Cognitive, Behavioral, and Systems NeuroscienceResearch PaperSynapse formation in adult barrel cortex following naturalistic environmental enrichment
Graphical Abstract
Laboratory mice that were placed in a naturalistic environment for two month show signs of synapse formation in the part of the cerebral cortex where sensory stimuli from their whiskers are processed. This study provides evidence that increased sensory experience alters cortical neuronal circuits in a specific manner and underlines the dynamic nature of the adult central nervous system.
Highlights
▶We studied neuronal plasticity induced by a naturalistic environment in adult mice. ▶Synapses were analyzed at the electron microscopic level 2 months after enrichment. ▶Excitatory and inhibitory synapse densities increased in enriched versus controls. ▶Sensory activation modifies cortical circuits in the adult somatosensory cortex. ▶Naturalistic whisker stimulation alters sensory processing in barrel cortex.
Section snippets
Animals
A total of 18, 6-week-old C57/Bl6J adult female mice were supplied by the “Stolbovaya” laboratory animal farm of the Russian Medical Academy of Sciences (RMAS). Upon arrival to the Chisti Les (“clear forest”) Biological Station (director: V.V. Pazhetnov), in Pozhnia Tvier Region in Russia, mice adapted for 1 week in standard mouse cages in the animal facility. This station is located 400 km west of Moscow and 400 km south of St. Petersburg (Lat 56.735474°; Long. 31.526772°).
Environmental enrichment
Twelve mice were
Morphometric analysis of synaptic density
For densities of excitatory and inhibitory synapses on spines and on dendritic shafts, differences in means between enriched (mean±SD: 1.887 per μm3±0.12) and control groups (mean±SD: 1.421 per μm3±0.17) were significant [MANOVA; F(4,3)=15.58, P=0.024]. The density of the total of all synapses (including excitatory and inhibitory synapses on spines and on the dendritic shaft) in the C2 barrel of the enriched group was significantly higher in the enriched group compared with the control group
Discussion
Environmental enrichment paradigms in the laboratory have been widely effective at modifying brain structure with lasting effects on synapses and spines in adult visual cortex (Connor et al., 1980, Diamond and Connor, 1983, Briones et al., 2004). In the present study, morphometric analyses performed on volumes of neuropil within the C2 barrel in adult somatosensory cortex found that the density of synapses, including inhibitory and excitatory synapses on shaft and spine, increased following an
Conclusions
In conclusion, layer IV of the somatosensory cortex is plastic not only when neurons are challenged in the laboratory (whisker stimulation, classical conditioning) but also when they are exposed to an enriched environment. Together these observations strengthen the notion that adult mice can modify their layer IV circuitry well beyond the critical period for barrel formation. Future work may study the links between whisker behavior in the natural environment, whisker cortical pathways, and
Acknowledgments
This work was supported by grants from the Swiss National Science Foundation (31003A_125379, 3100A0-108446 H.P.L.) and SCOPES 111081 to H.-P.L. and we are all very grateful to NCCR “Neural Plasticity and Repair.” We also thank Caroline Musetti, Rudolf Kraftsik, Nathalie Wenger and Christel Genoud for their valuable assistance at each step in EM, data analysis, statistics and discussion.
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