Role of non-NMDA receptors in osmotic and glutamate stimulation of vasopressin release: effect of rapid receptor desensitization.

Previous studies demonstrated that the increase in vasopressin (VP) release and induction of VPmRNA content by osmotic stimulation was blocked by kynurenic acid, a non-specific antagonist of excitatory amino acid (EAA) receptors. In order to identify the type of EAA receptor involved, perifused explants of the hypothalamo-neurohypophyseal system (HNS) were exposed to a ramp increase in osmolality (40 mOsm over 6 h achieved by increasing NaCl) in the presence and absence of 10 microM 6,7-dinitroquinoxaline-2,3-dione (DNQX), an antagonist of non-n-methyl-d-aspartate (NMDA) excitatory amino acid receptors. Vasopressin release and VP mRNA content were significantly increased by exposure to the osmotic stimulus. 6,7-dinitroquinoxaline-2,3-dione inhibited osmotically stimulated VP release (F=16.65, P=0.0008) without significantly reducing basal release. It also prevented the osmotically stimulated increase in VP mRNA content (P <0.05). Although these results implicated glutamate, the primary endogenous ligand for EAA receptors, in the regulation of VP, exogenous glutamate was ineffective in stimulating VP release from HNS explants in either low-Mg2+ or Mg2+-replete medium. However, blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor desensitization with cyclothiazide (100 microM) caused a marked increase in VP release in response to 100 microM glutamate, and blockade of kainate receptor desensitization with concanavalin A resulted in a small, but significant increase in VP release in response to 1 mM glutamate. These results support a role for non-NMDA receptor activation in osmotic regulation of VP release.

N-methyl-D-aspartic acid stimulation of vasopressin release: role in osmotic regulation and modulation by gonadal steroids.

Previous experiments demonstrated that excitatory amino acids participate in the osmotic regulation of vasopressin secretion, but the specific involvement of N-methyl-D-aspartic acid (NMDA) receptors was not evaluated. This was demonstrated in the present studies. NMDA stimulated vasopressin release from perifused explants of the hypothalamo-neurohypophyseal system (HNS), and osmotic stimulation of vasopressin release was inhibited by MK-801 (10 microM) and AP5 (100 microM) NMDA receptor antagonists. The effective concentration of NMDA was dependent upon the Mg2+ concentration of the perifusate with stimulation observed at 1 microM NMDA in Mg2+-replete compared with 5 microM in low-Mg2+ medium. Previous experiments also demonstrated that estradiol and dihydrotestosterone (DHT) inhibited osmotically stimulated vasopressin secretion, and a nongenomic mechanism of action was suggested by the ability of steroids conjugated to bovine serum albumin to replicate the effect. Experiments were performed to explore the potential role of NMDA receptors in this mechanism. Estradiol (50 pg/ml) and DHT (3 ng/ml) inhibited NMDA stimulated vasopressin release in perifused HNS explants. These results suggest a role of NMDA receptors in the mediation of vasopressin secretion in osmotically stimulated release. Furthermore, estradiol and DHT may exert their inhibitory effect on osmotically stimulated vasopressin release via the NMDA receptor.