JN - 00464 - 2004 Developmental changes in release properties of the CA 3 – CA 1 glutamate synapse in rat hippocampus

Developmental changes in release probability (P r) and paired–pulse plasticity at CA3 – CA1 glutamate synapses in hippocampal slices of neonatal rats were examined using field EPSP recordings. Paired–pulse facilitation (PPF) at these synapses was, on average, absent in the 1 st postnatal week but emerged and became successively larger during the 2 nd postnatal week. This developmental increase in PPF was associated with a reduction in P r , as indicated by the slower progressive block of the NMDA EPSP by the non-competitive NMDA receptor antagonist MK–801. This developmental reduction in P r was not homogenous among the synapses. As shown by the MK-801 analysis, the P r heterogeneity observed among adult CA3-CA1 synapses is present already during the 1 st postnatal week, and the developmental P r reduction was found to be largely selective for synapses with higher P r values, leaving P r of the vast majority of the synapses essentially unaffected. A reduction in P ves , the release probability of the individual vesicle, possibly caused by reduction in Ca 2+ influx, appears to explain the reduction in P r. In vivo injection of tetanus toxin at the end of the 1 st postnatal week did not prevent the increase in PPF indicating that this developmental change in release is not critically dependent on normal neural activity during the 2 nd postnatal week.