A role for an indoleamine other than 5-hydroxytryptamine in the hypothalamic thermoregulatory pathways of the rat

Abstract

1. Intrahypothalamic injection of either 5-hydroxytryptamine (5-HT) (20 mug) or tryptamine (1 mug) caused hypothermia and hyperthermia respectively in lightly restrained rats maintained at an ambient temperature of 20 +/- 1 degrees C.2. Both the 5-HT- and the tryptamine-sensitive sites were located within the same region of the preoptic area.3. When rats were tested at different ambient temperatures (4, 20 and 29 degrees C), intrahypothalamic injection of 5-HT caused a marked fall in core temperature (-1.3 degrees C) in rats maintained at 4 degrees C, but smaller responses were obtained at 20 and 29 degrees C (-0.9 and -0.5 degrees C respectively). Tryptamine caused a significant hyperthermia in rats kept at 20 degrees C, but had no significant effect in rats maintained at either 4 or 29 degrees C.4. The hypothermic effect of 5-HT was selectively antagonized by systemic pre-treatment with cyproheptadine (2.5 mg/kg), but not by methergoline (0.625 mg/kg) and methysergide (0.2 mg/kg). In contrast, the hyperthermic effect of tryptamine was blocked by methergoline and methysergide, but not by cyproheptadine.5. Cyproheptadine (2.5 mg/kg) reduced the ability of rats to cope with a heat load but had no effect on the response to cold. In contrast, methergoline (0.625 mg/kg) and methysergide (0.2 mg/kg) reduced the ability to cope with cold but the rats' ability to cope with a heat load remained intact.6. These results suggest the existence of two indoleamine pathways within the preoptic anterior hypothalamus involved in the control of body temperature: a serotonergic pathway mediating heat loss and a non-serotonergic pathway mediating heat gain. The non-serotonergic system may exert its effects by modulating the activity of a central serotonergic system.