Projection of scrotal thermal afferents to the preoptic and hypothalamic neurons in rats.

Effects of thermal stimulation (18--41.5 degrees C) of the scrotum were observed on neurons in the preoptic area and hypothalamus (POHY) in anesthetized rats. Changes in firing rate of POHY neurons occurred, with three exceptions, within the scrotal temperature range of 30--41 degrees C. There was no dynamic response and little or no sign of rapid adaptation. Out of 46 warm-sensitive neurons, 34 increased and 9 decreased in firing rate with a rise in scrotal temperature. Out of 26 cold-sensitive neurons, 19 were excited and 4 were suppressed by scrotal cooling. The 3 exceptions were excited by scrotal cooling. The 3 exceptions were excited by scrotal warming above 36 degrees C, and were also excited by cooling below 30 degrees C. Out of 54 thermally-insensitive neurons, 16 were excited by scrotal warming 7 by cooling and 2 by warming and cooling. The scrotal temperature change needed to produce full change in POHY neuronal activity varied from less than 1 degree C to 7 degrees C, and was usually less than 4 degrees C. Thermal stimulation of either half of the scrotum had identical influence on the same POHY neuron.