P 2.2. Neurokinin 3 receptor signaling in the release of vasopressin and oxytocin following osmotic and hypotensive challenges in rat

An increase in plasma osmolarity stimulates the release of vasopressin (VP) and oxytocin (OT), which act synergistically to restore bodily osmolarity. Tachykinin, neurokinin 3 receptors (NK3R) are heavily expressed by magnocellular neurons in the supraoptic and paraventricular nucleus of the hypothalamus (PVN). Exogenous injections of NK3R agonists stimulate VP and OT release into the circulation. The following studies determined if two physiological challenges that elicit VP and OT release activate and involve NK3R signaling. NK3 receptors are internalized to the cytoplasm following ligand binding. This trafficking can be followed immunohistochemically and is a pharmacologically specific marker of receptor activation. In Experiment 1, male rats were administered either isotonic saline or 2 M NaCl and sacrificed at varying times. The PVN region of the brains was double labeled for VP and NK3R immunohistochemistry and examined using confocal microscopy. The physiological role of NK3R activation in the release of VP and OT was next studied. In Experiment 2, male rats were administered an intraventricular pretreatment of saline or 250 pmol or 500 pmol SB222200, a selective NK3R antagonist, prior to an intravenous injection of 0.15 M or 2 M NaCl or hydralazine, which causes a fall in blood pressure. Blood samples were collected prior to and following the injections. NK3R immunohistochemistry showed that NK3R expressed by vasopressin magnocellular neurons are membrane-bound, but following intravenous injection of 2 M NaCl, NK3R were internalized within the cytoplasm of PVN VP neurons. This has now been confirmed by Western analysis. This result shows that osmotic challenge elicits the release of NK3R ligand, presumably NKB within the PVN. The functional role of endogenous NKB and NK3R activation was supported by the finding osmotic challenge and hypotension caused a significant elevation of both VP and OT, and pretreatment with SB222200 reduced osmotic-stimulated OT and VP levels by approximately 70-90%. Collectively the results indicate that hyperosmolarity causes the local release of an endogenous NK3R agonist, presumably NKB and that NOR signaling plays a significant role in the restoration of plasma osmolarity by stimulating the systemic release of both VP and OT.