Experience-dependent switch in sign and mechanisms for plasticity in layer 4 of primary visual cortex

Neural circuits are extensively refined by sensory experience during postnatal development. How the maturation of recurrent cortical synapses may contributetoevents regulating the postnatal refinement ofneocortical microcircuits remains controversial. Here we show that, in the main input layer of rat primary visual cortex, layer 4 (L4), recurrent excitatory synapses are endowed with multiple, devel-opmentally regulated mechanisms for induction and expression of excitatory synaptic plasticity. Maturation of L4 synapses and visual experience lead to a sharp switch in sign and mechanisms for plasticity at recurrent excitatory synapses in L4 at the onset of the critical period for visual cortical plasticity. The state of maturation of excitatory pyramidal neurons allows neurons to engage different mechanisms for plasticity inresponse to the same induction paradigm. Experience is determinant for the maturation of L4 synapses, as well as for the transition between formsofplasticity and the mechanisms they may engage. These results indicateatight correlation between the effects of sensory drive and maturation on cortical neurons and provide a new set of cellular mechanisms engaged in the postnatal refinement of cortical circuits.