Electrophysiologic evidence for neural connections between the paraventricular nucleus and neurons of the supraoptic nucleus in the rat.

These experiments were part of a study to examine the extrahypothalamic projections of the paraventricular nucleus in the rat. Thirty-nine cells, histologically identified as situated within the supraoptic nucleus, were recorded. Nine of those cells were classified as secreting vasopressin by their phasic patterns of electrical discharge. Six of the phasically firing cells were excited after electrical stimulation of the paraventricular nucleus and the other three were inhibited. Eight cells were antidromically identified as projecting to the paraventricular nucleus; most of these were not spontaneously active. Of the remaining 22 cells, the majority were spontaneously active, showing continuous patterns of firing, and most were inhibited after stimulation of the paraventricular nucleus. The data supply evidence for the presence of functional neural connections between the two major nuclei of the tuberohypophyseal neurosecretory system.