Contribution of the suprachiasmatic nucleus to the formation of a time memory for heat exposure in rats.

We have reported that after rats were acclimated to heat for about 5 h daily at a fixed time, the pattern of day-night variations of core temperature (Tcor) altered, i.e., their Tcor fell, especially during the period when they had previously been exposed to heat. The suprachiasmatic nucleus (SCN) of the hypothalamus is known to be indispensable for the genesis of circadian rhythms of Tcor. We therefore investigated the involvement of the SCN in the characteristic fall in Tcor in heat-acclimated rats. The rats were exposed to an ambient temperature of 33°C only in the last half of the dark phase for 10 consecutive days. After the heat exposure schedule, the nocturnal pattern of Tcor variations and Fos expression in the dorsomedial SCN altered so that the Tcor and the number of Fos immunoreactive cells decreased in the last half of the dark phase. The bilateral lesions of the SCN of rats were made electrically, and the electrical lesions of the SCNs abolished the daily cycle of Tcor. In the SCN-lesioned rats, theTcor levels were significantly lowered after the 10-day heat exposure schedule. However, their Tcor did not specifically drop during the period when they had previously been exposed to heat. These findings suggest that the SCN is crucial for establishing a time memory for heat stress, and it plays a minimal role in heat acclimation-induced changes in Tcor in rats.