Effects of apomorphine and pimozide on temperature regulation during exercise in the rat

The purpose of this study was to determine the effects of a specific dopamine receptor agonist (apomorphine) and antagonist (pimozide) on thermoregulation when the heat loss pathway was activated by the stress of exercise. Apomorphine or its control vehicle (0.9% wt/vol saline) was injected systemically (0.5, 1.0, 1.5 mg/kg ip) or within the preoptic-anterior hypothalamus (5, 10, 20 micrograms) immediately before the start of treadmill exercise at 21.5 m/min. Colonic, tailskin, and ambient temperatures were recorded each minute. Oxygen consumption was calculated from on-line measurements of percent O2 and CO2. Pimozide injected systemically (0.5 mg/kg ip) had no effect on resting colonic temperature, but caused a significant (P less than 0.05) hyperthermia during treadmill exercise compared to saline controls. Central and systemic injections of apomorphine caused a dose-dependent hypothermia that was blocked by pretreatment with pimozide. Oxygen uptake values during exercise following the central injection of apomorphine were virtually identical to those following the injection of saline, but colonic temperature was significantly (P less than 0.05) lower than saline controls, indicating that the hypothermia observed was not due to a reduction in metabolic rate. These data indicate that dopamine receptors in the preoptic-anterior hypothalamus of the rat participate in the mediation of heat dissipation when the animal is challenged with a heat stress.