Dopamine D1/D5 receptor activation fails to initiate an activity-independent late-phase LTP in rat hippocampus

The role of dopamine in the hippocampus remains poorly defined. Numerous studies have suggested that it acts as a neuromodulator of late-phase long-term potentiation (L-LTP) in CA1, while other reports controversially indicate that D1/D5 receptor (D1/D5R) activation may directly initiate activity-independent LTP. We have further investigated this putative role of dopamine in area CA1 in rat hippocampal slices using field potential recording techniques. Application of the dopamine D1/D5 receptor agonists SKF 38393 and 6-bromo-APB at 100 microM for 20 min did not induce an activity-independent L-LTP. Varying the incubation conditions still did not permit either SKF 38393 or an alternative D1/D5R agonist, 6-chloro-PB, to induce L-LTP. To further determine if intracellular mechanisms, which may act to limit the expression of LTP, were preventing D1/D5R-induced L-LTP expression, we inhibited protein phosphatase 1 activity by reducing cyclin-dependent kinase 5 (cdk5) inhibition of inhibitor 1. Inhibition of cdk5 by roscovitine (10 microM, 40 min) did not facilitate the ability of SKF 38393 to induce L-LTP in CA1. Biochemical experiments confirmed that the concentration of agonist used significantly elevated intracellular cAMP levels, suggesting that effective D1/D5R activation was achieved. Furthermore, coactivation with NMDA receptors (NMDAR) resulted in a synergistic increase in cAMP. These findings demonstrate that D1/D5R activation in CA1 initiates intracellular second messenger accumulation, but that this is insufficient to induce an activity-independent L-LTP.

Metabotropic Glutamate Receptor-Mediated Depression of the Slow Afterhyperpolarization Is Gated by Tyrosine Phosphatases in Hippocampal CA1 Pyramidal Neurons

Group I metabotropic glutamate receptor (mGluR) agonists increase the excitability of hippocampal CAl pyramidal neurons via depression of the postspike afterhyperpolarization. In adult rats, this is mediated by both mGluR1 and -5, but the signal transduction processes involved are unknown. In this study, we investigated whether altered levels of tyrosine phosphorylation of proteins are involved in the depression of the slow-duration afterhyperpolarization (sAHP) by the Group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) in CA1 pyramidal neurons of rat hippocampal slices. Preincubation with the tyrosine kinase inhibitors lavendustin A or genistein, or the Src-specific inhibitor 3-(4-chlorophenyl) 1-(1,1-dimethylethyl)-1 H-pyrazolo[3,4-d]pyrimidin-4-amine (PP2), did not inhibit the DHPG-mediated depression of the sAHP. However, preincubation with the tyrosine phosphatase inhibitor orthovanadate reduced the effects of DHPG. This effect of orthovanadate was prevented by simultaneous inhibition of tyrosine kinases with lavendustin A. Selective activation of either mGluR1 or -5 by application of DHPG plus either the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) or the mGluR1 antagonist (S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385) demonstrated that the effect of inhibiting tyrosine phosphatases is not specific to either subtype of mGluR. These results suggest that the depression of the sAHP induced by activation of mGluR1 and -5 is gated by a balance between tyrosine phosphorylation and dephosphorylation.

Long-term regulation of N-methyl-D-aspartate receptor subunits and associated synaptic proteins following hippocampal synaptic plasticity

Synaptic plasticity in the dentate gyrus is dependent on activation of the N-methyl-D-aspartate (NMDA)-subtype of glutamate receptors. In this study, we show that synaptic plasticity in turn regulates NMDA receptors, since subunits of the NMDA receptor complex are bidirectionally and independently regulated in the dentate gyrus following activation of perforant synapses in awake animals. Low-frequency stimulation that produced a mild synaptic depression resulted in a decrease in the NMDA receptor subunits NR1 and NR2B 48 h following stimulation. High-frequency stimulation that produced long-term potentiation resulted in an increase in NR1 and NR2B at the same time point. Further investigations revealed that in contrast to NR2B, NR1 levels increased gradually after long-term potentiation induction, reaching a peak level at 48 h, and were insensitive to the competitive NMDA receptor antagonist 3-3(2-carboxypiperazin-4-yl) propyl-1-phosphate. The increased levels of NR1 and NR2B at 48 h were found associated with synaptic membranes and with increased NMDA receptor-associated proteins, postsynaptic density protein 95, neuronal nitric oxide synthase and Ca(2+)/calmodulin-dependent protein kinase II, alpha subunit. These data suggest that the persistence of long-term potentiation is associated with an increase in the number of NMDA receptor complexes, which may be indicative of an increase in synaptic contact area.

Roles of amyloid precursor protein and its fragments in regulating neural activity, plasticity and memory

Amyloid-beta precursor protein (APP) is a membrane-spanning protein with a large extracellular domain and a much smaller intracellular domain. It is the source of the amyloid-beta (Abeta) peptide found in neuritic plaques of Alzheimer's disease (AD) patients. Because Abeta shows neurotoxic properties, and because familial forms of AD promote Abeta accumulation, a massive international research effort has been aimed at understanding the mechanisms of Abeta generation, catabolism and toxicity. APP, however, is an extremely complex molecule that may be a functionally important molecule in its full-length configuration, as well as being the source of numerous fragments with varying effects on neural function. For example, one fragment derived from the non-amyloidogenic processing pathway, secreted APPalpha (sAPPalpha), is neuroprotective, neurotrophic and regulates cell excitability and synaptic plasticity, while Abeta appears to exert opposing effects. Less is known about the neural functions of other fragments, but there is a growing interest in understanding the basic biology of APP as it has become recognized that alterations in the functional activity of the APP fragments during disease states will have complex effects on cell function. Indeed, it has been proposed that reductions in the level or activity of certain APP fragments, in addition to accumulation of Abeta, may play a critical role in the cognitive dysfunction associated with AD, particularly early in the course of the disease. To test and modify this hypothesis, it is important to understand the roles that full-length APP and its fragments normally play in neuronal structure and function. Here we review evidence addressing these fundamental questions, paying particular attention to the contributions that APP fragments play in synaptic transmission and neural plasticity, as these may be key to understanding their effects on learning and memory. It is clear from this literature that APP fragments, including Abeta, can exert a powerful regulation of key neural functions including cell excitability, synaptic transmission and long-term potentiation, both acutely and over the long-term. Furthermore, there is a small but growing literature confirming that these fragments correspondingly regulate behavioral learning and memory. These data indicate that a full account of cognitive dysfunction in AD will need to incorporate the actions of the full complement of APP fragments. To this end, there is an urgent need for a dedicated research effort aimed at understanding the behavioral consequences of altered levels and activity of the different APP fragments as a result of experience and disease.

How long will long-term potentiation last?

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

NMDA receptor regulation by amyloid-beta does not account for its inhibition of LTP in rat hippocampus

Accumulation of amyloid-beta peptide (Abeta) is widely believed to play a critical role in the pathogenesis of Alzheimer's disease. Although amyloid-containing plaques are a key neuropathological feature of AD, soluble forms of Abeta can interfere with synaptic plasticity in the brain, suggesting that this form of the peptide may be responsible for much of the memory deficit seen early in the disease. Here, we investigate the mechanism underlying the effects of Abeta on long-term potentiation (LTP) in area CA1 of rat hippocampus. Extracellular field recordings were made in area CA1 of hippocampal slices taken from young, adult male rats. A non-toxic concentration of Abeta (200 nM) produced a rapid inhibition of LTP induced by 100 Hz stimulation while having no long-term effect on normal synaptic transmission. The same dose of Abeta had no effect on long-term depression (LTD) induced by 1200 pulses at 1 or 3 Hz. Picrotoxin had no effect on the inhibition of LTP, suggesting Abeta does not act by enhancing GABAergic transmission. Since the LTP induction in this study was dependent on N-methyl-D-aspartate (NMDA) receptor activation, we looked at the effect of Abeta on isolated NMDA receptor-mediated field potentials. Abeta produced a small but significant inhibition of NMDA receptor-mediated synaptic potentials ( approximately 25%). However, a low dose of MK-801 (0.5 microM) that produced a similar inhibition of NMDA potentials had no effect on LTP induction but completely blocked LTD induction. These results suggest that Abeta does not inhibit LTP via effects on NMDA receptors, but rather interferes with a downstream pathway.

Induction and experience-dependent consolidation of stable long-term potentiation lasting months in the hippocampus

Long-term potentiation (LTP) is widely regarded as a memory mechanism, but it is not known whether it can last long enough to underlie very long-term memory. We report that high-frequency stimulation (HFS) paradigms applied to the rat dentate gyrus can elicit stable LTP lasting months and up to at least 1 year. The induction of stable LTP was sensitive to stimulation variables on the day of HFS and was associated with phosphorylation of cAMP response element-binding protein. The maintenance of stable LTP was also experience-dependent, because it was reversed when animals were exposed repeatedly to an enriched environment beginning 14 d post-HFS. However, stable LTP eventually consolidated over time and became resistant to reversal, because exposure to enriched environments 90 d post-HFS failed to influence stable LTP maintenance. Thus, LTP can be shown to meet one of the principal criteria for a very long-term memory storage mechanism. However, under naturalistic environmental conditions, LTP may normally be retained in the hippocampus for only short periods of time.

The AMPA receptor subunit GluR1 regulates dendritic architecture of motor neurons

The morphology of the mature motor neuron dendritic arbor is determined by activity-dependent processes occurring during a critical period in early postnatal life. The abundance of the AMPA receptor subunit GluR1 in motor neurons is very high during this period and subsequently falls to a negligible level. To test the role of GluR1 in dendrite morphogenesis, we reintroduced GluR1 into rat motor neurons at the end of the critical period and quantitatively studied the effects on dendrite architecture. Two versions of GluR1 were studied that differed by the amino acid in the "Q/R" editing site. The amino acid occupying this site determines single-channel conductance, ionic permeability, and other essential electrophysiologic properties of the resulting receptor channels. We found large-scale remodeling of dendritic architectures in a manner depending on the amino acid occupying the Q/R editing site. Alterations in the distribution of dendritic arbor were not prevented by blocking NMDA receptors. These observations suggest that the expression of GluR1 in motor neurons modulates a component of the molecular substrate of activity-dependent dendrite morphogenesis. The control of these events relies on subunit-specific properties of AMPA receptors.

Group I mGluRs increase excitability of hippocampal CA1 pyramidal neurons by a PLC-independent mechanism

Previous studies have implicated phospholipase C (PLC)-linked Group I metabotropic glutamate receptors (mGluRs) in regulating the excitability of hippocampal CA1 pyramidal neurons. We used intracellular recordings from rat hippocampal slices and specific antagonists to examine in more detail the mGluR receptor subtypes and signal transduction mechanisms underlying this effect. Application of the Group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) suppressed slow- and medium-duration afterhyperpolarizations (s- and mAHP) and caused a consequent increase in cell excitability as well as a depolarization of the membrane and an increase in input resistance. Interestingly, with the exception of the suppression of the mAHP, these effects were persistent, and in the case of the sAHP lasting for more than 1 h of drug washout. Preincubation with the specific mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), reduced but did not completely prevent the effects of DHPG. However, preincubation with both MPEP and the mGluR1 antagonist LY367385 completely prevented the DHPG-induced changes. These results demonstrate that the DHPG-induced changes are mediated partly by mGluR5 and partly by mGluR1. Because Group I mGluRs are linked to PLC via G-protein activation, we also investigated pathways downstream of PLC activation, using chelerythrine and cyclopiazonic acid to block protein kinase C (PKC) and inositol 1,4,5-trisphosphate-(IP(3))-activated Ca(2+) stores, respectively. Neither inhibitor affected the DHPG-induced suppression of the sAHP or the increase in excitability nor did an inhibitor of PLC itself, U-73122. Taken together, these results argue that in CA1 pyramidal cells in the adult rat, DHPG activates mGluRs of both the mGluR5 and mGluR1 subtypes, causing a long-lasting suppression of the sAHP and a consequent persistent increase in excitability via a PLC-, PKC-, and IP(3)-independent transduction pathway.

NMDA receptor-mediated metaplasticity during the induction of long-term depression by low-frequency stimulation

Metaplasticity refers to the activity-dependent modification of the ability of synapses to undergo subsequent synaptic plasticity. Here, we have addressed the question of whether metaplasticity contributes to the induction of long-term depression (LTD) by low-frequency stimulation (LFS). The experiments were conducted using standard extracellular recording techniques in stratum radiatum of area CA1 in hippocampal slices made from adult Sprague-Dawley rats. The degree of LTD induction was found to be a nonlinear function of the number of pulses during a 1-Hz LFS. Little LTD was observed following 600 or 900 pulses, but a significant LTD occurred following 1200 pulses of LFS, whether delivered in one episode, or in two bouts of 600 pulses given 10 min apart. A similar pattern was observed for 3 Hz LFS. The data support the suggestion that pulses occurring early in the LFS train prime synapses for LTD induction, as triggered by later occurring stimuli. The priming effect lasted at least 120 min, when tested by giving two bouts of 1 Hz LFS (600 pulses each) at different intervals. Neither heterosynaptic nor homosynaptic stimulation by itself was sufficient to prime LTD. However, a combination of the stimuli, induced by increased stimulus strength during the LFS, appeared necessary for inducing the effect. An N-methyl-d-aspartate (NMDA) receptor antagonist markedly reduced total LTD induction, regardless of whether it was administered during the first or second LFS in a protocol employing two bouts of 600 pulse LFS, 30 min apart. These findings strongly support the hypothesis that NMDA receptor-dependent metaplasticity processes contribute to the induction of LTD during standard LFS protocols.

Heterosynaptic metaplasticity in the hippocampus in vivo: a BCM-like modifiable threshold for LTP

The homeostatic maintenance of the "modification threshold" for inducing long-term potentiation (LTP) is a fundamental feature of the Bienenstock, Cooper, and Munro (BCM) model of synaptic plasticity. In the present study, two key features of the modification threshold, its heterosynaptic expression and its regulation by postsynaptic neural activity, were tested experimentally in the dentate gyrus of awake, freely moving rats. Conditioning stimulation ranging from 10 to 1,440 brief 400-Hz trains, when applied to medial perforant path afferents, raised the threshold for LTP induction heterosynaptically in the neighboring lateral perforant path synapses. This effect recovered slowly over a 7- to 35-day period. The same conditioning paradigms, however, did not affect the reversal of long-term depression. The inhibition of LTP by medial-path conditioning stimulation was N-methyl-D-aspartate (NMDA) receptor-dependent, but antidromic stimulation of the granule cells could also inhibit lateral path LTP induction, independently of NMDA receptor activation. Increased calcium buffering is a potential mechanism underlying the altered LTP threshold, but the levels of two important calcium-binding proteins did not increase after conditioning stimulation, nor was de novo protein synthesis required for generating the threshold shift. These data confirm, in an in vivo model, two key postulates of the BCM model regarding the LTP threshold. They also provide further evidence for the broad sensitivity of synaptic plasticity mechanisms to the history of prior activity, i.e., metaplasticity.

Priming stimulation of group II metabotropic glutamate receptors inhibits the subsequent induction of rat hippocampal long-term depression in vitro

The ability of priming activation of metabotropic glutamate receptors (mGluRs) to regulate long-term depression (LTD) was studied in area CA1 of hippocampal slices taken from young adult male rats. Pharmacological activation of Group I mGluRs 30-40 min prior to low-frequency stimulation at 3 Hz failed to affect LTD. Activation of Group II mGluRs, however, significantly inhibited the LTD by >50%, while activation of Group III mGluRs had no statistically significant effect on LTD. The inhibition of LTD by activation of Group II mGluRs was even stronger when the Group II agonist was applied during the low-frequency stimulation. Because activation of Group II mGluRs is also known to inhibit LTP, the net effect of such stimulation is the induction of a metaplasticity that greatly restricts the effective range of stimuli that can evoke synaptic plasticity in the hippocampus.

Tyrosine phosphorylation-dependent inhibition of hippocampal synaptic plasticity

We examined the effects of two protein tyrosine phosphatase inhibitors on the induction of synaptic plasticity in CA1 slices of rat hippocampus. Field potential recordings were made in stratum radiatum in response to stimulation of the Schaffer collateral afferents. Bath application of the tyrosine phosphatase inhibitors sodium orthovanadate or phenylarsine oxide for 30 min had little effect on basal synaptic transmission but blocked the induction of both long-term potentiation (LTP) and homosynaptic long-term depression (LTD). LTP could be partially recovered, and LTD fully recovered, when conditioning stimulation was given in conditions of reduced synaptic inhibition. The block of both forms of synaptic plasticity by the phosphatase inhibitors correlated with a concurrent depression of the N-methyl-D-aspartate (NMDA) receptor-mediated potential, as measured both extracellularly and intracellularly. This depression, which was also induced by peroxyvanadate, required synaptic stimulation to be induced, and was tyrosine kinase-dependent. Our results suggest that tyrosine phosphorylation of as yet unidentified proteins is responsible for a novel activity-dependent depression of NMDA receptor function that inhibits synaptic plasticity.

Sequential increase in Egr-1 and AP-1 DNA binding activity in the dentate gyrus following the induction of long-term potentiation

Establishment of long-term potentiation (LTP) at perforant path synapses is highly correlated with increased expression of Egr and AP-1 transcription factors in rat dentate gyrus granule cells. We have investigated whether increased transcription factor levels are reflected in increased transcription factor activity by assessing Egr and AP-1 DNA binding activity using gel shift assays. LTP produced an increase in binding to the Egr element, which was NMDA receptor-dependent and correlated closely with our previously reported increase in Egr-1 (zif/268) protein levels. Supershift analysis confirmed involvement of Egr-1, but not Egr-2 in the DNA binding activity. AP-1 DNA binding was also rapidly elevated in parallel with protein levels, however, the peak increase in activity was delayed until 4 h, a time point when we have previously shown that only jun-D protein was elevated. These data indicate that binding of Egr-1 and AP-1 to their response elements is increased in two phases. This may result in activation of distinct banks of target genes which contribute to the establishment of persistent LTP.

Metabotropic glutamate receptors trigger homosynaptic protein synthesis to prolong long-term potentiation

We investigated the mechanisms by which previous "priming" activation of group I metabotropic glutamate receptors (mGluRs) facilitates the persistence of long-term potentiation (LTP) in area CA1 of rat hippocampal slices. Priming of LTP was elicited by either pharmacological or synaptic activation of mGluRs before a weak tetanic stimulus that normally produced only a rapidly decaying phase of LTP that did not involve protein synthesis or mGluRs. Pharmacological priming of LTP persistence by a selective group I mGluR agonist was blocked by an inhibitor of group I mGluRs and by inhibitors of translation, but not by a transcriptional inhibitor. The same mGluR agonist increased (35)S-methionine incorporation into slice proteins. LTP could also be facilitated using a synaptic stimulation priming protocol, and this effect was similarly blocked by group I mGluR and protein synthesis inhibitors. Furthermore, using a two-pathway protocol, the synaptic priming of LTP was found to be input-specific. To test for the contribution of group I mGluRs and protein synthesis to LTP in nonprimed slices, a longer duration control tetanization protocol was used to elicit a more slowly decaying form of LTP than did the weak tetanus used in the previous experiments. The persistence of the LTP induced by this stronger tetanus was dependent on mGluR activation and protein synthesis but not on transcription. Together, these results suggest that mGluRs couple to nearby protein synthesis machinery to homosynaptically regulate an intermediate phase of LTP dependent on new proteins made from pre-existing mRNA.

Long-lasting increase in cellular excitability associated with the priming of LTP induction in rat hippocampus

The mechanisms underlying the facilitation (priming) of long-term potentiation (LTP) by prior activation of metabotropic glutamate receptors (mGluRs) were investigated in area CA1 of rat hippocampal slices. In particular, we focused on whether a long-lasting increase in postsynaptic excitability could account for the facilitated LTP. Administration of the mGluR agonist 1S, 3R-aminocyclopentanedicarboxylic acid (ACPD) produced rapid decreases in the amplitude of both the slow spike afterhyperpolarization (AHP(slow)) and spike frequency adaptation recorded intracellularly from CA1 pyramidal cells. These changes persisted after drug washout, showing only a slow decay over 20 min. ACPD also caused a leftward shift of the field EPSP-population spike relation and an overall increase in population spike amplitude, but this effect was not as persistent as the intracellularly measured alterations in cell excitability. ACPD-treated cells showed increased spike discharges during LTP-inducing tetanic stimulation, and the amplitude of the AHP(slow) was negatively correlated with the degree of initial LTP induction. The beta-adrenergic agonist isoproterenol also caused excitability changes as recorded intracellularly, whereas in extracellular experiments it weakly primed the induction but not the persistence of LTP. ACPD primed both LTP measures. Isoproterenol administration during the tetanus occluded the priming effect of ACPD on initial LTP induction but not its effect on LTP persistence. We conclude that the persistent excitability changes elicited by ACPD contributes to the priming of LTP induction but that other ACPD-triggered mechanisms must account for the facilitated persistence of LTP in the priming paradigm.

Immediate early gene transcription and synaptic modulation

Long-term changes in gene expression appear to be critical to the formation of memory, but little is known about its stimulus- transcription coupling. Numerous studies in the last decade, by focusing on unraveling this signal transduction pathway, have investigated the potential role of the immediate-early genes in this process. The krox family of immediate-early gene proteins are of particular interest because they may be involved in stabilizing the synaptic modifications that underlie hippocampal long-term potentiation (LTP). A potential upstream mediator of krox induction is cyclic AMP-responsive element binding protein (CREB), a posttranslationally activated transcription factor that has been implicated in numerous memory paradigms. In this study we investigated whether the activation of CREB by phosphorylation may have a role in the development of rat perforant- path-stimulated LTP and associated dentate granule cell krox-24 mRNA expression. Contrary to what was expected, we failed to show any difference in the levels of phosphorylated CREB after LTP or following endogenous synaptic facilitation stimulated by novelty. Using these same model systems we also investigated the protein levels of brain- derived neurotrophic factor (BDNF), another immediate-early gene that is induced following a durable form of LTP. However, BDNF protein was not induced within the hippocampus after LTP and was transiently decreased following novel environmental stimulation.

A double dissociation within the hippocampus of dopamine D1/D5 receptor and beta-adrenergic receptor contributions to the persistence of long-term potentiation

We compared the effects of the D1/D5 receptor antagonist SCH-23390 with the beta-adrenergic receptor antagonist propranolol on the persistence of long-term potentiation in the CA1 and dentate gyrus subregions of the hippocampus. In slices, SCH-23390 but not propranolol reduced the persistence of long-term potentiation in area CA1 without affecting its induction. The drugs exerted reverse effects in the dentate gyrus, although in this case the induction of long-term potentiation was also affected by propranolol. The lack of effect of SCH-23390 on the induction and maintenance of long-term potentiation in the dentate gyrus was confirmed in awake animals. The drug also had little or no effect on the expression of inducible transcription factors. In area CA1 of awake animals, SCH-23390 blocked persistence of long-term potentiation beyond 3 h, confirming the results in slices. To rule out a differential release of catecholamines induced by our stimulation protocols between brain areas, we compared the effects of the D1/D5 agonist SKF-38393 with the beta-adrenergic agonist isoproterenol on the persistence of a weakly induced, decremental long-term potentiation in CA1 slices. SKF-38393 but not isoproterenol promoted greater persistence of long-term potentiation over a 2-h period. In contrast, isoproterenol but not SKF-38392 facilitated the induction of long-term potentiation. These data demonstrate that there is a double dissociation of the catecholamine modulation of long-term potentiation between CA1 and the dentate gyrus, suggesting that long-term potentiation in these brain areas may be differentially consolidated according to the animal's behavioural state.

Biphasic changes in the levels of N-methyl-D-aspartate receptor-2 subunits correlate with the induction and persistence of long-term potentiation

N-Methyl-D-aspartate glutamate receptors (NMDAR) form ion channels made up of polypeptides from two classes of subunits; NR1 is obligatory for function whereas members of the NR2 class regulate the properties of the channel. Long-term potentiation (LTP) of synaptic transmission is an event largely dependent on NMDAR activation, and is studied as the primary cellular model of memory in the mammalian brain. While there has been a focus on non-NMDARs in mediating the expression of LTP, we report here biochemical evidence for plasticity of the NMDAR that is associated with LTP persistence in awake animals. Following the establishment of LTP in perforant path synapses of the dentate gyrus, we observed a rise in NR2B protein levels 48 h post-tetanus which was dependent upon activation of NMDARs during the tetanization, and which strongly correlated with the degree of LTP measured at this time-point. We also observed a transient increase in both NR2B and NR2A protein levels 20 min post-tetanus that returned to control levels by 4 h. These early increases were not observed in anaesthetized animals which do not sustain persistent LTP. Our data demonstrate a marked plasticity of NMDAR subunit expression, which may affect LTP persistence, as well as the subsequent ability to induce LTP at previously activated synapses.

Synaptic activity-dependent modulation of mitochondrial gene expression in the rat hippocampus

In order to identify genes that may underlie the maintenance of long-term potentiation (LTP) at perforant path synapses, complementary DNA libraries were synthesised from dentate gyrus total RNA extracts prepared 48 h after the induction of LTP and from control dentate gyrus extracts. Through differential screening of the LTP library we have identified the mitochondrial 12S rRNA (mt12SrRNA) as a transcript that was elevated at this late time. Northern blot analyses showed that the elevation in mt12SrRNA expression began around 8 h and persisted for at least 2 weeks post-tetanus. We then examined the expression patterns of other mitochondrially-encoded genes and demonstrated a similar elevation in their expression. mt12SrRNA levels were also elevated in other hippocampal regions, including areas CA3 and CA1 and were elevated following low-frequency stimulation or in the presence of an N-methyl-D-aspartate receptor antagonist where induction of LTP was precluded. Taken together, these observations suggest that a long-lasting up-regulation of energy production may be triggered by synaptic activity and this activity need not be of sufficient strength to induce LTP, but may be related to the induction of a metaplastic state.

Effects of endothelin-1 on hippocampal synaptic plasticity

We examined the effects of puff application of endothelin (ET)-1 on the induction of long-term potentiation (LTP) and heterosynaptic long-term depression (LTD) in hippocampal CA1 slices. ET-1 applied 2 min prior to tetanus blocked the induction of LTP, but facilitated the induction of heterosynaptic LTD. These ET-1 effects on synaptic plasticity were dose-dependent, and not due to a generalized depression of baseline responses. ET-1 did not alter NMDA receptor-mediated responses. These data provide the first evidence that endothelin modulates activity-dependent synaptic plasticity, and the potency of these effects suggests that endogenous ET-1 may play an important role in regulating memory storage processes.

Priming of long-term potentiation induced by activation of metabotropic glutamate receptors coupled to phospholipase C

Activation of metabotropic glutamate receptors (mGluRs) with 1-aminocyclopentane-1S,3R-dicarboxylic acid 20 min prior to tetanus facilitates, or "primes," subsequent induction of long-term potentiation (LTP; Cohen and Abraham, J Neurophysiol 1996;76:953-962). In the present study, we investigated the receptor specificity and associated second messenger pathways involved in the mGluR priming effect by using field potentials recorded from area CA1 of rat hippocampal slices. In controls, mild theta-burst or high-frequency (100 Hz) stimulation induced 16% and 21% LTP, respectively. A 10-min application of the group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) caused a transient depression of synaptic responses but a significant enhancement of subsequent LTP for both tetanus protocols (45% and 41% LTP, respectively). Maximal LTP, induced by stronger tetanization protocols, was not enhanced by DHPG, nor was mild LTP facilitated by post-tetanic application of DHPG. Priming with agonists selective for group II or III mGluRs had no effect on LTP. The mGluR antagonists L-2-amino-3-phosphonopropionic acid and 1-aminoindan-1,5-dicarboxylic acid inhibited the LTP facilitatory effect of DHPG but not the transient response depression, whereas alpha-methyl-4-carboxyphenylglycine produced the opposite effects. Priming with N-methyl-D-aspartate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid did not facilitate LTP induction. Prior activation of muscarinic acetylcholine receptors produced at best a weak priming effect. Inhibition of phospholipase C by U-73122 completely abolished the priming of LTP by DHPG. We conclude that mGluR priming of LTP results from biochemical cascades triggered by activation of phospholipase C coupled to group I mGluRs.

Glucocorticoid receptor activation lowers the threshold for NMDA-receptor-dependent homosynaptic long-term depression in the hippocampus through activation of voltage-dependent calcium channels

The effects of the glucocorticoid receptor agonist RU-28362 on homosynaptic long-term depression (LTD) were examined in hippocampal slices obtained from adrenal-intact adult male rats. Field excitatory postsynaptic potentials were evoked by stimulation of the Schaffer collateral/commissural pathway and recorded in stratum radiatum of area CA1. Low-frequency stimulation (LFS) was delivered at LTD threshold (2 bouts of 600 pulses, 1 Hz, at baseline stimulation intensity). LFS of the Schaffer collaterals did not produce significant homosynaptic LTD in control slices. However, identical conditioning in the presence of the glucocorticoid receptor agonist RU-28362 (10 microM) produced a robust LTD, which was blocked by the selective glucocorticoid antagonist RU-38486. The LTD induced by glucocorticoid receptor activation was dependent on N-methyl-D-aspartate (NMDA) receptor activity, because the specific NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) blocked the facilitation. However, the facilitation of LTD was not due to a potentiation of the isolated NMDA receptor potential by RU-28362. The facilitation of LTD by RU-28362 was also blocked by coincubation of the L-type voltage-dependent calcium channel (VDCC) antagonist nimodipine. Selective activation of the L-type VDCCs by the agonist Bay K 8644 also facilitated LTD induction. Both nimodipine and D-AP5 were effective in blocking the facilitation of LTD by Bay K 8644. These results indicate that L-type VDCCs can contribute to NMDA-receptor-dependent LTD induction.

Metaplasticity: a new vista across the field of synaptic plasticity

Over the past 20 years there has been an increasing understanding of the properties and mechanisms underlying long-term potentiation (LTP) and long-term depression (LTD) of synaptic efficacy, putative learning and memory mechanisms in the mammalian brain. More recently, however, it has become apparent that synaptic activity can also elicit persistent neuronal responses not manifest as changes in synaptic strength. Some of these changes may nonetheless modify the ability of synapses to undergo strength changes in response to subsequent episodes of synaptic activity. This kind of activity-dependent modulatory plasticity we have termed "metaplasticity". Metaplasticity has been observed physiologically as an inhibition of LTP and concomitant facilitation of LTD by prior N-methyl-D-aspartate receptor activation or, conversely, a facilitation of LTP induction by prior metabotropic glutamate receptor activation. The examples of metaplasticity described to date are input specific, and last as long as several hours. The mechanisms underlying such phenomena remain to be fully characterized, although some likely possibilities are an altered N-methyl-D-aspartate receptor function, altered calcium buffering, altered states of kinases or phosphatases, and a priming of protein synthesis machinery. While some details vary, experimentally observed metaplasticity bears some similarity to the "sliding threshold" feature of the Bienenstock, Cooper and Munro model of experience-dependent synaptic plasticity. Metaplasticity may serve several functions including (1) providing a way for synapses to integrate a response across temporally spaced episodes of synaptic activity and (2) keeping synapses within a dynamic functional range, and thus preventing them from entering states of saturated LTP or LTD.

TrkB expression in dentate granule cells is associated with a late phase of long-term potentiation

Recent studies have demonstrated that the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are induced in hippocampal neurons following the induction of long-term potentiation (LTP), a model of memory, and that BDNF and NT-3 (but not NGF) can induce LTP-like increases in synaptic efficacy. Receptors for these neurotrophins have been cloned and characterized and we investigated whether LTP alters the expression of two neurotrophin receptors, trkB (BDNF receptor) and trkC (NT-3 receptor) in dentate granule neurons of the hippocampus using in situ hybridization analysis. Results show that trkB is strongly induced in these neurons in an N-methyl-D-aspartate (NMDA) receptor-dependent manner. Moreover, the induction of trkB and trkC mRNAs was attenuated by sodium pentobarbital, which interferes with the durability of LTP. Low-frequency stimulation of the perforant path had no effect on trkB mRNA levels but significantly reduced trkC mRNA in dentate granule cells. Thus, both BDNF and its receptor trkB are induced in granule cells by stimulation that produces durable LTP, suggesting that this neurotrophin and its receptor play an important role in memory formation and may be suitable targets for the development of cognitive-enhancing drugs in the treatment of diseases, such as Alzheimer's.

Sequence-independent effects of phosphorothiolated oligonucleotides on synaptic transmission and excitability in the hippocampus in vivo

Antisense oligodeoxynucleotides (ODNs) have the potential to be a powerful tool for regulating gene expression and mRNA translation in spatially and temporally restricted domains. Prior to investigating the effects of antisense ODNs on hippocampal long-term potentiation, we investigated whether there are any non-specific effects of ODNs on perforant path synaptic transmission in the dentate gyrus of both pentobarbital-anaesthetized and awake, freely moving rats. Single injections of phosphorothioated antisense ODNs (4 nmol) to the immediate early gene zif/268 caused a rapid (within minutes) and long-lasting (>24 hr) profound depression of the perforant path evoked field potentials. This depressive effect was due to the phosphorothioate modification since a depression was not seen with unmodified antisense ODNs, relative to saline controls. Furthermore, the effect was not sequence-specific since modified sense ODNs caused the same degree of depression. The depression caused by the modified antisense ODNs was dose-dependent and specific to synaptic transmission, since antidromic population spikes elicited by mossy fibre stimulation were relatively unaffected compared to the orthodromic responses. A second unexpected side-effect of the modified ODNs was cellular hyperexcitability, such that bursts of epileptiform spikes in the EEG occurred both spontaneously and as a result of synaptic stimulation. While the mechanism of the synaptic depression remains unknown, these results indicate that phosphorothioate-modified ODNs exert profound non-specific effects on synaptic transmission in the hippocampus, that have the potential to seriously compromise any corresponding behavioural or electrophysiological studies.

Induction and reversal of long-term potentiation by repeated high-frequency stimulation in rat hippocampal slices

Field potential recordings were made from area CA1 of hippocampal slices from young adult rats to study the effects of repeated tetanic stimulation on the development of LTP. Stimulation was applied to the Schaffer collateral afferents, and field excitatory postsynaptic potentials were recorded in stratum radiatum. Theta-burst stimulation (TBS) resulted in variable amounts of long-term potentiation (LTP), depending on how many trains of stimulation were delivered. Peak amounts of LTP occurred after 8-16 trains of TBS, but virtually no LTP occurred after 24 or 32 trains of TBS. There was thus an inverted U-shaped relation between the amount of TBS and the degree of LTP. The temporal spacing of TBS trains was important for observing the lack of LTP after 32 trains ("over-stimulation"). If the trains were grouped into blocks of 8, with 10 min between blocks, LTP occurred normally. This finding suggests that a time-dependent LTP reversal process was occurring during the massed presentation of TBS trains. Over-stimulation inhibited for 60-90 min the subsequent induction of LTP by a normally efficient LTP-inducing protocol. This effect was input specific and dependent on activation of N-methyl-D-aspartate (NMDA) receptors. Lowering extracellular [Ca2+] from 2.5 to 2.0 mM, or adding the L-type calcium channel antagonist nimodipine, had only a small protective effect on the lack of LTP induced by 32 trains of TBS. Addition of an NMDA receptor antagonist to the bath solution shortly after the beginning of the over-stimulation protocol gave significantly more protection. Administration of an adenosine (A1) receptor antagonist during over-stimulation permitted robust LTP to occur, indicating that A1 receptor activation during TBS contributes to the depotentiation process. These findings confirm previous findings in the dentate gyrus that repeated afferent tetanization within a narrow time frame can lead to a loss or reversal of LTP. Activation of adenosine receptors appears to trigger this effect.

Facilitation of long-term potentiation by prior activation of metabotropic glutamate receptors

1. The influence of prior metabotropic glutamate receptor (mGluR) activation on subsequent long-term potentiation (LTP) induction was investigated with the use of the mGluR agonist 1-amino-cyclopentane-1S,3R-dicarboxylic acid (ACPD, 20 microM). Field potential recordings were made in the stratum radiatum of CA1 slices taken from young adult male rats and from which CA3 was routinely dissected. Theta burst stimulation (TBS) just above threshold was used to induce LTP. 2. A 10-min bath application of ACPD begun 30 min before the TBS facilitated the induction of LTP in a dose-dependent manner and resulted in an enhanced magnitude and stability of LTP. 3. ACPD did not enhance the degree of LTP induced by strong TBS, suggesting that it acts to lower the threshold for LTP induction but does not raise the ceiling on the amount of inducible LTP. 4. This priming effect by ACPD was stereo specific and lasted between 1 and 3 h. Synaptic stimulation during the ACPD application was not necessary for the enhancement of LTP. Blockade of N-methyl-D-aspartate receptors (NMDARs) during ACPD application also failed to affect the enhancement of LTP. 5. ACPD-induced priming of LTP was antagonized by L-2-amino-3-phosphonopropionic acid, suggesting an involvement of group I mGluRs. 6. ACPD-induced enhancement of LTP was not secondary to long-lasting changes in NMDAR activation or GABAAergic inhibition, because it was unaffected by the addition of picrotoxin, a gamma-aminobutyric acid-A (GABAA) receptor antagonist, and isolated NMDAR-mediated responses did not show a long-lasting enhancement in response to ACPD application. 7. These data demonstrate that activation of mGluRs can initiate persistent yet covert changes in synaptic function that facilitate the stable induction of LTP.

Low-frequency stimulation does not readily cause long-term depression or depotentiation in the dentate gyrus of awake rats

The ability of low-frequency stimulation (LFS) to induce either long-term depression (LTD) or depotentiation was assessed for perforant path synapses in the dentate gyrus of awake, adult rats. Neither LFS at 1 Hz (100 or 900 pulses) nor LFS at 3 Hz (900 pulses) was sufficient to produce either LTD or depotentiation. LFS at 3 Hz did produce a transient response depression of previously potentiated synapses, but this lasted less than 24 h and was secondary to seizure-like afterdischarges. We conclude that the LFS protocols so effective at eliciting LTD and depotentiation in area CA1 are ineffective for perforant path synapses in the dentate gyrus.

Bidirectional modification of CA1 synapses in the adult hippocampus in vivo

Memories are believed to be stored by synaptic modifications. One type of activity-dependent synaptic modification, long-term potentiation (LTP), has received considerable attention as a possible memory mechanism, particularly in hippocampus. However, use-dependent decreases in synaptic strength can store information as well. A form of homosynaptic long-term depression (LTD) has been described and widely studied in the CA1 region of the developing hippocampus in vitro. However, the relevance of this model of LTD to memory has been questioned because of failures to replicate it in the adult brain in vitro and, more recently, in vivo. Here we re-examine this important issue and find that homosynaptic LTD can in fact be elicited in the adult hippocampus in vivo, that it has all the properties described in immature CA1 in vitro, and that LTD and LTP are reversible modifications of the same Schaffer collateral synapses. Thus homosynaptic LTD is not peculiar to brain slices, nor is it only of developmental significance. Rather, out data suggest that the mechanisms of LTD and LTD may be equal partners in the mneumonic operations of hippocampal neural networks.

Metaplasticity: the plasticity of synaptic plasticity

In this paper, we review experimental evidence for a novel form of persistent synaptic plasticity we call metaplasticity. Metaplasticity is induced by synaptic or cellular activity, but it is not necessarily expressed as a change in the efficacy of normal synaptic transmission. Instead, it is manifest as a change in the ability to induce subsequent synaptic plasticity, such as long-term potentiation or depression. Thus, metaplasticity is a higher-order form of synaptic plasticity. Metaplasticity might involve alterations in NMDA-receptor function in some cases, but there are many other candidate mechanisms. The induction of metaplasticity complicates the interpretation of many commonly studied aspects of synaptic plasticity, such as saturation and biochemical correlates.

Long-term depression in hippocampus

Long-term depression (LTD) is a lasting decrease in synaptic effectiveness that follows some types of electrical stimulation in the hippocampus. Two broad types of LTD may be distinguished. Heterosynaptic LTD can occur at synapses that are inactive, normally during high-frequency stimulation of a converging synaptic input. Homosynaptic LTD can occur at synapses that are activated, normally at low frequencies. Here we discuss the mechanisms of LTD and their possible relevance to hippocampal function.

LTD: many means to how many ends?

Several forms of long-term synaptic depression (LTD) have been identified in the hippocampus. Although these LTDs differ in their methods of induction, it remains unclear whether they converge upon some final common mechanism(s) of expression. In the present paper we review critical aspects of heterosynaptic and homosynaptic LTD induction and consider those findings relevant to the expression of each. In addition, we suggest several experimental approaches which may be of benefit in addressing the commonality of LTD expression mechanisms.

Induction of heterosynaptic and homosynaptic LTD in hippocampal sub-regions in vivo

Establishing the conditions under which long-term depression (LTD) an be induced in vitro remains an important empirical issue. While heterosynaptic LTD is readily elicited in the hippocampus in vivo, the induction of homosynaptic LTD has been more problematic, both in CA1 and in the dentate gyrus. Our work has demonstrated, however, that 900 pulses at 1 Hz will reliably induce LTD in both area CA1 and the dentate gyrus of pentobarbital-anesthetized animals, when the stimulating and recording electrodes are in close proximity to each other.

Cooperative interactions among afferents govern the induction of homosynaptic long-term depression in the hippocampus

Prolonged periods of low-frequency stimulation have been shown to produce a robust, long-term synaptic depression (LTD) in both hippocampus and visual cortex. In the present study we have examined the extent to which interactions among afferents govern the induction of homosynaptic LTD in young-adult rats in hippocampal region CA1 in vitro. Field excitatory postsynaptic potentials were assessed before and after conditioning stimulation consisting of two 10-min trains of low-frequency stimulation (LFS; 1 Hz) of the Schaffer collateral/commissural pathway. LFS at an intensity producing a 0.5-mV response did not produce significant synaptic depression. However, LFS administered at a higher intensity resulted in significant input-specific LTD of a 0.5-mV test response. Picrotoxin, which also facilitates depolarization of CA1 neurons, significantly enhanced the magnitude of LTD after LFS at 0.5 mV. In addition, LFS at 0.5 mV in normal perfusion medium (no picrotoxin) produced only small changes in synaptic efficacy when either of two converging pathways was conditioned separately but produced a robust LTD when both pathways were conditioned simultaneously. This cooperative LTD was reversibly blocked by prior administration of 100 microM DL-aminophosphonovaleric acid but not by 20 microM nimodipine. Taken together, these results suggest that cooperative interactions among afferents contribute to voltage-dependent processes underlying the induction of homosynaptic LTD.

Evidence for common expression mechanisms underlying heterosynaptic and associative long-term depression in the dentate gyrus

1. The extent to which heterosynaptic and prime-associative stimulation protocols generate different forms of long-term depression (LTD) was assessed in the lateral perforant path synapses terminating on dentate gyrus granule cells in pentobarbital-anesthetized rats. 2. Heterosynaptic LTD was induced in the lateral path by repeated tetanization of the medial path. Prime-associative LTD of the lateral path was induced by alternating high-frequency conditioning trains to the medial path and single shocks to the lateral path at 100-ms intervals, all occurring 10 min after priming stimulation of the lateral path (5 Hz, 80 pulses). 3. Induction of LTD by one administration of the prime-associative protocol was normally greater in magnitude than the LTD induced by the heterosynaptic protocol. Saturation of LTD by repeated delivery of the prime-associative protocol completely occluded the subsequent induction of LTD by the heterosynaptic protocol. Saturation of LTD by repeated delivery of the heterosynaptic protocol produced an 80% occlusion of the LTD generated by the prime-associative protocol. 4. These data support the hypothesis that activity-dependent (associative) and activity-independent (heterosynaptic) LTD involve overlapping expression mechanisms, despite having demonstrably different induction mechanisms.

Analysis of the decremental nature of LTP in the dentate gyrus

The persistence of long-term potentiation (LTP) in the dentate gyrus was compared for two tetanization protocols: 50 trains on one day, or 50 trains on 5 consecutive days. LTP induction was significantly greater in the 250 train condition, but the LTP decay rate over weeks was similar between conditions. The decay of LTP could not be accounted for by deterioration of the preparation. Successive days of stimulation caused repetitive induction of immediate early genes, but did not prolong LTP, suggesting that either the effects of gene expression on LTP stabilization had saturated, or that these genes play other roles in synaptic plasticity.

Reduction of the threshold for long-term potentiation by prior theta-frequency synaptic activity

Activation of the lateral perforant path input to the dentate gyrus with theta-patterned conditioning trains produced LTP of synaptic efficacy that changed in magnitude as an inverted U-shaped function of the number of trains. The LTP induction function was not fixed, however, and could be shifted to the left by administering 5 Hz "priming" stimulation to the lateral path 10 min prior to the conditioning trains. The priming effect was input specific and selective to a narrow window of stimulus frequencies. The shift to the left of the LTP induction function by priming stimulation was blocked by the muscarinic receptor antagonist atropine sulphate. Nimodipine, an antagonist of L-type voltage-sensitive calcium channels, did not mimic the priming effect but instead produced a general facilitation of LTP induction. These data demonstrate that the degree to which LTP is induced in the lateral path is a non-linear function of afferent activity, and that this function, including LTP threshold, can be shifted to the left by prior synaptic activity at hippocampal theta-rhythm frequencies.

Krox20 may play a key role in the stabilization of long-term potentiation

Long-term potentiation-inducing stimulation of the perforant path was followed in dentate gyrus granule cells by a dramatic increase of mRNA and protein for Krox20, a zinc-finger-containing transcription factor. Induction of Krox20 required stimulation sufficient to induce LTP and was prevented by NMDA antagonists CPP and MK-801, which block LTP induction. Krox20 protein increased within 20 min of tetanization, was maximal between 1 and 8 h, and was still significantly elevated at 24 h after LTP induction. This prolonged appearance is in striking contrast with the more transient induction of the related molecule, Krox24. The elevation in the mRNA for Krox20 and Krox24 was of similar duration, suggesting that the Krox20 protein has a greater stability and may play a key role in the stabilization of long-term potentiation.

Immediate early gene expression associated with the persistence of heterosynaptic long-term depression in the hippocampus

Long-term depression (LTD) of synaptic efficacy is likely to be as important in memory processing as the more well-known long-term potentiation (LTP). The case for LTD serving as a memory mechanism, however, requires that it be shown to persist across days or weeks at least. Here we examined the persistence of heterosynaptic LTD in the medial and lateral perforant path inputs to the dentate gyrus in awake rats and correlated this persistence with the degree of immediate early gene expression as assessed immunohistochemically. Rats were chronically implanted with separate stimulating electrodes in the medial and lateral perforant paths and an extracellular field potential recording electrode in the dentate hilus. After recovery from surgery, either the medial or the lateral perforant path was tetanized with 400-Hz trains, and homosynaptic LTP and heterosynaptic LTD were followed across time. Heterosynaptic LTD was shown to occur readily in awake animals and to persist across days or weeks, depending on the stimulation protocol. The persistence of LTD and LTP was highly correlated within animals. Additional animals, given the same tetanization protocols, showed that the greatest immediate early gene expression occurred following that protocol which consistently gave the longest-lasting LTP and LTD. These data support the proposed role of LTD in memory processing but question whether immediate early genes are important for the persistence of LTP, LTD, or both.

Flip side of synaptic plasticity: long-term depression mechanisms in the hippocampus

There is growing interest in the phenomenon of long-term depression (LTD) of synaptic efficacy that, together with long-term potentiation (LTP), is a putative information storage mechanism in mammalian brain. In neural network models, multiple learning rules have been used for LTD induction. Similarly, in neurophysiological studies of hippocampal synaptic plasticity, a variety of activity patterns have been effective at inducing LTD, although experimental paradigms are still being optimized. In this review the authors summarize the major experimental paradigms and compare what is known about the mechanisms of LTD induction. Although all paradigms appear to initiate a cascade of events leading to an elevated level of Ca2+ postsynaptically, the extent to which these paradigms involve common expression mechanisms has not yet been tested. The authors discuss several critical experiments that would address this latter issue. Numerous questions about the properties and mechanisms of LTD(s) in the hippocampus remain to be answered, but it is clear that LTD has finally arrived, and will soon be attracting attention equal to its flip side, LTP.

L-type voltage-sensitive calcium channel antagonists block heterosynaptic long-term depression in the dentate gyrus of anaesthetized rats

We examined the role of dihydropyridine-sensitive Ca2+ channels in long-term potentiation (LTP) and long-term depression (LTD) in the dentate gyrus of pentobarbital-anaesthetized rats. Intrahippocampal infusions (1.0 microliter) of nimodipine (20 mM) and nifedipine (20 mM), two antagonists of L-type voltage-sensitive calcium channels (VSCC), 60 min prior to medial perforant path tetanization, completely blocked the appearance of LTD in the lateral perforant path, without significantly affecting the simultaneous induction of medial path LTP. Heterosynaptic LTD was not significantly different from control animals following infusions of the L-type calcium channel agonist BAY-K8644 (1 mM). These results suggest that L-type VSCC activity is necessary for the induction of heterosynaptic LTD in the dentate gyrus in vivo.

Differential regulation of paired-pulse plasticity following LTP in the dentate gyrus

The mechanisms underlying the expression of long-term synaptic potentiation (LTP) in the lateral and medial perforant path inputs to the dentate gyrus were examined in pentobarbital anaesthetized rats. Lateral path LTP was accompanied by a robust, long-lasting reduction in lateral path paired-pulse facilitation (PPF). The reduction in PPF lasted for at least 1 h, and remained even after post-tetanic EPSP slopes were reduced to pre-tetanus levels. The induction of lateral path LTP and the corresponding reduction in PPF were blocked by the competitive NMDA receptor antagonist CPP. Medial path paired-pulse depression was not affected by medial path LTP. These data suggest that changes in transmitter release may contribute more to lateral path than to medial path LTP.

Cysteamine potentiates entorhinal activation of dentate gyrus granule cells in rats

A dense plexus of somatostatin-positive fibers and varicosities is observed in the outer two-thirds of the dentate gyrus molecular layer where the glutamatergic perforant path afferents from the entorhinal cortex terminate. To test for a functional interaction between these two pathways, we examined the effects of cysteamine, which enhances somatostatin release for a few hours after administration but produces subsequent depletion of somatostatin lasting several days, on perforant path evoked potentials recorded in the dentate gyrus. Cysteamine (50-400 mg/kg, IP) increased the population spike dose-dependently both in anesthetized and in awake rats, but the slope of the population excitatory postsynaptic potential (EPSP) was left unchanged or even decreased. The antidromic population spike evoked by mossy fiber stimulation was not changed by cysteamine. The change is thought to be due to the increase in slope of the EPSP-spike relationship. In the hippocampal slice preparation, a similar effect of the drug (1-5 mM) on dentate evoked potentials was observed, suggesting that cysteamine acts through its effects on somatostatin in the hippocampus itself. In chronically implanted awake animals, the perforant path population spike was increased 1 h after cysteamine but returned to the predrug level by 24 h when somatostatin seemed to be depleted. These results suggest that hippocampal somatostatin released by cysteamine potentiates the response of dentate granule cells to perforant path input, without directly affecting synaptic transmission or general cell excitability.

Correlations between immediate early gene induction and the persistence of long-term potentiation

The duration of long-term potentiation in the dentate gyrus of awake rats was examined following systematic manipulation of the number of stimulus trains delivered. This was correlated with the induction of immediate early genes in separate groups of animals given identical stimulus regimes. Following 10 trains of stimulation, long-term potentiation decayed with a time constant of up to several days (long-term potentiation 2), and this correlated with the appearance of an increase in the messenger RNA and protein levels of zif/268. Increasing the number of stimulus trains resulted in a greater probability of eliciting long-term potentiation with a time constant of several weeks (long-term potentiation 3), as well as increasing the induction of zif/268, c-Jun, Jun-B, Jun-D and Fos-related proteins. When 10 trains were delivered repeatedly on up to five consecutive days, only the zif/268 protein levels showed associated changes. These data provide support for the hypothesis that long-term potentiation 3 involves mechanisms additional to those for long-term potentiation 2. One possible mechanism is altered gene expression, initiated by immediate early gene transcription factors such as zif/268 and possibly homo- or heterodimers of Fos and Jun family members, that then contributes to the stabilization or maintenance of long-term potentiation 3.

Comparison of associative and non-associative conditioning procedures in the induction of LTD in CA1 of the hippocampus

Recent reports have indicated that weak activity in a test input, negatively correlated (out-of-phase) with tetanization of a separate, converging input, produces an NMDA-independent, associative long-term depression (LTD) of the test input synapses, in hippocampal field CA1 (Stanton and Sejnowski, 1989; Stanton et al., 1991). Associative LTD has also been observed in the dentate gyrus, in vivo, but only following "priming" of the test path with 5 Hz stimulation prior to associative conditioning (Christie and Abraham, 1992b). We have used these stimulus protocols, in vitro, in order to compare the induction of non-associative and associative LTD in field CA1 of the adult rat hippocampus. Stimulation in normal solution evoked a small non-associative LTD, but no associative LTD. Addition of picrotoxin to the medium facilitated the induction of NMDA-dependent non-associative LTD, but not associative LTD. Previously potentiated pathways were not different from naive pathways in expression of LTD of either kind. Finally, 'priming' stimulation (5 Hz) of the test pathway produced a weak, selective enhancement of associative LTD that was, however, not significantly greater than non-associative LTD. These results indicate that, for our experimental conditions, negatively correlated co-activity during afferent tetanization does not induce a substantial associative LTD in area CA1.

Differential expression of immediate early genes after hippocampal long-term potentiation in awake rats

The pattern of expression of fos and jun family immediate early genes following the induction of long-term potentiation (LTP) was investigated in the dentate gyrus of awake rats. Rapid, transient increases in the levels of c-jun and jun-B mRNA and protein, and in the levels of Fos-related proteins (FRAs), occurred in the dentate gyrus after LTP-inducing tetanization of the perforant path. A delayed, and more prolonged induction occurred for jun-D mRNA and protein. The induction of c-Jun, Jun-B, Jun-D and Fos-related proteins was prevented by administration of an N-methyl-D-aspartate receptor antagonist, which also blocked LTP induction, and by pentobarbital, which reduced but did not block LTP. These findings show that differential expression of fos and jun gene family members occurs in a distinct pattern following LTP in awake rats. The responsive genes may participate in the biochemical cascade leading to the long-term stabilization of synaptic modifications.

Priming of associative long-term depression in the dentate gyrus by theta frequency synaptic activity

Associative long-term synaptic depression (LTD) was investigated utilizing negatively correlated activity patterns in the medial and lateral perforant path inputs to the dentate gyrus in anesthetized rats. Normally only nonassociative, or heterosynaptic, LTD is elicited in naive pathways. We report here, however, that associative LTD in the lateral path is readily induced after being "primed" by a brief period of lateral path synaptic activity at a theta rhythm frequency (5 Hz). Priming of associative LTD lasts at least 2 hr and is not seen following priming activity at non-theta frequencies (1 and 15 Hz). N-methyl-D-aspartate receptor activation is critical for establishing the priming effect, but not for the subsequent induction of the associative LTD. These data suggest that theta rhythm activity in the dentate gyrus may predispose the system to a specific form of synaptic plasticity, associative LTD.