Dopamine receptors: molecular biology, biochemistry and behavioural aspects

Topical review: Potential mechanisms of atropine for myopia control

SIGNIFICANCE

Atropine is effective at slowing myopia progression in children, but the mechanism of action by which it controls myopia remains unclear. This article is an evidenced-based review of potential receptor-based mechanisms by which atropine may act to slow the progression of myopia.The rising number of individuals with myopia worldwide and the association between myopia and vision-threatening ocular pathologies have made myopia control treatments one of the fastest growing areas of ophthalmic research. High-concentration atropine (1%) is the most effective treatment for slowing myopia progression to date; low concentrations of atropine (≤0.05%) appear partially effective and are currently being used to slow myopia progression in children. While significant progress has been made in the past few decades in understanding fundamental mechanisms by which atropine may control myopia, the precise characterization of how atropine works for myopia control remains incomplete. It is plausible that atropine slows myopia via its affinity to muscarinic receptors and influence on accommodation, but animal studies suggest that this is likely not the case. Other studies have shown that, in addition to muscarinic receptors, atropine can also bind, or affect the action of, dopamine, alpha-2-adrenergic, gamma-aminobutyric acid, and cytokine receptors in slowing myopia progression. This review summarizes atropine's effects on different receptor pathways of ocular tissues and discusses how these effects may or may not contribute to slowing myopia progression. Given the relatively broad array of receptor-based mechanisms implicated in atropine control of myopia, a single mode of action of atropine is unlikely; rather atropine may be exerting its myopia control effects directly or indirectly via several mechanisms at multiple levels of ocular tissues, all of which likely trigger the response in the same direction to inhibit eye growth and myopia progression.

Exploring the genetic contribution in obesity: An overview of dopaminergic system genes

Obesity is a widespread global health concern that affects a significant portion of the population and is associated with reduced quality of life, morbidity, and mortality. It is considered a pandemic, with its prevalence constantly rising in Western countries. As a result, numerous studies have focused on understanding the elements that contribute to obesity. Researchers have focused on neurotransmitters in the brain to develop weight management drugs that regulate food intake. This review explores the literature on genetic influences on dopaminergic processes to determine whether genetic variation has an association with obesity in reward-responsive regions, including mesolimbic efferent and mesocortical areas. Various neurotransmitters play an essential role in regulating food intake, such as dopamine which controls through mesolimbic circuits in the brain that modulate food reward. Appetite stimulation, including primary reinforcers such as food, leads to an increase in dopamine release in the reward centers of the brain. This release is related to motivation and reinforcement, which determines the motivational weighting of the reinforcer. Changes in dopamine expression can lead to hedonic eating behaviors and contribute to the development of obesity. Genetic polymorphisms have been investigated due to their potential role in modulating the risk of obesity and eating behaviors. Therefore, it is crucial to assess the impact of genetic alterations that disrupt this pathway on the obesity phenotype.

Aromatic linker variations in novel dopamine D2 and D3 receptor ligands

Dopamine D2-like receptors, especially D2 and D3 receptor subtypes, are important targets of antipsychotic agents. Many of these antipsychotics share an aliphatic linker element between a protonable amine group and an acyl-like moiety. Here, we have modified this aliphatic linker into phenylmethyl and phenylethyl linkers substituted in different positions. The design, synthesis, and in vitro evaluation of 18 dopamine D2 and D3 receptor ligands were performed in this study. Using a radioligand displacement assay, all ligands were found to have modest nanomolar affinity to D2R and D3R. N-(4-{2-[4-(2-Methoxyphenyl)piperazin-1-yl]ethyl}phenyl)acetamide (6c) demonstrates the highest D3R and D2R affinity values (pKi values of 7.83 [D2R] and 8.04 [D3R]), featuring a slight preference to D3R. This derivative can be taken as a reference structure for the development of a new class of D2R and D3R ligands.

Dopamine in the prefrontal cortex plays multiple roles in the executive function of patients with Parkinson's disease

Parkinson's disease can affect not only motor functions but also cognitive abilities, leading to cognitive impairment. One common issue in Parkinson's disease with cognitive dysfunction is the difficulty in executive functioning. Executive functions help us plan, organize, and control our actions based on our goals. The brain area responsible for executive functions is called the prefrontal cortex. It acts as the command center for the brain, especially when it comes to regulating executive functions. The role of the prefrontal cortex in cognitive processes is influenced by a chemical messenger called dopamine. However, little is known about how dopamine affects the cognitive functions of patients with Parkinson's disease. In this article, the authors review the latest research on this topic. They start by looking at how the dopaminergic system, is altered in Parkinson's disease with executive dysfunction. Then, they explore how these changes in dopamine impact the synaptic structure, electrical activity, and connection components of the prefrontal cortex. The authors also summarize the relationship between Parkinson's disease and dopamine-related cognitive issues. This information may offer valuable insights and directions for further research and improvement in the clinical treatment of cognitive impairment in Parkinson's disease.

A Comprehensive Review of Schizophrenia and Antipsychotic Metabolism as a Predictor of Treatment Response

Some patients with schizophrenia fail to respond to standard antipsychotics and are considered treatment-resistant. In these cases, clozapine is the only antipsychotic with proven efficacy, but its use is complicated by severe adverse effects, complex monitoring requirements, and non-response. Variation within the CYP450 enzymes CYP1A2, CYP2D6, CYP3A4, and CYP2C19 has been linked to the differential metabolism of antipsychotics. Testing for CYP450 single nucleotide polymorphisms may be a useful predictor of treatment resistance and could inform pharmacogenetic recommendations to identify potential treatment non-responders. Nonetheless, it remains uncertain whether differential antipsychotic metabolism is directly related to treatment efficacy. This comprehensive narrative review endeavours to delve into the molecular and genetic basis of schizophrenia, and discuss the current treatments available. In particular, we aim to examine the aetiology of treatment resistance in schizophrenia through available literature and discuss current challenges within the field.

InSpectro-Gadget: A Tool for Estimating Neurotransmitter and Neuromodulator Receptor Distributions for MRS Voxels

Magnetic resonance spectroscopy (MRS) is widely used to estimate concentrations of glutamate and γ -aminobutyric acid (GABA) in specific regions of the living human brain. As cytoarchitectural properties differ across the brain, interpreting these measurements can be assisted by having knowledge of such properties for the MRS region(s) studied. In particular, some knowledge of likely local neurotransmitter receptor patterns can potentially give insights into the mechanistic environment GABA- and glutamatergic neurons are functioning in. This may be of particular utility when comparing two or more regions, given that the receptor populations may differ substantially across them. At the same time, when studying MRS data from multiple participants or timepoints, the homogeneity of the sample becomes relevant, as measurements taken from areas with different cytoarchitecture may be difficult to compare. To provide insights into the likely cytoarchitectural environment of user-defined regions-of-interest, we produced an easy to use tool - InSpectro-Gadget - that interfaces with receptor mRNA expression information from the Allen Human Brain Atlas. This Python tool allows users to input masks and automatically obtain a graphical overview of the receptor population likely to be found within. This includes comparison between multiple masks or participants where relevant. The receptors and receptor subunit genes featured include GABA- and glutamatergic classes, along with a wide range of neuromodulators. The functionality of the tool is explained here and its use is demonstrated through a set of example analyses. The tool is available at https://github.com/lizmcmanus/Inspectro-Gadget .

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.

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.

Astrocytic insulin receptor controls circadian behavior via dopamine signaling in a sexually dimorphic manner

Mammalian circadian clocks respond to feeding and light cues, adjusting internal rhythms with day/night cycles. Astrocytes serve as circadian timekeepers, driving daily physiological rhythms; however, it's unknown how they ensure precise cycle-to-cycle rhythmicity. This is critical for understanding why mistimed or erratic feeding, as in shift work, disrupts circadian physiology- a condition linked to type 2 diabetes and obesity. Here, we show that astrocytic insulin signaling sets the free-running period of locomotor activity in female mice and food entrainment in male mice. Additionally, ablating the insulin receptor in hypothalamic astrocytes alters cyclic energy homeostasis differently in male and female mice. Remarkably, the mutants exhibit altered dopamine metabolism, and the pharmacological modulation of dopaminergic signaling partially restores distinct circadian traits in both male and female mutant mice. Our findings highlight the role of astrocytic insulin-dopaminergic signaling in conveying time-of-feeding or lighting cues to the astrocyte clock, thus governing circadian behavior in a sex-specific manner.

Bromocriptine monotherapy overcomes prostate cancer chemoresistance in preclinical models

Chemoresistance is a major obstacle in the clinical management of metastatic, castration-resistant prostate cancer (PCa). It is imperative to develop novel strategies to overcome chemoresistance and improve clinical outcomes in patients who have failed chemotherapy. Using a two-tier phenotypic screening platform, we identified bromocriptine mesylate as a potent and selective inhibitor of chemoresistant PCa cells. Bromocriptine effectively induced cell cycle arrest and activated apoptosis in chemoresistant PCa cells but not in chemoresponsive PCa cells. RNA-seq analyses revealed that bromocriptine affected a subset of genes implicated in the regulation of the cell cycle, DNA repair, and cell death. Interestingly, approximately one-third (50/157) of the differentially expressed genes affected by bromocriptine overlapped with known p53-p21- retinoblastoma protein (RB) target genes. At the protein level, bromocriptine increased the expression of dopamine D2 receptor (DRD2) and affected several classical and non-classical dopamine receptor signal pathways in chemoresistant PCa cells, including adenosine monophosphate-activated protein kinase (AMPK), p38 mitogen-activated protein kinase (p38 MAPK), nuclear factor kappa B  (NF-κB), enhancer of zeste homolog 2 (EZH2), and survivin. As a monotherapy, bromocriptine treatment at 15 mg/kg, three times per week, via the intraperitoneal route significantly inhibited the skeletal growth of chemoresistant C4-2B-TaxR xenografts in athymic nude mice. In summary, these results provided the first preclinical evidence that bromocriptine is a selective and effective inhibitor of chemoresistant PCa. Due to its favorable clinical safety profiles, bromocriptine could be rapidly tested in PCa patients and repurposed as a novel subtype-specific treatment to overcome chemoresistance.

Abnormally high expression of D1-like dopamine receptors on lupus CD4+ T cells promotes Tfh cell differentiation

OBJECTIVE

SLE is a chronic autoimmune disease that places a great burden on human society. T follicular helper (Tfh) cells play a critical role in the pathological process of SLE. Therefore, elucidating the mechanism of Tfh cell differentiation will contribute to SLE treatment. Dopamine receptors (DRDs) are members of the family of G protein-coupled receptors and are primarily divided into D1-like and D2-like receptors. Previous studies have found that DRDs can regulate differentiation of immune cells. However, there is currently a lack of research on DRDs and Tfh cells. We here explore the relationship between DRDs and Tfh cells, and analyse the relationship between DRD expression on Tfh cells and the course of SLE.

METHODS

We first detected plasma catecholamine concentrations in patients with SLE and healthy controls by mass spectrometry, followed by reverse transcription-quantitative PCR (RT-qPCR) to detect DRD messenger RNA (mRNA) expression in peripheral blood mononuclear cells (PBMCs) and CD4+ T cells, and flow cytometry to detect DRD expression in Tfh cells. Finally, in vitro experiments and RNA sequencing (RNA-seq) were used to explore the possible pathway by which DRDs regulate Tfh cell differentiation.

RESULTS

The plasma dopamine concentration in patients with SLE was significantly increased, and abnormal mRNA expression of DRDs was observed in both PBMCs and CD4+ T cells. The results of flow cytometry showed that D1-like receptors were highly expressed in Tfh cells of patients with SLE and associated with disease activity. In vitro induction experiments showed that differentiation of naïve T cells into Tfh cells was accompanied by an increase in D1-like receptor expression. RNA-seq and RT-qPCR results indicate that D1-like receptors might promote Tfh cell differentiation through the Phosphatidylinositol3-kinase (PI3K)/protein kinase B (AKT)/Forkhead box protein O1 (FOXO1)/Kruppel-like factor 2 (Klf2) pathway.

CONCLUSION

Tfh cells in patients with SLE highly express D1-like receptors, which correlate with disease activity. D1-like receptors may promote Tfh cell differentiation through the PI3K/AKT/FOXO1/Klf2 pathway.

Dopamine agonists for the treatment of pituitary tumours: From ergot extracts to next generation therapies

Dopamine agonists are a key tool in the therapeutic arsenal of endocrinologists worldwide. They exert their effects by binding to dopamine-2 (D2) receptors expressed by pituitary tumour cells to modulate hormonal secretion and tumour size. They are the established first-line treatment for prolactinomas which express high levels of D2 receptors. Growing data support their use as an adjuvant treatment option for other pituitary tumours including growth hormone, adrenocorticotrophic hormones, thyroid hormone secreting adenomas and nonfunctional pituitary tumours, all of which have been shown to express D2 receptors as well, albeit to varying extents. For those pituitary tumours inadequately treated by dopamine agonist alone, combined agonism of D2 and somatostatin receptors represent a new frontier in clinical development. Here we review the development and role of dopamine agonist for the treatment of prolactinomas, the literature supporting their adjuvant use for the treatment of all other pituitary tumours, and recent progress in the development of the next generation of chimeric compounds that target D2 and other receptor subtypes highly expressed on pituitary tumour cells.

Loss-of-Function Variants in DRD1 in Infantile Parkinsonism-Dystonia

The human dopaminergic system is vital for a broad range of neurological processes, including the control of voluntary movement. Here we report a proband presenting with clinical features of dopamine deficiency: severe infantile parkinsonism-dystonia, characterised by frequent oculogyric crises, dysautonomia and global neurodevelopmental impairment. CSF neurotransmitter analysis was unexpectedly normal. Triome whole-genome sequencing revealed a homozygous variant (c.110C>A, (p.T37K)) in DRD1, encoding the most abundant dopamine receptor (D1) in the central nervous system, most highly expressed in the striatum. This variant was absent from gnomAD, with a CADD score of 27.5. Using an in vitro heterologous expression system, we determined that DRD1-T37K results in loss of protein function. Structure-function modelling studies predicted reduced substrate binding, which was confirmed in vitro. Exposure of mutant protein to the selective D1 agonist Chloro APB resulted in significantly reduced cyclic AMP levels. Numerous D1 agonists failed to rescue the cellular defect, reflected clinically in the patient, who had no benefit from dopaminergic therapy. Our study identifies DRD1 as a new disease-associated gene, suggesting a crucial role for the D1 receptor in motor control.

Dopaminergic imaging in degenerative parkinsonisms, an established clinical diagnostic tool

Parkinson's disease (PD) and other neurodegenerative parkinsonisms are characterised by loss of striatal dopaminergic neurons. Dopamine functional deficits can be measured in vivo using positron emission tomography (PET) and single-photon emission computed tomography (SPECT) ligands assessing either presynaptic (e.g. dopamine synthesis and storage, transporter density) or postsynaptic terminals (i.e. D2 receptors availability). Nuclear medicine imaging thus helps the clinician to separate degenerative forms of parkinsonism with other neurological conditions, e.g. essential tremor or drug-induced parkinsonism. With the present study, we aimed at summarizing the current evidence about dopaminergic molecular imaging in the diagnostic evaluation of PD, atypical parkinsonian syndromes and dementia with Lewy bodies (DLB), as well as its potential to distinguish these conditions and to estimate disease progression. In fact, PET/SPECT methods are clinically validated and have been increasingly integrated into diagnostic guidelines (e.g. for PD and DLB). In addition, there is novel evidence on the classification properties of extrastriatal signal. Finally, dopamine imaging has an outstanding potential to detect neurodegeneration at the premotor stage, including REM-sleep behavior disorder and olfactory loss. Therefore, inclusion of subjects at an early stage for clinical trials can largely benefit from a validated in vivo biomarker such as presynaptic dopamine pathways PET/SPECT assessment.

Endocytosis of dopamine receptor: Signaling in brain

This chapter describes the physiological significance of dopamine receptor endocytosis and the consequence of the receptor signaling. Endocytosis of dopamine receptors is regulated by many components such as clathrin, β-arrestin, caveolin, and Rab family proteins. The dopamine receptors escape from lysosomal digestion, and their recycling occurs rapidly, reinforcing the dopaminergic signal transduction. In addition, the pathological impact of the receptors interacting with specific proteins has been the focus of much attention. Based on this background, this chapter provides an in-depth understanding of the mechanisms of molecules interacting with dopamine receptors and discusses the potential pharmacotherapeutic targets for α-synucleinopathies and neuropsychiatric disorders.

Therapeutic potential of dopamine agonists in the treatment of type 2 diabetes mellitus

Diabetes is a global health concern that has affected almost 415 million people globally. Bromocriptine is a dopamine D2 agonist, which is a Food and Drug Administration (FDA)-approved drug to treat type 2 diabetes mellitus (T2DM) patients. However, it is considered that a novel treatment therapy is required which can be used in the treatment of diabetes with or without other antidiabetic agents. Dopamine agonists are usually used in neurological disorders like Parkinson's disease (PD), restless leg syndrome, and hyperprolactinemia. However, dopamine agonists including bromocriptine and cabergoline are also effective in reducing the glycemic level in T2DM patients. Bromocriptine was formerly used for the treatment of PD, hyperprolactinemia, and restless leg syndrome, but now it is used for improving glycemic levels as well as reducing free fatty acids and triglycerides. In addition, cabergoline has been found to be effective in glycemic control, but this drug is yet to be approved by the FDA due to its limitations and lack of study. Findings of the clinical trials of bromocriptine have suggested that it reduces almost 0.4-0.8% glycated hemoglobin and cardiovascular risk by 40% in insulin-resistant patients. Moreover, the safe use of bromocriptine in obese T2DM patients makes it a more attractive option as it causes weight loss. Indeed, bromocriptine is a novel therapy for T2DM patients, as its mechanism of action is unique in T2DM patients with minimal adverse effects. This review summarizes the potential of dopamine agonists in the treatment of T2DM.

Systemic D1-R and D2-R antagonists in Non-Human Primates Differentially Impact Learning and Memory While Impairing Motivation and Motor Performance

Dopamine (DA) modulates cognition in part via differential activation of D1 and D2 receptors within the striatum and prefrontal cortex, yet evidence for cognitive impairments stemming from DA blockade or deficiency is inconsistent. Given the predominance of D1 over D2 receptors (R) in the prefrontal cortex of primates, D1-R blockade should more strongly influence frontal executive function (including working memory), while D2-R blockade should impair processes more strongly associated with the dorsal striatum (including cognitive flexibility, and learning). To test how systemic DA blockade disrupts cognition, we administered D1-R and D2-R like antagonists to healthy monkeys while they performed a series of cognitive tasks. Two selective DA receptor antagonist drugs (SCH-23390 hydrochloride: D1/D5-R antagonist; or Eticlopride hydrochloride: D2/D3-R antagonist) or placebo (0.9% saline) were systemically administered. Four tasks were used: (1) 'visually guided reaching', to test response time and accuracy, (2) 'reversal learning', to test association learning and attention, (3) 'self-ordered sequential search' to test spatial working memory, and (4) 'delayed match to sample' to test object working memory. Increased reach response times and decreased motivation to work for liquid reward was observed with both the D1/D5-R and D2/D3-R antagonists at the maximum dosages that still enabled task performance. The D2/D3-R antagonist impaired performance in the reversal learning task, while object and spatial working memory performance was not consistently affected in the tested tasks for either drug. These results are consistent with the theory that systemic D2/D3-R antagonists preferentially influence striatum processes (cognitive flexibility) while systemic D1/D5-R administration is less detrimental to frontal executive function.

Impact of the DRD2 Polymorphisms on the Effectiveness of the Training Program

Dopamine receptor D2 gene (DRD2) polymorphisms have been associated with cognitive abilities, obesity, addictions, and physical-activity-related behaviors, which may underlie differences in the effectiveness of training programs. What is not yet clear is the impact of DRD2 polymorphisms on the effectiveness of exercise programs. Thus, the aim of this study was to investigate the association between the DRD2 polymorphic sites (rs1076560, rs12364283, rs1799732, rs1800497, and rs1800498) and the body's response to regular physical activity. We studied genotypes and haplotypes distribution in a group of 165 females measured for body mass and body composition measurements, lipid profile, and glucose levels before and after realization of a 12-week training program. When tested individually, statistical analyses revealed one significant genotype by training interaction under the general model (for the basal metabolic rate, BMR, p = 0.033). Carriers of the rs1076560 CC genotype exhibited a decrease in BMR in response to training (p = 0.006). Haplotype analyses also showed that (i) the CACCC and CACTT haplotypes were associated with a post-training decrease in glucose level (β = -4.11, p = 0.032; β = -6.86, p = 0.020, respectively); (ii) the CGCCT with an increase in BMR (β = 0.65, p = 0.003) and fat free mass (FFM, β = 1.20, p = 0.009); (iii) the CA-CT with a decrease in low-density lipoprotein cholesterol (LDL, β = -17.26, p = 0.046). These results provide some evidence that the DRD2 polymorphisms may play a role in post-training changes in lipid and carbohydrate metabolism, and, as a consequence, in the effectiveness of training programs.

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.

Transcriptome Analysis Reveals miR-302a-3p Affects Granulosa Cell Proliferation by Targeting DRD1 in Chickens

Egg production is an important economic trait in laying chickens as higher yields bring higher profits. Small yellow follicle (SYFL) development is a key determinant of chicken reproductive performance; however, the majority of SYFLs are not selected during the process of chicken reproduction and thus, atresia occurs. Although there have been numerous omic studies focused on egg production, the molecular mechanisms involved are still not well-understood. In this study, we used high-throughput technology to analyze the differences between the SYFL mRNA transcriptomes of high– (H) and low–egg-yielding (L) Taihang layer hens, with the aim of identifying the potential candidate genes involved in controlling the rate of egg production. We constructed six cDNA libraries, three from H and three from L Taihang hens and then performed high-throughput sequencing. Comparison of the H and L groups showed 415 differentially expressed genes (DEGs). In the high-yield group, 226 were upregulated and 189 were downregulated. Differentially enriched biological functions and processes were identified using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) database analysis. Ten of the candidate DEGs we identified (DRD1, MC5R, PCK1, CTSA, TGFBR3, AGO4, SLIT2, RGS1, SCNN1B, and ZP3) have been identified in previous studies as being involved in the development of small yellow follicles. DRD1 was significantly enriched in the gap junction pathway, which is an important pathway in chicken granulosa cells (GCs) to pass nutrition to an oocyte. Homology analysis showed that DRD1 was highly conserved in numerous species, indicating that it may be a productive target for improving egg production. Evidence from bioinformatics analysis revealed that gga-miR-302a-3p putatively targets the 3′UTR region of DRD1. We then identified the functions of gga-miR-302a-3p in follicular granulosa cell proliferation by targeting DRD1. RT-qPCR analysis showed that DRD1 and miR-302a-3p expression were inversely related in the SYLs of high and low egg-yielding chickens. Luciferase assays showed that miR-302a-3p targets the 3′UTR of DRD1, and overexpression of miR-302a-3p significantly inhibits the expression of DRD1 in chicken GCs (p < 0.01). Functional experiments revealed that by targeting DRD1, miR-302a-3p acts as an inhibitor of GC proliferation. Taken together, we concluded that miR-302a-3p affects chicken GC proliferation by targeting DRD1. Our data expanded the knowledge base of genes whose functions are important in egg production and the molecular mechanisms of high-yield egg production in chicken small yellow follicles.

A test of context and sex-dependent dopaminergic effects on the behavior of a gregarious bird, the common waxbill Estrilda astrild

The Dopaminergic (DAergic) system has well known influences on behavioral and cognitive functions. Previous work with common waxbills (Estrilda astrild) reported context-specific DAergic effects that could have been due to social environment. Manipulating the dopamine D2-like receptor family (D2R) pathways had opposed effects on behavior depending on whether waxbills were tested alone or in a small cage with a mirror as social stimulus. Since waxbills are highly gregarious, it was hypothesized that being alone or perceiving to have a companion might explain this context-dependence. To test context-dependent DAergic effects, we compared behavioral effects of D2R manipulation in waxbills in the same familiar environment, but either alone or with a familiar, same-sex companion. We found that D2R agonism decreased movement and feeding, similarly to previous results when testing waxbills alone. However, contrary to the hypothesis of dependence on social context, we found that the behavioral effects of the D2R agonist were unchanged when waxbills were tested with a companion. The context-dependence reported earlier might thus be due to other factors, such as the stress of being in a novel environment (small cage) or with an unfamiliar social stimulus (mirror image). In tests with a companion, we also found a sex-specific social effect of D2R manipulation: D2R blocking tended to decrease aggression in males but to increase in females. Together with past work, our results suggest that DAergic effects on behavior involve different types of context- or sex-dependence.

Interactions of a boron-containing levodopa derivative on D2 dopamine receptor and its effects in a Parkinson disease model

Levodopa is a cornerstone in Parkinson's disease treatment. Beneficial effects are mainly by binding on D2 receptors. Docking simulations of a set of compounds including well-known D2-ligands and a pool of Boron-Containing Compounds (BCC), particularly boroxazolidones with a tri/tetra-coordinated boron atom, were performed on the D2 Dopamine receptor (D2DR). Theoretical results yielded higher affinity of the compound DPBX, a Dopaboroxazolidone, than levodopa on D2DR. Essential interactions with residues in the third and sixth transmembrane domains of the D2DR appear to be crucial to induce and stabilize interactions in the active receptor state. Results from a motor performance evaluation of a murine model of Parkinson's disease agree with theoretical results, as DPBX showed similar efficacy to that of levodopa for diminishing MPTP-induced parkinsonism. This beneficial effect was disrupted with prior Risperidone (D2DR antagonist) administration, supporting the role of D2DR in the biological effect of DPBX. In addition, DPBX limited neuronal loss in substantia nigra in a similar manner to that of levodopa administration.

α-Synuclein-induced dysregulation of neuronal activity contributes to murine dopamine neuron vulnerability

Pathophysiological damages and loss of function of dopamine neurons precede their demise and contribute to the early phases of Parkinson's disease. The presence of aberrant intracellular pathological inclusions of the protein α-synuclein within ventral midbrain dopaminergic neurons is one of the cardinal features of Parkinson's disease. We employed molecular biology, electrophysiology, and live-cell imaging to investigate how excessive α-synuclein expression alters multiple characteristics of dopaminergic neuronal dynamics and dopamine transmission in cultured dopamine neurons conditionally expressing GCaMP6f. We found that overexpression of α-synuclein in mouse (male and female) dopaminergic neurons altered neuronal firing properties, calcium dynamics, dopamine release, protein expression, and morphology. Moreover, prolonged exposure to the D2 receptor agonist, quinpirole, rescues many of the alterations induced by α-synuclein overexpression. These studies demonstrate that α-synuclein dysregulation of neuronal activity contributes to the vulnerability of dopaminergic neurons and that modulation of D2 receptor activity can ameliorate the pathophysiology. These findings provide mechanistic insights into the insidious changes in dopaminergic neuronal activity and neuronal loss that characterize Parkinson's disease progression with significant therapeutic implications.

Role of the Dopaminergic System in the Striatum and Its Association With Functional Recovery or Rehabilitation After Brain Injury

Disabilities are estimated to occur in approximately 2% of survivors of traumatic brain injury (TBI) worldwide, and disability may persist even decades after brain injury. Facilitation or modulation of functional recovery is an important goal of rehabilitation in all patients who survive severe TBI. However, this recovery tends to vary among patients because it is affected by the biological and physical characteristics of the patients; the types, doses, and application regimens of the drugs used; and clinical indications. In clinical practice, diverse dopaminergic drugs with various dosing and application procedures are used for TBI. Previous studies have shown that dopamine (DA) neurotransmission is disrupted following moderate to severe TBI and have reported beneficial effects of drugs that affect the dopaminergic system. However, the mechanisms of action of dopaminergic drugs have not been completely clarified, partly because dopaminergic receptor activation can lead to restoration of the pathway of the corticobasal ganglia after injury in brain structures with high densities of these receptors. This review aims to provide an overview of the functionality of the dopaminergic system in the striatum and its roles in functional recovery or rehabilitation after TBI.

In Parkinson's patient-derived dopamine neurons, the triplication of α-synuclein locus induces distinctive firing pattern by impeding D2 receptor autoinhibition

Pathophysiological changes in dopamine neurons precede their demise and contribute to the early phases of Parkinson's disease (PD). Intracellular pathological inclusions of the protein α-synuclein within dopaminergic neurons are a cardinal feature of PD, but the mechanisms by which α-synuclein contributes to dopaminergic neuron vulnerability remain unknown. The inaccessibility to diseased tissue has been a limitation in studying progression of pathophysiology prior to degeneration of dopamine neurons. To address these issues, we differentiated induced pluripotent stem cells (iPSCs) from a PD patient carrying the α-synuclein triplication mutation (AST) and an unaffected first-degree relative (NAS) into dopaminergic neurons. In human-like dopamine neurons α-synuclein overexpression reduced the functional availability of D2 receptors, resulting in a stark dysregulation in firing activity, dopamine release, and neuronal morphology. We back-translated these findings into primary mouse neurons overexpressing α-synuclein and found a similar phenotype, supporting the causal role for α-synuclein. Importantly, application of D2 receptor agonist, quinpirole, restored the altered firing activity of AST-derived dopaminergic neurons to normal levels. These results provide novel insights into the pre-degenerative pathophysiological neuro-phenotype induced by α-synuclein overexpression and introduce a potential mechanism for the long-established clinical efficacy of D2 receptor agonists in the treatment of PD.

The High Affinity Dopamine D2 Receptor Agonist MCL-536: A New Tool for Studying Dopaminergic Contribution to Neurological Disorders

The dopamine D₂ receptor exists in two different states, D₂^high and D_2low; the former is the functional form of the D₂ receptor and associates with intracellular G-proteins. The D₂ agonist [³H]MCL-536 has high affinity for the D₂ receptor (K_d 0.8 nM) and potently displaces the binding of (R-(-)-N-n-propylnorapomorphine (NPA; K_i 0.16 nM) and raclopride (K_i 0.9 nM) in competition binding assays. Here, we further characterize [³H]MCL-536. [³H]MCL-536 was metabolically stable, with about 75% of the compound remaining intact after 1 h incubation with human liver microsomes. Blood-brain barrier penetration in rats was good, attaining at 15 min a % injected dose per gram of wet tissue (%ID/g) of 0.28 in males versus 0.42 in females in the striatum. Specific uptake ratios ([%ID/g striatum]/[%ID/g cerebellum]) were stable in males during the first 60 min and in females up to 15-30 min. The D₂-rich striatum exhibited the highest uptake and slowest washout compared to D₂-poor cortex or cerebellum. In peripheral organs, uptake peaked at 15 min but declined to baseline at 60 min, indicating good clearance from the body. In vitro autoradiography on transaxial and coronal brain sections showed specific binding of [³H]MCL-536, which was abolished by preincubation with D₂/D₃ ligands sulpiride, NPA, and raclopride and in the presence of the stable GTP analogue guanylylimidodiphosphate. In amphetamine-sensitized animals, striatal binding was higher than in controls, indicating specificity for the D₂^high receptor state. [³H]MCL-536's unique properties make it a valuable tool for research on neurological disorders involving the dopaminergic system like Parkinson's disease or schizophrenia.

Dopamine D1 Receptor in Cancer

Simple Summary

Circulating hormones and their specific receptors play a significant role in the development and progression of various cancers. This review aimed to summarize current knowledge about the dopamine D1 receptor’s biological role in different cancers, including breast cancer, central nervous system tumors, lymphoproliferative disorders, and other neoplasms. Treatment with dopamine D1 receptor agonists was proven to exert a major anti-cancer effect in many preclinical models. We highlight this receptor’s potential as a target for the adjunct therapy of tumors and discuss possibilities and necessities for further research in this area.

Abstract

Dopamine is a biologically active compound belonging to catecholamines. It plays its roles in the human body, acting both as a circulating hormone and neurotransmitter. It acts through G-protein-coupled receptors divided into two subgroups: D1-like receptors (D1R and D5R) and D2-like receptors (D2R, D3R, D4R). Physiologically, dopamine receptors are involved in central nervous system functions: motivation or cognition, and peripheral actions such as blood pressure and immune response modulation. Increasing evidence indicates that the dopamine D1 receptor may play a significant role in developing different human neoplasms. This receptor’s value was presented in the context of regulating various signaling pathways important in tumor development, including neoplastic cell proliferation, apoptosis, autophagy, migration, invasiveness, or the enrichment of cancer stem cells population. Recent studies proved that its activation by selective or non-selective agonists is associated with significant tumor growth suppression, metastases prevention, and tumor microvasculature maturation. It may also exert a synergistic anti-cancer effect when combined with tyrosine kinase inhibitors or temozolomide. This review provides a comprehensive insight into the heterogeneity of dopamine D1 receptor molecular roles and signaling pathways in human neoplasm development and discusses possible perspectives of its therapeutic targeting as an adjunct anti-cancer strategy of treatment. We highlight the priorities for further directions in this research area.

Critical Impact of Different Conserved Endoplasmic Retention Motifs and Dopamine Receptor Interacting Proteins (DRIPs) on Intracellular Localization and Trafficking of the D2 Dopamine Receptor (D2-R) Isoforms

The type 2 dopamine receptor D₂ (D₂-R), member of the G protein-coupled receptor (GPCR) superfamily, exists in two isoforms, short (D_2S-R) and long (D_2L-R). They differ by an additional 29 amino acids (AA) in the third cytoplasmic loop (ICL3) of the D_2L-R. These isoforms differ in their intracellular localization and trafficking functionality, as D_2L-R possesses a larger intracellular pool, mostly in the endoplasmic reticulum (ER). This review focuses on the evolutionarily conserved motifs in the ICL3 of the D₂-R and proteins interacting with the ICL3 of both isoforms, specifically with the 29 AA insert. These motifs might be involved in D₂-R exit from the ER and have an impact on cell-surface and intracellular localization and, therefore, also play a role in the function of dopamine receptor signaling, ligand binding and possible homo/heterodimerization. Our recent bioinformatic data on potential new interaction partners for the ICL3 of D₂-Rs are also presented. Both are highly relevant, and have clinical impacts on the pathophysiology of several diseases such as Parkinson’s disease, schizophrenia, Tourette’s syndrome, Huntington’s disease, manic depression, and others, as they are connected to a variety of essential motifs and differences in communication with interaction partners.

First identification of dopamine receptors in pikeperch, Sander lucioperca, during the pre-ovulatory period

Dopamine (DA) is a ubiquitous neurotransmitter exerting a range of pleiotropic actions through two DA receptor families, the D1 and the D2. To date in vertebrates, a maximum of four receptor subtypes have been identified within the D1 family, D₁ (former D_1A), D₅ (former D_1B), D₆ (former D_1C and D_1D) and D₇ (former D_1E), while the D2 family encloses five subtypes, D₂, D₃, D₄, D₈ (former D_2like or D_2l) and D₉ (former D_{4-related sequence} or D_4-rs). In teleosts, no study has investigated in parallel all the DA receptors to identify and localize the whole receptor repertoire from both families. In pikeperch, Sander lucioperca, a species of interest for aquaculture development, the existence, number and location of the DA receptors are totally unknown. To address these questions, RNA-seq with de novo transcriptome reconstruction, functional annotation and phylogenetic analysis were performed to characterize the transcript repertoire of DA receptors in the brain of female pikeperch at the pre-ovulatory period. Ten different cDNA were identified and showed to belong to the D1 family: two D₁, one D_5a, one D_6a and one D_6b and to the D2 family: two spliced variants of D₂, one D₃, one D₈ and one D_9. Unlike zebrafish, the subtypes D₄ and D₇ have not yet been isolated in pikeperch. As expected D₁, D₃, D₈ and D₉ are mostly expressed in brain parts except for the cerebellum (D₁ and D₃). The inter-species differences in the number of DA receptors and the inter-organ differences in the gene expression of all receptors support the complexity of the dopaminergic actions in vertebrate.

Pathomechanisms of Non-Traumatic Acute Brain Injury in Critically Ill Patients

Delirium, an acute alteration in mental status characterized by confusion, inattention and a fluctuating level of arousal, is a common problem in critically ill patients. Delirium prolongs hospital stay and is associated with higher mortality. The pathophysiology of delirium has not been fully elucidated. Neuroinflammation and neurotransmitter imbalance seem to be the most important factors for delirium development. In this review, we present the most important pathomechanisms of delirium in critically ill patients, such as neuroinflammation, neurotransmitter imbalance, hypoxia and hyperoxia, tryptophan pathway disorders, and gut microbiota imbalance. A thorough understanding of delirium pathomechanisms is essential for effective prevention and treatment of this underestimated pathology in critically ill patients.

Parsing out the role of dopamine D4 receptor gene (DRD4) on alcohol-related phenotypes: A meta-analysis and systematic review

Genetics account for moderate variation of individual differences in developing alcohol use disorder (AUD), but it is unclear which genetic variations contribute to AUD risk. One candidate gene investigated due to its association with AUD is the dopamine D4 receptor gene (DRD4), which contains a 48-base pair variable number tandem repeat (VNTR) in exon 3 of its coding region. To date, no quantitative synthesis of the published literature on the effects of DRD4 VNTR variation on alcohol-related phenotypes has been conducted. MEDLINE, Embase, Web of Science, and PsycInfo were searched for studies that reported on alcohol craving, alcohol consumption, severity of AUD, and case-control (AUD versus no diagnosis of AUD) studies in DRD4L (seven repeats or more) carriers compared with DRD4S (six repeats or less) homozygotes. Random-effects meta-analysis was used for all analyses. A pooled sample size of 655 to 13,360 of 28 studies were included. Compared with DRD4S homozygotes, DRD4L carriers had increased number of drinking days (SMD: 0.205; 95% CI: 0.008 to 0.402), binge drinking days (SMD: 0.217; 95% CI: 0.0532 to 0.380), and severity of AUD (SMD: 0.143; 95% CI: 0.028 to 0.259). There was no difference between DRD4 VNTR genotypes on drinks per drinking day, largest number of drinks per day/occasion, and case-control analysis. It was not possible to conduct a meta-analysis of the craving data, but a systematic review of this literature found mixed results on DRD4 VNTR genotype effect. The present meta-analysis suggests DRD4 VNTR variation may be a risk factor for problematic alcohol use. Our findings are limited, however, by the absence of ancestry data from studies included in our analysis, precluding our ability to adjust for population stratification. Due to the likelihood of type I error in candidate gene approaches, our work highlights the critical need for studies with larger and more inclusive samples that account for sex and genetic ancestry to fully understand this relationship.

Circulating Triglycerides Gate Dopamine-Associated Behaviors through DRD2-Expressing Neurons

Energy-dense food alters dopaminergic (DA) transmission in the mesocorticolimbic (MCL) system and can promote reward dysfunctions, compulsive feeding, and weight gain. Yet the mechanisms by which nutrients influence the MCL circuitry remain elusive. Here, we show that nutritional triglycerides (TGs), a conserved post-prandial metabolic signature among mammals, can be metabolized within the MCL system and modulate DA-associated behaviors by gating the activity of dopamine receptor subtype 2 (DRD2)-expressing neurons through a mechanism that involves the action of the lipoprotein lipase (LPL). Further, we show that in humans, post-prandial TG excursions modulate brain responses to food cues in individuals carrying a genetic risk for reduced DRD2 signaling. Collectively, these findings unveil a novel mechanism by which dietary TGs directly alter signaling in the reward circuit to regulate behavior, thereby providing a new mechanistic basis by which energy-rich diets may lead to (mal)adaptations in DA signaling that underlie reward deficit and compulsive behavior.

Effects of Aberrant miR-384-5p Expression on Learning and Memory in a Rat Model of Attention Deficit Hyperactivity Disorder

Attention deficit hyperactivity disorder (ADHD) is a common neuropsychiatric disorder characterized by inattention, hyperactivity, and impulsivity. It may be accompanied by learning difficulties and working memory deficits. Few studies have examined the role of miRNAs in cognitive dysfunction in ADHD. This study investigated the effects of aberrant miR-384-5p expression on learning and memory in a widely used ADHD rat model. Lentiviral vectors were injected into the lateral ventricles of the rats to increase or decrease miR-384-5p level. To determine whether aberrant miR-384-5p expression affects learning and memory, spontaneous activity and cognitive function were assessed with the open field and Morris water maze tests. In the place navigation experiment of the Morris water maze test, time, and total swimming distance to reach the platform decreased compared to the control group when miR-384-5p was overexpressed, whereas down-regulation of miR-384-5p had the opposite effect. There were no obvious changes in brain tissue morphology following miR-384-5p overexpression or inhibition; however, dopamine (DA) receptor D1 (DRD1) level has decreased and increased, respectively, in the prefrontal cortex (PFC). The luciferase activity of the wild-type DRD1 group has decreased in luciferase reporter assay. Cyclic AMP response element-binding protein (CREB) phosphorylation has increased, and DA transporter (DAT) level has decreased in the PFC of spontaneously hypertensive rats (SHR) by miR-384-5p overexpression. On the other hand, miR-384-5p suppression increased DRD1 and decreased DAT and CREB protein levels relative to control rats. These findings suggest that miR-384-5p may play a critical role in learning and memory impairment in ADHD.

Advances in Dopamine D1 Receptor Ligands for Neurotherapeutics

The dopamine D1 receptor (D1R) is essential for neurotransmission in various brain pathways where it modulates key functions including voluntary movement, memory, attention and reward. Not surprisingly, the D1R has been validated as a promising drug target for over 40 years and selective activation of this receptor may provide novel neurotherapeutics for neurodegenerative and neuropsychiatric disorders. Several pharmacokinetic challenges with previously identified small molecule D1R agonists have been recently overcome with the discovery and advancement of new ligands, including drug-like non-catechol D1R agonists and positive allosteric modulators. From this, several novel molecules and mechanisms have recently entered clinical studies. Here we review the major classes of D1R selective ligands including antagonists, orthosteric agonists, non-catechol biased agonists and positive allosteric modulators, highlighting their structure-activity relationships and medicinal chemistry. Recent chemistry breakthroughs and innovative approaches to selectively target and activate the D1R also hold promise for creating pharmacotherapy for several neurological diseases.

Dopamine receptor antagonists effects on low-dimensional attractors of local field potentials in optogenetic mice

The goal of this study was to investigate the effects of acute cocaine injection or dopamine (DA) receptor antagonists on the medial prefrontal cortex (mPFC) gamma oscillations and their relationship to short term neuroadaptation that may mediate addiction. For this purpose, optogenetically evoked local field potentials (LFPs) in response to a brief 10 ms laser light pulse were recorded from 17 mice. D1-like receptor antagonist SCH 23390 or D2-like receptor antagonist sulpiride, or both, were administered either before or after cocaine. A Euclidian distance-based dendrogram classifier separated the 100 trials for each animal in disjoint clusters. When baseline and DA receptor antagonists trials were combined in a single trial, a minimum of 20% overlap occurred in some dendrogram clusters, which suggests a possible common, invariant, dynamic mechanism shared by both baseline and DA receptor antagonists data. The delay-embedding method of neural activity reconstruction was performed using the correlation time and mutual information to determine the lag/correlation time of LFPs and false nearest neighbors to determine the embedding dimension. We found that DA receptor antagonists applied before cocaine cancels out the effect of cocaine and leaves the lag time distributions at baseline values. On the other hand, cocaine applied after DA receptor antagonists shifts the lag time distributions to longer durations, i.e. increase the correlation time of LFPs. Fourier analysis showed that a reasonable accurate decomposition of the LFP data can be obtained with a relatively small (less than ten) Fourier coefficients.

Synthesis and Pharmacological Evaluation of Noncatechol G Protein Biased and Unbiased Dopamine D1 Receptor Agonists

Noncatechol heterocycles have recently been discovered as potent and selective G protein biased dopamine 1 receptor (D1R) agonists with superior pharmacokinetic properties. To determine the structure-activity relationships centered on G protein or β-arrestin signaling bias, systematic medicinal chemistry was employed around three aromatic pharmacophores of the lead compound 5 (PF2334), generating a series of new molecules that were evaluated at both D1R G_s-dependent cAMP signaling and β-arrestin recruitment in HEK293 cells. Here, we report the chemical synthesis, pharmacological evaluation, and molecular docking studies leading to the identification of two novel noncatechol D1R agonists that are a subnanomolar potent unbiased ligand 19 (PW0441) and a nanomolar potent complete G protein biased ligand 24 (PW0464), respectively. These novel D1R agonists provide important tools to study D1R activation and signaling bias in both health and disease.

Dopamine and Cognitive Control in Prefrontal Cortex

Cognitive control, the ability to orchestrate behavior in accord with our goals, depends on the prefrontal cortex. These cognitive functions are heavily influenced by the neuromodulator dopamine. We review here recent insights exploring the influence of dopamine on neuronal response properties in prefrontal cortex (PFC) during ongoing behaviors in primates. This review suggests three major computational roles of dopamine in cognitive control: (i) gating sensory input, (ii) maintaining and manipulating working memory contents, and (iii) relaying motor commands. For each of these roles, we propose a neuronal microcircuit based on known mechanisms of action of dopamine in PFC, which are corroborated by computational network models. This conceptual approach accounts for the various roles of dopamine in prefrontal executive functioning.

New Dopamine D3-Selective Receptor Ligands Containing a 6-Methoxy-1,2,3,4-tetrahydroisoquinolin-7-ol Motif

A series of analogues featuring a 6-methoxy-1,2,3,4-tetrahydroisoquinolin-7-ol unit as the arylamine "head" group of a classical D3 antagonist core structure were synthesized and evaluated for affinity at dopamine D1, D2, and D3 receptors (D1R, D2R, D3R). The compounds generally displayed strong affinity for D3R with very good D3R selectivity. Docking studies at D2R and D3R crystal structures revealed that the molecules are oriented such that their arylamine units are positioned in the orthosteric binding pocket of D3R, with the arylamide "tail" units residing in the secondary binding pocket. Hydrogen bonding between Ser 182 and Tyr 365 at D3R stabilize extracellular loop 2 (ECL2), which in turn contributes to ligand binding by interacting with the "tail" units of the ligands in the secondary binding pocket. Similar interactions between ECL2 and the "tail" units were absent at D2R due to different positioning of the D2R loop region. The presence of multiple H-bonds with the phenol moiety of the headgroup of 7 and Ser192 accounts for its stronger D3R affinity as compared to the 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline-containing analogue 8.

Modulatory effect of dopamine receptor 5 on the neurosecretory Dahlgren cells of the olive flounder, Paralichthys olivaceus

A neuromodulatory role for dopamine has been reported for magnocellular neuroendocrine cells in the mammalian hypothalamus. We examined its potential role as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) of the euryhaline flounder Paralichthys olivaceus. In vitro application of dopamine (DA) caused an increase in electrical activity (firing frequency, recorded extracellularly) of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The dopamine precursor, levodopa (L-DOPA), also increased firing frequency, cell recruitment and enhanced bursting and tonic activity. The effect of dopamine was blocked by the D1, D5 receptor antagonist SCH23390, but not by the D2, D3, D4 receptor antagonist amisulpride. Transcriptome sequencing revealed that all DA receptors (D1, D2, D3, D4, and D5) were present in the flounder CNSS. However, quantitative RT-PCR revealed that D5 receptor mRNA expression was significantly increased in the CNSS following dopamine superfusion. These findings suggest that dopamine may modulate CNSS activity in vivo, and therefore neurosecretory output, through D5 receptors.

New Dopamine D2 Receptor Agonist, [3H]MCL-536, for Detecting Dopamine D2high Receptors in Vivo

Increases in the D2 receptor high affinity state are associated with certain neurological disorders. We synthesized and characterized the high-affinity D2high ligand [³H]MCL-536 in competition binding against the D2/3 agonist R-(-)- N- n-propylnorapomorphine (NPA) and the D2/3 antagonist raclopride. The total binding of [³H]MCL-536 (minus that in the presence of 100 nM NPA) was measured by saturation binding in CHO cells expressing human D2long; the data yielded separable, nonsaturable nonspecific, and saturable specific components. The former represents an aporphine site common to NPA and [³H]MCL-536. The latter indicated specific binding to the total D2 receptors (both high and low-affinity states). [³H]MCL-536 had a K_d of 0.8 nM. In competition binding, NPA had a K_i of 0.16 nM, and raclopride had a K_i of 0.9 nM. Co-incubation with guanylylimidodiphosphate abolished binding to D2high. This unique profile makes radiolabeled MCL-536 a versatile tool for diagnostics and therapeutics, and may quantify D2high sites in schizophrenia and PD patients in vivo.

Physiological and Functional Basis of Dopamine Receptors and Their Role in Neurogenesis: Possible Implication for Parkinson's disease

Dopamine controls various physiological functions in the brain and periphery by acting on its receptors D1, D2, D3, D4, and D5. Dopamine receptors are G protein–coupled receptors involved in the regulation of motor activity and several neurological disorders such as schizophrenia, bipolar disorder, Parkinson’s disease (PD), Alzheimer’s disease, and attention-deficit/hyperactivity disorder. Reduction in dopamine content in the nigrostriatal pathway is associated with the development of PD, along with the degeneration of dopaminergic neurons in the substantia nigra region. Dopamine receptors directly regulate neurotransmission of other neurotransmitters, release of cyclic adenosine monophosphate, cell proliferation, and differentiation. Here, we provide an update on recent knowledge about the signalling mechanism, mode of action, and the evidence for the physiological and functional basis of dopamine receptors. We also highlight the pivotal role of these receptors in the modulation of neurogenesis, a possible therapeutic target that might help to slow down the process of neurodegeneration.

Dopamine D2-like receptors (DRD2 and DRD4) in chickens: Tissue distribution, functional analysis, and their involvement in dopamine inhibition of pituitary prolactin expression

Dopamine (DA) D2-like (and D1-like) receptors are suggested to mediate the dopamine actions in the anterior pituitary and/or CNS of birds. However, the information regarding the structure, functionality, and expression of avian D2-like receptors have not been fully characterized. In this study, we cloned two D2-like receptors (cDRD2, cDRD4) from chicken brain using RACE PCR. The cloned cDRD4 is a 378-amino acid receptor, which shows 57% amino acid (a.a.) identity with mouse DRD4. As in mammals, two cDRD2 isoforms, cDRD2L (long isoform, 437 a.a.) and cDRD2S (short isoform, 408 a.a.), which differ in their third intracellular loop, were identified in chickens. Using cell-based luciferase reporter assays or Western blot, we demonstrated that cDRD4, cDRD2L and cDRD2S could be activated by dopamine and quinpirole (a D2-like receptor agonist) dose-dependently, and their activation inhibits cAMP signaling pathway and stimulates MAPK/ERK signaling cascade, indicating that they are functional receptors capable of mediating dopamine actions. Quantitative real-time PCR revealed that cDRD2 and cDRD4 are widely expressed in chicken tissues with abundant expression noted in anterior pituitary, and their expressions are likely controlled by their promoters near exon 1, as demonstrated by dual-luciferase reporter assays in DF-1 cells. In accordance with cDRD2/cDRD4 expression in the pituitary, DA or quinpirole could partially inhibit vasoactive intestinal peptide-induced prolactin expression in cultured chick pituitary cells. Together, our data proves the functionality of DRD2 and DRD4 in birds and aids to uncover the conserved roles of DA/D2-like receptor system in vertebrates, such as its action on the pituitary.

Association of genetic variations in the serotonin and dopamine systems with aggressive behavior in the Chinese adolescent population: Single- and multiple-risk genetic variants

BACKGROUND

Genetic predisposition is an important factor leading to aggressive behavior. However, the relationship between genetic polymorphisms and aggressive behavior has not been elucidated.

METHODS

We identified candidate genes located in the dopaminergic and serotonin system (DRD3, DRD4, and FEV) that had been previously reported to be associated with aggressive behavior. We investigated 14 tag single-nucleotide polymorphisms (SNPs) using a multi-analytic strategy combining logistic regression (LR) and classification and regression tree (CART) to explore higher-order interactions between these SNPs and aggressive behavior in 318 patients and 558 controls.

RESULTS

Both LR and CART analyses suggested that the rs16859448 polymorphism is the strongest individual factor associated with aggressive behavior risk. In CART analysis, individuals carrying the combined genotypes of rs16859448TT/GT-rs11246228CT/TT-rs3773679TT had the highest risk, while rs16859448GG-rs2134655CT had the lowest risk (OR = 5.25, 95% CI: 2.53-10.86).

CONCLUSION

This study adds to the growing evidence on the association of single- and multiple-risk variants in DRD3, DRD4, and FEV with aggressive behavior in Chinese adolescents. However, the aggressive behavior scale used to diagnose aggression in this study did not account for comorbid conditions; therefore, further studies are needed to confirm our observations.

Mesocorticolimbic hemodynamic response in Parkinson's disease patients with compulsive behaviors

BACKGROUND

PD patients treated with dopamine therapy can develop maladaptive impulsive and compulsive behaviors, manifesting as repetitive participation in reward-driven activities. This behavioral phenotype implicates aberrant mesocorticolimbic network function, a concept supported by past literature. However, no study has investigated the acute hemodynamic response to dopamine agonists in this subpopulation.

OBJECTIVES

We tested the hypothesis that dopamine agonists differentially alter mesocortical and mesolimbic network activity in patients with impulsive-compulsive behaviors.

METHODS

Dopamine agonist effects on neuronal metabolism were quantified using arterial-spin-labeling MRI measures of cerebral blood flow in the on-dopamine agonist and off-dopamine states. The within-subject design included 34 PD patients, 17 with active impulsive compulsive behavior symptoms, matched for age, sex, disease duration, and PD severity.

RESULTS

Patients with impulsive-compulsive behaviors have a significant increase in ventral striatal cerebral blood flow in response to dopamine agonists. Across all patients, ventral striatal cerebral blood flow on-dopamine agonist is significantly correlated with impulsive-compulsive behavior severity (Questionnaire for Impulsive Compulsive Disorders in Parkinson's Disease- Rating Scale). Voxel-wise analysis of dopamine agonist-induced cerebral blood flow revealed group differences in mesocortical (ventromedial prefrontal cortex; insular cortex), mesolimbic (ventral striatum), and midbrain (SN; periaqueductal gray) regions.

CONCLUSIONS

These results indicate that dopamine agonist therapy can augment mesocorticolimbic and striato-nigro-striatal network activity in patients susceptible to impulsive-compulsive behaviors. Our findings reinforce a wider literature linking studies of maladaptive behaviors to mesocorticolimbic networks and extend our understanding of biological mechanisms of impulsive compulsive behaviors in PD. © 2017 International Parkinson and Movement Disorder Society.