Effects of intrahypothalamic and intraventricular norepinephrine and 5-hydroxytryptamine on hypothalamic blood flow in the conscious rabbit.

Hypothalamic blood flow (HBF) was measured in the conscious rabbit using a xenon clearance technique. The effects of intrahypothalamic norepinephrine (NE) and 5-hydroxytryptamine (5-HT) were determined by adding these compounds to the xenon-in-saline injectate, in varying concentrations. NE produced dose-dependent changes in flow; 1 /xg/ injection caused a mean increase in HBF of 55%, while doses of 10, 20, 40 and 200 fig/ injection reduced HBF by mean values of 26%, 17%, 20% and 29% respectively. The constrictor effect of NE 40 fig/injection was prevented by the addition of phenoxybenzamine 50 //.g/injection. Intrahypothalamic injections of 5-HT (20, 40 and 80 ^.g/injection) caused increases in mean HBF of 24%, 54% and 69% respectively. Injections of NE into the lateral cerebral ventricles caused an increase in HBF; 5-HT produced a variable response, but mean values for HBF tended to be lower after the injection. There were no changes in mean arterial blood pressure or arterial Pco2. It is concluded that NE and 5HT have an action on hypothalamic resistance vessels when applied from the adventitial side, and that, in the case of NE, the response is dose dependent, and the vasoconstrictor effects may be mediated by alpha receptors in hypothalamic vessels.