Transmission at the hippocampal mossy fibre (MF)–CA3 pyramidal cell synapse is characterized by prominent activity-dependent facilitation, which is thought to provide a wide dynamic range in hippocampal informational flow. At this synapse in mice the magnitude of paired-pulse facilitation and frequency-dependent facilitation markedly decreased with postnatal development from 3 weeks (3W) to 9 weeks (9W). Throughout this period the mean amplitude and variance of unitary EPSCs stayed constant. By altering extracellular Ca2+/Mg2+ concentrations the paired-pulse ratio could be changed to a similar extent as observed during development. However, this was accompanied by an over 30-fold change in EPSC amplitude, suggesting that the developmental change in facilitation ratio cannot simply be explained by a change in release probability. With paired-pulse stimulation the Ca2+ transients at MF terminals, monitored using mag-fura-5, showed a small facilitation, but its magnitude remained similar between 3W and 9W mice. Pharmacological tests using CNQX, adenosine, , H-7 or KN-62 suggested that neither presynaptic receptors (kainate, adenosine and metabotropic glutamate) nor protein kinases are responsible for the developmental change in facilitation. Nevertheless, loading the membrane-permeable form of BAPTA attenuated the paired-pulse facilitation in 3W mice to a much greater extent than in 9W mice, resulting in a marked reduction in age difference. These results suggest that the developmental decrease in the MF synaptic facilitation arises from a change associated with residual Ca2+, a decrease in residual Ca2+ itself or a change in Ca2+-binding sites involved in the facilitation. A developmental decline in facilitation ratio reduces the dynamic range of MF transmission, possibly contributing to the stabilization of hippocampal circuitry.
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