Low-voltage Activating K+ Channels in Cochlear Afferent Nerve Fiber Dendrites

Cochlear afferent nerve fibers (ANF) are the first neurons in the ascending auditory pathway. We investigated the low-voltage activating K+ channels expressed in ANF dendrites using isolated rat cochlear segments. Whole cell patch clamp recordings were made from the dendritic terminals of ANFs. Outward currents activating at membrane potentials as low as-64 mV were observed in all dendrites studied. These currents were inhibited by 4-aminopyridine (4-AP), a blocker known to preferentially inhibit low-voltage activating K+ currents (I-KL) in CNS auditory neurons and spiral ganglion neurons. When the dendritic I-KL was blocked by 4-AP, the EPSP decay time was significantly prolonged, suggesting that dendritic I-KL speeds up the decay of EPSPs and likely modulates action potentials of ANFs. To reveal molecular subtype of dendritic I-KL, alpha-dendrotoxin (alpha-DTX), a selective inhibitor for K(v)1.1, K(v)1.2, and K(v)1.6 containing channels, was tested. alpha-DTX inhibited 23 & PLUSMN;9% of dendritic I-KL. To identify the alpha-DTX-sensitive and alpha-DTX-insensitive components of I-KL, immunofluorescence labeling was performed. Strong K(v)1.1-and K(v)1.2-immunoreactivity was found at unmyelinated dendritic segments, nodes of Ranvier, and cell bodies of most ANFs. A small fraction of ANF dendrites showed K(v)7.2-immunoreactivity. These data suggest that dendritic I(KL )is conducted through K(v)1.1and K(v)1.2 channels, with a minor contribution from K(v)7.2 and other as yet unidentified channels.