The Effect of an Osmotic Stimulus on the Release of Neurohypophysial Hormones in the Cat: Preferential Release of Vasopressin with a Possible Involvement of the Area Postrema

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Abstract
In cats anaesthetized with intravenous chloralose, the injection of 0.05 to 0.4 ml 1.54 M NaCl solution (hypertonic saline, HS) into a lateral cerebral ventricle caused a large release of vasopressin. The concentration of vasopressin greatly exceeded that of oxytocin in the same samples of plasma. Vasopressin was also released when HS was injected into the fourth ventricle and into the cisterna magna from which there is no access in the cat to the ventricles, but it was less effective by these routes than when injected into a lateral ventricle in the same cat. This suggests a possible action of HS on circumventricular organs related to the third ventricle but also indicates an additional site of action reached from the subarachnoid space which would give access to the ventral and dorsal surfaces of the brainstem. Vasopressin was not released on topical application of HS to the ''nicotine sensitive area'' on the ventral surface of the brainstem where nicotine acts to release vasopressin without oxytocin. Vasopressin, however, was released without detectable oxytocin on topical application of HS to the dorsal surface of the brainstem either outside the fourth ventricle or to the floor of the ventricle at its distal extremity, in the region of the obex. A possible site where HS acts to cause a preferential release of vasopressin on injection into a lateral ventricle is the area postrema, a circumventricular organ which impinges on the walls of the fourth ventricle at the obex. Preferential release of vasopressin might then be mediated by a selective neural input, possibly through the nucleus of the tractus solitarius, from osmoreceptors in the area postrema to the vasopressin-secreting cells in the supraoptic and paraventricular nuclei.