Serotonergic and noradrenergic mechanisms involved in the cardiovascular effects of angiotensin II injected into the anterior hypothalamic preoptic region of rats

Administration of angiotensin II (AII: 100–500 ng) into the anterior hypothalamic/preoptic region (AH/PO) of rats produced an increase in arterial blood pressure (BP) and heart rate (HR). Serotonin (5-HT: 0.5–1 μg) administered into the AH/PO region induced an increase in BP and no significant change in HR. Norepinephrine (NE: 1–2 μg) infused into the same region elicited a decrease of both BP and HR. Intraperitoneal administration of l-tryptophan ( l-try: 300 mg/kg) increased the forebrain (FB) and brain stem (BS) 5-HT levels (50 and 60% respectively) and potentiated the pressor effect of AII infused into the AH/PO region. Administration of 5,7-dihydroxytryptamine (5,7-DHT: 8 μg) into the medial forebrain bundle produced a 78% decrease in FB 5-HT content and abolished the pressor effect of intrahypothalamic infused AII. In this condition the polypeptide induced a significant dose-dependent decrease in both BP and HR. Injection of 6-hydroxydopamine (6-OHDA: 200 μg) into the third ventricle produced a decrease in both FB and BS norepinephrine levels (80 and 50% respectively) and blunted the presser effects of both AII and 5-HT administered into the AH/PO region. The results suggest that serotonergic mechanisms are involved in the presser action of AII into the AH/PO region and that the presence of noradrenergic mechanisms seems to be necessary in the presser effect of both angitensin II and serotonin.