Evolutionary changes of the efferent hair cell receptor

MOGLIE M; LIPOVSEK M; ELGOYHEN AB

Baltimore

Congreso; 35th Midwinter Meeting, Association for Research in Otolaryngology; 2013

Association for Research in Otolaryngology

Background The á9 and á10 nicotinic acetylcholine receptor (nAChR) subunits are expressed in cochlear hair cells, where they form the receptor that participates in the efferent control of auditory function. This nAChR mediates the inhibitory synapse between efferent fibers and outer hair cells in mammals, or short hair cells in birds. This cholinergic efferent feedback to hair cells is a common feature among all vertebrates. Accordingly, one would expect the evolutionary history of the genes coding for the á9 and á10 subunits to be similar throughout the whole Subphylum. However, a detailed analysis of the phylogeny and sequences showed signatures of positive selective pressure only for the mammalian á10 subunits. These differences in the evolutionary history correlate with the divergent biophysical properties of rat and chicken á9á10 nAChRs, such as calcium permeability. Here, we compared the pharmacology of chicken and rat receptors looking for a better understanding of the effects of the non-synonymous substitutions accumulated on mammalian á10 subunits. Methods Recombinant á9 and á10 subunit cRNAs were synthesized in vitro and injected in Xenopus laevis oocytes. Responses to drugs were measured using the two microelectrode voltage-clamp technique. Results The effects of the classical nicotinic agonists choline and DMPP and the antagonist serotonin on chicken receptors differed from those on rat á9á10 receptors and resembled those on rat á9. Above all, choline showed higher efficacy on chicken á9á10 receptors (88±8% max response to ACh, n=5) compared to rat á9á10 receptors (34±4% max response to ACh, n=4). Moreover, in a Ratá9Chická10 hybrid receptor, responses to choline were 86±4% (n=3) of the maximal response to ACh, resembling chicken receptors and strongly suggesting that the sites involved have been altered in mammalian á10 subunits. Conclusions The aminoacid changes that accumulated on mammalian á10 subunits resulted in a mammalian a9a10 nAChR with reduced efficacy to the neurotransmitter metabolite choline. We propose that differential sensitivity to choline might result in differences in efferent synaptic kinetics and strength across species.