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A pharmacologically distinct nicotinic ACh receptor is found in a subset of frog semicircular canal hair cells. J Neurophysiol 90: 1526 -1536, 2003; 10.1152/jn.00273.2002. 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 Ϯ 1.2 mV). The current underlying this depolarization was typically small (80.1 Ϯ 21.6 pA) and showed an inward rectification starting around Ϫ45 mV. Given their respective EC 50 s (47 nM vs. 20 M), 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 (Ͼ3 mM). In frog, R DMPP most likely underlies the rapid excitatory response seen during efferent stimulation.

In the present work, we characterized the effects of serotonin type 3 receptor ligands on recombinant and native ␣9␣10-containing nicotinic acetylcholine receptors (nAChRs). Our results indicate that the recombinant ␣9␣10 nAChR shares striking pharmacological properties with 5-HT 3 ligand-gated ion channels. Thus, 5-HT 3 receptor antagonists block ACh-evoked currents in ␣9␣10-injected Xenopus laevis oocytes with a rank order of potency of tropisetron (IC 50 , 70.1 Ϯ 0.9 nM) Ͼ ondansetron (IC 50 , 0.6 Ϯ 0.1 M) ϭ MDL 72222 (IC 50 , 0.7 Ϯ 0.1 M).Although serotonin does not elicit responses in ␣9␣10-injected oocytes, it blocks recombinant ␣9␣10 receptors in a noncompetitive and voltage-dependent manner (IC 50 , 5.4 Ϯ 0.6 M).On the other hand, we demonstrate an in vivo correlate of these properties of the recombinant receptor, with those of the ␣9␣10-containing nAChR of frog saccular hair cells. The possibility that the biogenic amine serotonin might act as a neuromodulator of the cholinergic efferent transmission in the vestibular apparatus and in the organ of Corti is discussed.

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