Research ReportResponses of pigeon vestibular hair cells to cholinergic agonists and antagonists

Acetylcholine (ACh) is the major neurotransmitter released from vestibular efferent terminals onto hair cells and afferents. Previous studies indicate that the two classes of acetylcholine receptors, nicotinic (nAChRs) and muscarinic receptors (mAChRs), are expressed by vestibular hair cells (VHCs). To identify if both classes of receptors are present in VHCs, whole cell, voltage-clamp- and current-clamp-patch recordings were performed on isolated pigeon vestibular type I and type II HCs during the application of the cholinergic agonists, acetylcholine and carbachol, and the cholinergic antagonists, d-tubocurarine and atropine. By applying in different combinations, these compounds were used to selectively activate either nAChRs or mAChRs. The effects of nAChR and mAChR activation on HC currents and zero electrode current potential (Vz) were monitored. It was found that presumed mAChR activation decreased both inward and outward currents in both type I and type II HCs, resulting in either a depolarization or hyperpolarization. Conversely, nAChR activation mainly increased both inward and outward currents in type II HCs, resulting in a hyperpolarization of their Vz. nAChR activation also increased outward currents in type I HCs resulting in either a depolarization or hyperpolarization of their Vz. The decrease of inward and outward currents and the depolarization of the Vz in type I pigeon HCs by activation of mAChRs represents a new finding. Ion channel candidates in pigeon vestibular HCs that might underlie the modulation of the macroscopic ionic currents and Vz by different AChR activation are discussed.

Research Highlights► Pigeon type I and type II hair cells (HCs) have functional cholinergic receptors. ► Activation of these receptors modulates the current and Vzs in pigeon HCs. ► Modulations of the current and Vzs in pigeon type I HCs are previously unreported.