Selective D3 Receptor Antagonist SB-277011-A Potentiates the Effect of Cocaine on Extracellular Dopamine in the Nucleus Accumbens: a Dual Core-Shell Voltammetry Study in Anesthetized Rats

The nucleus accumbens in reward and aversion processing: insights and implications

The nucleus accumbens (NAc), a central component of the brain's reward circuitry, has been implicated in a wide range of behaviors and emotional states. Emerging evidence, primarily drawing from recent rodent studies, suggests that the function of the NAc in reward and aversion processing is multifaceted. Prolonged stress or drug use induces maladaptive neuronal function in the NAc circuitry, which results in pathological conditions. This review aims to provide comprehensive and up-to-date insights on the role of the NAc in motivated behavior regulation and highlights areas that demand further in-depth analysis. It synthesizes the latest findings on how distinct NAc neuronal populations and pathways contribute to the processing of opposite valences. The review examines how a range of neuromodulators, especially monoamines, influence the NAc's control over various motivational states. Furthermore, it delves into the complex underlying mechanisms of psychiatric disorders such as addiction and depression and evaluates prospective interventions to restore NAc functionality.

Nucleus accumbens core dopamine D2 receptors are required for performance of the odor span task in male rats

RATIONALE

The nucleus accumbens (NAc) core gates motivationally relevant behavioral action sequences through afferents from cortical and subcortical brain regions. While the role of the NAc core in reward and effort-based decision making is well established, its role in working memory (WM) processes is incompletely understood. The odor span task (OST) has been proposed as a measure of non-spatial working memory capacity (WMC) as it requires rodents to select a novel odor from an increasing number of familiar odors to obtain a food reward.

OBJECTIVE

To assess the role of the NAc core in the OST using (1) reversible chemical inactivation and (2) selective blockade of dopamine D1 and D2 receptors in the area.

METHODS

Well-trained male rats were tested on the OST following intra-NAc core infusions of muscimol/baclofen, the D1 receptor antagonist SCH-23390 (1 μg/hemisphere) and the D2 receptor antagonist eticlopride (1 μg/hemisphere). Behavioral measurements included the average odor span, maximum odor span, choice latency, searching vigor, and patterns of responding during foraging that may relate to impulsivity.

RESULTS

Chemical inactivation of the NAc core significantly decreased odor span relative to sham and vehicle conditions. Selective antagonism of D2, but not D1, receptors in the NAc core also produced deficits in odor span. We found that secondary behavioral measures of choice latency, searching vigor, and responding to the first odor stimulus encountered were largely unaffected by treatment.

CONCLUSIONS

These findings suggest that D2 receptors in the NAc core are required for OST performance.

Characterization of D3 Autoreceptor Function in Whole Zebrafish Brain with Fast-Scan Cyclic Voltammetry

Zebrafish (Danio rerio) are ideal model organisms for investigating nervous system function, both in health and disease. Nevertheless, functional characteristics of dopamine (DA) release and uptake regulation are still not well-understood in zebrafish. In this study, we assessed D3 autoreceptor function in the telencephalon of whole zebrafish brains ex vivo by measuring the electrically stimulated DA release ([DA]max) and uptake at carbon fiber microelectrodes with fast-scan cyclic voltammetry. Treatment with pramipexole and 7-OH-DPAT, selective D3 autoreceptor agonists, sharply decreased [DA]max. Conversely, SB277011A, a selective D3 antagonist, nearly doubled [DA]max and decreased k, the first-order rate constant for the DA uptake, to about 20% of its original value. Treatment with desipramine, a selective norepinephrine transporter blocker, failed to increase current, suggesting that our electrochemical signal arises solely from the release of DA. Furthermore, blockage of DA uptake with nomifensine-reversed 7-OH-DPAT induced decreases in [DA]max. Collectively, our data show that, as in mammals, D3 autoreceptors regulate DA release, likely by inhibiting uptake. The results of this study are useful in the further development of zebrafish as a model organism for DA-related neurological disorders such as Parkinson's disease, schizophrenia, and drug addiction.

Design, Synthesis and Pharmacological Evaluation of Novel Conformationally Restricted N-arylpiperazine Derivatives Characterized as D2/D3 Receptor Ligands, Candidates for the Treatment of Neurodegenerative Diseases

Most neurodegenerative diseases are multifactorial, and the discovery of several molecular mechanisms related to their pathogenesis is constantly advancing. Dopamine and dopaminergic receptor subtypes are involved in the pathophysiology of several neurological disorders, such as schizophrenia, depression and drug addiction. For this reason, the dopaminergic system and dopamine receptor ligands play a key role in the treatment of such disorders. In this context, a novel series of conformationally restricted N-arylpiperazine derivatives (5a–f) with a good affinity for D₂/D₃ dopamine receptors is reported herein. Compounds were designed as interphenylene analogs of the drugs aripiprazole (2) and cariprazine (3), presenting a 1,3-benzodioxolyl subunit as a ligand of the secondary binding site of these receptors. The six new N-arylpiperazine compounds were synthesized in good yields by using classical methodologies, and binding and guanosine triphosphate (GTP)-shift studies were performed. Affinity values below 1 μM for both target receptors and distinct profiles of intrinsic efficacy were found. Docking studies revealed that Compounds 5a–f present a different binding mode with dopamine D₂ and D₃ receptors, mainly as a consequence of the conformational restriction imposed on the flexible spacer groups of 2 and 3.

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

The dopamine D3 receptor (D3R) has emerged as a promising pharmacotherapeutic target for the treatment of several diseases including schizophrenia, Parkinson's disease, and substance use disorders. However, studies investigating the D3R's precise role in dopamine neurotransmission or how it may be exploited to modulate responses to drugs of abuse have produced contrasting results, in part because most D3R-targeted compounds often also interact with D2 receptors (D2R). To resolve this issue, we set out to systematically characterize and compare the consequences of selective D2R or D3R antagonists on the behavioral-stimulant properties of cocaine in mice, and to identify putative neurobiological mechanisms underlying their behavior-modifying effects. Pretreatment with the selective D2R antagonist L-741,626 attenuated, while pretreatment with the selective D3R antagonist PG01037 enhanced, the locomotor-activating effects of both acute cocaine administration as well as sensitization following repeated cocaine dosing. While both antagonists potentiated cocaine-induced increases in presynaptic dopamine release, we report for the first time that D3R blockade uniquely facilitated dopamine-mediated excitation of D1-expressing medium spiny neurons in the nucleus accumbens. Collectively, our results demonstrate that selective D3R antagonism potentiates the behavioral-stimulant effects of cocaine in mice, an effect that is in direct opposition to that produced by selective D2R antagonism or nonselective D2-like receptor antagonists, and is likely mediated by facilitating D1-mediated excitation in the nucleus accumbens. These findings provide novel insights into the neuropharmacological actions of D3R antagonists on mesolimbic dopamine neurotransmission and their potential utility as pharmacotherapeutics.

A cholecystokinin B receptor antagonist and cocaine interaction, phase I study

AIMS

RPR 102681, a cholecystokinin-B antagonist, increased dopamine (DA) release and reduced cocaine self-administration in animals. This pilot study sought to assess the safety and pharmacokinetics (PK) of co-administration of RPR 102681 and cocaine, and to confirm the DA release mechanism of RPR 102681.

METHODS

Sixteen cocaine-dependent participants were randomized to either placebo or RPR102681 at 3 ascending doses; cocaine was co-administered at steady state of RPR 102681. [¹¹ C]raclopride positron emission tomography scans were conducted at baseline and at each RPR102681 dose.

RESULTS

RPR 102681 was well tolerated, and safe to co-administer with cocaine. RPR 102681 did not alter the PK of either cocaine or its metabolite benzoylecgonine and showed no intrinsic abuse liability. There was a trend toward reduction of cocaine craving scores. In contrast to animal studies, RPR 102681 significantly increased the binding potential of [¹¹ C]raclopride in the ventral striatum (t test, P < .001) and caudate nucleus (t test, P < .0001) in a small subset of patients, suggesting that it may reduce intrasynaptic striatal DA.

CONCLUSION

Overall, this pilot study suggests that RPR 102681 would be unlikely candidate, as an agonist medication for the treatment for cocaine addiction but worth investigating further for possible role in reducing craving.

Functional and pharmacological MRI in understanding brain function at a systems level

Functional magnetic resonance imaging (fMRI) methods have been extensively applied to study the human brain and its functional organization in healthy and disease states. A strong rationale exists for the extension of this approach to animal models as a translational tool to bridge clinical and preclinical research. Specifically, the development of pharmacological MRI (phMRI), i.e., the use of fMRI to map spatiotemporal patterns of brain activity induced by pharmacological agents, has provided a robust and flexible tool to resolve brain circuits and mechanism-specific functional changes produced by selective intervention in different neurotransmitter systems in vivo. This chapter describes the methodological aspects of fMRI and phMRI in preclinical species, and some of the key findings, with a special emphasis on the translational potential of these methods in neuropharmacological research.

Further Electrochemical and Behavioural Evidence of a Direct Relationship Between Central 5-HT and Cytoskeleton in the Control of Mood

Reduced activity of CNS serotonin is reported in unipolar depression and serotonin is the major target of recent antidepressant drugs. However, an acute depletion of serotonin in healthy individuals does not induce depressive symptoms suggesting that depression does not correlate with the serotonin system only. Neuronal plasticity (structural adaptation of neurons to functional requirements) includes synthesis of microtubular proteins such as tyrosinated isoform of α-tubulin and presence of serotonin as regulator of synaptogenesis. In depression neuronal plasticity is modified.

Here, in rats submitted to a behavioural test widely used to predict the efficacy of antidepressant drugs (forced swimming test: FST) a significant decrease of both cerebral tyrosinated α-tubulin expression and serotonin levels is monitored. Moreover, treatment with para-chlorophenylalanine (PCPA, compound that specifically depletes brain serotonin) but not alpha-methyl para tyrosine (α-MPT, compound that blocks synthesis of catechols: chemicals also implicated in depression) significantly reduced tyrosinated α-tubulin. Thus, a direct relationship between serotonin and tyrosinated α-tubulin appears to be present both in “physiological” and in “pathological” states. In addition, data obtained in animals submitted to FST and/or treated with the selective serotonin reuptake inhibitor (SSRI) fluoxetine further support the interrelationship between central serotonin and cytoskeleton. These data propose that direct relationship between serotonin and tyrosinated α-tubulin could be considered within the mechanism(s) involved in the pathogenesis of depression.