Prefrontal dopamine D4 receptors are involved in encoding fear extinction

Dopamine D2-like receptors on conditioned and unconditioned fear: A systematic review of rodent pharmacological studies

Growing evidence supports dopamine's role in aversive states, yet systematic reviews focusing on dopamine receptors in defensive behaviors are lacking. This study presents a systematic review of the literature examining the influence of drugs acting on dopamine D2-like receptors on unconditioned and conditioned fear in rodents. The review reveals a predominant use of adult male rats in the studies, with limited inclusion of female rodents. Commonly employed tests include the elevated plus maze and auditory-cued fear conditioning. The findings indicate that systemic administration of D2-like drugs has a notable impact on both innate and learned aversive states. Generally, antagonists tend to increase unconditioned fear, while agonists decrease it. Moreover, both agonists and antagonists typically reduce conditioned fear. These effects are attributed to the involvement of distinct neural circuits in these states. The observed increase in unconditioned fear induced by D2-like antagonists aligns with dopamine's role in suppressing midbrain-mediated responses. Conversely, the reduction in conditioned fear is likely a result of blocking dopamine activity in the mesolimbic pathway. The study highlights the need for future research to delve into sex differences, explore alternative testing paradigms, and identify specific neural substrates. Such investigations have the potential to advance our understanding of the neurobiology of aversive states and enhance the therapeutic application of dopaminergic agents.

Psilocybin Facilitates Fear Extinction: Importance of Dose, Context, and Serotonin Receptors

A variety of classic psychedelics and MDMA have been shown to enhance fear extinction in rodent models. This has translational significance because a standard treatment for post-traumatic stress disorder (PTSD) is prolonged exposure therapy. However, few studies have investigated psilocybin's potential effect on fear learning paradigms. More specifically, the extents to which dose, timing of administration, and serotonin receptors may influence psilocybin's effect on fear extinction are not understood. In this study, we used a delay fear conditioning paradigm to determine the effects of psilocybin on fear extinction, extinction retention, and fear renewal in male and female mice. Psilocybin robustly enhances fear extinction when given acutely prior to testing for all doses tested. Psilocybin also exerts long-term effects to elevate extinction retention and suppress fear renewal in a novel context, although these changes were sensitive to dose. Analysis of sex differences showed that females may respond to a narrower range of doses than males. Administration of psilocybin prior to fear learning or immediately after extinction yielded no change in behavior, indicating that concurrent extinction experience is necessary for the drug's effects. Cotreatment with a 5-HT2A receptor antagonist blocked psilocybin's effects for extinction, extinction retention, and fear renewal, whereas 5-HT1A receptor antagonism attenuated only the effect on fear renewal. Collectively, these results highlight dose, context, and serotonin receptors as crucial factors in psilocybin's ability to facilitate fear extinction. The study provides preclinical evidence to support investigating psilocybin as a pharmacological adjunct for extinction-based therapy for PTSD.

65 years of research on dopamine's role in classical fear conditioning and extinction: A systematic review

Dopamine, a catecholamine neurotransmitter, has historically been associated with the encoding of reward, whereas its role in aversion has received less attention. Here, we systematically gathered the vast evidence of the role of dopamine in the simplest forms of aversive learning: classical fear conditioning and extinction. In the past, crude methods were used to augment or inhibit dopamine to study its relationship with fear conditioning and extinction. More advanced techniques such as conditional genetic, chemogenic and optogenetic approaches now provide causal evidence for dopamine's role in these learning processes. Dopamine neurons encode conditioned stimuli during fear conditioning and extinction and convey the signal via activation of D1-4 receptor sites particularly in the amygdala, prefrontal cortex and striatum. The coordinated activation of dopamine receptors allows for the continuous formation, consolidation, retrieval and updating of fear and extinction memory in a dynamic and reciprocal manner. Based on the reviewed literature, we conclude that dopamine is crucial for the encoding of classical fear conditioning and extinction and contributes in a way that is comparable to its role in encoding reward.

Behavioural effects of APH199, a selective dopamine D4 receptor agonist, in animal models

RATIONALE

The dopamine D4 receptors (DRD4) play a key role in numerous brain functions and are involved in the pathogenesis of various psychiatric disorders. DRD4 ligands have been shown to moderate anxiety, reward and depression-like behaviours, and cognitive impairments. Despite a series of promising but ambiguous findings, the therapeutic advantages of DRD4 stimulation remain elusive.

OBJECTIVES

The investigation focused on the behavioural effects of the recently developed DRD4 agonist, APH199, to evaluate its impact on anxiety, anhedonia, behavioural despair, establishment and retrieval of alcohol reinforcement, and amphetamine (AMPH)-induced symptoms.

METHODS

Male C57BL/6 J mice and Sprague-Dawley rats were examined in five independent experiments. We assessed APH199 (0.1-5 mg/kg, i.p.) effects on a broad range of behavioural parameters in the open field (OF) test, conditioned place preference test (CPP), elevated plus maze (EPM), light-dark box (LDB), novelty suppressed feeding (NSF), forced swim test (FST), sucrose preference test (SPT), AMPH-induced hyperlocomotion test (AIH), and prepulse inhibition (PPI) of the acoustic startle response in AMPH-sensitized rats.

RESULTS

APH199 caused mild and sporadic anxiolytic and antidepressant effects in EPM and FST, but no remarkable impact on behaviour in other tests in mice. However, we found a significant increase in AMPH-induced hyperactivity, suggesting an exaggeration of the psychotic-like responses in the AMPH-sensitized rats.

CONCLUSIONS

Our data challenged the hypothesis of the therapeutic benefits of DRD4 agonists, pointing out a possible aggravation of psychosis. We suggest a need for further preclinical studies to ensure the safety of antipsychotics with DRD4 stimulating properties.

Dopamine in Fear Extinction

The ability to extinguish fear memories when threats are no longer present is critical for adaptive behavior. Fear extinction represents a new learning process that eventually leads to the formation of extinction memories. Understanding the neural basis of fear extinction has considerable clinical significance as deficits in extinction learning are the hallmark of human anxiety disorders. In recent years, the dopamine (DA) system has emerged as one of the key regulators of fear extinction. In this review article, we highlight recent advances that have demonstrated the crucial role DA plays in mediating different phases of fear extinction. Emerging concepts and outstanding questions for future research are also discussed.

Efficacy and mechanisms of non-invasive brain stimulation to enhance exposure therapy: A review

Though cognitive behavioral techniques are generally effective in the treatment of anxiety disorders, some people fail to benefit from exposure therapy or experience a return of fear after terminating exposure therapy. The burgeoning field of non-invasive brain stimulation provides a potential method of augmenting exposure therapy so that it is more effective. Successful exposure therapy is hypothesized to occur due to inhibition, and research suggests that brain stimulation can alter inhibitory learning and related processes. As such, one can reasonably posit that brain stimulation could be used to test the inhibitory learning theory of exposure therapy and to increase the efficacy of exposure therapy by inducing stronger inhibitory learning during exposures. Four known studies that pair brain stimulation with exposure therapy have yielded promising preliminary evidence in support of the therapeutic use of brain stimulation. In this review we describe research illustrating the mechanisms and efficacy of non-invasive brain stimulation to enhance the understanding of and outcomes produced by exposure therapy.

Divergent prefrontal dopaminergic mechanisms mediate drug- and fear-associated cue extinction during adolescence versus adulthood

Cue-associated learning is vital to guiding behaviour for survival. Adolescence represents a key developmental stage for perturbations in cue-related learning, including a characteristic deficit in cue extinction learning. The present review summarizes evidence from animal and human literature that cue extinction is critically mediated by prefrontal dopamine, a system that undergoes dramatic reorganization during adolescence. We propose that extinction learning and memory is governed by a developmentally dynamic balance of dopamine receptors in the prefrontal cortex, which changes across adolescence into adulthood. This is contrary to the previous idea that extinction deficits during adolescence reflect inefficiency in the same neural circuitry as adults. This leads to proposal of the novel theory that cue extinction involves divergent prefrontal dopaminergic mechanisms depending on the age of extinction.

Traumatic stress causes distinctive effects on fear circuit catecholamines and the fear extinction profile in a rodent model of posttraumatic stress disorder

Central catecholamines regulate fear memory across the medial prefrontal cortex (mPFC), amygdala (AMYG), and hippocampus (HPC). However, inadequate evidence exists to address the relationships among these fear circuit areas in terms of the fear symptoms of posttraumatic stress disorder (PTSD). By examining the behavioral profile in a Pavlovian fear conditioning paradigm together with tissue/efflux levels of dopamine (DA) and norepinephrine (NE) and their reuptake abilities across the fear circuit areas in rats that experienced single prolonged stress (SPS, a rodent model of PTSD), we demonstrated that SPS-impaired extinction retrieval was concomitant with the changes of central DA/NE in a dissociable manner. For tissue levels, diminished DA and increased NE were both observed in the mPFC and AMYG. DA efflux and synaptosomal DA transporter were consistently reduced in the AMYG/vHPC, whereas SPS reduced NE efflux in the infralimbic cortex and synaptosomal NE transporter in the mPFC. Furthermore, a lower expression of synaptosomal VMAT2 was observed in the mPFC, AMYG, and vHPC after SPS. Finally, negative correlations were observed between retrieval freezing and DA in the mPFC/AMYG; nevertheless, the phenomena became invalid after SPS. Our results suggest that central catecholamines are crucially involved in the retrieval of fear extinction in which DA and NE play distinctive roles across the fear circuit areas.

Adolescent social defeat alters N-methyl-D-aspartic acid receptor expression and impairs fear learning in adulthood

Repeated social defeat of adolescent male rats results in adult mesocortical dopamine hypofunction, impaired working memory, and increased contextual anxiety-like behavior. Given the role of glutamate in dopamine regulation, cognition, and fear and anxiety, we investigated potential changes to N-methyl-D-aspartic acid (NMDA) receptors following adolescent social defeat. As both NMDA receptors and mesocortical dopamine are implicated in the expression and extinction of conditioned fear, a separate cohort of rats was challenged with a classical fear conditioning paradigm to investigate whether fear learning is altered by adolescent defeat. Quantitative autoradiography was used to measure 3H-MK-801 binding to NMDA receptors in regions of the medial prefrontal cortex, caudate putamen, nucleus accumbens, amygdala and hippocampus. Assessment of fear learning was achieved using an auditory fear conditioning paradigm, with freezing toward the auditory tone used as a measure of conditioned fear. Compared to controls, adolescent social defeat decreased adult NMDA receptor expression in the infralimbic region of the prefrontal cortex and central amygdala, while increasing expression in the CA3 region of the hippocampus. Previously defeated rats also displayed decreased conditioned freezing during the recall and first extinction periods, which may be related to the observed decreases and increases in NMDA receptors within the central amygdala and CA3, respectively. The alteration in NMDA receptors seen following adolescent social defeat suggests that dysfunction of glutamatergic systems, combined with mesocortical dopamine deficits, likely plays a role in the some of the long-term behavioral consequences of social stressors in adolescence seen in both preclinical and clinical studies.

Pharmacology of cognitive enhancers for exposure-based therapy of fear, anxiety and trauma-related disorders

Pathological fear and anxiety are highly debilitating and, despite considerable advances in psychotherapy and pharmacotherapy they remain insufficiently treated in many patients with PTSD, phobias, panic and other anxiety disorders. Increasing preclinical and clinical evidence indicates that pharmacological treatments including cognitive enhancers, when given as adjuncts to psychotherapeutic approaches [cognitive behavioral therapy including extinction-based exposure therapy] enhance treatment efficacy, while using anxiolytics such as benzodiazepines as adjuncts can undermine long-term treatment success. The purpose of this review is to outline the literature showing how pharmacological interventions targeting neurotransmitter systems including serotonin, dopamine, noradrenaline, histamine, glutamate, GABA, cannabinoids, neuropeptides (oxytocin, neuropeptides Y and S, opioids) and other targets (neurotrophins BDNF and FGF2, glucocorticoids, L-type-calcium channels, epigenetic modifications) as well as their downstream signaling pathways, can augment fear extinction and strengthen extinction memory persistently in preclinical models. Particularly promising approaches are discussed in regard to their effects on specific aspects of fear extinction namely, acquisition, consolidation and retrieval, including long-term protection from return of fear (relapse) phenomena like spontaneous recovery, reinstatement and renewal of fear. We also highlight the promising translational value of the preclinial research and the clinical potential of targeting certain neurochemical systems with, for example d-cycloserine, yohimbine, cortisol, and L-DOPA. The current body of research reveals important new insights into the neurobiology and neurochemistry of fear extinction and holds significant promise for pharmacologically-augmented psychotherapy as an improved approach to treat trauma and anxiety-related disorders in a more efficient and persistent way promoting enhanced symptom remission and recovery.

Mechanisms to medicines: elucidating neural and molecular substrates of fear extinction to identify novel treatments for anxiety disorders

The burden of anxiety disorders is growing, but the efficacy of available anxiolytic treatments remains inadequate. Cognitive behavioural therapy for anxiety disorders focuses on identifying and modifying maladaptive patterns of thinking and behaving, and has a testable analogue in rodents in the form of fear extinction. A large preclinical literature has amassed in recent years describing the neural and molecular basis of fear extinction in rodents. In this review, we discuss how this work is being harnessed to foster translational research on anxiety disorders and facilitate the search for new anxiolytic treatments. We begin by summarizing the anatomical and functional connectivity of a medial prefrontal cortex (mPFC)-amygdala circuit that subserves fear extinction, including new insights from optogenetics. We then cover some of the approaches that have been taken to model impaired fear extinction and associated impairments with mPFC-amygdala dysfunction. The principal goal of the review is to evaluate evidence that various neurotransmitter and neuromodulator systems mediate fear extinction by modulating the mPFC-amygdala circuitry. To that end, we describe studies that have tested how fear extinction is impaired or facilitated by pharmacological manipulations of dopamine, noradrenaline, 5-HT, GABA, glutamate, neuropeptides, endocannabinoids and various other systems, which either directly target the mPFC-amygdala circuit, or produce behavioural effects that are coincident with functional changes in the circuit. We conclude that there are good grounds to be optimistic that the progress in defining the molecular substrates of mPFC-amygdala circuit function can be effectively leveraged to identify plausible candidates for extinction-promoting therapies for anxiety disorders.

Fear extinction induces mGluR5-mediated synaptic and intrinsic plasticity in infralimbic neurons

Studies suggest that plasticity in the infralimbic prefrontal cortex (IL) in rodents and its homolog in humans is necessary for inhibition of fear during the recall of fear extinction. The recall of extinction is impaired by locally blocking metabotropic glutamate receptor type 5 (mGluR5) activation in IL during extinction training. This finding suggests that mGluR5 stimulation may lead to IL plasticity needed for fear extinction. To test this hypothesis, we recorded AMPA and NMDA currents, AMPA receptor (AMPAR) rectification, and intrinsic excitability in IL pyramidal neurons in slices from trained rats using whole-cell patch-clamp recording. We observed that fear extinction increases the AMPA/NMDA ratio, consistent with insertion of AMPARs into IL synapses. In addition, extinction training increased inward rectification, suggesting that extinction induces the insertion of calcium-permeable (GluA2-lacking) AMPARs into IL synapses. Consistent with this, selectively blocking calcium-permeable AMPARs with Naspm reduced the AMPA EPSCs in IL neurons to a larger degree after extinction. Extinction-induced changes in AMPA/NMDA ratio, rectification, and intrinsic excitability were blocked with an mGluR5 antagonist. These findings suggest that mGluR5 activation leads to consolidation of fear extinction by regulating the intrinsic excitability of IL neurons and modifying the composition of AMPARs in IL synapses. Therefore, impaired mGluR5 activity in IL synapses could be one factor that causes inappropriate modulation of fear expression leading to anxiety disorders.

Prefrontal dopamine and behavioral flexibility: shifting from an "inverted-U" toward a family of functions

Studies on prefrontal cortex (PFC) dopamine (DA) function have revealed its essential role in mediating a variety of cognitive and executive functions. A general principle that has emerged (primarily from studies on working memory) is that PFC DA, acting on D₁ receptors, regulates cognition in accordance to an “inverted-U” shaped function, so that too little or too much activity has detrimental effects on performance. However, contemporary studies have indicated that the receptor mechanisms through which mesocortical DA regulates different aspects of behavioral flexibility can vary considerably across different DA receptors and cognitive operations. This article will review psychopharmacological and neurochemical data comparing and contrasting the cognitive effects of antagonism and stimulation of different DA receptors in the medial PFC. Thus, set-shifting is dependent on a co-operative interaction between PFC D₁ and D₂ receptors, yet, supranormal stimulation of these receptors does not appear to have detrimental effects on this function. On the other hand, modification of cost/benefit decision biases in situations involving reward uncertainty is regulated in complex and sometimes opposing ways by PFC D₁ vs. D₂ receptors. When viewed collectively, these findings suggest that the “inverted-U” shaped dose-response curve underlying D₁ receptor modulation of working memory is not a one-size-fits-all function. Rather, it appears that mesocortical DA exerts its effects via a family of functions, wherein reduced or excessive DA activity can have a variety of effects across different cognitive domains.

Involvement of the ventral tegmental area in a rodent model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is an anxiety disorder of considerable prevalence in individuals who have experienced a traumatic event. Studies of the neural substrate of this disorder have focused on the role of areas such as the hippocampus, the amygdala and the medial prefrontal cortex. We show that the ventral tegmental area (VTA), which directly modulates these areas, is part of this circuitry. Using a rat model of PTSD, we show that a brief but intense foot shock followed by three brief reminders can cause long-term behavioral changes as shown by anxiety-like, nociception, and touch-sensitivity tests. We show that an intraperitoneal injection of a dopamine (DA) antagonist or a bilateral inactivation of the VTA administered immediately before the traumatic event decrease the occurrence or intensity of these behavioral changes. Furthermore, we show that there is a significant decrease of baseline VTA dopaminergic but not GABAergic cell firing rates 2 weeks after trauma. Our data suggest that VTA DA neurons undergo long-term physiological changes after trauma and that this brain area is a crucial part of the circuits involved in PTSD symptomatology.

Prefrontal stimulation of GABAA receptors counteracts the corticolimbic hyperactivity produced by NMDA antagonists in the prefrontal cortex of the rat

RATIONALE

The hypofunction of NMDA receptors in the prefrontal cortex (PFC) has been suggested to produce corticolimbic hyperactivity through the reduction of cortical GABA transmission.

OBJECTIVES

The present study investigates the effects of injections of the NMDA antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) into the PFC on (1) the release of dopamine and/or acetylcholine in the amygdala and hippocampus, (2) the levels of corticosterone in the hippocampus and (3) spontaneous motor activity. Also, the stimulation of GABA(A) receptors, by prefrontal injections of muscimol, on the effects produced by NMDA antagonists on these same neurochemical, hormonal and behavioural parameters was evaluated.

METHODS

Male Wistar rats were implanted with guide cannulae to perform bilateral microinjections into the PFC and microdialysis experiments in the amygdala and/or ventral hippocampus, simultaneously. Spontaneous motor activity was monitored in the open field.

RESULTS

Injections of CPP (1 μg/0.5 μl) into the PFC increased dialysate concentrations of dopamine and acetylcholine in the amygdala, acetylcholine and free corticosterone in the hippocampus and also motor activity. Simultaneous injections of muscimol (0.5 μg/0.5 μl) into the PFC counteracted the increases of dopamine and acetylcholine in the amygdala and hippocampus and also significantly reduced the peak increase of corticosterone in the hippocampus. Injections of muscimol (0.05 and 0.5 μg/0.5 μl) reduced the increases of motor activity produced by prefrontal NMDA antagonists.

CONCLUSIONS

These results suggest that the hypofunction of NMDA receptors in the PFC produces corticolimbic hyperactivity through the activation of prefrontal efferent projections to subcortical/limbic areas.

Pharmacological enhancement of memory and executive functioning in laboratory animals

Investigating how different pharmacological compounds may enhance learning, memory, and higher-order cognitive functions in laboratory animals is the first critical step toward the development of cognitive enhancers that may be used to ameliorate impairments in these functions in patients suffering from neuropsychiatric disorders. Rather than focus on one aspect of cognition, or class of drug, in this review we provide a broad overview of how distinct classes of pharmacological compounds may enhance different types of memory and executive functioning, particularly those mediated by the prefrontal cortex. These include recognition memory, attention, working memory, and different components of behavioral flexibility. A key emphasis is placed on comparing and contrasting the effects of certain drugs on different cognitive and mnemonic functions, highlighting methodological issues associated with this type of research, tasks used to investigate these functions, and avenues for future research. Viewed collectively, studies of the neuropharmacological basis of cognition in rodents and non-human primates have identified targets that will hopefully open new avenues for the treatment of cognitive disabilities in persons affected by mental disorders.

Glutamate receptors in extinction and extinction-based therapies for psychiatric illness

Some psychiatric illnesses involve a learned component. For example, in posttraumatic stress disorder, memories triggered by trauma-associated cues trigger fear and anxiety, and in addiction, drug-associated cues elicit drug craving and withdrawal. Clinical interventions to reduce the impact of conditioned cues in eliciting these maladaptive conditioned responses are likely to be beneficial. Extinction is a method of lessening conditioned responses and involves repeated exposures to a cue in the absence of the event it once predicted. We believe that an improved understanding of the behavioral and neurobiological mechanisms of extinction will allow extinction-like procedures in the clinic to become more effective. Research on the role of glutamate-the major excitatory neurotransmitter in the mammalian brain-in extinction has led to the development of pharmacotherapeutics to enhance the efficacy of extinction-based protocols in clinical populations. In this review, we describe what has been learned about glutamate actions at its three major receptor types (N-methyl-D-aspartate (NMDA) receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and metabotropic glutamate receptors) in the extinction of conditioned fear, drug craving, and withdrawal. We then discuss how these findings have been applied in clinical research.

Infralimbic D2 receptors are necessary for fear extinction and extinction-related tone responses

BACKGROUND

Fear extinction is dependent on plasticity in the infralimbic prefrontal cortex, an area heavily innervated by midbrain dopaminergic inputs. Dopamine D2 receptors are concentrated in infralimbic output neurons that are involved in the suppression of conditioned fear after extinction. Here, we examined the specific role of infralimbic D2 receptors in mediating associative learning underlying fear extinction using the selective D2 antagonist raclopride.

METHODS

Raclopride was administered systemically or infused into the infralimbic prefrontal cortex before fear extinction, and extinction retention was tested the following day. Rats were also prepared for single-unit recording in the infralimbic prefrontal cortex to assess the effect of raclopride on firing properties.

RESULTS

We found that systemic injection of raclopride given before extinction impaired retrieval of extinction when rats were tested drug-free the next day but also induced catalepsy during extinction training. To determine whether impaired extinction was due to impaired motor function or disruption of extinction consolidation, we infused raclopride directly into the infralimbic prefrontal cortex. Raclopride infused immediately before extinction training did not produce motor deficits but impaired recall of extinction when tested drug-free. Furthermore, in animals that underwent extinction training, systemic raclopride reduced the tone responsiveness of infralimbic prefrontal cortex neurons in layers 5/6, with no changes in average firing rate.

CONCLUSIONS

We suggest that D2 receptors facilitate extinction by increasing the signal-to-noise of infralimbic prefrontal cortex neurons that consolidate extinction.

Neuropeptide S stimulates dopaminergic neurotransmission in the medial prefrontal cortex

Neuropeptide S (NPS) is known to produce anxiolytic-like effects and facilitate extinction of conditioned fear. Catecholaminergic neurotransmission in the medial prefrontal cortex (mPFC) has been suggested to be crucially involved in these brain functions. In the current study, we investigated the effect of NPS on the release of dopamine and serotonin in the mPFC by in vivo microdialysis in rats. Central administration of NPS dose-dependently enhanced extracellular levels of dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid, with maximal effects lasting up to 120 min. In contrast, no effect on serotonergic neurotransmission was detected. Dopamine release in the mPFC has been previously linked to modulation of anxiety states and fear extinction. The present results may thus provide a physiological and anatomical basis for the reported effects of NPS on these behaviors.

Memory for fear extinction requires mGluR5-mediated activation of infralimbic neurons

Consolidation of fear extinction involves enhancement of N-methyl D aspartate (NMDA) receptor-dependent bursting in the infralimbic region (IL) of the medial prefrontal cortex (mPFC). Previous studies have shown that systemic blockade of metabotropic glutamate receptor type 5 (mGluR5) reduces bursting in the mPFC and mGluR5 agonists enhance NMDA receptor currents in vitro, suggesting that mGluR5 activation in IL may contribute to fear extinction. In the current study, rats injected with the mGluR5 antagonist 2-methyl-6-(phenylethyl)-pyridine (MPEP) systemically, or intra-IL, prior to extinction exhibited normal within-session extinction, but were impaired in their ability to recall extinction the following day. To directly determine whether mGluR5 stimulation enhances the burst firing of IL neurons, we used patch-clamp electrophysiology in prefrontal slices. The mGluR5 agonist, (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), increased intrinsic bursting in IL neurons. Increased bursting was correlated with a reduction in the slow after hyperpolarizing potential and was prevented by coapplication of MPEP. CHPG did not increase NMDA currents, suggesting that an NMDA receptor-independent enhancement of IL bursting via stimulation of mGluR5 receptors contributes to fear extinction. Therefore, the mGluR5 receptor could be a suitable target for pharmacological adjuncts to extinction-based therapies for anxiety disorders.

Extinction of drug- and withdrawal-paired cues in animal models: relevance to the treatment of addiction

Conditioned drug craving and withdrawal elicited by cues paired with drug use or acute withdrawal are among the many factors contributing to compulsive drug taking. Understanding how to stop these cues from having these effects is a major goal of addiction research. Extinction is a form of learning in which associations between cues and the events they predict are weakened by exposure to the cues in the absence of those events. Evidence from animal models suggests that conditioned responses to drug cues can be extinguished, although the degree to which this occurs in humans is controversial. Investigations into the neurobiological substrates of extinction of conditioned drug craving and withdrawal may facilitate the successful use of drug cue extinction within clinical contexts. While this work is still in the early stages, there are indications that extinction of drug- and withdrawal-paired cues shares neural mechanisms with extinction of conditioned fear. Using the fear extinction literature as a template, it is possible to organize the observations on drug cue extinction into a cohesive framework.

Microinfusion of the D1 receptor antagonist, SCH23390 into the IL but not the BLA impairs consolidation of extinction of auditory fear conditioning

In auditory fear conditioning, repeated presentation of the tone in the absence of the shock leads to extinction of the acquired fear response. Both the medial prefrontal cortex (mPFC) and the basolateral amygdala (BLA) are involved in extinction. Here we examined this involvement by antagonizing D1 receptors in both regions, in the rat. We microinfused the D1 receptor antagonist, SCH23390, into the infra-limbic part of the mPFC (IL) or BLA at different time points. SCH23390 mircoinfused into the IL either before extinction acquisition or following short extinction training resulted in impairment of extinction consolidation. Microinfusion of SCH23390 into the BLA, prior to acquisition of extinction caused impairment in acquisition of extinction without affecting extinction consolidation. This is supported by the results showing that microinfusion of SCH23390 into the BLA following a short-training session did not affect consolidation. These results further strengthen the role of mPFC in consolidation of extinction while highlighting the role of the D1 receptors in this process.

An unambiguous assay for the cloned human sigma1 receptor reveals high affinity interactions with dopamine D4 receptor selective compounds and a distinct structure-affinity relationship for butyrophenones

The ability of the sigma(1) receptor to interact with a huge range of drug structural classes coupled with its wide distribution in the body has contributed to it being implicated as a possible therapeutic target for a broad array of disorders ranging from substance abuse to depression to Alzheimer's disease. Surprisingly, the reported affinity values for some sigma(1) receptor ligands vary more than 50-fold. The potential of the sigma(1) receptor as a pharmacotherapeutic target prompted us to develop an unambiguous assay system for measuring the affinity of ligands to the cloned human sigma(1) receptor. In the course of characterizing this system and determining the true affinity values for almost three dozen compounds, it was discovered that some dopamine D(4) receptor selective compounds bind sigma(1) receptors with high affinity. A systematic analysis of haloperidol-like compounds revealed a clear structure-affinity relationship amongst clinically relevant butyrophenones. The antidepressant fluvoxamine, the drug of abuse methamphetamine, and the neurosteroid progesterone were amongst the many ligands whose interactions with the sigma(1) receptor were confirmed with our screening assay.

Noradrenergic signaling in infralimbic cortex increases cell excitability and strengthens memory for fear extinction

Emotional arousal strengthens memory. This is most apparent in aversive conditioning, in which the stress-related neurotransmitter norepinephrine (NE) enhances associations between sensory stimuli and fear-inducing events. In contrast to conditioning, extinction decreases fear responses, and is thought to form a new memory. It is not known, however, whether NE is necessary for extinction learning. Previous work has shown that the infralimbic prefrontal cortex (IL) is a site of extinction consolidation. Here, we show that blocking noradrenergic beta-receptors in IL before extinction training impaired retrieval of extinction the following day, consistent with a weakened extinction memory. We further found that the sequelae of beta-receptor activation, including protein kinase A (PKA), gene transcription and translation in IL, are necessary for extinction. To determine whether activation of this cascade modulates IL excitability, we measured the response of IL pyramidal neurons to injected current. NE increased the excitability of IL neurons in a beta-receptor- and PKA-dependent manner. We suggest that NE released in IL during fear extinction activates a PKA-mediated molecular cascade that strengthens extinction memory. Thus, emotional arousal evoked by conditioned fear paradoxically promotes the subsequent extinction of that fear, thereby ensuring behavioral flexibility.

Neural mechanisms of extinction learning and retrieval

Emotional learning is necessary for individuals to survive and prosper. Once acquired, however, emotional associations are not always expressed. Indeed, the regulation of emotional expression under varying environmental conditions is essential for mental health. The simplest form of emotional regulation is extinction, in which conditioned responding to a stimulus decreases when the reinforcer is omitted. Two decades of research on the neural mechanisms of fear conditioning have laid the groundwork for understanding extinction. In this review, we summarize recent work on the neural mechanisms of extinction learning. Like other forms of learning, extinction occurs in three phases: acquisition, consolidation, and retrieval, each of which depends on specific structures (amygdala, prefrontal cortex, hippocampus) and molecular mechanisms (receptors and signaling pathways). Pharmacological methods to facilitate consolidation and retrieval of extinction, for both aversive and appetitive conditioning, are setting the stage for novel treatments for anxiety disorders and addictions.

Dopaminergic regulation of inhibitory and excitatory transmission in the basolateral amygdala-prefrontal cortical pathway

Projections from the basolateral amygdala (BLA) and dopamine (DA) input from the ventral tegmental area (VTA) converge in the medial prefrontal cortex (mPFC), forming a neural circuit implicated in certain cognitive and emotional processes. However, the role that DA plays in modulating activity in the BLA-mPFC pathway is unknown. The present study investigated the mechanisms by which DA modulates BLA-evoked changes in mPFC neural activity, using extracellular single-unit recordings in urethane-anesthetized rats. BLA stimulation evoked two distinct types of responses in separate populations of mPFC neurons: monosynaptic, excitatory responses and, more commonly, inhibition of spontaneous firing. Stimulation of the VTA or local iontophoretic application of DA attenuated BLA-evoked inhibition of PFC neuron firing. Administration of selective DA receptor agonists revealed that these effects were mediated by D2 and D4 (but not D1) receptors. In addition, VTA stimulation or DA application attenuated BLA-evoked firing of a separate population of mPFC neurons in a frequency-dependent manner; firing evoked by higher-frequency stimulation of the BLA was less inhibited than that evoked by single-pulse stimulation. Attenuation of BLA-evoked firing was also induced by of D1 (but not D2 or D4) receptor agonists. These data indicate that dissociable DA receptor mechanisms regulate the balance of excitatory and inhibitory transmission in BLA-mPFC circuits, biasing toward an increase in the excitatory influence that the BLA exerts over populations of mPFC neurons. These findings may have important implications for understanding the pathophysiology underlying emotional and cognitive disturbances present in disorders such as depression and drug addiction.

Mechanisms of fear extinction

Excessive fear and anxiety are hallmarks of a variety of disabling anxiety disorders that affect millions of people throughout the world. Hence, a greater understanding of the brain mechanisms involved in the inhibition of fear and anxiety is attracting increasing interest in the research community. In the laboratory, fear inhibition most often is studied through a procedure in which a previously fear conditioned organism is exposed to a fear-eliciting cue in the absence of any aversive event. This procedure results in a decline in conditioned fear responses that is attributed to a process called fear extinction. Extensive empirical work by behavioral psychologists has revealed basic behavioral characteristics of extinction, and theoretical accounts have emphasized extinction as a form of inhibitory learning as opposed to an erasure of acquired fear. Guided by this work, neuroscientists have begun to dissect the neural mechanisms involved, including the regions in which extinction-related plasticity occurs and the cellular and molecular processes that are engaged. The present paper will cover behavioral, theoretical and neurobiological work, and will conclude with a discussion of clinical implications.