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 th e endbulb-bushy cell synapse of rats from 4 to 22 days of age were rec orded using whole-cell patch-clamp methods in in vitro slices of cochl ear nucleus. 2. The mean conductance of the AMPA component of evoked E PSCs increased by 68%, while that of the NMDA component decreased by 6 1%, 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 sp ontaneous EPSC conductance decreased by 83%, for 12- to 22-day-old rat s 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 g roup, after correction for the asynchrony of evoked quantal release. 4 . The decay time constant of spontaneous AMPA EPSCs showed a small dec rease in older animals, while the decay time constant of spontaneous N MDA EPSCs was markedly decreased in older animals. The decay time cons tants of evoked NMDA EPSCs showed a quantitatively similar decrease to that of spontaneous NMDA EPSCs. This suggests that AMPA receptor subu nit composition is unlikely to undergo developmental change, while NMD A receptor subunit composition may be substantially altered during syn aptic 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 transien t co-localization of NMDA receptors.