Firing properties and dendrotoxin-sensitive sustained potassium current investibular nuclei neurons of the hatchling chick

To understand the emergence of excitability in vestibular nuclei neurons, w e performed patch-clamp recordings on brain slices to characterize the firi ng pattern on depolarization and the underlying currents in principal cells of the chick tangential nucleus. This study, on 0- to 3-day-old hatchlings , distinguishes electrophysiologically one main group of principal cells ba sed on their response to depolarizing current pulses (300-400 ms) in curren t-clamp recordings. This group (90%: n=29) displayed nonaccommodating, repe titive firing on depolarization. The remaining cells fired one action poten tial at the beginning of the current pulse and then accommodated. In voltag e-clamp recordings, a low-threshold, sustained, dendrotoxin-sensitive (DTX; 200 nM) potassium current, I-DS, was identified in both cell groups. In th e repetitively firing principal cells, the mean proportion of the DTX-sensi tive sustained current contributing to the total outward current was less t han 20%. This percentage is significantly less than that reported (45%) in a previous study performed in late chick embryos (E16), in which most of th e cells (83%; n=89) were accommodating neurons. Tonic firing is an importan t electrophysiological feature characterizing most mature, second-order ves tibular neurons, since it allows the neurons to process signals from behavi orally relevant inputs. Accordingly, this study contributes toward defining the emergence of the mature pattern of neuronal excitability and the ionic currents involved.