Blood pressure and heart rate responses to microinjection of vasopressin into the nucleus tractus solitarius region of the rat.

The nucleus tractus solitarius (NTS) region in the rat has been shown to receive arginine vasopressin (AVP) and oxytocin (OT) neurophysin-containing neuronal projections from the suprachiasmatic (SNC) and paraventricular nucleus (PVN). Thus, vasopressin and oxytocin might have central influences on the circulation. This as investigated by measuring arterial blood pressure and heart rate (HR) responses following microinjection of vasopressin and oxytocin (0.1, 1.0 and 10.0 ng) into the right nucleus tractus solitarius region of rats anesthetized with urethane. Injections of vasopressin into nucleus tractus solitarius produced dose-related increases in blood pressure and heart rate. The effect of oxytocin on the blood pressure and heart rate was of a lesser magnitude without showing a dose-response relationship. Equivolumetric injections of vehicle and luteinizing hormone-releasing hormone (LH-RH) peptide had no detectable effect on blood pressure and minimal effect on heart rate. Injections of vasopressin into three different sites in the brain stem (1 mm anterior, posterior, and lateral to the tractus solitarius) did not produce significant hemodynamic changes. Intravenously injected vasopressin produced increments in blood pressure only at the highest dose level (10.0 ng) and a decrease rather than an increase in heart rate. Ganglionic blockade significantly reduced pressor responses to vasopressin injected into the nucleus tractus solitarius region and completely eliminated HR responses. Pretreatment of the nucleus tractus solitarius with a vasopressin antagonist abolished the blood pressure and heart rate responses produced by injection of vasopressin. These results suggest that vasopressin acts in the region of the nucleus tractus solitarius to exert a central action on the circulation.