Sleep homeostasis in rats assessed by a long-term intermittent paradoxical sleep deprivation protocol

Abstract

Numerous studies have evaluated the sleep homeostasis of rats after short- or long-periods of sleep deprivation, but none has assessed the effects of prolonged sleep restriction on the rat's sleep pattern. The purpose of the present study, therefore, was to evaluate the sleep homeostasis of rats under a protocol of chronic sleep restriction. Male Wistar rats were implanted with electrodes for EEG and EMG recordings. Using the single platform method, the animals were submitted to 18 h of sleep restriction, beginning at 16:00 h (lights on at 07:00 h), followed by a 6 h sleep window (from 10:00 h to 16:00 h) for 21 days. Immediately after this period, rats were allowed to sleep freely for 4 days (recovery period). The sleep–wake cycle was recorded throughout the entire experiment and the results showed that during the 6 h sleep window there was an increase on the percentage of sleep time, reflected by augmented time in high amplitude slow wave sleep and in paradoxical sleep, when compared to baseline sleep, whereas bouts of awakening longer than 1.5 min were greatly reduced, with the animals exhibiting a monophasic-type sleep pattern. During the deprivation period, paradoxical sleep was abolished. High amplitude slow wave sleep was also greatly affected by the protocol. Nonetheless, one day of recovery was sufficient to restore the normal sleep pattern. These findings indicate that this protocol was capable to induce many changes in the rat's sleep patterns, suggesting that during the 6 h sleep window there is a sleep adaptive homeostatic process.

Introduction

The sleep of rats is regulated by a combination of homeostatic and circadian mechanisms. Sleep rebound, which is observed after sleep deprivation, is the consequence of homeostatic sleep regulation [5] and appears to be proportional to the amount of sleep lost during the deprivation period. This regulation, however, is limited by a peak or a plateau effect [17], which, in many cases hinders the recovery of the full amount of sleep lost during the deprivation period [11], [14], [16].

Usually, short-term periods of sleep deprivation (6 h or less) results in rebound of slow wave sleep, but not of paradoxical sleep [28]. Total or partial sleep deprivation for longer periods (24 h or more) are associated with a robust rebound of paradoxical sleep and, in many cases, to a reduction of the time spent in slow wave sleep during the recovery period [11], [14], [21], [25]. This has been attributed by some authors, to an over-expression of paradoxical sleep [8], [10]. In a previous study we reported that 18 h of sleep deprivation by the modified multiple platform method produce a 20% increase of total sleep time and a 56% increase of paradoxical sleep, with a discrete increase of slow wave sleep (9%), assessed during the subsequent 6 h recovery [19].

However, there are but a few studies on sleep regulation resulting from protocols of repeated sleep restriction. For instance, in a study that used paradoxical sleep deprivation (and of high amplitude slow wave sleep) by the forced locomotion for 5 days, 12 h/day, showed a clear accumulation of sleep throughout the period of sleep restriction. It must be emphasized, however, that sleep rebound was more robust in rats deprived during the day, than in those deprived during the night, demonstrating an overlap between circadian and homeostatic processes [13]. Short-term intermittent sleep deprivation (20 × 10 min, for 3 h, during the light period) also results in sleep accumulation [18].

By using a protocol of intermittent sleep deprivation with the flower-pot technique, we believe that it is possible to produce, in the long-term, an overall restriction of sleep. Therefore, in the present study, we sought to produce a condition of sleep restriction in rats, using the flower-pot method for 18 h/day, for 21 consecutive days. During this period, rats were allowed to sleep for 6 h, in the light period. By means of electrophysiological recordings we sought to examine whether the animals would be capable of partially or completely compensate the sleep loss during the daily 6 h sleep window and to follow the homeostatic compensatory process throughout 21 days of sleep deprivation. In addition, we examined the sleep pattern of rats for 4 days after the end of the sleep restriction protocol, when they could sleep freely.