Activation and inhibition of tph2 serotonergic neurons operate in tandem to influence larval zebrafish preference for light over darkness.

Ruey-Kuang Cheng,Seetha Krishnan,Suresh Jesuthasan

doi:10.1038/srep20788

Abstract

Serotonergic neurons have been implicated in a broad range of processes, but the principles underlying their effects remain a puzzle. Here, we ask how these neurons influence the tendency of larval zebrafish to swim in the light and avoid regions of darkness. Pharmacological inhibition of serotonin synthesis reduces dark avoidance, indicating an involvement of this neuromodulator. Calcium imaging of tph2-expressing cells demonstrates that a rostral subset of dorsal raphe serotonergic neurons fire continuously while the animal is in darkness, but are inhibited in the light. Optogenetic manipulation of tph2 neurons by channelrhodopsin or halorhodopsin expression modifies preference, confirming a role for these neurons. In particular, these results suggest that fish prefer swimming in conditions that elicits lower activity in tph2 serotonergic neurons in the rostral raphe.

Highlights

Manipulations of serotonin signalling give conflicting results2,25 – e.g. an antagonist and agonist of 5HT1A both have the same effect in the light/dark assay25- the role of serotonergic neurons in controlling the preference for light versus darkness is unclear
There was no significant decrease in total distance traveled (Wilcoxon Rank Sum test, p = 0.346, Fig. 1e), a post-hoc visual analysis of swimming trajectory from video files indicated that pCPA-treated fish failed to turn away when they swim from the light towards darkness, in contrast to controls (Wilcoxon Rank Sum test, p = 0.034, Fig. 1f)
We have investigated the mechanism underlying the preference of larval zebrafish for light over darkness

Summary

Introduction

Manipulations of serotonin signalling give conflicting results2,25 – e.g. an antagonist and agonist of 5HT1A both have the same effect in the light/dark assay25- the role of serotonergic neurons in controlling the preference for light versus darkness is unclear. Treatment with pCPA, which inhibits serotonin synthesis, increased the time spent in the dark compartment (independent t-test, p = 0.006, Fig. 1d); this is again similar to the effect seen in adult zebrafish[25]. A subpopulation of tph[2] neurons in the dorsal raphe are inhibited by light and excited by darkness.

Results

Conclusion