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

Mechanisms contributing to the synchronisation of transmitter release during development were studied in synapses of the medial nucleus of the trapezoid body (MNTB) using patch recording and Ca2+ imaging techniques in a rat brainstem slice preparation. Excitatory postsynaptic currents (EPSCs) were generated in an all-or-none manner at immature synapses (postnatal days earlier than P6). Many delayed miniature EPSC (mEPSC)-like currents followed EPSCs at immature synapses, while observations of delayed mEPSC-like currents were rare at mature synapses (later than P9). At immature synapses bath application of either omega-conotoxin GVIA or omega-agatoxin-IVA reduced EPSCs (both to 40% of control), and Ca2+ currents in the presynaptic terminal (both to 70% of control). The frequency of delayed mEPSC-like currents was reduced by omega-conotoxin GVIA, but not by omega-agatoxin IVA. At immature synapses delayed mEPSC-like currents were rare after incubation of the slice with extrinsic Ca2+ buffers (EGTA AM). At mature synapses many mEPSC-like currents followed evoked EPSCs after partial block of Ca2+ channels by bath application of a low concentration of Cd2+ (3 microM) or omega-agatoxin IVA (50 nM) but not by low [Ca2+]o (0.5-1 mM). Ca2+ transients induced by action potentials in presynaptic terminals were monitored by adding a high concentration of fura-2 (200 microM) to the pipette. Their decay time course was slower at immature presynaptic terminals than at mature terminals. Both the Ca2+ extrusion rate and the endogenous Ca2+ binding capacity were estimated to be smaller at immature terminals than at mature terminals. These results suggest that the maturation of synaptic transmission in MNTB progresses with the capacity for Ca2+ clearance from the presynaptic terminal. The possible importance of developmental increases in both Ca2+ clearance capacity and Ca2+ currents is discussed in relation to the synchronisation of transmitter release.