Pattern of REM sleep occurrence in continuous darkness following the exposure to low ambient temperature in the rat

The occurrence of REM sleep episodes, separated by intervals >3 min (single episodes) and ≤3 min (sequential episodes), was determined in the rat during the recovery (ambient temperature (Ta) 23°C, L period of the LD [12 h:12 h]-cycle), which followed the exposure to low Ta (0 and −10°C) during the D period of the previous LD-cycle, either in normal light (DL) or in continuous darkness (DD). Both exposures were characterized by an almost complete disappearance of REM sleep, whilst the recoveries showed an increase in the amount of REM sleep in the form of sequential episodes, which in DD was particularly prominent and concomitant with a decrease in the amount of REM sleep in the form of single episodes. The initial 2 h-rate of REM sleep occurrence was lower following the exposure to Ta −10°C, than to Ta 0°C. In DD, such an effect was due to the large reduction in the occurrence of sequential REM sleep episodes. A functional correlate of this finding is that the accumulation capacity of a second messenger (cAMP) was found to be lower at the end of the exposure to Ta −10°C, with respect to both the control (Ta 23°C) and the end of exposure to Ta 0°C, in the preoptic–anterior hypothalamus, but not in the cerebral cortex.