Contribution of N-methyl-d-aspartate receptors in the anteroventral third ventricular region to vasopressin secretion, but not to cardiovascular responses provoked by hyperosmolality and prostaglandin E2 in conscious rats.

The aim of this study is to pursue roles of N-methyl-d-aspartate (NMDA) receptors in the anteroventral third ventricular region (AV3V; a pivotal area for autonomic functions) in controlling vasopressin (AVP) release and cardiovascular system. In conscious rats, we examined effects of AV3V infusion of MK-801 (a selective antagonist for NMDA receptor) on plasma AVP, osmolality, electrolytes, arterial pressure and heart rate, in the absence or presence of NMDA, hyperosmotic or prostaglandin (PG) E2 stimulus. The AV3V infusion of NMDA caused significant increases in plasma AVP, osmolality and sodium, hematocrit, arterial pressure and heart rate after 5 or 15min. When NMDA was administered into the cerebral ventricle, relatively smaller elevations were observed only in plasma AVP and arterial pressure. The effects of AV3V infusion of NMDA were nearly completely prevented by MK-801 applied to the same region before 15min. The application of MK-801 was also potent to block rises of plasma AVP elicited by AV3V injection of PGE2 or i.v. infusion of hypertonic saline. However, it inhibited neither increases of arterial pressure and heart rate due to the PGE2 treatment nor those of arterial pressure, plasma osmolality and sodium in response to the osmotic load. Histological analysis on the AV3V infusion sites of NMDA, MK-801 and PGE2 indicated that they had been located in the structures such as the median and medial preoptic nuclei, periventricular nucleus and medial preoptic area. These results suggest that stimulation of AV3V NMDA receptors in the basal state may facilitate AVP secretion and cause pressor and tachycardiac actions, and that these receptors may be involved in both the hyperosmolality- and PGE2-induced hormone release, but not in the cardiovascular responses to these stimuli.