The visual cortex has been used extensively as a model for the study of activity-dependent plasticity. Changes in the visual input are stimulus-dependent and result in functional reorganization at the cortical level. Alongside functional changes, the structure of cortical neurons is altered in animals subjected to different levels of visual drive. This structural rearrangement can be observed at the synaptic level in the structure of dendritic spines, which are subcellular structures that constitute the postsynaptic sites of most excitatory synapses in the CNS. Changes in spine density, morphology and motility have been described in visual cortex during normal development and with sensory deprivation.
In this study we examined the correlation between structural and functional changes in visual cortex in animals that have been deprived of sensory input from birth (dark rearing-DR) and that have been re-exposed to a normal light environment for different periods.
We addressed this issue in mice 28 days after birth (P28); we used time-lapse two-photon laser scanning microscopy to assay motility of spines on V1 neurons in vivo, in mice expressing GFP in a subset of cortical layer 5 neurons; for the measurement of cortical activity, we used optical imaging of intrinsic signals.
We found that dark rearing caused a significant increase in spine motility (about 20%) with respect to age matched controls; moreover the dark reared animals showed a significant increase (p-value <0.05 t-test) in thin spines (from 11% to 21%) and a decrease in stubby spines with respect to control (from 26% to 15%). The evoked cortical activity was four times lower in dark reared animals compared to controls and the organization of the retinotopic map was poor in absence of visual drive. Re-exposure to light for 2 days did not show any change in the motility of the whole population or in the magnitude of the visual evoked response, although the number of stubby spines returned to control levels and the retinotopic map was substantially improved. Only after 7 days of light re-exposure the structural as well as the functional measurements were comparable to those in control animals.
These results suggest that in visual cortex structural and functional plasticity are dependent on sensory experience and changes in spine dynamics and cortical activity have a consistent time course.