Potassium-induced long-term potentiation in area CA1 of the hippocampus involves phospholipase activation

Previous studies have shown that potassium-induced long-term potentiation (LTP) of the Schaffer collateral/commissural synapses in area CA1 of the hippocampus shares common properties with tetanus-induced LTP. In the present investigation, we performed electrophysiological and binding experiments on CA1 hippocampal slices to evaluate the location and nature of the changes underlying potassium-induced LTP. Paired-pulse facilitation, which represents an index of transmitter release, was markedly reduced by potassium-induced LTP. In addition, KC1-induced LTP was associated with an increase in ³H-AMPA ([³H]-amino-3-hydroxy-5-methylisoxazole-4-propionate) binding to CA1 synaptic membranes when measured 40 min after high-potassium exposure; however, no changes were detected in binding of an antagonist ([³H]-6-cyano-7-nitroquinoxaline-2,3-dione; ³H-CNQX) to AMPA receptors in slices expressing KC1-induced LTP. Administration of the phospholipase A₂ (PLA₂) inhibitor bromophenacyl bromide (BPB) prior to potassium application prevented LTP formation as well as the changes in paired-pulse facilitation and ³H-AMPA binding that characterized this type of potentiation. Taken together, these data indicate that potassium-in-duced LTP may be related to modifications in both pre-and postsynaptic properties and confirm the hypothesis that PLA₂ activation is an important mechanism in long-term changes of synaptic operation. © 1994 Wiley-Liss, Inc.     

Synaptic potentiation induces increased glial coverage of excitatory synapses in CA1 hippocampus

Patterns of activity that induce synaptic plasticity at excitatory synapses, such as long‐term potentiation, result in structural remodeling of the postsynaptic spine, comprising an enlargement of the spine head and reorganization of the postsynaptic density (PSD). Furthermore, spine synapses represent complex functional units in which interaction between the presynaptic varicosity and the postsynaptic spine is also modulated by surrounding astroglial processes. To investigate how activity patterns could affect the morphological interplay between these three partners, we used an electron microscopic (EM) approach and 3D reconstructions of excitatory synapses to study the activity‐related morphological changes underlying induction of synaptic potentiation by theta burst stimulation or brief oxygen/glucose deprivation episodes in hippocampal organotypic slice cultures. EM analyses demonstrated that the typical glia‐synapse organization described in in vivo rat hippocampus is perfectly preserved and comparable in organotypic slice cultures. Three‐dimensional reconstructions of synapses, classified as simple or complex depending upon PSD organization, showed significant changes following induction of synaptic potentiation using both protocols. The spine head volume and the area of the PSD significantly enlarged 30 min and 1 h after stimulation, particularly in large synapses with complex PSD, an effect that was associated with a concomitant enlargement of presynaptic terminals. Furthermore, synaptic activity induced a pronounced increase of the glial coverage of both pre‐ and postsynaptic structures, these changes being prevented by application of the NMDA receptor antagonist D‐2‐amino‐5‐phosphonopentanoic acid. These data reveal dynamic, activity‐dependent interactions between glial processes and pre‐ and postsynaptic partners and suggest that glia can participate in activity‐induced structural synapse remodeling. © 2009 Wiley‐Liss, Inc.     

Stimulus-dependent induction of long-term potentiation in CA1 area of the hippocampus: Experiment and model

In the CA1 area of the hippocampus, the magnitude of long-term potentiation (LTP) depends not only on the frequency of applied stimuli, but also on their number. With a slice preparation using extracellular recording in the hippocampus CA1 of a guinea pig, we investigate the magnitude of LTP induced by electrical stimuli with a range of frequencies and the number of applied stimuli. We find that the magnitude of the saturated potentiation obtained with periodic stimuli largely depends on the frequency and is insensitive to the number of stimuli, once the saturation level has been obtained. Furthermore, we investigated nonperiodic stimuli and found that the magnitude of the saturated potentiation is also sensitive to the statistical correlation between successive interstimulus intervals, even when their average frequency is held constant. In order to explain the LTP dependence on these various experimental parameters, we propose a simple mathematical model for the induction of LTP. In the model, an exponentially decaying element released as a result of previous stimuli is coupled with a new stimulus to act as the potentiation force, and the magnitude of potentiation is determined by this potentiation force. We can determine the decaying time constant of this hypothetical element as a model parameter by fitting the model to the experimental data. The time scale is found to be of the order of 200 msc. A molecular or cellular factor with this decaying time constant is likely to be induced in LTP induction. Hippocampus 7:416–426, 1997. © 1997 Wiley-Liss, Inc.     

Long-term potentiation is associated with increased [3H]AMPA binding in rat hippocampus.

The location and nature of the changes underlying long-term potentiation (LTP) remain controversial issues. In this study, we tested the possibility that changes in binding properties of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA/quisqualate and N-methyl-D-aspartate (NMDA) subtype of glutamate receptors are associated with LTP. LTP was elicited in vivo by stimulation of the perforant pathway in anesthetized rats. One hour following stimulation the animals were sacrificed. We performed quantitative ligand binding autoradiography on frozen brain sections using [3H]AMPA and [3H]N-(1-(2-thienyl)cyclohexyl)-3,4-piperidine ([3H]TCP) to label the AMPA/quisqualate and the NMDA receptors, respectively. No changes in [3H]TCP binding were detected in any of the treatment groups. However, increases in [3H]AMPA binding were observed only in animals that exhibited LTP. These increases were bilateral and present in several subfields of the hippocampus and cortical areas. Administration of the NMDA receptor antagonist, ketamine, prior to tetanic stimulation prevented both the increase in binding and the induction of LTP. These results suggest that changes in the characteristics of AMPA/quisqualate receptors are a biochemical correlate of LTP.     

High-frequency septal stimulation suppresses long-term potentiation (LTP) in the CA1 region of rat hippocampus

The effects of high-frequency stimulation (HFS) of the medial septum/diagonal band (MSDB) on long-term potentiation (LTP) of CA1 extracellular field potentials were assessed in anesthetized rats. Ten rats received HFS of the Schaffer collateral pathway alone, and 10 received MSDB HFS 10 min prior to hippocampal HFS. Septal HFS suppressed LTP development assessed by change in population spike (PS) amplitude 60 min after hippocampal HFS (ANOVA, P less than 0.03). Septal inhibition of LTP development was most prominent when septal HFS had little direct effect on the CA1 PS. These results provide a novel demonstration of the functional heterogeneity of septohippocampal pathways and in vivo modulation of hippocampal LTP by HFS of natural afferent inputs.     

Characterization of the integration time for the stabilization of long-term potentiation in area CA1 of the hippocampus.

In area CA1 of the hippocampus, synaptic activation of NMDA receptors during postsynaptic depolarization can generate either a decremental synaptic potentiation termed short-term potentiation (STP) or stable, long-term potentiation (LTP). Examining the relationship between these two forms of synaptic enhancement should provide information about the intracellular processes responsible for the stabilization of LTP. Using the hippocampal slice preparation, initial experiments confirmed that STP can be generated either by a weak tetanus or by pairing a single EPSP with postsynaptic depolarization. Following the generation of submaximal LTP, application of a weak, STP-inducing tetanus resulted in STP (not LTP), suggesting that the processes responsible for stabilizing LTP must be activated during induction and cannot be accessed at later times. To determine the interval over which processes activated during STP can be integrated and result in stable LTP (the "integration time" for the stabilization of LTP), a fixed number of afferent stimuli were given at varying intervals (5-60 sec) during postsynaptic depolarization. Using either extracellular or whole-cell recording, LTP was rarely (11% of experiments) elicited at 1 min intervals and frequently (76% of experiments) elicited at 10 sec intervals. These results indicate that following a single EPSP during postsynaptic depolarization, the processes responsible for the stabilization of LTP decay significantly within approximately 1 min, although this value may depend on the level of activation of the requisite intracellular processes.     

Long-term potentiation in the hippocampal CA1 area and dentate gyrus plays different roles in spatial learning

NMDA receptor-dependent long-term potentiation (LTP) at hippocampal synapses has been considered a crucial component of the cellular basis for learning and memory. This form of LTP occurs in excitatory synapses in both the CA1 area and the dentate gyrus in the hippocampus. However, differential roles of LTP in these areas have not yet been identified. To address this issue, we enhanced the degree of LTP by expressing Ca2+-permeable AMPA receptors at either hippocampal CA1 or dentate gyrus synapses using Sindbis viral vectors (SINs) encoding both green fluorescent proteins and unedited GluR2 (GluR2Q) subunits, and examined their effects on rat spatial learning. The viral vectors were locally injected into the 8-week-old-rat brain in vivo bilaterally. The postsynaptic expression of Ca2+-permeable AMPA receptors enhanced the degree of LTP, and induced NMDA receptor-independent LTP in the presence of the NMDA receptor antagonist in SIN-infected regions in both CA1 and dentate gyrus in hippocampal slice preparations. However, the regional expression of Ca2+-permeable AMPA receptors caused opposite behavioural consequences on the Morris water maze task: rats with SIN-infected CA1 pyramidal cells showed shorter escape latency and better probe test performance, whereas those with SIN-infected dentate gyrus granule cells showed impaired performance. Thus, it was demonstrated that CA1 and dentate gyrus synapses play different functional roles in spatial learning despite their similar mechanism for LTP induction.     

Trehalose protects from aggravation of amyloid pathology induced by isoflurane anesthesia in APP(swe) mutant mice

There is an open controversy about the role of surgery and anesthesia in the pathogenesis of Alzheimer's disease (AD). Clinical studies have shown a high prevalence of these procedures in subjects with AD but the interpretation of these studies is difficult because of the co-existence of multiple variables. Experimental studies in vitro and in vivo have shown that small molecular weight volatile anesthetics enhance amyloidogenesis in vitro and produce behavioral deficits and brain lesions similar to those found in patients with AD. We examined the effect of co-treatment with trehalose on isoflurane-induced amyloidogenesis in mice. WT and APP(swe) mice, of 11 months of age, were exposed to 1% isoflurane, 3 times, for 1.5 hours each time and sacrificed 24 hours after their last exposure to isoflurane. The right hemi-brain was used for histological analysis and the contra-lateral hemi-brain used for biochemical studies. In this study, we have shown that repetitive exposure to isoflurane in pre-symptomatic mature APP(swe) mice increases apoptosis in hippocampus and cerebral cortex, enhances astrogliosis and the expression of GFAP and that these effects are prevented by co-treatment with trehalose, a disaccharide with known effects as enhancer of autophagy. We have also confirmed that in our model the co-treatment with trehalose increases the expression of autophagic markers as well as the expression of chaperones. Cotreatment with trehalose reduces the levels of β amyloid peptide aggregates, tau plaques and levels of phospho-tau. Our study, therefore, provides new therapeutic avenues that could help to prevent the putative pro-amyloidogenic properties of small volatile anesthetics.     

Neuropathology of mice carrying mutant APP(swe) and/or PS1(M146L) transgenes: alterations in the p75(NTR) cholinergic basal forebrain septohippocampal pathway

Cholinergic basal forebrain (CBF) projection systems are defective in late Alzheimer's disease (AD). We examined the brains of 12-month-old singly and doubly transgenic mice overexpressing mutant amyloid precursor protein (APP(swe)) and/or presenilin-1 (PS1(M146L)) to investigate the effects of these AD-related genes on plaque and tangle pathology, astrocytic expression, and the CBF projection system. Two types of beta-amyloid (Abeta)-immunoreactive (ir) plaques were observed: type 1 were darkly stained oval and elongated deposits of Abeta, and type 2 were diffuse plaques containing amyloid fibrils. APP(swe) and PS1(M146L) mouse brains contained some type 1 plaques, while the doubly transgenic (APP(swe)/PS1(M146L)) mice displayed a greater abundance of types 1 and 2 plaques. Sections immunostained for the p75 NGF receptor (p75(NTR)) revealed circular patches scattered throughout the cortex and hippocampus of the APP(swe)/PS1(M146L) mice that contained Abeta, were innervated by p75(NTR)-ir neurites, but displayed virtually no immunopositive neurons. Tau pathology was not seen in any transgenic genotype, although a massive glial response occurred in the APP(swe)/PS1(M146L) mice associated with amyloid plaques. Stereology revealed a significant increase in p75(NTR)-ir medial septal neurons in the APP(swe) and PS1(M146L) singly transgenic mice compared to the APP(swe)/PS1(M146L) mice. No differences in size or optical density of p75(NTR)-ir neurons were observed in these three mutants. p75(NTR)-ir fibers in hippocampus and cortex were more pronounced in the APP(swe) and PS1(M146L) mice, while the APP(swe)/PS1(M146L) mice showed the least p75(NTR)-ir fiber staining. These findings suggest a neurotrophic role for mutant APP and PS1 upon cholinergic hippocampal projection neurons at 12 months of age.     

Long-term potentiation in rat hippocampus is inhibited by low concentrations of ethanol

Acute ethanol ingestion impairs memory in humans at concentrations associated with mild intoxication. A possible neurophysiological correlate of this effect is the suppression by ethanol of long-tem potentiation (LTP), a persistent increase in synaptic efficiency which has been proposed as a substrate for memory. However, in previous studies ethanol has been shown to impair LTP only at very high concentrations, near the lethal level in humans. We now report that ethanol can significantly reduce LTP in rat hippocampus at concentrations as low as 5 mM, a level attainable following ingestion of a single alcoholic drink. We also demonstrate that the potency of ethanol in depressing LTP correlates well with its potency in inhibiting the response to N-methyl-D-aspartate, an agonist at the glutamate receptors implicated in LTP induction. The influence of low ethanol concentrations on LTP may contribute to the memory impairment associated with its use in humans.     

Presynaptic ultrastructural correlates of long-term potentiation in the CA1 subfield of the hippocampus

To determine if long-term potentiation (LTP) is accompanied by changes in the ultrastructural distribution of calcium within presynaptic terminals, calcium was localized at the electron microscopic level using an oxalate/pyroantimonate histochemical technique. Following the induction of LTP at the Schaffer collateral/commissural synapses in the CA1 subfield of the rat hippocampal slice, there was a significant decrease (30%) in the percentage of synaptic vesicles containing calcium deposits. This effect could be accounted for by both a significant reduction in the average number of calcium deposit-bearing vesicles and a significant increase in the average number of synaptic vesicles per terminal profile in slices that displayed LTP. These changes persisted for at least one hour following the induction of LTP and were not observed in slices that received high-frequency stimulation in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid (APV, 50 microM), which blocked LTP. These data suggest that LTP may be accompanied by long-term changes in intraterminal calcium homeostasis and the number of synaptic vesicles. These effects may be related to the reported increase in transmitter release following the induction of LTP.     

Baclofen-induced disinhibition in area CA1 of rat hippocampus is resistant to extracellular Ba

The mechanism of disinhibition produced by (±)-baclofen was studied using intracellular recording in area CA1 of rat hippocampal slices. Baclofen reversibly depressed monosynaptic IPSPs evoked by direct activation of interneurons in the presence of the excitatory amino acid receptor antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) andd,l-2amino-5-phosphonovalerate (APV). Ba²⁺ prevented baclofen-induced hyperpolarization of pyramidal neurons but not depression of monosynaptic IPSPs by baclofen. Baclofen reversibly depressed monosynaptic IPSPs when applied close to the recording site, but was ineffective when applied close to the stimulating site in stratum radiatum. These results suggest that baclofen disinhibits pyramidal neurons in area CA1 of the rat hippocampus by activating receptors on the terminals of inhibitory neurons that are coupled to a Ba²⁺-insensitive effector mechanism.     

Long-term potentiation in vivo in the intact mouse hippocampus

We describe the characteristics of long-term potentiation (LTP) in the intact mouse. Perforant path stimulation evokes both a population excitatory postsynaptic potential (pop-EPSP) and a population spike potential (pop-spike) from the hippocampal dentate gyrus in urethane anesthetized animals. LTP, as measured by increased pop-spike amplitude and pop-EPSP slope, was successfully induced and reliably maintained at a stable level for at least 12 h, the longest time tested. The LTP-inducing stimulus (3 trains of 400 Hz, 8 0.4 ms pulses/train) used in two strains of mice was less by half than that used in rat. These parameters for inducing LTP were also successfully applied to obtain LTP in two different transgenic mouse strains: one bearing a F1/GAP-43 promoter-lacZ fusion gene and another which overexpresses the S100β gene. We also examined the effects of protein synthesis inhibitors, cycloheximide (CXM) and anisomycin (ANI). When CXM or ANI was given 30 min before LTP induction, there was no persistent loss of LTP at the 4 h time point. However, if CXM was given 4 h before LTP induction, significant decay of the potentiated responses occurred 90 min after induction. Half of the animals receiving CXM but not ANI showed a complete and sudden elimination of the entire response after the LTP-inducing stimulus. It was speculated that loss of a constitutively-expressed housekeeping protein, for example a calcium buffering protein, with an estimated half-life of 2 h would lead to an inability to buffer LTP-induced alterations, such as intracellular calcium elevation, increasing intracellular calcium to toxic levels. LTP can be reliably studied in the intact mouse hippocampus and can be usefully applied in both wild-type and transgenic preparations. It will afford the opportunity to study LTP in mouse mutants in their in vivo state rather than in vitro in the slice preparation permitting characterization of biochemical and molecular events after LTP and then to determine the explicit relation of these events with the physiology of synaptic enhancement.     

Neonatal exposure to novelty enhances long-term potentiation in CA1 of the rat hippocampus

Exposing rats to an enriched environment over an extended period of time has been shown to enhance hippocampal long-term potentiation (LTP). Whether such prolonged exposure to environmental manipulation is necessary for LTP enhancement and whether the environmentally induced enhancement can persist long after the cessation of the environmental manipulation remain unknown. Using a novelty exposure procedure modified from the method of neonatal handling, we exposed neonatal rats to a non-home environment for 3 min/day during the first 3 weeks of life. We examined the LTP of both population spikes and excitatory postsynaptic potentials (EPSPs), in vitro, in the CA1 of the hippocampus during adulthood (7-8 and 13-14 months of age). We found that both the LTP of population spikes and the LTP of EPSPs were enhanced among animals who experienced neonatal novelty exposure. These results demonstrate that effective environmental enhancement of LTP can be achieved by as brief and as transient a manipulation as a 3-min/day exposure over the first 3 weeks of life. The resulting enhancement can outlast the environmental manipulation by at least 1 year.     

Effects of extracellular potassium concentration and postsynaptic membrane potential on calcium-induced potentiation in area CA1 of rat hippocampus

Long-lasting potentiation can be induced in area CA1 of hippocampus by a relatively brief (7-10 min) exposure to a higher (4.0 mM) than normal (2.0 mM) extracellular calcium concentration. We have found that long-lasting calcium-induced potentiation is dependent on extracellular potassium concentration. Slices exposed to high extracellular calcium in the presence of normal extracellular potassium (3.35 mM) showed a transient facilitation. Long-lasting potentiation was induced by exposure to high calcium only in slices also exposed to higher than normal extracellular potassium (6.25 mM). In intracellular experiments we found that injection of depolarizing current into postsynaptic neurons could substitute for high extracellular potassium. These results suggest that calcium-induced potentiation involves a postsynaptic, voltage-dependent mechanism. A similar conclusion has been reached for tetanus-induced potentiation. We also found that calcium-induced potentiation, like tetanus-induced potentiation, is not accompanied by an increase in postsynaptic input resistance.     

Heterosynaptic long-term depression is facilitated by blockade of inhibition in area CA1 of the hippocampus

Non-associative long-term depression (LTD) of the stratum radiatum input to area CA1 was studied in rat hippocampal slices. Tetanization of either the alveus or stratum oriens produced > 30min depression of the radiatum field EPSP and population spike, but generally only in the presence of picrotoxin. The spike depression was accounted for by the EPSP depression, and could be blocked by prior administration of anN-methyl-d-aspartate receptor antagonist. These data suggest that the induction of non-associative LTD is depolarization-dependent and involves theN-methyl-d-aspartate receptor/channel complex.