Unravelling the molecular players at the cholinergic efferent synapse of the zebrafish lateral line

Abstract

AbstractThe lateral line (LL) is a sensory system that allows fish and amphibians to detect water currents. LL responsiveness to external stimuli is modulated by descending efferent neurons. LL efferent modulation aids the animal to distinguish between external and self-generated stimuli, maintaining sensitivity to relevant cues. One of the main components of the efferent system is cholinergic, the activation of which inhibits afferent activity. Since LL hair cells (HC) share structural, functional and molecular similarities with those of the cochlea, one could propose that the receptor at the LL efferent synapse is a α9α10 nicotinic cholinergic one (nAChR). However, the identity of the molecular players underlying acetylcholine (ACh)-mediated inhibition in the LL remain unknown. Surprisingly, through the analysis of single-cell expression data and in situ hybridization, we describe that α9, but not α10 subunits, are enriched in zebrafish HC. Moreover, the heterologous expression of zebrafish α9 subunits indicates that α9 homomeric receptors are functional and exhibit robust ACh-gated currents which are blocked by α-Bungarotoxin (α-Btx). In addition, in vivo Ca2+ imaging on mechanically-stimulated zebrafish LL HC showed that ACh elicits a decrease in evoked Ca2+ signals, irrespective of HC polarity. This effect was blocked by both α-Btx and apamin, indicating coupling of ACh mediated effects to SK potassium channels. Collectively, our results indicate that an α9-containing (α9*) nAChR operates at the zebrafish LL efferent synapse. Moreover, the activation of α9* nAChRs most likely leads to LL HC hyperpolarization served by the ACh-dependent activation of Ca2+-dependent SK potassium channels.Significance StatementFishes and amphibians have a mechanosensory system, the lateral line (LL), which serves to detect hydromechanical variations around the animal’s body. The LL receives descending efferent innervation from the brain that modulates its responsiveness to external stimuli. LL efferent inhibition is mediated by ACh, however the identity of the molecular players at the LL efferent synapse is unknown. Here we demonstrate that a nicotinic cholinergic receptor (nAChR) composed of α9 subunits operates at the LL efferent synapse. Activation of α9-containing (α9*) nAChRs leads to LL hair cell hyperpolarization. The inhibitory signature of this process is brought about by the subsequent activation of Ca2+-dependent potassium SK channels, functionally coupled to α9* nAChRs.