Fear extinction in 17 day old rats is dependent on metabotropic glutamate receptor 5 signaling

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.

The role of mGlu receptors in susceptibility to stress-induced anhedonia, fear, and anxiety-like behavior

Stress and trauma exposure contribute to the development of psychiatric disorders such as post-traumatic stress disorder (PTSD) and major depressive disorder (MDD) in a subset of people. A large body of preclinical work has found that the metabotropic glutamate (mGlu) family of G protein-coupled receptors regulate several behaviors that are part of the symptom clusters for both PTSD and MDD, including anhedonia, anxiety, and fear. Here, we review this literature, beginning with a summary of the wide variety of preclinical models used to assess these behaviors. We then summarize the involvement of Group I and II mGlu receptors in these behaviors. Bringing together this extensive literature reveals that mGlu5 signaling plays distinct roles in anhedonia, fear, and anxiety-like behavior. mGlu5 promotes susceptibility to stress-induced anhedonia and resilience to stress-induced anxiety-like behavior, while serving a fundamental role in the learning underlying fear conditioning. The medial prefrontal cortex, basolateral amygdala, nucleus accumbens, and ventral hippocampus are key regions where mGlu5, mGlu2, and mGlu3 regulate these behaviors. There is strong support that stress-induced anhedonia arises from decreased glutamate release and post-synaptic mGlu5 signaling. Conversely, decreasing mGlu5 signaling increases resilience to stress-induced anxiety-like behavior. Consistent with opposing roles for mGlu5 and mGlu2/3 in anhedonia, evidence suggests that increased glutamate transmission may be therapeutic for the extinction of fear learning. Thus, a large body of literature supports the targeting of pre- and post-synaptic glutamate signaling to ameliorate post-stress anhedonia, fear, and anxiety-like behavior.

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.

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.

Context fear learning and renewal of extinguished fear are dissociated in juvenile female rats

Extinction is the decrease in emotion to a cue that was previously associated with an emotionally significant event. It involves repeated presentation of the cue without any consequences. In adult animals, extinguished fear to a cue can return if the cue is presented in a different environment/context to where extinction occurred, referred to as renewal. We have previously reported that developing female, but not male, rats show renewal. This study investigates whether the ability of developing female rats to show renewal is related to their ability in fear conditioning to the context. Additionally, facilitation of context conditioning by weaning previously shown in male rats was tested in developing female rats. In experiment 1, postnatal day 25 (P25) and P18 female rats showed renewal. P25 rats show more fear overall, suggesting a weaker extinction recall in this age. Experiment 2 tested context- and cue-elicited fear either immediately or 24 hr following conditioning. At the immediate test, P18 rats showed less context-fear compared with P25 rats. All rats showed low levels of context-fear at the 24 hr test. There were no age differences in cued fear. Weaning at P21 did not affect context or cue memory in P25 female rats. These findings suggest that the ability to form contextual fear memory is unrelated to the expression of renewal in juvenile female rats.

Juvenile female rats, but not male rats, show renewal, reinstatement, and spontaneous recovery following extinction of conditioned fear

Anxiety disorders emerge early, and girls are significantly more likely to develop anxiety compared to boys. However, sex differences in fear during development are poorly understood. Therefore, we investigated juvenile male and female rats in the relapse behaviors following extinction of conditioned fear. In all experiments, 18-d-old rats first received three white-noise–footshock pairings on day 1. On day 2, extinction involved 60 white-noise alone trials. In experiment 1, we examined renewal by testing the rats in either the same or different context as extinction on day 3. Male rats did not show renewal, however, female rats showed renewal. Experiment 2 investigated reinstatement by giving rats either a mild reminder footshock or context exposure on day 3. When tested the next day, male rats did not show reinstatement, whereas female rats showed reinstatement. Experiment 3 investigated spontaneous recovery by testing the rats either 1 or 5 d following extinction. Male rats did not show any spontaneous recovery whereas female rats did. Taken together, fear regulation appear to be different in males versus females from early in development, which may explain why girls are more prone to suffer from anxiety disorders compared to boys.

Reducing Fear During Childhood to Prevent Anxiety Disorders Later: Insights From Developmental Psychobiology

Anxiety disorders are neurodevelopmental with the median age of onset 10 to 11 years, but developmental processes underlying fear and anxiety are rarely investigated. In the last decade, however, developmental rodent studies have increased our understanding of how to treat and prevent the persistence of anxiety. Behavioral findings from rodent studies match the observations in anxious children, and the neural and molecular findings help explain why anxiety disorders are indeed neurodevelopmental. Extinction processes that are involved in cognitive-behavioral therapy appear particularly effective in children compared with older populations. Policy should mandate school psychologists and government subsidies for therapy sessions to increase children’s mental-health-service utilization. Funding bodies also should challenge anxiety studies exclusively targeting adults to include younger people to investigate why anxiety disorders are developmental disorders and focus more on preventing their persistence later in life.

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.

A dissociation between renewal and contextual fear conditioning in juvenile rats

We investigated whether juvenile rats do not express renewal following extinction of conditioned fear due to their inability to form a long-term contextual fear memory. In experiment 1, postnatal day (P) 18 and 25 rats received 3 white-noise and footshock pairings, followed by 60 white-noise alone presentations the next day. When tested in a different context to extinction, P25 rats displayed renewal whereas P18 rats did not. Experiments 2A and 2B surprisingly showed that P18 and P25 rats do not show differences in contextual and cued fear, regardless of the conditioning-test intervals and the number of white-noise-footshock pairings received. Finally, we observed age differences in contextual fear when P25 rats were weaned at P21 in experiment 3. These results indicate that the developmental dissociation observed in renewal of extinguished fear is not related to the widely believed late emergence of contextual fear learning.

Prefrontal Dopaminergic Mechanisms of Extinction in Adolescence Compared to Adulthood in Rats

Adolescents with anxiety disorders attain poorer outcomes following extinction-based treatment compared to adults. Extinction deficit during adolescence has been identified to involve immaturity in the medial prefrontal cortex (mPFC). Findings from adult rodents suggest extinction involves dopamine signaling in the mPFC. This system changes dramatically during adolescence, but its role in adolescent extinction is unknown. Therefore, we investigated the role of prefrontal dopamine in extinction using Pavlovian fear conditioning in adolescent and adult rats. Using quantitative PCR (qPCR) analyses, we measured changes in dopamine receptor gene expression in the mPFC before and after extinction. We then enhanced dopamine 1 receptor (D1R) or dopamine 2 receptor (D2R) signaling in the infralimbic cortex (IL) of the mPFC using agonists at the time of extinction. Adolescent rats displayed a deficit in extinction retention compared to adults. Extinction induced a reduction in D1R compared to D2R gene expression in adolescent rats, whereas an increase of D1R compared to D2R gene expression was observed in adult rats. Acutely enhancing IL D1R signaling using SKF-81297 had no effect on extinction at either age. In contrast, acutely enhancing IL D2R signaling with quinpirole significantly enhanced long-term extinction in adolescents, and impaired within-session extinction in adults. Our results suggest a dissociated role for prefrontal dopamine in fear extinction during adolescence compared to adulthood. Findings highlight the dopamine system as a potential pharmacological target to improve extinction-based treatments for adolescents.

Biased allosteric agonism and modulation of metabotropic glutamate receptor 5: Implications for optimizing preclinical neuroscience drug discovery

Allosteric modulators, that exhibit no intrinsic agonist activity, offer the advantage of spatial and temporal fine-tuning of endogenous agonist activity, allowing the potential for increased selectivity, reduced adverse effects and improved clinical outcomes. Some allosteric ligands can differentially activate and/or modulate distinct signaling pathways arising from the same receptor, phenomena referred to as 'biased agonism' and 'biased modulation'. Emerging evidence for CNS disorders with glutamatergic dysfunction suggests the metabotropic glutamate receptor subtype 5 (mGlu₅) is a promising target. Current mGlu₅ allosteric modulators have largely been classified based on modulation of intracellular calcium (iCa²⁺) responses to orthosteric agonists alone. We assessed eight mGlu₅ allosteric modulators previously classified as mGlu₅ PAMs or PAM-agonists representing four distinct chemotypes across multiple measures of receptor activity, to explore their potential for engendering biased agonism and/or modulation. Relative to the reference orthosteric agonist, DHPG, the eight allosteric ligands exhibited distinct biased agonism fingerprints for iCa²⁺ mobilization, IP₁ accumulation and ERK1/2 phosphorylation in HEK293A cells stably expressing mGlu₅ and in cortical neuron cultures. VU0424465, DPFE and VU0409551 displayed the most disparate biased signaling fingerprints in both HEK293A cells and cortical neurons that may account for the marked differences observed previously for these ligands in vivo. Select mGlu₅ allosteric ligands also showed 'probe dependence' with respect to their cooperativity with different orthosteric agonists, as well as biased modulation for the magnitude of positive cooperativity observed. Unappreciated biased agonism and modulation may contribute to unanticipated effects (both therapeutic and adverse) when translating from recombinant systems to preclinical models. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

The metabotropic glutamate 5 receptor is necessary for extinction of cocaine-associated cues

BACKGROUND AND PURPOSE

There is currently no medication approved specifically to treat cocaine addiction. Behavioural interventions such as cue exposure therapy (CET) rely heavily on new learning. Antagonism of the metabotropic glutamate 5 (mGlu5 ) receptor has emerged as a potential treatment, by reducing the reinforcing properties of cocaine. However, mGlu5 receptor activity is necessary for learning; therefore, such agents could interfere with behavioural treatments. We used a novel rodent model of CET to test the effects of mGlu5 negative and positive allosteric modulators (NAM and PAM) on behavioural therapy.

EXPERIMENTAL APPROACH

Rats were trained to press a lever for cocaine in the presence of a discrete cue [conditioned stimulus (CS)] and then extinguished in the absence of the CS. Following lever extinction, half the rats received CS extinction in the same chambers but with the levers withdrawn; the remaining rats received no CS extinction. Before this session, rats received a systemic administration of either vehicle or a mGlu5 NAM (MTEP, experiment 1) or PAM (CDPPB, experiment 2). Cue-induced reinstatement was tested in a drug-free session the following day.

KEY RESULTS

At reinstatement, rats that had received CS extinction showed reduced responding. This effect was attenuated by MTEP treatment before CS extinction. In contrast, administration of CDPPB (PAM) led to decreased reinstatement the following day, regardless of extinction condition.

CONCLUSION AND IMPLICATIONS

These results suggest that mGlu5 receptor activity is both necessary and sufficient for efficient extinction of a cocaine-associated CS. Therefore, mGlu5 PAMs could enhance the efficacy of CET.

Metabotropic glutamate receptor subtype 5: molecular pharmacology, allosteric modulation and stimulus bias

The metabotropic glutamate receptor subtype 5 (mGlu5 ) is a family C GPCR that has been implicated in various neuronal processes and, consequently, in several CNS disorders. Over the past few decades, GPCR-based drug discovery, including that for mGlu5 receptors, has turned considerable attention to targeting allosteric binding sites. Modulation of endogenous agonists by allosteric ligands offers the advantages of spatial and temporal fine-tuning of receptor activity, increased selectivity and reduced adverse effects with the potential to elicit improved clinical outcomes. Further, with greater appreciation of the multifaceted nature of the transduction of mGlu5 receptor signalling, it is increasingly apparent that drug discovery must take into consideration unique receptor conformations and the potential for stimulus-bias. This novel paradigm proposes that different ligands may differentially modulate distinct signalling pathways arising from the same receptor. We review our current understanding of the complexities of mGlu5 receptor signalling and regulation, and how these relate to allosteric ligands. Ultimately, a deeper appreciation of these relationships will provide the foundation for targeted drug design of compounds with increased selectivity, not only for the desired receptor but also for the desired signalling outcome from the receptor. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

A dissociation between recognition and reactivation: The renewal effect at 3 months of age

Extinction allows organisms to adapt to an ever-changing environment. Despite its theoretical and applied significance, extinction has never been systematically studied with human infants. Using the operant mobile task, we examined whether 3-month-olds would exhibit evidence of original learning following extinction. In a recognition paradigm, infants exhibited renewal when tested in the acquisition context (ABA renewal) or a neutral context (ABC and AAB renewal) 1 day following extinction (Experiment 1a) and spontaneous recovery 3 days following extinction (Experiment 1b). In Experiments 2a-2b, we used a reminder paradigm to examine whether the extinguished response could be reinstated after the operant response had been forgotten. We failed, however, to find reinstatement of extinguished responding after spontaneous forgetting, regardless of the reminder and test contexts. We attributed this retention failure to competing responses at test. Although extinguished responding is recovered during infancy, this effect is elusive after the response has been forgotten.