Tuberal supraoptic neurons-I. Morphological and electrophysiological characteristics observed with intracellular recording and biocytin filling in vitro

Previous studies of the tuberal, or retrochiasmatic, portion of the supraoptic nucleus suggest its functional similarity to the more densely populated anterior supraoptic nucleus, but the basic electrophysiological and morphological features of tuberal supraoptic nucleus neurons have not been described. Using the hypothalamo-neurohypophysial explant preparation in the rat, intracellular recordings and biocytin injections were made in tuberal supraoptic nucleus neurons and the results indicate that the two parts of the nucleus are similar. The generally oval-shaped somata of tuberal supraoptic nucleus neurons exhibited short, irregularly shaped appendages, and possessed 2-5 varicose, sparsely branching dendrites oriented in the horizontal plane. Many tuberal supraoptic nucleus neurons could be antidromically stimulated (mean latency = 6.4 ms). Filled neurons had varicose axons which were traced to the median eminence and even as far as the neural stalk, but which did not bifurcate. Both axons and dendrites were sparsely invested with short, hair-like appendages. The input resistance of the recorded neurons (mean = 177.7 MΩ) was positively correlated with the membrane time constant (mean = 13.1 ms; r = 0.83). Tuberal supraoptic nucleus neurons displayed a prominent afterhyperpolarization following individual spikes or bursts of spikes, as well as firing frequency adaptation in response to positive current pulses. Although numbering far fewer than those of the anterior supraoptic nucleus, tuberal supraoptic nucleus neurons have axons which are more often intact in this preparation, and a dendritic tree which radiates within the plane of the explant. Thus these neurons should provide a useful model for further study of the electrophysiological and morphological characteristics of mammalian neurosecretory neurons.