5-hydroxytryptamine receptors in the hypothalamus mediate thermoregulatory responses in rabbits

Summary

1. The effects of microinjection of 5-hydroxytryptamine (5-HT) or its antagonists methysergide (a 5-HT₁ receptor antagonist), cyproheptadine (a mixed 5-HT₁/5-HT₂ receptor antagonist), or ketanserin (a 5-HT₂ receptor antagonist) into the preoptic anterior hypothalamus on thermoregulatory responses were assessed in conscious rabbits at different ambient temperatures (T _a). 2. Intrahypothalamic injection of 5-HT caused dose-dependent hypothermia in rabbits when the T _a was 2°C and 22°C. At 2°C the hypothermia was due to decreased metabolism, whereas at 22°C the hypothermia was due to increased peripheral blood flow and increased respiratory evaporative heat loss. 3. In contrast, administration of either cyproheptadine, methysergide or ketanserin into the 5-HT-sensitive sites in the preoptic anterior hypothalamus caused dose-dependent hyperthermia in rabbits when the T _a was 2°C, 22°C and 32°C. At 2°C the hyperthermia was due to increased metabolism, whereas at 32°C the hyperthermia was due to decreased peripheral blood flow and decreased respiratory evaporative heat loss. At 22°C, the hyperthermia was due to increased metabolism and decreased peripheral blood flow. 4. For a given intrahypothalamic dose (e.g.15–20 μg), either methysergide, cyproheptadine or ketanserin produced the same degree of rectal temperature elevation (e.g. about 1.4°C) in rabbits. Thus, there did not appear to be any association between hypothalamic 5-HT receptor types and thermoregulation. 5. However, the present results suggest that hypothalamic 5-HT receptors mediate thermoregulatory responses in the rabbit. Activation of hypothalamic 5-HT receptors decreases heat production and increases heat loss, whereas inhibition of hypothalamic 5-HT receptors increases heat production and decreases heat loss in the rabbit.