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1. Intrahypothalamic injection of either dopamine (10 microgram) or apomorphine (10 microgram) in a dose volume of 1 microliter. caused an almost immediate rise in tail skin temperature and a concomitant fall in core temperature in the conscious rat maintained at an ambient temperature of 17 +/- 1 degrees C. 2. The location of the dopamine-sensitive site was defined more accurately by reducing the dose volume to 0.5 microliter. and injecting dopamine at different points throughout the preoptic and anterior hypothalamic region. 3. The largest mean fall in core temperature (1.13 +/- 0.22 degrees C) was obtained after injection into the preoptic region. Injections with their perimeters more than 0.4 mm rostral or caudal to this site were ineffective. 4. Rats placed 0.65 m below a 250 W infra-red lamp responded to the imposed heat load by vasodilation of the tail skin blood vessels, indicated by an increased tail skin temperature. 5. Bilateral, but not unilateral, injection of either pimozide (0.5 microgram) or haloperidol (2.5 microgram) into the preoptic region significantly reduced the increase in tail skin temperature so that the rats were less able to withstand the imposed heat load. 6. Three serial sections (0.5 mm thick) were prepared from the preoptic anterior hypothalamic region of the rat brain, one anterior, one posterior and one corresponding to the dopamine-sensitive site. 7. Tissue from the middle slice increased its rate of synthesis of 3,5-cyclic AMP in response to addition of dopamine 20 or 100 micron to the incubation medium. The posterior slice was inactive, but the anterior slice had similar activity to the middle slice. 8. The effect of dopamine on the middle slice was specifically blocked by haloperidol (0.1 micron), whereas the effects on the anterior slice were partially blocked by both haloperidol (0.1 micron) and propranolol (0.1 micron). 9. These results indicate that there is within a well defined area of the preoptic region a population of dopamine receptors, which play a part in the transmission of information from warm sensors to heat loss effectors.
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