Activity of supraoptic nucleus neurons of the hypothalamus

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Abstract
"Osmosensitive" neurons of the supraoptic nuclei of the hypothalamus were identified in cats by recording the increase in rate of discharge following intra-arterial injections of 1 cc. of 1 mol. NaCl solution. Most cells increased their rate of firing following stimulation of the reticular formation or the thalamus, but some 19-23% of the neurons showed an initial slowing followed, after 30 seconds, by a minor acceleration. Somewhat over 10% of the cells showed no significant response. All the osmosensitive cells studied accelerated their firing after hypothalamic stimulation. No relationship was found between firing rate and blood pressure change following or during stimulations of reticular formation, thalamus, or hypothalamus. Isolation of the supraoptic nucleus region from surrounding brain tissues surgically did not abolish spontaneous firing of neurons but actually accelerated the discharge rate of these osmosensitive cells in many instances. These osmosensitive neurons responded to intra-arterial injections of acetylcholine as well as hypertonic sodium chloride before and after isolation from surrounding brain tissues by an acceleration of firing. In numerous instances, these cells appeared to be hypersensitive to acetylcholine after isolation. Supraoptic nucleus cells respond to many more forms or sources of stimulation than do neurons of the paraventricular nuclei.