Sleep Deprivation by Exposure to Novel Objects Increases Synapse Density and Axon–Spine Interface in the Hippocampal CA1 Region of Adolescent Mice

Abstract

Sleep has been hypothesized to rebalance overall synaptic strength after ongoing learning during waking leads to net synaptic potentiation. If so, because synaptic strength and size are correlated, synapses on average should be larger after wake and smaller after sleep. This prediction was recently confirmed in mouse cerebral cortex using serial block-face electron microscopy (SBEM). However, whether these findings extend to other brain regions is unknown. Moreover, sleep deprivation by gentle handling was reported to produce hippocampal spine loss, raising the question of whether synapse size and number are differentially affected by sleep and waking. Here we applied SBEM to measure axon-spine interface (ASI), the contact area between pre-synapse and post-synapse, and synapse density in CA1 stratum radiatum. Adolescent YFP-H mice were studied after 6-8 h of sleep (S ϭ 6), spontaneous wake at night (W ϭ 4) or wake enforced during the day by novelty exposure (EW ϭ 4; males/females balanced). In each animal Ն425 ASIs were measured and synaptic vesicles were counted in ϳ100 synapses/mouse. Reconstructed dendrites included many small, nonperforated synapses and fewer large, perforated synapses. Relative to S, ASI sizes in perforated synapses shifted toward higher values after W and more so after EW. ASI sizes in nonperforated synapses grew after EW relative to S and W, and so did their density. ASI size correlated with presynaptic vesicle number but the proportion of readily available vesicles decreased after EW, suggesting presynaptic fatigue. Thus, CA1 synapses undergo changes consistent with sleep-dependent synaptic renormalization and their number increases after extended wake.

Circadian Rhythms of Perineuronal Net Composition

“…We speculate that diurnal molecular modifications of PNNs may contribute to memory formation and consolidation mechanisms during the wake/sleep cycle, favoring activity-driven synaptogenesis and synaptic refinement, respectively. For instance, our results on the effects of 5 hour sleep deprivation on WFA+ PNNs in the mouse hippocampus are consistent with reports that 5 hours of sleep deprivation prevents changes in dendritic spine densities in the hippocampus occurring during sleep (Havekes et al, 2016;Raven et al, 2018;Spano et al, 2019;Gisabella et al, 2020). PNN rhythms observed in our study may reflect ongoing systemic and synaptic consolidation during sleep proposed by Rasch and Born (Rasch and Born, 2013).…”

Regulation of hippocampal dendritic spines following sleep deprivation

“…An additional limitation in our study is that we did not measure spines at the end of the dark period, thus we do not know if sleep deprivation caused an increase or if sleep allowed a decrease in spine measures. A recent study on the effect of sleep deprivation on the mouse hippocampus using electron microscopy analysis however, which included this control group, indicates that sleep reduces spine density and size (Spano et al, ). Taken together with evidence for synaptic downscaling during sleep in other brain regions (de Vivo et al, ; Tononi & Cirelli, ), we interpret our data in the context of the synaptic homeostasis hypothesis of downscaling during sleep.…”

“…Work by Cirelli et al found electron microscopic evidence of increases in axon‐spine interface and spine head volume selective for small and intermediate synapses in the motor cortex during prolonged wake, and reduction during subsequent sleep (de Vivo et al, ). Recently, increased axon‐spine interface and synapse density was observed in the hippocampus following sleep deprivation using electron microscopy (Spano et al, ).…”

Binary and analog variation of synapses between cortical pyramidal neurons

“…Previous studies of cortical synapses have found a continuum of synapse sizes (Arellano et al , 2007) that is well-modeled by a log-normal distribution (Loewenstein, Kuras and Rumpel, 2011;de Vivo et al , 2017;Santuy et al , 2018) . Even researchers who report bimodally distributed synapse size in hippocampus (Spano et al , 2019) still find log-normally distributed synapse size in neocortex (de Vivo et al , 2017) by the same methods. (a) Dendritic spine heads (yellow) and clefts (red) of dual connections between L2/3 PyCs.…”