Depression of learned thermoregulatory behavior by central injection of opioids in cats

Third cerebral ventricular administration of morphine (20 microgram), pentazocine (1 mg) or the synthetic opioid peptides D-Ala2-Met-enkephalinamide (25 microgram) and D-Ala2-D-Leu5-enkephalin (10 microgram) decreased heat-reinforcement activity and either reduced or did not increase heat-escape activity of cats trained to control thermal stimuli through behavior. If the behavioral changes had been coupled to the primary effects of these opioids on thermoregulation, one would have expected an inverse relationship between changes in heat-reinforcement and heat-escape activity. Motor incapacitation appeared not to have contributed to the reduction in responding except briefly after pentazocine injection. The opioids may have distracted the animals or otherwise reduced their reaction to thermal stimuli until body temperature increased to a potentially dangerous level.