Possible roles of prostaglandins in the anteroventral third ventricular region in the hyperosmolality-evoked vasopressin secretion of conscious rats

Abstract

This study explored the roles of prostaglandins in the anteroventral third ventricular region, a cerebral osmoreceptor site, in the osmoregulation mechanism of vasopressin release. We injected (1 μl) prostaglandin E₂ (12.8 nmol) or meclofenamate (78.3 nmol), an inhibitor of prostaglandin biosynthesis, into the brain region or the lateral cerebral ventricle of conscious rats, examining their effects on plasma vasopressin and its controlling factors in the presence or absence of an osmotic stimulus. The injection of prostaglandin E₂ into the anteroventral third ventricular region augmented plasma vasopressin and arterial pressure after 5 min and 15 min, without influencing plasma osmolality, sodium, potassium, or chloride. In contrast, intraventricular injection of prostaglandin E₂ did not cause any significant effect on those variables. The i.v. infusion (0.1 ml·kg⁻¹·min⁻¹) of hypertonic saline (2.5 mol/l) enhanced plasma vasopressin after 15 min and 30 min; this was accompanied by increased plasma osmolality, sodium, and chloride, and by unaltered or elevated arterial pressure. Meclofenamate given into the anteroventral third ventricular region 30 min before starting the hypertonic saline infusion abolished the osmotic vasopressin response without significantly changing the responses of the other variables. Histological analysis showed that the injection sites of meclofenamate in these rats were close to those of prostaglandin E₂ in the anteroventral third ventricular region and included the organum vasculosum of the lamina terminalis and the surrounding area, the medial preoptic area, and periventricular and median preoptic nuclei. When injection cannulae for meclofenamate deviated from those areas incidentally or when the drug was expressly administered into the cerebral ventricle, the osmotic vasopressin response was not inhibited. Plasma vasopressin and the other variables observed during the i.v. infusion of isotonic saline (0.15 mol/l) were not affected significantly by meclofenamate administration into the anteroventral third ventricular region or the cerebral ventricle. On the basis of these results, we concluded that prostaglandins synthesized in and/or near the anteroventral third ventricular region might contribute to the facilitation of vasopressin release in the hyperosmotic state.