Abstract
The circadian rhythmicity changes the density and shape of dendritic spines in mouse somatosensory barrel cortex, influencing their stability and maturation. In this study, we analyzed the main geometric parameters of dendritic spines reflecting the strength of synapses located on these spines under light/dark (12:12) and constant darkness conditions, in order to distinguish between endogenously regulated and light-driven parameters. Using morphological analysis of serial electron micrographs, as well as three-dimensional reconstructions, we found that the light induces elongation of single-synapse spine necks and increases in the diameter of double-synapse spine necks, increasing and decreasing the isolation of synapses from the parent dendrite, respectively. During the subjective night of constant darkness, we observed an enlargement of postsynaptic density area in inhibitory synapses and an increase in the number of polyribosomes inside double-synapse spines. The results show that both endogenous effect (circadian clock/locomotor activity) and light affect the morphological parameters of single- and double-synapse spines in the somatosensory cortex: light reduces the efficiency of excitatory synapses on single-synapse spines, increases the effect of synaptic transmission in double-synapse spines, and additionally masks the endogenous clock-driven enlargement of inhibitory synapses located on double-synapse spines. This indicates a special role of double-synapse spines and their inhibitory synapses in the regulation of synaptic transmission during both circadian and diurnal cycles in the mouse somatosensory cortex.
Highlights
The quantitative and qualitative changes of small dendritic protrusions—dendritic spines— associated with activity-dependent neural plasticity have been observed in various areas of the brain
In the present exploratory study, we investigate circadian changes of measurable geometric parameters, as well as postsynaptic density (PSD) area indicative of synapse strength and the number of polyribosomes reflecting the local protein synthesis, which plays a key role in the modification of synapses located on the spines
No significant differences in the length of single-synapse spines and spine necks between the subgroups REST and ACTIVE were observed in the DD group (Figures 3A,C)
Summary
The quantitative and qualitative changes of small dendritic protrusions—dendritic spines— associated with activity-dependent neural plasticity have been observed in various areas of the brain. While studies of the geometric parameters of the spines during sleep/wake cycle are more frequent (Frank and Cantera, 2014; van der Zee, 2015; Areal et al, 2017; Raven et al, 2018), the information on morphological changes of the spines in the circadian rhythm is relatively scarce. Investigation of such changes seems to be important, because circadian rhythmicity affects synaptic plasticity irrespective of sleep/wake state (Frank and Cantera, 2014)
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