Anoxia reveals a vulnerable period in the development of long-term potentiation

Contributions of site- and sex-specific LTPs to everyday memory

Commentaries about long-term potentiation (LTP) generally proceed with an implicit assumption that largely the same physiological effect is sampled across different experiments. However, this is clearly not the case. We illustrate the point by comparing LTP in the CA3 projections to CA1 with the different forms of potentiation in the dentate gyrus. These studies lead to the hypothesis that specialized properties of CA1-LTP are adaptations for encoding unsupervised learning and episodic memory, whereas the dentate gyrus variants subserve learning that requires multiple trials and separation of overlapping bodies of information. Recent work has added sex as a second and somewhat surprising dimension along which LTP is also differentiated. Triggering events for CA1-LTP differ between the sexes and the adult induction threshold is significantly higher in females; these findings help explain why males have an advantage in spatial learning. Remarkably, the converse is true before puberty: Females have the lower LTP threshold and are better at spatial memory problems. A mechanism has been identified for the loss-of-function in females but not for the gain-of-function in males. We propose that the many and disparate demands of natural environments, with different processing requirements across ages and between sexes, led to the emergence of multiple LTPs. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Synaptic and Network Contributions to Anoxic Depolarization in Mouse Hippocampal Slices

Ischemic stroke remains the third leading cause of death and leading cause of adult disability worldwide. A key event in the pathophysiology of stroke is the anoxic depolarization (AD) of neurons in the ischemic core. Previous studies have established that both the latency to AD and the time spent in AD prior to re-oxygenation are predictors of neuronal death. The present studies used hippocampal slices from male and female mice to investigate the electrophysiological events that affect latency to AD after oxygen deprivation. The results confirm that the epoch between AD and re-oxygenation largely determines the magnitude of synaptic recovery after anoxic challenge. Using a selective antagonist of adenosine A₁ receptors, we also confirmed that adenosine released during anoxia (ANOX) suppresses synaptic glutamate release; however, this action has no effect on AD latency or the potential for post-anoxic recovery of synaptic transmission. In contrast, antagonism of AMPA- and NMDA-type glutamate receptors significantly prolongs the latency to AD and alters the speed and synchrony of associated depolarizing waves. Experiments using slices with fields Cornu ammonis 3 (CA3) and Cornu ammonis 1 (CA1) disconnected showed that AD latency is longer in CA1 than in CA3; however, the early AD in CA3 is propagated to CA1 in intact slices. Finally, AD latency in CA1 was found to be longer in slices from female mice than in those from age-matched male mice. The results have implications for stroke prevention and for understanding brain adaptations in hypoxia-tolerant animals.

Retrograde enhancement of episodic learning by a postlearning stimulus

Evidence suggests encoding of recent episodic experiences may be enhanced by a subsequent salient event. We tested this hypothesis by giving rats a 3-min unsupervised experience with four odors and measuring retention after different delays. Animals recognized that a novel element had been introduced to the odor set at 24 but not 48 h. However, when odor sampling was followed within 5 min by salient light flashes or bedding odor, the memory lasted a full 2 d. These results describe a retroactive influence of salience to promote storage of episodic information and introduce a unique model for studying underlying plasticity mechanisms.

Activation of α7 nicotinic acetylcholine receptors protects potentiated synapses from depotentiation during theta pattern stimulation in the hippocampal CA1 region of rats

Long-term potentiation (LTP) shows memory-like consolidation and thus becomes increasingly resistant to disruption by low-frequency stimulation (LFS). However, it is known that nicotine application during LFS uniquely depotentiates consolidated LTP. Here, we investigated how nicotine contributes to the disruption of stabilized LTP in the hippocampal CA1 region. We found that nicotine-induced depotentiation is not due to masking LTP by inducing long-term depression and requires the activation of GluN2A-containing NMDARs. We further examined whether nicotine-induced depotentiation involves the reversal of LTP mechanisms. LTP causes phosphorylation of Ser-831 on GluA1 subunits of AMPARs that increases the single-channel conductance of AMPARs. This phosphorylation remained unchanged after depotentiation. LTP involves the insertion of new AMPARs into the synapse and the internalization of AMPARs is associated with dephosphorylation of Ser-845 on GluA1 and caspase-3 activity. Nicotine-induced depotentiation occurred without dephosphorylation of the Ser-845 and in the presence of a caspase-3 inhibitor. LTP is also accompanied by increased filamentous actin (F-actin), which controls spine size. Nicotine-induced depotentiation was prevented by jasplakinolide, which stabilizes F-actin, suggesting that nicotine depotentiates consolidated LTP by destabilizing F-actin. α7 nicotinic acetylcholine receptor (nAChR) antagonists mimicked the effect of nicotine and selective removal of hippocampal cholinergic input caused depotentiation in the absence of nicotine, suggesting that nicotine depotentiates consolidated LTP by inducing α7 nAChR desensitization. Our results demonstrate a new role for nicotinic cholinergic systems in protecting potentiated synapses from depotentiation by preventing GluN2A-NMDAR-mediated signaling for actin destabilization.

No Enhancement of 24-Hour Visuomotor Skill Retention by Post-Practice Caffeine Administration

Caffeine is widely consumed throughout the world and appears to indirectly facilitate learning and memory through effects on attention and motivation. Animal work indicates that post-training caffeine administration augments inhibitory avoidance memory, spatial memory, and object memory. In humans, post-training caffeine administration enhances the ability to discern between familiar images and new, similar images. However, the effect of post-training caffeine administration on motor memory has not been examined. Therefore, we tested two groups of low caffeine consumers (average weekly consumption ≤500 mg) in a double-blind, placebo-controlled study involving acquisition of a continuous isometric visuomotor tracking skill. On Day 1, subjects completed 5 blocks (150 repetitions) of training on the continuous isometric visuomotor skill and subsequently ingested either 200 mg of caffeine or placebo. On day 2, subjects completed an additional 5 blocks of training. Day 1 mean performance and performance variability were both similar between groups, suggesting that both groups acquired the motor skill similarly. For mean performance on Day 2, patterns of re-learning, mean performance learning magnitudes, mean performance learning rates, and mean performance retention magnitudes were all similar between groups. For performance variability on Day 2, there was a small trend towards increased variability in the caffeine group during re-learning, but performance variability learning magnitudes and performance variability retention magnitudes did not differ between groups. Because motor skill acquisition can also be conceptualized as a reduction in performance variability, these results suggest that there may be a small negative effect of post-practice caffeine administration on memory of a newly-learned visuomotor skill. Overall, we found no evidence to suggest that post-training caffeine administration enhances 24-hour retention of a newly-learned continuous visuomotor skill, and these results support the notion that memory-enhancing effects of post-training caffeine ingestion may be task-specific.

Adenosine: setting the stage for plasticity

It is widely accepted that Hebbian forms of plasticity mediate selective modifications in synaptic strength underlying information encoding in response to experience and circuit formation or refinement throughout development. Several complementary forms of homeostatic plasticity coordinate to keep Hebbian plasticity in check, frequently through the actions of conserved regulatory molecules. Recent evidence suggests that this may be the case for adenosine, which is ubiquitous in the brain and is released by both neurons and glial cells via constitutive and activity-dependent mechanisms. Through A1 and A2A receptor activation, adenosine modulates neuronal homeostasis and tunes the ability of synapses to undergo and/or sustain plasticity. Here, we review how adenosine equilibrates neuronal activity and sets the stage for synaptic plasticity.

The BCM theory of synapse modification at 30: interaction of theory with experiment

Thirty years have passed since the publication of Elie Bienenstock, Leon Cooper and Paul Munro's 'Theory for the development of neuron selectivity: orientation specificity and binocular interaction in visual cortex', known as the BCM theory of synaptic plasticity. This theory has guided experimentalists to discover some fundamental properties of synaptic plasticity and has provided a mathematical structure that bridges molecular mechanisms and systems-level consequences of learning and memory storage.

Physiological effects of enriched environment exposure and LTP induction in the hippocampus in vivo do not transfer faithfully to in vitro slices

A number of experimental paradigms use in vitro brain slices to test for changes in synaptic transmission and plasticity following a behavioral manipulation. For example, a number of previous studies have reported a variety of effects of environmental enrichment (EE) exposure on field potential responses in hippocampal slices, but in no study was is it known what changes had been elicited in vivo. In the present study, we recorded from the hippocampus in vivo while rats underwent a brief period of EE. There was no detectable EE-induced change in synaptic efficacy in the dentate gyrus in vivo, but there was an increase in cellular excitability. In slices prepared from the same animals, we failed to observe any evidence of the excitability increase. We next tested whether LTP induction in vivo was better preserved in vitro. However, when slices from these rats were examined, there was no observable change in perforant path synaptic strength, although there was a modest increase in excitability that correlated with the increased excitability observed in vivo. These findings suggest that synaptic changes induced in vivo either are not preserved faithfully or are difficult to detect in hippocampal slices, while changes in cellular excitability are better preserved.

Reactivation of fear memory renders consolidated amygdala synapses labile

It is believed that memory reactivation transiently renders consolidated memory labile and that this labile or deconsolidated memory is reconsolidated in a protein synthesis-dependent manner. The synaptic correlate of memory deconsolidation upon reactivation, however, has not been fully characterized. Here, we show that 3,5-dihydroxyphenylglycine (DHPG), an agonist for group I metabotropic glutamate receptors (mGluRI), induces synaptic depotentiation only at thalamic input synapses onto the lateral amygdala (T-LA synapses) where synaptic potentiation is consolidated, but not at synapses where synaptic potentiation is not consolidated. Using this mGluRI-induced synaptic depotentiation (mGluRI-depotentiation) as a marker of consolidated synapses, we found that mGluRI-depotentiation correlated well with the state of memory deconsolidation and reconsolidation in a predictable manner. DHPG failed to induce mGluRI-depotentiation in slices prepared immediately after reactivation when the reactivated memory was deconsolidated. DHPG induced mGluRI-depotentiation 1 h after reactivation when the reactivated memory was reconsolidated, but it failed to do so when reconsolidation was blocked by a protein synthesis inhibitor. To test the memory-specificity of mGluRI-depotentiation, conditioned fear was acquired twice using two discriminative tones (2.8 and 20 kHz). Under this condition, mGluRI-depotentiation was fully impaired in slices prepared immediately after reactivation with both tones, whereas mGluRI-depotentiation was partially impaired immediately after reactivation with the 20 kHz tone. Consistently, microinjection of DHPG into the LA 1 h after reactivation reduced fear memory retention, whereas DHPG injection immediately after reactivation failed to do so. Our findings suggest that, upon memory reactivation, consolidated T-LA synapses enter a temporary labile state, displaying insensitivity to mGluRI-depotentiation.

Extreme hypoxia tolerance of naked mole-rat brain

Mammalian brains have extremely high levels of aerobic metabolism and typically suffer irreversible damage after brief periods of oxygen deprivation such as occur during stroke or cardiac arrest. Here we report that brain tissue from naked mole-rats, rodents that live in a chronically low-oxygen environment, is remarkably resistant to hypoxia: naked mole-rat neurons maintain synaptic transmission much longer than mouse neurons and can recover from periods of anoxia exceeding 30 min. We suggest that brain tolerance to hypoxia may result from slowed or arrested brain development in these extremely long-lived animals.

Different Rho GTPase-dependent signaling pathways initiate sequential steps in the consolidation of long-term potentiation

The releasable factor adenosine blocks the formation of long-term potentiation (LTP). These experiments used this observation to uncover the synaptic processes that stabilize the potentiation effect. Brief adenosine infusion blocked stimulation-induced actin polymerization within dendritic spines along with LTP itself in control rat hippocampal slices but not in those pretreated with the actin filament stabilizer jasplakinolide. Adenosine also blocked activity-driven phosphorylation of synaptic cofilin but not of synaptic p21-activated kinase (PAK). A search for the upstream origins of these effects showed that adenosine suppressed RhoA activity but only modestly affected Rac and Cdc42. A RhoA kinase (ROCK) inhibitor reproduced adenosine's effects on cofilin phosphorylation, spine actin polymerization, and LTP, whereas a Rac inhibitor did not. However, inhibitors of Rac or PAK did prolong LTP's vulnerability to reversal by latrunculin, a toxin which blocks actin filament assembly. Thus, LTP induction initiates two synaptic signaling cascades: one (RhoA-ROCK-cofilin) leads to actin polymerization, whereas the other (Rac-PAK) stabilizes the newly formed filaments.

Leptin reverses long-term potentiation at hippocampal CA1 synapses

The hormone leptin crosses the blood brain barrier and regulates numerous neuronal functions, including hippocampal synaptic plasticity. Here we show that application of leptin resulted in the reversal of long-term potentiation (LTP) at hippocampal CA1 synapses. The ability of leptin to depotentiate CA1 synapses was concentration-dependent and it displayed a distinct temporal profile. Leptin-induced depotentiation was not associated with any change in the paired pulse facilitation ratio or the coefficient of variance, indicating a post-synaptic locus of expression. Moreover, the synaptic activation of NMDA receptors was required for leptin-induced depotentiation as the effects of leptin were blocked by the competitive NMDA receptor antagonist, D-aminophosphovaleric acid (D-AP5). The signaling mechanisms underlying leptin-induced depotentiation involved activation of the calcium/calmodulin-dependent protein phosphatase, calcineurin, but were independent of c-jun NH(2) terminal kinase. Furthermore, leptin-induced depotentiation was accompanied by a reduction in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor rectification indicating that loss of glutamate receptor 2 (GluR2)-lacking AMPA receptors underlies this process. These data indicate that leptin reverses hippocampal LTP via a process involving calcineurin-dependent internalization of GluR2-lacking AMPA receptors which further highlights the key role for this hormone in regulating hippocampal synaptic plasticity and neuronal development.

The substrates of memory: defects, treatments, and enhancement

Recent work has added strong support to the long-standing hypothesis that the stabilization of both long-term potentiation and memory requires rapid reorganization of the spine actin cytoskeleton. This development has led to new insights into the origins of cognitive disorders, and raised the possibility that a diverse array of memory problems, including those associated with diabetes, reflect disturbances to various components of the same mechanism. In accord with this argument, impairments to long-term potentiation in mouse models of Huntington's disease and in middle-aged rats have both been linked to problems with modulatory factors that control actin polymerization in spine heads. Complementary to the common mechanism hypothesis is the idea of a single treatment for addressing seemingly unrelated memory diseases. First tests of the point were positive: Brain-Derived Neurotrophic Factor (BDNF), a potent activator of actin signaling cascades in adult spines, rescued potentiation in Huntington's disease mutant mice, middle-aged rats, and a mouse model of Fragile-X syndrome. A similar reversal of impairments to long-term potentiation was obtained in middle-aged rats by up-regulating BDNF production with brief exposures to ampakines, a class of drugs that positively modulate AMPA-type glutamate receptors. Work now in progress will test if chronic elevation of BDNF enhances memory in normal animals.

Plasticity and repair in the post-ischemic brain

Stroke is the second commonest cause of death and the principal cause of adult disability in the world. In most cases ischemic injuries have been reported to induce mild to severe permanent deficits. Nevertheless, recovery is often dynamic, reflecting the ability of the injured neuronal networks to adapt. Plastic phenomena occurring in the cerebral cortex and in subcortical structures after ischemic injuries have been documented at the synaptic, cellular, and network level and several findings suggest that they may play a key role during neurorehabilitation in human stroke survivors. In particular, in vitro studies have demonstrated that oxygen and glucose deprivation (in vitro ischemia) exerts long-term effects on the efficacy of synaptic transmission via the induction of a post-ischemic long-term potentiation (i-LTP). i-LTP may deeply influence the plastic reorganization of cortical representational maps occurring after cerebral ischemia, inducing a functional connection of previously non-interacting neurons. On the other hand, there is evidence that i-LTP may exert a detrimental effect in the peri-infarct area, facilitating excitotoxic processes via the sustained, long-term enhancement of glutamate mediated neurotransmission. In the present work we will review the molecular and synaptic mechanisms underlying ischemia-induced synaptic plastic changes taking into account their potential adaptive and/or detrimental effects on the neuronal network in which they occur. Thereafter, we will consider the implications of brain plastic phenomena in the post-stroke recovery phase as well as during the rehabilitative and therapeutic intervention in human subjects.

Brain-derived neurotrophic factor promotes long-term potentiation-related cytoskeletal changes in adult hippocampus

Brain-derived neurotrophic factor (BDNF) is an extremely potent, positive modulator of theta burst induced long-term potentiation (LTP) in the adult hippocampus. The present studies tested whether the neurotrophin exerts its effects by facilitating cytoskeletal changes in dendritic spines. BDNF caused no changes in phalloidin labeling of filamentous actin (F-actin) when applied alone to rat hippocampal slices but markedly enhanced the number of densely labeled spines produced by a threshold level of theta burst stimulation. Conversely, the BDNF scavenger TrkB-Fc completely blocked increases in spine F-actin produced by suprathreshold levels of theta stimulation. TrkB-Fc also blocked LTP consolidation when applied 1-2 min, but not 10 min, after theta trains. Additional experiments confirmed that p21 activated kinase and cofilin, two actin-regulatory proteins implicated in spine morphogenesis, are concentrated in spines in mature hippocampus and further showed that both undergo rapid, dose-dependent phosphorylation after infusion of BDNF. These results demonstrate that the influence of BDNF on the actin cytoskeleton is retained into adulthood in which it serves to positively modulate the time-dependent LTP consolidation process.

LTP consolidation: Substrates, explanatory power, and functional significance

Long-term potentiation (LTP) resembles memory in that it is initially unstable and then, over about 30 min, becomes increasingly resistant to disruption. Here we present an hypothesis to account for this initial consolidation effect and consider implications that follow from it. Anatomical studies indicate that LTP is accompanied by changes in spine morphology and therefore likely involves cytoskeletal changes. Accordingly, theta bursts initiate calpain-mediated proteolysis of the actin cross-linking protein spectrin and trigger actin polymerization in spine heads, two effects indicative of cytoskeletal reorganization. Polymerization occurs within 2 min, has the same threshold as LTP, is dependent on integrins, and becomes resistant to disruption over 30 min. We propose that the stabilization of the new cytoskeletal organization, and thus of a new spine morphology, underlies the initial phase of LTP consolidation. This hypothesis helps explain the diverse array of proteins and signaling cascades implicated in LTP, as well as the often-contradictory results about contributions of particular molecules. It also provides a novel explanation for why LTP is potently modulated by factors likely to be released during theta trains (e.g., BDNF). Finally, building on evidence that normal patterns of activity reverse LTP, we suggest that consolidation provides a delay that allows brain networks to sculpt newly formed memories.

Synaptic plasticity in early aging

Studies of how aging affects brain plasticity have largely focused on old animals. However, deterioration of memory begins well in advance of old age in animals, including humans; the present review is concerned with the possibility that changes in synaptic plasticity, as found in the long-term potentiation (LTP) effect, are responsible for this. Recent results indicate that impairments to LTP are in fact present by early middle age in rats but only in certain dendritic domains. The search for the origins of these early aging effects necessarily involves ongoing analyses of how LTP is induced, expressed, and stabilized. Such work points to the conclusion that cellular mechanisms responsible for LTP are redundant and modulated both positively and negatively by factors released during induction of potentiation. Tests for causes of the localized failure of LTP during early aging suggest that the problem lies in excessive activity of a negative modulator. The view of LTP as having redundant and modulated substrates also suggests a number of approaches for reversing age-related losses. Particular attention will be given to the idea that induction of brain-derived neurotrophic factor, an extremely potent positive modulator, can be used to provide long periods of normal plasticity with very brief pharmacological interventions. The review concludes with a consideration of how the selective, regional deficits in LTP found in early middle age might be related to the global phenomenon of brain aging.

Nicotine reverses consolidated long-term potentiation in the hippocampal CA1 region

Long-term potentiation (LTP) has a memory-like consolidation period during which it becomes progressively stabilized. However, it is unknown how the consolidation is achieved. The present study demonstrates that nicotine reverses stabilized LTP in the hippocampal CA1 region, providing the first evidence that consolidated LTP can be reversed. The nicotine-induced reversal appeared to work by reversing cellular processes involved in stabilizing LTP, as LTP was readily induced again after reversal. The effect of nicotine was mediated, in large part, via desensitization of alpha7 nicotinic acetylcholine receptors (nAChRs), as an alpha7 nAChR-selective antagonist mimicked the nicotine effect. A non-selective N-methyl-d-aspartate receptor (NMDAR) antagonist completely abolished the nicotine-induced reversal, whereas an NR2B-containing NMDAR-selective antagonist had no effect. Furthermore, both the protein phosphatase 1/protein phosphatase 2A inhibitor okadaic acid and the protein phosphatase 2B (calcineurin) inhibitor cyclosporin A blocked the nicotine-induced reversal. Taken together, our results suggest that the reversal of stabilized LTP depends on the activation of NR2A-containing NMDARs and dephosphorylation. Thus, the consolidation of LTP appears to be the interruption of signaling leading to NR2A-containing NMDAR-dependent activation of protein phosphatases, which can be circumvented by nicotine-induced signaling. LTP induced in chronic nicotine-treated hippocampi contained a component that is immune to reversal, and thus acute nicotine was no longer effective to reverse consolidated LTP. These results demonstrate the differential effects of acute and chronic nicotine exposure on the cellular processes that are potentially involved in learning and memory.

AMPA receptor stimulation increases alpha5beta1 integrin surface expression, adhesive function and signaling

Integrin proteins are critical for stabilization of hippocampal long-term potentiation but the mechanisms by which integrin activities are involved in synaptic transmission are not known. The present study tested whether activation of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) class glutamate receptors increases surface expression of alpha5beta1 integrin implicated in synaptic potentiation. Surface protein biotinylation assays demonstrated that AMPA treatment of COS7 cells expressing GluR1 homomeric AMPA receptors increased membrane insertion and steady-state surface levels of alpha5 and beta1 subunits. Treated cells exhibited increased adhesion to fibronectin- and anti-alpha5-coated substrates and tyrosine kinase signaling elicited by fibronectin-substrate adhesion, as expected if new surface receptors are functional. Increased surface expression did not occur in calcium-free medium and was blocked by the protein kinase C inhibitor chelerythrine chloride and the exocytosis inhibitor brefeldin A. AMPA treatment similarly increased alpha5 and beta1 surface expression in dissociated neurons and cultured hippocampal slices. In both neuronal preparations AMPA-induced integrin trafficking was blocked by combined antagonism of NMDA receptor and L-type voltage-sensitive calcium channel activities but was not induced by NMDA treatment alone. These results provide the first evidence that glutamate receptor activation increases integrin surface expression and function, and suggest a novel mechanism by which synaptic activity can engage a volley of new integrin signaling in coordination with, and probably involved in, stabilization of synaptic potentiation.

Long-term potentiation is impaired in middle-aged rats: regional specificity and reversal by adenosine receptor antagonists

Memory loss in humans begins early in adult life and progresses thereafter. It is not known whether these losses reflect the failure of cellular processes that encode memory or disturbances in events that retrieve it. Here, we report that impairments in hippocampal long-term potentiation (LTP), a form of synaptic plasticity associated with memory, are present by middle age in rats but only in select portions of pyramidal cell dendritic trees. Specifically, LTP induced with theta-burst stimulation in basal dendrites of hippocampal field CA1 decayed rapidly in slices prepared from 7- to 10-month-old rats but not in slices from young adults. There were no evident age-related differences in LTP in the apical dendrites. Both the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and a positive AMPA receptor modulator (ampakine) offset age-related LTP deficits. Adenosine produced greater depression of synaptic responses in middle-aged versus young adult slices and in basal versus apical dendrites. These results were not associated with variations in A1 receptor densities and may instead reflect regional and age-related differences in adenosine clearance. Pertinent to this, brief applications of A1 receptor antagonists immediately after theta stimulation fully restored LTP in middle-aged rats. We hypothesize that the build-up of extracellular adenosine during theta activity persists into the postinduction period in the basal dendrites of middle-aged slices and thereby activates the A1 receptor-dependent LTP reversal effect. Regardless of the underlying mechanism, the present results provide a candidate explanation for memory losses during normal aging and indicate that, with regard to plasticity, different segments of pyramidal neurons age at different rates.

Integrins, synaptic plasticity and epileptogenesis

A number of processes are thought to contribute to the development of epilepsy including enduring increases in excitatory synaptic transmission, changes in GABAergic inhibition, neuronal cell death and the development of aberrant innervation patterns in part arising from reactive axonal growth. Recent findings indicate that adhesion chemistries and, most particularly, activities of integrin class adhesion receptors play roles in each of these processes and thereby are likely to contribute significantly to the cell biology underlying epileptogenesis. As reviewed in this chapter, studies of long-term potentiation have shown that integrins are important for stabilizing activity-induced increases in synaptic strength and excitability. Other work has demonstrated that seizures, and in some instances subseizure neuronal activity, modulate the expression of integrins and their matrix ligands and the activities of proteases which regulate them both. These same adhesion proteins and proteases play critical roles in axonal growth and synaptogenesis including processes induced by seizure in adult brain. Together, these findings indicate that seizures activate integrin signaling and induce a turnover in adhesive contacts and that both processes contribute to lasting changes in circuit and synaptic function underlying epileptogenesis.

Reversal of pentylenetetrazol-induced potentiation phenomenon by theta pulse stimulation in the CA1 region of rat hippocampal slices

The effect of theta pulse stimulation (TPS) on pentylenetetrazol (PTZ)-induced long-term potentiation of population spikes was studied in the CA1 region of rat hippocampal slices. The field excitatory postsynaptic potential (fEPSP) and population spikes (PS) were recorded from strata radiatum and pyramidale, respectively, following stimulation of Schaffer collaterals. A transient PTZ application produced a long-lasting enhancement of PS amplitude. A 3-min episode of TPS delivered at test-pulse intensity failed to reverse the PTZ potentiation. However, the same stimulation at a higher intensity produced complete reversal of the PTZ potentiation when delivered during the last minutes of PTZ application. Prior application of high-intensity TPS also decreased the amount of PTZ potentiation, whereas it had no long-lasting effect on baseline synaptic responses. High-intensity TPS induced reversal was blocked by adenosine A1 receptor antagonist and, furthermore, was reduced by protein phosphatase 1 inhibitor. The results suggest that mechanism of PTZ-induced LTP reversal involves activation of adenosine receptors and protein phosphatases.

Developmental and regional differences in the consolidation of long-term potentiation

The alpha5beta1 integrin is present in high concentrations in the apical dendrites of pyramidal neurons in adult rats but is virtually absent in the basal dendrites. Moreover, alpha5beta1 does not appear in apical dendritic branches until the third post-natal week. Given that integrins contribute to the consolidation of synaptic plasticity, these results raise the possibility of developmental and regional differences in the stability of long-term potentiation (LTP). The present study tested this point using a LTP reversal paradigm in field CA1 of hippocampal slices. In accord with earlier reports, low-frequency afferent stimulation (5 Hz) introduced 30 s after theta burst stimulation (TBS) completely reversed LTP but was ineffective 30 min and 60 min later in slices from adult rats. The same low-frequency trains caused a partial reversal of LTP when applied 30 and 60 min post-TBS in slices from 21-day-old rats and a complete reversal at all time points in slices from 10-day-old rats. LTP in the basal dendrites of adult rats did not fully consolidate; i.e. potentiation was partially reversed by low-frequency stimulation even after delays of 30 or 60 min. Moreover, spaced (10 min) applications of 5- Hz pulses beginning at 30 min post-TBS completely erased LTP. The reversal effect in both apical and basal dendrites was blocked by N-methyl-D-aspartic acid receptor antagonists but an integrin antagonist had differential effects across the two dendritic domains. These results constitute evidence that the stability of LTP increases with age in the apical dendrites but remains incomplete even in adulthood in the basal dendrites. The possibilities that the developmental and regional variations in LTP consolidation are correlated with integrin expression and linked to different types of memory processing are discussed.

Age-dependent loss of PTP and LTP in the hippocampus of PrP-null mice

We have investigated synaptic function in the hippocampus in mice of different ages carrying a null mutation in the PrP gene. Experiments carried out in vivo and in vitro in two laboratories revealed no differences in the ability of juvenile and young adult control and PrP-null mice to express long-term potentiation, paired-pulse facilitation, or posttetanic potentiation in either the dentate gyrus or in the CA1 region. However, we found a significant reduction in the level of posttetanic potentiation and long-term potentiation in the CA1 region of aged PrP-null mice. These results are discussed in relationship to reported increased levels of oxidative stress in older PrP-null mice.

Bidirectional synaptic plasticity: from theory to reality

Theories of receptive field plasticity and information storage make specific assumptions for how synapses are modified. I give a personal account of how testing the validity of these assumptions eventually led to a detailed understanding of long-term depression and metaplasticity in hippocampal area CA1 and the visual cortex. The knowledge of these molecular mechanisms now promises to reveal when and how sensory experience modifies synapses in the cerebral cortex.

How long will long-term potentiation last?

The paramount feature of long-term potentiation (LTP) as a memory mechanism is its characteristic persistence over time. Although the basic phenomenology of LTP persistence was established 30 years ago, new insights have emerged recently about the extent of LTP persistence and its regulation by activity and experience. Thus, it is now evident that LTP, at least in the dentate gyrus, can either be decremental, lasting from hours to weeks, or stable, lasting months or longer. Although mechanisms engaged during the induction of LTP regulate its subsequent persistence, the maintenance of LTP is also governed by activity patterns post-induction, whether induced experimentally or generated by experience. These new findings establish dentate gyrus LTP as a useful model system for studying the mechanisms governing the induction, maintenance and interference with long-term memory, including very long-term memory lasting months or longer. The challenge is to study LTP persistence in other brain areas, and to relate, if possible, the properties and regulation of LTP maintenance to these same properties of the information that is actually stored in those regions.

New life in an old idea: the synaptic plasticity and memory hypothesis revisited

The notion that changes in synaptic efficacy underlie learning and memory processes is now widely accepted, although definitive proof of the synaptic plasticity and memory hypothesis is still lacking. This article reviews recent evidence relevant to the hypothesis, with particular emphasis on studies of experience-dependent plasticity in the neocortex and hippocampus. In our view, there is now compelling evidence that changes in synaptic strength occur as a consequence of certain forms of learning. A major challenge will be to determine whether such changes constitute the memory trace itself or play a less specific supporting role in the information processing that accompanies memory formation.

Antagonists of the Platelet-activating Factor Receptor Block Long-term Potentiation in Hippocampal Slices

The effects of antagonists and agonists of the platelet-activating factor (PAF) receptor on the formation of long-term potentiation (LTP) were examined in slices of rat hippocampus. The antagonist rans-BTD (rans-2,5-bis-(3,4,5-trimethoxyphenyl)-1,3-dioxolane) at concentrations of 8 - 16 microM blocked LTP in field CA1 while the same concentration of a stereo isomer (cis-BTD) with low affinity for PAF receptors was without effect. CV3988, an antagonist structurally related to PAF, also attenuated LTP. The blockade of LTP by trans-BTD was partially reversed by simultaneous application of the non-metabolizable receptor agonist carbamyl-PAF. Trans-BTD did not change the following physiological measures: (i) paired-pulse facilitation, (ii) responses occurring during the short bursts given to induce LTP, (iii) N-methyl-d-aspartate receptor-mediated responses, and (iv) potentiation measured during the first minute after high-frequency stimulation. It thus appears that trans-BTD interferes with LTP at some step after induction and initial expression. These results suggest that activation of PAF receptors contributes to the stabilization of LTP, possibly via an effect on intracellular calcium levels.

Postsynaptic application of a cAMP analogue reverses long-term potentiation in hippocampal CA1 pyramidal neurons

The molecular mechanisms that underlie the maintenance of long-term potentiation (LTP) remain unclear. We have examined the influence of postsynaptic cAMP-dependent processes on LTP maintenance in CA1 hippocampal cells. After LTP induction, drugs affecting cAMP-dependent processes were perfused into the cell through a patch pipette. A cAMP analogue, Rp-cAMPS (4 mM), dramatically decreased the amplitude of potentiated synaptic responses. The amplitude of responses in the control pathway was also decreased but to a lesser extent, indicating a specific effect on the potentiation process. This specific effect was not due to the larger amplitude of potentiated responses, was not use-dependent and, unlike other factors that affect LTP maintenance, did not depend on the delay (2, 10, or 25 min) of drug application after LTP induction. Lower concentrations of Rp-cAMPS (1.0 and 0.4 mM) also produced an inhibitory effect but reduced the LTP and control pathways comparably. One possible action of Rp-cAMPS is competitive inhibition of protein kinase A (PKA). Surprisingly, a potent and noncompetitive PKA inhibitor, regulatory type II subunit of PKA, produced only a weak depression of potentiated and control responses indicating there must be other targets for Rp-cAMPS. Moreover, Sp-8-OH-cAMPS, which is an activator of PKA, and Rp-8-OH-cAMPS, which is a weak inhibitor of PKA, both produced effects similar to those of Rp-cAMPS. We conclude that there are postsynaptic cyclic nucleotide-dependent processes that can specifically alter the mechanisms that maintain LTP and that are not primarily dependent on PKA.

Alpha3 integrin receptors contribute to the consolidation of long-term potentiation

Several lines of evidence suggest that integrin receptors play a pivotal role in consolidation of long-term potentiation (LTP), but which of the many integrin dimers are involved remains to be discovered. The present study used an LTP reversal paradigm to test if alpha3 integrins make an important contribution. Function blocking alpha3 monoclonal antibodies or vehicle were locally infused into recording sites in field CA1 of rat hippocampal slices and LTP induced with theta burst stimulation. Low frequency trains of pulses were applied 30 min after the theta bursts. Previous work indicates that low frequency stimulation reverses LTP when applied immediately after induction but is largely ineffective after 30-45-min delays. If the antibodies were to block consolidation, then they should extend the period over which potentiation is vulnerable to disruption. There was no detectable difference between the two groups in the initial degree of LTP or within slice decay of potentiation 1-10 min after induction; a small but reliable decay occurred from 10 to 30 min with antibody treatment (P<0.01) but not in control slices. Percent potentiation was not statistically different for vehicle (55 +/- 19%, mean +/- S.D.) and anti-alpha3 (43 +/- 21%) slices at 30 min post-theta bursts. Five-Hz stimulation ("theta pulse" stimulation) 30 min after induction caused a reduction of LTP. The percent loss of potentiation after the 1-min trains was greater in the antibody-treated slices than in controls (98 +/- 4% vs. 62 +/- 28%, P<0.01, U-test) and correlated (r=0.84, alpha3 slices) with the percent LTP present prior to low frequency stimulation, as expected if the stimulation reversed potentiation. Recovery occurred in both groups but percent LTP was significantly smaller in experimental slices at 10 min post-theta pulses (5 +/- 11% vs. 36 +/- 15%, P<0.01). Recovery continued for 20 min after theta pulses and, in accordance with earlier work, was nearly complete for the control slices (50 +/- 19% vs 55 +/- 15%, 40 min post- vs. immediately pre-theta pulses). LTP remained depressed after 40 min of recovery in the anti-alpha3 slices (23 +/- 19% vs. 43 +/- 21%) at which point it was substantially less than that found in controls (P<0.01). Western blots with anti-alpha3 antibodies identified a polypeptide with the molecular mass (155 kDa) expected for the alpha3 subunit and further showed that it is broadly distributed in brain. Subcellular fractionation experiments demonstrated that alpha3 is concentrated in synaptic membranes over homogenates to about the same degree as the GluR1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptor. From these results we suggest that alpha3-containing integrins are localized to synapses and are needed to stabilize a slowly decaying form of LTP. The findings also show that vulnerability to reversal can be used in place of extended recording sessions in studying consolidation.

Pharmacological protection of synaptic function, spatial learning, and memory from transient hypoxia in rats

Hypoxia significantly reduced cholinergic theta activity in rat CA1 field and intracellular theta in the CA1 pyramidal cells, recorded in hippocampal slices. The hypoxic responses of the hippocampal CA1 pyramidal cells to a brief hypoxia consisted of a short period of "synaptic arrest", observed as an elimination of excitatory postsynaptic current under voltage clamp and recovered immediately as oxygenation was reinitiated. The hypoxic synaptic arrest was not associated with reduced postsynaptic responses of the pyramidal cells to externally applied L-glutamate, suggesting that the synaptic arrest might result from a presynaptic mechanism. The hypoxic synaptic arrest was abolished in the presence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a specific adenosine A(1) receptor antagonist. Blocking adenosine A(1) receptors also eliminated effects of hypoxia on the hippocampal CA1 field theta activity and intracellular theta of the CA1 pyramidal cells. In behaving rats, brief hypoxia impaired their water maze performance in both the escape latency and probe tests. The impairment was prevented by intralateral cerebroventricular injections of DPCPX. These results suggest that hypoxia releases adenosine and produces an inhibition of synaptic transmission and intracellular signal cascade(s) involved in generation/maintenance of hippocampal CA1 theta activity. This protection of synaptic efficacy and spatial learning through adenosine A(1) receptor antagonism may represent an effective therapeutic strategy to eliminate functional interruption due to transient hypoxic episodes and/or chronic hypoxia secondary to compromise of respiratory function.

Evidence that integrins contribute to multiple stages in the consolidation of long term potentiation in rat hippocampus

Three structurally distinct groups of antagonists were used to test the hypothesis that integrin adhesion receptors play an essential role in consolidating (stabilizing) long term potentiation of the Schaffer collaterals in rat hippocampus. Comparisons were made of percent potentiation at antagonist-treated versus control sites within CA1 stratum radiatum of the same hippocampal slice. Function blocking antibodies against the alpha5 subunit of the fibronectin receptor had no effect on baseline responses or initial potentiation but resulted in a >30% reduction, relative to within-slice control long term potentiation, 45 min later. Larger reductions were recorded in separate experiments continued for 4 h after the induction of potentiation. Alpha(v) and alpha2 subunit antibodies did not reliably affect the stabilization of potentiation. An antagonist peptide with preference for beta1 integrins produced a slowly developing decline of the type seen with alpha5 antibodies. A cyclic peptide antagonist reduced potentiation within 10 min of induction and caused an almost 40% decrease over 45 min. Two disintegrins (snake toxins that potently block integrins) were very effective in preventing the consolidation of long term potentiation: echistatin reduced potentiation by >70%, while triflavin caused approximately 50% decrease. The suppressing effects of echistatin were concentration-dependent, obtained with treatment after induction, and much more rapid than the effects of antibodies. Rapid declines in potentiation were particularly evident when the two disintegrins were applied together. These results indicate that hippocampal fibronectin receptors (alpha5/beta1 integrin) contribute importantly to a slowly developing phase of long term potentiation consolidation. They also suggest that other integrins are critical to aspects of consolidation occurring in the first few minutes after induction.

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

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

A transient increase in temperature induces persistent potentiation of synaptic transmission in rat hippocampal slices

Previous studies have shown that increasing the temperature of rat hippocampal brain slices from 32.5 to 38.5 degrees C initiates a profound, adenosine-mediated decrease in excitatory synaptic transmission in the CA1 region. Here we found that upon lowering the temperature back to 32.5 degrees C, the amplitude of the field excitatory postsynaptic potential often recovers to a level that is significantly potentiated with respect to the initial baseline. This potentiation is rapid in onset (< 5min following return to 32.5 degrees C) and long lasting (>60min following the termination of the increase in temperature). Similar effects could not be induced by superfusion with adenosine alone, and adenosine receptor antagonists did not block the potentiation. Therefore, although an adenosine-mediated decrease in excitatory synaptic transmission occurs during the temperature increase, it is unrelated to the potentiation. Likewise, N-methyl-D-aspartate receptor activation is not required, as N-methyl-D-aspartate receptor antagonists do not influence this form of potentiation. In summary, we propose that transiently increasing brain slice temperature represents a novel way to induce synaptic plasticity in the hippocampus, and may provide a paradigm to elucidate additional cellular mechanisms involved in functional plasticity.

Impairment of synaptic transmission by transient hypoxia in hippocampal slices: improved recovery in glutathione peroxidase transgenic mice

There is increasing evidence that oxygen free radicals contribute to ischemic brain injury. It is unclear, however, to what extent specific antioxidant enzymes can prevent or reverse the impairment of synaptic function caused by transient hypoxia. In this study, we investigated in transgenic (Tg) mice whether a moderate increase in glutathione peroxidase-1 (GPx1) may improve the capacity of CA1 pyramidal cells to recover synaptic transmission after a short period of hypoxia in vitro. In control hippocampal slices, transient hypoxia (7-9 min) produced irreversible loss of excitatory postsynaptic potentials. Complete recovery of synaptic transmission was observed with homozygous Tg-MT-GPx-6 mice after reoxygenation, and, after repeated episodes of hypoxia, synaptic transmission was still viable in most Tg slices, in contrast to non-Tg slices. Moreover, hypoxic episodes abolished the capacity of hippocampal slices to generate long-term potentiation in area CA1 of control mice, whereas a significant extent of long-term potentiation expression was still preserved in Tg tissues. We also demonstrated that susceptibility to N-methyl-d-aspartate-mediated oxidative injury was reduced in Tg hippocampal slices. In conclusion, our results suggest that a moderate GPx increase can be sufficient to prevent irreversible functional damage produced by transient hypoxia in the hippocampus and to help maintain basic electrophysiological mechanisms involved in memory formation.

Protein tyrosine kinase is required for the induction of long-term potentiation in the rat hippocampus

1. Protein tyrosine phosphorylation is thought to play an important role in the regulation of neuronal function. Previous work has shown that protein tyrosine kinase (PTK) inhibitors can inhibit the induction of long-term potentiation (LTP), a candidate synaptic mechanism involved in memory formation. However, how PTK activity might contribute to LTP induction remains elusive. To understand the role of PTK pathways in the development of LTP better, a set of studies was implemented in area CA1 of rat hippocampal slices using both intra- and extracellular recordings. We show here that bath application or injection into postsynaptic cells of the PTK inhibitors genistein and lavendustin A blocked the induction of LTP produced by high-frequency tetanic stimulation. 2. Application of lavendustin A 10 min before or 3 min after induction effectively blocked LTP. However, application at 10 or 30 min after induction had no detectable effect on potentiation. 3. PTK inhibitor pretreatment did not affect the long-lasting enhancement of synaptic response produced by phorbol 12,13-dibutyrate (PDBu), forskolin plus 3-isobutyl-L-methylxanthine (IBMX), or tetraethylammonium (TEA). In contrast, PTK inhibitors significantly blocked postanoxic LTP. 4. EPQ(pY)EEIPIA, an activator of Src family PTKs, produced a gradual and robust increase in the synaptic response and occluded LTP. 5. These results suggest that Src family kinases are potential candidates for the PTKs contributing to the molecular mechanism of LTP induction at Schaffer collateral-CA1 synapses.

Time-dependent reversal of long-term potentiation in area CA1 of the freely moving rat induced by theta pulse stimulation

Previous studies in slices have shown that low-frequency stimulation at 5 Hz, i.e., theta pulse stimulation (TPS), completely reverses long-term potentiation (LTP) in area CA1 when delivered within 1-2 min after induction but produces progressively less depotentiation at longer delays, until it has no longer any impact at 30 min after induction. The present study examined whether LTP in the freely moving rat exhibits a similar time-dependent susceptibility to reversal. Adult male Long-Evans rats with bilateral stimulating electrodes activating collateral/commissural projections to area CA1 were used. A 1 min episode of TPS, ineffective when applied to naive pathways, was found to permanently erase LTP when delivered to the test pathway either 30 sec or 15 min after induction. Administered at a delay of 30 min, however, the same treatment no longer had any impact on established LTP. Additional experiments examined the ability of shorter TPS episodes to erase LTP and found that a 30 sec treatment was effective at 30 sec but not 15 min after induction. When the duration of TPS was further reduced to 15 sec, a reversal was no longer obtained at any delay. These results provide the first demonstration that the limited vulnerability of LTP to reversal by TPS, originally observed in vitro, also holds true for LTP in the awake animal and occurs along the same time frame, supporting the notion that LTP stabilization mechanisms take less than 30 min to be complete.

The relationship between adhesion molecules and neuronal plasticity

1. It is presently widely assumed that structural reorganization of synaptic architectures subserves the functional gains that define certain neuronal plasticities. 2. While target molecules thought to participate in such morphological dynamics are not well defined, growing evidence suggests a pivotal role for cell adhesion molecules. 3. Herein, brief discussions are presented on (i) the history of how adhesion molecules became implicated in plasticity and memory processes, (ii) the general biology of some of the major classes of such molecules, and (iii) the future of the adhesion molecule/plasticity relationship.

Memory and the brain: unexpected chemistries and a new pharmacology

Efforts to characterize long-term potentiation (LTP) and to identify its substrates have led to the discovery of novel synaptic chemistries, computational algorithms, and, most recently, pharmacologies. Progress has also been made in using LTP to develop a "standard model" of how unusual, but physiologically plausible, levels of afferent activity create lasting changes in the operating characteristics of synapses in the cortical telencephalon. Hypotheses of this type typically distinguish induction, expression, and consolidation stages in the formation of LTP. Induction involves a sequence consisting of theta-type rhythmic activity, suppression of inhibitory currents, intense synaptic depolarization, NMDA receptor activation, and calcium influx into dendritic spines. Calcium-dependent lipases, kinases, and proteases have been implicated in LTP induction. Regarding the last group, it has been recently reported that theta pattern stimulation activates calpain and that translational suppression of the protease blocks potentiation. It is thus likely that proteolysis is readily driven by synaptic activity and contributes to structural reorganization. LTP does not interact with treatments that affect transmitter release, has a markedly differential effect on the currents mediated by colocalized AMPA vs NMDA synaptic receptors, changes the waveform of the synaptic current, modifies the effects of drugs that modulate AMPA receptors, and is sensitive to the subunit composition of those receptors. These results indicate that LTP is expressed by changes in AMPA receptor operations. LTP is accompanied by modifications in the anatomy of synapses and spines, something which accounts for its extreme duration (weeks). As with various types of memory, LTP requires about 30 min to consolidate (become resistant to disruption). Consolidation involves adhesion chemistries and, in particular, activation of integrins, a class of transmembrane receptors that control morphology in numerous cell types. Platelet activating factor and adenosine may contribute to consolidation by regulating the engagement of latent integrins. How consolidation stabilizes LTP expression is a topic of intense investigation but probably involves modifications to one or more of the following: membrane environment of AMPA receptors; access of regulatory proteins (e.g., kinases, proteases) to the receptors; receptor clustering; and space available for receptor insertion. Attempts to enhance LTP have focused on the induction phase and resulted in a class of centrally active drugs ("ampakines") that positively modulate AMPA receptors. These compounds promote LTP in vivo and improve the encoding of variety of memory types in animals. Positive results have also been obtained in preliminary studies with humans.

Mechanisms of emotional arousal and lasting declarative memory

Neuroscience is witnessing growing interest in understanding brain mechanisms of memory formation for emotionally arousing events, a development closely related to renewed interest in the concept of memory consolidation. Extensive research in animals implicates stress hormones and the amygdaloid complex as key, interacting modulators of memory consolidation for emotional events. Considerable evidence suggests that the amygdala is not a site of long-term explicit or declarative memory storage, but serves to influence memory-storage processes in other brain regions, such as the hippocampus, striatum and neocortex. Human-subject studies confirm the prediction of animal work that the amygdala is involved with the formation of enhanced declarative memory for emotionally arousing events.

Localization and seizure-regulation of integrin beta 1 mRNA in adult rat brain

Recent findings indicate that RGD-binding integrin receptors play a critical role in the maintenance of long-term potentiation but the identity and location of the integrin proteins involved are not known. The integrin beta1 is of particular interest in regard to synaptic plasticity because it is a component of many of the RGD-binding integrins and beta1-immunoreactivity has been localized within synaptic density fractions. The present study used in situ hybridization to evaluate the distribution of beta1 mRNA in adult rat brain and to determine if expression is altered by seizures. In untreated rats, beta1 mRNA is present at high levels in the ventricular epithelium and discrete neuronal groups including the magnocellular hypothalamic and efferent cranial nerve nuclei and the cerebellar Purkinje cells. Hybridization was less dense in the substantia nigra and hippocampal stratum pyramidale and low but present throughout the gray matter. Limbic seizures increased beta1 cRNA labeling of both neurons (e.g., hippocampal stratum pyramidale) and astroglial cells from 8 h through 48 h after seizure onset. These results indicate that in adult rat brain, beta1 mRNA is expressed by both neurons and glia; neuronal expression is highest in hypothalamic and peripherally projecting neurons capable of substantial morphological plasticity. Seizure effects demonstrate that beta1 is positively regulated by activity, and suggest that activity-dependent expression may play a role in synaptic plasticity in the adult brain.

Effects of aniracetam after LTP induction are suggestive of interactions on the kinetics of the AMPA receptor channel

The modulatory influence of aniracetam, a drug which reversibly modifies the kinetic properties of AMPA-type glutamate receptors, on synaptic responses is reported to be detectably changed by the induction of long-term potentiation (LTP). The present study used hippocampal slices to examine three issues arising from this result. First, possible contributions of inhibitory currents and postsynaptic spiking to the aniracetam/LTP interaction were investigated with infusions of GABA receptor antagonists and topical applications of tetrodotoxin. Second, tests were carried out to determine if the altered response to aniracetam is sufficiently persistent to be a plausible substrate for the extremely stable LTP effect. Third, the nature of the change responsible for the aniracetam/LTP interaction was explored with waveform analyses and a kinetic model of the AMPA receptor. The following results were obtained. LTP reduced the effect of aniracetam on the amplitude but increased its effect on the decay time constant of field EPSPs recorded under conditions in which local spiking and inhibitory responses were blocked. The LTP-induced change in the effect of aniracetam was extremely stable in that it was still evident 75 min after induction of potentiation. Finally, the waveform distortions introduced by LTP and aniracetam could be corrected by uniform stretching of the responses, suggesting that the changes introduced by each of the manipulations are unitary in nature. These distortions and the interactions between them could be reproduced in the AMPA receptor model by representing LTP as an acceleration of channel gating kinetics.

Induction and transient suppression of long-term potentiation in the peri- and postrhinal cortices following theta-related stimulation of hippocampal field CA1

During behavioral events associated with periods of likely mnemonic processing, CA1 pyramidal cells in rats typically discharge repetitively in either high-frequency bursts ('complex spikes') or single spikes, both of which are tightly phase-locked to the hippocampal theta rhythm. Interestingly, patterned stimulation which mimics the repetitive, learning-related complex spike discharges are optimal for inducing long-term potentiation (LTP) of excitatory field potentials in CA1, and patterned stimulation which mimics the theta-related single action potentials results in a robust and lasting depotentiation at these same synapses. The aim of the present study was to determine the extent to which these physiologically-relevant patterns of hippocampal stimulation have similar effects on synaptic efficacy in the monosynaptic projection from CA1 to the perirhinal and postrhinal cortices (PRh), areas thought to play a prominent role in many forms of learning and memory. Single-pulse stimulation of CA1 evoked a small amplitude, short latency population excitatory postsynaptic potential (EPSP) in the PRh. Theta-burst stimulation (TBS; n = 8) delivered to CA1 reliably potentiated the PRh EPSP slope for at least 30 min. Theta-pulse stimulation (TPS; 5 Hz; n = 4) delivered to CA1 5 min after TBS substantially but transiently suppressed EPSP slope relative to that of potentiated control preparations. Collectively these data suggest that theta-related patterns of hippocampal activation can reliably induce and transiently suppress LTP in PRh, and are consistent with the notion that behaviorally-relevant, theta-modulated patterns of CA1 unit activity may result in both long- and short-term alterations of synaptic strength within their rhinal cortical targets.

Oxygen deprivation produces delayed inhibition of long-term potentiation by activation of NMDA receptors and nitric oxide synthase

The acute and delayed effects of anoxia on synaptic transmission and long-term potentiation (LTP) were examined in the CA1 region of rat hippocampal slices. Oxygen deprivation for 20 minutes completely but reversibly depressed excitatory postsynaptic potentials mediated by both N-methyl-D-aspartate receptors (NMDAR) and non-NMDAR. Although LTP was reliably produced by a single tetanus delivered 30 minutes after reoxygenation, LTP could not be induced when a tetanus was delivered 70 to 100 minutes after reoxygenation. A tetanus delivered 100 minutes after reoxygenation produced lasting synaptic enhancement when 100 mumol/L D,L-amino-phosphonovaleric acid (APV), a competitive NMDAR antagonist, was administered during the period of oxygen deprivation. The delayed effects of oxygen deprivation were not blocked when APV was administered after oxygen deprivation. Similarly, the delayed effects on LTP induction were overcome by inhibitors of nitric oxide synthase when the nitric oxide synthase inhibitors were administered during anoxia, but not when administered after oxygen deprivation. These results suggest that untimely activation of NMDAR and nitric oxide release during anoxia produce delayed inhibition of LTP induction and may be involved in the memory defects that occur subsequent to cerebral hypoxia.

Interaction between tetanus long-term potentiation and hypoxia-induced potentiation in the rat hippocampus

Interaction between tetanus long-term potentiation and hypoxia-induced potentiation in the rat hippocampus. J. Neurophysiol. 78: 2475-2482, 1997. The interaction between tetanus-induced long-term potentiation (LTP) and hypoxia-induced potentiation was investigated by performing extracellular recordings in the CA1 region of rat hippocampus using a two-pathway design. Hippocampal slices were placed in an interface chamber containing artificial cerebrospinal fluid (ACSF) solution with high magnesium concentration. Hypoxia was induced by replacing the 5% CO2-95% O2 gas mixture with 5% CO2-95% N2 for 2 min. Tetanus-LTP was induced with 1-s, 100-Hz current pulses. Significant hypoxia-induced potentiation of the slope of the dendritic excitatory postsynaptic potential (EPSP) was found in ACSF containing 2 mM of magnesium 2, 27 +/- 10% (mean +/- SE; n = 16; P < 0.01) with no change in the mean amplitude of the presynaptic volley. All experiments in which a stable control baseline was obtained were used for data analysis. The data show that short episodes (2 min) of hypoxia can induce LTP of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-mediated synaptic transmission. The present study demonstrated that after tetanus-LTP, 33 +/- 3% (n = 10; P < 0.01), hypoxia further potentiated the field EPSP slopes by a mean value of 16 +/- 5% (n = 10; P < 0.05). Moreover, using a two-pathway design, we showed that hypoxia produced similar potentiation in both the control [19 +/- 5%; n = 10; P < 0.01) and tetanus-induced LTP pathway, and the total potentiation produced by a combination of tetanus then hypoxia, 63 +/- 13% (n = 10; p < 0.01), was significantly larger (P < 0.01) than hypoxia alone. These data suggest that hypoxia-induced potentiation is additive with tetanus-LTP. Occlusion experiments were performed to verify whether the mechanisms responsible for hypoxia-induced potentiation are independent of preexisting synaptic levels induced by high-frequency stimulation. Hypoxia produced significant potentiation (23 +/- 7%; n = 7; P < 0.05) after successful occlusion of the LTP pathway. Therefore, because the magnitude of hypoxia-induced potentiation is both independent of preexisting synaptic levels and also additive, synaptic specificity associated with LTP is preserved. The magnitude of tetanus-LTP induced 20 min after hypoxia (15 +/- 4%; n = 10) was significantly smaller (P < 0.01) relative to LTP after normoxic conditions (33 +/- 3%; n = 10). Additionally, hypoxia blocked the transient, robust potentiation occurring during the early phase of LTP induction. This study suggests that although hypoxia modifies neuronal processing by general excitation, synaptic specificity associated with tetanus-LTP still is preserved. However, hypoxia can disrupt neuronal processing by inhibiting new modification of synaptic transmission.

Evidence of very fast memory consolidation: an intracarotid amytal study

This study provides evidence from intracarotid amobarbital tests (IAT) in patients with epilepsy that complete suppression of the electrophysiological activity of the left language-dominant hemisphere through left IAT does not impair memory for verbal information given 1 min or immediately before the injection of the barbiturate. Although language functions were completely disrupted and patients were unable to encode new information during the left IAT, pre/post memory was as good as in the right IAT. This lack of retrograde amnesia leads to the conclusion that a relatively stable representation of events is achieved within seconds after encoding. The results contradict models which assume that short-term memory is based on purely electrophysiological processes.

The effects of metabolic stress on glutamate receptor-mediated depolarizations in the in vitro rat hippocampal slice

A grease-gap preparation for the in vitro rat hippocampal slice has been used to record field excitatory postsynaptic potentials (fEPSPs), extracellular d.c. potential and depolarizations in response to glutamate receptor agonists before, during and after hypoxic/ischaemic episodes in the CA1 region. Synaptic transmission was depressed by hypoxia in a temperature-dependent manner (t1/2 at 28 degrees C, 1.9 +/- 0.2 min; t1/2 at 36 degrees C, 1.0 +/- 0.1 min) but was unaffected by the absence of D-glucose during hypoxia (ischaemia) at 28 degrees C. The reappearance of the fEPSP during hypoxic/ischaemic episodes was a prelude to severe disruptions of synaptic transmission if control conditions were not reinstated within 1 min of the secondary depression of the fEPSP. For a 10 min episode of hypoxia, recovery of synaptic transmission at 28 degrees C (96 +/- 1.5% of control) was significantly better than recovery following either hypoxia at 36 degrees C or ischaemia at 28 degrees C (41 +/- 17.2% and 55 +/- 21% of control, respectively). Chart recordings of the d.c. potential during hypoxia revealed a predominate (67% of all episodes) triphasic sequence of events (i, hyperpolarization; ii, depolarization; iii, post-hypoxic hyperpolarization on reoxygenation). Depolarizing responses to N-methyl-D-aspartate (NMDA, 20-40 microM; in 1 mM extracellular Mg2+), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA, 2-10 microM) and L-glutamate (L-Glu, 2-5 mM) could be elicited at times when fEPSPs were completely depressed and up to 20 min into a hypoxic episode, the latest time-point examined. This implies, as others have suggested, that the hypoxic depression of excitatory synaptic transmission is presynaptic in origin. The application of AMPA or NMDA during the hypoxic depression of the fEPSP occasionally resulted in a short-lasting (12-45 min) potentiation (117-143% of control) of the fEPSP on return to normoxia. Furthermore, in other slices, which were exposed to severe metabolic stress, synaptic transmission was depressed to a significantly greater extent than AMPA depolarizations (mean depression; 76 +/- 5% and 28 +/- 8%, respectively).

Novel expression mechanism for synaptic potentiation: alignment of presynaptic release site and postsynaptic receptor

A combination of experimental and modeling approaches was used to study cellular-molecular mechanisms underlying the expression of short-term potentiation (STP) and long-term potentiation (LTP) of glutamatergic synaptic transmission in the hippocampal slice. Electrophysiological recordings from dentate granule cells revealed that high-frequency stimulation of perforant path afferents induced a robust STP and LTP of both (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptor-mediated synaptic responses. However, the decay time constant for STP of the AMPA receptor-mediated excitatory postsynaptic potential was approximately 6 min, whereas the decay time constant for STP of the NMDA receptor-mediated excitatory postsynaptic potential was only 1 min. In addition, focal application of agonists during the expression of STP revealed that the magnitude of conductance change elicited by NMDA application was significantly enhanced, whereas the magnitude of conductance change elicited by application of AMPA remained constant. These findings are most consistent with a postsynaptic mechanism of STP and LTP. Different putative mechanisms were evaluated formally using a computational model that included diffusion of glutamate within the synaptic cleft, different kinetic properties of AMPA and NMDA receptor/channels, and geometric relations between presynaptic release sites and postsynaptic receptor/channels. Simulation results revealed that the only hypothesis consistent with experimental data is that STP and LTP reflect a relocation of AMPA receptor/channels in the postsynaptic membrane such that they become more closely "aligned" with presynaptic release sites. The same mechanism cannot account for STP or LTP of NMDA receptor-mediated responses; instead, potentiation of the NMDA receptor subtype is most consistent with an increase in receptor sensitivity or number.