Intracellular injections of EGTA block induction of hippocampal long-term potentiation

Simultaneous NMDA-dependent long-term potentiation of EPSCs and long-term depression of IPSCs in cultured rat hippocampal neurons

A fundamental issue in understanding activity-dependent long-term plasticity of neuronal networks is the interplay between excitatory and inhibitory synaptic drives in the network. Using dual whole-cell recordings in cultured hippocampal neurons, we examined synaptic changes occurring as a result of a transient activation of NMDA receptors in the network. This enhanced transient activation led to a long-lasting increase in synchrony of spontaneous activity of neurons in the network. Simultaneous long-term potentiation of excitatory synaptic strength and a pronounced long-term depression of inhibitory synaptic currents (LTDi) were produced, which were independent of changes in postsynaptic potential and Ca2+ concentrations. Surprisingly, miniature inhibitory synaptic currents were not changed by the conditioning, whereas both frequency and amplitudes of miniature EPSCs were enhanced. LTDi was mediated by activation of a presynaptic GABAB receptor, because it was blocked by saclofen and CGP55845 [(2S)-3-{[(15)-1-(3, 4-dichlorophenyl)ethyl]amino-2-hydroxypropyl)(phenylmethyl)phosphinic acid]. The cAMP antagonist Rp-adenosine 3 ', 5 ' -cyclic monophosphothioate abolished all measured effects of NMDA-dependent conditioning, whereas a nitric oxide synthase inhibitor was ineffective. Finally, network-induced plasticity was not occluded by a previous spike-timing-induced plasticity, indicating that the two types of plasticity may not share the same mechanism. These results demonstrate that network plasticity involves opposite affects on inhibitory and excitatory neurotransmission.

Presynaptic and postsynaptic Ca(2+) and CamKII contribute to long-term potentiation at synapses between individual CA3 neurons

Long-term potentiation (LTP) in the Schaffer collateral pathway from the CA3 to the CA1 region of the hippocampus is thought to involve postsynaptic mechanisms including Ca(2+)- and CamKII-dependent alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor insertion. However, very little is known about possible presynaptic mechanisms. It is easier to address that question at synapses between individual neurons in the CA3 region, where both sides of the synapses are accessible to substances injected into the cell bodies. Previous studies using that method showed that CA3-CA3 LTP involves presynaptic protein kinases as well as postsynaptic receptor insertion. We have extended those findings by exploring the pre- and postsynaptic roles of Ca(2+) and CamKII, and we have also compared results with two induction protocols, 1-Hz-paired and -burst-paired, which may involve pre- and/or postsynaptic mechanisms in addition to receptor insertion in CA1. Similar to results in CA1, we find that CA3-CA3 LTP completely depends on postsynaptic Ca(2+) with the 1-Hz-paired protocol but depends only partially on postsynaptic Ca(2+) or CamKII with the -burst-paired protocol. Potentiation with that protocol also partially depends on presynaptic Ca(2+) or CamKII, suggesting that the additional mechanisms of potentiation, at least in part, are presynaptic. Furthermore, the pre- and postsynaptic mechanisms seem to act in series, suggesting coordinate regulation of the two sides of the synapses. CA3-CA3 LTP with the 1-Hz-paired protocol also partially depends on presynaptic Ca(2+), suggesting that it may involve presynaptic mechanisms as well.

Two signaling pathways regulate the expression and secretion of a neuropeptide required for long-term facilitation in Aplysia

Activation of several signaling pathways contributes to long-term synaptic plasticity, but how brief stimuli produce coordinated activation of these pathways is not understood. In Aplysia, the long-term facilitation (LTF) of sensory neuron synapses by 5-hydroxytryptamine (serotonin; 5-HT) requires the activation of several kinases, including mitogen-activated protein kinase (MAPK). The 5-HT-enhanced secretion of the sensory neuron-specific neuropeptide sensorin mediates the activation of MAPK. We find that stimulus-induced activation of two signaling pathways, phosphoinositide 3-kinase (PI3K) and type II protein kinase A (PKA), regulate sensorin secretion and responses. Treatment with 5-HT produces a rapid increase in sensorin synthesis, especially at varicosities, which precedes the secretion of sensorin. PI3K inhibitor and rapamycin block LTF and the rapid synthesis of sensorin at varicosities even in the absence of sensory neuron cell bodies. Secretion of the newly synthesized sensorin from the varicosities and activation of the autocrine responses of sensorin to produce LTF require type II PKA interaction with AKAPs (A-kinase anchoring proteins). Thus, long-term synaptic plasticity is produced when multiple signaling pathways that are important for regulating distinct cellular functions are activated in a specific sequence and recruit the secretion of a neuropeptide to activate additional critical pathways.

Local protein synthesis mediates a rapid increase in dendritic elongation factor 1A after induction of late long-term potentiation

The maintenance of long-term potentiation (LTP) requires a brief period of accelerated protein synthesis soon after synaptic stimulation, suggesting that an early phase of enhanced translation contributes to stable LTP. The mechanism regulating protein synthesis and the location and identities of mRNAs translated are not well understood. Here, we show in acute brain slices that the induction of protein synthesis-dependent hippocampal LTP increases the expression of elongation factor 1A (eEF1A), the mRNA of which contains a 5' terminal oligopyrimidine tract. This effect is blocked by rapamycin, indicating that the increase in EF1A expression is mediated by the mammalian target of rapamycin (mTOR) pathway. We find that mRNA for eEF1A is present in pyramidal cell dendrites and that the LTP-associated increase in eEF1A expression was intact in dendrites that had been severed from their cell bodies before stimulation. eEF1A levels increased within 5 min after stimulation in a translation-dependent manner, and this effect remained stable for 3 h. These results suggest a mechanism whereby synaptic stimulation, by signaling through the mTOR pathway, produces an increase in dendritic translational capacity that contributes to LTP maintenance.

Imaging LTP of presynaptic release of FM1-43 from the rapidly recycling vesicle pool of Schaffer collateral-CA1 synapses in rat hippocampal slices

Recent studies using the styryl dye FM1-43 and two-photon microscopy to directly visualize transmitter release at CA3-CA1 excitatory synapses in the hippocampus have demonstrated that activity-dependent long-term potentiation (LTP) and long-term depression are associated with alterations in vesicular release. It is not known whether particular vesicle pools preferentially express these alterations or what second messenger cascades are involved. To address these questions, we selectively loaded FM1-43 into the rapidly recycling pool (RRP) of vesicles by use of a brief hypertonic shock to release and load the RRP. We demonstrate here that the induction of LTP can lead to a selective long-lasting enhancement in presynaptic release from the RRP, while reserve pool kinetics remain unchanged. LTP of RRP release was N-methyl-d-aspartate receptor-dependent and also required production of the intercellular messenger NO and activation of receptor tyrosine kinase. Measurement of FM1-43 stimulus-evoked uptake rates following induction of LTP confirmed that LTP produces more rapid recycling of vesicles released by electrical stimulation, consistent with an enhanced release probability from the RRP.

Dissociating explicit and procedural-learning based systems of perceptual category learning

A fundamental question is whether people have available one category learning system, or many. Most multiple systems advocates postulate one explicit and one implicit system. Although there is much agreement about the nature of the explicit system, there is less agreement about the nature of the implicit system. In this article, we review a dual systems theory of category learning called competition between verbal and implicit systems (COVIS) developed by Ashby et al. The explicit system dominates the learning of verbalizable, rule-based category structures and is mediated by frontal brain areas such as the anterior cingulate, prefrontal cortex (PFC), and head of the caudate nucleus. The implicit system, which uses procedural learning, dominates the learning of non-verbalizable, information-integration category structures, and is mediated by the tail of the caudate nucleus and a dopamine-mediated reward signal. We review nine studies that test six a priori predictions from COVIS, each of which is supported by the data.

Ca2+ and synaptic plasticity

The induction and maintenance of synaptic plasticity is well established to be a Ca2+-dependent process. The use of fluorescent imaging to monitor changes [Ca2+]i in neurones has revealed a diverse array of signaling patterns across the different compartments of the cell. The Ca2+ signals within these compartments are generated by voltage or ligand-gated Ca2+ influx, and release from intracellular stores. The changes in [Ca2+]i are directly linked to the activity of the neurone, thus a neurone's input and output is translated into a dynamic Ca2+ code. Despite considerable progress in measuring this code much still remains to be determined in order to understand how the code is interpreted by the Ca2+ sensors that underlie the induction of compartment-specific plastic changes.

Frequency-dependent impairment of hippocampal LTP from NMDA receptors with reduced calcium permeability

Changes in postsynaptic Ca2+ levels are essential for alterations in synaptic strength. At hippocampal CA3-to-CA1 synapses, the Ca2+ elevations required for LTP induction are typically mediated by NMDA receptor (NMDAR) channels but a contribution of NMDAR-independent Ca2+ sources has been implicated. Here, we tested the sensitivity of different protocols modifying synaptic strength to reduced NMDAR-mediated Ca2+ influx by employing mice genetically programmed to express in forebrain principal neurons an NR1 form that curtails Ca2+ permeability. Reduced NMDAR-mediated Ca2+ influx did not facilitate synaptic depression in CA1 neurons of these genetically modified mice. However, we observed that LTP could not be induced by pairing low frequency synaptic stimulation (LFS pairing) with postsynaptic depolarization, a protocol that induced robust LTP in wild-type mice. By contrast to LFS pairing, similar LTP levels were generated in both genotypes when postsynaptic depolarization was paired with high frequency synaptic stimulation (HFS). This indicates that the postsynaptic Ca2+ elevation also reached threshold during HFS in the mutant, probably due to summation of NMDAR-mediated Ca2+ influx. However, only in wild-type mice did repeated HFS further enhance LTP. All tested forms of LTP were blocked by the NMDAR antagonist D-AP5. Collectively, our results indicate that only NMDAR-dependent Ca2+ sources (NMDARs and Ca2+-dependent Ca2+ release from intracellular stores) mediate LFS pairing-evoked LTP. Moreover, LTP induced by the first HFS stimulus train required lower Ca2+ levels than the additional LTP obtained by repeated trains.

A model of bidirectional synaptic plasticity: from signaling network to channel conductance

In many regions of the brain, including the mammalian cortex, the strength of synaptic transmission can be bidirectionally regulated by cortical activity (synaptic plasticity). One line of evidence indicates that long-term synaptic potentiation (LTP) and long-term synaptic depression (LTD), correlate with the phosphorylation/dephosphorylation of sites on the alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit protein GluR1. Bidirectional synaptic plasticity can be induced by different frequencies of presynaptic stimulation, but there is considerable evidence indicating that the key variable is calcium influx through postsynaptic N-methyl-d-aspartate (NMDA) receptors. Here, we present a biophysical model of bidirectional synaptic plasticity based on [Ca2+]-dependent phospho/dephosphorylation of the GluR1 subunit of the AMPA receptor. The primary assumption of the model, for which there is wide experimental support, is that the postsynaptic calcium concentration, and consequent activation of calcium-dependent protein kinases and phosphatases, is the trigger for phosphorylation/dephosphorylation at GluR1 and consequent induction of LTP/LTD. We explore several different mathematical approaches, all of them based on mass-action assumptions. First, we use a first order approach, in which transition rates are functions of an activator, in this case calcium. Second, we adopt the Michaelis-Menten approach with different assumptions about the signal transduction cascades, ranging from abstract to more detailed and biologically plausible models. Despite the different assumptions made in each model, in each case, LTD is induced by a moderate increase in postsynaptic calcium and LTP is induced by high Ca2+ concentration.

Protein kinase C and synaptic dysfunction after cardiac arrest

It is now understood that the mechanisms leading to neuronal cell death after cardiac arrest (CA) are highly complex. A well established fact in this field is that neurons continue to die over days and months after ischemia. It has been suggested that decreases in electrophysiological activities precede the morphologic deterioration in postischemic CA1 neurons and that this deterioration may be one cause for delayed cell death. The link between synaptic dysfunction and cardiac arrest is evident by the fact that about 50% of long-term survivors of cardiac arrest exhibit impaired mental abilities, manifested as learning impairment, memory disturbance. Since PKC is known to be a key player in synaptic function and has been implicated in promoting cell death after cerebral ischemia, it is a logical candidate as a modulator of synaptic derangements after CA. In this review, we provide an overview of synaptic dysfunction following CA and the putative role of PKC on this dysfunction.

Calcium signal-dependent plasticity of neuronal excitability developed postnatally

Neuronal plasticity and its development were investigated at pyramidal neurons in the cortical slices of rats. The threshold and probability of firing spikes were measured by using whole-cell recording to assess neuronal excitability. Postsynaptic high frequency activity (HFA) at the pyramidal neurons, evoked by 20 trains (250-ms interval) of five depolarization-pulses (1 ms) at 100 Hz, persistently lowered the threshold and increased the probability of firing spikes. After long-term enhancement of neuronal excitability by HFA was stable, another HFA induced further enhancement. Infusing 1 mM 1,2-bis(2-aminophenoxy)-ethane-N, N,N',N'-tetraacetic acid or 100 microM CaMKII(281-301) into the recording neurons prevented HFA-induced long-term enhancement of neuronal excitability. The infusion of 40 microM calcineurin autoinhibitory peptide enhanced neuronal excitability, which occluded HFA effect. HFA-induced long-term enhancement of intrinsic excitability expressed at most pyramidal neurons after postnatal day (PND) 14, but not at those before PND 9. Our results show a new type of neuronal plasticity induced by physiological activity at cortical neurons, which requires calcium-dependent protein phosphorylation and develops during postnatal period. An upregulation of intrinsic excitability at cortical neurons facilitates their activity and broadens signal codes; consequently, their computational ability is upgraded.

Mild cardiopulmonary arrest promotes synaptic dysfunction in rat hippocampus

Cardiac arrest (CA) patients exhibit learning and memory disabilities. These deficits suggest that synaptic dysfunction may underlie such disabilities. The hypothesis of the present study was that synaptic dysfunction occurs following CA and that this precedes cell death. To test this hypothesis, we used histopathological and electrophysiological markers in the hippocampus of rats subjected to CA. Evoked potentials (EP) were determined in the CA1 region of hippocampal slices harvested from animals subjected to CA or sham-operated rats by stimulating the Schaffer collaterals and recording in the CA1 pyramidal region. EP amplitudes were significantly attenuated by approximately 60% in hippocampal slices harvested from animals subjected to CA. Hippocampal slices harvested from sham rats exhibited normal long-term potentiation (LTP). In contrast, hippocampal slices harvested 24 h after CA exhibited no LTP response, even when no histopathological abnormalities were observed. These data suggest that synaptic dysfunction occurs before and without overt histopathology. We suggest that the synaptic dysfunction precedes and may be an early marker for delayed neuronal cell death in the hippocampus after CA.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Interactions Between LTP- and LTD-Inducing Stimulation in the Sensorimotor Cortex of the Awake Freely Moving Rat

Bidirectional modifications in synaptic efficacy are central components in models of cortical learning and memory. More recently, the regulation of synaptic plasticity according to the history of synaptic activation, termed “metaplasticity,” has become a focus of research on the physiology of memory. Here we explore such interactions between long-term potentiation (LTP) and long-term depression (LTD) in the chronically prepared rat. The effects of successive high- and low-frequency stimulation were examined in sensorimotor cortex in the adult, freely moving rat. High-frequency (300 Hz) stimulation (HFS) applied to the white matter was used to induce LTP, and prolonged, low-frequency (1 Hz) stimulation (LFS) was used to induce either depotentiation or LTD. Combined stimulation (HFS/LFS or LFS/HFS) during the induction phase attenuated potentiation effects only if the LFS followed the HFS. LTD induced by LFS alone was expressed as a reduction in the amplitude of both short- and long-latency field potential components, whereas depotentiation was primarily expressed as a decrease in the amplitude of the potentiated long-latency component. In other experiments, LTP (or LTD) was induced to asymptotic levels before applying LFS (or HFS). LFS caused depotentiation of the late component but had no measurable effect on the early component. HFS reversed previously induced LTD, but the potentiation decayed more rapidly than usual. LTP and LTD therefore modulate each other in the awake, behaving rat.

Cu,Zn superoxide dismutase increases intracellular calcium levels via a phospholipase C-protein kinase C pathway in SK-N-BE neuroblastoma cells

The superoxide dismutase isoenzymes (SOD) play a key role in scavenging, O*2- radicals. In contrast with previous studies, recent data have shown that human neuroblastoma cells are able to export the cytosolic Cu,Zn superoxide dismutase (SOD1), thus suggesting a paracrine role exerted by this enzyme in the nervous system. To evaluate whether SOD1 could activate intracellular signalling pathways, the functional interaction between SOD1 and human neuroblastoma SK-N-BE cells was investigated. By analyzing the surface binding of biotinylated SOD1 on SK-N-BE cells and by measuring intracellular calcium concentrations and PKC activity, we demonstrated that SOD1 specifically interacts in a dose-dependent manner with the cell surface membrane of SK-N-BE. This binding was able to activate a PLC-PKC-dependent pathway that increased intracellular calcium concentrations mainly deriving from the intracellular stores. Furthermore, we showed that this effect was independent of SOD1 dismutase activity and was totally inhibited by U73122, the PLC blocker. On the whole, these data indicate that SOD1 carries out a neuromodulatory role affecting calcium-dependent cellular functions.

Sustained extracellular signal-regulated kinase 1/2 phosphorylation in neonate 6-hydroxydopamine-lesioned rats after repeated D1-dopamine receptor agonist administration: implications for NMDA receptor involvement

Extracellular signal-regulated kinase (ERK) 1/2, a well known regulator of gene expression, is likely to contribute to signaling events underlying enduring neural adaptations. Phosphorylated (phospho)-ERK was examined immunohistochemically after both single and repeated (i.e., sensitizing) doses of the partial D1-dopamine (DA) receptor agonist SKF-38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benazepine HCl) to adult rats lesioned as neonates (neonate lesioned) with 6-hydroxydopamine. Remarkably, prolonged phospho-ERK accumulated primarily in layers II-III of medial prefrontal cortex (MPC), where it declined gradually yet remained significantly elevated for at least 36 d after repeated doses of SKF-38393. Sustained (> or =7 d) phospho-ERK was observed for shorter periods in various other cortical regions but was not detectable in striatum or nucleus accumbens. At 36 d, an additional injection of SKF-38393 to sensitized rats restored phospho-ERK to maximal levels only in MPC when examined 7 d later. Phosphorylated cAMP response element-binding protein (CREB), examined 7 d after the sensitizing regimen, was observed exclusively in MPC, where it was abundant throughout all layers. Systemic injections of SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile), an inhibitor of the upstream ERK activator mitogen ERK kinase, attenuated both ERK and CREB phosphorylation in layers II-III of MPC. Pretreatment with the D1 antagonist SCH-23390 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-OL maleate) inhibited the prolonged increase in MPC phospho-ERK, whereas the 5-HT2 receptor antagonist ketanserin (3-[2-[4-(4-fluorobenzoyl)-1-piperidinyl]ethyl]-2,4(1H,3H)-quinazolinedione tartrate) was ineffective. Competitive and noncompetitive NMDA receptor antagonists also blocked sustained ERK phosphorylation. Collectively, the present results demonstrate coupling of D1 and NMDA receptor function reflected in sustained activation of the ERK signaling pathway in MPC of SKF-38393-sensitized neonate-lesioned rats. Ultimately, long-lasting phosphorylation of ERK and CREB in MPC may play a pivotal role in any permanent adaptive change(s) in these animals.

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.

Coordinate high-frequency pattern of stimulation and calcium levels control the induction of LTP in striatal cholinergic interneurons

Current evidence appoints a central role to cholinergic interneurons in modulating striatal function. Recently, a long-term potentiation (LTP) of synaptic transmission has been reported to occur in these neurons. The relationship between the pattern of cortico/thalamostriatal fibers stimulation, the consequent changes in the intracellular calcium concentration ([Ca2+]i), and the induction of synaptic plasticity was investigated in striatal cholinergic interneurons from a rat corticostriatal slice preparation by means of combined electrophysiological intracellular recordings and microfluorometric techniques. Different protocols of stimulation were considered, varying both the frequency and the duration of the train of stimuli. High-frequency stimulation (HFS) (three trains at 100 Hz for 3 sec, 20-sec interval) induced a rise in [Ca2+]i, exceeding by fivefold the resting level, and caused a LTP of synaptic transmission. Tetanic stimulation delivered at lower frequencies (5-30 Hz) failed to induce long-term changes of synaptic efficacy. The observed elevation in [Ca2+]i during HFS was primarily mediated by L-type high-voltage activated (HVA)-Ca2+ channels, as it was fully prevented by nifedipine. Conversely, blockade of NMDA and AMPA glutamate receptor did not affect either LTP or the magnitude of the [Ca2+]i rise. Interestingly, the pharmacological analysis of the post-tetanic depolarizing postsynaptic potential (DPSP) revealed that LTP was attributable, to a large extent, to the potentiation of the GABA(A)-mediated component. In conclusion, the expression of LTP in striatal cholinergic interneurons is a selective response to a precise stimulation pattern of induction requiring a critical rise in [Ca2+]i.

Heterosynaptic co-activation of glutamatergic and dopaminergic afferents is required to induce persistent long-term potentiation

The persistence of protein synthesis-dependent long-term potentiation (late-LTP) is thought to require heterosynaptic activation of both glutamate and neuromodulatory receptors in the hippocampus. The present series of experiments contrasts two alternative accounts of heterosynaptic activation. The original version of the synaptic-tag hypothesis of the variable persistence of LTP implied that neuromodulatory and glutamatergic activation could occur independently, albeit within a critical time-window; an alternative view is that there needs to be simultaneous co-activation of both receptors to trigger the up-regulation of relevant protein synthesis (Neuron 34 (2002) 235). Our findings include a replication, over 6 h post-LTP-induction, of earlier findings showing heterosynaptic influences on LTP persistence. Specifically, 'strong' tetanisation with multiple trains of stimulation of one input pathway in a conventional hippocampal slice preparation induces a D1/D5 receptor-dependent form of late-LTP that enables 'weak' tetanic stimulation to induce late-LTP on an independent pathway. However, we also observed that when the first pathway was tetanised in the presence of AP5, not only was no LTP observed on that pathway, but there was also no rescue of late-LTP on the second pathway. Thus, it appears that DA receptors must be co-activated with NMDA receptors in a common pool of neurons to enable LTP persistence, although late-LTP can still be induced by selective activation of glutamatergic synapses if this occurs at time periods shortly before or shortly after this essential coactivation.

Subthreshold contribution of N-methyl-d-aspartate receptors to long-term potentiation induced by low-frequency pairing in rat hippocampal CA1 pyramidal cells

Long-term potentiation (LTP) is a use-dependent and persistent enhancement of synaptic strength. In the CA1 region of the hippocampus, LTP has Hebbian characteristics and requires precisely timed interaction between presynaptic firing and postsynaptic depolarization. Although depolarization is an absolute requirement for plasticity, it is still not clear whether the postsynaptic response during LTP induction should be subthreshold or suprathreshold for the generation of somatic action potential. Here, we use the whole-cell patch-clamp technique and different pairing protocols to examine systematically the postsynaptic induction requirements for LTP. We induce LTP by changes only in membrane potential while keeping the afferent stimulation constant and at minimal levels. This approach permits differentiation of two types of LTP: LTP induced with suprathreshold synaptic responses (LTP(AP)) and LTP induced with subthreshold excitatory postsynaptic current (EPSCs; LTP(EPSC)). We found that LTP(AP) (>40%) required pairing of depolarization (V(m)>or=-40 mV, for 40-60 s) with four to six (0.1 Hz) single synaptically initiated action potentials. LTP(EPSC) was of smaller magnitude (<30%) and required pairing of depolarization to -50 mV (60 s) with six subthreshold EPSCs. The N-methyl-d-aspartate receptor (NMDAR) antagonists aminophosphonovaleric acid and 7-chlorokynurenic acid consistently blocked LTP(EPSC) but were ineffective in preventing LTP(AP). Robust, NMDAR-independent LTP is obtained by stronger postsynaptic depolarization that converts the EPSCs to suprathreshold somatic action potentials. Purely NMDAR-dependent LTP is obtained by pairing mild somatic depolarization with subthreshold afferent pulses to the postsynaptic cell. Our results indicate that the degree of postsynaptic depolarization in the presence of single afferent pulses determines the type and magnitude of LTP.

A novel mechanism for the facilitation of theta-induced long-term potentiation by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) contributes to the induction of long-term potentiation (LTP) by theta-pattern stimulation, but the specific processes underlying this effect are not known. Experiments described here, using BDNF concentrations that have minor effects on baseline responses, show that the neurotrophin both reduces the threshold for LTP induction and elevates the ceiling on maximal potentiation. The enhanced LTP proved to be as stable and resistant to reversal as that recorded under control conditions. BDNF markedly increased the facilitation of burst responses that occurs within a theta train. This suggests that the neurotrophin acts on long-lasting events that (1) are set in motion by the first burst in a train and (2) regulate the amplitude of subsequent bursts. Whole-cell recordings established that BDNF causes a rapid reduction in the size of the long-lasting afterhyperpolarization (AHP) that follows individual theta bursts. Apamin, an antagonist of type 2 small-conductance Ca2+-activated potassium (SK2) channels, also reduced hippocampal AHPs and closely reproduced the effects of BDNF on theta-burst responses and LTP. The latter results were replicated with a newly introduced, highly selective inhibitor of SK2 channels. Immunoblot analyses indicated that BDNF increases SK2 serine phosphorylation in hippocampal slices. These findings point to the conclusion that BDNF-driven protein kinase cascades serve to depress the SK2 component, and possibly other constituents, of the AHP. It is likely that this mechanism, acting with other factors, promotes the formation and increases the magnitude of LTP.

A role for monomeric G-proteins in synaptic plasticity in the rat dentate gyrus in vitro

Recent studies have implicated Ras signalling in synaptic plasticity. In this study we have investigated a role for the low molecular weight G proteins Ras, Rap, Ra1 and Rac in long-term potentiation and depression using Clostridium Sordelli Lethal Toxin-82 (LT-82), which inactivates Ras, Rap, Ra1 and Rac, and manumycin A, a Ras inhibitor. Perfusion of hippocampal slices with LT-82 (200 ng/ml) attenuated LTP (83+/-10%, n=5, P<0.01, compared with controls of 160+/-11% at 60 min post HFS, n=5). LT-82 had no effect on LTD (63+/-1% at 100 ng/ml, n=5 and 66+/-1% at 200 ng/ml, n=4, compared to controls of 56+/-6%, n=6). Manumycin A (2 microM) had no effect on LTP (162+/-2%, n=5, compared to controls of 167+/-13%, n=5), but significantly attenuated LTD (88+/-6%, n=5, P<0.01, compared to controls of 63+/-9%, n=7). LT-82 (200 ng/ml) significantly increased the amplitude of the isolated NMDA-EPSP at 60 min post-drug application (240+/-40%, n=5, P<0.01, compared with controls of 100+/-4%, n=5). However, manumycin A, had no significant effect on NMDAR-EPSP amplitude (92+/-2%, n=5, compared with controls). These results demonstrate an important role for Ras in LTD and a role for Rap, Ra1 and Rac in LTP.

Small-Conductance Ca2+-Dependent K+ Channels Are the Target of Spike-Induced Ca2+ Release in a Feedback Regulation of Pyramidal Cell Excitability

Cooperative regulation of inosiol-1,4,5-trisphosphate receptors (IP3Rs) by Ca2+ and IP3 has been increasingly recognized, although its functional significance is not clear. The present experiments first confirmed that depolarization-induced Ca2+ influx triggers an outward current in visual cortex pyramidal cells in normal medium, which was mediated by apamin-sensitive, small-conductance Ca2+-dependent K+ channels (SK channels). With IP3-mobilizing neurotransmitters bath-applied, a delayed outward current was evoked in addition to the initial outward current and was mediated again by SK channels. Calcium turnover underlying this biphasic SK channel activation was investigated. By voltage-clamp recording, Ca2+ influx through voltage-dependent Ca2+ channels (VDCCs) was shown to be responsible for activating the initial SK current, whereas the IP3R blocker heparin abolished the delayed component. High-speed Ca2+ imaging revealed that a biphasic Ca2+ elevation indeed underlays this dual activation of SK channels. The first Ca2+ elevation originated from VDCCs, whereas the delayed phase was attributed to calcium release from IP3Rs. Such enhanced SK currents, activated dually by incoming and released calcium, were shown to intensify spike-frequency adaptation. We propose that spike-induced calcium release from IP3Rs leads to SK channel activation, thereby fine tuning membrane excitability in central neurons.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Up-regulation of NMDAR1 subunit gene expression in cortical neurons via a PKA-dependent pathway

Transcription mediated by protein kinase A and the cAMP response element binding protein (CREB) has been linked to the establishment of long-term memory and cell survival. However, all of the major targets for activated CREB have yet to be identified. Given the fact that CREB-mediated transcription is intimately involved in cellular processes of learning and memory and that CREB activity can be regulated by synaptic N-methyl-d-aspartate receptors (NMDARs) and metabotropic GABA receptors, we have studied the role of the cAMP-dependent signaling pathway in the regulation of the NMDA receptor subunit 1 (NMDAR1), a subunit required for functional receptor formation. We now report that levels of NMDAR1 subunit protein in primary neocortical cultures are increased 66% in response to forskolin, an activator of adenylyl cyclase. Up-regulation of NMDAR1 is paralleled by a twofold increase in mRNA levels and an 83% increase in NMDAR1 promoter/luciferase reporter activity that is dependent on protein kinase A. Three cAMP regulatory elements (CREs) in the rat NMDAR1 promoter (- 228, - 67, and - 39) bind CREB in vitro and forskolin increases binding to two of the sites (- 228 and - 67). Chromatin immunoprecipitation of neuronal rat genomic DNA reveals that CREB is bound in vivo to the endogenous NMDAR1 gene. Increased presence of the activated Ser133 phosphorylated form is dependent on the length of exposure to forskolin. Taken together with the results of mutational analysis, the findings strongly suggest that transcription of NMDAR1 is regulated by the c-AMP signaling pathway, most likely through the binding of CREB and its activation by signal-dependent phosphorylation.

Taurine regulates mitochondrial calcium homeostasis

We have investigated the protective role of taurine in glutamate-mediated cell death and the involvement of mitochondria in this process. In cultured cerebellar granule cells, glutamate induces a rapid and sustained elevation in cytoplasmic free calcium ([Ca2+]i), causing the collapse of the mitochondrial electrochemical gradient (MtECG) and subsequent cell death. We found that pre-treatment with taurine, did not affect the level of calcium uptake with glutamate but rather reduced its duration; the calcium increase was transient and returned to basal levels about 10 min after adding glutamate. Furthermore, taurine reduced mitochondrial calcium concentration under non-depolarizing conditions. Treatment of cerebellar granule cells with taurine enhanced mitochondrial activity as measured by rhodamine uptake, both in the presence or absence of glutamate. We conclude that taurine prevents or reduces glutamate excitotoxicity through both the enhancement of mitochondrial function and the regulation of intracellular (cytoplasmic and mitochondrial) calcium homeostasis.

Glial Cell Line-derived Neurotrophic Factor Increases Intracellular Calcium Concentration

Moderate increases of intracellular Ca2+ concentration ([Ca2+]i), induced by either the activation of tropomyosin receptor kinase (Trk) receptors for neurotrophins or by neuronal activity, regulate different intracellular pathways and neuronal survival. In the present report we demonstrate that glial cell line-derived neurotrophic factor (GDNF) treatment also induces [Ca2+]i elevation by mobilizing this cation from internal stores. The effects of [Ca2+]i increase after membrane depolarization are mainly mediated by calmodulin (CaM). However, the way in which CaM exerts its effects after tyrosine kinase receptor activation remains poorly characterized. It has been reported that phosphatidylinositol 3-kinase (PI 3-kinase) and its downstream target protein kinase B (PKB) play a central role in cell survival induced by neurotrophic factors; in fact, GDNF promotes neuronal survival through the activation of the PI 3-kinase/PKB pathway. We show that CaM antagonists inhibit PI 3-kinase and PKB activation as well as motoneuron survival induced by GDNF. We also demonstrate that endogenous Ca2+/CaM associates with the 85-kDa regulatory subunit of PI 3-kinase (p85). We conclude that changes of [Ca2+]i, induced by GDNF, promote neuronal survival through a mechanism that involves a direct regulation of PI 3-kinase activation by CaM thus suggesting a central role for Ca2+ and CaM in the signaling cascade for neuronal survival mediated by neurotrophic factors.

On the electrical function of dendritic spines

Dendritic spines mediate most excitatory inputs in the brain, yet their function is still unclear. Imaging experiments have demonstrated their role in biochemical compartmentalization at individual synapses, yet theoretical studies have suggested that they could serve an electrical function in transforming synaptic inputs and transmitting dendritic spikes. Recent data indicate that spines possess voltage-dependent conductances and that these channels can be spine-specific. Although direct experimental investigations of the electrical properties of spines have not yet taken place, spines could play a significant electrical role, greatly influencing dendritic integration and the function of neural circuits.

N-methyl-d-aspartate receptor-independent long-term depression and depotentiation in the sensorimotor cortex of the freely moving rat

Bidirectional modifications in synaptic efficacy are central components in recent models of cortical learning and memory, and we previously demonstrated both long-term synaptic potentiation (LTP) and long-term synaptic depression (LTD) in the neocortex of the unanaesthetized adult rat. Here, we have examined the effects of N-methyl-D-aspartate receptor (NMDAR) blockade on the induction of LTD, LTP, and depotentiation of field potentials evoked in sensorimotor cortex by stimulation of the white matter in the adult, freely moving rat. High frequency (300 Hz) stimulation (HFS) was used to induce LTP and prolonged, low-frequency (1 Hz) stimulation was used to induce either depotentiation or LTD. LTD was expressed as a reduction in the amplitude of the short and long-latency field potential components, while depotentiation was expressed as a decrease in the amplitude of a previously enhanced late component. Under NMDAR blockade, HFS failed to induce LTP and instead produced a depression effect similar to LTD. Following washout of the drug, HFS induced a normal LTP effect. Unlike LTP, LTD and depotentiation were found to be NMDAR-independent in the neocortex of the freely moving rat.

Effect of erythropoietin on nitric oxide production in the rat hippocampus using in vivo brain microdialysis

In order to investigate the role of erythropoietin (EPO) in the hippocampus, we studied the effect of EPO on nitric oxide (NO) production in the rat hippocampus using brain microdialysis. The dialysis probe was stereotaxically inserted into the rat hippocampus 24 h before the dialysis experiment. The perfusion fluid (Krebs-HEPES buffer, pH 7.4) was collected at 15-min intervals under freely moving conditions and NO metabolites (NOx) in the perfusate were immediately measured using a NOx-analyzing high performance liquid chromatography (HPLC)system. Following the collection of four fractions, 1 microl of EPO (10(-10) M, 10(-8) M and 10(-6) M) or vehicle (saline) was gently injected into the hippocampal tissue. The perfusion fluid was collected for 3 h after the injection. The NOx levels were unchanged by the injection of vehicle alone. After the injection of EPO, NOx levels gradually increased. The EPO-induced increase in NOx levels was significant at 10(-6) M EPO. The EPO-induced increases in NOx levels were eliminated in the presence of anti-EPO antibody. The increase in NOx levels induced by EPO was blunted by nicardipine, a Ca2+ channel blocker, but not by MK-801, an antagonist of N-methyl-D-aspartate (NMDA) receptors. These findings, taken together, suggest that EPO increased NO production in the rat hippocampus by activating voltage-gated Ca2+ channels, but not through NMDA receptors.

LTP in cultured hippocampal-entorhinal cortex slices from young adult (P25-30) rats

Cultured hippocampal neurons and immature organotypic slice cultures overcome temporal limitations of acute hippocampal slices and have been useful for investigating long-lasting plasticity. Difficulties with culturing adult neurons have restricted such studies to preparations from embryonic, perinatal, and juvenile tissue. By improving the methods for culturing and maintaining hippocampal-entorhinal cortex slices obtained from mature rats (P25-30), we show that their use in long-term electrophysiological investigations is feasible. Our cultured slices maintained an intact and functional trisynaptic cascade, normal synaptic function, and reliable long-term recording stability for at least 14 days in vitro. The electrophysiological properties and, in particular, the induction of long-term potentiation (LTP) in our mature organotypic slices were highly sensitive to dissection and tissue culture techniques. We present data describing the extracellular stimulation requirements for LTP-induction and its long-lasting maintenance (>4 h) at the Schaffer-collateral-CA1 synapse, and show that such changes in synaptic efficiency are NMDA receptor dependent. Our hippocampal-entorhinal cortex cultures from mature tissue can retain the electrophysiological properties required for long-term plasticity for several weeks in vitro.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Neuronal mechanism of nociceptin-induced modulation of learning and memory: involvement of N-methyl-D-aspartate receptors

Nociceptin (also called orphanin FQ) is an endogenous heptadecapeptide that activates the opioid receptor-like 1 (ORL1) receptor. Nociceptin system not only affects the nociception and locomotor activity, but also regulates learning and memory in rodents. We have previously reported that long-term potentiation and memory of ORL1 receptor knockout mice are enhanced compared with those in wild-type mice. Here, we show the neuronal mechanism of nociceptin-induced modulation of learning and memory. Retention of fear-conditioned contextual memory was significantly enhanced in the ORL1 receptor knockout mice without any changes in cued conditioned freezing. Inversely, in the wild-type mice retention of contextual, but not cued, conditioning freezing behavior was suppressed by exogenous nociceptin when it was administered into the cerebroventricle immediately after the training. ORL1 receptor knockout mice exhibited a hyperfunction of N-methyl-D-aspartate (NMDA) receptor, as evidenced by an increase in [3H]MK-801 binding, NMDA-evoked 45Ca2+ uptake and activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity and its phosphorylation as compared with those in wild-type mice. The NMDA-induced CaMKII activation in the hippocampal slices of wild-type mice was significantly inhibited by exogenous nociceptin via a pertussis toxin-sensitive pathway. However, the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor GluR1 subunit at Ser831 and Ser845, and NMDA receptor subunit NR2B at Thr286 were phosphorylated similarly after NMDA receptor stimulation in both type of mice. The expressions of GluR1 and GluR2 also did not change, but the levels of polysialylated form of neuronal cell adhesion molecule (N-CAM) were reduced in the ORL1 receptor knockout as compared with wild-type mice. These results suggest that nociceptin system negatively modulates learning and memory through the regulation of NMDA receptor function and the expression of N-CAM.

Effect of D-2 amino-5-phosphonopentanoate and nifedipine on postsynaptic calcium changes associated with long-term potentiation in hippocampal CA1 area

The induction of long-term potentiation (LTP) in CA1 hippocampal area requires a rise in intracellular postsynaptic calcium. Two major calcium mechanisms may mediate the transmembrane calcium influxes that contribute to this calcium accumulation: the N-methyl-D-aspartate (NMDA) receptor channels, which are voltage dependent and have large calcium permeability and voltage-dependent calcium channels (VDCCs). We have addressed the relative contribution of these routes of calcium entry before and during LTP expression, in synaptically evoked dendritic calcium transients from a population of CA1 pyramidal neurons. Combining the use of the fluorescent calcium indicator Fura-2 with field potential measurements, we observed that the calcium transients evoked by single stimuli, during the maintenance phase of LTP, were enhanced. These transients were not affected by D-2 amino-5-phosphonopentanoate (D-APV) (50 microM), an antagonist of NMDA receptors but were reduced by approximately one-quarter, in the presence of the L-type VDCCs blocker nifedipine (10 microM). During tetanic stimulation (100 Hz, 1 s) the components triggered by the activation of those two calcium mechanisms had comparable magnitudes representing the sum about half of the intracellular calcium accumulation. Thus, following both single and high frequency stimulation, a substantial fraction of calcium entry may occur through other types of VDCCs or be due to calcium release from intracellular stores.

Enhanced learning and memory in mice lacking Na+/Ca2+ exchanger 2

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) plays a role in regulation of intracellular Ca(2+) concentration via the forward mode (Ca(2+) efflux) or the reverse mode (Ca(2+) influx). To define the physiological function of the exchanger in vivo, we generated mice deficient for NCX2, the major isoform in the brain. Mutant hippocampal neurons exhibited a significantly delayed clearance of elevated Ca(2+) following depolarization. The frequency threshold for LTP and LTD in the hippocampal CA1 region was shifted to a lowered frequency in the mutant mice, thereby favoring LTP. Behaviorally, the mutant mice exhibited enhanced performance in several hippocampus-dependent learning and memory tasks. These results demonstrate that NCX2 can be a temporal regulator of Ca(2+) homeostasis and as such is essential for the control of synaptic plasticity and cognition.

Optical quantal analysis reveals a presynaptic component of LTP at hippocampal Schaffer-associational synapses

The mechanisms by which long-term potentiation (LTP) is expressed are controversial, with evidence for both presynaptic and postsynaptic involvement. We have used confocal microscopy and Ca(2+)-sensitive dyes to study LTP at individual visualized synapses. Synaptically evoked Ca(2+) transients were imaged in distal dendritic spines of pyramidal cells in cultured hippocampal slices, before and after the induction of LTP. At most synapses, from as early as 10 min to at least 60 min after induction, LTP was associated with an increase in the probability of a single stimulus evoking a postsynaptic Ca(2+) response. These observations provide compelling evidence of a presynaptic component to the expression of early LTP at Schaffer-associational synapses. In most cases, the store-dependent evoked Ca(2+) transient in the spine was also increased after induction, a novel postsynaptic aspect of LTP.

A beta 25-35-induced depression of long-term potentiation in area CA1 in vivo and in vitro is attenuated by verapamil

The effect of intracerebroventricular (icv) injection of A beta 25-35 and/or intraperitoneal (ip) application of the L-type calcium channel (VDCC) blockers verapamil or diltiazem were examined in vivo. To by-pass possible systemic actions of these agents, their effects on long-term potentiation (LTP) in the CA1 region of the in vitro hippocampal slice preparation were also examined. Application of A beta 25-35 (10 nmol in 5 microl, i.c.v.) significantly impaired LTP in vivo, as did IP injection of verapamil (1 or 10 mg/kg) or diltiazem (1 or 10 mg/kg). In the in vitro slice preparation, LTP was also depressed by prior application of A beta 25-35 (500 nmol), verapamil (20 microM), or diltiazem (50 microM). Combined application of A beta 25-35 and verapamil in either the in vivo or in vitro preparation resulted in a significant reversal of the LTP depression observed in the presence of either agent alone. However, co-application of diltiazem and A beta 25-35 failed to attenuate the depression of LTP observed in the presence of either agent alone in vivo or in vitro. Since LTP is a cellular correlate of memory and A beta is known to be involved in Alzheimer's disease (AD), these results indicate that verapamil, a phenylalkylamine, may be useful in the treatment of cognitive deficits associated with AD.

Ethanol-induced suppression of LTP can be attenuated with an angiotensin IV analog

Hippocampal slices taken from animals chronically or acutely treated with ethanol exhibit significant inhibition of long-term potentiation (LTP). This inhibition appears to be associated with impaired activity of N-methyl-D-aspartate (NMDA) receptors, perhaps via ethanol-induced increases in GABAergic synaptic transmission. Recently, a role for the octapeptide angiotensin II (AngII) in ethanol's inhibition of LTP has been reported. Complementary to these findings our laboratory has shown that the application of the hexapeptide metabolite of AngII, angiotensin IV (AngIV), significantly facilitated normal tetanic-induced LTP in the hippocampal slice. This facilitation is presumably by activation of the angiotensin receptor subtype, AT(4). The present study tested whether an AT(4) receptor agonist could overcome ethanol-induced suppression of LTP. The results indicate that Nle(1)-AngIV could offset ethanol-induced suppression of LTP in the CA(1) region of the hippocampus. Pretreatment with the specific AT(4) receptor antagonist Nle(1), Leual(3)-AngIV blocked this facilitation implicating the involvement of the AT(4) receptor subtype. These results suggest that an AT(4) receptor agonist is effective in overcoming ethanol's suppressing influence on LTP, and encourage further investigation of the cognitive enhancing properties of such compounds.

Long-term potentiation in the Eocene

The first ten years of long-term potentiation (LTP) research are reviewed. Surprisingly, given the intensity of current interest, the discovery paper did not trigger a wave of follow-on experiments. Despite this, the initial work laid out what ultimately became standard questions and paradigms. The application of the then still novel hippocampal slice technique oriented LTP towards basic neuroscience, perhaps somewhat at the cost of lesser attention to its functional significance. The use of slices led to the discovery of the events that trigger the formation of LTP and provided some first clues about its extraordinary persistence. Signs of the intense controversy over the nature of LTP expression (release vs receptors) emerged towards the end of the first decade of work. What appears to be lacking in the literature of that time is a widespread concern about LTP and memory. This may reflect a somewhat different attitude that neurobiologists then had towards memory research and a perceived need to integrate the new potentiation phenomenon into the web of established science before advancing extended arguments about its contributions to behaviour.

Long-term potentiation and memory

The discovery of long-term potentiation (LTP) transformed research on the neurobiology of learning and memory. This did not happen overnight, but the discovery of an experimentally demonstrable phenomenon reflecting activity-driven neuronal and synaptic plasticity changed discussions about what might underlie learning from speculation into something much more concrete. Equally, however, the relationship between the discovery of LTP and research on the neurobiology of learning and memory has been reciprocal; for it is also true that studies of the psychological, anatomical and neurochemical basis of memory provided a developing and critical intellectual context for the physiological discovery. The emerging concept of multiple memory systems, from 1970 onwards, paved the way for the development of new behavioural and cognitive tasks, including the watermaze described in this paper. The use of this task in turn provided key evidence that pharmacological interference with an LTP induction mechanism would also interfere with learning, a finding that was by no means a foregone conclusion. This reciprocal relationship between studies of LTP and the neurobiology of memory helped the physiological phenomenon to be recognized as a major discovery.

Expression mechanisms underlying long-term potentiation: a postsynaptic view

This review summarizes the various experiments that have been carried out to determine if the expression of long-term potentiation (LTP), in particular N-methyl-D-aspartate (NMDA) receptor-dependent LTP, is presynaptic or postsynaptic. Evidence for a presynaptic expression mechanism comes primarily from experiments reporting that glutamate overflow is increased during LTP and from experiments showing that the failure rate decreases during LTP. However, other experimental approaches, such as monitoring synaptic glutamate release by recording astrocytic glutamate transporter currents, have failed to detect any change in glutamate release during LTP. In addition, the discovery of silent synapses, in which LTP rapidly switches on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function at NMDA-receptor-only synapses, provides a postsynaptic mechanism for the decrease in failures during LTP. It is argued that the preponderance of evidence favours a postsynaptic expression mechanism, whereby NMDA receptor activation results in the rapid recruitment of AMPA receptors as well as a covalent modification of synaptic AMPA receptors.

Synaptic plasticity in the amygdala: comparisons with hippocampus

Long-term potentiation (LTP) is a widely studied form of synaptic plasticity, and a considerable amount of evidence indicates that it could be involved in learning and memory. Intensive investigation of this phenomenon in the hippocampus has yielded tremendous insight into the workings of synapses in the mammalian central nervous system, but important questions remain to be answered. The most important of these are: (1) whether LTP is the basis of learning and memory, and (2) how similar are the induction, maintenance, and expression mechanisms in the rest of the brain to those in the hippocampus. Because the most important strategy for linking LTP to learning involves disrupting the mechanisms of LTP and examining the consequences on behavior, it is likely that the first question cannot be answered until the second has been addressed. Recent evidence indicates that although the general processes have much in common, significant differences exist among forebrain structures, including the hippocampus, basolateral amygdala, and ventral striatum. It is clear that the roles of receptors and calcium channels, kinases, and transcription factors vary within these structures, reflecting the different functions of these brain regions.

Inhibition of L-type voltage dependent calcium channels causes impairment of long-term potentiation in the hippocampal CA1 region in vivo

Long-term potentiation (LTP), in the hippocampal CA1 region is dependent on postsynaptic calcium influx. It is generally accepted that calcium influx occurs via activation of the NMDA receptor channel complex. However, studies in vitro using a high-frequency stimulus protocol (> or =200 Hz) demonstrated previously an NMDA receptor-independent form of LTP that is dependent upon activation of L-type voltage-dependent calcium channels (VDCCs). Here we have investigated a role for L-type VDCCs in LTP in vivo. Two structurally different, L-type VDCC blockers, verapamil (1, 3 and 10 mg/kg) and diltiazem (1, 10 and 20 mg/kg), depressed the induction of LTP in a dose-dependent manner. Increased activation of L-type VDCCs by Bay K 8644, an L-type agonist, however, did not enhance LTP. The NMDA receptor antagonist D-AP5 (5 and 20 mM injected i.c.v) impaired, but failed to block fully LTP in vivo. A reduced level of LTP could still be recorded following co-administration of verapamil and D-AP5. The level of LTP recorded was similar to that observed in the presence of either verapamil (10 mg/kg) or D-AP5 alone. These results suggest that activation of the NMDA receptor/channel and L-type VDCCs are involved in the induction of LTP in area CA1 in vivo. However, it appears that activation of other receptor/channels may also play a role in this form of LTP.

Integrins modulate fast excitatory transmission at hippocampal synapses

The present study provides the first evidence that adhesion receptors belonging to the integrin family modulate excitatory transmission in the adult rat brain. Infusion of an integrin ligand (the peptide GRGDSP) into rat hippocampal slices reversibly increased the slope and amplitude of excitatory postsynaptic potentials. This effect was not accompanied by changes in paired pulse facilitation, a test for perturbations to transmitter release, or affected by suppression of inhibitory responses, suggesting by exclusion that alterations to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate receptors cause the enhanced responses. A mixture of function-blocking antibodies to integrin subunits alpha(3), alpha(5), and alpha(v) blocked ligand effects on synaptic responses. The ligand-induced increases were (i) blocked by inhibitors of Src tyrosine kinase, antagonists of N-methyl-d-aspartate receptors, and inhibitors of calcium calmodulin-dependent protein kinase II and (ii) accompanied by phosphorylation of both the Thr(286) site on calmodulin-dependent protein kinase II and the Ser(831) site on the GluR1 subunit of the AMPA receptor. N-Methyl-d-aspartate receptor antagonists blocked the latter two phosphorylation events, but Src kinase inhibitors did not. These results point to the conclusion that synaptic integrins regulate glutamatergic transmission and suggest that they do this by activating two signaling pathways directed at AMPA receptors.

Signaling from cAMP/PKA to MAPK and synaptic plasticity

The facilitation of hippocampus-based, long-lasting synaptic plasticity, which is frequently investigated in model systems such as long-term potentiation (LTP) and in learning paradigms such as the Morris water maze, is associated with several cellular key events: Ca(2+) influx through the N-methyl-D-aspartate (NMDA) receptor, generation of cyclic AMP (cAMP) and activation of protein kinase A (PKA), phosphorylation of mitogen-associated protein kinase (MAPK) and cAMP-response element-binding protein (CREB), and subsequent transcription of plasticity-associated genes. Recently, a signal-transduction cascade from cAMP/PKA to MAPK was discovered, which seems to be neuron-specific and comprises the critical events of hippocampus-based long-term plasticity described here into one single cascade. A major alternative to cAMP/PKA-MAPK signaling are the cascades from Ca(2+) to MAPK via Ras. However, Ras is inhibited by PKA. This article reviews the studies that argue for the existence of two competing pathways, and discusses their implication for the molecular mechanisms underlying synaptic plasticity.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

A large sustained Ca2+ elevation occurs in unstimulated spines during the LTP pairing protocol but does not change synaptic strength

Synapses in the CA1 region of the hippocampus undergo bidirectional synaptic modification in response to different patterns of activity. Postsynaptic Ca2+ elevation can trigger either synaptic strengthening or weakening, depending on the properties of the local Ca2+ signal. During the pairing protocol for long-term potentiation (LTP) induction, the cell is depolarized under voltage-clamp and is given low-frequency synaptic stimulation. As an initial step toward understanding the Ca2+ dynamics during this process, we used confocal microscopy to study the Ca2+ signals in spines evoked by the depolarization itself. This depolarization activates voltage-dependent Ca2+ channels (VDCC), but whether these channels inactivate rapidly or remain functional throughout the long depolarizations used in the pairing protocol remains unknown. Cells were depolarized to 0 mV for 2-3 min. This depolarization led to a large initial elevation of Ca2+ in spines that never decayed back to resting levels. The maintained signal was close to the Kd of the low-affinity (5 microM) Ca2+ dye, Magnesium Green. We attempted to determine the functional role of this elevation, using the Ca2+-channel blocker D-890. The addition of D-890 in the internal solution produced a nearly complete abolition of the Ca2+ elevation during depolarization. Under these conditions, the NMDA conductance was normal, but LTP was almost completely blocked. This might suggest the importance of VDCC in LTP; however, we found that high concentrations of D-890 can directly inhibit calmodulin protein kinase II (CaMKII), an enzyme required for LTP induction. Thus, whereas D-890 is a useful tool for blocking postsynaptic VDCC, it cannot be used to study the contribution of these channels to plasticity. We conclude that the activation of VDCC produces a large and persistent elevation of Ca2+ in all spines, but does not produce either LTP or long-term depression (LTD) in the absence of synaptic stimulation. The possible reasons for this are discussed.

Small conductance Ca2+-activated K+ channels modulate synaptic plasticity and memory encoding

Activity-dependent changes in neuronal excitability and synaptic strength are thought to underlie memory encoding. In hippocampal CA1 neurons, small conductance Ca2+-activated K+ (SK) channels contribute to the afterhyperpolarization, affecting neuronal excitability. In the present study, we examined the effect of apamin-sensitive SK channels on the induction of hippocampal synaptic plasticity in response to a range of stimulation frequencies. In addition, the role of apamin-sensitive SK channels on hippocampal-dependent memory encoding and retention was also tested. The results show that blocking SK channels with apamin increased the excitability of hippocampal neurons and facilitated the induction of synaptic plasticity by shifting the modification threshold to lower frequencies. This facilitation was NMDA receptor (NMDAR) dependent and appeared to be postsynaptic. Mice treated with apamin demonstrated accelerated hippocampal-dependent spatial and nonspatial memory encoding. They required fewer trials to learn the location of a hidden platform in the Morris water maze and less time to encode object memory in an object-recognition task compared with saline-treated mice. Apamin did not influence long-term retention of spatial or nonspatial memory. These data support a role for SK channels in the modulation of hippocampal synaptic plasticity and hippocampal-dependent memory encoding.