The principal cells of the chick tangential nucleus are vestibular nucleus neurons participating in the vestibuloocular and vestibulocollic reflexes. In birds and mammals, spontaneous and stimulusevoked firing of action potentials is essential for vestibular nucleus neurons to generate mature vestibular reflex activity. The emergence of spike-firing pattern and the underlying ion channels were studied in morphologically-identified principal cells using whole-cell patch-clamp recordings from brain slices of late-term embryos (embryonic day 16) and hatchling chickens (hatching day 1 and hatching day 5). Spontaneous spike activity emerged around the perinatal period, since at embryonic day 16 none of the principal cells generated spontaneous action potentials. However, at hatching day 1, 50% of the cells fired spontaneously (range, 3 to 32 spikes/s), which depended on synaptic transmission in most cells. By hatching day 5, 80% of the principal cells could fire action potentials spontaneously (range, 5 to 80 spikes/s), and this activity was independent of synaptic transmission and showed faster kinetics than at hatching day 1. Repetitive firing in response to depolarizing pulses appeared in the principal cells starting around embryonic day 16, when < 20% of the neurons fired repetitively. However, almost 90% of the principal cells exhibited repetitive firing on depolarization at hatching day 1, and 100% by hatching day 5. From embryonic day 16 to hatching day 5, the gain for evoked spike firing increased almost 10-fold. At hatching day 5, a persistent sodium channel was essential for the generation of spontaneous spike activity, while a small conductance, calciumdependent potassium current modulated both the spontaneous and evoked spike firing activity. Altogether, these in vitro studies showed that during the perinatal period, the principal cells switched from displaying no spontaneous spike activity at resting membrane potential and generating one spike on depolarization to the tonic firing of spontaneous and evoked action potentials.
Keywordsintrinsic membrane properties; spike activity; brain sliceThe vestibuloocular (VOR) and optokinetic (OKR) reflexes are involved in stabilizing visual gaze during head movements, whereas the vestibulocollic (VCR) reflex stabilizes the head position in space. To perform these functions, the VOR generates compensatory eye movements in a direction opposite to the head movements, while the OKR produces eye movements in the same direction as the image motion, and the VCR stimulates the muscles of the neck to support the head during movements. Defined as the eye speed divided by head speed, the VOR gain is an important property, which is subject to change during different types of movements. For example, during natural head rotations, the gain of compensatory eye movements is larger than 1, but when there is image motion during head turning, the gain is smaller than 1. Thus, the VOR is an adaptive response, rather than an automatic reflex (for . E-mail address: anakdp@gwumc....