ONTOGENY OF ELECTROPHYSIOLOGICAL PROPERTIES AND DENDRITIC PATTERN IN 2ND-ORDER CHICK VESTIBULAR NEURONS

The pattern of development of several subpopulations of second-order v estibular neurons was investigated by using intracellular recordings f rom chicken brain slices to define the timing of morphological and ele ctrophysiological changes occurring at 3 critical ages. Two embryonic stages, embryonic day 13 (E13) and E15-16, and also newborn chicks wer e selected according to previous anatomical findings showing the diffe rentiation of primary vestibular afferents and their synapses within a distinctive brainstem vestibular nucleus, the tangential nucleus. The responses of these cells to depolarizing and hyperpolarizing current pulses and their postsynaptic responses to vestibular nerve stimulatio n were recorded, while simultaneously biocytin was injected for subseq uent morphogenetic analysis. From this study, developmental schedules of membrane properties, synaptic responses, and dendritic differentiat ion were established for the 2 cell populations of the tangential nucl eus and other neurons located in the surrounding vestibular nuclei. Co mpared with all other second-order vestibular neurons, the principal c ells of the tangential nucleus exhibited a distinctive schedule. Mainl y, this includes their pattern of firing on depolarization, the shape and duration of the vestibular-evoked excitatory postsynaptic potentia l, and the time of onset of dendritic outgrowth. In regard to these ob servations, E15-16 appears to be a turning point in principal cell ont ogeny, whereas these features occur earlier in development for other s econd-order vestibular neurons. These findings, which indicate that th e principal cells may have distinct membrane properties at specific ag es, are discussed in light of their unique vestibular innervation and the possible consequences for vestibular signal processing. (C) 1997 W iley-Liss, Inc.