Comparison of the thermoregulatory responses to intracerebroventricularly injected dopamine and noradrenaline in the sheep.

The thermoregulatory effects of dopamine (DA), given by intracerebroventricular (i.c.v.) injection to sheep, have been examined and compared with those of i.c.v. noradrenaline (NA). At ambient temperatures (Ta) of 20 degrees and 30 degrees C both DA (200 nmol . kg-1) and NA (100 nmol . kg-1) induced constriction of the ear vessels, a decrease in respiratory frequency and an increase in rectal temperature (Tr). At Ta of 10 degrees and 0 degrees C both substances caused a decrease in heat production and a fall in Tr. The DA receptor blocker spiroperone (30 nmol . kg-1, i.c.v.), which itself had a vasodilatatory effect at 20 degrees C Ta, blocked the peripheral vasoconstriction and slightly attenuated the rise in Tr normally caused by i.c.v. DA or NA at this Ta, but did not eliminate the suppression of respiratory frequency. During i.c.v. infusion, at 20 degrees C Ta, with the DA-beta-hydroxylase inhibitor FLA-63, the effect of i.c.v. DA on Tr was attenuated, while that of NA was enhanced. These results suggest that in sheep central thermoregulatory system there are DA receptors which stimulate the pathway that controls peripheral vasomotor tone. The inhibitory effect of NA and DA on heat production and evaporative heat loss is probably mediated by noradrenergic receptors, which can also be activated by DA both directly and after its conversion to NA.