Allosteric mechanisms within the adenosine A2A-dopamine D2 receptor heterotetramer

Adenosine A2A and dopamine D2 receptor interaction controls fatigue resistance

Introduction: Caffeine and the selective A2A receptor antagonist SCH58261 both have ergogenic properties, effectively reducing fatigue and enhancing exercise capacity. This study investigates in male Swiss mice the interaction between adenosine A2A receptors and dopamine D2 receptors controlling central fatigue, with a focus on the striatum where these receptors are most abundant. Methods: We employed DPCPX and SCH58261 to antagonize A1 and A2A receptors, caffeine as a non-competitive antagonist for both receptors, and haloperidol as a D2 receptor antagonist; all compounds were tested upon systemic application and caffeine and SCH58261 were also directly applied in the striatum. Behavioral assessments using the open field, grip strength, and treadmill tests allowed estimating the effect of treatments on fatigue. Results and discussion: The results suggested a complex interplay between the dopamine and adenosine systems. While systemic DPCPX had little effect on motor performance or fatigue, the application of either caffeine or SCH58261 was ergogenic, and these effects were attenuated by haloperidol. The intra-striatal administration of caffeine or SCH58261 was also ergogenic, but these effects were unaffected by haloperidol. These findings confirm a role of striatal A2A receptors in the control of central fatigue but suggest that the D2 receptor-mediated control of the ergogenic effects of caffeine and of A2A receptor antagonists might occur outside the striatum. This prompts the need of additional efforts to unveil the role of different brain regions in the control of fatigue.

Effects of Intracerebral Aminophylline Dosing on Catalepsy and Gait in an Animal Model of Parkinson's Disease

Parkinson's disease (PD) is a progressive disorder characterized by the apoptosis of dopaminergic neurons in the basal ganglia. This study explored the potential effects of aminophylline, a non-selective adenosine A1 and A2A receptor antagonist, on catalepsy and gait in a haloperidol-induced PD model. Sixty adult male Swiss mice were surgically implanted with guide cannulas that targeted the basal ganglia. After seven days, the mice received intraperitoneal injections of either haloperidol (experimental group, PD-induced model) or saline solution (control group, non-PD-induced model), followed by intracerebral infusions of aminophylline. The assessments included catalepsy testing on the bar and gait analysis using the Open Field Maze. A two-way repeated-measures analysis of variance (ANOVA), followed by Tukey's post hoc tests, was employed to evaluate the impact of groups (experimental × control), aminophylline (60 nM × 120 nM × saline/placebo), and interactions. Significance was set at 5%. The results revealed that the systemic administration of haloperidol in the experimental group increased catalepsy and dysfunction of gait that paralleled the observations in PD. Co-treatment with aminophylline at 60 nM and 120 nM reversed catalepsy in the experimental group but did not restore the normal gait pattern of the animals. In the non-PD induced group, which did not present any signs of catalepsy or motor dysfunctions, the intracerebral dose of aminophylline did not exert any interference on reaction time for catalepsy but increased walking distance in the Open Field Maze. Considering the results, this study highlights important adenosine interactions in the basal ganglia of animals with and without signs comparable to those of PD. These findings offer valuable insights into the neurobiology of PD and emphasize the importance of exploring novel therapeutic strategies to improve patient's catalepsy and gait.

Repeated binge-like eating episodes in female rats alter adenosine A2A and dopamine D2 receptor genes regulation in the brain reward system

OBJECTIVE

Binge-eating disorder is an eating disorder characterized by recurrent binge-eating episodes, during which individuals consume excessive amounts of highly palatable food (HPF) in a short time. This study investigates the intricate relationship between repeated binge-eating episode and the transcriptional regulation of two key genes, adenosine A2A receptor (A2AAR) and dopamine D2 receptor (D2R), in selected brain regions of rats.

METHOD

Binge-like eating behavior on HPF was induced through the combination of food restrictions and frustration stress (15 min exposure to HPF without access to it) in female rats, compared to control rats subjected to only restriction or only stress or none of these two conditions. After chronic binge-eating episodes, nucleic acids were extracted from different brain regions, and gene expression levels were assessed through real-time quantitative PCR. The methylation pattern on genes' promoters was investigated using pyrosequencing.

RESULTS

The analysis revealed A2AAR upregulation in the amygdala and in the ventral tegmental area (VTA), and D2R downregulation in the nucleus accumbens in binge-eating rats. Concurrently, site-specific DNA methylation alterations at gene promoters were identified in the VTA for A2AAR and in the amygdala and caudate putamen for D2R.

DISCUSSION

The alterations on A2AAR and D2R genes regulation highlight the significance of epigenetic mechanisms in the etiology of binge-eating behavior, and underscore the potential for targeted therapeutic interventions, to prevent the development of this maladaptive feeding behavior. These findings provide valuable insights for future research in the field of eating disorders.

PUBLIC SIGNIFICANCE

Using an animal model with face, construct, and predictive validity, in which cycles of food restriction and frustration stress evoke binge-eating behavior, we highlight the significance of epigenetic mechanisms on adenosine A2A receptor (A2AAR) and dopamine D2 receptor (D2R) genes regulation. They could represent new potential targets for the pharmacological management of eating disorders characterized by this maladaptive feeding behavior.

G protein-coupled receptors in neurodegenerative diseases and psychiatric disorders

Neuropsychiatric disorders are multifactorial disorders with diverse aetiological factors. Identifying treatment targets is challenging because the diseases are resulting from heterogeneous biological, genetic, and environmental factors. Nevertheless, the increasing understanding of G protein-coupled receptor (GPCR) opens a new possibility in drug discovery. Harnessing our knowledge of molecular mechanisms and structural information of GPCRs will be advantageous for developing effective drugs. This review provides an overview of the role of GPCRs in various neurodegenerative and psychiatric diseases. Besides, we highlight the emerging opportunities of novel GPCR targets and address recent progress in GPCR drug development.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Presynaptic adenosine receptor heteromers as key modulators of glutamatergic and dopaminergic neurotransmission in the striatum

Adenosine plays a very significant role in modulating striatal glutamatergic and dopaminergic neurotransmission. In the present essay we first review the extensive evidence that indicates this modulation is mediated by adenosine A₁ and A_2A receptors (A₁Rs and A_2ARs) differentially expressed by the components of the striatal microcircuit that include cortico-striatal glutamatergic and mesencephalic dopaminergic terminals, and the cholinergic interneuron. This microcircuit mediates the ability of striatal glutamate release to locally promote dopamine release through the intermediate activation of cholinergic interneurons. A₁Rs and A_2ARs are colocalized in the cortico-striatal glutamatergic terminals, where they form A₁R-A_2AR and A_2AR-cannabinoid CB₁ receptor (CB₁R) heteromers. We then evaluate recent findings on the unique properties of A₁R-A_2AR and A_2AR-CB₁R heteromers, which depend on their different quaternary tetrameric structure. These properties involve different allosteric mechanisms in the two receptor heteromers that provide fine-tune modulation of adenosine and endocannabinoid-mediated striatal glutamate release. Finally, we evaluate the evidence supporting the use of different heteromers containing striatal adenosine receptors as targets for drug development for neuropsychiatric disorders, such as Parkinson's disease and restless legs syndrome, based on the ability or inability of the A_2AR to demonstrate constitutive activity in the different heteromers, and the ability of some A_2AR ligands to act preferentially as neutral antagonists or inverse agonists, or to have preferential affinity for a specific A_2AR heteromer.

Adenosine A2A receptor availability in patients with early- and moderate-stage Parkinson's disease

INTRODUCTION

Adenosine 2A (A_2A) receptors co-localize with dopamine D₂ receptors in striatopallidal medium spiny neurons of the indirect pathway. A_2A receptor activation in the striatum or pallidum decreases D₂ signaling. In contrast, A_2A receptor antagonism may help potentiate it. Furthermore, previous PET studies have shown increased A_2A receptor availability in striatum of late-stage PD patients with dyskinesia. However, human in vivo evidence for striatal A_2A receptor availability in early-stage PD is limited. This study aimed to investigate possible differences in A_2A receptor availability in the striatum and pallidum of early- and moderate-stage PD patients without dyskinesias.

METHODS

Brain MRI and PET with [¹¹C]TMSX radioligand, targeting A_2A receptors, was performed in 9 patients with early- and 9 with moderate-stage PD without dyskinesia and in 6 healthy controls. Distribution volume ratios (DVR) were calculated to assess specific [¹¹C]TMSX binding in caudate, putamen and pallidum.

RESULTS

A_2A receptor availability (DVR) was decreased in the bilateral caudate of early-stage PD patients when compared with healthy controls (P = 0.02). Conversely, DVR was increased bilaterally in the pallidum of moderate-stage PD patients compared to healthy controls (P = 0.03). Increased mean striatal DVR correlated with higher motor symptom severity ([Formula: see text] = 0.47, P = 0.02).

CONCLUSION

Our results imply regional and disease stage-dependent changes in A_2A receptor signaling in PD pathophysiology and in response to dopaminergic medication.

Quantitative Structure-Neurotoxicity Assessment and In Vitro Evaluation of Neuroprotective and MAO-B Inhibitory Activities of Series N'-substituted 3-(1,3,7-trimethyl-xanthin-8-ylthio)propanehydrazides

The neurotoxic, neuroprotective and MAO-B inhibitory effects of series N'-substituted 3-(1,3,7-trimethyl-xanthin-8-ylthio)propanehydrazides are evaluated. The results indicate compounds N'-(2,3-dimethoxybenzylidene)-3-(1,3,7-trimethyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-8-ylthio)propanehydrazide (6k) and N'-(2-hydroxybenzylidene)-3-(1,3,7-trimethyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-8-ylthio)propanehydrazide (6l) as most perspective. The performed QSTR analysis identified that the decreased lipophilicity and smaller dipole moments of the molecules are the structural features ensuring lower neurotoxicity. The obtained results may be used as initial information in the further design of (xanthinyl-8-ylthio)propanhydrazides with potential hMAOB inhibitory effect and pronounced neuroprotection.

Unmasking allosteric binding sites: Novel targets for GPCR drug discovery

INTRODUCTION

Unexpected non-apparent and hidden allosteric binding sites are non-classical and non-apparent allosteric centers in 3-D X-ray protein structures until orthosteric or allosteric ligands bind to them. The orthosteric center of one protomer that modulates binding centers of the other protomers within an oligomer is also an unexpected allosteric site. Furthermore, another partner protein can also produce these effects, acting as an unexpected allosteric modulator.

AREAS COVERED

This review summarizes both classical and non-classical allosterism. The authors focus on G protein-coupled receptor (GPCR) oligomers as a paradigm of allosteric molecules. Moreover, they show several examples of unexpected allosteric sites such as hidden allosteric sites in a protomer that appear after the interaction with other molecules and the allosterism exerted between orthosteric sites within GPCR oligomer, emphasizing on the allosteric modulations that can occur between binding sites.

EXPERT OPINION

The study of these new non-classical allosteric sites will expand the diversity of allosteric control on the function of orthosteric sites within proteins, whether GPCRs or other receptors, enzymes or transporters. Moreover, the design of new drugs targeting these hidden allosteric sites or already known orthosteric sites acting as allosteric sites in protein homo- or hetero-oligomers will increase the therapeutic potential of allosterism.

Potential Treat-to-Target Approach for Methamphetamine Use Disorder: A Pilot Study of Adenosine 2A Receptor Antagonist With Positron Emission Tomography

Introduction: The misuse of stimulant drugs such as methamphetamine is a global public health issue. One important neurochemical mechanism of methamphetamine use disorder may be altered dopaminergic neurotransmission. For instance, previous studies using positron emission tomography (PET) have consistently shown that striatal dopamine D2-type receptor availability (quantified as binding potential; BPND) is lower in methamphetamine use disorder. Further, methamphetamine use is known to induce chronic neuroinflammation through multiple physiological pathways. Upregulation of D2-type receptor and/or attenuation of neuroinflammation may therefore provide a therapeutic effect for this disorder. In vitro studies have shown that blockage of adenosine 2A (A2A) receptors may prevent D2-receptor downregulation and neuroinflammation-related brain damage. However, no study has examined this hypothesis yet.

Methods and Analysis: Using a within-subject design, this trial will assess the effect of the selective A2A receptor antagonist, istradefylline, primarily on D2-type BPND in the striatum, and secondarily on neuroinflammation in the whole brain in individuals with methamphetamine use disorder. The research hypotheses are that istradefylline will increase striatal D2-type BPND and attenuate neuroinflammation. Twenty participants with methamphetamine use disorder, aged 20–65, will be recruited to undergo [11C]raclopride PET (for every participant) and [11C]DAA1106 PET (if applicable) once before and once after administration of 40 mg/day istradefylline for 2 weeks. Neuropsychological measurements will be performed on the same days of the PET scans.

Heterodimer of A2A and Oxytocin Receptors Regulating Glutamate Release in Adult Striatal Astrocytes

BACKGROUND

Roles of astrocytes in the modulatory effects of oxytocin (OT) in central nervous system are increasingly considered. Nevertheless, OT effects on gliotransmitter release have been neglected.

METHODS

In purified astrocyte processes from adult rat striatum, we assessed OT receptor (OTR) and adenosine A2A receptor expression by confocal analysis. The effects of receptors activation on glutamate release from the processes were evaluated; A2A-OTR heteromerization was assessed by co-immunoprecipitation and PLA. Structure of the possible heterodimer of A2A and OT receptors was estimated by a bioinformatic approach.

RESULTS

Both A2A and OT receptors were expressed on the same astrocyte processes. Evidence for A2A-OTR receptor-receptor interaction was obtained by measuring the release of glutamate: OT inhibited the evoked glutamate release, while activation of A2A receptors, per se ineffective, abolished the OT effect. Biochemical and biophysical evidence for A2A-OTR heterodimers on striatal astrocytes was also obtained. The residues in the transmembrane domains 4 and 5 of both receptors are predicted to be mainly involved in the heteromerization.

CONCLUSIONS

When considering effects of OT in striatum, modulation of glutamate release from the astrocyte processes and of glutamatergic synapse functioning, and the interaction with A2A receptors on the astrocyte processes should be taken into consideration.

Allosteric Modulation of Adenosine A2A Receptors as a New Therapeutic Avenue

The therapeutic potential of targeting adenosine A_2A receptors (A_2ARs) is immense due to their broad expression in the body and central nervous system. The role of A_2ARs in cardiovascular function, inflammation, sleep/wake behaviors, cognition, and other primary nervous system functions has been extensively studied. Numerous A_2AR agonist and antagonist molecules are reported, many of which are currently in clinical trials or have already been approved for treatment. Allosteric modulators can selectively elicit a physiologic response only where and when the orthosteric ligand is released, which reduces the risk of an adverse effect resulting from A_2AR activation. Thus, these allosteric modulators have a potential therapeutic advantage over classical agonist and antagonist molecules. This review focuses on the recent developments regarding allosteric A_2AR modulation, which is a promising area for future pharmaceutical research because the list of existing allosteric A_2AR modulators and their physiologic effects is still short.

Adenosine Receptors in Neuropsychiatric Disorders: Fine Regulators of Neurotransmission and Potential Therapeutic Targets

Adenosine exerts an important role in the modulation of central nervous system (CNS) activity. Through the interaction with four G-protein coupled receptor (GPCR) subtypes, adenosine subtly regulates neurotransmission, interfering with the dopaminergic, glutamatergic, noradrenergic, serotoninergic, and endocannabinoid systems. The inhibitory and facilitating actions of adenosine on neurotransmission are mainly mediated by A₁ and A_2A adenosine receptors (ARs), respectively. Given their role in the CNS, ARs are promising therapeutic targets for neuropsychiatric disorders where altered neurotransmission represents the most likely etiological hypothesis. Activating or blocking ARs with specific pharmacological agents could therefore restore the balance of altered neurotransmitter systems, providing the rationale for the potential treatment of these highly debilitating conditions. In this review, we summarize and discuss the most relevant studies concerning AR modulation in psychotic and mood disorders such as schizophrenia, bipolar disorders, depression, and anxiety, as well as neurodevelopment disorders such as autism spectrum disorder (ASD), fragile X syndrome (FXS), attention-deficit hyperactivity disorder (ADHD), and neuropsychiatric aspects of neurodegenerative disorders.

Disrupting GPCR Complexes with Smart Drug-like Peptides

G protein-coupled receptors (GPCRs) are a superfamily of proteins classically described as monomeric transmembrane (TM) receptors. However, increasing evidence indicates that many GPCRs form higher-order assemblies made up of monomers pertaining to identical (homo) or to various (hetero) receptors. The formation and structure of these oligomers, their physiological role and possible therapeutic applications raise a variety of issues that are currently being actively explored. In this context, synthetic peptides derived from TM domains stand out as powerful tools that can be predictably targeted to disrupt GPCR oligomers, especially at the interface level, eventually impairing their action. However, despite such potential, TM-derived, GPCR-disrupting peptides often suffer from inadequate pharmacokinetic properties, such as low bioavailability, a short half-life or rapid clearance, which put into question their therapeutic relevance and promise. In this review, we provide a comprehensive overview of GPCR complexes, with an emphasis on current studies using GPCR-disrupting peptides mimicking TM domains involved in multimerization, and we also highlight recent strategies used to achieve drug-like versions of such TM peptide candidates for therapeutic application.

Elevated endogenous GDNF induces altered dopamine signalling in mice and correlates with clinical severity in schizophrenia

Presynaptic increase in striatal dopamine is the primary dopaminergic abnormality in schizophrenia, but the underlying mechanisms are not understood. Here, we hypothesized that increased expression of endogenous GDNF could induce dopaminergic abnormalities that resemble those seen in schizophrenia. To test the impact of GDNF elevation, without inducing adverse effects caused by ectopic overexpression, we developed a novel in vivo approach to conditionally increase endogenous GDNF expression. We found that a 2-3-fold increase in endogenous GDNF in the brain was sufficient to induce molecular, cellular, and functional changes in dopamine signalling in the striatum and prefrontal cortex, including increased striatal presynaptic dopamine levels and reduction of dopamine in prefrontal cortex. Mechanistically, we identified adenosine A2a receptor (A_2AR), a G-protein coupled receptor that modulates dopaminergic signalling, as a possible mediator of GDNF-driven dopaminergic abnormalities. We further showed that pharmacological inhibition of A_2AR with istradefylline partially normalised striatal GDNF and striatal and cortical dopamine levels in mice. Lastly, we found that GDNF levels are increased in the cerebrospinal fluid of first episode psychosis patients, and in post-mortem striatum of schizophrenia patients. Our results reveal a possible contributor for increased striatal dopamine signalling in a subgroup of schizophrenia patients and suggest that GDNF-A_2AR crosstalk may regulate dopamine function in a therapeutically targetable manner.

Effects of an Adenosine A2A Receptor Antagonist on Striatal Dopamine D2-Type Receptor Availability: A Randomized Control Study Using Positron Emission Tomography

Introduction: Altered dopaminergic neurotransmission, especially in the functioning of dopamine D2-type receptors, is considered central to the etiology of a variety of neuropsychiatric disorders. In particular, individuals with substance use disorders have been consistently observed to exhibit lower D2-type receptor availability (quantified as binding potential; BPND) using positron emission tomography (PET). Upregulation of D2-type receptor density thus may therefore provide a therapeutic effect for substance use disorders. Importantly, in vitro studies reveal that D2 receptors coexist with adenosine 2A (A2A) receptors to form the highest density of heteromers in the whole striatum, and there is a functional interaction between these two receptors. As such, blockade of A2A receptor's function may prevent D2 receptor downregulation, yet no study has currently examined this hypothesis in humans. Methods and Analysis: This double-blind, randomized controlled trial aims to evaluate the effect of the A2A receptor antagonist istradefylline (compared to placebo) on both dopamine D2-type receptor availability in the human brain and on neuropsychological measurements of impulsivity. It is hypothesized that istradefylline will both increase striatal D2-type BPND and improve control of impulsivity more than placebo. Forty healthy participants, aged 20-65 with no history of psychiatric or neurological disorders, will be recruited and randomized into two groups and will undergo [11C]raclopride PET, once before and once after administration of either 40 mg/day istradefylline or placebo for 2 weeks. Neuropsychological measurements will be administered on the same days of the PET scans. Ethics and Dissemination: The study protocol was approved by the Certified Review Boards (CRB) of National Center of Neurology and Psychiatry (CR18-011) and prospectively registered with the Japan Registry of Clinical Trials (jRCTs031180131; https://jrct.niph.go.jp/latest-detail/jRCTs031180131). The findings of this study will be disseminated through peer reviewed scientific journals and conferences. Clinical Trial Registration:www.ClinicalTrials.gov, identifier jRCTs031180131.

Post-weaning social isolation impairs purinergic signaling in rat brain

Early life stressors, such as social isolation (SI), can disrupt brain development contributing to behavioral and neurochemical alterations in adulthood. Purinergic receptors and ectonucleotidases are key regulators of brain development in embryonic and postnatal periods, and they are involved in several psychiatric disorders, including schizophrenia. The extracellular ATP drives purinergic signaling by activating P2X and P2Y receptors and it is hydrolyzed by ectonucleotidases in adenosine, which activates P1 receptors. The purpose of this study was to investigate if SI, a rodent model used to replicate abnormal behavior relevant to schizophrenia, impacts purinergic signaling. Male Wistar rats were reared from weaning in group-housed or SI conditions for 8 weeks. SI rats exhibited impairment in prepulse inhibition and social interaction. SI presented increased ADP levels in cerebrospinal fluid and ADP hydrolysis in the hippocampus and striatum synaptosomes. Purinergic receptor expressions were upregulated in the prefrontal cortex and downregulated in the hippocampus and striatum. A_2A receptors were differentially expressed in SI prefrontal cortex and the striatum, suggesting distinct roles in these brain structures. SI also presented decreased ADP, adenosine, and guanosine levels in the cerebrospinal fluid in response to D-amphetamine. Like patients with schizophrenia, uric acid levels were prominently increased in SI rats after D-amphetamine challenge. We suggest that the SI-induced deficits in prepulse inhibition might be related to the SI-induced changes in purinergic signaling. We provide new evidence that purinergic signaling is markedly affected in a rat model relevant to schizophrenia, pointing out the importance of purinergic system in psychiatry conditions.

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

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

Striatopallidal Pathway Distinctly Modulates Goal-Directed Valuation and Acquisition of Instrumental Behavior via Striatopallidal Output Projections

The striatopallidal pathway is specialized for control of motor and motivational behaviors, but its causal role in striatal control of instrumental learning remains undefined (partly due to the confounding motor effects). Here, we leveraged the transient and "time-locked" optogenetic manipulations with the reward delivery to minimize motor confounding effect, to better define the striatopallidal control of instrumental behaviors. Optogenetic (Arch) silencing of the striatopallidal pathway in the dorsomedial striatum (DMS) and dorsolateral striatum (DLS) promoted goal-directed and habitual behaviors, respectively, without affecting acquisition of instrumental behaviors, indicating striatopallidal pathway suppression of instrumental behaviors under physiological condition. Conversely, striatopallidal pathway activation mainly affected the acquisition of instrumental behaviors with the acquisition suppression achieved by either optogenetic (ChR2) or chemicogenetic (hM3q) activation, by strong (10 mW, but not weak 1 mW) optogenetic activation, by the time-locked (but not random) optogenetic activation with the reward and by the DMS (but not DLS) striatopallidal pathway. Lastly, striatopallidal pathway modulated instrumental behaviors through striatopallidal output projections into the external globus pallidus (GPe) since optogenetic activation of the striatopallidal pathway in the DMS and of the striatopallidal output projections in the GPe similarly suppressed goal-directed behavior. Thus, the striatopallidal pathway confers distinctive and inhibitory controls of animal's sensitivity to goal-directed valuation and acquisition of instrumental behaviors under normal and over-activation conditions, through the output projections into GPe.

The impact of caffeine consumption on clinical symptoms in patients with bipolar disorder: A systematic review

OBJECTIVES

In healthy populations, caffeine appears to have beneficial effects on health; however, patients with bipolar disorder (BD) are routinely advised to limit caffeine use in psychoeducation programmes. We aimed to examine all literature reporting whether caffeine intake/withdrawal impacts the natural course of BD, in terms of clinical outcomes.

METHODS

PubMed, Embase and PsycINFO were searched (up to 17/07/2020) and all studies reporting data on individuals with BD comparing a measure of caffeine use with illness severity (symptoms of mania, depression, psychosis, anxiety, sleep or suicidality) were included.

RESULTS

Of the 1678 records reviewed, 17 studies met inclusion criteria (10 case reports, 1 retrospective cohort study, 5 cross-sectional studies, 1 interventional study). Most case reports described people with BD switching to manic, hypomanic or mixed states after consuming caffeine in variable amounts and/or whose serum lithium concentrations increased after reducing caffeine consumption. The interventional study found that caffeine may suppress lithium concentrations, while the retrospective cohort study reported that participants who drank coffee had more suicidality than non-drinkers.

CONCLUSIONS

The literature examining clinical effects of caffeine in patients with BD is not conclusive. Acute increases in caffeine consumption may precede the occurrence of manic symptoms in patients with BD, potentially through a direct stimulant effect, affecting sleep patterns and/or the metabolism of lithium or other medications, although increases in caffeine intake could also be a consequence of an ongoing manic relapse or a prodromal sign. Further research is needed to determine whether caffeine use impacts the long-term prognosis of BD.

Experimental Evidence of A2A-D2 Receptor-Receptor Interactions in the Rat and Human Carotid Body

Adenosine A_2A receptors (A_2AR) and dopamine D₂ receptors (D₂R) are known to be involved in the physiological response to hypoxia, and their expression/activity may be modulated by chronic sustained or intermittent hypoxia. To date, A_2AR and D₂R can form transient physical receptor–receptor interactions (RRIs) giving rise to a dynamic equilibrium able to influence ligand binding and signaling, as demonstrated in different native tissues and transfected mammalian cell systems. Given the presence of A_2AR and D₂R in type I cells, type II cells, and afferent nerve terminals of the carotid body (CB), the aim of this work was to demonstrate here, for the first time, the existence of A_2AR–D₂R heterodimers by in situ proximity ligation assay (PLA). Our data by PLA analysis and tyrosine hydroxylase/S100 colocalization indicated the formation of A_2AR–D₂R heterodimers in type I and II cells of the CB; the presence of A_2AR–D₂R heterodimers also in afferent terminals is also suggested by PLA signal distribution. RRIs could play a role in CB dynamic modifications and plasticity in response to development/aging and environmental stimuli, including chronic intermittent/sustained hypoxia. Exploring other RRIs will allow for a broad comprehension of the regulative mechanisms these interactions preside over, with also possible clinical implications.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Adenosine A1 and A2A receptors are involved on guanosine protective effects against oxidative burst and mitochondrial dysfunction induced by 6-OHDA in striatal slices

6-Hydroxydopamine (6-OHDA) is the most used toxin in experimental Parkinson's disease (PD) models. 6-OHDA shows high affinity for the dopamine transporter and once inside the neuron, it accumulates and undergoes non-enzymatic auto-oxidation, promoting reactive oxygen species (ROS) formation and selective damage of catecholaminergic neurons. In this way, our group has established a 6-OHDA in vitro protocol with rat striatal slices as a rapid and effective model for screening of new drugs with protective effects against PD. We have shown that co-incubation with guanosine (GUO, 100 μM) prevented the 6-OHDA-induced damage in striatal slices. As the exact GUO mechanism of action remains unknown, the aim of this study was to investigate if adenosine A₁ (A₁R) and/or A_2A receptors (A_2AR) are involved on GUO protective effects on striatal slices. Pre-incubation with DPCPX, an A₁R antagonist prevented guanosine effects on 6-OHDA-induced ROS formation and mitochondrial membrane potential depolarization, while CCPA, an A₁R agonist, did not alter GUO effects. Regarding A_2AR, the antagonist SCH58261 had similar protective effect as GUO in ROS formation and mitochondrial membrane potential. Additionally, SCH58261 did not affect GUO protective effects. The A_2AR agonist CGS21680, although, completely blocked GUO effects. Finally, the A₁R antagonist DPCPX, and the A_2AR agonist CGS21680 also abolished the preventive guanosine effect on 6-OHDA-induced ATP levels decrease. These results reinforce previous evidence for a putative interaction of GUO with A₁R-A_2AR heteromer as its molecular target and clearly indicate a dependence on adenosine receptors modulation to GUO protective effect.

Detection of G Protein-Coupled Receptor Complexes in Postmortem Human Brain by Proximity Ligation Assay

Combining immunological and molecular biological methods, the antibody-based proximity ligation assay (PLA) has been used for more than a decade to detect and quantify protein-protein interactions, protein modification, and protein expression in situ, including in brain tissue. However, the transfer of this technology to human brain samples requires a number of precautions due to the nature of the specimens and their specific processing. Here, we used the PLA brightfield detection technique to assess the expression of dopamine D2 receptor and adenosine A2A receptor and their proximity in human postmortem brains, and we developed a systematic random sampling method to help quantify the PLA signals. © 2019 by John Wiley & Sons, Inc. Basic Protocol 1: Sample preparation and sectioning for PLA_BF Basic Protocol 2: PLA_BF staining of brain tissue Basic Protocol 3: Image acquisition and result analysis Support Protocol: Luxol fast blue/cresyl violet staining.

Involvement of adenosine A1 and A2A receptors on guanosine-mediated anti-tremor effects in reserpinized mice

Parkinson's disease (PD) signs and symptoms regularly include tremor. Interestingly, the nucleoside guanosine (GUO) has already proven to be effective in reducing reserpine-induced tremulous jaw movements (TJMs) in rodent models, thus becoming a promising antiparkinsonian drug. Here, we aimed at revealing the mechanism behind GUO antiparkinsonian efficacy by assessing the role of adenosine A₁ and A_2A receptors (A₁R and A_2AR) on GUO-mediated anti-tremor effects in the reserpinized mouse model of PD. Reserpinized mice showed elevated reactive oxygen species (ROS) production and cellular membrane damage in striatal slices assessed ex vivo and GUO treatment reversed ROS production. Interestingly, while the simultaneous administration of sub-effective doses of GUO (5 mg/kg) and SCH58261 (0.01 mg/kg), an A_2AR antagonist, precluded reserpine-induced TJMs, these were ineffective on reverting ROS production in ex vivo experiments. Importantly, GUO was able to reduce TJM and ROS production in reserpinized mouse lacking the A_2AR, thus suggesting an A_2AR-independent mechanism of GUO-mediated effects. Conversely, the administration of DPCPX (0.75 mg/kg), an A₁R antagonist, completely abolished both GUO-mediated anti-tremor effects and blockade of ROS production. Overall, these results indicated that GUO anti-tremor and antioxidant effects in reserpinized mice were A₁R dependent but A_2AR independent, thus suggesting a differential participation of adenosine receptors in GUO-mediated effects.

Xanthine Derivatives as Agents Affecting Non-dopaminergic Neuroprotection in Parkinson`s Disease

Parkinson's Disease (PD) is a neurodegenerative and debilitating disease that affects 1% of the elderly population. Patient's motor disability results in extreme difficulty to deal with daily activities. Conventional treatment is limited to dopamine replacement therapy, which fails to delay disease's progression and is often associated with a number of adverse reactions. Recent progress in understanding the mechanisms involved in PD has revealed new molecular targets for therapeutic approaches. Among them, caffeine and xanthine derivatives are promising drug candidates, because of the possible symptomatic benefits in PD. In fact, consumption of coffee correlates with a reduced risk of PD. Over the last decades, a lot of efforts have been made to uncover the therapeutic potential of xanthine structures. The substituted xanthine molecule is used as a scaffold for the synthesis of new compounds with protective effects in neurodegenerative diseases, including PD, asthma, cancer and others. The administration of the xanthines has been proposed as a non-dopaminergic strategy for neuroprotection in PD and the mechanisms of protection have been associated with antagonism of adenosine A2A receptors and Monoamine Oxidase type B (MAO-B) inhibition. The current review summarizes frequently suspected non-dopaminergic neuroprotective mechanisms and the possible beneficial effects of the xanthine derivatives in PD, along with some synthetic approaches to produce perspective xanthine derivatives as non-dopaminergic agents in PD treatment.

Adenosine Receptors in Modulation of Central Nervous System Disorders

The ubiquitous signaling nucleoside molecule, adenosine is found in different cells of the human body to provide its numerous pharmacological role. The associated actions of endogenous adenosine are largely dependent on conformational change of the widely expressed heterodimeric G-protein-coupled A1, A2A, A2B, and A3 adenosine receptors (ARs). These receptors are well conserved on the surface of specific cells, where potent neuromodulatory properties of this bioactive molecule reflected by its easy passage through the rigid blood-brainbarrier, to simultaneously act on the central nervous system (CNS). The minimal concentration of adenosine in body fluids (30-300 nM) is adequate to exert its neuromodulatory action in the CNS, whereas the modulatory effect of adenosine on ARs is the consequence of several neurodegenerative diseases. Modulatory action concerning the activation of such receptors in the CNS could be facilitated towards neuroprotective action against such CNS disorders. Our aim herein is to discuss briefly pathophysiological roles of adenosine on ARs in the modulation of different CNS disorders, which could be focused towards the identification of potential drug targets in recovering accompanying CNS disorders. Researches with active components with AR modulatory action have been extended and already reached to the bedside of the patients through clinical research in the improvement of CNS disorders. Therefore, this review consist of recent findings in literatures concerning the impact of ARs on diverse CNS disease pathways with the possible relevance to neurodegeneration.

PET Imaging of the Adenosine A2A Receptor in the Rotenone-Based Mouse Model of Parkinson's Disease with [18F]FESCH Synthesized by a Simplified Two-Step One-Pot Radiolabeling Strategy

The adenosine A_2A receptor (A_2AR) is regarded as a particularly appropriate target for non-dopaminergic treatment of Parkinson’s disease (PD). An increased A_2AR availability has been found in the human striatum at early stages of PD and in patients with PD and dyskinesias. The aim of this small animal positron emission tomography/magnetic resonance (PET/MR) imaging study was to investigate whether rotenone-treated mice reflect the aspect of striatal A_2AR upregulation in PD. For that purpose, we selected the known A_2AR-specific radiotracer [¹⁸F]FESCH and developed a simplified two-step one-pot radiosynthesis. PET images showed a high uptake of [¹⁸F]FESCH in the mouse striatum. Concomitantly, metabolism studies with [¹⁸F]FESCH revealed the presence of a brain-penetrant radiometabolite. In rotenone-treated mice, a slightly higher striatal A_2AR binding of [¹⁸F]FESCH was found. Nonetheless, the correlation between the increased A_2AR levels within the proposed PD animal model remains to be further investigated.

Zebrafish models: Gaining insight into purinergic signaling and neurological disorders

Zebrafish (Danio rerio) has been considered a complementary model for biomedical studies, especially due to advantages such as external and rapid development, and genetic manipulation. There is growing interest in this model in neuroscience research since the species has morphological and physiological similarities to mammals and a complex behavioral repertoire. The purinergic signaling has been described in zebrafish, and purinoceptors and nucleotide- and nucleoside-metabolizing enzymes have already been identified in the central nervous system (CNS) of this species. The involvement of the purinergic system in several models of neurological disorders, such as Alzheimers disease, Parkinson's disease, epilepsy, schizophrenia, and autism has been investigated in zebrafish. This mini review presents several studies describing purinergic signaling in the zebrafish CNS and the action of this neurotransmitter system in models of neurological disorders using this species as a biological model. The use of pharmacological approaches at different stages of development may be a useful tool for preclinical assays and the testing of purinergic compounds as new alternatives for the treatment of neurological disorders.

Detecting G protein-coupled receptor complexes in postmortem human brain with proximity ligation assay and a Bayesian classifier

Despite the controversy regarding the existence and physiological relevance of class A G protein-coupled receptor dimerization, there is substantial evidence for functional interactions between the dopamine D2 receptor (D2R) and the adenosine A2A receptor (A2AR). A2AR-D2R complexes have been detected in rodent brains by proximity ligation assay; however, their existence in the human brain has not been demonstrated. In this study, we used Brightfield proximity ligation assay, combined with a systematic sampling and a parameter-free naive Bayesian classifier, and demonstrated proximity between the D2R and the A2AR in the adult human ventral striatum, consistent with their colocalization within complexes and the possible existence of D2R-A2AR heteromers. These methods are applicable to the relative quantification of proximity of two proteins, as well as the expression levels of individual proteins.

Brightfield proximity ligation assay was used to assess the expression of G protein-coupled receptors and their proximity in postmortem adult human brains. A novel automated machine learning method (Bayesian optimized PLA signal sorting) was developed to automatically quantify Brightfield proximity ligation assay data.

Intravenous and oral caffeine self-administration in rats

Caffeine is widely consumed for its psychoactive effects worldwide. No pre-clinical study has established reliable caffeine self-administration, but we found that caffeine can enhance the reinforcing effects of non-drug rewards. The goal of the present studies was to determine if this effect of caffeine could result in reliable caffeine self-administration. In 2 experiments rats could make an operant response for caffeine delivered in conjunction with an oral 'vehicle' including saccharin (0.2% w/v) as a primary reinforcer. In Experiment 1, intravenous (IV) caffeine infusions were delivered in conjunction with oral saccharin for meeting the schedule of reinforcement. In control conditions, oral saccharin alone or presentations of IV caffeine alone served as the reinforcer. In Experiment 2, access to caffeine was provided in an oral vehicle containing water, decaffeinated instant coffee (0.5% w/v), or decaffeinated coffee and saccharin (0.2%). The concentration of oral caffeine was then manipulated across testing sessions. Oral and IV caffeine robustly increased responding for saccharin in a manner that was repeatable, reliable, and systematically related to unit IV dose. However, the relationship between oral caffeine dose and operant behavior was less systematic; the rats appeared to titrate their caffeine intake by reducing the consummatory response (drinking) rather than the appetitive response (lever pressing). These studies establish reliable volitional caffeine self-administration in rats. The reinforcement enhancing effects of caffeine may help to explain widespread caffeine use by humans, who ingest caffeine in complex vehicles with reinforcing properties.

Functional roles of ST8SIA3-mediated sialylation of striatal dopamine D2 and adenosine A2A receptors

Sialic acids are typically added to the end of glycoconjugates by sialyltransferases. Among the six ST8 α-N-acetyl-neuraminide α-2,8-sialyltransferases (ST8SIA) existing in adult brains, ST8SIA2 is a schizophrenia-associated gene. However, the in vivo substrates and physiological functions of most sialyltransferases are currently unknown. The ST8SIA3 is enriched in the striatum. Here, we showed that ablation of St8sia3 in mice (St8sia3-KO) led to fewer disialylated and trisialylated terminal glycotopes in the striatum of St8sia3-KO mice. Moreover, the apparent sizes of several striatum-enriched G-protein-coupled receptors (GPCRs) (including the adenosine A_2A receptor (A_2AR) and dopamine D₁/D₂ receptors (D₁R and D₂R)) were smaller in St8sia3-KO mice than in WT mice. A sialidase treatment removed the differences in the sizes of these molecules between St8sia3-KO and WT mice, confirming the involvement of sialylation. Expression of ST8SIA3 in the striatum of St8sia3-KO mice using adeno-associated viruses normalized the sizes of these proteins, demonstrating a direct role of ST8SIA3. The lack of ST8SIA3-mediated sialylation altered the distribution of these proteins in lipid rafts and the interaction between D₂R and A_2AR. Locomotor activity assays revealed altered pharmacological responses of St8sia3-KO mice to drugs targeting these receptors and verified that a greater population of D₂R formed heteromers with A_2AR in the striatum of St8sia3-KO mice. Since the A_2AR-D₂R heteromer is an important drug target for several basal ganglia diseases (such as schizophrenia and Parkinson's disease), the present study not only reveals a crucial role for ST8SIA3 in striatal functions but also provides a new drug target for basal ganglia-related diseases.

The acute effects of decaffeinated versus caffeinated coffee on reaction time, mood and skeletal muscle strength

Background Caffeinated coffee, a psycho-stimulant, is widely consumed throughout the globe. However, its chronic consumption has deleterious effects on human health. Meanwhile, decaffeinated coffee has low content of caffeine and thus can be an alternative to caffeinated coffee. Therefore, the study was undertaken to explore and compare the acute effects of decaffeinated and caffeinated coffee on reaction time, mood and skeletal muscle strength in healthy volunteers. Methods This was a prospective, interventional, comparative type of study. The study included 70 healthy adults divided into two groups (Caffeinated coffee group and Decaffeinated coffee group). The following parameters were assessed: reaction time was assessed by digital display multiple-choice apparatus, mood by Visual Analogue Scale (VAS) and Profile of Mood States revised version (POMS) and skeletal muscle strength by hand dynamometer. All parameters in both groups were assessed pre-intervention (baseline) and 30 min post-intervention. Results In both groups (decaffeinated and caffeinated coffee) post-intervention, there was a statistically significant (p < 0.05) improvement in the reaction time (VRT) and mood (VAS, POMS) from the baseline. However, both groups did not show any significant effects on the skeletal muscle strength. Upon comparing the two groups, we found that caffeinated coffee showed higher and significant improvement of mood than decaffeinated coffee. Conclusions Decaffeinated coffee exerts an acute significant stimulatory effect on the reaction time and mood. However, these effects in comparison to caffeinated coffee are low. Further randomized control clinical trials are thus needed to validate these interesting findings.

Diverse Mechanisms Lead to Common Dysfunction of Striatal Cholinergic Interneurons in Distinct Genetic Mouse Models of Dystonia

Clinical and experimental data indicate striatal cholinergic dysfunction in dystonia, a movement disorder typically resulting in twisted postures via abnormal muscle contraction. Three forms of isolated human dystonia result from mutations in the TOR1A (DYT1), THAP1 (DYT6), and GNAL (DYT25) genes. Experimental models carrying these mutations facilitate identification of possible shared cellular mechanisms. Recently, we reported elevated extracellular striatal acetylcholine by in vivo microdialysis and paradoxical excitation of cholinergic interneurons (ChIs) by dopamine D2 receptor (D2R) agonism using ex vivo slice electrophysiology in Dyt1 ^ΔGAG/+ mice. The paradoxical excitation was caused by overactive muscarinic receptors (mAChRs), leading to a switch in D2R coupling from canonical G_i/o to noncanonical β-arrestin signaling. We sought to determine whether these mechanisms in Dyt1 ^ΔGAG/+ mice are shared with Thap1 ^C54Y/+ knock-in and Gnal ^+/- knock-out dystonia models and to determine the impact of sex. We found Thap1 ^C54Y/+ mice of both sexes have elevated extracellular striatal acetylcholine and D2R-induced paradoxical ChI excitation, which was reversed by mAChR inhibition. Elevated extracellular acetylcholine was absent in male and female Gnal ^+/- mice, but the paradoxical D2R-mediated ChI excitation was retained and only reversed by inhibition of adenosine A2ARs. The G_i/o-preferring D2R agonist failed to increase ChI excitability, suggesting a possible switch in coupling of D2Rs to β-arrestin, as seen previously in a DYT1 model. These data show that, whereas elevated extracellular acetylcholine levels are not always detected across these genetic models of human dystonia, the D2R-mediated paradoxical excitation of ChIs is shared and is caused by altered function of distinct G-protein-coupled receptors.SIGNIFICANCE STATEMENT Dystonia is a common and often disabling movement disorder. The usual medical treatment of dystonia is pharmacotherapy with nonselective antagonists of muscarinic acetylcholine receptors, which have many undesirable side effects. Development of new therapeutics is a top priority for dystonia research. The current findings, considered in context with our previous investigations, establish a role for cholinergic dysfunction across three mouse models of human genetic dystonia: DYT1, DYT6, and DYT25. The commonality of cholinergic dysfunction in these models arising from diverse molecular etiologies points the way to new approaches for cholinergic modulation that may be broadly applicable in dystonia.

Distinct Dopamine D₂ Receptor Antagonists Differentially Impact D₂ Receptor Oligomerization

Dopamine D₂ receptors (D₂R) are known to form transient homodimer complexes, of which the increased formation has already been associated with development of schizophrenia. Pharmacological targeting and modulation of the equilibrium of these receptor homodimers might lead to a better understanding of the critical role played by these complexes in physiological and pathological conditions. Whereas agonist addition has shown to prolong the D₂R dimer lifetime and increase the level of dimer formation, the possible influence of D₂R antagonists on dimerization has remained rather unexplored. Here, using a live-cell reporter assay based on the functional complementation of a split Nanoluciferase, a panel of six D₂R antagonists were screened for their ability to modulate the level of D_2LR dimer formation. Incubation with the D₂R antagonist spiperone decreased the level of D_2LR dimer formation significantly by 40–60% in real-time and after long-term (≥16 h) incubations. The fact that dimer formation of the well-studied A_2a–D_2LR dimer was not altered following incubation with spiperone supports the specificity of this observation. Other D₂R antagonists, such as clozapine, risperidone, and droperidol did not significantly evoke this dissociation event. Furthermore, molecular modeling reveals that spiperone presents specific Tyr199^5.48 and Phe390^6.52 conformations, compared to clozapine, which may determine D₂R homodimerization.

Behavioral economic analysis of the reinforcing effects of "bath salts" mixtures: studies with MDPV, methylone, and caffeine in male Sprague-Dawley rats

RATIONALE

"Bath salts" preparations often contain combinations of synthetic cathinones (e.g., 3,4-methylenedioxymethcathinone [methylone], 3,4-methylenedioxypyrovalerone [MDPV]), and caffeine, and evidence suggests that mixtures of synthetic cathinones and caffeine (e.g., MDPV + caffeine or methylone + caffeine) can be more potent and/or effective reinforcers than predicted for an additive interaction.

OBJECTIVE

To use demand curve analyses to compare the reinforcing effectiveness of MDPV and methylone to mixtures of MDPV + caffeine and methylone + caffeine.

METHODS

Male Sprague-Dawley rats acquired methylone self-administration (0.32 mg/kg/inf) under a fixed ratio (FR) 1 schedule of reinforcement and generated full dose-response curves for methylone (0.01-1 mg/kg/inf) under an FR5 schedule of reinforcement. Demand curves were then obtained for methylone, MDPV, caffeine, and methylone + caffeine and MDPV + caffeine mixtures by increasing the FR across sessions according to the following series: 3, 10, 18, 32, 56, 100, 178, etc. RESULTS: Self-administration of methylone was rapidly acquired by 87.5% of rats and was maintained across a range of doses, producing an inverted U-shaped dose-response curve. Rank order demand for the individual constituents was MDPV > methylone > caffeine. Demand for the 3:1 (but not 10:1) methylone + caffeine mixture was greater than that for methylone alone, and demand for MDPV alone was similar to both MDPV + caffeine mixtures evaluated.

CONCLUSIONS

These studies provide additional evidence that although methylone is an effective reinforcer, combining methylone with caffeine results in an enhanced reinforcing effectiveness compared to methylone alone. Thus, abused "bath salts" preparations containing synthetic cathinones and caffeine may have higher abuse liability than preparations containing only synthetic cathinones.

The heterotetrameric structure of the adenosine A1-dopamine D1 receptor complex: Pharmacological implication for restless legs syndrome

Dopaminergic and purinergic signaling play a pivotal role in neurological diseases associated with motor symptoms, including Parkinson's disease (PD), multiple sclerosis, amyotrophic lateral sclerosis, Huntington disease, Restless Legs Syndrome (RLS), spinal cord injury (SCI), and ataxias. Extracellular dopamine and adenosine exert their functions interacting with specific dopamine (DR) or adenosine (AR) receptors, respectively, expressed on the surface of target cells. These receptors are members of the family A of G protein-coupled receptors (GPCRs), which is the largest protein superfamily in mammalian genomes. GPCRs are target of about 40% of all current marketed drugs, highlighting their importance in clinical medicine. The striatum receives the densest dopamine innervations and contains the highest density of dopamine receptors. The modulatory role of adenosine on dopaminergic transmission depends largely on the existence of antagonistic interactions mediated by specific subtypes of DRs and ARs, the so-called A_2AR-D₂R and A₁R-D₁R interactions. Due to the dopamine/adenosine antagonism in the CNS, it was proposed that ARs and DRs could form heteromers in the neuronal cell surface. Therefore, adenosine can affect dopaminergic signaling through receptor-receptor interactions and by modulations in their shared intracellular pathways in the striatum and spinal cord. In this work we describe the allosteric modulations between GPCR protomers, focusing in those of adenosine and dopamine within the A₁R-D₁R heteromeric complex, which is involved in RLS. We also propose that the knowledge about the intricate allosteric interactions within the A₁R-D₁R heterotetramer, may facilitate the treatment of motor alterations, not only when the dopamine pathway is hyperactivated (RLS, chorea, etc.) but also when motor function is decreased (SCI, aging, PD, etc.).

Shaky ground - The nature of metastable GPCR signalling complexes

How G protein-coupled receptors (GPCR) interact with one another remains an area of active investigation. Obligate dimers of class C GPCRs such as metabotropic GABA and glutamate receptors are well accepted, although whether this is a general feature of other GPCRs is still strongly debated. In this review, we focus on the idea that GPCR dimers and oligomers are better imagined as parts of larger metastable signalling complexes. We discuss the nature of functional oligomeric entities, their stabilities and kinetic features and how structural and functional asymmetries of such metastable entities might have implications for drug discovery. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Craving espresso: the dialetics in classifying caffeine as an abuse drug

Caffeine is the most consumed psychoactive substance in the world; in general, it is not associated to potentially harmful effects. Nevertheless, few studies were performed attempting to investigate the caffeine addiction. The present review was mainly aimed to answer the following question: is caffeine an abuse drug? To adress this point, the effects of caffeine in preclinical and clinical studies were summarized and critically analyzed taking account the abuse disorders described in the Diagnostic and Statistical Manual of Mental Disorders (DSM-V). We concluded that the diagnostic criteria evidenced on DSM-V to intoxication-continued use and abstinence are not well supported by clinical studies. The fact that diagnostic criteria is not widely supported by preclinical or clinical studies may be due specially to a controversy in its exactly mechanism of action: recent literature point to an indirect, rather than direct modulation of dopamine receptors, and auto-limitant consumption due to adverse sensations in high doses. On the other hand, it reports clear withdrawal-related symptoms. Thus, based on a classical action on reward system, caffeine only partially fits its mechanism of action as an abuse drug, especially because previous research does not report a clear effect of dopaminergic activity enhance on nucleus accumbens; despite this, there are reports concerning dopaminergic modulation by caffeine on the striatum. However, based on human and animal research, caffeine withdrawal evokes signals and symptoms, which are relevant enough to include this substance among the drugs of abuse.

Essential Control of the Function of the Striatopallidal Neuron by Pre-coupled Complexes of Adenosine A2A-Dopamine D2 Receptor Heterotetramers and Adenylyl Cyclase

The central adenosine system and adenosine receptors play a fundamental role in the modulation of dopaminergic neurotransmission. This is mostly achieved by the strategic co-localization of different adenosine and dopamine receptor subtypes in the two populations of striatal efferent neurons, striatonigral and striatopallidal, that give rise to the direct and indirect striatal efferent pathways, respectively. With optogenetic techniques it has been possible to dissect a differential role of the direct and indirect pathways in mediating "Go" responses upon exposure to reward-related stimuli and "NoGo" responses upon exposure to non-rewarded or aversive-related stimuli, respectively, which depends on their different connecting output structures and their differential expression of dopamine and adenosine receptor subtypes. The striatopallidal neuron selectively expresses dopamine D₂ receptors (D2R) and adenosine A_2A receptors (A2AR), and numerous experiments using multiple genetic and pharmacological in vitro, in situ and in vivo approaches, demonstrate they can form A2AR-D2R heteromers. It was initially assumed that different pharmacological interactions between dopamine and adenosine receptor ligands indicated the existence of different subpopulations of A2AR and D2R in the striatopallidal neuron. However, as elaborated in the present essay, most evidence now indicates that all interactions can be explained with a predominant population of striatal A2AR-D2R heteromers forming complexes with adenylyl cyclase subtype 5 (AC5). The A2AR-D2R heteromer has a tetrameric structure, with two homodimers, which allows not only multiple allosteric interactions between different orthosteric ligands, agonists, and antagonists, but also the canonical Gs-Gi antagonistic interaction at the level of AC5. We present a model of the function of the A2AR-D2R heterotetramer-AC5 complex, which acts as an integrative device of adenosine and dopamine signals that determine the excitability and gene expression of the striatopallidal neurons. The model can explain most behavioral effects of A2AR and D2R ligands, including the psychostimulant effects of caffeine. The model is also discussed in the context of different functional striatal compartments, mainly the dorsal and the ventral striatum. The current accumulated knowledge of the biochemical properties of the A2AR-D2R heterotetramer-AC5 complex offers new therapeutic possibilities for Parkinson's disease, schizophrenia, SUD and other neuropsychiatric disorders with dysfunction of dorsal or ventral striatopallidal neurons.

The Dynamics of GPCR Oligomerization and Their Functional Consequences

The functional importance of G protein-coupled receptor (GPCR) oligomerization remains controversial. Although obligate dimers of class C GPCRs are well accepted, the generalizability of this phenomenon is still strongly debated with respect to other classes of GPCRs. In this review, we focus on understanding the organization and dynamics between receptor equivalents and their signaling partners in oligomeric receptor complexes, with a view toward integrating disparate viewpoints into a unified understanding. We discuss the nature of functional oligomeric entities, and how asymmetries in receptor structure and function created by oligomers might have implications for receptor function as allosteric machines and for future drug discovery.

Allosterism within δ Opioid-κ Opioid Receptor Heteromers in Peripheral Sensory Neurons: Regulation of κ Opioid Agonist Efficacy

There is abundant evidence for formation of G protein-coupled receptor heteromers in heterologous expression systems, but little is known of the function of heteromers in native systems. Heteromers of δ and κ opioid receptors (DOR-KOR heteromers) have been identified in native systems. We previously reported that activation of DOR-KOR heteromers expressed by rat pain-sensing neurons (nociceptors) produces robust, peripherally mediated antinociception. Moreover, DOR agonist potency and efficacy is regulated by KOR antagonists via allosteric interactions within the DOR-KOR heteromer in a ligand-dependent manner. Here we assessed the reciprocal regulation of KOR agonist function by DOR antagonists in adult rat nociceptors in culture and in a behavioral assay of nociception. Naltrindole enhanced the potency of the KOR agonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-pyrrolidin-1-ylethyl]acetamide (ICI-199441) 10- to 20-fold, but did not alter responses to 2-(3,4-dichlorophenyl)-N-methyl-N-[(1R,2R)-2-pyrrolidin-1-ylcyclohexyl]acetamide (U50488). By contrast, the potency of U50488 was enhanced 20-fold by 7-benzylidenenaltrexone. The efficacy of 6'-guanidinonaltrindole (6'-GNTI) to inhibit nociceptors was blocked by small interfering RNA knockdown of DOR or KOR. Replacing 6'-GNTI occupancy of DOR with either naltrindole or 7-benzylidenenaltrexone abolished 6'-GNTI efficacy. Further, peptides derived from DOR transmembrane segment 1 fused to the cell membrane-penetrating HIV transactivator of transcription peptide also blocked 6'-GNTI-mediated responses ex vivo and in vivo, suggesting that 6'-GNTI efficacy in nociceptors is due to its positive allosteric regulation of KOR via occupancy of DOR in a DOR-KOR heteromer. Together, these results provide evidence for the existence of functional DOR-KOR heteromers in rat peripheral sensory neurons and that reciprocal, ligand-dependent allosteric interactions occur between the DOR and KOR protomers.

The D2 Dopamine Receptor Interferes With the Protective Effect of the A2A Adenosine Receptor on TDP-43 Mislocalization in Experimental Models of Motor Neuron Degeneration

The A2A adenosine receptor (A2AR) and D2 dopamine receptor (D2R) are two G-protein-coupled receptors that can form dimers and negatively regulate their partners. TAR DNA-binding protein (TDP-43) is a nuclear protein that has been implicated in amyotrophic lateral sclerosis (ALS). Mislocalization of TDP-43 from the nucleus to the cytoplasm is an early step of TDP-43 proteinopathy. Our previous studies indicated that A2AR is a potential drug target for ALS because treatment with an A2AR agonist (JMF1907; a T1-11 analog) prevents reactive oxygen species (ROS)-induced TDP-43 mislocalization in a motor neuron cell line (NSC34) and delays motor impairment in a TDP-43 transgenic ALS mouse model. Here, we set out to assess whether activation of D2R interferes with the beneficial effects of an A2AR agonist on motor neurons. We first demonstrated that A2AR and D2R are both located in motor neurons of mouse and human spinal cords and human iPSC-derived motor neurons. Expression of A2AR and D2R in NSC34 cells led to dimer formation without affecting the binding affinity of A2AR toward T1-11. Importantly, activation of D2R reduced T1-11-mediated activation of cAMP/PKA signaling and subsequent inhibition of TDP-43 mislocalization in NSC34 cells. Treatment with quinpirole (a D2 agonist) blunted the rescuing effect of T1-11 on TDP-43 mislocalization and impaired grip strength in a mouse model of ALS. Our findings suggest that D2R activation may limit the beneficial responses of an A2AR agonist in motor neurons and may have an important role in ALS pathogenesis.

CNS syndromes associated with antibodies against metabotropic receptors

PURPOSE OF REVIEW

Autoantibodies to Central nervous system (CNS) metabotropic receptors are associated with a growing family of autoimmune brain diseases, including encephalitis, basal ganglia encephalitis, Ophelia syndrome, and cerebellitis. The purpose of this review is to summarize the state of knowledge regarding the target receptors, the neurological autoimmune disorders, and the pathogenic mechanisms.

RECENT FINDINGS

Antibodies to the γ-aminobutyric acid B receptor are associate with limbic encephalitis and severe seizures, often with small cell lung cancers. Metabotropic glutamate receptor 5 (mGluR5) antibodies associate with Ophelia syndrome, a relatively mild form of encephalitis linked to Hodgkin lymphoma. mGluR1 antibodies associate with a form of cerebellar degeneration, and also Hodgkin lymphoma. Antibodies to Homer 3, a protein associated with mGluR1, have also been reported in two patients with cerebellar syndromes. Dopamine-2 receptor antibodies have been reported by one group in children with basal ganglia encephalitis and other disorders.

SUMMARY

CNS metabotropic receptor antibodies may exert direct inhibitory effects on their target receptors, but the evidence is more limited than with autoantibodies to ionotropic glutamate receptors. In the future, improved recognition of these patients may lead to better outcomes. Understanding the molecular mechanisms of the diseases may uncover novel treatment strategies.

Pivotal Role of Adenosine Neurotransmission in Restless Legs Syndrome

The symptomatology of Restless Legs Syndrome (RLS) includes periodic leg movements during sleep (PLMS), dysesthesias, and hyperarousal. Alterations in the dopaminergic system, a presynaptic hyperdopaminergic state, seem to be involved in PLMS, while alterations in glutamatergic neurotransmission, a presynaptic hyperglutamatergic state, seem to be involved in hyperarousal and also PLMS. Brain iron deficiency (BID) is well-recognized as a main initial pathophysiological mechanism of RLS. BID in rodents have provided a pathogenetic model of RLS that recapitulates the biochemical alterations of the dopaminergic system of RLS, although without PLMS-like motor abnormalities. On the other hand, BID in rodents reproduces the circadian sleep architecture of RLS, indicating the model could provide clues for the hyperglutamatergic state in RLS. We recently showed that BID in rodents is associated with changes in adenosinergic transmission, with downregulation of adenosine A₁ receptors (A1R) as the most sensitive biochemical finding. It was hypothesized that A1R downregulation leads to hypersensitive striatal glutamatergic terminals and facilitation of striatal dopamine release. Hypersensitivity of striatal glutamatergic terminals was demonstrated by an optogenetic-microdialysis approach in the rodent with BID, indicating that it could represent a main pathogenetic factor that leads to PLMS in RLS. In fact, the dopaminergic agonists pramipexole and ropinirole and the α₂δ ligand gabapentin, used in the initial symptomatic treatment of RLS, completely counteracted optogenetically-induced glutamate release from both normal and BID-induced hypersensitive corticostriatal glutamatergic terminals. It is a main tenet of this essay that, in RLS, a single alteration in the adenosinergic system, downregulation of A1R, disrupts the adenosine-dopamine-glutamate balance uniquely controlled by adenosine and dopamine receptor heteromers in the striatum and also the A1R-mediated inhibitory control of glutamatergic neurotransmission in the cortex and other non-striatal brain areas, which altogether determine both PLMS and hyperarousal. Since A1R agonists would be associated with severe cardiovascular effects, it was hypothesized that inhibitors of nucleoside equilibrative transporters, such as dipyridamole, by increasing the tonic A1R activation mediated by endogenous adenosine, could represent a new alternative therapeutic strategy for RLS. In fact, preliminary clinical data indicate that dipyridamole can significantly improve the symptomatology of RLS.