Discussion
Laboratory rats normally sleep for about 12 hrs/day, of which 15-20% of the sleep time corresponds to REM sleep stage.14 This study has shown that REM sleep deprivation is a stressor, which is evident from the elevated plasma corticosterone levels. This is in correlation with earlier reports.15,16
The inverted flowerpot technique is the most widely used method for REM sleep deprivation studies.17 This causes maximum REM sleep deprivation without significantly affecting non-REM sleep. It causes total loss of REM sleep in rats.18 Studies on behavioural evaluation of the stress induced by platform method for short term REM sleep deprivation in rats showed that the effect of stress induced by short term confinement to platform do not seem to be a remarkable confounding factor and large platform acts as an adequate stress control for the small platform.19 Hence large platform was used in the REMC group for all the comparisons. Plasma corticosterone level in the REMC group is almost near the control value for the Wistar strain animals which is in corroboration with earlier report.15
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Sleep seems to limit metabolic requirements. Therefore sleep deprivation could enhance metabolic rate and in turn increase oxidative stress. Increase in lipid peroxidation in the discrete regions of the brain following REM sleep deprivation in the current study suggests free radical generation and free radical induced neuronal damage. An increase in malonyldialdehyde level is related to an increase in the levels of lipid peroxidation in cell membrane.6 Mallick et al20 have shown that REM sleep deprivation decreases membrane fluidity in the rat brain. Deep destructive changes in the brain and erythrocyte mitochondria have also been demonstrated following REM sleep deprivation.21 Increase in lipid peroxidation was accompanied by decrease in total reduced glutathione following REM sleep deprivation. Ramanathan et al22 have shown similar biochemical changes in the hippocampal region of the brain in Wistar rats. D’Almeida et al23 showed that thalamus and hypothalamus are more susceptible to free radical damage following sleep deprivation as evidenced by decrease in GSH levels in these regions.
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Brain antioxidant enzymes provide a mechanism inherent to an organism for removing free radicals. Oxidised glutathione, GSSG, production from GSH occurs as an antioxidant defense when the formation of reactive oxygen species is observed.6 A decrease in total reduced glutathione following REM sleep deprivation could result in an increase in the level of GSSG. Several studies have experimentally also shown the sleep promoting effect of GSSG.24,25 If the organism accumulates free radicals during waking period, then GSSG would also accumulate which in turn would induce sleep. Honda et al26 have shown that GSSG has an inhibitory action on the excitatory synaptic membrane of rat brain. They have also speculated that the sleep-enhancing activity of GSSG was caused by its physiological modulation on the glutamatergic neurotransmission in the brain.
Further studies on the histopathological changes in these regions along with the biochemical changes would probably throw more light on the cellular level damages produced by REM sleep deprivation.
Restorative sleep following 96 hours of REM sleep deprivation returns lipid peroxidation and total reduced glutathione back to the base line values gradually by 24 hours of restorative sleep indicating that 96 hours of REM sleep deprivation does not cause permanent damage to the brain of Wistar rats. To emphasise on this fact, further investigation needs to be carried out in same lines along with histochemical and histopathological studies and by increasing durations of REM sleep deprivation.
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Increase in plasma corticosterone level in the initial stages of restorative sleep indicates that the body homeostatic mechanisms are impacted by the stress. However, plasma corticosterone level also returns back to base line value by 24 hours of restorative sleep. The decrease in lipid peroxidation following restorative sleep indicates that there is decrease in free radical production. The other possibility is that the free radicals are scavenged by the antioxidant mechanism. This is well correlated by increase in the total reduced glutathione level following restorative sleep. These results are in accordance with the study of Mallik et al20 where they have shown that norepinephrine activity and synaptosomal calcium levels returns to normal by 24 hrs of restorative sleep following 96hours of REMSD. Datta and Desarnaud27 have shown that recovery sleep following 3 hours of REMSD is due to activation of intracellular protein kinase A in the pedunculopontine tegmental nucleus. Mendelson and Bergmann28 have shown that there is age dependent change in the recovery sleep after 48 hrs of sleep deprivation in rats with recovery largely confined to the first 6 hours in the young and middle aged rats but maximum for the old rats occurred in the second six hours.
From this study it is clear that, REM sleep deprivation is a potent oxidative stressor. This could probably play a role in the behavioral and performance alteration seen in both experimental animals as well as humans following REM sleep deprivation.
Importance of REM sleep has been suggested by the study of Ranjan et al29 who have concluded that REMSD could lead to neurodegeneration memory loss and Alzheimer’s disease. Sleep deprivation or prolonged wakefulness leads to decrements in cognitive performance, which is recognized as a major hazard to public safety and implicated in vehicular accidents, industrial catastrophes and other incidents involving error in human performance.30 Though importance is given to the total duration of sleep, due importance is yet to be given to REM stage of sleep. Further investigations in this field are needed to highlight the importance of REM sleep.
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