1. By applying fura_2-based fluorometric calcium imaging to neonatal rat hippocampal slices we identified a developmentally regulated spontaneous neuronal activity in the CA1 region of the hippocampus. The activity consisted of bursts of intracellular Ca¥ transients recurring synchronously at a slow rate of 0·4-2 min¢ in the entire population of pyramidal neurones and interneurones. 2. These early network oscillations (ENOs) were present during a restricted period of postnatal development. Thus, they were not detected at the day of birth (P0), at P1-P4 they consisted of bursts of large (up to 1·5 ìÒ) Ca¥ transients, gradually transforming into regularly occurring, smaller Ca¥ transients during the subsequent week. Beyond P15-P16 no ENOs were detected. 3. The ENOs were blocked by tetrodotoxin (TTX) and by a reduction in temperature from 33-35°C to 20-22°C. By combining fluorometric imaging with whole-cell current-clamp recordings, we found that each ENO-related Ca¥ transient was associated with a highfrequency (up to 100 Hz) train of action potentials riding on a depolarizing wave. 4. Recordings in the voltage-clamp mode revealed barrages of synaptic currents that were strictly correlated with the ENO-associated Ca¥ transients in neighbouring pyramidal neurones. Perfusing the cells with an intracellular solution that allowed for a discrimination between GABAA and glutamate receptor-mediated currents showed that these barrages of synaptic currents were predominantly of GABAergic origin. 5. The ENOs were totally blocked by the GABAA receptor antagonist bicuculline and they were also substantially reduced by the glutamatergic antagonists d,l-2-amino-5-phosphonovaleric acid (d,l-APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 6. Synaptic stimulation and application of the GABAA receptor agonist muscimol mimicked the spontaneous Ca¥ transients in pyramidal neurones. The efficacy of muscimol in evoking Ca¥ transients decreased during development in parallel with the gradual disappearance of the ENOs. 7. The developmental decrease in the amplitude of ENO-associated Ca¥ transients occurred in parallel with the transformation of the excitatory synaptic transmission in the hippocampus from the immature GABAergic to the mature glutamatergic form. Thus, at the beginning of the first postnatal week single-shock synaptic stimulation produced Ca¥ transients that were completely blocked by bicuculline. At the end of the second postnatal week the same type of evoked synaptic stimulation produced a Ca¥ transient that was little affected by bicuculline but was abolished by the combined application of d,l-APV and CNQX. 8. These results demonstrate the presence of periodic and spontaneous Ca¥ transients in the majority of pyramidal cells and interneurones of the neonatal CA1 hippocampal network. These ENOs exhibit a highly region-specific developmental profile and may control the activity-dependent wiring of the synaptic connectivity during early postnatal development.

Brain-derived neurotrophic factor (BDNF) and other neurotrophins are critically involved in long-term potentiation (LTP). Previous reports point to a presynaptic site of neurotrophin action. By imaging dentate granule cells in mouse hippocampal slices, we identified BDNF-evoked Ca2+ transients in dendrites and spines, but not at presynaptic sites. Pairing a weak burst of synaptic stimulation with a brief dendritic BDNF application caused an immediate and robust induction of LTP. LTP induction required activation of postsynaptic Ca2+ channels and N-methyl-d-aspartate receptors and was prevented by the blockage of postsynaptic Ca2+ transients. Thus, our results suggest that BDNF-mediated LTP is induced postsynaptically. Our finding that dendritic spines are the exclusive synaptic sites for rapid BDNF-evoked Ca2+ signaling supports this conclusion.

Synapses are constantly generated at a high rate in the developing, prepubescent brain. Newly generated glutamatergic synapses lack functional AMPA receptor-mediated transmission. Most of these 'AMPA-silent' synapses are eliminated during the developmental period, but some are specifically selected for AMPA unsilencing by correlated pre-and postsynaptic activity as the first step in a process that leads to stabilization of the synapse. Premature, or delayed, unsilencing of AMPA-silent synapses has been implicated in neurodevelopmental disorders, and abnormal generation of AMPA-silent synapses is associated with brain trauma, addiction and neurodegenerative disorders, further highlighting the importance of AMPA-silent synapses in brain pathology.

Factors determining the release probability were examined using whole‐cell patch‐clamp recording and minimal stimulation (10 impulses, 50 Hz) of individual glutamatergic synapses, containing single release sites, on pyramidal neurones in the CA1 region of hippocampal slices from neonatal rats. Release probability following the first action potential in the burst (P1) varied among the synapses from 0 to 0.87 (mean ± s.d.; 0.35 ± 0.28, n= 52) and the average release during the burst (burst pool) varied from 0.4 to 4.1 events (1.7 ± 0.85, n= 52). Heterogeneity in P1 did not co‐vary with that of the burst pool. By selecting burst trials during which only one release event occurred, the vesicle release probability (Pves) at a release site could be determined. It was found to vary considerably among the synapses, from 0.04 to 0.94 (0.43 ± 0.28, n= 43). This heterogeneity correlated significantly with that of P1 such that more than half of the variation of P1 could be explained by a variation in Pves. The average number of vesicles directly available for release at the onset of the burst (the pre‐primed pool) was estimated as the cumulative release up to that point in the burst where a second release event did not produce higher initial release probability than that found in trials where only one vesicle was released. The average pre‐primed pool varied among the synapses from 0.4 to 2.1 (1.03 ± 0.42, n= 43). It co‐varied significantly with that of P1 such that it could explain the remaining variation in P1. The difference between the burst pool and the pre‐primed pool suggests the presence of a fast (< 100 ms), activity‐dependent priming of vesicles. Some synapses (9/52) did not show any initial release (P1= 0), but release occurred later during the burst (‘low frequency mute synapses’). Their behaviour was explained by an absence of a pre‐primed pool.

In the developing brain, many glutamate synapses have been found to transmit only NMDA receptor-mediated signaling, that is, they are AMPA-silent. This result has been taken to suggest that glutamate synapses are initially AMPA-silent when they are formed, and that AMPA signaling is acquired through activity-dependent synaptic plasticity. The present study on CA3-CA1 synapses in the hippocampus of the neonatal rat suggests that AMPA-silent synapses are created through a form of activity-dependent silencing of AMPA signaling. We found that AMPA signaling, but not NMDA signaling, could be very rapidly silenced by presynaptic electrical stimulation at frequencies commonly used to probe synaptic function (0.05-1 Hz). Although this AMPA silencing required a rise in postsynaptic Ca(2+), it did not require activation of NMDA receptors, metabotropic glutamate receptors or voltage-gated calcium channels. The AMPA silencing, possibly explained by a removal of postsynaptic AMPA receptors, could subsequently be reversed by paired presynaptic and postsynaptic activity.