Inhibition of nitric oxide synthase in the paraventricular nucleus prevents the hyperthermia-induced reduction of mesenteric blood flow in rats

Increasing body core temperature reflexly decreases mesenteric blood flow (MBF), and the hypothalamic paraventricular nucleus (PVN) plays an essential role in this response. Nitric oxide (NO) is involved in temperature regulation and is concentrated within the PVN. The present study investigated whether NO in the PVN contributes to the cardiovascular responses elicited by hyperthermia. Anesthetized rats were microinjected bilaterally in the PVN (100 nl/side) with saline or NG-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor (100 or 200 nmol/100 nl) (n = 5/group). Body core temperature was then elevated from 37°C to 39°C, and blood pressure (BP), heart rate (HR), MBF, and mesenteric vascular conductance (MVC) were monitored. In separate groups, L-NAME (200 nmol) (n = 5) or saline (n = 5) was microinjected in the PVN, but body core temperature was not elevated. In control rats, increasing body core temperature resulted in no marked change of BP but an increase in HR and significant decreases in MBF (15%) and MVC. Pretreatment with 100 nmol L-NAME did not affect the responses. In contrast, 200 nmol L-NAME prevented the normal reduction in MBF and MVC but did not significantly affect the BP and HR responses. In rats in which body core temperature was not increased, L-NAME reduced MBF by 19%. The present results suggest that endogenous NO in the PVN is important in mediating the reduction of MBF induced by hyperthermia. In the absence of hyperthermia, however, endogenous NO in the PVN may play a role in maintaining mesenteric vasodilation.