Coexpression of dopamine D1 and D3 receptors in islands of Calleja and shell of nucleus accumbens of the rat: opposite and synergistic functional interactions

Dissociable control of motivation and reinforcement by distinct ventral striatal dopamine receptors

Dopamine (DA) release in striatal circuits, including the nucleus accumbens medial shell (mNAcSh), tracks separable features of reward like motivation and reinforcement. However, the cellular and circuit mechanisms by which DA receptors transform DA release into distinct constructs of reward remain unclear. Here we show that DA D3 receptor (D3R) signaling in the mNAcSh drives motivated behavior in mice by regulating local microcircuits. Furthermore, D3Rs coexpress with DA D1 receptors, which regulate reinforcement, but not motivation. Paralleling dissociable roles in reward function, we report nonoverlapping physiological actions of D3R and DA D1 receptor signaling in mNAcSh neurons. Our results establish a fundamental framework wherein DA signaling within the same nucleus accumbens cell type is physiologically compartmentalized via actions on distinct DA receptors. This structural and functional organization provides neurons in a limbic circuit with the unique ability to orchestrate dissociable aspects of reward-related behaviors relevant to the etiology of neuropsychiatric disorders.

Licking microstructure in response to novel rewards, reward devaluation and dopamine antagonists: Possible role of D1 and D2 medium spiny neurons in the nucleus accumbens

Evidence on the effect of dopamine D1 and D2-like antagonists and of manipulations of reward value on licking microstructure is reanalysed considering recent findings on the role of nucleus accumbens (NAc) medium spiny neurons (MSNs) in the control of sugar intake. The results of this analysis suggest that D1 MSN activation, which is involved in the emission of licking bursts, might play a crucial role in response to novel rewards. D2 MSN activation, which results in reduction of burst size and suppression of licking, might mediate the response to reward devaluation. Elucidating the neural mechanisms underlying the licking response might lead to a better definition of its microstructural measures in behaviourally and psychologically meaningful functional terms. This could further support its use as a behavioural substrate in the study of the neural mechanisms of ingestive behaviour and motivation, as well as in animal models of pathological conditions such as eating disorders and obesity.

IRL790 modulated striatal D1 neurons synaptic plasticity ameliorating levodopa-induced dyskinesia in mouse

Objective

Levodopa (L-dopa) therapy is the principal pharmacological treatment for Parkinson's disease (PD). Nevertheless, prolonged use of this drug may result in different involuntary movement symptoms caused by the medication, referred to as levodopa-induced dyskinesia (LID). LID is associated with changes in synaptic plasticity of the D1 medium spiny neurons (MSNs) located in the dorsal striatum (dStr). Within the striatum, the amount of Dopamine D3 receptor (D3R) is notably increased in LID, demonstrating colocalization with D1R expression in neurons, and the level of D3R expression is directly related to the intensity of LID. IRL 790, as a D3R antagonist, can ameliorate LID. This study aims to explore if IRL 790 improves LID by regulating the synaptic plasticity of D1+ MSNs in dStr.

Methods

The electrophysiology and synaptic spine density of D1+ MSNs in dStr were recorded for sham mice, LID mice, and LID mice treated with IRL 790. The regulation of synaptic plasticity in LID D1+ MSNs by IRL 790 was analyzed. Behavioral tests were conducted to confirm the treatment effect of IRL 790 on LID.

Results

In LID D1+ MSNs, there was persistent abnormal LTP, absence of LTD, and an increase in spontaneous excitatory postsynaptic currents (sEPSCs). IRL 790 treatment restored normal LTP, LTD, and sEPSCs. Treatment with IRL 790 also restored the reduced dendritic spine density in D1+ MSNs of LID mice. IRL790 improved dyskinetic manifestations in LID mice.

Conclusion

IRL790 ameliorates LID by regulating the synaptic structure and functional plasticity of striatal D1+ MSNs.

Modulation of D3R Splicing, Signaling, and Expression by D1R through PKA→PTB Phosphorylation

The D1R and D3R receptors functionally and synergistically interact in striatonigral neurons. Dopaminergic denervation turns this interaction antagonistic, which is correlated with a decrement in D3nf isoform and an increment in D3R membranal expression. The mechanisms of such changes in D3R are attributed to the dysregulation of the expression of their isoforms. The cause and mechanism of this phenomenon remain unknown. Dopaminergic denervation produces a decrement in D1R and PKA activity; we propose that the lack of phosphorylation of PTB (regulator of alternative splicing) by PKA produces the dysregulation of D3R splicing and changes D3R functionality. By using in silico analysis, we found that D3R mRNA has motifs for PTB binding and, by RIP, co-precipitates with PTB. Moreover, D1R activation via PKA promotes PTB phosphorylation. Acute and 5-day D1R blockade decreases the expression of D3nf mRNA. The 5-day treatment reduces D3R, D3nf, and PTB protein in the cytoplasm and increases D3R in the membrane and PTB in the nucleus. Finally, the blockade of D1R mimics the effect of dopaminergic denervation in D1R and D3R signaling. Thus, our data indicate that through PKA→PTB, D1R modulates D3R splicing, expression, and signaling, which are altered during D1R blockade or the lack of stimulation in dopaminergic denervation.

Ventral striatal islands of Calleja neurons bidirectionally mediate depression-like behaviors in mice

The ventral striatum is a reward center implicated in the pathophysiology of depression. It contains islands of Calleja, clusters of dopamine D3 receptor-expressing granule cells, predominantly in the olfactory tubercle (OT). These OT D3 neurons regulate self-grooming, a repetitive behavior manifested in affective disorders. Here we show that chronic restraint stress (CRS) induces robust depression-like behaviors in mice and decreases excitability of OT D3 neurons. Ablation or inhibition of these neurons leads to depression-like behaviors, whereas their activation ameliorates CRS-induced depression-like behaviors. Moreover, activation of OT D3 neurons has a rewarding effect, which diminishes when grooming is blocked. Finally, we propose a model that explains how OT D3 neurons may influence dopamine release via synaptic connections with OT spiny projection neurons (SPNs) that project to midbrain dopamine neurons. Our study reveals a crucial role of OT D3 neurons in bidirectionally mediating depression-like behaviors, suggesting a potential therapeutic target.

Dissociable control of motivation and reinforcement by distinct ventral striatal dopamine receptors

Dopamine release in striatal circuits, including the nucleus accumbens (NAc), tracks separable features of reward such as motivation and reinforcement. However, the cellular and circuit mechanisms by which dopamine receptors transform dopamine release into distinct constructs of reward remain unclear. Here, we show that dopamine D3 receptor (D3R) signaling in the NAc drives motivated behavior by regulating local NAc microcircuits. Furthermore, D3Rs co-express with dopamine D1 receptors (D1Rs), which regulate reinforcement, but not motivation. Paralleling dissociable roles in reward function, we report non-overlapping physiological actions of D3R and D1R signaling in NAc neurons. Our results establish a novel cellular framework wherein dopamine signaling within the same NAc cell type is physiologically compartmentalized via actions on distinct dopamine receptors. This structural and functional organization provides neurons in a limbic circuit with the unique ability to orchestrate dissociable aspects of reward-related behaviors that are relevant to the etiology of neuropsychiatric disorders.

Unveiling the Differences in Signaling and Regulatory Mechanisms between Dopamine D2 and D3 Receptors and Their Impact on Behavioral Sensitization

Dopamine receptors are classified into five subtypes, with D2R and D3R playing a crucial role in regulating mood, motivation, reward, and movement. Whereas D2R are distributed widely across the brain, including regions responsible for motor functions, D3R are primarily found in specific areas related to cognitive and emotional functions, such as the nucleus accumbens, limbic system, and prefrontal cortex. Despite their high sequence homology and similar signaling pathways, D2R and D3R have distinct regulatory properties involving desensitization, endocytosis, posttranslational modification, and interactions with other cellular components. In vivo, D3R is closely associated with behavioral sensitization, which leads to increased dopaminergic responses. Behavioral sensitization is believed to result from D3R desensitization, which removes the inhibitory effect of D3R on related behaviors. Whereas D2R maintains continuous signal transduction through agonist-induced receptor phosphorylation, arrestin recruitment, and endocytosis, which recycle and resensitize desensitized receptors, D3R rarely undergoes agonist-induced endocytosis and instead is desensitized after repeated agonist exposure. In addition, D3R undergoes more extensive posttranslational modifications, such as glycosylation and palmitoylation, which are needed for its desensitization. Overall, a series of biochemical settings more closely related to D3R could be linked to D3R-mediated behavioral sensitization.

Ferulic acid-loaded nanostructure prevents morphine reinstatement: the involvement of dopamine system, NRF2, and ΔFosB in the striatum brain area of rats

Morphine is among the most powerful analgesics and pain-relieving agents. However, its addictive properties limit their medical use because patients may be susceptible to abuse and reinstatement. Morphine addiction occurs because of dopamine release in the mesolimbic brain area, implying in an increase in oxidative stress. Ferulic acid (FA), a phenolic phytochemical found in a variety of foods, has been reported to exert antioxidant and neuroprotective effects; however, its low bioavailability makes its nano-encapsulated form a promising alternative. This study aimed to evaluate the protective effects of a novel nanosystem with FA on morphine reinstatement and the consequent molecular neuroadaptations and oxidative status in the mesolimbic region. Rats previously exposed to morphine in conditioned place preference (CPP) paradigm were treated with ferulic acid-loaded nanocapsules (FA-Nc) or nonencapsulated FA during morphine-preference extinction. Following the treatments, animals were re-exposed to morphine to induce the reinstatement. While morphine-preference extinction was comparable among all experimental groups, FA-Nc treatment prevented morphine reinstatement. In the dorsal striatum, while morphine exposure increased lipid peroxidation (LP) and reactive species (RS), FA-Nc decreased LP and FA decreased RS levels. Morphine exposure increased the dopaminergic markers (D1R, D3R, DAT) and ΔFosB immunoreactivity in the ventral striatum; however, FA-Nc treatment decreased D1R, D3R, and ΔFosB and increased D2R, DAT, and NRF2. In conclusion, FA-Nc treatment prevented the morphine reinstatement, promoted antioxidant activity, and modified the dopaminergic neurotransmission, NRF2, and ΔFosB, what may indicate a neuroprotective and antioxidant role of this nanoformulation.

Serotonin receptors contribute to dopamine depression of lateral inhibition in the nucleus accumbens

Dopamine modulation of nucleus accumbens (NAc) circuitry is central to theories of reward seeking and reinforcement learning. Despite decades of effort, the acute dopamine actions on the NAc microcircuitry remain puzzling. Here, we dissect out the direct actions of dopamine on lateral inhibition between medium spiny neurons (MSNs) in mouse brain slices and find that they are pathway specific. Dopamine potently depresses GABAergic transmission from presynaptic dopamine D2 receptor-expressing MSNs (D2-MSNs), whereas it potentiates transmission from presynaptic dopamine D1 receptor-expressing MSNs (D1-MSNs) onto other D1-MSNs. To our surprise, presynaptic D2 receptors mediate only half of the depression induced by endogenous and exogenous dopamine. Presynaptic serotonin 5-HT1B receptors are responsible for a significant component of dopamine-induced synaptic depression. This study clarifies the mechanistic understanding of dopamine actions in the NAc by showing pathway-specific modulation of lateral inhibition and involvement of D2 and 5-HT1B receptors in dopamine depression of D2-MSN synapses.

Dopamine D3 receptor ligand suppresses the expression of levodopa-induced dyskinesia in nonhuman primate model of parkinson's disease

Parkinson's disease (PD) is a complex multisystem, chronic and so far incurable disease with significant unmet medical needs. The incidence of PD increases with aging and the expected burden will continue to escalate with our aging population. Since its discovery in the 1961 levodopa has remained the gold standard pharmacotherapy for PD. However, the progressive nature of the neurodegenerative process in and beyond the nigrostriatal system causes a multitude of side effects, including levodopa-induced dyskinesia within 5 years of therapy. Attenuating dyskinesia has been a significant challenge in the clinical management of PD. We report on a small molecule that eliminates the expression of levodopa-induced dyskinesia and significantly improves PD-like symptoms. The lead compound PD13R we discovered is a dopamine D3 receptor partial agonist with high affinity and selectivity, orally active and with desirable drug-like properties. Future studies are aimed at developing this lead compound for treating PD patients with dyskinesia.

Recent Advances in Dopamine D3 Receptor Heterodimers: Focus on Dopamine D3 and D1 Receptor-Receptor Interaction and Striatal Function

G protein-coupled receptors (GPCR) heterodimers represent new entities with unique pharmacological, signalling, and trafficking properties, with specific distribution restricted to those cells where the two interacting receptors are co-expressed. Like other GPCR, dopamine D3 receptors (D3R) directly interact with various receptors to form heterodimers: data showing the D3R physical interaction with both GPCR and non-GPCR receptors have been provided including D3R interaction with other dopamine receptors. The aim of this chapter is to summarize current knowledge of the distinct roles of heterodimers involving D3R, focusing on the D3R interaction with the dopamine D1 receptor (D1R): the D1R-D3R heteromer, in fact, has been postulated in both ventral and motor striatum. Interestingly, since both D1R and D3R have been implicated in several pathological conditions, including schizophrenia, motor dysfunctions, and substance use disorders, the D1R-D3R heteromer may represent a potential drug target for the treatment of these diseases.

Ventral striatal islands of Calleja neurons control grooming in mice

The striatum comprises multiple subdivisions and neural circuits that differentially control motor output. The islands of Calleja (IC) contain clusters of densely packed granule cells situated in the ventral striatum, predominantly in the olfactory tubercle (OT). Characterized by expression of the D3 dopamine receptor, the IC are evolutionally conserved, but have undefined functions. Here, we show that optogenetic activation of OT D3 neurons robustly initiates self-grooming in mice while suppressing other ongoing behaviors. Conversely, optogenetic inhibition of these neurons halts ongoing grooming, and genetic ablation reduces spontaneous grooming. Furthermore, OT D3 neurons show increased activity before and during grooming and influence local striatal output via synaptic connections with neighboring OT neurons (primarily spiny projection neurons), whose firing rates display grooming-related modulation. Our study uncovers a new role of the ventral striatum's IC in regulating motor output and has important implications for the neural control of grooming.

G-protein coupled receptors and synaptic plasticity in sleep deprivation

Insufficient sleep has been correlated to many physiological and psychoneurological disorders. Over the years, our understanding of the state of sleep has transcended from an inactive period of rest to a more active state involving important cellular and molecular processes. In addition, during sleep, electrophysiological changes also occur in pathways in specific regions of the mammalian central nervous system (CNS). Activity mediated synaptic plasticity in the CNS can lead to long-term and sometimes permanent strengthening and/or weakening synaptic strength affecting neuronal network behaviour. Memory consolidation and learning that take place during sleep cycles, can be affected by changes in synaptic plasticity during sleep disturbances. G-protein coupled receptors (GPCRs), with their versatile structural and functional attributes, can regulate synaptic plasticity in CNS and hence, may be potentially affected in sleep deprived conditions. In this review, we aim to discuss important functional changes that can take place in the CNS during sleep and sleep deprivation and how changes in GPCRs can lead to potential problems with therapeutics with pharmacological interventions.

BDNF Overexpression Increases Striatal D3 Receptor Level at Striatal Neurons and Exacerbates D1-Receptor Agonist-Induced Dyskinesia

BACKGROUND

We recently showed that striatal overexpression of brain derived neurotrophic factor (BDNF) by adeno-associated viral (AAV) vector exacerbated L-DOPA-induced dyskinesia (LID) in 6-OHDA-lesioned rats. An extensive sprouting of striatal serotonergic terminals accompanied this effect, accounting for the increased susceptibility to LID.

OBJECTIVE

We set to investigate whether the BDNF effect was restricted to LID, or extended to dyskinesia induced by direct D1 receptor agonists.

METHODS

Unilaterally 6-OHDA-lesioned rats received a striatal injection of an AAV vector to induce BDNF or GFP overexpression. Eight weeks later, animals received daily treatments with a low dose of SKF82958 (0.02 mg/kg s.c.) and development of dyskinesia was evaluated. At the end of the experiment, D1 and D3 receptors expression levels and D1 receptor-dependent signaling pathways were measured in the striatum.

RESULTS

BDNF overexpression induced significant worsening of dyskinesia induced by SKF82958 compared to the GFP group and increased the expression of D3 receptor at striatal level, even in absence of pharmacological treatment; by contrast, D1 receptor levels were not affected. In BDNF-overexpressing striata, SKF82958 administration resulted in increased levels of D1-D3 receptors co-immunoprecipitation and increased phosphorylation levels of Thr34 DARPP-32 and ERK1/2.

CONCLUSION

Here we provide evidence for a functional link between BDNF, D3 receptors and D1-D3 receptor close interaction in the augmented susceptibility to dyskinesia in 6-OHDA-lesioned rats. We suggest that D1-D3 receptors interaction may be instrumental in driving the molecular alterations underlying the appearance of dyskinesia; its disruption may be a therapeutic strategy for treating dyskinesia in PD patients.

The role of dopamine pharmacotherapy and addiction-like behaviors in Parkinson's disease

Addictions involve a spectrum of behaviors that encompass features of impulsivity and compulsivity, herein referred to as impulsive-compulsive spectrum disorders (ICSDs). The etiology of ICSDs likely involves a complex interplay among neurobiological, psychological and social risk factors. Neurobiological risk factors include the status of the neuroanatomical circuits that govern ICSDs. These circuits can be altered by disease, as well as exogenous influences such as centrally-acting pharmacologics. The 'poster child' for this scenario is Parkinson's disease (PD) medically managed by pharmacological treatments. PD is a progressive neurodegenerative disease that involves a gradual loss of dopaminergic neurons largely within nigrostriatal projections. Replacement therapy includes dopamine receptor agonists that directly activate postsynaptic dopamine receptors (bypassing the requirement for functioning presynaptic terminals). Some clinically useful dopamine agonists, e.g., pramipexole and ropinirole, exhibit high affinity for the D2/D3 receptor subtypes. These agonists provide excellent relief from PD motor symptoms, but some patients exhibit debilitating ICSD. Teasing out the neuropsychiatric contribution of PD-associated pathology from the drugs used to treat PD motor symptoms is challenging. In this review, we posit that modern clinical and preclinical research converge on the conclusion that dopamine replacement therapy can mediate addictions in PD and other neurological disorders. We provide five categories of evidences that align with this position: (i) ICSD prevalence is greater with D2/D3 receptor agonist therapy vs PD alone. (ii) Capacity of dopamine replacement therapy to produce addiction-like behaviors is independent of disease for which the therapy is being provided. (iii) ICSD-like behaviors are recapitulated in laboratory rats with and without PD-like pathology. (iv) Behavioral pathology co-varies with drug exposure. (v) ICSD Features of ICSDs are consistent with agonist pharmacology and neuroanatomical substrates of addictions. Considering the underpinnings of ICSDs in PD should not only help therapeutic decision-making in neurological disorders, but also apprise ICSDs in general.

Coexistence of D3 R typical and atypical signaling in striatonigral neurons during dopaminergic denervation. Correlation with D3 nf expression changes

Dopamine D₃ R are widely expressed in basal ganglia where interact with D₁ R. D₃ R potentiate cAMP accumulation and GABA release stimulated by D₁ R in striatonigral neurons through "atypical" signaling. During dopaminergic denervation, D₃ R signaling changes to a "typical" in which antagonizes the effects of D₁ R, the mechanisms of this switching are unknown. D₃ nf splice variant regulates membrane anchorage and function of D₃ R and decreases in denervation; thus, it is possible that D₃ R signaling switching correlates with changes in D₃ nf expression and increases of membranal D₃ R that mask D₃ R atypical effects. We performed experiments in unilaterally 6-hydroxydopamine lesioned rats and found a decrease in mRNA and protein of D₃ nf, but not of D₃ R in the denervated striatum. Proximity ligation assay showed that D₃ R-D₃ nf interaction decreased after denervation, whereas binding revealed an increased B_max in D₃ R. The new D₃ R antagonized cAMP accumulation and GABA release stimulated by D₁ R; however, in the presence of N-Ethylmaleimide (NEM), to block G_i protein signaling, activation of D₃ R produced its atypical signaling stimulating D₁ R effects. Finally, we investigated if the typical and atypical effects of D₃ R modulating GABA release are capable of influencing motor behavior. Injections of D₃ R agonist into denervated nigra decreased D₁ R agonist-induced turning behavior but potentiated it in the presence of NEM. Our data indicate the coexistence of D₃ R typical and atypical signaling in striatonigral neurons during denervation that correlated with changes in the ratio of expression of D₃ nf and D₃ R isoforms. The coexistence of both atypical and typical signaling during denervation influences motor behavior.

Biased G Protein-Independent Signaling of Dopamine D1-D3 Receptor Heteromers in the Nucleus Accumbens

Several studies found in vitro evidence for heteromerization of dopamine D₁ receptors (D1R) and D₃ receptors (D3R), and it has been postulated that functional D1R-D3R heteromers that are normally present in the ventral striatum mediate synergistic locomotor-activating effects of D1R and D3R agonists in rodents. Based also on results obtained in vitro, with mammalian transfected cells, it has been hypothesized that those behavioral effects depend on a D1R-D3R heteromer-mediated G protein-independent signaling. Here, we demonstrate the presence on D1R-D3R heteromers in the mouse ventral striatum by using a synthetic peptide that selectively destabilizes D1R-D3R heteromers. Parallel locomotor activity and ex vivo experiments in reserpinized mice and in vitro experiments in D1R-D3R mammalian transfected cells were performed to dissect the signaling mechanisms of D1R-D3R heteromers. Co-administration of D1R and D3R agonists in reserpinized mice produced synergistic locomotor activation and a selective synergistic AKT phosphorylation in the most ventromedial region of the striatum in the shell of the nucleus accumbens. Application of the destabilizing peptide in transfected cells and in the shell of the nucleus accumbens allowed demonstrating that both in vitro and in vivo co-activation of D3R induces a switch from G protein-dependent to G protein-independent D1R-mediated signaling determined by D1R-D3R heteromerization. The results therefore demonstrate that a biased G protein-independent signaling of D1R-D3R heteromers localized in the shell of the nucleus accumbens mediate the locomotor synergistic effects of D1R and D3R agonists in reserpinized mice.

Pharmacological insights into impulsive-compulsive spectrum disorders associated with dopaminergic therapy

Impulsive-compulsive spectrum disorders are associated with dopamine agonist therapy in some patients. These untoward outcomes occur with direct-acting, full and partial agonists at D2 dopamine family receptors. The disorders typically emerge during chronic treatment, and exhibit common features that are independent of the neurological or psychiatric pathology for which the initial therapy was indicated. It is well-documented that the brain is 'plastic', changing in response to alterations to internal factors (e.g., disease processes), as well as external factors (e.g., therapies). The complexities of these clinical scenarios have eluded a clear depiction of the neurobiology for impulsive-compulsive spectrum disorders and engendered considerable debate regarding the mechanistic underpinnings of the disorders. In this opinion, we use pharmacological concepts related to homeostatic compensation subsequent to chronic receptor activation to provide a unifying construct. This construct helps explain the occurrence of impulsive-compulsive spectrum disorders across disease states, and during therapy with full and partial agonists.

(-)-Stepholidine reduces cue-induced reinstatement of cocaine seeking and cocaine self-administration in rats

BACKGROUND

Dopamine receptors are implicated in cocaine reward and seeking. We hypothesize that (-)-stepholidine, a dopamine D1/D2/D3 multi-receptor agent, would be effective in reducing cocaine reward and seeking in an animal model. We investigated the effects of (-)-stepholidine in cue-induced reinstatement of cocaine seeking and cocaine self-administration (reward).

METHODS

Cue-induced reinstatement experiment: Rats were trained to press a lever reinforced by cocaine (1 mg/kg/injection) for 15 consecutive daily sessions, after which the response was extinguished by withholding cocaine and cocaine-paired cues (light and pump activation). This was followed by a cue-induced reinstatement test where subjects were exposed to two cocaine cue presentations and presses on the active lever produced cues. Subjects were treated with one of four (-)-stepholidine doses prior to the reinstatement test. Cocaine self-administration (reward) experiment: Rats were trained to self-administer cocaine under a progressive ratio schedule of reinforcement. After stable breakpoints were established, rats were injected with four doses of (-)-stepholidine prior to testing; each dose was injected prior to a separate test session with no-treatment sessions intervening to re-establish break points.

RESULTS

(-)-Stepholidine significantly reduced cue-induced reinstatement of cocaine seeking in a dose-related manner. Additionally, (-)-stepholidine significantly reduced break points for cocaine reward. (-)-Stepholidine did not significantly affect locomotor activity.

CONCLUSIONS

(-)-Stepholidine reduces cue-induced reinstatement of cocaine seeking and cocaine reward, suggesting that it may be useful in treating relapse in cocaine addiction.

Behavioral and cellular dopamine D1 and D3 receptor-mediated synergy: Implications for L-DOPA-induced dyskinesia

Individually, D1 and D3 dopamine receptors (D₁R and D₃R, respectively) have been implicated in L-DOPA-induced dyskinesia (LID). Of late, direct D₁R-D₃R interactions have been linked to LID yet remain enigmatic. Therefore, the current research sought to characterize consequences of putative D₁R-D₃R interactions in dyskinesia expression and in LID-associated downstream cellular signaling. To do so, adult male Sprague-Dawley hemi-parkinsonian rats were given daily L-DOPA (6 mg/kg; s.c.) for 2 weeks to establish stable LID, as measured via the abnormal voluntary movements (AIMs) scale. Thereafter, rats underwent dose-response AIMs testing for the D₁R agonist SKF38393 (0, 0.3, 1.0, 3.0 mg/kg) and the D₃R agonist, PD128907 (0, 0.1, 0.3, 1.0 mg/kg). Each agonist dose-dependently induced dyskinesia, implicating individual receptor involvement. More importantly, when threshold doses were co-administered, rats displayed synergistic exacerbation of dyskinesia. Interestingly, this observation was not mirrored in general locomotor behaviors, highlighting a potentially dyskinesia-specific effect. To illuminate the mechanisms by which D₁R-D₃R co-stimulation led to in vivo synergy, levels of striatal phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) were quantified after administration of SKF38393 and/or PD128907. Combined agonist treatment synergistically drove striatal pERK1/2 expression. Together, these results support the presence of a functional, synergistic interaction between D₁R and D₃R that manifests both behaviorally and biochemically to drive dyskinesia in hemi-parkinsonian rats.

Dopamine D1 and D3 receptor polypharmacology as a potential treatment approach for substance use disorder

In the search for efficacious pharmacotherapies to treat cocaine addiction much attention has been given to agents targeting dopamine D1 or D3 receptors because of the involvement of these receptors in drug-related behaviors. D1-like and D3 receptor partial agonists and antagonists have been shown to reduce drug reward, reinstatement of drug seeking and conditioned place preference in rodents and non-human primates. However, translation of these encouraging results to clinical settings has been limited due to a number of factors including toxicity, poor pharmacokinetic properties and extrapyramidal and sedative side effects. This review highlights the role of D1 and D3 receptors in drug reward and seeking, the discovery of D1-D3 heteromers and their potential as targets in the treatment of addiction.

Cariprazine for the Treatment of Bipolar Disorder

PURPOSE

To review the data regarding a new antipsychotic, cariprazine.

CONCLUSIONS

Cariprazine is a dopamine D3, D2 partial agonist, with greater affinity to D3. It has been examined for schizophrenia, bipolar mania, bipolar depression, and unipolar depression. It has demonstrated efficacy in schizophrenia and mania, and has recently been approved by the U.S. Food and Drug Administration. However, it has a more inconsistent effect in depression, both unipolar and bipolar. Adverse effects include extrapyramidal symptoms, akathisia, and gastrointestinal distress.

PRACTICE IMPLICATIONS

Cariprazine will be a promising addition in the treatment of patients with acute mania and schizophrenia.

Baseline Perfusion Alterations Due to Acute Application of Quetiapine and Pramipexole in Healthy Adults

BACKGROUND

The dopaminergic system is implicated in many mental processes and neuropsychiatric disorders. Pharmacologically, drugs with dopamine receptor antagonistic and agonistic effects are used, but their effects on functional brain metabolism are not well known.

METHODS

In this randomized crossover, placebo-controlled, and rater-blinded study, 25 healthy adults received an acute dose placebo substance (starch), quetiapine (dopamine receptor antagonist), or pramipexole (dopamine agonist of the nonergoline class) 1 hour before the experiment. Background-suppressed 2D pseudo-continuous arterial spin labeling was used to examine whole-brain baseline cerebral blood flow differences induced by the 3 substances.

RESULTS

We found that quetiapine reduced perfusion in the occipital (early visual areas) and bilateral cerebellar cortex relative to placebo. In contrast, quetiapine enhanced cerebral blood flow (relative to placebo) in the striatal system (putamen and caudate nucleus) but also in the supplementary motor area, insular-, prefrontal- as well as in the pre- and postcentral cortex. Pramipexole increased cerebral blood flow compared with placebo in the caudate nucleus, putamen, middle frontal, supplementary motor area, and brainstem (substantia nigra), but reduced cerebral blood flow in the posterior thalamus, cerebellum, and visual areas. Pramipexole administration resulted in stronger cerebral blood flow relative to quetiapine in the hypothalamus, cerebellum, and substantia nigra.

CONCLUSIONS

Our results indicate that quetiapine and pramipexole differentially modulate regional baseline cerebral blood flow. Both substances act on the dopaminergic system, although they affect distinct regions. Quetiapine altered dopaminergic function in frontal, striatal, and motor regions. In contrast, pramipexole affected cerebral blood flow of the nigrostriatal (striatum and substantia nigra) dopaminergic, but less the fronto-insular system.

Update on schizophrenia and bipolar disorder: focus on cariprazine

Schizophrenia and bipolar disorder are severe psychiatric disorders that are frequently associated with persistent symptoms and significant dysfunction. While there are a multitude of psychopharmacologic agents are available for treatment of these illnesses, suboptimal response and significant adverse consequences limit their utility. Cariprazine is a new, novel antipsychotic medication with dopamine D2 and D3 partial agonist effects. Its safety and efficacy have been investigated in acute psychosis of schizophrenia, bipolar mania, bipolar depression, and unipolar depression. Efficacy has been demonstrated in schizophrenia and mania. It is unclear if cariprazine is effective in depression associated with unipolar or bipolar illness. Adverse consequences include extrapyramidal symptoms including akathisia, and various gastrointestinal symptoms. The US Food and Drug Administration (FDA) has recently approved cariprazine. This review will provide clinicians with basic information regarding the research program of cariprazine.

Targeting the dopamine D3 receptor: an overview of drug design strategies

INTRODUCTION

Dopamine is a neurotransmitter widely distributed in both the periphery and the central nervous system (CNS). Its physiological effects are mediated by five closely related G protein-coupled receptors (GPCRs) that are divided into two major subclasses: the D1-like (D1, D5) and the D2-like (D2, D3, D4) receptors. D3 receptors (D3Rs) have the highest density in the limbic areas of the brain, which are associated with cognitive and emotional functions. These receptors are therefore attractive targets for therapeutic management.

AREAS COVERED

This review summarizes the functional and pharmacological characteristics of D3Rs, including the design and clinical relevance of full agonists, partial agonists and antagonists, as well as the capacity of these receptors to form active homodimers, heterodimers or higher order receptor complexes as pharmacological targets in several neurological and neurodegenerative disorders.

EXPERT OPINION

The high sequence homology between D3R and the D2-type challenges the development of D3R-selective compounds. The design of new D3R-preferential ligands with improved physicochemical properties should provide a better pharmacokinetic/bioavailability profile and lesser toxicity than is found with existing D3R ligands. It is also essential to optimize D3R affinity and, especially, D3R vs. D2-type binding and functional selectivity ratios. Developing allosteric and bitopic ligands should help to improve the D3R selectivity of these drugs. As most evidence points to the ability of GPCRs to form homomers and heteromers, the most promising therapeutic strategy in the future is likely to involve the application of heteromer-selective drugs. These selective ligands would display different affinities for a given receptor depending on the receptor partners within the heteromer. Therefore, designing novel compounds that specifically target and modulate D1R-D3R heteromers would be an interesting approach for the treatment of levodopa (L-DOPA)-induced dyskinesias.

Dopamine D3 receptor agonists as pharmacological tools

Dysregulation of the dopaminergic innervation in the central nervous system plays a key role in different neurological disorders like Parkinson´s disease, restless legs syndrome, schizophrenia etc. Although dopamine D3 receptors have been recognized as an important target in these diseases, their full pharmacological properties need further investigations. With focus on dopamine D3 receptor full agonists, this review has divided the ergoline and non-ergoline ligands in dissimilar chemical subclasses describing their pharmacodynamic properties on different related receptors, on species differences and their functional properties on different signaling mechanism. This is combined with a short description of structure-activity relationships for each class. Therefore, this overview should support the rational choice for the optimal compound selection based on affinity, selectivity and efficacy data in biochemical and pharmacological studies.

Dopaminergic denervation switches dopamine D3 receptor signaling and disrupts its Ca(2+) dependent modulation by CaMKII and calmodulin in striatonigral projections of the rat

In striatonigral projections activation of dopamine D3 receptors (D3Rs) potentiates the stimulation of GABA release and cAMP production caused by activation of dopamine D1 receptors (D1Rs). Cytoplasmic [Ca(2+)] in the terminals controls this response by modulating CaMKII, an enzyme that depresses D3R action. To examine the effects of dopamine deprivation on D3R signaling we investigated their function in striatonigral terminals of hemiparkinsonian rats. Denervation switched the signaling cascade initiated by D3R activation. In the non-lesioned side activation of D3R potentiated the stimulatory effects of D1R activation on cAMP production and K(+)-depolarization induced [(3)H] GABA release. In contrast, in the denervated side the stimulatory effects of both D1R activation and forskolin administration were blocked by D3R activation. In non-lesioned slices, D3R responses were inhibited by the activation of CaMKII produced by K(+)-depolarization (via increased Ca(2+) entry). The CaMKII-induced inhibition was blocked by the selective inhibitor KN-62. In denervated tissues the response to D3R stimulation was not modified either by K(+) depolarization or by blocking CaMKII with KN-62. Immunoblotting studies showed that depolarization-induced CaMKII binding to the D3 receptor and CaMKII phosphorylation were suppressed in denervated tissues. We also determined calmodulin expression with PCR and immunoblot techniques. Both techniques showed that calmodulin expression was depressed in the lesioned side. In sum, our studies show that dopaminergic denervation switches the D3R signaling cascade and depresses CaMKII signaling through a process that appears to involve reduced calmodulin levels. Since calmodulin is a major cytoplasmic Ca(2+) buffer our findings suggest that abnormal Ca(2+) buffering may be an important component of the abnormalities observed during dopaminergic denervation.

Dopamine heteroreceptor complexes as therapeutic targets in Parkinson's disease

INTRODUCTION

Several types of D2R and D1R heteroreceptor complexes were discovered in the indirect and direct pathways of the striatum, respectively. The hypothesis is given that changes in the function of the dopamine heteroreceptor complexes may help us understand the molecular mechanisms underlying the motor complications of long-term therapy in Parkinson's disease (PD) with l-DOPA and dopamine receptor agonists.

AREAS COVERED

In the indirect pathway, the potential role of the A2AR-D2R, A2AR-D2R-mGluR5 and D2R-NMDAR heteroreceptor complexes in PD are covered and in the direct pathway, the D1R-D3R, A1R-D1R, D1R-NMDAR and putative A1R-D1R-D3R heteroreceptor complexes.

EXPERT OPINION

One explanation for the more powerful ability of l-DOPA treatment versus treatment with the partial dopamine receptor agonist/antagonist activity to induce dyskinesias, may be that dopamine formed from l-DOPA acts as a full agonist. The field of D1R and D2R heteroreceptor complexes in the CNS opens up a new understanding of the wearing off of the antiparkinson actions of l-DOPA and dopamine receptor agonists and the production of l-DOPA-induced dyskinesias. It can involve a reorganization of the D1R and D2R heteroreceptor complexes and a disbalance of the D1R and D2R homomers versus non-dopamine receptor homomers in the direct and indirect pathways.

Canadian Association of Neurosciences Review: The Role of Dopamine Receptor Function in Neurodegenerative Diseases

Dopamine (DA) receptors, which are heavily expressed in the caudate/putamen of the brain, represent the molecular target of several drugs used in the treatment of various neurological disorders, such as Parkinson's disease. Although most of the drugs are very effective in alleviating the symptoms associated with these conditions, their long-term utilization could lead to the development of severe side-effects. In addition to uncovering novel mediators of physiological DA receptor functions, recent research advances are suggesting a role of these receptors in toxic effects on neurons. For instance, accumulating evidence indicates that DA receptors, particularly D1 receptors, are central in the neuronal toxicity induced by elevated synaptic levels of DA. In this review, we will discuss recent findings on DA receptors as regulators of long term neuronal dysfunction and neurodegenerative processes.

Stronger Dopamine D1 Receptor-Mediated Neurotransmission in Dyskinesia

Radioligand binding assays to rat striatal dopamine D1 receptors showed that brain lateralization of the dopaminergic system were not due to changes in expression but in agonist affinity. D1 receptor-mediated striatal imbalance resulted from a significantly higher agonist affinity in the left striatum. D1 receptors heteromerize with dopamine D3 receptors, which are considered therapeutic targets for dyskinesia in parkinsonian patients. Expression of both D3 and D1-D3 receptor heteromers were increased in samples from 6-hydroxy-dopamine-hemilesioned rats rendered dyskinetic by treatment with 3, 4-dihydroxyphenyl-L-alanine (L-DOPA). Similar findings were obtained using striatal samples from primates. Radioligand binding studies in the presence of a D3 agonist led in dyskinetic, but not in lesioned or L-DOPA-treated rats, to a higher dopamine sensitivity. Upon D3-receptor activation, the affinity of agonists for binding to the right striatal D1 receptor increased. Excess dopamine coming from L-DOPA medication likely activates D3 receptors thus making right and left striatal D1 receptors equally responsive to dopamine. These results show that dyskinesia occurs concurrently with a right/left striatal balance in D1 receptor-mediated neurotransmission.

Intranasal delivery of a peptide with antidepressant-like effect

A critical issue in drug development is developing effective, noninvasive delivery routes to the central nervous system (CNS). Major depressive disorder (MDD) is an illness associated with significant morbidity. Even with multiple antidepressant trials, 10-15% of patients continue to experience persistent depressive symptoms. We previously developed an interfering peptide that has antidepressant-like effects in rats when injected directly into the brain. To be clinically viable, it must demonstrate efficacy via a noninvasive administration route. We report here that the interfering peptide designed to disrupt the interaction between the D1 and D2 dopamine receptors can be delivered to relevant brain areas using the Pressurized Olfactory Device (POD), a novel intranasal delivery system developed by Impel NeuroPharma. We validate this delivery method by demonstrating that, at doses ⩾1.67 nmol/g, the D1-D2 interfering peptide has a significant antidepressant-like effect comparable to that of imipramine in the forced swimming test (FST), a common test for antidepressant efficacy. The antidepressant-like effect of the interfering peptide can be detected for 2 h after intranasal administration. Furthermore, we show that the interfering peptide disrupts the D1-D2 interaction and it can be detected in the prefrontal cortex after intranasal administration. This study provides strong preclinical support for intranasal administration of the D1-D2 interfering peptide as a new treatment option for patients suffering from MDD.

Inhibitory modulation of CART peptides in accumbal neuron through decreasing interaction of CaMKIIα with dopamine D3 receptors

Previous studies in rats have shown that microinjections of cocaine- and amphetamine-regulated transcript (CART) peptide into the nucleus accumbens (NAc; the area of the brain that mediates drug reward and reinforcement) attenuate the locomotor effects of psychostimulants. CART peptide has also been shown to induce decreased intracellular concentrations of calcium (Ca(2+)) in primary cultures of hippocampus neurons. The purpose of this study was to characterize the interaction of Ca(2+)/calmodulin-dependent kinases (CaMKIIα) with dopamine D3 (D3) receptors (R) in primary cultures of accumbal neurons. This interaction is involved in inhibitory modulation of CART peptides. In vitro, CART (55-102) peptide (0.1, 0.5 or 1μM) was found to dose-dependently inhibit K(+) depolarization-elicited Ca(2+) influx and CaMKIIα phosphorylation in accumbal neurons. Moreover, CART peptides were also found to block cocaine (1μM)-induced Ca(2+) influx, CaMKIIα phosphorylation, CaMKIIα-D3R interaction, and CREB phosphorylation. In vivo, repeated microinjections of CART (55-102) peptide (2μg/1μl/side) into the NAc over a 5-day period had no effect on behavioral activity but blocked cocaine-induced locomotor activity. These results indicate that D3R function in accumbal neurons is a target of CART (55-102) peptide and suggest that CART peptide by dephosphorylating limbic D3Rs may have potential as a treatment for cocaine abuse.

Heteromeric dopamine receptor signaling complexes: emerging neurobiology and disease relevance

The pharmacological modification of dopamine transmission has long been employed as a therapeutic tool in the treatment of many mental health disorders. However, as many of the pharmacotherapies today are not without significant side effects, or they alleviate only a particular subset of symptoms, the identification of novel therapeutic targets is imperative. In light of these challenges, the recognition that dopamine receptors can form heteromers has significantly expanded the range of physiologically relevant signaling complexes as well as potential drug targets. Furthermore, as the physiology and disease relevance of these receptor heteromers is further understood, their ability to exhibit pharmacological and functional properties distinct from their constituent receptors, or modulate the function of endogenous homomeric receptor complexes, may allow for the development of alternate therapeutic strategies and provide new avenues for drug design. In this review, we describe the emerging neurobiology of the known dopamine receptor heteromers, their physiological relevance in brain, and discuss the potential role of these receptor complexes in neuropsychiatric disease. We highlight their value as targets for future drug development and discuss innovative research strategies designed to selectively target these dopamine receptor heteromers in the search for novel and clinically efficacious pharmacotherapies.

Disease-specific heteromerization of G-protein-coupled receptors that target drugs of abuse

Drugs of abuse such as morphine or marijuana exert their effects through the activation of G-protein-coupled receptors (GPCRs), the opioid and cannabinoid receptors, respectively. Moreover, interactions between either of these receptors have been shown to be involved in the rewarding effects of drugs of abuse. Recent advances in the field, using a variety of approaches, have demonstrated that many GPCRs, including opioid, cannabinoid, and dopamine receptors, can form associations between different receptor subtypes or with other GPCRs to form heteromeric complexes. The formation of these complexes, in turn, leads to the modulation of the properties of individual protomers. The development of tools that can selectively disrupt GPCR heteromers as well as monoclonal antibodies that can selectively block signaling by specific heteromer pairs has indicated that heteromers involving opioid, cannabinoid, or dopamine receptors may play a role in various disease states. In this review, we describe evidence for opioid, cannabinoid, and dopamine receptor heteromerization and the potential role of GPCR heteromers in pathophysiological conditions.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Fos expression in response to dopamine D3-preferring phenylpiperazine drugs given with and without cocaine

WC 44 and WC 10 are phenylpiperazines with low (23 fold) to moderate (42 fold) selectivity for dopamine D3 receptors (D3Rs) over D2Rs, respectively. WC 44 is a full D3R agonist in the forskolin-stimulated adenylyl cyclase (AC) assay, whereas WC 10 has little efficacy. In contrast to their opposite effects in the AC assay, these drugs often produce similar behavioral effects, suggesting that the AC assay does not predict the efficacy of these drugs in vivo. Here, we examined whether Fos protein expression induced by these drugs would be more consistent with their behavioral effects in vivo. Rats received either vehicle, WC 10 (5.6 mg/kg, i.p.), WC 44 (10.0 mg/kg, i.p), cocaine (10.0 mg/kg, i.p.), or cocaine with WC 10 (5.6 mg/kg, i.p.) or with WC 44 (10.0 mg/kg, i.p). Locomotion was monitored for 90 min and the brains were harvested for immunohistochemistry. Both WC 10 and WC 44 decreased spontaneous and cocaine-induced locomotion. Both compounds also increased Fos expression relative to saline in the dorsal striatum and nucleus accumbens core and shell, and relative to cocaine alone in the nucleus accumbens shell. The findings suggest that even though these compounds have different efficacy in the AC bioassy, they produce similar brain activation and attenuation of cocaine hyperlocomotion. Together with our previous research demonstrating that these compounds down-shift the cocaine self-administration dose-effect function, the findings support the idea that D3R-selective compounds may be useful for cocaine dependence medications development.

Cholinergic interneurons suppress action potential initiation of medium spiny neurons in rat nucleus accumbens shell

Acetylcholine plays a crucial role in the regulation of neural functions, including dopamine release, synaptic activity, and intrinsic electrophysiological properties of the nucleus accumbens (NAc) shell. Although the effects of acetylcholine on the action potential properties of NAc medium spiny (MS) neurons have been reported, how intrinsic acetylcholine released from NAc cholinergic interneurons regulates the neural activity of MS neurons is still an open issue. To explore the cholinergic effects on the subthreshold responses and action potential properties of MS neurons in the NAc shell, we first tested the effects of carbachol, a non-selective cholinergic agonist, on MS neuronal activity. Then, we tested the effects of the activation of cholinergic interneurons on the electrophysiological properties of MS neurons via multiple whole-cell patch-clamp recordings. Bath application of carbachol induced resting membrane potential depolarization accompanied by an increase in the voltage response to negative current injection. These increases were blocked by the pre-application of pirenzepine, an M1 muscarinic receptor antagonist. In spite of the facilitative effect on voltage responses of negative current injection, carbachol diminished the characteristic slowly-depolarizing ramp potentials, which respond to positive current pulse injection. Thus, carbachol increased the rheobase and shifted the frequency-current curve toward the right. Repetitive spike firing of a cholinergic interneuron following positive current injection induced a similar increase in the rheobase, which delayed the action potential initiation in 38.9% MS neurons. In contrast to the bath application of carbachol, cholinergic interneuronal stimulation had little effect on the resting membrane potential in MS neurons. These results suggest that the acetylcholine released from a cholinergic interneuron is sufficient to suppress the repetitive spike firing of the adjacent MS neurons, although the depolarization of the resting membrane potential may require simultaneous activation of multiple cholinergic interneurons.

The functional DRD3 Ser9Gly polymorphism (rs6280) is pleiotropic, affecting reward as well as movement

Abnormalities of motivation and behavior in the context of reward are a fundamental component of addiction and mood disorders. Here we test the effect of a functional missense mutation in the dopamine 3 receptor (DRD3) gene (ser9gly, rs6280) on reward-associated dopamine (DA) release in the striatum. Twenty-six healthy controls (HCs) and 10 unmedicated subjects with major depressive disorder (MDD) completed two positron emission tomography (PET) scans with [¹¹C]raclopride using the bolus plus constant infusion method. On one occasion subjects completed a sensorimotor task (control condition) and on another occasion subjects completed a gambling task (reward condition). A linear regression analysis controlling for age, sex, diagnosis, and self-reported anhedonia indicated that during receipt of unpredictable monetary reward the glycine allele was associated with a greater reduction in D2/3 receptor binding (i.e., increased reward-related DA release) in the middle (anterior) caudate (p<0.01) and the ventral striatum (p<0.05). The possible functional effect of the ser9gly polymorphism on DA release is consistent with previous work demonstrating that the glycine allele yields D3 autoreceptors that have a higher affinity for DA and display more robust intracellular signaling. Preclinical evidence indicates that chronic stress and aversive stimulation induce activation of the DA system, raising the possibility that the glycine allele, by virtue of its facilitatory effect on striatal DA release, increases susceptibility to hyperdopaminergic responses that have previously been associated with stress, addiction, and psychosis.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

Dopamine D1, D2, D3 receptors, vesicular monoamine transporter type-2 (VMAT2) and dopamine transporter (DAT) densities in aged human brain

The dopamine D(1), D(2), D(3) receptors, vesicular monoamine transporter type-2 (VMAT2), and dopamine transporter (DAT) densities were measured in 11 aged human brains (aged 77-107.8, mean: 91 years) by quantitative autoradiography. The density of D(1) receptors, VMAT2, and DAT was measured using [(3)H]SCH23390, [(3)H]dihydrotetrabenazine, and [(3)H]WIN35428, respectively. The density of D(2) and D(3) receptors was calculated using the D(3)-preferring radioligand, [(3)H]WC-10 and the D(2)-preferring radioligand [(3)H]raclopride using a mathematical model developed previously by our group. Dopamine D(1), D(2), and D(3) receptors are extensively distributed throughout striatum; the highest density of D(3) receptors occurred in the nucleus accumbens (NAc). The density of the DAT is 10-20-fold lower than that of VMAT2 in striatal regions. Dopamine D(3) receptor density exceeded D(2) receptor densities in extrastriatal regions, and thalamus contained a high level of D(3) receptors with negligible D(2) receptors. The density of dopamine D(1) linearly correlated with D(3) receptor density in the thalamus. The density of the DAT was negligible in the extrastriatal regions whereas the VMAT2 was expressed in moderate density. D(3) receptor and VMAT2 densities were in similar level between the aged human and aged rhesus brain samples, whereas aged human brain samples had lower range of densities of D(1) and D(2) receptors and DAT compared with the aged rhesus monkey brain. The differential density of D(3) and D(2) receptors in human brain will be useful in the interpretation of PET imaging studies in human subjects with existing radiotracers, and assist in the validation of newer PET radiotracers having a higher selectivity for dopamine D(2) or D(3) receptors.

Cocaine-induced dendritic remodeling occurs in both D1 and D2 dopamine receptor-expressing neurons in the nucleus accumbens

Repeated exposure to cocaine can induce persistent alterations in the brain's reward system, including increases in the number of dendrites and spine density on medium-sized spiny neurons (MSNs) in the nucleus accumbens (NAc). The structural remodeling of dendrites and spines in the NAc is thought to play a critical role in cocaine addiction. MSNs in the NAc can be classified by expression of either D1 or D2 dopamine receptors, which are localized to the direct and indirect pathway, respectively. It is unknown whether the dendritic changes induced by repeated cocaine treatment occur in MSNs of the direct or indirect pathway. Because the traditional Golgi-Cox impregnation of neurons precludes identifying particular subpopulations of MSNs, we performed dendritic morphology analysis after biocytin-labeling and Golgi-Cox impregnation. We found that the biocytin staining MSNs showed higher dendritic spine density and higher number of dendrites than that in Golgi impregnation group. In addition, we found that the increasing spine density induced by repeated cocaine treatment in female mice was higher than that in male mice. Next we used biocytin staining and dynorphin/D2 receptor colocalization to determine which cell type(s) displayed dendritic changes after repeated cocaine treatment. We found that cocaine-induced changes in dendritic parameters occurred in MSNs of both the direct (D1-expressing) and indirect (D2-expressing) pathways.

Sensitization to cocaine is inhibited after intra-accumbal GR103691 or rimonabant, but it is enhanced after co-infusion indicating functional interaction between accumbens D(3) and CB1 receptors

RATIONALE

Dopamine D(3) receptors and cannabinoid CB(1) receptors are both expressed in the nucleus accumbens, and they have been involved in motor sensitization to cocaine. The objectives were: (1) to study the effects of blockade of these receptors on sensitization to repeated cocaine, by using GR103691, D(3) receptor blocker, and rimonabant, CB(1) receptor ligand, and (2) to discern if both receptors interact by co-infusing them.

MATERIALS AND METHODS

Cocaine (10 mg/kg) was injected daily for 3 days (induction phase) and later on day 8 (expression phase), and locomotor activity was measured during 2 h after cocaine. GR103691 and rimonabant were bilaterally injected (0.5 μl volume of each infusion) in the nucleus accumbens through cannulae (GR103691, 0, 4.85, and 9.7 μg/μl; rimonabant, 0, 0.5, and 1.5 μg/μl), before cocaine, during either induction or expression phases of sensitization.

RESULTS

The findings indicated that sensitizing effects of cocaine were abolished after D(3) receptor blocking during both induction and expression phases, as well as rimonabant infusion during the expression (not induction) phase. A functional interaction between both receptors was also observed, because if GR103691 was injected during induction and rimonabant during expression, sensitizing effects of cocaine were observed to be normal or further enhanced.

CONCLUSION

Dopamine D(3) receptors within the nucleus accumbens are critical for the development and consolidation of sensitization, and cannabinoid CB(1) receptors are critical for the expression of sensitization. Co-blockade of D(3) and CB(1) receptors exert opposite effects to blockade of these receptors separately, revealing the existence of a functional interaction between them.

Upregulation of D2-class signaling in dopamine-denervated striatum is in part mediated by D3 receptors acting on Ca V 2.1 channels via PIP2 depletion

The loss of dopaminergic neurons in the substantia nigra compacta followed by striatal dopamine depletion is a hallmark of Parkinson's disease. After dopamine depletion, dopaminergic D(2) receptor (D(2)R)-class supersensitivity develops in striatal neurons. The supersensitivity results in an enhanced modulation of Ca(2+) currents by D(2)R-class receptors. However, the relative contribution of D(2)R, D(3)R, and D(4)R types to the supersensitivity, as well as the mechanisms involved, have not been elucidated. In this study, whole cell voltage-clamp recordings were performed to study Ca(2+) current modulation in acutely dissociated striatal neurons obtained from rodents with unilateral 6-hydroxydopamine lesions in the substantia nigra compacta. Selective antagonists for D(2)R, D(3)R, and D(4)R types were used to identify whether the modulation by one of these receptors experiences a selective change after dopaminergic denervation. It was found that D(3)R-mediated modulation was particularly enhanced. Increased modulation targeted Ca(V)2.1 (P/Q) Ca(2+) channels via the depletion of phosphatidylinositol 4,5-bisphosphate, an intracellular signaling cascade hard to detect in control neurons and hypothesized as being amplified by dopamine depletion. An imbalance in the striatal expression of D(3)R and its splice variant, D(3)nf, accompanied enhanced D(3)R activity. Because Ca(V)2.1 Ca(2+) channels mediate synaptic GABA release from the terminals of striatal neurons, reinforcement of their inhibition by D(3)R may explain in part the profound decrease in synaptic strength in the connections among striatal projection neurons observed in the dopamine-depleted striatum.

The neurobiology of dopamine receptors: evolution from the dual concept to heterodimer complexes

CONTEXT

G protein-coupled receptors (GPCRs) have been classically thought to work as monomeric entities. The current view of their organization, however, assumes that they are part of highly organized molecular complexes, where different receptors and interacting proteins are clustered. These heteromers have peculiar pharmacological, signaling, and trafficking properties. GPCR heteromerization, raising different combinatorial possibilities, thus underlies an unexpected level of diversity within this receptor family.

METHODS

In this paper, we summarize recent data, reported by different research groups, suggesting that the dopamine (DA) D1 receptor forms heteromers with receptors of the same family and with structurally and functionally divergent receptors.

RESULTS AND DISCUSSION

DA D1 and D3 receptors and glutamate NMDA receptors regulate rewarding mechanisms and motivated behavior, modulate emotional and cognitive processes and regulate locomotor activity by extensive cross-talk mechanisms. Co-localization of D1 and D3 receptors and D1 and NMDA receptors in specific neuronal populations in the striatum and nucleus accumbens, moreover, suggested that their cross-talk may involve direct interactions. By using different experimental approaches various groups have, in fact, demonstrated the existence of D1-NMDA and D1-D3 heteromers, in both transfected cell systems and in the straitum, with peculiar pharmacological, signaling, and functional properties. The putative role of the D1-D3 and D1-NMDA heteromers in the physiological regulation of striatal function and in the development of motor dysfunctions will be discussed.