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

Abstract

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 66 %, 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.

Figure 1. Reciprocal developmental changes in AMPA vs. NMDA receptor-mediated components of EPSCs evoked in bushy cells of anteroventral cochlear nucleus by single auditory nerve fibre stimulation

A, averaged traces of the nerve-evoked EPSC in a 4-day-old animal. The EPSC exhibits a large NMDA receptor-mediated component and a small AMPA receptor-mediated component (indicated with horizontal arrows), compared with the corresponding dual components of the EPSC in a 16-day-old animal (B). Stimulus artifacts have been removed and the stimulus is marked with a vertical arrow in A and B. C, grouped data (means ±s.e.m.) show that there is a significant increase in the AMPA receptor-mediated component of the evoked EPSC (* P < 0.05) in 12- to 18-day-old rats (), compared with that in 4- to 11-day-old rats (▪). In contrast, there is a significant decrease in the amplitude of the NMDA receptor-mediated component of the evoked EPSC for the same two age groups (** P < 0.01). D and E, plots of evoked EPSC conductance at +50 mV (D, AMPA component; E, NMDA component) for individual cells vs. animal age, with linear regression fits which show significant age-related trends. Amplitude of the evoked EPSC has been converted to conductance, g (nS), in this and all subsequent figures.

Figure 2. Quantal EPSCs at the endbulb synapse change significantly during development

A, the reciprocal change in amplitude of the fast (AMPA) and slow (NMDA) components of averaged spontaneous EPSCs recorded in a cell from a 7-day-old animal cf. a 16-day-old animal. B, grouped data (means ±s.e.m.) show that there is a significant increase in the mean amplitude of the AMPA receptor-mediated component of spontaneous EPSCs in 12- to 22-day-old animals () compared with that in 4- to 11-day-old animals (▪). In contrast, the NMDA receptor-mediated component from spontaneous EPSCs in the same age groups shows a significant decrease in mean amplitude. *** P < 0.001. C and D, the mean amplitude of the AMPA and NMDA components, respectively, of spontaneous EPSCs is plotted against postnatal age for individual cells. Between 4 days and 3 weeks after birth, there is a significant increase in the AMPA receptor-mediated component of spontaneous EPSCs, and a simultaneous decrease in the amplitude of the NMDA receptor-mediated component. Data are fitted with a linear regression fit (C) and a single-exponential function (D).

Figure 3. Developmental changes in EPSC kinetics

The decay time constant of the AMPA receptor-mediated EPSC shows a small decrease in spontaneous EPSCs and a larger decrease in evoked EPSCs in older animals, while the decay time constant of the NMDA receptor-mediated component of spontaneous and evoked EPSCs shows similar large decreases in older animals. A, averaged spontaneous EPSCs from neurones at -70 mV, from a 4-day-old animal (filled symbols) and a 14-day-old animal (open symbols), are plotted, together with single-exponential fits to the decay phase of the AMPA component of the EPSCs. The decay time constant for the 4-day-old animal is slightly larger than that for the 14-day-old animal (τ_decay= 0.47 cf. 0.39 ms). B, grouped data (means ±s.e.m.) showing mean decreases in the AMPA receptor-mediated EPSC τ_decay of spontaneous (* P < 0.05) and evoked EPSCs (** P < 0.01) from 4- to 11-day-old rats (▪) cf. 12- to 18-day-old rats (). C, averaged spontaneous EPSCs from neurones at +50 mV from a 4-day-old animal (filled symbols) and a 14-day-old animal (open symbols) are plotted, together with single-exponential fits to the decay phase of the NMDA component of the EPSCs; note that for clarity, the full record length has not been plotted and, after the first 27 ms, each data point plotted is an average of 4 consecutive data points in the original fitted record. The decay time course is slower for the 4-day-old than for the 14-day-old rat (τ_decay= 108 cf. 37 ms). D, grouped data (means ±s.e.m.) showing mean decreases in the NMDA receptor-mediated EPSC τ_decay of spontaneous and evoked EPSCs from 4- to 11-day-old rats (▪) cf. 12- to 18-day-old rats (). ** P < 0.01.