Hormonal secretion patterns but not autonomic effector responses elicited by hypothalamic heating and cooling are altered in febrile rabbits.

The effects of hypothalamic heating and cooling on thermoregulatory effector activities, lipid and carbohydrate metabolism, insulin, glucagon, thyroxine, arginine vasopressin (AVP) and cortisol were investigated in conscious rabbits and compared with those obtained in the febrile state. The study shows that under control conditions hypothalamic heating lowers, and cooling raises core temperature. Core temperature always rose to similar degrees in response to bacterial lipopolysaccharide (LPS) during an observation time of 150 min, but it started to rise from lower and higher levels, respectively, during hypothalamic heating and cooling. The effects of hypothalamic thermal stimulation on specific thermoregulatory effector activities support the conclusion that, within 60 min after LPS, the hypothalamic warm signal input is reduced relative to the cold signal input. The increase of thyroxine levels following LPS suggests that the elevation of the thermoregulatory setpoint was caused by an increased input of hypothalamic TRH neurons, known to induce the full autonomic pattern of cold defense also in response to non-thermal stimuli. With the exception of an increase of glucagon during hypothalamic cooling at control conditions, hypothalamic thermal stimulation alone did not alter lipid and carbohydrate metabolism, insulin, thyroid hormone, AVP and cortisol secretion. A spontaneous heat loss effector response separated the first from the second fever phase 60 min after LPS. Subsequently AVP and cortisol plasma levels rose in febrile animals, irrespective of hypothalamic heating and cooling, presumably as a consequence of pyrogenic activation of corticotropin releasing factor (CRF) producing neurons and their reciprocal interaction with TRH neurons on the one hand, and by a reciprocal interaction of the latter with AVP neurons on the other.