Silent Synapses Research Articles

Maturation of glutamatergic neurotransmission in dentate gyrus granule cells

We studied the development of glutamatergic neurotransmission in dentate gyrus granule cells (GCs) in hippocampal slices from 5 to 12-day-old rats. The active postnatal neuronogenesis in dentate permits GCs with staggered birthdates to be studied in situ in a single preparation. We recorded evoked responses to medial perforant path stimulation using visually-guided whole-cell patch clamping to select immature GCs, and biocytin filling to correlate electrophysiologic responses with maturational stage. Even within this immature cell population we found four distinct electrophysiologic patterns. Type 1 cells had no glutamatergic current; Type 2 cells had only N-methyl- d-aspartate receptor (NMDA) current; Type 3 cells had both NMDA and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) current although the NMDA component could be isolated at low stimulus intensity (NMDA threshold<AMPA threshold); Type 4 cells had both AMPA and NMDA currents with NMDA threshold≥AMPA threshold. Type 1 cells were least mature, and Type 4 cells most mature as assessed by cell properties, dendritic arborization, and penetration of dendrites into the molecular layer. Thus NMDA-mediated currents predominate early in GC development as is consistent with their role in processes that determine dentate architecture — neuronal migration, dendritic outgrowth and regression, and synapse stabilization. By analogy with ‘silent synapses’ (i.e. synapses that contain only NMDA receptors), Type 2 cells are candidate ‘silent cells’ that may undergo activity-dependent acquisition of functional fast-conducting AMPA receptors with maturation.

Dendritic mechanisms in brain function and developmental disabilities.

The papers assembled in this special issue of Cerebral Cortex are part of a continuing dialogue on cellular and molecular aspects of synaptic development and their impact on brain function, which was stimulated by a meeting sponsored by the National Institute of Child Health and Human Development (April 30– May 1, 1998 at the NIH). In recent years, various types of dendritic abnormalities have been described in Down syndrome, fragile-X syndrome, Rett syndrome, autism and other neurodevelopmental disorders. Subtle alterations in dendritic structure or number are precisely the types of abnormalities that would account for a brain that is largely functional, but has subtle deficits in particular cognitive and/or behavioral domains. Remarkable technical advances allow us to focus on the key cellular and molecular events in brain development, synaptic transmission and information processing. This provides us with a unique opportunity to understand the cognitive and behavioral deficits associated with these disorders, and to develop improved therapeutic strategies. Several papers in this issue address the determinants of dendritic structure during development. Tashiro, Minden and Yuste examine the role of small signaling molecules in hippocampal development. They use two-photon microscopy to examine the effects of GTPases of the Rho family transfected into cultured pyramidal cells, and describe the acute effects on the density and morphology of dendritic spines. Hormones and other trophic factors also shape dendritic structure, as evidenced by the profound alterations seen in thyroid hormone deficiency disorders. Thompson and Potter examine the classes of thyroid hormone-responsive genes in the brain as a basis for understanding the effect of hormonal deficiencies on neuronal development. Frotscher, Drakew and Heimrich examine the effect of afferent input on dendritic structure. Using hippocampal neurons co-cultured with entorhinal explants, they show that the granule cell dendritic arbor develops the same laminar specificity in vitro as in vivo, but it is reduced in the absence of entorhinal co-culture. Blockade of afferent activity by tetrodotoxin affects spine morphology on granule cell dendrites but not their density. For several decades, researchers have sought the structural basis of learning and memory. Classic debates have focused on increases in synaptic number versus increasing efficacy of specific synaptic interactions. Three papers in this issue highlight current thinking on structural changes that may provide the basis for functional alterations in neurological pathways. Geinisman reviews changes in the hippocampus associated with long-term potentiation (LTP), a simplified model of memory. Evidence from a number of researchers indicates that LTP is associated with a maturation of synaptic interactions, rather than a net increase in the number of synaptic contacts. Histological studies show a shift from less mature to more mature synaptic profiles, consistent with more efficacious transmission across existing sites and possibly even recruitment of formerly silent synapses. The structural basis of learning is less clear; some studies suggest increases in synaptic number and while others suggest maturation of existing synapses. In a related paper, McAllister reviews studies on the interplay of electrical activity and dendritic structure, which has become a dominant theme in developmental neurobiology. She examines some of the key cellular and molecular mechanisms that drive this dynamic process, and the possible role of neurotrophic factors. Lambe, Goldman-Rakic and Aghajanian examine the effects of serotonin on different regions of pyramidal dendrites within the laminate structure of the prefrontal cortex. They find evidence for layer-specific and compartment-specific effects on synaptic transmission, and discuss these finding in terms of dendritic architecture and the regulation of thalamo-cortical neuro

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

At early developmental stages, silent synapses have been commonly found in different brain areas. These synapses are called silent because they do not respond at rest but are functional at positive membrane potentials. A widely accepted interpretation is that N-methyl-d-aspartate (NMDA) but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are functionally expressed on the subsynaptic membrane. Here we show that, in both CA3 and CA1 hippocampal regions, AMPA-mediated synaptic responses can be detected already at early stages of postnatal development. However, some synapses appear silent because of a very low probability of glutamate release. They can be converted into functional ones by factors that enhance release probability such as paired-pulse stimulation, increasing the temperature or cyclothiazide (CTZ), a drug that blocks AMPA receptor desensitization and increases transmitter release. Conversely, conducting synapses can be switched off by increasing the frequency of stimulation. Although we cannot exclude that "latent AMPA receptors" can become functional after activity-dependent processes, our results clearly indicate that, in the neonatal hippocampus, a proportion of glutamatergic synaptic connections are presynaptically rather than postsynaptically silent.

Competition at silent synapses in reinnervated skeletal muscle.

Synaptic connections are made and broken in an activity-dependent manner in diverse regions of the nervous system. However, whether activity is strictly necessary for synapse elimination has not been resolved directly. Here we report that synaptic terminals occupying motor endplates made electrically silent by tetrodotoxin and alpha-bungarotoxin block were frequently displaced by regenerating axons that were also both inactive and synaptically ineffective. Thus, neither evoked nor spontaneous activation of acetylcholine receptors is required for competitive reoccupation of neuromuscular synaptic sites by regenerating motor axons.

Postfusional regulation of cleft glutamate concentration during LTP at 'silent synapses'.

'Silent synapses' show responses from high-affinity NMDA receptors (NMDARs) but not low-affinity AMPA receptors (AMPARs), but gain AMPAR responses upon long-term potentiation (LTP). Using the rapidly reversible NMDAR antagonist l-AP5 to assess cleft glutamate concentration ([glu]cleft), we found that it peaked at <<170 microM at silent neonatal synapses, but greatly increased after potentiation. Cyclothiazide (CTZ), a potentiator of AMPAR, revealed slowly rising AMPA EPSCs at silent synapses; LTP shortened their rise times. Thus, LTP at silent synapses increased rate-of-rise and peak amplitude of [glu]cleft. Release probability reported by NMDARs remained unchanged during LTP, implying that [glu]cleft increases arose from immediately presynaptic terminals. Our data suggest that changes in the dynamics of fusion-pore opening contribute to LTP.

Silent glutamatergic synapses and long-term facilitation in spinal dorsal horn neurons

Neurons in the superficial dorsal horn of the spinal cord are important for conveying sensory information from the periphery to the central nervous system. It has been proposed that some synapses between primary afferent fibers and spinal dorsal horn neurons are inefficient or silent. Ineffective sensory transmission could result from a small postsynaptic current that fails to depolarize the cell to threshold for an action potential or a cell with a normal postsynaptic current but an increased threshold for action potentials. One possible mechanism for ineffective synapses is the existent of silent glutamatergic synapses. In silent synapses, postsynaptic dorsal horn neurons lack functional AMPA/KA receptors and no synaptic responses are detected even when glutamate is released from presynaptic sensory afferent fibers. Serotonin (5-HT), a major neurotransmitter of the raphe-spinal projecting pathway, transforms silent glutamatergic synapses into functional ones by recruiting postsynaptic functional AMPA receptors. AMPA receptor interaction with PDZ-containing proteins is critical for 5-HT induced recruitment. Silent synapses, as well as their recruitment mechanisms, are developmentally regulated and can still be found in adult synapses. However, they are “functional” and contribute to some sensory transmission. The recruitment of AMPA receptors into pure NMDA receptor containing sensory synapses requires coactivation of 5-HT and postsynaptic adenylyl cyclases. The recruitment of silent glutamatergic synapses provides a synaptic mechanism for spinal senstization in pathological pain, including possible pheonotype switching of dorsal horn neurons. These results suggest that the transformation of silent glutamatergic synapses may serve as a cellular mechanism for central plasticity in the spinal cord.

Involvement of BDNF in activating the silent synapses during neonatal development of mouse barrel cortex

Involvement of BDNF in activating the silent synapses during neonatal development of mouse barrel cortex

Synaptic plasticity at thalamocortical synapses in developing rat somatosensory cortex: LTP, LTD, and silent synapses

Thalamocortical synaptic transmission in the rat's primary somatosensory (S1) cortex is modified by sensory experience during a critical period early in life. Despite the importance of such plasticity for the maturation of thalamocortical circuits, the synaptic basis of this plasticity is unknown. Here, we review evidence suggesting that long-term potentiation and depression (LTP and LTD) of thalamocortical synaptic transmission may be involved in this plasticity. In an in vitro slice preparation, thalamocortical synaptic responses exhibit N-methyl-D-aspartate (NMDA) receptor-dependent LTP and LTD during a developmental period similar to the critical period in vivo. The inability to induce LTP and LTD after the critical period may result in part from a developmental reduction in the duration of NMDA receptor currents. In addition, during the critical period many thalamocortical synapses exhibit NMDA receptor currents but no detectable AMPA receptor currents, and thus may be functionally silent at resting membrane potentials. LTP converts silent synapses to functional ones by causing the rapid appearance of AMPA currents. These observations suggest that thalamocortical synapses may be formed as silent synapses which are subsequently made functional by LTP. LTP and LTD may then regulate the efficacy of these functional synapses and thereby contribute to experience-dependent changes in S1 thalamocortical circuits. © 1999 John Wiley & Sons, Inc. J Neurobiol 41: 92–101, 1999

Synaptic plasticity at thalamocortical synapses in developing rat somatosensory cortex: LTP, LTD, and silent synapses

Thalamocortical synaptic transmission in the rat's primary somatosensory (S1) cortex is modified by sensory experience during a critical period early in life. Despite the importance of such plasticity for the maturation of thalamocortical circuits, the synaptic basis of this plasticity is unknown. Here, we review evidence suggesting that long-term potentiation and depression (LTP and LTD) of thalamocortical synaptic transmission may be involved in this plasticity. In an in vitro slice preparation, thalamocortical synaptic responses exhibit N-methyl-D-aspartate (NMDA) receptor-dependent LTP and LTD during a developmental period similar to the critical period in vivo. The inability to induce LTP and LTD after the critical period may result in part from a developmental reduction in the duration of NMDA receptor currents. In addition, during the critical period many thalamocortical synapses exhibit NMDA receptor currents but no detectable AMPA receptor currents, and thus may be functionally silent at resting membrane potentials. LTP converts silent synapses to functional ones by causing the rapid appearance of AMPA currents. These observations suggest that thalamocortical synapses may be formed as silent synapses which are subsequently made functional by LTP. LTP and LTD may then regulate the efficacy of these functional synapses and thereby contribute to experience-dependent changes in S1 thalamocortical circuits.

Speaking Out of Turn: A Role for Silent Synapses in Pain

Speaking Out of Turn: A Role for Silent Synapses in Pain

Excitatory amino acids and synaptic transmission in embryonic rat brainstem motoneurons in organotypic culture.

We used brainstem motoneurons recorded in organotypic slice co-cultures maintained for more than 18 days in vitro, together with multibarrel ionophoretic applications of glutamate receptor agonists and bath applications of specific blocking agents, to study the responses of rat brainstem motoneurons to glutamate receptor activation, and the contribution of these receptors to synaptic transmission. Differentiated brainstem motoneurons in vitro are depolarized by glutamate, N-methyl-d-aspartate (NMDA) and dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) iontophoresis, and express NMDA, AMPA and also specific kainate receptors, as evidenced by (+/-)2-amino-5-phosphonovaleric acid (APV)- and (-)1-(4-aminophenyl)-3-methyl-carbamoyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzo-diazepine [GYKI 53784 (LY303070)]-resistant depolarizations. Electrical stimulations applied to the dorsal part of the explant trigger excitatory synaptic potentials with latencies distributed in three regularly spaced groups. Excitatory postsynaptic potentials (EPSPs) in the earliest group have a similar latency and time course and correspond to monosynaptic activation. EPSPs in later groups have more scattered latencies and time courses and correspond to polysynaptic activation. Monosynaptic EPSPs are insensitive to the specific NMDA blocker APV, and are completely and reversibly suppressed by the non-competitive AMPA receptor antagonist GYKI 53784 (LY303070). Detailed analysis of the spontaneous excitatory synaptic activity shows that APV decreases the frequency of spontaneous EPSPs without modifying their shape or amplitude. We conclude that excitatory synapses on brainstem motoneurons in vitro are mainly activated through AMPA receptors (AMPA-Rs). NMDA receptors (NMDA-Rs) are present in the membrane, but are located either at extrasynaptic sites or silent synapses, and are not directly involved in synaptic transmission on motoneurons. On the contrary, NMDA receptors contribute to synaptic transmission within the premotor interneuronal network.

Regulation of Synaptic Vesicle Recycling by Calcium and Serotonin

Serotonin, a neuromodulator at the crayfish neuromuscular junction, regulates neurotransmission without changing intracellular calcium levels. However, the mechanism of this regulation remains unclear. By analysis of synaptic depression using a depletion model and measurement of vesicle recycling using the styryl dye FM1-43, we show that serotonin increases the number of vesicles available for transmitter release (total synaptic vesicle pool size). This regulation is due either to an increase in the number of vesicles at each release site or to an activation of previously nonsecreting or silent synapses. We also observed that low calcium medium rendered part of the vesicle pool unavailable for release. These results suggest a new mechanism for regulating synaptic transmission.

Silent glutamatergic synapses and nociception in mammalian spinal cord.

Neurons in the superficial dorsal horn of the spinal cord are important for conveying sensory information from the periphery to the central nervous system. Some synapses between primary afferent fibres and spinal dorsal horn neurons may be inefficient or silent. Ineffective sensory transmission could result from a small postsynaptic current that fails to depolarize the cell to threshold for an action potential or from a cell with a normal postsynaptic current but an increased threshold for action potentials. Here we show that some cells in the superficial dorsal horn of the lumbar spinal cord have silent synapses: they do not respond unless the holding potential is moved from -70 mV to +40 mV. Serotonin (5-hydroxytryptamine, 5-HT), an important neurotransmitter of the raphe-spinal projecting pathway, transforms silent glutamatergic synapses into functional ones. Therefore, transformation of silent glutamatergic synapses may serve as a cellular mechanism for central plasticity in the spinal cord.

Suppression of Sprouting: An Early Function of NMDA Receptors in the Absence of AMPA/Kainate Receptor Activity

Electrophysiological studies have documented the existence of synapses showing only NMDA ionotropic glutamate receptor function that are therefore presumably "silent" at resting membrane potentials. Silent synapses are more prevalent in young than in older neurons, and NMDA receptor activity at such contacts may facilitate the appearance of functional AMPA receptors. However, it is uncertain whether such silent synapses actually have a function in young neurons independent of AMPA receptor induction. Using a newly characterized culture system for neurons from larval Xenopus tecta, we show that blocking NMDA receptors or preventing changes in intracellular free Ca2+ concentration with BAPTA AM significantly increases neurite sprouting and elongation in contacted but not in isolated neurons. Blocking AMPA/KA receptors or Na+-dependent action potentials does not mimic this effect. Moreover, in these young neurons, NMDA receptor-dependent Ca2+ responses to glutamate measured with confocal fluo-3 imaging are retained during AMPA/KA receptor blockade. The data suggest that many of the young contacts in these cultures are active even though they use only NMDA ionotropic glutamate receptors. Calcium influx through the NMDA receptor at these contacts seems to reduce neurite motility. This effect should lead to the accumulation of glutamatergic inputs on NMDA receptor-expressing dendrites, which could facilitate the onset of AMPA/KA receptor function and the action potential-dependent phase of synaptogenesis.

Short-term facilitation evoked during brief afferent tetani is not altered by long-term potentiation in the guinea-pig hippocampal CA1 region.

1. The aim was to examine whether long-term potentiation (LTP) had effects on short-term synaptic plasticity outside those predicted from its effect on single volley-induced responses. Field recordings from the CA1 region of guinea-pig hippocampal slices were used, and short- term plasticity was evoked by five-impulse trains of 20 and 50 Hz. 2. The five-impulse trains were evoked in the presence of D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 20-50 microM), picrotoxin (100 microM), and 2-OH-saclofen (200 microM), and care was taken to avoid initiation of postsynaptic spike activation. Field responses were thus considered to reflect non-NMDA receptor-mediated activity only, and demonstrated a net facilitation during the trains. 3. The facilitation was found, on average, to be unaffected by LTP, evoked by strong afferent tetanization. This was true also when release probability had been altered either by the adenosine agonist N-cyclohexyladenosine (CHA; 100 nM) or the antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 200 nM). When examined for individual experiments, cases with increases, or decreases, of facilitation following LTP were observed. These deviations showed no relation to initial release probability or to LTP magnitude, and they were also observed in control inputs not subjected to LTP. 4. Impairment of non-NMDA receptor desensitization by cyclothiazide (30 microM) increased facilitation observed during a 50 Hz, but not a 20 Hz, train. LTP had no effect on facilitation, in the presence of this drug, either during 20 or 50 Hz trains. 5. The results suggest that the effect of LTP in the hippocampal CA1 region on non-NMDA receptor-mediated synaptic responses to a brief afferent tetanus does not differ from that on a low-frequency, single volley-induced response. They do not support the notion that LTP is based on changes in release probability of previously active synapses. If LTP is based on recruitment of previously, pre- or postsynaptically, silent synapses, these synapses must have, on average, release characteristics similar to the previously active ones.

Brain‐derived neurotrophic factor transiently stabilizes silent synapses on developing neuromuscular junctions

Brain‐derived neurotrophic factor transiently stabilizes silent synapses on developing neuromuscular junctions

Vibrissectomy induced changes in GAP-43 immunoreactivity in the adult rat barrel cortex.

Within the rat primary somatosensory cortex, neurons responding principally to movement of each individual mystacial vibrissa are grouped together in structures termed barrels. Previous studies have examined changes in the area of cortex showing increased 2-deoxyglucose uptake in response to vibrissal stimulation. These studies have shown that chronic removal of all but the central (C3) vibrissa in adult rats induces an enlarged representation of the remaining C3 barrel in the contralateral cortex. This increase is prevented by cortical norepinephrine depletion. The major question raised by such studies is whether such plasticity is due to structural rearrangement or unmasking of otherwise silent synapses. In this study, antibodies to GAP-43, a presynaptic protein whose synthesis is related to neuronal development and regeneration, were used to investigate this issue. In adult rat brain, tangential sections through layer IV of the barrel receptor field normally show moderate levels of GAP-43 immunoreactivity (GAP-IR) in the inter-barrel septa and low levels within the barrels themselves. The present study examined changes in the pattern of GAP-IR from 1 to 8 weeks after vibrissectomy with sparing of C3 as an index of possible physical reorganization of cortical circuits. Quantitative analysis of the cortices of animals with unilateral vibrissectomy with sparing of C3 showed that the area of low GAP-IR within the barrels surrounding C3 was decreased at 1 week (8.4% shrinkage; P less than 0.01) and 8 weeks (12.0% shrinkage; P less than 0.015), relative to the cortex ipsilateral to the surgery. Both bilateral vibrissectomy with sparing of C3 and ibotenic acid lesions of the ventrobasal thalamus produced similar results. Some evidence was also seen that the area of low GAP-IR in the C3 barrel shrank to a similar degree after such manipulations. Cortical norepinephrine depletion had no apparent effect on vibrissectomy-induced GAP-IR changes. These results suggest that removal of vibrissal input to the adult rat barrel cortex produces transynaptic induction of axonal sprouting within the barrel cortex.

Quantal analysis of synaptic potentials in neurons of the central nervous system.

This review concentrates on the results of quantal analysis at central synapses, particularly where the morphology and the electrophysiology of the same synaptic connection has been investigated. Results from quantal analysis at peripheral synapses are drawn on to help resolve some issues. The major topics covered in this review are the properties of the quantal synaptic potential and the junctional mechanisms involved in its generation

An attempt to demonstrate subliminal ‘silent synapses’ in the visual cortex of deprived cats

An attempt to demonstrate subliminal ‘silent synapses’ in the visual cortex of deprived cats