Synchronisation of neurotransmitter release during postnatal development in a calyceal presynaptic terminal of rat

1. Mechanisms contributing to the synchronisation of transmitter release du ring development were studied in synapses of the medial nucleus of the trap ezoid body (MNTB) using patch recording and Ca2+ imaging techniques in a ra t brainstem slice preparation. 2. Excitatory postsynaptic currents (EPSCs) were generated in an all-or-non e manner at immature synapses (postnatal days earlier than P6). Many delaye d miniature EPSC (mEPSC)-like currents followed EPSCs at immature synapses, while observations of delayed mEPSC-like currents were rare at mature syna pses (later than P9). 3. At immature synapses bath application of either omega -conotoxin GVIA or omega -agatoxin-IVA reduced EPSCs (both to 40% of control), and Ca2+ curre nts in the presynaptic terminal (both to 70% of control). The frequency of delayed mEPSC-like currents was reduced by w-conotoxin GVIA, but not by ome ga -agatoxin IVA. 4. At immature synapses delayed mEPSC-like currents were rare after incubat ion of the slice with extrinsic Ca2+ buffers (EGTA AM). 5. At mature synapses many mEPSC-like currents followed evoked EPSCs after partial block of Ca2+ channels by bath application of a low concentration o f Cd2+ (3 muM) or omega -agatoxin IVA (50 nM) but not by low [Ca2+](o) (0.5 -1 mM). 6. Ca2+ transients induced by action potentials in presynaptic terminals we re monitored by adding a high concentration of fura-2 (200 muM) to the pipe tte. Their decay time course was slower at immature presynaptic terminals t han at mature terminals. Both the Ca2+ extrusion rate and the endogenous Ca 2+ binding capacity were estimated to be smaller at immature terminals than at mature terminals. 7. These results suggest that the maturation of synaptic transmission in MN TB progresses with the capacity for Ca2+ clearance from the presynaptic ter minal. The possible importance of developmental increases in both Ca2+ clea rance capacity and Ca2+ currents is discussed in relation to the synchronis ation of transmitter release.