Memory consolidation of Pavlovian fear conditioning: a cellular and molecular perspective

That dog is smarter than you know: advances in understanding canine learning, memory, and cognition

Tests of canine cognition are now receiving much deserved attention. Not only are dogs excellent models for human anxiety-related conditions and those involving brain aging, but how dogs learn and problem solve are interesting stand-alone topics. A number of factors can affect learning at the molecular level including stress or distress, factors that affect olfaction, effects of breed and nutritional factors including that may affect available brain energy. This review provides an overview of how these factors may affect baseline learning and brain aging.

Acquisition of Pavlovian fear conditioning using β-adrenoceptor activation of the dorsal premammillary nucleus as an unconditioned stimulus to mimic live predator-threat exposure

In the present work, we sought to mimic the internal state changes in response to a predator threat by pharmacologically stimulating the brain circuit involved in mediating predator fear responses, and explored whether this stimulation would be a valuable unconditioned stimulus (US) in an olfactory fear conditioning paradigm (OFC). The dorsal premammillary nucleus (PMd) is a key brain structure in the neural processing of anti-predatory defensive behavior and has also been shown to mediate the acquisition and expression of anti-predatory contextual conditioning fear responses. Rats were conditioned by pairing the US, which was an intra-PMd microinjection of isoproterenol (ISO; β-adrenoceptor agonist), with amyl acetate odor-the conditioned stimulus (CS). ISO (10 and 40 nmol) induced the acquisition of the OFC and the second-order association by activation of β-1 receptors in the PMd. Furthermore, similar to what had been found for contextual conditioning to a predator threat, atenolol (β-1 receptor antagonist) in the PMd also impaired the acquisition and expression of OFC promoted by ISO. Considering the strong glutamatergic projections from the PMd to the dorsal periaqueductal gray (dPAG), we tested how the glutamatergic blockade of the dPAG would interfere with the OFC induced by ISO. Accordingly, microinjections of NMDA receptor antagonist (AP5, 6 nmol) into the dPAG were able to block both the acquisition, and partially, the expression of the OFC. In conclusion, we have found that PMd β-1 adrenergic stimulation is a good model to mimic predatory threat-induced internal state changes, and works as a US able to mobilize the same systems involved in the acquisition and expression of predator-related contextual conditioning.

Early growth response gene 1 (Egr-1) is required for new and reactivated fear memories in the lateral amygdala

The immediate-early gene early growth response gene-1 (EGR-1, zif-268) has been extensively studied in synaptic plasticity and memory formation in a variety of memory systems. However, a convincing role for EGR-1 in amygdala-dependent memory consolidation processes has yet to emerge. In the present study, we have examined the role of EGR-1 in the consolidation and reconsolidation of amygdala-dependent auditory Pavlovian fear conditioning. In our first series of experiments, we show that EGR-1 is regulated following auditory fear conditioning in the lateral nucleus of the amygdala (LA). Next, we use antisense oligodeoxynucleotide (ODN) knockdown of EGR-1 in the LA to show that training-induced expression of EGR-1 is required for memory consolidation of auditory fear conditioning; that is, long-term memory (LTM) is significantly impaired while acquisition and short-term memory (STM) are intact. In a second set of experiments, we show that EGR-1 is regulated in the LA by retrieval of an auditory fear memory. We then show that retrieval-induced expression of EGR-1 in the LA is required for memory reconsolidation of auditory fear conditioning; that is, post-retrieval (PR)-LTM is significantly impaired while memory retrieval and PR-STM are intact. Additional experiments show these effects to be restricted to the LA, to be temporally graded, and unlikely to be due to nonspecific toxicity within the LA. Collectively, our findings strongly implicate a role for EGR-1 in both the initial consolidation and in the reconsolidation of auditory fear memories in the LA.

Interaction between N-ethylmaleimide-sensitive factor and GluR2 is essential for fear memory formation in lateral amygdala

Long-term memory formation is believed to involve alterations of synaptic efficacy. It has been shown that GluR1-containing AMPA receptors are inserted into synapses following stimuli leading to plasticity and that GluR2/GluR3-containing receptors replace existing synaptic AMPA receptors continuously and may act to maintain synaptic efficacy. Maintaining GluR2/GluR3 receptors level in synapse requires interactions of N-ethylmaleimide-sensitive factor (NSF) with GluR2. To assess possible roles of NSF-GluR2 interaction in rat lateral amygdala (LA) in fear memory formation we used a specific GluR2-NSF interaction inhibitory peptide (pep-R845A). This inhibitory peptide, composed of a modified NSF binding site of GluR2, was previously shown to interact specifically with NSF and to affect AMPA-mediated synaptic efficacy. The inhibitory peptide was linked to a TAT peptide (TAT-pep-R845A) to facilitate internalization into LA cells. Infusion of the TAT-pep-R845A inhibitory peptide into LA 30 min before fear conditioning led to a significant impairment of long-term fear memory formation. In contrast, the control TAT peptide alone had no effect on fear memory. Injection of TAT-pep-R845A peptide into LA had no effect on short-term fear memory. In addition, the inhibitory peptide had no effect on memory retrieval when injected into LA 30 min before fear memory test. Furthermore, maintenance of memory was not impaired when the peptide was injected 24 h after fear conditioning and fear memory was tested 48 h afterward. These results show that GluR2-NSF interaction in LA is necessary for fear memory consolidation but not retrieval or persistence.

Amygdala regulates risk of predation in rats foraging in a dynamic fear environment

In a natural environment, foragers constantly face the risk of encountering predators. Fear is a defensive mechanism evolved to protect animals from danger by balancing the animals' needs for primary resources with the risk of predation, and the amygdala is implicated in mediating fear responses. However, the functions of fear and amygdala in foraging behavior are not well characterized because of the technical difficulty in quantifying prey-predator interaction with real (unpredictable) predators. Thus, the present study investigated the rat's foraging behavior in a seminaturalistic environment when confronted with a predator-like robot programmed to surge toward the animal seeking food. Rats initially fled into the nest and froze (demonstrating fear) and then cautiously approached and seized the food as a function of decreasing nest-food and increasing food-robot distances. The likelihood of procuring food increased and decreased via lesioning/inactivating and disinhibiting the amygdala, respectively. These results indicate that the amygdala bidirectionally regulates risk behavior in rats foraging in a dynamic fear environment.

Homeostatic switch in hebbian plasticity and fear learning after sustained loss of Cav1.2 calcium channels

Ca(2+) influx through postsynaptic Ca(v)1.x L-type voltage-gated channels (LTCCs) is particularly effective in activating neuronal biochemical signaling pathways that might be involved in Hebbian synaptic plasticity (i.e., long-term potentiation and depression) and learning and memory. Here, we demonstrate that Ca(v)1.2 is the functionally relevant LTCC isoform in the thalamus-amygdala pathway of mice. We further show that acute pharmacological block of LTCCs abolishes Hebbian plasticity in the thalamus-amygdala pathway and impairs the acquisition of conditioned fear. On the other hand, chronic genetic loss of Ca(v)1.2 triggers a homeostatic change of the synapse, leading to a fundamental alteration of the mechanism of Hebbian plasticity by synaptic incorporation of Ca(2+)-permeable, GluA2-lacking AMPA receptors. Our results demonstrate for the first time the importance of the Ca(v)1.2 LTCC subtype in synaptic plasticity and fear memory acquisition.

Temporal modulation of spike-timing-dependent plasticity

Spike-timing-dependent plasticity (STDP) has attracted considerable experimental and theoretical attention over the last decade. In the most basic formulation, STDP provides a fundamental unit – a spike pair – for quantifying the induction of long-term changes in synaptic strength. However, many factors, both pre- and postsynaptic, can affect synaptic transmission and integration, especially when multiple spikes are considered. Here we review the experimental evidence for multiple types of nonlinear temporal interactions in STDP, focusing on the contributions of individual spike pairs, overall spike rate, and precise spike timing for modification of cortical and hippocampal excitatory synapses. We discuss the underlying processes that determine the specific learning rules at different synapses, such as postsynaptic excitability and short-term depression. Finally, we describe the success of efforts toward building predictive, quantitative models of how complex and natural spike trains induce long-term synaptic modifications.

Synaptic plasticity and NO-cGMP-PKG signaling coordinately regulate ERK-driven gene expression in the lateral amygdala and in the auditory thalamus following Pavlovian fear conditioning

We have recently hypothesized that NO-cGMP-PKG signaling in the lateral nucleus of the amygdala (LA) during auditory fear conditioning coordinately regulates ERK-driven transcriptional changes in both auditory thalamic (MGm/PIN) and LA neurons that serve to promote pre- and postsynaptic alterations at thalamo-LA synapses, respectively. In the present series of experiments, we show that N-methyl-D-aspartate receptor (NMDAR)-driven synaptic plasticity and NO-cGMP-PKG signaling in the LA regulate the training-induced expression of ERK and the ERK-driven immediate early genes (IEGs) Arc/Arg3.1, c-Fos, and EGR-1 in the LA and the MGm/PIN. Rats receiving intra-LA infusion of the NR2B selective antagonist Ifenprodil, the NOS inhibitor 7-Ni, or the PKG inhibitor Rp-8-Br-PET-cGMPS exhibited significant decreases in ERK activation and in the training-induced expression of all three IEGs in the LA and MGm/PIN while intra-LA infusion of the PKG activator 8-Br-cGMP had the opposite effect. Remarkably, those rats given intra-LA infusion of the membrane impermeable NO scavenger c-PTIO exhibited significant decreases in ERK activation and ERK-driven IEG expression in the MGm/PIN, but not in the LA. Together with our previous experiments, these results suggest that synaptic plasticity and the NO-cGMP-PKG signaling pathway promote fear memory consolidation, in part, by regulating ERK-driven transcription in both the LA and the MGm/PIN. They further suggest that synaptic plasticity in the LA during fear conditioning promotes ERK-driven transcription in MGm/PIN neurons via NO-driven "retrograde signaling."

Understanding the cognitive consequences of critical illness through experimental animal models

Tuon and colleagues have developed an animal model to examine the impact of sepsis on memory in rats. They report important data that expand the understanding of the cognitive consequences of critical illness. Future research should follow this path of inquiry and extend animal models beyond aversive conditioning to include recently developed paradigms that will permit assessment of complex and cognitive processes, such as attention, episodic memory and orientation to time and place. This has the potential to greatly increase the putative understanding of the homologous neurocognitive dysfunctions acquired during critical illness.

New findings on extinction of conditioned fear early in development: theoretical and clinical implications

Research with adult animals suggests that extinction depends, at least partly, on new inhibitory learning that is specific to the context in which it is learned. However, several recent studies show that extinction processes are dissociated across development. The present article reviews research on the behavioral and neurobiological mechanisms underlying extinction in developing rats. To summarize, postweanling aged rats (i.e., 24-day-olds) display adult-like extinction in that they show renewal, reinstatement, spontaneous recovery, and compound summation of extinguished stimuli. However, preweanling aged rats (i.e., 17-day-olds) do not show any of those behavioral phenomena. Pharmacological studies also show that reducing N-methyl-D-aspartate, gamma-aminobutryic acid, and opioid neurotransmission impairs extinction in 24-day-old rats, but extinction in P17 rats is only affected by the blocking of opioid neurotransmission. Lastly, extinction in 24-day-old rats involves the amygdala and the ventromedial prefrontal cortex (vmPFC), which are critical brain areas in the neural circuitry of extinction in adult rats. Interestingly, extinction in 17-day-old rats involves the amygdala but not the vmPFC. The existing models of extinction cannot account for these developmental differences. These findings showing that distinct processes mediate extinction at different stages of development may have significant clinical implications, which are discussed in this review.

Early-life stress disrupts attachment learning: the role of amygdala corticosterone, locus ceruleus corticotropin releasing hormone, and olfactory bulb norepinephrine

Infant rats require maternal odor learning to guide pups' proximity-seeking of the mother and nursing. Maternal odor learning occurs using a simple learning circuit including robust olfactory bulb norepinephrine (NE), release from the locus ceruleus (LC), and amygdala suppression by low corticosterone (CORT). Early-life stress increases NE but also CORT, and we questioned whether early-life stress disrupted attachment learning and its neural correlates [2-deoxyglucose (2-DG) autoradiography]. Neonatal rats were normally reared or stressed-reared during the first 6 d of life by providing the mother with insufficient bedding for nest building and were odor-0.5 mA shock conditioned at 7 d old. Normally reared paired pups exhibited typical odor approach learning and associated olfactory bulb enhanced 2-DG uptake. However, stressed-reared pups showed odor avoidance learning and both olfactory bulb and amygdala 2-DG uptake enhancement. Furthermore, stressed-reared pups had elevated CORT levels, and systemic CORT antagonist injection reestablished the age-appropriate odor-preference learning, enhanced olfactory bulb, and attenuated amygdala 2-DG. We also assessed the neural mechanism for stressed-reared pups' abnormal behavior in a more controlled environment by injecting normally reared pups with CORT. This was sufficient to produce odor aversion, as well as dual amygdala and olfactory bulb enhanced 2-DG uptake. Moreover, we assessed a unique cascade of neural events for the aberrant effects of stress rearing: the amygdala-LC-olfactory bulb pathway. Intra-amygdala CORT or intra-LC corticotropin releasing hormone (CRH) infusion supported aversion learning with intra-LC CRH infusion associated with increased olfactory bulb NE (microdialysis). These results suggest that early-life stress disturbs attachment behavior via a unique cascade of events (amygdala-LC-olfactory bulb).

Amygdala mechanisms of Pavlovian psychostimulant conditioning and relapse

Psychostimulant addiction often consists of periods of sustained drug abstinence disrupted by periods of relapse and renewed heavy drug use. Prevention of relapse remains the greatest challenge to the successful treatment of drug addiction. Drug-associated cues are a primary trigger for relapse, as they can elicit intense craving for the drug. These cues become associated with the drug reward through Pavlovian learning processes that develop over multiple drug-cue pairings. The amygdala (AMY) is critical for such drug-related learning. Intrinsic and extrinsic circuitry position the AMY to integrate cue and drug-related information and influence drug-seeking and drug-taking behaviors. Animal models of conditioned drug reward, drug use, and relapse have confirmed the necessary role of the AMY for drug conditioned cues to control motivated behavior. Neurons within the AMY are responsive to the primary effects of psychostimulants, and more critically, they also respond to the presentation of drug-associated cues. The mechanisms by which conditioned cues come to influence drug-seeking behavior likely involve long-term plasticity and neuroadaptations within the AMY. A greater understanding of the associative learning mechanisms that depend upon the AMY and related limbic and cortical structures, and the process by which drug cues come to gain control over behavior that maintains the addictive state, will facilitate the development of more effective addiction treatments.

Long-term synaptic changes in two input pathways into the lateral nucleus of the amygdala underlie fear extinction

Plasticity in two input pathways into the lateral nucleus of the amygdala (LA), the medial prefrontal cortex (mPFC) and the sensory thalamus, have been suggested to underlie extinction, suppression of a previously acquired conditioned response (CR) following repeated presentations of the conditioned stimulus (CS). However, little is known about the joint dynamics of the relevant synaptic changes within the LA that accompany fear extinction. Employing a novel training procedure, in which stimulation of the medial geniculate nucleus (MGm) of the thalamus served as the CS, we tested necessary and sufficient conditions for extinction in anesthetized rats. Repeatedly applying the brain-stimulation CS was neither sufficient to produce activation of the mPFC nor behavioral extinction when the animal was under anesthesia. Only when the CS was combined with contingent stimulation of the infralimbic cortex (IL) of the mPFC was the CR markedly reduced, emulating extinction. To elucidate the nature of synaptic alterations linking the extinction procedure with CR suppression, evoked field potentials to IL and MGm stimulations were recorded in the LA. The results showed that paired stimulations of the IL and MGm significantly enhanced the neural response at the IL-LA synapses and reversed conditioning-induced synaptic potentiation at the MGm-LA synapses. Taken together, our results provide strong evidence that dual plasticity within the LA underlies suppression of conditioned fear response following extinction.

Identification of plasticity-associated genes regulated by Pavlovian fear conditioning in the lateral amygdala

Most recent studies aimed at defining the cellular and molecular mechanisms of Pavlovian fear conditioning have focused on protein kinase signaling pathways and the transcription factor cAMP-response element binding protein (CREB) that promote fear memory consolidation in the lateral nucleus of the amygdala (LA). Despite this progress, there still remains a paucity of information regarding the genes downstream of CREB that are required for long-term fear memory formation in the LA. We have adopted a strategy of using microarray technology to initially identify genes induced within the dentate gyrus following in vivo long-term potentiation (LTP) followed by analysis of whether these same genes are also regulated by fear conditioning within the LA. In the present study, we first identified 34 plasticity-associated genes that are induced within 30 min following LTP induction utilizing a combination of DNA microarray, qRT-PCR, and in situ hybridization. To determine whether these genes are also induced in the LA following Pavlovian fear conditioning, we next exposed rats to an auditory fear conditioning protocol or to control conditions that do not support fear learning followed by qRT-PCR on mRNA from microdissected LA samples. Finally, we asked whether identified genes induced by fear learning in the LA are downstream of the extracellular-regulated kinase/mitogen-activated protein kinase signaling cascade. Collectively, our findings reveal a comprehensive list of genes that represent the first wave of transcription following both LTP induction and fear conditioning that largely belong to a class of genes referred to as 'neuronal activity dependent genes' that are likely calcium, extracellular-regulated kinase/mitogen-activated protein kinase, and CREB-dependent.

Impaired long-term potentiation and enhanced neuronal excitability in the amygdala of Ca(V)1.3 knockout mice

Previously, we demonstrated that mice in which the gene for the L-type voltage-gated calcium channel Ca(V)1.3 is deleted (Ca(V)1.3 knockout mice) exhibit an impaired ability to consolidate contextually-conditioned fear. Given that this form of Pavlovian fear conditioning is critically dependent on the basolateral complex of the amygdala (BLA), we were interested in the mechanisms by which Ca(V)1.3 contributes to BLA neurophysiology. In the present study, we used in vitro amygdala slices prepared from Ca(V)1.3 knockout mice and wild-type littermates to explore the role of Ca(V)1.3 in long-term potentiation (LTP) and intrinsic neuronal excitability in the BLA. We found that LTP in the lateral nucleus (LA) of the BLA, induced by high-frequency stimulation of the external capsule, was significantly reduced in Ca(V)1.3 knockout mice. Additionally, we found that BLA principal neurons from Ca(V)1.3 knockout mice were hyperexcitable, exhibiting significant increases in firing rates and decreased interspike intervals in response to prolonged somatic depolarization. This aberrant increase in neuronal excitability appears to be at least in part due to a concomitant reduction in the slow component of the post-burst afterhyperpolarization. Together, these results demonstrate altered neuronal function in the BLA of Ca(V)1.3 knockout mice which may account for the impaired ability of these mice to consolidate contextually-conditioned fear.

Temporary inactivation of the dorsal entorhinal cortex impairs acquisition and retrieval of spatial information

We tested the effects of temporary inactivation of the dorsal entorhinal cortex on spatial discrimination using a conditioned cue preference (CCP) paradigm. The three phases of the procedure were: pre-exposure: unreinforced exploration of the center platform and two adjacent arms of an eight-arm radial maze; training: rats were confined to the ends of the two arms on alternate days - one arm always contained food and the other never contained food; testing: unreinforced exploration of the center platform and the two arms. Rats that received bilateral infusions of saline into the dorsal entorhinal cortex before the training trials or before the test trial spent significantly more time in the arm that previously contained food than in the arm that never contained food, demonstrating that they had acquired and were able to express information that discriminated between the two adjacent maze arms. In contrast, rats that received bilateral, intra-entorhinal infusions of muscimol, a gamma-aminobutyric acid(a) (GABA(a)) agonist, before either training or testing spent equal amounts of time in the two arms, indicating that they failed to acquire and were unable to express this information. Interactions between the entorhinal cortex and hippocampus in the acquisition and expression of the information required for this discrimination are discussed.

Opioid receptors regulate retrieval of infant fear memories: effects of naloxone on infantile amnesia

The authors examined the role of the endogenous opioid system in infantile amnesia for contextual fear conditioning. Rats that were 18 days of age received an aversive footshock in a novel context. Rats displayed conditioned fear when tested 1 min after training but not 24 hr after training. Systemic injection of the opioid receptor antagonist naloxone prior to test, but not immediately after training, alleviated infantile amnesia. Naloxone also alleviated infantile amnesia when injected prior to test 7 days after training. These effects of naloxone were due to actions on central rather than peripheral opioid receptors and were not due to any tendency of the drug to produce fear or freezing. These results show that central opioid receptors regulate retrieval of fear memories in infant rats.

The amygdala is not necessary for unconditioned stimulus inflation after Pavlovian fear conditioning in rats

The basolateral complex (BLA) and central nucleus (CEA) of the amygdala play critical roles in associative learning, including Pavlovian conditioning. However, the precise role for these structures in Pavlovian conditioning is not clear. Recent work in appetitive conditioning paradigms suggests that the amygdala, particularly the BLA, has an important role in representing the value of the unconditioned stimulus (US). It is not known whether the amygdala performs such a function in aversive paradigms, such as Pavlovian fear conditioning in rats. To address this issue, Experiments 1 and 2 used temporary pharmacological inactivation of the amygdala prior to a US inflation procedure to assess its role in revaluing shock USs after either overtraining (Experiment 1) or limited training (Experiment 2), respectively. Inactivation of the BLA or CEA during the inflation session did not affect subsequent increases in conditioned freezing observed to either the tone conditioned stimulus (CS) or the conditioning context in either experiment. In Experiment 3, NBQX infusions into the BLA impaired the acquisition of auditory fear conditioning with an inflation-magnitude US, indicating that the amygdala is required for associative learning with intense USs. Together, these results suggest that the amygdala is not required for revaluing an aversive US despite being required for the acquisition of fear to that US.

Chronic infusion of PCP via osmotic mini-pumps: a new rodent model of cognitive deficit in schizophrenia characterized by impaired attentional set-shifting (ID/ED) performance

The identification of animal disease-like models for cognitive symptoms in schizophrenia is of central importance to the successful development of pharmacological therapies for psychosis resulting in a functional outcome in patients. Executive function is one of the most severely affected cognitive domains in schizophrenia that remains inadequately treated by existing therapies. The rat attentional set-shifting (or intra-dimensional-extra-dimensional (ID/ED)) task has been developed to test executive function in rodents and successful translation of pre-clinical data into the clinical setting now depends on the identification of a predictive animal disease-like model. The present study investigates whether a continuous 14-day mini-pump infusion of the non-competitive NMDA receptor antagonist phencyclidine (PCP) leads to a deficit in the ID/ED task, and subsequently evaluates the effect of modafinil in this model. Lister hooded rats were implanted subcutaneously with osmotic mini-pumps containing saline or PCP (15 mg/kg/day) for 14 days followed by a 7-day drug-free recovery phase. Rats were then tested in the ID/ED task following an acute injection of either vehicle or modafinil. PCP-treated animals displayed a selective deficit at the ED shift stage resembling that observed in schizophrenic patients. This deficit was reversed by an acute injection of modafinil. The PCP-induced impairment and its reinstatement by modafinil are quantitatively and qualitatively similar to that described earlier by our group following sub-chronic intraperitoneal PCP administration, indicative that sub-chronic PCP infusion via osmotic mini-pumps may represent an attractive alternative to the systemic administration protocols generally employed to date.

TrkB modulates fear learning and amygdalar synaptic plasticity by specific docking sites

Understanding the modulation of the neural circuitry of fear is clearly one of the most important aims in neurobiology. Protein phosphorylation in response to external stimuli is considered a major mechanism underlying dynamic changes in neural circuitry. TrkB (Ntrk2) neurotrophin receptor tyrosine kinase potently modulates synaptic plasticity and activates signal transduction pathways mainly through two phosphorylation sites [Y515/Shc site; Y816/PLCgamma (phospholipase Cgamma) site]. To identify the molecular pathways required for fear learning and amygdalar synaptic plasticity downstream of TrkB, we used highly defined genetic mouse models carrying single point mutations at one of these two sites (Y515F or Y816F) to examine the physiological relevance of pathways activated through these sites for pavlovian fear conditioning (FC), as well as for synaptic plasticity as measured by field recordings obtained from neurons of different amygdala nuclei. We show that a Y816F point mutation impairs acquisition of FC, amygdalar synaptic plasticity, and CaMKII signaling at synapses. In contrast, a Y515F point mutation affects consolidation but not acquisition of FC to tone, and also alters AKT signaling. Thus, TrkB receptors modulate specific phases of fear learning and amygdalar synaptic plasticity through two main phosphorylation docking sites.

Bilateral phosphorylation of ERK in the lateral and centrolateral amygdala during unilateral storage of fear memories

We previously showed that when rats were trained to fear an auditory conditioned stimulus (CS) by pairing it with a mild unilateral shock to the eyelid (the unconditioned stimulus, or US), conditioned freezing depended upon the amygdala contralateral but not ipsilateral from the US. It was proposed that convergent activation of amygdala neurons by the CS and US occurred mainly in the amygdala contralateral from US delivery, causing memories of the CS-US association to be stored primarily by that hemisphere. In the present study, we further tested this interpretation by administering unilateral infusions of U0126 (in 50% dimethyl sulfoxide (DMSO) vehicle) to block phosphorylation of extracellular signal-responsive kinase (ERK) in the amygdala prior to CS-US pairings. Conditioned freezing was impaired 24 h after training when U0126 was infused contralaterally-but not ipsilaterally-from the US, suggesting that fear memories were consolidated mainly by the contralateral amygdala. However, immunostaining experiments revealed that ERK phosphorylation was elevated in both hemispheres of the amygdale's lateral (LA) and centrolateral (CeL) nuclei after paired (but not unpaired (UNP)) presentations of the CS and US. Thus, fear acquisition induced ERK phosphorylation bilaterally in the amygdala, even though the ipsilateral hemisphere did not appear to participate in conditioned freezing. These findings suggest that associative plasticity may occur in both amygdala hemispheres even when only one hemisphere is involved in freezing behavior. Conditioning-induced ERK phosphorylation was identical in both hemispheres of LA, but was slightly greater in the contralateral than ipsilateral hemisphere of CeL. Hence, asymmetric induction of plasticity in CeL might help to explain why conditioned freezing depends preferentially upon the amygdala contralateral from the US in our fear conditioning paradigm.

Fear conditioning induces distinct patterns of gene expression in lateral amygdala

The lateral nucleus of the amygdala (LA) has been implicated in the formation of long-term associative memory (LTM) of stimuli associated with danger through fear conditioning. The current study aims to detect genes that are expressed in LA following associative fear conditioning. Using oligonucleotide microarrays, we monitored gene expression in rats subjected to paired training where a tone co-terminates with a footshock, or unpaired training where the tone and footshock are presented in a non-overlapping manner. The paired protocol consistently leads to auditory fear conditioning memory formation, whereas the unpaired protocol does not. When the paired group was compared with the unpaired group 5 h after training, the expression of genes coding for the limbic system-associated membrane protein (Lsamp), kinesin heavy chain member 2 (Kif2), N-ethylmaleimide-sensitive fusion protein (NSF) and Hippocalcin-like 4 protein (Hpcal4) was higher in the paired group. These genes encode proteins that regulate neuronal axonal morphology (Lsamp, Kif2), presynaptic vesicle cycling and release (Hpcal4 and NSF), and AMPA receptor maintenance in synapses (NSF). Quantitative real-time PCR (qPCR) showed that Kif2 and Lsamp are expressed hours following fear conditioning but minutes after unpaired training. Hpcal4 is induced by paired stimulation only 5 h after the training. These results show that fear conditioning induces a unique temporal activation of molecular pathways involved in regulating synaptic transmission and axonal morphology in LA, which is different from non-associative stimulation.

Cellular and systems mechanisms of memory strength as a constraint on auditory fear reconsolidation

Memory reconsolidation has been demonstrated in various tasks and species, suggesting it is a fundamental process. However, there are experimental parameters that can inhibit reconsolidation from occurring (boundary conditions). These conditions and their mechanisms remain poorly defined. Here, we characterize the ability of strong training to inhibit reconsolidation at the behavioral, systems and molecular levels. We demonstrate that strong memories in rats initially are resistant to reconsolidation, but after sufficient time will undergo reconsolidation, suggesting that boundary conditions can be transient. At the systems level, we show that the hippocampus is necessary for inhibiting reconsolidation in the amygdala. At the molecular level, we demonstrate that NR2B NMDA-receptor subunits which are critical for the induction of reconsolidation of auditory memories in the amygdala, are downregulated only under conditions when strong memories do not undergo reconsolidation. This suggests that one molecular mechanism for mediating boundary conditions is through downregulation of reconsolidation induction mechanisms.

Neuromodulatory property of standardized extract Ginkgo biloba L. (EGb 761) on memory: behavioral and molecular evidence

Although it has been suggested that the standardized Ginkgo biloba leaf extract (Egb 761) may have a beneficial effect on memory, the cellular and molecular changes that underlie this process are not yet well defined. The present study evaluated the effects of acute (one dose) or subacute treatments (one daily dose/seven days) with EGb 761 (0.5 g kg(-1) and 1.0 g kg(-1)) on rats submitted to a conditioned emotional response (CER) in comparison with positive (4 mg kg(-1) Diazepam) and negative (12%Tween 80) control groups. To this end, eighty (n=10/group) adult, male, Wistar rats (+/-250-300 g) were used in an off-baseline CER procedure. We here observed that the rats submitted to an acute and subacute EGb 761 treatments had acquisition of fear conditioning. Additionally, we investigate if the expression of genes previously associated with classical conditioning (CREB-1 and GAP-43) and new candidate genes (GFAP) are modulated following EGb 761 acute treatment. CREB-1, GAP-43 and GFAP mRNA and protein expressions were evaluated using both quantitative PCR (qPCR) and immunohistochemical analysis, respectively. We here show, for the first time, that EGb 761 modulated GAP-43, CREB-1 and GFAP expression in the prefrontal cortex, amygdala and hippocampus. We observed an underexpression of GAP-43 in all structures evaluated and over-expression of GFAP in the amygdala and hippocampus following acute G. biloba treatment when compared to control group (Tween; p<0.01). GAP-43 expression was decreased in prefrontal cortex and hippocampus in the subacute treatment with EGb 761. Subacute treatment with EGb 761 lead to a decreased CREB-1 in mPFC (p<0.001) and increased in the hippocampus to 1.0 g kg(-1)G. biloba group (p<0.001). The results obtained from immunohistochemical analysis support our aforementioned findings and revealed that the changes in expression occurred within specific regions in the areas evaluated. All together, our findings not only provide new evidence for a role of EGb 761 on memory but also identify molecular changes that underlie the fear memory consolidation.

Signaling through cGMP-dependent protein kinase I in the amygdala is critical for auditory-cued fear memory and long-term potentiation

Long-term potentiation (LTP) of inputs relaying sensory information from cortical and thalamic neurons to principal neurons in the lateral amygdala (LA) is thought to serve as a cellular mechanism for associative fear learning. Nitric oxide (NO), a messenger molecule widely implicated in synaptic plasticity and behavior, has been shown to enhance LTP in the LA as well as consolidation of associative fear memory. Additional evidence suggests that NO-induced enhancement of LTP and amygdala-dependent learning requires signaling through soluble guanylyl cyclase (sGC) and cGMP-dependent protein kinase (cGK). Mammals possess two genes for cGK: the prkg1 gene gives rise to the cGK type I isoforms, cGKIalpha and cGKIbeta, and the prkg2 gene encodes the cGK type II. Reportedly, both cGKI and cGKII are expressed in the amygdala, and cGKII is involved in controlling anxiety-like behavior. Because selective pharmacological tools for individual cGK isoforms are lacking, we used different knock-out mouse models to examine the function of cGKI and cGKII for LTP in the LA and pavlovian fear conditioning. We found robust expression of the cGKI specifically in the LA with cGKIbeta as the prevailing isoform. We further show a marked reduction of LTP at both thalamic and cortical inputs to the LA and a selective impairment of auditory-cued fear memory in cGKI-deficient mutants. In contrast, cGKII null mutants lack these phenotypes. Our data suggest a function of cGKI, likely the beta isoform, in the LA, supporting synaptic plasticity and consolidation of fear memory.

Towards a theory of chronic pain

In this review, we integrate recent human and animal studies from the viewpoint of chronic pain. First, we briefly review the impact of chronic pain on society and address current pitfalls of its definition and clinical management. Second, we examine pain mechanisms via nociceptive information transmission cephalad and its impact and interaction with the cortex. Third, we present recent discoveries on the active role of the cortex in chronic pain, with findings indicating that the human cortex continuously reorganizes as it lives in chronic pain. We also introduce data emphasizing that distinct chronic pain conditions impact on the cortex in unique patterns. Fourth, animal studies regarding nociceptive transmission, recent evidence for supraspinal reorganization during pain, the necessity of descending modulation for maintenance of neuropathic behavior, and the impact of cortical manipulations on neuropathic pain is also reviewed. We further expound on the notion that chronic pain can be reformulated within the context of learning and memory, and demonstrate the relevance of the idea in the design of novel pharmacotherapies. Lastly, we integrate the human and animal data into a unified working model outlining the mechanism by which acute pain transitions into a chronic state. It incorporates knowledge of underlying brain structures and their reorganization, and also includes specific variations as a function of pain persistence and injury type, thereby providing mechanistic descriptions of several unique chronic pain conditions within a single model.

Pavlovian fear conditioning as a behavioral assay for hippocampus and amygdala function: cautions and caveats

Pavlovian fear conditioning has become an important model for investigating the neural substrates of learning and memory in rats, mice and humans. The hippocampus and amygdala are widely believed to be essential for fear conditioning to contexts and discrete cues, respectively. Indeed, this parsing of function within the fear circuit has been used to leverage fear conditioning as a behavioral assay of hippocampal and amygdala function, particularly in transgenic mouse models. Recent work, however, blurs the anatomical segregation of cue and context conditioning and challenges the necessity for the hippocampus and amygdala in fear learning. Moreover, nonassociative factors may influence the performance of fear responses under a variety of conditions. Caution must therefore be exercised when using fear conditioning as a behavioral assay for hippocampal- and amygdala-dependent learning.

Challenges in developing novel treatments for childhood disorders: lessons from research on anxiety

Alterations in brain development may contribute to chronic mental disorders. Novel treatments targeted toward the early-childhood manifestations of such chronic disorders may provide unique therapeutic opportunities. However, attempts to develop and deliver novel treatments face many challenges. Work on pediatric anxiety disorders illustrates both the inherent challenges as well as the unusual opportunities for therapeutic advances. The present review summarizes three aspects of translational research on pediatric anxiety disorders as the work informs efforts to develop novel interventions. First, the review summarizes data on developmental conceptualizations of anxiety from both basic neuroscience and clinical perspectives. This summary is integrated with a discussion of the two best-established treatments, cognitive behavioral therapy and selective serotonin reuptake inhibitors. Second, the review summarizes work on attention bias to threat, considering implications for both novel treatments and translational research on neural circuitry functional development. This illustrates the manner in which clinical findings inform basic systems neuroscience research. Finally, the review summarizes work in basic science on fear learning, as studied in fear conditioning, consolidation, and extinction paradigms. This summary ends by describing potential novel treatments, illustrating the manner in which basic neuroscience informs therapeutics.

Activation of ERK/MAPK in the lateral amygdala of the mouse is required for acquisition of a fear-potentiated startle response

There is considerable interest in examining the genes that may contribute to anxiety. We examined the function of ERK/MAPK in the acquisition of conditioned fear, as measured by fear-potentiated startle (FPS) in mice as a model for anticipatory anxiety in humans. We characterized the following for the first time in the mouse: (1) the expression of the ERK/MAPK signaling pathway components at the protein level in the lateral amygdala (LA); (2) the time course of activation of phospho-activated MAPK in the LA after fear conditioning; (3) if pharmacological inhibition of pMAPK could modulate the acquisition of FPS; (4) the cell-type specificity of pMAPK in the LA after fear conditioning. Using western blot and immunohistochemistry techniques and injecting the MEK inhibitor U0126 in the LA, we showed the following: (1) both MEK1/MEK2 and ERK1/ERK2 were co-expressed in the LA of the adult mouse brain; (2) there is a peak of pMAPK at 60 min after fear conditioning; (3) the ERK/MAPK signaling pathway activation is essential for the acquisition of an FPS response; (4) at 60 min, the pMAPK are exclusively neuronal and not glial. These results emphasize the importance of this signaling pathway in the acquisition of conditioned fear in the mouse. Given the widely held view that conditioned fear models the essential aspects of anxiety disorders, the results confirm the ERK/MAPK signaling pathway as a molecular target for the treatment of anxiety disorders in the clinic.

Extracellular recordings of rodents in vivo: their contribution to integrative neuroscience

The prevalent theory in learning and memory processes is that they are underlain by short and long-term changes in synaptic weight, which continuously modulates neural networks during acquisition and recall. This synaptic plasticity has been revealed by recording extracellular field potentials. The enhancement of synaptic transmission was primarily noted in the hippocampus and was named long-term potentiation (LTP). The opposite mechanism, long-term depression (LTD), a reduction of synaptic transmission, was first discovered in the cerebellum. Since then, the LTP-model has been studied mainly using in vitro and acute anesthetized in vivo preparations. This approach has led to remarkable progress in the comprehension of intracellular molecular processes during LTP and LTD. In this review, we focus mainly on what we can learn about molecular events using extracellular field potential recordings with a more ecological model, i.e., studies using the freely behaving animal, with animals that are genetically modified or not, in several behavioral paradigms aimed at gaining insight into some of the conflicting results obtained with in vitro and in vivo preparations.

Associative structure of fear memory after basolateral amygdala lesions in rats

The authors have recently demonstrated that rats with basolateral amygdala (BLA) lesions acquire Pavlovian fear conditioning after overtraining. However, it is not known whether the associative basis of Pavlovian fear memory acquired by rats with BLA lesions is similar to that of intact rats. Associations are typically formed between the conditional (CS) and unconditional (US) stimuli (stimulus-stimulus; S-S), although it is possible for stimuli to enter into association with the responses they produce (stimulus-response; S-R). Indeed, the central nucleus of the amygdala, which is essential for fear conditioning in rats with BLA lesions, may mediate S-R associations in some Pavlovian tasks. The authors therefore used a postconditioning US inflation procedure (i.e., exposure to intense footshock USs) to assess the contribution of S-S associations to fear conditioning after overtraining in rats with BLA lesions. In Experiment 1, intact rats that were overtrained and later inflated displayed elevated freezing levels when tested, indicating that S-S associations contribute to overtrained fear memories. Interestingly, neither neurotoxic BLA lesions nor temporary inactivation of the BLA during overtraining prevented the inflation effect (Experiment 2 and 3, respectively). These results reveal that S-S associations support Pavlovian fear memories after overtraining in both intact rats and rats with BLA lesions, and imply that the central nucleus of the amygdala encodes CS-US associations during fear conditioning.

The activity-regulated cytoskeletal-associated protein (Arc/Arg3.1) is required for memory consolidation of pavlovian fear conditioning in the lateral amygdala

The activity-regulated cytoskeletal-associated protein (Arc/Arg3.1) is an immediate early gene that has been widely implicated in hippocampal-dependent learning and memory and is believed to play an integral role in synapse-specific plasticity. Here, we examined the role of Arc/Arg3.1 in amygdala-dependent Pavlovian fear conditioning. We first examined the regulation of Arc/Arg3.1 mRNA and protein after fear conditioning and LTP-inducing stimulation of thalamic inputs to the lateral amygdala (LA). Quantitative real-time PCR, in situ hybridization, Western blotting and immunohistochemistry revealed a significant upregulation of Arc/Arg3.1 mRNA and protein in the LA relative to controls. In behavioral experiments, intra-LA infusion of an Arc/Arg3.1 antisense oligodeoxynucleotide (ODN) was observed to be anatomically restricted to the LA, taken up by LA cells, and to promote significant knockdown of Arc/Arg3.1 protein. Rats given intra-LA infusions of multiple doses of the Arc/Arg3.1 ODN showed an impairment of LTM (tested approximately 24 later), but no deficit in STM (tested 3 h later) relative to controls infused with scrambled ODN. Finally, to determine whether upregulation of Arc/Arg3.1 occurs downstream of ERK/MAPK activation, we examined Arc/Arg3.1 expression in rats given intra-LA infusion of the MEK inhibitor U0126. Relative to vehicle controls, infusion of U0126 impaired training-induced increases in Arc/Arg3.1 expression. These findings suggest that Arc/Arg3.1 expression in the amygdala is required for fear memory consolidation, and further suggest that Arc/Arg3.1 regulation in the LA is downstream of the ERK/MAPK signaling pathway.

The NO-cGMP-PKG signaling pathway regulates synaptic plasticity and fear memory consolidation in the lateral amygdala via activation of ERK/MAP kinase

Recent studies have shown that nitric oxide (NO) signaling plays a crucial role in memory consolidation of Pavlovian fear conditioning and in synaptic plasticity in the lateral amygdala (LA). In the present experiments, we examined the role of the cGMP-dependent protein kinase (PKG), a downstream effector of NO, in fear memory consolidation and long-term potentiation (LTP) at thalamic and cortical input pathways to the LA. In behavioral experiments, rats given intra-LA infusions of either the PKG inhibitor Rp-8-Br-PET-cGMPS or the PKG activator 8-Br-cGMP exhibited dose-dependent impairments or enhancements of fear memory consolidation, respectively. In slice electrophysiology experiments, bath application of Rp-8-Br-PET-cGMPS or the guanylyl cyclase inhibitor LY83583 impaired LTP at thalamic, but not cortical inputs to the LA, while bath application of 8-Br-cGMP or the guanylyl cyclase activator YC-1 resulted in enhanced LTP at thalamic inputs to the LA. Interestingly, YC-1-induced enhancement of LTP in the LA was reversed by concurrent application of the MEK inhibitor U0126, suggesting that the NO-cGMP-PKG signaling pathway may promote synaptic plasticity and fear memory formation in the LA, in part by activating the ERK/MAPK signaling cascade. As a test of this hypothesis, we next showed that rats given intra-LA infusion of the PKG inhibitor Rp-8-Br-PET-cGMPS or the PKG activator 8-Br-cGMP exhibit impaired or enhanced activation, respectively, of ERK/MAPK in the LA after fear conditioning. Collectively, our findings suggest that an NO-cGMP-PKG-dependent form of synaptic plasticity at thalamic input synapses to the LA may underlie memory consolidation of Pavlovian fear conditioning, in part, via activation of the ERK/MAPK signaling cascade.

De novo mRNA synthesis is required for both consolidation and reconsolidation of fear memories in the amygdala

Memory consolidation is the process by which newly learned information is stabilized into long-term memory (LTM). Considerable evidence indicates that retrieval of a consolidated memory returns it to a labile state that requires it to be restabilized. Consolidation of new fear memories has been shown to require de novo RNA and protein synthesis in the lateral nucleus of the amygdala (LA). We have previously shown that de novo protein synthesis in the LA is required for reconsolidation of auditory fear memories. One key question is whether protein synthesis during reconsolidation depends on already existing mRNAs or on synthesis of new mRNAs in the amygdala. In the present study, we examined the effect of mRNA synthesis inhibition during consolidation and reconsolidation of auditory fear memories. We first show that intra-LA infusion of two different mRNA inhibitors dose-dependently impairs long-term memory but leaves short-term memory (STM) intact. Next, we show that intra-LA infusion of the same inhibitors dose-dependently blocks post-reactivation long-term memory (PR-LTM), whereas post-reactivation short-term memory (PR-STM) is left intact. Furthermore, the same treatment in the absence of memory reactivation has no effect. Together, these results show that both consolidation and reconsolidation of auditory fear memories require de novo mRNA synthesis and are equally sensitive to disruption of de novo mRNA synthesis in the LA.

Enhancement of presynaptic glutamate release and persistent inflammatory pain by increasing neuronal cAMP in the anterior cingulate cortex

Both presynaptic and postsynaptic alterations are associated with plastic changes of brain circuits, such as learning and memory, drug addiction and chronic pain. However, the dissection of the relative contributions of pre- and postsynaptic components to brain functions is difficult. We have previously shown peripheral inflammation caused both presynaptic and postsynaptic changes and calcium-stimulated cyclic AMP (cAMP) pathway in the anterior cingulate cortex (ACC) is critical in the synaptic plasticity and behavioral sensitization to pain. It remains to be elucidated whether presynaptic or postsynaptic modulation by cAMP in the ACC could be sufficient for enhancing inflammatory pain. In order to address this question, we took advantage of a novel transgenic mouse model, heterologously expressing an Aplysia octopamine receptor (Ap oa₁). This receptor is G protein-coupled and selectively activates the cAMP pathway. We found that activation of Ap oa₁ by octopamine enhanced glutamatergic synaptic transmission in the ACC by increasing presynaptic glutamate release in vitro. Bilateral microinjection of octopamine into the ACC significantly facilitated behavioral responses to inflammatory pain but not acute pain. The present study provides the first evidence linking enhanced presynaptic glutamate release in the ACC to behavioral sensitization caused by peripheral inflammation.

Molecular mechanisms underlying a cellular analog of operant reward learning

Operant conditioning is a ubiquitous but mechanistically poorly understood form of associative learning in which an animal learns the consequences of its behavior. Using a single-cell analog of operant conditioning in neuron B51 of Aplysia, we examined second-messenger pathways engaged by activity and reward and how they may provide a biochemical association underlying operant learning. Conditioning was blocked by Rp-cAMP, a peptide inhibitor of PKA, a PKC inhibitor, and by expressing a dominant-negative isoform of Ca2+-dependent PKC (apl-I). Thus, both PKA and PKC were necessary for operant conditioning. Injection of cAMP into B51 mimicked the effects of operant conditioning. Activation of PKC also mimicked conditioning but was dependent on both cAMP and PKA, suggesting that PKC acted at some point upstream of PKA activation. Our results demonstrate how these molecules can interact to mediate operant conditioning in an individual neuron important for the expression of the conditioned behavior.

Intra-amygdala injections of CREB antisense impair inhibitory avoidance memory: role of norepinephrine and acetylcholine

Infusions of CREB antisense into the amygdala prior to training impair memory for aversive tasks, suggesting that the antisense may interfere with CRE-mediated gene transcription and protein synthesis important for the formation of new memories within the amygdala. However, the amygdala also appears to modulate memory formation in distributed brain sites, through mechanisms that include the release of norepinephrine and acetylcholine within the amygdala. Thus, CREB antisense injections may affect memory by interfering with mechanisms of modulation, rather than storage, of memory. In the present experiment, rats received bilateral intra-amygdala infusions of CREB antisense (2 nmol/1 microL) 6 h prior to inhibitory avoidance training. In vivo microdialysis samples were collected from the right amygdala before, during, and following training. CREB antisense produced amnesia tested at 48 h after training. In addition, CREB antisense infusions dampened the training-related release of norepinephrine, and to a lesser extent of acetylcholine, in the amygdala. Furthermore, intra-amygdala infusions of the beta-adrenergic receptor agonist clenbuterol administered immediately after training attenuated memory impairments induced by intra-amygdala injections of CREB antisense. These findings suggest that intra-amygdala treatment with CREB antisense may affect processes involved in modulation of memory in part through interference with norepinephrine and acetylcholine neurotransmission in the amygdala.

Vulnerability to lasting anxiogenic effects of brief exposure to predator stimuli: sex, serotonin and other factors-relevance to PTSD

Lasting anxiogenic effects of predator stress in rodents may model aspects of post-traumatic stress disorder (PTSD). There is a link between genetic variation in the serotonin (5-HT) transporter (SERT) and anxiety in humans, prompting the generation of SERT knockout mice. This review brings together studies of SERT knockout male mice, normal female mice, and different 5-HT receptors in predator stress effects on anxiety. These studies provide for a link between vulnerability to the anxiogenic effects of predator stress and abnormalities of 5-HT transmission induced by a life long reduction in 5-HT reuptake in male mice, which creates a vulnerability like that seen in normal female mice. Data reviewed suggest abnormalities in 5-HT transmission contribute to vulnerability to lasting anxiogenic effects of species relevant stressors. To the extent to which predator stress effects model aspects of PTSD, and in the light of relevant human literature, these considerations implicate abnormalities of 5-HT transmission in vulnerability to PTSD per se, and as a potential contributor to enhanced female vulnerability to PTSD.

Metabotropic glutamate receptors subtype 5 are necessary for the enhancement of auditory evoked potentials in the lateral nucleus of the amygdala by tetanic stimulation of the auditory thalamus

The lateral nucleus of the amygdala (LA) receives axonal projections from the auditory thalamus, the medial geniculate nucleus (MGN), and mediates auditory fear conditioning. Tetanic electrical stimulation of the MGN can induce long-term potentiation of acoustically-evoked responses (AEPs) recorded in the LA of anesthetized rats. The present study investigated the temporal development of tetanus-induced AEP potentiation recorded in the LA of anesthetized rats during the recording time up to 120 min after tetanization. In addition, the present study investigated whether the artificially-induced AEP potentiation is mediated by the metabotropic glutamate receptors subtype 5 (mGluR5). The results show that AEPs recorded in the LA to a broadband-noise burst were significantly enhanced immediately after tetanic but not low-frequency stimulation of the MGN. The AEP potentiation was well retained up to 120 min after tetanization. High-dose (1.5 microg/4 microl) microinjection of the selective antagonist of mGluR5, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), into the ipsilateral lateral ventricle 30 min before tetanization completely blocked the AEP potentiation without affecting the baseline AEP. Low-dose (0.5 microg/4 microl) microinjection partially suppressed the AEP potentiation. When the high-dose MPEP was injected 40 min after tetanization, the AEP potentiation was not affected. These results indicate that in anesthetized rats mGluR5 receptors are necessary for the induction or early maintenance (40 min) of AEP potentiation in the LA by tetanic stimulation of the MGN.