Ultrastructural effects of sleep and wake on the parallel fiber synapses of the cerebellum

Abstract

Multiple evidence in rodents shows that the strength of excitatory synapses in the cerebral cortex and hippocampus is greater after wake than after sleep. The widespread synaptic weakening afforded by sleep is believed to keep the cost of synaptic activity under control, promote memory consolidation, and prevent synaptic saturation, thus preserving the brain’s ability to learn day after day. The cerebellum is highly plastic and the Purkinje cells, the sole output neurons of the cerebellar cortex, are endowed with a staggering number of excitatory parallel fiber synapses. However, whether these synapses are affected by sleep and wake is unknown. Here, we used serial block face scanning electron microscopy to obtain the full 3D reconstruction of more than 7000 spines and their parallel fiber synapses in the mouse posterior vermis. This analysis was done in mice whose cortical and hippocampal synapses were previously measured, revealing that average synaptic size was lower after sleep compared to wake with no major changes in synapse number. Here, instead, we find that while the average size of parallel fiber synapses does not change, the number of branched synapses is reduced in half after sleep compared to after wake, corresponding to ~16% of all spines after wake and ~8% after sleep. Branched synapses are harbored by two or more spines sharing the same neck and, as also shown here, are almost always contacted by different parallel fibers. These findings suggest that during wake, coincidences of firing over parallel fibers may translate into the formation of synapses converging on the same branched spine, which may be especially effective in driving Purkinje cells to fire. By contrast, sleep may promote the off-line pruning of branched synapses that were formed due to spurious coincidences.

Disrupted sleep in dystonia depends on cerebellar function but not motor symptoms in mice

“…Existing studies are split, some suggesting that abnormal muscle activity in dystonic patients disappears during sleep [23] while other indicate that it might persist during sleep [24]. On the one hand, it is possible that our results indicate that dysfunction of the mechanisms involved in synaptic renormalization are affected in dystonia, which are believed to occur during sleep and mediate muscle recovery and atonia during sleep [32,49,50]. On the other hand, as a reconciling interpretation, the work we presented here could also suggest that cerebellar dysfunction, in the presence or absence of dystonic motor dysfunction, is sufficient to drive nonmotor impairments in sleep in mouse models of dystonia (Figure 6B).…”

Disrupted sleep in dystonia depends on cerebellar function but not motor symptoms in mice

“…Existing studies are split, some suggesting that abnormal muscle activity in dystonic patients disappears during sleep 23 while other indicate that it might persist during sleep 24 . On the one hand, it is possible that our results indicate that dysfunction of the mechanisms involved in synaptic renormalization are affected in dystonia, which are believed to occur during sleep and mediate muscle recovery and atonia during sleep 32,45,46 . On the other hand, as a reconciling interpretation, the work we presented here could also suggest that cerebellar dysfunction, in the presence or absence of dystonic motor dysfunction, is sufficient to drive nonmotor impairments in sleep in mouse models of dystonia (Figure 6B).…”