Patterned stimulation at the theta frequency is optimal for the induction of hippocampal long-term potentiation

Extracellular phosphorylation of membrane protein modifies theta burst-induced long-term potentiation in CA1 neurons of guinea-pig hippocampal slices

The involvement of ecto-protein kinase activity in activity-dependent long-term potentiation (LTP) was studied in CA1 neurons of guinea-pig hippocampal slices. Application of 5 microM K-252b, an ecto-protein kinase inhibitor, blocked LTP induced by a theta-burst stimulation (3 bursts composed of 5 pulses at 100 Hz with inter-burst intervals of 200 ms). On the other hand, under 10 microM RK682, an ecto-phosphatase inhibitor, a robust LTP was induced by a weak theta-burst stimulation (3 bursts composed of 3 pulses) which was just at the threshold for the induction of LTP in the control perfusate. These findings suggest that ATP released from presynaptic terminals during the burst stimulation plays an important role in the induction of LTP through phosphorylation of extracellular domains of synaptic membrane proteins, as the substrate for ecto-protein kinase.

A genetic deficiency in calpastatin and isovalerylcarnitine treatment is associated with enhanced hippocampal long-term potentiation

The Milan hypertensive strain (MHS) of rats, in addition to having hypertension, is also characterized by a genetic deficiency in calpastatin, the endogenous inhibitor of calpain. Since this protease has been implicated in long-term potentiation (LTP), we have investigated whether induction of this form of plasticity was altered in this strain of rats as compared to control animals (Milan normotensive strain, MNS). Progressive induction of LTP by increasing numbers of high frequency trains resulted in a greater degree of potentiation measured with all inducing protocols in MHS as compared with MNS animals. This difference was not related to the hypertension, since another hypertensive strain (the SHR strain) and a segregated Milan hypertensive strain, expressing only the hypertension but not the calpastatin deficiency (the MHNE strain), exhibited an LTP indistinguishable from control rats. Treatment of MHNE rats for 2 months with isovalerylcarnitine, a compound that increases calpain activity, also resulted in a greater amount of LTP induced by high frequency trains. These effects were not related to an enhancement of the NMDA receptor dependent component of responses to burst stimulation. These results are consistent with the idea that conditions under which calpain activation is facilitated are associated with a greater degree of synaptic potentiation.

Spontaneous potentiation of focal potentials of the CA1 field in long-surviving hippocampal slices of the rat in the absence of electrical stimulation

The changes in the efficiency of the synaptic transmission in the Schaffer collaterals-field CA1 neurons pathways following prolonged deprivation of afferentation, the cessation (up to four hours) of stimulation of the tested input with brief series of tests every hour (the control afferent inputs, were stimulated throughout the entire experiment at a frequency of 0.05/sec), were investigated in surviving slices of the hippocampus of rats. The evoked focal potentials, population spikes (PS) and population excitatory postsynaptic potentials (pEPSP) were recorded. The prolonged cessation of stimulation led to a significant (p < 0.001) increase in the amplitude of the PS (up to 208% in relation to the baseline level). This phenomenon of potentiation was specific in relation to input and exhibited the properties of "E-S potentiation". A hypothesis is advanced regarding the association of mechanisms responsible for the development of "deprivational" potentiation and of the late phase of long-term posttetanic potentiation.

Alterations in the immunoreactivity for muscarinic acetylcholine receptors and colocalized PKC gamma in mouse hippocampus induced by spatial discrimination learning

This study describes changes in the immunoreactivity for muscarinic acetylcholine receptors (mAChRs) in the hippocampus of mice in relation to spatial discrimination behavior, employing the monoclonal antibody M35 raised against purified bovine mAChR protein. Performance in a hole board in which the animals learned the pattern of 4 baited holes out of 16 holes served as the measure of spatial discrimination learning and memory. Twenty-six adult male house mice were used, divided into four groups. Three groups served as various controls: group N (naive; blank controls); group H (habituated; animals were introduced to the hole board with all holes baited for 5 consecutive days), and group P (pseudo-trained; the animals were admitted to the hole board for 13 consecutive days with all holes baited). The T group (trained) was subjected to the hole board for 5 consecutive habituation days with all holes baited (similar to the H and P groups), followed by 8 successive training days with only four holes baited in a fixed pattern. During the 8 training days, the T group gradually acquired a pattern to visit the baited holes, whereas the P group continued visiting holes in a random fashion. The mice were killed 24 h after the last behavioral session. All principal cells in teh cornu ammonis (CA) and dentate gyrus (DG) of the habituated animals revealed increased levels of mAChR immunoreactivity (mAChR-ir) over the naive mice. A minor increase in mAChR-ir was found in the apical dendrites of the CA1 pyramidal cells. Pseudotraining resulted in a CA1-CA2 region with a low level of mAChR-ir, resembling naive animals, whereas the trained mice showed a further increase in mAChR-ir in the CA1-CA2 pyramidal cell bodies and apical dendrites. Optical density measures of the mAChR-ir in the CA1 region revealed a significant (P < 0.05) increase in the pyramidal cell bodies of the H and T group over the N and P group, and a significant (P < 0.05) increase in the apical dendrites of the T group over all other groups. In contrast to the CA1-CA2 region, both pseudotrained and trained mice revealed high mAChR staining in the CA3-CA4 region and the DG. These results indicate that prolonged exposure to the hole board is sufficient for an enhanced mAChR-ir in the CA3-CA4 and DG, whereas the increase in CA1-CA2 pyramidal cells is a training-specific feature related to spatial orientation. Nonpyramidal neurons within the CA1-CA2 region with enhanced mAChR-ir in the pyramidal cells, however, revealed a decreased level of mAChR-ir. The opposing effect of pyramidal and nonpyramidal cells suggests a shift in the excitability of the hippocampal microcircuitry. Previously we demonstrated an increase and redistribution of hippocampal protein kinase C gamma-immunoreactivity (PKC gamma-ir) induced by hole board learning in mice (Van der Zee et al., 1992, J Neurosci 12:4808-4815). Immunofluorescence double-labeling experiments conducted in the present study in naive and trained animals revealed that the principal cells and DG interneurons co-express mAChRs and PKC gamma, and that the immunoreactivity for both markers increased in relation to spatial orientation within these neurons. The mAChR-positive nonpyramidal cells of the CA1-CA2 region were devoid of PKC gamma and revealed an opposite training-induced effect. These results suggest that the postsynaptic changes in mAChR- and PKC gamma-ir reflect functional alterations of the hippocampal formation induced by spatial learning.

Effect of stress and long-term potentiation (LTP) on subsequent LTP and the theta burst response in the dentate gyrus

Exposure to an aversive and stressful event is reported to have similar effects on hippocampal plasticity and behavior as does exposure to high-frequency stimulation of the hippocampus. Here we directly compared the effects of exposure to a stressor vs. a previous induction of LTP on a subsequent induction of LTP and the extracellular response to a tetanus patterned after endogenous theta rhythms. Stimulating the dentate gyrus via the perforant path, Sprague-Dawley rats (n = 65) were tetanized 2 h after exposure to a stressor consisting of restraint and 60, 1, s, 1 mA tail shocks. Unstressed controls were tetanized once and then again 2 h later. Exposure to the stressor impaired LTP of the EPSP 2 h later, as did a previous induction of LTP. In addition, exposure to the stressor altered the extracellular response to subsequent theta burst stimulation (10, 40 ms bursts at 100 Hz, each separated by 200 ms), as did a previous induction of LTP. Whereas unstressed rats exposed to the first tetanus exhibited a marked decline in the amplitude across successive bursts, stressed rats exhibited no such decline, a response pattern similar to that observed in unstressed rats exposed to a second tetanus. The similarity between the effects of stress and tetanic stimulation on hippocampal plasticity support the hypothesis that stress and LTP are converging on similar neuronal mechanisms.

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.

Centrally active modulators of glutamate receptors facilitate the induction of long-term potentiation in vivo

An experimental drug, 1-(1,3-benzodioxol-5-ylcarbonyl)piperidine, that facilitates glutamatergic transmission in brain after systemic administration was tested for its effects on the induction of long-term potentiation in the hippocampus of rats. Intraperitoneal injections of the drug markedly increased the degree and duration of long-term potentiation; similar results were obtained with an analogue of 1-(1,3-benzodioxol-5-ylcarbonyl)piperidine that was also found to improve retention of memory in a radial maze task and in an odor-matching problem. These results define tools for enhancing long-term potentiation in vivo and confirm an important prediction from the hypothesis that long-term potentiation is a substrate of memory.

EPSP-spike potentiation during primed burst-induced long-term potentiation in the CA1 region of rat hippocampal slices

Long-term potentiation induced by high-frequency stimulation in the CA1 region of the hippocampus exhibits EPSP-spike potentiation. This consists of an increase in population spike amplitude exceeding that predicted by EPSP potentiation alone. This phenomenon is apparently due to an increase in pyramidal cell excitability. Patterns of afferent stimuli which activate pyramidal cells to reproduce the theta rhythm observed in the hippocampus under physiological conditions, have been shown to induce LTP-like enhancement of synaptic responses in vitro. The aim of this study was to investigate the presence of EPSP-spike potentiation and/or changes in pyramidal cell excitability during the long-term potentiation induced in the CA1 region of rat hippocampal slices by theta-like patterns of stimuli: the primed burst and the patterned stimulation. Using extracellular recording, a significant leftward shift in the EPSP-spike relationship was found 30 min after primed burst or patterned stimulation. The magnitude of EPSP-spike potentiation induced by patterned stimulation was similar to that produced by high-frequency stimulation. Both were significantly greater than that induced by a primed burst, indicating that only a subset of pyramidal cells were potentiated by this kind of afferent activation. Modifications in synaptic efficacy and cell excitability brought about by a primed burst were investigated in 25 intracellularly recorded pyramidal cells. Consistent with extracellular results, it was found that only 11 out of 25 neurons receiving a primed burst were potentiated. In these cells the increase in probability of firing action potentials elicited by synaptic activation with test shocks was accompanied by enhanced cell excitability, but not by an increase in EPSP slope. High-frequency stimulation delivered 40 min after a primed burst invariably increased the EPSP slope, the probability of firing upon synaptic stimulation, and the excitability of cells. The presence of EPSP-spike potentiation and of increased excitability of potentiated cells during the primed burst-induced long-term potentiation strengthen the suggestion that theta pattern-induced synaptic potentiation can be considered similar to high-frequency stimulation and long-term potentiation and supports the notion that the EPSP-spike potentiation is a constitutive characteristic of long-term potentiation.

Dendritic Ca2+ accumulations and metabotropic glutamate receptor activation associated with an N-methyl-D-aspartate receptor-independent long-term potentiation in hippocampal CA1 neurons

Bathing hippocampal slices in the potassium channel blocker tetraethylammonium (TEA), while stimulating the Schaffer collaterals at a low frequency, induces Ca(2+)-dependent, N-methyl-D-aspartate (NMDA) receptor-independent long-term potentiation of synaptic transmission (LTPk) in CA1 neurons. We have combined ratio imaging of fura-2 and mag-fura-5 in hippocampal CA1 neurons with intracellular and field recordings to evaluate postsynaptic Ca2+ changes that occur in the induction of LTPk. Test stimuli were applied at 0.05 Hz to stratum radiatum in the presence of the NMDA receptor antagonists D,L-2-amino-5-phosphonovaleric acid (100 microM) or MK-801 (10 microM). During TEA exposure (15-25 mM; 10 min), cells fired prolonged action potentials both spontaneously and in response to test stimuli resulting in transient, micromolar Ca2+ accumulations in both somata and dendrites. The initial EPSP slope, measured 60 min after TEA wash-out, was potentiated to approximately 200% of control. The Ca2+ channel blocker nimodipine (10 microM) greatly reduced Ca2+ transients in both magnitude and duration and prevented LTPk induction. Pretreatment of slices with compounds that block metabotropic glutamate receptor (mGluR)-stimulated phosphoinositide hydrolysis, L-2-amino-3-phosphonopropionic acid (L-AP3, 50-200 microM) or L-aspartate-beta-hydroxamate (50-100 microM), as well as protein kinase C (PKC) inhibitors (sphingosine, 20 microM; RO-31-8220, 0.2 microM; or calphostin C, 2 microM) also blocked LTPk. Ca2+ transients were unaffected by L-AP3 or RO-31-8220. These findings suggest that Ca2+ influx through voltage-gated channels and co-activation of PKC by mGluRs are both necessary for induction of LTPk. Activation of mGluRs must also occur in NMDA receptor-dependent induction paradigms, but is possibly of lesser importance owing to the much greater gating of Ca2+ directly into the dendritic spines.

Fimbria-fornix lesions impair spatial performance and induce epileptic-like activity but do not affect long-term potentiation in the CA1 region of rat hippocampal slices

Groups of rats were given bilateral fimbria-fornix lesions and one month later grafted into the hippocampus with fetal cholinergic and non-cholinergic (hippocampal) neural tissue. Three weeks and 3 months after transplantation the animals were trained to find and then to retain the location of a hidden platform in the Morris water maze. After the final behavioral testing phase, electrophysiological studies of the short- and long-term potentiation (STP and LTP) and epileptiform activity of evoked responses were performed in vitro in the CA1 region of the hippocampus. The lesions produced a marked deficit in spatial function in the early testing phase which showed some recovery at the three month time point. Neither the cholinergic nor the non-cholinergic grafts improved spatial performance; indeed, on some measures these groups showed a significantly greater deficit than the lesion-alone group. Epileptiform activity, which was defined as the ratio of the sum of amplitudes of second and third population spikes to the amplitude of the first, before tetanization was not significantly different for all groups. After tetanization of the radiatum input, however, the epileptiform activity in the FFL group was significantly higher in comparison to that of the control groups. Grafting of cholinergic tissue decreased this parameter to the control level, but non-cholinergic grafts did not modify the lesion-induced epileptiform activity. Epileptiform activity after tetanization of the oriens input was approximately equal for all groups. There were no significant differences between surgical groups in STP and LTP for both the oriens and radiatum inputs.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of maternal lead exposure on functional plasticity in the visual cortex and hippocampus of immature rats

We examined the amount of long-term potentiation (LTP) in slices from the visual cortex and hippocampus of pre- and postnatally lead-exposed rats and controls at postnatal days (PND) 12-20. A dietary lead intake of 750 ppm by the dams resulted in a mean blood lead concentration in the suckling offspring of about 17' micrograms/dl. While high-frequency stimulation (HFS) of the white matter induced LTP of the field potentials in layers II/III in cortical slices of ten out of the 14 control rats, only three of the twelve lead-exposed rats showed a small amount of LTP. However, in slices from seven of the twelve lead-exposed rats a long-term depression was found following HFS. Furthermore, paired-pulse inhibition was weaker in cortical slices from the lead-exposed as compared to the control rats. In the CA1 hippocampal region the amount of LTP was significantly reduced in the lead-exposed group only in slices taken from rats at PND 16-20, while no differences were seen in slices from younger animals. It is concluded that even low level lead exposure impairs functions of the visual cortex in the immature rat. We suggest that the developing hippocampus is able to compensate for lead-induced functional deficits in the 2nd postnatal week, being more vulnerable at older ages.

Endothelial NOS and the blockade of LTP by NOS inhibitors in mice lacking neuronal NOS

Long-term potentiation (LTP) is a persistent increase in synaptic strength implicated in certain forms of learning and memory. In the CA1 region of the hippocampus, LTP is thought to involve the release of one or more retrograde messengers from the postsynaptic cell that act on the presynaptic terminal to enhance transmitter release. One candidate retrograde messenger is the membrane-permeant gas nitric oxide (NO), which in the brain is released after activation of the neuronal-specific NO synthase isoform (nNOS). To assess the importance of NO in hippocampal synaptic plasticity, LTP was examined in mice where the gene encoding nNOS was disrupted by gene targeting. In nNOS- mice, LTP induced by weak intensity tetanic stimulation was normal except for a slight reduction in comparison to that in wild-type mice and was blocked by NOS inhibitors, just as it was in wild-type mice. Immunocytochemical studies indicate that in the nNOS- mice as in wild-type mice, the endothelial form of NOS (eNOS) is expressed in CA1 neurons. These findings suggest that eNOS, rather than nNOS, generates NO within the postsynaptic cell during LTP.

Serotonin controls the magnitude of LTP induced by theta bursts via an action on NMDA-receptor-mediated responses

The present studies examined the inhibitory effects of serotonin (5-HT) on LTP in the context of the theta burst stimulation paradigm and its known relationship to the induction chemistries of LTP. Comparisons were made between the effects of various dosages of 5-HT on: (i) the extent to which the second member of a pair of theta bursts was facilitated over the first member of the pair; and (ii) the degree of LTP produced by the paired bursts. Both LTP and burst facilitation were affected in a graded manner by the drug: at high concentrations LTP was completely blocked and burst enhancement was minimal, at lower dosages LTP stabilized at a reduced level while burst responses showed substantial but still impaired facilitation. The competitive antagonist AP-5 was then used to test if 5-HT blocked the NMDA receptor mediated synaptic currents which normally occur during the facilitated burst responses. AP-5 had no effect on the size of burst responses in slices pre-treated with 5-HT indicating that serotonin suppressed the activation of the NMDA receptors by theta stimulation. Serotonin depressed the facilitation of burst responses in slices pre-treated with AP-5 indicating that it also reduces the enhanced AMPA receptor mediated currents that occur during theta pattern stimulation. These results are discussed in terms of the known effects of serotonin on hippocampal physiology and how these might interact with the machinery whereby theta stimulation activates NMDA receptor mediated currents.

(RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) does not block theta burst-induced long-term potentiation in area CA1 of rat hippocampal slices

We have used the selective metabotropic glutamate receptor antagonist (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) to investigate in the CA1 hippocampal subregion in vitro whether coactivation of N-methyl-D-aspartate (NMDA) and metabotropic glutamate receptors is necessary for the induction of long-term potentiation (LTP) when LTP is induced by theta burst stimulation (TBS). When MCPG (500 microM) was bath applied 14-30 min prior to a triple high-frequency tetanization (100 Hz, 1 s) and washed out immediately afterwards the potentiation of the extracellularly recorded field potentials decayed gradually to baseline (P < 0.05) over 2-3 h. However, when MCPG was applied in the same manner before a triple TBS (10 bursts at 5 Hz, 100 Hz within the bursts) the resulting potentiation remained stable for at least 4 h. MCPG had no effect on baseline synaptic transmission or post-tetanic potentiation. These results demonstrate a clear difference in the mechanisms underlying these two different forms of LTP.

Baclofen produces dose-related working memory impairments after intraseptal injection

Altering the activity of the septohippocampal pathway can impair spatial memory in rats. Pharmacological manipulation of septal GABA-A receptors with the agonist, muscimol, or the benzodiazepine agonist, chlordiazepoxide, also impairs spatial memory and depresses hippocampal cholinergic activity. The present experiment examined the effects of intraseptal infusion of the GABA-B agonist baclofen on the performance of rats on a working memory radial arm maze (RAM) task. Post-training administration of baclofen (3 nmol, but not 1.5 or 0.75 nmol) produced a significant impairment of RAM performance. Baclofen significantly reduced the number of correct choices and increased the number of errors committed during testing without affecting latency per arm choice or the ability of the rats to navigate the maze and consume food pellets. The data suggest that baclofen impaired retention of the task without producing proactive performance deficits. Furthermore, the present data are consistent with the hypothesis that a GABAergic mechanism in the medial septum modulates the maintenance or retrieval of spatial working memory.

Resonant activation of calcium signal transduction in neurons

The relevant parameters of calcium fluxes mediating activation of immediate-early genes and the collapse of growth cones in mouse DRG neurons in response to action potentials delivered in different temporal patterns were measured in a multicompartment cell culture preparation using digital fluorescence videomicroscopy. Growth cone collapse was produced by trains of action potentials causing a large rise in [Ca2+]i, but after chronic exposure to patterned stimulation growth cones regenerated and became insensitive to the stimulus-induced increase in [Ca2+]i. Calcium reached similar peak concentrations, but the [Ca2+]i increased more slowly than in naive growth cones (time constant of 6.0 s versus 1.4 s in naive growth cones). Semiquantitative PCR measurements of gene expression showed that pulsed stimulation delivered at 1-min intervals for 30 min induced expression of c-fos, but the same total number of action potentials delivered at 2-min intervals failed to induce c-fos expression, even though this stimulus induces a larger peak [Ca2+]i than the effective stimulus pattern. The experiments suggest that the kinetics of calcium fluxes produced by different patterns of stimulation, and changes in the kinetics of calcium flux in neurons under different states of activation, are critical in determining the effects of action potentials on growth cone motility or expression of IE genes during development of neuronal circuits. We propose that differences in kinetics of individual reactions in the stimulus-response pathway may lead to resonance of activation in the neuron, such that certain processes will be selectively activated by particular temporal patterns of stimulation.

Origins of the variations in long-term potentiation between synapses in the basal versus apical dendrites of hippocampal neurons

Responses to theta pattern stimulation, and the long-term potentiation (LTP) they induce, were compared in the basal versus apical dendrites of neurons in field CA1 of hippocampus. A series of 10 theta bursts produced more than twice as much LTP in basal synapses as in their apical counterparts as measured with field EPSPs. This confirms earlier field potential studies showing that the maximum degree of potentiation (the LTP ceiling) is considerably greater in stratum oriens than stratum radiatum. Experiments with whole-cell clamp recording obtained similar results, indicating that synapses at different loci on the same neuron reach different LTP ceilings following prolonged theta burst stimulation. The basal synapses also required fewer theta bursts to reach their LTP ceiling than did the apical synapses. Tests with paired-pulse facilitation and an antagonist of the NMDA receptor gave no indication that the greater LTP in basal synapses was qualitatively different from the lesser effect obtained in apical contacts. Intracellular recording revealed significant differences between basal versus apical responses to single theta bursts and trains of bursts: the within-burst depolarization was greater and the between-burst hyperpolarization was smaller for the basal dendritic responses. These two variables have previously been proposed to influence the magnitude of LTP and the observed differences between basal versus apical synapses are in accord with this hypothesis. Together with recently described immunocytochemical results, the findings reported here suggest that variations in LTP across dendritic subfields of hippocampus reflect a differential distribution of a subclass of GABAergic interneurons.

Targeting learning

Novel transgenic approaches provide an exciting opportunity to assess the impact of the loss of specific genes in the biochemistry and electrophysiology of neurons involved in a learned behavior. Recent studies describing mice harboring mutations in five kinase genes expressed in the hippocampus found that two of these kinases, the alpha-Ca(2+)-calmodulin-dependent kinase II and the Fyn tyrosine kinase are necessary for the establishment of long-term potentiation. In addition to providing a new tool for the dissection of the molecular mechanisms of synaptic plasticity, these mutants will be important in determining how changes in synaptic strength affect not only learning and memory, but also a host of other processes thought to be associated with plasticity.

2,3-diphosphoglyceric acid blocks long-term potentiation of excitatory postsynaptic currents in hippocampal CA1 neurons of the rat

The dependence of long-term potentiation on an intact inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] metabolism was investigated with the whole-cell voltage-clamp method in the CA1 region of hippocampal slices. The intracellular application of 2,3-diphosphoglyceric acid (1 mM), an inhibitor of the Ins(1,4,5)P3 5-phosphatase and of Ins(1,4,5)P3 3-kinase eliminated the potentiation of postsynaptic currents in pyramidal cells 30 min after paired pre- and postsynaptic activation. These data suggest a possible role of postsynaptic inositol 1,4-bisphosphate and/or inositol 1,3,4,5-tetra-kisphosphate in synaptic plasticity.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

The characteristics and pharmacology of olfactory cortical LTP induced by theta-burst high frequency stimulation and 1S,3R-ACPD

Long-term potentiation (LTP) of monosynaptic excitations in the olfactory cortex slice by theta burst high frequency stimulation (theta-HFS) and application of the metabotropic glutamate receptor-selective agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) has been studied. Theta-HFS optimally induced LTP when given 4 times at intervals of 20-30 min. The degree of LTP was significantly potentiated by the inclusion of picrotoxin in the perfusion medium but induction was prevented by D-2-amino-5-phosphonopentanoate (25 microM), L-2-amino-3-phosphonopropionate (125 microM), 5-(isoquinolinyl-sulphonyl)-2-methylpiperazine (50 microM), sangivamycin (25 microM) and thapsigargin (1 microM). Of the drugs tested, only D-2-amino-5-phosphonopentanoate failed to depotentiate established LTP. Application of 1S,3R-ACPD (100 microM) repeated 4 times at intervals of 20-30 min also optimally induced an LTP which was significantly less in unstimulated preparations and showed the same pattern of sensitivity to the drugs tested as LTP induced by theta-HFS. It is concluded that the induction of LTP by theta-HFS and 1S,3R-ACPD requires activation of both N-methyl-D-aspartate and metabotropic glutamate receptors and that a protein kinase is essential for the induction and maintenance of LTP. The likely mechanisms of induction and maintenance of olfactory cortical and hippocampal LTP are contrasted.

Temporal pattern sensitivity of long-term potentiation in hippocampal CA1 neurons

High-frequency electrical stimulation in the hippocampus leads to an increase in synaptic efficacy that lasts for many hours. This long-term potentiation (LTP) of synaptic transmission is presumed to play a crucial role in learning and memory in the brain. However, the frequency of stimulation generally used to obtain LTP is beyond the normal physiological range of activity of hippocampal neurons. We found that LTP can be induced by an electrical stimulation whose frequency is comparable to that of the naturally occurring firing activity of hippocampal neurons if the stimulating pulse-interval train has a special time structure. In the present experiment, we compared the magnitude of LTP induced by the four types of stimuli which have the same pulse number and the same mean frequency but different time structure in interstimulus intervals. One type of stimuli has regular intervals, and this served as a control stimulus. In the other three types of stimuli, the adjacent interstimulus interval had the following statistical properties: in type 1, their correlations are positive; in type 2, negative; and in type 3, independent. The magnitude of LTP induced by these four types of stimuli showed clear order relationships: type 3/type 1 >> control > type 2. Detailed analysis of the evoked potential during a period of temporal pattern stimulation revealed that the amplitude of the population spikes of repetitive firing, especially of the second and third population spikes, had the same order relationship as the LTP.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA-receptor-dependent synaptic plasticity: multiple forms and mechanisms

Long-term potentiation in the CA1 region of the hippocampus is the most extensively studied model of activity-dependent synaptic plasticity in the mammalian brain. Its induction normally involves activation of postsynaptic N-methyl-D-aspartate (NMDA) receptors, which are thought to control the occurrence of long-term potentiation at individual synapses. Recent work in the hippocampus indicates that NMDA receptor activation does not necessarily lead to induction of long-term potentiation but instead may elicit a repertoire of distinct forms of synaptic plasticity including short-term potentiation or long-term depression. Furthermore, mechanisms exist such that the induction of long-term potentiation can be inhibited by modest activation of NMDA receptors. Experimental results are beginning to clarify the mechanistic relationships between these different phenomena, although much remains unknown. Whatever their underlying mechanisms, these additional forms of NMDA-receptor-dependent synaptic plasticity confer increased flexibility to neural circuits involved in information processing and storage.

Hippocampus, theta, and spatial memory

The past 18 months have witnessed interesting developments in several areas of hippocampal research. First, the mechanisms of hippocampal theta are becoming clear, as is its role in spatial coding; each theta cycle appears to act as a clock mechanism against which the firing of the place cells can be timed. Second, there has been a continued strengthening in the support for the spatial theory of hippocampal function from single unit and lesion experiments; particularly important is the finding that the deficit in (non-spatial) delayed non-match to sample memory experiments in the monkey following medial temporal lobe damage stems from the part of the cortex which surrounds the hippocampus, and not from the hippocampus itself. Third, in contrast, it is proving more difficult than originally thought to show a causal relationship between long-term potentiation at the synaptic level and place learning-induced changes in hippocampal synapses.

Neuronal transmission of hippocampal CA1 neurones is modulated by corticotropin-like intermediate lobe peptide [CLIP; ACTH(18-39)]

The study was conducted to test whether CLIP [ACTH(18-39)] influences the neuronal transmission and the induction of long-term potentiation (LTP) in the hippocampus. The population spike was recorded in the hippocampal CA1 region of freely moving rats before and after intracerebroventricular (ICV) administration of CLIP in comparison to ACTH and saline (controls). After infusion of CLIP, the population spike amplitude (PSA) rose to about 200% of baseline values. After reaching this level, it was impossible to induce a further increase of PSA by tetanization. However, if the stimulus intensity was reduced to a new baseline level, electrically induced LTP could be observed. There were no significant changes after infusion of ACTH. Our results indicate that the ICV administration of CLIP leads to an enhancement of excitability in the hippocampal CA1 region, which might be independent of LTP.

The suppression of long-term potentiation in rat hippocampus by inhibitors of nitric oxide synthase is temperature and age dependent

At room temperature (23 degrees C-25 degrees C), the induction of long-term potentiation (LTP) in area CA1 of slices from young male Sprague-Dawley rats was depressed by preincubation with the nitric oxide synthase inhibitors NG-nitro-L-arginine (L-NA, 100 microM) and NG-nitro-L-arginine methyl ester (L-NAME, 100 microM). The D isomers were ineffective under the same conditions. Hemoglobin (20 microM) reduced but did not completely block LTP. Neither L-NA (at concentrations up to 1 mM) nor hemoglobin (20 microM) had any significant effect on LTP in slices from adult rats at room temperature, or in young rats at 29 degrees C-30 degrees C. These results suggest that nitric oxide is unlikely to play a role in the induction of LTP under physiological conditions.

Differential effects of benzodiazepine receptor agonists on hippocampal long-term potentiation and spatial learning in the Morris water maze

The amnesic effect of benzodiazepine drugs has been well documented, though the mechanisms mediating this effect are unknown. Long-term potentiation (LTP) has been proposed as a mechanism by which information is stored in the mammalian central nervous system. This experiment sought to determine if benzodiazepines impair mnemonic processes by blocking LTP. Rats implanted with a stimulating electrode in the perforant path and a recording electrode in the dentate gyrus were given high-frequency stimulation after the administration of either chlordiazepoxide (5 mg/kg), diazepam (5 mg/kg) or CL 218,872 (10 mg/kg). None of these drugs completely blocked the induction of LTP as measured by changes in the magnitude of the population spike amplitude, though CL 218,872 significantly suppressed potentiation over the duration of recording (24 h). Moreover, the potentiation observed in diazepam-treated rats returned to baseline after 24 h. Two weeks after the last recording, the same implanted rats were given their previous drug and dose and then tested for spatial learning ability in the Morris water maze. Each drug resulted in a severe impairment of spatial learning, but had no effect on cue learning. Two days later, in the absence of drugs, the same rats readily acquired a reversed platform location. Together these results suggest that CL 218,872 may impair spatial learning by suppressing LTP in the perforant path but that chlordiazepoxide and diazepam can impair spatial learning in the absence of LTP suppression in this pathway.

Long-term potentiation of hippocampal afferents and efferents to prefrontal cortex: implications for associative learning

It has been proposed that the physical substrate of memory resides in alterations of the strengths or weights of modifiable synaptic connections. In recent years, the hypothesis that the mechanisms underlying a particular form of synaptic plasticity, known as long-term potentiation, or LTP, are activated during learning and may actually subserve the formation of associative memories, has gained much empirical support. This paper reviews experimental studies suggesting that changes in synapse physiology and chemistry are involved in the formation of neural associative representation in hippocampal networks during classical conditioning. Recent experiments investigating LTP and learning-induced synaptic changes at hippocampal outputs to the prefrontal cortex are reported. The results provide a working framework within which the dynamics of information storage in hippocampal and prefrontal cortical networks is profiled.

Waveform analysis suggests that LTP alters the kinetics of synaptic receptor channels

The waveform of an isolated excitatory monosynaptic response reflects the kinetics of transmitter release, the kinetics of synaptic receptor channels and the filtering properties of neurons. Results reported here indicate that long-term potentiation (LTP) causes correlated decreases in the rise time and decay time constant of synaptic potentials recorded in hippocampal slices in which inhibitory currents and post-synaptic spiking were suppressed. Statistical comparisons of waveforms revealed that the distortions introduced by LTP could be corrected by stretching the time-scale of potentiated responses according to the percent change in the decay time constant. The LTP associated decrease in the decay time constant also obtained in slices from immature hippocampus which contain spines and dendrites greatly simplified from those of the adult. Hence, filtering properties of spines are not likely involved in the effect. Paired-pulse facilitation (PPF), a transient increase in transmitter release, did not reproduce the waveform effects of LTP but did cause a slight leftward shift of the response. These results suggest that LTP modifies the kinetics of receptor channels, and that PPF accelerates release.

Heightened synaptic plasticity of hippocampal CA1 neurons during a cholinergically induced rhythmic state

Brain cholinergic neurons are critical for memory function and their loss may contribute to memory impairment in Alzheimer's disease. One role of cholinergic neurons is to elicit an oscillatory activity called theta rhythm in the hippocampus, a brain region involved in memory processing. Theta rhythm occurs during periods of learning, but its effect on the synaptic plasticity that underlies learning remains unclear. We have studied synaptic plasticity in hippocampal slices during theta-frequency oscillations induced by a cholinergic agonist. Here we report that during these oscillations, synapses are in a state of heightened plasticity and can be modified by what would otherwise be ineffective stimulation. This heightened plasticity is sensitive to the timing of incoming stimuli with respect to the oscillatory activity. The results suggest that cholinergic systems may affect memory formation through the induction of an oscillatory state in which the requirements for synaptic plasticity are dramatically altered.

Common forms of synaptic plasticity in the hippocampus and neocortex in vitro

Activity-dependent synaptic plasticity in the superficial layers of juvenile cat and adult rat visual neocortex was compared with that in adult rat hippocampal field CA1. Stimulation of neocortical layer IV reliably induced synaptic long-term potentiation (LTP) and long-term depression (LTD) in layer III with precisely the same types of stimulation protocols that were effective in CA1. Neocortical LTP and LTD were specific to the conditioned pathway and, as in the hippocampus, were dependent on activation of N-methyl-D-aspartate receptors. These results provide strong support for the view that common principles may govern experience-dependent synaptic plasticity in CA1 and throughout the superficial layers of the mammalian neocortex.

Heterosynaptic short-term depression of population spike amplitude in the pyramidal layer of the CA1 hippocampal region evoked by a theta-like tetanization

Heterosynaptic short-term depression (STD) of the stratum radiatum and stratum oriens inputs to the CA1 region was studied in rat hippocampal slices. STD was evoked by trains of 1050 impulses with interstimulus interval (ISI) variable from 10 to 700 ms. The STD was found to be very pronounced for tetanizations with ISI around 200 ms, and almost absent for ISI less than 50 ms or more than 500 ms. These data show that theta-like tetanization is an effective pattern not only for induction of the long-term potentiation (LTP), as has been shown previously, but for production of the heterosynaptic STD as well. This implies that heterosynaptic STD can effectively modulate induction of LTP by theta-like tetanization, and plays an important role in differentiation of potentiated pathways. It is discussed that the theta-like tetanization-induced release of ACh is a possible mechanism of the STD.

Theta modulation of neurons of the hippocampus of the rabbit and its interrelationship with other parameters of spontaneous and evoked activity

The reliability of the existing functional criteria of the differentiation of pyramidal ("neurons with complex spikes") and inhibitory ("theta neurons") cells of the hippocampus is examined on the basis of a statistical analysis of the spontaneous and evoked activity of neurons of the hippocampus of the awake rabbit. The analysis shows that the parameters of average frequency, the presence of theta modulation of activity, the behavior of the neurons in situations evoking theta rhythm in the EEG of the hippocampus (inhibition or activation during the effect of sensory stimuli), and the character of the influences of stimulation of the medial septal region of the internal connections of the hippocampus do not permit the reliable identification of different types of neurons of the hippocampus in the awake rabbit. The available data on the functional classification of neurons of the hippocampus are discussed in connection with notions regarding their state in situations associated with the generation of theta rhythm.

The biology and function of rapid eye movement sleep

Rapid eye movement sleep, a stage of sleep that appears to be present in all marsupial and terrestrial placental mammals, was first identified in 1953. Although the brainstem mechanisms responsible for its generation have been clarified, the function of rapid eye movement sleep remains elusive. Recent findings suggest a role in memory processing.

Further studies concerning the role of nitric oxide in LTP induction and maintenance

Nitric oxide (NO) has recently been proposed to act as a retrograde messenger to produce long-term potentiation (LTP) in hippocampal area CA1. This notion is based largely on the absence of LTP when hippocampal slices are incubated in the presence of inhibitors of NO synthase (NOS) or of NO scavengers. In the present study, we tested the effects of such compounds on both the induction and maintenance of LTP in field CA1 of hippocampal slices. Incubation of slices in the presence of N-methyl-L-arginine (MLA) or L-nitro-arginine (LNA), two inhibitors of NOS, or in the presence of hemoglobin (Hb), a NO scavenger, produced a large reduction in the magnitude of LTP induced by a theta burst stimulation (TBS) paradigm. These compounds had no effect on the degree of paired-pulse facilitation but produced a significant reduction of the facilitation of postsynaptic responses occurring during TBS. On the other hand, MLA did not prevent the potentiation induced by application of tetraethylammonium (TEA). These results suggest that the inhibition of LTP produced by these agents could be due to an effect on a physiological mechanism that triggers LTP and not necessarily on an event that follows the triggering step.

Time course of synaptic development in hippocampal organotypic cultures

Using electrophysiological recordings of field potentials, we investigated the time course of synapse formation and maturation in organotypic cultures prepared from neonate animals of different ages. Following explanation, the size of the maximal synaptic responses elicited in area CA1 by stimulation of a small group of CA3 neurons increased progressively during the first three weeks in culture in a way that corresponded to the changes observed in synaptic contact density. Growth of synaptic responses was found to occur much more rapidly in cultures prepared from 8-day-old as compared with 2-day-old rats. Development of synaptic connections between CA3 and CA1 neurones was also faster than between granule cells and CA3 neurones. Acquisition of mature synaptic properties occurred in vitro as indicated by changes in degree of paired-pulse facilitation and the onset of long-term potentiation (LTP) after a few days in culture. The onset of LTP was much faster in cultures prepared from 8-day-old as compared with 2-day-old neonates and corresponded approximately to the 12-14th postnatal day. It is concluded that development proceeds in the cultures with a time course that resembles the in situ situation.

Reversal of LTP by theta frequency stimulation

Reversal of long-term potentiation (LTP) by physiological stimulation was tested in the CA1 field of hippocampal slices. In control medium, a one minute episode of 5 Hz (theta frequency) stimulation beginning 1-3 min after LTP had no effect on the degree of potentiation measured 30 min later. However, in the presence of norepinephrine (200 microM), 5 Hz stimulation reduced LTP by about 30%. Theta frequency stimulation was only effective when administered within 10 min of LTP induction and had no lasting effects on non-potentiated synapses. Stimulation at 1 Hz did not reverse LTP and stimulation at 10 Hz was no more effective than 5 Hz stimulation. LTP could be nearly completely reversed by theta frequency stimulation when potentiation was induced by milder and more naturalistic stimulation patterns. Under these conditions, LTP reversal was blocked by an antagonist of adenosine A1 receptors. These results suggest that the hippocampal theta rhythm promotes both the induction of LTP and its subsequent reversal with the latter process involving activation of adenosine receptors. Reversal of LTP may function to refine or sharpen recently encoded representations.

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

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

Biochemical correlates of long-term potentiation in hippocampal synapses

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

Intraseptal injection of GABA and benzodiazepine receptor ligands alters high-affinity choline transport in the hippocampus

Injection of GABA and benzodiazepine (BDZ) agonists and antagonists into the medial septum produced bidirectional alterations in hippocampal high-affinity choline transport (HAChT). Male Sprague-Dawley rats were injected in the medial septum with either drug vehicle, a BDZ agonist, antagonist, or inverse agonist, or with a GABA-A or GABA-B agonist or antagonist and sacrificed 1 h later for assessment of HAChT in hippocampal synaptosomes. The GABA-A agonist muscimol, the GABA-B agonist baclofen, and the BDZ agonist chlordiazepoxide (CDP) produced dose-related decreases in HAChT 1 h following injection into the septum. The muscimol-induced decrease in HAChT was prevented by prior intraseptal injection of the GABA-A antagonist, bicuculline. Intraseptal injection of GABA-A (bicuculline) or GABA-B (2-hydroxysaclofen) antagonists did not alter HAChT, whereas the BDZ antagonist flumazenil (RO15,1788) and the BDZ inverse agonist methyl-beta-carboline-3-carboxylate (beta-CCM) increased this measure up to 30% in a dose-dependent manner. These results demonstrate that cholinergic neurons in the medial septum can be modulated in a bidirectional way through the pharmacological manipulation of GABA-A, GABA-B, and BDZ receptors. The potential functional and therapeutic consequences of these interactions are discussed.

Hippocampal plasticity induced by primed burst, but not long-term potentiation, stimulation is impaired in area CA1 of aged Fischer 344 rats

The effect of two types of electrical stimulation designed to induce long-lasting plasticity of the Schaffer/commissural inputs to CA1 pyramidal neurons was investigated using in vitro hippocampal slices made from young (3-6 month) and old (24-27 month) Fischer 344 rats. The first stimulation paradigm, primed burst (PB) stimulation, consisted of a total of five physiologically patterned stimuli: a single priming pulse followed 170 ms later by a burst of four pulses at 200 Hz. The second stimulation paradigm, long-term potentiation (LTP) stimulation, consisted of a 200 Hz/1 second train (a total of 200 stimuli). Primed burst and LTP stimulation were equally effective at inducing a lasting increase in the population spike recorded from slices made from young rats. However, the enhancement of population spike amplitude produced by PB, but not LTP, stimulation was significantly less in slices made from old rats. These results suggest that the capacity of the hippocampus to demonstrate long-lasting synaptic plasticity is not altered with age, but that engaging plasticity-inducing mechanisms becomes more difficult. Furthermore, these data suggest that physiologically patterned paradigms for inducing long-lasting synaptic plasticity may more accurately assess the functional status of hippocampal memory encoding mechanisms than does conventional LTP stimulation.

Factors regulating the magnitude of long-term potentiation induced by theta pattern stimulation

Electrical stimulation patterned after the hippocampal theta rhythm produces a robust and stable long-term potentiation (LTP) effect. Pharmacological manipulations were used in the present studies in an effort to relate characteristics of the responses occurring during theta stimulation to the magnitude of potentiation which follows it. Comparisons were made using five or ten bursts of stimulation which respectively induce sub-maximal or near maximal degrees of LTP. DPCPX, a drug that increases release by blocking adenosine A1 receptors, was used to enhance the depolarization produced by individual theta bursts. This resulted in a marked increase in the amount of stable LTP induced by five theta bursts but did not affect that resulting from ten bursts. This finding suggested that depolarization occurring during a burst response influences per burst potentiation but not the ceiling on maximum LTP. Aniracetam, a nootropic drug that enhances responses via an action on glutamate (AMPA) receptors, was used to test this conclusion. Like DPCPX, aniracetam increased the size of the burst response and enhanced the degree of LTP caused by five but not ten theta bursts. Forskolin, an activator of adenylate cyclase, was used to test the effects of blocking the hyperpolarization normally present between theta bursts on the induction of LTP. The drug augmented the degree of LTP resulting from five theta bursts and, in contrast to DPCPX and aniracetam, nearly doubled that obtained with ten bursts. Thus the drug affected both per burst potentiation and the ceiling on LTP. These results are discussed in terms of an hypothesis in which the magnitude of NMDA receptor mediated currents affects the degree of potentiation produced by individual theta bursts while the duration of the currents is related to the limit on the maximum LTP induced by a series of bursts. The possible implications of the findings for learning are also considered.

Muscimol infused into the medial septal area impairs long-term memory but not short-term memory in inhibitory avoidance, water maze place learning and rewarded alternation tasks

These experiments investigated the effects of injections of muscimol (1 or 5 nmol), administered into the medial septal area prior to training, on memory tested at different retention delays after training in 3 tasks: an inhibitory avoidance task, a one-trial place learning task, and a rewarded alternation task. In all 3 tasks, intraseptal injections of muscimol did not impair memory performance at short retention delays, but impaired memory at the longer retention delays. These findings are consistent with the view that GABAergic regulation of the septohippocampal cholinergic system plays a selective role in the establishment of long-term memory.

Theta burst stimulation is optimal for induction of LTP at both apical and basal dendritic synapses on hippocampal CA1 neurons

The efficacy of stimulation patterns consisting of brief high frequency bursts repeated at various intervals to induce long-term potentiation (LTP) at synapses on apical and basal dendrites of CA1 hippocampal neurons was tested in vitro. Both apical and basal dendritic synapses exhibited maximal LTP after bursts repeated at 5-10 Hz, i.e. close to the frequency of the endogenous hippocampal theta rhythm. As at apical dendritic synapses, LTP at basal dendritic synapses was blocked by an antagonist of NMDA receptors. Basal dendritic LTP was significantly greater in magnitude than apical dendritic LTP, although the reason for this is unknown.

Increasing binding affinity of agonists to glutamate receptors increases synaptic responses at glutamatergic synapses

This study examined the relationship between the affinity of glutamate agonists for the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and the characteristics of the physiological responses elicited by endogenous activation of the AMPA receptors. We tested the effects of chaotropic ions on [3H]AMPA binding in synaptic membranes as well as on synaptic responses elicited in CA1 by electrical stimulation of the Schaffer/commissural pathway in the in vitro hippocampal slice preparation. Of the chaotropic ions tested, only perchlorate and thiocyanate produced large increases in [3H]AMPA binding to synaptic membranes. The effect was due to an increase in affinity for agonists, as shown by a shift of the displacement curves of 6-cyano-7-nitro[3H]-quinoxaline-2,3-dione binding by AMPA or glutamate. The effect of thiocyanate on [3H]AMPA binding was extremely sensitive to temperature, as the binding was increased almost 10-fold at 0 degree C but only 2- to 3-fold at 35 degrees C. The effect of perchlorate was only weakly temperature dependent. Similarly, thiocyanate and perchlorate were the only chaotropic ions tested that increased the initial slope and amplitude of the extracellularly recorded potentials evoked in CA1 dendritic field. Both ions did not change paired-pulse facilitation, an index of transmitter release, or fiber volley amplitude, an index of afferent recruitment. The chaotropic ions had no significant effects on either [3H]glutamate binding to the N-methyl-D-aspartate receptor or N-methyl-D-aspartate receptor-mediated synaptic responses. Finally, the effect of perchlorate on synaptic responses was significantly reduced after induction of long-term potentiation. These results indicate that an increase in affinity of the AMPA receptors for their agonists results in increased synaptic responses and strongly suggest that characteristics of the AMPA receptor are modified following long-term potentiation.

Acute stress impairs (or induces) synaptic long-term potentiation (LTP) but does not affect paired-pulse facilitation in the stratum radiatum of rat hippocampus

Rats were exposed to restraint coupled with 60, 1-sec, 1-mA, 60-Hz tail shocks. One hippocampus was immediately dissected for in vitro measurement of paired-pulse facilitation and LTP of the excitatory postsynaptic potential (EPSP) recording from the stratum radiatum of field CA1. There was no change in paired-pulse facilitation, suggesting that acute exposure to the stressor does not result in a decrease in presynaptic neurotransmitter release. There was, however, a significant decrease in the percent LTP produced by theta burst stimulation relative to naive controls. These results are consistent with the hypothesis that the stress-induced impairment of LTP is a result of changes in the postsynaptic glutamate receptors, specifically the AMPA type.