Actions of adenosine A2A receptor antagonist KW-6002 on drug-induced catalepsy and hypokinesia caused by reserpine or MPTP

Effects of adenosine receptor antagonists on the in vivo LPS-induced inflammation model of Parkinson's disease

The study shows effects of the nonselective adenosine A1/A2A receptor antagonist caffeine and the selective A2A receptor antagonist KW6002 on LPS-induced changes in the extracellular levels of dopamine (DA), glutamate, adenosine, hydroxyl radical, and A2A receptor density in the rat striatum. Intrastriatal LPS (10 μg) injection decreased extracellular level of DA and increased the level of adenosine, glutamate, and hydroxyl radical on the ipsilateral side 24 h after LPS administration. Caffeine (10 and 20 mg/kg i.p.) and KW6002 (1.5 and 3 mg/kg i.p.) given once daily for 6 days and on the 7th day 2 h before and 4 h after LPS injection reversed the LPS-induced changes in extracellular levels of DA, adenosine, glutamate, and hydroxyl radical production. Moreover, LPS-induced decrease in the striatal A2A receptor density was increased by caffeine and KW6002. In order to show the late LPS effect on oxidative damage of DA neurons, the contents of DA, DOPAC, HVA, and hydroxyl radical were determined 72 h after LPS (10 μg) administration into both striata. LPS decreased striatal and substantia nigra content of DA, DOPAC, and HVA while increased striatal but not nigral content of hydroxyl radical. Caffeine (20 mg/kg) and KW60002 (3 mg/kg) given once daily for 6 days and on the 7th day 2 h before and 4 h after intrastriatal injection of LPS normalized the content of DA and its metabolites in both brain regions as well as decreased LPS-induced increase in the striatal level of hydroxyl radical. In conclusion, our data demonstrated antioxidant effects of caffeine and KW6002 in the inflammatory model of PD.

In vitro pharmacological profile of the A2A receptor antagonist istradefylline

Adenosine A2A receptors are suggested to be a promising non-dopaminergic target for the treatment of Parkinson's disease (PD). Istradefylline is an adenosine A2A receptor antagonist that has been reported to exhibit antiparkinsonian activities in PD patients as well as both rodents and nonhuman primate models of PD. The aim of this study was to evaluate the in vitro pharmacological profile of istradefylline as an A2A receptor antagonist. Istradefylline exhibited high affinity for A2A receptors in humans, marmosets, dogs, rats, and mice. The affinities for the other subtypes of adenosine receptors (A1, A2B, and A3) were lower than that for A2A receptors in each species. Istradefylline demonstrated no significant affinity for other neurotransmitter receptors, including dopamine receptors (D1, D2, D3, D4, and D5). In addition, istradefylline hardly inhibited monoamine oxidase-A, monoamine oxidase-B, or catechol-O-methyl transferase. A kinetic analysis indicated that istradefylline reversibly binds to the human A2A receptors: The association reached equilibrium within 1 min, and the binding was also almost completely dissociated within 1 min. Istradefylline inhibited the A2A agonist CGS21680-induced accumulation of cAMP in the cultured cells and then shifted the concentration-response curve of CGS21680 to the right without affecting the maximal response of the agonist. These results indicate that istradefylline is a potent, selective, and competitive A2A receptor antagonist. The in vitro pharmacological profile of istradefylline helps to explain the in vivo profile of istradefylline and may be useful for clinical pharmacokinetic-pharmacodynamic considerations of efficacy and safety.

Adenosine A2A receptor antagonist istradefylline reduces daily OFF time in Parkinson's disease

BACKGROUND

We evaluated the efficacy and safety of istradefylline, a selective adenosine A2A receptor antagonist administered as adjunctive treatment to levodopa for 12 weeks in a double-blind manner in Parkinson's disease patients with motor complications in Japan.

METHODS

A total of 373 subjects were randomized to receive placebo (n=126), istradefylline 20 mg/day (n=123), or istradefylline 40 mg/day (n=124). The primary efficacy variable was the change in daily OFF time. Other secondary variables were also evaluated.

RESULTS

The change in daily OFF time was significantly reduced in the istradefylline 20 mg/day (-0.99 hours, P=.003) and istradefylline 40 mg/day (-0.96 hours, P=.003) groups compared with the placebo group (-0.23 hours). The most common adverse event was dyskinesia (placebo, 4.0%; istradefylline 20 mg/day, 13.0%; istradefylline 40 mg/day, 12.1%).

CONCLUSIONS

Istradefylline reduced daily OFF time and was well tolerated in Japanese PD patients with motor complications on levodopa treatment.

G protein-coupled receptor heterocomplexes in neuropsychiatric disorders

G protein-coupled receptors (or GPCRs) represent the largest family of membrane proteins in the human genome and are the target of approximately half of all therapeutic drugs. GPCRs contain a conserved structure of seven transmembrane domains. Their amino terminus is located extracellularly, whereas the carboxy terminus extends into the cytoplasm. Accumulating evidence suggests that GPCRs exist and function as monomeric entities. Nevertheless, more recent findings indicate that GPCRs can also form dimers or even higher order oligomers. The differential pharmacological and signaling properties of GPCR heteromeric complexes hint that their physiological effects may be different as compared to those obtained in tissue cultures that express a particular GPCR. In this chapter, we review current data on the role of GPCR heteromerization in receptor signaling, as well as its potential implication in neuropsychiatric disorders such as schizophrenia, depression, and Parkinson's disease.

Adenosine A2A antagonists in Parkinson's disease: what's next?

Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder, affecting up to 10 million people worldwide. Current treatment primarily involves symptom management with dopaminergic replacement therapy. Levodopa remains the most effective oral treatment, although long-term use is associated with complications such as wearing off, dyskinesias, and on-off fluctuations. Non-dopaminergic medications that improve PD symptoms and motor fluctuations are in demand. Adenosine A2A receptors are abundantly expressed within the basal ganglia and offer a unique target to modify abnormal striatal signaling associated with PD. Preclinical animal models have shown the ability of adenosine A2A receptor antagonists to improve PD motor symptoms, reduce motor fluctuations and dyskinesia, as well as protect against toxin-induced neuronal degeneration. Both istradefylline and preladenant have demonstrated moderate efficacy in reducing off time in PD patients with motor fluctuations. The safety and efficacy of this class of compounds continues to be defined and future studies should focus on non-motor symptoms, dyskinesias, and neuroprotection.

Room-temperature super-extraction system (RTSES) optimizes the anxiolytic- and antidepressant-like behavioural effects of traditional Xiao-Yao-San in mice

Background

Xiao-Yao-San (XYS) is a Chinese medicinal formula for treating anxiety and depression. This study aims to evaluate the use of a room-temperature super-extraction system (RTSES) to extract the major active components of XYS and enhance their psycho-pharmacological effects.

Methods

The neuroprotective roles of XYS/RTSES against reserpine-derived neurotoxicity were evaluated using a glial cell injury system (in vitro) and a depression-like C57BL/6 J mouse model (in vivo). The anxiolytic-behavioural effects were measured by the elevated plus-maze (EPM) test and the antidepressant effects were evaluated by the forced swimming test (FST) and tail suspension test (TST). Glucose tolerance and insulin resistance were assayed by ELISA. The expression of 5-HT_1A receptors in the prefrontal cortex was examined by western blotting.

Results

XYS/RTSES (300 μg/mL) diminished reserpine-induced glial cell death more effectively than either XYS (300 μg/mL) or fluoxetine (30 μM) at 24 h (P = 0.0481 and P = 0.054, respectively). Oral administration of XYS/RTSES (500 mg/kg/day) for 4 consecutive weeks significantly elevated the ratios of entries (open arms/closed arms; P = 0.0177) and shuttle activity (P = 0.00149) on the EPM test, and reduced the immobility time by 90% on the TST (P = 0.00000538) and FST (P = 0.0000053839). XYS/RTSES also improved the regulation of blood glucose (P = 0.0305) and increased the insulin sensitivity (P = 0.0093). The Western blot results indicated that the activation of cerebral 5-HT_1A receptors may be involved in the mechanisms of XYS/RTSES actions.

Conclusion

The RTSES could provide a novel method for extracting effective anxiolytic- and antidepressant-like substances. XYS/RTSES improved the regulation of blood glucose and increased the insulin sensitivity in reserpine-induced anxiety and depression. Neuroprotection of glial cells and activation of cerebral 5-HT_1A receptors were also involved.

Istradefylline, an adenosine A₂A receptor antagonist, for patients with Parkinson's Disease: a meta-analysis

OBJECTIVES

To assess the efficacy and safety of istradefylline as an adjunct to levodopa in patients with Parkinson's Disease (PD).

METHODS

In this study, we searched the Cochrane Library, MEDLINE, Embase, China Academic Journal Full-text Database (CNKI), China Biomedical Literature Database (CBM), Chinese Scientific Journals Database (VIP), and Wanfang Database. The quality of included studies was strictly evaluated. Data analyses were performed by the Cochrane Collaboration's RevMan5.0 software.

RESULTS

Five randomized controlled trials (RCTs) were included. The result showed a significant reduction of the awake time per day spent in the OFF state and improvement of the Unified Parkinson's Disease Rating Scale (UPDRS) Part III in the ON state when receiving istradefylline compared with patients receiving placebo. There was no significant difference between the istradefylline 20mg and the istradefylline 40 mg groups in the UPDRS Part III in the ON state (WMD=1.27, 95% CI [-0.40, 2.95]). The results showed significant differences in dyskinesia (RR=1.63, 95% CI [1.16, 2.29]) compared to istradefylline 40 mg with placebo. There was no significant statistical difference with regard to other adverse events.

CONCLUSIONS

The present study showed that istradefylline is safe and effective as an adjunct to levodopa in patients with PD. Future large-scale, higher-quality, long-treatment, and placebo-controlled trials are needed.

Safety, tolerability and pharmacokinetics after single and multiple doses of preladenant (SCH420814) administered in healthy subjects

WHAT IS KNOWN AND OBJECTIVE

Preladenant (SCH420814, MK-3814) is a highly selective orally bioavailable non-methylxanthine adenosine 2A (A(2A) ) receptor antagonist under investigation for the treatment for Parkinson's disease. This study evaluated the safety, tolerability and pharmacokinetics of preladenant at single and multiple doses for the first time in humans.

METHODS

These were two randomized, double-blind, placebo-controlled, ascending-dose studies, one evaluating single rising preladenant doses (5-200 mg) compared with placebo and the other evaluating multiple rising preladenant doses (10-200 mg once daily over 10 days) compared with placebo. Safety was the primary end point of both studies. Safety evaluations, physical examinations, electrocardiograms, vital signs determinations and routine laboratory tests were performed before and at intervals throughout the studies. Blood samples were collected immediately before study drug administration and at various time points after dosing. Pharmacokinetic assessments of plasma preladenant and metabolites SCH434748 and SCH446637 were performed.

RESULTS AND DISCUSSION

One hundred and eight healthy adult men were randomly assigned in a 3 : 1 ratio to receive oral preladenant or matching placebo capsules under fasting conditions. Preladenant reached peak plasma concentrations in ∼1 h and then declined rapidly. Dose-related increases in exposure were observed up to 100 mg/day; accumulation was negligible at all doses. Transient mild increases in blood pressure occurred within a few hours after preladenant administration; blood pressure changes were neither cumulative nor dose-related nor associated with clinical sequelae.

WHAT IS NEW AND CONCLUSION

Preladenant was generally well tolerated up to the maximum dose tested (200 mg/day).

Therapeutic potential of targeting metabotropic glutamate receptors for Parkinson's disease

Parkinson's disease (PD) is a progressive neurological disorder predominantly characterized by motor symptoms including bradykinesia and resting tremor. The gold standard of treatment for PD remains dopamine replacement therapy, which eventually fails due to continued progression of the disease and the development of debilitating side effects. Recent breakthroughs are providing the first major advances in the development of fundamentally new pharmacological strategies for the treatment of PD that do not rely on dopamine replacement strategies, but rather aim to reduce the overactive indirect pathway within the basal ganglia. In this article, we will review the role of metabotropic glutamate receptors within the basal ganglia and discuss the potential for modulation of metabotropic glutamate receptors as a treatment for PD.

Animal models of Parkinson's disease: a source of novel treatments and clues to the cause of the disease

Animal models of Parkinson's disease (PD) have proved highly effective in the discovery of novel treatments for motor symptoms of PD and in the search for clues to the underlying cause of the illness. Models based on specific pathogenic mechanisms may subsequently lead to the development of neuroprotective agents for PD that stop or slow disease progression. The array of available rodent models is large and ranges from acute pharmacological models, such as the reserpine- or haloperidol-treated rats that display one or more parkinsonian signs, to models exhibiting destruction of the dopaminergic nigro-striatal pathway, such as the classical 6-hydroxydopamine (6-OHDA) rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse models. All of these have provided test beds in which new molecules for treating the motor symptoms of PD can be assessed. In addition, the emergence of abnormal involuntary movements (AIMs) with repeated treatment of 6-OHDA-lesioned rats with L-DOPA has allowed for examination of the mechanisms responsible for treatment-related dyskinesia in PD, and the detection of molecules able to prevent or reverse their appearance. Other toxin-based models of nigro-striatal tract degeneration include the systemic administration of the pesticides rotenone and paraquat, but whilst providing clues to disease pathogenesis, these are not so commonly used for drug development. The MPTP-treated primate model of PD, which closely mimics the clinical features of PD and in which all currently used anti-parkinsonian medications have been shown to be effective, is undoubtedly the most clinically-relevant of all available models. The MPTP-treated primate develops clear dyskinesia when repeatedly exposed to L-DOPA, and these parkinsonian animals have shown responses to novel dopaminergic agents that are highly predictive of their effect in man. Whether non-dopaminergic drugs show the same degree of predictability of response is a matter of debate. As our understanding of the pathogenesis of PD has improved, so new rodent models produced by agents mimicking these mechanisms, including proteasome inhibitors such as PSI, lactacystin and epoximycin or inflammogens like lipopolysaccharide (LPS) have been developed. A further generation of models aimed at mimicking the genetic causes of PD has also sprung up. Whilst these newer models have provided further clues to the disease pathology, they have so far been less commonly used for drug development. There is little doubt that the availability of experimental animal models of PD has dramatically altered dopaminergic drug treatment of the illness and the prevention and reversal of drug-related side effects that emerge with disease progression and chronic medication. However, so far, we have made little progress in moving into other pharmacological areas for the treatment of PD, and we have not developed models that reflect the progressive nature of the illness and its complexity in terms of the extent of pathology and biochemical change. Only when this occurs are we likely to make progress in developing agents to stop or slow the disease progression. The overarching question that draws all of these models together in the quest for better drug treatments for PD is how well do they recapitulate the human condition and how predictive are they of successful translation of drugs into the clinic? This article aims to clarify the current position and highlight the strengths and weaknesses of available models.

Neuroprotective effect of kaempferol against a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced mouse model of Parkinson's disease

In the present study, we investigated the neuroprotective effects of kaempferol in the mouse model of Parkinson's disease, which was induced by neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We confirmed that MPTP led to behavioral deficits, depletion of dopamine and its metabolites, reduction in superoxide dismutase (SOD) and glutathione peroxidase (GSH-PX) activity, and the elevation of malondialdehyde (MDA) levels in the substantia nigra. When administered prior to MPTP, kaempferol improved motor coordination, raised striatal dopamine and its metabolite levels, increased SOD and GSH-PX activity, and reduced the content of MDA compared with mice treated with MPTP alone. Immunohistochemical studies using anti-tyrosine hydroxylase (TH) antibody showed that medication of kaempferol could prevent the loss of TH-positive neurons induced by MPTP. Taken together, we propose that kaempferol has shown anti-parkinsonian properties in our studies. More work is needed to explore detailed mechanisms of action.

New pharmacological avenues for the treatment of L-DOPA-induced dyskinesias in Parkinson's disease: targeting glutamate and adenosine receptors

INTRODUCTION

Parkinson's disease (PD) therapy is still centered on the use of L-3,4-dihydroxyphenylalanine (L-DOPA), which is hampered by numerous side effects, including abnormal involuntary movements known as L-DOPA-induced dyskinesias (LIDs). LIDs are the result of pre- and postsynaptic changes at the corticostriatal level, induced by chronic and pulsatile stimulation of striatal dopaminergic receptors. These changes impact on synaptic plasticity and involve also selected, nondopaminergic receptors expressed by striatal projection neurons.

AREAS COVERED

Among nondopaminergic receptors, glutamate receptors - NMDA and mGluR5 subtypes in particular - and adenosine A(2A) receptors are those most likely involved in LIDs. The aim of the present review is to summarize results of studies undertaken with specific antagonists of these receptors, first conducted in animal models of LIDs, which in selected cases have been translated into clinical trials.

EXPERT OPINION

Selected antagonists of glutamate and adenosine receptors have been proposed as anti-dyskinetic agents. Promising results have been obtained in preclinical investigations and in initial clinical trials, but long-term safety, tolerability and efficacy studies in patients are still required. The current development of novel antagonists, including tools able to act on receptor mosaics, may provide innovative tools for LIDs management in the next future.

Adenosine A(2A) Receptor Antagonists Do Not Disrupt Rodent Prepulse Inhibition: An Improved Side Effect Profile in the Treatment of Parkinson's Disease

Parkinson's disease (PD) is characterized by loss of dopaminergic neurons in the substantia nigra. Current treatments for PD focus on dopaminergic therapies, including L-dopa and dopamine receptor agonists. However, these treatments induce neuropsychiatric side effects. Psychosis, characterized by delusions and hallucinations, is one of the most serious such side effects. Adenosine A_2A receptor antagonism is a nondopaminergic treatment for PD with clinical and preclinical efficacy. The present studies assessed A_2A antagonists SCH 412348 and istradefylline in rodent prepulse inhibition (PPI), a model of psychosis. Dopamine receptor agonists pramipexole (0.3–3 mg/kg), pergolide (0.3–3 mg/kg), and apomorphine (0.3–3 mg/kg) significantly disrupted PPI; ropinirole (1–30 mg/kg) had no effect; L-dopa (100–300 mg/kg) disrupted rat but not mouse PPI. SCH 412348 (0.3–3 mg/kg) did not disrupt rodent PPI; istradefylline (0.1–1 mg/kg) marginally disrupted mouse but not rat PPI. These results suggest that A_2A antagonists, unlike dopamine agonists, have an improved neuropsychiatric side effect profile.

Adenosine A2A-receptor antagonism and pathophysiology of Parkinson's disease and drug-induced movement disorders

Parkinson's disease and drug-induced movement disorders (DIMDs) have commonalities in etiology based on impaired dopamine-based neurotransmission. Adenosine A(2A)-receptor antagonism may provide a new mechanism through which these disorders can be managed. In the motor circuit, tonic output from the globus pallidus and substantia nigra regulates movement via opposing excitatory and inhibitory inputs to the cerebral cortex through the direct and indirect pathways. Increased activity of the direct pathway increases movement via an inhibitory effect on thalamocortical projection neurons; increased activity of the indirect pathway has the opposite effect. Regulation of these pathways is mediated primarily by reciprocal inhibitory interactions between dopamine and adenosine receptors on neurons of these pathways. Adenosine A(2A) receptors are colocalized with dopamine D(2) receptors on the indirect pathway neurons, with A(2A) activation opposing the effect of D(2) activation. The A(2A) receptors' role in the pathophysiology of Parkinson's disease and DIMDs is evidenced by the upregulation of A(2A) receptors in patients with Parkinson's disease and patients receiving long-term administration of dopamine blockers. Further, A(2A)-receptor antagonists are effective in reversing parkinsonian motor deficits and extrapyramidal symptoms in animal models of Parkinson's disease and DIMDs. Understanding the role of A(2A)-receptor antagonism in the pathophysiology of Parkinson's disease and DIMD has therapeutic implications.

Adenosine A2a receptor antagonists attenuate striatal adaptations following dopamine depletion

The motor symptoms of Parkinson's disease (PD) are widely thought to arise from an imbalance in the activity of the two major striatal efferent pathways following the loss of dopamine (DA) signaling. In striatopallidal, indirect pathway spiny projection neurons (iSPNs), intrinsic excitability rises following the loss of inhibitory D2 receptor signaling. Because these receptors are normally counterbalanced by adenosine A2a adenosine receptors, antagonists of these receptors are being examined as an adjunct to conventional pharmacological therapies. However, little is known about the effects of sustained A2a receptor antagonism on striatal adaptations in PD models. To address this issue, the A2a receptor antagonist SCH58261 was systemically administered to DA-depleted mice. After 5 days of treatment, the effects of SCH58261 on iSPNs were examined in brain slices using electrophysiological and optical approaches. SCH58261 treatment did not prevent spine loss in iSPNs following depletion, but did significantly attenuate alterations in synaptic currents, spine morphology and dendritic excitability. In part, these effects were attributable to the ability of SCH58261 to blunt the effects of DA depletion on cholinergic interneurons, another striatal cell type that co-expresses A2a and D(2) receptors. Collectively, these results suggest that A2a receptor antagonism improves striatal function in PD models by attenuating iSPN adaptations to DA depletion.

The role of extracellular adenosine in chemical neurotransmission in the hippocampus and Basal Ganglia: pharmacological and clinical aspects

Now there is general agreement that the purine nucleoside adenosine is an important neuromodulator in the central nervous system, playing a crucial role in neuronal excitability and synaptic/non-synaptic transmission in the hippocampus and basal ganglia. Adenosine is derived from the breakdown of extra- or intracellular ATP and is released upon a variety of physiological and pathological stimuli from neuronal and non-neuronal sources, i.e. from glial cells and exerts effects diffusing far away from release sites. The resultant elevation of adenosine levels in the extracellular space reaches micromolar level, and leads to the activation A₁, A_2A, A_2B and A₃ receptors, localized to pre- and postsynaptic as well as extrasynaptic sites. Activation of presynaptic A₁ receptors inhibits the release of the majority of transmitters including glutamate, acetylcholine, noradrenaline, 5-HT and dopamine, whilst the stimulation of A_2A receptors facilitates the release of glutamate and acetylcholine and inhibits the release of GABA. These actions underlie modulation of neuronal excitability, synaptic plasticity and coordination of neural networks and provide intriguing target sites for pharmacological intervention in ischemia and Parkinson’s disease. However, despite that adenosine is also released during ischemia, A₁ adenosine receptors do not participate in the modulation of excitotoxic glutamate release, which is nonsynaptic and is due to the reverse operation of transporters. Instead, extrasynaptic A1 receptors might be responsible for the neuroprotection afforded by A₁ receptor activation.

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A(2A) receptors (A(2A)R) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A(2A) receptors in the basal ganglia. At present it is believed that A(2A)R antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A(2A)R antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A(2A) adenosine receptors. Thus, the development of heteromer-specific A(2A) receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.

Impacts of methylxanthines and adenosine receptors on neurodegeneration: human and experimental studies

Neurodegenerative disorders are some of the most feared illnesses in modern society, with no effective treatments to slow or halt this neurodegeneration. Several decades after the earliest attempt to treat Parkinson's disease using caffeine, tremendous amounts of information regarding the potential beneficial effect of caffeine as well as adenosine drugs on major neurodegenerative disorders have accumulated. In the first part of this review, we provide general background on the adenosine receptor signaling systems by which caffeine and methylxanthine modulate brain activity and their role in relationship to the development and treatment of neurodegenerative disorders. The demonstration of close interaction between adenosine receptor and other G protein coupled receptors and accessory proteins might offer distinct pharmacological properties from adenosine receptor monomers. This is followed by an outline of the major mechanism underlying neuroprotection against neurodegeneration offered by caffeine and adenosine receptor agents. In the second part, we discuss the current understanding of caffeine/methylxantheine and its major target adenosine receptors in development of individual neurodegenerative disorders, including stroke, traumatic brain injury Alzheimer's disease, Parkinson's disease, Huntington's disease and multiple sclerosis. The exciting findings to date include the specific in vivo functions of adenosine receptors revealed by genetic mouse models, the demonstration of a broad spectrum of neuroprotection by chronic treatment of caffeine and adenosine receptor ligands in animal models of neurodegenerative disorders, the encouraging development of several A(2A) receptor selective antagonists which are now in advanced clinical phase III trials for Parkinson's disease. Importantly, increasing body of the human and experimental studies reveals encouraging evidence that regular human consumption of caffeine in fact may have several beneficial effects on neurodegenerative disorders, from motor stimulation to cognitive enhancement to potential neuroprotection. Thus, with regard to neurodegenerative disorders, these potential benefits of methylxanthines, caffeine in particular, strongly argue against the common practice by clinicians to discourage regular human consumption of caffeine in aging populations.

Adenosine receptors and brain diseases: neuroprotection and neurodegeneration

Adenosine acts in parallel as a neuromodulator and as a homeostatic modulator in the central nervous system. Its neuromodulatory role relies on a balanced activation of inhibitory A(1) receptors (A1R) and facilitatory A(2A) receptors (A2AR), mostly controlling excitatory glutamatergic synapses: A1R impose a tonic brake on excitatory transmission, whereas A2AR are selectively engaged to promote synaptic plasticity phenomena. This neuromodulatory role of adenosine is strikingly similar to the role of adenosine in the control of brain disorders; thus, A1R mostly act as a hurdle that needs to be overcame to begin neurodegeneration and, accordingly, A1R only effectively control neurodegeneration if activated in the temporal vicinity of brain insults; in contrast, the blockade of A2AR alleviates the long-term burden of brain disorders in different neurodegenerative conditions such as ischemia, epilepsy, Parkinson's or Alzheimer's disease and also seem to afford benefits in some psychiatric conditions. In spite of this qualitative agreement between neuromodulation and neuroprotection by A1R and A2AR, it is still unclear if the role of A1R and A2AR in the control of neuroprotection is mostly due to the control of glutamatergic transmission, or if it is instead due to the different homeostatic roles of these receptors related with the control of metabolism, of neuron-glia communication, of neuroinflammation, of neurogenesis or of the control of action of growth factors. In spite of this current mechanistic uncertainty, it seems evident that targeting adenosine receptors might indeed constitute a novel strategy to control the demise of different neurological and psychiatric disorders.

A randomized, double-blind, placebo-controlled study to investigate the safety, tolerability, and pharmacokinetics of single enantiomer (+)-mefloquine compared with racemic mefloquine in healthy persons

Racemic mefloquine is a highly effective antimalarial whose clinical utility has been compromised by its association with neuropsychiatric and gastrointestinal side effects. It is hypothesized that the cause of the side effects may reside in the (-) enantiomer. We sought to compare the safety, tolerability and pharmacokinetic profile of (+)-mefloquine with racemic mefloquine in a randomized, ascending-dose, double-blind, active and placebo-controlled, parallel cohort study in healthy male and female adult volunteers. Although differing in its manifestations, both study drugs displayed a substantially worse tolerability profile compared with placebo. The systemic clearance was slower for (-)-mefloquine than (+)-mefloquine. Thus, (+)-mefloquine has a different safety and tolerability profile compared with racemic mefloquine but its global safety profile is not superior and replacement of the currently used antimalarial drug with (+)-mefloquine is not warranted.

Preladenant, a selective A(2A) receptor antagonist, is active in primate models of movement disorders

Parkinson's Disease (PD) and Extrapyramidal Syndrome (EPS) are movement disorders that result from degeneration of the dopaminergic input to the striatum and chronic inhibition of striatal dopamine D(2) receptors by antipsychotics, respectively. Adenosine A(2A) receptors are selectively localized in the basal ganglia, primarily in the striatopallidal ("indirect") pathway, where they appear to operate in concert with D(2) receptors and have been suggested to drive striatopallidal output balance. In cases of dopaminergic hypofunction, A(2A) receptor activation contributes to the overdrive of the indirect pathway. A(2A) receptor antagonists, therefore, have the potential to restore this inhibitor imbalance. Consequently, A(2A) receptor antagonists have therapeutic potential in diseases of dopaminergic hypofunction such as PD and EPS. Targeting the A(2A) receptor may also be a way to avoid the issues associated with direct dopamine agonists. Recently, preladenant was identified as a potent and highly selective A(2A) receptor antagonist, and has produced a significant improvement in motor function in rodent models of PD. Here we investigate the effects of preladenant in two primate movement disorder models. In MPTP-treated cynomolgus monkeys, preladenant (1 or 3 mg/kg; PO) improved motor ability and did not evoke any dopaminergic-mediated dyskinetic or motor complications. In Cebus apella monkeys with a history of chronic haloperidol treatment, preladenant (0.3-3.0 mg/kg; PO) delayed the onset of EPS symptoms evoked by an acute haloperidol challenge. Collectively, these data support the use of preladenant for the treatment of PD and antipsychotic-induced movement disorders.

Adenosine A(2A) receptors in psychopharmacology: modulators of behavior, mood and cognition

The adenosine A(2A) receptor (A(2A)R) is in the center of a neuromodulatory network affecting a wide range of neuropsychiatric functions by interacting with and integrating several neurotransmitter systems, especially dopaminergic and glutamatergic neurotransmission. These interactions and integrations occur at multiple levels, including (1) direct receptor- receptor cross-talk at the cell membrane, (2) intracellular second messenger systems, (3) trans-synaptic actions via striatal collaterals or interneurons in the striatum, (4) and interactions at the network level of the basal ganglia. Consequently, A(2A)Rs constitute a novel target to modulate various psychiatric conditions. In the present review we will first summarize the molecular interaction of adenosine receptors with other neurotransmitter systems and then discuss the potential applications of A(2A)R agonists and antagonists in physiological and pathophysiological conditions, such as psychostimulant action, drug addiction, anxiety, depression, schizophrenia and learning and memory.

Population pharmacokinetic analysis of istradefylline in healthy subjects and in patients with Parkinson's disease

This model-based analysis quantifies the population pharmacokinetics (PK) of orally administered istradefylline, a selective adenosine A(2A) receptor antagonist, in healthy subjects and patients with Parkinson's disease, including the estimation of covariate effects on istradefylline PK parameters. Istradefylline plasma concentration data from 8 phase 1 and 8 phase 2/3 studies conducted in 1449 patients and normal, healthy volunteers aged from 18 to 87 years were best described by a 2-compartment model with first-order absorption parameterized in terms of apparent oral clearance (CL/F), apparent central volume of distribution (V2/F), apparent intercompartmental clearance (Q/F), apparent peripheral volume of distribution (V3/F) and a first-order absorption rate-constant (Ka). The typical population PK parameters were CL/F (5.76 L/h), V2/F (198 L), Q (21.6 L/h), V3/F (307 L), and Ka (0.464 h(-1)) for a 70-kg, nonsmoking Caucasian who had 55.6 kg of lean body mass, no presence of CYP3A4 inhibitors, and unknown food status. Smoking and CYP3A4 inhibitors as concomitant medications were important predictors of istradefylline exposure. Istradefylline area under the concentration-time curve at steady-state increased 35% (95% confidence interval, 18%-55%) in the presence of CYP3A4 inhibitors and decreased 38% (95% confidence interval, 26%-50%) in smokers. The population PK model described the observed concentration data well and was deemed appropriate for further evaluation of the istradefylline exposure-response relationship in patients with Parkinson's disease.

Novel 8-(furan-2-yl)-3-substituted thiazolo [5,4-e][1,2,4] triazolo[1,5-c] pyrimidine-2(3H)-thione derivatives as potential adenosine A(2A) receptor antagonists

Novel thiazolotriazolopyrimidine derivatives (23-33) designed as potential adenosine A(2A) receptor (A(2A)R) antagonists were synthesized. Molecular docking studies revealed that all compounds (23-33) exhibited strong interaction with A(2A)R. The strong interaction of the compounds (23-33) with A(2A)R in silico was confirmed by their high binding affinity with human A(2A)R stably expressed in HEK293 cells using radioligand-binding assay. The compounds 24-26 demonstrated substantial binding affinity and selectivity for A(2A)R as compared to SCH58261, a standard A(2A)R antagonist. Decrease in A(2A)R-coupled release of endogenous cAMP in treated HEK293 cells demonstrated in vitro A(2A)R antagonist potential of the compounds 24-26. Attenuation in haloperidol-induced motor impairments (catalepsy and akinesia) in Swiss albino male mice pre-treated with compounds 24-26 further supports their role in the alleviation of PD symptoms.

Synthesis of novel 7-imino-2-thioxo-3,7-dihydro-2H-thiazolo [4,5-d] pyrimidine derivatives as adenosine A2A receptor antagonists

Novel bicyclic thiazolopyrimidine compounds (15-26) were synthesized to develop adenosine A(2A) receptor (A(2A)R) antagonist for the treatment of Parkinson's disease (PD). The binding affinity of the compounds (15-26) with A(2A)R was evaluated using radioligand binding assay on isolated membranes from stably transfected HEK293 cells. Selectivity of the compounds towards A(2A)R was assessed by comparing their binding affinities with A(1) receptors (A(1)R). cAMP concentrations were measured from HEK293 cells treated with compounds (15-26) as compared to NECA (A(2A)R agonist). The compound (16) possessed strongest A(2A)R binding affinity (K(i) value=0.0038 nM) and selectivity (737-fold) versus A(1)R. Decrease in A(2A)R-coupled release of endogenous cAMP from HEK293 cells treated with compounds (15-26) is evocative of their potential as A(2A)R antagonist.

Istradefylline as monotherapy for Parkinson disease: results of the 6002-US-051 trial

OBJECTIVE

6002-US-051 was a 12-week, double-blind study evaluating the safety and efficacy of istradefylline, a selective A(2A) adenosine receptor antagonist, as monotherapy in patients with Parkinson's disease (PD).

METHODS

Patients with Hoehn-Yahr stages 1-2.5 who had not received dopaminergic drugs in the past 30 days or levodopa for >30 days at anytime were randomized to 40 mg/day istradefylline or placebo. The primary efficacy outcome was the change from Baseline to Endpoint in the Unified Parkinson's Disease Rating Scale (UPDRS) Subscale III score. Safety was assessed by physical examination, laboratory tests, electrocardiograms, and adverse event monitoring.

RESULTS

176 patients comprised the intent-to-treat population. Although istradefylline showed numerically greater improvements in UPDRS Subscale III at each time point and reached statistical significance at Week 2 (LS mean difference = -1.47), it did not show statistically significant improvement from placebo for the primary endpoint (least square [LS] mean difference = -1.11). Similar proportions of patients in each group experienced treatment-emergent adverse events (63% istradefylline, 65% placebo).

CONCLUSIONS

Istradefylline, as monotherapy in patients with PD, is safe and well tolerated. However, efficacy in improving motor symptoms in early PD was not statistically demonstrated by this study.

Antagonists of the human A(2A) receptor. Part 5: Highly bio-available pyrimidine-4-carboxamides

A novel series of antagonists of the human A(2A) receptor have been identified and have been shown to display good potency and high degrees of selectivity over other receptor sub-types. Displaying in vivo potency in commonly used disease models and high oral bio-availability, this class of compounds may serve as clinically useful treatments for the relief of the symptoms associated with Parkinson's disease.

Characterization of the potent and highly selective A2A receptor antagonists preladenant and SCH 412348 [7-[2-[4-2,4-difluorophenyl]-1-piperazinyl]ethyl]-2-(2-furanyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine] in rodent models of movement disorders and depression

The adenosine A(2A) receptor has been implicated in the underlying biology of various neurological and psychiatric disorders, including Parkinson's disease (PD) and depression. Preladenant and SCH 412348 [7-[2-[4-2,4-difluorophenyl]-1-piperazinyl]ethyl]-2-(2-furanyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine] are potent competitive antagonists of the human A(2A) receptor (K(i) = 1.1 and 0.6 nM, respectively) and have >1000-fold selectivity over all other adenosine receptors, making these compounds the most selective A(2A) receptor antagonists reported to date. Both compounds attenuate hypolocomotion induced by the A(2A) receptor agonist CGS-21680 [2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine], suggesting that they inhibit A(2A) receptor activity in vivo. Their high degree of selectivity and robust in vivo activity make preladenant and SCH 412348 useful tools to investigate the role of the A(2A) receptor system in animal models of PD and depression. Oral administration of preladenant and SCH 412348 (0.1-1 mg/kg) to rats potentiated 3,4-dihydroxy-L-phenylalanine (L-Dopa)-induced contralateral rotations after 6-hydroxydopamine lesions in the medial forebrain bundle and potently attenuated the cataleptic effects of haloperidol. Preladenant (1 mg/kg) inhibited L-Dopa-induced behavioral sensitization after repeated daily administration, which suggests a reduced risk of the development of dyskinesias. Finally, preladenant and SCH 412348 exhibited antidepressant-like profiles in models of behavioral despair, namely the mouse tail suspension test and the mouse and rat forced swim test. These studies demonstrate that preladenant and SCH 412348 are potent and selective A(2A) receptor antagonists and provide further evidence of the potential therapeutic benefits of A(2A) receptor inhibition in PD (with reduced risk of dyskinesias) and depression (one of the primary nonmotor symptoms of PD).

Dual-target-directed drugs that block monoamine oxidase B and adenosine A(2A) receptors for Parkinson's disease

Inadequacies of the current pharmacotherapies to treat Parkinson's disease (PD) have prompted efforts to identify novel drug targets. The adenosine A(2A) receptor is one such target. Antagonists of this receptor (A(2A) antagonists) are considered promising agents for the symptomatic treatment of PD. Evidence suggests that A(2A) antagonists may also have neuroprotective properties that may prevent the development of the dyskinesia that often complicates levodopa treatment. Because the therapeutic benefits of A(2A) antagonists are additive to that of dopamine replacement therapy, it may be possible to reduce the dose of the dopaminergic drugs and therefore the occurrence of side effects. Inhibitors of monoamine oxidase (MAO)-B also are considered useful tools for the treatment of PD. When used in combination with levodopa, inhibitors of MAO-B may enhance the elevation of dopamine levels after levodopa treatment, particularly when used in early stages of the disease when dopamine production may not be so severely compromised. Furthermore, MAO-B inhibitors may also possess neuroprotective properties in part by reducing the damaging effect of dopamine turnover in the brain. These effects of MAO-B inhibitors are especially relevant when considering that the brain shows an age-related increase in MAO-B activity. Based on these observations, dual-target-directed drugs, compounds that inhibit MAO-B and antagonize A(2A) receptors, may have value in the management of PD. This review summarizes recent efforts to develop such dual-acting drugs using caffeine as the lead compound.

The effects of systemic, intrastriatal, and intrapallidal injections of caffeine and systemic injections of A2A and A1 antagonists on forepaw stepping in the unilateral 6-OHDA-lesioned rat

RATIONALE AND OBJECTIVES

Given that adenosine A2A antagonists appear to be therapeutic in several animal models of Parkinson's disease (PD), we examined the extent to which caffeine and selective A2A and A1 antagonists could enhance contralateral forepaw stepping in the unilateral 6-OHDA-lesioned rat.

MATERIALS AND METHODS

Following unilateral injections of 12 microg 6-OHDA into the medial forebrain bundle (MFB), frequency of stepping with both front paws was counted separately as the paws were dragged anteriorally and laterally by a treadmill.

RESULTS

The MFB lesions decreased contralateral stepping by 74-83%, and 8 mg/kg 3,4-dihydroxy-L-phenylalanine (L-DOPA) increased contralateral stepping by 25-26%. Caffeine given systemically (15 mg/kg) or into the dorsal striatum or external globus pallidus (GPE; 20-40 microg) increased contralateral forepaw stepping by 14%, 27%, and 26%, respectively, and enhanced the effect of 8 mg/kg L-DOPA on stepping. The selective A(2A) antagonist SCH-58261 (2 mg/kg) also increased stepping by 13% and enhanced the therapeutic effect of L-DOPA, whereas the selective A(1) [corrected] antagonist 8-cyclopentyltheophylline (3-7 mg/kg) and A(1) agonist N(6)-cyclopentyladenosine (0.03-0.2 mg/kg) had no effect. None of these drugs appeared to produce dyskinesic effects.

CONCLUSIONS

In this well-validated animal model of the akinesic effects of PD, caffeine and a selective A2A, but not an A1, antagonist were able to provide both monotherapeutic and adjunctive therapeutic effects. These data are consistent with the hypothesis that A2A antagonists may be therapeutic in human PD patients and indicate that the dorsal striatum and GPE are critical sites of therapeutic action.

Adenosine A2A receptors and Parkinson's disease

The drug treatment of Parkinson's disease (PD) is accompanied by a loss of drug efficacy, the onset of motor complications, lack of effect on non-motor symptoms, and a failure to modify disease progression. As a consequence, novel approaches to therapy are sought, and adenosine A(2A) receptors (A(2A)ARs) provide a viable target. A(2A)ARs are highly localized to the basal ganglia and specifically to the indirect output pathway, which is highly important in the control of voluntary movement. A(2A)AR antagonists can modulate gamma-aminobutyric acid (GABA) and glutamate release in basal ganglia and other key neurotransmitters that modulate motor activity. In both rodent and primate models of PD, A(2A)AR antagonists produce alterations in motor behavior, either alone or in combination with dopaminergic drugs, which suggest that they will be effective in the symptomatic treatment of PD. In clinical trials, the A(2A)AR antagonist istradefylline reduces "off" time in patients with PD receiving optimal dopaminergic therapy. However, these effects have proven difficult to demonstrate on a consistent basis, and further clinical trials are required to establish the clinical utility of this drug class. Based on preclinical studies, A(2A)AR antagonists may also be neuroprotective and have utility in the treatment of neuropsychiatric disorders. We are only now starting to explore the range of potential uses of A(2A)AR antagonists in central nervous system disorders, and their full utility is still to be uncovered.

The effects of adenosine A2A receptor antagonists on haloperidol-induced movement disorders in primates

RATIONALE

Adenosine and dopamine interact within the striatum to control striatopallidal output and globus pallidus GABA release. Manipulating striatal adenosine transmission via blockade of the A2A receptor subtype can compensate for the reduced dopamine activity within the striatum that underlies movement disorders such as antipsychotic-induced extrapyramidal syndrome (EPS) and Parkinson's disease (PD). Preclinical studies in the rat have demonstrated that adenosine A2A receptor antagonists can attenuate behaviors reflecting reduced dopamine activity, such as haloperidol-induced catalepsy and hypoactivity.

OBJECTIVES

In the present studies using nonhuman primates, adenosine antagonists were tested against haloperidol-induced EPS in Cebus apella and haloperidol-induced catalepsy in Saimiri sciureus (squirrel monkey). Specifically, the A2A receptor antagonists, SCH 412348 (0.3-30 mg/kg PO) and KW-6002 (3-100 mg/kg PO); the A1/A2A receptor antagonist, caffeine (1-30 mg/kg PO and IM); and the A1 receptor antagonist, DPCPX (3-30 mg/kg PO) were tested in at least one of these models.

RESULTS

SCH 412348 (10-30 mg/kg), KW-6002 (57-100 mg/kg), and caffeine (30 mg/kg) significantly increased the time to EPS onset. Additionally, SCH 412348, KW-6002, and caffeine afforded protection from the onset of EPS for at least 6 h in some of the primates. SCH 412348 (10 mg/kg) and caffeine (10 mg/kg) significantly reduced haloperidol-induced catalepsy. DPCPX produced a very slight attenuation of EPS at 30 mg/kg, but had no effect on catalepsy.

CONCLUSIONS

These findings suggest that adenosine A2A receptor antagonists may represent an effective treatment for the motor impairments associated with both antipsychotic-induced EPS and PD.

Potential therapeutic interest of adenosine A2A receptors in psychiatric disorders

The interest on targeting adenosine A(2A) receptors in the realm of psychiatric diseases first arose based on their tight physical and functional interaction with dopamine D(2) receptors. However, the role of central A(2A) receptors is now viewed as much broader than just controlling D(2) receptor function. Thus, there is currently a major interest in the ability of A(2A) receptors to control synaptic plasticity at glutamatergic synapses. This is due to a combined ability of A(2A) receptors to facilitate the release of glutamate and the activation of NMDA receptors. Therefore, A(2A) receptors are now conceived as a normalizing device promoting adequate adaptive responses in neuronal circuits, a role similar to that fulfilled, in essence, by dopamine. This makes A(2A) receptors particularly attractive targets to manage psychiatric disorders since adenosine may act as go-between glutamate and dopamine, two of the key players in mood processing. Furthermore, A(2A) receptors also control glia function and brain metabolic adaptation, two other emerging mechanisms to understand abnormal processing of mood, and A(2A) receptors are important players in controlling the demise of neurodegeneration, considered an amplificatory loop in psychiatric disorders. Current data only provide an indirect confirmation of this putative role of A(2A) receptors, based on the effects of caffeine (an antagonist of both A(1) and A(2A) receptors) in psychiatric disorders. However, the introduction of A(2A) receptors antagonists in clinics as anti-parkinsonian agents is hoped to bolster our knowledge on the role of A(2A) receptors in mood disorders in the near future.

The hyperactive syndrome: metanalysis of genetic alterations, pharmacological treatments and brain lesions which increase locomotor activity

The large number of transgenic mice realized thus far with different purposes allows addressing new questions, such as which animals, over the entire set of transgenic animals, show a specific behavioural abnormality. In the present study, we have used a metanalytical approach to organize a database of genetic modifications, brain lesions and pharmacological interventions that increase locomotor activity in animal models. To further understand the resulting data set, we have organized a second database of the alterations (genetic, pharmacological or brain lesions) that reduce locomotor activity. Using this approach, we estimated that 1.56% of the genes in the genome yield to hyperactivity and 0.75% of genes produce hypoactivity when altered. These genes have been classified into genes for neurotransmitter systems, hormonal, metabolic systems, ion channels, structural proteins, transcription factors, second messengers and growth factors. Finally, two additional classes included animals with neurodegeneration and inner ear abnormalities. The analysis of the database revealed several unexpected findings. First, the genes that, when mutated, induce hyperactive behaviour do not pertain to a single neurotransmitter system. In fact, alterations in most neurotransmitter systems can give rise to a hyperactive phenotype. In contrast, fewer changes can decrease locomotor activity. Specifically, genetic and pharmacological alterations that enhance the dopamine, orexin, histamine, cannabinoids systems or that antagonize the cholinergic system induce an increase in locomotor activity. Similarly, imbalances in the two main neurotransmitters of the nervous system, GABA and glutamate usually result in hyperactive behaviour. It is remarkable that no genetic alterations pertaining to the GABA system have been reported to reduce locomotor behaviour. Other neurotransmitters, such as norepinephrine and serotonin, have a more complex influence. For instance, a decrease in norepinephrine synthesis usually results in hypoactive behaviour. However, a chronic increase in norepinephrine may result in hypoactivity too. Similarly, changes in both directions of serotonin levels may reduce locomotor activity, whereas alterations in specific serotonin receptors can induce hyperactivity. The lesion of at least 12 different brain regions can increase locomotor activity too. Comparatively, few focal lesions decrease locomotor activity. Finally, a large number of toxic events can increase locomotor activity, particularly if delivered during the prepuberal time window. These data show that there is a net imbalance in the number of altered genes/brain lesions/toxics that induce hyperactivity versus hypoactive behaviour. Although some of these data may be explained in terms of the activating role of subcortical systems (such as catecholamines), the larger number of alterations that induce hyperactivity suggests a different scenario. Specifically, we hypothesize (i) the existence of a control system that continuously inhibit a basally hyperactive locomotor tone and (ii) that this control system is highly vulnerable (intrinsic fragility) to any change in the genetic asset or to any toxic/drug delivered during prepuberal stages. Brain lesion studies suggest that the putative control system is located along an axis that connects the olfactory bulb and the enthorhinal cortex (enthorhinal-hippocampal-septal-prefrontal cortex-olfactory bulb axis). We suggest that the increased locomotor activity in many psychiatric diseases may derive from the interference with the development of this brain axis during a specific postnatal time window.

Dopamine/adenosine interactions related to locomotion and tremor in animal models: possible relevance to parkinsonism

Adenosine A(2A) antagonists can exert antiparkinsonian effects in animal models. Recent experiments studied the ability of MSX-3 (an adenosine A(2A) antagonist) to reverse the locomotor suppression and tremor produced by dopamine antagonists in rats. MSX-3 reversed haloperidol-induced suppression of locomotion, and reduced the tremulous jaw movements induced by haloperidol, pimozide, and reserpine. Infusions of MSX-3 into the nucleus accumbens core increased locomotion in haloperidol-treated rats, but there were no effects of infusions into the accumbens shell or ventrolateral neostriatum. In contrast, MSX-3 injected into the ventrolateral neostriatum reduced pimozide-induced tremulous jaw movements. Dopamine/adenosine interactions in different striatal subregions are involved in distinct aspects of motor function.

Brain adenosine A2A receptor occupancy by a novel A1/A2A receptor antagonist, ASP5854, in rhesus monkeys: relationship to anticataleptic effect

The purpose of the present study was to measure adenosine A(2A) receptor (A(2A)R) occupancy in the brain by a novel adenosine A(1)/A(2A) antagonist, 5-[5-amino-3-(4fluorophenyl)pyrazin-2-yl]-1-isopropylpyridine-2(1H)-one (ASP5854), and to determine the degree of receptor occupancy necessary to inhibit haloperidol-induced catalepsy in rhesus monkeys.

METHODS

A(2A)R occupancy by ASP5854 (0.001-0.1 mg/kg) was examined in the striatum using an A(2A)R-specific radiotracer, (11)C-SCH442416, and PET in conscious rhesus monkeys. A(2A)R occupancy was monitored after a single intravenous administration of ASP5854 in 3 animals, and a dynamic PET scan was performed at 1, 4, and 8 h after an intravenous bolus injection of the tracer for approximately 740 MBq. Catalepsy was induced by haloperidol (0.03 mg/kg, intramuscularly) and examined for incidence and duration.

RESULTS

ASP5854 dose-dependently increased A(2A)R occupancy in the striatum and showed long-lasting occupancy even after the reduction of plasma concentration. Haloperidol induced severe catalepsy at 40 min after intramuscular injection. The incidence and duration of cataleptic posture were dose-dependently reduced by ASP5854 at 1 h after oral administration, and the minimum ED(50) value was 0.1 mg/kg. Administration of a dose of 0.1 mg/kg yielded a plasma concentration of 97 +/- 16.3 ng/mL, which corresponded to 85%-90% of A(2A)R occupancy.

CONCLUSION

These results showed that ASP5854 antagonized A(2A)R in the striatum, and the dissociation from A(2A)R was relatively slow. In addition, more than 85% A(2A)R occupancy by ASP5854 resulted in an inhibition of haloperidol-induced catalepsy. Thus, such a pharmacodynamic study directly demonstrates both the kinetics of a drug in the brain and the relationship between dose-dependent receptor occupancy and plasma level.

Blockade of globus pallidus adenosine A(2A) receptors displays antiparkinsonian activity in 6-hydroxydopamine-lesioned rats treated with D(1) or D(2) dopamine receptor agonists

We have recently demonstrated how antagonism of adenosine A(2A) receptors within the globus pallidus (GP) ipsilateral to dopaminergic denervation potentiates contralateral rotational behavior induced by the dopamine precursor L-DOPA in 6-hydroxydopamine-lesioned hemiparkinsonian rats. To further characterize the influence of pallidal A(2A) receptor blockade on the motor stimulant effects elicited by dopamine receptor activation, hemiparkinsonian rats were infused with the water-soluble A(2A) antagonist SCH BT2 in the GP, alone or in combination with systemic administration of either SKF 38393 or quinpirole, to stimulate dopamine D(1) or D(2) receptors, respectively. SCH BT2 alone (5 mug/1 mul) neither altered motor behavior nor produced postural asymmetry. In contrast, the contralateral rotations elicited by SKF 38393 (1.5 mg/kg) as well as quinpirole (0.05 mg/kg) were potentiated by the concomitant intrapallidal infusion of SCH BT2. The results of this study demonstrate that blockade of pallidal A(2A) receptors exerts a facilitatory influence on the motor effects produced by the selective stimulation of either D(1) or D(2) dopamine receptors in hemiparkinsonian rats and suggest an involvement of GP in the antiparkinsonian activity of A(2A) receptor antagonists.