Induction of multiple synapses by experience in the visual cortex of adult rats.

This study examined experience effects upon the formation of multiple synaptic contacts among individual dendritic and axonal elements. Axonal boutons and dendritic spines forming contacts with more than one process were assessed within layer IV of the visual cortex in adult rats following 60 days of housing in standard laboratory cages (IC) or in complex environments (EC). Multiple synaptic boutons (MSBs) that formed synaptic contacts with both a dendritic spine and a dendritic shaft were found to be markedly increased in number per neuron in EC rats in comparison to those in IC rats. In contrast, single-synaptic contacts were not increased, indicating that the formation of new single-synaptic boutons is, at most, merely sufficient to replace boutons that may have been recruited into the population of MSBs. This apparent tendency to reutilize presynaptic processes may indicate a constraint upon the formation of neural circuitry and a fundamental form of plastic synaptic change.