Activation of central warm-sensitive neurons and the tail vasomotor response in rats during brain and scrotal thermal stimulation.

The effects of preoptic and hypothalamic thermal stimulation on tail skin temperature were observed at different scrotal temperatures. The threshold hypothalamic temperature for tail vasodilation at a scrotal temperature of 40 degrees C was significantly lower than that at a scrotal temperature of either 25 degrees C or 33 degrees C. The effects of scrotal thermal stimulation on tail skin vasodilated by higher hypothalamic temperatures were observed. Cooling the scrotum from 42 to 30 degrees C invariably caused a rapid fall in tail temperature, whereas scrotal cooling from 30 to 25 degrees C did not cause any significant change. Cooling of either the left or right half of the scrotum caused a similar fall in tail temperature. The temperature characteristics of the preoptic hypothalamic thermo-sensitive neurons were determined at scrotal temperatures of 32, 36 and 26 degrees C. The firing rate of warm-sensitive neurons at a given hypothalamic temperature was highest at a scrotal temperature of 36 degrees C, while that of cold-sensitive neurons was lowest at that temperature. The scrotal temperature range over which the number of neurons activated by scrotal warming increased rapidly was between 36 and 39 degrees C when hypothalamic temperature was held at 36-37 degrees C.