Selective D2 and D3 receptor antagonists oppositely modulate cocaine responses in mice via distinct postsynaptic mechanisms in nucleus accumbens

Presynaptic and Postsynaptic Mesolimbic Dopamine D3 Receptors Play Distinct Roles in Cocaine Versus Opioid Reward in Mice

BACKGROUND

Past research has illuminated pivotal roles of dopamine D3 receptors (D3R) in the rewarding effects of cocaine and opioids. However, the cellular and neural circuit mechanisms that underlie these actions remain unclear.

METHODS

We employed Cre-LoxP techniques to selectively delete D3R from presynaptic dopamine neurons or postsynaptic dopamine D1 receptor (D1R)-expressing neurons in male and female mice. We utilized RNAscope in situ hybridization, immunohistochemistry, real-time polymerase chain reaction, voltammetry, optogenetics, microdialysis, and behavioral assays (n ≥ 8 animals per group) to functionally characterize the roles of presynaptic versus postsynaptic D3R in cocaine and opioid actions.

RESULTS

Our results revealed D3R expression in ∼25% of midbrain dopamine neurons and ∼70% of D1R-expressing neurons in the nucleus accumbens. While dopamine D2 receptors (D2R) were expressed in ∼80% dopamine neurons, we found no D2R and D3R colocalization among these cells. Selective deletion of D3R from dopamine neurons increased exploratory behavior in novel environments and enhanced pulse-evoked nucleus accumbens dopamine release. Conversely, deletion of D3R from D1R-expressing neurons attenuated locomotor responses to D1-like and D2-like agonists. Strikingly, deletion of D3R from either cell type reduced oxycodone self-administration and oxycodone-enhanced brain-stimulation reward. In contrast, neither of these D3R deletions impacted cocaine self-administration, cocaine-enhanced brain-stimulation reward, or cocaine-induced hyperlocomotion. Furthermore, D3R knockout in dopamine neurons reduced oxycodone-induced hyperactivity and analgesia, while deletion from D1R-expressing neurons potentiated opioid-induced hyperactivity without affecting analgesia.

CONCLUSIONS

We dissected presynaptic versus postsynaptic D3R function in the mesolimbic dopamine system. D2R and D3R are expressed in different populations of midbrain dopamine neurons, regulating dopamine release. Mesolimbic D3R are critically involved in the actions of opioids but not cocaine.

Substance Addiction Rehabilitation Drugs

The relapse rate of substance abusers is high, and addiction rehabilitation adjunct drugs need to be developed urgently. There have been numerous reports on blocking the formation of substance addiction, but studies on drugs that can alleviate withdrawal symptoms are very limited. Both the dopamine transporter (DAT) hypothesis and D3 dopamine receptor (D3R) hypothesis are proposed. DAT activators reduce the extracellular dopamine level, and D3R antagonists reduce the neuron's sensitivity to dopamine, both of which may exacerbate the withdrawal symptoms subsequently. The D3R partial agonist SK608 has biased signaling properties via the G-protein-dependent pathway but did not induce D3R desensitization and, thus, may be a promising drug for the withdrawal symptoms. Drugs for serotoninergic neurons or GABAergic neurons and anti-inflammatory drugs may have auxiliary effects to addiction treatments. Drugs that promote structural synaptic plasticity are also discussed.

Effect of "magic chlorine" in drug discovery: an in silico approach

The chlorine atom plays a vital role in drug design, yet the benefits of chlorine in 250 FDA-approved chlorine-containing drugs have not been studied properly. To see the "magic chloro" effect, computational studies have been carried out for 35 inhibitors, which are numbered as 12 complexes with (parent (-H), one chlorine, or two chlorine) substituents. The physicochemical properties are studied by conceptual density functional theory (CDFT). The pharmacokinetics, toxicity and metabolic properties of the studied inhibitors are estimated using chemoinformatics tools. SwissTargetPrediction is used to predict the multitarget activities of the studied inhibitors. Four FDA-approved drugs, diazepam, chloroquine, chloramphenicol, and bendamustine, are referenced to validate the studies. A higher HOMO-LUMO gap predicted high stability for the studied one and two chlorine-substituted analogues. Most of the studied inhibitors show "drug likeliness", nontoxicity, and high gastrointestinal (GI) absorption. The addition of one or two chloro substituents has increased the physicochemical properties and stability of most of the inhibitors compared to the parent analogues, whereas the toxicity is not affected. No change in metabolic properties is observed on addition of one or two chlorine substituents. The multi-target activities of all the studied inhibitors are validated by the reference drugs and experimental results.

"Magic Chloro": Profound Effects of the Chlorine Atom in Drug Discovery

Chlorine is one of the most common atoms present in small-molecule drugs beyond carbon, hydrogen, nitrogen, and oxygen. There are currently more than 250 FDA-approved chlorine-containing drugs, yet the beneficial effect of the chloro substituent has not yet been reviewed. The seemingly simple substitution of a hydrogen atom (R = H) with a chlorine atom (R = Cl) can result in remarkable improvements in potency of up to 100,000-fold and can lead to profound effects on pharmacokinetic parameters including clearance, half-life, and drug exposure in vivo. Following the literature terminology of the "magic methyl effect" in drugs, the term "magic chloro effect" has been coined herein. Although reports of 500-fold or 1000-fold potency improvements are often serendipitous discoveries that can be considered "magical" rather than planned, hypotheses made to explain the magic chloro effect can lead to lessons that accelerate the cycle of drug discovery.

Presynaptic adenosine receptor heteromers as key modulators of glutamatergic and dopaminergic neurotransmission in the striatum

Adenosine plays a very significant role in modulating striatal glutamatergic and dopaminergic neurotransmission. In the present essay we first review the extensive evidence that indicates this modulation is mediated by adenosine A₁ and A_2A receptors (A₁Rs and A_2ARs) differentially expressed by the components of the striatal microcircuit that include cortico-striatal glutamatergic and mesencephalic dopaminergic terminals, and the cholinergic interneuron. This microcircuit mediates the ability of striatal glutamate release to locally promote dopamine release through the intermediate activation of cholinergic interneurons. A₁Rs and A_2ARs are colocalized in the cortico-striatal glutamatergic terminals, where they form A₁R-A_2AR and A_2AR-cannabinoid CB₁ receptor (CB₁R) heteromers. We then evaluate recent findings on the unique properties of A₁R-A_2AR and A_2AR-CB₁R heteromers, which depend on their different quaternary tetrameric structure. These properties involve different allosteric mechanisms in the two receptor heteromers that provide fine-tune modulation of adenosine and endocannabinoid-mediated striatal glutamate release. Finally, we evaluate the evidence supporting the use of different heteromers containing striatal adenosine receptors as targets for drug development for neuropsychiatric disorders, such as Parkinson's disease and restless legs syndrome, based on the ability or inability of the A_2AR to demonstrate constitutive activity in the different heteromers, and the ability of some A_2AR ligands to act preferentially as neutral antagonists or inverse agonists, or to have preferential affinity for a specific A_2AR heteromer.

Presynaptic Gq-coupled receptors drive biphasic dopamine transporter trafficking that modulates dopamine clearance and motor function

Extracellular dopamine (DA) levels are constrained by the presynaptic DA transporter (DAT), a major psychostimulant target. Despite its necessity for DA neurotransmission, DAT regulation in situ is poorly understood, and it is unknown whether regulated DAT trafficking impacts dopaminergic signaling and/or behaviors. Leveraging chemogenetics and conditional gene silencing, we found that activating presynaptic Gq-coupled receptors, either hM3Dq or mGlu5, drove rapid biphasic DAT membrane trafficking in ex vivo striatal slices, with region-specific differences between ventral and dorsal striata. DAT insertion required D2 DA autoreceptors and intact retromer, whereas DAT retrieval required PKC activation and Rit2. Ex vivo voltammetric studies revealed that DAT trafficking impacts DA clearance. Furthermore, dopaminergic mGlu5 silencing elevated DAT surface expression and abolished motor learning, which was rescued by inhibiting DAT with a subthreshold CE-158 dose. We discovered that presynaptic DAT trafficking is complex, multimodal, and region specific, and for the first time, we identified cell autonomous mechanisms that govern presynaptic DAT tone. Importantly, the findings are consistent with a role for regulated DAT trafficking in DA clearance and motor function.

Mechanisms Underlying Dopaminergic Regulation of Nicotine-Induced Kinetic Tremor

Nicotine induces kinetic tremor, which resembles pharmacological features of essential tremors, via activating the inferior olive (IO) neurons. Since nicotine is known to enhance dopamine release by stimulating α4β2 and/or α6 nACh receptors, we examined the effects of various dopamine receptor ligands on nicotine-induced tremor to clarify the role of the dopaminergic system in modulating nicotine tremor. A tremorgenic dose of nicotine increased the dopamine level in the pons and medulla oblongata (P/MO), and the levels of dopamine metabolites in the hippocampus, P/MO, and striatum. Treatment of animals with the D1/5 agonist SKF-38393 inhibited the induction of nicotine tremor, whereas the D3 agonist PD-128,907 facilitated nicotine-induced tremor. The D2 agonist sumanirole showed no effect. In addition, nicotine tremor was significantly enhanced by the D1/5 antagonist SCH-23390 and inhibited by the D3 antagonist U-99194. Neither the D2 (L-741,626) nor D4 (L-745,870) antagonist affected the generation of nicotine tremor. Furthermore, microinjection of U-99194 into the cerebellum significantly inhibited nicotine-induced tremor, whereas its injection into IO or the striatum did not affect tremor generation. Although intrastriatal injection of SCH-23390 showed no effects, its injection into IO tended to enhance nicotine-induced tremor. The present study suggests that dopamine D3 and D1/5 receptors regulate the induction of nicotine tremor in an opposite way, D3 receptors facilitately and D1/5 receptors inhibitorily. In addition, the cerebellar D3 receptors may play an important role in modulating the induction of nicotine tremor mediated by the olivo-cerebellar system.

Consequences of Antipsychotic Medications on Mental Health

Individuals suffering from mental illnesses, unfortunately, have a shorter lifespan. The increase in mortality rates is primarily due to physical illness, unhealthy lifestyle, and associated comorbidities. Antipsychotic medications, previously known as tranquilizers, antipsychotics, or neuroleptics, can alleviate or attenuate symptoms related to psychosis, delusion, and/or hallucinations and are used in the treatment of psychosis, schizophrenia, bipolar disorder, depression, or Alzheimer's disease. Within hours to days, these medications cause calm and reduce confusion in individuals with psychosis however may take longer for full effect. Importantly, these drugs are not curing, but only treat the disease symptoms. The treatment is adjusted to reduce any psychotic symptoms while keeping the adverse effects to a minimum level. Antipsychotics may lead to increased risk of diseases, including but not limited to, diabetes, obesity, metabolic disorders, cardiovascular, renal, or respiratory disorders. Improved dosages, polypharmacy, and age-specific treatment play an important role in limiting the comorbidities as well as the side effects. Further research and clinical attention are required to understand the functioning of these medications. The review focuses on the use of antipsychotic medications in different diseases and their effect on mental health.

A Historical Perspective on the Dopamine D3 Receptor

Before 1990, the multiplicity of dopamine receptors beyond D1 and D2 had remained a controversial concept, despite its substantial clinical implications, at a time when it was widely accepted that dopamine interacted with only two receptor subtypes, termed D1 and D2, differing one from the other by their pharmacological specificity and opposite effects on adenylyl cyclase. It was also generally admitted that the therapeutic efficacy of antipsychotics resulted from blockade of D2 receptors. Thanks to molecular biology techniques, the D3 receptor could be characterized as a distinct molecular entity having a restricted anatomical gene expression and different signaling, which could imply peculiar functions in controlling cognitive and emotional behaviors. Due to the structural similarities of D2 and D3 receptors, the search for D3-selective compounds proved to be difficult, but nevertheless led to the identification of fairly potent and in vitro and in vivo selective compounds. The latter permitted to confirm a role of D3 receptors in motor functions, addiction, cognition, and schizophrenia, which paved the way for the development of new drugs for the treatment of psychiatric disorders.

Effects of the selective dopamine D3 receptor antagonist PG01037 on morphine-induced hyperactivity and antinociception in mice

Recent preclinical studies have reported that pretreatment with the novel and highly-selective dopamine D₃ receptor (D₃R) antagonists R-VK4-40 or VK4-116 attenuates the abuse-related behavioral effects of oxycodone while enhancing its analgesic properties. However, whether these observed effects are generalizable to the broad class of D₃R antagonists and/or extend to opioids other than oxycodone has not been extensively explored. The present study sought to assess the impact of pretreatment with another selective D₃R antagonist, PG01037, on several behavioral effects of morphine in mice. C57Bl/6 J mice were pretreated with PG01037 (0-10 mg/kg) and tested for 1) hyperlocomotion induced by acute morphine (5.6-56 mg/kg), 2) locomotor sensitization following repeated morphine (56 mg/kg), 3) antinociception following acute morphine (18 mg/kg), and 4) catalepsy following administration of PG01037 alone or in combination with morphine (56 mg/kg). PG01037 dose-dependently attenuated morphine-induced hyperlocomotion and morphine-induced antinociception at doses that did not alter basal locomotion or nociception alone, but did not prevent the induction of locomotor sensitization following repeated morphine administration. Moreover, PG01037 did not induce catalepsy either alone or in combination with morphine. These results suggest that attenuation of acute opioid-induced hyperactivity may be a behavioral effect shared among D₃R-selective antagonists, thus supporting continued investigations into their use as potential treatments for opioid use disorder. However, PG01037 is unlike newer, highly-selective D₃R antagonists in its capacity to reduce opioid-induced antinociception, indicating that modulation of opioid analgesia may vary across different D₃R antagonists.

RGS14 modulates locomotor behavior and ERK signaling induced by environmental novelty and cocaine within discrete limbic structures

RATIONALE

In rodents, exposure to novel environments or psychostimulants promotes locomotion. Indeed, locomotor reactivity to novelty strongly predicts behavioral responses to psychostimulants in animal models of addiction. RGS14 is a plasticity-restricting protein with unique functional domains that enable it to suppress ERK-dependent signaling as well as regulate G protein activity. Although recent studies show that RGS14 is expressed in multiple limbic regions implicated in psychostimulant- and novelty-induced hyperlocomotion, its function has been examined mostly in the context of hippocampal physiology and memory.

OBJECTIVE

We investigated whether RGS14 modulates novelty- and cocaine-induced locomotion (NIL and CIL, respectively) and neuronal activity.

METHODS

We assessed Rgs14 knockout (RGS14 KO) mice and wild-type (WT) littermate controls using NIL and CIL behavioral tests, followed by quantification of c-fos and phosphorylated ERK (pERK) induction in limbic regions that normally express RGS14.

RESULTS

RGS14 KO mice were less active than WT controls in the NIL test, driven by avoidance of the center of the novel environment. By contrast, RGS14 KO mice demonstrated augmented peripheral locomotion in the CIL test conducted in either a familiar or novel environment. RGS14 KO mice exhibited increased thigmotaxis, as well as greater c-fos and pERK induction in the central amygdala and dorsal hippocampus, when cocaine and novelty were paired.

CONCLUSIONS

RGS14 KO mice exhibited anti-correlated locomotor responses to novelty and cocaine, but displayed increased thigmotaxis in response to either stimuli which was augmented by their combination. Our findings also suggest RGS14 may reduce neuronal activity in limbic subregions by inhibiting ERK-dependent signaling.

Cognitive Deficit in Schizophrenia: From Etiology to Novel Treatments

Schizophrenia is a major mental illness characterized by positive and negative symptoms, and by cognitive deficit. Although cognitive impairment is disabling for patients, it has been largely neglected in the treatment of schizophrenia. There are several reasons for this lack of treatments for cognitive deficit, but the complexity of its etiology-in which neuroanatomic, biochemical and genetic factors concur-has contributed to the lack of effective treatments. In the last few years, there have been several attempts to develop novel drugs for the treatment of cognitive impairment in schizophrenia. Despite these efforts, little progress has been made. The latest findings point to the importance of developing personalized treatments for schizophrenia which enhance neuroplasticity, and of combining pharmacological treatments with non-pharmacological measures.

Cocaine Augments Dopamine-Mediated Inhibition of Neuronal Activity in the Dorsal Bed Nucleus of the Stria Terminalis

The dorsal region of the bed nucleus of the stria terminalis (dBNST) receives substantial dopaminergic input which overlaps with norepinephrine input implicated in stress responses. Using ex vivo fast scan cyclic voltammetry in male C57BL6 mouse brain slices, we demonstrate that electrically stimulated dBNST catecholamine signals are of substantially lower magnitude and have slower uptake rates compared with caudate signals. Dopamine terminal autoreceptor activation inhibited roughly half of the catecholamine transient, and noradrenergic autoreceptor activation produced an ∼30% inhibition. Dopamine transporter blockade with either cocaine or GBR12909 significantly augmented catecholamine signal duration. We optogenetically targeted dopamine terminals in the dBNST of transgenic (TH:Cre) mice of either sex and, using ex vivo whole-cell electrophysiology, we demonstrate that optically stimulated dopamine release induces slow outward membrane currents and an associated hyperpolarization response in a subset of dBNST neurons. These cellular responses had a similar temporal profile to dopamine release, were significantly reduced by the D2/D3 receptor antagonist raclopride, and were potentiated by cocaine. Using in vivo fiber photometry in male C57BL/6 mice during training sessions for cocaine conditioned place preference, we show that acute cocaine administration results in a significant inhibition of calcium transient activity in dBNST neurons compared with saline administration. These data provide evidence for a mechanism of dopamine-mediated cellular inhibition in the dBNST and demonstrate that cocaine augments this inhibition while also decreasing net activity in the dBNST in a drug reinforcement paradigm.SIGNIFICANCE STATEMENT The dorsal bed nucleus of the stria terminalis (dBNST) is a region highly implicated in mediating stress responses; however, the dBNST also receives dopaminergic inputs from classically defined drug reward pathways. Here we used various techniques to demonstrate that dopamine signaling within the dBNST region has inhibitory effects on population activity. We show that cocaine, an abused psychostimulant, augments both catecholamine release and dopamine-mediated cellular inhibition in this region. We also demonstrate that cocaine administration reduces population activity in the dBNST, in vivo Together, these data support a mechanism of dopamine-mediated inhibition within the dBNST, providing a means by which drug-induced elevations in dopamine signaling may inhibit dBNST activity to promote drug reward.

RGS14 Regulation of Post-Synaptic Signaling and Spine Plasticity in Brain

The regulator of G-protein signaling 14 (RGS14) is a multifunctional signaling protein that regulates post synaptic plasticity in neurons. RGS14 is expressed in the brain regions essential for learning, memory, emotion, and stimulus-induced behaviors, including the basal ganglia, limbic system, and cortex. Behaviorally, RGS14 regulates spatial and object memory, female-specific responses to cued fear conditioning, and environmental- and psychostimulant-induced locomotion. At the cellular level, RGS14 acts as a scaffolding protein that integrates G protein, Ras/ERK, and calcium/calmodulin signaling pathways essential for spine plasticity and cell signaling, allowing RGS14 to naturally suppress long-term potentiation (LTP) and structural plasticity in hippocampal area CA2 pyramidal cells. Recent proteomics findings indicate that RGS14 also engages the actomyosin system in the brain, perhaps to impact spine morphogenesis. Of note, RGS14 is also a nucleocytoplasmic shuttling protein, where its role in the nucleus remains uncertain. Balanced nuclear import/export and dendritic spine localization are likely essential for RGS14 neuronal functions as a regulator of synaptic plasticity. Supporting this idea, human genetic variants disrupting RGS14 localization also disrupt RGS14’s effects on plasticity. This review will focus on the known and unexplored roles of RGS14 in cell signaling, physiology, disease and behavior.

Akathisia and Restless Legs Syndrome: Solving the Dopaminergic Paradox

Akathisia is an urgent need to move that is associated with treatment with dopamine receptor blocking agents (DRBAs) and with restless legs syndrome (RLS). The pathogenetic mechanism of akathisia has not been resolved. This article proposes that it involves an increased presynaptic dopaminergic transmission in the ventral striatum and concomitant strong activation of postsynaptic dopamine D₁ receptors, which form complexes (heteromers) with dopamine D₃ and adenosine A₁ receptors. It also proposes that in DRBA-induced akathisia, increased dopamine release depends on inactivation of autoreceptors, whereas in RLS it depends on a brain iron deficiency-induced down-regulation of striatal presynaptic A₁ receptors.

Dopaminergic modulation of reward discounting in healthy rats: a systematic review and meta-analysis

RATIONALE

Although numerous studies have suggested that pharmacological alteration of the dopamine (DA) system modulates reward discounting, these studies have produced inconsistent findings.

OBJECTIVES

Here, we conducted a systematic review and pre-registered meta-analysis to evaluate DA drug-mediated effects on reward discounting of time, probability, and effort costs in studies of healthy rats. This produced a total of 1343 articles to screen for inclusion/exclusion. From the literature, we identified 117 effects from approximately 1549 individual rats.

METHODS

Using random effects with maximum-likelihood estimation, we meta-analyzed placebo-controlled drug effects for (1) DA D1-like receptor agonists and (2) antagonists, (3) D2-like agonists and (4) antagonists, and (5) DA transporter-modulating drugs.

RESULTS

Meta-analytic effects showed that DAT-modulating drugs decreased reward discounting. While D1-like and D2-like antagonists both increased discounting, agonist drugs for those receptors had no significant effect on discounting behavior. A number of these effects appear contingent on study design features like cost type, rat strain, and microinfusion location.

CONCLUSIONS

These findings suggest a nuanced relationship between DA and discounting behavior and urge caution when drawing generalizations about the effects of pharmacologically manipulating dopamine on reward-based decision-making.

An autophagy-related protein Becn2 regulates cocaine reward behaviors in the dopaminergic system

Drug abuse is a foremost public health problem. Cocaine is a widely abused drug worldwide that produces various reward-related behaviors. The mechanisms that underlie cocaine-induced disorders are unresolved, and effective treatments are lacking. Here, we found that an autophagy-related protein Becn2 is a previously unidentified regulator of cocaine reward behaviors. Becn2 deletion protects mice from cocaine-stimulated locomotion and reward behaviors, as well as cocaine-induced dopamine accumulation and signaling, by increasing presynaptic dopamine receptor 2 (D2R) autoreceptors in dopamine neurons. Becn2 regulates D2R endolysosomal trafficking, degradation, and cocaine-induced behaviors via interacting with a D2R-bound adaptor GASP1. Inactivating Becn2 by upstream autophagy inhibitors stabilizes striatal presynaptic D2R, reduces dopamine release and signaling, and prevents cocaine reward in normal mice. Thus, the autophagy protein Becn2 is essential for cocaine psychomotor stimulation and reward through regulating dopamine neurotransmission, and targeting Becn2 by autophagy inhibitors is a potential strategy to prevent cocaine-induced behaviors.

Exploring the structural basis and atomistic binding mechanistic of the selective antagonist blockade at D3 dopamine receptor over D2 dopamine receptor

More recently, there has been a paradigm shift toward selective drug targeting in the treatment of neurological disorders, including drug addiction, schizophrenia, and Parkinson's disease mediated by the different dopamine receptor subtypes. Antagonists with higher selectivity for D₃ dopamine receptor (D3DR) over D₂ dopamine receptor (D2DR) have been shown to attenuate drug-seeking behavior and associated side effects compared to non-subtype selective antagonists. However, high conservations among constituent residues of both proteins, particularly at the ligand-binding pockets, remain a challenge to therapeutic drug design. Recent studies have reported the discovery of two small-molecules R-VK4-40 and Y-QA31 which substantially inhibited D3DR with >180-fold selectivity over D2DR. Therefore, in this study, we seek to provide molecular and structural insights into these differential binding mechanistic using meta-analytic computational simulation methods. Findings revealed that R-VK4-40 and Y-QA31 adopted shallow binding modes and were more surface-exposed at D3DR while on the contrary, they exhibited deep hydrophobic pocket binding at D2DR. Also, two non-conserved residues; Tyr36^1.39 and Ser182^45.51 were identified in D3DR, based on their crucial roles and contributions to the selective binding of R-VK4-40 and Y-QA31. Importantly, both antagonists exhibited high affinities in complex with D3DR compared to D2DR, while van der Waals energies contributed majorly to their binding and stability. Structural analyses also revealed the distinct stabilizing effects of both compounds on D3DR secondary architecture relative to D2DR. Therefore, findings herein pinpointed the origin and mechanistic of selectivity of the compounds, which may assist in the rational design of potential small molecules of the D₂ -like dopamine family receptor subtype with improved potency and selectivity.

SKF83959, an agonist of phosphatidylinositol-linked dopamine receptors, prevents renewal of extinguished conditioned fear and facilitates extinction

Fear-related anxiety disorders, such as social phobia and post-traumatic stress disorder, are partly explained by an uncontrollable state of fear. An emerging literature suggests dopamine receptor-1 (D₁ receptor) in the amygdala is involved in the regulation of fear memory. An early study has reported that amygdaloid D₁ receptor (D₁R) is not coupled to the classic cAMP-dependent signal transduction. Here, we investigated whether SKF83959, a typical D₁R agonist that mainly activates a D₁-like receptor-dependent phosphatidylinositol (PI) signal pathway, facilitates fear extinction and reduces the return of extinguished fear. Interestingly, long-term loss of fearful memories can be induced through a combination of SKF83959 (1 mg/kg/day, i.p., once daily for one week) pharmacotherapy and extinction training. Furthermore, sub-chronic administration of SKF83959 after fear conditioning reduced fear renewal and reinstatement in the mice. We found that the activation D₁R and PI signaling in the amygdala was responsible for the effect of SKF83959 on fear extinction. Additionally, SKF83959 significantly promoted the elevation of brain-derived neurotrophic factor (BDNF) expression, possibly by the cAMP response element binding protein (CREB) -directed gene transcription. Given the beneficial effects on extinction, SKF83959 may emerge as a candidate pharmacological approach for improving cognitive-behavioral therapy on fear-related anxiety disorders.

Loss of β-arrestin2 in D2 cells alters neuronal excitability in the nucleus accumbens and behavioral responses to psychostimulants and opioids

Psychostimulants and opioids increase dopamine (DA) neurotransmission, activating D1 and D2 G protein-coupled receptors. β-arrestin2 (βarr2) desensitizes and internalizes these receptors and initiates G protein-independent signaling. Previous work revealed that mice with a global or cell-specific knockout of βarr2 have altered responses to certain drugs; however, the effects of βarr2 on the excitability of medium spiny neurons (MSNs), and its role in mediating the rewarding effects of drugs of abuse are unknown. D1-Cre and D2-Cre transgenic mice were crossed with floxed βarr2 mice to eliminate βarr2 specifically in cells containing either D1 (D1^βarr2-KO ) or D2 (D2^βarr2-KO ) receptors. We used slice electrophysiology to characterize the role of βarr2 in modulating D1 and D2 nucleus accumbens MSN intrinsic excitability in response to DA and tested the locomotor-activating and rewarding effects of cocaine and morphine in these mice. Eliminating βarr2 attenuated the ability of DA to inhibit D2-MSNs and altered the DA-induced maximum firing rate in D1-MSNs. While D1^βarr2-KO mice had mostly normal drug responses, D2^βarr2-KO mice showed dose-dependent reductions in acute locomotor responses to cocaine and morphine, attenuated locomotor sensitization to cocaine, and blunted cocaine reward measured with conditioned place preference. Both D2^βarr2-KO and D1^βarr2-KO mice displayed an enhanced conditioned place preference for the highest dose of morphine. These results indicate that D1- and D2-derived βarr2 functionally contribute to DA-induced changes in MSN intrinsic excitability and behavioral responses to psychostimulants and opioids dose-dependently.