Pharmacology of acetylcholine-mediated cell signaling in the lateral line organ following efferent stimulation.
Cholinergic efferent fibers modify hair cell responses to mechanical stimulation. It is hypothesized that calcium entering the hair cell through a nicotinic receptor activates a small-conductance (SK), calcium-activated potassium channel to hyperpolarize the hair cell. The calcium signal may be amplified by calcium-induced calcium release from the synaptic cisternae. Pharmacological tests of these ideas in the intact cochlea have been technically difficult because of the complex and fragile structure of the mammalian inner ear. We turned to the Xenopus laevis lateral line organ, whose simplicity and accessibility make it a model for understanding hair cell organ function in a relatively intact system. Drugs were applied to the inner surface of the skin while monitoring the effects of efferent stimulation on afferent fiber discharge rate. Efferent effects were blocked by antagonists of SK channels including apamin (EC50 = 0.5 microM) and dequalinium (EC50 = 12 microM). The effect of apamin was not enhanced by co-administration of phenylmethylsulfonyl fluoride, a proteolysis inhibitor. Efferent effects were attenuated by ryanodine, an agent that can interfere with calcium-induced calcium release, although relatively high (mM) concentrations of ryanodine were required. Fluorescent cationic styryl dyes, 4-di-2-asp and fm 1-43, blocked efferent effects, although it was not possible to observe specific entry of the dye into the base of hair cells. These pharmacological findings in the Xenopus lateral line organ support the hypothesis that effects of efferent stimulation are mediated by calcium entry through the nicotinic receptor via activation of SK channels and suggest the generality of this mechanism in meditating cholinergic efferent effects.
Dawkins
Rosie
R
Eaton-Peabody Lab., Massachusetts Eye and Ear Infirmary, 243 Charles St., Boston, MA 02114, USA.
Keller
Sarah L
SL
Sewell
William F
WF
D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D000891:Anti-Infective Agents, Local / Q000494:pharmacology; D001030:Apamin / Q000494:pharmacology; D001285:Atropine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D003868:Dequalinium / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005456:Fluorescent Dyes / Q000494:pharmacology; D006198:Hair Cells, Auditory / Q000187:drug effects* / Q000502:physiology; D066298:In Vitro Techniques; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009476:Neurons, Efferent / Q000187:drug effects* / Q000502:physiology; D010664:Phenylmethylsulfonyl Fluoride / Q000494:pharmacology; D011480:Protease Inhibitors / Q000494:pharmacology; D011726:Pyridinium Compounds / Q000494:pharmacology; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology; D009435:Synaptic Transmission / Q000187:drug effects* / Q000502:physiology; D013997:Time Factors; D014982:Xenopus laevis
D000891:Anti-Infective Agents, Local; C073804:FM1 43; D005456:Fluorescent Dyes; D018727:Muscarinic Antagonists; D011480:Protease Inhibitors; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; C052247:4-(4-diethylaminostyryl)-N-methylpyridinium; D012433:Ryanodine; D001030:Apamin; D010664:Phenylmethylsulfonyl Fluoride; D001285:Atropine; D003868:Dequalinium; D000109:Acetylcholine; D002118:Calcium