Dopamine Receptor Ligands Research Articles

Discovery of Selective Dopamine Receptor Ligands Derived from (-)-Stepholidine via C-3 Alkoxylation and C-3/C-9 Dialkoxylation.

We evaluated C-3 alkoxylated and C-3/C-9 dialkoxylated (-)-stepholidine analogues to probe the tolerance at the C-3 and C-9 positions of the tetrahydroprotoberberine (THPB) template toward affinity for dopamine receptors. A C-9 ethoxyl substituent appears optimal for D1R affinity since high D1R affinities were observed for compounds that contain an ethyl group at C-9, with larger C-9 substituents tending to decrease D1R affinity. A number of novel ligands were identified, such as compounds 12a and 12b, with nanomolar affinities for D1R and no affinity for either D2R or D3R, with compound 12a being identified as a D1R antagonist for both G-protein- and β-arrestin-based signaling. Compound 23b was identified as the most potent and selective D3R ligand containing a THPB template to date and functions as an antagonist for both G-protein- and β-arrestin-based signaling. Molecular docking and molecular dynamics studies validated the D1R and D3R affinity and selectivity of 12a, 12b, and 23b.

Novel multi-target ligands of dopamine and serotonin receptors for the treatment of schizophrenia based on indazole and piperazine scaffolds–synthesis, biological activity, and structural evaluation

Schizophrenia is a chronic mental disorder that is not satisfactorily treated with available antipsychotics. The presented study focuses on the search for new antipsychotics by optimising the compound D2AAK3, a multi-target ligand of G-protein-coupled receptors (GPCRs), in particular D2, 5-HT1A, and 5-HT2A receptors. Such receptor profile may be beneficial for the treatment of schizophrenia. Compounds 1–16 were designed, synthesised, and subjected to further evaluation. Their affinities for the above-mentioned receptors were assessed in radioligand binding assays and efficacy towards them in functional assays. Compounds 1 and 10, selected based on their receptor profile, were subjected to in vivo tests to evaluate their antipsychotic activity, and effect on memory and anxiety processes. Molecular modelling was performed to investigate the interactions of the studied compounds with D2, 5-HT1A, and 5-HT2A receptors on the molecular level. Finally, X-ray study was conducted for compound 1, which revealed its stable conformation in the solid state.

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

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

Mechanisms Underlying Dopaminergic Regulation of Nicotine-Induced Kinetic Tremor.

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

Structure-Functional Selectivity Relationship Studies on A-86929 Analogs and Small Aryl Fragments toward the Discovery of Biased Dopamine D1 Receptor Agonists.

Dopamine regulates normal functions such as movement, reinforcement learning, and cognition, and its dysfunction has been implicated in multiple psychiatric and neurological disorders. Dopamine acts through D1- (D1R and D5R) and D2-class (D2R, D3R, and D4R) receptors and activates both G protein- and β-arrestin-dependent signaling pathways. Current dopamine receptor-based therapies are used to ameliorate motor deficits in Parkinson's disease or as antipsychotic medications for schizophrenia. These drugs show efficacy for ameliorating only some symptoms caused by dopamine dysfunction and are plagued by debilitating side effects. Studies in primates and rodents have shown that shifting the balance of dopamine receptor signaling toward the arrestin pathway can be beneficial for inducing normal movement, while reducing motor side effects such as dyskinesias, and can be efficacious at enhancing cognitive function compared to balanced agonists. Several structure-activity relationship (SAR) studies have embarked on discovering β-arrestin-biased dopamine agonists, focused on D2 partial agonists, noncatechol D1 agonists, and mixed D1/D2R dopamine receptor agonists. Here, we describe an SAR study to identify novel D1R β-arrestin-biased ligands using A-86929, a high-affinity D1R catechol agonist, as a core scaffold to identify chemical motifs responsible for β-arrestin-biased activity at both D1 and D2Rs. Most of the A-86929 analogs screened were G protein-biased, but none of them were exclusively arrestin-biased. Additionally, various small-fragment molecular probes displayed weak bias toward the β-arrestin pathway. Continued in-depth SFSR (structure-functional selectivity relationship) studies informed by structure determination, molecular modeling, and mutagenesis studies will facilitate the discovery of potent and efficacious arrestin-biased dopamine receptor ligands.

Dopamine and Dopamine-Related Ligands Can Bind Not Only to Dopamine Receptors.

The dopaminergic system is one of the most important neurotransmitter systems in the central nervous system (CNS). It acts mainly by activation of the D1-like receptor family at the target cell. Additionally, fine-tuning of the signal is achieved via pre-synaptic modulation by the D2-like receptor family. Some dopamine drugs (both agonists and antagonists) bind in addition to DRs also to α2-ARs and 5-HT receptors. Unfortunately, these compounds are often considered subtype(s) specific. Thus, it is important to consider the presence of these receptor subtypes in specific CNS areas as the function virtually elicited by one receptor type could be an effect of other—or the co-effect of multiple receptors. However, there are enough molecules with adequate specificity. In this review, we want to give an overview of the most common off-targets for established dopamine receptor ligands. To give an overall picture, we included a discussion on subtype selectivity. Molecules used as antipsychotic drugs are reviewed too. Therefore, we will summarize reported affinities and give an outline of molecules sufficiently specific for one or more subtypes (i.e., for subfamily), the presence of DR, α2-ARs, and 5-HT receptors in CNS areas, which could help avoid ambiguous results.

In silico Screening and Behavioral Validation of a Novel Peptide, LCGA-17, With Anxiolytic-Like Properties.

The aim of the study was to develop better anxiolytics and antidepressants. We focused on GABAA receptors and the α2δ auxiliary subunit of V-gated Ca2+ channels as putative targets because they are established as mediators of efficacious anxiolytics, antidepressants, and anticonvulsants. We further focused on short peptides as candidate ligands because of their high safety and tolerability profiles. We employed a structural bioinformatics approach to develop novel tetrapeptides with predicted affinity to GABAA receptors and α2δ. In silico docking studies of one of these peptides, LCGA-17, showed a high binding score for both GABAA receptors and α2δ, combined with anxiolytic-like properties in a Danio rerio behavioral screen. LCGA-17 showed anxiolytic-like effects in the novel tank test, the light–dark box, and the social preference test, with efficacy comparable to fluvoxamine and diazepam. In binding assays using rat brain membranes, [3H]-LCGA-17 was competed more effectively by gabapentinoid ligands of α2δ than ligands of GABAA receptors, suggesting that α2δ represents a likely target for LCGA-17. [3H]-LCGA-17 binding to brain lysates was unaffected by competition with ligands for GABAB, glutamate, dopamine, serotonin, and other receptors, suggesting specific interaction with α2δ. Dose-finding studies in mice using acute administration of LCGA-17 (i.p.) demonstrated anxiolytic-like effects in the open field test, elevated plus maze, and marble burying tests, as well as antidepressant-like properties in the forced swim test. The anxiolytic effects were effectively blocked by bicuculline. Therefore, LCGA-17 is a novel candidate anxiolytic and antidepressant that may act through α2δ, with possible synergism by GABAA receptors.

Measuring mania-like elevated mood through amphetamine-induced 50-kHz ultrasonic vocalizations in rats.

Rats emit 50-kHz ultrasonic vocalizations (USV) in appetitive situations, reflecting a positive affective state. Particularly high rates of 50-kHz USV are elicited by the psychostimulant d-amphetamine. Exaggerated 50-kHz USV emission evoked by d-amphetamine is modulated by dopamine, noradrenaline and 5-hydroxytyrptamine receptor ligands and inhibited by the mood stabilizer lithium, the gold standard anti-manic drug for treating bipolar disorder. This indicates that exaggerated 50-kHz USV emission can serve as a reliable and valid measure for assessing mania-like elevated mood in rats with sufficient translational power for gaining a better understanding of relevant pathophysiological mechanisms and the identification of new therapeutic targets. The improved capacity to study the effects of anti-manic pharmacological interventions on a broader range of behaviours by including exaggerated 50-kHz USV emission as preclinical outcome measure complementary to locomotor hyperactivity will refine rodent models for mania. LINKED ARTICLES: This article is part of a themed issue on New discoveries and perspectives in mental and pain disorders. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.17/issuetoc.

Synthesis, in silico, and in vitro studies of novel dopamine D2 and D3 receptor ligands.

Dopamine is an important neurotransmitter in the human brain and its altered concentrations can lead to various neurological diseases. We studied the binding of novel compounds at the dopamine D2 (D2 R) and D3 (D3 R) receptor subtypes, which belong to the D2 -like receptor family. The synthesis, in silico, and in vitro characterization of 10 dopamine receptor ligands were performed. Novel ligands were docked into the D2 R and D3 R crystal structures to examine the precise binding mode. A quantum mechanics/molecular mechanics study was performed to gain insights into the nature of the intermolecular interactions between the newly introduced pentafluorosulfanyl (SF5 ) moiety and D2 R and D3 R. A radioligand displacement assay determined that all of the ligands showed moderate-to-low nanomolar affinities at D2 R and D3 R, with a slight preference for D3 R, which was confirmed in the in silico studies. N-{4-[4-(2-Methoxyphenyl)piperazin-1-yl]butyl}-4-(pentafluoro-λ6-sulfanyl)benzamide (7i) showed the highest D3 R affinity and selectivity (pKi values of 7.14 [D2 R] and 8.42 [D3 R]).

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 (D2S-R) and long (D2L-R). They differ by an additional 29 amino acids (AA) in the third cytoplasmic loop (ICL3) of the D2L-R. These isoforms differ in their intracellular localization and trafficking functionality, as D2L-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.

Filtering out Low-Affinity Bitropic Ligands for Dopamine Receptors Based on Ligand Conformation.

We investigated the correlation between the conformations of a set of published 90 bitopic compounds on their affinity for two subtypes of the human dopamine receptor, D2R and D3R. Using molecular dynamics simulations, we showed that the compounds with large populations of compact conformation in the free solution are weak binders to both subtypes of the receptor. Our study provides a computational approach to quickly filter out low-affinity dopamine receptor ligands before their costly chemical synthesis.

Synthesis, in Silico and in Vitro Study on Phase I Metabolism of the Potent 5-Ht7/5-Ht1a/D2 Receptor Ligand: 4-Fluoron -(1-{2-[2-(Methylsulfanyl)- Phenoxy]Ethyl}Pyrrolidin-3-Yl) Benzene Sulfonamide

In the present study we analyze the synthesis as well as biotransformation pathways and metabolic stability of 4-fluoro- N -(1-{2-[2-(methylsulfanyl)phenoxy]ethyl}pyrrolidin-3-yl) benzene sulfonamide – thioether compound 5, with the affinity for several subtypes of the serotonin and dopamine receptor. Studies were conducted in vitro on liver microsomes from three species – mouse, rat, and human. The biotransformation was analyzed using liquid chromatography coupled with mass spectrometry. Two major metabolites (M1 and M2) – products of S-oxidation of the parent molecule – were generated both by in silico and in vitro methods. Use of three species allowed us to determine the interspecies differences in metabolic stability of tested compound. Two parameters, t0.5 and Clint , were calculated. Compound 5 was the most stable in human liver microsomes. Results are useful for understanding the interspecies differences in metabolism of the serotonin and dopamine receptor ligand associated with sulfur atom.

Application of Fragment Molecular Orbital Method to investigate dopamine receptors

GPCRs are a vast family of seven-domain transmembrane proteins. This family includes dopamine receptors (D1, D2, D3, D4, and D5), which mediate the variety of dopamine-controlled physiological functions in the brain and periphery. Ligands of dopamine receptors are used for managing several neuropsychiatric disorders, including bipolar disorder, schizophrenia, anxiety, and Parkinson’s disease. Recent studies have revealed that dopamine receptors could be part of multiple signaling cascades, rather than of a single signaling pathway. For these targets, a variety of experimental and computational drug design techniques are utilized. In this work, dopamine receptors D2, D3, and D4 were investigated using molecular dynamic method as well as computational ab initio Fragment Molecular Orbital method (FMO), which can reveal atomistic details about ligand binding. The results provided useful insights into the significances of amino acid residues in ligand binding sites. Moreover, similarities and differences between active-sites of three studied types of receptors were examined.

Benzyl Phenylsemicarbazides: A Chemistry-Driven Approach Leading to G Protein-Biased Dopamine D4 Receptor Agonists with High Subtype Selectivity.

Many subtype-selective dopamine receptor ligands developed for the D2-D4 family incorporate a 1-arylpiperazine-derived primary recognition motif, which is connected to a lipophilic moiety occupying an extended binding pocket (EBP) of the receptor via an aliphatic linker of variable lengths. The evaluation of a novel group of dopamine receptor ligands now showed that highly subtype-selective ligands [up to Ki(D4.4) = 0.25 nM, D2L/D4.4 = 320, D3/D4.4 = 710 for APH199 (17)] can be obtained by choosing a relatively large and conformationally flexible 1-benzyl-1-phenylsemicarbazide substructure to fill the EBP. The novel chemotype APH199 (17) was found to act as a full agonist at the D4 receptor showing significant bias toward G protein activation over β-arrestin recruitment in comparison to quinpirole.

Design, synthesis and biological evaluation of novel serotonin and dopamine receptor ligands being 6-bromohexyl saccharine derivatives.

Due to numerous side effects of current antidepressants, the search for new, safer bioactive compounds is still a valid research topic in medical chemistry. In our research we decided to synthesize and determine SAR for new hexyl arylpiperazines (LACPs) derivated with saccharin moiety. High biological activity has been explained using molecular modelling methods. The compounds obtained show high affinity for the 5-HT1A (compound 18, Ki = 4 nM - antagonist mode) and D2 (compound 15, Ki = 7 nM - antagonist mode) receptor, and in some cases also 5-HT7 receptor (compound 17, Ki = 20 nM). A preliminary ADME analysis showed that the compounds exhibit CNS drugability properties. We have proved that carbon-chain lengthening may have a beneficial effect on increasing the activity towards serotonin and dopamine receptors.

Talipexole variations as novel bitopic dopamine D2 and D3 receptor ligands

We linked 5,6,7,8-tetrahydro-4H-thiazoloazepine scaffolds with phenylpiperazine pharmacophores to generate bitopic dopamine receptor ligands.

Indole-substituted 2,4-diamino-5,8-dihydropyrido[2,3-d]pyrimidines from one-pot process and evaluation of their ability to bind dopamine receptors

A series of novel 7-indole substituted 2,4-diamino-5,8-dihydropyrido[2,3-d]pyrimidine analogous to the 2,4-diaminopteridine core were synthesized by the three-component one-pot cyclocondensation between 2,4,6-triaminopyrimidine, 3-(2-cyanoacetyl)indole and aromatic aldehydes. The reactions, which exhibited good performance, proceeded in EtOH using indium (III) chloride as catalyst under microwave irradiation, in short reaction times. On the basis of certain structural similarity of these compounds with known ligands of the D2 dopamine receptors (D2DR), the study of these compounds as possible ligands of dopamine D2 and D1 receptors was carried out. Three of them showed moderate affinity to D2-DR since the Ki D1/D2 ratio reached values of 40, 65 and 31 for compounds 4c, 4k and 4j, respectively. Finally, molecular modeling studies revealed stronger molecular interactions of such derivatives with the D2DR than with D1DR, what agrees with the experimental data, and gives an additional support to the observed selectivity to the D2DR.

Novel Therapeutics in Glaucoma Management.

Background: Glaucoma is a progressive optic neuropathy characterized by retinal ganglion cell death and altera-tions of visual field. Elevated intraocular pressure (IOP) is considered the main risk factor of glaucoma, even though other factors cannot be ruled out, such as epigenetic mechanisms.Objective: An overview of the ultimate promising experimental drugs to manage glaucoma has been provided.Results: In particular, we have focused on purinergic ligands, KATP channel activators, gases (nitric oxide, carbon monoxide and hydrogen sulfide), non-glucocorticoid steroidal compounds, neurotrophic factors, PI3K/Akt activators, citicoline, histone deacetylase inhibitors, cannabinoids, dopamine and serotonin receptors ligands, small interference RNA, and Rho kinase in-hibitors.Conclusions: The review has been also endowed of a brief chapter on last reports about potential neuroprotective benefits of anti-glaucoma drugs already present in the market.

Dopamine Receptor Subtypes Differentially Regulate Autophagy.

Some dopamine receptor subtypes were reported to participate in autophagy regulation, but their exact functions and mechanisms are still unclear. Here we found that dopamine receptors D2 and D3 (D2-like family) are positive regulators of autophagy, while dopamine receptors D1 and D5 (D1-like family) are negative regulators. Furthermore, dopamine and ammonia, the two reported endogenous ligands of dopamine receptors, both can induce dopamine receptor internalization and degradation. In addition, we found that AKT (protein kinase B)-mTOR (mechanistic target of rapamycin) and AMPK (AMP-activated protein kinase) pathways are involved in DRD3 (dopamine receptor D3) regulated autophagy. Moreover, autophagy machinery perturbation inhibited DRD3 degradation and increased DRD3 oligomer. Therefore, our study investigated the functions and mechanisms of dopamine receptors in autophagy regulation, which not only provides insights into better understanding of some dopamine receptor-related neurodegeneration diseases, but also sheds light on their potential treatment in combination with autophagy or mTOR pathway modulations.

Effects of apomorphine, a dopamine agonist, on Daphnia magna: Imaging of swimming track density as a novel tool in the assessment of swimming activity

Apomorphine (APO) is a non-selective agonist of dopamine receptor activating D2-like receptors. Although Daphnia has been used in neurotoxicology in toxicity testing, little is known on its behavioural and physiological responses to dopamine receptors ligands. Therefore, the aim of our study was to determine swimming behaviour (swimming track density, speed, turning activity) and physiological parameters such as heart rate, thoracic limb activity and post-abdominal claw movement frequency in daphnids exposed for 1, 2 and 4h to concentrations of 0.3, 3 and 30mg/L of APO. The results showed the most significant decrease of behavioural endpoints such as swimming track density, speed and degree of turning angles of daphnids exposed for 4h to the highest concentrations of APO. The study also showed that a decrease of thoracic limb activity was found after 2 and 4h but only at the highest concentration. Heart rate was not affected by APO which may be a result of a lack of signalling with dopamine receptors in the heart of Daphnia. Therefore, activity of this organ seems to be not a valuable physiological biomarker in the assessment of effects induced by dopamine receptor ligands. The study also showed that our new methodological approach, imaging of swimming track density may be a promising tool for studying the effects of neuroactive substances on locomotor system activity of Daphnia magna.