Spontaneous "phasic-firing' in supraoptic neurons recorded from hypothalamo-neurohypophysial explants in vitro

Single unit, extracellular electrical activity of 97 hypothalamic supraoptic neurons was recorded with micropipettes from rat hypothalamo-neurohypophysial explants maintained acutely in vitro. Spontaneously firing as well as quiescent neurons were encountered in roughly in roughly equal numbers. Of the spontaneously active neurons, 36.3% fired in a phasic mode quantitatively similar to that of "vasopressin-secreting' neurons recorded in vivo under conditions of enhanced hormone release. A small portion (approximately 14%) of the recorded supraoptic neurons had spikes which showed constant latency and high frequency following responses to stimulation of the neural stalk, responses characteristic of antidromically stimulated neurons. The mean latency and the range of latencies to these responses were similar to that recorded in vivo. Hypothalamo-neurohypophysial explants maintained in vitro thus provide a useful tool for investigating the electrical activity of supraoptic neurosecretory cells. Since the explant contains both the supraoptic neurons and their neurohypophysial projection path, this preparation offers an opportunity for characterizing further the relationship between electrical activity and hormone release.