Selective antagonism of medial prefrontal cortex D4 receptors decreases fear-related behaviour in rats

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.

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.

Norepinephrine and dopamine contribute to distinct repetitive behaviors induced by novel odorant stress in male and female mice

Exposure to unfamiliar odorants induces an array of repetitive defensive and non-defensive behaviors in rodents which likely reflect adaptive stress responses to the uncertain valence of novel stimuli. Mice genetically deficient for dopamine β-hydroxylase (Dbh-/-) lack the enzyme required to convert dopamine (DA) into norepinephrine (NE), resulting in globally undetectable NE and supranormal DA levels. Because catecholamines modulate novelty detection and reactivity, we investigated the effects of novel plant-derived odorants on repetitive behaviors in Dbh-/- mice and Dbh+/- littermate controls, which have catecholamine levels comparable to wild-type mice. Unlike Dbh+/- controls, which exhibited vigorous digging in response to novel odorants, Dbh-/- mice displayed excessive grooming. Drugs that block NE synthesis or neurotransmission suppressed odorant-induced digging in Dbh+/- mice, while a DA receptor antagonist attenuated grooming in Dbh-/- mice. The testing paradigm elicited high circulating levels of corticosterone regardless of Dbh genotype, indicating that NE is dispensable for this systemic stress response. Odorant exposure increased NE and DA abundance in the prefrontal cortex (PFC) of Dbh+/- mice, while Dbh-/- animals lacked NE and had elevated PFC DA levels that were unaffected by novel smells. Together, these findings suggest that novel odorant-induced increases in central NE tone contribute to repetitive digging and reflect psychological stress, while central DA signaling contributes to repetitive grooming. Further, we have established a simple method for repeated assessment of stress-induced repetitive behaviors in mice, which may be relevant for modeling neuropsychiatric disorders like Tourette syndrome or obsessive-compulsive disorder that are characterized by stress-induced exacerbation of compulsive symptoms.

Comparative Analysis of Pathobiochemical Changes in Major Depression and Post-Traumatic Stress Disorder

Comparative analysis of available literature data on the pathogenetic neuroendocrine mechanisms of depression and post-traumatic stress disorder (PTSD) is provided in this review to identify their common features and differences. We discuss the multidirectional modifications of the activity of cortical and subcortical structures of the brain, levels of neurotransmitters and their receptors, and functions of the hypothalamic-pituitary-adrenocortical axis in depression and PTSD. The analysis shows that these disorders are examples of opposite failures in the system of adaptive stress response of the body to stressful psychotraumatic events. On this basis, it is concluded that the currently widespread use of similar approaches to treat these disorders is not justified, despite the significant similarity of their anxiety-depressive symptoms; development of differential therapeutic strategies is required.

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.

Involvement of catecholaminergic and GABAAergic mediations in the anxiety-related behavior in long-term powdered diet-fed mice

Dietary habits are important factors which affect metabolic homeostasis and the development of emotion. We have previously shown that long-term powdered diet feeding in mice increases spontaneous locomotor activity and social interaction (SI) time. Moreover, that diet causes changes in the dopaminergic system, especially increased dopamine turnover and decreased dopamine D4 receptor signals in the frontal cortex. Although the increased SI time indicates low anxiety, the elevated plus maze (EPM) test shows anxiety-related behavior and impulsive behavior. In this study, we investigated whether the powdered diet feeding causes changes in anxiety-related behavior. Mice fed a powdered diet for 17 weeks from weaning were compared with mice fed a standard diet (control). The percentage (%) of open arm time and total number of arm entries were increased in powdered diet-fed mice in the EPM test. We also examined the effects of diazepam, benzodiazepine anti-anxiety drug, bicuculline, GABA-A receptor antagonist, methylphenidate, dopamine transporter (DAT) and noradrenaline transporter (NAT) inhibitor, atomoxetine, selective NAT inhibitor, GBR12909, selective DAT inhibitor, and PD168077, selective dopamine D4 receptor agonist, on the changes of the EPM in powdered diet-fed mice. Methylphenidate and atomoxetine are clinically used to treat attention deficit/hyperactivity disorder (ADHD) symptoms. The % of open arm time in powdered diet-fed mice was decreased by treatments of atomoxetine, methylphenidate and PD168077. Diazepam increased the % of open arm time in control diet-fed mice, but not in powdered diet-fed mice. The powdered diet feeding induced a decrease in GABA transaminase, GABA metabolic enzymes, in the frontal cortex. Moreover, the powdered diet feeding induced an increase in NAT expression, but not DAT expression, in the frontal cortex. These results suggest that the long-term powdered diet feeding may cause low anxiety or impulsivity, possibly via noradrenergic and/or dopaminergic, and GABAAergic mediations and increase the risk for onset of ADHD-like behaviors.

Bidirectional modulation of hippocampal synaptic plasticity by Dopaminergic D4-receptors in the CA1 area of hippocampus

Long-term potentiation (LTP) is the persistent increase in the strength of the synapses. However, the neural networks would become saturated if there is only synaptic strenghthening. Synaptic weakening could be facilitated by active processes like long-term depression (LTD). Molecular mechanisms that facilitate the weakening of synapses and thereby stabilize the synapses are also important in learning and memory. Here we show that blockade of dopaminergic D4 receptors (D4R) promoted the formation of late-LTP and transformed early-LTP into late-LTP. This effect was dependent on protein synthesis, activation of NMDA-receptors and CaMKII. We also show that GABA_A-receptor mediated mechanisms are involved in the enhancement of late-LTP. We could show that short-term plasticity and baseline synaptic transmission were unaffected by D4R inhibition. On the other hand, antagonizing D4R prevented both early and late forms of LTD, showing that activation of D4Rs triggered a dual function. Synaptic tagging experiments on LTD showed that D4Rs act as plasticity related proteins rather than the setting of synaptic tags. D4R activation by PD 168077 induced a slow-onset depression that was protein synthesis, NMDAR and CaMKII dependent. The D4 receptors, thus exert a bidirectional modulation of CA1 pyramidal neurons by restricting synaptic strengthening and facilitating synaptic weakening.

Activation of type 4 dopaminergic receptors in the prelimbic area of medial prefrontal cortex is necessary for the expression of innate fear behavior

The prelimbic area (PL) of the medial Prefrontal cortex (mPFC) is involved in the acquisition and expression of conditioned and innate fear. Both types of fear share several neuronal pathways. It has been documented that dopamine (DA) plays an important role in the regulation of aversive memories in the mPFC. The exposure to an aversive stimulus, such as the smell of a predator odor or the exposure to footshock stress is accompanied by an increase in mPFC DA release. Evidence suggests that the type 4 dopaminergic receptor (D4R) is the molecular target through which DA modulates fear expression. In fact, the mPFC is the brain region with the highest expression of D4R; however, the role of D4R in the expression of innate fear has not been fully elucidated. Therefore, the principal objective of this work was to evaluate the participation of mPFC D4R in the expression of innate fear. Rats were exposed to the elevated plus-maze (EPM) and to the cat odor paradigm after the intra PL injection of L-745,870, selective D4R antagonist, to measure the expression of fear-related behaviors. Intra PL injection of L-745,870 increased the time spent in the EPM open arms and decreased freezing behavior in the cat odor paradigm. Our results also showed that D4R is expressed in GABAergic and pyramidal neurons in the PL region of PFC. Thus, D4R antagonism in the PL decreases the expression of innate fear-behavior indicating that the activation of D4R in the PL is necessary for the expression of innate fear-behavior.

The ADHD-linked human dopamine D4 receptor variant D4.7 induces over-suppression of NMDA receptor function in prefrontal cortex

The human dopamine D4 receptor (hD4R) variants with long tandem repeats in the third intracellular loop have been strongly associated with attention deficit hyperactivity disorder (ADHD) and risk taking behaviors. To understand the potential molecular mechanism underlying the connection, we have investigated the synaptic function of human D4R polymorphism by virally expressing the ADHD-linked 7-repeat allele, hD4.7, or its normal counterpart, hD4.4, in the prefrontal cortex (PFC) of D4R knockout mice. We found that hD4R bound to the SH3 domain of PSD-95 in a state-dependent manner. Activation of hD4.7 caused more reduction of NR1/PSD-95 binding and NR1 surface expression than hD4.4 in PFC slices. Moreover, the NMDAR-mediated excitatory postsynaptic currents (NMDAR-EPSC) in PFC pyramidal neurons were suppressed to a larger extent by hD4.7 than hD4.4 activation. Direct stimulation of NMDARs with the partial agonist d-cycloserine prevented the NMDAR hypofunction induced by hD4.7 activation. Moreover, hD4.7-expressing mice exhibited the increased exploratory and novelty seeking behaviors, mimicking the phenotypic hallmark of human ADHD. d-cycloserine administration ameliorated the ADHD-like behaviors in hD4.7-expressing mice. Our results suggest that over-suppression of NMDAR function may underlie the role of hD4.7 in ADHD, and enhancing NMDAR signaling may be a viable therapeutic strategy to ADHD humans carrying the D4.7 allele.

Antidepressant-like cognitive and behavioral effects of acute ketamine administration associated with plasticity in the ventral hippocampus to medial prefrontal cortex pathway

RATIONALE

Acute low-dose administration of the N-methyl-D-aspartate (NMDA) receptor antagonist, ketamine, produces rapid and sustained antidepressant-like effects in humans and rodents. Recently, we found that the long-lasting effect of ketamine on the forced swim test requires ventral hippocampal (vHipp) activity at the time of drug administration. The medial prefrontal cortex (mPFC), a target of the vHipp dysregulated in depression, is important for cognitive flexibility and response strategy selection. Deficits in cognitive flexibility, the ability to modify thoughts and behaviors in response to changes in the environment, are associated with depression. We have shown that chronic stress impairs cognitive flexibility on the attentional set-shifting test (AST) and induces a shift from active to passive response strategies on the shock-probe defensive burying test (SPDB).

OBJECTIVE

In this study, we tested the effects of ketamine on chronic stress-induced changes in cognitive flexibility and coping behavior on the AST and SPDB, respectively. Subsequently, we investigated vHipp-mPFC plasticity as a potential mechanism of ketamine's therapeutic action.

RESULTS

Ketamine reversed deficits in cognitive flexibility and restored active coping behavior in chronically stressed rats. Further, high frequency stimulation in the vHipp replicated ketamine's antidepressant-like effects on the forced swim test and AST, but not on the SPDB.

CONCLUSION

These results show that ketamine restores cognitive flexibility and coping response strategy compromised by stress. Activity in the vHipp-mPFC pathway may represent a neural substrate for some of the antidepressant-like behavioral effects of ketamine, including cognitive flexibility, but other circuits may mediate the effects of ketamine on coping response strategy.

Mesocortical dopamine depletion and anxiety-related behavior in the rat: sex and hemisphere differences

The mesocortical dopamine (DA) system of the rat plays an important role in prefrontal cortex (PFC) regulation of stress and emotion and exhibits functional hemispheric asymmetry for such processing. Since few studies examine sex differences in this context, we compared the effects of left vs. right unilateral PFC DA depletion in males and females in several behavioral situations associated with anxiety or aversion. Adult rats received unilateral injections of 6-hydroxydopamine (6-OHDA) or vehicle in the ventromedial (vm) PFC. Behavioral tests included a predator odor burying test, elevated plus maze and sucrose consumption with simple taste aversion. Tissue analysis confirmed that vmPFCs injected with 6-OHDA were depleted of DA (75-85%) compared to controls. Burying behavior and sucrose consumption were affected only by left lesions, similarly in both sexes. However, risk assessment behaviors were affected by right lesions in opposite directions in males and females. Behaviors modified preferentially by the left cortex thus showed less evidence of sex differences than those modulated by the right. While mesocortical DA depletion effects are lateralized, the nature of these effects can vary with sex and specific behavior. Such findings may be clinically significant, given the large gender differences in the incidence of mood and anxiety disorders, which also show many lateralized prefrontal abnormalities.

Dopamine, cognitive function, and gamma oscillations: role of D4 receptors

Cognitive deficits in individuals with schizophrenia (SCZ) are considered core symptoms of this disorder, and can manifest at the prodromal stage. Antipsychotics ameliorate positive symptoms but only modestly improve cognitive symptoms. The lack of treatments that improve cognitive abilities currently represents a major obstacle in developing more effective therapeutic strategies for this debilitating disorder. While D4 receptor (D4R)-specific antagonists are ineffective in the treatment of positive symptoms, animal studies suggest that D4R drugs can improve cognitive deficits. Moreover, recent work from our group suggests that D4Rs synergize with the neuregulin/ErbB4 signaling pathway, genetically identified as risk factors for SCZ, in parvalbumin (PV)-expressing interneurons to modulate gamma oscillations. These high-frequency network oscillations correlate with attention and increase during cognitive tasks in healthy subjects, and this correlation is attenuated in affected individuals. This finding, along with other observations indicating impaired GABAergic function, has led to the idea that abnormal neural activity in the prefrontal cortex (PFC) in individuals with SCZ reflects a perturbation in the balance of excitation and inhibition. Here we review the current state of knowledge of D4R functions in the PFC and hippocampus, two major brain areas implicated in SCZ. Special emphasis is given to studies focusing on the potential role of D4Rs in modulating GABAergic transmission and to an emerging concept of a close synergistic relationship between dopamine/D4R and neuregulin/ErbB4 signaling pathways that tunes the activity of PV interneurons to regulate gamma frequency network oscillations and potentially cognitive processes.

Amygdala-prefrontal pathways and the dopamine system affect nociceptive responses in the prefrontal cortex

Background

We previously demonstrated nociceptive discharges to be evoked by mechanical noxious stimulation in the prefrontal cortex (PFC). The nociceptive responses recorded in the PFC are conceivably involved in the affective rather than the sensory-discriminative dimension of pain. The PFC receives dense projection from the limbic system. Monosynaptic projections from the basolateral nucleus of the amygdala (BLA) to the PFC are known to produce long-lasting synaptic plasticity. We examined effects of high frequency stimulation (HFS) delivered to the BLA on nociceptive responses in the rat PFC.

Results

HFS induced long lasting suppression (LLS) of the specific high threshold responses of nociceptive neurons in the PFC. Microinjection of N-methyl-D-aspartic acid (NMDA) receptor antagonists (2-amino-5-phosphonovaleric acid (APV), dizocilpine (MK-801)) and also metabotropic glutamate receptor (mGluR) group antagonists (α-methyl-4-carboxyphenylglycine (MCPG), and 2-[(1S,2S)-2-carboxycyclopropyl]-3-(9H-xanthen-9-yl)-D-alanine (LY341495)), prevented the induction of LLS of nociceptive responses. We also examined modulatory effects of dopamine (DA) on the LLS of nociceptive responses. With depletion of DA in response to 6-hydroxydopamine (6-OHDA) injection into the ipsilateral forebrain bundle, LLS of nociceptive responses was decreased, while nociceptive responses were normally evoked. Antagonists of DA receptor subtypes D2 (sulpiride) and D4 (3-{[4-(4-chlorophenyl) piperazin-1-yl] methyl}-1H-pyrrolo [2, 3-b] pyridine (L-745,870)), microinjected into the PFC, inhibited LLS of nociceptive responses.

Conclusions

Our results indicate that BLA-PFC pathways inhibited PFC nociceptive cell activities and that the DA system modifies the BLA-PFC regulatory function.

Mineralocorticoid receptors in the medial prefrontal cortex and hippocampus mediate rats' unconditioned fear behaviour

Corticosterone is released from the adrenal cortex in response to stress, and binds to glucocorticosteroid receptors (GRs) and mineralocorticosteroid receptors (MRs) in the brain. Areas such as the dorsal hippocampus (DH), ventral hippocampus (VH) and medial prefrontal cortex (mPFC) all contain MRs and have been previously implicated in fear and/or memory. The purpose of the following experiments was to examine the role of these distinct populations of MRs in rats' unconditioned fear and fear memory. The MR antagonist (RU28318) was microinfused into the DH, VH, or mPFC of rats. Ten minutes later, their unconditioned fear was tested in the elevated plus-maze and the shock-probe tests, two behavioral models of rat "anxiety." Twenty-four hours later, conditioned fear of a non-electrified probe was assessed in rats re-exposed the shock-probe apparatus. Microinfusions of RU28318 into each of the three brain areas reduced unconditioned fear in the shock-probe burying test, but only microinfusions into the VH reduced unconditioned fear in the plus-maze test. RU28318 did not affect conditioned fear of the shock-probe 24hr later. MRs in all three areas of the brain mediated unconditioned fear to a punctate, painful stimulus (probe shock). However, only MRs in the ventral hippocampus seemed to mediate unconditioned fear of the more diffuse threat of open spaces (open arms of the plus maze). In spite of the known roles of the hippocampus in spatial memory and conditioned fear memory, MRs within these sites did not appear to mediate memory of the shock-probe.

A critical test of the hippocampal theta model of anxiolytic drug action

Hippocampal theta rhythms have been associated with a number of behavioural processes, including learning, memory and arousal. Recently it has been argued that the suppression of hippocampal theta is a valid indicator of anxiolytic drug action. Like all such models, however, it has relied almost exclusively on the experimental effects of well-known, clinically proven anxiolytic compounds for validation. The actual predictive validity of putative models of anxiolytic drug action, however, cannot be rigorously tested with this approach alone. The present study provides a stringent test of the predictive validity of the theta suppression model, using the drug phenytoin (50 mg/kg and 10 mg/kg), and a positive comparison compound, diazepam (2 mg/kg). Phenytoin has two important properties that are advantageous for assessing the validity of the theta suppression model: 1) it is a standard antiepileptic drug with no known anxiolytic effects, and 2) its primary mechanism of action is through suppression of the persistent sodium current, an effect that should also suppress hippocampal theta. Because of the latter property, we also directly compared the effects of phenytoin in the theta suppression model with its effects in the most widely tested behavioural model of anxiolytic drug action, the elevated plus-maze. While an anxiolytic-like effect of phenytoin in the theta suppression model might be expected simply due to its suppressive effects on sodium channel currents, anxiolytic effects in both tests would provide strong support for the predictive validity of the theta suppression model. Surprisingly, phenytoin produced clear anxiolytic-like effects in both neurophysiological and behavioural models, thus providing strong evidence of the predictive validity of the theta suppression model. This article is part of a Special Issue entitled 'Anxiety and Depression'.

The effects of dopaminergic drugs in the dorsal hippocampus of mice in the nicotine-induced anxiogenic-like response

RATIONALE

Nicotine, an active alkaloid of tobacco has an acetylcholine property that alters anxiety-like behaviors in rodents. Moreover, several investigations suggest that the mesolimbic/cortical dopamine systems to be involved in the drugs affecting anxiety. The dopaminergic modulation of acetylcholine synaptic transmission has also been also suggested by different studies. Furthermore, modulation of such behaviors in rodents may be mediated through the dorsal hippocampus.

OBJECTIVES

In the present study, a possible role of the dorsal hippocampal acetylcholine receptor mechanism in nicotine's influence on anxiogenic-like responses has been investigated.

METHODS

During test sessions, the hole-board was used to investigate the effects of SCH23390, sulpiride, SKF38393 and quipirole on nicotine response in mice.

RESULTS

Intraperitoneal (i.p.) administration of nicotine (0.5 mg/kg) decreased the number of head dips but had no effect on other behaviors. Intra-dorsal hippocampal injections of ineffective doses of SCH23390 (SCH; 0.125 and 0.25 μg/mouse) or sulpiride (SUL; 0.5 and 0.75 μg/mouse) reversed head dips induced by nicotine but did not impact other exploratory behaviors. Furthermore, co-administration of ineffective doses of SKF38393 (SKF; 4 μg/mouse, dorsal hippocampus) or quipirole (QUI; 0.5 μg/mouse) in conjunction with an ineffective dose of nicotine (0.25 mg/kg, i.p.) decreased head dips induced by nicotine, but were otherwise ineffective.

CONCLUSION

These results may indicate a modulatory effect for the dorsal hippocampus dopamine receptors (D₁ and D₂) on an anxiogenic-like response induced by nicotine.

The effects of dopaminergic drugs in the ventral hippocampus of rats in the nicotine-induced anxiogenic-like response

Nicotine an active alkaloid of tobacco has dopaminergic properties. The drug alters anxiety-like behavior in rodents. Ventral hippocampus (VHC) may be a site for modulation of anxiety-like behaviors. The possible involvement of ventral hippocampal dopaminergic receptor mechanism in the nicotine influence on anxiogenic-like response has been investigated in the present study. The effects of apomorphine, sulpiride and SCH23390 on nicotine response in elevated plus maze in rats have been investigated. Intraperitoneal administration of nicotine (0.6mg/kg) decreased percentage of open arm time (%OAT) but not percentage of open arm entries (%OAE) and locomotor activity, indicating an anxiogenic-like response. Intra-hippocampal injection (intra-VHC) of apomorphine, a D(1)/D(2) dopamine receptor agonist (0.1 and 0.2microg/rat) also caused anxiogenic-like effects, but the drug blocked that of nicotine. Intra-VHC administration of the D(2) receptor antagonist, sulpiride (1, 2.5 and 5microg/rat) or the D(1) receptor antagonist, SCH23390 (0.01, 0.1 and 1microg/rat) did not elicit any response. However, pretreatment with sulpiride (1microg/rat) or SCH23390 (0.1microg/rat) decreased nicotine's effect. The results may indicate a modulatory effect for the D(1) and D(2) dopamine receptors of VHC in the anxiogenic-like response induced by nicotine.

Dopamine D4-receptor modulation of cortical neuronal network activity and emotional processing: Implications for neuropsychiatric disorders

Dopamine (DA) transmission within cortical and subcortical structures is involved critically in the processing of emotionally relevant sensory information. Three interconnected neural regions, the medial prefrontal cortex (mPFC), basolateral nucleus of the amygdala (BLA) and the ventral tegmental area (VTA) have received considerable experimental attention, both in animal and clinical research models, as essential interconnected processors of emotional information. Neuronal network activity within both the mPFC and BLA are strongly modified by DA inputs from the VTA through both DA D(2)-like and D(1)-like receptors. However, emerging evidence from clinical, genetic, behavioral and electrophysiological investigations demonstrates a critical role for the DA D(4)-receptor subtype as a crucial modulator of emotional memory encoding and expression, both at the level of the single neuron, and at the systems level. In this review, we will examine recent evidence at the neuronal, behavioral and genetic levels of analysis that increasingly demonstrates an important role for DA D(4) transmission within cortical and subcortical emotional processing circuits. We will present evidence and some theoretical frameworks suggesting how disturbances in D(4)-receptor related neural circuitry may be involved in the neuropathological manifestations common in many neuropsychiatric disorders including schizophrenia, attention-deficit hyperactivity disorder (ADHD) and addiction.

Dopamine release during human emotional processing

Involvement of dopamine neurotransmission in human emotional processing is unclear but animal studies have indicated that it is critical for processing of fear response. In this experiment we examined dopaminergic involvement in the processing of human emotions. We used a novel dynamic molecular imaging technique to detect and map dopamine released during presentation of emotional stimuli. The technique exploited the competition between endogenously released dopamine and its ligand for receptor occupancy and involved dynamic voxel-wise measurement of the rate at which a dopamine receptor ligand ((18)F-Fallypride) was displaced from receptor sites during emotional processing. An increase in the rate indicated dopamine release. We found that the rate of ligand displacement increased significantly in the left amygdala, left medial temporal lobe (MTL) and left inferior frontal gyrus. The results provide the first direct evidence of dopaminergic modulation of human emotional processing and suggest that the modulation occurs at multiple levels of processing. This finding indicates that the neurocognitive models of human emotion should take into account dopaminergic effects, and that, there is a need to investigate whether manipulation of the dopaminergic system could be an alternate strategy for treatment of conditions in which emotional processing is impaired.

Dopaminergic modulation of risk-based decision making

Psychopharmacological studies have implicated the mesolimbic dopamine (DA) system in the mediation of cost/benefit evaluations about delay or effort-related costs associated with larger rewards. However, the role of DA in risk-based decision making remains relatively unexplored. The present study investigated the effects of systemic manipulations of DA transmission on risky choice using a probabilistic discounting task. Over discrete trials, rats chose between two levers; a press on the 'small/certain' lever always delivered one reward pellet, whereas a press on the other, 'large/risky' lever delivered four pellets, but the probability of receiving reward decreased across the four trial blocks (100, 50, 25, 12.5%). In separate groups of well-trained rats we assessed the effects of the DA releaser amphetamine, as well as receptor selective agonists and antagonists. Amphetamine consistently increased preference for the large/risky lever; an effect that was blocked or attenuated by co-administration of either D(1) (SCH23390) or D(2) (eticlopride) receptor antagonists. Blockade of either of these receptors alone induced risk aversion. Conversely, stimulation of D(1) (SKF81297) or D(2) (bromocriptine) receptors also increased risky choice. In contrast, activation of D(3) receptors with PD128,907 reduced choice of the large/risky lever. Likewise, D(3) antagonism with nafadotride potentiated the amphetamine-induced increase in risky choice. Blockade or stimulation of D(4) receptors did not reliably alter behavior. These findings indicate that DA has a critical role in mediating risk-based decision making, with increased activation of D(1) and D(2) receptors biasing choice toward larger, probabilistic rewards, whereas D(3) receptors appear to exert opposing effects on this form of decision making.

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.

Dopamine modulation of emotional processing in cortical and subcortical neural circuits: evidence for a final common pathway in schizophrenia?

The neural regulation of emotional perception, learning, and memory is essential for normal behavioral and cognitive functioning. Many of the symptoms displayed by individuals with schizophrenia may arise from fundamental disturbances in the ability to accurately process emotionally salient sensory information. The neurotransmitter dopamine (DA) and its ability to modulate neural regions involved in emotional learning, perception, and memory formation has received considerable research attention as a potential final common pathway to account for the aberrant emotional regulation and psychosis present in the schizophrenic syndrome. Evidence from both human neuroimaging studies and animal-based research using neurodevelopmental, behavioral, and electrophysiological techniques have implicated the mesocorticolimbic DA circuit as a crucial system for the encoding and expression of emotionally salient learning and memory formation. While many theories have examined the cortical-subcortical interactions between prefrontal cortical regions and subcortical DA substrates, many questions remain as to how DA may control emotional perception and learning and how disturbances linked to DA abnormalities may underlie the disturbed emotional processing in schizophrenia. Beyond the mesolimbic DA system, increasing evidence points to the amygdala-prefrontal cortical circuit as an important processor of emotionally salient information and how neurodevelopmental perturbances within this circuitry may lead to dysregulation of DAergic modulation of emotional processing and learning along this cortical-subcortical emotional processing circuit.

Mesocortical dopamine modulation of executive functions: beyond working memory

RATIONALE

Dopamine (DA) neurotransmission in the prefrontal cortex (PFC) is known to play an essential role in mediating executive functions such as the working memory. DA exerts these effects by acting on D1 receptors because blockade or stimulation of these receptors in the PFC can impair performance on delayed response tasks. However, comparatively less is known about dopaminergic mechanisms that mediate other executive functions regulated by the PFC. Furthermore, the functional importance of other DA receptor subtypes that reside on PFC neurons (D2 and D4) is unclear.

OBJECTIVES

This review will summarize previous findings and previously unpublished data addressing the contribution of PFC DA to higher-order cognition. We will compare the DA receptor mechanisms, which regulate executive functions such as working memory, behavioral flexibility, and decision-making.

RESULTS AND CONCLUSIONS

Whereas PFC D1 receptor activity is of primary importance in working memory, D1 and D2 receptors act in a cooperative manner to facilitate behavioral flexibility. We note that the principle of the "inverted U-shaped" function of D1 receptor activity mediating working memory does not necessarily apply to other PFC functions. DA in different subregions of the PFC also mediates decision-making assessed with delay discounting or effort-based procedures, and we report that D1, D2, and D4 receptors in the medial PFC contribute to decision-making when animals must bias the direction of behavior to avoid aversive stimuli, assessed with a conditioned punishment procedure. Thus, mesocortical DA modulation of distinct executive functions is subserved by dissociable profiles of DA receptor activity in the PFC.

Prefrontal dopamine D4 receptors are involved in encoding fear extinction

The medial prefrontal cortex is known to be crucially involved in the memory for fear extinction. It has been shown that the dopamine D4 receptor is abundantly localized in the medial prefrontal cortex of rodents and that this receptor subtype is involved in the mediation of fear-related behaviour. In the present study, we tested whether dopamine D4 receptors within the medial prefrontal cortex are involved in the learning and encoding of fear extinction. Infusions of the specific dopamine D4 receptor antagonist L-741,741 into the medial prefrontal cortex immediately before fear extinction did not affect fear extinction learning but attenuated the memory on fear extinction on the following day. These results suggest that medial prefrontal cortex dopamine D4 receptors are involved in the consolidation of fear extinction memory.

Mesocortical dopamine and HPA axis regulation: Role of laterality and early environment

The infralimbic (IL) cortex is importantly involved in regulating behavioral and physiological responses to stress, including those of the hypothalamic-pituitary-adrenal (HPA) axis. The mesocortical dopamine (DA) system is an important afferent modulator of this region, is highly stress sensitive and frequently shows functional hemispheric asymmetry. Postnatal handling stimulation facilitates development of cortical asymmetry and is also associated with optimal HPA axis regulation. The present study examines the poorly understood role of the mesocortical DA system in regulating HPA axis function in adult rats which were handled (H) or nonhandled (NH) postnatally. In the first experiment, unilateral intra-IL cortex injection of the DA (D1/D2) antagonist alpha-flupenthixol into either hemisphere significantly exaggerated the restraint stress-induced increases in plasma adrenocorticotrophic hormone and corticosterone in NH rats. In H rats, the same effect was lateralized to the right IL cortex. In a second experiment, post mortem neurochemical analysis of DAergic measures in the IL cortex was conducted in H and NH animals following either acute or repeated (5 times) restraint stress. DAergic measures in the right IL cortex were significantly correlated with reduced stress hormone activation in both H and NH rats, especially in repeatedly restrained rats. However, while H rats showed a significant rightward shift in DA metabolism with repeated stress experience, NH rats shifted DA metabolism to the left. It is suggested that, during stress, mesocortical DA release normally acts in an adaptive, negative feedback capacity preventing excessive HPA activation and, with repeated stress, the right IL cortex is particularly important in this capacity. As well, the selective enhancement of DA metabolism in the right IL cortex of H rats may underlie, in part, their typically superior ability to adapt to stress and constrain HPA activity.

A subpopulation of neurons in the medial prefrontal cortex encodes emotional learning with burst and frequency codes through a dopamine D4 receptor-dependent basolateral amygdala input

The basolateral nucleus of the amygdala (BLA) and medial prefrontal cortex (mPFC) are involved importantly in the processing and encoding of emotionally salient learned associations. Here, we examined the possible role of the mPFC in the acquisition and encoding of emotional associative learning at the behavioral and single-neuron level. A subpopulation of neurons in the mPFC that received monosynaptic and orthodromic inputs from the BLA demonstrated strong associative responding to odors paired previously with footshock by increasing spontaneous activity and bursting activity. This occurred specifically in response to postconditioning presentations of the footshock-paired odors but not to odors presented in the absence of footshock. In contrast, mPFC neurons that failed to respond to BLA stimulation showed no associative responding. Systemic or intra-mPFC blockade of dopamine (DA) D4 receptors prevented this emotional associative learning in neurons of the mPFC and blocked the expression of olfactory conditioned fear. These results demonstrate that individual neurons in the mPFC that receive a functional input from the BLA actively encode emotional learning and that this process depends on DA D4 receptor stimulation in the mPFC.

Multiple dopamine receptor subtypes in the medial prefrontal cortex of the rat regulate set-shifting

Dopamine (DA) input to the prefrontal cortex (PFC), acting on D1 receptors, plays an essential role in mediating working memory functions. In comparison, less is known about the importance of distinct PFC DA receptor subtypes in mediating executive functions such as set-shifting. The present study assessed the effects of microinfusion of D2 and D4 receptor antagonists, and D1, D2, and D4 receptor agonists into the PFC on performance of a maze-based set-shifting task. In Experiment 1, rats were trained on a response discrimination task, and then on a visual-cue discrimination task requiring rats to suppress the use of the response strategy and approach the previously irrelevant cue to locate food. In Experiment 2, the order of training was reversed. Infusions of the D2 antagonist eticlopride, or the D4 agonist PD-168,077, impaired shifting from a response to a visual-cue discrimination strategy and vice versa, and caused a selective increase in perseverative errors. In contrast, infusions of the D4 antagonist L-745,870 improved set-shifting. Infusions of the D1 agonist SKF81297 or the D2 agonist quinpirole caused no reliable effect. These data, in combination with previous reports of impaired set-shifting following D1 receptor blockade, suggest that multiple receptors in the PFC are essential for set-shifting and that the mechanisms by which PFC DA mediates behavioral flexibility may be different from those underlying working memory. These findings may have important implications for developing novel treatments for cognitive deficits observed in disorders such as attentional deficit and hyperactivity disorder and schizophrenia.

The L-type calcium channel blocker nifedipine impairs extinction, but not reduced contingency effects, in mice

We recently reported that fear extinction, a form of inhibitory learning, is selectively blocked by systemic administration of L-type voltage-gated calcium channel (LVGCC) antagonists, including nifedipine, in mice. We here replicate this finding and examine three reduced contingency effects after vehicle or nifedipine (40 mg/kg) administration. In the first experiment, contingency reduction was achieved by adding USs to the training protocol (degraded contingency), a phenomenon thought to be independent of behavioral inhibition. In the second experiment, contingency reduction was achieved by varying the percentage of CS-US pairing, a phenomenon thought to be weakly dependent on behavioral inhibition. In the third and fourth experiments, contingency reduction was achieved by adding CSs to the training protocol (partial reinforcement), a phenomenon thought to be completely dependent on behavioral inhibition. We found that none of these reduced contingency effects was impaired by nifedipine. In a final experiment, we found that extinction conducted 1 or 3 h post-acquisition, but not immediately, was LVGCC-dependent. Taken together, the results suggest that reduced contingency effects and extinction depend on different molecular mechanisms and that LVGCC dependence of behavioral inhibition develops with time after associative CS-US learning.

Activation of dopaminergic neurotransmission in the medial prefrontal cortex by N-methyl-d-aspartate stimulation of the ventral hippocampus in rats

Many behavioral functions-including sensorimotor, attentional, memory, and emotional processes-have been associated with hippocampal processes and with dopamine transmission in the medial prefrontal cortex (mPFC). This suggests a functional interaction between hippocampus and prefrontal dopamine. The anatomical substrate for such an interaction is the intimate interconnection between the ventral hippocampus and the dopamine innervation of the mPFC. The present study yielded direct neurochemical evidence for an interaction between ventral hippocampus and prefrontal dopamine transmission in rats by demonstrating that subconvulsive stimulation of the ventral hippocampus with N-methyl-d-aspartate (NMDA; 0.5 mug/side) activates dopamine transmission in the mPFC. Postmortem measurements revealed that bilateral NMDA stimulation of the ventral hippocampus, resulting in locomotor hyperactivity, increased the homovanillic acid/dopamine ratio, an index of dopamine transmission, in the mPFC; indices of dopamine transmission in any of five additionally examined forebrain regions (amygdala, nucleus accumbens shell/core, lateral prefrontal cortex, caudate putamen) were unaltered. In vivo microdialysis measurements in freely moving rats corroborated the suggested activation of prefrontal dopamine transmission by demonstrating that unilateral NMDA stimulation of the ventral hippocampus increased extracellular dopamine in the ipsilateral mPFC. The suggested influence of the ventral hippocampus on prefrontal dopamine may be an important mechanism for hippocampo-prefrontal interactions in normal behavioral processes. Moreover, it indicates that aberrant hippocampal activity, as found in neuropsychiatric diseases, such as schizophrenia and mood disorders, may contribute to disruption of certain cognitive and emotional functions which are extremely sensitive to imbalanced prefrontal dopamine transmission.

Systemic blockade of D2-like dopamine receptors facilitates extinction of conditioned fear in mice

Extinction of conditioned fear in animals is the explicit model of behavior therapy for human anxiety disorders, including panic disorder, obsessive-compulsive disorder, and post-traumatic stress disorder. Based on previous data indicating that fear extinction in rats is blocked by quinpirole, an agonist of dopamine D2 receptors, we hypothesized that blockade of D2 receptors might facilitate extinction in mice, while agonists should block extinction, as they do in rats. One day after fear conditioning mice with three pairings of a white noise conditional stimulus (CS) with moderate footshock, we injected the D2 antagonist, sulpiride, the D2 agonist, quinpirole, or vehicle, just before repeated CS presentations to generate extinction. We assayed fear by measuring behavioral freezing during extinction presentations and then drug-free during CS presentations 1 d later. We found that sulpiride injections before extinction training facilitated extinction memory 24 h later, while quinpirole partially blocked extinction memory compared with vehicle-injected controls. Notably, sulpiride treatment yielded significant extinction after spaced CS presentations, which yield no extinction at all in vehicle-treated mice. These findings suggest that dopamine D2-mediated signaling contributes physiological inhibition of extinction, and that D2 antagonists may be useful adjuncts to behavior therapy of human anxiety disorders.