Dopamine receptor pharmacology

Dopamine D2-like receptors on conditioned and unconditioned fear: A systematic review of rodent pharmacological studies

Growing evidence supports dopamine's role in aversive states, yet systematic reviews focusing on dopamine receptors in defensive behaviors are lacking. This study presents a systematic review of the literature examining the influence of drugs acting on dopamine D2-like receptors on unconditioned and conditioned fear in rodents. The review reveals a predominant use of adult male rats in the studies, with limited inclusion of female rodents. Commonly employed tests include the elevated plus maze and auditory-cued fear conditioning. The findings indicate that systemic administration of D2-like drugs has a notable impact on both innate and learned aversive states. Generally, antagonists tend to increase unconditioned fear, while agonists decrease it. Moreover, both agonists and antagonists typically reduce conditioned fear. These effects are attributed to the involvement of distinct neural circuits in these states. The observed increase in unconditioned fear induced by D2-like antagonists aligns with dopamine's role in suppressing midbrain-mediated responses. Conversely, the reduction in conditioned fear is likely a result of blocking dopamine activity in the mesolimbic pathway. The study highlights the need for future research to delve into sex differences, explore alternative testing paradigms, and identify specific neural substrates. Such investigations have the potential to advance our understanding of the neurobiology of aversive states and enhance the therapeutic application of dopaminergic agents.

Demotion of canonical/non-canonical inflammasome and pyroptosis alleviates ischemia/reperfusion-induced acute kidney injury: Novel role of the D2/D3 receptor agonist ropinirole

Ropinirole used to treat Parkinson's disease highly targets the dopaminergic receptor D3 over the D2 receptor but although both are expressed in the kidneys the ropinirole potential to treat kidney injury provoked by ischemia/reperfusion (I/R) is undraped. We investigated whether ropinirole can alleviate renal I/R by studying its anti-inflammatory, antioxidant, and anti-pyroptotic effects targeting its aptitude to inhibit the High-mobility group box 1/Toll-like receptor 4/Nuclear factor-kappa B (HMGB1/TLR4/NF-κB) cue and the canonical/non-canonical NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome trajectories. Herein, bilateral I/R surgery was induced in animals to be either untreated or treated with ropinirole for three days after the insult. Ropinirole successfully improved the histopathological picture and renal function which was confirmed by reducing cystatin C and the standard parameters creatinine and blood urea nitrogen (BUN). Ropinirole achieved this through its anti-inflammatory capacity mediated by reducing the HMGB1/TLR4 axis and inactivating NF-κB, which are upstream regulators of the NLRP3 pathway. As a result, the injurious inflammasome markers (NLRP3, apoptosis-associated speck-like protein (ASC), active caspase-1) and their target cytokines interleukin-1 beta (IL-1β) and IL-18 were decreased. Ropinirole also reduced the pyroptotic cell death markers caspase-11 and gasdermin-D. Furthermore, ropinirole by replenishing antioxidants and decreasing malondialdehyde helped to reduce oxidative stress in the kidneys. The docking findings confirmed that ropinirole highly binds to the dopaminergic D3 receptor more than to the D2 receptor. In conclusion, ropinirole has the potential to be a reno-therapeutic treatment against I/R insult by abating the inflammatory NLRP3 inflammasome signal, pyroptosis, and oxidative stress.

Apomorphine is a potent inhibitor of ferroptosis independent of dopaminergic receptors

Originally, apomorphine was a broad-spectrum dopamine agonist with an affinity for all subtypes of the Dopamine D1 receptor to the D5 receptor. We previously identified apomorphine as a potential therapeutic agent for mitochondrial diseases by screening a chemical library of fibroblasts from patients with mitochondrial diseases. In this study, we showed that apomorphine prevented ferroptosis in fibroblasts from various types of mitochondrial diseases as well as in normal controls. Well-known biomarkers of ferroptosis include protein markers such as prostaglandin endoperoxide synthase 2 (PTGS2), a key gene for ferroptosis-related inflammation PTGS2, lipid peroxidation, and reactive oxygen species. Our findings that apomorphine induced significant downregulation of PTSG2 and suppressed lipid peroxide to the same extent as other inhibitors of ferroptosis also indicate that apomorphine suppresses ferroptosis. To our knowledge, this is the first study to report that the anti-ferroptosis effect of apomorphine is not related to dopamine receptor agonist action and that apomorphine is a potent inhibitor of ferroptotic cell death independent of dopaminergic receptors.

Trace Amine-Associated Receptors and Monoamine-Mediated Regulation of Insulin Secretion in Pancreatic Islets

Currently, metabolic syndrome treatment includes predominantly pharmacological symptom relief and complex lifestyle changes. Trace amines and their receptor systems modulate signaling pathways of dopamine, norepinephrine, and serotonin, which are involved in the pathogenesis of this disorder. Trace amine-associated receptor 1 (TAAR1) is expressed in endocrine organs, and it was revealed that TAAR1 may regulate insulin secretion in pancreatic islet β-cells. For instance, accumulating data demonstrate the positive effect of TAAR1 agonists on the dynamics of metabolic syndrome progression and MetS-associated disease development. The role of other TAARs (TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9) in the islet's function is much less studied. In this review, we summarize the evidence of TAARs' contribution to the metabolic syndrome pathogenesis and regulation of insulin secretion in pancreatic islets. Additionally, by the analysis of public transcriptomic data, we demonstrate that TAAR1 and other TAAR receptors are expressed in the pancreatic islets. We also explore associations between the expression of TAARs mRNA and other genes in studied samples and demonstrate the deregulation of TAARs' functional associations in patients with metabolic diseases compared to healthy donors.

Evidence for the existence of facilitatory interactions between the dopamine D2 receptor and the oxytocin receptor in the amygdala of the rat. Relevance for anxiolytic actions

Introduction: The amygdala is a limbic region of high value for understanding anxiety and its treatment. Dopamine D2 receptors (D2Rs) and oxytocin receptors (OXTRs) have both been shown to participate in modulating anxiety involving effects in the amygdala. The goal is to understand if D2R-OXTR heterocomplexes exist in the central amygdala and if, through enhancing allosteric receptor-receptor interactions, may enhance anxiolytic actions. Methods: The methods used involve the shock-probe burying test, the in situ proximity ligation assay (PLA), image acquisition and analysis, and the BRET2 assay. Bilateral cannulas were introduced into the amygdala, and the effects of the coadministration of oxytocin and the D2R-like agonist quinpirole into the amygdala were studied. Results: The combination treatment enhanced the anxiolytic effects compared to the single treatment. The D2R/D3R antagonist raclopride blocked the effects of the combination treatment of oxytocin and the D2R agonist, although oxytocin is regarded as a distinct modulator of fear-mediating anxiolytic effects. In situ PLA results indicate the existence of D2R-OXTR heteroreceptor complexes and/or the co-location of OXTR and D2R within the same cell membrane nanodomains in the central amygdala. With BRET2, evidence is given for the existence of D2R-OXTR heteromers in HEK293 cells upon co-transfection. Discussion: The enhanced behavioral effects observed upon co-treatment with OXTR and D2R agonists may reflect the existence of improved positive receptor-receptor interactions in the putative D2R-OXTR heterocomplexes in certain neuronal populations of the basolateral and central amygdala. The D2R-OXTR heterocomplex, especially upon agonist co-activation in the central amygdala, may open a new pharmacological venue for the treatment of anxiety.

The neurobiology of social play behaviour: Past, present and future

Social play behaviour is a highly energetic and rewarding activity that is of great importance for the development of brain and behaviour. Social play is abundant during the juvenile and early adolescent phases of life, and it occurs in most mammalian species, as well as in certain birds and reptiles. To date, the majority of research into the neural mechanisms of social play behaviour has been performed in male rats. In the present review we summarize studies on the neurobiology of social play behaviour in rats, including work on pharmacological and genetic models for autism spectrum disorders, early life manipulations and environmental factors that influence play in rats. We describe several recent developments that expand the field, and highlight outstanding questions that may guide future studies.

An Exploratory Data Analysis from Ovine and Bovine RNA-Seq Identifies Pathways and Key Genes Related to Cervical Dilatation

The present study developed a review and exploration of data in public and already validated repositories. The main objective was to identify the pathways involved in ruminants' cervical dilatation, which are conserved between cattle and sheep in the follicular and luteal phases of the reproductive cycle. In cattle, 1961 genes were more differentially expressed in the follicular phase and 1560 in the luteal phase. An amount of 24 genes were considered exclusively expressed from these. A total of 18 genes were in the follicular phase and 6 genes were in the luteal phase. In sheep, 2126 genes were more differentially expressed in the follicular phase and 2469 genes were more differentially expressed in the luteal phase. Hoxb genes were identified in both species and are correlated with the PI3K/Akt pathway. PI3K/Akt was also found in both cattle and sheep, appearing prominently in the follicular and luteal phases of both species. Our analyses have pointed out that the PI3K/Akt pathway and the Hoxb genes appear in prominence in modulating mechanisms that involve estrus alterations in the cervix. PI3K/Akt appears to be an important pathway in the cervical relaxation process.

Role of dopamine agonists in Parkinson's disease therapy

Dopamine agonists are an important component of Parkinson's therapy. When weighing up the various therapy options, therapy with levodopa has recently been increasingly preferred due to its stronger efficacy and the ostensibly lower rate of side effects. The advantage of the lower incidence of motor complications during therapy with dopamine agonists was neglected. The occurrence of side effects can be explained by the different receptor affinity to the individual dopaminergic and non-dopaminergic receptors of the individual dopamine agonists. However, the different affinity to individual receptors also explains the different effect on individual Parkinson symptoms and can, therefore, contribute to a targeted use of the different dopamine agonists. Since comparative studies on the differential effect of dopamine agonists have only been conducted for individual substances, empirical knowledge of the differential effect is of great importance. Therefore, the guidelines for the treatment of Parkinson's disease do not consider the differential effect of the dopamine agonists. The historical consideration of dopamine agonists within Parkinson's therapy deserves special attention to be able to classify the current discussion about the significance of dopamine agonists.

Revisiting PFA-mediated tissue fixation chemistry: FixEL enables trapping of small molecules in the brain to visualize their distribution changes

Various small molecules have been used as functional probes for tissue imaging in medical diagnosis and pharmaceutical drugs for disease treatment. The spatial distribution, target selectivity, and diffusion/excretion kinetics of small molecules in structurally complicated specimens are critical for function. However, robust methods for precisely evaluating these parameters in the brain have been limited. Herein, we report a new method termed "fixation-driven chemical cross-linking of exogenous ligands (FixEL)," which traps and images exogenously administered molecules of interest (MOIs) in complex tissues. This method relies on protein-MOI interactions and chemical cross-linking of amine-tethered MOI with paraformaldehyde used for perfusion fixation. FixEL is used to obtain images of the distribution of the small molecules, which addresses selective/nonselective binding to proteins, time-dependent localization changes, and diffusion/retention kinetics of MOIs such as the scaffold of PET tracer derivatives or drug-like small molecules.

Spinal dopaminergic D1 and D5 receptors contribute to reserpine-induced fibromyalgia-like pain in rats

Fibromyalgia is a complex pain syndrome without a precise etiology. Reduced monoamines levels in serum and cerebrospinal fluid in fibromyalgia patients has been reported and could lead to a dysfunction of descending pain modulatory system producing the painful syndrome. This study evaluated the role of D₁-like dopamine receptors in the reserpine-induced fibromyalgia-like pain model in female Wistar rats. Reserpine-treated animals were intrathecally injected with different dopamine receptors agonists and antagonists, and small interfering RNAs (siRNAs) against D₁ and D₅ receptor subtypes. Withdrawal and muscle pressure thresholds were assessed with von Frey filaments and the Randall-Selitto test, respectively. Expression of D₁-like receptors in lumbar spinal cord and dorsal root ganglion was determined using real time polymerase chain reaction (qPCR). Reserpine induced tactile allodynia and muscle hyperalgesia. Intrathecal dopamine and D₁-like receptor agonist SKF-38393 induced nociceptive hypersensitivity in naïve rats, whilst this effect was prevented by the D₁-like receptor antagonist SCH-23390. Moreover, SCH-23390 induced a sex-dependent antiallodynic effect in reserpine-treated rats. Furthermore, transient silencing of D₁ and D₅ receptors significantly reduced reserpine-induced hypersensitivity in female rats. Reserpine slightly increased mRNA D₅ receptor expression in dorsal spinal cord, but not in DRG. This work provides new insights about the involvement of the spinal dopaminergic D₁/D₅ receptors in reserpine-induced hypersensitivity in rats.

The multiple roles of dopamine receptor activation in the modulation of gastrointestinal motility and mucosal function

Dopamine (DA) is a catecholamine regulatory molecule with potential role in physiology and physiopathology of the intestinal tract. Various cellular sources of DA have been indicated as enteric neurons, immune cells, intestinal flora and gastrointestinal epithelium. Moreover, DA is produced by nutritional tyrosine. All the five DA receptors, actually described, are present throughout the gut. Current knowledge of DA in this area is reviewed, focusing on gastrointestinal function in health and during inflammation. Research on animal models and humans are reported. A major obstacle to understanding the physiologic and/or pharmacological roles of enteric DA is represented by the multiplicity of receptors involved in the responses together with many signalling pathways related to each receptor subtype. It is mandatory to map precisely the distributions of DA receptors, to determine the relevance of a receptor in a specific location in order to explore novel therapies directed to dopaminergic targets that may be useful in the control of intestinal inflammation.

Methylation pattern and mRNA expression of synapse-relevant genes in the MAM model of schizophrenia in the time-course of adolescence

Schizophrenia is highly heritable and aggregating in families, but genetics alone does not exclusively explain the pathogenesis. Many risk factors, including childhood trauma, viral infections, migration, and the use of cannabis, are associated with schizophrenia. Adolescence seems to be the critical period where symptoms of the disease manifest. This work focuses on studying an epigenetic regulatory mechanism (the role of DNA methylation) and its interaction with mRNA expression during development, with a particular emphasis on adolescence. The presumptions regarding the role of aberrant neurodevelopment in schizophrenia were tested in the Methyl-Azoxy-Methanol (MAM) animal model. MAM treatment induces neurodevelopmental disruptions and behavioral deficits in off-springs of the treated animals reminiscent of those observed in schizophrenia and is thus considered a promising model for studying this pathology. On a gestational day-17, adult pregnant rats were treated with the antimitotic agent MAM. Experimental animals were divided into groups and subgroups according to substance treatment (MAM and vehicle agent [Sham]) and age of analysis (pre-adolescent and post-adolescent). Methylation and mRNA expression analysis of four candidate genes, which are often implicated in schizophrenia, with special emphasis on the Dopamine hypothesis i.e., Dopamine receptor D₂ (Drd2), and the "co-factors" Disrupted in schizophrenia 1 (DISC1), Synaptophysin (Syp), and Dystrobrevin-binding protein 1 (Dtnbp1), was performed in the Gyrus cingulum (CING) and prefrontal cortex (PFC). Data were analyzed to observe the effect of substance treatment between groups and the impact of adolescence within-group. We found reduced pre-adolescent expression levels of Drd2 in both brain areas under the application of MAM. The "co-factor genes" did not show high deviations in mRNA expression levels but high alterations of methylation rates under the application of MAM (up to ~20%), which diminished in the further time course, reaching a comparable level like in Sham control animals after adolescence. The pre-adolescent reduction in DRD2 expression might be interpreted as downregulation of the receptor due to hyperdopaminergic signaling from the ventral tegmental area (VTA), eventually even to both investigated brain regions. The notable alterations of methylation rates in the three analyzed co-factor genes might be interpreted as attempt to compensate for the altered dopaminergic neurotransmission.

Inverse Salt Sensitivity of Blood Pressure Is Associated with an Increased Renin-Angiotensin System Activity

High and low sodium diets are associated with increased blood pressure and cardiovascular morbidity and mortality. The paradoxical response of elevated BP in low salt diets, aka inverse salt sensitivity (ISS), is an understudied vulnerable 11% of the adult population with yet undiscovered etiology. A linear relationship between the number of single nucleotide polymorphisms (SNPs) in the dopamine D₂ receptor (DRD2, rs6276 and 6277), and the sodium myo-inositol cotransporter 2 (SLC5A11, rs11074656), as well as decreased expression of these two genes in urine-derived renal proximal tubule cells (uRPTCs) isolated from clinical study participants suggest involvement of these cells in ISS. Insight into this newly discovered paradoxical response to sodium is found by incubating cells in low sodium (LS) conditions that unveil cell physiologic differences that are then reversed by mir-485-5p miRNA blocker transfection and bypassing the genetic defect by DRD2 re-expression. The renin-angiotensin system (RAS) is an important counter-regulatory mechanism to prevent hyponatremia under LS conditions. Oversensitive RAS under LS conditions could partially explain the increased mortality in ISS. Angiotensin-II (AngII, 10 nmol/L) increased sodium transport in uRPTCs to a greater extent in individuals with ISS than SR. Downstream signaling of AngII is verified by identifying lowered expression of nuclear factor erythroid 2-related factor 2 (NRF2), CCCTC-binding factor (CTCF), and manganese-dependent mitochondrial superoxide dismutase (SOD2) only in ISS-derived uRPTCs and not SR-derived uRPTCs when incubated in LS conditions. We conclude that DRD2 and SLC5A11 variants in ISS may cause an increased low sodium sensitivity to AngII and renal sodium reabsorption which can contribute to inverse salt-sensitive hypertension.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

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.

Dopamine, Erectile Function and Male Sexual Behavior from the Past to the Present: A Review

Early and recent studies show that dopamine through its neuronal systems and receptor subtypes plays different roles in the control of male sexual behavior. These studies show that (i) the mesolimbic/mesocortical dopaminergic system plays a key role in the preparatory phase of sexual behavior, e.g., in sexual arousal, motivation and reward, whereas the nigrostriatal system controls the sensory-motor coordination necessary for copulation, (ii) the incertohypothalamic system is involved in the consummatory aspects of sexual behavior (penile erection and copulation), but evidence for its role in sexual motivation is also available, (iii) the pro-sexual effects of dopamine occur in concert with neural systems interconnecting the hypothalamus and preoptic area with the spinal cord, ventral tegmental area and other limbic brain areas and (iv) D₂ and D₄ receptors play a major role in the pro-sexual effects of dopamine. Despite some controversy, increases or decreases, respectively, of brain dopamine activity induced by drugs or that occur physiologically, usually improves or worsens, respectively, sexual activity. These findings suggest that an altered central dopaminergic tone plays a role in mental pathologies characterized by aberrant sexual behavior, and that pro-erectile D₄ receptor agonists may be considered a new strategy for the treatment of erectile dysfunction in men.

Prokinetic Agents

Gastrointestinal (GI) prokinetic agents are drugs that increase GI motility and promote the movement of contents in the GI tract by amplifying and controlling the contraction of GI smooth muscle. Currently used prokinetics increase GI motility by acting as a dopamine D₂ receptor antagonist (e.g., metoclopramide, domperidone, levosulpiride) and 5-HT₄ receptor agonist (e.g., mosapride, prucalopride). Some prokinetics also have a cholinesterase inhibitory property (e.g., itopride), and herb-derived prokinetics (e.g., motilitone) affect multiple receptors. Depending on the type and distribution of receptors on which the prokinetics bind, the effect(s) may be regional or throughout the GI tract. Most prokinetics have been used for functional dyspepsia and gastroparesis because they mainly affect upper GI motility. However, prucalopride, a highly selective 5-HT₄ receptor agonist, is used primarily to treat chronic constipation and pseudo-obstruction. Dopamine D₂ receptor antagonists also inhibit the D₂ receptor in the medulla oblongata chemoreceptor trigger zone; therefore, they can treat nausea and vomiting. However, short term use of dopamine D₂ antagonists at an appropriate dose is recommended because of their potential for central nervous system side effects by penetrating the blood-brain barrier. It is necessary to know the mechanism of action, each clinical trial’s characteristics, and the side effects of prokinetics to obtain the best clinical outcomes. This article aims to summarize the results of clinical studies related to the impact of currently available prokinetic agents in Korea on GI motility.

Dysfunctional Heteroreceptor Complexes as Novel Targets for the Treatment of Major Depressive and Anxiety Disorders

Among mental diseases, major depressive disorder (MDD) and anxiety deserve a special place due to their high prevalence and their negative impact both on society and patients suffering from these disorders. Consequently, the development of novel strategies designed to treat them quickly and efficiently, without or at least having limited side effects, is considered a highly important goal. Growing evidence indicates that emerging properties are developed on recognition, trafficking, and signaling of G-protein coupled receptors (GPCRs) upon their heteromerization with other types of GPCRs, receptor tyrosine kinases, and ionotropic receptors such as N-methyl-D-aspartate (NMDA) receptors. Therefore, to develop new treatments for MDD and anxiety, it will be important to identify the most vulnerable heteroreceptor complexes involved in MDD and anxiety. This review focuses on how GPCRs, especially serotonin, dopamine, galanin, and opioid heteroreceptor complexes, modulate synaptic and volume transmission in the limbic networks of the brain. We attempt to provide information showing how these emerging concepts can contribute to finding new ways to treat both MDD and anxiety disorders.

Pramipexole regulates depression-like behavior via dopamine D3 receptor in a mouse model of Parkinson's disease

Depression is one of the strongest predictors of quality of life in patients with Parkinson's disease (PD). Despite the high prevalence of depression, there is no clear guidance for its treatment in PD because the evidence for the efficacy of most antidepressants remains insufficient. Pramipexole, a dopamine agonist, is one of the few drugs that has proven to be clinically useful. However, the underlying mechanisms of antidepressive effects of pramipexole are still unknown. A 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model, dopamine D2 receptor (DRD2) and D3 receptor (DRD3) knockout mice were used in our study. Compared with other dopamine D2-like receptor agonists and madopar, pramipexole improved depression-like behavior and alleviate bradykinesia in an MPTP-induced mouse model of PD. Pramipexole significantly improved depression-like behavior in DRD2^-/- mice but not in DRD3^-/- mice. These results demonstrate that the antidepressive effect of pramipexole is mediated by DRD3 but not DRD2. Our findings highlight the need to develop novel dopamine agonists specifically targeting DRD3 for the treatment of depression in PD in the future.

Bidirectional role of dopamine in learning and memory-active forgetting

Dopaminergic neurons projecting from the Substantia Nigra to the Striatum play a critical role in motor functions while dopaminergic neurons originating in the Ventral Tegmental Area (VTA) and projecting to the Nucleus Accumbens, Hippocampus and other cortical structures regulate rewarding learning. While VTA mainly consists of dopaminergic neurons, excitatory (glutamate) and inhibitory (GABA) VTA-neurons have also been described: these neurons may also modulate and contribute to shape the final dopaminergic response, which is critical for memory formation. However, given the large amount of information that is handled daily by our brain, it is essential that irrelevant information be deleted. Recently, apart from the well-established role of dopamine (DA) in learning, it has been shown that DA plays a critical role in the intrinsic active forgetting mechanisms that control storage information, contributing to the deletion of a consolidated memory. These new insights may be instrumental to identify therapies for those disorders that involve memory alterations.

Humour-based interventions for people with schizophrenia

BACKGROUND

Humour-based interventions are defined as any intervention that promotes health and wellness by stimulating a playful discovery, expression, or appreciation of the absurdity or incongruity of life's situations. Humour-based interventions can be implemented in different settings, including hospitals, nursing homes and day care centres. They have been posed as an adjunct to usual care for people with schizophrenia, but a summary of the evidence is lacking.

OBJECTIVES

To examine the effects of humour-based interventions as an add-on intervention to standard care for people with schizophrenia.

SEARCH METHODS

On 31 July 2019 and 10 February 2021 we searched the Cochrane Schizophrenia Group's study-based register of trials, which is based on CENTRAL, CINAHL, ClinicalTrials.Gov, Embase, ISRCTN, MEDLINE, PsycINFO, PubMed, and WHO ICTRP.

SELECTION CRITERIA

We included all randomised controlled trials comparing humour-based interventions with active controls, other psychological interventions, or standard care for people with schizophrenia. We excluded studies fulfilling our prespecified selection criteria but without useable data from further quantitative synthesis.

DATA COLLECTION AND ANALYSIS

Two review authors independently inspected citations, selected studies, extracted data and appraised study quality, following the guidance from the Cochrane Handbook for Systematic Reviews of Interventions. For binary outcomes we calculated risk ratios (RRs) and their 95% confidence intervals (CIs). For continuous outcomes we calculated the mean differences (MDs) and their 95% CIs. We assessed risks of bias for included studies and created summary of findings tables using the GRADE approach.

MAIN RESULTS

We included three studies in this review for qualitative synthesis, although one study did not report any relevant outcomes. We therefore include two studies (n = 96) in our quantitative synthesis. No data were available on the following prespecified primary outcomes: clinically-important change in general mental state, clinically-important change in negative symptoms, clinically-important change in overall quality of life, and adverse effects. As compared with active control, humour-based interventions may not improve the average endpoint score of a general mental state scale (Positive and Negative Syndrome Scale (PANSS) total score: MD -1.70, 95% CI -17.01 to 13.61; 1 study, 30 participants; very low certainty of evidence); positive symptoms (PANSS positive symptom score: MD 0.00, 95% CI -2.58 to 2.58; 1 study, 30 participants; low certainty of evidence), negative symptoms (PANSS negative symptom score: MD -0.70, 95% CI -4.22 to 2.82; 1 study, 30 participants; very low certainty of evidence) and anxiety (State-Trait Anxiety Inventory (STAI): MD -2.60, 95% CI -5.76 to 0.56; 1 study, 30 participants; low certainty of evidence). Due to the small sample size, we remain uncertain about the effect of humour-based interventions on leaving the study early as compared with active control (no event, 1 study, 30 participants; very low certainty of evidence). On the other hand, humour-based interventions may reduce depressive symptoms (Beck Depression Inventory (BDI): MD -6.20, 95% CI -12.08 to -0.32; 1 study, 30 participants; low certainty of evidence). Compared with standard care, humour-based interventions may not improve  depressive symptoms (BDI second edition: MD 0.80, 95% CI -2.64 to 4.24; 1 study, 59 participants; low certainty of evidence). We are uncertain about the effect of humour-based interventions on leaving the study early for any reason compared with standard care (risk ratio 0.38, 95% CI 0.08 to 1.80; 1 study, 66 participants; very low certainty of evidence).

AUTHORS' CONCLUSIONS

We are currently uncertain whether the evidence supports the use of humour-based interventions in people with schizophrenia. Future research with rigorous and transparent methodology investigating clinically important outcomes is warranted.

Protective Role of Sarpogrelate in Combination with Bromocriptine and Cabergoline for Treatment of Diabetes in Alloxan-induced Diabetic Rats

BACKGROUND

Although dopamine D2 receptor agonists, bromocriptine and cabergoline, are notable medications in the treatment of Parkinsonism, hyperprolactinemia, and hyperglycemia, there is an identified relationship between the utilization of D2-like R agonists and the progress of myocardial injury, especially in the early phase of therapy.

OBJECTIVE

This investigation aimed to examine the potential activity of sarpogrelate (a 5-hydroxytryptamine 2A [5-HT2A] receptor blocker) in reducing myocardial injury prompted by extended haul utilization of D2 receptor agonists in a model of diabetic rats.

METHODS

In the in vivo studies, both bromocriptine and cabergoline were managed independently and combined with sarpogrelate for about a month in diabetic nephropathy rats. Blood glucose level and other myocardial biochemical parameters were estimated. The probable mechanism for insulin secretagogue action was evaluated through in vitro isolated islets study. Sodium/potassium-adenosine triphosphatase activity was assayed in an isolated microsomal fraction of the renal cortex. Isolated perfused rat hearts were treated with different doses of dopamine before and after being subjected to the tested drugs, dose response of heart rate, and heart contractility were recorded.

RESULTS

Bromocriptine and cabergoline created a significant reduction in blood glucose level without any action on insulin secretagogues. Bromocriptine prevented the loss of sodium/potassium-adenosine triphosphatase activity in the cortex of an ischemic kidney. Treatment of bromocriptine or cabergoline with sarpogrelate altogether decreased the levels of the elevated myocardial biomarkers in serum. Administration of different doses of dopamine in presence of bromocriptine or capergoline resulted in significantly rising in the heart rate percentage comparing to dopamine alone. A mix of bromocriptine or cabergoline with sarpogrelate diminished both heart rate and contractility, respectively.

CONCLUSIONS

The examination demonstrated that the combined use of sarpogrelate with bromocriptine or cabergoline decreased the potential adverse effects of these 2 drugs on myocardial tissues.

Interaction of clozapine with metformin in a schizophrenia rat model

The low efficacy of antipsychotic drugs (e.g., clozapine) for negative symptoms and cognitive impairment has led to the introduction of adjuvant therapies. Because previous data suggest the procognitive potential of the antidiabetic drug metformin, this study aimed to assess the effects of chronic clozapine and metformin oral administration (alone and in combination) on locomotor and exploratory activities and cognitive function in a reward-based test in control and a schizophrenia-like animal model (Wisket rats). As impaired dopamine D1 receptor (D₁R) function might play a role in the cognitive dysfunctions observed in patients with schizophrenia, the second goal of this study was to determine the brain-region-specific D₁R-mediated signaling, ligand binding, and mRNA expression. None of the treatments affected the behavior of the control animals significantly; however, the combination treatment enhanced D₁R binding and activation in the cerebral cortex. The Wisket rats exhibited impaired motivation, attention, and cognitive function, as well as a lower level of cortical D₁R binding, signaling, and gene expression. Clozapine caused further deterioration of the behavioral parameters, without a significant effect on the D₁R system. Metformin blunted the clozapine-induced impairments, and, similarly to that observed in the control animals, increased the functional activity of D₁R. This study highlights the beneficial effects of metformin (at the behavioral and cellular levels) in blunting clozapine-induced adverse effects.

Dopaminergic mechanisms underlying the expression of antipsychotic-induced dopamine supersensitivity in rats

Antipsychotic treatment can produce a dopamine-supersensitive state, potentiating the response to dopamine receptor stimulation. In both schizophrenia patients and rats, this is linked to tolerance to ongoing antipsychotic treatment. In rodents, dopamine supersensitivity is often confirmed by an exaggerated psychomotor response to d-amphetamine after discontinuation of antipsychotic exposure. Here we examined in rats the dopaminergic mechanisms mediating this enhanced behavioural response, as this could uncover pathophysiological processes underlying the expression of antipsychotic-evoked dopamine supersensitivity. Rats received 0.5 mg/kg/day haloperidol via osmotic minipump for 2 weeks, before treatment was discontinued. After cessation of antipsychotic treatment, rats showed a supersensitive psychomotor response to the D2 agonist quinpirole, but not to the D1 partial agonist SKF38393 or the dopamine reuptake blocker GBR12783. Furthermore, acute D1 receptor blockade (using SCH39166) decreased the exaggerated psychomotor response to d-amphetamine in haloperidol-pretreated rats, whereas acute D2 receptor blockade (using sulpiride) enhanced it. Thus, after discontinuation of antipsychotic treatment, D1- and D2-mediated transmission differentially modulate the expression of a supersensitive response to d-amphetamine. This supersensitive behavioural response was accompanied by enhanced GSK3β activity and suppressed ERK1/2 activity in the nucleus accumbens (but not caudate-putamen), suggesting increased mesolimbic D2 transmission. Finally, after discontinuing haloperidol treatment, neither increasing ventral midbrain dopamine impulse flow nor infusing d-amphetamine into the cerebral ventricles triggered the expression of already established dopamine supersensitivity, suggesting that peripheral effects are required. Thus, while dopamine receptor-mediated signalling regulates the expression of antipsychotic-evoked dopamine supersensitivity, a simple increase in central dopamine neurotransmission is insufficient to trigger this supersensitivity.

Pharmacological blockade of dopamine D1- or D2-receptor in the prefrontal cortex induces attentional impairment in the object-based attention test through different neuronal circuits in mice

Dopamine is a key neurotransmitter that regulates attention through dopamine D1 and D2-receptors in the prefrontal cortex (PFC). We previously developed an object-based attention test (OBAT) to evaluate attention in mice. Disruption of the dopaminergic neuronal system in the PFC induced attentional impairment in the OBAT. However, previous studies have not systematically examined which specific brain regions are associated with the blockade of PFC dopamine D1 and D2-receptors in the OBAT. In this study, we investigated the association of dopamine D1 and D2-receptors in the PFC with attention and neuronal activity in diverse brain regions. We found that both dopamine D1 and D2-receptor antagonists induced attentional impairment in the OBAT by bilateral microinjection into the PFC of mice, suggesting that both dopamine D1 and D2-receptors were associated with attention in the OBAT. Our analysis of the neuronal activity as indicated by c-Fos expression in 11 different brain regions showed that based on the antagonist types, there was selective activation of several brain regions. Overall, this study suggests that both dopamine D1 and D2-receptors play a role in attention through different neuronal circuits in the PFC of mice.

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 (pK_i values of 7.14 [D₂ R] and 8.42 [D₃ R]).

Nicotine-induced enhancement of a sensory reinforcer in adult rats: antagonist pretreatment effects

RATIONALE AND OBJECTIVES

The reinforcement-enhancing effect (REE) of nicotine refers to the drug's ability to enhance the strength of other primary and conditioned reinforcers. The main aim was to investigate neuropharmacological mechanisms underlying nicotine's strengthening of a primary visual reinforcer (i.e., a light cue), using a subcutaneous (SC) dose previously shown to provide plasma nicotine levels associated with habitual smoking.

METHODS

Adult male rats pressed an "active" lever to illuminate a brief cue light during daily 60-min sessions. Rats that showed a clear REE were tested with systemically administered pretreatment drugs followed by nicotine (0.1 mg/kg SC) or saline challenge, in within-subject counterbalanced designs. Pretreatments were mecamylamine (nicotinic, 0.1-1 mg/kg SC), SCH 39166 (D1-like dopaminergic, 0.003-0.2 mg/kg SC), naloxone (opioid, 1 and 5 mg/kg SC), prazosin (alpha1-adrenergic antagonist, 1 and 2 mg/kg IP), rimonabant (CB1 cannabinoid inverse agonist, 3 mg/kg IP), sulpiride (D2-like dopaminergic antagonist, 40 mg/kg SC), or propranolol (beta-adrenergic antagonist, 10 mg/kg IP).

RESULTS

The nicotine REE was abolished by three antagonists at doses that did not impact motor output, i.e., mecamylamine (1 mg/kg), SCH 39166 (0.01 and 0.03 mg/kg), and naloxone (5 mg/kg). Prazosin and rimonabant both attenuated the nicotine REE, but rimonabant also suppressed responding more generally. The nicotine REE was not significantly altered by sulpiride or propranolol.

CONCLUSIONS

In adult male rats, the reinforcement-enhancing effect of low-dose nicotine depends on nicotinic receptor stimulation and on neurotransmission via D1/D5 dopaminergic, opioid, alpha1-adrenergic, and CB1 cannabinoid receptors.

Application of quinpirole in the paraventricular thalamus facilitates emergence from isoflurane anesthesia in mice

BACKGROUND AND PURPOSE

Dopamine is well-known to contribute to emergence from anesthesia. Previous studies have demonstrated that the paraventricular thalamus (PVT) in the midline nuclei is crucial for wakefulness. Moreover, the PVT receives dopaminergic projections from the brainstem. Therefore, we hypothesize that the dopaminergic signaling in the PVT plays a role in emergence from isoflurane anesthesia.

METHODS

We used c-Fos immunohistochemistry to reveal the activity of PVT neurons in three groups: The first group (iso⁺ EM^- ) underwent the anesthesia protocol and was sacrificed before emergence. The second group (iso⁺ EM⁺ ) underwent passive emergence from the same anesthesia protocol. The last group (oxy⁺ ) received oxygen. D2-like agonist quinpirole (2 or 4 mM) or D2-like antagonist raclopride (2 or 5 mM) was microinjected into the PVT, and their effects on emergence and induction time were analyzed. Surface cortical electroencephalogram (EEG) recordings were used to explore the effects of quinpirole injection into the PVT on cortical excitability during isoflurane anesthesia. The activity of PVT neurons after quinpirole injection was assessed by c-Fos immunohistochemistry.

RESULTS

The number of c-Fos-positive nuclei for the iso⁺ EM⁺ group was significantly higher than the oxy⁺ and iso⁺ EM^- groups. Application of quinpirole (4 mM) into the PVT shortened emergence time compared with the saline group (p < .01). In contrast, administration of raclopride (2 mM) delayed emergence time (p < .05). Neither quinpirole nor raclopride exerted an effect on induction time. EEG analyses showed that quinpirole (4 mM) decreased the burst suppression ratio during isoflurane anesthesia (p < .01). The number of c-Fos-positive nuclei for the quinpirole (4 mM) group was significantly higher than saline group (p < .01).

CONCLUSIONS

Our findings suggest that the activity of PVT neurons is enhanced after emergence from anesthesia, and the dopaminergic signaling in the PVT may facilitate emergence from isoflurane anesthesia.

Dopamine, Alpha-Synuclein, and Mitochondrial Dysfunctions in Parkinsonian Eyes

Parkinson’s disease (PD) is characterized by motor dysfunctions including bradykinesia, tremor at rest and motor instability. These symptoms are associated with the progressive degeneration of dopaminergic neurons originating in the substantia nigra pars compacta and projecting to the corpus striatum, and by accumulation of cytoplasmic inclusions mainly consisting of aggregated alpha-synuclein, called Lewy bodies. PD is a complex, multifactorial disorder and its pathogenesis involves multiple pathways and mechanisms such as α-synuclein proteostasis, mitochondrial function, oxidative stress, calcium homeostasis, axonal transport, and neuroinflammation. Motor symptoms manifest when there is already an extensive dopamine denervation. There is therefore an urgent need for early biomarkers to apply disease-modifying therapeutic strategies. Visual defects and retinal abnormalities, including decreased visual acuity, abnormal spatial contrast sensitivity, color vision defects, or deficits in more complex visual tasks are present in the majority of PD patients. They are being considered for early diagnosis together with retinal imaging techniques are being considered as non-invasive biomarkers for PD. Dopaminergic cells can be found in the retina in a subpopulation of amacrine cells; however, the molecular mechanisms leading to visual deficits observed in PD patients are still largely unknown. This review provides a comprehensive analysis of the retinal abnormalities observed in PD patients and animal models and of the molecular mechanisms underlying neurodegeneration in parkinsonian eyes. We will review the role of α-synuclein aggregates in the retina pathology and/or in the onset of visual symptoms in PD suggesting that α-synuclein aggregates are harmful for the retina as well as for the brain. Moreover, we will summarize experimental evidence suggesting that the optic nerve pathology observed in PD resembles that seen in mitochondrial optic neuropathies highlighting the possible involvement of mitochondrial abnormalities in the development of PD visual defects. We finally propose that the eye may be considered as a complementary experimental model to identify possible novel disease’ pathways or to test novel therapeutic approaches for PD.

Inhibition of type 4 cAMP-phosphodiesterases (PDE4s) in mice induces hypothermia via effects on behavioral and central autonomous thermoregulation

Inhibitors of Type 4 cAMP-phosphodiesterases (PDE4s) exert a number of promising therapeutic benefits, including potent anti-inflammatory, memory- and cognition-enhancing, metabolic, and antineoplastic effects. We report here that treatment with a number of distinct PDE4 inhibitors, including Rolipram, Piclamilast, Roflumilast and RS25344, but not treatment with the PDE3-selective inhibitor Cilostamide, induces a rapid (10-30 min), substantial (-5 °C) and long-lasting (up to 5 h) decrease in core body temperature of C57BL/6 mice; thus, identifying a critical role of PDE4 also in the regulation of body temperature. As little as 0.04 mg/kg of the archetypal PDE4 inhibitor Rolipram induces hypothermia. As similar or higher doses of Rolipram were used in a majority of published animal studies, most of the reported findings are likely paralleled by, or potentially impacted by hypothermia induced by these drugs. We further show that PDE4 inhibition affects central body temperature regulation and acts by lowering the cold-defense balance point of behavioral (including posture and locomotion) and autonomous (including cutaneous tail vasodilation) cold-defense mechanisms. In line with the idea of an effect on central body temperature regulation, hypothermia is induced by moderate doses of various brain-penetrant PDE4 inhibitors, but not by similar doses of YM976, a PDE4 inhibitor that does not efficiently cross the blood-brain barrier. Finally, to begin delineating the mechanism of drug-induced hypothermia, we show that blockade of D2/3-type dopaminergic, but not β-adrenergic, H1-histaminergic or opiate receptors, can alleviate PDE4 inhibitor-induced hypothermia. We thus propose that increased D2/3-type dopaminergic signaling, triggered by PDE4 inhibitor-induced and cAMP-mediated dopamine release in the thermoregulatory centers of the hypothalamus, is a significant contributor to PDE4 inhibitor-induced hypothermia.

Synthesis of MOF525/PEDOT Composites as Microelectrodes for Electrochemical Sensing of Dopamine

Dopamine (DA) is an important neurotransmitter responsible for the functions and activities of multiple systems in human. Electrochemical detection of DA has the advantages of fast analysis and cost-effectiveness, while a regular electrode probe is restricted to laboratory use because the probe size is too large to be suitable for an in vivo or in vitro analysis. In this study, we have developed porphyrin-based metal organic framework (MOF525) and poly(3,4-ethylenedioxythiophene) (PEDOT)-based composites to modify microelectrode for DA detection. Two types of PEDOT monomers with different functional groups were investigated in this study. By varying the monomer ratios, electrolyte concentrations, and electropolymerization temperature, it was found that the PEDOT monomer containing carboxylic group facilitated the formation of regular morphology during the electropolymerization process. The uniform morphology of the PEDOT promoted the electron transmission efficiency in the same direction, while the MOF525 provided a large reactive surface area for electrocatalysis of DA. Thus, the MOF525/PEDOT composite improved the sensitivity-to-noise ratio of DA signaling, where the sensitivity reached 11 nA/μM in a good linear range of 4–100 µM. In addition, porphyrin-based MOF could also increase the selectivity to DA against other common clinical interferences, such as ascorbic acid and uric acid. The as-synthesized microelectrode modified with MOF525/PEDOT in this study exhibited great potential in real time analysis.

Dopamine D4 receptor subtype activation reduces the rat cardiac parasympathetic discharge

The dopaminergic system influences the heart rhythm by inhibiting the rat cardiac sympathetic and parasympathetic neurotransmissions through activation of D₂-like receptors (encompassing the D₂, D₃, and D₄ subtypes). Whereas D₂ receptor subtype activation results in cardiac sympatho-inhibition, the dopamine receptor subtypes involved in rat cardiac vago-inhibition remain unknown. Hence, this study investigated the specific functional role of the D₂-like receptor subtypes (D₂, D₃, and/or D₄) inhibiting the rat heart cholinergic drive. For this purpose, male Wistar rats were pithed and prepared for cardiac vagal stimulation. Bradycardic responses were obtained by electrical stimulation of vagal fibres (3, 6, 9 Hz; n = 100) or i.v. acetylcholine (ACh; 1, 5, 10 μg/kg; n = 15). Expression of D₂, D₃, and D₄ receptors was studied in left and right atrium samples by PCR (n = 4). Intravenous injections of quinpirole (D₂-like agonist; 1-30 μg/kg), but not of SFK-38393 (D₁-like agonist; 1-30 μg/kg), dose-dependently inhibited the vagally induced bradycardia. The vago-inhibition induced by quinpirole (which failed to affect the bradycardia to i.v. ACh) was unchanged after i.v. injections of the antagonists L-741,626 (D₂; 100 μg/kg) or SB-277011-A (D₃; 100 μg/kg), but it was abolished by L-745,870 (D₄; 100 μg/kg). mRNA levels of D₂, D₃, and D₄ receptor subtype were detected in the left and right rat atria. Our results suggest that the quinpirole-induced vagolytic effect involves prejunctional D₄ receptor subtypes, located in the left and right atria. This provides new evidence on the relevance of D₄ receptor modulating the heart parasympathetic control.

Dopamine receptors in emesis: Molecular mechanisms and potential therapeutic function

Dopamine is a member of the catecholamine family and is associated with multiple physiological functions. Together with its five receptor subtypes, dopamine is closely linked to neurological disorders such as schizophrenia, Parkinson's disease, depression, attention deficit-hyperactivity, and restless leg syndrome. Unfortunately, several dopamine receptor-based agonists used to treat some of these diseases cause nausea and vomiting as impending side-effects. The high degree of cross interactions of dopamine receptor ligands with many other targets including G-protein coupled receptors, transporters, enzymes, and ion-channels, add to the complexity of discovering new targets for the treatment of nausea and vomiting. Using activation status of signaling cascades as mechanism-based biomarkers to foresee drug sensitivity combined with the development of dopamine receptor-based biased agonists may hold great promise and seems as the next step in drug development for the treatment of such multifactorial diseases. In this review, we update the present knowledge on dopamine and dopamine receptors and their potential roles in nausea and vomiting. The pre- and clinical evidence provided in this review supports the implication of both dopamine and dopamine receptor agonists in the incidence of emesis. Besides the conventional dopaminergic antiemetic drugs, potential novel antiemetic targeting emetic protein signaling cascades may offer superior selectivity profile and potency.

Synthesis and dopamine receptor pharmacological evaluations on ring C ortho halogenated 1-phenylbenzazepines

A series of 1-phenylbenzazepines containing bromine or chlorine substituents at the ortho position of the appended phenyl ring (2'-monosubstituted or 2',6'- disubstituted patterns) were synthesized and evaluated for affinity towards dopamine D₁R, D₂R and D₅R. As is typical of the 1-phenylbenzazepine scaffold, the compounds displayed selectivity towards D₁R and D₅R; analogs generally lacked affinity for D₂R. Interestingly, 2',6'-dichloro substituted analogs showed modest D₅R versus D₁R selectivity whereas this selectivity was reversed in compounds with a 2'-halo substitution pattern. Compound 10a was identified as a D₁R antagonist (K_i = 14 nM; IC₅₀ = 9.4 nM).

Structure of a D2 dopamine receptor-G-protein complex in a lipid membrane

The D₂ dopamine receptor (DRD2) is a therapeutic target for Parkinson’s disease¹ and antipsychotic drugs². DRD2 is activated by the endogenous neurotransmitter dopamine and synthetic agonist drugs such as bromocriptine³, leading to stimulation of G_i and inhibition of adenylyl cyclase. We used cryo-electron microscopy to elucidate the structure of an agonist-bound activated DRD2-G_i complex reconstituted into a phospholipid membrane. The extracellular ligand binding site of DRD2 is remodeled in response to agonist binding, with conformational changes in extracellular loop 2 (ECL2), transmembrane domain 5 (TM5), TM6, and TM7 propagating to opening of the intracellular G_i binding site. The DRD2-G_i structure represents the first experimental model of a GPCR-G protein complex embedded in a phospholipid bilayer, which serves as a benchmark to validate the interactions seen in previous detergent-bound structures. The structure also reveals interactions that are unique to the membrane-embedded complex, including helix 8 burial in the inner leaflet, ordered lysine and arginine sidechains in the membrane interfacial regions, and lipid anchoring of the G protein in the membrane. Our model of the activated DRD2 will help inform the design of subtype-selective DRD2 ligands for multiple human CNS disorders.

Cell adhesion molecule close homolog of L1 binds to the dopamine receptor D2 and inhibits the internalization of its short isoform

Cell adhesion molecule close homolog of L1 (CHL1) and the dopamine receptor D2 (DRD2) are associated with psychiatric and mental disorders. We here show that DRD2 interacts with CHL1 in mouse brain, as evidenced by co-immunostaining, proximity ligation assay, co-immunoprecipitation, and pull-down assay with recombinant extracellular CHL1 domain fused to Fc (CHL1-Fc). Direct binding of CHL1-Fc to the first extracellular loop of DRD2 was shown by ELISA. Using HEK cells transfected to co-express CHL1 and the short (DRD2-S) or long (DRD2-L) DRD2 isoforms, co-localization of CHL1 and both isoforms was observed by immunostaining and proximity ligation assay. Moreover, CHL1 inhibited agonist-triggered internalization of DRD2-S. Proximity ligation assay showed close interaction between CHL1 and DRD2 in neurons expressing dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP32) or tyrosine hydroxylase (TH) in tissue sections of adult mouse striatum. In cultures of striatum or ventral midbrain, CHL1 was also closely associated with DRD2 in DARPP32- or TH-immunopositive cells, respectively. In the dorsal striatum of CHL1-deficient mice, lower levels of DRD2 and phosphorylated TH were observed, when compared to wild-type littermates. In the ventral striatum of CHL1-deficient mice, levels of phosphorylated DARPP32 were reduced. We propose that CHL1 regulates DRD2-dependent presynaptic and postsynaptic functions.

Structure-Functional-Selectivity Relationship Studies of Novel Apomorphine Analogs to Develop D1R/D2R Biased Ligands

Loss of dopamine neurons is central to the manifestation of Parkinson's disease motor symptoms. The dopamine precursor L-DOPA, the most commonly used therapeutic agent for Parkinson's disease, can restore normal movement yet cause side-effects such as dyskinesias upon prolonged administration. Dopamine D1 and D2 receptors activate G-protein- and arrestin-dependent signaling pathways that regulate various dopamine-dependent functions including locomotion. Studies have shown that shifting the balance of dopamine receptor signaling toward the arrestin pathway can be beneficial for inducing normal movement, while reducing dyskinesias. However, simultaneous activation of both D1 and D2Rs is required for robust locomotor activity. Thus, it is desirable to develop ligands targeting both D1 and D2Rs and their functional selectivity. Here, we report structure-functional-selectivity relationship (SFSR) studies of novel apomorphine analogs to identify structural motifs responsible for biased activity at both D1 and D2Rs.

The prefrontal cortex and the caudate nucleus respond conjointly to methylphenidate (Ritalin). Concomitant behavioral and neuronal recording study

Methylphenidate (MPD) is commonly used to treat attention-deficit hyperactivity disorder (ADHD). Recently, it is being abused for cognitive enhancement and recreation leading to concerns regarding its addictive potential. The prefrontal cortex (PFC) and caudate nucleus (CN) are two of the brain structures involved in the motive/reward circuit most affected by MPD and are also thought to be responsible for ADHD phenomena. This study is unique in that it investigated acute and chronic, dose-response MPD exposure on animals' behavior activity concomitantly with PFC and CN neuronal circuitry in freely behaving adult animals without the interference of anesthesia. Further, it compared acute and chronic MPD action on over 1,000 subcortical and cortical neurons simultaneously, allowing for a more accurate interpretation of drug action on corticostriatal neuronal circuitry. For this experiment, four groups of animals were used: saline (control), 0.6, 2.5, and 10.0 mg/kg MPD following acute and repetitive exposure. The data shows that the same MPD dose elicits behavioral sensitization in some animals and tolerance in others and that the PFC and CN neuronal activity correlates with the animals' behavioral responses to MPD. The expression of sensitization and tolerance are experimental biomarkers indicating that a drug has addictive potential. In general, a greater percentage of CN units responded to both acute and chronic MPD exposure as compared to PFC units. Dose response differences between the PFC and the CN units were observed. The dichotomy that some PFC and CN units responded to the same MPD dose by excitation and other units by attenuation in neuronal firing rate is discussed. In conclusion, to understand the mechanism of action of the drug, it is essential to study, simultaneously, on more than one brain site, the electrophysiological and behavioral effects of acute and chronic drug exposure, as sensitization and tolerance are experimental biomarkers indicating that a drug has addictive potential. The behavioral and neuronal data obtained from this study indicates that chronic MPD exposure results in behavioral and biochemical changes consistent with a substance abuse disorder.

The role of dopamine in eye growth responses to color and luminance flicker in chicks

Stroboscopic luminance flicker has been found to prevent the increase in eye growth normally associated with form deprivation through the release of retinal dopamine. In this study, we sought to investigate whether dopamine plays a role in the decreased growth observed with 2 Hz sine-wave luminance flicker and increased growth with color flicker. Starting 5-7 days after hatching, chicks were exposed to 2 Hz sinusoidally modulated illumination (Mean: 680 lux) for 4 days and were otherwise in the dark. Chicks were exposed to color-modulated red and green (RG) light, to luminance modulated RGB components (LUM), or to a no-flicker (NF) control. Chicks received daily 10 μL intravitreal injections of apomorphine, spiperone, or saline. Fellow eyes received no injection. Spiperone injections prevented the decrease in eye growth typically seen with LUM flicker, with a relative increase in eye length, but no other significant effects compared with saline controls. Apomorphine injections prevented the increase in eye growth typically seen with RG flicker, with a relative decrease in eye length compared to saline controls. These results indicate a role for the activation of D2-receptor types in the inhibition of eye growth in response to luminance flicker, and a lack of dopamine receptor activation associated with the increase in eye growth with color flicker.

Activation of Dopamine D3 Receptor Subtypes Inhibits the Neurogenic Systemic Vasodilation Induced by Stimulation of the Perivascular CGRPergic Discharge

The sensory nervous system controls cardiovascular homeostasis via capsaicin-sensitive neurons that release calcitonin gene-related peptide (CGRP), which subsequently activates CGRP receptors. How this perivascular CGRPergic discharge is modulated, nevertheless, remains unclear. In pithed rats, systemic vasodilation induced by CGRPergic discharge stimulation results in diastolic blood pressure (BP) decrements that are inhibited by the dopamine D₂-like receptor agonist quinpirole. Since this inhibition is mediated by raclopride- or haloperidol-sensitive D₂-like receptors (comprising the D₂, D₃, and D₄ subtypes), the present study pharmacologically investigated the specific contribution of these subtypes to the modulation of the systemic CGRPergic vasodilation, using highly specific antagonists. To that end, 55 male Wistar rats were pithed for thoracic (T₉-T₁₂) spinal stimulation of the perivascular CGRPergic discharge. The resulting frequency-dependent decrements in diastolic BP were inhibited by quinpirole, and this sensory-inhibition was (a) unchanged after i.v. injections of the antagonists L-741,626 (D₂) or L-745,870 (D₄) and (b) completely blocked by SB-277011-A (D₃). Accordingly, we suggest the main role of the D₃ receptor subtypes in the inhibition by quinpirole of the neurogenic CGRPergic systemic vasodilation. These findings contribute to a better understanding of the dopaminergic modulation of the rat perivascular CGRPergic discharge producing systemic vasodilation.

Conventional and Novel Pharmacological Approaches to Treat Dopamine-Related Disorders: Focus on Parkinson's Disease and Schizophrenia

The dopaminergic system integrated by cell groups distributed in several brain regions exerts a modulatory role in brain. Particularly important for this task are the mesencephalic dopamine neurons, which from the substantia nigra and ventral tegmental area project to the dorsal striatum and the cortical/subcortical limbic systems, respectively. Dopamine released from these neurons operates mainly via the short distance extrasynaptic volume transmission and activates five different dopaminergic receptor subtypes modulating synaptic GABA and glutamate transmission. To accomplish this task dopaminergic neurons keep mutual modulating interactions with neurons of other neurotransmitter systems, including allosteric receptor-receptor interactions in heteroreceptor complexes. As a result of its modulatory role dopaminergic mechanisms are involved in either the etiology or physiopathology of many brain diseases such as Parkinsońs disease and schizophrenia. The aim of this work is to review some novel and conventional approaches that either have been used or are currently employed to treat these diseases. Particular attention is paid to the approaches derived from the knowledge recently acquired in the realm of receptor-receptor interactions taking place through multiple dopamine heteroreceptor complexes in the plasma membrane. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Dopaminergic modulation of retinal processing from starlight to sunlight

Neuromodulators such as dopamine, enable context-dependent plasticity of neural circuit function throughout the central nervous system. For example, in the retina, dopamine tunes visual processing for daylight and nightlight conditions. Specifically, high levels of dopamine release in the retina tune vision for daylight (photopic) conditions, while low levels tune it for nightlight (scotopic) conditions. This review covers the cellular and circuit-level mechanisms within the retina that are altered by dopamine. These mechanisms include changes in gap junction coupling and ionic conductances, both of which are altered by the activation of diverse types of dopamine receptors across diverse types of retinal neurons. We contextualize the modulatory actions of dopamine in terms of alterations and optimizations to visual processing under photopic and scotopic conditions, with particular attention to how they differentially impact distinct cell types. Finally, we discuss how transgenic mice and disease models have shaped our understanding of dopaminergic signaling and its role in visual processing. Cumulatively, this review illustrates some of the diverse and potent mechanisms through which neuromodulation can shape brain function.

Understanding the role of dopamine in conditioned and unconditioned fear

Pharmacological and molecular imaging studies in anxiety disorders have primarily focused on the serotonin system. In the meantime, dopamine has been known as the neurotransmitter of reward for 60 years, particularly for its action in the nervous terminals of the mesocorticolimbic system. Interest in the mediation by dopamine of the well-known brain aversion system has grown recently, particularly given recent evidence obtained on the role of D2 dopamine receptors in unconditioned fear. However, it has been established that excitation of the mesocorticolimbic pathway, originating from dopaminergic (DA) neurons from the ventral tegmental area (VTA), is relevant for the development of anxiety. Among the forebrain regions innervated by this pathway, the amygdala is an essential component of the neural circuitry of conditioned fear. Current findings indicate that the dopamine D2 receptor-signaling pathway connecting the VTA to the basolateral amygdala modulates fear and anxiety, whereas neural circuits in the midbrain tectum underlie the expression of innate fear. The A13 nucleus of the zona incerta is proposed as the origin of these DA neurons projecting to caudal structures of the brain aversion system. In this article we review data obtained in studies showing that DA receptor-mediated mechanisms on ascending or descending DA pathways play opposing roles in fear/anxiety processes. Dopamine appears to mediate conditioned fear by acting at rostral levels of the brain and regulate unconditioned fear at the midbrain level.

Role of dopamine D2-like receptors and their modulation by adenosine receptor stimulation in the reinstatement of methamphetamine seeking

RATIONALE AND OBJECTIVE

Previous work has demonstrated that dopamine and adenosine receptors are involved in drug-seeking behaviors, yet the pharmacological interactions between these receptors in methamphetamine (MA) seeking are not well characterized. The present studies examined the role of the dopamine D₂-like receptors in MA seeking and identified the interactive effects of adenosine receptor stimulation.

METHODS

Adult male Sprague-Dawley rats were trained to lever press for MA in daily 2-h self-administration sessions on a fixed-ratio 1 schedule for 10 consecutive days. After 1 day of abstinence, lever pressing was extinguished in six daily extinction sessions. Treatments were administered systemically prior to a 2-h reinstatement test session.

RESULTS

An increase in MA seeking was observed following the administration of the dopamine D₂-like agonist, quinpirole, or the D₃ receptor agonist, 7-OH-DPAT. Stimulation of D₂ or D₄ receptors was ineffective at inducing MA seeking. Quinpirole-induced MA seeking was inhibited by D₃ receptor antagonism (SB-77011A or PG01037), an adenosine A₁ agonist, CPA, and an adenosine A_2A agonist, CGS 21680. MA seeking induced by a MA priming injection or D₃ receptor stimulation was inhibited by a pretreatment with the adenosine A₁ agonist, CPA, but not the adenosine A_2A agonist, CGS 21680.

CONCLUSIONS

These results demonstrate the sufficiency of dopamine D₃ receptors to reinstate MA seeking that is inhibited when combined with adenosine A₁ receptor stimulation.