Responses of supraoptic neurones in the intact and deafferented rat hypothalamus to injections of hypertonic sodium chloride.

1. Recordings were made from a total of fifty-three neurones in the supraoptic nuclei of four groups of rats: intact rats, animals in which the hypothalamus had been partly denervated by anteriorly or posteriorly placed semicircular cuts, and rats with a totally deafferented hypothalamus. 2. When first encountered, cells from intact animals fired at a mean rate of 5.08 +/- 0.78 spikes/sec, those from posteriorly isolated hypothalami at 3.93 +/- 0.63 spikes/sec, those from the anteriorly isolated hypothalami at 2.05 +/- 0.83 spikes/sec, and those from totally isolated hypothalami at 0.99 +/- 0.46 spikes/sec. 3. When stimulated osmotically by an intraperitoneal injection of ml. 1.5 M-NaCl, eight out of eight cells in intact rats showed a significant increase in firing rate between 20 and 30 min after the injection. Six out of nine cells in posteriorly isolated hypothalami showed significant but smaller responses. No increase in firing rate could be detected in seven cells from totally isolated hypothalami or from eight cells in hypothalami partly isolated by anterior cuts. 4. The results imply that under the conditions of these experiments by the spontaneous activity of the supraoptic nucleus in intact animals was maintained by an extrahypothalamic excitatory input, that partial hypothalamic isolation reduced its intensity, possibly by unmasking an inhibitory input, and that total isolation reduced it to an even greater extent. Osmotic activation of supraoptic cells was only possible when the anterior connexions of the hypothalamus were intact. Thus the cerebral osmo-receptors for vasopressin release may be situated outside the supraoptic nuclei.