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

Open Access
Publisher Website
Google Scholar
Abstract
1. Intrahypothalamic injection of either 5‐hydroxytryptamine (5‐HT) (20 μg) or tryptamine (1 μg) caused hypothermia and hyperthermia respectively in lightly restrained rats maintained at an ambient temperature of 20 ± 1 °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 °C), intrahypothalamic injection of 5‐HT caused a marked fall in core temperature (‐1·3 °C) in rats maintained at 4 °C, but smaller responses were obtained at 20 and 29 °C (‐0·9 and ‐0·5 °C respectively). Tryptamine caused a significant hyperthermia in rats kept at 20 °C, but had no significant effect in rats maintained at either 4 or 29 °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.