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A Pharmacologically Distinct Nicotinic ACh Receptor Is Found in a Subset of Frog Semicircular Canal Hair Cells

Department of Pharmacology (SL83), Tulane University School of Medicine, New Orleans, Louisiana 70112

Submitted 12 April 2002; accepted in final form 28 May 2003

Frog vestibular organs are endowed with a prominent cholinergic efferent innervation whose stimulation results in several different effects, thereby suggesting diversity in the expression of postsynaptic acetylcholine (ACh) receptors. The application of ACh can mimic efferent stimulation in producing both an inhibition and a facilitation of afferent discharge which are thought to be mediated by at least two distinct ACh receptors present on vestibular hair cells, i.e., 9-containing nicotinic receptors (9nAChR) and muscarinic receptors (mAChR), respectively. Using patch-clamp and multiunit vestibular afferent recordings, we demonstrate the presence of an additional excitatory hair cell nicotinic ACh receptor pharmacologically distinct from both 9nAChR and mAChR. In order of increasing potency, this distinct receptor was activated by ACh, carbachol, and particularly by the selective nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP). This DMPP-sensitive nicotinic receptor (R_DMPP) was antagonized by the classic nicotinic antagonist d-tubocurarine, but refractory to strychnine, atropine, and propylbenzilylcholine mustard, at concentrations that completely block 9nAChR and/or mAChR. Activation of R_DMPP on application of ACh or DMPP to a subpopulation of isolated posterior semicircular canal (SCC) hair cells resulted in a large depolarization (18.0 ± 1.2 mV). The current underlying this depolarization was typically small (80.1 ± 21.6 pA) and showed an inward rectification starting around –45 mV. Given their respective EC₅₀s (47 nM vs. 20 µM), R_DMPP was nearly 400 times more sensitive to ACh than 9nAChR and thus responded to concentrations of ACh considered too low to be effective at stimulating 9nAChR. Despite this remarkable sensitivity, exogenous ACh readily stimulated the mAChR in the intact posterior SCC preparation but failed to activate R_DMPP unless the acetylcholinesterase inhibitor physostigmine was present, or high concentrations of ACh were used (>3 mM). In frog, R_DMPP most likely underlies the rapid excitatory response seen during efferent stimulation.

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