Conversion Of Silent Synapses Research Articles

How Are Synapses Born? A Functional and Molecular View of the Role of the Wnt Signaling Pathway

Synaptic transmission is a dynamic process that requires precise regulation. Early in life, we must be able to forge appropriate connections (add and remove) to control our behavior. Neurons must recognize appropriate targets, and external soluble factors that activate specific signaling cascades provide the regulation needed to achieve this goal. Wnt signaling has been implicated in several forms of synaptic plasticity, including functional and structural changes associated with brain development. The analysis of synapses from an electrophysiological perspective allows us to characterize the functional role of cellular signaling pathways involved in brain development. The application of quantal theory to principles of developmental plasticity offers the possibility of dissecting the function of structural changes associated with the birth of new synapses as well as the maturation of immature silent synapses. Here, we focus on electrophysiological and molecular evidence that the Wnt signaling pathway regulates glutamatergic synaptic transmission, specifically N-methyl-d-aspartate receptors (NMDARs), to control the birth of new synapses. We also focus on the role of Wnts in the conversion of silent synapses into functional synapses.

Control of GABA release at single mossy fiber-CA3 connections in the developing hippocampus

In this review some of the recent work carried out in our laboratory concerning the functional role of GABAergic signalling at immature mossy fibres (MF)-CA3 principal cell synapses has been highlighted. While in adulthood MF, the axons of dentate gyrus granule cells release onto CA3 principal cells and interneurons glutamate, early in postnatal life they release GABA, which exerts into targeted cells a depolarizing and excitatory action. We found that GABAA-mediated postsynaptic currents (MF-GPSCs) exhibited a very low probability of release, were sensitive to L-AP4, a group III metabotropic glutamate receptor agonist, and revealed short-term frequency-dependent facilitation. Moreover, MF-GPSCs were down regulated by presynaptic GABAB and kainate receptors, activated by spillover of GABA from MF terminals and by glutamate present in the extracellular medium, respectively. Activation of these receptors contributed to the low release probability and in some cases to synapses silencing. By pairing calcium transients, associated with network-driven giant depolarizing potentials or GDPs (a hallmark of developmental networks thought to represent a primordial form of synchrony between neurons), generated by the synergistic action of glutamate and GABA with MF activation increased the probability of GABA release and caused the conversion of silent synapses into conductive ones suggesting that GDPs act as coincident detector signals for enhancing synaptic efficacy. Finally, to compare the relative strength of CA3 pyramidal cell output in relation to their MF glutamatergic or GABAergic inputs in adulthood or in postnatal development, respectively, a realistic model was constructed taking into account different biophysical properties of these synapses.

Conversion of Silent Synapses Into the Active Pool by Selective GluR1-3 and GluR4 AMPAR Trafficking During In Vitro Classical Conditioning

The conversion of silent synapses into active sites is hypothesized to be a primary mechanism underlying learning and memory processes. Here we used an in vitro model of classical conditioning from turtles that demonstrates a neural correlate of eyeblink conditioning to examine whether the conversion of silent synapses has a role in this form of associative learning. This was accomplished by direct visualization of AMPA receptor (AMPAR) and N-methyl-d-aspartate receptor (NMDAR) subunits colocalized with synaptophysin (Syn) using immunofluorescence and confocal microscopy. In naive preparations, there was a relatively high level of synapses immunopositive for NR1-Syn alone interpreted to be silent synapses. After early stages of conditioning during acquisition of conditioned responses (CRs), there was a significant increase in the colocalization of GluR1-3 AMPAR subunits at NR1-immunopositive synaptic sites. Later in conditioning, levels of GluR1-3 declined and enhanced colocalization of GluR4-containing AMPAR subunits at synapses was observed. The trafficking of these subunits during conditioning was NMDAR mediated and was accompanied by protein synthesis of GluR4 subunits. Examination of the postsynaptic density fraction confirmed the early and late synaptic insertion of GluR1-3 and GluR4, respectively, during conditioning. These findings suggest that there is differential trafficking of synaptic AMPARs during classical conditioning. Existing GluR1-3 AMPAR subunits are initially delivered to silent synapses early in conditioning to unsilence them followed by synthesis and insertion of GluR4 AMPAR subunits that are required for acquisition and expression of CRs.

Distinct Transmitter Release Properties Determine Differences in Short-Term Plasticity at Functional and Silent Synapses

Recent evidence suggests that functional and silent synapses are not only postsynaptically different but also presynaptically distinct. The presynaptic differences may be of functional importance in memory formation because a proposed mechanism for long-term potentiation is the conversion of silent synapses into functional ones. However, there is little direct experimentally evidence of these differences. We have investigated the transmitter release properties of functional and silent Schaffer collateral synapses and show that on the average functional synapses displayed a lower percentage of failures and higher excitatory postsynaptic current (EPSC) amplitudes than silent synapses at +60 mV. Moreover, functional but not silent synapses show paired-pulse facilitation (PPF) at +60 mV and thus presynaptic short-term plasticity will be distinct in the two types of synapse. We examined whether intraterminal endoplasmic reticulum Ca2+ stores influenced the release properties of these synapses. Ryanodine (100 microM) and thapsigargin (1 microM) increased the percentage of failures and decreased both the EPSC amplitude and PPF in functional synapses. Caffeine (10 mM) had the opposite effects. In contrast, silent synapses were insensitive to both ryanodine and caffeine. Hence we have identified differences in the release properties of functional and silent synapses, suggesting that synaptic terminals of functional synapses express regulatory molecular mechanisms that are absent in silent synapses.

Memory-Making Events?

Nerve cells communicate with one another through specialized cell-cell junctions called synapses, and changes in how efficiently information is transferred across these junctions are believed to underlie memory. Antonova et al. examined the clustering of proteins at synapses in culture as they underwent simulated learning. Within minutes of the teaching stimulus, the amount of a key protein required for sending information, synaptophysin, increased on the presynaptic side of the synapse, and there was a parallel increase in GluR1, the postsynaptic receptor that received the information. The change was unexpectedly rapid (appearing within 5 to 10 minutes) and depended on an intact actin cytoskeleton, which suggests that this clustering may reflect the conversion of silent synapses into active ones. I. Antonova, O. Arancio, A.-C. Trillat, H.-G. Wang, L. Zablow, H. Udo, E. R. Kandel, R. D. Hawkins, Rapid increase in clusters of presynaptic proteins at onset of long-lasting potentiation. Science 294 , 1547-1550 (2001). [Abstract] [Full Text]

Dynamic actin filaments are required for stable long-term potentiation (LTP) in area CA1 of the hippocampus.

The hypothesis that dynamic actin filaments participate in specific aspects of synaptic plasticity was investigated at the Schaffer-collateral-CA1 pyramidal cell synapse of mouse hippocampus. Low concentrations (0.01-1 microM) of compounds that inhibit actin filament assembly were bath applied to hippocampal slices during extracellular recording of field excitatory postsynaptic potentials. Cytochalasin D, cytochalasin B, and latrunculin A all impaired the maintenance of LTP induced by brief high-frequency stimulation. This effect on LTP maintenance was specific, because none of the compounds affected basal synaptic transmission, paired-pulse facilitation, LTP induction, or post-tetanic potentiation. The effect of cytochalasin B was reversible. The results are consistent with a model in which dynamic actin filaments play an essential role in the molecular mechanisms underlying the early maintenance phase of LTP, such as growth of new synaptic connections or conversion of silent synapses.