Activation of a K-252b-Sensitive Protein Kinase is Necessary for a Post-Synaptic Phase of Long-Term Potentiation in Area CA1 of Rat Hippocampus

Presynaptic mechanisms involved in the expression of STP and LTP at CA1 synapses in the hippocampus

The study of long-term potentiation (LTP) has for many years been the centre of a raging debate as to whether the process is expressed by presynaptic or postsynaptic mechanisms. Here we present evidence that two forms of synaptic plasticity at CA3-CA1 synapses in the hippocampus are expressed by presynaptic changes. One form is short-term potentiation (STP) and the other a neonatal form of early-LTP (E-LTP). We review recent experimental data that suggests that this latter form of LTP involves an increase in the probability of neurotransmitter release (Pr). We describe how this is caused by the rapid down-regulation of a high affinity kainate receptor, which otherwise responds to ambient levels of l-glutamate by depressing Pr.

PDZ proteins interacting with C-terminal GluR2/3 are involved in a PKC-dependent regulation of AMPA receptors at hippocampal synapses

We investigated the role of PDZ proteins (GRIP, ABP, and PICK1) interacting with the C-terminal GluR2 by infusing a ct-GluR2 peptide ("pep2-SVKI") into CA1 pyramidal neurons in hippocampal slices using whole-cell recordings. Pep2-SVKI, but not a control or PICK1 selective peptide, caused AMPAR-mediated EPSC amplitude to increase in approximately one-third of control neurons and in most neurons following the prior induction of LTD. Pep2-SVKI also blocked LTD; however, this occurred in all neurons. A PKC inhibitor prevented these effects of pep2-SVKI on synaptic transmission and LTD. We propose a model in which the maintenance of LTD involves the binding of AMPARs to PDZ proteins to prevent their reinsertion. We also present evidence that PKC regulates AMPAR reinsertion during dedepression.

A role for protein kinase C in a form of metaplasticity that regulates the induction of long-term potentiation at CA1 synapses of the adult rat hippocampus

The possibility that protein kinase C (PKC) is involved in the induction of N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) at CA1 synapses in the hippocampus has been the subject of considerable investigation. However, many of the conclusions have been drawn from the use of relatively nonspecific PKC inhibitors. In the present study we have examined the role of PKC in tetanus-induced LTP of AMPA receptor-mediated synaptic transmission in hippocampal slices obtained from adult rats. In particular, we have investigated the possible role of PKC in a molecular switch process that is triggered by the synaptic activation of metabotropic glutamate receptors and regulates the induction of LTP. We find that the three PKC inhibitors examined, chelerythrine, Ro-31-8220 and Gö 6983, all block the setting of the molecular switch at concentrations consistent with inhibition of PKC. In contrast, these inhibitors are without affect on the induction of LTP, even when applied in very much higher concentrations. A PKA inhibitor, Rp-cAMPS, had no effect on either process. We suggest that neither PKC nor PKA is required to induce LTP at this synapse. However, PKC is involved in the regulation of LTP induction, via the molecular switch process.

Interaction between paired-pulse facilitation and long-term potentiation of minimal excitatory postsynaptic potentials in rat hippocampal slices: a patch-clamp study

Long-term potentiation is an experimental paradigm used to study synaptic plasticity and memory mechanisms. One similarity between long-term potentiation and memory is the existence of several distinct phases. However, our preliminary quantal analysis did not reveal essential differences in expression mechanisms of the early (< 1 h) and later (up to 3 h) phases of long-term potentiation. The data were compatible with presynaptic mechanisms of both phases. Another approach to distinguish between presynaptic and postsynaptic mechanisms is analysis of interaction between long-term potentiation and presynaptic paired-pulse facilitation. Such analysis had been previously done mainly with recordings of field potentials reflecting the activity of large neuronal populations. Only the early potentiation phase had been previously analysed with recordings from single neurons. The results from different groups were contradictory. In the present study, minimal excitatory postsynaptic potentials were recorded from CA1 pyramidal neurons of rat hippocampal slices. Paired-pulse facilitation ratios were calculated for various periods (up to 2-3 h) following induction of long-term potentiation. The ratio persistently decreased in the majority of neurons following long-term potentiation induction. The decrease in the paired-pulse facilitation ratio correlated with the magnitude of long-term potentiation and with the initial (pretetanic) facilitation ratio. Therefore, the general results of the present analysis was similar with the results of the quantal analysis: it is consistent with a strong involvement of presynaptic mechanisms in maintenance of both early and late phases of long-term potentiation. However, individual neurons could show variable changes in the paired-pulse facilitation, e.g., increases at late (> 0.5-1 h) periods after tetanus. Calculations of partial correlations and regression analysis indicated that positive correlation between potentiation magnitude and initial (pretetanic) paired-pulse facilitation tended to increase in the late potentiation phase (1.5-2.5 h post-tetanus) indicating that different mechanisms are involved in the early (0.5 h post-tetanus) and the late phase of long-term potentiation. The findings are compatible with involvement of presynaptic mechanisms in both the early and late phases of long-term potentiation. However, the results suggest that contribution of changes in release probability and in effective number of transmitter release sites may differ during the two phases. It is suggested that activation of silent synapses and increases in the number of transmission zones due to pre- and postsynaptic structural rearrangements represent important mechanisms of the late phase of long-term potentiation.

Postsynaptic inhibitors of calcium/calmodulin-dependent protein kinase type II block induction but not maintenance of pairing-induced long-term potentiation

The role of postsynaptic kinases in the induction and maintenance of long-term potentiation (LTP) was studied in the CA1 region of the rat hippocampal slice. A peptide inhibitor for the catalytic domain of calcium/calmodulin-dependent protein kinase type II (CaM-kinase) was applied through a perfused patch pipette. The inhibitor completely blocked both the short-term potentiation and LTP induced by a pairing protocol. This indicates that the kinase or kinases affected by the peptide are downstream from depolarization in the LTP cascade. The ability to block LTP required that measures be taken to interfere with degradation of the peptide kinase inhibitor by endogenous proteases; either addition of protease inhibitors or modifications of the peptide itself greatly enhanced the effectiveness of the peptide. Protease inhibitors by themselves or control peptide did not block LTP induction. To study the effect of kinase inhibitor on LTP maintenance, we induced LTP in one pathway. Subsequent introduction of the kinase inhibitor blocked the induction of LTP in a second pathway, but it did not affect maintenance of LTP in the first. The implications for the role of kinases in LTP maintenance are discussed.

Protein kinase C in synaptic plasticity: changes in the in situ phosphorylation state of identified pre- and postsynaptic substrates

1. Long-term potentiation and its counterpart long-term depression are two forms of activity dependent synaptic plasticity, in which protein kinases and protein phosphatases are essential. 2. B-50/GAP-43 and RC3/neurogranin are two defined neuronal PKC substrates with different synaptic localization. B-50/GAP-43 is a presynaptic protein and RC3/neurogranin is only found at the postsynaptic site. Measuring their phosphorylation state in hippocampal slices, allows us to simultaneously monitor changes in pre- and postsynaptic PKC mediated phosphorylation. 3. Induction of LTP in the CA1 field of the hippocampus is accompanied with an increase in the in situ phosphorylation of both B-50/GAP-43 and RC3/neurogranin, during narrow, partially overlapping, time windows. 4. Pharmacological data show that mGluR stimulation results in an increase in the in situ phosphorylation of B-50/GAP-43 and RC3/neurogranin.

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.

A biochemist's view of long-term potentiation

This review surveys the molecular mechanisms of long-term potentiation (LTP) from the point of view of a biochemist. On the basis of available data, LTP in area CA1 of the hippocampus is divided into three phases--initial, early, and late--and the mechanisms contributing to the induction and expression of each phase are examined. We focus on evidence for the involvement of various second messengers and their effectors as well as the biochemical strategies employed in each phase to convert a transient signal into a lasting change in the neuron. We also consider, from a biochemical perspective, the implications of a multiphase model for LTP.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

Ionotropic glutamate receptors. Their possible role in the expression of hippocampal synaptic plasticity

In the brain, most fast excitatory synaptic transmission is mediated through L-glutamate acting on postsynaptic ionotropic glutamate receptors. These receptors are of two kinds--the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (non-NMDA) and the N-methyl-D-aspartate (NMDA) receptors, which are thought to be colocalized onto the same postsynaptic elements. This excitatory transmission can be modulated both upward and downward, long-term potentiation (LTP) and long-term depression (LTD), respectively. Whether the expression of LTP/LTD is pre-or postsynaptically located (or both) remains an enigma. This article will focus on what postsynaptic modifications of the ionotropic glutamate receptors may possibly underly long-term potentiation/depression. It will discuss the character of LTP/ LTD with respect to the temporal characteristics and to the type of changes that appears in the non-NMDA and NMDA receptor-mediated synaptic currents, and what constraints these findings put on the possible expression mechanism(s) for LTP/LTD. It will be submitted that if a modification of the glutamate receptors does underly LTP/LTD, an increase/ decrease in the number of functional receptors is the most plausible alternative. This change in receptor number will have to include a coordinated change of both the non-NMDA and the NMDA receptors.

Glutamate-dependent mechanisms in the induction of a calcium long-term potentiation-like phenomenon

The electric synaptic efficacy, in terms of extracellular electrical potentials, and the intracellular postsynaptic efficacy, in terms of inositol phosphate (IP) accumulation, were evaluated in rat hippocampal slices exposed for a brief period (10 min) to a high concentration of calcium (+2.7 mM). In addition, the effects of N-methyl-D-asparate (NMDA) ionotropic and metabotropic glutamate receptor (mGluR) antagonists on the induction and the establishment or maintenance of enhanced synaptic efficacy of CA1 pyramidal neurons due to high-calcium exposure were also tested. Elevation of the calcium concentration from 1.3-4 mM in the medium bathing hippocampal slices produced a long-lasting (80 over 90 min) increase in the slope of the CA1 somatic excitatory postsynaptic potential and the amplitude of the population spike (PS). Slice perfusion with NMDA antagonists cyclazocine and cis-4-phosphonomethyl-2-piperidine-carboxylic acid (CGS 19755) or with mGluR antagonists L-2-amino-3-phosphonopropionic acid (AP3) or alpha-methyl-4-carboxyphenyl-glycine (all 0.1 mM), during the 10-min period of exposure to high-calcium prevented the induction of such changes. By contrast, slice perfusion with the same concentration of CGS 19755 or L-AP3 did not affect the already established long-lasting increase in amplitude of CA1 PS induced by high-calcium. Moreover, high-calcium failed to produce any significant modification of the basal IP accumulation or of the IP accumulation elicited by mGluR agonist 1S,3R-trans-amino cyclo-pentane-1,3-dicarboxylic acid (ACPD). In conclusion, the results confirm that high-calcium induces a long-lasting increase in synaptic efficacy in rat hippocampal slices. Both NMDA ionotropic and mGluR receptors are involved in the induction, but not in the maintenance, of this phenomenon. In line with these data no modifications of basal or ACPD-induced phosphoinositide hydrolysis have been found during the maintenance stage.

Long-term potentiation and N-methyl-D-aspartate receptors: foundations of memory and neurologic disease?

Understanding the physiology of learning and memory is one of the great challenges of neuroscience. The discovery in recent years of long-term potentiation (LTP) of synaptic transmission and the elaboration of the mechanisms involved, in particular the NMDA receptor, offers the prospect not only of improving our understanding of normal memory storage and retrieval, but may also yield insights about various neurological and psychiatric clinical disorders. In this review, we begin by examining the different forms, properties, and methods of inducing LTP, followed by a description of molecular mechanisms thought to underlie the phenomenon. Molecular structure of the receptor is discussed, along with the roles of Ca2+ second messenger systems, synaptic morphology changes, and retrograde messengers in LTP. Finally, implications of the NMDA receptor and LTP in learning, memory, and certain clinical conditions such as epilepsy, Alzheimer's disease, and schizophrenia are discussed.

On the mechanism of long-term potentiation induced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) in rat hippocampal slices

We have reported previously that transient application of a specific metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) can induce a slow-onset form of long-term potentiation (LTP) of synaptic transmission in the CA1 region of rat hippocampal slices [Bortolotto Z. A. and Collingridge G. L. (1993) Neuropharmacology 32, 1-9]. Here we have investigated further the mechanisms involved in the induction and expression of ACPD-induced LTP. Unless otherwise stated, field excitatory postsynaptic potentials (EPSPs) were recorded in stratum radiatum in response to low frequency (0.033 Hz stimulation) of the Schaffer collateral-commissural pathway and 10 microM ACPD was added for 20 min to the perfusate. ACPD-induced LTP was still observed following blockade of GABAA receptor-mediated synaptic inhibition using picrotoxin (50 microM) and was not the result of a change in the presynaptic fibre volley. Intracellular recording from area CA1 revealed an increase in the size of the EPSP but no associated change in membrane potential or input resistance. However, ACPD-induced potentiation was never seen when intracellular electrodes contained the Ca(2+)-chelating agent 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 0.5 M). In area CA3, ACPD elicited a slow-onset LTP of the intracellularly recorded EPSP, evoked by stimulation of associational fibres. In contrast to area CA1, 10 microM ACPD depolarized CA3 neurones. Unlike certain other forms of tetanus- and chemically-induced potentiation, ACPD-induced LTP was not affected by the L-type Ca2+ channel antagonist nimodipine (50 microM). It was, however, prevented by delivering low frequency stimulation (900 shocks at 1 Hz) immediately following termination of the application of ACPD; an effect which was inhibited by the specific N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (AP5; 50 microM). ACPD failed to induce LTP of pharmacologically-isolated NMDA receptor-mediated EPSPs. The induction of ACPD-induced LTP was blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), in a reversible manner. In slices in which area CA3 had been removed ACPD failed to induce LTP when applied alone or together with AMPA. However, a slow-onset form of LTP was induced, in slices lacking area CA3, when a tetanus (100 Hz, 1 sec) was delivered in the presence of ACPD and 50 microM AP5 (the latter applied to prevent conventional tetanus-induced LTP). ACPD-induced LTP was associated with a parallel increase in the sensitivity of CA1 neurones to AMPA. Considered together, these data suggest that ACPD-induced LTP is due to a direct increase in the AMPA receptor-mediated synaptic conductance and involves postsynaptic induction and expression mechanisms.

A critical period of protein kinase activity after tetanic stimulation is required for the induction of long-term potentiation

A critical period of protein kinase activity required for the induction of long-term potentiation (LTP) was determined in area CA1 or hippocampal slices using the broad-range and potent protein kinase inhibitors K-252a and staurosporine. As reported previously, K-252a and staurosporine blocked LTP induction when applied before, during, and after high-frequency stimulation (HFS). In contrast, K-252a did not block LTP when applied only before and during HFS and washed out immediately after HFS. K-252a and staurosporine both attenuated LTP magnitude when applied immediately after or as late as 5 min after HFS. However, K-252a applications beginning 30-45 min after HFS did not affect LTP expression significantly. K-252a had no detectable effect on isolated N-methyl-D-aspartate (NMDA) receptor-mediated EPSPs but significantly inhibited the in situ phosphorylation of specific hippocampal proteins (synapsin I, MARCKS, and B-50). In addition, K-252a attenuated 4 beta-phorbol-12,13-dibutyrate (PDBu)-enhanced synaptic transmission. Our results indicate that there is a critical period of protein kinase activity required for LTP induction that extends for approximately 20 min after HFS. In addition, our results suggest that protein kinase activity during and immediately after HFS is not sufficient for LTP induction. These results provide new information about the mechanisms that underlie LTP induction and expression and provide evidence for persistent and/or Ca(2+)-independent protein kinase activity involvement in LTP.

Long-term potentiation and synaptic protein phosphorylation

Long-term potentiation (LTP) is a well known experimental model for studying the activity-dependent enhancement of synaptic plasticity, and because of its long duration and its associative properties, it has been proposed as a system to investigate the molecular mechanisms of memory formation. At present, there are several lines of evidence that indicate that pre- and postsynaptic kinases and their specific substrates are involved in molecular mechanisms underlying LTP. Many studies focus on the involvement of protein kinase C (PKC). One way to investigate the role of PKC in long-term potentiation is to determine the degree of phosphorylation of its substrates after in situ phosphorylation in hippocampal slices. Two possible targets are the presynaptic membrane-associated protein B-50 (a.k.a. GAP 43, neuromodulin and F1), which has been implicated in different forms of synaptical plasticity in the brain such as neurite outgrowth, hippocampal LTP and neurotransmitter release, and the postsynaptic protein neurogranin (a.k.a. RC3, BICKS and p17) which function remains to be determined. This review will focus on the protein kinase C activity in pre- and postsynaptic compartment during the early phase of LTP and the possible involvement of its substrates B-50 and neurogranin.

Intra-amygdala kinase inhibitors disrupt retention of a learned avoidance response in rats

To assess the involvement of intra-amygdala kinase activity in aversively motivated learning, rats received intracranial injections of polymixin B sulfate (PMXB)--a protein kinase C (PKC) and calcium/calmodulin-dependent kinase II inhibitor--immediately after inhibitory avoidance training. When tested 48 h later, retention was significantly impaired relative to vehicle-injected controls. Delayed injections (2 h posttraining) and injections made dorsal to the amygdala were ineffective. Immediate posttraining injections of the more selective PKC inhibitor, NPC 15437, also impaired retention. These results suggest that intra-amygdala protein phosphorylation must occur soon after training for learned avoidance responses to be successfully retained.

Onset and stabilization of NMDA receptor-dependent hippocampal long-term potentiation

This article discusses recent data concerning the temporal development of N-methyl-D-aspartate (NMDA) receptor-dependent potentiation in the hippocampus. It argues against a mechanistic subdivision of NMDA receptor-dependent potentiation into an early short-term potentiation (STP) and a slowly developing long-term potentiation (LTP). Thus, the article proposes that LTP starts a few seconds after the induction event, that it is fully developed within a minute, and that its subsequent stabilization is controlled by the degree of NMDA receptor activation, and associated increase of calcium concentration in the spine, during the induction event. It is suggested that most biochemical interventions that have been reported to interfere with the LTP process, for example application of protein kinase inhibitors, might have acted through an impairment of the induction mechanism rather than through an impairment of specific stabilization or maintenance mechanisms.

A molecular switch activated by metabotropic glutamate receptors regulates induction of long-term potentiation

Pharmacological studies of long-term potentiation (LTP) in the hippocampus are starting to provide a molecular understanding of synaptic plastic processes which are believed to be important for learning and memory in vertebrates. In the CA1 region of the hippocampus, the synaptic activation of glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype is necessary for the induction of LTP under most experimental conditions. The synaptic activation of metabotropic glutamate receptors (mGluRs) is also needed for the induction of LTP. We now show that the role of mGluRs in the induction of LTP is fundamentally different from that of NMDA receptors. NMDA receptors initiate a molecular event that needs to be triggered each time a tetanus is delivered to induce LTP. In contrast, mGluRs activate a molecular switch which then negates the need for mGluR stimulation during the induction of LTP. This mGluR-activated switch is input-specific and can be turned off by a train of low-frequency stimulation. The molecular switch is a new feature of LTP which has fundamental consequences for our understanding of synaptic plastic mechanisms.

Dissociation between long-term potentiation and associated changes in field EPSP waveform in the hippocampal CA1 region: an in vitro study in guinea pig brain slices

The present study examines changes in field excitatory postsynaptic potential (EPSP) waveform in association with long-term potentiation (LTP) in the CA1 region of the hippocampal slice preparation. Experiments were performed in the presence of the GABAA-antagonist picrotoxin. With test field EPSP about one-third the size of that evoking spike activity (measured in the cell body layer along the same somatodendritic axis as the dendritic recording) a decreased decay time constant (approximately 8%) was seen in association with LTP. This change in field EPSP waveform was not associated with any apparent spike activity in the cell body recording. Nevertheless, several findings suggest that increased spike activity explains the change in the decay time constant of the field EPSP during LTP. First, when reducing the stimulation strength after the induction of LTP to obtain a field EPSP of the same magnitude as the pretetanus one, the change of the decay time constant was reduced to only 2.8%. Second, when using small test field EPSP (about one-fourth the size of that evoking spike activity) the decay time constant was not significantly affected in association with LTP. Third, when cutting the slice in such a manner that spike activity originating from somatic regions closer to the stimulating electrode was removed, the EPSPs decay time constant was not significantly affected in association with LTP. It is concluded that LTP is not associated with a change in the shape of the field EPSP.

Metabotropic glutamate response in acutely dissociated hippocampal CA1 pyramidal neurones of the rat

1. The metabotropic glutamate (mGlu) response was investigated in dissociated rat hippocampal CA1 pyramidal neurones using conventional and nystatin-perforated whole-cell modes of the patch recording configuration. 2. In the perforated patch recording configuration, the application of glutamate (Glu), quisqualate (QA), aspartate (Asp) and N-methyl-D-aspartate (NMDA) induced a slow outward current superimposed on a fast ionotropic inward current, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate (KA) induced only an ionotropic inward current at a holding potential (VH) of -20 mV. A specific agonist of the mGlu receptor (mGluR), trans-1-aminocyclopentane-1,3-dicarboxylate (tACPD), induced an outward current in approximately 80% of the neurones tested. Asp- and NMDA-induced outward currents were antagonized by D-2-amino-5-phosphonopentanoate (D-AP5) whereas Glu-, QA- and tACPD-induced outward currents were not antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D-AP5, indicating that the mGlu response is an outward current component. 3. L-2-Amino-3-phosphonopropionate (L-AP3) and DL-2-amino-4-phosphonobutyrate (AP4) did not block the mGlu response. 4. The relative potencies of mGlu agonists were QA > Glu > tACPD. The threshold and EC50 values of metabotropic outward currents were 10-100 times lower than those of the ionotropic inward current (iGlu response). 5. The reversal potential of the mGlu response (EmGlu) was close to EK (K+ equilibrium potential), and it shifted 59.5 mV for a tenfold change in extracellular K+ concentration. 6. In Ca(2+)-free external solution, the mGlu response was elicited by an initial application of Glu, but subsequent applications failed to induce the response. There was also an increase in the intracellular free Ca2+ concentration ([Ca2+]i) during the application of Glu and QA but not of AMPA, indicating Ca2+ release from an intracellular Ca2+ store. 7. During the activation of a Ca(2+)-dependent K+ current (IK(Ca)) by inositol trisphosphate (IP3) in the internal solution, the mGlu response was suppressed. Addition of GDP-beta-S, neomycin or heparin to the internal solution also suppressed the mGlu response, but staurosporine had no effect. The mGlu response was abolished by pretreatment with either caffeine or ryanodine, but treatment with pertussis toxin (IAP) for 6-8 h had no effect. 8. The mGlu response was suppressed by tetraethylammonium, but not by either apamin or iberiotoxin, suggesting that intermediate-conductance Ca(2+)-dependent K+ (KCa+) channels are involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Staurosporine impairs both short-term and long-term potentiation in the dentate gyrus in vitro

The present study shows that the protein kinase inhibitor staurosporine impairs the transient (< 60 min) potentiation (short-term potentiation) evoked by a weak tetanus to about the same extent as the more stable potentiation (long-term potentiation) evoked by a strong tetanus. This effect on short-term and long-term potentiation was seen both as a reduced magnitude and an increased decay rate, the latter being increased by about 50% compared to that seen under normal conditions. Comparison with potentiations evoked at different strengths in control solution suggested that much, but not all, of the increased decay rate observed in the presence of staurosporine could be explained by an impared induction. Staurosporine did not affect the N-methyl-D-aspartate-mediated field excitatory postsynaptic potential evoked by low-frequency stimulation or the magnitude of N-methyl-D-aspartate-mediated currents during high-frequency tetanization. This result suggests that the induction is impaired at a stage not related to the N-methyl-D-aspartate-mediated calcium influx. The present results suggest that short-term and long-term potentiation cannot be separated on the basis of protein kinase dependence. They do not support the common notion that short-term and long-term potentiation are mechanistically separate entities. Instead, the results support the view that long-term potentiation has a variable duration/stability dependent on the induction conditions and that protein kinase activation, via an action on induction mechanisms, contributes to its stabilization.

An antagonist of glutamate metabotropic receptors, (RS)-alpha-methyl-4-carboxyphenylglycine, prevents the LTP-related increase in postsynaptic AMPA sensitivity in hippocampal slices

(RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), a compound which selectively antagonizes metabotropic glutamate receptors (mGlu-R), presents the LTP of field excitatory postsynaptic potentials (fEPSP) as well as the tetanus-induced increase in AMPA-evoked responses (fPRs) in the CA1 region of hippocampal slices. This effects of MCPG provide evidence for the involvement of mGlu-Rs in mechanisms underlying the postsynaptic maintenance of LTP, which appears to be mediated, at least partially, by an increase in sensitivity and/or number of postsynaptic AMPA receptors.

Time-dependent reversal of long-term potentiation by brief cooling shocks in rat hippocampal slices

Using a recording chamber built with peltier elements, we studied the effects of fast and brief reductions in temperature on synaptic transmission and plasticity in area CA1 of rat hippocampal slices. Cooling shocks consisted of a drop in temperature from 33 degrees C to 30 degrees C, 27 degrees C or 24 degrees C for 2-5 min. Equilibrium to the new temperature was reached in about 30 s. During these cooling episodes, marked modifications of the size and time course of synaptic responses were observed. Changing the temperature for 4-5 min from 33 degrees C to 24 degrees C resulted in a 75% reduction in amplitude and 158% prolongation of the rise time of excitatory postsynaptic potentials (EPSPs). These changes were followed by a complete recovery of synaptic transmission. This recovery was very fast for the EPSP rise time (about 30 s), but much slower for the amplitude or initial slope (20-30 min). This slow recovery was correlated with changes in size of the presynaptic fiber volley, thereby indicating transient modifications of cell excitability. Application of cooling episodes of 4-5 min from 33 degrees C to 24 degrees C during the first 20 min that followed induction of long-term potentiation resulted in a complete reversal of synaptic potentiation. The LTP abolished by a cooling shock could be reinstated by re-applying high frequency trains. Several sequential induction/abolition effects could thus be obtained. In contrast, cooling episodes applied later than 25 min after LTP induction did not affect synaptic potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal transduction pathways involved in the acute potentiation of NMDA responses by 1S,3R-ACPD in rat hippocampal slices

1. A grease-gap recording technique has been used to investigate the mechanisms underlying the acute potentiation of N-methyl-D-aspartate (NMDA) responses by aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD) in area CA1 of rat hippocampal slices. 2. 1S,3R-ACPD (10 microM), but not 1R,3S- ACPD (10 microM), potentiated submaximal responses to NMDA (dose-ratio of 0.81 +/- 0.02 (mean +/- s.e.mean); n = 55), but not to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), in a readily reversible manner. Potentiation also occurred in slices treated with 0.2 microM tetrodotoxin, and in slices perfused with Mg(2+)-free medium. 3. 1S,3R-ACPD-induced potentiation was unaffected by the protein kinase inhibitors K-252b (0.1 microM) and staurosporine (1 microM) and the intracellular Ca2+ store depletor, thapsigargin (10 microM). Coapplication of staurosporine and thapsigargin was also without effect. 4. 1S,3R-ACPD-induced potentiation was unaffected by inhibitors of arachidonic acid formation, bromophenacyl bromide (50 microM) and RG80267 (100 microM). Arachidonic acid (10-50 microM) depressed reversibly NMDA-induced responses. The potentiation was unaffected by 8-bromo cyclic AMP (500 microM) or the PKA inhibitor Rp-adenosine 3,5-cyclic monophosphothioate triethylamine (Rp-cAMPS; 50 microM). 5. 1S,3R-ACPD-induced potentiation was abolished in slices perfused with Ca(2+)-free medium. The potentiation was also blocked by phorbol-12,13-diacetate (1 microM), in a staurosporine-sensitive manner. 6. It is concluded that the potentiation of NMDA responses by 1S,3R-ACPD is not mediated by protein kinase A or C, by release of Ca2+ from intracellular stores or by arachidonic acid. It involves a Ca2+-sensitive process and is negatively regulated by protein kinase C.

Blockade of long-term potentiation and of NMDA receptors by the protein kinase C antagonist calphostin C

We have studied the effects of calphostin C, an antagonist of the regulatory subunit of protein kinase C, on the induction and expression of long-term potentiation (LTP) and on responses mediated by activation of N-methyl-D-aspartate (NMDA) receptors in rat hippocampal slices. No effect of calphostin C was observed on pre-established LTP, even at concentrations of 2-3 mumol/l. In contrast, the drug was found to prevent LTP induction. This effect was concentration-dependent, although high concentrations were needed (1-2 mumol/l), and, at the lower concentrations, it could be partially antagonized by using coactivation of two pathways instead of single input activation. While calphostin C did not alter synaptic transmission mediated by activation of alpha-amino-3-hydroxy-5- methylisoxazole-4-propionic acid (AMPA) receptors, it considerably interfered with the function of NMDA receptors. The drug blocked the NMDA receptor-mediated component of burst responses, significantly antagonized the NMDA receptor-mediated synaptic responses recorded in the presence of an AMPA receptor antagonist, and blocked the effect of iontophoretic application of NMDA on regular synaptic transmission. These results are consistent with the idea that calphostin C prevents the induction of long-term potentiation by interfering with the function of NMDA receptors.

Metabotropic receptor stimulation coupled to weak tetanus leads to long-term potentiation and a rapid elevation of cytosolic protein kinase C activity

We have previously shown that short-term potentiation (STP) inducing weak tetanus induces long-term potentiation (LTP) when it is coupled with activation of metabotropic glutamate (mGlu) receptors by trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD) in rat CA1 slices. In the present study, we examined if this conversion of STP to LTP involves activation of protein kinase C (PKC). Two minutes but not 30 min after coupling, there was a significant increase in the activator-dependent PKC activity in the cytosolic fraction. STP induction or t-ACPD application did not change PKC activity. There was no activity increase in the membrane fraction. STP was also induced by a co-application of gamma-amino-3-hydroxy-5-methyllisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA). Coupling this STP with t-ACPD, however, did not result in an LTP or PKC activity increase, indicating a requirement for synaptic activity. A rapid and transient (< 5 min) increase in cytosolic PKC activity was also seen after the induction of LTP by stronger tetanic stimulation. No LTP tested in the present study was accompanied by activator-independent, persistent increases in PKC activity. STP induction depends on NMDA receptor activation, and the activation of mGlu receptors results in the production of intracellular second messengers. Our results therefore indicate that these separate components may add and bring about PKC activation and LTP.

Facilitatory effect of phorbol ester on 2-deoxyglucose uptake in rat hippocampal slices

It is well known that synaptic potentiation in the hippocampus can be produced by phorbol ester, a protein kinase C activator. The 2-deoxyglucose uptake is an index of regional glucose utilization which predominantly reflects activity in the axonal terminal of neuronal pathways. In the present experiment, therefore, we examined whether application of phorbol ester produces a facilitatory effect on 2-deoxyglucose uptake by the rat hippocampus in vitro. The application of phorbol-12,13-dibutyrate (PdBU) produced an elevation of 2-deoxyglucose uptake, while pretreatment with PdBU for 60 min eliminated the pdBU-induced elevation. Pretreatment with protein kinase C inhibitors, K252a (0.1 and 1 microM) or staurosporine (0.1 and 1 microM), was found to block significantly the PdBU-induced elevation of 2-deoxyglucose uptake. In addition, the facilitatory effect of glutamate, quisqualate and carbachol on 2-deoxyglucose uptake was reduced by pretreatment with PdBU. In the present experiment, we demonstrated that application of phorbol ester caused an elevation of 2-deoxyglucose uptake, which is linked in turn to neuronal activity, suggesting a positive relationship between protein kinase C activation and energy consumption.

Co-activation of metabotropic glutamate and N-methyl-D-aspartate receptors is involved in mechanisms of long-term potentiation maintenance in rat hippocampal CA1 neurons

Slices of hippocampal area CA1 in the rat were employed to test the hypothesis that the activation of metabotropic glutamate receptors during tetanization is necessary for the late maintenance of long-term potentiation. If the metabotropic glutamate receptor antagonist L-2-amino-3-phosphonopropionate was present during tetanization, post-tetanic and early long-term potentiation of the population spike as well as field excitatory postsynaptic potential developed almost normally. However, 100 min after tetanization, long-term potentiation of the field excitatory postsynaptic potential decreased in an irreversible manner. The same concentration of D-2-amino-3-phosphonopropionate was ineffective. If L-2-amino-3-phosphonopropionate was applied 120 min after tetanization, it did not influence long-term potentiation. The presence of the metabotropic glutamate receptor agonist trans-D,L-1-aminocyclopentane-1,3-dicarboxylic acid during tetanization weakly enhanced the slope of field excitatory postsynaptic potential long-term potentiation. The influence of L-2-amino-3-phosphonopropionate and D,L-1-aminocyclopentane-1,3-dicarboxylic acid on ionotropic glutamate receptors was studied using whole-cell voltage-clamp and pressure application techniques. No effect of L-2-amino-3-phosphonopropionate on either early or late components of excitatory postsynaptic currents could be detected at the concentration used to block long-term potentiation. It is therefore unlikely that the effect of L-2-amino-3-phosphonopropionate on long-term potentiation is due to an interaction with N-methyl-D-aspartate receptors or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors. However, bath-applied 1S,3R-D,L-1-aminocyclopentane-1,3-dicarboxylic acid facilitated the N-methyl-D-aspartate-induced depolarization in response to N-methyl-D-aspartate pressure application in a reversible manner. These data suggest that besides the involvement of N-methyl-D-aspartate receptors the activation of a 2-amino-3-phosphonopropionate-sensitive metabotropic glutamate receptors during or immediately after tetanization is necessary for subsequent mechanisms responsible for the maintenance of long-term potentiation. A link between metabotropic glutamate receptors and protein kinase C activation during long-term potentiation is discussed considering the similar time course of long-term potentiation blockade after application of L-2-amino-3-phosphonopropionate and protein kinase C inhibitors.

Long-lasting potentiation of synaptic transmission in the Schaffer collateral-commissural pathway of the guinea pig hippocampus by activation of postsynaptic N-methyl-D-aspartate receptor

The effects of short-period (2 min) perfusion of conditioning solution, which contains N-methyl-D-aspartate (NMDA), glycine, and spermine, on the synaptic transmission in the Schaffer collateral-commissural pathway were examined in hippocampal slices with the intracellular recording technique. Long-lasting potentiation of excitatory postsynaptic potentials (EPSPs) was induced (as long as the records lasted, up to 3 h in the longest observation) after membrane potentials of postsynaptic neurons were depolarized by current injection during perfusion of the conditioning solution. D-2-amino-5-phosphonovaleric acid (D-AP5), a specific antagonist of NMDA receptors, block the induction of the long-lasting potentiation by perfusion of NMDA containing solution. This potentiation was accompanied by a decrease in the relative magnitude of EPSP amplitude fluctuation (coefficient of variation, CV). The reciprocals of squared CVs (= mean2/variance) were almost proportional to the magnitude of the potentiation, and the ratios of 1/CV2 and the magnitudes of potentiation were not different from those of long-term potentiation (LTP) induced by tetanic stimulation. These findings suggest that long-lasting potentiation is induced solely by activation of postsynaptic NMDA receptors, and transmitter release from presynaptic terminals may be modified by the activation of postsynaptic receptors.

A synaptic model of memory: long-term potentiation in the hippocampus

Long-term potentiation of synaptic transmission in the hippocampus is the primary experimental model for investigating the synaptic basis of learning and memory in vertebrates. The best understood form of long-term potentiation is induced by the activation of the N-methyl-D-aspartate receptor complex. This subtype of glutamate receptor endows long-term potentiation with Hebbian characteristics, and allows electrical events at the postsynaptic membrane to be transduced into chemical signals which, in turn, are thought to activate both pre- and postsynaptic mechanisms to generate a persistent increase in synaptic strength.

Characterisation of LTP induced by the activation of glutamate metabotropic receptors in area CA1 of the hippocampus

The transient activation of the N-methyl-D-aspartate (NMDA) receptor system by high frequency (tetanic) stimulation results in a rapidly developing and long-lasting potentiation of synaptic transmission in the CA1 region of the hippocampus. This potentiation can be divided into an early decremental component, known as short-term potentiation (STP), and a more slowly developing persistent phase, termed long-term potentiation (LTP). Here we describe how activation of metabotropic glutamate receptors (mGluRs), by aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD), can induce the same stable form of LTP, but without the STP component. 1S,3R-ACPD-induced LTP does not require electrical stimulation during its induction, but is dependent on an intact connection between the CA3 and CA1 regions of the hippocampus. 1S,3R-ACPD-induced LTP circumvents the need for the activation of NMDA receptors and is likely to involve both the stimulation of protein kinase C (PKC) and the release of Ca2+ from intracellular stores.

Biochemical correlates of long-term potentiation in hippocampal synapses

Figure 2 summarizes biochemical events which are currently known or hypothesized to participate in LTP induction/maintenance. Current evidence strongly suggests that postsynaptic Ca2+, both entered from the outside of cells and released from intracellular stores, is the initial key substance for the induction of LTP. A rise of [Ca2+]i triggers a variety of enzymatic reactions and initiates the enhancement of synaptic transmission. This first step may be achieved by direct/indirect phosphorylations of protein molecules in postsynaptic receptors/ion channels. This would result in an increase in receptor sensitivity. An immediate increase in the number of available postsynaptic receptors by modifications of spine morphology is another candidate. Such modifications may be accomplished by cytoskeleton rearrangements or changes in extracellular environments. A change in spine structure may also cause an increase in spine neck conductance. Although it is unknown to what extent the increase in [Ca2+]i affects cellular chemistry, Ca2+ probably also directly/indirectly stimulates cascades which exert effects more slowly. A delayed increase in metabotropic receptor sensitivity may occur. New synthesis of protein molecules may be involved in late periods of LTP by replacing turnovered molecules and/or by supplying new materials. Some of these chains of biochemical events may also apply to presynaptic terminals, although the existence of retrograde messenger substances must still be confirmed. In addition, interactions between different protein kinases and second messengers appear to occur to bring about final effects.

Long-term potentiation as synaptic dialogue

We have proposed a testable model of the physiological and biochemical events underlying LTP that offers the following novel features. (1) The focus is not on a single mechanism or synaptic site, but rather on the integration and interaction of mechanisms occurring on both sides of the synapse, (2) beta PKC plays a critical presynaptic role in LTP, while gamma PKC functions postsynaptically. (3) These stages can be ordered in a time-delimited sequence of post- then presynaptic molecular events based on the period of effectiveness of inhibitor compounds. (4) The distinction is made between the time when kinase activation occurs and the time when the potentiated response requiring this kinase activation is observed.

A role for protein kinase C in long term potentiation of nicotinic transmission in the superior cervical ganglion of the rat

The superior cervical ganglion of rats was perfused with Ringer solution containing hexamethonium to produce a steady, partial, nicotinic block. The compound action potential (CAP) evoked by supramaximal single shock stimulation of the cervical sympathetic trunk (CST) was recorded from the internal carotid nerve. Bolus injection of the protein kinase C (PKC) activators 4 beta-phorbol-12,13-dibutyrate (PDBu) or 4 beta-phorbol-12,13-diacetate (PDAc) produced a marked, prolonged, dose-dependent potentiation of the CAP amplitude (e.g. 90% decay 2 h). A non-PKC activating phorbol ester (PE), 4 alpha-phorbol-12,13-didecanoate, produced no potentiation. The PE-induced potentiation was antagonized by the PKC inhibitor H-7. In addition, after 1 h exposure to PDBu (3 microM) and recovery from the potentiation (e.g. 2-4 h), a second exposure to PDBu or PDAc produced no potentiation. A 5 s 40 Hz supramaximal train to the CST produced a long lasting potentiation of the CAP (long-term potentiation, LTP) as described previously. However, a similar train did not evoke LTP after perfusion for 1 h with PDBu. The train-evoked LTP was depressed by the PKC inhibitor H-7 at a concentration which antagonized the PE-evoked potentiation. These data suggest that (i) PKC activation potentiates nicotinic transmission, and (ii) a component of the train-evoked LTP is mediated by PKC.

Protein kinases and phosphatase inhibitors mediating long-term desensitization of glutamate receptors in cerebellar Purkinje cells

Long-term desensitization of the AMPA-selective glutamate receptors in Purkinje cells was examined in rat cerebellar slices by means of the wedge recording method. It was not induced by application of AMPA alone, but occurred regularly when slices were conditioned by perfusion with 0.5 mM 8-bromo-cGMP (but not cAMP derivatives) or the protein phosphatase inhibitors, okadaic acid and calyculin A. Phorbol esters also showed a similar effect. The 8-bromo-cGMP desensitization was antagonized by KT5823, an inhibitor of protein kinase G, while the effect of calyculin A was inhibited by polymyxin B, H-7, or K252a. These results suggest that AMPA receptors are persistently desensitized due to concerted action of both an agonist and an enzymatic system involving protein kinases G and C and a protein phosphatase inhibitor.

Activation of glutamate metabotropic receptors induces long-term potentiation

The specific glutamate metabotropic receptor agonist 1S,3R-aminocyclopentane dicarboxylate (ACPD), but not its inactive enantiomer 1R,3S-ACPD, induced a slowly developing potentiation of synaptic transmission in rat hippocampal slices. This effect was independent of its ability to potentiate responses mediated by the activation of N-methyl-D-aspartate receptors. Perfusion with 1S,3R-ACPD provides, therefore, a means of chemically inducing a form of long-term potentiation.

Postsynaptic protein kinase C essential to induction and maintenance of long-term potentiation in the hippocampal CA1 region

Previous studies on the effects of protein kinase C (PKC) inhibitors intracellularly introduced into the postsynaptic neuron on long-term potentiation (LTP) in the hippocampal CA1 region showed that given before the tetanic stimulation they only blocked the development of the maintenance phase of LTP and that given after the tetanus they did not affect the continued maintenance of established LTP. We now report different results in such experiments obtained by looking into the dose-effect relationship of the inhibitors given to the postsynaptic neuron and making use of a synergistic effect of two inhibitors given together. We used the following three PKC inhibitors: polymyxin B (PMB), PKC-(19-31), and H7. With the intracellular delivery of the inhibitor(s) beginning 30 min before the tetanus, PMB in adequate dosage or a combination of PMB and PKC-(19-31), each at a low dosage, could block the development of LTP completely including its initial induction phase. With the delivery beginning at the time of the tetanus, PKC-(19-31) or H7 slowly caused the established LTP to decline to the baseline; this decline was greatly accelerated when PMB and PKC-(19-31) or PMB and H7 were given together. PMB and PKC-(19-31) given together 75-90 min or even 3 h after the tetanus caused a decline of the maintained LTP similar to the decline observed when both inhibitors were given at the time of the tetanus. These results show that postsynaptic PKC is essentially involved in both the initial induction and the subsequent maintenance of LTP, contrary to current views on the subject.

Induction of stable long-term potentiation in the presence of the protein kinase C antagonist staurosporine

We have studied the effects of staurosporine, an antagonist of the catalytic subunit of protein kinase C, on the mechanisms of long-term potentiation (LTP) in rat hippocampal slices maintained in vitro. Application of staurosporine did not affect pre-established LTP, but resulted in a decaying potentiation when high frequency stimulation was delivered in its presence. However, coactivation of two inputs to the same group of CA1 neurons during high frequency stimulation transformed the decaying potentiation into stable LTP. Staurosporine also reduced the NMDA receptor-mediated component of synaptic responses to burst stimulation. It is concluded that the PKC antagonist interferes with LTP induction, but not expression mechanisms.

Persistent enhancement of transmitter release accompanying long-term potentiation in the guinea pig hippocampus

In order to examine temporal changes in enhancement of transmitter release during long-term potentiation (LTP), we examined amplitude fluctuation of excitatory postsynaptic potentials (EPSPs) for longer periods than 2 h after tetanic stimulation (up to 4 h in the longest observation). The relative magnitude of excitatory postsynaptic potentiation (EPSP) fluctuation (coefficient of variation, CV) reduced throughout the observation periods in association with an increase in EPSP amplitude after tetanic stimulation. The reciprocals of squared CVs (= mean2/variance) were almost in proportion to the magnitude of LTP, and the ratio of 1/CV2 and the LTP magnitude did not change significantly for up to 4 h. These findings suggest that a prolonged enhancement of transmitter release from presynaptic terminals underlies LTP, and the relative contribution of this presynaptic enhancement does not change significantly for 2 h (maybe up to 4 h, or longer) after tetanic stimulation.

The effect of dopaminergic D1 receptor blockade during tetanization on the expression of long-term potentiation in the rat CA1 region in vitro

The effect of dopaminergic D1 receptor blockade on the expression of long-term potentiation (LTP) was investigated in the rat hippocampal CA1 region in vitro by extracellular recordings (by measuring the population spike amplitude and the field EPSP). The presence of the very selective D1 receptor blocker SCH 23390 at a concentration of 0.1 microM during tetanization with 3 trains of 100 impulses (100 Hz) resulted in a prevention of late LTP stages (greater than 1-2 h). When SCH 23390 was added to the bath medium immediately after tetanization, an influence on established LTP could not be observed during the first 3 h investigated.

Long-term potentiation, protein kinase C, and glutamate receptors

Among the various molecular events that have been proposed to contribute to the mechanisms of long-term potentiation (LTP), one of the most cited possibilities has been the activation of protein kinase C (PKC). Here we review various aspects of the cellular actions of PKC activation and inhibition, with special emphasis on the effects of the kinase on synaptic transmission and the N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated components of synaptic responses. We discuss the implications of these effects for interpretations of the role of PKC in the mechanisms of LTP induction and maintenance.

Differential effects of protein kinase inhibitors on pre-established long-term potentiation in rat hippocampal neurons in vitro

The possibility that a permanent protein kinase C (PKC) activity is necessary for the maintenance of long-term potentiation (LTP) was investigated in rat hippocampal slices. The action of the potent kinase inhibitors K-252a, K-252b and staurosporine on LTP of orthodromic population excitatory postsynaptic potentials (EPSPs) recorded from CA1 pyramidal cells was tested both during tetanization and after establishment of LTP. Confirming earlier studies, all inhibitors applied during tetanization at a concentration of 50 nM eliminate late LTP. Only staurosporine, but not K-252a or K-252b, blocked already established late LTP (i.e. late application). Normal synaptic transmission was influenced only weakly by staurosporine. Considering that all inhibitors have similar potencies against PKC and were all effective if applied during tetanization these data suggest that the late maintenance of LTP depends on a staurosporine/H7-sensitive process (or kinase) rather than permanent activation of PKC.

Postsynaptic factors control the duration of synaptic enhancement in area CA1 of the hippocampus

In area of CA1 of the hippocampus, at least two phases of long-term potentiation (LTP) can be isolated: an early decremental component referred to as short-term potentiation (STP), which precedes a long-lasting, nondecremental component commonly considered to be stable LTP. Utilizing the hippocampal slice preparation, experiments were performed to determine the physiological factors controlling the conversion of STP to LTP. The duration of NMDA receptor-dependent synaptic enhancement was influenced by several factors, including the degree of postsynaptic NMDA receptor activation and the magnitude and timing of postsynaptic membrane depolarization during synaptic transmission. It was possible to convert STP to LTP by manipulations that increased the influx of calcium into the postsynaptic cell. These results demonstrate that NMDA receptor activation can result in distinct forms of synaptic potentiation and imply that the magnitude of postsynaptic calcium increase is a critical variable controlling the duration of synaptic enhancement.

Excitatory amino acid receptors and synaptic plasticity

Excitatory amino acid receptors are the mediators of synaptic transmission at many synapses that can undergo use-dependent modifications of synaptic efficiency. They also play an essential role in the induction of these plastic changes. Graham Collingridge and Wolf Singer describe how NMDA receptors can endow synapses with hebbian-like properties and discuss how these may be used by vertebrates for associative learning and experience-dependent modifications of synaptic connections during development. The role of AMPA receptors in the maintenance of long-term potentiation is also discussed.