Fast cholinergic efferent inhibition in guinea pig outer hair cells.
Hair cells of inner ear are suggested to be inhibited by the activation of the alpha9-containing nicotinic acetylcholine (ACh) receptors (alpha9-containing nAChRs). Several studies have suggested that the native nicotinic-like ACh receptors (nAChRs) in hair cells display a significant permeability of Ca(2+) ions and unusual pharmacological properties. The activation of native nAChRs will initiate the hyperpolarization of hair cells by activation of the small conductance, Ca(2+)-activated K(+) channels (SK). In this work, the properties of the ACh-sensitive potassium current (IK(ACh)) in outer hair cells (OHCs) of guinea pigs were investigated by employing whole-cell patch-clamp. Followed by perfusion of ACh, OHCs displayed a rapid desensitized current with an N-shaped current-voltage curve (I-V) and a reversal potential of - 66 +/- 7 mV. The IK(ACh) was still present during perfusion of either iberiotoxin (IBTX, 200 nM) or TEA (5 mM) but was potently inhibited by apamin (1 muM), TEA (30 mM). The IK(ACh) demonstrated a strong sensitivity to alpha-bungarotoxin (alpha-BgTx), bicuculline and strychnine. These results suggested that OHCs display the well-known SK current, which might be gated by the alpha9-containing nAChRs. Two important changes were present after lowering the Ca(2+) concentration in the external conditions from 2 mM to 0.2 mM: one was a flattened N-shape I-V relationship with a maximum shifted toward hyperpolarized potentials from -20 approximately -30 mV approximately -40 to -50 mV, the other was a significant reduction in the agonist maximal response (percentage of maximal response 10.5 +/- 5.4). These results indicated that native nAChRs are both permeable to and modulated by extracellular Ca(2+) ions. Taken together, this work provides direct evidences that SK channels in OHCs of guinea pigs are gated by alpha9-containing nAChRs, which play an important role in the fast cholinergic efferent inhibition. This fast inhibition is both potently dependent on the permeability of Ca(2+) ions through the native nAChRs and modulated by Ca(2+) ions.
Kong
Wei-Jia
WJ
Department of Otolaryngology, Union Hospital of Tongji Medical College, Hua-Zhong University of Science and Technology, Wu Han, People's Republic of China. wjkong889@yahoo.com
Guo
Chang-Kai
CK
Zhang
Song
S
Zhang
Xiao-Wen
XW
Wang
Yan-Jun
YJ
Li
Zhi-Wang
ZW
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002038:Bungarotoxins / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D018678:Cholinergic Agents / Q000494:pharmacology; D002799:Cholinergic Fibers / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D004347:Drug Interactions; D004558:Electric Stimulation / Q000379:methods; D018684:Glycine Agents / Q000494:pharmacology; D006168:Guinea Pigs; D018072:Hair Cells, Auditory, Outer / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D009925:Organ of Corti / Q000166:cytology; D018408:Patch-Clamp Techniques / Q000379:methods; D010455:Peptides / Q000494:pharmacology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D013331:Strychnine / Q000494:pharmacology; D019789:Tetraethylammonium / Q000494:pharmacology
D002038:Bungarotoxins; D018678:Cholinergic Agents; D018684:Glycine Agents; D010455:Peptides; D026902:Potassium Channel Blockers; D001030:Apamin; D019789:Tetraethylammonium; C064719:iberiotoxin; D013331:Strychnine; D000109:Acetylcholine; D002118:Calcium