The expression of muscarinc acetylcholine receptors on the avian vestibular hair cells and the modulation effects on KIR2.1
Gang Q Li
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The major neurotransmitter released from the vestibular efferent terminals onto hair cells is acetylcholine (ACh). Two types of acetylcholine receptors (AChRs) have been suggested to be located on hair cells (HCs). They were nicotinic receptors (nAChRs) and muscarinic receptors (mAChRs). Even though pioneering physiological and pharmacological studies have shown evidence of mAChR located on the HCs, the protein expression and the role of muscarinic acetylcholine receptor (mAChR) in HCs have not been carefully studied. Our preliminary data indicated that the activation of mAChRs on pigeon vestibular type II HCs produced an inhibitory effect on an inward rectifier potassium channel, Kir2.1. Using immunohistochemical, immunocytochemical, and Western blot techniques, we demonstrated for the first time, the expression and co-expression of mAChR subtypes M1-M5 on the peripheral vestibular structures, including hair cells, supporting cells, ganglion, and other neural elements. To better understand the modulation effects of mAChR activation on Kir2.1 channels, the heterologous expression system (HES) was used to express a single mAChR subtype and Kir2.1. The expression plasmids for M3 or M5 mAChR subtype were co-transfected with pKir2.1 into the mammalian cell line tsA201 cells and electrophysiological studies including whole cell voltage clamp and current clamp were performed. Muscarinic receptor agonist (CCh) application to the tsA201 cells transfected with M3&pKir2.1 or M5&pKir2.1 exhibited concentration- and time- dependent facilitatory and/or inhibitory effects on the pKir2.1 channels. These effects depolarized or hyperpolarized the cell membrane potential, respectively. Further electrophysiological experiments were performed on native isolated\r\nvii\r\navian (pigeon) vestibular HCs. Whole cell patch clamp on both type I and type II HCs indicated that mAChR activation decreased both inward and outward current (type I & type II) which consequently produced hyperpolarization (type II) and depolarization (type I & type II). We also noticed that nAChR activation increased the outward current (type I & type II) and inward current (type II) which hyperpolarized the membrane potential (type I & type II) or depolarized the membrane potential (type II). In conclusion, mAChR played a role in the control of membrane potentials by modulating the ionic channels. The mechanism of mAChR induced effects is still unknown.