Fear Conditioning In Mice Research Articles

Extinction of Blood Pressure and Heart Rate Responses to Conditioned Fear

BackgroundPost‐traumatic stress disorder (PTSD) is characterized by an impaired ability to extinguish fear associations that trigger exaggerated neurophysiological responses, which may contribute to increased cardiovascular (CV) risk in these individuals. The objective of this study was to evaluate the effects of learned inhibition or extinction on the conditioned cardio‐autonomic response during fear memory recall. We hypothesized that extinction training would lead to a temporary reduction of conditioned heart rate (HR) and systolic blood pressure (SBP) responses.MethodsIn male C57BL/6 mice, Pavlovian fear conditioning combined with simultaneous in‐vivo telemetry was used to assess CV and behavioral measures in both home‐cage and testing contexts. Mice were fear conditioned with five tone/shock pairings followed by two days of extinction training, each consisting of 30 conditioned stimulus (CS) presentations. The conditioned CV response was evaluated during short‐term (STR) (1‐hour post‐extinction) and long‐term recall (LTR) (2 weeks post‐extinction) of the CS in the home‐cage.ResultsFreezing behavior, systolic blood pressure (SBP) and heart rate (HR) were simultaneously measured over two days of extinction training. Freezing in response to the CS from Day 1 to Day 2 was reduced in fear conditioned mice (80% ± 7 vs. 57% ± 11; p<0.05), whereas non‐conditioned animals did not respond to the CS on either day. SBP and HR were elevated to near‐physiological maximums throughout each session, regardless of whether or not the animals had been previously fear conditioned. However two‐weeks after extinction training, fear‐conditioned animals in the home‐cage context exhibited a strong conditioned CV response to the CS, which remained elevated over the course of four CS presentations (change in SBP 18 mmHg ± 2 and HR 166 bpm ± 42; n= 8; p<0.05) (Fig. 1). We then assessed short‐term CV responses before and after extinction training. 24 hours after fear conditioning, we observed increased CV activity in response to the CS (change in SBP 17 mmHg ± 4.3 and HR 101 bpm ± 34.3; n= 9–12; p<0.05) over 4CS trials. Interestingly, this STR was similar to that of animals 2 weeks after extinction training. Conditioned mice were then split into two groups: (1) extinction (2) no extinction. The extinction group was repeatedly re‐exposed to the CS over two days of extinction training. At the end of this two‐day period, both groups went through a final conditioned CV response test in the home cage. In non‐extinguished controls, there was a significant increase in SBP during CS exposure (114 mmHG ± 2.7 vs. 124 mmHG ± 4.3; p<0.05), while in the extinction group, the SBP response was completely abolished. Heart rate did not change in response to CS exposure in either group at this time point.ConclusionsThese data demonstrate for the first time that both short and long‐term recall of fear memory elicit a conditioned blood pressure response that can be temporarily attenuated by extinction training. Furthermore, these data suggest that conditioned blood pressure and heart rate responses may vary in their sensitivity to inhibitory learning. Understanding the effects of extinction training on autonomic regulation of blood pressure and heart rate may help identify or improve interventions that lead to better PTSD‐CVD comorbidity outcomes.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Nicotine exposure leads to deficits in differential cued fear conditioning in mice and humans: A potential role of the anterior cingulate cortex

Stress and anxiety disorders such as posttraumatic stress disorder (PTSD) are characterized by disrupted safety learning. Tobacco smoking has been strongly implicated in stress and anxiety disorder symptomatology, both as a contributing factor and as a vulnerability factor. Rodent studies from our lab have recently shown that acute and chronic nicotine exposure disrupts safety learning. However, it is unknown if these effects of nicotine translate to humans. The present studies addressed this gap by administering a translational differential cued fear conditioning paradigm to both mice and humans. In mice, we found that chronic nicotine exposure reduced discrimination between a conditioned stimulus (CS) that signals for danger (CS+) and another CS that signals for safety (CS−) during both acquisition and testing. We then employed a similar differential cued fear conditioning paradigm in human smokers and non-smokers undergoing functional magnetic resonance imaging (fMRI). Smokers showed reduced CS+/CS− discrimination during fear conditioning compared to non-smokers. Furthermore, using fMRI, we found that subgenual and dorsal anterior cingulate cortex activations were lower in smokers than in non-smokers during differential cued fear conditioning. These results suggest a potential biological mechanism underlying a dysregulated ability to discriminate between danger and safety cues. Our results indicate a clear parallel between the effects of nicotine exposure on safety learning in mice and humans and therefore suggest that smoking might represent a risk factor for inability to process information related to danger and safety related cues.

Intranasal Cotinine Plus Krill Oil Facilitates Fear Extinction, Decreases Depressive-Like Behavior, and Increases Hippocampal Calcineurin A Levels in Mice.

Failure in fear extinction is one of the more troublesome characteristics of posttraumatic stress disorder (PTSD). Cotinine facilitates fear memory extinction and reduces depressive-like behavior when administered 24h after fear conditioning in mice. In this study, it was investigated the behavioral and molecular effects of cotinine, and other antidepressant preparations infused intranasally. Intranasal (IN) cotinine, IN krill oil, IN cotinine plus krill oil, and oral sertraline were evaluated on depressive-like behavior and fear retention and extinction after fear conditioning in C57BL/6 mice. Since calcineurin A has been involved in facilitating fear extinction in rodents, we also investigated changes of calcineurin in the hippocampus, a region key on contextual fear extinction. Short-term treatment with cotinine formulations was superior to krill oil and oral sertraline in reducing depressive-like behavior and fear consolidation and enhancing contextual fear memory extinction in mice. IN krill oil slowed the extinction of fear. IN cotinine preparations increased the levels of calcineurin A in the hippocampus of conditioned mice. In the light of the results, the future investigation of the use of IN cotinine preparations for the extinction of contextual fear memory and treatment of treatment-resistant depression (TRD) in PTSD is discussed.

Neural circuits: Shock signals.

In fear conditioning in mice, neurons in the lateral central amygdala that express protein kinase Cδ convey the unconditioned stimulus to the lateral amygdala to enable the encoding of aversive memory.

Neuronally-directed effects of RXR activation in a mouse model of Alzheimer\u2019s disease

Alzheimer’s disease (AD) is characterized by extensive neuron loss that accompanies profound impairments in memory and cognition. We examined the neuronally directed effects of the retinoid X receptor agonist bexarotene in an aggressive model of AD. We report that a two week treatment of 3.5 month old 5XFAD mice with bexarotene resulted in the clearance of intraneuronal amyloid deposits. Importantly, neuronal loss was attenuated by 44% in the subiculum in mice 4 months of age and 18% in layer V of the cortex in mice 8 months of age. Moreover, bexarotene treatment improved remote memory stabilization in fear conditioned mice and improved olfactory cross habituation. These improvements in neuron viability and function were correlated with significant increases in the levels of post-synaptic marker PSD95 and the pre-synaptic marker synaptophysin. Moreover, bexarotene pretreatment improved neuron survival in primary 5XFAD neurons in vitro in response to glutamate-induced excitotoxicity. The salutary effects of bexarotene were accompanied by reduced plaque burden, decreased astrogliosis, and suppression of inflammatory gene expression. Collectively, these data provide evidence that bexarotene treatment reduced neuron loss, elevated levels of markers of synaptic integrity that was linked to improved cognition and in an aggressive model of AD.

Abstract # 1831 Repeated social defeat stress-induced neuroinflammation contributes to later fear sensitization and impaired extinction recall of fear conditioning in mice: Involvement of the endocannabinoid system

Repeated social defeat (RSD) stress promotes myeloid cell trafficking, neuroinflammation and prolonged anxiety-like behavior. RSD could be useful to mimic PTSD-like symptoms in mice. Endocannabinoid (eCB) system molecules are expressed by immune and neuronal cells, modulating their functions, including behavior. The purpose of this study was to test if eCB system activation in mice would attenuate neuroinflammatory and behavioral effects of RSD. To address that, C57BL/6 mice received a nonselective cannabinoid agonist, WIN55,2122 (WIN; 1 mg/kg, i.p.), 30 min prior to each of six social defeat stress exposure. In the next morning, the anxiety-like behavior was evaluated. One week later, independent groups of mice were submitted to the contextual fear conditioning (3 footshock, 0.75 mA, 2s/each). Fear expression and extinction acquisition were evaluated 24 h later and fear extinction recall after additional 24 h. WIN attenuated the RSD-induced immune system and HPA axis activation, neuroinflammation and anxiety-like behavior. Brain CD11b + cells from stressed mice exhibited increased cytokines and eCB-related genes mRNA expression. Moreover, those cells stimulated with LPS ex-vivo presented higher supernatant IL-6. RSD also induced later fear sensitization, impaired extinction recall and increased frontal cortex and hippocampus IL-1 β mRNA expression. WIN attenuated RSD effects. Our data showing that WIN prevents conditioned fear processing alteration following RSD-induced neuroinflammation suggest that the eCB system could be a potential target in stress-related disorders with a neuroinflammatory component, like PTSD.

Toxoplasma gondii Infection in Mice Impairs Long-Term Fear Memory Consolidation through Dysfunction of the Cortex and Amygdala

Chronic infection with Toxoplasma gondii becomes established in tissues of the central nervous system, where parasites may directly or indirectly modulate neuronal function. Epidemiological studies have revealed that chronic infection in humans is a risk factor for developing mental diseases. However, the mechanisms underlying parasite-induced neuronal dysfunction in the brain remain unclear. Here, we examined memory associated with conditioned fear in mice and found that T. gondii infection impairs consolidation of conditioned fear memory. To examine the brain pathology induced by T. gondii infection, we analyzed the parasite load and histopathological changes. T. gondii infects all brain areas, yet the cortex exhibits more severe tissue damage than other regions. We measured neurotransmitter levels in the cortex and amygdala because these regions are involved in fear memory expression. The levels of dopamine metabolites but not those of dopamine were increased in the cortex of infected mice compared with those in the cortex of uninfected mice. In contrast, serotonin levels were decreased in the amygdala and norepinephrine levels were decreased in the cortex and amygdala of infected mice. The levels of cortical dopamine metabolites were associated with the time spent freezing in the fear-conditioning test. These results suggest that T. gondii infection affects fear memory through dysfunction of the cortex and amygdala. Our findings provide insight into the mechanisms underlying the neurological changes seen during T. gondii infection.

Adiponectin regulates contextual fear extinction and intrinsic excitability of dentate gyrus granule neurons through AdipoR2 receptors

Post-traumatic stress disorder (PTSD) is characterized by exaggerated fear expression and impaired fear extinction. The underlying molecular and cellular mechanisms of PTSD are largely unknown. The current pharmacological and non-pharmacological treatments for PTSD are either ineffective or temporary with high relapse rates. Here we report that adiponectin-deficient mice exhibited normal contextual fear conditioning but displayed slower extinction learning. Infusions of adiponectin into the dentate gyrus (DG) of the hippocampus in fear-conditioned mice facilitated extinction of contextual fear. Whole-cell patch-clamp recordings in brain slices revealed that intrinsic excitability of DG granule neurons was enhanced by adiponectin deficiency and suppressed after treatment with the adiponectin mimetic AdipoRon, which were associated with increased input resistance and hyperpolarized resting membrane potential, respectively. Moreover, deletion of AdipoR2, but not AdipoR1 in the DG, resulted in augmented fear expression and reduced extinction, accompanied by intrinsic hyperexcitability of DG granule neurons. Adiponectin and AdipoRon failed to induce facilitation of fear extinction and elicit inhibition of intrinsic excitability of DG neurons in AdipoR2 knockout mice. These results indicated that adiponectin action via AdipoR2 was both necessary and sufficient for extinction of contextual fear and intrinsic excitability of DG granule neurons, implying that enhancing or dampening DG neuronal excitability may cause resistance to or facilitation of extinction. Therefore, our findings provide a functional link between adiponectin/AdipoR2 activation, DG neuronal excitability and contextual fear extinction, and suggest that targeting adiponectin/AdipoR2 may be used to strengthen extinction-based exposure therapies for PTSD.

Behavioral verification of associative learning in whiskers-related fear conditioning in mice.

Fear-conditioning is one of the most widely used paradigms in attempts to unravel the processes and mechanisms underlying learning and plasticity. In most of the Pavlovian conditioning paradigms auditory stimulus is used as a conditioned stimulus (CS), but conditioning can be accomplished also to tactile CS. The whisker-to-barrel tactile system in mice offers convenient way to investigate the brain pathways and mechanisms of learning, and plasticity of the brain cortex. To support a claim that an animal learns during conditioning session and that the plastic changes are associative in nature, objective measures of behavior are necessary. Multiple types of conditioned responses can develop, depending on the training situation, CS and unconditioned stimulus (UCS) characteristics. These include physiological responses, such as salivation, heart rate, galvanic skin reaction, and also behavioral responses, such as startle reflex potentiation or suppression of the ongoing behavior. When studying learning with the whisker system in behaving mice, stimulation of individual whiskers in a well-controlled manner may require animal restrain with a disadvantage of only limited behavioral responses observed. Stimulation of whiskers in the neck-restraining apparatus evokes head movements. When whiskers stimulation (CS) is paired with an aversive UCS during conditioning, the head movements decrease in the course of the training. This reaction, called minifreezing, resembles freezing response, frequently used behavioral measure, however applicable only in freely moving animals. This article will review experimental evidences confirming that minifreezing is a relevant index of association formation between the neutral CS and the and the aversive UCS.

Activity-induced synaptic delivery of the GluN2A-containing NMDA receptor is dependent on endoplasmic reticulum chaperone Bip and involved in fear memory.

The N-methyl-D-aspartate receptor (NMDAR) in adult forebrain is a heterotetramer mainly composed of two GluN1 subunits and two GluN2A and/or GluN2B subunits. The synaptic expression and relative numbers of GluN2A- and GluN2B-containing NMDARs play critical roles in controlling Ca(2+)-dependent signaling and synaptic plasticity. Previous studies have suggested that the synaptic trafficking of NMDAR subtypes is differentially regulated, but the precise molecular mechanism is not yet clear. In this study, we demonstrated that Bip, an endoplasmic reticulum (ER) chaperone, selectively interacted with GluN2A and mediated the neuronal activity-induced assembly and synaptic incorporation of the GluN2A-containing NMDAR from dendritic ER. Furthermore, the GluN2A-specific synaptic trafficking was effectively disrupted by peptides interrupting the interaction between Bip and GluN2A. Interestingly, fear conditioning in mice was disrupted by intraperitoneal injection of the interfering peptide before training. In summary, we have uncovered a novel mechanism for the activity-dependent supply of synaptic GluN2A-containing NMDARs, and demonstrated its relevance to memory formation.

Dissociation of learned helplessness and fear conditioning in mice: a mouse model of depression.

The state of being helpless is regarded as a central aspect of depression, and therefore the learned helplessness paradigm in rodents is commonly used as an animal model of depression. The term ‘learned helplessness’ refers to a deficit in escaping from an aversive situation after an animal is exposed to uncontrollable stress specifically, with a control/comparison group having been exposed to an equivalent amount of controllable stress. A key feature of learned helplessness is the transferability of helplessness to different situations, a phenomenon called ‘trans-situationality’. However, most studies in mice use learned helplessness protocols in which training and testing occur in the same environment and with the same type of stressor. Consequently, failures to escape may reflect conditioned fear of a particular environment, not a general change of the helpless state of an animal. For mice, there is no established learned helplessness protocol that includes the trans-situationality feature. Here we describe a simple and reliable learned helplessness protocol for mice, in which training and testing are carried out in different environments and with different types of stressors. We show that with our protocol approximately 50% of mice develop learned helplessness that is not attributable to fear conditioning.

Fear conditioning suppresses large-conductance calcium-activated potassium channels in lateral amygdala neurons

It was previously shown that depression-like behavior is accompanied with suppression of the large-conductance calcium activated potassium (BK) channel in cingulate cortex pyramidal cells. To test whether BK channels are also involved in fear conditioning, we studied neuronal properties of amygdala principal cells in fear conditioned mice. After behavior, we made brain slices containing the amygdala, the structure critically relevant to fear memory. The resting membrane potential in lateral amygdala (LA) neurons obtained from fear conditioned mice (FC group) was more depolarized than in neurons from naïve controls. The frequencies of spikes evoked by current injections were higher in neurons from FC mice, demonstrating that excitability of LA neurons was elevated by fear conditioning. The depolarization in neurons from FC mice was shown to depend on BK channels by using the BK channel blocker charybdotoxin. Suppression of BK channels in LA neurons from the FC group was further confirmed on the basis of the spike width, since BK channels affect the descending phase of spikes. Spikes were broader in the FC group than those in the naïve control in a manner dependent on BK channels. Consistently, quantitative real-time PCR revealed a decreased expression of BK channel mRNA. The present findings suggest that emotional disorder manifested in the forms of fear conditioning is accompanied with BK channel suppression in the amygdala, the brain structure critical to this emotional disorder.

Effects of Acute Radiation Exposure on Extinction of Previously Conditioned Fear in Mice

Effects of Acute Radiation Exposure on Extinction of Previously Conditioned Fear in Mice

Bidirectional switch of the valence associated with a hippocampal contextual memory engram.

The valence of memories is malleable because of their intrinsic reconstructive property1. This property of memory has been used clinically to treat maladaptive behaviours2. However, the neuronal mechanisms and brain circuits that enable the switching of the valence of memories remain largely unknown. Here, we investigated these mechanisms by applying the recently developed memory engram cell-labelling and -manipulation technique 3,4. We labelled, with Channelrhodopsin-2 (ChR2), a population of cells in either the dorsal dentate gyrus (DG) of the hippocampus or the basolateral complex of the amygdala (BLA) that were specifically activated during contextual fear or reward conditioning. Both groups of fear-conditioned mice displayed aversive light-dependent responses in an optogenetic place avoidance test, whereas both DG- and BLA-labelled mice that underwent reward conditioning exhibited an appetitive response in an optogenetic place preference test. Next, in an attempt to reverse the valence of memory within a subject, mice whose DG or BLA engram had initially been labelled by contextual fear or reward conditioning were subjected to a second conditioning of the opposite valence while their original DG or BLA engram was reactivated by blue light. Subsequent optogenetic place avoidance and preference tests revealed that while the DG-engram group displayed a response indicating a switch of the memory valence, the BLA-engram group did not. This switch was also evident at the cellular level by a change in functional connectivity between DG engram-bearing cells and BLA engram-bearing cells. Thus, we found that in the DG, the neurons carrying the memory engram of a given neutral context have plasticity such that the valence of a conditioned response evoked by their reactivation can be reversed by re-associating this contextual memory engram with a new US of an opposite valence. Our present work provides new insight into the functional neural circuit underlying the malleability of emotional memory.

Requirement of Mammalian target of rapamycin complex 1 downstream effectors in cued fear memory reconsolidation and its persistence.

Memory retrieval, often termed reconsolidation, can render previously consolidated memories susceptible to manipulation that can lead to alterations in memory strength. Although it is known that reconsolidation requires mammalian target of rapamycin complex 1 (mTORC1)-dependent translation, the specific contributions of its downstream effectors in reconsolidation are unclear. Using auditory fear conditioning in mice, we investigated the role of eukaryotic translation initiation factor 4E (eIF4E)-eIF4G interactions and p70 S6 kinase polypeptide 1 (S6K1) in reconsolidation. We found that neither 4EGI-1 (2-[(4-(3,4-dichlorophenyl)-thiazol-2-ylhydrazono)-3-(2-nitrophenyl)]propionic acid), an inhibitor of eFI4E-eIF4G interactions, nor PF-4708671 [2-((4-(5-ethylpyrimidin-4-yl)piperazin-1-yl)methyl)-5-(trifluoromethyl)-1H-benzo[d]imidazole], an inhibitor of S6K1, alone blocked the reconsolidation of auditory fear memory. In contrast, using these drugs in concert to simultaneously block eIF4E-eIF4G interactions and S6K1 immediately after memory reactivation significantly attenuated fear memory reconsolidation. Moreover, the combination of 4EGI-1 and PF-4708671 further destabilized fear memory 10 d after memory reactivation, which was consistent with experiments using rapamycin, an mTORC1 inhibitor. Furthermore, inhibition of S6K1 immediately after retrieval resulted in memory destabilization 10 d after reactivation, whereas inhibition of eIF4E-eIF4G interactions did not. These results indicate that the reconsolidation of fear memory requires concomitant association of eIF4E to eIF4G as well as S6K1 activity and that the persistence of memory at longer intervals after memory reactivation also requires mTORC1-dependent processes that involve S6K1. These findings suggest a potential mechanism for how mTORC1-dependent translation is fine tuned to alter memory persistence.

Trace Fear Conditioning in Mice

In this experiment we present a technique to measure learning and memory. In the trace fear conditioning protocol presented here there are five pairings between a neutral stimulus and an unconditioned stimulus. There is a 20 sec trace period that separates each conditioning trial. On the following day freezing is measured during presentation of the conditioned stimulus (CS) and trace period. On the third day there is an 8 min test to measure contextual memory. The representative results are from mice that were presented with the aversive unconditioned stimulus (shock) compared to mice that received the tone presentations without the unconditioned stimulus. Trace fear conditioning has been successfully used to detect subtle learning and memory deficits and enhancements in mice that are not found with other fear conditioning methods. This type of fear conditioning is believed to be dependent upon connections between the medial prefrontal cortex and the hippocampus. One current controversy is whether this method is believed to be amygdala-independent. Therefore, other fear conditioning testing is needed to examine amygdala-dependent learning and memory effects, such as through the delay fear conditioning.

Trace Fear Conditioning in Mice

Trace Fear Conditioning in Mice

Palmitoylation of δ-catenin by DHHC5 mediates activity-induced synapse plasticity

Synaptic cadherin adhesion complexes are known to be key regulators of synapse plasticity. However, the molecular mechanisms that coordinate activity-induced modifications in cadherin localization and adhesion and subsequent changes in synapse morphology and efficacy, remain unanswered. We demonstrate that the intracellular cadherin binding protein, δ-catenin, is transiently palmitoylated by DHHC5 following enhanced synaptic activity, and that palmitoylation increases δ-catenin/cadherin interactions at synapses. Both the palmitoylation of δ-catenin and its binding to cadherin are required for activity-induced stabilization of N-cadherin at synapses, the enlargement of postsynaptic spines, as well as insertion of GluA1 and GluA2 subunits into the synaptic membrane and the concomitant increase in mEPSC amplitude. Importantly, context-dependent fear conditioning in mice results in increased δ-catenin palmitoylation as well as increased δ-catenin/cadherin associations at hippocampal synapses. Together, this suggests a role for palmitoylated δ-catenin in coordinating activity-dependent changes in synaptic adhesion molecules, synapse structure, and receptor localization that are involved in memory formation.

N-methyl-D-aspartate receptors and protein synthesis are necessary for reinstatement of conditioned fear

Conditioned fear is extinguished if a conditioned animal receives the conditioned stimulus without an unconditioned stimulus. The extinguished fear response can be reinstated after the animal experiences a mild unconditioned stimulus. Although extensive studies on the neuronal circuitry and neurochemical mechanisms leading to fear acquisition and extinction have been carried out, few studies have focused on reinstatement. In this study, we investigated the effects of N-methyl-D-aspartic acid receptor (NMDAR) antagonists, protein synthesis inhibitors, cannabinoid receptor type 1 (CB1R) antagonists, and benzodiazepine on reinstatement of conditioned fear in mice. An intraperitoneal injection of the NMDAR antagonist MK-801 or the protein synthesis inhibitor anisomycin before the reminder shock attenuated fear reinstatement tested the next day. However, anisomycin had no effect on fear reinstatement tested 2 h after the reminder shock. CB1R antagonists, SR141716, and a benzodiazepine, diazepam, had no effect on fear reinstatement. These results suggested that NMDAR and protein synthesis-dependent plasticity contributed toward the reinstatement of conditioned fear and that protein synthesis was involved in consolidation of reinstated fear.

Effects of D-amino acid oxidase inhibition on memory performance and long-term potentiation in vivo.

N-methyl-d-aspartate receptor (NMDAR) activation can initiate changes in synaptic strength, evident as long-term potentiation (LTP), and is a key molecular correlate of memory formation. Inhibition of d-amino acid oxidase (DAAO) may increase NMDAR activity by regulating d-serine concentrations, but which neuronal and behavioral effects are influenced by DAAO inhibition remain elusive. In anesthetized rats, extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded before and after a theta frequency burst stimulation (TBS) of the Schaffer collateral pathway of the CA1 region in the hippocampus. Memory performance was assessed after training with tests of contextual fear conditioning (FC, mice) and novel object recognition (NOR, rats). Oral administration of 3, 10, and 30 mg/kg 4H-furo[3,2-b]pyrrole-5-carboxylic acid (SUN) produced dose-related and steady increases of cerebellum d-serine in rats and mice, indicative of lasting inhibition of central DAAO. SUN administered 2 h prior to training improved contextual fear conditioning in mice and novel object recognition memory in rats when tested 24 h after training. In anesthetized rats, LTP was established proportional to the number of TBS trains. d-cycloserine (DCS) was used to identify a submaximal level of LTP (5× TBS) that responded to NMDA receptor activation; SUN administered at 10 mg/kg 3–4 h prior to testing similarly increased in vivo LTP levels compared to vehicle control animals. Interestingly, in vivo administration of DCS also increased brain d-serine concentrations. These results indicate that DAAO inhibition increased NMDAR-related synaptic plasticity during phases of post training memory consolidation to improve memory performance in hippocampal-dependent behavioral tests.