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In the rat the exposure to an ambient temperature (Ta) of -10 degrees C induces an almost total REM sleep deprivation that results in a proportional rebound in the following recovery at normal laboratory Ta when the exposure lasts for 24 h, but in a rebound much lower than expected when the exposure lasts 48 h. The possibility that this may be related to plastic changes in the nervous structures involved in the control of thermoregulation and REM sleep has been investigated by measuring changes in the concentration of adenosine 3':5'-cyclic monophosphate (cAMP) and D-myo-inositol 1,4,5-trisphosphate (IP(3)) in the preoptic-anterior hypothalamic area (PO-AH), the ventromedial hypothalamic nucleus (VMH) and, as a control, the cerebral cortex (CC). Second messenger concentration was determined in animals either stimulated by being exposed to hypoxia, a depolarizing condition that induces maximal second messenger accumulation or unstimulated, at the end of a 24-h and a 48-h exposure to -10 degrees C and also between 4 h 15 min and 4 h 30 min into recovery (early recovery). At the end of both exposure conditions, cAMP concentration significantly decreased in PO-AH-VMH, but did not change in CC, whilst changes in IP(3) concentration were similar in all these regions. The low cAMP concentration in PO-AH-VMH was concomitant with a significantly low accumulation in hypoxia. The normal capacity of cAMP accumulation was only restored in the early recovery following 24 h of exposure, but not following 48 h of exposure, suggesting that this may be a biochemical equivalent of the REM sleep inhibition observed during 48 h of exposure and which is carried over to the recovery.

Serotonin [5-hydroxytryptamine (5-HT)] plays an inhibitory role in rapid-eye-movement (REM) sleep although the exact mechanism(s) and site(s) of action are not known. It is commonly assumed that 5-HT exerts its influence on REM sleep via input from the dorsal raphe nucleus (DRN) directly onto cholinergic neurons involved in the generation of REM sleep. 5-HT(2) receptor sites have been found on cholinergic neurons in the laterodorsal tegmental nucleus (LDT) and pedunculopontine tegmental nucleus (PPT). We locally microinjected the 5-HT(2) agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl) and the 5-HT(2) antagonist, ketanserin, in LDT in rats to determine whether these receptor sites are involved in the regulation of behavioral states. DOI and ketanserin primarily affected REM sleep, by significantly decreasing or increasing, respectively, the number, but not the duration, of REM sleep episodes. DOI specifically decreased the occurrence of clusters of REM sleep episodes appearing at intervals less than or equal to 3 min (sequential episodes) without affecting single episodes separated by more than 3 min. An opposite effect of ketanserin on REM sleep clusters, although not statistically significant, was observed.

The concentration of adenosine 3':5' cyclic monophosphate (cAMP) was determined in the anteroventro-medial hypothalamus, the cerebral cortex, the pineal gland and the interscapular brown adipose tissue, during the different stages of the wake-sleep cycle of rats kept, under a 12-12-h light-dark cycle, in different environmental conditions, i.e., control (47-52 h at ambient temperature (Ta) 23 +/- 0.5 degrees C), exposure (47-52 h at Ta 0 +/- 1 degree C) and recovery (1-4 h at Ta 23 degrees C after 48 h at Ta 0 degree C). The results show that cAMP concentration consistently changed: (1) during the wake-sleep cycle in the anteroventro-medial hypothalamus, decreasing from wakefulness to sleep; (2) during the dark-light transition in the pineal gland, increasing with the onset of the light phase; and (3) with the environmental condition in the interscapular brown adipose tissue increasing, with respect to the control condition, in exposure and recovery. No significant changes in cAMP concentration were observed in the cerebral cortex.

The concentration of adenosine 3',5'-cyclic monophosphate was measured, during the wake-sleep cycle, in the preoptic region and the cerebral cortex of rats kept in normal laboratory conditions (ambient temperature 22 +/- 0.5 degrees C, 12 h:12 h light-dark cycle) and, during wakefulness, in the preoptic region of rats exposed to extended light and dark periods (i.e. dark in the light hours of the normal photoperiod, and light in the dark hours of the normal photoperiod). The results show that the concentration of cAMP in the preoptic region changes according to the ultradian wake-sleep cyclic, decreasing from wakefulness, through synchronized sleep and to desynchronized sleep. This pattern of change was found to occur both in light and dark hours, however, in the dark hours the levels of preoptic cAMP are higher than those observed in the light hours. In contrast, no significant modification in cAMP concentration was found in the cerebral cortex. In the extended light and dark periods preoptic cAMP concentration increases above the levels found during wakefulness in normal photoperiods. These results show that preoptic cAMP concentration is influenced by ultradian and circadian factors which also appear to be related to sleep processes.