REM sleep enhancement due to rhythmical auditory stimulation in the rat.

From a physiological viewpoint, REM sleep (REMS) is a period during which homeostatic physiological regulations are impaired. In the rat, REMS occurs in two forms respectively characterized by episodes separated by long intervals (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 body homeostatic processes interact in the control of REMS occurrence, we have 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 been previously observed by different authors to enhance REMS in the absence of a previous sleep deprivation. Stimuli were delivered to pairs of animals and triggered by the occurrence of REMS in one rat (REMS-selective stimulation), whilst the other animal received the same stimulus irrespectively of the stage of the wake-sleep cycle (REMS-unselective stimulation). The results showed that the REMS-selective stimulation did not change the overall amount of REMS, since an increase in the occurrence of REMS clusters was concomitant with a decrease in the occurrence of single REMS episodes. In contrast, 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 interval), the results show that the occurrence of REMS is more consistently affected when the stimuli are delivered in a period during which homeostatic physiological regulations are fully operant.

Changes in REM sleep occurrence due to rhythmical auditory stimulation in the rat.

The effects of the rhythmical delivery of an auditory stimulus (1000 Hz, from 50 to 100 dB, 20 ms, every 20 s) on the pattern of rapid eye movement (REM) sleep occurrence was studied in the rat. The stimulation was simultaneously carried out on pairs of rats over 5 consecutive days (10-h recording sessions), during which a tone of increasing intensity (50, 63, 75, 88, 100 dB) was used. In each experimental session, auditory stimulation was triggered by the REM sleep occurrence of one rat (REMS-selective stimulation) whilst the other rat simultaneously received the same stimuli, but during any stage of the wake-sleep cycle (REMS-unselective stimulation). The results showed that the total amount of REM sleep in the 10-h recording session was increased over the 5 days of stimulation in the REMS-unselective group. This effect was due to an increase in the mean duration of REM sleep episodes. However, no significant changes were observed in animals under REMS-selective stimulation, nor in a third group of animals in which the spontaneous evolution of REM sleep occurrence (REMS-spontaneous) was studied. Since 86% of the stimuli under the REMS-unselective auditory stimulation fell outside REM sleep, the result would suggest that REM sleep occurrence is affected when the stimuli are delivered during a time period (i.e. during wakefulness or non-REM sleep) in which it is well known that physiological regulations are fully operant.

Firing of inferior colliculus neurons in response to low-frequency sound stimulation during sleep and waking.

In vivo extracellular recordings of 102 units in the central nucleus of the inferior colliculus (IC), were made in chronically implanted guinea-pigs during the sleep/wake cycle. During wakefulness, the units were classified according to their response characteristics. Most neurons (63%) recorded showed changes, increasing or decreasing in the number of evoked discharges during the animal's transitions between wakefulness and slow-wave sleep. In the paradoxical sleep phase, the result was similar; changes were observed in most neurons, while only 11% of units did not shift their discharge pattern during ipsilateral sound stimulation. The post-stimulus time histogram of the overall evoked pattern of discharge showed sleep/wake dependency, i.e. changed in 35% of the units recorded during the 50 ms of sound stimulation. Fifty-five percent of auditory neurons did not show any change in the spontaneous firing rate during slow-wave sleep as compared to the previous waking period, while 22% exhibited a discharge increase and 23% decreased their firing. During paradoxical sleep, 14 out of 17 cells increased their spontaneous firing rate. The IC auditory neurons send descending connections to regions such as the dorsal pontine nuclei, known to mediate sleep processes. Thus, for constant auditory input, the firing rate or number of discharge variations are due to functional shifts in the sleeping brain. Auditory processing is present during sleep and differs from that observed during wakefulness. Differences were observed in the evoked firing number and/or spontaneous rate, as well as in the pattern of discharge. The ultimate reason for auditory unit shifts during sleep remains yet unexplained.

Effects of sleep on the responses of single cells in the lateral superior olive.

The effects of behavioral shifts on auditory lateral superior olive neurons were analyzed in guinea-pigs during the sleep-waking cycle with single unit extracellular recordings at the unit characteristic frequency and with low sound intensity. Shifts in the number of spikes in response to pure tones and in spontaneous firing proved to be closely related to waking, slow wave and paradoxical sleep. All of the recorded lateral superior olive (LSO) auditory neurons showed sleep-related firing shifts. Moreover, changes in the pattern of discharge over time were observed in 15% of the LSO cells on passing from waking to sleep. Sleep may determine either an increase or a decrease of the firing number in response to sound. The most important change observed in decreasing firing units was the near-absence of units responding to sound in the paradoxical sleep phase during the last 40 ms of the response. The waking cues for binaural detection, studied with our experimental paradigm, disappeared during slow wave sleep. We thus conclude that the binaural function of some lateral superior olive neurons (11.5%) was impaired during this sleep period in the present experimental conditions. Auditory efferent pathways are postulated to impinge on the auditory processing at LSO nucleus level during the sleep-waking cycle. Thus, auditory unitary activity appears to be dependent on both incoming information, and a CNS descending action closely related to the waking and sleep periods. Functional interactions between pontine sleep-related groups of neurons and auditory system units are suggested.