Silent synapses in the developing hippocampus: lack of functional AMPA receptors or low probability of glutamate release?

Quantal variability at glutamatergic synapses in area CA1 of the rat neonatal hippocampus

Quantal variability was determined at glutamatergic synapses in the neonatal (postnatal days 1-7) rat hippocampal slice preparation. Synaptic AMPA and NMDA quantal EPSCs were recorded from CA1 pyramidal neurones using the whole-cell, or perforated, patch-clamp technique. Release was evoked by minimal stimulation using brief trains (10 impulses, 50 Hz), and various tests ascertained that this stimulation activated single release sites releasing single vesicles. Both AMPA and NMDA quantal responses at a given release site varied considerably in magnitude, the coefficient of variation (CV) among the synapses averaging 0.39 and 0.30, respectively. This variability differed among the synapses, from 0.2 to 0.7, and 0.10 to 0.50, respectively, and CV values of AMPA responses co-varied with those of the NMDA responses. Both for AMPA and NMDA, low CV values were associated with a Gaussian distribution of EPSC peak values, whereas synapses with high CV values displayed distributions skewed towards lower values. Analysis of successive NMDA responses during a train revealed a considerable degree of non-saturation of NMDA receptors by a single vesicle. The results are compatible with a quantal variability based, to a large extent, on non-saturated AMPA and NMDA responses fluctuating as a function of the amount of transmitter released from each vesicle.

Pair recordings reveal all-silent synaptic connections and the postsynaptic expression of long-term potentiation

The activation of silent synapses is a proposed mechanism to account for rapid increases in synaptic efficacy such as long-term potentiation (LTP). Using simultaneous recordings from individual pre- and postsynaptic neurons in organotypic hippocampal slices, we show that two CA3 neurons can be connected entirely by silent synapses. Increasing release probability or application of cyclothiazide does not produce responses from these silent synapses. Direct measurement of NMDAR-mediated postsynaptic responses in all-silent synaptic connections before and after LTP induction show no change in failure rate, amplitude, or area. These data do not support hypotheses that synapse silent results from presynaptic factors or that LTP results from increases in presynaptic glutamate release. LTP is also associated with an increase in postsynaptic responsiveness to exogenous AMPA. We conclude that synapse silence, activation, and expression of LTP are postsynaptic.

A developmental switch in neurotransmitter flux enhances synaptic efficacy by affecting AMPA receptor activation

Formation of glutamatergic synapses entails development of "silent" immature contacts into mature functional synapses. To determine how this transformation occurs, we investigated the development of neurotransmission at single synapses in vitro. Maturation of presynaptic function, assayed with endocytotic markers, followed accumulation of synapsin I. During this period, synaptic transmission was primarily mediated by activation of NMDA receptors, suggesting that most synapses were functionally silent. However, local glutamate application to silent synapses indicated that these synapses contained functional AMPA receptors, suggesting a possible presynaptic locus for silent transmission. Interference with presynaptic vesicle fusion by exposure to tetanus toxin reverted functional to silent transmission, implicating SNARE-mediated fusion as a determinant of the ratio of NMDA:AMPA receptor activation. This work reveals that functional maturation of synaptic transmission involves transformation of presynaptic silent secretion into mature synaptic transmitter release.

Regulation of AMPA receptor endocytosis by a signaling mechanism shared with LTD

The endocytosis of AMPA receptors is thought to be important in the expression of long-term depression (LTD) triggered by NMDA receptor activation. Although signaling pathways necessary for LTD induction have been identified, those responsible for the regulated internalization of AMPA receptors are unknown. Here we show that activation of NMDA receptors alone can trigger AMPA receptor endocytosis through calcium influx and activation of the calcium-dependent protein phosphatase calcineurin. A distinct signaling mechanism mediates the AMPA receptor endocytosis stimulated by insulin. These results demonstrate that although multiple signaling pathways can induce AMPA receptor internalization, NMDA receptor activation enhances AMPA receptor endocytosis via a signaling mechanism required for the induction of LTD.