Adaptive and pathological inhibition of neuroplasticity associated with circadian rhythms and sleep.

The circadian system organizes sleep and wake through imposing a daily cycle of sleep propensity on the organism. Sleep has been shown to play an important role in learning and memory. Apart from the daily cycle of sleep propensity, however, direct effects of the circadian system on learning and memory also have been well documented. Many mechanistic components of the memory consolidation process ranging from the molecular to the systems level have been identified and studied. The question that remains is how do these various processes and components work together to produce cycles of increased and decreased learning abilities, and why should there be times of day when neural plasticity appears to be restricted? Insights into this complex problem can be gained through investigations of the learning disabilities caused by circadian disruption in Siberian hamsters and by aneuploidy in Down's syndrome mice. A simple working hypothesis that has been explored in this work is that the observed learning disabilities are due to an altered excitation/inhibition balance in the CNS. Excessive inhibition is the suspected cause of deficits in memory consolidation. In this article we present the evidence that excessive inhibition in these cases of learning disability involves GABAergic neurotransmission, that treatment with GABA receptor inhibitors can reverse the learning disability, and that the efficacy of the treatment is time sensitive coincident with the major daily sleep phase, and that it depends on sleep. The evidence we present leads us to hypothesize that a function of the circadian system is to reduce neuroplasticity during the daily sleep phase when processes of memory consolidation are taking place.