M-Like K+Currents in Type I Hair Cells and Calyx Afferent Endings of the Developing Rat Utricle

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Abstract
Type I vestibular hair cells have large K+currents that, like neuronal M currents, activate negative to resting potential and are modulatable. In rodents, these currents are acquired postnatally. In perforated-patch recordings from rat utricular hair cells, immature hair cells [younger than postnatal day 7 (P7)] had a steady-state K+conductance (g−30) with a half-activation voltage (V1/2) of −30 mV. The size and activation range did not change in maturing type II cells, but, by P16, type I cells had added a K conductance that was on average fourfold larger and activated much more negatively. This conductance may comprise two components:g−60(V1/2of −60 mV) andg−80(V1/2of −80 mV).g−80washed out during ruptured patch recordings and was blocked by a protein kinase inhibitor.M currents can include contributions from KCNQ and ether-a-go-go-related (erg) channels. KCNQ and erg channel blockers both affected the K+currents of type I cells, with KCNQ blockers being more potent at younger than P7 and erg blockers more potent at older than P16. Single-cell reverse transcription-PCR and immunocytochemistry showed expression of KCNQ and erg subunits. We propose that KCNQ channels contribute tog−30andg−60and erg subunits contribute tog−80.Type I hair cells are contacted by calyceal afferent endings. Recordings from dissociated calyces and afferent endings revealed large K+conductances, including a KCNQ conductance. Calyx endings were strongly labeled by KCNQ4 and erg1 antisera. Thus, both hair cells and calyx endings have large M-like K+conductances with the potential to control the gain of transmission.