Possible contribution of dopaminergic receptors in the anteroventral third ventricular region to hyperosmolality-induced vasopressin secretion in conscious rats.

Abstract

We have reported previously that regions encompassing the cerebral ventricle may contain dopamine receptors responsible for facilitatory roles in the osmotic release of vasopressin in conscious rats. In order to explore the location of these receptors, we injected (0.5 mul) the dopamine antagonist haloperidol (13.3 nmol) or dopamine (26.4 nmol) topically into the anteroventral third ventricular region or the paraventricular nucleus of rats, and their effects on the levels of plasma vasopressin and its controlling factors were examined in the presence or absence of an osmotic stimulus. The effects of haloperidol injections into the ventral tegmental area were also tested to study whether information associated with drinking behavior may affect the osmotic vasopressin secretion. Intravenous infusion (0.1 ml kg-1 body wt min-1) of hypertonic saline (2.5 mol/l) enhanced plasma vasopressin 15 and 30 min later, and this was accompanied by an augmentation of plasma osmolality, sodium and chloride, and by elevated or unaltered arterial pressure. The vasopressin response was abolished by haloperidol injection into the anteroventral third ventricular region 10 min before the beginning of the hypertonic saline infusion. The injection sites were confirmed histologically to have been in or near the organum vasculosum of the laminae terminalis and a ventral part of the median preoptic nucleus. Similarly, a partial but significant reduction of the vasopressin response was noted after bilateral injections of haloperidol into the ventral tegmental area, whereas bilateral haloperidol injections into the paraventricular nucleus had no appreciable effect. The responses of plasma osmolality, electrolytes and arterial pressure to the osmotic load were not affected significantly by haloperidol injections into the anteroventral third ventricular region, ventral tegmental area or the paraventricular nucleus. The iv infusion of isotonic saline (0.15 mol/l) did not change plasma vasopressin and the other variables significantly, and this was also the case when preceded by application of haloperidol into the anteroventral third ventricular region, ventral tegmental area or the paraventricular nucleus. Dopamine injection into the anteroventral third ventricular region increased plasma vasopressin 5 min later, without affecting plasma osmolality, electrolytes or arterial pressure. On the basis of these results, we concluded that dopamine receptors responsible for facilitatory roles in osmotically stimulated vasopressin secretion may exist in the anteroventral third ventricular region and ventral tegmental area.