Heterosynaptic long-term depression is facilitated by blockade of inhibition in area CA1 of the hippocampus

Fear Memory

Fear memory is the best-studied form of memory. It was thoroughly investigated in the past 60 years mostly using two classical conditioning procedures (contextual fear conditioning and fear conditioning to a tone) and one instrumental procedure (one-trial inhibitory avoidance). Fear memory is formed in the hippocampus (contextual conditioning and inhibitory avoidance), in the basolateral amygdala (inhibitory avoidance), and in the lateral amygdala (conditioning to a tone). The circuitry involves, in addition, the pre- and infralimbic ventromedial prefrontal cortex, the central amygdala subnuclei, and the dentate gyrus. Fear learning models, notably inhibitory avoidance, have also been very useful for the analysis of the biochemical mechanisms of memory consolidation as a whole. These studies have capitalized on in vitro observations on long-term potentiation and other kinds of plasticity. The effect of a very large number of drugs on fear learning has been intensively studied, often as a prelude to the investigation of effects on anxiety. The extinction of fear learning involves to an extent a reversal of the flow of information in the mentioned structures and is used in the therapy of posttraumatic stress disorder and fear memories in general.

Astrocytes: Orchestrating synaptic plasticity?

Synaptic plasticity is the capacity of a preexisting connection between two neurons to change in strength as a function of neural activity. Because synaptic plasticity is the major candidate mechanism for learning and memory, the elucidation of its constituting mechanisms is of crucial importance in many aspects of normal and pathological brain function. In particular, a prominent aspect that remains debated is how the plasticity mechanisms, that encompass a broad spectrum of temporal and spatial scales, come to play together in a concerted fashion. Here we review and discuss evidence that pinpoints to a possible non-neuronal, glial candidate for such orchestration: the regulation of synaptic plasticity by astrocytes.

GABA promotes the competitive selection of dendritic spines by controlling local Ca2+ signaling

Activity-dependent competition of synapses plays a key role in neural organization and is often promoted by GABA; however, its cellular bases are poorly understood. Excitatory synapses of cortical pyramidal neurons are formed on small protrusions known as dendritic spines, which exhibit structural plasticity. We used two-color uncaging of glutamate and GABA in rat hippocampal CA1 pyramidal neurons and found that spine shrinkage and elimination were markedly promoted by the activation of GABAA receptors shortly before action potentials. GABAergic inhibition suppressed bulk increases in cytosolic Ca(2+) concentrations, whereas it preserved the Ca(2+) nanodomains generated by NMDA-type receptors, both of which were necessary for spine shrinkage. Unlike spine enlargement, spine shrinkage spread to neighboring spines (<15 μm) and competed with their enlargement, and this process involved the actin-depolymerizing factor ADF/cofilin. Thus, GABAergic inhibition directly suppresses local dendritic Ca(2+) transients and strongly promotes the competitive selection of dendritic spines.

Heterosynaptic long-term depression mediated by ATP released from astrocytes

Heterosynaptic long-term depression (hLTD) at untetanized synapses accompanying the induction of long-term potentiation (LTP) spatially sharpens the activity-induced synaptic potentiation; however, the underlying mechanism remains unclear. We found that hLTD in the hippocampal CA1 region is caused by stimulation-induced ATP release from astrocytes that suppresses transmitter release from untetanized synaptic terminals via activation of P2Y receptors. Selective stimulation of astrocytes expressing channelrhodopsin-2, a light-gated cation channel permeable to Ca(2+) , resulted in LTD of synapses on neighboring neurons. This synaptic modification required Ca(2+) elevation in astrocytes and activation of P2Y receptors, but not N-methyl-D-aspartate receptors. Furthermore, blocking P2Y receptors or buffering astrocyte intracellular Ca(2+) at a low level prevented hLTD without affecting LTP induced by SC stimulation. Thus, astrocyte activation is both necessary and sufficient for mediating hLTD accompanying LTP induction, strongly supporting the notion that astrocytes actively participate in activity-dependent synaptic plasticity of neural circuits.

Amygdala-prefrontal pathways and the dopamine system affect nociceptive responses in the prefrontal cortex

Background

We previously demonstrated nociceptive discharges to be evoked by mechanical noxious stimulation in the prefrontal cortex (PFC). The nociceptive responses recorded in the PFC are conceivably involved in the affective rather than the sensory-discriminative dimension of pain. The PFC receives dense projection from the limbic system. Monosynaptic projections from the basolateral nucleus of the amygdala (BLA) to the PFC are known to produce long-lasting synaptic plasticity. We examined effects of high frequency stimulation (HFS) delivered to the BLA on nociceptive responses in the rat PFC.

Results

HFS induced long lasting suppression (LLS) of the specific high threshold responses of nociceptive neurons in the PFC. Microinjection of N-methyl-D-aspartic acid (NMDA) receptor antagonists (2-amino-5-phosphonovaleric acid (APV), dizocilpine (MK-801)) and also metabotropic glutamate receptor (mGluR) group antagonists (α-methyl-4-carboxyphenylglycine (MCPG), and 2-[(1S,2S)-2-carboxycyclopropyl]-3-(9H-xanthen-9-yl)-D-alanine (LY341495)), prevented the induction of LLS of nociceptive responses. We also examined modulatory effects of dopamine (DA) on the LLS of nociceptive responses. With depletion of DA in response to 6-hydroxydopamine (6-OHDA) injection into the ipsilateral forebrain bundle, LLS of nociceptive responses was decreased, while nociceptive responses were normally evoked. Antagonists of DA receptor subtypes D2 (sulpiride) and D4 (3-{[4-(4-chlorophenyl) piperazin-1-yl] methyl}-1H-pyrrolo [2, 3-b] pyridine (L-745,870)), microinjected into the PFC, inhibited LLS of nociceptive responses.

Conclusions

Our results indicate that BLA-PFC pathways inhibited PFC nociceptive cell activities and that the DA system modifies the BLA-PFC regulatory function.

Reduced spine density in specific regions of CA1 pyramidal neurons in two transgenic mouse models of Alzheimer's disease

One major hallmark of Alzheimer's disease (AD) is the massive loss of synapses that occurs at an early clinical stage of the disease. In this study, we characterize alterations in spine density and the expression of synapse-associated immediate early gene Arc (activity-regulated cytoskeleton-associated protein) in the hippocampal CA1 regions of two different amyloid precursor protein (APP) transgenic mouse lines before plaque development and their connection to performance in hippocampus-dependent memory tests. The density of mushroom-type spines was reduced by 34% in the basal dendrites proximal to the soma of CA1 pyramidal neurons in 5.5-month-old Tg2576 mice, carrying the Swedish mutation, compared with wild-type littermates. A similar reduction of 42% was confirmed in the same region of 8-month-old APP/Lo mice, carrying the London mutation. In this strain, the reduction extended to the distal dendritic spines (28%), although no differences were found in apical dendrites in either transgenic mouse line. Both transgenic mice lines presented a significant increase in Arc protein expression in CA1 compared with controls, suggesting rather an overactivity and increased spine turnover that was supported by a significant decrease in number of somatostatin-immunopositive inhibitory interneurons in the stratum oriens of CA1. Behaviorally, the transgenic mice showed decrease freezing in the fear contextual conditioning test and impairment in spatial memory assessed by Morris water maze test. These data indicate that cognitive impairment in APP transgenic mice is correlated with impairment of synaptic connectivity in hippocampal CA1, probably attributable to loss of inhibitory interneurons and subsequent hyperactivity.

"Heterosynaptic" LTD in the dentate gyrus of anesthetized rat requires homosynaptic activity

Heterosynaptic long-term depression (LTD) is conventionally defined as occurring at synapses that are inactive during a time when neighboring synapses are activated by high-frequency stimulation. A new model that combines computational properties of both the Bienenstock, Cooper and Munro model and spike timing-dependent plasticity, however, suggests that such LTD actually may require presynaptic activity in the depressed pathway. We tested experimentally whether presynaptic activity is in fact necessary for previously described heterosynaptic LTD in lateral perforant path synapses in the dentate gyrus of urethane-anesthetized rats. As predicted by the model, procaine infusion into the lateral path fibers, sufficient to transiently block neural activity in this pathway, prevented the induction of LTD in the lateral path following medial path high-frequency stimulation. These data indicate that the previously described heterosynaptic LTD in the dentate gyrus in vivo is actually a form of homosynaptic LTD, requiring presynaptic activity in the depressed pathway.

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

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

Competitive interactions between endogenous LTD and LTP in the hippocampus underlie the storage of emotional memories and stress-induced amnesia

This speculative review serves two purposes. First, it as an extension of the ideas we developed in a previous review (Diamond et al., Hippocampus, 2004;14:281-291), and second, it is a rebuttal to Abraham's (Hippocampus, 2004;14:675-676) critique of that review. We had speculated on the functional significance of the finding that post-training LTP induction produces retrograde amnesia. We noted the similarities between the findings that strong tetanizing stimulation can produce LTP and retrograde amnesia, and that a strong emotional experience can produce a long-lasting memory and retrograde amnesia, as well. The commonalities between LTP induction and emotional learning provided the basis of our hypothesis that an emotional experience generates endogenous LTD/depotentiation, which reverses synaptic plasticity formed during previous learning experiences, and endogenous LTP, which underlies the storage of new information. Abraham raised several concerns with our review, including the criticism that our speculation "falters because there is no evidence that stress causes LTD or depotentiation," and that research on stress and hippocampus has "failed to report any LTP-like changes." Abraham's points are well-taken because stress, in isolation, does not appear to generate long-lasting changes in baseline measures of hippocampal excitability. Here, within the context of a reply to Abraham's critique, we have provided a review of the literature on the influence of stress, novelty, fear conditioning, and the retrieval of emotional memories on cognitive and physiological measures of hippocampal functioning. An emphasis of this review is our hypothesis that endogenous forms of depotentiation, LTD and LTP are generated only when arousing experiences occur in conjunction with memory-related activation of the hippocampus and amygdala. We conclude with speculation that interactions among the different forms of endogenous plasticity underlie a form of competition by synapses and memories for access to retrieval resources.

Presynaptic induction of heterosynaptic associative plasticity in the mammalian brain

The induction of associative synaptic plasticity in the mammalian central nervous system classically depends on coincident presynaptic and postsynaptic activity. According to this principle, associative homosynaptic long-term potentiation (LTP) of excitatory synaptic transmission can be induced only if synaptic release occurs during postsynaptic depolarization. In contrast, heterosynaptic plasticity in mammals is considered to rely on activity-independent, non-associative processes. Here we describe a novel mechanism underlying the induction of associative LTP in the lateral amygdala (LA). Simultaneous activation of converging cortical and thalamic afferents specifically induced associative, N-methyl-D-aspartate (NMDA)-receptor-dependent LTP at cortical, but not at thalamic, inputs. Surprisingly, the induction of associative LTP at cortical inputs was completely independent of postsynaptic activity, including depolarization, postsynaptic NMDA receptor activation or an increase in postsynaptic Ca2+ concentration, and did not require network activity. LTP expression was mediated by a persistent increase in the presynaptic probability of release at cortical afferents. Our study shows the presynaptic induction and expression of heterosynaptic and associative synaptic plasticity on simultaneous activity of converging afferents. Our data indicate that input specificity of associative LTP can be determined exclusively by presynaptic properties.

Long-term depression in horizontal slices of the rat lateral amygdala

Long-term depression (LTD) is an enduring decrease in synaptic efficacy and is thought to underlie memory. In contrast to investigations of plasticity mechanisms in the amygdala in rat coronal slices, this study was done in horizontal slices. Field excitatory postsynaptic potentials (fEPSPs) and EPSPs, respectively, were recorded extracellularly and intracellularly from the lateral nucleus of the amygdala (LA). We show that low-frequency stimulation (LFS) induces LTD in the LA, when stimulation electrodes were located in the LA. No significant differences were found between females and males. In dependence of strain variations, a reduction of GABAergic inhibition either reduced the magnitude of LTD or was a prerequisite for the induction of extracellularly recorded LA-LTD. Theta pulse stimulation (TPS) of afferents within the LA caused a weaker LTD than LFS. Theta burst stimulation (TBS) given 20 min after the end of LFS reversed LTD, whereas high-frequency stimulation (HFS) resulted in long-term potentiation (LTP) that was significantly stronger than that obtained in naive slices. Therefore, primed induction of LTD facilitates high-frequency-induced LTP in the rat lateral amygdala. NMDARs as well as group II mGluRs were involved in the mediation of LA-LTD. In contrast to data obtained by stimulation of afferents running within the LA, LFS of the external capsule fibers induced a weak LA-LTD, and TPS was not able to induce LTD. This study showed for the first time that LTD can be induced in the LA by standard LFS (900 pulses at 1 Hz) and that LTP stimuli reversed LTD. The results also provide further evidence for the broad sensitivity of synaptic plasticity mechanisms to the history of prior activity.

Conservation of total synaptic weight through balanced synaptic depression and potentiation

Memory is believed to depend on activity-dependent changes in the strength of synapses. In part, this view is based on evidence that the efficacy of synapses can be enhanced or depressed depending on the timing of pre- and postsynaptic activity. However, when such plastic synapses are incorporated into neural network models, stability problems may develop because the potentiation or depression of synapses increases the likelihood that they will be further strengthened or weakened. Here we report biological evidence for a homeostatic mechanism that reconciles the apparently opposite requirements of plasticity and stability. We show that, in intercalated neurons of the amygdala, activity-dependent potentiation or depression of particular glutamatergic inputs leads to opposite changes in the strength of inputs ending at other dendritic sites. As a result, little change in total synaptic weight occurs, even though the relative strength of inputs is modified. Furthermore, hetero- but not homosynaptic alterations are blocked by intracellular dialysis of drugs that prevent Ca2+ release from intracellular stores. Thus, in intercalated neurons at least, inverse heterosynaptic plasticity tends to compensate for homosynaptic long-term potentiation and depression, thus stabilizing total synaptic weight.

NMDA Receptor-dependent Transient Homo- and Heterosynaptic Depression in Picrotoxin-treated Hippocampal Slices

Extracellular recording was used to study the effects of high-frequency (tetanic) stimulation on excitatory synaptic transmission in the CA1 region of rat hippocampal slices in the presence of the gamma-aminobutyric acid (GABA) type A antagonist, picrotoxin (50 microM). Under these conditions tetanic stimulation (100 Hz, 1 s) at the test intensity resulted in homosynaptic long-term potentiation (LTP). In contrast, tetanic stimulation of higher intensity (100 Hz, 1 s, double test intensity) resulted in homo- and heterosynaptic depression which recovered within 45 min. A transient (1 - 3 min) negative shift in DC potential and a transient (5 - 10 min) depression of the homosynaptic fibre volley occurred immediately following the higher intensity tetanus. The DC shift, induction of homo- and heterosynaptic depression and depression of the fibre volley were reversibly prevented by the N-methyl-d-aspartate (NMDA) receptor antagonist, d-2-amino-5-phosphonopentanoate (AP5; 20 microM) but were not prevented by a variety of L-type calcium channel antagonists. Transient (30 - 45 min) synaptic depression of pharmacologically isolated NMDA receptor-mediated field excitatory postsynaptic potentials also occurred following tetanic stimulation (100 Hz, 1 s) at double test intensity. These results demonstrate an NMDA receptor-dependent form of reversible synaptic depression in the CA1 region of the hippocampus.

Hebbian synapses in cortical and hippocampal pathways

Use-dependent alterations in synaptic efficacy are believed to form the basis for such complex brain functions as learning and memory and significantly contribute to the development of neuronal networks. The algorithm of synapse modification proposed by Hebb as early as 1949 is the coincident activation of pre- and postsynaptic neurons. The present review considers the evolution of experimental protocols in which postsynaptic cell depolarization through the recording microelectrode was used to reveal the manifestation of Hebb-type plasticity in the synaptic inputs of the neocortex and hippocampus. Special attention is focused on the inhibitory control of the Hebb-type plasticity. Disinhibition within the local neuronal circuits is considered to be an important factor in Hebbian plasticity, contributing to such phenomena as priming, primed burst potentiation, hippocampal theta-rhythm and cortical arousal. The role of various transmitters (acetylcholine, norepinephrine, gamma-amino-butyric acid) in disinhibition is discussed with a special emphasis on the brain noradrenergic system. Possible mechanisms of Hebbian synapse modification and their modulation by memory enhancing substances are considered. It is suggested that along with their involvement in disinhibition processes these substances may control Hebb-type plasticity through intracellular second messenger systems.

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.

Long-term plasticity at excitatory synapses on aspinous interneurons in area CA1 lacks synaptic specificity

The synaptic specificity of long-term potentiation (LTP) was examined at synapses formed on aspinous dendrites of interneurons whose somata were located in the pyramidal cell layer of hippocampal area CA1. Intracellular recordings from slices prepared from rats were used to monitor excitatory postsynaptic potentials (EPSPs) elicited by extracellular stimulation in stratum radiatum. Two synaptic inputs were evoked at 0.5 Hz by stimulating axons adjacent to stratum pyramidale and s. lacunosum-moleculare. After obtaining baseline recordings (>/=10 min), one of the EPSPs was conditioned. The protocol involved tetanic stimulation, sometimes combined with somatic depolarization. Low-frequency stimulation of the two pathways was then resumed and EPSPs were recorded for <30 min. We observed both homosynaptic and heterosynaptic changes in synaptic strength. LTP and long-term depression (LTD) were seen in both pathways and all possible combinations of changes in the two EPSPs were observed, including heterosynaptic LTP associated with either homosynaptic LTP or LTD. Intracellular 1,2-bis (2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (10 mM) abolished alterations in synaptic strength. When axons in s. radiatum synapse onto a spiny pyramidal cell, synaptic specificity of LTP is preserved. However the results obtained from aspinous interneurons show that synaptic specificity of LTP is lost. These results are consistent with the hypothesis that spines provide postsynaptic mechanism(s) for conferring specificity to LTP.

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.

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.

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.

Dendritic calcium channels and hippocampal long-term depression

The authors examine the potential role of the different sources of dendritic Ca2+ influx in long-term depression (LTD) of synaptic efficacy. Recent data on the location and functional distribution of voltage-gated Ca2+ channels obtained from fluorescence imaging and patch clamp experiments are presented. This is followed by a discussion of the existing evidence for different sources of Ca2+ playing a role in the induction of LTD. The authors conclude that a number of key issues need to be resolved before any conclusions are drawn as to the involvement of any specific route of Ca2+ entry in LTD.

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.

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.

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.

Hippocampal homosynaptic long-term depression/depotentiation induced by adrenal steroids

The effects of adrenal steroids on synaptic plasticity were investigated in the dentate gyrus of the hippocampus. Experiments were performed in either adrenalectomized or intact (non-adrenalectomized), anesthetized rats. High-frequency stimulation was applied to the medial perforant pathway at three different frequencies; 100, 200 or 400 Hz, either post- or pre- and post-administration of the specific Type-II adrenal steroid receptor agonist RU 28362. High-frequency stimulation prior to RU 28362 administration produced a frequency-dependent long-term potentiation of the population spike, with 100 Hz showing no long-term potentiation and 400 Hz the highest degree of potentiation. In contrast, following administration of RU 28362, high-frequency stimulation produced a long-term depression (in comparison to baseline). In the experiments in which high-frequency stimulation was applied both pre- and post-RU 28362 administration, the size of the population spike was initially potentiated and then depotentiated after the RU 28362 injection. This effect was also frequency dependent, although opposite to the long-term potentiation effect. That is, 400 Hz was the least effective frequency for inducing long-term depression/depotentiation, while 100 Hz was the most effective. Long-term depression/depotentiation was immediate following high-frequency stimulation and lasted for the extent of the recording session, in some cases longer than 1 h. Similar to the finding reported in the accompanying paper, induction of long-term potentiation was substantially suppressed by RU 28362. However, in a number of experiments long-term potentiation could still be induced after RU 28362 administration, even after long-term depression/depotentiation had been established. In these cases, stimulation at the higher frequencies was necessary.(ABSTRACT TRUNCATED AT 250 WORDS)

Associative EPSP--spike potentiation induced by pairing orthodromic and antidromic stimulation in rat hippocampal slices

1. Pairing low-frequency orthodromic stimulation with high-frequency antidromic conditioning of pyramidal cells in area CA1 of the rat hippocampus resulted in long-lasting potentiation of the extracellular population spike of the cells, without an accompanying increase in the extracellular excitatory postsynaptic potential (EPSP), indicating an increase in EPSP-spike (E-S) coupling, also called E-S potentiation. 2. The amplitude of the antidromically conditioned E-S potentiation took up to 60 min to reach its peak, much longer than synaptic long-term potentiation (LTP) induced by orthodromic tetanic stimulation. 3. The population spike amplitude of a control orthodromic input, which stimulated a separate set of fibres and which was inactive during the pairing, was also increased in over half the slices tested. That it can affect a silent pathway suggests that antidromically conditioned E-S potentiation is not generated locally at tetanized synapses. 4. Bath application of 50 microM D,L-2-amino-5-phosphonovaleric acid (AP5) blocked induction of antidromically conditioned E-S potentiation. After washing out the AP5, the same stimulation resulted in population spike increases. This suggests that activation of the NMDA subtype of glutamate receptor is necessary for the induction of this form of E-S potentiation. 5. Application of 10 microM picrotoxin and/or 10 microM bicuculline, which block inhibition mediated by gamma-aminobutyric acid A (GABAA) receptors, did not reduce antidromically conditioned E-S potentiation. Thus, plasticity in GABAA-mediated inhibition cannot account for the increased population spike amplitude. 6. E-S potentiation did not increase the amplitude of either extracellular or intracellular EPSPs recorded at the cell body.

Long-term potentiation and evidence for novel synaptic association in CA1 stratum oriens of rat hippocampus

In CA1 stratum oriens of hippocampal slices, a robust long-term potentiation (LTP) induced by tetanic stimulation (20 pulses at 100 Hz, 10 trains delivered at 0.1 Hz) was accompanied by a slowly developing potentiation in a second, untetanized pathway. N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (D-APV, 50 or 100 microM) reduced the homosynaptic LTP by 80% but failed to affect heterosynaptic LTP. The metabotropic receptor antagonist DL-2-amino-3-phosphonopropionic acid DL-AP3, 300 microM) or (+)-alpha-methyl-4-carboxyphenylglycine (MCPG, 500 microM), applied with DL-APV, further reduced homosynaptic LTP and blocked heterosynaptic LTP. The inhibitor of Ca(2+)-induced Ca2+ release dantrolene (20 microM), also applied with DL-APV, blocked both components of LTP. Importantly, when low-frequency test stimulation (0.017 Hz) to the untetanized, heterosynaptic pathway was interrupted for 30 min after homosynaptic tetanization, heterosynaptic LTP did not develop. These results demonstrate homosynaptic and heterosynaptic LTP inductions in stratum oriens of the area CA1 and suggest that the heterosynaptic LTP is induced by NMDA-independent, novel associative processes between tetanized and untetanized pathways.

Neurobiology of the integrative activity of the brain: some properties of long-term posttetanic heterosynaptic depression in the motor cortex of the cat

It has been demonstrated that long-term posttetanic heterosynaptic depression (LTHD), manifested in the form of a prolonged decrease in the probability of monosynaptic responses of the cell to stimulation of that afferent pathway which was not activated during conditioning tetanization of another input, takes place in the neocortex, as it does in the hippocampus. LTHD is characterized by such properties as its long-term character, cooperativity, and nonspecificity of input. LTHD in the nonconditioned input and long-term posttetanic potentiation or long-term posttetanic homosynaptic depression in the conditioned input may develop both in parallel or independantly of one another. It is hypothesized on the basis of the results obtained that LTHD (as is the case with LTP and LTD) is a calcium-dependant phenomenon, and that the achievement of a specific level of depolarization of the membrane in the region of the disposition of the inactive synapses is required for its occurrence. "Contrasting," i.e., a relative increase in the efficiency of transmission in the activating synapse, may be effected through LTHD; LTHD may be one of the mechanisms underlying forgetting.

Long-term potentiation and depression in the dentate gyrus, and effects of nicotine

Effects of a brief high-frequency stimulation or a prolonged low-frequency stimulation in the presence and absence of nicotine were studied in thin transverse slices of the dentate gyrus prepared from the guinea pig. Test and conditioning stimulations were delivered to the middle one-third of the molecular layer, and the slope of the population excitatory postsynaptic potentials (EPSPs) elicited by the test stimulation was taken as an indicator of potentiation or depression. Nicotine was without effects on the N-methyl-D-aspartate (NMDA) component of EPSPs at 50 or 100 microM. A brief high-frequency stimulation induced long-term potentiation (LTP) in the presence of bicuculline. Nicotine (50 microM) almost doubled the magnitude of LTP. In the absence of bicuculline, an identical high-frequency stimulation induced a brief depression, the duration and magnitude of which were increased by nicotine. The increase was not statistically significant, however. In contrast to observations in the region CA1, 720 pulses at 1 Hz delivered after induction of LTP failed to induce long-term depression. Nicotine did not modify the after-effect of the low-frequency stimulation. It was discussed that the facilitation of LTP by nicotine probably resulted from suppression of inhibitory processes.

Synaptic plasticity: LTP and LTD

Long-term potentiation (LTP) is a synaptic enhancement that follows brief, high-frequency electrical stimulation in the hippocampus and neocortex. Recent evidence suggests that induction of LTP may require, in addition to postsynaptic Ca2+ entry, activation of metabotropic glutamate receptors and the generation of diffusible intercellular messengers. A new form of synaptic plasticity, homosynaptic long-term depression (LTD) has also recently been documented, which, like LTP, requires Ca2+ entry through the NMDA receptor. Current work suggests that this LTD is a reversal of LTP, and vice versa, and that the mechanisms of LTP and LTD may converge at the level of specific phosphoproteins.

Postsynaptic induction and presynaptic expression of hippocampal long-term depression

Long-term depression (LTD) is an activity-dependent decrease in synaptic efficacy that together with its counterpart, long-term potentiation, is thought to be an important cellular mechanism for learning and memory in the mammalian brain. The induction of LTD in hippocampal CA1 pyramidal neurons in neonatal rats is shown to depend on postsynaptic calcium ion entry through L-type voltage-gated calcium channels paired with the activation of metabotropic glutamate receptors. Although induced postsynaptically, LTD is due to a long-term decrease in transmitter release from presynaptic terminals. This suggests that LTD is likely to require the production of a retrograde messenger.

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.

Metabotropic glutamate response in acutely dissociated hippocampal CA1 pyramidal neurones of the rat

1. The metabotropic glutamate (mGlu) response was investigated in dissociated rat hippocampal CA1 pyramidal neurones using conventional and nystatin-perforated whole-cell modes of the patch recording configuration. 2. In the perforated patch recording configuration, the application of glutamate (Glu), quisqualate (QA), aspartate (Asp) and N-methyl-D-aspartate (NMDA) induced a slow outward current superimposed on a fast ionotropic inward current, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate (KA) induced only an ionotropic inward current at a holding potential (VH) of -20 mV. A specific agonist of the mGlu receptor (mGluR), trans-1-aminocyclopentane-1,3-dicarboxylate (tACPD), induced an outward current in approximately 80% of the neurones tested. Asp- and NMDA-induced outward currents were antagonized by D-2-amino-5-phosphonopentanoate (D-AP5) whereas Glu-, QA- and tACPD-induced outward currents were not antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D-AP5, indicating that the mGlu response is an outward current component. 3. L-2-Amino-3-phosphonopropionate (L-AP3) and DL-2-amino-4-phosphonobutyrate (AP4) did not block the mGlu response. 4. The relative potencies of mGlu agonists were QA > Glu > tACPD. The threshold and EC50 values of metabotropic outward currents were 10-100 times lower than those of the ionotropic inward current (iGlu response). 5. The reversal potential of the mGlu response (EmGlu) was close to EK (K+ equilibrium potential), and it shifted 59.5 mV for a tenfold change in extracellular K+ concentration. 6. In Ca(2+)-free external solution, the mGlu response was elicited by an initial application of Glu, but subsequent applications failed to induce the response. There was also an increase in the intracellular free Ca2+ concentration ([Ca2+]i) during the application of Glu and QA but not of AMPA, indicating Ca2+ release from an intracellular Ca2+ store. 7. During the activation of a Ca(2+)-dependent K+ current (IK(Ca)) by inositol trisphosphate (IP3) in the internal solution, the mGlu response was suppressed. Addition of GDP-beta-S, neomycin or heparin to the internal solution also suppressed the mGlu response, but staurosporine had no effect. The mGlu response was abolished by pretreatment with either caffeine or ryanodine, but treatment with pertussis toxin (IAP) for 6-8 h had no effect. 8. The mGlu response was suppressed by tetraethylammonium, but not by either apamin or iberiotoxin, suggesting that intermediate-conductance Ca(2+)-dependent K+ (KCa+) channels are involved.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Origins of the variations in long-term potentiation between synapses in the basal versus apical dendrites of hippocampal neurons

Responses to theta pattern stimulation, and the long-term potentiation (LTP) they induce, were compared in the basal versus apical dendrites of neurons in field CA1 of hippocampus. A series of 10 theta bursts produced more than twice as much LTP in basal synapses as in their apical counterparts as measured with field EPSPs. This confirms earlier field potential studies showing that the maximum degree of potentiation (the LTP ceiling) is considerably greater in stratum oriens than stratum radiatum. Experiments with whole-cell clamp recording obtained similar results, indicating that synapses at different loci on the same neuron reach different LTP ceilings following prolonged theta burst stimulation. The basal synapses also required fewer theta bursts to reach their LTP ceiling than did the apical synapses. Tests with paired-pulse facilitation and an antagonist of the NMDA receptor gave no indication that the greater LTP in basal synapses was qualitatively different from the lesser effect obtained in apical contacts. Intracellular recording revealed significant differences between basal versus apical responses to single theta bursts and trains of bursts: the within-burst depolarization was greater and the between-burst hyperpolarization was smaller for the basal dendritic responses. These two variables have previously been proposed to influence the magnitude of LTP and the observed differences between basal versus apical synapses are in accord with this hypothesis. Together with recently described immunocytochemical results, the findings reported here suggest that variations in LTP across dendritic subfields of hippocampus reflect a differential distribution of a subclass of GABAergic interneurons.

Activation of the medial septal area attenuates LTP of the lateral perforant path and enhances heterosynaptic LTD of the medial perforant path in aged rats

Age-related memory impairments may be due to dysfunction of the septohippocampal system. The medial septal area (MSA) provides the major cholinergic projection to the hippocampus and is critical for memory. Knowledge of the neurobiological mechanisms by which the cholinergic system can attenuate age-related memory loss can facilitate the development of effective cognitive enhancers. At present, one of the best neurobiological models of memory formation is long-term potentiation/long-term depression (LTP/LTD). In previous studies, intraseptal infusion of the muscarinic agonist oxotremorine, which excites MSA neurons, improved memory in aged rats. The present study examined LTP and LTD in aged Fisher 344 rats following intraseptal infusion of oxotremorine. LTP and LTD were assessed using the slope of the EPSP recorded from the hilar region of the dentate gyrus. Induction of LTP was blocked in the lateral perforant path, but not in the medial perforant path, following intraseptal infusions of oxotremorine. The generation and amplitude of heterosynaptic LTD was enhanced in the medial perforant path, but not in the lateral perforant path. The results provide evidence that pharmacological activation of the MSA can modulate LTP and LTD in the hippocampus of aged rats. The implications of these results with respect to memory and synaptic plasticity in the hippocampus are discussed.

Synaptic inhibition regulates associative interactions between afferents during the induction of long-term potentiation and depression

The induction of long-term potentiation and depression depends upon associative interactions between synapses that converge on individual dendrites. The distance over which these associative interactions occur is limited. The present study evaluates whether this limitation is regulated by synaptic inhibition. We evaluated the associative interactions between two inputs that terminate on different proximo-distal locations along the dendrites of dentate granule cells in the presence of the gamma-aminobutyric acid (GABA) antagonist bicuculline methiodide. Local blockade of GABAergic inhibition enhanced associative interactions between nonoverlapping inputs, compared to within-animal control sites, where inhibitory transmission was intact. The results suggest that synaptic inhibition limits interactions between excitatory synapses by creating current shunts that limit the spread of depolarization within the dendritic tree.

Long-term depression of excitatory synaptic transmission and its relationship to long-term potentiation

In many brain areas, including the cerebellar cortex, neocortex, hippocampus, striatum and nucleus accumbens, brief activation of an excitatory pathway can produce long-term depression (LTD) of synaptic transmission. In most preparations, induction of LTD has been shown to require a minimum level of postsynaptic depolarization and a rise in the intracellular Ca2+ concentration [Ca2+]i in the postsynaptic neurone. Thus, induction conditions resemble those described for the initiation of associative long-term potentiation (LTP). However, data from structures susceptible to both LTD and LTP suggest that a stronger depolarization and a greater increase in [Ca2+]i are required to induce LTP than to initiate LTD. The source of Ca2+ appears to be less critical for the differential induction of LTP and LTD than the amplitude of the Ca2+ surge, since the activation of voltage- and ligand-gated Ca2+ conductances as well as the release from intracellular stores have all been shown to contribute to both LTD and LTP induction. LTD is induceable even at inactive synapses if [Ca2+]i is raised to the appropriate level by antidromic or heterosynaptic activation, or by raising the extracellular Ca2+ concentration [Ca2+]o. These conditions suggest a rule (called here the ABS rule) for activity-dependent synaptic modifications that differs from the classical Hebb rule and that can account for both homosynaptic LTD and LTP as well as for heterosynaptic competition and associativity.

Failure to induce long-term depression by an anti-correlation procedure in area CA1 of the rat hippocampal slice

We have independently in our two laboratories re-examined the report by Stanton and Sejnowski (Nature, 339, 215-218, 1989) that single stimuli to a test pathway in area CA1 of the hippocampal slice, when delivered between short bursts of stimuli to a second, convergent pathway, produce an associative long-term depression (LTD) in the test pathway. While robust associative LTP was observed when stimuli to the two inputs were correlated in time, the anti-correlation procedure failed to induce LTD; rather, a trend towards potentiation was observed. This result was obtained using both submerged and interface chambers, and in two different strains of rat. A transient depression lasting for a few minutes could usually be elicited by strong tetanic stimulation; this depression was not restricted to activated pathways.

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.

Bicuculline permits the induction of long-term depression by heterosynaptic, translaminar conditioning in the hippocampal dentate gyrus

Using intact, urethane-anesthetized albino rats, we examine long-term depression (LTD) of the ipsilateral medial entorhinal cortical (MEC) response in the dentate gyrus (DG) that is induced by conditioning of the ipsilateral lateral entorhinal cortex (LEC). Although the LEC-DG response potentiates, conditioning the LEC induces little or no LTD of the MEC-DG response. However, bicuculline methiodide, and the correlated presence of population spikes, enable the LEC input to induce LTD of the MEC-DG response. This result suggests that inhibition can control the selectivity of translaminar interactions.

Heterosynaptic short-term depression of population spike amplitude in the pyramidal layer of the CA1 hippocampal region evoked by a theta-like tetanization

Heterosynaptic short-term depression (STD) of the stratum radiatum and stratum oriens inputs to the CA1 region was studied in rat hippocampal slices. STD was evoked by trains of 1050 impulses with interstimulus interval (ISI) variable from 10 to 700 ms. The STD was found to be very pronounced for tetanizations with ISI around 200 ms, and almost absent for ISI less than 50 ms or more than 500 ms. These data show that theta-like tetanization is an effective pattern not only for induction of the long-term potentiation (LTP), as has been shown previously, but for production of the heterosynaptic STD as well. This implies that heterosynaptic STD can effectively modulate induction of LTP by theta-like tetanization, and plays an important role in differentiation of potentiated pathways. It is discussed that the theta-like tetanization-induced release of ACh is a possible mechanism of the STD.

Mechanisms underlying induction of homosynaptic long-term depression in area CA1 of the hippocampus

The mechanisms responsible for long-lasting, activity-dependent decreases in synaptic efficacy are not well understood. We have examined the initial steps required for the induction of long-term depression (LTD) in CA1 pyramidal cells by repetitive low frequency (1 Hz) synaptic stimulation. This form of LTD was synapse specific, was saturable, and required activation of post-synaptic NMDA receptors. Loading CA1 cells with the Ca2+ chelator BAPTA prevented LTD, whereas lowering extracellular Ca2+ resulted in the induction of LTD by stimulation that previously elicited long-term potentiation. Following LTD, synaptic strength could be increased to its original maximal level, indicating that LTD is reversible and not due to deterioration of individual synapses. Induction of homosynaptic LTD therefore requires an NMDA receptor-dependent change in postsynaptic Ca2+ which may be distinct from that required for long-term potentiation.

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.

NMDA-dependent heterosynaptic long-term depression in the dentate gyrus of anaesthetized rats

This report examines the inductive mechanisms involved in long-term heterosynaptic depression (LTD) in the dentate gyrus of anaesthetized rats. Associative and non-associative stimulus protocols were implemented, using the ipsilateral medial and lateral perforant path inputs to the dentate gyrus as the test pathways. In all experiments, the medial perforant path (MPP) received the conditioning stimuli which consisted of eight stimulus trains of 2 s duration, spaced 1 minute apart. Within each train the stimuli occurred as a burst of 5 pulses at 100 Hz, repeated at 200 ms intervals. The lateral perforant path (LPP) served as the test pathway in all of the initial experiments. In the associative condition, it received single pulses equally spaced between the medial path bursts. In the non-associative condition, no lateral path stimuli were given during the medial path trains. In both conditions, the application of the conditioning stimuli resulted in a long-term potentiation (LTP) of the medial path evoked responses (P less than 0.001), while the lateral path responses showed LTD (P less than 0.001). A two-way analyses of variance revealed there to be no difference between the two paradigms in the expression of LTP or LTD in naive pathways or in their ability to depress a potentiated pathway (P greater than 0.05) An occlusion test also showed there to be no further decreases in synaptic efficacy with the associative paradigm after the lateral path synapses were saturated with non-associative LTD.(ABSTRACT TRUNCATED AT 250 WORDS)

The involvement of L-type calcium channels in heterosynaptic long-term depression in the hippocampus

The involvement of L-type calcium channels in heterosynaptic long-term depression (LTD) of the stratum radiatum input to area CA1 was studied in rat hippocampal slices. LTD of the radiatum field excitatory postsynaptic potential (EPSP) and population spike, produced by tetanization of the alveus in the presence of picrotoxin, was blocked by the calcium antagonist nimodipine and by a monoclonal antibody to the L-type calcium channel. LTD was produced in the absence of picrotoxin when the L-type calcium channel agonist, BAY-K8644, was applied. This effect was also blocked by nimodipine. These results indicate that L-type calcium channels are involved in heterosynaptic long-term depression.