Developmental changes in release properties of the CA3-CA1 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 excitatory postsynaptic potential (EPSP) recordings. Paired-pulse facilitation (PPF) at these synapses was, on average, absent in the first postnatal week but emerged and became successively larger during the second postnatal week. This developmental increase in PPF was associated with a reduction in P-r, as indicated by the slower progressive block of the N-methyl-D-aspartate (NMDA) EPSP by the noncompetitive 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 first 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 Ca2+ influx, seems to explain the reduction in P-r. In vivo injection of tetanus toxin at the end of the first 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 second postnatal week.