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

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Abstract
A total of 53 neurons in the supraoptic nuclei of the following 4 groups of rats were recorded: intact rats, animals with a partly denervated hypothalamus by anteriorly or posteriorly placed semicircular cuts and rats with a totally deafferented hypothalamus. Cells from intact animals initially fired at a mean rate of 5.08 .+-. 0.78 spikes/s, cells from posteriorly isolated hypothalami at 3.93 .+-. 0.63 spikes/s, cells from the anteriorly isolated hypothalami at 2.05 .+-. 0.83 spikes/s and cells from totally isolated hypothalami at 0.99 .+-. 0.46 spikes/s. When stimulated osmotically by an i.p. injection of 1 ml 1.5 M NaCl, of 8 cells in intact rats showed a significant increase in firing rate between 20-30 min after the injection. Of 9 cells, 6 in posteriorly isolated hypothalami showed significant but smaller responses. No increase in firing rate was detected in 7 cells from totally isolated hypothalami or 8 cells in hypothalami partly isolated by anterior cuts. The spontaneous activity of the supraoptic nucleus in intact animals apparently was maintained by an extrahypothalamic excitatory input and partial hypothalamic isolation reduced its intensity, possibly by unmasking an inhibitory input. Total isolation reduced it to an even greater extent. Osmotic activation of supraoptic cells was only possible when the anterior connections of the hypothalamus were intact. The cerebral osmoreceptors for vasopressin release may be situated outside the supraoptic nuclei.