Single-unit responses of serotonergic dorsal raphe nucleus neurons to environmental heating and pyrogen administration in freely moving cats

Single-unit activity of serotonergic neurons in the dorsal raphe nucleus was examined in response to environmental heating and pyrogen-induced fever in freely moving cats. In the heating study, ambient temperature was rapidly raised from a baseline of 25° to 43 ± 1°C and maintained at this level for 2 h. Cats displayed hyperthermia, intense panting, and signs of heat stress, however, the discharge rate of serotonergic neurons of the dorsal raphe nucleus was not significantly different from baseline at any time during heat exposure. Similarly, the activity of these neurons was not significantly altered at any time during an approximately 6-h long febrile response induced by the synthetic pyrogen muramyl dipeptide (50 μg/kg, i.v.). These results indicate that serotonergic unit activity in the dorsal raphe nucleus is not related to either the activation of behavioral or physiological mechanisms underlying heat defense, or to alterations in thermoregulatory mechanisms during the febrile response to exogenous pyrogen. Furthermore, because these neurons do not respond to an elevation in body temperature induced by either ambient heating or pyrogen, they do not appear to be directly temperature-sensitive. These results do not support a specific role for serotonergic neurons of the dorsal raphe nucleus in thermoregulation.