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

Working and Reference Memory Tasks Trigger Opposed Long-Term Synaptic Changes in the Rat Dentate Gyrus

Long-term storage of information into memory is supposed to rely on long-term synaptic plasticity processes. The detection of such synaptic changes after training in long-term/reference memory (RM) tasks has yet been scarce, variable and only studied on a short time scale. Short-term or working memory (WM) is largely known to depend on persistent neuronal activity or short-term plasticity. However, processing information into WM could also involve long-term synaptic changes that could be responsible for the erasure/forgetting of items previously stored in WM and acting as proactive interference. In order to study long-term synaptic changes associated with RM or WM, we trained chronically implanted rats in 3 different radial maze tasks: a classical RM task and 2 WM tasks involving different levels of proactive interference. Synaptic responses in the dentate gyrus were recorded during 2 × 24 h in freely moving rats after training. We found that consolidation of long-term information leads first to a delayed synaptic potentiation, occurring 9 h after RM training that is replaced by a synaptic depression once the RM rule is fully acquired. In contrast, optimal information processing into WM triggers a synaptic depression immediately after training and lasting 3 h that could act as a mechanism for interference erasure/forgetting.

mGluR5 Facilitates Long-Term Synaptic Depression in a Stress-Induced Depressive Mouse Model

BACKGROUND

Glutamatergic system has been known to play a role in the pathogenesis of major depression disorder by inducing N-methyl-d-aspartate receptor-dependent long-term depression (LTD) or metabotropic glutamate receptors (mGluR)-dependent LTD. Here, we characterized the LTD in a chronic social defeat stress (CSDS)-induced depressive mouse model.

METHODS

CSDS was used to induce the depressive-like behaviors in C57BL/6 male mice, which were assessed using sucrose preference test and social interaction test. The synaptic strength including LTD and long-term potentiation (LTP) induced by paired-pulse low frequency stimulation (PP-LFS) was measured using whole-cell recording technique.

RESULTS

CSDS induced depressive-like behaviors and facilitated PP-LFS-induced LTD in hippocampal CA3-CA1 pathway in the susceptible mice. Interestingly, mGluR5 but not N-methyl-d-aspartate receptor mediated the PP-LFS-induced LTD. In addition, mGluR5 agonist dihydroxyphenylglycine promoted PP-LFS-induced LTD specifically in susceptible mice, which was diminished by activating the BDNF/TrkB signaling pathway.

CONCLUSIONS

Our results suggest that mGluR5-dependent LTD might be responsible for the development of depressive-like behaviors in CSDS-induced depression mice model.

Memory Susceptibility to Retroactive Interference Is Developmentally Regulated by NMDA Receptors

Retroactive interference (RI) occurs when new incoming information impairs an existing memory, which is one of the primary sources of forgetting. Although long-term potentiation (LTP) reversal shows promise as the underlying neural correlate, the key molecules that control the sensitivity of memory circuits to RI are unknown, and the developmental trajectory of RI effects is unclear. Here we found that depotentiation in the hippocampal dentate gyrus (DG) depends on GluN2A-containing NMDA receptors (NMDARs). The susceptibility of LTP to disruption progressively increases with the rise in the GluN2A/GluN2B ratio during development. The vulnerability of hippocampus-dependent memory to interference from post-learning novelty exploration is subject to similar developmental regulation by NMDARs. Both GluN2A overexpression and GluN2B downregulation in the DG promote RI-induced forgetting. Altogether, our results suggest that a switch in GluN2 subunit predominance may confer age-related differences to depotentiation and underlie the developmental decline in memory resistance to RI.

Hippocampal LTP modulation and glutamatergic receptors following vestibular loss

Vestibular dysfunction strongly impairs hippocampus-dependent spatial memory performance and place cell function. However, the hippocampal encoding of vestibular information at the synaptic level, remains sparsely explored and controversial. We investigated changes in in vivo long-term potentiation (LTP) and NMDA glutamate receptor (NMDAr) density and distribution after bilateral vestibular lesions (BVL) in adult rats. At day 30 (D₃₀) post-BVL, the LTP of the population spike recorded in the dentate gyrus (DG) was higher in BVL rats, for the entire 3 h of LTP recording, while no difference was observed in the fEPSP slope. However, there was an increase in EPSP-spike (E-S) potentiation in lesioned rats. NMDArs were upregulated at D₇ and D₃₀ predominantly within the DG and CA1. At D₃₀, we observed a higher NMDAr density in the left hippocampus. NMDArs were overexpressed on both neurons and non-neuronal cells, suggesting a decrease of the entorhinal glutamatergic inputs to the hippocampus following BVL. The EPSP-spike (E-S) potentiation increase was consistent with the dorsal hippocampus NMDAr upregulation. Such an increase could reflect a non-specific enhancement of synaptic efficacy, leading to a disruption of memory encoding, and therefore might underlie the memory deficits previously reported in rats and humans following vestibular loss.

Properties and Mechanisms of LTP Maintenance

Memory is fundamentally important to everyday life, and memory loss has devastating consequences to individuals and society. Understanding the neurophysiological and cellular basis of memory paves the way for gaining insights into the molecular steps involved in memory formation, thereby revealing potential therapeutic targets for neurological diseases. For three decades, long-term potentiation (LTP) has been the gold standard synaptic model for mammalian memory mechanisms, in large part because of its long-lasting nature. Here, the authors summarize the characteristics of LTP persistence in the dentate gyrus of the hippocampus, comparing this with other hippocampal subregions and neocortex. They consider how long LTP can last and how its persistence is affected by subsequent behavioral experiences. Next, they review the molecular mechanisms known to contribute to LTP induction and persistence, in particular the role of new gene expression and protein synthesis and how they may be associated with potential structural reorganization of the synapse. A temporal schema for the processes important for consolidating LTP into a persistent form is presented. The parallels between the molecular aspects of LTP and memory strongly support the continuation with LTP as a model system for studying the mechanisms underlying long-term memory consolidation and retention.

A Voltage-Based STDP Rule Combined with Fast BCM-Like Metaplasticity Accounts for LTP and Concurrent "Heterosynaptic" LTD in the Dentate Gyrus In Vivo

Long-term potentiation (LTP) and long-term depression (LTD) are widely accepted to be synaptic mechanisms involved in learning and memory. It remains uncertain, however, which particular activity rules are utilized by hippocampal neurons to induce LTP and LTD in behaving animals. Recent experiments in the dentate gyrus of freely moving rats revealed an unexpected pattern of LTP and LTD from high-frequency perforant path stimulation. While 400 Hz theta-burst stimulation (400-TBS) and 400 Hz delta-burst stimulation (400-DBS) elicited substantial LTP of the tetanized medial path input and, concurrently, LTD of the non-tetanized lateral path input, 100 Hz theta-burst stimulation (100-TBS, a normally efficient LTP protocol for in vitro preparations) produced only weak LTP and concurrent LTD. Here we show in a biophysically realistic compartmental granule cell model that this pattern of results can be accounted for by a voltage-based spike-timing-dependent plasticity (STDP) rule combined with a relatively fast Bienenstock-Cooper-Munro (BCM)-like homeostatic metaplasticity rule, all on a background of ongoing spontaneous activity in the input fibers. Our results suggest that, at least for dentate granule cells, the interplay of STDP-BCM plasticity rules and ongoing pre- and postsynaptic background activity determines not only the degree of input-specific LTP elicited by various plasticity-inducing protocols, but also the degree of associated LTD in neighboring non-tetanized inputs, as generated by the ongoing constitutive activity at these synapses.

The vast majority of computational studies that model synaptic plasticity neglect the fact that in vivo neurons exhibit an ongoing spontaneous spiking which affects the dynamics of synaptic changes. Here we study how key components of learning mechanisms in the brain, namely spike timing-dependent plasticity and metaplasticity, interact with spontaneous activity in the input pathways of the neuron. Using biologically realistic simulations we show that ongoing background activity is a key determinant of the degree of long-term potentiation and long-term depression of synaptic transmission between nerve cells in the hippocampus of freely moving animals. This work helps better understand the computational rules which drive synaptic plasticity in vivo.

Biochemical Computation for Spine Structural Plasticity

The structural plasticity of dendritic spines is considered to be essential for various forms of synaptic plasticity, learning, and memory. The process is mediated by a complex signaling network consisting of numerous species of molecules. Furthermore, the spatiotemporal dynamics of the biochemical signaling are regulated in a complicated manner because of geometrical restrictions from the unique morphology of the dendritic branches and spines. Recent advances in optical techniques have enabled the exploration of the spatiotemporal aspects of the signal regulations in spines and dendrites and have provided many insights into the principle of the biochemical computation that underlies spine structural plasticity.

l-DOPA reverses the impairment of Dentate Gyrus LTD in experimental parkinsonism via β-adrenergic receptors

Parkinson's disease (PD) patients exhibit motor and non-motor symptoms that severely affect quality of life. Cognitive alterations in PD subjects have been related to both structural and functional hippocampal changes. Here we investigated the effects of the 6-hydroxydopamine (6-OHDA) lesion in the Medial Forebrain Bundle (MFB) on the hippocampus focusing on the Dentate Gyrus (DG). In vivo microdialysis measurements revealed that the 6-OHDA injection disrupts both dopaminergic and noradrenergic transmission in rat DG. In vitro electrophysiological recordings showed that these neurochemical alterations were accompanied by impairment of long-term depression (LTD) at medial perforant path/DG synapses. Furthermore, this alteration was reversed by l-DOPA treatment. Notably, the therapeutic effect of l-DOPA on LTD was blocked by the antagonism of β-noradrenergic receptors, but not by dopamine D1 or D2 receptor antagonists. Thus, while the dopaminergic transmission does not seem to be implicated in this therapeutic effect of l-DOPA, the noradrenergic system plays a central role in the synaptic dysfunction of the DG in experimental PD. Our work provides new evidence on the role of catecholamines in DG synaptic plasticity and sheds light on the possible synaptic mechanisms underlying cognitive deficits in PD. Furthermore, our results indicate that l-DOPA exerts a therapeutic effect on the parkinsonian brain through different, coexistent, mechanisms.

Redistribution of ionotropic glutamate receptors detected by laser microdissection of the rat dentate gyrus 48 h following LTP induction in vivo

The persistence and input specificity of long-term potentiation (LTP) make it attractive as a mechanism of information storage. In its initial phase, both in vivo and in vitro studies have shown that LTP is associated with increased membrane localization of AMPA receptor subunits, but the molecular basis of LTP maintenance over the long-term is still unclear. We have previously shown that expression of AMPA and NMDA receptor subunits is elevated in whole homogenates prepared from dentate gyrus 48 h after LTP induction in vivo. In the present study, we utilized laser microdissection (LMD) techniques to determine whether AMPA and NMDA receptor upregulation occurs specifically in the stimulated regions of the dentate gyrus dendritic arbor. Receptor proteins GluN1, GluA1 and GluA2, as well as postsynaptic density protein of 95 kDa and tubulin were detected by Western blot analysis in microdissected samples. Gradients of expression were observed for GluN1 and GluA2, decreasing from the inner to the outer zones of the molecular layer, and were independent of LTP. When induced at medial perforant path synapses, LTP was associated with an apparent specific redistribution of GluA1 and GluN1 to the middle molecular layer that contains these synapses. These data indicate that glutamate receptor proteins are delivered specifically to dendritic regions possessing LTP-expressing synapses, and that these changes are preserved for at least 48 h.

Timing and expectation of reward: a neuro-computational model of the afferents to the ventral tegmental area

Neural activity in dopaminergic areas such as the ventral tegmental area is influenced by timing processes, in particular by the temporal expectation of rewards during Pavlovian conditioning. Receipt of a reward at the expected time allows to compute reward-prediction errors which can drive learning in motor or cognitive structures. Reciprocally, dopamine plays an important role in the timing of external events. Several models of the dopaminergic system exist, but the substrate of temporal learning is rather unclear. In this article, we propose a neuro-computational model of the afferent network to the ventral tegmental area, including the lateral hypothalamus, the pedunculopontine nucleus, the amygdala, the ventromedial prefrontal cortex, the ventral basal ganglia (including the nucleus accumbens and the ventral pallidum), as well as the lateral habenula and the rostromedial tegmental nucleus. Based on a plausible connectivity and realistic learning rules, this neuro-computational model reproduces several experimental observations, such as the progressive cancelation of dopaminergic bursts at reward delivery, the appearance of bursts at the onset of reward-predicting cues or the influence of reward magnitude on activity in the amygdala and ventral tegmental area. While associative learning occurs primarily in the amygdala, learning of the temporal relationship between the cue and the associated reward is implemented as a dopamine-modulated coincidence detection mechanism in the nucleus accumbens.

Sleep and synaptic homeostasis

In the last decades a substantial knowledge about sleep mechanisms has been accumulated. However, the function of sleep still remains elusive. The difficulty with unraveling sleep's function may arise from the lack of understanding of how the multitude of processes associated with waking and sleep-from gene expression and single neuron activity to the whole brain dynamics and behavior-functionally and mechanistically relate to each other. Therefore, novel conceptual frameworks, which integrate and take into account the variety of phenomena occurring during waking and sleep at different levels, will likely lead to advances in our understanding of the function of sleep, above and beyond what merely descriptive or correlative approaches can provide. One such framework, the synaptic homeostasis hypothesis, focuses on wake- and sleep-dependent changes in synaptic strength. The core claim of this hypothesis is that learning and experience during wakefulness are associated with a net increase in synaptic strength. In turn, the proposed function of sleep is to provide synaptic renormalization, which has important implications with respect to energy needs, intracranial space, metabolic supplies, and, importantly, enables further plastic changes. In this article we review the empirical evidence for this hypothesis, which was obtained at several levels-from gene expression and cellular excitability to structural synaptic modifications and behavioral outcomes. We conclude that although the mechanisms behind the proposed role of sleep in synaptic homeostasis are undoubtedly complex, this conceptual framework offers a unique opportunity to provide mechanistic and functional explanation for many previously disparate observations, and define future research strategies.

Low-frequency stimulation induces long-term depression and slow onset long-term potentiation at perforant path-dentate gyrus synapses in vivo

The expression of homosynaptic long-term depression (LTD) is thought to mediate a crucial role in sustaining memory function. Our in vivo investigations of LTD expression at lateral (LPP) and medial perforant path (MPP) synapses in the dentate gyrus (DG) corroborate prior demonstrations that PP-DG LTD is difficult to induce in intact animals. In freely moving animals, LTD expression occurred inconsistently among LPP-DG and MPP-DG responses. Interestingly, following acute electrode implantation in anesthetized rats, low-frequency stimulation (LFS; 900 pulses, 1 Hz) promotes slow-onset LTP at both MPP-DG and LPP-DG synapses that utilize distinct induction mechanisms. Systemic administration of the N-methyl-d-aspartate (NMDA) receptor antagonist (+/-)-cyclopiperidine-6-piperiperenzine (CPP; 10 mg/kg) 90 min before LFS selectively blocked MPP-DG but not LPP-DG slow onset LTP, suggesting MPP-DG synapses express a NMDA receptor-dependent slow onset LTP whereas LPP-DG slow onset LTP induction is NMDA receptor independent. In experiments where paired-pulse LFS (900 paired pulses, 200-ms paired-pulse interval) was used to induce LTD, paired-pulse LFS of the LPP resulted in rapid onset LTP of DG responses, whereas paired-pulse LFS of the MPP induced slow onset LTP of DG responses. Although LTD observations were very rare following acute electrode implantation in anesthetized rats, LPP-DG LTD was demonstrated in some anesthetized rats with previously implanted electrodes. Together, our data indicate in vivo PP-DG LTD expression is an inconsistent phenomenon that is primarily observed in recovered animals, suggesting perturbation of the dentate through surgery-related tissue trauma influences both LTD incidence and LTP induction at PP-DG synapses in vivo.

Multiple spine boutons are formed after long-lasting LTP in the awake rat

The formation of multiple spine boutons (MSBs) has been associated with cognitive abilities including hippocampal-dependent associative learning and memory. Data obtained from cultured hippocampal slices suggest that the long-term maintenance of synaptic plasticity requires the formation of new synaptic contacts on pre-existing synapses. This postulate however, has never been tested in the awake, freely moving animals. In the current study, we induced long-term potentiation (LTP) in the dentate gyrus (DG) of awake adult rats and performed 3-D reconstructions of electron micrographs from thin sections of both axonal boutons and dendritic spines, 24 h post-induction. The specificity of the observed changes was demonstrated by comparison with animals in which long-term depression (LTD) had been induced, or with animals in which LTP was blocked by an N-methyl-D-aspartate (NMDA) antagonist. Our data demonstrate that whilst the number of boutons remains unchanged, there is a marked increase in the number of synapses per bouton 24 h after the induction of LTP. Further, we demonstrate that this increase is specific to mushroom spines and not attributable to their division. The present investigation thus fills the gap existing between behavioural and in vitro studies on the role of MSB formation in synaptic plasticity and cognitive abilities.

Interaction between long-term potentiation and depression in CA1 synapses: temporal constrains, functional compartmentalization and protein synthesis

Information arriving at a neuron via anatomically defined pathways undergoes spatial and temporal encoding. A proposed mechanism by which temporally and spatially segregated information is encoded at the cellular level is based on the interactive properties of synapses located within and across functional dendritic compartments. We examined cooperative and interfering interactions between long-term synaptic potentiation (LTP) and depression (LTD), two forms of synaptic plasticity thought to be key in the encoding of information in the brain. Two approaches were used in CA1 pyramidal neurons of the mouse hippocampus: (1) induction of LTP and LTD in two separate synaptic pathways within the same apical dendritic compartment and across the basal and apical dendritic compartments; (2) induction of LTP and LTD separated by various time intervals (0–90 min). Expression of LTP/LTD interactions was spatially and temporally regulated. While they were largely restricted within the same dendritic compartment (compartmentalized), the nature of the interaction (cooperation or interference) depended on the time interval between inductions. New protein synthesis was found to regulate the expression of the LTP/LTD interference. We speculate that mechanisms for compartmentalization and protein synthesis confer the spatial and temporal modulation by which neurons encode multiplex information in plastic synapses.

Differential effects of strain, circadian cycle, and stimulation pattern on LTP and concurrent LTD in the dentate gyrus of freely moving rats

Because long-term potentiation (LTP) and long-term depression (LTD) are thought to be involved in learning and memory, it is important to delineate factors that modulate their induction and persistence, especially as studied in freely moving animals. Here, we investigated the effects of rat strain, circadian cycle, and high-frequency stimulation (HFS) pattern on LTP and concurrently induced LTD in the dentate gyrus (DG). Comparison of two commonly used rat strains revealed that medial perforant path field EPSP-population spike (E-S) coupling and LTP were greater in Long-Evans than Sprague-Dawley rats. Circadian cycle experiments conducted in Long-Evans rats revealed greater E-S coupling and enhanced LTP during the dark phase. Interestingly, concurrent LTD in the lateral perforant path did not significantly differ across strains or circadian cycle. Testing HFS protocols during the dark phase revealed that theta burst stimulation (100 Hz bursts at 5 Hz intervals) was ineffective in eliciting either LTP or concurrent LTD in DG, whereas 400 Hz bursts delivered at theta (5 Hz) or delta (1 Hz) frequencies produced substantial LTP and concurrent LTD. Thus, these natural and experimental factors regulate granule cell excitability, and differentially affect LTP and concurrent LTD in the DG of freely moving rats. © 2011 Wiley Periodicals, Inc.

Medial and lateral perforant path evoked potentials are selectively modulated by pairing with glutamatergic activation of locus coeruleus in the dentate gyrus of the anesthetized rat

Norepinephrine (NE) in vitro produces long-lasting potentiation of medial perforant path input and depression of lateral perforant path input to dentate gyrus in the rat. Similar, but highly transient, effects have been reported in vivo using paragigantocellular stimulation to release NE. The present study uses alternate stimulation of the medial perforant path and lateral olfactory tract (eliciting a lateral perforant path-evoked potential) to examine the effects of glutamatergic activation of locus coeruleus (LC) on the two pathways for up to 3 h post-LC activation. In the first experiment, the expected potentiation of the medial perforant path population spike in dentate gyrus was observed, but without accompanying depression of the lateral perforant path-mediated evoked potential (lateral olfactory tract stimulation, 60 s ISI). In a second experiment, with more frequent pairing of input with NE release (10 s ISI), significant potentiation of lateral perforant path-mediated input to dentate gyrus occurred, but potentiation of medial perforant path input was not seen. A third experiment with a 30 s ISI again produced potentiation of lateral perforant path-mediated input without potentiation of the medial perforant path population spike. The size of effects with the 30 s ISI was intermediate between that seen with 10 s and 60 s ISI. Potentiation of lateral perforant path over medial perforant path input has previously been reported with acute nicotinic activation of the LC. This outcome also resembles heterosynaptic modulation previously reported with tetanic potentiation. The data argue for a competitive relationship between medial and lateral perforant path inputs to dentate gyrus and suggest pairing with increased NE produces a bias favoring one or the other pathway depending on parameters such as strength and frequency. NE potentiating effects on lateral perforant path input here may also have occurred in entorhinal cortex (EC) given the system-wide NE release with LC activation.

Differential regulation of synaptic plasticity of the hippocampal and the hypothalamic inputs to the anterior thalamus

The hippocampus projects to the anterior thalamic nuclei both directly and indirectly via the mammillary bodies, but little is known about the electrophysiological properties of these convergent pathways. Here we demonstrate, for the first time, the presence of long-term plasticity in anterior thalamic nuclei synapses in response to high- and low-frequency stimulation (LFS) in urethane-anesthetized rats. We compared the synaptic changes evoked via the direct vs. the indirect hippocampal pathways to the anterior thalamus, and found that long-term potentiation (LTP) of the thalamic field response is induced predominantly through the direct hippocampal projections. Furthermore, we have estimated that that long-term depression (LTD) can be induced only after stimulation of the indirect connections carried by the mammillothalamic tract. Interestingly, basal synaptic transmission mediated by the mammillothalamic tract undergoes use-dependent, BDNF-mediated potentiation, revealing a distinct form of plasticity specific to the diencephalic region. Our data indicate that the thalamus does not passively relay incoming information, but rather acts as a synaptic network, where the ability to integrate hippocampal and mammillary body inputs is dynamically modified as a result of previous activity in the circuit. The complementary properties of these two parallel pathways upon anterior thalamic activity reveal that they do not have duplicate functions.

Electrophysiological correlates of sleep homeostasis in freely behaving rats

The electrical activity of the brain does not only reflect the current level of arousal, ongoing behavior, or involvement in a specific task but is also influenced by what kind of activity, and how much sleep and waking occurred before. The best marker of sleep-wake history is the electroencephalogram (EEG) spectral power in slow frequencies (slow-wave activity, 0.5-4 Hz, SWA) during sleep, which is high after extended wakefulness and low after consolidated sleep. While sleep homeostasis has been well characterized in various species and experimental paradigms, the specific mechanisms underlying homeostatic changes in brain activity or their functional significance remain poorly understood. However, several recent studies in humans, rats, and computer simulations shed light on the cortical mechanisms underlying sleep regulation. First, it was found that the homeostatic changes in SWA can be fully accounted for by the variations in amplitude and slope of EEG slow waves, which are in turn determined by the efficacy of corticocortical connectivity. Specifically, the slopes of sleep slow waves were steeper in early sleep compared to late sleep. Second, the slope of cortical evoked potentials, which is an established marker of synaptic strength, was steeper after waking, and decreased after sleep. Further, cortical long-term potentiation (LTP) was partially occluded if it was induced after a period of waking, but it could again be fully expressed after sleep. Finally, multiunit activity recordings during sleep revealed that cortical neurons fired more synchronously after waking, and less so after a period of consolidated sleep. The decline of all these electrophysiological measures-the slopes of slow waves and evoked potentials and neuronal synchrony-during sleep correlated with the decline of the traditional marker of sleep homeostasis, EEG SWA. Taken together, these data suggest that homeostatic changes in sleep EEG are the result of altered neuronal firing and synchrony, which in turn arise from changes in functional neuronal connectivity.

Alterations in synaptic curvature in the dentate gyrus following induction of long-term potentiation, long-term depression, and treatment with the N-methyl-D-aspartate receptor antagonist CPP

Alterations in curvature of the post synaptic density (PSD) and apposition zone (AZ), are believed to play an important role in determining synaptic efficacy. In the present study we have examined curvature of PSDs and AZs 24 h following homosynaptic long-term potentiation (LTP), and heterosynaptic long-term depression (LTD) in vivo, in awake adult rats. High frequency stimulation (HFS) applied to the medial perforant path to the dentate gyrus induced LTP while HFS stimulation of the lateral perforant path induced LTD in the middle molecular layer of the dentate gyrus (DG). Curvature changes were analysed in this area using three dimensional (3-D) reconstructions of electron microscope images of ultrathin serial sections. Very large and significant changes in 3-D measurements of AZ and PSD curvature occurred 24 h following both LTP and LTD, with a flattening of the normal concavity of mushroom spine heads and a change to convexity for thin spines. An N-methyl-D-aspartate (NMDA) receptor antagonist CPP (3-[(R)-2-Carboxypiperazin-4-yl]-propyl-1-phosphonic acid) blocked the changes in curvature of mushroom and thin spine PSDs and apposition zones, actually increasing the concavity of mushroom spines as the spine engulfed the presynaptic bouton. In order to establish whether these changes resulted from the effect of the NMDA antagonist or from its coincidence with synaptic activation during testing we examined the effects of CPP alone on PSD and apposition zone curvature. It was found that CPP alone also caused a small decrease in curvature of both PSD and apposition zone of mushroom and thin spines.

N-methyl-D-aspartate receptor independent changes in expression of polysialic acid-neural cell adhesion molecule despite blockade of homosynaptic long-term potentiation and heterosynaptic long-term depression in the awake freely behaving rat dentate gyrus

Investigations examining the role of polysialic acid (PSA) on the neural cell adhesion molecule (NCAM) in synaptic plasticity have yielded inconsistent data. Here, we addressed this issue by determining whether homosynaptic long-term potentiation (LTP) and heterosynaptic long-term depression (LTD) induce changes in the distribution of PSA-NCAM in the dentate gyrus (DG) of rats in vivo. In addition, we also examined whether the observed modifications were initiated via the activation of N-methyl-D-aspartate (NMDA) receptors. Immunocytochemical analysis showed an increase in PSA-NCAM positive cells both at 2 and 24 h following high-frequency stimulation of either medial or lateral perforant paths, leading to homosynaptic LTP and heterosynaptic LTD, respectively, in the medial molecular layer of the DG. Analysis of sub-cellular distribution of PSA-NCAM by electron microscopy showed decreased PSA dendritic labelling in LTD rats and a sub-cellular relocation towards the spines in LTP rats. Importantly, these modifications were found to be independent of the activation of NMDA receptors. Our findings suggest that strong activation of the granule cells up-regulates PSA-NCAM synthesis which then incorporates into activated synapses, representing NMDA-independent plastic processes that act synergistically on LTP/LTD mechanisms without participating in their expression.

State-, timing-, and pattern-dependent neuromodulation of synaptic strength by a serotonergic interneuron

Here we report that a serotonergic neuron evokes two distinct neuromodulatory actions with different state, timing, and firing pattern dependencies. These neuromodulatory actions may have important behavioral functions. In the mollusc, Tritonia diomedea, EPSCs evoked by ventral swim interneuron B (VSI) exhibited intrinsic plasticity; after a spike train, EPSC amplitude increased from a basal state to a potentiated state, which usually lasted >10 min. While the synapse was in a potentiated state, stimulation of a serotonergic dorsal swim interneuron (DSI) decreased VSI synaptic strength, returning it to a basal state. The extent of the DSI-evoked decrement was strongly correlated with the magnitude of the homosynaptic potentiation. This synaptic reset, or depotentiation, by DSI was blocked by the serotonin receptor antagonist methysergide and mimicked by a serotonin puff. In contrast to this state-dependent neuromodulatory action, we found that a previously described DSI-evoked transient enhancement of VSI synaptic strength was state-independent, producing the same multiplicative increase in EPSC amplitude regardless of whether the synapse was in a potentiated or basal state. These two actions also differed in their dependencies on the firing pattern of DSI and VSI action potentials. Results suggest that state-independent synaptic enhancement by DSI may play a short-term role during a swim motor pattern, whereas state-dependent actions may have longer-lasting consequences, resetting VSI synaptic strength after a swim bout. Thus, differences in two neuromodulatory actions at one synapse may allow a serotonergic neuron to play distinct roles at different stages of a motor pattern.

Roles of endocannabinoids in heterosynaptic long-term depression of excitatory synaptic transmission in visual cortex of young mice

Tetanic stimulation of one of two afferent pathways converging to neurons in the visual cortex induces long-term depression (LTD) of synaptic transmission in the other, nonactivated pathway under a certain condition. This form of synaptic plasticity called heterosynaptic LTD (hetero-LTD) was not systematically investigated in previous studies, whereas homosynaptic LTD has been extensively studied. To determine whether hetero-LTD is induced in visual cortical slices of mice and, if so, through what mechanisms, we recorded EPSPs evoked in layer II/III neurons by alternating test stimulation of two sites in layer IV at 0.05 Hz. After theta-burst stimulation of one site, EPSPs evoked by test stimulation of the other site were depressed for a long time in most of the neurons, whereas homosynaptic long-term potentiation was induced at activated synapses. Such a hetero-LTD was induced in most mice at postnatal day 7-20 (P7-P20), but not induced in mice at P35-P41. Tests using the paired-pulse stimulation protocol and coefficient of variation analysis suggested that hetero-LTD was expressed at presynaptic sites. Pharmacological analysis indicated that this form of LTD was induced through activation of the type 5 of metabotropic glutamate receptors, not through the NMDA type of glutamate receptors. Additional analysis using a cannabinoid type 1 receptor agonist and an antagonist suggested that endocannabinoids (eCBs) are involved in this type of LTD. Moreover, results suggest that brain-derived neurotrophic factor, which may be released from strongly activated presynaptic sites, prevents eCBs from suppressing the release of transmitters from these sites.

Modeling the nonlinear dynamic interactions of afferent pathways in the dentate gyrus of the hippocampus

The dentate gyrus is the first region of the hippocampus that receives and integrates sensory information (e.g., visual, auditory, and olfactory) via the perforant path, which is composed of two distinct neuronal pathways: the Lateral Perforant Path (LPP) and the Medial Perforant Path (MPP). This paper examines the nonlinear dynamic interactions among arbitrary stimulation patterns at these two afferent pathways and their combined effect on the resulting response of the granule cells at the dentate gyrus. We employ non-parametric Poisson-Volterra models that serve as canonical quantitative descriptors of the nonlinear dynamic transformations of the neuronal signals propagating through these two neuronal pathways. These Poisson-Volterra models are estimated in the so-called "reduced form" with experimental data from in vitro hippocampal slices and provide excellent predictions of the electrophysiological activity of the granule cells in response to arbitrary stimulation patterns. The data are acquired through a custom-made multi-electrode-array system, which stimulated simultaneously the two pathways with random impulse trains and recorded the neuronal postsynaptic activity at the granule cell layer. The results of this study show that significant nonlinear interactions exist between the LPP and the MPP that may be critical for the integration of sensory information performed by the dentate gyrus of the hippocampus.

Brain plasticity mechanisms and memory: a party of four

A defining characteristic of the brain is its remarkable capacity to undergo activity-dependent functional and morphological remodeling via mechanisms of plasticity that form the basis of our capacity to encode and retain memories. Today, it is generally accepted that the neurobiological substrate of memories resides in activity-driven modifications of synaptic strength and structural remodeling of neural networks activated during learning. Since the discovery of long-term potentiation, the role of synaptic strengthening in learning and memory has been the subject of considerable investigation, and numerous studies have provided new insights into how this form of plasticity can subserve memory function. At the same time, other studies have explored the contribution of synaptic elimination or weakening; synaptogenesis, the growth of new synaptic connections and synapse remodeling; and more recently, neurogenesis, the birth and growth of new neurons in the adult brain. In this review, based on work in the hippocampus, the authors briefly outline recent advances in their understanding of the mechanisms and functional role of these four types of brain plasticity in the context of learning and memory. While they have long been considered as alternative mechanisms of plasticity underlying the storage of long-term memories, recent evidence suggests that they are functionally linked, suggesting the mechanisms underlying plasticity in the brain required for the formation and retention of memories are multifaceted.

"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.

Theta driving both inhibits and potentiates the effects of nicotine on dentate gyrus responses

The medial septal area of conscious rats was stimulated through previously implanted electrodes at a frequency of 7.7 Hz for 20 min each day for 7 days to evoke rhythmic slow activity in CA1 at a similar frequency to spontaneous theta. Two weeks later in the anaesthetized rats the effects of a single subcutaneous injection of nicotine (0.4 mg/kg) on fEPSPs, evoked in the dentate gyrus by separate stimulation of the MPP and LPP, were studied and compared with those obtained in controls. Nicotine increased the firing of locus coeruleus neurons and the slope of the fEPSPs evoked by LPP stimulation, but not by MPP stimulation. Prior theta driving considerably increased the effect of nicotine on the responses evoked by stimulation of the MPP and abolished the nicotine-induced potentiation of the responses evoked by stimulation of the LPP. The results are attributed to theta driving increasing the amount of noradrenaline released by nicotine and to noradrenaline producing a beta-adrenoceptor long-lasting potentiation at the medial perforant path synapse and a long-lasting depression at the lateral perforant path synapse.

Hippocampal long-term depression: master or minion in declarative memory processes?

The neural mechanisms for the formation of declarative memory (memory for facts and events) are believed to be integrated from processes mediated by hippocampal long-term potentiation (LTP) and long-term depression (LTD). Traditionally, LTP has been designated as the main mediator of spatial memory storage in the hippocampus, whereas LTD has been assigned an auxiliary role in signal-to-noise regulation or in forgetting. It has recently become apparent, however, that LTD contributes directly to hippocampal information storage. In fact, LTD could dominate in the processing of precise spatial characteristics. Accumulating evidence supports the idea that LTP and LTD enable distinct and separate forms of information storage, which together facilitate the generation of a spatial cognitive map.

Plastic processes in the dentate gyrus: a computational perspective

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

Comparison of cellular mechanisms of long-term depression of synaptic strength at perforant path-granule cell and Schaffer collateral-CA1 synapses

This chapter compares the cellular mechanisms that have been implicated in the induction and expression of long-term depression (LTD) at Schaffer collateral-CA1 synapses to perforant path-dentate gyrus (DG) synapses. In general, Schaffer collateral LTD and long-term potentiation (LTP) both appear to be a complex combination of many alterations in synaptic transmission that occur at both presynaptic and postsynaptic sites, while at perforant path synapses, most evidence has focused on postsynaptic long-term alterations. Within the DG, the medial perforant path is far more studied than lateral perforant path synapses, where most evidence relates to the induction of heterosynaptic LTD at lateral perforant path synapses when LTP is induced in the medial perforant path. Of course, there remain many other classes of synapses in the DG where synaptic plasticity, including LTD, have been largely neglected. It is clear that a better understanding of the range of DG loci where long-lasting activity-dependent plasticity, both LTD and LTP, are expressed will be essential to improve our understanding of the cognitive roles of such DG plasticity.

Long-lasting synaptic loss after repeated induction of LTD: independence to the means of LTD induction

Short- and long-lasting synaptic plasticity is assumed to be the cellular basis of short- and long-lasting memory, respectively. However, the cellular consequences leading to the long-lasting synaptic plasticity, assumed to include the processes of synapse formation and elimination, remain unknown. Using hippocampal slices maintained stably in culture, we found previously that the repeated induction of long-term potentiation (LTP) triggered a slowly developing long-lasting enhancement in synaptic transmission strength accompanied by synapse formation, which was separate from LTP itself. We recently reported a phenomenon apparently of a mirror-image effect. The repeated activations of metabotropic glutamate receptor (mGluR), which induces long-term depression (LTD), triggered a long-lasting reduction in synaptic strength accompanied by synapse elimination. To clarify whether the reported long-lasting effect was specific to the drugs used previously and whether the effect was specific to mGluR-mediated LTD, we exposed the cultured slices repeatedly to another Group I metabotropic glutamate receptor (mGluR) agonist, an N-methyl-d-aspartate receptor agonist, and a Na+/K+-pump inhibitor. All these treatments resulted in an equivalent long-lasting synaptic reduction/elimination when repeated three times, indicating that the repeated LTD induction leads to synapse elimination. The independence of synapse elimination to the means of LTD induction suggests that the signals leading to short-term plasticity and long-term plasticity are independent. Detailed inspections in the representative case of mGluR activation revealed that the reduction in synaptic strength developed with a approximately 1-week delay from the decrease in the number of synaptic structures. This synapse elimination should be unique as it is activity-dependent rather than inactivity-dependent.

Role of cellular prion protein on LTP expression in aged mice

Cellular prion protein (PrP(c)) has been associated with some physiological functions in the last few years. In a previous paper, we have demonstrated an increased hippocampal synaptic transmission in adult mice lacking this protein. In the present study, we investigate the impact of aging on the generation and maintenance of hippocampal long-term Potentiation (LTP) in 9-month-old mice devoid of PrP(c) protein (Prnp(0/0)). We observed a lower threshold for inducing LTP in 9-month-old Prnp(0/0) mice compared to wild-type ones at the same age. The maintenance of dentate gyrus LTP was more persistent in hippocampal slices from Prnp(0/0) mice. Furthermore, the expression of mRNA for NR2A and NR2B subunits of the NMDA glutamatergic receptor in hippocampus of aged Prnp(0/0) animals showed an increase compared to the wild type. We propose that increased hippocampal glutamatergic transmission in Prnp(0/0) mice is related to the enhanced plasticity and persistence of the dentate LTP.

Induction and activity-dependent reversal of persistent LTP and LTD in lateral perforant path synapses in vivo

The reversibility of long-term potentiation (LTP) and heterosynaptic long-term depression (LTD) lasting weeks was examined in the lateral perforant path of freely moving adult Sprague-Dawley rats. LTP lasting weeks was rapidly reversed within minutes by high-frequency heterosynaptic stimulation of the medial perforant path, in an N-methyl-D-aspartate receptor-dependent manner. LTP reversal also occurred, albeit more slowly and to a lesser extent, when animals were given 1-3 weeks of overnight exposure to an enriched environment (EE). LTD likewise was reversed upon repeated EE exposure. A covert similarity between the degrees of LTP and LTD reversal was revealed when the small potentiation effect of EE treatment by itself on lateral path responses was taken into account. Despite its ability to reverse previously acquired synaptic plasticity, two weeks of EE treatment had no effect on animals' retention of the platform location in a spatial watermaze task, although it did facilitate new learning. These data are in agreement with the hypothesis that hippocampal synapses retain the capacity for rapid synaptic change even when otherwise relatively stable plasticity has previously been induced. Slow reversal of such plasticity did not correlate with a loss of memory retention, possibly because either slow changes permit reorganization of representations such that both old and new information can be accommodated, or else the new information is synaptically represented in orthogonal fashion to the old information.

NMDA receptor antagonists block heterosynaptic long-term depression (LTD) but not long-term potentiation (LTP) in the CA3 region following lateral perforant path stimulation

High-frequency stimulation of lateral perforant path is accompanied by a heterosynaptic long-term depression (LTD) of medial perforant path synaptic responses in both the dentate gyrus and the CA3 region of the hippocampus. We reported previously that LTP induction at lateral perforant path-CA3 synapses is unaffected by NMDA antagonists. However, it is not known if heterosynaptic LTD that is observed in the CA3 region following lateral perforant path stimulation also is independent from NMDA receptors. We address this question in anesthetized adult rats using systemic administration of the competitive NMDA receptor antagonist CPP. Induction of lateral perforant path-CA3 LTP produced a sustained heterosynaptic depression of medial perforant path-CA3 responses. Systemic administration of CPP (10 mg/kg) was ineffective in blocking the induction of LTP at lateral perforant path-CA3 responses. However, heterosynaptic LTD of medial perforant path-CA3 responses was blocked completely by CPP. These data indicate that NMDA receptors are not required for the induction of lateral perforant path-CA3 LTP, but are involved in the induction of heterosynaptic LTD that accompanies lateral perforant path activity. The requirement for NMDA receptors for heterosynaptic LTD suggests one functional role of NMDA receptors at termination fields of the lateral perforant path.

Reinforcement of rat hippocampal LTP by holeboard training

Hippocampal long-term potentiation (LTP) can be dissociated in early-LTP lasting 4-5 h and late-LTP with a duration of more than 8 h, the latter of which requires protein synthesis and heterosynaptic activity during its induction. Previous studies in vivo have shown that early-LTP in the dentate gyrus can protein synthesis-dependently be transformed (reinforced) into late-LTP by the association of arousing novel environmental stimuli. Here we show that consolidation of spatial memory also reinforces early-LTP in the dentate gyrus. Both memory consolidation and LTP-reinforcement depend on protein synthesis. Four groups of animals were trained by five, seven, eight or 10 trials, respectively, to recognize a fixed pattern of baited holes. The last trial was performed 15 min after tetanus. Errors of long-term reference memory during the last trial were significantly decreased only in the eight- and 10-trial experimental groups compared to pseudo-trained animals. In correlation to this learning effect we found a reinforcement of early-LTP only in these experimental groups compared to controls. The data suggest that the synthesis of new proteins required for spatial reference-memory formation also contributes to LTP maintenance in the hippocampal dentate gyrus.

Modulation of memory processes and cellular excitability in the dentate gyrus of freely moving rats by a 5-HT4 receptors partial agonist, and an antagonist

Firstly, olfactory association learning was used to determine the modulating effect of 5-HT4 receptor involvement in learning and long-term memory. Secondly, the effects of systemic injections of a 5-HT4 partial agonist and an antagonist on long-term potentiation (LTP) and depotentiation in the dentate gyrus (DG) were tested in freely moving rats. The modulating role of the 5-HT4 receptors was studied by using a potent, 5-HT4 partial agonist RS 67333 [1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-n-butyl-4-piperidinyl)-1-propanone] and a selective 5-HT4 receptor antagonist RS 67532 [1-(4-amino-5-chloro-2-(3,5-dimethoxybenzyloxyphenyl)-5-(1-piperidinyl)-1-propanone]. Agonist or antagonist systemic chronic injections prior to five training sessions yielded a facilitatory effect on procedural memory during the first session only with the antagonist. Systemic injection of the antagonist only before the first training session improved procedural memory during the first session and associative memory during the second session. Similar injection with the 5-HT4 partial agonist had an opposite effect. The systemic injection of the 5-HT4 partial agonist prior to the induction of LTP in the dentate gyrus by high-frequency stimulation was followed by a population spike increase, while the systemic injection of the antagonist accelerated the depotentiation 48 h later. The behavioural and physiological results pointed out the involvement of 5-HT4 receptors in processing related to the long-term hippocampal-dependent memory system, and suggest that specific 5-HT4 agonists could be used to treat amnesic patients with a dysfunction in this particular system.

Reversal and consolidation of activity-induced synaptic modifications

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

Long-term synaptic morphometry changes after induction of long-term potentiation and long-term depression in the dentate gyrus of awake rats are not simply mirror phenomena

Mechanisms of expression of long-term synaptic plasticity are believed to involve morphological changes of the activated synapses and remodelling of connectivity. Here, we investigated changes in synaptic and neuronal parameters in the dentate gyrus 24 h after induction of long-term potentiation (LTP) and long-term depression (LTD) in awake rats. In dentate granule cells, tetanization of the medial or lateral perforant paths induces LTP in specific synaptic bands along the dendrites in the middle and outer molecular layers, respectively, and tetanization of the lateral path induces robust LTD heterosynaptically in the middle molecular layer. This functional segregation allowed us to assess morphological changes associated with LTP and LTD in each pathway in the same population of neurons. Electron microscopy and unbiased counting methods were used to estimate neuronal density, axospinous, axodendritic and perforated synapse density, multiple synapse bouton density and postsynaptic density (PSD) area. Whereas there was no change in neuronal density, PSD area and multiple synapse boutons 24 h after either LTP or LTD, there was a noninput-specific increase in unperforated axospinous synapses after both LTP and LTD. However, we found that LTP of the medial, but not lateral, perforant path is associated with a specific increase in perforated axospinous synapses in the potentiated area. We also show that heterosynaptic LTD is associated with an input-specific increase in axodendritic synapse density. These results suggest that each perforant pathway may differ with respect to the nature of LTP-induced long-term changes and show that morphologically LTD is not simply the converse of LTP.

Effects of stimulus frequency and age on bidirectional synaptic plasticity in the dentate gyrus of freely moving rats

We investigated the frequency-dependent transition from homosynaptic long-term depression (LTD) to long-term potentiation (LTP) at the lateral perforant pathway/dentate gyrus synapse in adult (90 days of age) and immature (15 days of age) awake, freely moving rats. Dentate-evoked field potentials were recorded and analyzed using the population spike amplitude and the field EPSP slope measures following sustained stimulation (900 pulses) of the lateral perforant pathway at various frequencies (1, 3, 7, 30, 50, or 200 Hz). Our results indicate that both the strength and the direction (LTP or LTD) of synaptic plasticity vary as a function of activation frequency: sustained low-frequency stimulation ranging from 1 to 7 Hz results in depression of activated synapses, whereas high-frequency stimulation (30-200 Hz) produces potentiation. In addition, a significant (P < 0.01) ontogenetic shift in the frequency of transition from LTD to LTP was observed; the transition frequency in immature animals was significantly lower than that obtained in adult animals. These observations agree strongly with the prediction of the Bienenstock-Cooper-Munro theory of synapse modification, indicating perhaps a neurophysiological basis for this theoretical model of learning in the dentate gyrus of awake behaving rats.

Input and output stations of the entorhinal cortex: superficial vs. deep layers or lateral vs. medial divisions?

Based on the results of recent electrophysiological and anatomical studies, we argue that the classical division of the entorhinal cortex (EC) into a superficial layer input station and deep layer output station is no longer tenable. We point out that the anatomical data suggest that the medial and lateral divisions of EC are separate, and recent studies of the propagation of signals originating in the lateral olfactory tract and perirhinal cortex to the EC [J. Neurophysiol. 83 (2000) 1924-1931; Biella and de Curtis, 2000) indicate that the lateral division is the input station, and the medial division the output station for information processed in the hippocampus and subiculum.

Long-term potentiation in freely moving rats reveals asymmetries in thalamic and cortical inputs to the lateral amygdala

Long-term memory underlying Pavlovian fear conditioning is believed to involve plasticity at sensory input synapses in the lateral nucleus of the amygdala (LA). A useful physiological model for studying synaptic plasticity is long-term potentiation (LTP). LTP in the LA has been studied only in vitro or in anaesthetized rats. Here, we tested whether LTP can be induced in auditory input pathways to the LA in awake rats, and if so, whether it persists over days. In chronically implanted rats, extracellular field potentials evoked in the LA by stimulation of the auditory thalamus and the auditory association cortex, using test simulations and input/output (I/O) curves, were compared in the same animals after tetanization of either pathway alone or after combined tetanization. For both pathways, LTP was input-specific and long lasting. LTP at cortical inputs exhibited the largest change at early time points (24 h) but faded within 3 days. In contrast, LTP at thalamic inputs, though smaller initially than cortical LTP, remained stable until at least 6 days. Comparisons of I/O curves indicated that the two pathways may rely on different mechanisms for the maintenance of LTP and may benefit differently from their coactivation. This is the first report of LTP at sensory inputs to the LA in awake animals. The results reveal important characteristics of synaptic plasticity in neuronal circuits of fear memory that could not have been revealed with in vitro preparations, and suggest a differential role of thalamic and cortical auditory afferents in long-term memory of fear conditioning.