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 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.

Possible Biological Mechanisms Linking Mental Health and Heat—A Contemplative Review

“…These nuclei send serotonergic nerve fibers to many other brain areas, such as the olfactory bulb, hypothalamus, thalamus, hippocampus, amygdala, and cerebral cortex [ 45 ]. Since the 1960s, several studies have attempted to clarify the role of serotonin signaling in the POAH for body temperature with contradictory results i.e., [ 44 , 47 ]. In 2004, Ishiwata et al concluded that POAH may not be the main location for thermoregulatory signaling of serotonin, as external infusion of serotonin in the POAH did not affect body temperature, nor did heat exposure change the serotonin release in this brain area in their experiment [ 48 ].…”

Changes of body temperature and extracellular serotonin level in the preoptic area and anterior hypothalamus after thermal or serotonergic pharmacological stimulation of freely moving rats

“…However, there is contradiction that the same Tb change is not obtained in spite of having medicated PO/AH with the same 5-HT agents (Clark and Lipton, 1986). For example, although Feldberg and Myers (1963) reported Tb increase after 5-HT microinjection into the PO/AH, Cox et al (1983) reported Tb decrease after the same procedure. Thus, the exact role of 5-HT in the PO/AH in thermoregulation is not established yet due to conflicting results probably related to experimental techniques or conditions such as the use of anesthesia.…”

“…For example, Cox et al (1983) reported Tb decrease after 5-HT microinjection into the PO/AH. Clark and Lipton (1986) reviewed different results of Tb response after microinjection of 5-HT into the PO/AH and concluded that the role of 5-HT in the PO/AH in thermoregulation is obscure.…”

The Role of the Selective Serotonin Reuptake Inhibitor Fluoxetine in Temperature Regulation in Ovariectomized Rat Models

“…Blockade of 5-HT synthesis resulted in the inability to regulate CBT in rats [34]. Additionally, intrahypothalamic injection of 5-HT caused a marked decreased in CBT in rats [35]. However, another study found no direct correlation between changes in CBT and subsequent changes in 5-HT or levels of its metabolite, 5-HIAA, in the hypothalamus [36].…”