Synapse formation in adult barrel cortex following naturalistic environmental enrichment

Environmental enrichment paradigms in adult laboratory animals, consisting of physical, perceptual, and social stimulation, have been shown to affect synapse and cell morphology in sensory cortex and enhance learning ability, whereas enrichment, which is in harmony with the animal's natural habitat may have even greater implications for plasticity. Previous studies in our laboratory have shown that whisker stimulation induced the formation of synapses and spines in the corresponding barrel. In the present study adult C57/Bl6J female laboratory mice at 6 weeks of age were placed during 2 months in a protected enrichment enclosure in a forest clearing at the Chisti Les Biological Station, Tvier, Russia. We analyzed neuropil ultrastructure in the C2 barrel using serial-section electron microscopy on a total of eight mice (n=4 enriched, n=4 standard cagemate controls). Quantitative analyses of volumes of neuropil showed a significant increase in excitatory and inhibitory synapses on spines and excitatory synapses on dendritic shafts in the C2 barrel in the enriched group compared with standard cagemate controls. These results demonstrate that naturalistic experience alters the synaptic circuitry in layer IV of the somatosensory cortex, the first cortical relay of sensory information, leaving a lasting trace that may guide subsequent behavior.

Graphical AbstractLaboratory 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.Figure optionsDownload high-quality image (160 K)Download as PowerPoint slide