Bilateral equalization of synaptic output in olfactory glomeruli of Xenopus tadpoles

Reviewer #1 (Public review):

In this study, the authors investigate LFP responses to methionine in the olfactory system of the Xenopus tadpole. They show that this response is local to the glomerular layer, arises ipsilaterally, and is blocked by pharmacological blockade of AMPA and NMDA receptors, with little modulation during blockade of GABA-A receptors. They then show that this response is translently enlarged following transection of the contralateral olfactory nerve, but not the optic lobe nerve. Measurement of ROS- a marker of inflammation- was not affected by contralateral nerve transection, and LFP expansion was not affected by pharmacological blockade of ROS production. Imaging biased towards presynaptic terminals suggests that the enlargement of the LFP has a presynaptic component. A D2 antagonist increases the LFP size and variability in intact tadpoles, while a GABA-B antagonist does not. On this basis, the authors conclude that the increase driven by contralateral nerve transection is due to DA signaling.

Overall, I found the array of techniques and approaches applied in this study to be creatively and effectively employed. However, several of the conclusions made in the Discussion are too strong, given the evidence presented. For example, the authors state that "The observed potentiation was not related to inflammatory mediators associated to inury, because it was caused by a release of the inhibition made by D2 dopamine receptor present in OSN axon terminals." This statement is too strong - the authors have shown that D2 receptors are sufficient to cause an increase in LFP, but not that they are required for the potentiation evoked by nerve transection. The right experiment here would be to get rid of the D2 receptors prior to transection and show that the potentiation is now abolished. In addition, the authors have not shown any data localizing D2 receptors to OSN axon terminals.

Similarly, the authors state, "the onset of LFP changes detected in glomeruli is determined by glutamate release from OSNs." Again, the authors have shown that blockade of AMPA/NMDA receptors decreases the LFP, and that uncaging of glutamate can evoke small negative deflections, but not that the intact signal arises from glutamate release from OSNs. The conclusions about the in vivo contribution of this contralateral pathway are also rather speculative. Acute silencing of one hemisphere would likely provide more insight into the moment-to-moment contributions of bilateral signals to those recorded in one hemisphere.

Author response:

Thank you for your time and for considering our manuscript as a Reviewed Preprint. We also would like to thank Reviewer 1 for their evaluation of our manuscript.

Here, we present a provisional response to reviewer comments and following their suggestions we will make an effort to: i) increase evidence for the role of dopamine in olfactory glomeruli and ii) delineate the circuit involved mediating the observed potentiation. Next, we briefly describe the set of experiments that are in progress or will be performed to improve our paper.

We will carry out immunostainings for tyrosine hydroxylase to certify that dopamine can be released on the genetically labelled glomerulus. There is a lack of good commercial antibodies for Xenopus (we already tried one and did not work, PA1-4679, Thermofisher scientific), but we will look for alternatives. In a previous set of experiments, we attempted to measure dopamine release in the glomerular layer by electroporating olfactory sensory neurons or olfactory bulb neurons with the dopamine sensors dLight1.1 (Addgene #111053) or dLight1.3 (Addgene # 111056). In our hands, fluorescence signals were extremely weak, barely undetectable. Similar results were obtained after electroporating the tectum or the rhombencephalon. We propose to repeat experiments using a more sensitive sensor such as GRAB_DA2m. Other approaches, such as performing single cell transcriptomics of olfactory sensory neurons might be considered to confirm the expression of D2 receptors.

We agree with the reviewer that we should obtain more lines of evidence in support for a presynaptic inhibition mediated by D2 receptors.To gain insight on the bilateral circuit mediating the observed potentiation of glomerular responses we are currently investigating the role of dorsolateral pallium neurons. In Xenopus tadpoles the lateral pallium plays an analogous role to the olfactory cortex in amniotes. Preliminary observations show that neurons located in this pallial region respond to ipsilateral stimulation of the olfactory epithelium and if damaged, a contralateral potentiation of glomerular output occurs. We aim to conclude this set of experiments and include it in the paper as we believe it clarifies the circuitry involved.