Opiates influence behavioral thermoregulation in the curly-tailed lizard, Leiocephalus carinatus

Low doses of the opioid agonist, morphine (1-3 mg kg-1), significantly increased the behaviorally selected body temperatures of the curly-tailed lizard, Leiocephalus carinatus. A higher dose of morphine (10 mg kg-1) resulted in an initial behavioral hyperthermia that was followed by a significant decrease in preferred temperatures and a subsequent behavioral hypothermia which declined to control levels within 3-4 hr. These effects could be blocked and reversed by the opiate antagonist, naloxone, with naloxone (1 and 10 mg kg-1) by itself causing significant dose-dependent decreases in preferred temperatures. These morphine and naloxone induced changes occurred regardless of initial body temperatures. These results suggest that opiates are involved in the control of behavioral thermoregulation in lizards.