Current drug treatments targeting dopamine D3 receptor

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.

Effects and potential mechanisms of exercise and physical activity on eye health and ocular diseases

In the field of eye health, the profound impact of exercise and physical activity on various ocular diseases has become a focal point of attention. This review summarizes and elucidates the positive effects of exercise and physical activities on common ocular diseases, including dry eye disease (DED), cataracts, myopia, glaucoma, diabetic retinopathy (DR), and age-related macular degeneration (AMD). It also catalogues and offers exercise recommendations based on the varying impacts that different types and intensities of physical activities may have on specific eye conditions. Beyond correlations, this review also compiles potential mechanisms through which exercise and physical activity beneficially affect eye health. From mitigating ocular oxidative stress and inflammatory responses, reducing intraocular pressure, enhancing mitochondrial function, to promoting ocular blood circulation and the release of protective factors, the complex biological effects triggered by exercise and physical activities reveal their substantial potential in preventing and even assisting in the treatment of ocular diseases. This review aims not only to foster awareness and appreciation for how exercise and physical activity can improve eye health but also to serve as a catalyst for further exploration into the specific mechanisms and key targets through which exercise impacts ocular health. Such inquiries are crucial for advancing innovative strategies for the treatment of eye diseases, thereby holding significant implications for the development of new therapeutic approaches.

Alterations in type 2 dopamine receptors across neuropsychiatric conditions: A large-scale PET cohort

PURPOSE

Aberrant dopaminergic function is linked with motor, psychotic, and affective symptoms, but studies have typically compared a single patient group with healthy controls.

METHODS

Here, we investigated the variation in striatal (caudate nucleus, nucleus accumbens, and putamen) and thalamic type 2 dopamine receptor (D2R) availability using [11C]raclopride positron emission tomography (PET) data from a large sample of 437 humans including healthy controls, and subjects with Parkinson's disease (PD), antipsychotic-naïve schizophrenia, severe violent behavior, pathological gambling, depression, and overweight. We analyzed regional group differences in D2R availability. We also analyzed the interregional correlation in D2R availability within each group.

RESULTS

Subjects with PD showed the clearest decline in D2R availability. Overall, the groups showed high interregional correlation in D2R availability, while this pattern was weaker in violent offenders. Subjects with schizophrenia, pathological gambling, depression, or overweight did not show clear changes in either the regional receptor availability or the interregional correlation.

CONCLUSION

We conclude that the dopaminergic changes in neuropsychiatric conditions might not only affect the overall receptor availability but also how coupled regions are across people. The region-specific receptor availability more profoundly links to the motor symptoms, while the between-region coupling might be disrupted in violence.

In silico exploration of venlafaxine, a potential non-tricyclic antidepressant in a liposomal formulation for nose-to-brain drug delivery

OBJECTIVE

Non-tricyclic antidepressants (non-TCAs) work by preventing the intake of norepinephrine and serotonin. Therefore, the aim of this study was to identify a potent non-TCAs and to develop liposomal formulation, characterize and to determine the drug release study across model of dialysis membrane via in vitro and in silico techniques.

METHODS

The in silico docking analysis identified venlafaxine (VLF) as the best non-TCAs with the depressant targets (PDB ID: 3PBL and 4BVN). VLF-loaded liposomal formulation was prepared by the thin-film hydration technique and characterized by physicochemical properties, including entrapment efficacy, in vitro drug release, particle size analysis, and FTIR. Moreover, this article also compares VLF and VLF-loaded with liposome carriers (LPs) based on nose-to-brain drug delivery approaches to treating depression.

RESULTS

Drug release profiles of the optimal liposomal formulation of VLF-LPs were examined in the high entrapment efficiency 94.13 ± 1.20% was attained at 224 nm, composed of spherical particles having a mean particle size of 191 ± 2.0 nm, a polydispersity index of 0.281 ± 0.06 and zeta potential of -20.3 mV. The best formulation of VLF-LPs was more effective than oral VLF treatment, as shown by the in vitro drug release data.

CONCLUSION

The results show that the VLF-LPs formulation is a promising potential platform for application in nose-to-brain drug delivery. Thus, highlighting the robustness of the intranasal drug delivery system with enhanced pharmaceutical properties, efficacy, and bioavailability for the anti-depression effect.

Design and Synthesis of D3R Bitopic Ligands with Flexible Secondary Binding Fragments: Radioligand Binding and Computational Chemistry Studies

A series of bitopic ligands based on Fallypride with a flexible secondary binding fragment (SBF) were prepared with the goal of preparing a D3R-selective compound. The effect of the flexible linker ((R,S)-trans-2a-d), SBFs ((R,S)-trans-2h-j), and the chirality of orthosteric binding fragments (OBFs) ((S,R)-trans-d, (S,R)-trans-i, (S,S)-trans-d, (S,S)-trans-i, (R,R)-trans-d, and (R,R)-trans-i) were evaluated in in vitro binding assays. Computational chemistry studies revealed that the interaction of the fragment binding to the SBF increased the distance between the pyrrolidine nitrogen and ASP1103.32 of the D3R, thereby reducing the D3R affinity to a suboptimal level.

Synthesis of bitopic ligands based on fallypride and evaluation of their affinity and selectivity towards dopamine D2 and D3 receptors

The difference in the secondary binding site (SBS) between the dopamine 2 receptor (D2R) and dopamine 3 receptor (D3R) has been used in the design of compounds displaying selectivity for the D3R versus D2R. In the current study, a series of bitopic ligands based on Fallypride were prepared with various secondary binding fragments (SBFs) as a means of improving the selectivity of this benzamide analog for D3R versus D2R. We observed that compounds having a small alkyl group with a heteroatom led to an improvement in D3R versus D2R selectivity. Increasing the steric bulk in the SBF increase the distance between the pyrrolidine N and Asp110, thereby reducing D3R affinity. The best-in-series compound was (2S,4R)-trans-27 which had a modest selectivity for D3R versus D2R and a high potency in the β-arrestin competition assay which provides a measure of the ability of the compound to compete with endogenous dopamine for binding to the D3R. The results of this study identified factors one should consider when designing bitopic ligands based on Fallypride displaying an improved affinity for D3R versus D2R.

The role of the D3 dopamine receptor and its partial agonist cariprazine in patients with schizophrenia and substance use disorder

INTRODUCTION

Comorbidity of substance use disorder (SUD) with schizophrenia, referred to as dual disorder (DD), significantly increases morbidity and mortality compared to schizophrenia alone. A dopaminergic dysregulation seems to be a common pathophysiological basis of the comorbidity.

AREAS COVERED

This article reports the current evidence on the role of dopamine dysregulations in DD, the pharmacological profile of cariprazine, a partial agonist of D3 and D2 dopamine receptors, and first clinical observations that may support its usefulness in the therapy of DD. PubMed/MEDLINE was searched for the keywords 'cariprazine,' 'schizophrenia,' 'dual disorder,' 'dopamine,' and 'dopamine receptor.' Preclinical and clinical studies, and reviews published in English were retrieved.

EXPERT OPINION

Although the management of DD remains challenging, and the evidence for pharmacologic treatments is still unsatisfactory, cariprazine may be a candidate medication in DD due to its unique mechanism of action. Preliminary clinical experiences suggest that cariprazine has both antipsychotic and anticraving properties and should be considered early in patients with DD.

Using novel click chemistry algorithm to design D3R inhibitors as blood-brain barrier permeants

Dopamine receptor D3 (D3R) has gained attention as a promising therapeutic target for neurological disorders. In this study, an innovative in silico click reaction strategy was employed to identify potential D3R binders. The ligand template, 1-phenyl-4-[4-(1H-1,2,3-triazol-5-yl)butyl]piperazine, with substitution at the 1,2,3-triazole ring, served as the starting point. Generated compounds underwent filtration based on their brain-to-blood concentration ratio (logBB), leading to the identification of 1-{4-[1-(decahydronaphthalen-1-yl)-1H-1,2,3-triazol-5-yl]butyl}-4-phenylpiperazine as the most promising candidate, displaying superior D3R affinity and blood-brain barrier (BBB) permeability compared to the reference ligand, eticlopride. Molecular dynamics simulations further supported these findings. This study presents a novel hit for designing D3R ligands and establishes a workflow utilizing in silico click chemistry to screen compounds with BBB permeability. The proposed click reaction-based algorithm holds significant potential as a valuable tool in the development of effective antipsychotic compounds.

The Role of Dopamine D3 Receptors, Dysbindin, and Their Functional Interaction in the Expression of Key Genes for Neuroplasticity and Neuroinflammation in the Mouse Brain

Cognitive impairment in schizophrenia remains a clinically and pharmacologically unsolved challenge. Clinical and preclinical studies have revealed that the concomitant reduction in dysbindin (DYS) and dopamine receptor D3 functionality improves cognitive functions. However, the molecular machinery underlying this epistatic interaction has not yet been fully elucidated. The glutamate NMDA receptors and the neurotrophin BDNF, with their established role in promoting neuroplasticity, may be involved in the complex network regulated by the D3/DYS interaction. Furthermore, as inflammation is involved in the etiopathogenesis of several psychiatric diseases, including schizophrenia, the D3/DYS interaction may affect the expression levels of pro-inflammatory cytokines. Thus, by employing mutant mice bearing selective heterozygosis for D3 and/or DYS, we provide new insights into the functional interactions (single and synergic) between these schizophrenia susceptibility genes and the expression levels of key genes for neuroplasticity and neuroinflammation in three key brain areas for schizophrenia: the prefrontal cortex, striatum, and hippocampus. In the hippocampus, the epistatic interaction between D3 and DYS reversed to the wild-type level the downregulated mRNA levels of GRIN1 and GRIN2A were observed in DYS +/- and D3 +/- mice. In all the areas investigated, double mutant mice had higher BDNF levels compared to their single heterozygote counterparts, whereas D3 hypofunction resulted in higher pro-inflammatory cytokines. These results may help to clarify the genetic mechanisms and functional interactions involved in the etiology and development of schizophrenia.

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.

Insights into the Involvement and Therapeutic Target Potential of the Dopamine System in the Posttraumatic Stress Disorder

Posttraumatic stress disorder (PTSD) is a neuropsychiatric disease closely related to life-threatening events and psychological stress. Re-experiencing, hyperarousal, avoidance, and numbness are the hallmark symptoms of PTSD, but their underlying neurological processes have not been clearly elucidated. Therefore, the identification and development of drugs for PTSD that targets brain neuronal activities have stalled. Considering that the persistent fear memory induced by traumatic stimulation causes high alertness, high arousal, and cognitive impairment of PTSD symptoms. While the midbrain dopamine system can affect physiological processes such as aversive fear memory learning, consolidation, persistence, and extinction, by altering the functions of the dopaminergic neurons, our viewpoint is that the dopamine system plays a considerable role in the PTSD occurrence and acts as a potential therapeutic target of the disorder. This paper reviews recent findings on the structural and functional connections between ventral tegmental area neurons and the core synaptic circuits involved in PTSD, gene polymorphisms related to the dopamine system that confer susceptibility to clinical PTSD. Moreover, the progress of research on medications that target the dopamine system as PTSD therapies is also discussed. Our goal is to offer some hints for early detection and assist in identifying novel, efficient approaches for treating PTSD.

Fluorescence based HTS-compatible ligand binding assays for dopamine D3 receptors in baculovirus preparations and live cells

Dopamine receptors are G-protein-coupled receptors that are connected to severe neurological disorders. The development of new ligands targeting these receptors enables gaining a deeper insight into the receptor functioning, including binding mechanisms, kinetics and oligomerization. Novel fluorescent probes allow the development of more efficient, cheaper, reliable and scalable high-throughput screening systems, which speeds up the drug development process. In this study, we used a novel Cy3B labelled commercially available fluorescent ligand CELT-419 for developing dopamine D3 receptor-ligand binding assays with fluorescence polarization and quantitative live cell epifluorescence microscopy. The fluorescence anisotropy assay using 384-well plates achieved Z’ value of 0.71, which is suitable for high-throughput screening of ligand binding. The assay can also be used to determine the kinetics of both the fluorescent ligand as well as some reference unlabeled ligands. Furthermore, CELT-419 was also used with live HEK293-D3R cells in epifluorescence microscopy imaging for deep-learning-based ligand binding quantification. This makes CELT-419 quite a universal fluorescence probe which has the potential to be also used in more advanced microscopy techniques resulting in more comparable studies.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

Current Perspectives on Selective Dopamine D3 Receptor Antagonists/Partial Agonists as Pharmacotherapeutics for Opioid and Psychostimulant Use Disorders

Over three decades of evidence indicate that dopamine (DA) D3 receptors (D3R) are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders (SUD). The expectation that a selective D3R antagonist/partial agonist would be efficacious for the treatment of SUD is based on the following key observations. First, D3R are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and ventral pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse produces neuroadaptations in the D3R system. Third, the synthesis and characterization of highly potent and selective D3R antagonists/partial agonists have further strengthened the role of the D3R in SUD. Based on extensive preclinical and preliminary clinical evidence, the D3R shows promise as a target for the development of pharmacotherapies for SUD as reflected by their potential to (1) regulate the motivation to self-administer drugs and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, drug-associated environmental cues, or stress. The availability of PET ligands to assess clinically relevant receptor occupancy by selective D3R antagonists/partial agonists, the definition of reliable dosing, and the prospect of using human laboratory models may further guide the design of clinical proof of concept studies. Pivotal clinical trials for more rapid progression of this target toward regulatory approval are urgently required. Finally, the discovery that highly selective D3R antagonists, such as R-VK4-116 and R-VK4-40, do not adversely affect peripheral biometrics or cardiovascular effects alone or in the presence of oxycodone or cocaine suggests that this class of drugs has great potential in safely treating psychostimulant and/or opioid use disorders.

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.

Dopamine Receptors: Is It Possible to Become a Therapeutic Target for Depression?

Dopamine and its receptors are currently recognized targets for the treatment of several neuropsychiatric disorders, including Parkinson’s disease, schizophrenia, some drug use addictions, as well as depression. Dopamine receptors are widely distributed in various regions of the brain, but their role and exact contribution to neuropsychiatric diseases has not yet been thoroughly studied. Based on the types of dopamine receptors and their distribution in different brain regions, this paper reviews the current research status of the molecular, cellular and circuit mechanisms of dopamine and its receptors involved in depression. Multiple lines of investigation of these mechanisms provide a new future direction for understanding the etiology and treatment of depression and potential new targets for antidepressant treatments.

Updated perspectives on the clinical significance of negative symptoms in patients with schizophrenia

INTRODUCTION

Negative symptoms in schizophrenia are associated with poor response to available treatments, poor quality of life, and functional outcome. Therefore, they represent a substantial burden for people with schizophrenia, their families, and health-care systems.

AREAS COVERED

In this manuscript, we will provide an update on the conceptualization, assessment, and treatment of this complex psychopathological dimension of schizophrenia.

EXPERT OPINION

Despite the progress in the conceptualization of negative symptoms and in the development of state-of-the-art assessment instruments made in the last decades, these symptoms are still poorly recognized, and not always assessed in line with current conceptualization. Every effort should be made to disseminate the current knowledge on negative symptoms, on their assessment instruments and available treatments whose efficacy is supported by research evidence. Longitudinal studies should be promoted to evaluate the natural course of negative symptoms, improve our ability to identify the different sources of secondary negative symptoms, provide effective interventions, and target primary and persistent negative symptoms with innovative treatment strategies. Further research is needed to identify pathophysiological mechanisms of primary negative symptoms and foster the development of new treatments.

Knockout of Dopamine D3 Receptor Gene Blocked Methamphetamine-Induced Distinct Changes of Dopaminergic and Glutamatergic Synapse in the Nucleus Accumbens Shell of Mice

Structural plasticity changes in the brain are thought to underlie, at least partially, drug-induced persistent changes in behavior. Our previous study reported that increased synaptic density in the nucleus accumbens shell (NAcsh) correlates with and may contribute to behavioral sensitization induced by methamphetamine (METH). However, the distinct changes of dopaminergic and glutamatergic synapses and the modulating effects of dopamine D3 receptor remain unclear. In the current study, we used immunohistochemistry electron-microscopy and immunofluorescence to detect the changes of dopamine D1, D2, and glutamate NR2B-positive synapses and cells in the NAcsh of METH-sensitized wild type (WT) and knockout of dopamine D3 receptor gene (D3^–/–) mice. We found that METH induced long-term behavioral sensitization in WT mice, which was accompanied by an increased number and rate of dopamine D1 receptor-positive synapses and cells, as well as glutamate NR2B-positive synapses and cells. In contrast, the number and rate of dopamine D2 receptor-positive synapses and cells were significantly decreased in the NAcsh of METH-sensitized WT mice. D3^–/– mice exhibited attenuated acute locomotor responses and behavioral sensitization to METH compared with WT mice. Moreover, the knockout of dopamine D3 receptor gene inhibited METH-induced changes of dopaminergic and glutamatergic synapses in the NAcsh of METH-sensitized mice. Taken together, our results suggest that METH induced distinct changes of dopaminergic and glutamatergic synapses and cells in the NAcsh of mice, which was blocked by the knockout of dopamine D3 receptor gene, and may contribute to, at least partially, METH-induced behavior sensitization as well as the modulating effect of the dopamine D3 receptor.

Atlas of type 2 dopamine receptors in the human brain: Age and sex dependent variability in a large PET cohort

BACKGROUND

The dopamine system contributes to a multitude of functions ranging from reward and motivation to learning and movement control, making it a key component in goal-directed behavior. Altered dopaminergic function is observed in neurological and psychiatric conditions. Numerous factors have been proposed to influence dopamine function, but due to small sample sizes and heterogeneous data analysis methods in previous studies their specific and joint contributions remain unresolved.

METHODS

In this cross-sectional register-based study we investigated how age, sex, body mass index (BMI), as well as cerebral hemisphere and regional volume influence striatal type 2 dopamine receptor (D₂R) availability in the human brain. We analyzed a large historical dataset (n=156, 120 males and 36 females) of [11C]raclopride PET scans performed between 2004 and 2018.

RESULTS

Striatal D₂R availability decreased through age for both sexes (2-5 % in striatal ROIs per 10 years) and was higher in females versus males throughout age (7-8% in putamen). BMI and striatal D₂R availability were weakly associated. There was no consistent lateralization of striatal D₂R. The observed effects were independent of regional volumes. These results were validated using two different spatial normalization methods, and the age and sex effects also replicated in an independent sample (n=135).

CONCLUSIONS

D₂R availability is dependent on age and sex, which may contribute to the vulnerability of neurological and psychiatric conditions involving altering D₂R expression.

Dopamine D3 receptor in the nucleus accumbens alleviates neuroinflammation in a mouse model of depressive-like behavior

We recently reported that dopamine D3 receptor (D3R) was involved in inflammation-related depression. Nucleus accumbens (NAc) inflammation is implicated in the development and progression of depression, but its regulatory mechanism remains largely unknown. In a mouse model of NAc neuroinflammation induced by bilateral NAc injection of lipopolysaccharide (LPS), we observed that NAc neuroinflammation triggered depressive-like behaviors, and D3R expression decline and microglial activation in the NAc. A selective knockdown of D3R in the NAc elicited depressive-like behaviors, while re-expression of D3R in the NAc of global D3RKO mice alleviated depressive-like behaviors induced by D3R deficiency. D3R downregulation in the NAc shifted microglia toward a proinflammatory state, which was validated with cultured mouse microglial cultures. Further in vitro results demonstrated that D3R inhibition induced microglia to enter a proinflammatory state primarily through the Akt signaling pathway. In conclusion, our results suggest that D3R expression in the NAc may inhibit microglial proinflammatory responses in the NAc, thus alleviating NAc neuroinflammation and subsequent depressive-like behaviors through the Akt signaling pathway.

Switching Clozapine to Cariprazine in Three Patients with Persistent Symptoms of Schizophrenia: A Case Series

Despite many available treatments for schizophrenia, several unmet needs persist in treating individuals with this disorder, and the response rate to first-line antipsychotics remains relatively low. Clozapine has shown efficacy in treating schizophrenia patients who failed to respond to previous antipsychotics. However, side effects and the need for routine blood tests have limited its use as a first-line treatment. Cariprazine is a D₂/D₃ partial agonist antipsychotic with a mechanism of action that differs from other antipsychotics due to its higher affinity for D₃ receptors. Several trials have demonstrated the efficacy of cariprazine on positive and negative symptoms of schizophrenia and have shown that it is a well-tolerated treatment. In this series, we present 3 cases of patients diagnosed with schizophrenia who were initially under treatment with clozapine. Despite some initial improvement, the patients showed persisting positive and negative symptoms or developed limiting side effects while in treatment with clozapine. Cariprazine treatment was titrated concurrently with clozapine tapering until its discontinuation. Significant improvement in both positive and negative symptoms was observed up to 14 months after starting cariprazine, and resolution of side effects was reported in all cases. Our case series supports cariprazine as an effective treatment for positive and negative symptoms in patients who failed to adequately respond or poorly tolerated treatment with clozapine, as well as a potential treatment in dual disorders, specifically psychotic disorders and cocaine use disorder.

Dopamine D3 Receptor, Cognition and Cognitive Dysfunctions in Neuropsychiatric Disorders: From the Bench to the Bedside

The dopamine D3 receptor (D3R) plays a prominent role in the modulation of cognition in healthy individuals, as well as in the pathophysiological mechanism underlying the cognitive deficits affecting patients suffering from neuropsychiatric disorders. At a therapeutic level, a growing body of evidence suggests that the D3R blockade enhances cognitive and thus it may be an optimal therapeutic strategy against cognitive dysfunctions. However, this is not always the case because other ligands targeting the D3R, and behaving as partial agonists or biased agonists, may exert their pro-cognitive effect by maintaining adequate level of dopamine in key brain areas tuning cognitive performances. In this chapter, we review and discuss preclinical and clinical findings with the aim to remark the crucial role of the D3R in cognition and to strengthen the message that drugs targeting D3R may be excellent cognitive enhancers for the treatment of several neuropsychiatric and neurological disorders.

Potential Mechanisms for Why Not All Antipsychotics Are Able to Occupy Dopamine D3 Receptors in the Brain in vivo

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D₂ receptors (D₂Rs) and D₃ receptors (D₃Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D₂Rs and D₃Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D₃Rs and D₂Rs, only very few can significantly occupy D₃Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D₃Rs receptor affinity and selectivity as determinant factors for in vivo D₃Rs occupancy by antipsychotics, are also discussed.

Chiral Cyclic Aliphatic Linkers as Building Blocks for Selective Dopamine D2 or D3 Receptor Agonists

Linkers are emerging as a key component in regulating the pharmacology of bitopic ligands directed toward G-protein coupled receptors (GPCRs). In this study, the role of regio- and stereochemistry in cyclic aliphatic linkers tethering well-characterized primary and secondary pharmacophores targeting dopamine D2 and D3 receptor subtypes (D2R and D3R, respectively) is described. We introduce several potent and selective D2R (rel-trans-16b; D2R Ki = 4.58 nM) and D3R (rel-cis-14a; D3R Ki = 5.72 nM) agonists while modulating subtype selectivity in a stereospecific fashion, transferring D2R selectivity toward D3R via inversion of the stereochemistry around these cyclic aliphatic linkers [e.g., (-)-(1S,2R)-43 and (+)-(1R,2S)-42]. Pharmacological observations were supported with extensive molecular docking studies. Thus, not only is it an innovative approach to modulate the pharmacology of dopaminergic ligands described, but a new class of optically active cyclic linkers are also introduced, which can be used to expand the bitopic drug design approach toward other GPCRs.

Low-dose polypharmacology targeting dopamine D1 and D3 receptors reduces cue-induced relapse to heroin seeking in rats

Chemical compounds that target dopamine (DA) D1 or D3 receptors have shown promise as potential interventions in animal models of cue-induced relapse. However, undesirable side effects or pharmacodynamic profiles have limited the advancement of new compounds in preclinical studies when administered as independent treatments. In this series of experiments, we explored the effects of coadministration of a D1-receptor partial agonist (SKF 77434) and a D3-receptor antagonist (NGB 2904) in heroin-seeking rats within a "conflict" model of abstinence and cue-induced relapse. Rats were first trained to press a lever to self-administer heroin, and drug delivery was paired contingently with cues (e.g., light and pump noise). Self-initiated abstinence was facilitated by applying electrical current to the flooring in front of the levers. Lastly, a relapse response was provoked by noncontingent presentation of conditioned cues. Prior to provocation, rats received a systemic injection of SKF 77434, NGB 2904, or a combination of both compounds to assess treatment effects on lever pressing. Results indicated that the coadministration of low (i.e., independently ineffective) doses of both compounds was more effective in reducing cue-induced relapse to heroin seeking than either compound alone, with some evidence of drug synergism. Follow-up studies indicated that this reduction was not due to motoric impairment nor enhanced sensitivity to the electrified flooring and that this treatment did not significantly affect motivation for food. Implications for the treatment of opiate use disorder and recommendations for further research are discussed.

Dopaminergic neuromodulation of prefrontal cortex activity requires the NMDA receptor coagonist d-serine

Prefrontal control of cognitive functions critically depends upon glutamatergic transmission and N-methyl D-aspartate (NMDA) receptors, the activity of which is regulated by dopamine. Yet whether the NMDA receptor coagonist d-serine is implicated in the dopamine-glutamate dialogue in the prefrontal cortex (PFC) and other brain areas remains unexplored. Here, using electrophysiological recordings, we show that d-serine is required for the fine-tuning of glutamatergic neurotransmission, neuronal excitability, and synaptic plasticity in the PFC through the actions of dopamine at D₁ and D₃ receptors. Using in vivo microdialysis, we show that D₁ and D₃ receptors exert a respective facilitatory and inhibitory influence on extracellular levels and activity of d-serine in the PFC, with actions expressed primarily via the cAMP/protein kinase A (PKA) signaling cascade. Further, using functional magnetic resonance imaging (fMRI) and behavioral assessment, we show that d-serine is required for the potentiation of cognition by D₃R blockade as revealed in a test of novel object recognition memory. Collectively, these results unveil a key role for d-serine in the dopaminergic neuromodulation of glutamatergic transmission and PFC activity, findings with clear relevance to the pathogenesis and treatment of diverse brain disorders involving alterations in dopamine-glutamate cross-talk.

Chirality of Novel Bitopic Agonists Determines Unique Pharmacology at the Dopamine D3 Receptor

The dopamine D2/D3 receptor (D₂R/D₃R) agonists are used as therapeutics for Parkinson's disease (PD) and other motor disorders. Selective targeting of D₃R over D₂R is attractive because of D₃R's restricted tissue distribution with potentially fewer side-effects and its putative neuroprotective effect. However, the high sequence homology between the D₂R and D₃R poses a challenge in the development of D₃R selective agonists. To address the ligand selectivity, bitopic ligands were designed and synthesized previously based on a potent D₃R-preferential agonist PF592,379 as the primary pharmacophore (PP). This PP was attached to various secondary pharmacophores (SPs) using chemically different linkers. Here, we characterize some of these novel bitopic ligands at both D₃R and D₂R using BRET-based functional assays. The bitopic ligands showed varying differences in potencies and efficacies. In addition, the chirality of the PP was key to conferring improved D₃R potency, selectivity, and G protein signaling bias. In particular, compound AB04-88 exhibited significant D₃R over D₂R selectivity, and G protein bias at D₃R. This bias was consistently observed at various time-points ranging from 8 to 46 min. Together, the structure-activity relationships derived from these functional studies reveal unique pharmacology at D₃R and support further evaluation of functionally biased D₃R agonists for their therapeutic potential.

The epistatic interaction between the dopamine D3 receptor and dysbindin-1 modulates higher-order cognitive functions in mice and humans

The dopamine D2 and D3 receptors are implicated in schizophrenia and its pharmacological treatments. These receptors undergo intracellular trafficking processes that are modulated by dysbindin-1 (Dys). Indeed, Dys variants alter cognitive responses to antipsychotic drugs through D2-mediated mechanisms. However, the mechanism by which Dys might selectively interfere with the D3 receptor subtype is unknown. Here, we revealed an interaction between functional genetic variants altering Dys and D3. Specifically, both in patients with schizophrenia and in genetically modified mice, concomitant reduction in D3 and Dys functionality was associated with improved executive and working memory abilities. This D3/Dys interaction produced a D2/D3 imbalance favoring increased D2 signaling in the prefrontal cortex (PFC) but not in the striatum. No epistatic effects on the clinical positive and negative syndrome scale (PANSS) scores were evident, while only marginal effects on sensorimotor gating, locomotor functions, and social behavior were observed in mice. This genetic interaction between D3 and Dys suggests the D2/D3 imbalance in the PFC as a target for patient stratification and procognitive treatments in schizophrenia.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Partial Agonists and Dual Disorders: Focus on Dual Schizophrenia

Dual disorder is a term applied to patients with an addictive disorder and other mental disorder. Epidemiological studies have established that dual disorders are an expectation rather than an exception. They are difficult to diagnose and treat and constitute a huge burden for both patients and their relatives and society. Current treatments are a combination of those needed to treat the addictive disorder with those focused on the co-occurring psychiatric disorder. Focusing specifically on schizophrenia, growing scientific evidence supports the existence of a shared vulnerability for substance use in these patients and those at risk. Various antipsychotics have been found to be useful in the treatment of psychotic symptoms and disorders; however, few effective treatments have been identified until now for substance use disorders in patients with dual schizophrenia. Partial agonism stands as a new pharmacological option available in recent years. Molecules with this kind of action may act as functional agonists or as antagonists, depending on the surrounding levels of the neurotransmitter. Studies have found their efficacy in schizophrenia, addiction, anxiety and depression. Certain partial agonist antipsychotics seem to have a role in the treatment of dual schizophrenia. That could be the case with cariprazine. Because of its higher affinity for dopaminergic D3 receptors compared to D2, a potential to prevent relapse to addiction, added to its antipsychotic efficacy, has been suggested. Here we briefly review current advances and future directions and introduce some personal insights into the role of partial agonists in co-occurring schizophrenia and substance use.

Preferential Effects of Cariprazine on Counteracting the Disruption of Social Interaction and Decrease in Extracellular Dopamine Levels Induced by the Dopamine D3 Receptor Agonist, PD-128907 in Rats: Implications for the Treatment of Negative and Depressive Symptoms of Psychiatric Disorders

The negative and cognitive symptoms of schizophrenia and related disorders may be due to reduced dopaminergic tone in cortical brain areas. Alteration in the function of dopamine (DA) D₃ receptors may play a role in this cortical hypofunctionality and underlie the deficits in social behaviors and cognitive functions in schizophrenia. Cariprazine is a potent DA D₃-preferring D₃/D₂ receptor partial agonist that is approved for the treatment of schizophrenia and bipolar disorder. The objective of the study was to compare the abilities of cariprazine, aripiprazole (another DA receptor partial agonist with more D₂ receptor preference), and ABT-925 (a selective DA D₃ antagonist) to counteract the social deficit and neurochemical alterations induced by the D₃ receptor-preferring agonist (+)-PD 128907 (PD) in rats. Administration of PD (0.16 mg/kg; s.c.) induced a marked (-72%) but short-lasting disruption of the defensive social aggregation behavior (huddling) in the first 10-min period. Cariprazine at all doses (0.1, 0.3, 1 mg/kg; p.o.) almost completely abolished the PD-induced disruption of huddling. Likewise, ABT-925 (3 mg/kg; p.o.) and to a lesser extent aripiprazole (20 mg/kg; p.o.) were effective in blocking the PD-induced disruption of huddling. As measured by microdialysis, the highest dose of cariprazine prevented a PD-induced decrease in DA levels (40-80 min post PD dose) in the medial prefrontal cortex (mPFC), whereas aripiprazole did not have a significant effect. ABT-925 significantly counteracted the effect of PD at 80 min post-dose. In the nucleus accumbens (nAcc) shell, the highest dose of cariprazine, as well as ABT-925 and aripiprazole, significantly reversed the PD-induced decrease in DA levels. Taken together, these data provide behavioral and in vivo neurochemical evidence for the preferential DA D₃ receptor action of cariprazine in the rat. This property of cariprazine may offer therapeutic benefits against the cognitive deficits and negative/depressive symptoms of schizophrenia and related disorders.

Dopaminergic System Alteration in Anxiety and Compulsive Disorders: A Systematic Review of Neuroimaging Studies

Objective: The dopaminergic system is involved in many psychiatric disorders as a GABAergic, serotonergic, and glutamatergic system. A systematic review and meta-analysis was performed to elucidate the alteration of the dopaminergic system in anxiety and compulsive disorders.

Methods: The databases of Pubmed, Embase, and ScienceDirect were searched and articles reporting the involvement of the dopaminergic system in patients with anxiety disorder and obsessive compulsive disorder (OCD) were recognized. The key research data were extracted from the included articles and standardized mean differences were calculated using meta-analyses if there were more than two studies with obtainable data. Sensitivity analyses were further performed to detect the stability of results, and the qualities of all the included studies were assessed using the Newcastle Ottawa scale.

Results: Finally, we identified 8 and 11 studies associated with anxiety disorder and OCD for further analysis, respectively. Most consistently, the striatal dopamine D₂ receptor (D₂R) of OCD patients had decreased while no significant correlation was found between striatal D2R and disease severity. The striatal dopamine transporter (DAT) had not been significantly altered in both the anxiety disorder and OCD patients. The heterogeneity values from the meta-analyses were extremely high while those results remained stable after sensitivity analyses. Inconsistent data were found in the striatal D₂R of patients with anxiety disorder. Limited data had suggested that dopamine synthesis increased in most regions of the cerebral cortex and cerebellum in OCD patients.

Conclusions: The most convincing finding was that the D₂ receptor decreased in patients with obsessive compulsive disorder. The dopamine transporter may have no relationship with anxiety and compulsive disorder.

Dopamine, Cognitive Impairments and Second-Generation Antipsychotics: From Mechanistic Advances to More Personalized Treatments

The pharmacological treatment of cognitive impairments associated with schizophrenia is still a major unmet clinical need. Indeed, treatments with available antipsychotics generate highly variable cognitive responses among patients with schizophrenia. This has led to the general assumption that antipsychotics are ineffective on cognitive impairment, although personalized medicine and drug repurposing approaches might scale down this clinical issue. In this scenario, evidence suggests that cognitive improvement exerted by old and new atypical antipsychotics depends on dopaminergic mechanisms. Moreover, the newer antipsychotics brexpiprazole and cariprazine, which might have superior clinical efficacy on cognitive deficits over older antipsychotics, mainly target dopamine receptors. It is thus reasonable to assume that despite more than 50 years of elusive efforts to develop novel non-dopaminergic antipsychotics, dopamine receptors remain the most attractive and promising pharmacological targets in this field. In the present review, we discuss preclinical and clinical findings showing dopaminergic mechanisms as key players in the cognitive improvement induced by both atypical antipsychotics and potential antipsychotics. We also emphasize the concept that these mechanistic advances, which help to understand the heterogeneity of cognitive responses to antipsychotics, may properly guide treatment decisions and address the unmet medical need for the management of cognitive impairment associated with schizophrenia.

An Overview of the Heterogeneity of Major Depressive Disorder: Current Knowledge and Future Prospective

Major depressive disorder (MDD) is estimated to impose maximum debilitating effects on the society by 2030, with its critical effects on health, functioning, quality of life and concomitant high levels of morbidity and mortality. Yet, the disease is inadequately understood, diagnosed and treated. Moreover, with the recent drastic rise in the pace of life, stress has materialized as one of the most potent environmental factors for depression. In this scenario, it is important to understand the modern pathogenetic hypotheses and mechanisms, and possibly try to shift from the traditional approaches in depression therapy. These include the elaboration of pathophysiological changes in heterogeneous systems such as genetic, epigenetic, serotonergic, noradrenergic, gamma-aminobutyric acid, glutamatergic and endocannabinoid systems, neurotrophic factors, HPA axis, immune system as well as cellular stress mechanisms. These components interact with each other in a complex matrix and further elucidation of their mechanism and cascade pathways are needed. This might aid in the identification of MDD subtypes as well as the development of sophisticated biomarkers. Further, characterization might also aid in developing multitargeted therapies that hold much promise as compared to the conventional monoamine based treatment. New candidate pharmacons, refined psychotherapeutic modalities, advanced neuro-surgical and imaging techniques as well as the implementation of pharmacokinetic, pharmacogenetic prescribing guidelines constitute the emerging expanses of MDD treatment.

Impaired working memory, cognitive flexibility and reward processing in mice genetically lacking Gpr88: Evidence for a key role for Gpr88 in multiple cortico-striatal-thalamic circuits

The GPR88 orphan G protein-coupled receptor is expressed throughout the striatum, being preferentially localised in medium spiny neurons. It is also present in lower densities in frontal cortex and thalamus. Rare mutations in humans suggest a role in cognition and motor function, while common variants are associated with psychosis. Here we evaluate the influence of genetic deletion of GPR88 upon performance in translational tasks interrogating motivation, reward evaluation and cognitive function. In an automated radial arm maze 'N-back' working memory task, Gpr88 KO mice showed impaired correct responding, suggesting a role for GPR88 receptors in working memory circuitry. Associative learning performance was similar to wild-type controls in a touchscreen task but performance was impaired at the reversal learning stage, suggesting cognitive inflexibility. Gpr88 KO mice showed higher breakpoints, reduced latencies and lengthened session time in a progressive ratio task consistent with enhanced motivation. Simultaneously, locomotor hyperactivity was apparent in this task, supporting previous findings of actions of GPR88 in a cortico-striatal-thalamic motor loop. Evidence for a role of GPR88 in reward processing was demonstrated in a touchscreen-based equivalent of the Iowa gambling task. Although both Gpr88 KO and wild-type mice showed a preference for an optimum contingency choice, Gpr88 KO mice selected more risky choices at the expense of more advantageous lower risk options. Together these novel data suggest that striatal GPR88 receptors influence activity in a range of procedures integrated by prefrontal, orbitofrontal and anterior cingulate cortico-striatal-thalamic loops leading to altered cognitive, motivational and reward evaluation processes.

Improving translation of animal models of addiction and relapse by reverse translation

Critical features of human addiction are increasingly being incorporated into complementary animal models, including escalation of drug intake, punished drug seeking and taking, intermittent drug access, choice between drug and non-drug rewards, and assessment of individual differences based on criteria in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). Combined with new technologies, these models advanced our understanding of brain mechanisms of drug self-administration and relapse, but these mechanistic gains have not led to improvements in addiction treatment. This problem is not unique to addiction neuroscience, but it is an increasing source of disappointment and calls to regroup. Here we first summarize behavioural and neurobiological results from the animal models mentioned above. We then propose a reverse translational approach, whose goal is to develop models that mimic successful treatments: opioid agonist maintenance, contingency management and the community-reinforcement approach. These reverse-translated 'treatments' may provide an ecologically relevant platform from which to discover new circuits, test new medications and improve translation.

New roles for dopamine D2 and D3 receptors in pancreatic beta cell insulin secretion

Although long-studied in the central nervous system, there is increasing evidence that dopamine (DA) has important roles in the periphery including in metabolic regulation. Insulin-secreting pancreatic β-cells express the machinery for DA synthesis and catabolism, as well as all five DA receptors. In these cells, DA functions as a negative regulator of glucose-stimulated insulin secretion (GSIS), which is mediated by DA D₂-like receptors including D₂ (D2R) and D₃ (D3R) receptors. However, the fundamental mechanisms of DA synthesis, storage, release, and signaling in pancreatic β-cells and their functional relevance in vivo remain poorly understood. Here, we assessed the roles of the DA precursor L-DOPA in β-cell DA synthesis and release in conjunction with the signaling mechanisms underlying DA's inhibition of GSIS. Our results show that the uptake of L-DOPA is essential for establishing intracellular DA stores in β-cells. Glucose stimulation significantly enhances L-DOPA uptake, leading to increased DA release and GSIS reduction in an autocrine/paracrine manner. Furthermore, D2R and D3R act in combination to mediate dopaminergic inhibition of GSIS. Transgenic knockout mice in which β-cell D2R or D3R expression is eliminated exhibit diminished DA secretion during glucose stimulation, suggesting a new mechanism where D₂-like receptors modify DA release to modulate GSIS. Lastly, β-cell-selective D2R knockout mice exhibit marked postprandial hyperinsulinemia in vivo. These results reveal that peripheral D2R and D3R receptors play important roles in metabolism through their inhibitory effects on GSIS. This opens the possibility that blockade of peripheral D₂-like receptors by drugs including antipsychotic medications may significantly contribute to the metabolic disturbances observed clinically.

Drugs-Induced Pathological Gambling: An Analysis of Italian Spontaneous Reporting System

Pathological gambling has been reported as a direct complication of Parkinson’s disease and its pharmacological treatment based on dopamine agonists. Moreover, further medications (not dopamine agonists) were associated to the occurrence of gambling disorder. We aim to analyze the spontaneous reports of gambling disorder on the whole Italian territory with a focus on Campania Region (Southern Italy) from January 1st 2002 to July 31st 2018. We analyzed gambling disorder’s reports across the 2002–2018 period in the Italian spontaneous reporting database (Rete Nazionale di Farmacovigilanza—RNF), with a focus on Campania region. 94 suspected cases of gambling disorder associated to apomorphine, aripiprazole, cabergoline, levodopa, levodopa and derivatives in association with entacapone/benserazide and carbidopa, pergolide, pramipexole, ropinirole, and rotigotine were reported into the RNF. Of these cases, two related to pramipexole and one to aripiprazole were sent to Campania Pharmacovigilance Regional Centre. Although it is widely recognized that dopamine agonists may induce behavioral disorders, Parkinson’s disease is itself associated to pathological gambling, compulsive shopping and eating. Since our results could not clarify the correlation between Parkinson’s disease, its pharmacological treatment and pathological gambling, in order to better define this correlation there is a need to conduct further ad hoc observational studies.

Levosulpiride Increases the Levels of Prolactin and Antiangiogenic Vasoinhibin in the Vitreous of Patients with Proliferative Diabetic Retinopathy

Purpose

High circulating levels of the hormone prolactin (PRL) protect against experimental diabetic retinopathy (DR) due to the retinal accumulation of vasoinhibin, a PRL fragment that inhibits blood vessel permeability and growth. A phase 2 clinical trial is investigating a new therapy for DR based on elevating serum PRL levels with levosulpiride, a prokinetic dopamine D2 receptor blocker. Here, we tested whether levosulpiride-induced hyperprolactinemia elevates PRL and vasoinhibin in the vitreous of volunteer patients with proliferative DR (PDR) undergoing elective pars plana vitrectomy.

Methods

Patients were randomized to receive placebo (lactose pill, orally TID; n = 19) or levosulpiride (25 mg orally TID; n = 18) for the 7 days before vitrectomy. Vitreous samples from untreated non-diabetic (n = 10) and PDR (n = 17) patients were also studied.

Results

Levosulpiride elevated the systemic (101 ± 13 [SEM] vs. 9.2 ± 1.3 ng/mL, P < 0.0001) and vitreous (3.2 ± 0.4 vs. 1.5 ± 0.2 ng/mL, P < 0.0001) levels of PRL, and both levels were directly correlated (r = 0.58, P < 0.0002). The vitreous from non-diabetic patients or from PDR patients treated with levosulpiride, but not from placebo-treated PDR patients, inhibited the basic fibroblast growth factor (bFGF)- and vascular endothelial growth factor (VEGF)-induced proliferation of endothelial cells in culture. Vasoinhibin-neutralizing antibodies reduced the vitreous antiangiogenic effect. Matrix metalloproteases (MMPs) in the vitreous cleaved PRL to vasoinhibin, and their activity was higher in non-diabetic than in PDR patients.

Conclusions

Levosulpiride increases the levels of PRL in the vitreous of PDR patients and promotes its MMP-mediated conversion to vasoinhibin, which can inhibit angiogenesis in DR.

Translational Relevance

These findings support the potential therapeutic benefit of levosulpiride against vision loss in diabetes. 

P2X7 receptor antagonism preserves retinal ganglion cells in glaucomatous mice

To investigate the role of P2X7 receptor to preserve retinal ganglion cells (RGCs) structure and function in a genetic mouse model (DBA/2J mouse) of age-related glaucomatous neurodegeneration. Chronic treatment with P2X7 receptor antagonist eye drops was carried out in order to assess RGCs function and density by pattern electroretinogram (PERG) and RBPMS immunostaining, respectively. Further, microglia activation was assessed in flat-mounted retina by using Iba-1 immunostaining. Untreated glaucomatous eyes displayed significant microglia activation, alteration of PERG signal, and RGCs loss. In the P2X7 receptor antagonist-treated eyes, the PERG signal was significantly (p < 0.05) improved compared to controls, along with a significant (p < 0.05) reduction in terms of retinal microglial activation, and remarkable preservation of RGCs density. Altogether, these findings demonstrated that topical treatment with a P2X7 receptor antagonist has a neuroprotective effect on RGCs in glaucomatous mice, suggesting an appealing pharmacological approach to prevent retinal degenerative damage in optic neuropathy.

Amide Bond Bioisosteres: Strategies, Synthesis, and Successes

The amide functional group plays a key role in the composition of biomolecules, including many clinically approved drugs. Bioisosterism is widely employed in the rational modification of lead compounds, being used to increase potency, enhance selectivity, improve pharmacokinetic properties, eliminate toxicity, and acquire novel chemical space to secure intellectual property. The introduction of a bioisostere leads to structural changes in molecular size, shape, electronic distribution, polarity, pK_a, dipole or polarizability, which can be either favorable or detrimental to biological activity. This approach has opened up new avenues in drug design and development resulting in more efficient drug candidates introduced onto the market as well as in the clinical pipeline. Herein, we review the strategic decisions in selecting an amide bioisostere (the why), synthetic routes to each (the how), and success stories of each bioisostere (the implementation) to provide a comprehensive overview of this important toolbox for medicinal chemists.

Negative Symptoms of Schizophrenia and Dopaminergic Transmission: Translational Models and Perspectives Opened by iPSC Techniques

Negative symptoms (NS) represent a heterogeneous dimension of schizophrenia (SCZ), associated with a poor functional outcome. A dysregulated dopamine (DA) system, including a reduced D1 receptor activation in the prefrontal cortex, DA hypoactivity in the caudate and alterations in D3 receptor activity, seems to contribute to the pathogenesis of NS. However, failure to take into account the NS heterogeneity has slowed down progress in research on their neurobiological correlates and discoveries of new effective treatments. A better neurobiological characterization of NS is needed, and this requires objective quantification of their features that can be applied in translational models, such as animal models and human inducible pluripotent stem cells (iPSC). In this review we summarize the evidence for dopaminergic alterations relevant to NS in translational animal models focusing on dysfunctional motivation, a core aspect of NS. Among others, experiments on mutant rodents with an overexpression of DA D2 or D3 receptors and the dopamine deficient mice are discussed. In the second part we summarize the findings from recent studies using iPSC to model the pathogenesis of SCZ. By retaining the genetic background of risk genetic variants, iPSC offer the possibility to study the effect of de novo mutations or inherited polymorphisms from subgroups of patients and their response to drugs, adding an important tool for personalized psychiatry. Given the key role of DA in NS, we focus on findings of iPSC-derived DA neurons. Since implementation of iPSC-derived neurons to study the neurobiology of SCZ is a relatively recent acquisition, the available data are limited. We highlight some methodological aspects of relevance in the interpretation of in vitro testing results, including limitations and strengths, offering a critical viewpoint for the implementation of future pharmacological studies aimed to the discovery and characterization of novel treatments for NS.

Dual-acting agents for improving cognition and real-world function in Alzheimer's disease: Focus on 5-HT6 and D3 receptors as hubs

To date, there are no interventions that impede the inexorable progression of Alzheimer's disease (AD), and currently-available drugs cholinesterase (AChE) inhibitors and the N-Methyl-d-Aspartate receptor antagonist, memantine, offer only modest symptomatic benefit. Moreover, a range of mechanistically-diverse agents (glutamatergic, histaminergic, monoaminergic, cholinergic) have disappointed in clinical trials, alone and/or in association with AChE inhibitors. This includes serotonin (5-HT) receptor-6 antagonists, despite compelling preclinical observations in rodents and primates suggesting a positive influence on cognition. The emphasis has so far been on high selectivity. However, for a multi-factorial disorder like idiopathic AD, 5-HT6 antagonists possessing additional pharmacological actions might be more effective, by analogy to "multi-target" antipsychotics. Based on this notion, drug discovery programmes have coupled 5-HT6 blockade to 5-HT4 agonism and inhibition of AchE. Further, combined 5-HT6/dopamine D3 receptor (D3) antagonists are of especial interest since D3 blockade mirrors 5-HT6 antagonism in exerting broad-based pro-cognitive properties in animals. Moreover, 5-HT6 and dopamine D3 antagonists promote neurocognition and social cognition via both distinctive and convergent actions expressed mainly in frontal cortex, including suppression of mTOR over-activation and reinforcement of cholinergic and glutamatergic transmission. In addition, 5-HT6 blockade affords potential anti-anxiety, anti-depressive and anti-epileptic properties, and antagonising 5-HT6 receptors may be associated with neuroprotective ("disease-modifying") properties. Finally D3 antagonism may counter psychotic episodes and D3 receptors themselves offer a promising hub for multi-target agents. The present article reviews the status of "R and D" into multi-target 5-HT6 and D3 ligands for improved treatment of AD and other neurodegenerative disorders of aging. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Neuroprotection: A versatile approach to combat glaucoma

In most retinal diseases, neuronal loss is the main cause of vision loss. Neuroprotection is the alteration of neurons and/or their environment to encourage the survival and function of the neurons, especially in environments that are deleterious to the neuronal health. The area of neuroprotection progresses with a therapeutically-based hope of improving vision and clinical outcomes for patients through the developments in neurotrophic therapy, antioxidative therapy, anti-excitotoxic, anti-ischemic, anti-inflammatory, and anti-apoptotic care. In this review, we summarize the various neuroprotection strategies for the treatment of glaucoma, genetics of glaucoma and the role of various nanoplatforms in the treatment of glaucoma.

Altered dopamine D3 receptor gene expression in MAM model of schizophrenia is reversed by peripubertal cannabidiol treatment

Gestational methylazoxymethanol acetate (MAM) treatment produces offspring with adult phenotype relevant to schizophrenia, including positive- and negative-like symptoms, cognitive deficits, dopaminergic dysfunction, structural and functional abnormalities. Here we show that adult rats prenatally treated with MAM at gestational day 17 display significant increase in dopamine D3 receptor (D3) mRNA expression in prefrontal cortex (PFC), hippocampus and nucleus accumbens, accompanied by increased expression of dopamine D2 receptor (D2) mRNA exclusively in the PFC. Furthermore, a significant change in the blood perfusion at the level of the circle of Willis and hippocampus, paralleled by the enlargement of lateral ventricles, was also detected by magnetic resonance imaging (MRI) techniques. Peripubertal treatment with the non-euphoric phytocannabinoid cannabidiol (30 mg/kg) from postnatal day (PND) 19 to PND 39 was able to reverse in MAM exposed rats: i) the up-regulation of the dopamine D3 receptor mRNA (only partially prevented by haloperidol 0.6 mg/kg/day); and ii) the regional blood flow changes in MAM exposed rats. Molecular modelling predicted that cannabidiol could bind preferentially to dopamine D3 receptor, where it may act as a partial agonist according to conformation of ionic-lock, which is highly conserved in GPCRs. In summary, our results demonstrate that the mRNA expression of both dopamine D2 and D3 receptors is altered in the MAM model; however only the transcript levels of D3 are affected by cannabidiol treatment, likely suggesting that this gene might not only contribute to the schizophrenia symptoms but also represent an unexplored target for the antipsychotic activity of cannabidiol.

Design and Synthesis of Bitopic 2-Phenylcyclopropylmethylamine (PCPMA) Derivatives as Selective Dopamine D3 Receptor Ligands

2-Phenylcyclopropylmethylamine (PCPMA) analogues have been reported as selective serotonin 2C agonists. On the basis of the same scaffold, we designed and synthesized a series of bitopic derivatives as dopamine D3R ligands. A number of these new compounds show a high binding affinity for D3R with excellent selectivity. Compound (1R,2R)-22e and its enantiomer (1S,2S)-22e show a comparable binding affinity for the D3R, but the former is a potent D3R agonist, while the latter acts as an antagonist. Molecular docking studies revealed different binding poses of the PCPMA moiety within the orthosteric binding pocket of the D3R, which might explain the different functional profiles of the enantiomers. Compound (1R,2R)-30q shows a high binding affinity for the D3R (K_i = 2.2 nM) along with good selectivity, as well as good bioavailability and brain penetration properties in mice. These results reveal that the PCPMA scaffold may serve as a privileged scaffold for the design of aminergic GPCR ligands.

Retinal biomarkers and pharmacological targets for Hermansky-Pudlak syndrome 7

Deletion of dystrobrevin binding protein 1 has been linked to Hermansky-Pudlak syndrome type 7 (HPS-7), a rare disease characterized by oculocutaneous albinism and retinal dysfunction. We studied dysbindin-1 null mutant mice (Dys^−/−) to shed light on retinal neurodevelopment defects in HPS-7. We analyzed the expression of a focused set of miRNAs in retina of wild type (WT), Dys^+/− and Dys^−/− mice. We also investigated the retinal function of these mice through electroretinography (ERG). We found that miR-101-3p, miR-137, miR-186-5p, miR-326, miR-382-5p and miR-876-5p were up-regulated in Dys^−/−mice retina. Dys^−/− mice showed significant increased b-wave in ERG, compared to WT mice. Bioinformatic analysis highlighted that dysregulated miRNAs target synaptic plasticity and dopaminergic signaling pathways, affecting retinal functions of Dys^−/− mice. Overall, the data indicate potential mechanisms in retinal neurodevelopment of Dys^−/− mice, which may have translational significance in HSP-7 patients, both in terms of diagnostic/prognostic biomarkers and novel pharmacological targets.

Artificial Intelligence to Identify Retinal Fundus Images, Quality Validation, Laterality Evaluation, Macular Degeneration, and Suspected Glaucoma

Purpose

To assess the performance of deep learning algorithms for different tasks in retinal fundus images: (1) detection of retinal fundus images versus optical coherence tomography (OCT) or other images, (2) evaluation of good quality retinal fundus images, (3) distinction between right eye (OD) and left eye (OS) retinal fundus images,(4) detection of age-related macular degeneration (AMD) and (5) detection of referable glaucomatous optic neuropathy (GON).

Patients and Methods

Five algorithms were designed. Retrospective study from a database of 306,302 images, Optretina’s tagged dataset. Three different ophthalmologists, all retinal specialists, classified all images. The dataset was split per patient in a training (80%) and testing (20%) splits. Three different CNN architectures were employed, two of which were custom designed to minimize the number of parameters with minimal impact on its accuracy. Main outcome measure was area under the curve (AUC) with accuracy, sensitivity and specificity.

Results

Determination of retinal fundus image had AUC of 0.979 with an accuracy of 96% (sensitivity 97.7%, specificity 92.4%). Determination of good quality retinal fundus image had AUC of 0.947, accuracy 91.8% (sensitivity 96.9%, specificity 81.8%). Algorithm for OD/OS had AUC 0.989, accuracy 97.4%. AMD had AUC of 0.936, accuracy 86.3% (sensitivity 90.2% specificity 82.5%), GON had AUC of 0.863, accuracy 80.2% (sensitivity 76.8%, specificity 83.8%).

Conclusion

Deep learning algorithms can differentiate a retinal fundus image from other images. Algorithms can evaluate the quality of an image, discriminate between right or left eye and detect the presence of AMD and GON with a high level of accuracy, sensitivity and specificity.

Dopamine D3 receptor: A neglected participant in Parkinson Disease pathogenesis and treatment?

Parkinson disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms which relentlessly and progressively lead to substantial disability and economic burden. Pathologically, these symptoms follow the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) associated with abnormal α-synuclein (α-Syn) deposition as cytoplasmic inclusions called Lewy bodies in pigmented brainstem nuclei, and in dystrophic neurons in striatal and cortical regions (Lewy neurites). Pharmacotherapy for PD focuses on improving quality of life and primarily targets dopaminergic pathways. Dopamine acts through two families of receptors, dopamine D1-like and dopamine D2-like; dopamine D3 receptors (D3R) belong to dopamine D2 receptor (D2R) family. Although D3R's precise role in the pathophysiology and treatment of PD has not been determined, we present evidence suggesting an important role for D3R in the early development and occurrence of PD. Agonist activation of D3R increases dopamine concentration, decreases α-Syn accumulation, enhances secretion of brain derived neurotrophic factors (BDNF), ameliorates neuroinflammation, alleviates oxidative stress, promotes neurogenesis in the nigrostriatal pathway, interacts with D1R to reduce PD associated motor symptoms and ameliorates side effects of levodopa (L-DOPA) treatment. Furthermore, D3R mutations can predict PD age of onset and prognosis of PD treatment. The role of D3R in PD merits further research. This review elucidates the potential role of D3R in PD pathogenesis and therapy.