Thermoregulatory responses to intraventricular norepinephrine in normal and hypothalamic-damaged rats.

We examined the effects of intraventricular injection of low doses of norepinephrine (NE) on internal temperature and on behavioral and reflexive thermoregulatory responses in unrestrained rats. NE lowered the body temperature of the rats in cold and neutral environments but had little effect in the heat. The hypothermia was blocked by alpha-adrenergic antagonists but unchanged by beta-adrenergic antagonists. In the cold, the hypothermia was caused primarily by a lowered metabolic rate. At ambient temperature 25 degrees C, it was caused primarily by vasodilatation while metabolic rate increased. Thus, reflexive responses were not integrated to lower body temperature. Behavioral responding compensated for the hypothermia. In the cold, rats increased responding to get heat after NE. In a warm environment, they did not increase responding to escape heat. Thus, both reflexive and behavioral results support the idea that the set-point is unchanged after intraventricular NE. Wherever in the brain the NE may be acting, it is not in the preoptic-anterior hypothalamic area, because when this area was ablated, the hypothermia in response to intraventricular NE was greatly exaggerated.