The effect of the mGlu5 negative allosteric modulator MTEP and NMDA receptor partial agonist D-cycloserine on Pavlovian conditioned fear

Minimizing Variability in Developmental Fear Studies in Mice: Toward Improved Replicability in the Field

In rodents, the first weeks of postnatal life feature remarkable changes in fear memory acquisition, retention, extinction, and discrimination. Early development is also marked by profound changes in brain circuits underlying fear memory processing, with heightened sensitivity to environmental influences and stress, providing a powerful model to study the intersection between brain structure, function, and the impacts of stress. Nevertheless, difficulties related to breeding and housing young rodents, preweaning manipulations, and potential increased variability within that population pose considerable challenges to developmental fear research. Here we discuss several factors that may promote variability in studies examining fear conditioning in young rodents and provide recommendations to increase replicability. We focus primarily on experimental conditions, design, and analysis of rodent fear data, with an emphasis on mouse studies. The convergence of anatomical, synaptic, physiological, and behavioral changes during early life may increase variability, but careful practice and transparency in reporting may improve rigor and consensus in the field. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC.

Tailored behavioural tests reveal early and progressive cognitive deficits in M1000 prion disease

Prion diseases are pathogenically linked to the normal cellular prion protein (PrP^C) misfolding into abnormal conformers (PrP^Sc), with PrP^Sc accumulation underpinning both transmission and neurotoxicity. Despite achieving this canonical understanding, however fundamental questions remain incompletely resolved, including the level of pathophysiological overlap between neurotoxic and transmitting species of PrP^Sc and the temporal profiles of their propagation. To further investigate the likely time of occurrence of significant levels of neurotoxic species during prion disease development, the well characterised in vivo M1000 murine model was employed. Following intracerebral inoculation, detailed serial cognitive and ethological testing at specified time points suggested subtle transition to early symptomatic disease from ∼50% of the overall disease course. In addition to observing a chronological order for impaired behaviours, different behavioural tests also showed distinctive profiles of evolving cognitive impairments with the Barnes maze demonstrating a relatively simple linear worsening of spatial learning and memory over an extended period while in contrast a conditioned fear memory paradigm previously untested in murine prion disease demonstrated more complex alterations during disease progression. These observations support the likely production of neurotoxic PrPSc from at least just prior to the mid-point of murine M1000 prion disease and illustrate the likely need to tailor the types of behavioural testing across the time course of disease progression for optimal detection of cognitive deficits.

Control of Theta Oscillatory Activity Underlying Fear Expression by mGlu5 Receptors

Metabotropic glutamate 5 receptors (mGlu₅) are thought to play an important role in mediating emotional information processing. In particular, negative allosteric modulators (NAMs) of mGlu₅ have received a lot of attention as potential novel treatments for several neuropsychiatric diseases, including anxiety-related disorders. The aim of this study was to assess the influence of pre- and post-training mGlu₅ inactivation in cued fear conditioned mice on neuronal oscillatory activity during fear retrieval. For this study we used the recently developed mGlu₅ NAM Alloswicth-1 administered systemically. Injection of Alloswicth-1 before, but not after, fear conditioning resulted in a significant decrease in freezing upon fear retrieval. Mice injected with Alloswicth-1 pre-training were also implanted with recording microelectrodes into both the medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC). The recordings revealed a reduction in theta rhythmic activity (4-12 Hz) in both the mPFC and vHPC during fear retrieval. These results indicate that inhibition of mGlu₅ signaling alters local oscillatory activity in principal components of the fear brain network underlying a reduced response to a predicted threat.

Faecal microbiota transplant ameliorates gut dysbiosis and cognitive deficits in Huntington’s disease mice

Huntington’s disease is a neurodegenerative disorder involving psychiatric, cognitive and motor symptoms. Huntington’s disease is caused by a tandem-repeat expansion in the huntingtin gene, which is widely expressed throughout the brain and body, including the gastrointestinal system. There are currently no effective disease-modifying treatments available for this fatal disorder. Despite recent evidence of gut microbiome disruption in preclinical and clinical Huntington’s disease, its potential as a target for therapeutic interventions has not been explored. The microbiota–gut–brain axis provides a potential pathway through which changes in the gut could modulate brain function, including cognition. We now show that faecal microbiota transplant (FMT) from wild-type into Huntington’s disease mice positively modulates cognitive outcomes, particularly in females. In Huntington’s disease male mice, we revealed an inefficiency of FMT engraftment, which is potentially due to the more pronounced changes in the structure, composition and instability of the gut microbial community, and the imbalance in acetate and gut immune profiles found in these mice. This study demonstrates a role for gut microbiome modulation in ameliorating cognitive deficits modelling dementia in Huntington’s disease. Our findings pave the way for the development of future therapeutic approaches, including FMT and other forms of gut microbiome modulation, as potential clinical interventions for Huntington’s disease.

Sex-dependent effects of chronic exercise on cognitive flexibility but not hippocampal Bdnf in aging mice

Cognitive impairments associated with advanced age involve alterations in the hippocampus that changes with experience throughout life. The hippocampus is critical for cognitive flexibility involved with extinction and reinstatement of conditioned fear. It is widely accepted that regular exercise can be beneficial for hippocampal function. Therefore, we asked whether chronic voluntary exercise in middle-aged mice can improve extinction and/or reinstatement of conditioned fear compared with standard-housing. Eight-month-old male and female C57Bl/6J mice had access to a running wheel or remained in standard-housing until 11 months of age. Alongside control standard-housed young adult (3-month-old) mice, they received tone-footshock pairings, which were subsequently extinguished with tone-alone presentations the next day. Half of the mice then received a reminder in the form of a single footshock. Male and female 11-month-old mice housed in standard conditions exhibited impaired reinstatement compared with young adult mice. However, for males that had access to a running wheel from 8 months of age, the reminder treatment rescued reinstatement ability. This was not observed in females. Additionally, exercise during middle age in both sexes increased expression of brain-derived neurotrophic factor (Bdnf) mRNA in the hippocampus, specifically exon 4 mRNA. These results show that, at least for males, physical exercise is beneficial for reducing age-related decline in cognitive abilities. Despite not affecting reinstatement, exercise also increased Bdnf gene expression in the female hippocampus, which could potentially benefit other forms of hippocampus-dependent cognition.

Prelimbic cortical stimulation disrupts fear memory consolidation through ventral hippocampal dopamine D2 receptors

BACKGROUND AND PURPOSE

Anxiety disorders pose one of the biggest threats to mental health worldwide, yet current therapeutics have been mostly ineffective due to issues with relapse, efficacy and toxicity of the medications. Deep brain stimulation (DBS) is a promising therapy for treatment-resistant psychiatric disorders including anxiety, but very little is known about the effects of deep brain stimulation on fear memories.

EXPERIMENTAL APPROACH

In this study, we employed a standard tone-footshock fear conditioning paradigm and modified plus maze discriminative avoidance task to probe the effects of prelimbic cortex deep brain stimulation on various stages of memory.

KEY RESULTS

We identified memory consolidation stage as a critical time point to disrupt fear memory via prelimbic cortex deep brain stimulation. The observed disruption was partially modulated by the inactivation of the ventral hippocampus and the transient changes in ventral hippocampus dopamine (D₂ ) receptors expression upon prelimbic cortex deep brain stimulation. We also observed wide-scale changes of various neurotransmitters and their metabolites in ventral hippocampus, confirming its important role in response to prelimbic cortex deep brain stimulation.

CONCLUSION AND IMPLICATIONS

These findings highlight the molecular mechanism in the ventral hippocampus in response to prelimbic cortex stimulation and may have translational value, indicating that targeting the prelimbic cortex in the memory consolidation stage via non-invasive neuromodulation techniques may be a feasible therapeutic strategy against anxiety disorders.

mGlu5: A double-edged sword for aversive learning related therapeutics

Aversive memories underlie many types of anxiety disorders. One area of research to more effectively treat anxiety disorders has therefore been identifying pharmacological targets to affect memory processes. Among these targets, the metabotropic glutamate 5 receptor (mGlu5) has received attention due to the availability of drugs to utilize its role in learning and memory. In this review, we highlight preclinical studies examining the role of mGlu5 at various stages of aversive learning and its inhibition via extinction in order to gain a better understanding of its therapeutic potential. We suggest that mGlu5 has distinct roles at different stages of memory that not only makes it a tricky target, but a double-edged sword as a therapeutic. However, the selective involvement of mGlu5 in different memory stages allows for certain precision that could be harnessed clinically. We therefore suggest potential applications, limitations, and pitfalls when considering use of mGlu5 modulators as therapeutics. In addition, we recommend future studies to address important gaps in this literature, such as sex and age factors in light of anxiety disorders being more prevalent in those demographics.

A practical approach to the ethical use of memory modulating technologies

Background

Recent advancements in neuroscientific techniques have allowed us to make huge progress in our understanding of memories, and in turn has paved the way for new memory modification technologies (MMTs) that can modulate memories with a degree of precision, which was not previously possible. With advancements in such techniques, new and critical ethical questions have emerged. Understanding and framing these ethical questions within the current philosophical theories is crucial in order to systematically examine them as we translate these techniques to the clinic.

Main body

In this paper, we discuss the ethical implications of modern neuroscience techniques that aim to disrupt or enhance memories. We attempt to frame the MMTs in the context of existing ethical philosophical theories to provide a cohesive analysis of the myriad of ethical quagmires that might emerge from such technologies. We argue the application of Aristotle’s Golden Mean and multiple accounts of authenticity are useful in approaching the ethical questions surrounding MMTs. We then propose a framework in which ethical considerations can be systematically examined. Lastly, we provide caveats and considerations for the use of this framework. Overall, we provide a practical approach for the ethical use of MMTs depending on the situation.

Conclusion

While at face value, our model appears to put severe limitations on the application of MMTs, we are not completely opposed to their use, but rather our framework guides the agent to consider the implications before making any decisions. Most importantly, we argue that the use of MMTs does not reduce the responsibility of the initial decision, and the agent must accept the post-MMT self as the new “true self” regardless of the outcome. As the developmental trajectory of MMTs suggests we are getting closer to practical clinical applications, ethical concerns across a wide range of disciplines need to be addressed to develop best strategies and policies when dealing with MMTs. If this can be achieved, we believe the ethical use of MMTs is not only possible but would also be of tremendous benefit to many people suffering from memory-related mental disorders.

Amygdala GluN2B-NMDAR dysfunction is critical in abnormal aggression of neurodevelopmental origin induced by St8sia2 deficiency

Aggression is frequently observed in neurodevelopmental psychiatric disorders such as schizophrenia, autism, and bipolar disorder. Due to a lack of understanding of its underlying mechanisms, effective treatments for abnormal aggression are still missing. Recently, genetic variations in Sialyltransferase 2 (St8sia2) have been linked to these disorders and aggression. Here we identify abnormal aggressive behaviors and concomitant blunted fear learning in St8sia2 knockout (-/-) mice. It is worth noting that the amygdala of St8sia2-/- mice shows diminished threat-induced activation, as well as alterations in synaptic structure and function, including impaired GluN2B-containing NMDA receptor-mediated synaptic transmission and plasticity. Pharmacological rescue of NMDA receptor activity in the amygdala of St8sia2-/- mice with the partial agonist D-cycloserine restores synaptic plasticity and normalizes behavioral aberrations. Pathological aggression and associated traits were recapitulated by specific amygdala neonatal St8sia2 silencing. Our results establish a developmental link between St8sia2 deficiency and a pathological aggression syndrome, specify synaptic targets for therapeutic developments, and highlight D-cycloserine as a plausible treatment.

Context and topography determine the role of basolateral amygdala metabotropic glutamate receptor 5 in appetitive Pavlovian responding

Preclinical data have shown that the excitatory metabotropic Gα_q-coupled glutamate receptor, mGluR5, has a role in substance abuse and relapse. However, little is known about the contribution of mGluR5 to the expression of conditioned responding elicited by appetitive Pavlovian cues. We investigated this question in rats that were trained to associate a discrete, auditory conditioned stimulus (CS) with a fructose-glucose solution (5.5% fructose/4.5% glucose; "sugar"). In subsequent tests for the expression of conditioned responding without sugar delivery, CS-elicited fluid port entries were elevated in a context associated with sugar, relative to an equally familiar, neutral context. Inhibiting mGluR5 via systemic injections of a negative allosteric modulator (MTEP; 5 mg/kg) reduced CS port entries in both the sugar context and neutral context. Targeting MTEP microinjections (3 µg/side; 0.3 µl/min) to the nucleus accumbens (Acb) core had no effect on CS port entries at test, whereas the same manipulation in the basolateral amygdala (BLA) produced effects that were topographically dependent. Specifically, microinjecting MTEP in the posterior BLA had no effect on behavior, whereas inhibiting mGluR5 in the anterior BLA enhanced the contextual discrimination of CS port entries. These data are the first to show a role of mGluR5 in the context-dependent expression of appetitive Pavlovian conditioned responding, with a topographically defined arrangement of mGluR5 in the BLA being particularly important for context-based responding to a discrete, appetitive cue.

Effects of Methamphetamine Exposure on Fear Learning and Memory in Adult and Adolescent Rats

Methamphetamine (meth) use is often comorbid with anxiety disorders, with both conditions predominant during adolescence. Conditioned fear extinction is the most widely used model to study the fear learning and regulation that are relevant for anxiety disorders. The present study investigates how meth binge injections or meth self-administration affect subsequent fear conditioning, extinction and retrieval in adult and adolescent rats. In experiment 1, postnatal day 35 (P35-adolescent) and P70 (adult) rats were intraperitoneally injected with increasing doses of meth across 9 days. At P50 or P85, they underwent fear conditioning followed by extinction and test. In experiments 2a-c, P35 or P70 rats self-administered meth for 11 days then received fear conditioning at P50 or P85, followed by extinction and test. We observed that meth binge exposure caused a significant disruption of extinction retrieval in adult but not adolescent rats. Interestingly, meth self-administration in adolescence or adulthood disrupted acquisition of conditioned freezing in adulthood. Meth self-administration in adolescence did not affect conditioned freezing in adolescence. These results suggest that intraperitoneal injections of high doses of meth and meth self-administration have dissociated effects on fear conditioning and extinction during adulthood, while adolescent fear conditioning and extinction are unaffected.

Eternal sunshine of the neuromodulated mind: Altering fear memories through neuromodulation

Anxiety disorders pose one of the greatest threats to mental health. Modern treatment methods exist but are hindered by relapse, toxicity, and low efficacy. The use of neuromodulation to treat anxiety disorders has shown promising results, yet its underpinning mechanisms remain poorly understood. In this review, we make the case for further development of neuromodulation techniques to alter fear memories, with particular regard to future clinical applications in treating anxiety disorders. We start by briefly summarizing the neural circuitry of fear while identifying the pros and cons of possible neuromodulation targets. We then highlight recent advances in neuromodulation techniques that have been used to alter fear memories. Next, we apply a novel network-based approach to elucidate possible mechanisms of neuromodulation which may disrupt the consolidation of fear memory. Finally, we emphasize the need for more systematic neuromodulation studies on animal models and the developing brain. Overall, we aim to provide an integrated framework for future action, identifying key research priorities that must be addressed before effective neuromodulation-based treatments can be developed for practical use.

Touchscreen testing reveals clinically relevant cognitive abnormalities in a mouse model of schizophrenia lacking metabotropic glutamate receptor 5

Metabotropic glutamate receptor 5 (mGlu5) has been implicated in certain forms of synaptic plasticity and cognitive function. mGlu5 knockout (KO) mice and mGlu5 antagonists have been previously used to study the pathophysiology of schizophrenia as they have been shown respectively to display or induce endophenotypes relevant to schizophrenia. While schizophrenia presents with generalized cognitive impairments, the cognitive phenotype of mice lacking mGlu5 has so far only been explored using largely hippocampal-dependent spatial and contextual memory tasks. To address this, we used a touchscreen system to assess mGlu5 KO mice for pairwise visual discrimination, reversal learning, and extinction of an instrumental response requiring no discrimination. Furthermore, we tested the role of mGlu5 in working memory using the Trial-Unique Non-Matching to Location (TUNL) task utilizing pharmacological ablation. mGlu5 KO mice were impaired on discrimination learning, taking longer to reach criterion and requiring more correction learning trials. Performance on reversal learning was also impaired, with mGlu5 KO mice demonstrating a perseverative phenotype. The mGlu5 KO mice responded at a higher rate during extinction, consistent with this perseverative profile. In contrast, wildtype mice treated acutely with an mGlu5 antagonist (MTEP) showed no deficits in a touchscreen task assessing working memory. The present study demonstrates learning and memory deficits as well as an increased perseverative phenotype following constitutive loss of mGlu5 in this mouse model of schizophrenia. These findings will inform translational approaches using this preclinical model and the pursuit of mGlu5 as therapeutic target for schizophrenia and other brain disorders.

Dissociated Role of D-Serine in Extinction During Consolidation vs. Reconsolidation of Context Conditioned Fear

Extinction-based exposure therapy is widely used for the treatment of anxiety disorders, such as post-traumatic stress disorder (PTSD). D-serine, an endogenous co-agonist at the glycine-binding site of the N-methyl-D-aspartate-type glutamate receptor (NMDAR), has been shown to be involved in extinction of fear memory. Recent findings suggest that the length of time between the initial learning and an extinction session is a determinant of neural mechanism involved in fear extinction. However, how D-serine is involved in extinction of fear memory at different timings remains unclear. In the present study, we investigated the role of D-serine in immediate, delayed and post-retrieval extinction (P-RE) of contextual fear memory using wild-type (WT) and serine racemase (SRR) knockout (KO) mice that exhibit 90% reduction in D-serine content in the hippocampus. We found that SRR disruption impairs P-RE, facilitates immediate extinction (IE), but has no effect on delayed extinction (DE) of contextual fear memories. The impaired P-RE of contextual fear memory in SRRKO mice was associated with increased expression of the GluA1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor (AMPAR) in the hippocampal synaptic membrane fraction after P-RE, and this increase of AMPAR and impaired P-RE were rescued by the administration of D-serine to SRRKO mice. Our findings suggest that D-serine is differentially involved in the regulation of contextual fear extinction depending on the timing of behavioral intervention, and that combining D-serine or other drugs, enhancing the NMDAR function, with P-RE may achieve optimal outcomes for the treatment of PTSD.

Aripiprazole Facilitates Extinction of Conditioned Fear in Adolescent Rats

Anxiety disorders are the most common type of mental disorder during adolescence, which is at least partly due to the resistance to extinction exhibited at this age. The dopaminergic system is known to be dysregulated during adolescence; therefore, we aimed to facilitate extinction in adolescent rats using the dopamine receptor 2 partial agonist aripiprazole (Abilify™), and examine the behavioral and neural outcomes. Adolescent rats were conditioned to fear a tone. The next day, rats received extinction 30 min after a systemic injection of either 5 mg/kg aripiprazole or vehicle, and then were tested the following day. For the immunohistochemistry experiment, naïve and "no extinction" conditions were added and rats were perfused either on the extinction day or test day. To assess the activation of neurons receiving dopaminergic input, c-Fos, and dopamine- and cAMP-regulated neuronal phosphoprotein (DARPP-32) labeled neurons were quantified in the amygdala and the medial prefrontal cortex (mPFC). Systemic treatment with aripiprazole at the time of extinction significantly reduced freezing at test the next day. This effect was not observed in rats that were fear conditioned but did not receive any extinction. Aripiprazole's facilitation of extinction was accompanied by increased activation of neurons in the mPFC. Taken together, aripiprazole represents a novel pharmacological adjunct to exposure therapy worthy of further examination. The effect of aripiprazole is related to enhanced activation of mPFC neurons receiving dopaminergic innervation.

Exercise alters mouse sperm small noncoding RNAs and induces a transgenerational modification of male offspring conditioned fear and anxiety

There is growing evidence that the preconceptual lifestyle and other environmental exposures of a father can significantly alter the physiological and behavioral phenotypes of their children. We and others have shown that paternal preconception stress, regardless of whether the stress was experienced during early-life or adulthood, results in offspring with altered anxiety and depression-related behaviors, attributed to hypothalamic-pituitary-adrenal axis dysregulation. The transgenerational response to paternal preconceptual stress is believed to be mediated by sperm-borne small noncoding RNAs, specifically microRNAs. As physical activity confers physical and mental health benefits for the individual, we used a model of voluntary wheel-running and investigated the transgenerational response to paternal exercise. We found that male offspring of runners had suppressed reinstatement of juvenile fear memory, and reduced anxiety in the light-dark apparatus during adulthood. No changes in these affective behaviors were observed in female offspring. We were surprised to find that running had a limited impact on sperm-borne microRNAs. The levels of three unique microRNAs (miR-19b, miR-455 and miR-133a) were found to be altered in the sperm of runners. In addition, we discovered that the levels of two species of tRNA-derived RNAs (tDRs)-tRNA-Gly and tRNA-Pro-were also altered by running. Taken together, we believe this is the first evidence that paternal exercise is associated with an anxiolytic behavioral phenotype of male offspring and altered levels of small noncoding RNAs in sperm. These small noncoding RNAs are known to have an impact on post-transcriptional gene regulation and can thus change the developmental trajectory of offspring brains and associated affective behaviors.

Knockdown of corticotropin-releasing factor 1 receptors in the ventral tegmental area enhances conditioned fear

The neuropeptide corticotropin-releasing factor (CRF) coordinates the physiological and behavioural responses to stress. CRF receptors are highly expressed in the ventral tegmental area (VTA), an important region for motivated behaviour. Therefore, we examined the role of CRF receptor type 1 (CRFR1) in the VTA in conditioned fear, using a viral-mediated RNA interference approach. Following stereotaxic injection of a lentivirus that contained either shCRF-R1 or a control sequence, mice received tone-footshock pairings. Intra-VTA shCRF-R1 did not affect tone-elicited freezing during conditioning. Once conditioned fear was acquired, however, shCRF-R1 mice consistently showed stronger freezing to the tone even after extinction and reinstatement. These results implicate a novel role of VTA CRF-R1 in conditioned fear, and suggest how stress may modulate aversive learning and memory.

Elevated paternal glucocorticoid exposure alters the small noncoding RNA profile in sperm and modifies anxiety and depressive phenotypes in the offspring

Recent studies have suggested that physiological and behavioral traits may be transgenerationally inherited through the paternal lineage, possibly via non-genomic signals derived from the sperm. To investigate how paternal stress might influence offspring behavioral phenotypes, a model of hypothalamic-pituitary-adrenal (HPA) axis dysregulation was used. Male breeders were administered water supplemented with corticosterone (CORT) for 4 weeks before mating with untreated female mice. Female, but not male, F1 offspring of CORT-treated fathers displayed altered fear extinction at 2 weeks of age. Only male F1 offspring exhibited altered patterns of ultrasonic vocalization at postnatal day 3 and, as adults, showed decreased time in open on the elevated-plus maze and time in light on the light-dark apparatus, suggesting a hyperanxiety-like behavioral phenotype due to paternal CORT treatment. Interestingly, expression of the paternally imprinted gene Igf2 was increased in the hippocampus of F1 male offspring but downregulated in female offspring. Male and female F2 offspring displayed increased time spent in the open arm of the elevated-plus maze, suggesting lower levels of anxiety compared with control animals. Only male F2 offspring showed increased immobility time on the forced-swim test and increased latency to feed on the novelty-supressed feeding test, suggesting a depression-like phenotype in these animals. Collectively, these data provide evidence that paternal CORT treatment alters anxiety and depression-related behaviors across multiple generations. Analysis of the small RNA profile in sperm from CORT-treated males revealed marked effects on the expression of small noncoding RNAs. Sperm from CORT-treated males contained elevated levels of three microRNAs, miR-98, miR-144 and miR-190b, which are predicted to interact with multiple growth factors, including Igf2 and Bdnf. Sustained elevation of glucocorticoids is therefore involved in the transmission of paternal stress-induced traits across generations in a process involving small noncoding RNA signals transmitted by the male germline.

Apelin-13 Impaires Acquisition but Not Consolidation or Expression of Contextual Fear in Rats

Apelin-13, as an endogenous neuropeptide, is the ligand for the G-protein-coupled receptor, APJ, which has recently been demonstrated to be involved in the process that contributes to learning and memory. Previous studies showed that apelin may be required for certain forms of learning and memory. Up to date, the role of apelin in fear memory has not been explored. In the present study, we tested the effects of apelin-13 (1.0, 2.0 and 4.0 µg/rat) on contextual fear conditioning (experiment 1), consolidation (experiment 2) and expression (experiment 3) in rats. A well established fear conditioning protocol was used, which contained three training phases: habituation, fear conditioning and test. Apelin-13 was i.c.v injected 10 min before conditioning (experiment 1), immediately after conditioning (experiment 2) or 10 min before testing (experiment 3). The values of percent freezing were used to measure fear. We found that only 2.0 µg apelin-13 administrations produced a decrease freezing in experiment 1. The most effective dose of apelin-13 (2.0 µg) was selected, but it had no effect on freezing in experiment 2 and 3. Furthermore, the decreased freezing in experiment 1 was not attributed to the deficits of locomotor activity and foot-shock sensitivity. These results, for the first time, indicated that apelin-13 impaired fear acquisition but not fear consolidation or expression.

The metabotropic glutamate receptor, mGlu5, is required for extinction learning that occurs in the absence of a context change

The metabotropic glutamate (mGlu) receptors and, in particular, mGlu5 are crucially involved in multiple forms of synaptic plasticity that are believed to underlie explicit memory. MGlu5 is also required for information transfer through neuronal oscillations and for spatial memory. Furthermore, mGlu5 is involved in extinction of implicit forms of learning. This places this receptor in a unique position with regard to information encoding. Here, we explored the role of this receptor in context-dependent extinction learning under constant, or changed, contextual conditions. Animals were trained over 3 days to take a left turn under 25% reward probability in a T-maze with a distinct floor pattern (Context A). On Day 4, they experienced either a floor pattern change (Context B) or the same floor pattern (Context A) in the absence of reward. After acquisition of the task, the animals were returned to the maze once more on Day 5 (Context A, no reward). Treatment with the mGlu5 antagonist, 2-methyl-6-(phenylethynyl) pyridine, before maze exposure on Day 4 completely inhibited extinction learning in the AAA paradigm but had no effect in the ABA paradigm. A subsequent return to the original context (A, on Day 5) revealed successful extinction in the AAA paradigm, but impairment of extinction in the ABA paradigm. These data support that although extinction learning in a new context is unaffected by mGlu5 antagonism, extinction of the consolidated context is impaired. This suggests that mGlu5 is intrinsically involved in enabling learning that once-relevant information is no longer valid.