Opposing effects of hypothalamic cooling on threshold and sensitivity of metabolic response to body cooling in rabbits.

1. Extrahypothalamic thermosensitivity at normal (39 degrees C) and lowered (33.8 degrees C) hypothalamic clamp temperatures has been measured in cold exposed rabbits equipped with chronically implanted hypothalamic perfusion thermodes. The animals were subjected to body core cooling by means of cold perfused thermodes placed in the colon. 2. At lowered hypothalamic temperature the threshold core temperature for increased metabolic heat production due to body core cooling was significantly increased as compared to that at normal hypothalamic temperature. However, the slope of the response relating increased metabolic rate to body core temperature was significantly reduced. 3. The experimental results compare well with those predicted from a mathematical model, which takes into account the effect of temperature on synaptic transmission of afferent information in the hypothalamus.