Developmental changes in EPSC quantal size and quantal content at a central glutamatergic synapse in rat

1. Developmental changes in amplitude and time course of single-fibre-evoked and spontaneous EPSCs mediated by AMPA and NMDA receptors at the endbulb-bushy cell synapse of rats from 4 to 22 days of age were recorded using whole-cell patch-clamp methods in in vitro slices of cochlear nucleus. 2. The mean conductance of the AMPA component of evoked EPSCs increased by 68%, while that of the NMDA component decreased by 61%, for 12- to 18-day-old rats cf. 4- to 11-day old rats. 3. The mean AMPA spontaneous EPSC conductance increased by 54%, while mean NMDA spontaneous EPSC conductance decreased by 83%, for 12- to 22-day-old rats cf. 4- to 11-day-old rats. The mean number of quanta contributing to peak evoked AMPA conductance also increased by 78% in the older age group, after correction for the asynchrony of evoked quantal release. 4. The decay time constant of spontaneous AMPA EPSCs showed a small decrease in older animals, while the decay time constant of spontaneous NMDA EPSCs was markedly decreased in older animals. The decay time constants of evoked NMDA EPSCs showed a quantitatively similar decrease to that of spontaneous NMDA EPSCs. This suggests that AMPA receptor subunit composition is unlikely to undergo developmental change, while NMDA receptor subunit composition may be substantially altered during synaptic maturation. 5. These data are consistent with a developmentally increased efficacy of AMPA receptor-mediated synaptic transmission at the endbulb-bushy cell synapse, due to an increase in underlying AMPA-mediated quantal size and content during the same period as a transient co-localization of NMDA receptors.