Interaction of apomorphine and stressors in the production of hyperthermia in the rabbit.

Apomorphine-induced hyperthermia in the rabbit was shown to be dependent upon the presence of stressors. Two types of stressors were used: 1) a physical stressor, foot shock; and 2) an emotional stressor, conditioned fear. Apomorphine (4 mg/kg i.v.), in the absence of a stressor, produced behavioral responses of arousal, stereotypic gnawing and hind foot thumping. Foot shock, which in control animals exerted no temperature effects, when paired with apomorphine administration, produced a hyperthermic response of 1.3 degrees C. This hyperthermia was abolished by pretreatment with diazepam (5 mg/kg i.v.) or haloperidol (50 micrograms/kg i.v.). Classical conditioning was used to provide the emotional stressor and was accomplished by pairing a tone with foot shock. After conditioning the tone combined with apomorphine produced an increase in temperature of 1.3 degrees C, whereas the tone paired with saline (1 ml/kg i.v.) produced no hyperthermia. An intensity vs. effect curve was determined, and foot shock alone elicited increases of temperature in a dose-dependent manner. At a given shock intensity, apomorphine elicited a much greater response than shock alone and also decreased the threshold for response to shock. Our findings firmly establish the hypothesis that apomorphine elicits hyperthermia in the presence of stressors by increasing sensitivity and decreasing the threshold for response to stressors.