Hypoxia-induced hypothermia mediated by noradrenaline and nitric oxide in the rostromedial preoptic area

Central nitric oxide (NO) has an important role in hypothermia induced by hypoxia as well as in that elicited by noradrenaline (NA) microinjected into the rostromedial preoptic area (POA) of the hypothalamus. Here, I tested the hypothesis that activation of adrenoceptors and NO in the rostromedial POA is involved in hypoxia-induced hypothermia in urethane-chloralose-anesthetized, neuromuscularly blocked, artificially ventilated rats. Hypoxic ventilation (10% O2-90% N2, 5 min) evoked an increase in the tail skin temperature and a decrease in the colonic temperature, though these changes occurred at 30 s to 7 min after returning the rats to ventilation with room air. These responses were eliminated by prior bilateral transection of the carotid sinus nerves, but not by bilateral cervical vagotomy, suggesting the involvement of activated carotid chemoreceptors in the hypoxic ventilation-induced hypothermia. Such responses were also greatly attenuated by the microinjection of an NO synthase (NOS) inhibitor, NG-monomethyl-L-arginine (L-NMMA, 25 nmol), but not by that of its inactive enantiomer, NG-monomethyl-D-arginine (D-NMMA, 25 nmol), into the NA-sensitive, hypothermia-inducing site in the rostromedial POA. Pretreatment with the α1-adrenoceptor blocker prazosin (50 pmol), but not vehicle saline, also greatly attenuated the hypoxic ventilation-induced heat loss responses. These results suggest that this hypoxia-induced hypothermia was mediated, at least in part, by activation of α1-adrenoceptors and NOS in the rostromedial POA.