The effects of repetitive low frequency stimulation on control and "potentiated" synaptic responses in the hippocampus

Impaired fear memory, altered object memory and modified hippocampal synaptic plasticity in split-brain mice

The hippocampus is critical for memory formation. However, the contributions of the hippocampal commissure (HC) and the corpus callosum (CC) are less clear. To elucidate the role of the forebrain commissures in learning and memory, we performed a behavioural and electrophysiological characterization of an inbred mouse strain that displays agenesis of the CC and congenitally reduced HC (BTBR T+ tf/J; 'BTBR'). Compared to a control strain, BTBR mice have severely impaired contextual fear memory, with normal object recognition memory. Interestingly, continuous environmental "enrichment" significantly increased object recognition in BTBR, but not in control C57BL/6 ('BL/6') mice. In area CA1 of hippocampal slices, BTBR displayed intact expression of long-term potentiation (LTP), paired-pulse facilitation (PPF) and basal synaptic transmission, compared to BL/6 mice. However, BTBR hippocampal slices show an increased susceptibility to depotentiation (DPT), an activity-induced reversal of LTP. We conclude that the HC and CC are critical for some forms of hippocampal memory and for synaptic resistance to DPT. Agenesis of the CC and HC may unmask some latent ability to encode, store or retrieve certain forms of recognition memory. We suggest that the increased susceptibility to DPT in BTBR may underlie the memory phenotype reported here.

Synapse elimination accompanies functional plasticity in hippocampal neurons

A critical component of nervous system development is synapse elimination during early postnatal life, a process known to depend on neuronal activity. Changes in synaptic strength in the form of long-term potentiation (LTP) and long-term depression (LTD) correlate with dendritic spine enlargement or shrinkage, respectively, but whether LTD can lead to an actual separation of the synaptic structures when the spine shrinks or is lost remains unknown. Here, we addressed this issue by using concurrent imaging and electrophysiological recording of live synapses. Slices of rat hippocampus were cultured on multielectrode arrays, and the neurons were labeled with genes encoding red or green fluorescent proteins to visualize presynaptic and postsynaptic neuronal processes, respectively. LTD-inducing stimulation led to a reduction in the synaptic green and red colocalization, and, in many cases, it induced a complete separation of the presynaptic bouton from the dendritic spine. This type of synapse loss was associated with smaller initial spine size and greater synaptic depression but not spine shrinkage during LTD. All cases of synapse separation were observed without an accompanying loss of the spine during this period. We suggest that repeated low-frequency stimulation simultaneous with LTD induction is capable of restructuring synaptic contacts. Future work with this model will be able to provide critical insight into the molecular mechanisms of activity- and experience-dependent refinement of brain circuitry during development.

Synaptic plasticity, memory and the hippocampus: a neural network approach to causality

Two facts about the hippocampus have been common currency among neuroscientists for several decades. First, lesions of the hippocampus in humans prevent the acquisition of new episodic memories; second, activity-dependent synaptic plasticity is a prominent feature of hippocampal synapses. Given this background, the hypothesis that hippocampus-dependent memory is mediated, at least in part, by hippocampal synaptic plasticity has seemed as cogent in theory as it has been difficult to prove in practice. Here we argue that the recent development of transgenic molecular devices will encourage a shift from mechanistic investigations of synaptic plasticity in single neurons towards an analysis of how networks of neurons encode and represent memory, and we suggest ways in which this might be achieved. In the process, the hypothesis that synaptic plasticity is necessary and sufficient for information storage in the brain may finally be validated.

Inhibition of associative long-term depression by activation of β-adrenergic receptors in rat hippocampal CA1 synapses

The aim of this study was to investigate the role of beta-adrenergic receptors in modulating associative long-term depression (LTD) at CA1 synapses in rat hippocampal slices. Standard extracellular electrophysiological techniques were employed to record field excitatory post-synaptic potential (fEPSP) activity and to induce associative LTD. Two independent Schaffer collateral pathways were elicited in hippocampal CA1 areas. In one (weak) pathway, the stimulating intensity was adjusted to elicit small fEPSP activity (20-30% of the maximum response). In contrast, 80-90% of the maximum response was evoked in the other (strong) pathway. Associative LTD of weak pathway could be induced by paired stimulation of weak and the strong pathways, repeated 100 times at 0.167 Hz. The associative LTD of weak pathway was NMDA receptor- and phosphatase 2B dependent, because bath application of 50 microM D, L-AP5 or 10 microM cypermethrin blocked its induction. Bath application of 1 microM isoproterenol inhibited associative LTD, and this effect was blocked by timolol, suggesting the involvement of beta-adrenergic receptors. The inhibitory effect of beta-adrenergic receptors on LTD induction was blocked in slices pretreated with inhibitors of protein kinase A and mitogen-activated protein kinase, suggesting that these signal cascades are downstream effectors following activation of beta-adrenergic receptors. Nevertheless, bath application of timolol or cypermethrin alone did not have significant effect on associative LTD induction, suggesting neither endogenous function of beta-adrenergic receptor nor endogenous PKA activity does have a role in associative LTD induction.

Role of NMDA receptor subtypes in different forms of NMDA-dependent synaptic plasticity

Background

The involvement of different NMDA receptor (NMDAR) subunits has been implicated in several forms of synaptic plasticity. However, it is still controversial to what extent the involvement is specific, and little is known about the role of NMDAR subunits in certain "non-conventional" forms of plasticity. In this study we used subunit-specific blockers to test the roles of NR2A- and NR2B-containing NMDARs in a type of chemical long-term depression (LTD) induced by brief bath application of the NMDAR agonist NMDA to hippocampal slices from 12–18 days old rats. For comparison, we also examined other forms of plasticity, including a "slow LTD" induced by 0.1 Hz stimulation under low Mg²⁺ conditions as well as long-term potentiation (LTP).

Results

A blocker of NR2A-containing NMDARs, NVP-AAM077 (NVP), substantially reduced the two forms of studied depression whereas blockers of NR2B-containing NMDARs, Ro25-6981 (Ro) or Ifenprodil (Ife), had no significant effect on them. LTP appeared to be more sensitive as it was fully blocked by NVP and partially blocked by Ro or Ife. However, the blocking effects of NVP could be counteracted by general amplification of NMDA responses by lowering Mg²⁺ concentration in the perfusion solution. Applying NVP or Ro/Ife on isolated NMDA-EPSPs recorded in low Mg²⁺ solution reduced responses to about 70% and 20% of initial size, respectively, whereas coapplication of both blockers almost completely abolished the responses. Additionally, NMDA application caused depotentiation of a pathway with prior tetanus-induced LTP, and NVP but not Ro/Ife substantially prevented that depotentiation as well as the chemical LTD of the control pathway. A second tetanus on the LTP pathway induced repotentiation which was fully blocked by NVP but partially blocked by Ro/Ife.

Conclusion

All of these results on hippocampal slices from young rats can be explained by a simple model, in which NR2A subunits dominate over NR2B subunits with respect to both plasticity and NMDAR-mediated responses. The model suggests that Ca²⁺ influx into the postsynaptic spine via different subtypes of NMDARs makes up a "final common pathway", controlling synaptic plasticity by its magnitude and temporal pattern regardless of the source.

Transcranial magnetic stimulation and synaptic plasticity: experimental framework and human models

Interest in the therapeutic potential of non-invasive human brain stimulation has been boosted by an improved understanding of the mechanisms of synaptic plasticity and the stimulus protocols that can induce plasticity in experimental preparations. A range of transcranial magnetic stimulation (TMS) protocols are available that have the potential to mimic these experimental protocols in the human. Repetitive TMS emulates aspects of activity-dependent plasticity, and theta-burst refinements may be able to take into account excitatory and inhibitory networks, paired associative stimulation can extend network considerations to incorporate sensorimotor integration, inhibitory networks may be targeted with short-interval paired stimulation and finally even the precision of spike-timing dependent plasticity may be accessible through I-(indirect)wave dynamics. This review will provide a synthesis of current concepts of activity- and time-dependent plasticity and their homeostatic regulation based on experimental studies, and relate these concepts to the promising range of TMS interventions that are available to target human brain plasticity.

Aging effects on the limits and stability of long-term synaptic potentiation and depression in rat hippocampal area CA1

Altered hippocampal synaptic plasticity may underlie age-related memory impairment. In acute hippocampal slices from aged (22-24 mo) and young adult (1-12 mo) male Brown Norway rats, extracellular excitatory postsynaptic field potentials were recorded in CA1 stratum radiatum evoked by Schaffer collateral stimulation. We used enhanced Ca(2+) to Mg(2+) ratio and paired-pulse stimulation protocol to induce maximum changes in the synaptic plasticity. Six episodes of theta-burst stimulation (TBS) or nine episodes of paired low-frequency stimulation (pLFS) were used to generate asymptotic long-term potentiation (LTP) and long-term depression (LTD), respectively. In addition, long-term depotentiation (LTdeP) or de-depression (LTdeD) from maximal LTP and LTD were examined using two episodes of pLFS or TBS. Multiple episodes of TBS or pLFS produced significant LTP or LTD in aged and young adult rats; this was not different between age groups. Moreover, there was no significant difference in the amount of LTdeP or LTdeD between aged and young adult rats. Our results show no age differences in the asymptotic magnitude of LTP or LTD, rate of synaptic modifications, development rates, reversal, or decay after postconditioning. Thus impairment of the basic synaptic mechanisms responsible for expression of these forms of plasticity is not likely to account for decline in memory function within this age range.

Capabilities of the WinLTP data acquisition program extending beyond basic LTP experimental functions

WinLTP is a Windows data acquisition program designed for the investigation of long-term potentiation (LTP), long-term depression (LTD), and synaptic responses in general. The capabilities required for basic LTP and LTD experiments include alternating two-input extracellular pathway stimulation, LTP induction by single train, theta burst, and primed burst stimulation, and LTD induction by low frequency stimulation. WinLTP provides on-line analyses of synaptic waveforms including measurement of slope, peak amplitude, population-spike amplitude, average amplitude, area, rise time, decay time, duration, cell input resistance, and series resistance. WinLTP also has many advanced capabilities that extend beyond basic LTP experimental capabilities: (1) analysis of all the evoked synaptic potentials individually within a sweep, and the analysis of the entire train-evoked synaptic response as a single entity. (2) Multitasking can be used to run a Continuous Acquisition task (saving data to a gap-free Axon Binary File), while concurrently running the Stimulation/Acquisition Sweeps task. (3) Dynamic Protocol Scripting can be used to make more complicated protocols involving nested Loops (with counters), Delays, Sweeps (with various stimulations), and Run functions (which execute a block of functions). Protocol flow can be changed while the experiment is running. WinLTP runs on National Instruments M-Series and Molecular Devices Digidata 132x boards, and is available at www.winltp.com.

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.

Long-term depression: multiple forms and implications for brain function

Long-term potentiation (LTP) and long-term depression (LTD) remain widely accepted vertebrate models for the cellular and molecular mechanisms that underlie synaptic changes during learning and memory. Although LTD is a phenomenon that occurs in many regions of the CNS, it is clear that the mechanisms recruited in its induction and expression can vary, depending on many factors, including brain region and developmental time point. LTD in the hippocampus and cerebellum is probably the best characterized, although there are also other brain areas where mechanisms of LTD are well understood, and where it is thought to have a functional role.

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.

Plastic processes in the dentate gyrus: a computational perspective

The dentate gyrus has the capacity for numerous types of synaptic plasticity that use diverse mechanisms and are thought essential for the storage of information in the hippocampus. Here we review the various forms of synaptic plasticity that involve afferents and efferents of the dentate gyrus, and, from a computational perspective, relate how these plastic processes might contribute to sparse, orthogonal encoding, and the selective recall of information within the hippocampus.

Coexistence of muscarinic long-term depression with electrically induced long-term potentiation and depression at CA3-CA1 synapses

Our laboratory recently characterized a form of long-term depression (LTD) at CA3-CA1 synapses mediated by M1 muscarinic receptors (mAChRs), termed muscarinic LTD (mLTD). mLTD is both activity and NMDAR dependent, characteristics shared by forms of synaptic plasticity thought to be relevant to learning and memory, including long-term potentiation (LTP) induced by high-frequency stimulation (HFS-LTP) and long-term depression induced by low-frequency stimulation (LFS-LTD). However, it remains unclear whether mLTD can occur sequentially with these electrically induced forms of hippocampal plasticity or whether mLTD might interact with them. The first goal of this study was to examine the interplay of mLTD and HFS-LTP. We report that mLTD expression does not alter subsequent induction of HFS-LTP and, further, at synapses expressing HFS-LTP, mLTD can mediate a novel form of depotentiation. The second goal was to determine whether mLTD would alter LFS-LTD induction and/or expression. Although we show that mLTD is occluded by saturation of LFS-LTD, suggesting mechanistic similarity between these two plasticities, saturation of mLTD does not occlude LFS-LTD. Surprisingly, however, the LFS-LTD that follows cholinergic receptor activation is NMDAR independent, indicating that application of muscarinic agonist induces a change in the induction mechanism required for LFS-LTD. These data demonstrate that mLTD can coexist with electrically induced forms of synaptic plasticity and support the hypothesis that mLTD is one of the mechanisms by which the cholinergic system modulates hippocampal function.

Extinction training in conjunction with a partial agonist of the glycine site on the NMDA receptor erases memory trace

Much evidence indicates that extinction training does not erase memory traces but instead forms inhibitory learning that prevents the expression of original memory. Fear conditioning induces long-term potentiation and drives synaptic insertion of AMPA receptors into the amygdala. Here we show that extinction training applied 1 h after training reversed the conditioning-induced increase in surface glutamate receptor subunit 1 (GluR1) in parallel with the inhibition of startle potentiation. However, if applied 24 h after training, extinction training reduced startle potentiation without influencing the GluR1 increase. We infused D-cycloserine (DCS), a partial agonist of the glycine site on the NMDA receptor, bilaterally into the amygdala 30 min before extinction training. This augmented the extinction training-elicited reduction in startle and reversed the conditioning-induced increase in GluR1. Delivery of five sets of tetanic stimulation (TS) to the external capsule produced a robust enhancement of synaptic responses in the lateral amygdala neurons that persisted for >2 h. Low-frequency stimulation applied 1 h after TS had no long-lasting effect on synaptic responses. The same treatments, however, induced depotentiation in the presence of DCS and reversed TS-induced increase in surface GluR1. Together, this study has two important findings: (1) whether a memory trace remains intact or is erased depends on the interval between conditioning and extinction training and (2) DCS facilitates the reversal of memory trace. DCS-induced augmentation of extinction and reversal of GluR1 surface expression are likely mediated by DCS-facilitated endocytosis of AMPA receptors.

Persistent (>24h) long-term depression in the dentate gyrus of freely moving rats is not dependent on activation of NMDA receptors, L-type voltage-gated calcium channels or protein synthesis

Hippocampal long-term depression (LTD) comprises a persistent reduction of synaptic strength that is typically induced by low frequency stimulation (LFS). Although LTD has been described for the dentate gyrus in vitro, this phenomenon in the dentate gyrus of the intact animal is less well understood. In the current study, we investigated the contribution of NMDA receptors, L-type voltage gated calcium channels and protein synthesis to LFS-induced LTD in the dentate gyrus of freely moving rats. Animals were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. LTD persisted for at least 24h, and was unaffected by prior treatment with the NMDA receptor antagonists AP5 or ifenprodil, which, in contrast, prevented LTP. Neither the L-type voltage-gated calcium channel antagonist, methoxyverapamil, nor the protein translation inhibitors, anisomycin or emetine had an effect on the profile of LTD. Our results suggest that NMDA receptors and L-type voltage-gated calcium channels are not involved in the induction of LTD in the dentate gyrus in vivo. Intriguingly, persistent LTD can be established without the synthesis of new proteins, suggesting that in the dentate gyrus, alternative mechanisms exist for the sustainment of enduring LTD.

Ampakines and the threefold path to cognitive enhancement

Ampakines are the first peripherally administered drugs that increase excitatory monosynaptic responses in the brain. Because of this effect, the compounds improve communication in complex networks, potently facilitate long-term potentiation (LTP) and induce the expression of neurotrophic factors. Ampakines are effective in animal models of neuropsychiatric disorders and have produced positive results in a small number of human studies. Neurobiological considerations and modeling studies suggest that the drugs, in addition to their effects on disturbed behavior, will alter the encoding and organization of information in normal brains. Results from physiological and behavioral studies accord with this prediction. Building on these findings, this article considers how the threefold effects of ampakines will modify, and enhance, cognition.

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.

Low-frequency stimulation induces a new form of LTP, metabotropic glutamate (mGlu5) receptor- and PKA-dependent, in the CA1 area of the rat hippocampus

Low frequency-induced short-term synaptic plasticity was investigated in hippocampal slices with 60-electrode recording array. Remarkably, the application of low-frequency stimulation (1 Hz) for a short duration (3-5 min) resulted in the induction of a slow-onset long-term potentiation (LTP) in the immediate vicinity of the stimulated electrode. This phenomenon was observed exclusively in the CA1 subfield, neither in the CA3 area nor in the dentate gyrus. The induction of this slow-onset LTP required neither N-methyl-D-aspartate (NMDA) nor non-NMDA ionotropic receptor activation but was strongly dependent on metabotropic glutamate mGlu(5) receptor stimulation and [Ca(2+)]i increase. In addition, this form of synaptic plasticity was associated with an increase in cAMP concentration and required protein kinase A activation. Paired-pulse facilitation ratio and presynaptic fiber volley amplitude were unaffected when this LTP was triggered, suggesting the involvement of postsynaptic modifications. Although mitogen activated protein kinase pathway was stimulated after the application of low frequency, the induction and maintenance of this slow-onset LTP were not dependent on the activation of this intracellular pathway. The direct activation of adenylyl cyclase with forskolin also induced a synaptic enhancement displaying similar features. This new form of LTP could represent the mnesic engram of mild and repetitive stimulation involved in latent learning.

Long-lasting modulation of the induction of LTD and LTP in rat hippocampal CA1 by behavioural stress and environmental enrichment

Behavioural experience (e.g. chronic stress, environmental enrichment) can have long-lasting effects on cognitive functions. Because activity-dependent persistent changes in synaptic strength are believed to mediate memory processes in brain areas such as hippocampus, we tested whether behaviour has also long-lasting effects on synaptic plasticity by examining the induction of long-term potentiation (LTP) and long-term depression (LTD) in slices of hippocampal CA1 obtained from rats either 7-9 months after social defeat (behavioural stress) or 3-5 weeks after 5-week exposure to environmental enrichment. Compared with age-matched controls, defeated rats showed markedly reduced LTP. LTP was even completely impaired but LTD was enhanced in defeated and, subsequently, individually housed (during the 7-9-month period after defeat) rats. However, increasing stimulus intensity during 100-Hz stimulation resulted in significant LTP. This suggests that the threshold for LTP induction is still raised and that for LTD lowered several months after a short stressful experience. Both LTD and LTP were enhanced in environmentally enriched rats, 3-5 weeks after enrichment, as compared with age-matched controls. Because enrichment reduced paired-pulse facilitation, an increase in presynaptic release, facilitating both LTD and LTP induction, might contribute to enhanced synaptic changes. Consistently, enrichment reduced the number of 100-Hz stimuli required for inducing LTP. But enrichment may also actually enhance the range of synaptic modification. Repeated LTP and LTD induction produced larger synaptic changes in enriched than in control rats. These data reveal that exposure to very different behavioural experiences can produce long-lasting effects on the susceptibility to synaptic plasticity, involving pre- and postsynaptic processes.

Metaplasticity of the late-phase of long-term potentiation: a critical role for protein kinase A in synaptic tagging

The late-phase of long-term potentiation (L-LTP) in hippocampal area CA1 requires gene expression and de novo protein synthesis but it is expressed in an input-specific manner. The 'synaptic tag' theory proposes that gene products can only be captured and utilized at synapses that have been 'tagged' by previous activity. The mechanisms underlying synaptic tagging, and its activity dependence, are largely undefined. Previously, we reported that low-frequency stimulation (LFS) decreases the stability of L-LTP in a cell-wide manner by impairing synaptic tagging. We show here that a phosphatase inhibitor, okadaic acid, blocked homosynaptic and heterosynaptic inhibition of L-LTP by prior LFS. In addition, prior LFS homosynaptically and heterosynaptically impaired chemically induced synaptic facilitation elicited by forskolin/3-isobutyl-1-methylxanthine, suggesting that there is a cell-wide dampening of cAMP/protein kinase A (PKA) signaling concurrent with phosphatase activation. We propose that prior LFS impairs expression of L-LTP by inhibiting synaptic tagging through its actions on the cAMP/PKA pathway. In support of this notion, we show that hippocampal slices from transgenic mice that have genetically reduced hippocampal PKA activity display impaired synaptic capture of L-LTP. An inhibitor of PKA, KT-5720, also blocked synaptic capture of L-LTP. Moreover, pharmacological activation of the cAMP/PKA pathway can produce a synaptic tag to capture L-LTP expression, resulting in persistent synaptic facilitation. Collectively, our results show that PKA is critical for synaptic tagging and for input-specific L-LTP. PKA-mediated signaling can be constrained by prior episodes of synaptic activity to regulate subsequent L-LTP expression and perhaps control the integration of multiple synaptic events over time.

Integrin-driven actin polymerization consolidates long-term potentiation

Long-term potentiation (LTP), like memory, becomes progressively more resistant to disruption with time after its formation. Here we show that threshold conditions for inducing LTP cause a rapid, long-lasting increase in polymerized filamentous actin in dendritic spines of adult hippocampus. Two independent manipulations that reverse LTP disrupted this effect when applied shortly after induction but not 30 min later. Function-blocking antibodies to beta1 family integrins selectively eliminated both actin polymerization and stabilization of LTP. We propose that the initial stages of consolidation involve integrin-driven events common to cells engaged in activities that require rapid morphological changes.

Beta-adrenergic receptor activation facilitates induction of a protein synthesis-dependent late phase of long-term potentiation

Long-term potentiation (LTP) is activity-dependent enhancement of synaptic strength that can critically regulate long-term memory storage. Like memory, LTP exhibits at least two mechanistically distinct temporal phases. Early LTP (E-LTP) does not require protein synthesis, whereas the late phase of LTP (L-LTP), like long-term memory, requires protein synthesis. Hippocampal beta-adrenergic receptors can regulate expression of both E-LTP and long-term memory. Although beta-adrenergic receptor activation enhances the ability of subthreshold stimuli to induce E-LTP, it is unclear whether such activation can facilitate induction of L-LTP. Here, we use electrophysiological recording methods on mouse hippocampal slices to show that when synaptic stimulation that is subthreshold for inducing L-LTP is paired with beta-adrenergic receptor activation, the resulting LTP persists for over 6 h in area CA1. Like L-LTP induced by multiple trains of high-frequency electrical stimulation, this LTP requires protein synthesis. Unlike tetanus-induced L-LTP, however, L-LTP induced by beta-adrenergic receptor activation during subthreshold stimulation appears to involve dendritic protein synthesis but not somatic transcription. Maintenance of this LTP also requires activation of extracellular signal-regulated kinases (ERKs). Thus, beta-adrenergic receptor activation elicits a type of L-LTP that requires translation and ERK activation but not transcription. This form of L-LTP may be a cellular mechanism for facilitation of behavioral long-term memory during periods of heightened emotional arousal that engage the noradrenergic modulatory system.

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.

Homosynaptic and heterosynaptic inhibition of synaptic tagging and capture of long-term potentiation by previous synaptic activity

Long-term potentiation (LTP) is an enhancement of synaptic strength that may contribute to information storage in the mammalian brain. LTP expression can be regulated by previous synaptic activity, a process known as "metaplasticity." Cell-wide occurrence of metaplasticity may regulate synaptic strength. However, few reports have demonstrated metaplasticity at synapses that are silent during activity at converging synaptic inputs. We describe a novel form of cell-wide metaplasticity in hippocampal area CA1. Low-frequency stimulation (LFS) decreased the stability of long-lasting LTP ["late" LTP (L-LTP)] induced later at the same inputs (homosynaptic inhibition) and at other inputs converging on the same postsynaptic cells (heterosynaptic inhibition). Significantly, heterosynaptic inhibition of L-LTP also occurred across basal and apical dendrites ("heterodendritic" inhibition). Because transient early LTP (E-LTP) was not affected by previous LFS, we examined the effects of LFS on the consolidation of E-LTP to L-LTP. The duration of E-LTP induced at one set of inputs can be extended by capturing L-LTP-associated gene products generated by previous activity at other inputs to the same postsynaptic neurons. LFS applied homosynaptically or heterosynaptically before L-LTP induction did not impair synaptic capture by subsequent E-LTP stimulation, suggesting that LFS does not impair L-LTP-associated transcription. In contrast, LFS applied just before E-LTP (homosynaptically or heterosynaptically) prevented synaptic tagging, and capture of L-LTP expression. Thus, LFS inhibits synaptic tagging to impair expression of subsequent L-LTP. Such anterograde inhibition represents a novel way in which synaptic activity can regulate the expression of future long-lasting synaptic plasticity in a cell-wide manner.

Do stress and long-term potentiation share the same molecular mechanisms?

Stress is a biological, significant factor shown to influence hippocampal synaptic plasticity and cognitive functions. Although numerous studies have reported that stress produces a suppression in long-term potentiation (LTP; a putative synaptic mechanism underlying learning and memory), little is known about the mechanism by which this occurs. Because the effects of stress on LTP and its converse process, long-term depression (LTD), parallel the changes in synaptic plasticity that occur following the establishment of LTP with tetanic stimulation (i.e., occluding LTP and enhancing LTD induction), it has been proposed that stress affects subsequent hippocampal plasticity by sharing the same molecular machinery required to support LTP. This article summarizes recent findings from ours and other laboratories to assess this view and discusses relevant hypotheses in the study of stress-related modifications of synaptic plasticity.

Developmental switch from LTD to LTP in low frequency-induced plasticity

The stimulation of the Schaffer collateral/commissural fibers at low frequency (1 Hz) for 3-5 min can trigger a slow-onset form of low-frequency stimulation (LFS)-long-term potentiation (LTP) (LFS-LTP) in the CA1 area of the adult rat hippocampus. Here we have examined the developmental profile of this plasticity. In 9-15 day-old rats, the application of 1 Hz for 5 min induced long-term depression (LFS-LTD). In 17-21 day-old rats, 1 Hz stimulation had no effect when applied for 5 min but mediated LTD when stimulus duration was increased to 15 min. Over 25 day-old, 1 Hz stimulation mediated LFS-LTP. LFS-LTD was dependent on both N-methyl-D-aspartate (NMDA) and mGlu5 receptor activation. Antagonists of mGlu1alpha and cannabinoid type 1 receptor were ineffective to block LTD induction. LFS-LTD was not associated with a change in paired-pulse facilitation ratio, suggesting a postsynaptic locus of expression of this plasticity. Next, we examined whether LFS-LTD was related to 'chemical' LTDs obtained by the direct stimulation of mGlu5 and NMDA receptors. The saturation of LFS-LTD completely occluded NMDA- and (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG)-induced LTD. CHPG-LTD and NMDA-LTD occluded each other. In addition, we observed that NMDA-LTD was dependent on mGlu5 receptor activation in 9-12 day old rats while it was not in animals older than 15 day-old. Therefore we postulate that during LFS application, NMDA and mGlu5 receptor could interact to trigger LTD. Low-frequency-mediated synaptic plasticity is subject to a developmental switch from NMDA- and mGlu5 receptor-dependent LTD to mGlu5 receptor-dependent LTP with a transient period (17-21 day-old) during which LFS is ineffective.

Properties of long-term synaptic plasticity and metaplasticity in organotypic slice cultures of rat hippocampus

The aim of this study was to investigate whether synaptic plasticity and metaplasticity in slice cultures of the young rat hippocampus were comparable to previously reported synaptic plasticity and metaplasticity in acute adult hippocampal slices. This is relevant since differences do exist between the preparations as a result of age and the ex vivo maintenance. We prepared and maintained slice cultures from 5- to 6-day-old rats according to the porous membrane method. After 12-16 days in vitro, extracellular low-frequency stimulation (LFS) and high-frequency stimulation (HFS) protocols were applied to the Schaffer collaterals, and extracellular field potentials were recorded in area CA1. LFS and HFS induced long-term depression (LTD) and long-term potentiation (LTP), respectively. LTP could be reversed by LFS, as could LTD by HFS 60 min after induction. Plotting the amount of LTD and LTP versus stimulation protocol demonstrated frequency-dependence of the sign and extent of plasticity. Priming activation of group 1 metabotropic glutamate receptors (mGluRs) with DHPG facilitated subsequent LTP, revealing a metaplastic effect similar to that observed in acute slices. Immunohistochemistry for group 1 mGluR subtypes mGluR1alpha and mGluR5 showed both receptors to be present in these cultures. We conclude that synaptic plasticity and mGluR-mediated metaplasticity are largely comparable to those effects found in acute in vitro techniques.

Theta stimulation polymerizes actin in dendritic spines of hippocampus

It has been proposed that the endurance of long-term potentiation (LTP) depends on structural changes entailing reorganization of the spine actin cytoskeleton. The present study used a new technique involving intracellular and extracellular application of rhodamine-phalloidin to conventional hippocampal slices to test whether induction of LTP by naturalistic patterns of afferent activity selectively increases actin polymerization in juvenile to young adult spines. Rhodamine-phalloidin, which selectively binds to polymerized actin, was detected in perikarya and proximal dendrites of CA1 pyramidal cells that received low-frequency afferent activity but was essentially absent in spines and fine dendritic processes. Theta pattern stimulation induced LTP and caused a large (threefold), reliable increase in labeled spines and spine-like puncta in the proximal dendritic zone containing potentiated synapses. The spines frequently occurred in the absence of labeling to other structures but were also found in association with fluorescent dendritic processes. These effects were replicated (>10-fold increase in labeled spines) using extracellular applications of rhodamine-phalloidin. Increases in labeling appeared within 2 min, were completely blocked by treatments that prevent LTP induction, and occurred in slices prepared from young adult rats. These results indicate that near-threshold conditions for inducing stable potentiation cause the rapid polymerization of actin in mature spines and suggest that the effect is both sufficiently discrete to satisfy the synapse-specificity rule of LTP as well as rapid enough to participate in the initial stages of LTP consolidation.

Long-term potentiation persists in an occult state following mGluR-dependent depotentiation

Depotentiation, the reversal of long-term potentiation (LTP), can be induced by activation of metabotropic glutamate receptors (mGluRs) or NMDA receptors (NMDARs). Although NMDAR-dependent depotentiation is due to a protein phosphatase-dependent erasure of LTP, the notion that mGluR-dependent depotentiation also involves LTP erasure is controversial. To address this issue we used electrophysiological and biochemical approaches to investigate mGluR-dependent depotentiation in hippocampal slices. Activating group I mGluRs with (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced robust depotentiation in both the CA1 and CA3 regions of hippocampal slices. Western immunoblotting of samples prepared from DHPG-treated slices revealed, however, that activation of group I mGluRs causes a transient increase in phosphorylation of AMPA receptor GluR1 subunits at sites crucial for LTP and under some conditions causes persistent activation of alphaCamKII. The paradoxical ability of DHPG to induce depotentiation while at the same time activating signaling pathways involved in LTP suggests that LTP might not be erased by mGluR-dependent depotentiation. Consistent with this, DHPG-induced depotentiation did not restore the ability of high-frequency stimulation to induce LTP at synapses that had previously undergone saturating levels of LTP. In addition, blocking the expression of DHPG-induced LTD revealed hidden LTP at depotentiated synapses. Our results indicate that LTP and mGluR-dependent LTD can co-exist at excitatory synapses.

Dissection of Bidirectional Synaptic Plasticity Into Saturable Unidirectional Processes

In populations of synapses, overall synaptic strength can undergo either a net strengthening (long-term potentiation) or weakening (long-term depression). These phenomena have distinct induction pathways, but the functional outcome is usually measured as a single lumped quantity. In hippocampal CA3-CA1 synapses, we took two approaches to study the activity dependence of each phenomenon in isolation. First, we selectively blocked one process by applying kinase or phosphatase inhibitors known, respectively, to block potentiation or depression. Second, we saturated depression or potentiation and examined the activity dependence of the converse process. The resulting unidirectional learning rules could be recombined to give a well-known bidirectional frequency-dependent learning rule under the assumption that when both pathways are activated kinases dominate, resulting in potentiation. Saturation experiments revealed an additional process in which potentiated synapses can be locked at high strength. Saturability of the components of plasticity implies that the amount of plasticity contributed by each pathway depends on the initial level of strength of the synapses. Variation in the distribution of initial synaptic strengths predicts a form of metaplasticity and can account for differences in learning rules observed under several physiological and genetic manipulations.

Diabetes mellitus concomitantly facilitates the induction of long-term depression and inhibits that of long-term potentiation in hippocampus

Memory impairments, which occur regularly across species as a result of ageing, disease (such as diabetes mellitus) and psychological insults, constitute a useful area for investigating the neurobiological basis of learning and memory. Previous studies in rats found that induction of diabetes (with streptozotocin, STZ) impairs long-term potentiation (LTP) but enhances long-term depression (LTD) induced by high- (HFS) and low-frequency stimulations (LFS), respectively. Using a pairing protocol under whole-cell recording conditions to induce synaptic plasticity at Schaffer collateral synapses in hippocampal CA1 slices, we show that LTD and LTP have similar magnitudes in diabetic and age-matched control rats. But, in diabetic animals, LTD is induced at more polarized and LTP more depolarized membrane potentials (V(ms)) compared with controls: diabetes produces a 10 mV leftward shift in the threshold for LTD induction and 10 mV rightward shift in the LTD-LTP crossover point of the voltage-response curve for synaptic plasticity. Prior repeated short-term potentiations or LTP are known to similarly, though reversibly, lower the threshold for LTD induction and raise that for LTP induction. Thus, diabetes- and activity-dependent modulation of synaptic plasticity (referred to as metaplasticity) display similar phenomenologies. In addition, compared with naïve synapses, prior induction of LTP produces a 10 mV leftward shift in Vms for inducing subsequent LTD in control but not in diabetic rats. This could indicate that diabetes acts on synaptic plasticity through mechanisms involved in metaplasticity. Persistent facilitation of LTD and inhibition of LTP may contribute to learning and memory impairments associated with diabetes mellitus.

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.

Up-regulation of peripherin is associated with alterations in synaptic plasticity in CA1 and CA3 regions of hippocampus

Peripherin is a type III intermediate filament protein normally undetectable in most brain neurons. Here, we report a similar pattern of peripherin expression in the brains of both mice treated with systemic injections of kainic acid (KA) and in peripherin transgenic mice (Per mice) over-expressing the normal peripherin gene under its own promoter. Double-immunofluorescence labeling revealed a partial co-localization of peripherin with the microtubule-associated protein MAP2, but not with neurofilament proteins. Electrophysiological studies revealed that synaptic plasticity was markedly altered in Per mice: in CA1, long-term potentiation (LTP) was decreased in Per slices (+29 +/- 2.0%, vs. +58 +/- 5.4%, in WT); while in CA3, LTP was increased in Per (+63 +/- 3.5% vs. +43 +/- 2.4.0%). In the hippocampus of Per mice, the levels of MAP2 were decreased, though synaptophysin and PSD95 remained unchanged. These intriguing findings suggest a role of peripherin in the alteration of hippocampal synaptic plasticity.

Activation of group II metabotropic glutamate receptors induces depotentiation in amygdala slices and reduces fear-potentiated startle in rats

There is a close correlation between long-term potentiation (LTP) in the synapses of lateral amygdala (LA) and fear conditioning in animals. We predict that reversal of LTP (depotentiation) in this area of the brain may ameliorate conditioned fear. Activation of group II metabotropic glutamate receptors (mGluR II) with DCG-IV induces depotentiation in the LA. The induction of depotentiation is independent of NMDA receptors, L-type Ca++ channels, and calcineurin activity, but requires presynaptic activity and extracellular Ca++. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) depotentiation is accompanied by a decrease in the frequency but not the amplitude of miniature excitatory post-synaptic currents (mEPSCs) and could be mimicked by endogenously released glutamate. DCG-IV inhibited the release of glutamate evoked by 4-AP but not that evoked by ionomycin, suggesting that the effect of DCG-IV is not mediated by an action downstream of Ca++ entry. Intra-amygdala infusion of mGluR II agonist blocks the consolidation of fear memory measured with fear-potentiated startle. Taken together, the present results characterize the properties of DCG-IV depotentiation and reveal a close parallel between depotentiation in the amygdala slice and the reduction of conditioned fear in animals.

The metabotropic glutamate receptor mGluR3 is critically required for hippocampal long-term depression and modulates long-term potentiation in the dentate gyrus of freely moving rats

Group II metabotropic glutamate receptors (mGluRs) play an important role in the regulation of hippocampal synaptic plasticity in vivo: long-term potentiation (LTP) is inhibited and long-term depression (LTD) is enhanced by activation of these receptors. The contribution, in vivo, of the individual group II mGluR subtypes has not been characterized. We analysed the involvement of the subtype mGluR3 in LTD and LTP. Rats were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. Neither the selective mGluR3 agonist, N-acetylaspartylglutamate (NAAG), nor the antagonist beta-NAAG, given intracerebrally, affected basal synaptic transmission. beta-NAAG significantly inhibited LTD expression. NAAG exhibited transient inhibitory effects on the intermediate phase of LTD. Whereas NAAG altered paired-pulse responses, beta-NAAG had no effect, suggesting that antagonism of mGluR3 prevents LTD via a postsynaptic mechanism, whereas agonist activation of mGluR3 modulates LTD at a presynaptic locus. NAAG impaired the expression of LTP, whereas beta-NAAG had no effect. NAAG effects on LTP were blocked by EGLU, a selective group II mGluR antagonist. Our data suggest an essential role for mGluR3 in LTD, and a modulatory role for mGluR3 in LTP, with effects being mediated by distinct pre- and post-synaptic loci.

Group II mGluR-induced long term depression in the dentate gyrus in vivo is NMDA receptor-independent and does not require protein synthesis

Long term depression (LTD) can be induced by low frequency stimulation (LFS) as well as by agonist activation of neurotransmitter receptors. Group II metabotropic glutamate receptors (mGluRs) play an essential role in the regulation of electrically-induced LTD in the hippocampus in vivo: LTD is inhibited by antagonists, and enhanced by agonists of group II mGluRs. Here we investigated induction of LTD by activation of group II mGluRs as well as the cellular mechanisms which might mediate group II mGluR-induced LTD. Rats were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. Drug application was made through a cannula implanted into the ipsilateral cerebral ventricle. LTD could be induced by agonist activation of either group II mGluRs, or the group II mGluR subtype, mGluR3. Both, group II mGluR-induced LTD and mGluR3-induced LTD were not abolished by mRNA/protein synthesis inhibition. Furthermore, mGluR3-induced LTD was not inhibited by NMDA receptor antagonists or altered by L-type voltage-gated calcium channel blockers. Our data suggest that sole activation of group II mGluRs can mediate LTD in vivo. Intriguingly, this form of LTD is not dependent on protein synthesis or activation of NMDA receptors.

Reversal of Hippocampal LTP by Spontaneous Seizure-Like Activity: Role of Group I mGluR and Cell Depolarization

Memory impairment is a common consequence of epileptic seizures. The hippocampal formation is particularly prone to seizure-induced amnesia due to its prominent role in mnemonic processes. We used the isolated CA1 slice preparation to examine effects of seizure-like activity on hippocampal plasticity, long-term potentiation (LTP), and long-term depression (LTD). Repeated spontaneous ictal events, generated in the presence of antagonists of GABAA receptor function, led to a stepwise erasure of LTP (termed spontaneous depotentiation, SDP). SDP could be initiated at various stages of LTP consolidation (tested ≤120 min after the induction of LTP). Renewed tetanic stimulation re-established LTP. SDP was remarkably specific: baseline transmission and other forms of hippocampal plasticity, i.e., Ca2+-induced LTP and two forms of LTD [(RS)-3,5-dihydroxyphenyglycine (DHPG) mediated and low-frequency stimulation mediated] were not affected by the same type of seizure activity. SDP was blocked in the presence of the group I mGluR antagonist ( S)-4-carboxyphenylglycine. The mGluR1 antagonist ( S)-(+)-α-amino-methylbenzeneacetic acid blocked ∼80%, the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)-pyridine ∼30% of SDP. Most efficient implementation of SDP was observed during seizures in the combined presence of the group I mGluR agonist DHPG and the GABAA antagonist bicuculline. However, similar ictal activity generated in the presence of DHPG alone did not lead to SDP in the vast majority of recordings. Complete disinhibition and at least partial activation of group I mGluR were necessary conditions for the induction of SDP. The depotentiating pharmacological conditions were accompanied by tonic membrane depolarization of CA1 pyramidal cells. Since hyperpolarization (by negative current injection) prevented intracellular SDP under depotentiating pharmacological conditions and depolarization (by positive current injection) led to selective intracellular SDP in the non-depotentiating seizure protocol of DHPG, it is concluded that cell depolarization was a sufficient condition for seizure-like activity to reverse hippocampal LTP.

Spontaneous Waves in the Dentate Gyrus of Slices From the Ventral Hippocampus

Spontaneous negative-going potentials occurring at an average frequency of 0.7 Hz were recorded from the dentate gyrus of slices prepared from the temporal hippocampus of young adult rats. These events (here termed “dentate waves”) in several respects resembled the dentate spikes described for freely moving rats during immobile behaviors and slow-wave sleep. Action potentials were observed on the descending portion of the in vitro waves and, as expected from this, whole cell recordings established that the waves were composed of depolarizing currents. Dentate waves appeared to be locally generated within the granule cell layer and were greatly reduced by antagonists of AMPA-type glutamate receptors or by lesions to the entorhinal cortex. Simultaneous recordings indicated that the waves were often synchronized in the inner and outer blades of the dentate gyrus. Knife cuts through the perforant path and the commissural/associational system did not eliminate synchronization, leaving electrotonic propagation via gap junctions as its probable cause. In accord with this, cuts that separated the two blades of the dentate eliminated synchronization between them, and a compound that inhibits gap junctions reduced wave activity. Dentate waves were regularly accompanied by sharp waves in field CA3 and were reduced in size by the acetylcholinesterase inhibitor, physostigmine. It is hypothesized that dentate waves occur when spontaneous glutamate release from dentate afferents produces action potentials in neighboring granule cells that then summate electrotonically into a population event; once initiated, the waves propagate, again electrotonically, and thereby engage a significant portion of the granule cell population.

LTP and LTD: an embarrassment of riches

LTP and LTD, the long-term potentiation and depression of excitatory synaptic transmission, are widespread phenomena expressed at possibly every excitatory synapse in the mammalian brain. It is now clear that "LTP" and "LTD" are not unitary phenomena. Their mechanisms vary depending on the synapses and circuits in which they operate. Here we review those forms of LTP and LTD for which mechanisms have been most firmly established. Examples are provided that show how these mechanisms can contribute to experience-dependent modifications of brain function.

Bi-directional modulation of AMPA receptor unitary conductance by synaptic activity

Background

Knowledge of how synapses alter their efficiency of communication is central to the understanding of learning and memory. The most extensively studied forms of synaptic plasticity are long-term potentiation (LTP) and its counterpart long-term depression (LTD) of AMPA receptor-mediated synaptic transmission. In the CA1 region of the hippocampus, it has been shown that LTP often involves a rapid increase in the unitary conductance of AMPA receptor channels. However, LTP can also occur in the absence of any alteration in AMPA receptor unitary conductance. In the present study we have used whole-cell dendritic recording, failures analysis and non-stationary fluctuation analysis to investigate the mechanism of depotentiation of LTP.

Results

We find that when LTP involves an increase in unitary conductance, subsequent depotentiation invariably involves the return of unitary conductance to pre-LTP values. In contrast, when LTP does not involve a change in unitary conductance then depotentiation also occurs in the absence of any change in unitary conductance, indicating a reduction in the number of activated receptors as the most likely mechanism.

Conclusions

These data show that unitary conductance can be bi-directionally modified by synaptic activity. Furthermore, there are at least two distinct mechanisms to restore synaptic strength from a potentiated state, which depend upon the mechanism of the previous potentiation.

Reversal and consolidation of activity-induced synaptic modifications

Persistent activity-induced synaptic modification is generally regarded as the cellular basis for developmental refinement of neuronal connections and for learning and memory. It has long been recognized that synaptic modifications can be reversed by subsequent stimuli. Recent in vivo studies indicate that reversal of synaptic modifications is a natural process that can be triggered by physiological activity. Long-term potentiation (LTP) of hippocampal synapses in adult rats was reversed as rats entered a novel environment. LTP of retinotectal synapses in developing Xenopus was also reversed by subsequent spontaneous activity. Repetitive stimulation with spaced patterns, however, can overcome this reversal, leading to stabilized synaptic modifications. The requirement of spaced stimulus patterns for stable synaptic modifications could ensure appropriate refinement of developing connections.

Multidisciplinary approaches for investigating the mechanisms of hippocampus-dependent memory: a focus on inbred mouse strains

Inbred mouse strains differ in genetic makeup and display diverse learning and memory phenotypes. Mouse models of memory impairment can be identified by examining hippocampus-dependent memory in multiple strains. These mouse models may be used to establish the genetic, molecular, and cellular correlates of deficits in learning or memory. In this article, we review research that has characterized hippocampal learning and memory in inbred mouse strains. We focus on two well-established behavioral tests, contextual fear conditioning and the Morris water maze (MWM). Selected cellular and molecular correlates of good and poor memory performance in inbred strains are highlighted. These include hippocampal long-term potentiation, a type of synaptic plasticity that can influence hippocampal learning and memory. Further methods that might help to pinpoint the anatomical loci, and genetic and cellular/molecular factors that contribute to memory impairments in inbred mice, are also discussed. Characterization of inbred mouse strains, using multidisciplinary approaches that combine cellular, genetic, and behavioral techniques, can complement directed mutagenesis to help identify molecular mechanisms for normal and abnormal memory functions.

Aging impairs the late phase of long-term potentiation at the medial perforant path-CA3 synapse in awake rats

The effects of aging on long-term potentiation (LTP) in the dentate gyrus (DG) and CA1 are well documented, but LTP at the medial perforant path (MPP)-CA3 synapse of aged animals has remained unexplored. Because the MPP-DG and Schaffer-collateral-CA1 synapses account for only about 20% of total hippocampal synapses, global understanding of how aging affects hippocampal plasticity has remained limited. Much is known about LTP induction in the hippocampal formation, whereas the mechanisms that regulate LTP maintenance are less understood, especially during aging. We investigated the effects of aging on MPP-CA3 LTP induction and maintenance in awake rats. As is the case in the DG and CA1, high-frequency stimulation-induced LTP at the MPP-CA3 synapse is normal in aged rats. These data indicate that N-methyl-D-aspartate (NMDA) receptor-mediated processes are intact at the MPP-CA3 synapse in aged rats. In contrast, aging impaired the magnitude and duration of MPP-CA3 LTP over a period of days. Also, these data are consistent with reports that area CA3 is especially susceptible to age-related changes. Our data suggest that aging impairs mechanisms that regulate the late phase of MPP-CA3 LTP and contribute to a more global understanding of how aging affects hippocampal plasticity.

Novel form of LTD induced by transient, partial inhibition of the Na,K-pump in rat hippocampal CA1 cells

We tested the hypothesis that transient, partial inhibition of the Na,K-pumps could produce lasting effects on synaptic efficacy in brain tissue by applying a low concentration of the ouabain analogue, dihydroouabain (DHO), to hippocampal slices for 15 min and studying the effects on field excitatory postsynaptic potentials (fEPSPs). DHO caused a suppression of fEPSPs during the application period, but this recovered only partially, to approximately 80% of control levels, after washout lasting as long as 2 h. The lasting suppression had several properties in common with low-frequency stimulation induced long-term depression (LFS-LTD), including an ability to depotentiate long-term potentiated responses. However, DHO-LTD was insensitive to blockade of N-methyl-d-aspartate or mGlu receptors or to inhibitors of protein kinase C or p38 MAP kinase. DHO-LTD did not co-occlude with LFS-LTD and therefore appears to represent a novel form of LTD. Interestingly, DHO-LTD could be prevented by pretreating slices with iberiotoxin, the selective blocker of large, Ca(2+)-dependent K+ channels ("big K," BK channels), although this toxin did not affect basal fEPSPs. Certain pathological conditions, including hypoxia and ischemia, are associated with a decrease in Na,K-pump activity and hence DHO-LTD may serve as a model for the effects on neuronal function in these conditions.

Role of the group III metabotropic glutamate receptor in LTP, depotentiation and LTD in the dentate gyrus of freely moving rats

We investigated whether group III metabotropic glutamate (mGlu) receptors are critically involved in the expression of long-term potentiation (LTP), depotentiation, or long-term depression (LTD) in the dentate gyrus of freely moving rats. Male Wistar rats (7 8 weeks) underwent implantation of stimulating and recording electrodes in the medial perforant path and dentate gyrus granule cell layer, respectively. A cannula was permanently implanted into the ipsilateral cerebral ventricle to enable drug administration. Intracerebral injection of the group III mGlu receptor agonist, L(+)-2-amino-4-phosphonobutanoic acid (AP4), significantly inhibited LTP at a concentration which unaffects basal synaptic transmission. Depotentiation. short-term depression (STD) and LTDwere unaffected by the agonist. The antagonist. (R.S)-r-cyclopropyl-4-phosphonophenylglycine (CPPG), inhibited agonist effects. but had no independent effects on basal synaptic transmission. CPPG did not affect the profile of LTP, depotentiation or STD elicited by low frequency stimulation (LFS) at 0.5 or 3 Hz. but significantly impaired LTD expression (at I Hz) and STD elicited at 5 Hz. These findings suggest that activation of group III mGlu receptors is critically required for LTD. but not LTP or depotentiation in the dentate gyrus and provide evidence for the involvement of separate mechanisms underlying LTD and depotentiation.

Synaptically stimulated induction of group i metabotropic glutamate receptor-dependent long-term depression and depotentiation is inhibited by prior activation of metabotropic glutamate receptors and PROTEIN KINASE C

We have investigated metaplasticity of the group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) and depotentiation (DP) induced by physiological synaptic stimulation in the medial perforant path of the dentate gyrus in vitro. Group I mGluR-LTD/DP was inhibited by prior preconditioning brief high frequency stimulation (HFS) if the preconditioning HFS induced long-term potentiation (LTP) or if the induction of LTP was inhibited by an NMDA receptor antagonist. The inhibitory effect of the preconditioning HFS on LTD/DP was dependent upon activation of mGluRs, as it was blocked by the presence of the mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine during the preconditioning stimulation. The inhibitory effect of the preconditioning HFS involved stimulation of PKC, as the presence of the PKC inhibitor bisindolylmaleimide (BIS) during the preconditioning stimulation prevented the inhibitory effect of such preconditioning stimulation. Activation of PKC was also necessary for the induction of mGluR-LTD itself, as the PKC inhibitor BIS prevented the induction of the mGluR-LTD. We suggest that the physiological stimulation of mGluRs by the preconditioning stimulation produces a PKC-dependent inactivation of subsequent group I mGluR functioning and thereby an inhibition of induction of group I mGluR-dependent LTD/DP induction.