Motility of dendritic spines in visual cortex in vivo: changes during the critical period and effects of visual deprivation

Critical period regulation

Neuronal circuits are shaped by experience during critical periods of early postnatal life. The ability to control the timing, duration, and closure of these heightened levels of brain plasticity has recently become experimentally accessible, especially in the developing visual system. This review summarizes our current understanding of known critical periods across several systems and species. It delineates a number of emerging principles: functional competition between inputs, role for electrical activity, structural consolidation, regulation by experience (not simply age), special role for inhibition in the CNS, potent influence of attention and motivation, unique timing and duration, as well as use of distinct molecular mechanisms across brain regions and the potential for reactivation in adulthood. A deeper understanding of critical periods will open new avenues to "nurture the brain"-from international efforts to link brain science and education to improving recovery from injury and devising new strategies for therapy and lifelong learning.

Enriched environment and acceleration of visual system development

Rearing mice from birth in an enriched environment leads to a conspicuous acceleration of visual system development appreciable at behavioral, electrophysiological and molecular level. Little is known about the possible mechanisms of action through which enriched environment affects visual system development. It has been suggested that differences in maternal behavior between enriched and non-enriched conditions could contribute to the earliest effects of enriched environment on visual development and that neurotrophins, BDNF in particular, might be involved. Here, we examined Brain Derived Neurotrophic Factor (BDNF) levels in the visual cortex during development and showed that an increase occurs in the first week of life in enriched pups compared to standard reared pups; BDNF levels at birth were equal in the two groups. This suggests a postnatal rather than a prenatal effect of environment on BDNF. A detailed analysis of maternal care behavior showed that pups raised in a condition of social and physical enrichment experienced higher levels of licking behavior and physical contact compared to standard reared pups and that enhanced levels of licking were also provided to pups in an enriched environment where no adult females other than the mother were present. Thus, different levels of maternal care in different environmental conditions could act as indirect mediator for the earliest effects of enrichment on visual system development. Some of the effects of different levels of maternal care on the offspring behavior are long lasting. We measured the visual acuity of differentially reared mice at the end of the period of visual acuity development (postnatal day 45) and at 12 months of age, using a behavioral discrimination task. We found better learning abilities and higher visual acuity in enriched compared to standard reared mice at both ages.