The expansion and pruning of ion channel families has played a crucial role in the evolution of nervous systems. Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels with distinct roles in synaptic transmission at the neuromuscular junction, the central and peripheral nervous system, and the inner ear. Remarkably, the complement of nAChR subunits has been highly conserved along vertebrate phylogeny. To ask whether the different subtypes of receptors underwent different evolutionary trajectories, we performed a comprehensive analysis of vertebrate nAChRs coding sequences, mouse single-cell expression patterns, and comparative functional properties of receptors from three representative tetrapod species. We found significant differences between hair cell and neuronal receptors that were most likely shaped by the differences in coexpression patterns and coassembly rules of component subunits. Thus, neuronal nAChRs showed high degree of coding sequence conservation, coupled to greater coexpression variance and conservation of functional properties across tetrapod clades. In contrast, hair cell α9α10 nAChRs exhibited greater sequence divergence, narrow coexpression pattern, and great variability of functional properties across species. These results point to differential substrates for random change within the family of gene paralogs that relate to the segregated roles of nAChRs in synaptic transmission.

The 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 Ca 2+ imaging on mechanically-stimulated zebrafish LL HC showed that ACh elicits a decrease in evoked Ca 2+ 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 Ca 2+ -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 .

The α9α10 nicotinic acetylcholine receptor (nAChR) plays a fundamental role in inner ear physiology. It mediates synaptic transmission between efferent olivocochlear fibers that descend from the brainstem and hair cells of the auditory sensory epithelium. The α9 and α10 subunits have undergone a distinct evolutionary history within the family of nAChRs. Predominantly in mammalian vertebrates, the α9α10 receptor has accumulated changes at the protein level that may ultimately relate to the evolutionary history of the mammalian hearing organ. In the present work, we investigated the responses of α9α10 nAChRs to choline, the metabolite of acetylcholine degradation at the synaptic cleft. Whereas choline is a full agonist of chicken α9α10 receptors it is a partial agonist of the rat receptor. Making use of the expression of α9α10 heterologous receptors, encompassing wild-type, heteromeric, homomeric, mutant, chimeric, and hybrid receptors, and in silico molecular docking, we establish that the mammalian (rat) α10 nAChR subunit underscores the reduced efficacy of choline. Moreover, we show that whereas the complementary face of the α10 subunit does not play an important role in the activation of the receptor by ACh, it is strictly required for choline responses. Thus, we propose that the evolutionary changes acquired in the mammalian α9α10 nAChR resulted in the loss of choline acting as a full agonist at the efferent synapse, without affecting the triggering of ACh responses. This may have accompanied the fine-tuning of hair cell post-synaptic responses to the high-frequency activity of efferent medial olivocochlear fibers that modulate the cochlear amplifier.

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.