Effects of Intrahypothalamic and Intraventricular Norepinephrine and 5-Hydroxytryptamine on Hypothalamic Blood Flow in the Conscious Rabbit

Abstract

Hypothalamic blood flow (HBF) was measured in the conscious rabbit using a ¹³³xenon clearance technique. The effects of intrahypothalamic norepinephrine (NE) and 5-hydroxytryptamine (5-HT) were determined by adding these compounds to the ¹³³xenon-in-saline injectate, in varying concentrations. NE produced dose-dependent changes in flow; 1 µg/m injection caused a mean increase in HBF of 55%, while doses of 10, 20, 40 and 200 µg/injection reduced HBF by mean values of 26%, 17%, 20% and 29% respectively. The constrictor effect of NE 40 µg/injection was prevented by the addition of phenoxybenzamine 50 µg/injection. Intrahypothalamic injections of 5-HT (20, 40 and 80 µg/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 5- HT 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.