Coactivation of Local Circuit NMDA Receptor Mediated epsps Induces Lasting Enhancement of Minimal Schaffer Collateral epsps in Slices of Rat Hippocampus

Circuits and Synapses: Hypothesis, Observation, Controversy and Serendipity - An Opinion Piece

More than a century of dedicated research has resulted in what we now know, and what we think we know, about synapses and neural circuits. This piece asks to what extent some of the major advances - both theoretical and practical - have resulted from carefully considered theory, or experimental design: endeavors that aim to address a question, or to refute an existing hypothesis. It also, however, addresses the important part that serendipity and chance have played. There are cases where hypothesis driven research has resulted in important progress. There are also examples where a hypothesis, a model, or even an experimental approach - particularly one that seems to provide welcome simplification - has become so popular that it becomes dogma and stifles advance in other directions. The nervous system rejoices in complexity, which should neither be ignored, nor run from. The emergence of testable "rules" that can simplify our understanding of neuronal circuits has required the collection of large amounts of data that were difficult to obtain. And although those collecting these data have been criticized for not advancing hypotheses while they were "collecting butterflies," the beauty of the butterflies always enticed us toward further exploration.

Phenomenological models of synaptic plasticity based on spike timing

Synaptic plasticity is considered to be the biological substrate of learning and memory. In this document we review phenomenological models of short-term and long-term synaptic plasticity, in particular spike-timing dependent plasticity (STDP). The aim of the document is to provide a framework for classifying and evaluating different models of plasticity. We focus on phenomenological synaptic models that are compatible with integrate-and-fire type neuron models where each neuron is described by a small number of variables. This implies that synaptic update rules for short-term or long-term plasticity can only depend on spike timing and, potentially, on membrane potential, as well as on the value of the synaptic weight, or on low-pass filtered (temporally averaged) versions of the above variables. We examine the ability of the models to account for experimental data and to fulfill expectations derived from theoretical considerations. We further discuss their relations to teacher-based rules (supervised learning) and reward-based rules (reinforcement learning). All models discussed in this paper are suitable for large-scale network simulations.

Activity-Dependent Depression of Local Excitatory Connections in the CA1 Region of Mouse Hippocampus

The existence of recurrent excitatory synapses between pyramidal cells in the hippocampal CA1 region has been known for some time yet little is known about activity-dependent forms of plasticity at these synapses. Here we demonstrate that under certain experimental conditions, Schaffer collateral/commissural fiber stimulation can elicit robust polysynaptic excitatory postsynaptic potentials due to recurrent synaptic inputs onto CA1 pyramidal cells. In contrast to CA3 pyramidal cell inputs, recurrent synapses onto CA1 pyramidal cells exhibited robust paired-pulse depression and a sustained, but rapidly reversible, depression in response to low-frequency trains of Schaffer collateral fiber stimulation. Blocking GABAB receptors abolished paired-pulse depression but had little effect on low-frequency stimulation (LFS)-induced depression. Instead, LFS-induced depression was significantly attenuated by an inhibitor of A1 type adenosine receptors. Blocking the postsynaptic effects of GABAB and A1 receptor activation on CA1 pyramidal cell excitability with an inhibitor of G-protein-activated inwardly rectifying potassium channels had no effect on either paired-pulse depression or LFS-induced depression. Thus activation of presynaptic GABAB and adenosine receptors appears to have an important role in activity-dependent depression at recurrent synapses. Together, our results indicate that CA3-CA1 and CA1-CA1 synapses exhibit strikingly different forms of short-term synaptic plasticity and suggest that activity-dependent changes in recurrent synaptic transmission can transform the CA1 region from a sparsely connected recurrent network into a predominantly feedforward circuit.

Estradiol increases delayed, N-methyl-D-aspartate receptor-mediated excitation in the hippocampal CA1 region

Hippocampal functions, e.g. synaptic plasticity and hippocampal-dependent behavior, are influenced by the circulating levels of ovarian steroids in adult, female rats. The mechanisms underlying this estradiol-dependent modulation, however, are poorly understood. One possibility is that estradiol alters N-methyl-D-aspartate (NMDA)-receptor functioning in the hippocampus. Here, using the in vitro hippocampal slice preparation, we evaluate estradiol-dependent changes in the NMDA receptor- and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated components of excitatory postsynaptic potentials (EPSPs) evoked in CA1 by Schaffer collateral test stimulation. Using established experimental conditions [J Neurosci 17 (1997) 1848], we replicate the observation that estradiol pretreatment of ovariectomized rats increases a pharmacologically isolated NMDA receptor-mediated EPSP evoked by Schaffer collateral stimulation. However, using different conditions that optimize study of this evoked response, the estradiol-dependent increase in the monosynaptic NMDA receptor-mediated EPSP is eliminated. Low-intensity test stimulation of the Schaffer collaterals in this optimized medium reveals a novel, late NMDA receptor-mediated EPSP in CA1 from estradiol-pretreated rats. The mechanism(s) underlying this estradiol-dependent increase in a late, NMDA receptor-mediated EPSP is not known, but enhanced CA1-CA1 excitatory circuitry and glutamate spillover could contribute to this response. We conclude that estradiol pretreatment enhances NMDA receptor function in the female hippocampus by increasing not the monosynaptic, but rather a late NMDA receptor-mediated response. Variations in the magnitude of this late response may well contribute to ovarian steroid-dependent modulation of hippocampal synaptic plasticity.

NMDA receptor-mediated depolarizing after-potentials in the basal dendrites of CA1 pyramidal neurons

It was shown recently that the basal dendrites of cortical pyramidal neurons generate NMDA-spikes. In the present study, we made whole-cell recordings from hippocampal CA1 pyramidal neurons and examined whether NMDA receptor activation was involved in synaptic responses. At low input stimulus intensity, EPSPs with a fast decay time were induced. As the intensity of stimulation was increased in the presence of GABA receptor antagonists, a depolarizing after-potential (DAP) was generated in addition to a fast decaying potential. A DAP was never observed when the input was applied to the apical dendrites. The DAP was suppressed by hyperpolarization or by NMDA receptor antagonists, but not by Na+, K+, or Ca2+ channel blockers. One possible mechanism is that the morphology of the basal dendrites favors DAP generation. A compartmental model simulation showed that synaptic inputs to thinner shorter dendrites generated a potential that resembled a DAP. Our study shows that a synaptic input to the basal dendrites of a hippocampal pyramidal neuron can generate a NMDA receptor-mediated potential in the presence of GABA receptor blockade.

Neuropathology provides clues to the pathophysiology of Gaucher disease

To better understand the pathogenesis of brain dysfunction in Gaucher disease (GD), we studied brain pathology in seven subjects with type 1 GD (four also exhibited parkinsonism and dementia), three with type 2 GD and four with type 3 GD. Unique pathologic patterns of disease involving the hippocampal CA2-4 regions and layer 4b of the calcarine cortex were identified. While these findings were common to all three GD phenotypes, the extent of the changes varied depending on the severity of disease. Cerebral cortical layers 3 and 5, hippocampal CA2-4, and layer 4b were involved in all GD patients. Neuronal loss predominated in both type 2 and type 3 patients with progressive myoclonic encephalopathy, whereas patients classified as type 1 GD had only astrogliosis. Adjacent regions and lamina, including hippocampal CA1 and calcarine lamina 4a and 4c were spared of pathology, highlighting the specificity of the vulnerability of selective neurons. Elevated glucocerebrosidase expression by immunohistochemistry was found in CA2-4. Hippocampal (45)Ca(2+) uptake autoradiography in rat brain was performed demonstrating that hippocampal CA2-4 neurons, rather than CA1 neurons, were calcium-induced calcium release sensitive (CICR-sensitive). These findings match recent biochemical studies linking elevated glucosylceramide levels to sensitization of CA2-4 RyaR receptors and 300% potentiation of neuronal CICR sensitivity. In two patients with type 1 GD and parkinsonism, numerous synuclein positive inclusions, similar to brainstem-type Lewy bodies found in Parkinson disease, were also found hippocampal CA2-4 neurons. These findings argue for a common cytotoxic mechanism linking aberrant glucocerebrosidase activity, neuronal cytotoxicity, and cytotoxic Lewy body formation in GD.

Endogenous 5-HT, released by MDMA through serotonin transporter- and secretory vesicle-dependent mechanisms, reduces hippocampal excitatory synaptic transmission by preferential activation of 5-HT1B receptors located on CA1 pyramidal neurons

A multitude of different serotonin (5-HT) receptor types are expressed in the hippocampus, but the identity of receptors actually mediating the physiological response to endogenous 5-HT has not been determined. We combined pharmacologically induced release of 5-HT with patch-clamp recordings on disinhibited rat CA1 minislices to determine effects of endogenous 5-HT on the excitability of pyramidal neurons and synaptic transmission among them. We found that application of 5-HT releasers, 3,4-methylenedioxy-methamphetamine (MDMA) or p-methylthioamphetamine, at concentrations ranging from 2 to 50 microm, reduced the excitatory synaptic transmission between CA1 pyramidal neurons without altering their basal electrical properties. This effect of MDMA was blocked by the selective 5-HT1B antagonist GR 55562, was dependent on endogenous 5-HT content and was mediated by presynaptically located, pertussis-toxin sensitive mechanisms. We found no other MDMA effects in our preparation, which indicates that the release of endogenous 5-HT preferentially stimulates 5-HT1B receptors on CA1 pyramidal neurons. Therefore, 5-HT1B receptor activation may represent a predominant component of the physiological response to endogenous 5-HT in the CA1. The high sensitivity of the 5-HT1B receptor-mediated reduction of polysynaptic excitatory responses to the extracellular 5-HT level enabled us to study mechanisms of the 5-HT releasing action of MDMA. Block of the serotonin transporter (SERT) with citalopram slowed the time course and reduced overall 5-HT release by MDMA. Depletion of vesicular 5-HT, by inhibition of vesicular monoamine transporter type 2 with tetrabenazine prevented the release. Thus although the SERT reversal contributes, a direct vesicle-depleting action is essential for MDMA release of 5-HT.

Pharmacological characterization of 5-HT(1B) receptor-mediated inhibition of local excitatory synaptic transmission in the CA1 region of rat hippocampus

1 In the hippocampus, axon collaterals of CA1 pyramidal cells project locally onto neighbouring CA1 pyramidal cells and interneurones, forming a local excitatory network which, in disinhibited conditions, feeds polysynaptic epscs (poly-epscs). 5-hydroxytryptamine (5-HT) has been shown to inhibit poly-epscs through activation of a presynaptic receptor. The aim of the present work was the pharmacological characterization of the 5-HT receptor involved in this 5-HT action. 2 Poly-epscs, evoked by electrical stimulation of the stratum radiatum and recorded in whole-cell voltage-clamp from CA1 pyramidal neurones, were studied in mini-slices of the CA1 region under pharmacological block of GABA(A), GABA(B), and 5-HT(1A) receptors. 3 The 5-HT(1B) receptor selective agonist 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one dihydrochloride (CP 93129) inhibited poly-epscs (EC(50)=55 nM), an effect mimicked by the 5-HT(1B) ligands 5-carboxamidotryptamine (5-CT; EC(50)=14 nM) and methylergometrine (EC(50)=78 nM), but not by 1-(3-chlorophenyl)piperazine dihydrochloride (mCPP; 10 micro M) or 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline dimaleate (CGS 12066B; 10 micro M). 4 The effects of CP 93129 and 5-CT were blocked by the selective 5-HT(1B) receptor antagonist 3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR 55562; K(B) approximately 100 nM) and by cyanopindolol (K(B)=6 nM); methiothepin (10 micro M) and dihydroergotamine (1 micro M). For both GR 55562 and methiothepin, application times of at least two hours were required in order to achieve their full antagonistic effects. 5 Our results demonstrate that 5-HT(1B) receptors are responsible for the presynaptic inhibition of neurotransmission at CA1/CA1 local excitatory synapses exerted by 5-HT.

Target and temporal pattern selection at neocortical synapses

We attempt to summarize the properties of cortical synaptic connections and the precision with which they select their targets in the context of information processing in cortical circuits. High-frequency presynaptic bursts result in rapidly depressing responses at most inputs onto spiny cells and onto some interneurons. These 'phasic' connections detect novelty and changes in the firing rate, but report frequency of maintained activity poorly. By contrast, facilitating inputs to interneurons that target dendrites produce little or no response at low frequencies, but a facilitating-augmenting response to maintained firing. The neurons activated, the cells they in turn target and the properties of those synapses determine which parts of the circuit are recruited and in what temporal pattern. Inhibitory interneurons provide both temporal and spatial tuning. The 'forward' flow from layer-4 excitatory neurons to layer 3 and from 3 to 5 activates predominantly pyramids. 'Back' projections, from 3 to 4 and 5 to 3, do not activate excitatory cells, but target interneurons. Despite, therefore, an increasing complexity in the information integrated as it is processed through these layers, there is little 'contamination' by 'back' projections. That layer 6 acts both as a primary input layer feeding excitation 'forward' to excitatory cells in other layers and as a higher-order layer with more integrated response properties feeding inhibition to layer 4 is discussed.

Selective inhibition of local excitatory synaptic transmission by serotonin through an unconventional receptor in the CA1 region of rat hippocampus

1. The modulation of synaptic transmission by serotonin (5-HT) was studied using whole-cell voltage-clamp and sharp-electrode current-clamp recordings from CA1 pyramidal neurones in transverse rat hippocampal slices in vitro. 2. With GABA(A) receptors blocked, polysynaptic transmission evoked by stratum radiatum stimulation was inhibited by submicromolar concentrations of 5-HT, while monosynaptic excitatory transmission and CA1 pyramidal neurone excitability were unaffected. The effect persisted following pharmacological blockade of 5-HT(1A) and 5-HT(4) receptors, which directly affect CA1 pyramidal neurone excitability. 3. Concentration-response relationships for 5-HT were determined in individual neurones; the EC(50) values for block of polysynaptic excitation and inhibition by 5-HT were approximately 230 and approximately 160 nM, respectively. The 5-HT receptor type responsible for the observed effect does not fall easily into the present classification of 5-HT receptors. 4. 5-HT inhibition of polysynaptic EPSCs persisted following complete block of GABAergic transmission and in CA1 minislices, ruling out indirect effects through interneurones and non-CA1 pyramidal neurones, respectively. 5. Monosynaptic EPSCs evoked by stimulation of CA1 afferent pathways appeared to be unaffected by 5-HT. Monosynaptic EPSCs evoked by stimulation of the alveus, which contains CA1 pyramidal neurone axons, were partially inhibited by 5-HT. 6. We conclude that 5-HT inhibited synaptic transmission by acting at local recurrent collaterals of CA1 pyramidal neurones. This may represent an important physiological action of 5-HT in the hippocampus, since it occurs over a lower concentration range than the 5-HT effects reported so far.

Differences in amplitude-voltage relations between minimal and composite mossy fibre responses of rat CA3 hippocampal neurons support the existence of intrasynaptic ephaptic feedback in large synapses

Computer simulations and electrophysiological experiments have been performed to test the hypothesis on the existence of an ephaptic interaction in purely chemical synapses. According to this hypothesis, the excitatory postsynaptic current would depolarize the presynaptic release site and further increase transmitter release, thus creating an intrasynaptic positive feedback. For synapses with the ephaptic feedback, computer simulations predicted non-linear amplitude-voltage relations and voltage dependence of paired-pulse facilitation. The deviation from linearity depended on the strength of the feedback determined by the value of the synaptic cleft resistance. The simulations showed that, in the presence of the intrasynaptic feedback, recruitment of imperfectly clamped synapses and synapses with linear amplitude-voltage relations tended to reduce the non-linearity and voltage dependence of paired-pulse facilitation. Therefore, the simulations predicted that the intrasynaptic feedback would particularly affect small excitatory postsynaptic currents induced by activation of electrotonically close synapses with long synaptic clefts. In electrophysiological experiments performed on hippocampal slices, the whole-cell configuration of the patch-clamp technique was used to record excitatory postsynaptic currents evoked in CA3 pyramidal cells by activation of large mossy fibre synapses. In accordance with the simulation results, minimal excitatory postsynaptic currents exhibited "supralinear" amplitude-voltage relations at hyperpolarized membrane potentials, decreases in the failure rate and voltage-dependent paired-pulse facilitation. Composite excitatory postsynaptic currents evoked by activation of a large amount of presynaptic fibres typically bear linear amplitude-voltage relationships and voltage-independent paired-pulse facilitation. These data are consistent with the hypothesis on a strong ephaptic feedback in large mossy fibre synapses. The feedback would provide a mechanism whereby signals from large synapses would be amplified. The ephaptic feedback would be more effective on synapses activated in isolation or together with electrotonically remote inputs. During synchronous activation of a large number of neighbouring inputs, suppression of the positive intrasynaptic feedback would prevent abnormal boosting of potent signals.

Molecular frequency filters at central synapses

During the 1950s to 70s most of the mechanisms that control transmitter release from presynaptic nerve terminals were described at the neuromuscular junction. It was not, however, until the 1990s that the multiplicity of protein-protein interactions that govern this process began to be identified. The sheer numbers of proteins and the complexity of their interactions at first appears excessive, even redundant. However, studies of identified central synapses indicate that this molecular diversity may underlie a important functional diversity. The task of the neuromuscular junction is to relay faithfully the rate and pattern code generated by the motoneurone. To demonstrate phenomena such as facilitation and augmentation that are apparent only when the probability of release is low, experimental manipulation is required. In the cortex, however, low probability synapses displaying facilitation can be recorded in parallel with high probability synapses displaying depression. The mechanisms are largely the same as those displayed by the neuromuscular junction, but some are differentially expressed and controlled. Central synapses demonstrate exquisitely fine tuned information transfer, each of the many types displaying its own repertoire of pattern- and frequency-dependent properties. These appear tuned to match both the discharge pattern in the presynaptic neurone and the integrative requirements of the postsynaptic cell. The molecular identification of these differentially expressed frequency filters is now just coming into sight. This review attempts to correlate these two aspects of synaptic physiology and to identify the components of the release process that are responsible for the diversity of function.

Termination of epileptic afterdischarge in the hippocampus

The mechanism of afterdischarge termination in the various hippocampal regions was examined in the rat. Stimulation of the perforant path or the commissural system was used to elicit afterdischarges. Combination of multiple site recordings with silicon probes, current source density analysis, and unit recordings in the awake animal allowed for a high spatial resolution of the field events. Interpretation of the field observations was aided by intracellular recordings from anesthetized rats. Irrespective of the evoking conditions, afterdischarges always terminated first in the CA1 region. Termination of the afterdischarge was heralded by a large DC shift initiated in dendritic layers associated with a low amplitude "afterdischarge termination oscillation" (ATO) at 40 to 80 Hz in the cell body layer. ATOs were also observed in the CA3 region and the dentate gyrus. The DC shift spread at the same velocity (0. 1-0.2 mm/sec) in all directions and could cross the hippocampal fissure. All but 1 of the 25 putative interneurons in the CA1 and dentate regions ceased to fire before the onset of ATO. Intracellularly, ATO and the emerging DC potential were associated with fast depolarizing potentials and firing of pyramidal cells and depolarization block of spike initiation, respectively. Both field ATO and the intracellular depolarization shift were replicated by focal microinjection of potassium. We hypothesize that [K+]o lost by the intensely discharging neurons during the afterdischarge triggers propagating waves of depolarization in the astrocytic network. In turn, astrocytes release potassium, which induces a depolarization block of spike generation in neurons, resulting in "postictal depression" of the EEG.

Blocking GABA(A) inhibition reveals AMPA- and NMDA-receptor-mediated polysynaptic responses in the CA1 region of the rat hippocampus

We have investigated the conditions required to evoke polysynaptic responses in the isolated CA1 region of hippocampal slices from Wistar adult rats. Experiments were performed with extracellular and whole cell recording techniques. In the presence of bicuculline (10 microM), 6-cyano-7-nitroquinoxaline-2-3-dione (10 microM), glycine (10 microM), and a low external concentration of Mg2+ (0.3 mM), electrical stimulation of the Schaffer collaterals/commissural pathway evoked graded N-methyl-D-aspartate (NMDA)-receptor-mediated late field potentials in the stratum radiatum of the CA1 region. These responses were generated via polysynaptic connections because their latency varied strongly and inversely with the stimulation intensity and they were abolished by a high concentration of divalent cations (7 mM Ca2+). These responses likely were driven by local collateral branches of CA1 pyramidal cell axons because focal application of tetrodotoxin (30 microM) in the stratum oriens strongly reduced the late synaptic component and antidromic stimulation of CA1 pyramidal cells could evoke the polysynaptic response. Current-source density analysis suggested that the polysynaptic response was generated along the proximal part of the apical dendrites of CA1 pyramidal cells (50-150 microm below the pyramidal cell layer in the stratum radiatum). In physiological concentration of Mg2+ (1.3 mM), the pharmacologically isolated NMDA-receptor-mediated polysynaptic response was abolished. In control artificial cerebrospinal fluid (with physiological concentration of Mg2+), bicuculline ( 10 microM) generated a graded polysynaptic response. Under these conditions, this response was mediated both by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/NMDA receptors. In the presence of D-2-amino-5-phosphonovalerate (50 microM), the polysynaptic response could be mediated by AMPA receptors, although less efficiently. In conclusion, suppression of gamma-aminobutyric acid-A inhibition reveals glutamate receptor-mediated network-driven events in the isolated CA1 region. These polysynaptic responses are mediated by AMPA and/or NMDA receptors depending on the pharmacological conditions and the external concentration of Mg2+ used. We suggest that these responses are driven by local recurrent collaterals of CA1 pyramidal cells.

Spatiotemporal distribution of intracellular calcium transients during epileptiform activity in guinea pig hippocampal slices

Calcium ions are known to play an important role in epileptogenesis. Although there is clear evidence for increased neuronal calcium influx during epileptiform potentials, direct measurements of the corresponding intracellular calcium transients are rare and the origin of calcium influx is not known. Therefore the spatial and temporal distribution of intracellular calcium transients during epileptiform activity in guinea pig hippocampal slices was monitored with the use of the indicator Calcium-Green and a fast optical recording method. Two models of epilepsy (bicuculline and low Mg2+) were compared. In both models, single epileptiform events were evoked by electrical stimulation of the Schaffer collaterals in CA1 or of stratum pyramidale in area CA3. Intracellular calcium transients during epileptiform activity were approximately 5 times larger than during control stimulation. Calcium transients during epileptiform activity were present across at least the entire CA1 area, whereas presynaptic calcium transients from stimulated fibers were only seen at a distance up to 1 mm from the stimulation site. DL-2-amino-5-phosphonovaleric acid (APV), a specific antagonist of the N-methyl-D-aspartate (NMDA) receptor, abolished low-Mg2+ epileptiform activity and reduced bicuculline-induced epileptiform activity; it reduced calcium transients following stimulation of CA1 by only 29% (bicuculline) and 38% (low Mg2+). For comparison, calcium transients during control stimulation were 78% (bicuculline) and 69% (low Mg2+) smaller than epileptiform calcium transients. At a distance from the stimulation site, calcium transients and their NMDA-receptor-dependent components were largest in stratum pyramidale in the bicuculline model and in stratum oriens in the low-Mg2+ model. In both models, minimal onset latencies of calcium influx shifted with increasing distance to the stimulation electrode from stratum radiatum to stratum oriens. APV reduced the extent of spread of calcium transients in the low-Mg2+ model. In the bicuculline model, the spatial extent of spread of epileptiform calcium transients was not affected by application of APV; however, the mean velocity of spread was reduced from 0.20 to 0.12 m/s. In conclusion, the large size of calcium transients and of their NMDA-receptor-dependent components in stratum pyramidale or stratum oriens as well as shortest onset latencies of calcium transients at these sites suggest an important role of cell somata, basal dendrites, and possibly local circuit excitatory interactions for the generation and spread of epileptiform activity.

A role for synaptic and network plasticity in controlling epileptiform activity in CA1 in the kainic acid-lesioned rat hippocampus in vitro

1. Stimulation of the surviving afferents in the stratum radiatum of the CA1 area in kainic acid-lesioned hippocampal slices produced graded epileptiform activity, part of which (> 20%) involved the activation of N-methyl-D-aspartate (NMDA) receptors. There was also a failure of synaptic inhibition in this region. In this preparation, we have tested the effects of low-frequency stimulation (LFS; 1 Hz for 15 min) on synaptic responses and epileptiform activity. 2. LFS resulted in long-term depression (LTD) of excitatory synaptic potentials (EPSPs), long-term decrease of population spike amplitudes (PSAs) and EPSP-spike (E-S) potentiation. Evoked epileptiform activity was reduced but neurons had a higher probability of discharge. LTD could be reversed by subsequent tetanic stimulation whereas E-S dissociation remained unchanged. Synaptic and network responses could be saturated towards either potentiation or depression. However, E-S potentiation was maximal following the first conditioning stimulus. 3. NMDA receptor-mediated responses were pharmacologically isolated. LFS resulted in LTD of synaptic responses, long-term decrease of PSAs and E-S depression. These depressions could not be reversed by subsequent tetanic stimulation. alpha-Amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and NMDA receptor-mediated responses were then measured in isolation before and following conditioning stimuli. LFS was shown to simultaneously produce LTD of AMPA and NMDA receptor-mediated responses. E-S potentiation of the AMPA component and E-S depression of the NMDA component occurred coincidentally. 4. LTD of AMPA and NMDA receptor-mediated responses were shown to be NMDA dependent. In contrast, E-S potentiation and depression occurred even when NMDA receptors were pharmacologically blocked. 5. These findings indicate that synaptic responses could be modified bidirectionally in the CA1 area of kainic acid-lesioned rat hippocampus in an NMDA receptor-dependent manner. However, E-S dissociations were independent of the activation of NMDA receptors, hinting at mechanisms different from those of synaptic LTD. We suggest that changes in E-S coupling were caused by a modification of the firing threshold of the CA1 pyramidal neurons. Furthermore, the firing mechanisms controlling NMDA and AMPA receptor-mediated network activity appeared to be different. The possible use of LFS applied to the hippocampus as a clinical intervention to suppress epileptiform activity is discussed.

Latent N-methyl-D-aspartate receptors in the recurrent excitatory pathway between hippocampal CA1 pyramidal neurons: Ca(2+)-dependent activation by blocking A1 adenosine receptors

When performed at increased external [Ca2+]/[Mg2+] ratio (2.5 mM/0.5 mM), temporary block of A1 adenosine receptors in hippocampus [by 8-cyclopentyltheophylline (CPT)] leads to a dramatic and irreversible change in the excitatory postsynaptic current (EPSC) evoked by Schaffer collateral/commissural (SCC) stimulation and recorded by in situ patch clamp in CA1 pyramidal neurons. The duration of the EPSC becomes stimulus dependent, increasing with increase in stimulus strength. The later occurring component of the EPSC is carried through N-methyl-D-aspartate (NMDA) receptor-operated channels but disappears under either the NMDA antagonist 2-amino-5-phosphonovaleric acid (APV) or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). These findings indicate that the late component of the SCC-evoked EPSC is polysynaptic: predominantly non-NMDA receptor-mediated SCC inputs excite CA1 neurons that recurrently excite each other by predominantly NDMA receptor-mediated synapses. These recurrent connections are normally silent but become active after CPT treatment, leading to enhancement of the late component of the EPSC. The activity of these connections is maintained for at least 2 hr after CPT removal. When all functional NMDA receptors are blocked by dizocilpine maleate (MK-801), subsequent application of CPT leads to a partial reappearance of NMDA receptor-mediated EPSCs evoked by SCC stimulation, indicating that latent NMDA receptors are recruited. Altogether, these findings indicate the existence of a powerful system of NMDA receptor-mediated synaptic contacts in SCC input to hippocampal CA1 pyramidal neurons and probably also in reciprocal connections between these neurons, which in the usual preparation are kept latent by activity of A1 receptors.

High-resolution immunogold localization of AMPA type glutamate receptor subunits at synaptic and non-synaptic sites in rat hippocampus

The cellular and subcellular localization of the GluRA, GluRB/C and GluRD subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type glutamate receptor was determined in the rat hippocampus using polyclonal antipeptide antibodies in immunoperoxidase and immunogold procedures. For the localization of the GluRD subunit a new polyclonal antiserum was developed using the C-terminal sequence of the protein (residues 869-881), conjugated to carrier protein and absorbed to colloidal gold for immunization. The purified antibodies immunoprecipitated about 25% of 3[H]AMPA binding activity from the hippocampus, cerebellum or whole brain, but very little from neocortex. These antibodies did not precipitate a significant amount of 3[H]kainate binding activity. The antibodies also recognize the GluRD subunit, but not the other AMPA receptor subunits, when expressed in transfected COS-7 cells and only when permeabilized with detergent, indicating an intracellular epitope. All subunits were enriched in the neuropil of the dendritic layers of the hippocampus and in the molecular layer of the dentate gyrus. The cellular distribution of the GluRD subunit was studied more extensively. The strata radiatum, oriens and the dentate molecular layer were more strongly immunoreactive than the stratum lacunosum moleculare, the stratum lucidum and the hilus. However, in the stratum lucidum of the CA3 area and in the hilus the weakly reacting dendrites were surrounded by immunopositive rosettes, shown in subsequent electron microscopic studies to correspond to complex dendritic spines. In the stratum radiatum, the weakly reacting apical dendrites contrasted with the surrounding intensely stained neuropil. The cell bodies of pyramidal and granule cells were moderately reactive. Some non-principal cells and their dendrites in the pyramidal cell layer and in the alveus also reacted very strongly for the GluRD subunit. At the subcellular level, silver intensified immunogold particles for the GluRA, GluRB/C and GluRD subunits were present at type 1 synaptic membrane specializations on dendritic spines of pyramidal cells throughout all layers of the CA1 and CA3 areas. The most densely labelled synapses tended to be on the largest spines and many smaller spines remained unlabelled. Immunoparticle density at type 1 synapses on dendritic shafts of some non-principal cells was consistently higher than at labelled synapses of dendritic spines of pyramidal cells. Synapses established between dendritic spines and mossy fibre terminals, were immunoreactive for all studied subunits in stratum lucidum of the CA3 area. The postembedding immunogold method revealed that the AMPA type receptors are concentrated within the main body of the anatomically defined type 1 (asymmetrical) synaptic junction. Often only a part of the membrane specialization showed clustered immunoparticles. There was a sharp decrease in immunoreactive receptor density at the edge of the synaptic specialization. Immunolabelling was consistently demonstrated at extrasynaptic sites on dendrites, dendritic spines and somata. The results demonstrate that the GluRA, B/C and D subunits of the AMPA type glutamate receptor are present in many of the glutamatergic synapses formed by the entorhinal, CA3 pyramidal and mossy fibre terminals. Some interneurons have a higher density of AMPA type receptors in their asymmetrical afferent synapses than pyramidal cells. This may contribute to a lower activation threshold of interneurons as compared to principal cells by the same afferents in the hippocampal formation.

Simultaneous expression of excitatory postsynaptic potential/spike potentiation and excitatory postsynaptic potential/spike depression in the hippocampus

Tetanic stimulation of afferents in the stratum radiatum of the CA1 area of the rat hippocampus results in long-term potentiation of excitatory synaptic responses in pyramidal cells. Previous studies have reported a greater increase in the population spike amplitude following the induction of long-term potentiation than could be accounted for by the increase of the slope of the population excitatory postsynaptic potential. Two hypotheses have been proposed to explain this phenomenon (called excitatory postsynaptic potential/spike potentiation): a modification of the firing threshold and/or a modification of the inhibitory drive. Previous studies have not, however, addressed the question of possible changes in spike threshold in association with long-term depression. This paper examines whether the concomitant long-term potentiation of pharmacologically isolated N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials, reported previously, is also associated with a change in spike threshold. When the amplitude of the population spike is plotted as a function of the slope of the population excitatory postsynaptic potential (excitatory postsynaptic potential/spike curve), excitatory postsynaptic potential/spike potentiation (depression) is seen as a shift of the excitatory postsynaptic potential/spike curve to the left (right) following a conditioning stimulus. In this study, using kainic acid lesioned hippocampus, we have shown that tetanic stimulation produced excitatory postsynaptic potential/spike potentiation of the control synaptic response and excitatory postsynaptic potential/spike depression of the isolated N-methyl-D-aspartate receptor-mediated responses.

Neurophysiological analysis of long-term potentiation in mammalian brain

Long-term potentiation (LTP) is a persistent increase in postsynaptic response following a high-frequency presynaptic activation. Characteristic LTP features, including input specificity and associativity, make it a popular model to study memory mechanisms. Mechanisms of LTP induction and maintenance are briefly reviewed. Increased intracellular Ca2+ concentration is shown to be critical for LTP induction. This increase is believed to be based on Ca2+ influx secondary to activation of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. Existence of other sources of Ca2+ increase and other critical factors is now becoming evident. They include voltage-dependent Ca2+ channels, Ca2+ intracellular stores, metabotropic glutamate receptors, 'modulatory' transmitters. An example of an involvement of voltage-dependent Ca2+ channels is potentiation induced by intracellular depolarizing pulses. LTP can be divided into decremental earlier (E-LTP) and non-decremental late (L-LTP) phases which explains some inconsistencies in studies of LTP mechanisms. E-LTP is suggested to be based on a transient increase in presynaptic release probabilities. A hypothesis is considered which explains L-LTP by suggesting that Ca2+ activates structural changes leading to an increase in the synaptic gap resistance. This enhances positive synaptic electrical feedback and augments release probability. The hypothesis predicts specific morphological changes, synchronous transmitter release of two or several quanta in some central synapses and the amplification of such synchronization following LTP induction. Data are discussed which maintain these predictions.

Amplitude fluctuations of dual-component EPSCs in hippocampal pyramidal cells: implications for long-term potentiation

Quantal analysis has provided evidence for a presynaptic contribution to long-term potentiation in hippocampal CA1 cells. This however leaves unexplained the observation that long-term potentiation has little or no effect on the NMDA receptor-mediated component of the synaptic signal. Here, I report that, in baseline conditions, the coefficient of variation of the AMPA/kainate receptor-mediated signal (CVA/K) is consistently larger than that of the NMDA component (CVNMDA), a result which can be explained if AMPA/kainate receptors are absent or nonfunctional at a proportion of synapses. Long-term potentiation is associated with a reduction in CVA/K, but no change in either the average amplitude of the NMDA component or CVNMDA. This is consistent with the proposal that long-term potentiation induction uncovers clusters of latent AMPA/kainate receptors, with no change in transmitter release.

Temporal and spatial properties of local circuits in neocortex

A large body of anatomical data has detailed many complexities of neocortical circuitry, and physiological studies have indicated some roles for this circuitry in the complex functions of the cortex. Until recently, however, we have little precise information about the spatio-temporal properties of synaptic connections between individual neocortical neurones. Studies of synaptic responses elicited in one neocortical neurone by action potentials in another, and parallel morphological studies that have identified these neurones and the synaptic connections between them, have now described these parameters for certain types of local circuit connection in the neocortex. Some of these studies confirmed previous observations and inferences, but others provided major surprises. Evidence indicates that the class of both the presynaptic and postsynaptic neurone together determine a wide range of synaptic properties, such as the type of postsynaptic receptors involved and the temporal pattern of transmitter release, so that each type of synapse displays unique properties. A role for retrograde diffusable messages in determining the temporal properties of these circuits is postulated.

Fluctuations in pyramid-pyramid excitatory postsynaptic potentials modified by presynaptic firing pattern and postsynaptic membrane potential using paired intracellular recordings in rat neocortex

Single axon excitatory connections between pairs of neocortical pyramidal neurons were studied using paired intracellular recordings in layers II/III and IV of coronal slices of adult rat somatosensory/motor cortex. Excitatory postsynaptic potentials evoked with different presynaptic firing patterns and at different postsynaptic membrane potentials were compared. Two methods of statistical analysis were used in attempts to determine whether changes in mean excitatory postsynaptic potential amplitude were due to presynaptic or postsynaptic modifications. Analysis of the decrease in mean excitatory postsynaptic potential amplitude associated with increases in presynaptic firing rate were consistent with a change in probability of transmitter release. Paired pulse depression appeared to exhibit both presynaptic and postsynaptic components when the interspike interval was < 10 ms, but could be explained simply by a decrease in probability of release with interspike intervals between 10 and 80 ms. Previous studies had demonstrated that these excitatory postsynaptic potentials are partially mediated by N-methyl-D-aspartate receptors. In contrast to the apparently presynaptic effects of firing pattern, postsynaptic membrane depolarization appeared to produce an increase in quantal amplitude. In addition to this increase at low frequencies, a form of frequency-dependent, self-potentiation involving the recruitment of an additional, longer-latency postsynaptic component occurred at higher presynaptic firing rates. The possibility is discussed that two different mechanisms are involved in the replacement of vesicles at release sites. Over a few tens of milliseconds (paired-pulse depression) availability of releasable transmitter may be determined by the rate of replacement of discharged vesicles from a readily releasable pool of vesicles. Over longer periods of firing at 0.33-2 Hz, the readily releasable pool may become exhausted and require replenishment. Postsynaptic depolarization increases the duration of these excitatory postsynaptic potentials, facilitating summation and enables two components of excitatory postsynaptic potential enhancement at N-methyl-D-aspartate receptor-mediated synapses; one that is present at all firing rates and relates simply to voltage dependent events and one that occurs at higher firing rates and involves a gradual, time dependent event. These data also indicate that the optimal pyramidal firing pattern if another pyramid is to be activated is a tonic, or brief burst pattern at relatively low repetition rates. Long bursts of many presynaptic spikes recruit little that is not activated by pairs of spikes. This situation is in stark contrast to the results obtained in the following paper in which excitatory inputs from pyramids to non-pyramids are described.

Biochemical and immunocytochemical characterization of antipeptide antibodies to a cloned GluR1 glutamate receptor subunit: cellular and subcellular distribution in the rat forebrain

Antibodies were made to synthetic peptides corresponding to residues 253-367, 757-771 and 877-889 of the published amino acid sequence of the rat brain glutamate receptor GluR1 subunit [Hollmann et al. (1989) Nature 342, 643-648]. The peptides were synthesized both as multiple copies on a branching lysyl matrix (multiple antigenic peptides) and conventional linear peptides using solid-phase synthesis. Rabbits were immunized with these peptides either without conjugation (multiple antigenic peptides) or following coupling to ovalbumin with glutaraldehyde (monomeric peptides). The antibodies from immune sera were then purified by affinity chromatography using reactigel coupled monomeric peptides. All the rabbits produced good antipeptide responses, and were characterized by immunoprecipitation of solubilized alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and kainate binding activity and by their staining patterns on immunoblots. Antibody to peptide 253-267 specifically immunoprecipitated 12 +/- 3, 50 +/- 3 and 44 +/- 4% of solubilized alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding activity from cortex, hippocampus and cerebellum, respectively. Under identical conditions, antibody against the 877-889 peptide removed 23 +/- 4, 9 +/- 4 and 15 +/- 9% of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding sites from these areas. On immunoblots of rat brain membrane samples separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, antibodies labelled a 105,000 mol. wt immunoreactive band. GluR1 was immunoaffinity-purified using subunit-specific antibodies against both N-terminal (253-267) and C-terminal (877-889) residues, covalently attached to protein A-agarose. Analysis of the purified product from each column showed a major immunoreactive band, recognized by both sera at 105,000 mol. wt and silver staining identified the same major protein. After exhaustive immunoprecipitation of solubilized membrane samples with antibody against the C-terminal of the subunit, a subpopulation of GluR1 was labelled with antibodies specific for the N-terminal part of the receptor. These observations suggest that the GluR1 subunit consists of at least two isoforms possessing a common N-terminal region but a distinct C-terminus. Immunocytochemistry, using immunoperoxidase staining, was performed for the GluR1 subunit in rat forebrain with antisera raised against the N-terminal (253-267) and the C-terminal parts (877-889) of the molecule. Both antisera gave a similar distribution of immunoreactivity at the light-microscopic level. Immunoreactivity for the GluR1 subunit was selectively distributed throughout the rat forebrain. The hippocampus, septum, amygdala and olfactory bulb exhibited the strongest immunoreactivity.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic enhancement induced by NMDA and Qp receptors and presynaptic activity

Activity-dependent changes in synaptic efficacy may underlie the acquisition of memory. Much interest has centred on the involvement of NMDA (N-methyl-D-aspartate) receptors in the induction of long-term potentiation (LTP), but since activation of this receptor alone results only in short-term enhancement, other receptors are clearly involved. This paper describes a novel form of lasting synaptic enhancement requiring concomitant activation of both NMDA and metabotropic quisqualate, or Qp receptors for induction. Neither high frequency presynaptic activity, co-activation of many presynaptic axons, postsynaptic depolarization, nor blockade of inhibition were required. However, repetitive, though relatively low frequency activity of the presynaptic axon appeared to be necessary. We suggest, tentatively, that presynaptic activity may be directly involved in priming the recently active terminal to respond to postsynaptic changes.