Evidence for a functional relationship between noradrenaline and neurohypophyseal peptides in the brainstem of rats

The possibility of a functional relationship between noradrenaline and neurohypophyseal peptides in the control of cardiovascular function in the nucleus tractus solitarius of rats has been investigated. The hypotensive response to microinjections of noradrenaline (20 nmol) was abolished by simultaneous (but not prior) microinjections of [Arg8]vasopressin (AVP) and oxytocin at doses (0.9 pmol) which alone had no effects on cardiovascular parameters. AVP plus noradrenaline actually resulted in a transient pressor effect. Simultaneous administration of [deamino-D-Arg8]vasopressin, a selective agonist of AVP V2 receptors, did not modify the effect of noradrenaline, whereas the specific V1 antagonist D-(CH2)5-Tyr(Me)-AVP partially decreased its intensity. When subthreshold doses of both AVP (0.9 pmol) and noradrenaline (10 pmol) were administered simultaneously, a pressor response was observed. In vasopressin-deficient Brattleboro rats, microinjections of 0.9 pmol AVP had no effects, but a marked pressor response was observed after the administration of a higher dose (9 pmol). In parent strain Long-Evans rats, noradrenaline (20 nmol) also produced a hypotensive response, but in Brattleboro rats microinjections of this amine elicited a marked pressor effect. In these rats, simultaneous administration of a subthreshold dose of AVP (0.9 pmol) reversed this response in such a way that a fall in blood pressure, similar to that observed in Long-Evans rats after injection of noradrenaline alone, was registered. These results provide evidence for a functional interaction between noradrenaline and neurohypophyseal peptides in the control of cardiovascular function in the brainstem.