Formation of time memory for temperature stimuli in rats

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After daily temperature stimulus for 4 〜 5 hours limited to a fixed time, time memory for temperature stimuli can be formed in the thermoregulatory system of rats and humans and, according to the memory, thermoregulatory functions alter during the period when subjects were previously exposed to heat without actual temperature stress. We investigated the Fos protein expression in various nuclei in the brain in rats after subjection to daily timed temperature stress. Rats, kept in a 12 : 12 LD cycle, were exposed to heat or cold only in the last half of the dark phase of the day. Control rats were not exposed to temperature stress. After a 12-day temperature exposure schedule, they were anesthetized and their brains were removed in the first half (M), at the middle (N) or in the last half of the dark phase (A) of a day. The brain samples were then sliced and immunohistochemical staining for Fos protein was performed. In the control and cold-exposed rats, the number of Fos-positive cells in the preoptic anterior hypothalamus increased during the periods N and A compared to that of the period M, while it did not increase in the period A in the heat-exposed rats. The number of Fos-positive cells in the suprachiasmatic nucleus did not change in the dark phase in the controls, whereas it gradually decreased in the dark phase in the heat- and cold-exposed rats. A similar pattern of Fos expression was seen in the paraventricular nucleus. There were no differences in the Fos expression in the supraoptic, supramammilary and lateral mammilary nuclei among the groups. It seems that timed daily temperature stimuli alter the pattern of day-night variations of neuronal activities in the several brain nuclei in rats.