Silent synapses in the developing XXXX visual cortex: evidence for postsynaptic expression of synaptic plasticity.

@entity35 @entity2227 @entity140 @entity143
In the developing visual cortex activity-dependent refinement of synaptic connectivity is thought to involve synaptic plasticity processes analogous to long-term potentiation (LTP). The recently described conversion of so-called silent synapses to functional ones might underlie some forms of LTP. Using whole-cell recording and minimal stimulation procedures in immature pyramidal neurons, we demonstrate here the existence of functionally silent synapses, i.e., glutamatergic synapses that show only @entity143 receptor-mediated transmission, in the neonatal @entity35 visual cortex. The incidence of silent synapses strongly decreased during early postnatal development. After pairing presynaptic stimulation with postsynaptic depolarization, silent synapses were converted to functional ones in an LTP-like manner, as indicated by the long-lasting induction of @entity140 receptor-mediated synaptic transmission. This conversion was dependent on the activation of @entity143 receptors during the pairing protocol. The selective activation of @entity143 receptors at silent synapses could be explained presynaptically by assuming a lower @entity2227 concentration compared with functional ones. However, we found no differences in @entity2227 concentration-dependent properties of @entity143 receptor-mediated PSCs, suggesting that synaptic @entity2227 concentration is similar in silent and functional synapses. Our results thus support a postsynaptic mechanism underlying silent synapses, i.e., that they do not contain functional @entity140 receptors. Synaptic plasticity at silent synapses might be expressed postsynaptically by modification of nonfunctional @entity140 receptors or rapid membrane insertion of @entity140 receptors. This conversion of silent synapses to functional ones might play a major role in activity-dependent synaptic refinement during development of the visual cortex.
