Circadian rhythmicity of synapses in mouse somatosensory cortex

Abstract

The circadian rhythmicity displayed by motor behavior of mice: activity at night and rest during the day; and the associated changes in the sensory input are reflected by cyclic synaptic plasticity in the whisker representations located in the somatosensory (barrel) cortex. It was not clear whether diurnal rhythmic changes in synapse density previously observed in the barrel cortex resulted from changes in the activity of the animals, from daily light/dark (LD) rhythm or are driven by an endogenous clock. These changes were investigated in the barrel cortex of C57BL/6 mouse strain kept under LD 12 : 12 h conditions and in constant darkness (DD). Stereological analysis of serial electron microscopic sections was used to assess numerical density of synapses. In mice kept under LD conditions, the total density of synapses and the density of excitatory synapses located on dendritic spines was higher during the light period (rest phase). In contrast, the density of inhibitory synapses located on dendritic spines increased during the dark period (activity phase). Under DD conditions, the upregulation of the inhibitory synapses during the activity phase was retained, but the cyclic changes in the density of excitatory synapses were not observed. The results show that the circadian plasticity concerns only synapses located on spines (and not those on dendritic shafts), and that excitatory and inhibitory synapses are differently regulated during the 24 h cycle: the excitatory synapses are influenced by light, whilst the inhibitory synapses are driven by the endogenous circadian clock.

Daily Fluctuation of Orexin Neuron Activity and Wiring: The Challenge of “Chronoconnectivity”

“…The synaptic homeostasis hypothesis is centered on the variation of synaptic strength leading to synaptic potentiation during wake and synaptic depression during sleep ( Tononi and Cirelli, 2003 , 2014 ), but does not involve neuronal firing rates ( Cirelli, 2017 ). Synaptic homeostatic changes during sleep and wake could be subserved by circadian synaptic changes, as observed in synaptic morphology in the somatosensory cortex ( Jasinska et al, 2015 ), and in the size of cortical dendritic spines and area of axon-spine interface during sleep and wake ( De Vivo et al, 2017 ). “Chronoconnectivity” implies instead time of day-dependent and sleep-wake-dependent fluctuation of firing rate and connectivity.…”

Circadian Mechanisms Underlying Reward-Related Neurophysiology and Synaptic Plasticity

“…Functional effects of circadian oscillations on synaptic plasticity in the somatosensory cortex of mice have been examined in constant darkness conditions where the cyclic changes in the density of excitatory synapses on spines were abolished. These findings suggest that the density of excitatory synapses shows daily changes while the density of inhibitory synapses in this cortical region shows circadian changes ( 130 , 131 ). Recent important work from the laboratory of Takao Hensch demonstrates that the Clock gene regulates critical period plasticity in the primary visual cortex of mice.…”

Diurnal Variation Induces Neurobehavioral and Neuropathological Differences in a Rat Model of Traumatic Brain Injury

“…In previous works, it has been reported that neuronal body loss (which we observed in this work) is related to white matter atrophy due to DAI ( Armstrong et al, 2016 ). Diurnal variation in parameters depending of time at which TBI was induced may be related to changes in neurotransmitters such as GABA; for example, some studies have shown the diurnal variation in inhibitory synapses, which increase gradually in the active phase (night) in the mouse somatosensory cortex ( Jasinska et al, 2014 , 2015 ). Righting reflex and contralateral reflex were not affected by either TBI or its time of occurrence.…”