Postsynaptic Ca2+ influx mediated by three different pathways during synaptic transmission at a calyx-type synapse

Whole-cell recordings and Ca2+ flux measurements were made at a giant calyx -type synapse in rat brainstem slices to determine the contribution of glut amate receptor (GluR) channels and voltage-dependent Ca2+ channels (VDCCs) to postsynaptic Ca2+ influx during synaptic transmission. A single presynap tic action potential (AP) evoked an EPSP, followed by a single AP. The EPSP -AP sequence caused a postsynaptic Ca2+ influx of similar to 3.0 pC, primar ily through VDCCs (similar to 70%) and NMDA-type (up to 30%) channels but a lso through AMPA-type (<5%) GluR channels. At -80 mV, the fractional Ca2+ c urrent (P-f) mediated by AMPA receptor (AMPAR) and NMDA receptor (NMDAR) ch annels was 1.3 and 11-12%, respectively. Simulations of the time course of Ca2+ influx through GluR channels suggested that the small contribution of AMPAR channels occurred only during the first few milliseconds of an EPSP, whereas influx through NMDAR channels dominated later. The NMDAR-mediated C a2+ influx was localized in regions covered by the presynaptic terminal, wh ereas the Ca2+ influx mediated by VDCCs was more homogeneously distributed. Because of the temporal and spatial differences, calcium ions entering thr ough the three different pathways are likely to activate different intracel lular targets in the postsynaptic cell.