 asks for additional clarification and analysis on various experiments, which we intend to address in a revised manuscript. We concur with reviewer #3 that, with the existing data, it is not possible to conclude with certainty that the IPN projections from gsc2 and rln3 NI neurons are solely axonal in nature. Additional experiments with axon- and dendrite- specific markers will be used to resolve this point in future work.