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

Proteolysis of cell adhesion molecules by serine proteases: a role in long term potentiation?

Tissue plasminogen activator (tPA), a serine protease endogenous to hippocampal neurons, is shown to recognize a highly conserved sequence in the extracellular domain of cell adhesion molecules (CAMs). When added to brain homogenates, tPA generated a CAM fragment similar in size to that produced in hippocampal slices by brief periods of NMDA receptor stimulation. The serine protease inhibitor 4-(2-Aminoethyl)-benzenesulfonyl fluoride blocked the effects of tPA with an approximately 50% suppression at 250 microM. The inhibitor at this concentration had no evident effect on synaptic responses but caused long term potentiation to decay back to baseline over a 1 h period. These results suggest that extracellular breakdown of cell adhesion molecules initiated by NMDA receptors and mediated by serine proteases contributes to the formation of stable potentiation.

The relationship between adhesion molecules and neuronal plasticity

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

Requirement of rapid Ca2+ entry and synaptic activation of metabotropic glutamate receptors for the induction of long-term depression in adult rat hippocampus

1. During block of gamma-aminobutyric acid-A-mediated inhibition, low-frequency stimulation (2 Hz, 900 pulses) to Schaffer collateral-CA1 neuron synapses of adult rat hippocampus induced an N-methyl-D-aspartate receptor-independent, postsynaptic Ca2+-dependent depression of synaptic strength (long-term depression; LTD). 2. Ratio imaging with fura-2 revealed moderate dendritic [Ca2+] increases (approximately 500 nM) during only the initial approximately 30 s of the 7.5 min stimulation period. Conditioning for 30 s was, however, insufficient to induce LTD. 3. The [Ca2+] changes were insensitive to the metabotropic glutamate receptor (mGluR) antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG). MCPG, however, completely blocked LTD when present during conditioning. 4. The [Ca2+] changes were abolished by postsynaptic hyperpolarization (-110 mV at the soma). Hyperpolarizing neurons to -110 mV during conditioning significantly attenuated LTD induction. 5. LTD induction was also blocked by the postsynaptic presence of the protein kinase C inhibitor peptide PKC(19-36). 6. These results suggest that LTD induction in adult hippocampus by prolonged low-frequency stimulation depends on both a rapid Ca2+ influx through voltage-sensitive channels and synaptic stimulation of mGluRs which may be coupled to phospholipase C.

Memory and the brain: unexpected chemistries and a new pharmacology

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

Sustained long term potentiation and anxiety in mice lacking the Mas protooncogene

The Mas protooncogene is a maternally imprinted gene encoding an orphan G protein-coupled receptor expressed mainly in forebrain and testis. Here, we provide evidence for a function of Mas in the central nervous system. Targeted disruption of the Mas protooncogene leads to an increased durability of long term potentiation in the dentate gyrus, without affecting hippocampal morphology, basal synaptic transmission, and presynaptic function. In addition, Mas-/- mice show alterations in the onset of depotentiation. The permissive influence of Mas ablation on hippocampal synaptic plasticity is paralleled by behavioral changes. While spatial learning in the Morris water maze is not significantly influenced, Mas-deficient animals display an increased anxiety as assessed in the elevated-plus maze. Thus, Mas is an important modulating factor in the electrophysiology of the hippocampus and is involved in behavioral pathways in the adult brain.

Time-dependent reversal of long-term potentiation by an integrin antagonist

The integrin antagonist Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) was applied by local ejection to one of two recording sites in hippocampal slices at various times before and after long-term potentiation (LTP) was induced at both sites with theta burst stimulation. Applications 10 min before, immediately after, and 10 min after induction caused LTP at the experimental site to decay steadily relative to that at the within-slice control site. However, application at 25 min or more after induction had no detectable effect on potentiation. Similar results were obtained when the integrin antagonist was perfused into the slice rather than applied locally. The time period after induction during which GRGDSP interfered with LTP consolidation corresponds to that during which LTP is susceptible to reversal by low-frequency afferent stimulation and newly formed memories are vulnerable to various disruptive treatments. Comparable experiments using a peptide that blocks an extracellular binding site of neural cell adhesion molecules (NCAMs) did not yield time-dependent reversal of LTP; i.e., an antagonist that interacts with the fourth immunoglobulin-like domain reduced LTP when applied before induction but not afterward. Moreover, LTP formation occurred normally in the presence of an antibody against the fibronectin repeat domain of NCAM. These results suggest that integrin activation and signaling occurring over several minutes after LTP induction are necessary for stabilizing synaptic potentiation and by inference may be required for the conversion of new memories into a not readily disrupted state.

Increasing age reduces expression of long-term depression and dynamic range of transmission plasticity in CA1 field of the rat hippocampus

Long-term depression, depotentiation and long-term potentiation of field excitatory postsynaptic potentials in the CA1 field of the hippocampus were studied in slices from two-, 12-, 24- and 36-week-old rats. Long-term potentiation was induced by stimulating afferent fibres for 1 s at 100 Hz. Long-term depression was induced either by stimulating the afferent pathways twice for 15 min at 1 Hz (protocol 1), giving in total 1800 pulses, or by stimulating the fibres at 5 min intervals twice at 1 Hz for 5 min followed by 5 min stimulation at 5 Hz (protocol 2), giving in total 2100 pulses. We found significant long-term depression in slices of all groups stimulated with protocol 1; however, the magnitude of long-term depression in slices from 24- and 36-week-old rats was significantly lower than that in slices from two- and 12-week old rats, although there was no such difference in the magnitude of long-term potentiation between slices. Stimulation protocol 2 induced long-term depression only in slices from two- and 12-week-old rats. Comparison of the dynamic range of transmission plasticity in slices from two- and 36-week-old rats, calculated as the difference between the nearly saturated long-term potentiation and nearly saturated depotentiation, revealed a significantly smaller dynamic range in slices from 36-week-old rats in comparison with slices from two-week-old animals. The decrease in the dynamic range in slices from 36-week-old rats was due to a diminished capacity to depotentiate the nearly saturated long-term potentiation and not due to a decreased long-term potentiation expression in these slices. In contrast to long-term depression, in which the slope of the field excitatory postsynaptic potentials consistently and significantly decreased below the baseline level, the nearly saturated depotentiation did not decrease below the original, pre-long potentiation baseline level. The results demonstrate that increasing age reduces expression of long-term depression and the dynamic range of transmission plasticity.

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

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

Long-term synaptic plasticity between pairs of individual CA3 pyramidal cells in rat hippocampal slice cultures

1. Long-term potentiation (LTP) and depression (LTD) were investigated at synapses formed by pairs of monosynaptically connected CA3 pyramidal cells in rat hippocampal slice cultures. 2. An N-methyl-D-aspartate (NMDA) receptor-mediated component of the unitary EPSP, elicited at the resting membrane potential in response to single action potentials in an individual CA3 cell, could be isolated pharmacologically. 3. Associative LTP was induced when single presynaptic action potentials were repeatedly paired with 240 ms postsynaptic depolarizing pulses that evoked five to twelve action potentials or with single postsynaptic action potentials evoked near the peak of the unitary EPSP. LTP induction was prevented by an NMDA receptor antagonist. 4. Associative LTD was induced when single presynaptic action potentials were repeatedly elicited with a certain delay after either 240 ms postsynaptic depolarizing pulses or single postsynaptic action potentials. The time window within which presynaptic activity had to occur for LTD induction was dependent on the amount of postsynaptic depolarization. LTD was induced if single pre- and postsynaptic action potentials occurred synchronously. 5. Homosynaptic LTD was induced by 3 Hz tetanization of the presynaptic neuron for 3 min and was blocked by an NMDA receptor antagonist. 6. Depotentiation was produced with stimulation protocols that elicit either homosynaptic or associative LTD. 7. Recurrent excitatory synapses between CA3 cells display associative potentiation and depression. The sign of the change in synaptic strength is a function of the relative timing of pre- and postsynaptic action potentials.

Age-associated changes in Ca(2+)-dependent processes: relation to hippocampal synaptic plasticity

Altered calcium (Ca2+) homeostasis is thought to play a key role in aging and neuropathology resulting in memory deficits. Several forms of hippocampal synaptic plasticity are dependent on Ca2+, providing a potential link between altered Ca2+ homeostasis and memory deficits associated with aging. The current study reviews evidence for Ca2+ dysregulation during aging which could interact with Ca(2+)-dependent synaptic plasticity. The authors suggest that changes in Ca2+ regulation could adjust the thresholds for synaptic modification, favoring processes for depression of synaptic strength during aging.

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

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

Conditions for the induction of long-term potentiation and long-term depression by conjunctive pairing in the dentate gyrus in vitro

Conditions for the induction of long-term potentiation and long-term depression by conjunctive pairing in the dentate gyrus in vitro. J. Neurophysiol. 78: 2569-2573, 1997. The conditions under which long-term potentiation (LTP) and long-term depression (LTD) of excitatory postsynaptic currents were induced by the conjunctive pairing-type protocol of afferent stimulation and postsynaptic depolarization were studied in the medial perforant pathway-granule cell synapse of the dentate gyrus in vitro. The conjunctive pairing of 1-Hz afferent stimulation and steady state postsynaptic depolarization to 0 mV did not induce LTP or LTD. Inhibition of LTD induction with a phosphatase inhibitor or ruthenium red resulted in induction of LTP after the conjunctive pairing. Such LTP induction was N-methyl--aspartate dependent. Conversely, inhibition of LTP induction with a kinase inhibitor resulted in LTD induction after the conjunctive pairing. Thus the failure to induce LTP or LTD with the pairing protocol involving depolarization to 0 mV membrane potential was due to simultaneous activation of intracellular processes that generate the induction of LTP and LTD. Increasing the frequency of afferent stimulation to 200 Hz, even for just eight stimuli, resulted in LTP induction. The studies show that two factors govern the induction of LTP/LTD, membrane potential and frequency of afferent stimulation, with either increased depolarization or increased afferent stimulation favoring LTP induction.

Effects of prenatal protein malnutrition on hippocampal long-term potentiation in freely moving rats

It has been demonstrated that prenatal protein malnutrition significantly affects hippocampal plasticity, as measured by long-term potentiation, throughout development. This paper focuses on the hippocampal dentate granule cell population response to two separate paradigms of tetanization of the medial perforant pathway in prenatally protein-malnourished and normally nourished adult male rats. The 100-pulse paradigm consisted of the application of ten 25-ms-duration bursts of 400 Hz stimulation with an interburst interval of 10 s. The 1000-pulse paradigm consisted of the application of five 500-ms bursts of 400 Hz stimulation with an interburst interval of 5 s. No between-group differences were obtained for input/output response measures prior to tetanization. No between-group, nor between-paradigm, differences were obtained in the degree of population EPSP slope enhancement. However, in response to both paradigms, prenatally malnourished animals showed significantly less enhancement of the population spike amplitude (PSA) measure than normally nourished animals. Normally nourished animals showed a significantly greater level of PSA enhancement in response to the 100-pulse paradigm than the 1000-pulse paradigm. Prenatally malnourished animals showed no significant differences in the degree of PSA enhancement between the two paradigms. Results indicate that short duration bursts (< or = 25 ms) are more effective in inducing maximal PSA enhancement in normally nourished rats than longer duration stimulus bursts. The apparent inability of prenatally malnourished rats to transfer enhanced cellular activation (population EPSP slope enhancement) into enhanced cellular discharge (PSA enhancement) suggests that a preferential enhancement of GABAergic inhibitory modulation of granule cell excitability may result from the prenatal dietary insult. Such potentiation of inhibitory activity would significantly lower the probability of granule cell population discharge, resulting in the significantly lower level of PSA enhancement obtained from these animals.

The group I mGlu receptor agonist DHPG induces a novel form of LTD in the CA1 region of the hippocampus

The group I specific metabotropic glutamate (mGlu) receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) (100 microM, 10 min) induced long-term depression (LTD) of synaptic transmission in the CA1 region of adult rat hippocampal slices, measured using a grease-gap recording technique. In "normal" (1 mM Mg2+-containing) medium, LTD (measured 30 min after washout of DHPG) was small (13+/-3%), but LTD was enhanced if DHPG was applied when the tissue was made hyperexcitable, either by omitting Mg2+ from the perfusate (35+/-3%) or by adding the GABA(A) receptor antagonist picrotoxin (29+/-2%). The N-methyl-D-aspartate (NMDA) receptor antagonist AP5 (100 microM) substantially reduced the generation of DHPG-induced LTD in Mg2+-free medium, but had little effect on LTD induced in the presence of picrotoxin. In Mg2+-free medium, the threshold concentration of DHPG required to induce LTD was between 1 and 3 microM. Neither agonists specific for group II (100 nM DCG-IV or 1 microM LY354740) or group III (10 microM L-AP4) mGlu receptors or a combined group I and II agonist (30-100 microM (1S,3R)-ACPD) induced LTD. However, an agonist (1 mM CHPG) which activates mGlu5 but not mGlu1 receptors did induce LTD. Surprisingly, DHPG-induced LTD was reversed by mGlu receptor antagonists, applied hours after washout of DHPG. DHPG-induced LTD did not occlude with LTD induced by synaptic activation (1200 stimuli delivered at 2 Hz), in Mg2+-free medium. These data show that activation of group I mGlu receptors (probably mGlu5) can induce LTD and that this mGlu receptor-mediated LTD may, or may not, require activation of NMDA receptors, depending on the experimental conditions.

(R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) fails to block long-term potentiation under urethane anaesthesia in vivo

The effects of the metabotropic glutamate receptor antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) on the induction of long-term potentiation (LTP) in the dentate gyrus were examined under urethane anaesthesia in vivo. In experiment 1, bilateral intraventricular infusion of either 20 mM or 200 mM (R,S)-MCPG (5 microl each side) failed to block LTP in the perforant path-granule cell projection, relative to vehicle-infused controls; 30 mM D-AP5 (5 microl each side) infused in the same way as MCPG completely blocked LTP. Experiment 2, in which the contralateral perforant path-dentate gyrus pathway was used as a non-tetanized control, revealed that slight baseline changes induced by MCPG infusion were transient; again no block of LTP was obtained. The efficacy of mGluR blockade was confirmed in experiment 3, in which MCPG antagonized an increase in spontaneous activity induced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD). In experiment 4, significant depotentiation was induced by low frequency stimulation (5 Hz for 1 min) given 2 min after high frequency tetanization, but MCPG remained ineffective in blocking LTP after a second tetanus. In experiment 5, increasing the period of low frequency stimulation from 1 to 10 min produced greater depotentiation, but still did not unmask an MCPG-sensitive component of LTP. These experiments fail to support a role for mGluRs in the induction of LTP in the dentate gyrus under urethane anaesthesia in vivo, nor do they support the idea that a metabotropic switch controlling sensitivity to MCPG is reset by depotentiation.

Reversal of LTP in the hippocampal afferent fiber system to the prefrontal cortex in vivo with low-frequency patterns of stimulation that do not produce LTD

We examined the efficacy of several patterns of low-frequency stimulation for producing long-term depression (LTD) or depotentiation in the hippocampal fiber pathway to the prefrontal cortex in the anesthetized rat. Field potentials elicited by stimulation of the CA1/subicular region of the ventral hippocampus were recorded in the prelimbic area of the prefrontal cortex. We found no evidence that low-frequency trains (0.5-1 Hz), consisting of either single pulses, paired pulses (35-ms interpulse interval), or two-pulse bursts (5-ms interval), produce LTD in the prefrontal cortex. In contrast, all three stimulus protocols were found to induce a small-amplitude, persistent potentiation of the amplitude of the negative wave of the field response recorded in the prefrontal cortex. We also examined the ability of patterns of low-frequency stimulation to produce depotentiation of previously established long-term potentiation (LTP). Although low-frequency stimulation with single pulses or paired pulses was ineffective, we found that the two-pulse burst protocol selectively produced a rapid reversal of LTP in the hippocampo-prefrontal cortex pathway. Depotentiation is reversible and can be induced >2 h after the induction of LTP. Repeated trains failed to decrease the prefrontal cortex response below the original, unpotentiated level. These findings demonstrate the existence of a depotentiation mechanism that is capable of exerting powerful control over ongoing or recently induced synaptic plasticity in hippocampocortical connections in vivo.

8-cyclopentyltheophylline, an adenosine A1 receptor antagonist, inhibits the reversal of long-term potentiation in hippocampal CA1 neurons

The effects of an adenosine A1 receptor antagonist, 8-cyclopentyltheophylline (8-CPT, 1 microM), on the reduction of long-term potentiation were studied in CA1 neurons of guinea pig hippocampal slices. Reduction of long-term potentiation (depotentiation) was achieved by delivering a train of low-frequency afferent stimuli (low-frequency stimulation, 1000 pulses, 1 Hz) 20 min after the tetanus (100 Hz, 100 pulses). In control experiments, low-frequency stimulation reduced the potentiated component of the slope of the field EPSP and the amplitude of the population spike by 68.5 +/- 14.4% and 80.1 +/- 8.8%, respectively (n = 6); these values were significantly reduced to 13.4 +/- 9.7% and 9.0 +/- 10.9% (n = 7) when the low-frequency stimulation was applied during the perfusion with 8-CPT (1 microM). These results indicate that activation of adenosine A1 receptors enhances the depotentiation of long-term potentiation.

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.

Studies on long-term depression in area CA1 of the anesthetized and freely moving rat

Homosynaptic long-term depression (LTD) is reported to occur in field CA1 of hippocampal slices collected from immature brains. Because the effect has been postulated to be a memory storage mechanism, it is of interest to test for its presence in adult, awake animals. Unfortunately, not only has hippocampal LTD proved difficult to obtain reliably in vivo, but the few successful studies vary with respect to protocols and evidence that the depression is input-specific. The present study tested for input-specific (homosynaptic) LTD in field CA1 after application of various stimulation protocols to the Schaffer collateral/commissural projections in freely moving, adult rats. The results indicate that although low-frequency trains do induce decrements in synaptic transmission lasting for hours to several days, the success rate of eliciting input-specific LTD in the awake rat is very modest compared with the ease with which stable potentiation is obtained in the same synapses. Moreover, it is questionable that the effective protocols represent patterns of activity likely to occur during behavior. The stronger the afferent activation during low-frequency stimulation, the greater was the probability of eliciting LTD accompanied by persistent heterosynaptic depression. Clear evidence for the occurrence of LTD, irrespective of stimulation protocol and current intensity, could not be obtained in rats under barbiturate anesthesia. In all, the results do not accord with the suggestion that LTD occurs routinely in the hippocampus in vivo as part of memory encoding.

Noradrenergic regulation of synaptic plasticity in the hippocampal CA1 region

The effects of norepinephrine (NE) and related agents on long-lasting changes in synaptic efficacy induced by several patterns of afferent stimuli were investigated in the CA1 region of rat hippocampal slices. NE (10 microM) showed little effect on the induction of long-term potentiation (LTP) triggered by theta-burst-patterned stimulation, whereas it inhibited the induction of long-term depression (LTD) triggered by 900 pulses of 1-Hz stimulation. In nontreated slices, 900 pulses of stimuli induced LTD when applied at lower frequencies (1-3 Hz), and induced LTP when applied at a higher frequency (30 Hz). NE (10 microM) caused a shift of the frequency-response relationship in the direction preferring potentiation. The effect of NE was most prominent at a stimulus frequency of 10 Hz, which induced no changes in control slices but clearly induced LTP in the presence of NE. The facilitating effect of NE on the induction of LTP by 10-Hz stimulation was blocked by the beta-adrenergic receptor antagonist timolol (50 microM), but not by the alpha receptor antagonist phentolamine (50 microM), and was mimicked by the beta-agonist isoproterenol (0.3 microM), but not by the alpha1 agonist phenylephrine (10 microM). The induction of LTD by 1-Hz stimulation was prevented by isoproterenol but not by phenylephrine, indicating that the activation of beta-receptors is responsible for these effects of NE. NE (10 microM) also prevented the reversal of LTP (depotentiation) by 900 pulses of 1-Hz stimulation delivered 30 min after LTP induction. In contrast to effects on naive (nonpotentiated) synapses, the effect of NE on previously potentiated synapses was only partially mimicked by isoproterenol, but fully mimicked by coapplication of phenylephrine and isoproterenol. In addition, the effect of NE was attenuated either by phentolamine or by timolol, indicating that activation of both alpha1 and beta-receptors is required. These results show that NE plays a modulatory role in the induction of hippocampal synaptic plasticity. Although beta-receptor activation is essential, alpha1 receptor activation is also necessary in determining effects on previously potentiated synapses.

Behavioural stress facilitates the induction of long-term depression in the hippocampus

The induction of activity-dependent persistent increases in synaptic efficacy, such as long-term potentiation (LTP), is inhibited by behavioural stress. The question arises whether stress also affects the ability to induce persistent decreases in synaptic efficacy, such as long-term depression (LTD). We now report that the induction of stable homosynaptic LTD in the CA1 area of the hippocampus of awake adult rats is facilitated, rather than inhibited, by exposure to mild naturalistic stress. The same stress blocked the induction of LTP. The effects of such stress were short lasting: acclimatization to, or removal from, the conditions that facilitated LTD induction led to a rapid loss of the ability to elicit this form of plasticity. The time window in which LTD could be reliably elicited was prolonged by inducing anaesthesia immediately after the stress. These data reveal that even brief exposure to mild stress can produce a striking shift in the susceptibility to synaptic plasticity in the awake animal.

Bidirectional associative plasticity of unitary CA3-CA1 EPSPs in the rat hippocampus in vitro

Associative long-term potentiation (LTP) and depression of compound and unitary CA3-CA excitatory postsynaptic potentials (EPSPs) were investigated in rat hippocampal slice cultures. The induction of LTP with synchronous pairing of synaptic activation and postsynaptic depolarization resulted in an increase in the amplitude of EPSPs to the same absolute level, regardless of whether the input was naive or had been previously depressed by asynchronous pairing of pre- and postsynaptic activity. Saturated LTP of compound and unitary EPSPs was reversed by asynchronous pairing and could be reinduced by synchronous pairing. The likelihood that an action potential in a presynaptic CA3 cell failed to trigger an unitary EPSP in a postsynaptic CA1 cell decreased after induction of associative potentiation and increased after induction of associative depotentiation. These changes in the rate of transmission failures were accompanied by large changes in the amplitude of nonfailure EPSPs. We conclude that the same CA3-CA1 synapses can alternatively undergo associative potentiation and depression, perhaps through opposite changes in a single expression mechanism.

Evidence for involvement of the neuronal isoform of nitric oxide synthase during induction of long-term potentiation and long-term depression in the rat dentate gyrus in vitro

The possible role of nitric oxide in the induction of long-term potentiation and long-term depression of field excitatory postsynaptic potentials in the dentate gyrus of the hippocampal slice has been investigated, in the rat, using two novel nitric oxide synthase inhibitors, 1-(2-trifluoromethylphenyl)imidazole, which is selective for the neuronal isoform in vitro, and 3-bromo-7-nitro-indazole. Long-term potentiation was induced by a series of high-frequency trains, and long-term depression was induced by prolonged low-frequency stimulation at 1 Hz. The induction of long-term potentiation was inhibited by both 1-(2-trifluoromethylphenyl)imidazole and 3-bromo-7-nitro-indazole at concentrations which did not alter the amplitude of the test excitatory postsynaptic potential. The inhibitory effect of 1-(2-trifluoromethylphenyl)imidazole on the induction of long-term potentiation was prevented by pretreatment with L-arginine, the substrate amino acid used by nitric oxide synthase for nitric oxide production. The induction of long-term depression was inhibited by both 3-bromo-7-nitro-indazole and 1-(2-trifluoromethylphenyl)imidazole at concentrations which did not affect the test excitatory postsynaptic potential. The inhibitory effect of 1-(2-trifluoromethylphenyl)imidazole was prevented by pretreatment with L-arginine. The present experiments provide strong support for the involvement of the neuronal isoform of nitric oxide synthase in the induction of long-term potentation and long-term depression.

Long-term potentiation and long-term depression are induced through pharmacologically distinct NMDA receptors

N-Methyl-D-aspartate (NMDA) receptor activation initiates both homosynaptic long-term depression (LTD) and long-term potentiation (LTP) in the CA1 region of the hippocampus. The mechanism by which two opposing forms of synaptic plasticity can be initiated through the activation of a single receptor is not known. We examined the effects of two competitive antagonists on the induction of LTP and LTD, D-2-amino-5-phosphonovaleric acid (D-AP5), a broad spectrum inhibitor of the NMDA receptor, and 3-((RS)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), an antagonist that binds with high affinity to conventional NMDA receptors subtypes, but not to atypical subtypes that are relatively independent of voltage-dependent Mg2+-blockade. As has previously been reported, LTP, LTD, and depotentiation were all blocked by applications of D-AP5. In contrast, only LTP, but not LTD or depotentiation, was blocked by CPP. These observations suggest that decreases and increases of synaptic strength are mediated by the activation of distinct NMDA receptor subpopulations.

Arg-Gly-Asp-Ser-selective adhesion and the stabilization of long-term potentiation: pharmacological studies and the characterization of a candidate matrix receptor

Peptides known to block the extracellular interactions of adhesion receptors belonging to a subclass of the integrin family were tested for their effects on the stabilization of long-term potentiation (LTP) in hippocampal slices. Theta burst stimulation delivered after infusions of Gly-Ala-Val-Ser-Thr-Ala (GAVSTA) resulted in a potentiation effect that decayed steadily over a period of 40 min; LTP elicited in the presence of inactive control peptides remained stable over this time period. GAVSTA had no detectible influence on baseline responses, induction processes, or the initial degree of potentiation. Infusions of integrin antagonists after application of theta bursts also resulted in the occurrence of a decremental form of LTP. Affinity chromatography was then used in an effort to identify targets of the structurally dissimilar integrin blockers that disrupt LTP stabilization. Both integrin antagonists Gly-Arg-Gly-Asp-Ser-Pro and GAVSTA eluted a major species of 55 kDa (synaptegrin-1) from GRGDSP-affinity columns that had been loaded with solubilized synaptic membranes; lesser concentrations of three polypeptides of approximately 20, 27, and 30 kDa were also collected. Synaptegrin-1 was labeled by antibodies to the RGDS-binding integrin alpha5beta1. In addition, the synaptegrin, as well as the 27 kDa, protein was found to copurify with pre- and postsynaptic markers during the isolation of forebrain synaptosomes. These results indicate that a matrix recognition event occurring several minutes after induction of LTP is a necessary step in the stabilization of potentiated synapses; they also identify an integrin-like matrix receptor of 55 kDa that may contribute to this event.

Low-frequency stimulation induces homosynaptic depotentiation but not long-term depression of synaptic transmission in the adult anaesthetized and awake rat hippocampus in vivo

The induction of homosynaptic long-term depression and depotentiation of previously established long-term potentiation was investigated in the CA1 hippocampal region of anaesthetized and awake adult rats following prolonged ipsilateral low-frequency stimulation of the Schaffer collateral/ commissural pathway. Prolonged low-frequency stimulation at 1-10 Hz failed to induce long-term depression of field excitatory postsynaptic potentials in the anaesthetized or awake adult rat. However, prolonged low-frequency stimulation at 5 and 10 Hz, although not at 1 or 2 Hz, did induce depotentiation of previously established long-term potentiation in anaesthetized animals. Thus, in the anaesthetized animals, 900 pulses at 10 Hz induced a depotentiation of 68%, 59% and 66% when given 10, 30 and 40 min following long-term potentiation induction. Depotentiation could also be induced at much longer times following the induction of long-term potentiation. Thus, in anaesthetized rats, depotentiation measuring 34% was induced by 10-Hz stimulation 4 h following long-term potentiation induction, and depotentiation measuring 60% was induced in two sets of experiments 24 h after long-term potentiation induction in awake animals. The results of the present study show that homosynaptic long-term depression was not induced in the adult hippocampus in vivo using stimulation protocols which are effective in hippocampal slices. However, erasure of long-term potentiation by the process of depotentiation has been shown to occur in the adult hippocampus in vivo, both at short times and at prolonged times after the induction of long-term potentiation.

Heterosynaptic LTD and depotentiation in the medial perforant path of the dentate gyrus in the freely moving rat

We examined the characteristics of heterosynaptic long-term depression (LTD) and depotentiation of previously established long-term potentiation (LTP) in the medial and lateral entorhinal afferents to the dentate gyrus in the awake rat. Rats were prepared for chronic recording of dentate gyrus evoked potentials to activation of the medial and lateral perforant paths. This study in awake rats confirms that heterosynaptic LTD can be induced at inactive medial perforant path synapses in conjunction with the induction of LTP produced by high-frequency stimulation of the lateral perforant path. This form of LTD was long lasting and reversible by tetanic stimulation delivered to the depressed pathway. In contrast, tetanic stimulation of the medial perforant path had only a small heterosynaptic effect on the lateral pathway, suggesting that the two input pathways to the dentate gyrus are not symmetrical in their ability to induce heterosynaptic LTD. We also examined the ability of high-frequency stimulation of one pathway to produce depotentiation of the other pathway. We found that when LTP was first induced in the medial perforant path, depotentiation was induced heterosynaptically by tetanization of the lateral pathway. Both newly established LTP (30 min) and LTP induced and saturated by repeated tetanic stimulation over several days can be depotentiated heterosynaptically. Moreover, depotentiation of the medial perforant path synapses was found to be linearly correlated with the magnitude of LTP induced in the lateral perforant path synapses, and subsequent tetanic stimulation of the depotentiated medial perforant path restored LTP to an extent that counterbalanced depotentiation. The saturation and repotentiation experiments provide clear support for the conclusion that the rapid reversal of LTP reflects true depotentiation of the medial input. Again, as with heterosynaptic LTD, tetanization of the medial perforant path had little effect on previously induced LTP in the lateral path. These results provide evidence that medial perforant path synapses can be depressed and depotentiated heterosynaptically. They suggest that in the intact rat synaptic changes in the afferents to the dentate gyrus from the lateral entorhinal cortex exert powerful control over ongoing or recent synaptic plasticity in the medial entorhinal afferents.

Nitric-oxide-guanylyl-cyclase-dependent and -independent components of multiple forms of long-term synaptic depression

Long-term depression (LTD) of synaptic strength is induced by glutamate-triggered increases in postsynaptic [Ca2+], through either influx or release from intracellular stores. Induction of LTD has also been reported to require release of Ca2+ from presynaptic stores and activation of presynaptic Ca2+/calmodulin-dependent protein kinase II. This finding leads to the hypothesis that the intercellular messenger nitric oxide (NO) may be a means by which postsynaptic Ca2+ triggers changes expressing LTD in presynaptic terminals. We report that bath application of the oxadiazoloquinoxalone derivative ODQ (4 microM), a selective inhibitor of NO-sensitive guanylyl cyclase (NOGC), markedly attenuated (90%) the magnitude of LTD induced by low-frequency stimulation (LFS; 1 Hz/15 min) of Schaffer collateral-CA1 synapses in hippocampal slices in vitro. Both the NO donor S-nitroso-N-acetylpenicillamine (100 microM) and the membrane-permeant cyclic guanine 3',5'-monophosphate (cGMP) analogue 8-(-4-chlorophenylthio) guanosine (8-pCPT)-cGMP (50 microM) enhanced the magnitude of LTD, which is consistent with he hypothesis that activation of NOGC plays a role in the induction of LTD. Nicotinamide (20 mM), an inhibitor of NO-activated ADP ribosyltransferase, did not impair the induction of LTD. In contrast to de novo LTD, the reversal of long-term potentiation by LFS (depotentiation) was only partially blocked (55%) by ODQ, and heterosynaptic LTD was not impaired at all, suggesting that there are both NOGC-dependent and -independent forms of LTD. Because postsynaptic intracellular infusion of ODQ (500 microM) failed to block the induction of LTD, we conclude that activation of presynaptic NOGC is a necessary step in the induction of an NOGC-dependent component of LTD.

Induction of hippocampal long-term depression requires release of Ca2+ from separate presynaptic and postsynaptic intracellular stores

Studies have suggested that an increase in intracellular [Ca2+] is necessary for the induction of both long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission, and that release of Ca2+ from intracellular storage pools can be necessary to induce LTP. We investigated whether release of Ca2+ from intracellular stores also is required for the induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices. Both thapsigargin (1 microM) and cyclopiazonic acid (1 microM), compounds that deplete all intracellular Ca2+ pools by blocking LTP-dependent Ca2+ uptake into intracellular compartments, blocked the induction, but not maintenance, of LTD by low-frequency stimulation (LFS) (1 Hz/15 min) without affecting baseline synaptic transmission. Washout of the reversible inhibitor cyclopiazonic acid restored the ability to induce LTD. In contrast, thapsigargin did not block depotentiation of LTP by 1 Hz LFS, suggesting that LTP causes a reduction in the threshold [Ca2+] necessary for LTD. Selective depletion of the ryanodine receptor-gated Ca2+ pool by bath application of ryanodine (10 microM) also blocked the induction of LTD, indicating a requirement for Ca(2+)-induced Ca2+ release. Impalement of CA1 pyramidal neurons with microelectrodes containing thapsigargin (500 nM to 200 microM) prevented the induction of LTD at synapses on that neuron without blocking LTD in the rest of the slice. In contrast, similar filling of CA1 pyramidal neurons with ryanodine (2 microM to 5 mM) did not block the induction of LTD. From these data, we conclude that the induction of LTD requires release of Ca2+ both from a presynaptic ryanodine-sensitive pool and from postsynaptic (presumably IP3-gated) stores.

Increased susceptibility to induction of long-term depression and long-term potentiation reversal during aging

Homosynaptic long-term depression (LTD) and reversal of long-term potentiation (LTP) were examined extracellularly at CA3-CA1 synapses in stratum radiatum of slices from adult (6-9 months) and aged (20-24 months) Fischer 344 rats. Prolonged low-frequency stimulation (LFS) (900 pulses/1 Hz) of the Schaffer collaterals depressed the initial slope of the excitatory postsynaptic potential (EPSP) in aged but not adult rats. LTD at aged synapses was pathway-specific, persistent, and sensitive to the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5). Adult slices exhibited AP5-sensitive LTD in high [Ca2+] medium, whereas LTD in aged slices was blocked by high [Mg2+], suggesting that differences in Ca2+ regulation may underlie susceptibility to LTD. Despite age-related differences in LTD induction, no age difference in LTP magnitude was revealed. Additionally, LFS delivered 60 min after LTP induction resulted in similar LTP reversal for both age groups. Susceptibility differences to LTP reversal were indicated after multiple short-duration LFS bursts (30 pulses/1 Hz), with each burst separated by 10 min. Aged synapses exhibited significant reversal after a single burst and complete reversal after three LFS episodes. In adult slices, LTP reversal appeared after the fourth burst, and at no time was LTP depressed to initial baseline levels. This study provides the first characterization of homosynaptic LTD/LTP reversal in the aged animal and demonstrates that one form of plasticity, depression attributable to LFS, is increased during aging.

Activity-dependent decay of early LTP revealed by dual EPSP recording in hippocampal slices from young rats

The early maintenance of long-term potentiation (LTP) was studied in the CA1 region of hippocampal slices from 12- to 18-day-old rats in a low-magnesium solution (0.1 mM). The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components of the field excitatory postsynaptic potential were estimated in parallel using early and late measurements of the composite potential. At the normal test stimulus frequency of 0.1 Hz, LTP was seen initially as a predominant increase in the AMPA component, but converted, via a substantial decay of this component and a gradual growth of the NMDA component, into nearly equal changes of the two components. Interrupting the test stimulation for 10 min, changing the test stimulus frequency to 1/60 Hz after LTP induction, or using a test stimulus frequency of 1/60 Hz during the entire experiment significantly reduced the decay of the potentiation of the AMPA component while enhancing the potentiation of the NMDA one. The ratio between the magnitudes of the two excitatory postsynaptic potential (EPSP) components showed a decaying time course that was independent of the manipulations used. Application of the NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (50 microM) after LTP induction stabilized the LTP of the AMPA component until washout was started. On the other hand, the phosphatase inhibitor okadaic acid (1 microM) resulted in decay of the potentiation of both EPSP components back to around baseline and altered the time course of the ratio between the components. Our results show that the early maintenance of LTP is controlled in an activity-dependent and NMDA-dependent manner. This process accelerates the decay of LTP of both AMPA and NMDA components in parallel, suggesting that it is similar to homosynaptic long-term depression, although it operates at the normal test stimulus frequency. The data support a scenario in which LTP ensues as a selective AMPA receptor modification and subsequently converts to another modification, possibly a presynaptic one.

Low-frequency stimulation cancels the high-frequency-induced long-lasting effects in the rat medial vestibular nuclei

In rat brainstem slices, we investigated the effects of low-frequency stimulation (LFS) of the primary vestibular afferents on the amplitude of the field potentials evoked in the medial vestibular nuclei (MVN). LFS induced long-term effects, the sign of which depended on whether the vestibular neurons were previously conditioned by HFS. In unconditioned slices, LFS evoked modifications of the responses that were similar to those observed after HFS but had a smaller extension. In fact, LFS caused long-lasting potentiation of the N1 wave in the MVN ventral portion (Vp) and long-lasting depression of the N2 wave in the MVN dorsal portion (Dp), whereas it provoked small and variable effects on the N1 wave. By contrast, when the synaptic transmission was already conditioned, LFS influenced the synaptic responses oppositely, reducing or annulling the HFS long-term effects. This phenomenon was specifically induced by LFS, because HFS was not able to cause it. The involvement of NMDA receptors in mediating the LFS long-term effects was supported by the fact that AP-5 prevented their induction. In addition, the annulment of HFS long-term effects by LFS was also demonstrated by the shift in the latency of the evoked unitary potentials after LFS. In conclusion, we suggest that the reduction of the previously induced conditioning could represent a cancellation mechanism, useful to quickly adapt the vestibular system to continuous different needs and to avoid saturation.

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.

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.

The induction of homo- vs. heterosynaptic LTD in area CA1 of hippocampal slices from adult rats

The induction of long-term depression (LTD) was investigated in area CA1 of hippocampal slices from adult rats. To produce LTD, prolonged low-frequency stimulation (LFS, 900 stimuli at 1 Hz) was delivered to one of two independent Schaffer-collateral/commissural projections, while the second input served as a control to monitor heterosynaptic effects. The depression was calculated as percent decrease in the slope of the dendritic field EPSP relative to baseline values, and LTD was considered established if the response decrement was at least 15% in magnitude and stable for 30-60 min. By delivering LFS in conditions of different relative baseline response magnitudes, it was revealed that the intensity of afferent low-frequency activity has a significant impact on the induction frequency, magnitude and input-specificity of the depression: the rate of LTD occurrence and the effect of LFS on the absolute response decrement increased successively as the stimulation strength was raised, but the impact of LFS on the relative LTD magnitude decreased at higher stimulation intensities; the depression was specific to the stimulated input (homosynaptic LTD) when baseline responses were spike-free, but spread to the pathway which was silent during LFS (heterosynaptic LTD) in experiments conducted above spiking threshold. The results indicate that in the adult rat (i) the induction of input-specific LTD is dependent on the level of synaptic activation during LFS, and (ii) LTD can easily be obtained in strongly stimulated pathways but may be the result of a generalized decrease in the postsynaptic response.

A novel synaptic interaction underlying induction of long-term depression in the area CA1 of adult rat hippocampus

1. We describe a novel synaptic property that regulates induction of homosynaptic long-term depression (LTD), and slowly developing heterosynaptic LTD, of Schaffer collateral-pyramidal cell synapses in adult rat hippocampus. 2. Two independent pathways converging on the same neuron were alternately tested with 0.017 Hz single pulses, and LTD was induced by 900 conditioning stimuli delivered at 2 Hz. All experiments were performed in the presence of the GABA(A) antagonists picrotoxin or bicuculline. 3. After delivery of the 2 Hz stimulation to the homosynaptic pathway, the 0.017 Hz test pulses to the heterosynaptic pathway were interrupted for 25 min. When the test stimulations were resumed, heterosynaptic LTD could not be observed. Homosynaptic LTD also failed to be induced in this protocol. Interruption of test pulses did not itself cause a general increase of synaptic responses. 4. Doubling the frequency of homosynaptic test pulses (to 0.033 Hz) during a 25 min interruption of heterosynaptic stimulus did not preserve homosynaptic LTD. This suggests that the failure of homosynaptic LTD induction seen when the test pulses were interrupted was not caused by a decrease in the number of synaptic inputs at the postsynaptic neuron following conditioning. 5. When only the homosynaptic pathway was involved, with no heterosynaptic stimulation, as in conventional experiments, 2 Hz conditioning successfully induced homosynaptic LTD. 6. We propose that when a heterosynaptic pathway has been recently used, continuous input to that pathway following conditioning is necessary for induction of homosynaptic LTD on the same neuron.

Impaired hippocampal plasticity in mice lacking the Cbeta1 catalytic subunit of cAMP-dependent protein kinase

Neural pathways within the hippocampus undergo use-dependent changes in synaptic efficacy, and these changes are mediated by a number of signaling mechanisms, including cAMP-dependent protein kinase (PKA). The PKA holoenzyme is composed of regulatory and catalytic (C) subunits, both of which exist as multiple isoforms. There are two C subunit genes in mice, Calpha and Cbeta, and the Cbeta gene gives rise to several splice variants that are specifically expressed in discrete regions of the brain. We have used homologous recombination in embryonic stem cells to introduce an inactivating mutation into the mouse Cbeta gene, specifically targeting the Cbeta1-subunit isoform. Homozygous mutants showed normal viability and no obvious pathological defects, despite a complete lack of Cbeta1. The mice were analyzed in electrophysiological paradigms to test the role of this isoform in long-term modulation of synaptic transmission in the Schaffer collateral-CA1 pathway of the hippocampus. A high-frequency stimulus produced potentiation in both wild-type and Cbeta1-/- mice, but the mutants were unable to maintain the potentiated response, resulting in a late phase of long-term potentiation that was only 30% of controls. Paired pulse facilitation was unaffected in the mutant mice. Low-frequency stimulation produced long-term depression and depotentiation in wild-type mice but failed to produce lasting synaptic depression in the Cbeta1 -/- mutants. These data provide direct genetic evidence that PKA, and more specifically the Cbeta1 isoform, is required for long-term depression and depotentiation, as well as the late phase of long-term potentiation in the Schaffer collateral-CA1 pathway.

Associative synaptic plasticity in hippocampus and visual cortex: cellular mechanisms and functional implications

Synchronous pre- and postsynaptic neuronal activity results in long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus and the neocortex. Induction of this form of potentiation requires calcium influx mediated by NMDA receptors. Experimental evidence is reviewed for induction of long-term depression (LTD) of synaptic transmission in the hippocampus in vitro and neocortical neurons in vivo, when the discharge of the postsynaptic neuron is temporally decorrelated from the presynaptic stimulation. Homosynaptic LTD induced by low frequency tetani in the hippocampus in vitro requires NMDA receptor activation and a moderate calcium influx. The role of postsynaptic calcium as a key parameter in the encoding of temporal contiguity of neural activity and its possible implications in the formation of engrams during specific learning tasks are discussed.

Frequency dependence of long-term potentiation and depression in the dentate gyrus of the freely moving rat

Long-term potentiation (LTP) or long-term depression (LTD) can be produced in the dentate gyrus (DG) of the hippocampus with high-or low-frequency stimulation trains, respectively. Although LTP can be elicited in a variety of preparations, we know of no reports of LTD unaccompanied by seizure activity in the awake rat. In this experiment, test pulses at alternating high (95% of maximum response) and moderate (50-75% of maximum) intensities were presented at 0.05 Hz to the perforant path of freely moving rats in order to assess changes in DG population spike amplitude. Trains were delivered at 10-min intervals, and intratrain frequency was adjusted either upward from 3 Hz or downward from 400 Hz until all subjects had received three consecutive tetani at each of 3, 6, 12.5, 25, 50, 100, 200, and 400 Hz. Potentiation was observed at high frequencies regardless of whether ascending (ASC) or descending (DES) test order was used. Depression occurred at low frequencies, but only in ASC rats. The LTD observed in this preparation was not very robust and was clearly seen only when moderate-intensity test pulses were used. The threshold frequency (at which depression gives way to potentiation) was approximately 6-9 Hz for DES rats but was 100-120 Hz for ASC animals. Prior stimulation therefore affected the response to subsequent trains. These results are generally consistent with the hypothesis of a variable threshold for LTP induction. Our findings can also be explained by postulating a wide "labile range" at moderate frequencies within which no plastic changes occur.

NMDA receptor-dependent LTD in different subfields of hippocampus in vivo and in vitro

In simulations with artificial neural networks, efficient information processing and storage has been shown to require that the strength of connections between network elements has the capacity to both increase and decrease in a use-dependent manner. In contrast to long-term potentiation (LTP) of excitatory synaptic transmission, activity-dependent long-term depression (LTD) has been difficult to demonstrate in forebrain in vivo. Theoretical arguments indicate that coincidence of presynaptic excitation and low-magnitude postsynaptic activation are the necessary prerequisites for LTD induction. Here we report that stimulation paradigms which cause 1) sufficient excitation to result in NMDA receptor activation and simultaneously 2) attenuate the level of postsynaptic activation by recruitment of GABAA receptor-mediated inhibition consistently produce LTD of commissural input to area CA1 in the hippocampus of anesthetized adult rats, and of the perforant path input to the dentate gyrus in the hippocampus of anesthetized and unanesthetized adult rabbits. A functionally similar pre- and postsynaptic activation pattern applied to the hippocampal slice preparation by injecting hyperpolarizing current into the postsynaptic cell during NMDA receptor-mediated excitation also was effective in consistently inducing LTD. Results of studies in vitro show that Ca2+ influx through the NMDA channel is necessary for the induction of LTD, and moreover, that NMDA receptors also participate in the expression of LTD. Our findings demonstrate a general mechanism for the implementation of a theoretically derived learning rule in adult forebrain in vivo and in vitro and provide justification for the inclusion of use-dependent decreases of connection weights in formal models of cognitive processing.

Associative long-term depression in the hippocampus in vitro

An association between the test and conditioning stimuli is critical for determining the nature of their interaction during learning and memory. Two experimental protocols which result in the induction of associative forms of long-term depression (LTD) at Schaffer collateral synapses onto CA1 pyramidal cells in vitro are reviewed in this article. The requirements for the induction of LTD with these protocols are discussed, as well as the relationship between these forms of associative LTD and so-called homosynaptic LTD. In particular, the biological basis of the experimentally demonstrated necessity for temporal and spatial conjunction between the test and conditioning stimuli is examined.

Low-frequency trains of paired stimuli induce long-term depression in area CA1 but not in dentate gyrus of the intact rat

We have examined the efficacy of a recently introduced protocol for inducing homosynaptic long-term depression (LTD) in area CA1 of the anesthetized rat (Thiels et al. [1994] J Neurophysiol 72:3009-3116.). In area CA1 of the awake animal, this protocol, consisting of 200 pairs of pulses delivered at 0.5 Hz, with an interpulse interval of 25 ms, consistently produced LTD, provided the initial pulse was sufficiently strong to produce significant paired-pulse depression of the evoked response. We extended these experiments to the dentate gyrus, using either paired pulses given to the perforant path in the awake adult rat, or, in the anesthetized adult, a two-pathway pairing procedure, in which the first pulse was delivered to the commissural input to the dentate gyrus and the second to the perforant path. In both cases, the first pulse led to substantial suppression of the response evoked by the second pulse. With neither protocol, however, was there any evidence for LTD or depotentiation. Paired-pulse stimulation of the perforant path of young rats (10-11 days) also failed to induce LTD or depotentiation of the population excitatory postsynaptic potential (EPSP). Thus, the dentate gyrus in the intact animal appears to be less susceptible to LTD and depotentiation than area CA1, a conclusion consistent with previous experiments in which we found that stimulation at 1-5 Hz produced LTD/depotentiation in area CA1 of young (but not adult) rats in vivo but was ineffective at any age in the dentate gyrus. Our results do not rule out the possibility that other, untested protocols may produce homosynaptic LTD and/or depotentiation in the dentate gyrus in vivo.

Homosynaptic LTD and depotentiation: do they differ in name only?

Long-term depression (LTD) now occupies a major place in theories of the cellular basis of learning and memory and other nervous system phenomena involving persistent changes in synaptic responsiveness. LTD can be induced using a variety of stimulation paradigms. Homosynaptic LTD in this review refers to a depression of basal responses that is restricted to the pathway that has been stimulated by a low-frequency (1 Hz) stimulus train. Despite the intensive interest in LTD, there has been controversy about the ease with which LTD can be induced and reports range from no success to routine success. There has been much less controversy about a related form of response depression now called "depotentiation" which shares many similarities with LTD. Depotentiation is the response reduction that affects, not the basal responses affected by LTD, but responses that have been increased by the process of long-term potentiation (LTP). LTD and depotentiation can be induced by similar stimulation and have many biochemical properties in common, but it has not been clear whether or not they represent the same phenomenon, in part because it often occurs that the same preparation that does not undergo LTD readily expresses depotentiation. We review work that indicates that the major differences between LTD and depotentiation involve age-dependence, the need for priming stimulation and sensitivity to GABA receptor antagonists. We present a hypothetical model that can reconcile the apparent disparities between LTD and depotentiation.

LTD, LTP, and the sliding threshold for long-term synaptic plasticity

LTD of synaptic transmission is a form of long-term synaptic plasticity with the potential to be as significant as LTP to both the activity-dependent development of neural circuitry and adult memory storage. In addition, interactions between LTP and LTD and the dynamic regulation of the gain of synaptic plasticity mechanisms are also very important. In particular, the computational ability of LTD to properly counterbalance LTP may be essential to maintaining synaptic strengths in the linear range, and to maximally sharpen the ability of synapses to compute and store frequency-based information about the phase relation between synapses. Experimental data confirm the presence of an activity-dependent "sliding threshold" with the expected properties. That is, when levels of neuronal activity are high, indicating circumstances increasing the likelihood of inducing LTP, compensatory changes cause the suppression of LTP and an enhanced likelihood of LTD. Conversely, we would predict that low levels of synaptic activity would shift the threshold in favor of greater LTP and less LTD, a hypothesis which has yet to be tested. The sliding threshold for LTP and LTD also has implications for underlying cellular mechanisms of both forms of long-term synaptic plasticity. If the thresholds for LTP and LTD are tightly and reciprocally co-regulated, that could imply that at least one component of LTD is a true depotentiation caused by reversal of a change mediating LTP. If so, the intuitively simplest hypothesis is that phosphorylation of AMPA glutamate receptors causes LTP of synaptic e.p.s.p.s, while dephosphorylation of the same site or sites causes depotentiation LTD. Of course, this hypothesis would refer only to a postsynaptic component of both LTP and LTD. There has been a recent report that, in neonatal rat hippocampus, a form of LTD that is expressed developmentally earlier than LTP appears to have a postsynaptic induction site, but is expressed as decreased presynaptic transmitter release (Bolshakov and Siegelbaum, 1994). Whether these properties will be retained as LTD matures is unknown, as is the likelihood that, if a component of LTP is expressed presynaptically, depotentiation of that presynaptic component can also occur. Equally unclear is the persistence of LTD relative to LTP. The few rigorous long-term anatomical studies available suggest that the latest phases of LTP may be expressed as changes in dendritic spine shapes and/or synaptic morphology. While heterosynaptic LTD has been reported to have a duration of weeks in vivo (Abraham et al., 1994), we do not know whether LTP-induced morphological changes that take many days to appear can be reversed in an activity-dependent manner. An important feature of the consolidation of memories may turn out to be the slow development of LTP that is resistant to reversal by LTD. While we still at an earlier stage in our understanding of the mechanisms underlying LTD compared to LTP, some things are becoming clear. LTD is induced by afferent neuronal activity that is relatively ineffective in exciting the postsynaptic cell--an "anti-hebbian" condition. This property, coupled with the hebbian properties of LTP and the dynamic nature of membrane conductances, necessarily confers upon synapses the ability to compute and store the results of a covariance function. However, the role of such a computation in processing and/or memory is unclear. In addition, LTD appears to require the activation of NMDA and metabotropic subtypes of glutamate receptors, release of Ca2+ from intracellular stores, and an increase in intracellular [Ca2+] that is lower than that necessary to induce LTP. The early evidence is consistent with some overlap of targets for modification by LTP and LTD, with some forms of LTD likely to be a reversal, or "depotentiation," of previous LTP, perhaps through dephosphorylation of AMPA receptors.

Only 'de novo' long-term depression (LTD) in the rat hippocampus in vitro is blocked by the same low concentration of FK506 that blocks LTD in the visual cortex

It has been proposed that the long-term depression (LTD) seen following low frequency stimulation (LFS) in the rat hippocampus involves calcineurin. We have tested this by examining the effect of FK506, a macrolide which blocks calcineurin at nanomolar concentrations, on synaptic transmission in the rat hippocampal slice at a concentration of 1 microM which has been shown to block LTD in the visual cortex. The effect of FK506 on long-term potentiation (LTP) and spontaneous transmitter release was also studied. The magnitude of LTD induced by LFS was 16.7 +/- 2.4% in control which was not significantly different from the 22.3 +/- 3.0% seen in the same preparations after exposure to FK506 for 25-30 min. In contrast the magnitude of LTD induced 'de novo' in preparations exposed to FK506 was significantly reduced. FK506 had no significant effect on LTP, miniature EPSP frequency, miniature EPSP amplitude, resting membrane potential or input resistance. These results, therefore, support the hypothesis that calcineurin is involved in 'de novo' LTD but it appears that an event is triggered by LFS whereby FK506-insensitive LTD can subsequently be activated by a second episode of LFS.

Bidirectional synaptic plasticity induced by a single burst during cholinergic theta oscillation in CA1 in vitro

In standard protocols, the frequency of synaptic stimulation determines whether CA1 hippocampal synapses undergo long-term potentiation or depression. Here we show that during cholinergically induced theta oscillation (theta) synaptic plasticity is greatly sensitized and can be induced by a single burst (4 pulses, 100 Hz). A burst given at the peak of theta induces homosynaptic LTP; the same burst at a trough induces homosynaptic LTD of previously potentiated synapses. Heterosynaptic LTD is produced at inactive synapses when others undergo LTP. The synaptic modifications during theta require NMDA receptors and muscarinic receptors. The enhancement is cooperative and occludes with standard LTP. These results suggest that the similar bursts observed during theta rhythm in vivo may be a natural stimulus for inducing LTP/LTD.

The complementary nature of long-term depression and potentiation revealed by dual component excitatory postsynaptic potentials in hippocampal slices from young rats

Homosynaptic long-term depression and long-term potentiation were studied in hippocampal slices from 12-18-day-old rats using field excitatory postsynaptic potentials recorded in the CA1 subfield (stratum radiatum). Independent estimates of the alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and the N-methyl-D-aspartate receptor-mediated components of the field excitatory postsynaptic potential were obtained in parallel using early and late measurements of a dual component excitatory postsynaptic potential in a solution containing low (0.1 mM) magnesium and 1 microM of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Long-term depression, induced by 2 Hz stimulation for 10 min, was observed as an equal relative depression of the AMPA and N-methyl-D-aspartate receptor-mediated components of the field excitatory postsynaptic potential, whereas long-term potentiation induced by single or repeated high-frequency stimulation, was seen initially as a predominant potentiation of the AMPA receptor-mediated component. Within the first 30-60 min, long-term potentiation gradually changed to more equal increases of the two components of the excitatory postsynaptic potential. During alternating induction of long-term depression and long-term potentiation, the AMPA and N-methyl-D-aspartate receptor-mediated components could both be repeatedly regulated up and down. Long-term depression and long-term potentiation also showed several signs of interaction with each other during such experiments; e.g., long-term depression removed the occlusive effect of large long-term potentiation on a subsequent long-term potentiation, and long-term potentiation applied after the induction of long-term depression was found to be more stable than otherwise. The results support the notion that long-term depression and long-term potentiation employ changes in a common synaptic property. A tentative mechanism for this modification, expressed as equal changes of AMPA and N-methyl-D-aspartate receptor-mediated components of the excitatory postsynaptic potential, is an alteration in transmitter release, while the initial asymmetric part of long-term potentiation indicates involvement of an additional short-term modification.