REM sleep enhancement due to rhythmical auditory stimulation in the rat

From a physiological viewpoint, REM sleep (REMS) is a period during which h omeostatic physiological regulations are impaired. In the rat, REMS occurs in two forms respectively characterized by episodes separated by long inter vals (single REMS episodes) and by episodes which have short intervals and occur in sequences (REMS clusters). Since the partition of REMS in the form of either single or clustered episodes may reveal how the REMS drive and b ody homeostatic processes interact in the control of REMS occurrence, we ha ve used this approach to clarify the effects of the rhythmical delivery of an auditory stimulus (1000 Hz, 63 or 88 dB, 50 ms, every 20 s), which has b een previously observed by different authors to enhance REMS in the absence of a previous sleep deprivation. Stimuli were delivered to pairs of animal s and triggered by the occurrence of REMS in one rat (REMS-selective stimul ation), whilst the other animal received the same stimulus irrespectively o f the stage of the wake-sleep cycle (REMS-unselective stimulation). The res ults showed that the REMS-selective stimulation did not change the overall amount of REMS, since an increase in the occurrence of REMS clusters was co ncomitant with a decrease in the occurrence of single REMS episodes. In con trast, under the REMS-unselective stimulation, the total amount of REMS was increased during the second day of stimulation through an increase in the duration of both types of REMS episodes. Since during the REMS-unselective stimulation 87%,, of the stimuli fell outside REMS (i.e., during the REMS i nterval), the results show that the occurrence of REMS is more consistently affected when the stimuli are delivered in a period during which homeostat ic physiological regulations are fully operant. (C) 2001 Elsevier Science B .V. All rights reserved.