Vasopressin micro-injections into the nucleus tractus solitarii decrease heart rate and blood pressure in anaesthetized rats.

Bilateral micro-injections of arginine vasopressin into the nucleus tractus solitarii caused a dose-dependent (1-100 pg) decrease in blood pressure and heart rate in anaesthetized rats. Higher doses (1-20 ng) caused an increase in both parameters. Subcutaneous injection of an antagonist [d(CH2)5Tyr(Me)-arginine vasopressin] failed to block these effects. Micro-injections of the antagonist (0:1-100 pg) into the nucleus tractus solitarii decreased blood pressure and the heart rate similarly to arginine vasopressin. The effect of the antagonist disappeared after about 30 min. At that time administration of arginine vasopressin into the nucleus tractus solitarii was ineffective in inducing changes in blood pressure and the heart rate. When a low-dose (0.1 or 1 pg) antagonist was used the effect of 100 pg arginine vasopressin was partially inhibited. We conclude that administration of arginine vasopressin into the nucleus tractus solitarii is effective at low doses, and may be involved in inhibitory cardiovascular control at the level of nucleus tractus solitarii.