Temperature regulation and dopaminergic systems in the brain: does the substantia nigra play a role?

Male Sprague-Dawley rats (250-300 g) were stereotaxically implanted above the substantia nigra (SN) and preoptic/anterior hypothalamus (PO/AH) with 23 gauge stainless-steel guide tubes. Microinjections of apomorphine (APO), pimozide, or 0.9% saline were made bilaterally in 0.5 or 1.0 microliter vols. using a Harvard infusion pump. Oxygen consumption, and colonic (Tc), tail-skin, and ambient temperatures were monitored each minute. Microinjections of APO into the SN produced a dose dependent hypothermia that was antagonized by the central (1.0 microgram) or systemic (0.5mg/kg i.p.) injection of pimozide. This hypothermia was associated with increased heat loss and decreased heat production and occurred at ambient temperatures of 15, 23 and 25 degree C indicating that APO did not produce a poikilothermic state. Injecting 20 microgram APO into the PO/AH or SN produced similar hypothermic responses; Tc fell 0.87 /+- 0.10 degree C and 1.02 /+- 0.08 degree C, respectively. Pimozide injected into the SN failed to alter thermoregulation during exercise or exogenous heating. Moreover, systemic injections of APO before and after electrolytic lesioning of the SN produced similar hypothermic responses. However, when the SN was lesioned, (a) resting Tc was significantly reduced, and (b) during exposure to a 35 degree C environment for 55 min, Tc rose to 39.5 /+- 0.68 degree C before the lesion compared with a rise to only 38.5 /+- 0.13 degree C after the lesion. We conclude that the pharmacological data implicate a thermoregulatory role for dopamine receptors in the rat, but the functional significance of this central location in temperature regulation remains to be elucidated.