Behaviour of NMDA and AMPA receptor-mediated miniature EPSCs at rat cortical neuron synapses identified by calcium imaging

1. Simultaneous recording of intracellular calcium concentration at a synap se and synaptic currents from the cell body allows mapping of miniature exc itatory postsynaptic currents (mEPSCs) to single synapses. 2. In the absence of extracellular Mg2+, 77% of synapses had mEPSCs with fa st and slow components, attributed to AMPA- and NMDA-type glutamate recepto rs, respectively. The remainder of synapses (23 %) had mEPSCs that lacked a fast component; these responses were attributed to NMDA receptors. 3. A strong positive correlation between the amplitude of the calcium trans ient and the NMDA receptor-mediated mEPSC was observed, indicating that the mEPSCs originate from an identified synapse. 4. At synapses that had both mEPSC components, the AMPA receptor component was positively correlated with charge influx mediated by NMDA receptors dur ing repeated synaptic events. No periodic failure in the AMPA receptor mEPS C was observed at synapses expressing both receptor components. 5. A significant positive correlation between the mean amplitudes of NMDA a nd AMPA receptor components of mEPSCs is observed across different synapses . 6. We suggest that factors effecting both receptor classes, such as the amo unt of transmitter in synaptic vesicles, might contribute to the variation in mEPSC amplitude during repeated miniature events at a single synapse. Al though the average postsynaptic response at different synapses can vary in amplitude, there appears to be a mechanism to keep the ratio of each recept or subtype within a narrow range.