RELATION BETWEEN THE THERMOREGULATORY EFFECTS OF INTRACEREBROVENTRICULARLY INJECTED DOPAMINE AND 5-HYDROXYTRYPTAMINE IN THE RABBIT

The thermoregulatory effects of dopamine (DA, 400 μg), injected intracerebroventricularly (i.c.v.) in rabbits, were investigated at different ambient temperatures (Ta). The effects at 20° C Ta were compared with those of 5-hydroxytryptamine (5-HT, 200 μg) and noradrenaline (NA, 200 μg). At 20° C Ta DA induced a dilatation of the ear vessels but did not change rectal temperature (Tr). I.c.v. injection of 5-HT induced panting and fall in Tr while after NA a rise in Tr and constriction of the ear vessels were observed. At 32° C Ta no change in Tr was observed after DA. At 5° C Ta DA caused a short-lasting decrease in Tr. After treatment with the DA antagonist haloperidol (400 μg, i.c.v.) a hypothermia was observed after DA at 20° C Ta. Haloperidol did not influence the response after 5-HT or NA. The 5-HT antagonist methysergide (500 μg, i.c.v.) blocked panting and the hypothermia normally caused by i.c.v. 5-HT at 20° C Ta; instead of decreasing, Tr tended to increase after 5-HT. After pretreatment with methysergide Tr rose immediately after i.c.v. injection of DA. These results suggest that the hypothermia after i.c.v. DA in the rabbit is secondarily mediated by serotonergic structures in the pathway between heat sensors and heat loss effectors. The hyperthermic effect of i.c.v. DA, which showed after pretreatment with methysergide and was apparently blocked by haloperidol, is probably the result of an activation of DA receptors in the pathway from cold sensors to heat production effectors. The hyperthermia after i.c.v. NA could be due to an inhibition of the heat loss pathway. © 1979 Springer-Verlag.