Effects of antipyretics on normothermic rabbits

The effects of antipyretics on the ear skin and rectal temperature (Te and Tre) were investigated in non-febrile rabbits placed in a cold environment or when they were cooled locally in the hypothalamus. Intravenous injections of sulpyrin (25-500 mg/kg) or sodium salicylate (25-100 mg/kg) produced the dose-related increase in Te and slight fall in Tre. Decrease in Te induced by hypothalamic cooling was also reversed by the intravenous injection of antipyretics. Intraventricular injections of sulpyrin and salicylate (0.5-5.0 mg/kg) also caused the vasodilatation of the pinna skin and the fall in Tre. Bilateral microinjections of sulpyrin were made into various areas of the brain stem in order to locate the site of action. The injection sites where the vasodilatatory effect was elicited were the anterior hypothalamus, ventromedial nucleus of hypothalamus and the midbrain reticular formation. The responses elicited from the ventromedial nucleus were usually accompanied by restless and violent struggling. Injection of antipyretics into the mammillary body had no effect on Te. Intrahypothalamic injection of procaine depressed the sulpyrin-induced vasodilatation. These findings suggest that action of antipyretics on the normal body temperature is mediated through anterior hypothalamus and midbrain reticular formation.