Paradoxical effects of exogenous norepinephrine on cold-induced thermogenesis in the rat

The effect of norepinephrine (NE, 250 microgram/kg IP) on thermoregulatory behavior of rats in a cold environment was tested. Norepinephrine produced an increase in operant responding for heat reward but a decrease in core temperature. Since animals compensate behaviorally for alterations in autonomic thermoregulation, this suggested that NE might inhibit cold-induced thermogenesis, an effect contrary to the expected action of this agent. A second experiment showed that NE increased oxygen consumption when rats were tested at 25 degrees C as expected, but decreased oxygen consumption when tested at 5 degrees C. The beta-adrenoceptor antagonist propranolol decreased oxygen consumption both at 25 and at 5 degrees C as expected. These results suggest that the thermogenic effect of NE is highly dependent on ambient temperature.