Thermoresponsiveness of posterior hypothalamic (PH) neurons of rats to scrotal and abdominal thermal stimulation.

The thermoresponsiveness of posterior hypothalamic (PH) neurons to localized, incremental thermal heating and cooling between 10-40 degrees C of the abdomen or scrotum was determined in urethane anesthetized, male Sprague-Dawley rats whose core temperature was maintained at 37 degrees C during testing. PH extracellular neuronal activity was recorded along with changes in gastrocnemius muscle EMG activity and temperature (Tms, indicative of shivering thermogenesis) and intrascapular brown adipose tissue temperature (TIBATs, indicative of non-shivering thermogenesis). Seventy-five PH neurons were recorded following both scrotal and abdominal trials of thermal stimulation. Nine percent of PH neurons were classified as warm responsive neurons (WRNs), 20% as cold responsive (CRNs), and 71% as temperature nonresponsive neurons (TNRNs), based on their thermal coefficients (TCs). Mean TC for warm PH neurons was significantly increased with scrotal warming between 30-40 degrees C from the mean TC of the same PH WRNs following abdominal warming. Similarly, the thermal coefficient was increased (i.e., was more negative) for cold responsive PH neurons to scrotal cooling (20-10 degrees C) as opposed to the TC of the same PH CRNs for abdominal cooling. No shivering thermogenesis (no change in temperature or EMG activity from gastrocnemius muscle) or non-shivering thermogenesis (no significant increase in IBAT temperatures) occurred with scrotal or abdominal cooling in these 21 degrees C acclimatized rats. The results indicate that a small population of PH neurons are thermoresponsive to localized physiological changes in temperature of the scrotum and abdomen with greater thermoresponsiveness shown of both warm and cold PH neurons to scrotal vs. abdominal thermal stimulation.