ACETYLCHOLINE-INDUCED POTASSIUM CURRENT OF GUINEA-PIG OUTER HAIR-CELLS - ITS DEPENDENCE ON A CALCIUM INFLUX THROUGH NICOTINIC-LIKE RECEPTORS

The cholinergic efferent inhibition of mammalian outer hair cells (OHC s) is mediated by a hyperpolarizing K+ current. We have made whole-cel l tight-seal recordings from single OHCs isolated from the guinea pig cochlea to characterize the mechanism by which acetylcholine (ACh) act ivates K+ channels. After ACh application, OHCs exhibited a biphasic r esponse: an early depolarizing current preceding the predominant hyper polarizing K+ current. The current-voltage (I-V) relationship of the A Ch-induced response displayed an N-shape, suggesting the involvement o f Ca2+ influx. When whole-cell recording was combined with confocal ca lcium imaging, we simultaneously observed the ACh-induced K+ current ( I-K(ACh)) and a Ca2+ response restricted to the synaptic area of the c ell. This I-K(ACh) could be prevented by loading OHCs with 10 mM of th e fast Ca2+ buffer bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic ac id (or BAPTA), therefore allowing the observation of the ACh-induced e arly current in isolation. This early current revealed nicotinic featu res because it activated with an intrinsic delay in the millisecond ra nge, reversed nearly in between potassium and sodium equilibrium poten tials, and was blocked by curare. However, it was strongly reduced in the absence of external Ca2+, and its I-V relationship displayed an un usual outward rectification at positive membrane potentials and an inw ard rectification below -60 mV. The results indicate that the choliner gic response of mammalian OHCs involves a ''nicotinic-like'' nonspecif ic cation channel through which Ca2+ enters and triggers activation of nearby Ca2+-dependent K+ channels.