Activation of postsynaptically silent synapses during pairing-induced LTP in CA1 region of hippocampal slice

NMDA-R1 subunit of the cerebral cortex co-localizes with neuronal nitric oxide synthase at pre- and postsynaptic sites and in spines

The majority of nitric oxide's (NO) physiologic and pathologic actions in the brain has been linked to NMDA receptor activation. In order to determine how the NO-synthesizing enzyme within brain, neuronal NO synthase (nNOS), and NMDA receptors are functionally linked, previous studies have used in situ hybridization techniques in combination with light microscopic immunocytochemistry to show that the two are expressed within single neurons. However, this light microscopic finding does not guarantee that NMDA receptors are distributed sufficiently close to nNOS within single neurons to allow direct interaction of the two. Thus, in this study, dual immuno-electron microscopy was performed to determine whether nNOS and NMDA receptors co-exist within fine neuronal processes. We show that nNOS and the obligatory subunit of functional NMDA receptors, i.e. the NMDA-R1, co-exist within dendritic shafts, spines and terminals of the adult rat visual cortex. Axon terminals form asymmetric synaptic junctions with the dually labeled dendrites, suggesting that the presynaptic terminals release glutamate. Axons and dendrites expressing one without the other also are detected. These results indicate that it is possible for the generation of NO to be temporally coordinated with glutamatergic synaptic transmission at axo-dendritic and axo-axonic junctions and that NO may be generated independently of glutamatergic synaptic transmission. Together, our observations point to a greater complexity than previously recognized for glutamatergic neurotransmission, based on the joint versus independent actions of NO relative to NMDA receptors at pre- versus postsynaptic sites.

Transient synaptic potentiation in the visual cortex. I. Cellular mechanisms

The cellular mechanisms that underlie transient synaptic potentiation were studied in visual cortical slices of adult guinea pigs (> or = age 5 wk postnatal). Postsynaptic potentials (PSPs) elicited by stimulation of the white matter/layer VI border were recorded with conventional intracellular techniques from layer II/III neurons. Transient potentiation (average duration 23 +/- 3 min, mean +/- SE) was evoked by 60 low-frequency (0.1 Hz) pairings of weak afferent stimulation with coincident intracellular depolarizing pulses (80 ms) of the postsynaptic cell. Fifty-one percent (47 of 92) of the pairing protocols led to significant enhancement (+26 +/- 3%) of the PSP peak amplitude. Blockade of action potential output from the recorded neuron during pairing with Lidocaine, N-ethyl bromide quaternary salt in the recording micropipette did not reduce the likelihood of potentiation (7 of 14 protocols = 50%). Thus transient synaptic potentiation does not require action potential output from the paired cell or recurrent synaptic activation in the local cortical circuit. Rather, the modification occurs at synaptic sites that directly impinge onto the activated neuron. Intracellular postsynaptic blockade of inhibitory PSPs only onto the paired cell with the chloride channel blocker 4,4'-dinitro-stilbene-2,2'-disulfonic acid and the potassium channel blocker cesium in he micropipette also did not reduce the likelihood of induction of potentiation (6 of 9 protocols = 67%). These results suggest that the potentiation is due to a true upregulation of excitatory synaptic transmission and that it does not require a reduction of inhibitory components of the compound PSP for induction. Chelation of postsynaptic intracellular calcium with 1,2-bis-2-aminophenoxy ethane-N,N,N',N'-tetraacetic acid (BAPTA) in all cases effectively blocked the induction of potentiation (no change in the PSP, 9 of 13 protocols; induction of synaptic depression, 4 of 13 protocols), suggesting that a rise in the intracellular postsynaptic calcium level is critical for the pairing-induced synaptic potentiation to occur. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV) reversibly blocked potentiation of the PSP peak amplitude in most cells (14 of 16) that were capable of significant potentiation of control solution. Blockade of nitric oxide production with bath application of the competitive inhibitor of nitric oxide synthase, L-nitro-arginine (LNA), did not significantly affect the likelihood of synaptic potentiation (11 of 20 cells). It did, however, block subsequent enhancement for several cells (2 of 4) that had previously had their inputs potentiated. Moreover, LNA increased the overall average magnitude of synaptic potentiation (with an additional +28%) when induction was successful. These results suggest that endogenous cortical nitric oxide production can both positively and negatively modulate this NMDA receptor-mediated type of synaptic plasticity.

Silent synapses during development of thalamocortical inputs

During development, activity-dependent mechanisms are thought to contribute to the refinement of topographical projections from the thalamus to the cortex. Because activity-dependent increases in synaptic strength may contribute to the stabilization of synaptic connections, we have explored the mechanisms of long-term potentiation (LTP) at thalamocortical synapses in rat somatosensory (barrel) cortex. During early postnatal development (postnatal days 2-5), we find that a significant proportion of thalamocortical synapses are functionally silent and that these are converted to functional synapses during LTP. Silent synapses disappear by postnatal day 8-9, the exact time at which the susceptibility of these synapses to LTP is lost. These findings suggest that the activity-dependent conversion of silent to functional synapses due to correlated pre- and postsynaptic activity may contribute to the early development and refinement of thalamocortical inputs to cortex.

Changes in paired-pulse facilitation correlate with induction of long-term potentiation in area CA1 of rat hippocampal slices

The phenomenon of long-term potentiation is widely used as an experimental model of memory. An approach that has been used to study its underlying mechanisms is to analyse its interaction with presynaptic paired-pulse facilitation. Several studies found no evidence for an interaction in the CA1 hippocampal area, whereas other data, for example from quantal analysis, suggested that presynaptic mechanisms contribute to the maintenance of long-term potentiation. In the present study, initial slopes of field potentials in area CA1 were measured in rat hippocampal slices. "Conventional" long-term potentiation was induced by high-frequency (100 Hz) afferent tetanization of the testing input. "Associative" long-term potentiation was induced by combining lower frequency (40 Hz) tetanization of a testing input with high-frequency tetanization of a second input. The paired-pulse facilitation ratio decreased in the majority of experiments in which long-term potentiation was induced conventionally, but it decreased, increased or did not change after inducing associative potentiation. Decreases in the paired-pulse facilitation correlated inversely with the initial (pre-tetanic) facilitation ratio. A more detailed regression analysis suggests that this correlation results from two other correlations: (i) that between changes in paired-pulse facilitation and the magnitude of long-term potentiation, and (ii) that between initial paired-pulse facilitation and the magnitude of long-term potentiation. The first correlation prevailed during the initial 10 min following tetanization, while the second prevailed 40-60 min later. A post-tetanic decrease in paired-pulse facilitation is evidence for an involvement of presynaptic mechanisms in the maintenance of long-term potentiation. The lack of significant changes in some studies could be due to the inclusion in the analyses of experiments with long-term potentiation of small magnitude, in which changes in paired-pulse facilitation ratios would have been inconsistent. The present study suggests that the early (10-20 min) and late (40-50 min) phases of long-term potentiation were mediated by different mechanisms, with a mixture of these mechanisms during the intermediate period. On the basis of the present and previous studies, the following scheme of involvement of several mechanisms in long-term potentiation maintenance is proposed. The early phase includes two major mechanisms: an increase in the probability of transmitter release, leading to an apparent increase in the number of effective release sites, and an increase in efficacy of one transmitter quantum, probably due to an increased number of postsynaptic receptors. The later phase of long-term potentiation is attributed to an increase in the number of transmitter zones, presumably due to structural modifications.

Extrasynaptic glutamate spillover in the hippocampus: dependence on temperature and the role of active glutamate uptake

At excitatory synapses on CA1 pyramidal cells of the hippocampus, a larger quantal content is sensed by N-methyl-D-aspartic acid receptors (NMDARs) than by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). A novel explanation for this discrepancy is that glutamate released from terminals presynaptic to one cell can diffuse to and activate NMDARs, but not AMPARs, on a neighboring cell. If this occurs in the living brain, it could invalidate the view that glutamatergic synapses function as private communication channels between neurons. Here, we show that the discrepancy in quantal content mediated by the two receptors is greatly decreased at physiological temperature, compared with conventional recording conditions. This effect of temperature is not due to changes in release probability or uncovering of latent AMPARs. It is, however, partially reversed by the glutamate uptake inhibitor dihydrokainate. The results suggest that glutamate transporters play a critical role in limiting the extrasynaptic diffusion of glutamate, thereby minimizing cross-talk between neighboring excitatory synapses.

A synaptically controlled, associative signal for Hebbian plasticity in hippocampal neurons

The role of back-propagating dendritic action potentials in the induction of long-term potentiation (LTP) was investigated in CA1 neurons by means of dendritic patch recordings and simultaneous calcium imaging. Pairing of subthreshold excitatory postsynaptic potentials (EPSPs) with back-propagating action potentials resulted in an amplification of dendritic action potentials and evoked calcium influx near the site of synaptic input. This pairing also induced a robust LTP, which was reduced when EPSPs were paired with non-back-propagating action potentials or when stimuli were unpaired. Action potentials thus provide a synaptically controlled, associative signal to the dendrites for Hebbian modifications of synaptic strength.

Postnatal changes in the laminar and subcellular distribution of NMDA-R1 subunits in the cat visual cortex as revealed by immuno-electron microscopy

Although it is recognized that nearly all synapses in the cerebral cortex form postnatally, little is known about the emergence of molecules necessary to render these synapses functional. This study visualized the emergence of synaptically localized NMDA receptors by immuno-electron microscopic labeling of the receptor's obligatory subunit, NMDA-R1, in the developing cat visual cortex. Prior to eye-opening (postnatal day 2-10), NMDA-R1 immunoreactivity is already present within dendritic and growth cones, even though these profiles are devoid of synaptic specializations. This indicates that synthesis and incorporation of NMDA-R1 into plasma membranes are independent of form vision. During the next 2-3 weeks, i.e., preceding the onset of the critical period for ocular dominance plasticity (around the fourth week), NMDA-R1 immunoreactivity changes from a diffuse distribution within dendrites to a more discrete aggregation over postsynaptic densities of axo-spinous junctions. Such clustering of NMDA-R1 at synapses may be a prerequisite for stabilization and strengthening of synapses activated by visual stimulation during the critical period. Furthermore, only during the first several weeks, intensely NMDA-R1-immunoreactive neurons are present in the infragranular layers and the white matter. Enrichment of NMDA-R1 in the deep-layer neurons may reflect the neurons' supportive role in the development of cortical circuitry, serving as transient synaptic targets for geniculate and cortico-cortical afferents while these afferents 'wait' in the infragranular for their ultimate, life-long target neurons to become receptive in the upper layers.

Postnatal development of zinc-containing cells and neuropil in the hippocampal region of the mouse

The present study describes the postnatal development of zinc-containing boutons and their neurons of origin in the hippocampal region of the mouse. Ages investigated for the development of zinc-containing neuropil were postnatal days 0 (P0), P3, P7, P11, P15, P21, and P28. For zinc-containing cell bodies P7, P15, P21, and P28 were studied. In the area dentata, zinc-containing neuropil appeared first by P3 adjacent to the suprapyramidal limb of the granule cell layer and extended later toward the infrapyramidal limb. By P15, inter- and intralaminar gradients corresponded to those seen in adult animals. The appearance of labeled granule cells followed closely, although temporally delayed, the pattern of granule cell neurogenesis. All granule cells were labeled by P28. In the hippocampus proper, zinc-containing neuropil was seen by P0, but staining of the incipient mossy fiber zone was first visible by P3. Staining pattern and intensity developed gradually until they reached their mature appearance by P15. The distribution of labeled cells was identical to that seen in mature animals by P7 in CA3, but first by P21 in CA1. In the subiculum, neuropil staining first appeared proximally by P7, included all of this area by P11, and appeared mature by P21. A few labeled cells were seen in the proximal subiculum at all ages at which labeled cells were present in CA1. Labeled cells which extended further distally became first visible by P21. Their number and labeling intensity reached mature levels by P28. In the presubiculum, retrosplenial area 29e, and parasubiculum, neuropil staining first appeared by P3. The retrosplenial area 29e could be distinguished by P11. This area and the presubiculum reached their adult appearance by P21. This occurred first by P28 in the parasubiculum due to the late maturation of the parasubiculum a. Labeled cells were first seen by P7 in layer III of the presubiculum and by P15 in the retrosplenial area 29e and the parasubiculum. Cell labeling appeared mature by the same times as the neuropil staining. In the entorhinal areas a very light neuropil stain was apparent in the deeper layers by P0. A distinct rise in staining intensity was first observed by P7 in layers I-III. Thereafter, mature characteristics developed gradually and were attained by P21. Cell labeling was not seen in the medial entorhinal area. A few labeled cells were apparent by P7 in the lateral entorhinal area. After a slight increase by P15, numerous labeled cells were found in layer II and layer VI by P21. Their distribution and labeling intensity appeared mature by P28. Zinc-containing cells appear to represent cells formed late in the course of neurogenesis in all areas aside from the lateral entorhinal area. As far as intrinsic connections are concerned, it is the development of projections from this subset of neurons which is monitored in this study. We suggest that the appearance of zinc may contribute via its different effects on N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors to the end of a developmental phase that is permissive to changes in synaptic efficacy. Species differences and alternative functions of zinc are considered.

Brain-derived neurotrophic factor and nerve growth factor potentiate excitatory synaptic transmission in the rat visual cortex

1. The effect of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) on excitatory synaptic transmission in the developing visual cortex was studied by whole-cell patch-clamp recordings from rat brain slices. 2. Both neurotrophins induced a rapid increase in the amplitude of impulse-evoked excitatory postsynaptic currents (EPSCs). BDNF also increased the frequency of spontaneous EPSCs. 3. Analysis of the currents revealed that alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components contributing to the EPSC peak amplitude were equally potentiated by the neurotrophins. 4. When synaptic transmission was studied by minimal stimulation of intracortical afferents, neurotrophins induced a decrease in the occurrence of release failures. 5. A number of neurones were insensitive to the effects of the neurotrophins, possibly related to the considerable heterogeneity of neuronal types and to the uneven distribution of neurotrophin receptors in the visual cortex. 6. The probability of neurotransmitter release represents a rapidly modifiable synaptic feature by which neurotrophins can potentiate the efficacy of excitatory synaptic transmission in the visual cortex.

Role of neurotrophins in synapse development and plasticity

Neurotrophic factors are traditionally viewed as secretory proteins that regulate long-term survival and differentiation of neurons. The role of neurotrophic factors in the structural integrity of the nervous system makes them attractive candidates as therapeutic agents for neurodegenerative diseases. However, the fact that expression of many neurotrophic factors in the central nervous system is rapidly enhanced by neuronal activity suggests a new role for these factors in activity-dependent processes, such as synaptic development and plasticity. A series of recent studies has provided strong evidence for this novel function of neurotrophic factors. The neurotrophin family of proteins has been shown to acutely potentiate synaptic transmission at the neuromuscular junction and in the brain. These factors are also involved in the maturation of the neuromuscular synapses and in the development of synapses in the visual system. Gene targeting and physiological experiments demonstrate that brain-derived neurotrophic factor (BDNF) plays an important role in long-term potentiation (LTP), a cellular model for learning and memory. These findings have brought together two hotly pursued areas of neuroscience, namely, the function of neurotrophic factors and the mechanisms for synaptic plasticity. Continuous studies in this new field will help understand how synapses develop and function in the brain, and may have significant implications in treating learning disorders in both children and adults.

Nitric oxide-related species inhibit evoked neurotransmission but enhance spontaneous miniature synaptic currents in central neuronal cultures

Nitric oxide (NO.) does not react significantly with thiol groups under physiological conditions, whereas a variety of endogenous NO donor molecules facilitate rapid transfer to thiol of nitrosonium ion (NO+, with one less electron than NO.). Here, nitrosonium donors are shown to decrease the efficacy of evoked neurotransmission while increasing the frequency of spontaneous miniature excitatory postsynaptic currents (mEPSCs). In contrast, pure NO donors have little effect (displaying at most only a slight increase) on the amplitude of evoked EPSCs and frequency of spontaneous mEPSCs in our preparations. These findings may help explain heretofore paradoxical observations that the NO moiety can either increase, decrease, or have no net effect on synaptic activity in various preparations.

Nitric oxide acts directly in the presynaptic neuron to produce long-term potentiation in cultured hippocampal neurons

Nitric oxide (NO) has been proposed to act as a retrograde messenger during long-term potentiation (LTP) in the CA1 region of hippocampus, but the inaccessibility of the presynaptic terminal has prevented a definitive test of this hypothesis. Because both sides of the synapse are accessible in cultured hippocampal neurons, we have used this preparation to investigate the role of NO. We examined LTP following intra- or extracellular application of an NO scavenger, an inhibitor of NO synthase, and a membrane-impermeant NO donor that releases NO only upon photolysis with UV light. Our results indicate that NO is produced in the postsynaptic neuron, travels through the extracellular space, and acts directly in the presynaptic neuron to produce long-term potentiation, supporting the hypothesis that NO acts as a retrograde messenger during LTP.

Multiple actions of 1S,3R-ACPD in modulating endogenous synaptic transmission to spinal respiratory motoneurons

To determine physiological roles of metabotropic glutamate receptors (mGluRs) affecting breathing, we examined the effects of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) on synaptic transmission and excitability of phrenic motoneurons (PMNs) in an in vitro neonatal rat brainstem/spinal cord preparation. The effects of 1S,3R-ACPD were multiple, including reduction of inspiratory-modulated synaptic currents and increase of neuronal excitability via an inward current (Iacpd) associated with a decrease of membrane conductance. The mechanism underlying synaptic depression was examined. We found that 1S,3R-ACPD reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents. The current induced by exogenous AMPA was not significantly affected by 1S,3R-ACPD. These results suggest that 1S,3R-ACPD-induced reduction of inspiratory synaptic currents is mediated by presynaptic mGluRs. We also examined the ionic basis for Iacpd. We found that Iacpd had a reversal potential of approximately -100 mV, close to the estimated, EK+ (-95 mV). Elevating extracellular [K+] to 9 mM reduced the Iacpd reversal potential to -75 mV. The K+ channel blocker Ba2+ induced an inward current with a reversal potential at -93 mV associated with a decrease of membrane conductance, closely resembling the effect of 1S,3R-ACPD. Moreover, Ba2+, occluded 1S,3R-ACPD effects. In the presence of Ba2+, Iacpd and the 1S,3R-ACPD-induced decrease of membrane conductance were diminished. Our data indicate that the dominant component of Iacpd results from the blockade of a Ba(2+)-sensitive resting K+ conductance. We conclude that the activation of mGluRs affects the inspiratory-modulated activity of PMNs via distinct mechanisms at pre- and postsynaptic sites.

Long-term potentiation is reduced in mice that are doubly mutant in endothelial and neuronal nitric oxide synthase

Nitric oxide (NO) has been implicated in hippocampal long-term potentiation (LTP), but LTP is normal in mice with a targeted mutation in the neuronal form of NO synthase (nNOS-). LTP was also normal in mice with a targeted mutation in endothelial NOS (eNOS-), but LTP in stratum radiatum of CA1 was significantly reduced in doubly mutant mice (nNOS-/eNOS-). By contrast, LTP in stratum oriens was normal in the doubly mutant mice. These results provide the first genetic evidence that NOS is involved in LTP in stratum radiatum and suggest that the neuronal and endothelial forms can compensate for each other in mice with a single mutation. They further suggest that there is also a NOS-independent component of LTP in stratum radiatum and that LTP in stratum oriens is largely NOS independent.

Long-term potentiation in distinct subtypes of hippocampal nonpyramidal neurons

We have investigated NMDA receptor-dependent long-term potentiation (LTP) in distinct subtypes of nonpyramidal neurons of the CA1 hippocampus using induction protocols that permitted the differentiation between a direct form of LTP and plasticity resulting simply from the "passive propagation" of LTP occurring on CA1 pyramidal neurons. Two types of stratum (st.) oriens/ alveus interneurons received passive propagation of synaptic potentiation via the recurrent collaterals of CA1 pyramidal cells, but neither subtype possessed direct plasticity. In st. radiatum, two distinct classes of cells were observed: st. radiatum interneurons that showed neither direct nor propagated forms of synaptic plasticity, and "giant cells" for which EPSPs were robustly potentiated after a pairing protocol. This potentiation is similar to the LTP described in pyramidal cells, and its induction requires NMDA receptor activation. Thus, a large heterogeneity of synaptic plasticity exists in morphologically distinct neurons and suggests that complex changes in the CA1 network properties will occur after the induction of LTP.

Bidirectional regulation of protein kinase M zeta in the maintenance of long-term potentiation and long-term depression

Long-term potentiation (LTP) and long-term depression (LTD) are persistent modifications of synaptic efficacy that may contribute to information storage in the CA1 region of the hippocampus. Persistently enhanced phosphorylation has been implicated in the maintenance phase of LTP. This hypothesis is supported by our previous observation that protein kinase M zeta (PKM zeta), the constitutively active catalytic fragment of a single protein kinase C isoform (PKC zeta), increases in LTP maintenance. In contrast, dephosphorylation may be important in LTD maintenance, because phosphatase inhibitors reverse established LTD, in addition to blocking its induction. Because phosphorylation is determined by a balance of phosphatases and kinases, both increases in phosphatase activity and decreases in kinase activity could contribute to LTD. We now report that the reduction of protein kinase activity by H7, as well as selective inhibition of PKC by chelerythrine, mimics and occludes the maintenance phase of homosynaptic LTD in rat hippocampal slices. Conversely, saturated LTD occludes the synaptic depression caused by chelerythrine. Biochemical analysis demonstrates a decrease of PKM zeta, as well as PKCs gamma and epsilon, in LTD maintenance and a concomitant loss of constitutive PKC activity. LTD and the downregulation of PKM zeta are prevented by NMDA receptor antagonists and Ca(2+)-dependent protease inhibitors. Both LTD and the downregulation of PKM zeta are reversible by high-frequency afferent stimulation. Our findings indicate that the molecular mechanisms of LTP and LTD maintenance are inversely related through the bidirectional regulation of PKC.

Effect of phosphatidylserine on the binding properties of glutamate receptors in brain sections from adult and neonatal rats

The effects of phosphatidylserine (PS) on the binding properties of the AMPA (alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid) and NMDA (N-methyl-D-aspartate) subtypes of glutamate receptors were analyzed by quantitative autoradiography of [3H]AMPA, [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and [3H]glutamate binding on rat brain tissue sections. Preincubation of brain sections with PS produced an increase in [3H]AMPA binding without modifying the binding properties of [3H]CNQX, an antagonist of AMPA receptors. This effect of PS appeared to be specific for the AMPA subtype of glutamate receptors as the same treatment did not modify [3H]glutamate binding to the NMDA receptors. Furthermore, the PS-induced increase in [3H]AMPA binding was different in various brain structures, being larger in the molecular layer of the cerebellum and almost absent in the striatum. Preincubation with calcium also augmented [3H]AMPA binding, and the lack of additivity of the effects of calcium and PS on [3H]AMPA binding strongly suggests that both treatments share a common mechanism(s) for producing increased agonist binding. Finally, the effect of PS on AMPA receptor properties was markedly reduced in rat brain sections prepared from neonatal rats at a developmental stage that is normally characterized by the absence of LTP expression in certain brain regions. The present data are consistent with the hypothesis that alteration in the lipid composition of synaptic membranes may be an important mechanism for regulating AMPA receptor properties, which could be involved in producing long-lasting changes in synaptic operation.

Maturation of a central glutamatergic synapse

Whole-cell recordings from optic tectal neurons in Xenopus tadpoles were used to study the maturation of a glutamatergic synapse. The first glutamatergic transmission is mediated only by N-methyl-D-aspartate (NMDA) receptors and is silent at resting potentials. More mature synapses acquire transmission by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors. This maturational program is mimicked by postsynaptic expression of constitutively active calcium-calmodulin-dependent protein kinase II (CaMKII). Newly formed synapses may be silent unless sufficient depolarization is provided by coincident activity that could activate postsynaptic CaMKII, resulting in the appearance of AMPA responses.

Localization of the glutamate receptor subunit GluR1 on the surface of living and within cultured hippocampal neurons

The distribution of the glutamate receptor subunit GluR1 was investigated in cultured hippocampal neurons by confocal microscopy, using polyclonal antibodies directed against either the N- or C-terminal region. On living neurons, GluR1 immunofluorescence was detected with the N-terminal antibody only. GluR1 was localized in a highly punctate manner on the surface of neuronal soma and throughout the dendritic tree. Many GluR1 puncta co-localized with the synaptic marker synaptophysin, although extrasynaptic GluR1 puncta were also observed. A comparison of GluR1 subunit distribution of living neurons labelled with N-terminal antibody with that obtained after the cells had been fixed, permeabilized and subsequently reacted with C-terminal or additional N-terminal antibody showed a number of differences. In permeabilized cells additional, diffuse labelling was observed which was very pronounced in the soma and extended into the proximal dendrites. Furthermore, some spines showed little or no labelling of their membrane surface, but labelled strongly after the cells had been fixed and permeabilized. Such spines may be the postsynaptic components of silent or suboptimal synapses.

Mechanism of altered synaptic strength due to experience: relation to long-term potentiation

An increase in medial perforant synaptic strength can be observed for hippocampal slices from rats exposed to environmental enrichment. The expression of enhanced synaptic strength exhibits properties similar to long-term potentiation (LTP), a physiological model of memory storage. Similarities include an increase in strength of the synaptic response in the absence of an altered paired-pulse ratio and an increase in the binding of the glutamate agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. Furthermore, environmental enrichment interacts with the mechanisms responsible for the induction of LTP by inhibiting further increases in synaptic strength following LTP-inducing stimulation. The results provide evidence for experience-mediated influences on postsynaptic mechanisms regulating medial perforant path synaptic strength.

Binding of the inward rectifier K+ channel Kir 2.3 to PSD-95 is regulated by protein kinase A phosphorylation

Dynamic regulation of ion channel interactions with the cytoskeleton mediates aspects of synaptic plasticity, yet mechanisms for this process are largely unknown. Here, we report that two inwardly rectifying K+ channels, Kir 2.1 and 2.3, bind to PSD-95, a cytoskeletal protein of postsynaptic densities that clusters NMDA receptors and voltage-dependent K+ channels. Kir 2.3 colocalizes with PSD-95 in neuronal populations in forebrain, and a PSD-95/Kir 2.3 complex occurs in hippocampus. Within the C-terminal tail of Kir 2.3, a serine residue critical for interaction with PSD-95, is also a substrate for phosphorylation by protein kinase A (PKA). Stimulation of PKA in intact cells causes rapid dissociation of the channel from PSD-95. This work identifies a physiological mechanism for regulating ion channel interactions with the postsynaptic density.

The hippocampus, long-term potentiation and memory

1. The induction of long-term potentiation (LTP) at excitatory synapses on CA1 pyramidal neurons occurs partly through an increase in the quantal AMPA current. It is also accompanied by an increase in the number of active sites. Any presynaptic modification, requiring the presence of a retrograde messenger, remains uncertain. 2. There are no definitive data that establish a causal link between LTP and the formation of memories. Pharmacological blockage of NMDA and mGLU receptors and genetic mutants with proteins deleted that are thought to be involved in LTP induction or maintenance or in the formation of memories have all linked deficits in LTP with impairments to behavioural learning, but these links are not necessarily causal. The development of tissue- and time-specific lesion of gene expression for multiple gene products may overcome the present limitations of gene deletion experiments and provide more revealing insights into the relationship between LTP and memory.

Estradiol increases the frequency of multiple synapse boutons in the hippocampal CA1 region of the adult female rat

The effect of estradiol to increase the density of dendritic spines and axospinous synapses on hippocampal CA1 pyramidal cells in the adult female rat has been well-documented. However, presynaptic involvement in this process of synapse elimination and formation in the adult is unknown. To address this issue, we have reconstructed 410 complete presynaptic boutons through coded serial electron micrographs of CA1 stratum radiatum to determine the: (1) frequency of multiple (MSB) vs. single (SSB) synapse boutons; (2) number of synaptic contacts per MSB; (3) bouton volume and surface area; and (4) types of spines in synaptic contact with MSBs and SSBs in ovariectomized, estradiol-treated animals (OVX + E) versus ovariectomized oil-treated controls (OVX + O). Quantitative analysis of this tissue revealed that, in OVX + E animals, 45.0% of presynaptic boutons form multiple synaptic contacts with dendritic spines compared to 27.3% in controls (P < 0.01); the average number of synapses per dendritic spines compared to 27.3% in controls (P < 0.01); the average number of synapses per MSB was 2.7 in OVX + E animals compared to 2.3 in controls (P < 0.05). This represents a 25.5% increase in the number of synapses formed by a given number of presynaptic boutons in estradiol-treated animals (P < 0.01) which largely accounts for the previously observed estradiol-induced increase in axospinous synapse density. There was no treatment effect on bouton size; however, because MSBs are larger than SSBs, the increased frequency of MSBs in estradiol-treated tissue results in a trend toward an estradiol-induced increase in average bouton size. Additionally, MSBS were found to be more irregular in shape, i.e., significantly less spherical, than SSBs. Our results indicate that estradiol-induced dendritic spines form synapses primarily with preexisting boutons in stratum radiatum and that these boutons enlarge and change shape as they accommodate new synapses. Such findings suggest a relatively active role for dendrites in the process of adult synapse formation.

Activity-dependent decay of early LTP revealed by dual EPSP recording in hippocampal slices from young rats

The early maintenance of long-term potentiation (LTP) was studied in the CA1 region of hippocampal slices from 12- to 18-day-old rats in a low-magnesium solution (0.1 mM). The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components of the field excitatory postsynaptic potential were estimated in parallel using early and late measurements of the composite potential. At the normal test stimulus frequency of 0.1 Hz, LTP was seen initially as a predominant increase in the AMPA component, but converted, via a substantial decay of this component and a gradual growth of the NMDA component, into nearly equal changes of the two components. Interrupting the test stimulation for 10 min, changing the test stimulus frequency to 1/60 Hz after LTP induction, or using a test stimulus frequency of 1/60 Hz during the entire experiment significantly reduced the decay of the potentiation of the AMPA component while enhancing the potentiation of the NMDA one. The ratio between the magnitudes of the two excitatory postsynaptic potential (EPSP) components showed a decaying time course that was independent of the manipulations used. Application of the NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (50 microM) after LTP induction stabilized the LTP of the AMPA component until washout was started. On the other hand, the phosphatase inhibitor okadaic acid (1 microM) resulted in decay of the potentiation of both EPSP components back to around baseline and altered the time course of the ratio between the components. Our results show that the early maintenance of LTP is controlled in an activity-dependent and NMDA-dependent manner. This process accelerates the decay of LTP of both AMPA and NMDA components in parallel, suggesting that it is similar to homosynaptic long-term depression, although it operates at the normal test stimulus frequency. The data support a scenario in which LTP ensues as a selective AMPA receptor modification and subsequently converts to another modification, possibly a presynaptic one.

Involvement of silent synapses in the induction of long-term potentiation and long-term depression in neocortical and hippocampal neurons

Changes in the latency of small excitatory postsynaptic potentials were observed in association with induction of long-term modifications of synaptic transmission in slices of rat neocortex and guinea-pig hippocampus. After potentiation response latency decreased in 3/10 cases in the neocortex and in 6/24 cases in the hippocampus, and increased after depression in 4/8 cases in the neocortex. These latency changes could not be attributed to changes in presynaptic fibre excitability, monosynaptic inhibition, release kinetics or activation kinetics of postsynaptic ion channels. We conclude therefore that potentiation led to the activation of previously silent synapses of fast-conducting afferents and depression to the inactivation of previously functional synapses. Thus, neocortical and hippocampal synapses can be in a non-functional state, and regimes that induce long-term potentiation and depression not only change the efficacy of synapses but also alter their functional state.

Retrograde messengers and long-term potentiation: a progress report

Long term potentiation (LTP) is a widely studied form of synaptic plasticity. Brief tetanic stimulation of synaptic afferents in several areas of the brain, most notably the hippocampus, produces long lasting changes in the synaptic strength. The induction of LTP requires in the limit a significant activation of the NMDA receptor and the subsequent entry of calcium into the post synaptic cell. The maintenance of LTP requires at least in part a change in presynaptic function. This review addresses the current thinking in the literature on how a post synaptic induction event may be communicated to the presynaptic terminal and subsequently lead to a series of poorly defined biochemical process that ultimately lead to an enhancement in the efficiency of the potentiated terminal.

Platelet-activating factor as a potential retrograde messenger

Previously the involvement of platelet-activating factor (PAF) as a retrograde messenger in the induction of long-term potentiation (LTP) in CA1 region of rat hippocampus has been reported (Kato, K. et al. (1994) Nature 367, 175-179). In this report, the effect of PAF receptor antagonist on LTP in the dentate gyrus has been demonstrated and suggests the existence of common mechanisms of LTP in Ca1 and dentate gyrus. The site responsible for the expression of LTP induced by PAF has also been examined. The frequency of miniature EPSCs were enhanced by application of a PAF analog, but median amplitude were unaffected, suggesting that PAF preferentially affects presynaptic sites. Evoked synaptic responses were also monitored and the potentiation by the PAF analog was observed. Interestingly, there is a discrepancy between the results observed in the miniature and evoked potential studies in terms of dose effect of PAF. This discrepancy could have resulted from repeated synaptic stimulations required for studying evoked responses. The requirement for a monitoring stimulation in the evoked response measurement could activate additional biological process that enhance LTP expression.

Upregulation of spinal glutamate receptors in chronic pain

Recent studies indicate that glutamate binding to N-methyl-D-aspartate receptors in the spinal cord is involved in triggering the development of chronic pain However, the processes which directly underlie the increased pain remain unclear. Here we report that, following peripheral nerve injury (ligation of the sciatic nerve) in the rat, there is an increase in immunoreactive labelling of non-N-methyl-D-asparatate, AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate), glutamate receptors in the superficial laminae of the lumbar spinal cord ipsilateral to the ligation. The increase in AMPA receptor expression peaks 14 days after nerve ligation and decreases 35 days post-ligation, corresponding to the time-course of heightened sensitivity to mechanical and thermal noxious stimuli (hyperalgesia) induced by the ligation. Given evidence that AMPA receptors in the superficial laminae mediate fast nociceptive transmission in the spinal cord, our findings suggest that an upregulation of spinal AMPA receptors contributes to hyperalgesia following peripheral nerve injury.

LTP of AMPA and NMDA receptor-mediated signals: evidence for presynaptic expression and extrasynaptic glutamate spill-over

We have addressed the expression of long-term potentiation (LTP) in hippocampal CA1 by comparing AMPA and NMDA receptor-(AMPAR- and NMDAR-) mediated postsynaptic signals. We find that potentiation of NMDAR-mediated signals accompanies LTP of AMPAR-mediated signals, and is associated with a change in variability implying an increase in quantal content. Further, tetanic LTP of NMDAR-mediated signals can be elicited when LTP of AMPAR-mediated signals is prevented. We propose that LTP is mainly expressed presynaptically, and that, while AMPARs respond only to glutamate from immediately apposed terminals, NMDARs also sense glutamate released from terminals presynaptic to neighboring cells. We also find that tetanic LTP increases the rate of depression of NMDAR-mediated signals by the use-dependent blocker MK-801, implying an increase in the glutamate release probability. These findings argue for a presynaptic contribution to LTP and for extrasynaptic spill-over of glutamate onto NMDARs.