Maison SF, Pyott SJ, Meredith AL, Liberman MC. Olivocochlear suppression of outer hair cells in vivo: evidence for combined action of BK and SK2 channels throughout the cochlea. J Neurophysiol 109: 1525-1534, 2013. First published January 2, 2013; doi: 10.1152/jn.00924.2012.-Cholinergic inhibition of cochlear hair cells via olivocochlear (OC)-efferent feedback is mediated by Ca2+ entry through alpha(9)-/alpha(10)-nicotinic receptors, but the nature of the K+ channels activated by this Ca2+ entry has been debated (Yoshida N, Hequembourg SJ, Atencio CA, Rosowski JJ, Liberman MC. J Neurophysiol 85: 84-88, 2001). A recent in vitro study (Wersinger E, McLean WJ, Fuchs PA, Pyott SJ. PLoS One 5: e13836, 2010) suggests that small-conductance (SK2) channels mediate cholinergic effects in the apical turn, whereas large-conductance (BK) channels mediate basal turn effects. Here, we measure, as a function of cochlear frequency, the magnitude of BK and SK2 expression in outer hair cells and the strength of in vivo OC suppression in BK+/+ mice vs. BK-/- lacking the obligatory alpha-subunit (Meredith AL, Thorneloe KS, Werner ME, Nelson MT, Aldrich RW. J Biol Chem 279: 3674636752, 2004). Except at the extreme apical tip, we see immunostaining for both BK and SK2 in BK+/+. Correspondingly, at all testable frequencies (8-45 kHz), we see evidence for both SK2 and BK contributions to OC effects evoked by electrically stimulating the OC bundle: OC-mediated suppression was reduced, but not eliminated, at all frequencies in the BK-/- ears. The suppression remaining in BK nulls was blocked by strychnine, suggesting involvement of alpha(9)-/alpha(10) cholinergic receptors, coupled to activation of the remaining SK2 channels.
