THE DIFFERENTIAL EXPRESSION OF LOW-THRESHOLD SUSTAINED POTASSIUM CURRENT CONTRIBUTES TO THE DISTINCT FIRING PATTERNS IN EMBRYONIC CENTRAL VESTIBULAR NEURONS

The principal cells of the chick tangential nucleus are second-order s ensory neurons that participate in the three-neuron vestibule-ocular a nd vestibulocollic reflexes. In postnatal animals, second-order vestib ular neurons fire repetitively on depolarization. Previous studies hav e shown that, although this is an important feature for normal reflex function, it is only acquired gradually during embryonic development. Whereas at 13 embryonic days (E13) the principal cells accommodate aft er firing a single spike, at E16 a few principal cells repetitively ca n fire multiple action potentials on depolarization, Finally, in the h atchling, the vast majority of principal cells is capable of nonaccomm odating firing on depolarization, As a first step in understanding the mechanisms underlying developmental change in excitability of these s econd-order vestibular neurons, we analyzed the outward potassium curr ents and their role in accommodation, using brainstem slices at E16. T he principal cells exhibited transient and sustained potassium current s, with both of these containing calcium-dependent components. Further , both high-and low-threshold sustained potassium currents have been d istinguished. The low-threshold dendrotoxin-sensitive sustained potass ium current (I-DS) is associated with principal cells that accommodate and is not expressed in those that fire repetitively. Finally, blocki ng of I-DS transforms accommodating cells into neurons capable of firi ng trains of action potentials on depolarization. These findings indic ate that suppression of I-DS during development is sufficient to trans form accommodating principal cells into nonaccommodating firing neuron s and suggests that developmental regulation of this current is necess ary for the establishment of normal vestibular function.