Selective acquisition of AMPA receptors over postnatal development suggests a molecular basis for silent synapses

Early in postnatal development, glutamatergic synapses transmit primarily t hrough NMDA receptors. As development progresses, synapses acquire AMPA rec eptor function. The molecular basis of these physiological observations is not known. Here we examined single excitatory synapses with immunogold elec tron-microscopic analysis of AMPA and NMDA receptors along with electrophys iological measurements. Early in postnatal development, a significant fract ion of excitatory synapses had NMDA receptors and lacked AMPA receptors. As development progressed, synapses acquired AMPA receptors with little chang e in NMDA receptor number. Thus, synapses with NMDA receptors but no AMPA r eceptors can account for the electrophysiologically observed 'silent synaps e'.