Intracerebroventricular injection of sympathomimetic drugs inhibits both heat production and heat loss mechanisms in the rat

Abstract

The effects of intracerebroventricular (i.c.v.) injections of sympathomimetic drugs on thermoregulatory functions in conscious rats maintained at low (8 decrees C), moderate (22 degrees C), and high (30 degrees C) ambient temperatures were assessed. Norepinephrine, tyramine, and ephedrine each produced hypothermia at ambient temperature (Ta) 8 degrees C and hyperthermia at Ta 22 and 30 degrees C. At Ta 8 degrees, the hypothermia in response to norepinephrine, tyramine, and ephedrine was due to decreased metabolic rate (M) whereas at Ta 22 degrees C the hyperthermia was due to cutaneous vasoconstriction. AT Ta 22 degrees C, the hyperthermia in response to norepinephrine and tyramine was due to cutaneous vasoconstriction whereas the hyperthermia in response to ephedrine was brought about by increased M (due to behavioral excitation). Intracerebroventricular injection of epinephrine produced hypothermia followed by hyperthermia at Ta 8 and 22 degrees C. The hypothermia was due to decreased M whereas the hyperthermia was due to cutaneous vasoconstriction and increased M. AT Ta 30 degrees C, epinephrine led to a reduction in cutaneous temperature and hyperthermia. Furthermore, i.c.v. administration of phenylephrine produced a decreased M and hypothermia Ta 8 degrees C and an increased M (due to behavioral excitation) and hyperthermia at Ta 30 degrees C. At Ta 22 degrees C, phenylephrine produced hyperthermia (due to cutaneous vasoconstriction and increased M) preceded by hypothermia (due to decreased M). Moreover, the temperature effects induced by norepinephrine were antagonized by pretreatment with the adrenoceptor antagonist phentolamine. In general, the data indicate that activation of central adrenoceptors with sympathomimetic drugs inhibits both heat production and heat loss mechanisms in the rat.