D(1)-like receptors in the nucleus accumbens shell regulate the expression of contextual fear conditioning and activity of the anterior cingulate cortex in rats

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.

Emotion in action: When emotions meet motor circuits

The brain is a remarkably complex organ responsible for a wide range of functions, including the modulation of emotional states and movement. Neuronal circuits are believed to play a crucial role in integrating sensory, cognitive, and emotional information to ultimately guide motor behavior. Over the years, numerous studies employing diverse techniques such as electrophysiology, imaging, and optogenetics have revealed a complex network of neural circuits involved in the regulation of emotional or motor processes. Emotions can exert a substantial influence on motor performance, encompassing both everyday activities and pathological conditions. The aim of this review is to explore how emotional states can shape movements by connecting the neural circuits for emotional processing to motor neural circuits. We first provide a comprehensive overview of the impact of different emotional states on motor control in humans and rodents. In line with behavioral studies, we set out to identify emotion-related structures capable of modulating motor output, behaviorally and anatomically. Neuronal circuits involved in emotional processing are extensively connected to the motor system. These circuits can drive emotional behavior, essential for survival, but can also continuously shape ongoing movement. In summary, the investigation of the intricate relationship between emotion and movement offers valuable insights into human behavior, including opportunities to enhance performance, and holds promise for improving mental and physical health. This review integrates findings from multiple scientific approaches, including anatomical tracing, circuit-based dissection, and behavioral studies, conducted in both animal and human subjects. By incorporating these different methodologies, we aim to present a comprehensive overview of the current understanding of the emotional modulation of movement in both physiological and pathological conditions.

Valence and salience encoding by parallel circuits from the paraventricular thalamus to the nucleus accumbens

The anterior and posterior subregions of the paraventricular thalamus (aPVT and pPVT, respectively) play unique roles in learned behaviors, from fear conditioning to alcohol/drug intake, potentially through differentially organized projections to limbic brain regions including the nucleus accumbens medial shell (mNAcSh). Here, we found that the aPVT projects broadly to the mNAcSh and that the aPVT-mNAcSh circuit encodes positive valence, such that in vivo manipulations of the circuit modulated both innately programmed and learned behavioral responses to positively and negatively valenced stimuli, particularly in females. Further, the endogenous activity of aPVT presynaptic terminals in the mNAcSh was greater in response to positively than negatively valenced stimuli, and the probability of synaptic glutamate release from aPVT neurons in the mNAcSh was higher in females than males. In contrast, we found that the pPVT-mNAcSh circuit encodes stimulus salience regardless of valence. While pPVT-mNAcSh circuit inhibition suppressed behavioral responses in both sexes, circuit activation increased behavioral responses to stimuli only in males. Our results point to circuit-specific stimulus feature encoding by parallel PVT-mNAcSh circuits that have sex-dependent biases in organization and function.

Exogenous Cannabinoids Impair Effort-Related Decision-Making via Affecting Neural Synchronization between the Anterior Cingulate Cortex and Nucleus Accumbens

Humans and animals frequently make an endeavor-based choice based on assessing reinforcement value and response costs. The cortical-limbic-striatal pathway mediates endeavor-based choice behavior, including the nucleus accumbens (NAc) and the anterior cingulate cortex (ACC). Furthermore, cannabinoid agonists demonstratively impairs decision-making processes. In this study, neural synchronization and functional connectivity between the NAc and ACC while endeavor-related decision-making and reaching reward were evaluated. The effect of exogenous cannabinoids on this synchronization was then assessed. A T-maze decision-making task with a differential expense (low vs. high endeavor) and remuneration (low vs. high remuneration) was performed and local field potentials (LFP) from the ACC and NAc were registered simultaneously. Results showed functional connectivity during endeavor-related decision-making while the animals chose the high endeavor/high remuneration in both regions’ delta/beta (1–4 and 13–30 Hertz) frequency bands. Furthermore, functional connectivity existed between both areas in delta/theta (1–4 and 4–12) frequencies while reaching a remuneration. However, neural simultaneity was not observed while the animals received cannabinoid agonists, making a decision and reaching remuneration. The obtained results demonstrated that functional connectivity and neural simultaneity between the NAc and ACC in delta/beta and delta/theta frequencies have a role in endeavor-related decision-making and reaching remuneration, respectively. The effect of exogenous cannabinoids on decision-making impairment is relevant to changes in the ACC and NAC brain wave frequencies.

HANDLING AND NOVEL OBJECT RECOGNITION MODULATE FEAR RESPONSE AND ENDOCANNABINOID SIGNALING IN NUCLEUS BASALIS MAGNOCELLULARIS

Storage of aversive memories is of utmost importance for survival, allowing animals to avoid upcoming similar stimuli. However, without reinforcement, the learned avoidance response gradually decreases over time. Although the molecular mechanisms controlling this extinction process are not well known, there is evidence that the endocannabinoid system plays a key role through CB₁ receptor‐mediated modulation of cholinergic signaling. In this study, we measured fear extinction throughout 7 months using naïve rats, assessed in passive avoidance (PA) test in a non‐reinforced manner. Then, we evaluated the effect of gentle handling and non‐aversive novel object recognition test (NORT) on the extinction and expression of fear memories by measuring passive avoidance responses. Neurochemical correlates were analyzed by functional autoradiography for cannabinoid, cholinergic, and dopaminergic receptors. Despite results showing a gradual decrease of passive avoidance response, it did not fully disappear even after 7 months, indicating the robustness of this process. Meanwhile, in rats that received gentle handling or performed NORT after receiving the PA aversive stimulus, extinction occurred within a week. In contrast, gentle handling performed before receiving the aversive stimulus exacerbated fear expression and triggered escape response in PA. The neurochemical analysis showed increased cannabinoid and cholinergic activity in the nucleus basalis magnocellularis (NBM) in rats that had performed only PA, as opposed to rats that received gentle handling before PA. Additionally, a correlation between CB₁ mediated‐signaling in the NBM and freezing in PA was found, suggesting that the endocannabinoid system might be responsible for modulating fear response induced by aversive memories.

Reduction of DNMT3a and RORA in the nucleus accumbens plays a causal role in post-traumatic stress disorder-like behavior: reversal by combinatorial epigenetic therapy

Post-traumatic stress disorder (PTSD) is an incapacitating trauma-related disorder, with no reliable therapy. Although PTSD has been associated with epigenetic alterations in peripheral white blood cells, it is unknown where such changes occur in the brain, and whether they play a causal role in PTSD. Using an animal PTSD model, we show distinct DNA methylation profiles of PTSD susceptibility in the nucleus accumbens (NAc). Data analysis revealed overall hypomethylation of different genomic CG sites in susceptible animals. This was correlated with the reduction in expression levels of the DNA methyltransferase, DNMT3a. Since epigenetic changes in diseases involve different gene pathways, rather than single candidate genes, we next searched for pathways that may be involved in PTSD. Analysis of differentially methylated sites identified enrichment in the RAR activation and LXR/RXR activation pathways that regulate Retinoic Acid Receptor (RAR) Related Orphan Receptor A (RORA) activation. Intra-NAc injection of a lentiviral vector expressing either RORA or DNMT3a reversed PTSD-like behaviors while knockdown of RORA and DNMT3a increased PTSD-like behaviors. To translate our results into a potential pharmacological therapeutic strategy, we tested the effect of systemic treatment with the global methyl donor S-adenosyl methionine (SAM), for supplementing DNA methylation, or retinoic acid, for activating RORA downstream pathways. We found that combined treatment with the methyl donor SAM and retinoic acid reversed PTSD-like behaviors. Thus, our data point to a novel approach to the treatment of PTSD, which is potentially translatable to humans.

Dopamine D2 receptors in the expression and extinction of contextual and cued conditioned fear in rats

Dopamine seems to mediate fear conditioning through its action on D2 receptors in the mesolimbic pathway. Systemic and local injections of dopaminergic agents showed that D2 receptors are preferentially involved in the expression, rather than in the acquisition, of conditioned fear. To further examine this issue, we evaluated the effects of systemic administration of the dopamine D2-like receptor antagonists sulpiride and haloperidol on the expression and extinction of contextual and cued conditioned fear in rats. Rats were trained to a context-CS or a light-CS using footshocks as unconditioned stimuli. After 24 h, rats received injections of sulpiride or haloperidol and were exposed to the context-CS or light-CS for evaluation of freezing expression (test session). After another 24 h, rats were re-exposed to the context-CS or light-CS, to evaluate the extinction recall (retest session). Motor performance was assessed with the open-field and catalepsy tests. Sulpiride, but not haloperidol, significantly reduced the expression of contextual and cued conditioned fear without affecting extinction recall. In contrast, haloperidol, but not sulpiride, had cataleptic and motor-impairing effects. The results reinforce the importance of D2 receptors in fear conditioning and suggest that dopaminergic mechanisms mediated by D2 receptors are mainly involved in the expression rather than in the extinction of conditioned freezing.

An Emergent Discriminative Learning Is Elicited During Multifrequency Testing

In auditory-conditioned fear learning, the freezing response is independent of the sound frequencies used, but the frequency of the conditioned sound is considered distinct from those of unrelated sounds based on electrophysiological responses in the auditory system. Whether an emergent discriminative learning underlies auditory fear conditioning and which nuclei and pathways are involved in it remain unclear. Using behavioral and electrophysiological assays, we found that the response of medial prefrontal cortex (mPFC) neurons to a conditioned auditory stimulus (CS) was enhanced relative to the response to unrelated frequencies (UFs) after auditory fear conditioning, and mice could distinguish the CS during multifrequency testing, a phenomenon called emergent discriminative learning. After silencing the mPFC with muscimol, emergent discriminative learning was blocked. In addition, the pure tone responses of mPFC neurons were inhibited after injection of lidocaine in the ipsilateral primary auditory cortex (A1), and the emergent discriminative learning was blocked by silencing both sides of A1 with muscimol. This study, therefore, provides evidence for an emergent discriminative learning mediated by mPFC and A1 neurons after auditory fear conditioning.

Understanding the role of dopamine in conditioned and unconditioned fear

Pharmacological and molecular imaging studies in anxiety disorders have primarily focused on the serotonin system. In the meantime, dopamine has been known as the neurotransmitter of reward for 60 years, particularly for its action in the nervous terminals of the mesocorticolimbic system. Interest in the mediation by dopamine of the well-known brain aversion system has grown recently, particularly given recent evidence obtained on the role of D2 dopamine receptors in unconditioned fear. However, it has been established that excitation of the mesocorticolimbic pathway, originating from dopaminergic (DA) neurons from the ventral tegmental area (VTA), is relevant for the development of anxiety. Among the forebrain regions innervated by this pathway, the amygdala is an essential component of the neural circuitry of conditioned fear. Current findings indicate that the dopamine D2 receptor-signaling pathway connecting the VTA to the basolateral amygdala modulates fear and anxiety, whereas neural circuits in the midbrain tectum underlie the expression of innate fear. The A13 nucleus of the zona incerta is proposed as the origin of these DA neurons projecting to caudal structures of the brain aversion system. In this article we review data obtained in studies showing that DA receptor-mediated mechanisms on ascending or descending DA pathways play opposing roles in fear/anxiety processes. Dopamine appears to mediate conditioned fear by acting at rostral levels of the brain and regulate unconditioned fear at the midbrain level.

Intracerebral injection of R-(-)-Apomorphine into the nucleus accumbens decreased carbachol-induced 22-kHz ultrasonic vocalizations in rats

Rats can produce ultrasonic vocalizations (USVs) in a variety of different contexts that signal their emotional state to conspecifics. Under distress, rats can emit 22-kHz USVs, while during positive pro-social interactions rats can emit frequency-modulated (FM) 50-kHz USVs. It has been previously reported that rats with increasing emission of FM 50-kHz USVs in anticipation of rewarding electrical stimulation or positive pro-social interaction decrease the number of emitted 22-kHz USVs. The purpose of the present investigation was to determine, in a pharmacological-behavioural experiment, if the positive emotional arousal of the rat indexed by the number of emitted FM 50-kHz USVs can decrease the magnitude of a subsequent negative emotional state indexed by the emission of 22-kHz USVs. To induce a positive emotional state, an intracerebral injection of a known D₁/D₂ agonist R-(-)-apomorphine (3.0 μg/0.3 μl) into the medial nucleus accumbens shell was used, while a negative emotional state was induced by intracerebral injection of carbachol (1.0 μg/0.3 μl), a known broad-spectrum muscarinic agonist, into the anterior hypothalamic-medial preoptic area. Our results demonstrated that initiation of a positive emotional state was able to significantly decrease the magnitude of subsequently expressed negative emotional state measured by the number of emitted 22-kHz USVs. The results suggest the neurobiological substrates that initiate positive emotional state indirectly antagonize the brain regions that initiate negative emotional states.

Dopamine D1-like receptors in the dorsomedial prefrontal cortex regulate contextual fear conditioning

RATIONALE

Dopamine D1 receptor (D1R) signalling is involved in contextual fear conditioning. The D1R antagonist SCH23390 impairs the acquisition of contextual fear when administered systemically or infused locally into the dorsal hippocampus or basolateral amygdala.

OBJECTIVES

We determined if state dependency may account for the impairment in contextual fear conditioning caused by systemic SCH23390 administration. We also examined if the dorsomedial prefrontal cortex (dmPFC), nucleus accumbens (NAc), and ventral hippocampus (VH) are involved in mediating the effect of systemic SCH23390 treatment on contextual fear conditioning.

METHODS

In experiment 1, SCH23390 (0.1 mg/kg) or vehicle was given before contextual fear conditioning and/or retrieval. In experiment 2, SCH23390 (2.5 μg/0.5 uL) or vehicle was infused locally into dmPFC, NAc, or VH before contextual fear conditioning, and retrieval was tested drug-free. Freezing was quantified as a measure of contextual fear.

RESULTS

In experiment 1, SCH23390 given before conditioning or before both conditioning and retrieval decreased freezing at retrieval, whereas SCH23390 given only before retrieval had no effect. In experiment 2, SCH23390 infused into dmPFC before conditioning decreased freezing at retrieval, while infusion of SCH23390 into NAc or VH had no effect.

CONCLUSIONS

The results of experiment 1 confirm those of previous studies indicating that D1Rs are required for the acquisition but not retrieval of contextual fear and rule out state dependency as an explanation for these findings. Moreover, the results of experiment 2 provide evidence that dmPFC is also part of the neural circuitry through which D1R signalling regulates contextual fear conditioning.

Rate and Temporal Coding Mechanisms in the Anterior Cingulate Cortex for Pain Anticipation

Pain is a complex sensory and affective experience. Through its anticipation, animals can learn to avoid pain. Much is known about passive avoidance during a painful event; however, less is known about active pain avoidance. The anterior cingulate cortex (ACC) is a critical hub for affective pain processing. However, there is currently no mechanism that links ACC activities at the cellular level with behavioral anticipation or avoidance. Here we asked whether distinct populations of neurons in the ACC can encode information for pain anticipation. We used tetrodes to record from ACC neurons during a conditioning assay to train rats to avoid pain. We found that in rats that successfully avoid acute pain episodes, neurons that responded to pain shifted their firing rates to an earlier time, whereas neurons that responded to the anticipation of pain increased their firing rates prior to noxious stimulation. Furthermore, we found a selected group of neurons that shifted their firing from a pain-tuned response to an anticipatory response. Unsupervised learning analysis of ensemble spike activity indicates that temporal spiking patterns of ACC neurons can indeed predict the onset of pain avoidance. These results suggest rate and temporal coding schemes in the ACC for pain avoidance.

Regulation and function of MeCP2 Ser421 phosphorylation in U50488-induced conditioned place aversion in mice

RATIONALE

Phosphorylation of the methyl DNA-binding protein MeCP2 at Ser421 (pMeCP2-S421) is induced in corticolimbic brain regions during exposure to drugs of abuse and modulates reward-driven behaviors. However, whether pMeCP2-S421 is also involved in behavioral adaptations to aversive drugs is unknown.

OBJECTIVES

Our goal was to establish the role and regulation of pMeCP2-S421 in corticolimbic brain regions of mice upon acute treatment with the kappa opioid receptor agonist U50488 and during the expression of U50488-induced conditioned place aversion (CPA).

METHODS

pMeCP2-S421 levels were measured in the nucleus accumbens (NAc), prelimbic cortex, infralimbic cortex (ILC), and basolateral amygdala (BLA) of male mice after intraperitoneal administration of U50488 and upon the expression of U50488-induced CPA. Fos was measured as marker of neural activity in the same brain regions. U50488-induced CPA and Fos levels were compared between knockin (KI) mice that lack pMeCP2-S421 and their wild-type (WT) littermates.

RESULTS

U50488 administration acutely induced pMeCP2-S421 and Fos selectively in the NAc but did not alter MeCP2 levels in any brain region. U50488-induced CPA was associated with decreased pMeCP2-S421 in the ILC and BLA and induced Fos in the BLA. MeCP2 KI mice showed CPA indistinguishable from their WT littermates, but they also showed less BLA Fos induction upon CPA.

CONCLUSION

These data are the first to show that pMeCP2-S421 is induced in the brain acutely after U50488 administration but not upon U50488-induced CPA. Although pMeCP2-S421 is not required for U50488-induced CPA, this phosphorylation event may contribute to molecular plasticities in brain regions that govern aversive behaviors.

Effects of dopamine D1 modulation of the anterior cingulate cortex in a fear conditioning procedure

The anterior cingulate cortex (AC) component of the medial prefrontal cortex (mPFC) has been implicated in attention and working memory as measured by trace conditioning. Since dopamine (DA) is a key modulator of mPFC function, the present study evaluated the role of DA receptor agents in rat AC, using trace fear conditioning. A conditioned stimulus (CS, noise) was followed by an unconditioned stimulus (US, shock) with or without a 10s trace interval interposed between these events in a between-subjects design. Conditioned suppression of drinking was assessed in response to presentation of the CS or an experimental background stimulus (flashing lights, previously presented for the duration of the conditioning session). The selective D1 agonist SKF81297 (0.05μg/side) or D1 antagonist SCH23390 (0.5μg/side) was administered by intra-cerebral microinfusion directly into AC. It was predicted that either of these manipulations should be sufficient to impair trace (but not delay) conditioning. Counter to expectation, there was no effect of DA D1 modulation on trace conditioning as measured by suppression to the noise CS. However, rats infused with SKF81297 acquired stronger conditioned suppression to the experimental background stimulus than those infused with SCH23390 or saline. Thus, the DA D1 agonist SKF81297 increased conditioned suppression to the contextual background light stimulus but was otherwise without effect on fear conditioning.

Rapid Activation of Glucocorticoid Receptors in the Prefrontal Cortex Mediates the Expression of Contextual Conditioned Fear in Rats

The aim of the present study was to investigate the role of glucocorticoids in medial prefrontal cortex (mPFC) activity and the expression of contextual conditioned fear (freezing). Rats were pretreated with vehicle or metyrapone, a corticosterone synthesis blocker, and exposed to a context previously paired with footshocks. Freezing and Fos-protein expression in different mPFC regions were assessed. Exposure to the aversive context led to increased freezing and Fos expression in the prelimbic (PrL), anterior cingulate areas 1 and 2 (Cg1/Cg2). Pretreatment with metyrapone decreased freezing and Fos expression in these areas. Administration of spironolactone, an MR antagonist, in the PrL before the test decreased freezing. Pretreatment with RU38486, a glucocorticoid receptor (GR) antagonist, reduced this effect of spironolactone, suggesting that the effects of this MR antagonist may be attributable to a redirection of endogenous corticosterone actions to GRs. Consistent with this result, the decrease in freezing that was induced by intra-PrL injections of corticosterone was attenuated by pretreatment with RU38486 but not spironolactone. These findings indicate that corticosterone release during aversive conditioning influences mPFC activity and the retrieval of conditioned fear memory indicating the importance of balance between MR:GR-mediated effects in this brain region in this process.

The importance of the context in the hippocampus and brain related areas throughout the performance of a fear conditioning task

The importance context has been broadly studied in the management of phobias and in the drug addiction literature. The way in which changes to a context influence behavior after the simple acquisition of a passive avoidance task remains unclear. The hippocampus has long been implicated in the contextual and spatial processing required for contextual fear, but its role in encoding the aversive component of a contextual fear memory is still inconclusive. Our work tries to elucidate whether a change in context, represented as differences in the load of the stimuli, is critical for learning about the context-shock association and whether this manipulation of the context could be linked to any change in metabolic brain activity requirements. For this purpose, we used an avoidance conditioning task. Animals were divided into three different experimental conditions. In one group, acquisition was performed in an enriched stimuli environment and retention was performed in a typically lit chamber (the PA-ACQ-CONTX group). In another group, acquisition was performed in the typically lit chamber and retention was undertaken in the highly enriched chamber (the PA-RET-CONTX group). Finally, for the control group, PA-CN-CONTX, acquisition, and retention were performed in the enriched stimuli environment. Our results showed that the PA-ACQ-CONTX group had longer escape latencies and poorer retention than the PA-RET-CONTX and PA-CN-CONTX groups after 24 h of acquisition under contextual changes. To study metabolic brain activity, histochemical labelling of cytochrome c-oxidase (CO) was performed. CO results suggested a neural circuit including the hippocampus, amygdala, thalamus, parahippocampal cortices, and mammillary nuclei that is involved in the learning and memory processes that enable context-dependent behavior. These results highlight how dysfunction in this network may be involved in the contextualization of fear associations that underlie several forms of psychopathology, including post-traumatic stress disorder, schizophrenia, and substance abuse disorders.

Brain transcriptome profiles in mouse model simulating features of post-traumatic stress disorder

Background

Social-stress mouse model, based on the resident-intruder paradigm was used to simulate features of human post-traumatic stress disorder (PTSD). The model involved exposure of an intruder (subject) mouse to a resident aggressor mouse followed by exposure to trauma reminders with rest periods. C57BL/6 mice exposed to SJL aggressor mice exhibited behaviors suggested as PTSD-in-mouse phenotypes: intermittent freezing, reduced locomotion, avoidance of the aggressor-associated cue and apparent startled jumping. Brain tissues (amygdala, hippocampus, medial prefrontal cortex, septal region, corpus striatum and ventral striatum) from subject (aggressor exposed: Agg-E) and control C57BL/6 mice were collected at one, 10 and 42 days post aggressor exposure sessions. Transcripts in these brain regions were assayed using Agilent’s mouse genome-wide arrays.

Results

Pathways and biological processes associated with differentially regulated genes were mainly those thought to be involved in fear-related behavioral responses and neuronal signaling. Expression-based assessments of activation patterns showed increased activations of pathways related to anxiety disorders (hyperactivity and fear responses), impaired cognition, mood disorders, circadian rhythm disruption, and impaired territorial and aggressive behaviors. In amygdala, activations of these pathways were more pronounced at earlier time-points, with some attenuation after longer rest periods. In hippocampus and medial prefrontal cortex, activation patterns were observed at later time points. Signaling pathways associated with PTSD-comorbid conditions, such as diabetes, metabolic disorder, inflammation and cardiac infarction, were also significantly enriched. In contrast, signaling processes related to neurogenesis and synaptic plasticity were inhibited.

Conclusions

Our data suggests activations of behavioral responses associated with anxiety disorders as well as inhibition of neuronal signaling pathways important for neurogenesis, cognition and extinction of fear memory. These pathways along with comorbid-related signaling pathways indicate the pervasive and multisystem effects of aggressor exposure in mice, potentially mirroring the pathologic conditions of PTSD patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0104-3) contains supplementary material, which is available to authorized users.

A role for phasic dopamine release within the nucleus accumbens in encoding aversion: a review of the neurochemical literature

Survival is dictated by an organism's fitness in approaching positive stimuli and avoiding harm. While a rich literature outlines a role for mesolimbic dopamine in reward and appetitive behaviors, dopamine's involvement in aversion and avoidance behaviors remains controversial. Debate surrounding dopamine's function in the processing of negative stimuli likely stems from conflicting results reported by single-unit electrophysiological studies. Indeed, a number of studies suggest that midbrain dopaminergic cells are inhibited by the presentation of negative or fearful stimuli, while others report no change, or even an increase, in their activity. These disparate results may be due to population heterogeneity. Recent evidence demonstrates that midbrain dopamine neurons are heterogeneous in their projection targets, responses to environmental stimuli, pharmacology, and influences on motivated behavior. Thus, in order to assemble an accurate account of dopamine function during aversive stimulus experience and related behavior, it is necessary to examine the functional output of dopamine neural activity at mesolimbic terminal regions. This Review presents a growing body of evidence that dopamine release in the nucleus accumbens encodes not only reward, but also aversion. For example, our laboratory recently utilized fast-scan cyclic voltammetry to show that real-time changes in accumbal dopamine release are detected when animals are presented with predictors of aversion and its avoidance. These data, along with other reports, support a considerably more nuanced view of dopamine neuron function, wherein accumbal dopamine release is differentially modulated by positive and negative affective stimuli to promote adaptive behaviors.

Dopamine D1-like receptor signalling in the hippocampus and amygdala modulates the acquisition of contextual fear conditioning

RATIONALE

Dopamine D1-like receptor signalling is involved in contextual fear conditioning, but the brain regions involved and its role in other contextual fear memory processes remain unclear.

OBJECTIVES

The objective of this study was to investigate (1) the effects of SCH 23390, a dopamine D1/D5 receptor antagonist, on contextual fear memory encoding, retrieval and reconsolidation, and (2) if the effects of SCH 23390 on conditioning involve the dorsal hippocampus (DH) and/or basolateral amygdala (BLA).

METHODS

Rats were used to examine the effects of systemically administering SCH 23390 on the acquisition, consolidation, retrieval and reconsolidation of contextual fear memory, and on locomotor activity and shock sensitivity. We also determined the effects of MK-801, an NMDA receptor antagonist, on contextual fear memory reconsolidation. The effects of infusing SCH 23390 locally into DH or BLA on contextual fear conditioning and locomotor activity were also examined.

RESULTS

Systemic administration of SCH 23390 impaired contextual fear conditioning but had no effects on fear memory consolidation, retrieval or reconsolidation. MK-801 was found to impair reconsolidation, suggesting that the behavioural parameters used allowed for the pharmacological disruption of memory reconsolidation. The effects of SCH 23390 on conditioning were unlikely the result of any lasting drug effects on locomotor activity at memory test or any acute drug effects on shock sensitivity during conditioning. SCH 23390 infused into either DH or BLA impaired contextual fear conditioning and decreased locomotor activity.

CONCLUSIONS

These findings suggest that dopamine D1-like receptor signalling in DH and BLA contributes to the acquisition of contextual fear memory.

Prefrontal neuronal circuits of contextual fear conditioning

Over the past years, numerous studies have provided a clear understanding of the neuronal circuits and mechanisms involved in the formation, expression and extinction phases of conditioned cued fear memories. Yet, despite a strong clinical interest, a detailed understanding of these memory phases for contextual fear memories is still missing. Besides the well-known role of the hippocampus in encoding contextual fear behavior, growing evidence indicates that specific regions of the medial prefrontal cortex differentially regulate contextual fear acquisition and storage in both animals and humans that ultimately leads to expression of contextual fear memories. In this review, we provide a detailed description of the recent literature on the role of distinct prefrontal subregions in contextual fear behavior and provide a working model of the neuronal circuits involved in the acquisition, expression and generalization of contextual fear memories.

Further evidence for involvement of the dorsal hippocampus serotonergic and γ-aminobutyric acid (GABA)ergic pathways in the expression of contextual fear conditioning in rats

Intra-dorsal hippocampus (DH) injections of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a serotonin-1A (5-hydroxytryptamine (5-HT)-1A) receptor agonist, were previously shown to inhibit the expression of contextual fear when administered six hours after conditioning. However, further understanding of the consolidation and expression of aversive memories requires investigations of these and other mechanisms at distinct time points and the regions of the brain to which they are transferred. Thus, the purpose of the present study was to investigate the role of DH serotonergic and γ-aminobutyric acid (GABA)ergic mechanisms in the expression of contextual fear 24 h after conditioning, reflected by fear-potentiated startle (FPS) and freezing behavior. The recruitment of the amygdala and medial prefrontal cortex (mPFC) in these processes was also evaluated by measuring Fos protein immunoreactivity. Although intra-DH injections of 8-OH-DPAT did not produce behavioral changes, muscimol reduced both FPS and the freezing response. Fos protein immunoreactivity revealed that contextual fear promoted wide activation of the mPFC, which was significantly reduced after intra-DH infusions of muscimol. The present findings, together with previous data, indicate that in contrast to 5-HT, which appears to play a role during the early phases of contextual aversive memory consolidation, longer-lasting GABA-mediated mechanisms are recruited during the expression of contextual fear memories.

Dopamine D2 receptors modulate the expression of contextual conditioned fear: role of the ventral tegmental area and the basolateral amygdala

Although dopaminergic systems are more commonly associated with the reinforcing effects of various stimuli, numerous reports have demonstrated a relationship between changes in dopaminergic transmission and aversive situations. In the present study, we examined the involvement of D1-like and D2-like receptors in the expression of conditioned freezing using the context as the conditioned stimulus. Intraperitoneal injections of the D1 agonist SKF38393 or the D1 antagonist SCH23390 did not change the conditioned freezing in rats subjected to the contextual fear paradigm. In contrast, intraperitoneal injections of the D2 agonist quinpirole and the D2 antagonist sulpiride caused a significant dose-dependent reduction in the expression of contextual conditioned freezing. As these data may reflect that the systemic manipulations acted on dopaminergic receptors in different brain areas, the effects of administration of quinpirole and sulpiride into the ventral tegmental area (VTA) and the basolateral amygdala complex (BLA) on the expression of contextual conditioned freezing were also evaluated. Intra-VTA quinpirole and intra-BLA sulpiride injections reduced the conditioned freezing response; intra-VTA sulpiride and intra-BLA quinpirole injections had no significant effects. These data suggest that D2-like receptors, but not D1-like receptors, play an important role in the expression of contextual conditioned freezing. Quinpirole may act at D2 presynaptic receptors located in the VTA, decreasing dopamine levels in the terminal fields of the mesolimbic pathway. The effects of sulpiride, in contrast, appear to be triggered by an action on postsynaptic dopaminergic receptors located in the BLA. However, it cannot be totally excluded that the injected solutions did not also affect neighboring amygdalar regions. Together with previous findings, the present data suggest the need to consider dopaminergic mechanisms in the mesolimbic circuit as novel targets for the pharmacological treatment of fear-related disorders, especially post-traumatic stress disorder.

Analogous responses in the nucleus accumbens and cingulate cortex to pain onset (aversion) and offset (relief) in rats and humans

In humans, functional magnetic resonance imaging (fMRI) activity in the anterior cingulate cortex (ACC) and the nucleus accumbens (NAc) appears to reflect affective and motivational aspects of pain. The responses of this reward-aversion circuit to relief of pain, however, have not been investigated in detail. Moreover, it is not clear whether brain processing of the affective qualities of pain in animals parallels the mechanisms observed in humans. In the present study, we analyzed fMRI blood oxygen level-dependent (BOLD) activity separately in response to an onset (aversion) and offset (reward) of a noxious heat stimulus to a dorsal part of a limb in both humans and rats. We show that pain onset results in negative activity change in the NAc and pain offset produces positive activity change in the ACC and NAc. These changes were analogous in humans and rats, suggesting that translational studies of brain circuits modulated by pain are plausible and may offer an opportunity for mechanistic investigation of pain and pain relief.