Separate mechanisms of long-term potentiation in two input systems to CA3 pyramidal neurons of rat hippocampal slices as revealed by the whole-cell patch-clamp technique

Evolutionary conservation of hippocampal mossy fiber synapse properties

Various specialized structural/functional properties are considered essential for contextual memory encoding by hippocampal mossy fiber (MF) synapses. Although investigated to exquisite detail in model organisms, synapses, including MFs, have undergone minimal functional interrogation in humans. To determine the translational relevance of rodent findings, we evaluated MF properties within human tissue resected to treat epilepsy. Human MFs exhibit remarkably similar hallmark features to rodents, including AMPA receptor-dominated synapses with small contributions from NMDA and kainate receptors, large dynamic range with strong frequency facilitation, NMDA receptor-independent presynaptic long-term potentiation, and strong cyclic AMP (cAMP) sensitivity of release. Array tomography confirmed the evolutionary conservation of MF ultrastructure. The astonishing congruence of rodent and human MF core features argues that the basic MF properties delineated in animal models remain critical to human MF function. Finally, a selective deficit in GABAergic inhibitory tone onto human MF postsynaptic targets suggests that unrestrained detonator excitatory drive contributes to epileptic circuit hyperexcitability.

Synaptic weights that correlate with presynaptic selectivity increase decoding performance

The activity of neurons in the visual cortex is often characterized by tuning curves, which are thought to be shaped by Hebbian plasticity during development and sensory experience. This leads to the prediction that neural circuits should be organized such that neurons with similar functional preference are connected with stronger weights. In support of this idea, previous experimental and theoretical work have provided evidence for a model of the visual cortex characterized by such functional subnetworks. A recent experimental study, however, have found that the postsynaptic preferred stimulus was defined by the total number of spines activated by a given stimulus and independent of their individual strength. While this result might seem to contradict previous literature, there are many factors that define how a given synaptic input influences postsynaptic selectivity. Here, we designed a computational model in which postsynaptic functional preference is defined by the number of inputs activated by a given stimulus. Using a plasticity rule where synaptic weights tend to correlate with presynaptic selectivity, and is independent of functional-similarity between pre- and postsynaptic activity, we find that this model can be used to decode presented stimuli in a manner that is comparable to maximum likelihood inference.

Differentially Altered NMDAR Dependent and Independent Long-Term Potentiation in the CA3 Subfield in a Model of Anti-NMDAR Encephalitis

Purpose: Autoantibodies against NMDA receptors (NMDAR) in the cerebrospinal fluid (CSF) from anti-NMDAR encephalitis patients have been suggested to be pathogenic since in previous studies using patient CSF, NMDAR-dependent processes such as long-term potentiation (LTP) were compromised. However, autoantibodies may represent a family of antibodies targeted against different epitopes, and CSF may contain further autoantibodies. Here, we tested the specificity of the autoantibody by comparing NMDAR-dependent and NMDAR-independent LTP within the same hippocampal subfield, CA3, using CSF samples from four anti-NMDAR encephalitis patients and three control patients.

Methods: We performed a stereotactic injection of patient-derived cell-free CSF with proven presence or absence of NMDAR-antibodies into the rat hippocampus in vivo. Hippocampal brain slices were prepared 1–8 days after intrahippocampal injection, and NMDAR-dependent LTP at the associational-commissural (A/C) fiber-CA3 synapse was compared to NMDAR-independent LTP at the mossy fiber (MF)-CA3 synapse.

Results: The LTP magnitude at A/C fiber-CA3 synapses in slices from control-CSF-treated animals (168 ± 8% n = 54) was significantly higher than LTP in slices from NMDAR-CSF-treated animals (139 ± 9%, n = 40; P = 0.015), although there was some variation between the individual CSF samples. We found residual LTP in NMDAR-CSF-treated tissue which could be abolished by the NMDAR inhibitor D-AP5. Moreover, the CA3 field excitatory postsynaptic potential (fEPSP) was followed by epileptiform afterpotentials in 5% of slices (4/78) from control-CSF-treated animals, but in 26% of slices (12/46) from NMDAR-CSF-treated animals (P = 0.002). Application of the LTP-inducing paradigm increased the proportion of slices with epileptiform afterpotentials, but D-AP5 significantly reduced the occurrence of epileptiform afterpotentials only in NMDAR-CSF-treated, but not in control tissue. At the MF synapse, no significant difference in LTP values of control-CSF and in NMDAR-CSF-treated tissue was observed indicating that NMDAR-independent MF-LTP is intact in NMDAR-CSF-treated tissue.

Conclusion: These findings indicate that anti-NMDAR containing CSF impairs LTP at the A/C fiber-CA3 synapse, although there is substantial variation among CSF samples suggesting different epitopes among patient-derived antibodies. The differential inhibition of LTP at this synapse in contrast to the MF-CA3 synapse suggests the specificity and underlines the pathophysiological role of the NMDAR-antibody.

The Role of mGlu Receptors in Hippocampal Plasticity Deficits in Neurological and Psychiatric Disorders: Implications for Allosteric Modulators as Novel Therapeutic Strategies

Long-term potentiation (LTP) and long-term depression (LTD) are two distinct forms of synaptic plasticity that have been extensively characterized at the Schaffer collateral-CA1 (SCCA1) synapse and the mossy fiber (MF)-CA3 synapse within the hippocampus, and are postulated to be the molecular underpinning for several cognitive functions. Deficits in LTP and LTD have been implicated in the pathophysiology of several neurological and psychiatric disorders. Therefore, there has been a large effort focused on developing an understanding of the mechanisms underlying these forms of plasticity and novel therapeutic strategies that improve or rescue these plasticity deficits. Among many other targets, the metabotropic glutamate (mGlu) receptors show promise as novel therapeutic candidates for the treatment of these disorders. Among the eight distinct mGlu receptor subtypes (mGlu1-8), the mGlu1,2,3,5,7 subtypes are expressed throughout the hippocampus and have been shown to play important roles in the regulation of synaptic plasticity in this brain area. However, development of therapeutic agents that target these mGlu receptors has been hampered by a lack of subtype-selective compounds. Recently, discovery of allosteric modulators of mGlu receptors has provided novel ligands that are highly selective for individual mGlu receptor subtypes. The mGlu receptors modulate the multiple forms of synaptic plasticity at both SC-CA1 and MF synapses and allosteric modulators of mGlu receptors have emerged as potential therapeutic agents that may rescue plasticity deficits and improve cognitive function in patients suffering from multiple neurological and psychiatric disorders.

Kainate receptors: multiple roles in neuronal plasticity

Ionotropic glutamate receptors of the N-methyl-d-aspartate (NMDA)- and AMPA-type, as well as metabotropic glutamate receptors have been extensively invoked in plasticity. Until relatively recently, however, kainate-type receptors (KARs) had been the most elusive to study because of the lack of appropriate pharmacological tools to specifically address their roles. With the development of selective glutamate receptor antagonists, and knockout mice with specific KAR subunits deleted, the functions of KARs in neuromodulation and synaptic transmission, together with their involvement in some types of plasticity, have been extensively probed in the central nervous system. In this review, we summarize the findings related to the roles of KARs in short- and long-term forms of plasticity, primarily in the hippocampus, where KAR function and synaptic plasticity have received avid attention.

The use of mouse models to understand and improve cognitive deficits in Down syndrome

Remarkable advances have been made in recent years towards therapeutics for cognitive impairment in individuals with Down syndrome (DS) by using mouse models. In this review, we briefly describe the phenotypes of mouse models that represent outcome targets for drug testing, the behavioral tests used to assess impairments in cognition and the known mechanisms of action of several drugs that are being used in preclinical studies or are likely to be tested in clinical trials. Overlaps in the distribution of targets and in the pathways that are affected by these diverse drugs in the trisomic brain suggest new avenues for DS research and drug development.

Differential induction of long-term potentiation in the horizontal versus columnar superficial connections to layer II cells of the entorhinal cortex

The entorhinal cortex (EC) is a nodal and independent mnemonic element of the medial temporal lobe memory circuit as it forms a bidirectional interface between the neocortex and hippocampus. Within the EC, intra- and inter-lamellar associational connections occur via horizontal and columnar projections, respectively. We undertook a comparative study of these two inputs as they converge upon EC layer II cells using whole-cell patch techniques in an adult rat EC horizontal slice preparation in which the deepest layers (V-VI) had been dissected out. Electrical stimulation of layers I and III during GABA blockade allowed us to study excitatory synaptic properties and plasticity in the horizontal and columnar fibre systems, respectively. Both pathways exhibited AMPA- and NMDA-receptor mediated transmission and both exhibited long-term potentiation (LTP) after high-frequency (tetanic) stimulation. LTP in the horizontal, but not in the columnar pathway, was blocked by NMDA receptor antagonism. Intriguingly, LTP in both appeared to be mediated by post synaptic increases in Ca²⁺ that may be coupled to differing second messenger pathways. Thus, the superficial excitatory horizontal and columnar associative pathways to layer II have divergent mechanisms for LTP which may endow the EC with even more complex and dynamic processing characteristics than previously thought.

Synaptic plasticity at hippocampal mossy fibre synapses

The dentate gyrus provides the main input to the hippocampus. Information reaches the CA3 region through mossy fibre synapses made by dentate granule cell axons. Synaptic plasticity at the mossy fibre-pyramidal cell synapse is unusual for several reasons, including low basal release probability, pronounced frequency facilitation and a lack of N-methyl-D-aspartate receptor involvement in long-term potentiation. In the past few years, some of the mechanisms underlying the peculiar features of mossy fibre synapses have been elucidated. Here we describe recent work from several laboratories on the various forms of synaptic plasticity at hippocampal mossy fibre synapses. We conclude that these contacts have just begun to reveal their many secrets.

Active zones for presynaptic plasticity in the brain

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

Input- and subunit-specific AMPA receptor trafficking underlying long-term potentiation at hippocampal CA3 synapses

Hippocampal CA3 pyramidal neurons receive synaptic inputs from both mossy fibres (MFs) and associational fibres (AFs). Long-term potentiation (LTP) at these synapses differs in its induction sites and N-methyl-D-aspartate receptor (NMDAR) dependence. Most evidence favours the presynaptic and postsynaptic mechanisms for induction of MF LTP and AF LTP, respectively. This implies that molecular and functional properties differ between MF and AF synapses at both presynaptic and postsynaptic sites. In this study, we focused on the difference in the postsynaptic trafficking of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) between these synapses. To trace the subunit-specific trafficking of AMPARs at each synapse, GluR1 and GluR2 subunits were introduced into CA3 pyramidal neurons in hippocampal organotypic cultures using the Sindbis viral expression system. The electrophysiologically-tagged GluR2 AMPARs, produced by the viral-mediated transfer of the unedited form of GluR2 (GluR2Q), were inserted into both MF and AF postsynaptic sites in a neuronal activity-independent manner. Endogenous Ca(2+)-impermeable AMPARs at these synapses were replaced with exogenous Ca(2+)-permeable receptors, and Ca(2+) influx via the newly expressed postsynaptic AMPARs induced NMDAR-independent LTP at AF synapses. In contrast, no GluR1 AMPAR produced by the gene transfer was constitutively incorporated into AF postsynaptic sites, and only a small amount into MF postsynaptic sites. The synaptic trafficking of GluR1 AMPARs was triggered by the activity of Ca(2+)/calmodulin-dependent kinase II or high-frequency stimulation to induce LTP at AF synapses, but not at MF synapses. These results indicate that MF and AF postsynaptic sites possess distinct properties for AMPAR trafficking in CA3 pyramidal neurons.

Phosphorylation of RIM1α by PKA Triggers Presynaptic Long-Term Potentiation at Cerebellar Parallel Fiber Synapses

Presynaptic activation of protein kinase A (PKA) induces LTP in cerebellar parallel fiber synapses. Presynaptic LTP is known to require the active zone protein RIM1alpha, but the underlying induction mechanism remains unclear. We now show that PKA directly phosphorylates RIM1alpha at two sites. Using paired recordings from cultured cerebellar granule and Purkinje neurons, we demonstrate that LTP is absent in neurons from RIM1alpha KO mice but is rescued by presynaptic expression of RIM1alpha. Mutant RIM1alpha lacking the N-terminal phosphorylation site is unable to rescue LTP in RIM1alpha knockout neurons but selectively suppresses LTP in wild-type neurons. Our findings suggest that PKA-mediated phosphorylation of the active zone protein RIM1alpha at a single N-terminal site induces presynaptic LTP.

A role for Ca2+ stores in kainate receptor-dependent synaptic facilitation and LTP at mossy fiber synapses in the hippocampus

Compared with NMDA receptor-dependent LTP, much less is known about the mechanism of induction of NMDA receptor-independent LTP; the most extensively studied form of which is mossy fiber LTP in the hippocampus. In the present study we show that Ca2+-induced Ca2+ release from intracellular stores is involved in the induction of mossy fiber LTP. This release also contributes to the kainate receptor-dependent component of the pronounced synaptic facilitation that occurs during high-frequency stimulation. We also present evidence that the trigger for this Ca2+ release is Ca2+ permeation through kainate receptors. However, these novel synaptic mechanisms can be bypassed when the Ca2+ concentration is raised (from 2 to 4 mM), via a compensatory involvement of L-type Ca2+ channels. These findings suggest that presynaptic kainate receptors at mossy fiber synapses can initiate a cascade involving Ca2+ release from intracellular stores that is important in both short-term and long-term plasticity.

Temporal and regional expression of NMDA receptor subunit NR3A in the mammalian brain

NR3A is a developmentally regulated N-methyl-D-aspartate receptor (NMDAR) subunit that was previously known as NMDAR-L or chi-1. Unlike other NMDAR subunits, NR3A inhibits the NMDAR-associated ion channel in a novel manner, and a role in synaptogenesis has been suggested for this subunit. Here, we report a comprehensive study to delineate the temporal and anatomic expression of NR3A protein in the mammalian brain by using a monoclonal anti-NR3A antibody. NR3A protein was found to peak at postnatal day (P) 8, and to decrease gradually from P12 to adulthood in the rat central nervous system. Moreover, NR3A protein was heavily expressed in all areas of the isocortex, portions of the amygdaloid nuclei, and selective cell layers and nuclei of the hippocampus, thalamus, hypothalamus, brainstem, and spinal cord. NR3A protein was also expressed in the cerebellar cortex, whereas only weak signal was detected in the previous in situ studies by using riboprobes. At an ultrastructural level, NR3A was associated specifically with asymmetrical synapses and localized to postsynaptic membranes. This information will facilitate future research on NMDARs by providing clues to possible inclusion of the NR3A subunit in NMDARs in many brain regions.

Postsynaptic expression of a new calcium pathway in hippocampal CA3 neurons and its influence on mossy fiber long-term potentiation

Long-term potentiation (LTP) in the CA1 region of the hippocampus is induced by postsynaptic Ca(2+) influx via NMDA receptors (NMDARs). However, this synaptic plasticity occurs independently of NMDARs when Ca(2+)-permeable AMPA receptors (AMPARs) are expressed at postsynaptic sites using various genetic techniques, indicating that an increase in Ca(2+) level at critical postsynaptic sites, regardless of its entry pathway, triggers the induction of LTP at CA1 synapses. In contrast, NMDARs are sparsely distributed on mossy fiber (MF) synapses in CA3 hippocampal neurons, and most evidence favors the presynaptic mechanism for LTP induction, although some reports suggested a postsynaptic mechanism. In this study, we examined whether Ca(2+) influx through the newly produced postsynaptic receptors during high-frequency stimulation affects the induction of MF LTP. For this purpose, we expressed Ca(2+)-permeable AMPARs in CA3 pyramidal neurons by Sindbis viral-mediated gene transfer of the unedited form of the glutamate receptor 2 (GluR2Q) subunit, as a new pathway for postsynaptic Ca(2+) entry, in rat hippocampal organotypic cultures. Virally expressed myc-tagged GluR2Q was detected at the complex spines known as the thorny excrescences, which serve as postsynaptic targets for MF synaptic input, on the proximal apical dendrites of CA3 pyramidal cells. Furthermore, endogenous Ca(2+)-impermeable AMPARs at MF synapses were converted into Ca(2+)-permeable receptors by GluR2Q expression. However, the postsynaptic expression of Ca(2+)-permeable AMPARs had no significant influence on the two types of MF LTP induced by different stimulus protocols. These results supported the notion that MF LTP is independent of postsynaptic Ca(2+).

Trans-synaptic Eph receptor-ephrin signaling in hippocampal mossy fiber LTP

The site of induction of long-term potentiation (LTP) at mossy fiber-CA3 synapses in the hippocampus is unresolved, with data supporting both pre- and postsynaptic mechanisms. Here we report that mossy fiber LTP was reduced by perfusion of postsynaptic neurons with peptides and antibodies that interfere with binding of EphB receptor tyrosine kinases (EphRs) to the PDZ protein GRIP. Mossy fiber LTP was also reduced by extracellular application of soluble forms of B-ephrins, which are normally membrane-anchored presynaptic ligands for the EphB receptors. The application of soluble ligands for presynaptic ephrins increased basal excitatory transmission and occluded both tetanus and forskolin-induced synaptic potentiation. These findings suggest that PDZ interactions in the postsynaptic neuron and trans-synaptic interactions between postsynaptic EphB receptors and presynaptic B-ephrins are necessary for the induction of mossy fiber LTP.

A critical role of a facilitatory presynaptic kainate receptor in mossy fiber LTP

The mechanisms involved in mossy fiber LTP in the hippocampus are not well established. In the present study, we show that the kainate receptor antagonist LY382884 (10 microM) is selective for presynaptic kainate receptors in the CA3 region of the hippocampus. At a concentration at which it blocks mossy fiber LTP, LY382884 selectively blocks the synaptic activation of a presynaptic kainate receptor that facilitates AMPA receptor-mediated synaptic transmission. Following the induction of mossy fiber LTP, there is a complete loss of the presynaptic kainate receptor-mediated facilitation of synaptic transmission. These results identify a central role for the presynaptic kainate receptor in the induction of mossy fiber LTP. In addition, these results suggest that the pathway by which kainate receptors facilitate glutamate release is utilized for the expression of mossy fiber LTP.

Immunoelectron microscopic localization of the neural recognition molecules L1, NCAM, and its isoform NCAM180, the NCAM-associated polysialic acid, beta1 integrin and the extracellular matrix molecule tenascin-R in synapses of the adult rat hippocampus

We have investigated the possibility that morphologically different excitatory glutamatergic synapses of the "trisynaptic circuit" in the adult rodent hippocampus, which display different types of long-term potentiation (LTP), may express the immunoglobulin superfamily recognition molecules L1 and NCAM, the extracellular matrix molecule tenascin-R, and the extracellular matrix receptor constituent beta1 integrin in a differential manner. The neural cell adhesion molecules L1, NCAM (all three major isoforms), NCAM180 (the largest major isoform with the longest cytoplasmic domain), beta1 integrin, polysialic acid (PSA) associated with NCAM, and tenascin-R were localized by pre-embedding immunostaining procedures in the CA3/CA4 region (mossy fiber synapses) and in the dentate gyrus (spine synapses) of the adult rat hippocampus. Synaptic membranes of mossy fiber synapses where LTP is expressed presynaptically did not show detectable levels of immunoreactivity for any of the molecules/epitopes studied. L1, NCAM, and PSA, but not NCAM180 or beta1 integrin, were detectable on axonal membranes of fasciculating mossy fibers. In contrast to mossy fiber synapses, spine synapses in the outer third of the molecular layer of the dentate gyrus, which display postsynaptic expression mechanisms of LTP, were both immunopositive and immunonegative for NCAM, NCAM180, beta1 integrin, and PSA. Those spine synapses postsynaptically immunoreactive for NCAM or PSA also showed immunoreactivity on their presynaptic membranes. NCAM180 was not detectable presynaptically in spine synapses. L1 could not be found in spine synapses either pre- or postsynaptically. Also, the extracellular matrix molecule tenascin-R was not detectable in synaptic clefts of all synapses tested, but was amply present between fasciculating axons, axon-astrocyte contact areas, and astrocytic gap junctions. Differences in expression of the membrane-bound adhesion molecules at both types of synapses may reflect the different mechanisms for induction and/or maintenance of synaptic plasticity.

Time-dependent reversal of long-term potentiation by low-frequency stimulation at the hippocampal mossy fiber-CA3 synapses

Using mouse hippocampal slices, we studied the induction of depotentiation of long-term potentiation (LTP) at the mossy fiber synapses onto CA3 pyramidal neurons. A long train of low-frequency (1 Hz/900 pulses) stimulation (LFS) induced a long-term depression of baseline synaptic transmission or depotentiation of previously established LTP, which was reversible and was independent of NMDA receptor activation. This LFS-induced depotentiation was observed when the stimulus was delivered 1 or 10 min after LTP induction. However, when LFS was applied at 30 min after induction, significantly less depotentiation was found. The induction of depotentiation on one input was associated with a heterosynaptic reverse of the LTP induced previously on a separate pathway. In addition, this LFS-induced depotentiation appeared to be mediated by the activation of group 2 metabotropic glutamate receptors (mGluRs), because it was mimicked by the bath-applied group 2 agonist (2S,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl) glycine and was specifically inhibited by the group 2 antagonists (S)-alpha-methyl-4-carboxyphenylglycine and (alphaS)-alpha-amino-alpha-(1S,2S)-2-carboxycyclopropyl-9H-xanthine-9-propanic acid. Moreover, the induction of depotentiation was entirely normal when synaptic transmission is blocked by glutamate receptor antagonist kynurenic acid and was associated with a reversal of paired-pulse facilitation attenuation during LTP expression. Pretreatment of the hippocampal slices with G(i/o)-protein inhibitor pertussis toxin (PTX) prevented the LFS-induced depotentiation. These results suggest that the activation of presynaptic group 2 mGluRs and in turn triggering a PTX-sensitive G(i/o)-protein-coupled signaling cascade may contribute to the LFS-induced depotentiation at the mossy fiber-CA3 synapses.

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.

Input-specific targeting of NMDA receptor subtypes at mouse hippocampal CA3 pyramidal neuron synapses

Hippocampal CA3 pyramidal neurons receive synaptic inputs from commissural and associational fibers on both apical and basal dendrites. NMDA receptors at these synapses were examined in hippocampal slices of wild-type mice and GluRvarepsilon1 (NR2A) subunit knockout mice. Electrical stimulations at the CA3 stratum radiatum or stratum oriens activate both commissural and associational (C/A) synapses, whereas stimulations at ventral fimbria mainly activate commissural synapses. Ro 25-6981 and ifenprodil, the GluRepsilon2 (NR2B) subunit-selective NMDA receptor antagonists, suppressed NMDA receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) at the commissural-CA3 synapses on basal dendrites more strongly than those at the C/A-CA3 synapses on apical or basal dendrites. However, glutamate-evoked NMDA receptor currents were reduced by the GluRepsilon1 subunit knockout to a similar extent at both apical and basal dendrites. The GluRepsilon1 subunit knockout also reduced NMDA EPSCs at the C/A-CA3 synapses on basal dendrites, but did not affect NMDA EPSCs at the commissural-CA3 synapses on basal dendrites. These results confirmed our previous findings that NMDA receptors operating at different synapses in CA3 pyramidal cells have different GluRepsilon subunit compositions, and further show that the GluRepsilon subunit composition may be regulated depending on the types of synaptic inputs, even within a single CA3 pyramidal neuron.

2-Deoxyglucose-induced long-term potentiation in CA1 is not prevented by intraneuronal chelator

In hippocampal slices, temporary (10-20 min) replacement of glucose with 10 mM 2-deoxyglucose is followed by marked and very sustained potentiation of EPSPs (2-DG LTP). To investigate its mechanism, we examined 2-DG's effect in CA1 neurons recorded with sharp 3 M KCl electrodes containing a strong chelator, 50 or 100 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). In most cases, field EPSPs were simultaneously recorded and conventional LTP was also elicited in some cells by tetanic stimulation of stratum radiatum. 2-DG potentiated intracellular EPSP slopes by 48 +/- 5.1% (SE) in nine cells recorded with plain KCl electrodes and by 52 +/- 6.2% in seven cells recorded with EGTA-containing electrodes. In four of the latter cells, tetanic stimulation (twice 100 Hz for 1 s) failed to evoke LTP (2 +/- 1.1%), although field EPSPs were clearly potentiated (by 28 +/- 6.9%). Thus unlike tetanic LTP, 2-DG LTP is not readily prevented by postsynaptic intraneuronal injection of EGTA. These findings agree with other evidence that the rise in postsynaptic (somatic) [Ca(2+)](i) caused by 2-DG is not the principal trigger for the subsequent 2-DG LTP and that it may be a purely presynaptic phenomenon.

Activation of presynaptic cAMP-dependent protein kinase is required for induction of cerebellar long-term potentiation

Cerebellar long-term potentiation (LTP) is a persistent increase in the strength of the granule cell-Purkinje neuron synapse that occurs after brief stimulation of granule cell axons at 2-8 Hz. Previous work has indicated that cerebellar LTP induction requires presynaptic Ca influx, stimulation of Ca-sensitive adenylyl cyclase, and activation of PKA. The evidence implicating PKA has come from bath application of drugs during LTP induction, an approach that does not distinguish between PKA activation in the presynaptic or postsynaptic cell. Although bath application of PKA inhibitor drugs (KT5720, Rp-8CPT-cAMP-S) blocked LTP induction in granule cell-Purkinje neuron pairs in culture, selective application to granule cell or Purkinje neuron somata via patch pipettes did not. We hypothesized that presynaptic PKA activation is required for LTP induction but that drugs applied to the granule cell soma cannot diffuse to the terminal within this timescale. To test this hypothesis, we transfected cerebellar cultures with an expression vector encoding a peptide inhibitor of PKA [Rous sarcoma virus (RSV)-protein kinase A inhibitor (PKI)]. Transfection of RSV-PKI into presynaptic granule cells, but not postsynaptic Purkinje neurons or glial cells, blocked LTP induction produced by either synaptic stimulation or an exogenous cAMP analog. An expression vector encoding a control peptide with no PKA inhibitory activity was ineffective. These results show that induction of cerebellar LTP requires a presynaptic signaling cascade, including Ca influx, stimulation of Ca-sensitive adenylyl cyclase, and activation of PKA, and argue against a requirement for postsynaptic Ca signals or their sequelae.

Multiple forms of LTP in hippocampal CA3 neurons use a common postsynaptic mechanism

We investigated long-term potentiation (LTP) at mossy fiber synapses on CA3 pyramidal neurons in the hippocampus. Using Ca2+ imaging techniques, we show here that when postsynaptic Ca2+ was sufficiently buffered so that [Ca2+]i did not rise during synaptic stimulation, the induction of mossy fiber LTP was prevented. In addition, induction of mossy fiber LTP was suppressed by postsynaptic injection of a peptide inhibitor of cAMP-dependent protein kinase. Finally, when ionotropic glutamate receptors were blocked, LTP depended on the postsynaptic release of Ca2+ from internal stores triggered by activation of metabotropic glutamate receptors. These results support the conclusion that mossy fiber LTP and LTP at other hippocampal synapses share a common induction mechanism involving an initial rise in postsynaptic [Ca2+].

Calcium-dependent mechanisms involved in presynaptic long-term depression at the hippocampal mossy fibre-CA3 synapse

Long-term potentiation (LTP) and long-term depression (LTD) are induced presynaptically at the hippocampal mossy fibre-CA3 synapse. Activation of presynaptic metabotropic glutamate receptors (mGluRs) is necessary, but not sufficient for the LTD induction. Using mouse hippocampal slices, we attempted to identify additional presynaptic factors involved in the induction of mossy fibre LTD. Suppression of a rise in the presynaptic intracellular Ca2+ concentration ([Ca2+]i) with a membrane-permeable Ca2+ chelator, 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), reduced the magnitude of LTD, whereas an increase in Ca2+ influx induced LTD, suggesting that an elevation of presynaptic [Ca2+]i is crucial for the LTD induction. A broad-spectrum protein kinase inhibitor, H-7, blocked LTD without affecting a presynaptic inhibition induced by an mGluR agonist. Furthermore, LTD was reduced by an inhibitor of calmodulin or Ca2+/calmodulin-dependent protein kinases. Thus, we conclude that mossy fibre LTD requires an increase in presynaptic [Ca2+]i and subsequent activation of Ca2+/calmodulin-dependent protein kinases. Because mossy fibre LTP may also require a rise in presynaptic [Ca2+]i, bidirectional long-term plasticity at the mossy fibre synapse is likely to be regulated by presynaptic Ca2+-dependent processes.

FK960, a novel potential anti-dementia drug, augments long-term potentiation in mossy fiber-CA3 pathway of guinea-pig hippocampal slices

Our previous studies have demonstrated that FK960 (FR59960; N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate), a novel antidementia piperazine derivative, exerts beneficial effects on memory deficits in various animal models of amnesia in rats [M. Yamazaki, N. Matsuoka, N. Maeda, Y. Ohkubo, I. Yamaguchi, FK960 N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate ameliorates the memory deficits in rats through a novel mechanism of action, J. Pharmacol. Exp. Ther., 279 (1996) 1157-1173.] and in rhesus monkeys [N. Matsuoka, T.G. Aigner, FK960 [N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate], a novel potential antidementia drug, improves visual recognition memory in rhesus monkeys: comparison with physostigmine, J. Pharmacol. Exp. Ther., 280 (1997) 1201-1209]. To clarify the synaptic mechanisms of its antiamnesic action, FK960 was investigated for its effects on the development of long-term potentiation (LTP) in guinea-pig hippocampal slices. The magnitude of LTP of population spike recorded in CA3 pyramidal neurons was significantly augmented by perfusing FK960 (10-9-10-6 M) for 25 min before and during tetanic stimulation of the mossy fibers, whereas the basal amplitude of population spikes before tetanus was hardly affected by the drug. The dose-response curve was bell-shaped with a maximal augmentation at 10-7 M. Scopolamine (10-6 M) per se had little effect on the magnitude of LTP in the mossy fiber-CA3 pathway, but significantly attenuated its enhancement by FK960 (10-7 M). In hippocampal slices from animals treated with cysteamine (200 mg/kg, s.c.), which was shown to deplete the hippocampal somatostatin, FK960 (10-7 M) hardly affected the LTP. These results suggest that FK960 enhances the magnitude of LTP in the mossy fiber-CA3 pathway through an activation of the cholinergic-somatostatinergic link in the hippocampal formation. Furthermore, it can be postulated that the drug regulates the cognitive function by modulating directly synaptic plasticity in the hippocampal neuronal network.

5-Hz stimulation of CA3 pyramidal cell axons induces a beta-adrenergic modulated potentiation at synapses on CA1, but not CA3, pyramidal cells

In mouse hippocampal slices, long-term potentiation (LTP) at Schaffer collateral fiber synapses onto CA1 pyramidal cells could be induced by brief trains of 5-Hz synaptic stimulation (30 s) or by longer trains of 5-Hz stimulation (3 min) delivered during beta-adrenergic receptor activation. In contrast, 5-Hz stimulation, either alone or in the presence of the beta-adrenergic receptor agonist isoproterenol, failed to induce LTP at associational-commissural (assoc-com) fiber synapses onto CA3 pyramidal cells. Our results suggest that although CA3 pyramidal cells give rise to both the Schaffer collateral fiber synapses in CA1 and the assoc-com fiber synapses in CA3, the induction of LTP at these synapses may be regulated by different activity- and modulatory neurotransmitter-dependent processes.

L-Type calcium channels are required for one form of hippocampal mossy fiber LTP

The requirement of postsynaptic calcium influx via L-type channels for the induction of long-term potentiation (LTP) of mossy fiber input to CA3 pyramidal neurons was tested for two different patterns of stimulation. Two types of LTP-inducing stimuli were used based on the suggestion that one of them, brief high-frequency stimulation (B-HFS), induces LTP postsynaptically, whereas the other pattern, long high-frequency stimulation (L-HFS), induces mossy fiber LTP presynaptically. To test whether or not calcium influx into CA3 pyramidal neurons is necessary for LTP induced by either pattern of stimulation, nimodipine, a L-type calcium channel antagonist, was added during stimulation. In these experiments nimodipine blocked the induction of mossy fiber LTP when B-HFS was given [34 +/- 5% (mean +/- SE) increase in control versus 7 +/- 4% in nimodipine, P < 0.003]; in contrast, nimodipine did not block the induction of LTP with L-HFS (107 +/- 10% in control vs. 80 +/- 9% in nimodipine, P > 0.05). Administration of nimodipine after the induction of LTP had no effect on the expression of LTP. In addition, B- and L-HFS delivered directly to commissural/associational fibers in stratum radiatum failed to induce a N-methyl--aspartate-independent form of LTP, obviating the possibility that the presumed mossy fiber LTP resulted from potentiation of other synapses. Nimodipine had no effect on calcium transients recorded from mossy fiber presynaptic terminals evoked with the B-HFS paradigm but reduced postsynaptic calcium transients. Our results support the hypothesis that induction of mossy fiber LTP by B-HFS is mediated postsynaptically and requires entry of calcium through L-type channels into CA3 neurons.

[3H]MK-801 binding and the mRNA for the NMDAR1 subunit of the NMDA receptor are differentially distributed in human and rat forebrain

The distributions of [3H]MK-801 binding and the NMDA NR1 subunit mRNA were studied using receptor autoradiography and in-situ hybridization in rat and human brain whole-hemisphere coronal sections. Receptor protein detected by radioligand autoradiography and the mRNA for the key subunit of the receptor presented similar distributions in the forebrain, with a few areas showing an imbalance between the levels of mRNA and receptor protein. Human frontal cortex showed a relative abundance of NMDAR1 mRNA as compared to [3H]MK-801 binding. The same area in rat brain did not show any difference in the two distributions. In comparison, the rat claustrum presented a relative excess of NMDAR1 mRNA which was not detected in human sections. Human caudate nucleus exhibited relatively high levels of [3H]MK-801 binding that were unmatched in rat caudate. The hippocampi of either species presented similar levels of [3H]MK-801 binding and NMDAR1 mRNA, but when the two signals were measured in specific subfields of the hippocampal formation, the differential distribution of the two signals reflected the anatomy of hippocampal connections assuming a preferential dendritic distribution for MK-801 binding. Interestingly, rat and human hippocampi also showed some important species-dependent difference in the relative distribution of the receptor protein and mRNA. The data presented show an overall good correlation between the mRNA for the key subunit of the NMDA receptor and the functional receptor detected with radioligand binding and highlight the presence of local differences in their ratio. This may reflect different splicing of the mRNA for the NMDAR1 subunit in specific brain areas of rat and human. The species-dependent differences in the relative distribution of the mRNA for the key subunit of the NMDA receptor and that of a marker of functional receptors also highlights important differences in the NMDA function in rat and human brain.

Origin of the apparent asynchronous activity of hippocampal mossy fibers

Fiber volleys (FVs) from the stratum lucidum of rat hippocampal area CA3 were recorded extracellularly from in vitro slices in the presence of 10 mM kynurenic acid. In agreement with previous work, bulk stimulation of the dentate gyrus (DG) near the hilar border leads to an asynchronous FV. Transection of the stratum lucidum between the DG stimulation site and the CA3 recording site reduced or eliminated the early components of the asynchronous FV, indicating that they are of mossy fiber (MF) origin. In contrast, moving the stimulating electrode away from the hilus toward the hippocampal fissure reduced or eliminated the late components of the FV. Subsequently, we found that bulk stimulation on the DG/hilar border induces an antidromic population spike in CA3 pyramidal cells. Finally, the MFs and associational collaterals have different conduction velocities (0.51 and 0.37 m/s, respectively; temperature = 33 degrees C). From these data, we conclude that the late components of the asynchronous FV are due to antidromic activation of CA3 collaterals that have been shown to be present in the DG and hilus. A corollary of these findings is that bulk stimulation on the DG/hilar border can lead to at least two different monosynaptic inputs to CA3 pyramidal cells: the MFs and the antidromically activated associational collaterals. We suggest that when MF synaptic responses are being evoked with the use of bulk stimulation, stimulating electrodes should be placed in the outer molecular layer of the DG to prevent the activation of hilar-projecting associational collaterals. This procedure should be added to the previously proposed criteria for preventing polysynaptic contamination of the intracellularly recorded evoked MF synaptic response.

Two forms of hippocampal long-term depression, the counterpart of long-term potentiation

In the hippocampus there are two distinct forms of long-term depression (LTD) of excitatory synaptic transmission. In the CA1 region, prolonged low-frequency stimulation induces LTD by activating postsynaptic NMDA receptors, which causes a moderate rise in Ca2+ concentrations. In mossy fiber synapses of the CA3 region, similar low-frequency stimulation also gives rise to LTD. However, this form of LTD (mossy fiber LTD) does not require activation of NMDA receptors, but is mediated by activation of presynaptic metabotropic glutamate receptors. Induction of mossy fiber LTD is not dependent on postsynaptic depolarization or activation of postsynaptic ionotropic glutamate receptors, thus it is likely to be mediated by purely presynaptic mechanisms. This conclusion is confirmed by the analysis of mutant mice lacking presynaptic mGluR2, in which mossy fiber LTD is almost absent. Since long-term potentiation at mossy fiber synapses is also induced presynaptically, the synaptic efficacy may be regulated through common mechanisms bidirectionally, which may contribute to neural information processing in the hippocampus.

Direct evidence for increase in excitatory amino acids release during mossy fiber LTP in rat hippocampal slices as revealed by the patch sensor methods

We had previously established a patch sensor system for detecting overflow of excitatory amino acids (EAA) from hippocampal slices induced by a single electric stimulus. We performed simultaneous recordings of patch sensor current and whole-cell current in rat hippocampal slices. Tetanus of mossy fibers produced long-term potentiation (LTP) of excitatory postsynaptic current (EPSC) recorded from a CA3 pyramidal cell with an increase in single electric stimulus-evoked patch current amplitude recorded with a patch sensor positioned in the stratum lucidum. In Schaffer collateral-CA1 synapses, however, LTP of EPSC was accompanied by an increase in AMPA-evoked whole-cell current, but not with change in patch current in the stratum radiatum. These results indicate that EAA release is increased during mossy fiber LTP but not CA1 LTP.

Large amplitude miniature excitatory postsynaptic currents in hippocampal CA3 pyramidal neurons are of mossy fiber origin

Neonatal (P0) gamma-irradiation was used to lesion selectively the mossy fiber (MF) synaptic input to CA3 pyramidal cells. This lesion caused a > 85% reduction in the MF input as determined by quantitative assessment of the number of dynorphin immunoreactive MF boutons. The gamma-irradiation lesion caused a reduction in the mean number of miniature excitatory postsynaptic currents (mEPSCs) recorded from CA3 pyramidal cells (2,292 vs. 1,429/3-min period; n = 10). The lesion also caused a reduction in the mean mEPSC peak amplitude from 19.1 +/- 0.45 to 14.6 +/- 0.49 pA (mean +/- SE; peak conductance 238.8 +/- 5.6 to 182.0 +/- 6.1 pS). Similarly, there was a reduction in the mean 10-85% rise time from 1.72 +/- 0.02 ms to 1.42 +/- 0.04 ms. The effects of the gamma-irradiation on both mEPSC amplitude and 10-85% rise time were significant at P < 0.002 and P < 0.005 (2-tailed Kolmogorov-Smirnov test). Based on the selectively of the gamma-irradiation, MF and non-MF mEPSC amplitude and 10-85% rise-time distributions were calculated. Both the amplitude and 10-85% rise-time distributions showed extensive overlap between the MF and non-MF mediated mEPSCs. The MF mEPSC distributions had a mean peak amplitude of 24.6 pA (307.5 pS) and a mean 10-85% rise time of 2.16 ms. THe non-MF mEPSC distributions had a mean peak amplitude of 12.2 pA (152.5 pS) and 10-85% rise time of 1.26 ms. The modes of the amplitude distributions were the same at 5 pA (62 pS). The MF and non-MF mEPSC amplitude and 10-85% rise-time distributions were significantly different at P << 0.001 (1-tailed, large sample Kolmogorov-Smirnov test). The data demonstrate that the removal of the MF synaptic input to CA3 pyramidal cells leads to the absence of the large amplitude mEPSCs that are present in control recordings.

Cognitive enhancers and hippocampal long-term potentiation in vitro

We review our works on the pharmacological modulation of long-term potentiation (LTP) at guinea pig hippocampal mossy fiber-CA3 synapses in vitro. The magnitude of tetanus-induced LTP at the mossy fiber synapse was augmented by perfusion of slices with several cognitive enhancers, such as bifemelane (1 microM). The mossy fiber LTP was enhanced by somatostatin (0.32 microM) and inhibited in somatostatin-depleted slices from cysteamine-treated guinea pigs. An involvement of the 5-HT3 receptor also showed that granisetron (0.1 microM) enhanced the mossy fiber LTP. The above-mentioned enhancements by perfused agents were commonly reversed, at least in part, by muscarinic antagonists. However, the magnitude of mossy fiber LTP was bidirectionally modulated by muscarinic stimulations of slices with physostigmine or carbachol at different concentrations. The enhancing effects of high-concentration carbachol was antagonized by pirenzepine, and in contrast, the inhibition by low-concentration carbachol was antagonized in the presence of AF-DX116. When guinea pigs were preinjected with the cholinotoxin AF64A, the magnitude of LTP was decreased in the slices prepared from AF64A-treated animals. These results suggest that endogenous acetylcholine dominantly plays facilitatory roles through muscarinic M1 receptors in the induction of mossy fiber LTP. The pharmacological characterization of mossy fiber LTP may be of help to the evaluation of cognitive enhancers at a neuronal circuit level.

Induction of hebbian and non-hebbian mossy fiber long-term potentiation by distinct patterns of high-frequency stimulation

The synapse made by hippocampal mossy fibers onto pyramidal neurons of hippocampal area CA3 displays a form of long-term potentiation (LTP) that is independent of the activation of NMDA receptors. Considerable controversy exists as to whether the induction of mossy fiber LTP requires postsynaptic activation and, thus, whether mossy fiber LTP is Hebbian or non-Hebbian. Here we report the induction of both Hebbian and non-Hebbian forms of long-term potentiation at the mossy fiber-CA3 synapse in in vitro slice preparation. These two forms of potentiation can be induced selectively by different induction conditions. Sustained presynaptic activation is sufficient to induce the non-Hebbian form of mossy fiber LTP, whereas brief presynaptic activation coincident with postsynaptic depolarization is required to induce the Hebbian form. We suggest that non-Hebbian forms of plasticity may play an important role in dynamically regulating the thresholds for inducing Hebbian forms of plasticity.

Presynaptic long-term depression at the hippocampal mossy fiber-CA3 synapse

Long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength may underlie learning and memory in the brain. The induction of LTP occurs in postsynaptic cells in the hippocampal CA1 region but is presynaptic in CA3. LTD is also well characterized in CA1 but not in CA3. Low-frequency stimulation of mouse hippocampal slices caused homosynaptic LTD at the mossy fiber-CA3 synapse, which may be induced presynaptically by activation of metabotropic glutamate receptors. Thus, the efficacy of mossy fiber-CA3 synapses can be regulated bidirectionally, which may contribute to neuronal information processing.

Long-term potentiation in cultures of single hippocampal granule cells: a presynaptic form of plasticity

We have explored the mechanisms of mossy fiber long-term potentiation (LTP) at autapses in single-cell cultures of guinea pig hippocampal dentate granule cells. L-AP4-sensitive, but not insensitive, cells responded to a brief tetanus with a sustained potentiation in the synaptic responses. The induction of this LTP appeared identical to that observed in hippocampal mossy fiber synapses in situ, in that it required a rise in presynaptic Ca2+ and activation of protein kinase A. Its expression also appeared to be presynaptic and was due, at least in part, to events that occurred after the entry of Ca2+ and to the switching on of previously silent release sites.

Cyclic AMP mediates a presynaptic form of LTP at cerebellar parallel fiber synapses

The N-methyl-D-aspartate receptor-independent form of long-term potentiation (LTP) at hippocampal mossy fiber synapses requires presynaptic Ca(2+)-dependent activation of adenylyl cyclase. To determine whether this form of LTP might occur at other synapses, we examined cerebellar parallel fibers that, like hippocampal mossy fiber synapses, express high levels of the Ca2+/calmodulin-sensitive adenylyl cyclase I. Repetitive stimulation of parallel fibers caused a long-lasting increase in synaptic strength that was associated with a decrease in paired-pulse facilitation. Blockade of glutamate receptors did not prevent LTP induction, nor did loading of Purkinje cells with a Ca2+ chelator. LTP was occluded by forskolin-induced potentiation and blocked by the protein kinase A inhibitor Rp-8-CPT-cAMPS. These findings suggest that parallel fiber synapses express a form of LTP that is dependent on the activation of a presynaptic adenylyl cyclase and is indistinguishable from LTP at hippocampal mossy fiber synapses.

In vitro study of afferent synaptic transmission in the rostral gustatory zone of the rat nucleus of the solitary tract

The synaptic responses of rostral nucleus of the solitary tract (rNST) neurons to electrical stimulation of the solitary tract (ST) fibers were investigated using whole-cell recordings in brain slices of adult rat medulla. Most neurons of the rNST (47%) responded to stimulation of the ST with excitatory postsynaptic potentials (EPSPs), 28% responded with mixed excitatory and inhibitory postsynaptic potentials (PSPs) and 25% responded with inhibitory postsynaptic potentials (IPSPs). The estimated reversal potentials for the EPSPs (EEPSP) was -7 mV and for the IPSPs (EIPSP) was -69 mV. The glutamate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) acting at the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)/kainate receptor, either reduced or blocked all EPSPs tested. D-2-Amino-5-phosphonovalerate (APV), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, also reduced the amplitude of the EPSPs. These results suggest that glutamate is released following stimulation of afferent fibers in the ST and acts on both AMPA/kainate and NMDA glutamate receptors. The IPSPs result from release of gamma-aminobutyric acid (GABA) since superfusion of the GABAA receptor antagonist, bicuculline reversibly blocked the IPSPs. The GABAB receptor antagonist, phaclofen, also reduced the IPSP components in some neurons, indicating that both GABAA and GABAB receptors are involved in inhibitory transmission in the rNST. When the morphology of the recorded neurons was examined by filling the neurons with biocytin and reconstructing the neurons, each morphological type of rNST neuron responded with excitatory and inhibitory PSPs following stimulation of the ST.

Contrasting properties of two forms of long-term potentiation in the hippocampus

Activity-dependent enhancement of synaptic transmission, referred to as long-term potentiation (LTP), is observed at many synapses in the central nervous system. In the hippocampus two distinct forms of LTP have been identified. One involves the activation of the NMDA (N-methyl-D-aspartate) subtype of glutamate receptor and a rise in postsynaptic Ca2+, whereas the other, which is found at mossy fibre synapses, is independent of NMDA receptors but does require a rise in presynaptic Ca2+. Although it is now generally accepted that mossy fibre LTP is expressed presynaptically, the locus of expression for NMDA-receptor-dependent LTP is controversial. Here the two forms of LTP are compared and it is argued that the balance of evidence favours a postsynaptic locus for NMDA-receptor-dependent LTP.

Overlapping and differential expression of BIG-2, BIG-1, TAG-1, and F3: four members of an axon-associated cell adhesion molecule subgroup of the immunoglobulin superfamily

Axon-associated cell adhesion molecules (AxCAMs) play crucial roles in the formation, maintenance, and plasticity of functional neuronal networks. We report here a molecular cloning of a novel AxCAM, BIG-2. BIG-2 is a member of TAG-1/F3 subgroup of the immunoglobulin (Ig) superfamily, with six Ig-like domains, four fibronectin type III-like repeats, and a glycosyl phosphatidylinositol-anchoring domain. Recombinant BIG-2 protein had a neurite outgrowth-promoting activity when used as a substrate for neurons in vitro. To survey the spatial expression pattern of BIG-2 in comparison with other TAG-1/F3 subgroup members, an in situ hybridization analysis was performed in adult and developing rat brain sections with riboprobes specific for BIG-2, BIG-1, TAG-1, and F3. The four AxCAM transcripts displayed cell type-specific expression patterns with overlapping and distinct profiles. In adult hippocampus, for example, we observed BIG-1 mRNA specifically in granule cells of the dentate gyrus, BIG-2 mRNA highly in the CA1 pyramidal cells, TAG-1 mRNA predominantly in the CA3 pyramidal cells, and F3 mRNA in neurons in all of these fields. These results suggest that BIG-2, BIG-1, TAG-1, and F3 may play important roles in the formation and maintenance of specific neuronal networks in the brain.

Patch sensor detection of glutamate release evoked by a single electrical shock

We tried to detect minimal stimulation-induced glutamate overflow from the surface of a hippocampal slice using an outside-out patch electrode excised from pyramidal cell membranes. The amplitude of the stimulation-induced patch current was dependent on the distance between the slice surface and the tip of patch sensor. The current-voltage relations of the stimulation-induced patch current were similar to those of the current evoked puff by application of L-glutamate to the patch. This indicates that the stimulation-induced patch current was produced by glutamate released from presynaptic terminals, and thus this technique may be useful in the study of transmitter release evoked by minimal electrical stimulation in brain slices.

Presynaptic changes during mossy fibre LTP revealed by NMDA receptor-mediated synaptic responses

Activity-dependent changes in synaptic strength are important for learning and memory. Long-term potentiation (LTP) of glutamatergic excitatory synapses following brief repetitive stimulation provides a compelling cellular model for such plasticity. In the CA1 region of the hippocampus, anatomical studies have revealed large numbers of NMDA (N-methyl-D-aspartate) receptor sites at excitatory synapses, which express primarily an NMDA receptor-dependent form of LTP. In contrast, these studies have suggested that mossy fibre synapses activate primarily or exclusively alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and, indeed, these synapses express a form of LTP that is entirely independent of NMDA receptors. Here we present physiological data demonstrating that mossy fibres activate a substantial NMDA receptor synaptic component that expresses LTP. The presence of an NMDA receptor response allowed us to use the open-channel NMDA receptor antagonist MK-801 to establish directly that the probability of transmitter release is enhanced during the expression of mossy fibre LTP.

NMDA receptor-dependent and -independent immediate early gene expression induced by focal mechanical brain injury

In the present study we analysed, by in situ hybridization, the effects of an extremely localized mechanical brain injury, obtained by the simple needle insertion (30 g) in rat hippocampus or cortex, on the expression of several immediate early genes (c-fos, fosB, c-jun, junB, junD, zif/268). When the needle is deepened into the hippocampus through the cortex, a simultaneous ipsilateral activation of all examined IEGs is observed in both the cerebral cortex and in the dentate gyrus of hippocampus. Maximal effects are detected between 30 and 60 min with the following rank order of induction: zif/268 > c-fos- > junB > fosB > c-jun > junD. On the other hand, when the penetration of the needle is limited to the cerebral cortex the activation of the IEGs (c-fos, fosB, junB and zif/268) spreads throughout the ipsilateral cortex but does not involve the hippocampal region. Systemic administration of ketamine, a non-competitive antagonist of N-methyl-D-aspartate (NMDA) receptors, blocks IEG expression induced by brain injury in the cerebral cortex and in the hippocampal dentate gyrus. Pretreatment with the anticonvulsant diazepam, the anaesthetic urethane, or the muscarinic receptor antagonist scopolamine do not affect the injury-induced genomic response. An important regional difference in the sensitivity to the blocking effect of ketamine can be observed analysing the results regarding the zif/268 gene expression in the hippocampus. A clear induction of this gene by needle insertion can be detected both in the dentate gyrus and in the hippocampal layers. However, the dentate gyrus induction is completely blocked by the ketamine pretreatment, while the induction in the hippocampal layers is not affected by this NMDA antagonist. The zif/268 induction in the hippocampal layers is not blocked even if the intracerebroventricular administration of a non-NMDA glutamate receptor antagonist is associated to the systemic pretreatment with ketamine. This result represents the first observation of injury-induced neuronal genomic responses that are not critically dependent on the NMDA receptor activity.

Posttetanic potentiation and presynaptically induced long-term potentiation at the mossy fiber synapse in rat hippocampus

A form of long-term potentiation (LTP) is induced at the mossy fiber (MF) synapse in the hippocampus by high-frequency presynaptic stimulation (HFS). It is generally accepted that induction of this form of LTP (MF LTP) does not depend on postsynaptic Ca2+ current gated by N-methyl-D-aspartate receptors, but it has remained controversial whether induction depends on postsynaptic depolarization and voltage-gated entry of Ca2+. There are also contradictory data on the time course of both LTP and post-tetanic potentiation (PTP), a shorter duration form of potentiation observed at MF synapses immediately following HFS. It has been proposed that some of these differences in results may have arisen because of difficulties in isolating monosynaptic responses to MF input. In the present study, whole cell recording was used to observe excitatory postsynaptic currents (EPSCs) elicited in CA3 pyramidal cells by input from MFs. Postsynaptic cells were dialyzed with 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) and F- to inhibit postsynaptic mechanisms that required Ca2+, cells were under voltage clamp during HFS, and conditions were selected to minimize the likelihood of polysynaptic contamination. Under these conditions, HFS nevertheless induced robust LTP (mean magnitude, 62%). The possibility that EPSCs were contaminated by polysynaptic components was investigated by exposing the slices to a suppressing medium (one that partially blocked neurotransmission). EPSC waveforms did not change shape during suppression, indicating that contamination was absent. The LTP observed always was accompanied by prominent PTP that lasted through the first 5 to 15 min following HFS (mean decay time constant, 3.2 min). Induction of this LTP was not cooperative; there was no relationship between the size of responses and the magnitude of the LTP induced. LTP magnitude also was unrelated to the extent to which postsynaptic cells depolarized during HFS. These results show that high rates of presynaptic MF activity elicit robust LTP whether or not there is accompanying postsynaptic depolarization or increase in the concentration of postsynaptic Ca2+. High-frequency MF activity also results in a PTP that is unusually large and long.

Further characteristics of long-term potentiation in piriform cortex

Mechanisms for the induction and expression of long-term potentiation (LTP) were studied in slices of piriform cortex. Cooperativity among afferent inputs as a controlling factor for induction of LTP was tested by pairing stimulation of one input that normally does not induce LTP with stimulation of another input. Combined stimulation, given either to two weak inputs with simultaneous bursts or by pairing single pulses with bursts, did effectively induce LTP. Tests for expression of LTP by NMDA vs. non-NMDA receptors indicated that non-NMDA receptor-mediated responses expressed much greater LTP than NMDA receptor-mediated responses. Ratios for paired-pulse facilitation and depression were not altered after induction of LTP. These characteristics are comparable to those exhibited by synapses in the CA1 field of hippocampus.

Mediation of hippocampal mossy fiber long-term potentiation by cyclic AMP

Repetitive activation of hippocampal mossy fibers evokes a long-term potentiation (LTP) of synaptic responses in pyramidal cells in the CA3 region that is independent of N-methyl-D-aspartate receptor activation. Previous results suggest that the site for both the induction and expression of this form of LTP is presynaptic. Experimental elevation of cyclic adenosine 3',5'-monophosphate (cAMP) both mimics and interferes with tetanus-induced mossy fiber LTP, and blockers of the cAMP cascade block mossy fiber LTP. It is proposed that calcium entry into the presynaptic terminal may activate Ca(2+)-calmodulin-sensitive adenylyl cyclase I which, through protein kinase A, causes a persistent enhancement of evoked glutamate release.