'Deaf, mute and whispering' silent synapses: their role in synaptic plasticity

Paired-pulse inhibition in the auditory cortex in Parkinson's disease and its dependence on clinical characteristics of the patients

We aimed to determine the value of the paired-pulse inhibition (PPI) in the auditory cortex in patients with Parkinson's disease (PD) and analyze its dependence on clinical characteristics of the patients. The central (Cz) auditory evoked potentials were recorded in 58 patients with PD and 22 age-matched healthy subjects. PPI of the N1/P2 component was significantly (P < .001) reduced for interstimulus intervals 500, 700, and 900 ms in patients with PD compared to control subjects. The value of PPI correlated negatively with the age of the PD patients (P < .05), age of disease onset (P < .05), body bradykinesia score (P < .01), and positively with the Mini Mental State Examination (MMSE) cognitive score (P < .01). Negative correlation between value of PPI and the age of the healthy subjects (P < .05) was also observed. Thus, results show that cortical inhibitory processes are deficient in PD patients and that the brain's ability to carry out the postexcitatory inhibition is age-dependent.

Rapid activation of dormant presynaptic terminals by phorbol esters

Presynaptic stimulation stochastically recruits transmission according to the release probability (P(r)) of synapses. The majority of central synapses have relatively low P(r), which includes synapses that are completely quiescent presynaptically. The presence of presynaptically dormant versus active terminals presumably increases synaptic malleability when conditions demand synaptic strengthening or weakening, perhaps by triggering second messenger signals. However, whether modulator-mediated potentiation involves recruitment of transmission from dormant terminals remains unclear. Here, by combining electrophysiological and fluorescence imaging approaches, we uncovered rapid presynaptic awakening by select synaptic modulators. A phorbol ester phorbol 12,13-dibutyrate (PDBu) (a diacylglycerol analog), but not forskolin (an adenylyl cyclase activator) or elevated extracellular calcium, recruited neurotransmission from presynaptically dormant synapses. This effect was not dependent on protein kinase C activation. After PDBu-induced awakening, these previously dormant terminals had a synaptic P(r) spectrum similar to basally active synapses naive to PDBu treatment. Dormant terminals did not seem to have properties of nascent or immature synapses, judged by NR2B NMDAR (NMDA receptor) receptor subunit contribution after PDBu-stimulated awakening. Strikingly, synapses rendered inactive by prolonged depolarization, unlike basally dormant synapses, were not awakened by PDBu. These results suggest that the initial release competence of synapses can dictate the acute response to second messenger modulation, and the results suggest multiple pathways to presynaptic dormancy and awakening.

Neuronal pentraxins mediate silent synapse conversion in the developing visual system

Neuronal pentraxins (NPs) are hypothesized to play important roles in the recruitment of AMPA receptors (AMPARs) to immature synapses, yet a physiological role for NPs at nascent synapses in vivo has remained elusive. Here we report that the loss of NP1 and NP2 (NP1/2) leads to a dramatic and specific reduction in AMPAR-mediated transmission at developing visual system synapses. In thalamic slices taken from early postnatal mice (<P10) NP1/2 knock-out (KO) neurons displayed severely reduced AMPAR-mediated retinogeniculate transmission. The reduced currents reflected an increased number of silent synapses with no change in quantal amplitude or presynaptic release. These are the first data to demonstrate that NP1/2 are required in vivo for the normal development of AMPAR-mediated transmission. In addition, they suggest a novel role for NP1/2 in silent synapse conversion during a discrete developmental period when visual circuit connections are undergoing eye-specific refinement. After this period, retinogeniculate transmission not only recovered in the knock-outs but became excessive. The enhanced currents were attributable, at least in part, to a deficit in the characteristic elimination of functional inputs that occurs in the developing dLGN. These data indicate that the loss of NP1/2 disrupts several aspects of retinogeniculate development including the initial establishment of AMPAR transmission and the subsequent elimination of inappropriate circuit connections.

Maturation of Schaffer collateral synapses generates a phenotype of unreliable basal evoked release and very reliable facilitated release

Short-term synaptic plasticity undergoes important age-dependent changes that have crucial implications during the development of the nervous system. Paired-pulse facilitation is a form of short-term synaptic plasticity by which the response to the second of two temporally-paired stimuli is larger and more reliable than the response to the first stimulus. In this study, a paired-pulse minimal stimulation technique was used to measure the probability and quantal amplitude of synaptic release at hippocampal synapses from 12-16-day-old (young) and 7-9-week-old (adult) rats. In order to assess the contribution of temperature-dependent processes, we carried out experiments at both room temperature and at near physiological temperature. We report here that neither temperature nor maturation affected the low basal evoked release probability and quantal amplitude of release. However, the warmer temperature revealed a unique developmental increase in facilitated evoked release probability and quantal amplitude of release. As a result, although both basal evoked release and facilitated release are rather unreliable in synapses from young animals, the maturation process at near physiological temperature generates a phenotype with unreliable basal evoked release and highly reliable facilitated release.

Properties of glutamatergic synapses in immature layer Vb pyramidal neurons: coupling of pre- and postsynaptic maturational states

Following initial contact formation, glutamatergic synapses in cortical neurons undergo pronounced functional maturation. These maturational events, occurring both pre- and postsynaptically, have been well described in the developing hippocampus. In this paper, we characterized glutamatergic synapses in immature layer Vb pyramidal neurons of the mouse somatosensory cortex during early postnatal development. At postnatal day 7, a significant subpopulation of glutamatergic synapses exhibited a low release probability that was accompanied by strong paired-pulse facilitation of AMPA EPSCs (paired-pulse ratio C > or = 2). Increasing extracellular Ca(2+) concentration increased release probability and led to paired-pulse depression. During further postnatal development, these functionally immature synapses disappeared. As shown pharmacologically,these synapses expressed postsynaptic NMDA receptors containing NR2B subunits, while NMDA receptors with NR2A subunits were lacking. Taken together, a low release probability presynaptically was coupled to postsynaptic NR2B signaling. This subpopulation of neocortical synapses thus differed from the majority of synapses in the developing hippocampus, where high release probability is coupled to NR2B signaling. The novel type of functionally immature glutamatergic synapse described here might play an important role in early developmental synapse elimination and in the activity-dependent refinement of the neocortical synaptic microcircuitry.

Control of GABA Release at 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 GABA_A-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 GABA_B 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.

Rapid and long-lasting increase in sites for synapse assembly during late-phase potentiation in rat hippocampal neurons

Long-term potentiation in hippocampal neurons has stages that correspond to the stages of learning and memory. Early-phase (10–30 min) potentiation is accompanied by rapid increases in clusters or puncta of presynaptic and postsynaptic proteins, which depend on actin polymerization but not on protein synthesis. We have now examined changes in pre- and postsynaptic puncta and structures during glutamate-induced late-phase (3 hr) potentiation in cultured hippocampal neurons. We find that (1) the potentiation is accompanied by long-lasting maintenance of the increases in puncta, which depends on protein synthesis, (2) most of the puncta and synaptic structures are very dynamic, continually assembling and disassembling at sites that are more stable than the puncta or structures themselves, (3) the increase in presynaptic puncta appears to be due to both rapid and more gradual increases in the number of sites where the puncta may form, and also to the stabilization of existing puncta, (4) under control conditions, puncta of postsynaptic proteins behave similarly to puncta of presynaptic proteins and share sites with them, and (5) the increase in presynaptic puncta is accompanied by a similar increase in presumably presynaptic structures, which may form at distinct as well as shared sites. The new sites could contribute to the transition between the early and late phase mechanisms of plasticity by serving as seeds for the formation and maintenance of new synapses, thus acting as local “tags” for protein synthesis-dependent synaptic growth during late-phase plasticity.

BDNF signaling in the formation, maturation and plasticity of glutamatergic and GABAergic synapses

In the past 15 years numerous reports provided strong evidence that brain-derived neurotrophic factor (BDNF) is one of the most important modulators of glutamatergic and GABAergic synapses. Remarkable progress regarding localization, kinetics, and molecular mechanisms of BDNF secretion has been achieved, and a large number of studies provided evidence that continuous extracellular supply of BDNF is important for the proper formation and functional maturation of glutamatergic and GABAergic synapses. BDNF can play a permissive role in shaping synaptic networks, making them more susceptible for the occurrence of plastic changes. In addition, BDNF appears to be also an instructive factor for activity-dependent long-term synaptic plasticity. BDNF release just in response to synaptic stimulation might be a molecular trigger to convert high-frequency synaptic activity into long-term synaptic memories. This review attempts to summarize the current knowledge in synaptic secretion and synaptic action of BDNF, including both permissive and instructive effects of BDNF in synaptic plasticity.

beta-Phorbol ester-induced enhancement of exocytosis in large mossy fiber boutons of mouse hippocampus

beta-Phorbol esters (BPE), synthetic analogues of diacylglycerol (DAG), induce the potentiation of transmission in many kinds of synapses through activating the C(1) domain-containing receptors. However, their effects on synaptic vesicle exocytosis have not yet been investigated. Here, we evaluated the vesicular exocytosis directly from individual large mossy fiber boutons (LMFBs) in hippocampal slices from transgenic mice that selectively express synaptopHluorin (SpH). We found that the activity-dependent increment of SpH fluorescence (DeltaSpH) was enhanced by 4beta-phorbol 12,13-diacetate (PDAc), one of the BPEs, without influencing the recycled component of SpH. These PDAc effects on DeltaSpH were almost completely inhibited by staurosporine, a non-selective antagonist of protein kinases. However, intermittent synaptic transmission was still potentiated through a staurosporine-resistant mechanism. The staurosporine-sensitive cascade may facilitate the vesicle replenishment, thus maintaining the fidelity of transmission at a high level during repetitive firing of the presynaptic neuron.

Regulation of postsynaptic structure and function by an A-kinase anchoring protein-membrane-associated guanylate kinase scaffolding complex

A-kinase anchoring protein (AKAP) 79/150 is a scaffold protein found in dendritic spines that recruits the cAMP-dependent protein kinase (PKA) and protein phosphatase 2B-calcineurin (CaN) to membrane-associated guanylate kinase (MAGUK)-linked AMPA receptors (AMPARs) to control receptor phosphorylation and synaptic plasticity. However, AKAP79/150 may also coordinate regulation of AMPAR activity with spine structure directly through MAGUK binding and membrane-cytoskeletal interactions of its N-terminal targeting domain. In cultured hippocampal neurons, we observed that rat AKAP150 expression was low early in development but then increased coincident with spine formation and maturation. Overexpression of human AKAP79 in immature or mature neurons increased the number of dendritic filopodia and spines and enlarged spine area. However, RNA interference knockdown of AKAP150 decreased dendritic spine area only in mature neurons. Importantly, AKAP79 overexpression in immature neurons increased AMPAR postsynaptic localization and activity. Neither the AKAP79 PKA nor CaN anchoring domain was required for increasing dendritic protrusion numbers, spine area, or AMPAR synaptic localization; however, an internal region identified as the MAGUK binding domain was found to be essential as shown by expression of a MAGUK binding mutant that formed mainly filopodia and decreased AMPAR synaptic localization and activity. Expression of the AKAP79 N-terminal targeting domain alone also increased filopodia numbers but not spine area. Overall, these results demonstrate a novel structural role for AKAP79/150 in which the N-terminal targeting domain induces dendritic filopodia and binding to MAGUKs promotes spine enlargement and AMPAR recruitment.

Expression of long-term plasticity at individual synapses in hippocampus is graded, bidirectional, and mainly presynaptic: optical quantal analysis

Key aspects of the expression of long-term potentiation (LTP) and long-term depression (LTD) remain unresolved despite decades of investigation. Alterations in postsynaptic glutamate receptors are believed to contribute to the expression of various forms of LTP and LTD, but the relative importance of presynaptic mechanisms is controversial. In addition, while aggregate synaptic input to a cell can undergo sequential and graded (incremental) LTP and LTD, it has been suggested that individual synapses may only support binary changes between initial and modified levels of strength. We have addressed these issues by combining electrophysiological methods with two-photon optical quantal analysis of plasticity at individual active (non-silent) Schaffer collateral synapses on CA1 pyramidal neurons in acute slices of hippocampus from adolescent rats. We find that these synapses sustain graded, bidirectional long-term plasticity. Remarkably, changes in potency are small and insignificant; long-term plasticity at these synapses is expressed overwhelmingly via presynaptic changes in reliability of transmitter release.

Chloride accumulation drives volume dynamics underlying cell proliferation and migration

During brain development, progenitor cells migrate over long distances through narrow and tortuous extracellular spaces posing significant demands on the cell's ability to alter cell volume. This phenotype is recapitulated in primary brain tumors. We demonstrate here that volume changes occurring spontaneously in these cells are mediated by the flux of Cl- along with obligated water across the cell membrane. To do so, glioma cells accumulate Cl- to approximately 100 mM, a concentration threefold greater than predicted by the Nernst equation. Shunting this gradient through the sustained opening of exogenously expressed GABA-gated Cl- channels caused a 33% decrease in cell volume and impaired the ability of cells to migrate in a spatially constrained environment. Further, dividing cells condense their cytoplasm prior to mitosis, a phenomenon which is associated with the release of intracellular Cl- as indicated by a 40-mM decrease in [Cl-]i. These findings provide a new framework for considering the role of intracellular Cl- in glioma cells. Here, Cl- serves as an important osmotically active regulator of cell volume being the energetic driving force for volume changes required by immature cells in cell migration and proliferation. This mechanism that was studied in CNS malignancies may be shared with other immature cells in the brain as well.

Synaptic strength and postsynaptically silent synapses through advanced aging in rat hippocampal CA1 pyramidal neurons

Synaptic dysfunction is thought to contribute to age-related learning impairments. Detailed information regarding the presence of silent synapses and the strength of functional ones through advanced aging, however, is lacking. Here we used paired-pulse minimal stimulation techniques in CA1 stratum radiatum to determine whether the amplitude of spontaneous and evoked miniature excitatory postsynaptic currents (sEPSCs and eEPSCs, respectively) changes over the lifespan of rats in hippocampal CA1 pyramidal neurons, and whether silent synapses are present in adult and aged rats. The amplitudes of both sEPSCs and eEPSCs at resting membrane potential (i.e., clamped at -65 mV) initially increased between 2 weeks and 3 months, but then remained constant through 36 months of age. The potency of the eEPSCs at depolarized membrane potentials (i.e., clamped at +40 mV), however, was highest among 36-month old rats. Additionally, presynaptically silent synapses in CA1 stratum radiatum disappeared between 2 weeks and 3 months, but postsynaptically silent synapses were present through advanced aging. The similarity of silent and functional synapses in CA1 hippocampus at resting membrane potentials throughout adulthood in rats may indicate that impairments in the mechanisms of synaptic plasticity and its subsequent stabilization, rather than deficient synaptic transmission, underlie age-related cognitive decline. Such a notion is consistent with the increased amplitude of synaptic currents at depolarized potentials, perhaps suggesting an upregulation in the expression of synaptic NMDA receptors once rats reach advanced age.

An unexpected role for ion channels in brain tumor metastasis

Over the past two decades it has become apparent that essentially all living cells express voltage-activated ion channels. While the role of ion channels for electrical signaling between excitable cells is well known, their function in non-excitable cells is somewhat enigmatic. Research on cancer cells suggests that certain ion channels, K+ channels in particular, may be involved in aberrant tumor growth and channel inhibitors often lead to growth arrest. An unsuspected role for K+ and Cl(-) channels has now been documented for primary brain tumors, glioma, where the concerted activity of these channels promotes cell invasion and the formation of brain metastasis. Specifically, Ca2+-activated K+ (BK) channels colocalize with ClC-3 Cl(-) channels to the invading processes of these tumor cells. Upon a rise in intracellular Ca2+, these channels activate and release K+ and Cl(-) ions together with obligated water causing a rapid shrinkage of the leading process. This in turn facilitates the invasion of the cell into the narrow and tortuous extracellular brain spaces. The NKCC1 cotransporter accumulates intracellular Cl(-) to unusually high concentrations, thereby establishing an outward directed gradient for Cl(-) ions. This allows glioma cells to utilize Cl(-) as an osmotically active anion during invasion. Importantly, the inhibition of Cl(-) channels retards cell volume changes, and, in turn, compromises tumor cell invasion. These findings have led to the clinical evaluation of a Cl(-) channel blocking peptide, chlorotoxin, in patients with malignant glioma. Data from this clinical trial shows remarkable tumor selectivity for chlorotoxin. The experimental therapeutic was well tolerated and is now evaluated in a multi-center phase II clinical trial. A similar role for Cl(-) and K+ channels is suspected in other metastatic cancers, and lessons learned from studies of gliomas may pave the way towards the development of novel therapeutics targeting ion channels.

Silent synapses in developing rat nucleus tractus solitarii have AMPA receptors

NMDA-only synapses, called silent synapses, are thought to be the initial step in synapse formation in several systems. However, the underlying mechanism and the role in circuit construction are still a matter of dispute. Using combined morphological and electrophysiological approaches, we searched for silent synapses at the level of the nucleus tractus solitarii (NTS), a brainstem structure that is a gateway for many visceral sensory afferent fibers. Silent synapses were detected at birth by using electrophysiological recordings and minimal stimulation protocols. However, anatomical experiments indicated that nearly all, if not all, NTS synapses had AMPA receptors. Based on EPSC fluctuation measurements and differential blockade by low-affinity competitive and noncompetitive glutamate antagonists, we then demonstrated that NTS silent synapses were better explained by glutamate spillover from neighboring fibers and/or slow dynamic of fusion pore opening. Glutamate spillover at immature NTS synapses may favor crosstalk between active synapses during development when glutamate transporters are weakly expressed and contribute to synaptic processing as well as autonomic circuit formation.

Oxytocin-induced antinociception in the spinal cord is mediated by a subpopulation of glutamatergic neurons in lamina I-II which amplify GABAergic inhibition

BACKGROUND

Recent evidence suggests that oxytocin (OT), secreted in the superficial spinal cord dorsal horn by descending axons of paraventricular hypothalamic nucleus (PVN) neurons, produces antinociception and analgesia. The spinal mechanism of OT is, however, still unclear and requires further investigation. We have used patch clamp recording of lamina II neurons in spinal cord slices and immunocytochemistry in order to identify PVN-activated neurons in the superficial layers of the spinal cord and attempted to determine how this neuronal population may lead to OT-mediated antinociception.

RESULTS

We show that OT released during PVN stimulation specifically activates a subpopulation of lamina II glutamatergic interneurons which are localized in the most superficial layers of the dorsal horn of the spinal cord (lamina I-II). This OT-specific stimulation of glutamatergic neurons allows the recruitment of all GABAergic interneurons in lamina II which produces a generalized elevation of local inhibition, a phenomenon which might explain the reduction of incoming Adelta and C primary afferent-mediated sensory messages.

CONCLUSION

Our results obtained in lamina II of the spinal cord provide the first clear evidence of a specific local neuronal network that is activated by OT release to induce antinociception. This OT-specific pathway might represent a novel and interesting therapeutic target for the management of neuropathic and inflammatory pain.

Repetitive induction of late-phase LTP produces long-lasting synaptic enhancement accompanied by synaptogenesis in cultured hippocampal slices

Long-term plasticity of synaptic transmission is assumed to underlie the formation of long-term memory. Although the cellular mechanisms underlying short-term plasticity have been analyzed in detail, the mechanisms underlying the transformation from short-term to long-term plasticity remain largely unrevealed. We propose the novel long-lasting phenomenon as a model system for the analysis of long-term plasticity. We previously reported that the repetitive activation of cAMP-dependent protein kinase (PKA) by forskolin application led to an enhancement in synaptic strength coupled with synaptogenesis that lasted more than 3 weeks in cultured rat hippocampal slices. To elucidate whether this long-lasting synaptic enhancement depended on the induction of long-term potentiation (LTP) or on the pharmacological effect of forskolin, we applied glutamate (Glu) and correlated its dose with the production of the long-lasting synaptic enhancement. When the dose of Glu was low (10, 30 muM), only transient excitation or early-phase LTP (E-LTP) was induced by a single application and no long-lasting synaptic enhancement was produced by three applications. When the dose was raised to 100 or 300 muM, late-phase LTP (L-LTP) was induced by a single application and long-lasting synaptic enhancement was produced by three applications. The Glu-produced enhancement was accompanied by an increase in the frequency (but not the amplitude) of miniature EPSC and the number of synaptic structures. The enhancement depended on the interval of repetition and protein synthesis immediately after the Glu applications. These results indicate that the repetitive induction of L-LTP, but not E-LTP or transient excitation, triggers cellular processes leading to the long-lasting synaptic enhancement and the formation of new synapses.

Pyramidal neurons: dendritic structure and synaptic integration

Pyramidal neurons are characterized by their distinct apical and basal dendritic trees and the pyramidal shape of their soma. They are found in several regions of the CNS and, although the reasons for their abundance remain unclear, functional studies--especially of CA1 hippocampal and layer V neocortical pyramidal neurons--have offered insights into the functions of their unique cellular architecture. Pyramidal neurons are not all identical, but some shared functional principles can be identified. In particular, the existence of dendritic domains with distinct synaptic inputs, excitability, modulation and plasticity appears to be a common feature that allows synapses throughout the dendritic tree to contribute to action-potential generation. These properties support a variety of coincidence-detection mechanisms, which are likely to be crucial for synaptic integration and plasticity.

Brain-derived neurotrophic factor but not forced arm use improves long-term outcome after photothrombotic stroke and transiently upregulates binding densities of excitatory glutamate receptors in the rat brain

BACKGROUND AND PURPOSE

Both application of neurotrophic factors like brain-derived neurotrophic factor (BDNF) and constraint-induced movement therapy like forced arm use have been shown to potentially improve outcome after stroke. The aim of the present study was to check whether postischemic long-term outcome correlates to specific modifications in the abundance of various neurotransmitter receptors.

METHODS

Adult male Wistar rats were subjected to photothrombotic ischemia and assigned to various treatment groups (n=5 each) with end points at 3 and 6 weeks: (1) ischemic control (saline); (2) BDNF (ischemia, 20 microg BDNF); (3) forced arm use (ischemia, saline, and ipsilateral plaster cast for 5 or 14 days for the 3- and 6-week groups, respectively); and (4) combined treatment (combi; ischemia, 20 microg BDNF, forced arm use). Animals received intravenous bolus infusions of saline or BDNF 1 hour 3 and 5 days after ischemia, respectively. A group of sham rats (n=2) served as a control. A battery of behavioral tests was performed before and up to 6 weeks after ischemia. Quantitative in vitro receptor autoradiography was performed on 12-microm-thick cryostat sections using [(3)H]MK-801, [(3)H]AMPA, and [(3)H]muscimol for labeling of NMDA, AMPA, and GABA(A) receptors, respectively.

RESULTS

Best functional outcome was seen after BDNF treatment, whereas vice versa rats with forced arm use did worse in behavioral performance. Improved behavioral outcome was associated with increased perilesional binding densities of NMDA and AMPA receptors 3 weeks after stroke.

CONCLUSIONS

Our findings suggest that transient enhanced neurotransmission as reflected by increased ligand binding of NMDA and AMPA receptors may participate in successful postlesional reorganization processes.

Developmental presence and disappearance of postsynaptically silent synapses on dendritic spines of rat layer 2/3 pyramidal neurons

Silent synapses are synapses whose activation evokes NMDA-type glutamate receptor (NMDAR) but not AMPA-type glutamate receptor (AMPAR) mediated currents. Silent synapses are prominent early in postnatal development and are thought to play a role in the activity- and sensory-dependent refinement of neuronal circuits. The mechanisms that account for their silent nature have been controversial, and both presynaptic and postsynaptic mechanisms have been proposed. Here, we use two-photon laser uncaging of glutamate to directly activate glutamate receptors and measure AMPAR- and NMDAR-dependent currents on individual dendritic spines of rat somatosensory cortical layer 2/3 pyramidal neurons. We find that dendritic spines lacking functional surface AMPARs are commonly found before postnatal day 12 (P12) but are absent in older animals. Furthermore, AMPAR-lacking spines are contacted by release-competent presynaptic terminals. After P12, the AMPAR/NMDAR current ratio at individual spines continues to increase, consistent with continued addition of AMPARs to postsynaptic terminals. Our results confirm the existence of postsynaptically silent synapses and demonstrate that the morphology of the spine is not strongly predictive of its AMPAR content.

The continuing case for the Renshaw cell

Renshaw cell properties have been studied extensively for over 50 years, making them a uniquely well-defined class of spinal interneuron. Recent work has revealed novel ways to identify Renshaw cells in situ and this in turn has promoted a range of studies that have determined their ontogeny and organization of synaptic inputs in unprecedented detail. In this review we illustrate how mature Renshaw cell properties and connectivity arise through a combination of activity-dependent and genetically specified mechanisms. These new insights should aid the development of experimental strategies to manipulate Renshaw cells in spinal circuits and clarify their role in modulating motor output.

Functional maturation of excitatory synapses in layer 3 pyramidal neurons during postnatal development of the primate prefrontal cortex

In the primate dorsolateral prefrontal cortex (DLPFC), the density of excitatory synapses decreases by 40-50% during adolescence. Although such substantial circuit refinement might underlie the adolescence-related maturation of working memory performance, its functional significance remains poorly understood. The consequences of synaptic pruning may depend on the properties of the eliminated synapses. Are the synapses eliminated during adolescence functionally immature, as is the case during early brain development? Or do maturation-independent features tag synapses for pruning? We examined excitatory synaptic function in monkey DLPFC during postnatal development by studying properties that reflect synapse maturation in rat cortex. In 3-month-old (early postnatal) monkeys, excitatory inputs to layer 3 pyramidal neurons had immature properties, including higher release probability, lower alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/N-methyl-D-aspartate (NMDA) ratio, and longer duration of NMDA-mediated synaptic currents, associated with greater sensitivity to the NMDA receptor subunit B (NR2B) subunit-selective antagonist ifenprodil. In contrast, excitatory synaptic inputs in neurons from preadolescent (15 months old) and adult (42 or 84 months old) monkeys had similar functional properties. We therefore conclude that the contribution of functionally immature synapses decreases significantly before adolescence begins. Thus, remodeling of excitatory connectivity in the DLPFC during adolescence may occur in the absence of widespread maturational changes in synaptic strength.

Activation of silent synapses with sustained but not decremental long-term potentiation

Silent synapses display no excitatory post-synaptic currents (EPSCs) at resting potentials, but can conduct at depolarized potentials. In the hippocampal CA1 region of young animals, conversion of silent synapses to functional synapses occurs rapidly after pairing post-synaptic depolarization with 1Hz pre-synaptic stimulation, a protocol that also induces long-term potentiation (LTP). LTP appears to have a decremental phase and a sustained phase. Many studies have shown that decremental LTP can be pharmacologically isolated from sustained LTP, suggesting that they represent two distinct forms, rather than "phases" of LTP that are expressed simultaneously through different mechanisms. We investigated whether silent synapse activation (SSA) is associated specifically with the expression of sustained or decremental LTP. We found that under control conditions, in which sustained and decremental LTP were induced, SSA was observed. However, under conditions in which only decremental LTP was expressed (in the presence of a protein kinase antagonist), SSA did not occur. We conclude that SSA is associated with the expression of sustained LTP, not decremental LTP, and requires protein kinase activation. These findings support the hypothesis that decremental and sustained LTP are expressed through different mechanisms.

Primary afferent synapses on developing and adult Renshaw cells

The mechanisms that diversify adult interneurons from a few pools of embryonic neurons are unknown. Renshaw cells, Ia inhibitory interneurons (IaINs), and possibly other types of mammalian spinal interneurons have common embryonic origins within the V1 group. However, in contrast to IaINs and other V1-derived interneurons, adult Renshaw cells receive motor axon synapses and lack proprioceptive inputs. Here, we investigated how this specific pattern of connectivity emerges during the development of Renshaw cells. Tract tracing and immunocytochemical markers [parvalbumin and vesicular glutamate transporter 1 (VGLUT1)] showed that most embryonic (embryonic day 18) Renshaw cells lack dorsal root inputs, but more than half received dorsal root synapses by postnatal day 0 (P0) and this input spread to all Renshaw cells by P10-P15. Electrophysiological recordings in neonates indicated that this input is functional and evokes Renshaw cell firing. VGLUT1-IR bouton density on Renshaw cells increased until P15 but thereafter decreased because of limited synapse proliferation coupled with the enlargement of Renshaw cell dendrites. In parallel, Renshaw cell postsynaptic densities apposed to VGLUT1-IR synapses became smaller in adult compared with P15. In contrast, vesicular acetylcholine transporter-IR motor axon synapses contact embryonic Renshaw cells and proliferate postnatally matching Renshaw cell growth. Like other V1 neurons, Renshaw cells are thus competent to receive sensory synapses. However, after P15, these sensory inputs appear deselected through arrested proliferation and synapse weakening. Thus, Renshaw cells shift from integrating sensory and motor inputs in neonates to predominantly motor inputs in adult. Similar synaptic weight shifts on interneurons may be involved in the maturation of motor reflexes and locomotor circuitry.

Protein kinase C activity affects neurotransmitter release at polyinnervated neuromuscular synapses

By using intracellular recording, we studied how protein kinase C (PKC) activity affected transmitter release in singly and dually innervated endplates of the Levator auris longus muscle of 5-6-day-old rats during axonal competition in the postnatal synaptic elimination period. In dually innervated fibers, a second endplate potential (EPP) may appear after the first one when the stimulation intensity is increased. The nerve terminals that generate the lowest and the highest EPP amplitudes are designated "small-EPP generating ending" (SEGE) and "large-EPP generating ending" (LEGE), respectively. Blocking PKC with calphostin C, staurosporine, or chelerythrine results in an increased release from SEGE ( approximately 80%), whereas release from LEGE and from endings generating only one EPP (OEGE) is not significantly affected. Blocking PKC also leads to the recruitment of silent synapses (acetylcholine cannot be released before PKC inhibition). The mean number of functional axon terminals per synapse increases by approximately 47%, and these are now designated the "recruited-EPP generating endings" (REGE). This suggests that axonal PKC can modulate postnatal synaptic elimination by favoring the nerve terminal disconnection of certain weak axonal endings (REGE and SEGE). We conclude that a PKC-mediated mechanism should occupy a pivotal place in neonatal synapse elimination, because functional axonal withdrawal can indeed be turned back by PKC block.

Simultaneous monitoring of three key neuronal functions in primary neuronal cultures

The coupling of Ca²⁺ influx to synaptic vesicle (SV) recycling in nerve terminals is essential for neurotransmitter release and thus neuronal communication. Both of these parameters have been monitored using fluorescent reporter dyes such as fura-2 and FM1-43 in single central nerve terminals. However, their simultaneous monitoring has been hampered by the proximity of their fluorescence spectra, resulting in significant contamination of their signals by bleedthrough. We have developed an assay that simultaneously monitors both SV recycling and changes in intracellular free Ca²⁺ ([Ca²⁺]_i) in cultured neurons using the reporter dyes FM4-64 and fura-2AM. By monitoring both fura-2 and FM4-64 emission in the far red range, we were able to visualize functionally independent readouts of both SV recycling and [Ca²⁺]_i independent of fluorescence bleedthrough. We were also able to incorporate an assay of cell viability without any fluorescence bleedthrough from either fura-2 or FM4-64 signals, using the dye SYTOX Green. We propose that this assay of three key neuronal functions could be simply translated into a high content screening format for studies investigating small molecule inhibitors of these processes.

Synapse development: still looking for the forest, still lost in the trees

Synapse development in the vertebrate central nervous system is a highly orchestrated process occurring not only during early stages of brain development, but also (to a lesser extent) in the mature nervous system. During development, the formation of synapses is intimately linked to the differentiation of neuronal cells, the extension of their axons and dendrites, and the course wiring of the nervous system. Subsequently, the stabilization, elimination, and strengthening of synaptic contacts is coupled to the refinement of axonal and dendritic arbors, to the establishment of functionally meaningful connections, and probably also to the day-to-day acquisition, storage, and retrieval of memories, higher order thought processes, and behavioral patterns.

Protein kinase Mzeta enhances excitatory synaptic transmission by increasing the number of active postsynaptic AMPA receptors

Protein kinase Mzeta (PKMzeta), a constitutively active, atypical PKC isoform, enhances synaptic strength during the maintenance of long-term potentiation (LTP). Here we examine the mechanism by which PKMzeta increases synaptic transmission. Postsynaptic perfusion of PKMzeta during whole-cell recordings of CA1 pyramidal cells strongly potentiated the amplitude of AMPA receptor (AMPAR)-mediated miniature EPSCs (mEPSCs). Nonstationary fluctuation analysis of events recorded before and after PKMzeta enhancement showed that the kinase doubled the number of functional postsynaptic AMPAR channels. After sustained potentiation, application of a PKMzeta inhibitor reversed the increase in functional channel number to basal levels, suggesting that persistent increase of PKMzeta is required to maintain the postsynaptic localization of a mobile subpopulation of receptors. The kinase did not affect other sites of LTP expression, including presynaptic transmitter release, silent synapse conversion, or AMPAR unit conductance. Thus PKMzeta functions specifically to establish and maintain long-term increases in active postsynaptic AMPAR number.

Quantifying the magnitude of changes in synaptic level parameters with long-term potentiation

Experimental evidence supports a number of mechanisms for the synaptic change that occurs with long-term potentiation (LTP) including insertion of AMPA receptors, an increase in AMPA receptor single channel conductance, unmasking silent synapses, and increases in vesicle release probability. Here we combine experimental and modeling studies to quantify the magnitude of the change needed at the synaptic level to explain LTP with these proposed mechanisms. Whole cell patch recordings were used to measure excitatory postsynaptic potential (EPSP) amplitude in response to near minimal afferent stimulation before and after LTP induction in CA1 pyramidal cells. Detailed neuron and synapse level models were constructed to estimate quantitatively the changes needed to explain the experimental results. For cells in normal artificial cerebrospinal fluid (ACSF), we found a 60% average increase in EPSP amplitude with LTP. This was explained in the models by a 63% increase in the number of activated synapses, a 64% increase in the AMPA receptor single channel conductance, or a 73% increase in the number of AMPA receptors per potentiated synapse. When the percentage LTP was above the average, the required increases through the proposed mechanisms became nonlinear, particularly for increases in the number of receptors. Given constraints from other experimental studies, our quantification suggests that neither unmasking silent synapses nor increasing the numbers of AMPA receptors at synapses is sufficient to explain the magnitude of LTP we observed, but increasing AMPA single channel conductance or vesicle release probability can be sufficient. Our results are most compatible with a combination of mechanisms producing LTP.

Associative memory stored by functional novel pathway rather than modifications of preexisting neuronal pathways

Associative conditioning involves changes in the processing pathways activated by sensory information to link the conditioned stimulus (CS) to the conditioned behavior. Thus, conditioning can recruit neuronal elements to form new pathways for the processing of the CS and/or can change the strength of existing pathways. Using a behavioral and systems level electrophysiological approach on a tractable invertebrate circuit generating feeding in the mollusk Lymnaea stagnalis, we identified three independent pathways for the processing of the CS amyl acetate used in appetitive conditioning. Two of these pathways, one suppressing and the other stimulating feeding, mediate responses to the CS in naive animals. The effects of these two pathways on feeding behavior are unaltered by conditioning. In contrast, the CS response of a third stimulatory pathway is significantly enhanced after conditioning, becoming an important contributor to the overall CS response. This is unusual because, in most of the previous examples in which naive animals already respond to the CS, memory formation results from changes in the strength of pathways that mediate the existing response. Here, we show that, in the molluscan feeding system, both modified and unmodified pathways are activated in parallel by the CS after conditioning, and it is their integration that results in the conditioned response.

GABAergic signaling at mossy fiber synapses in neonatal rat hippocampus

In the adult rat hippocampus, granule cell mossy fibers (MFs) form excitatory glutamatergic synapses with CA3 principal cells and local inhibitory interneurons. However, evidence has been provided that, in young animals and after seizures, the same fibers can release in addition to glutamate GABA. Here we show that, during the first postnatal week, stimulation of granule cells in the dentate gyrus gave rise to monosynaptic GABAA-mediated responses in principal cells and in interneurons. These synapses were indeed made by MFs because they exhibited strong paired-pulse facilitation, high sensitivity to the metabotropic glutamate receptor agonist l-AP-4, and short-term frequency-dependent facilitation. MF responses were potentiated by blocking the plasma membrane GABA transporter GAT-1 with NO-711 or by allosterically modulating GABAA receptors with flurazepam. Chemical stimulation of granule cell dendrites with glutamate induced barrages of GABAA-mediated postsynaptic currents into target neurons. Furthermore, immunocytochemical experiments demonstrated colocalization of vesicular GABA transporter with vesicular glutamate transporter-1 and zinc transporter 3, suggesting that GABA can be taken up and stored in synaptic vesicles of MF terminals. Additional fibers releasing both glutamate and GABA into principal cells and interneurons were recruited by increasing the strength of stimulation. Both the GABAergic and the glutamatergic component of synaptic currents occurred with the same latency and were reversibly abolished by l-AP-4, indicating that they originated from the MFs. GABAergic signaling may play a crucial role in tuning hippocampal network during postnatal development. Low-threshold GABA-releasing fibers may undergo elimination, and this may occur when GABA shifts from the depolarizing to the hyperpolarizing direction.

[Brain synaptogenesis and epigenesis]

Synaptic plasticity, or epigenesis, is present and varies throughout the whole life of the cerebral cortex. The adult synapse is formed of large and variable proteins assemblies acting as molecular switches leading to many distinct functional states. In the flow of activity circulating through the synaptic circuits, these multiple synaptic states transitions are modulated by the levels and sequences of activations of the pre- and post-synaptic domains. The efficiency of synaptic transmission is also modulated by competition and/or cooperativity with neighbouring synapses, and by many neuromodulations. Some transitions eventually lead to synaptogenesis. In the adult cerebral cortex, synaptogenesis remains a local event; axonal and dendritic arbors are not reshaped. On the contrary, during pre- and post-natal synaptogenesis, the same molecular mechanisms lead to a significant reorganization of the axonal and dendritic arbors. Early in the development, synapses are generated and differentiate under the control of robust mechanisms governed by genes. Then, during the critical periods, extending from the end of gestation to the end of puberty, the refinement of the synaptic architecture becomes experience-expectant. This "epigenetic opening" of synaptogenesis to environment is maximal in the human brain. It is the source of the exceptional cognitive adaptability of our species, and possibly one of its major fragility. Epigenetic manipulations of these critical periods are undertaken, allowing restoration of synaptic plasticity also in the adult brain.

AMPA signalling in nascent glutamatergic synapses: there and not there!

Nascent glutamatergic synapses are thought to be equipped with only NMDA receptors and to mature in a stepwise fashion when AMPA receptors are acquired later, through specific patterns of activity. We review recent data suggesting that AMPA receptors are in fact present in the nascent synapse but in a labile state. The nascent synapse can easily switch between AMPA-signalling and AMPA-silent states in a manner not requiring activation of NMDA receptors or metabotropic glutamate receptors. NMDA receptor activation by correlated presynaptic and postsynaptic activity can switch the nascent synapse to a mature, more stable state, in which AMPA receptor signalling is modified only through conventional plasticity processes. Thus, the AMPA receptor silence of nascent glutamatergic synapses depends on the synaptic activation history rather than on the nascent state itself.

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.

Recruitment of synapses in the neurosecretory process during long-term facilitation at the lobster neuromuscular junction

We investigated long-term facilitation at the lobster neuromuscular synapse employing a combination of FM1-43 staining of synaptic vesicles, electron microscopy analysis, and electrical recordings of synaptic activity. Synaptic terminals were loaded with the fluorescent dye FM1-43 producing clusters of activity-dependent fluorescent spots. Electron microscopy analysis of synaptic ultrastructure suggested that fluorescent spots represent compartments of synaptic terminals filled with vesicles. Excitatory postsynaptic currents were recorded from the stained synaptic terminals using focal macropatch electrodes. Terminals were stained during the nerve stimulation at a low stimulation frequency (2, 5 or 10 Hz) before and after long-term facilitation was elicited by high-frequency stimulation (20 or 30 Hz for 5 min). We found that staining after long-term facilitation results in the appearance of new fluorescent spots, as well as in the increase in fluorescence of the spots that appeared before long-term facilitation. This increase in fluorescence accounted for the increase in quantal release. Activation of individual fluorescent spots was found to be non-uniform. In spite of overall increase in fluorescence, some synaptic compartments decreased their staining after long-term facilitation. Thus, our study demonstrates that long-term facilitation produces non-uniform activation of FM1-43 uptake in synaptic compartments that correlates with the increase in quantal neurosecretion.

Synapse formation and plasticity: the roles of local protein synthesis

From simple reflexes in lower animals to complex motor patterns and learning and memory in higher animals, all nervous system functions hinge upon fundamental, albeit specialized, neuronal units termed synapses. The term synapse denotes the structural and functional building block upon which pivots the enormous information-processing capabilities of our brain. It is the neuronal communications through synapses that ultimately determine who we are and how we react and adapt to our ever-changing environment. Synapses are not only the epic center of our intellect, but they also control myriad traits of our personality, ranging from sinfulness to sainthood (see, e.g., Hamer 2004). Simply put-we are what our synapses deem us to be (LeDoux 2003)! Notwithstanding the reasoning that some aspects of the synaptic arrangement may be genetically hardwired, an overwhelming body of knowledge does nevertheless provide ample plausible evidence that synapses are highly plastic entities undergoing rapid adaptive changes throughout life. It is this adaptability that endows our brain with its "uncanny" powers.

Olfactory learning-induced morphological modifications in single dendritic spines of young rats

Learning-related morphological modifications in single dendritic spines were studied quantitatively in the brains of young Sprague-Dawley rats. We have previously shown that olfactory discrimination rule-learning results in transient physiological and morphological modifications in piriform cortex pyramidal neurons. In particular, spine density along the apical dendrites of neurons from trained rats is increased after learning. The aim of the present study was to identify and describe olfactory learning-induced modifications in the morphology of single spines along apical dendrites of the same type of neurons. By using laser-scanning confocal microscopy, we show that 3 days after training completion spines on neurons from olfactory discrimination trained rats are shorter as compared to spines on neurons from control rats. Further analysis revealed that spine shortening attributed to olfactory discrimination learning derives from shortening of spine head and not from shortening of spine neck. In addition, detailed analysis of spine head volume suggests that spines with large heads are absent after learning. As spine head size may be related to the efficacy of the synapse it bears, we suggest that modifications in spine head dimensions following olfactory rule-learning enhance the cortical network ability to enter into a 'learning mode', in which memories of new odours can be acquired rapidly and efficiently.

Dendritic spines and long-term plasticity

A recent flurry of time-lapse imaging studies of live neurons have tried to address the century-old question: what morphological changes in dendritic spines can be related to long-term memory? Changes that have been proposed to relate to memory include the formation of new spines, the enlargement of spine heads and the pruning of spines. These observations also relate to a more general question of how stable dendritic spines are. The objective of this review is to critically assess the new data and to propose much needed criteria that relate spines to memory, thereby allowing progress in understanding the morphological basis of memory.

Active zones for presynaptic plasticity in the brain

Some of the most abundant synapses in the brain such as the synapses formed by the hippocampal mossy fibers, cerebellar parallel fibers and several types of cortical afferents express presynaptic forms of long-term potentiation (LTP), a putative cellular model for spatial, motor and fear learning. Those synapses often display presynaptic mechanisms of LTP induction, which are either NMDA receptor independent of dependent of presynaptic NMDA receptors. Recent investigations on the molecular mechanisms of neurotransmitter release modulation in short- and long-term synaptic plasticity in central synapses give a preponderant role to active zone proteins as Munc-13 and RIM1-alpha, and point toward the maturation process of synaptic vesicles prior to Ca(2+)-dependent fusion as a key regulatory step of presynaptic plasticity.

Microelectrode array recordings of cultured hippocampal networks reveal a simple model for transcription and protein synthesis-dependent plasticity

A simplified cell culture system was developed to study neuronal plasticity. As changes in synaptic strength may alter network activity patterns, we grew hippocampal neurones on a microelectrode array (MEA) and monitored their collective behaviour with 60 electrodes simultaneously. We found that exposure of the network for 15 min to the GABA(A) receptor antagonist bicuculline induced an increase in synaptic efficacy at excitatory synapses that was associated with an increase in the frequency of miniature AMPA receptor-mediated EPSCs and a change in network activity from uncoordinated firing of neurones (lacking any recognizable pattern) to a highly organized, periodic and synchronous burst pattern. Induction of recurrent synchronous bursting was dependent on NMDA receptor activation and required extracellular signal-regulated kinase (ERK)1/2 signalling and translation of pre-existing mRNAs. Once induced, the burst pattern persisted for several days; its maintenance phase (> 4 h) was dependent on gene transcription taking place in a critical period of 120 min following induction. Thus, cultured hippocampal neurones display a simple, transcription and protein synthesis-dependent form of plasticity. The non-invasive nature of MEA recordings provides a significant advantage over traditional assays for synaptic connectivity (i.e. long-term potentiation in brain slices) and facilitates the search for activity-regulated genes critical for late-phase plasticity.