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

Adenosine A2A antagonism reverses levodopa-induced motor alterations in hemiparkinsonian rats

To evaluate the possible involvement of adenosine A(2A) receptor-mediated mechanisms in levodopa-induced motor fluctuations, we investigated the effects of CSC (8-(3-chlorostryryl) caffeine), a selective adenosine A(2A) receptor antagonist, on levodopa-induced motor alterations in rats with unilateral 6-OHDA lesion. Acute and chronic administration of CSC was studied to evaluate the possible reversion or prevention of these levodopa effects. In a first set of experiments, rats were treated with levodopa (25 mg/kg with benserazide, twice daily, i.p.) for 22 days and on day 23 CSC (5 mg/kg, i.p.) was administered immediately before levodopa. In a second set of experiments, rats were treated daily for 22 days with levodopa and CSC (5 mg/kg/day, i.p.). The duration of the rotational behavior induced by chronic levodopa decreased after 22 days (P < 0.05). Acute administration of CSC on day 23 reversed levodopa-induced shortening in motor response duration (P < 0.01). Chronic CSC administration did not prevent the shortening in response duration induced by levodopa. Our results demonstrate that the adenosine A(2A) receptor antagonist CSC reverses but does not prevent levodopa-induced motor alterations in parkinsonian rats. These results suggest a role for adenosine A(2A) receptor-mediated mechanisms in the pathophysiology of levodopa-induced motor response complications. These findings suggest that the antagonism of adenosine A(2A) receptors might confer clinical benefit to parkinsonian patients under levodopa therapy suffering from motor complication syndrome.

Distribution of adenosine A(2A) receptor antagonist KW-6002 and its effect on gene expression in the rat brain

A novel adenosine A(2A) receptor selective antagonist, KW-6002 [(E)-1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methyl-3,7-dihydro-1H-purine-2,6-dione], possesses antiparkinsonian activities in rodent and primate models. In the present study, we investigated the distribution of [14C]KW-6002 in forebrain after oral administration at pharmacologically effective doses. Also, we monitored the effects of the compound on preproenkephalin (PPE) and preprotachykinin (PPT) gene expression in rat striatum. The highest level of radioactivity was observed in the striatum after oral administration of [14C]KW-6002; 30 min after 0.1 and 0.3 mg/kg, the density values in the striatum were 2.45 and 2.43 times higher than those in a reference region (frontal cortex), respectively. At the dose of 3 mg/kg, p.o., the ratio was only 1.58 and the compound was distributed more extensively in the brain. The distribution pattern and intensity of radioactivity were maintained even 90 min after the administration of [14C]KW-6002. Oral administration of KW-6002 (0.3 and 3 mg/kg/day) to rats for 14 days reversed the increased gene expression of PPE in striatum that had been depleted of dopamine by prior treatment with 6-hydroxydopamine (6-OHDA). On the other hand, KW-6002 did not alter the decreased gene expression of PPT in 6-OHDA-treated rats. These results are the first to show directly that orally administered KW-6002 is distributed selectively to the striatum and that it modulates the activity of striatopallidal enkephalin-containing neurons but not striatonigral substance P-containing neurons.

Caffeinated clues and the promise of adenosine A(2A) antagonists in PD

Large prospective epidemiologic studies have linked the consumption of coffee and other caffeinated beverages to a reduced risk of subsequently developing PD. Caffeine as well as more specific antagonists of the adenosine A(2A) receptor have also now been found to attenuate neurotoxicity in a mouse model of PD. The convergence of these epidemiologic and laboratory data supports the possibility that caffeine may reduce the risk of developing PD. However, a neuroprotective effect of caffeine in PD remains unproven; current evidence does not provide a rational basis for recommending caffeine consumption to modify the risk or progression of PD. In addition to possessing neuroprotective potential, caffeine and other A(2A) antagonists have long been known to acutely reverse motor deficits in a variety of PD models. This symptomatic antiparkinsonian benefit of blocking A(2A) receptors, coupled with their remarkably restricted expression in the basal ganglia, have made A(2A) antagonists attractive targets for drug development. Now, with the prospect of a neuroprotective bonus, the novel therapeutic potential of A(2A) antagonists appears all the more promising just as they are entering clinical trials for PD.

Current evidence for neuroprotective effects of nicotine and caffeine against Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder affecting 1 to 3% of individuals over the age of 65 years. While effective therapy exists for treating the bradykinesia, rigidity and tremor associated with the disease, the cause is unknown. There is no treatment available to prevent or slow the progressive neuronal loss in the substantia nigra and associated decreased levels of dopamine in the striatum that underlie the cardinal features of the disease. Both retrospective and prospective epidemiological studies have consistently demonstrated an inverse association between cigarette smoking and PD, leading to theories that smoking in general and nicotine in particular might be neuroprotective. Nicotine has been shown in animals to stimulate the release of dopamine in the striatum, and to preserve nigral neurons and striatal dopamine levels in laboratory animals with lesioned nigrostriatal pathways. Coffee and caffeine consumption have also been shown in epidemiological studies to be inversely related to PD risk. Caffeine is an adenosine A(2A) receptor antagonist that enhances locomotor activity in animal models of parkinsonism. Theophylline, a related compound that has A(2A) receptor blocking properties, has been shown in one small trial to improve motor function in patients with PD. Recently, potent and highly selective A(2A) receptor antagonists have been developed that have demonstrated improvement in motor function in animal models of parkinsonism. Exciting findings are emerging that demonstrate attenuation of dopaminergic neurotoxicity with caffeine and other adenosine receptor antagonists in mice given the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), suggesting that these compounds may be neuroprotective. Evidence for the neuroprotective potential of nicotine and caffeine is compelling, but further work is needed before testing these and related compounds in clinical trials for both individuals at high risk of developing PD and those with early, untreated disease.

Evaluation of [4-O-methyl-(11)C]KW-6002 as a potential PET ligand for mapping central adenosine A(2A) receptors in rats

KW-6002, a xanthine-based adenosine A(2A) antagonist, was labelled with the positron emitter carbon-11 by O-methylation of its precursor, KF23325, using [(11)C]iodomethane and was evaluated in rats as a putative in vivo radioligand for positron emission tomography (PET). Following intravenous injection of [(11)C]KW-6002, radioactivity was measured in blood, plasma, peripheral tissues, and in discrete brain tissues over a 2-h time period commensurate with PET scanning. In brain, [(11)C]KW-6002 showed highest retention in striata, with evidence of saturable binding, and lowest retention in frontal cortex (a tissue low in adenosine A(2A) receptors). PET scanning with [(11)C]KW-6002 demonstrated a specific signal in the striata which could be described using compartmental modelling. Specific binding was, however, also detected in extrastriatal regions, including brain areas reported to have low adenosine A(2A) receptor density. Blocking studies with the A(1) selective antagonist KF15372 and the non xanthine-type A(2A) antagonist ZM 241385 failed to elucidate the nature of this binding. Thus, although [(11)C]KW-6002 shows some potential for development as a PET ligand for quantifying striatal adenosine A(2A) receptor function, its in vivo selectivity requires further investigation.

Adenosine A(2a) receptor antagonists: potential therapeutic and neuroprotective effects in Parkinson's disease

The most effective treatment of Parkinson's disease (PD) is, at present, the dopamine precursor L-3,4-dihydroxyphenylalanine (L-DOPA), however a number of disadvantages such as a loss of drug efficacy and severe side-effects (psychoses, dyskinesias and on-off phenomena) limit long-term effective utilisation of this drug. Recent experimental studies in which selective antagonists of adenosine A(2A) receptors were used, have shown an improvement in motor disabilities in animal models of PD. The A(2A) antagonist [7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-(4,3-e)-1,2,4-triazolo(1,5-c) pyrimidine] (SCH 58261) potentiated the contralateral turning behavior induced by a threshold dose of L-DOPA or direct dopamine receptor agonists in unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, an effect accompanied by an increase in Fos-like-immunoreactivity in neurons of the lesioned striatum. Likewise, other A(2A) receptor antagonists such as (3,7-dimethyl-1-propargylxanthine) (DMPX), [E-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine] (KF 17837) and [E-1,3-diethyl-8(3,4-dimethoxystyryl-7-methyl-3,7-dihydro-1H-purine-2,6-dione] (KW 6002) antagonized catalepsy induced by haloperidol or reserpine in the rat, whereas in non-human primate models of PD, KW 6002 reduced the rigidity and improved the disability score of MPTP-treated marmosets and cynomolgus monkeys. Moreover, in contrast to L-DOPA, selective A(2A) receptor antagonists administered chronically did not produce dyskinesias and did not evoke tolerance in 6-OHDA and MPTP models of PD. An additional therapeutic potential of adenosine A(2A) antagonists emerged from studies showing neuroprotective properties of these compounds in animal models of cerebral ischemia and excitotoxicity, as well as in the MPTP model of PD. Adenosine A(2A) receptor antagonists by reversing motor impairments in animal models of PD and by contrasting cell degeneration are some of the most promising compounds for the treatment of PD.

Behavioral phenotyping of the MPTP mouse model of Parkinson's disease

In mice, the systemical or intracranial application of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can lead to severe damage to the nigrostriatal dopaminergic system. This can result in a variety of symptoms concerning motor control resembling those in human Parkinson's disease, such as akinesia, rigidity, tremor, gait and posture disturbances. The aim of this work is to review a variety of behavioral paradigms for these and other symptoms, which have been used to characterize behavioral changes in mice after MPTP treatment. Main results are summarized, and general influential factors as well as potential problems in the experimental procedures are discussed, which should be taken into account when conducting behavioral analyses in mice with parkinsonian symptoms. Since there is reliable evidence (e.g. from strain comparisons) that the susceptibility of the nigrostriatal pathway to neurodegeneration is probably genetically influenced, relevant genes can be expected to be identified in the future. Therefore, the points discussed here will be useful not only for further applications in the MPTP mouse model, but also more generally for the behavioral characterization of future mouse models of PD, e.g. mice with a manipulation of genes relevant to the function of the basal ganglia.

Catalepsy induced by a blockade of dopamine D1 or D2 receptors was reversed by a concomitant blockade of adenosine A(2A) receptors in the caudate-putamen of rats

The present study sought to determine, in more detail, the effects of an unselective and a selective adenosine A(2A) receptor blockade on catalepsy induced by a blockade of dopamine D1 or D2 receptors in rats. The results demonstrated that systemic administration of the unselective A1/A2 receptor antagonist, theophylline and the selective A(2A) receptor antagonist, CSC potently reversed catalepsy induced by a systemic D2 receptor blockade with raclopride or by a bilateral blockade of D2 receptors in the caudate-putamen (CPu) with S(-)sulpiride. Likewise, systemic administration of theophylline and CSC reversed catalepsy induced by a systemic D1 receptor blockade with SCH23390; theophylline also counteracted catalepsy after an intra-CPu D1 receptor blockade with SCH23390. Intracerebral co-microinfusions of the selective A(2A) receptor antagonist, MSX-3 together with a D1 (SCH23390) or D2 receptor [S(-) sulpiride] antagonist revealed that catalepsy due to intra-CPu D1 or D2 receptor blockade can be potently reversed by an intra-CPu A2A receptor blockade. In conclusion, our results with systemic and intra-CPu drug administration demonstrate that D1 and D2 receptor-mediated catalepsy can both be reversed by a concomitant blockade of A(2A) receptors. Our results implicate that the CPu is a critical neural substrate for antagonistic interactions of a D1/D2 receptor blockade and an A(2A) receptor blockade in control of motor activity. The present results provide further support for the view that A(2A) receptor antagonists may be potential therapeutics for the treatment of Parkinson's disease.

Adenosine A2A receptor antagonists are potential antidepressants: evidence based on pharmacology and A2A receptor knockout mice

1. Adenosine, an ubiquitous neuromodulator, and its analogues have been shown to produce 'depressant' effects in animal models believed to be relevant to depressive disorders, while adenosine receptor antagonists have been found to reverse adenosine-mediated 'depressant' effect. 2. We have designed studies to assess whether adenosine A2A receptor antagonists, or genetic inactivation of the receptor would be effective in established screening procedures, such as tail suspension and forced swim tests, which are predictive of clinical antidepressant activity. 3. Adenosine A2A receptor knockout mice were found to be less sensitive to 'depressant' challenges than their wildtype littermates. Consistently, the adenosine A2A receptor blockers SCH 58261 (1 - 10 mg kg(-1), i.p.) and KW 6002 (0.1 - 10 mg kg(-1), p.o.) reduced the total immobility time in the tail suspension test. 4. The efficacy of adenosine A2A receptor antagonists in reducing immobility time in the tail suspension test was confirmed and extended in two groups of mice. Specifically, SCH 58261 (1 - 10 mg kg(-1)) and ZM 241385 (15 - 60 mg kg(-1)) were effective in mice previously screened for having high immobility time, while SCH 58261 at 10 mg kg(-1) reduced immobility of mice that were selectively bred for their spontaneous 'helplessness' in this assay. 5. Additional experiments were carried out using the forced swim test. SCH 58261 at 10 mg kg(-1) reduced the immobility time by 61%, while KW 6002 decreased the total immobility time at the doses of 1 and 10 mg kg(-1) by 75 and 79%, respectively. 6. Administration of the dopamine D2 receptor antagonist haloperidol (50 - 200 microg kg(-1) i.p.) prevented the antidepressant-like effects elicited by SCH 58261 (10 mg kg(-1) i.p.) in forced swim test whereas it left unaltered its stimulant motor effects. 7. In conclusion, these data support the hypothesis that A2A receptor antagonists prolong escape-directed behaviour in two screening tests for antidepressants. Altogether the results support the hypothesis that blockade of the adenosine A2A receptor might be an interesting target for the development of effective antidepressant agents.

SCH 58261, an A(2A) adenosine receptor antagonist, counteracts parkinsonian-like muscle rigidity in rats

The aim of the present study was to find out whether blockade of adenosine A(2A) receptors by a selective antagonist, SCH 58261, influenced parkinsonian-like muscle rigidity. Muscle tone was examined using a combined mechano- and electromyographic method which simultaneously measured muscle resistance (MMG) of a rat hindfoot to passive extension and flexion in the ankle joint and electromyographic activity (EMG) of the antagonistic muscles of that joint: gastrocnemius and tibialis anterior. Muscle rigidity produced by reserpine (5 mg/kg + alpha-methyl-p-tyrosine, 250 mg/kg) was antagonized by SCH 58261 (0.1-5 mg/kg). SCH 58261 (5 mg/kg) also reduced reserpine-enhanced tonic and reflex EMG activities in both the gastrocnemius and the tibialis muscles. Moreover, SCH 58261 in doses of 1 and 5 mg/kg abolished muscle resistance induced by haloperidol (0.5 mg/kg). However, only the highest dose of SCH 58261 (5 mg/kg) decreased tonic EMG activity enhanced by haloperidol. Administration of L-DOPA (75 and 100 mg/kg) dose-dependently decreased the muscle resistance as well as tonic EMG activity evoked by haloperidol. Combined administration of SCH 58261 (0.1 mg/kg) and L-DOPA (50 mg/kg) in doses which did not affect the haloperidol-induced muscle rigidity produced a pronounced synergistic effect. The ability of SCH 58261 to diminish the parkinsonian-like muscle rigidity and to potentiate the effect of L-DOPA in this model seems to indicate a therapeutic value of this compound in the treatment of Parkinson's disease.

Adenosine/dopamine interaction: implications for the treatment of Parkinson's disease

Evidence for a role of dopaminergic neurotransmission in the motor effects of adenosine antagonists, such as caffeine, is reviewed, based on the existence of specific antagonistic interactions between specific subtypes of adenosine and dopamine receptors in the striatum. Both adenosine A(1) and adenosine A(2A) receptor antagonists induce motor activation in rodents. At least a certain degree of dopaminergic activity is required to obtain adenosine antagonist-induced motor activation, with adenosine A(1) antagonists being the most sensitive and non-selective adenosine antagonists the most resistant to striatal dopamine depletion. When considering long-term treatment with adenosine antagonists concomitant administration of dopamine agonists might be required in order to obtain strong motor effects (cross-sensitization) and to avoid the development of telerance.

Adenosine A2A receptor knockout mice are partially protected against drug-induced catalepsy

Catalepsy assessed using the bar test was measured in both adenosine A2A receptor knockout (A2AR KO) and wild-type (A2AR WT) mice submitted to acute administration of the dopamine D2 receptor antagonist haloperidol (0.5, 2, 4, 6 mg/kg i.p.), the dopamine D1 antagonist SCH 23390 (0.3-3 mg/kg, s.c.), the vesicular monoamine transporter blocker reserpine (3-5 mg/kg, s.c.) or the acetylcholine muscarinic receptor agonist pilocarpine (25-50 mg/kg, i.p.). Except for reserpine, catalepsy scores were significantly lower in A2AR KO mice than in A2AR WT mice following low doses of these cataleptogenic agents. These results suggest that adenosine A2A receptors influence not only dopamine D2 and D1 receptor-mediated neurotransmission but also acetylcholine muscarinic receptor-mediated neurotransmission.

Systemic administration of adenosine A(2A) receptor antagonist reverses increased GABA release in the globus pallidus of unilateral 6-hydroxydopamine-lesioned rats: a microdialysis study

The ability of adenosine A(2A) receptor antagonists to exhibit antiparkinsonian activity has recently been reported, but the mechanisms of action are still unknown. Since A(2A) receptors have been localized to GABAergic striatopallidal neurons, it is probable that these antagonists affect the activity of these neurons. In the present study, extracellular GABA basal levels were increased in the ipsilateral striatum and globus pallidus following a unilateral 6-hydroxydopamine lesion of the nigrostriatal pathway. The A(2A) receptor-selective antagonist KW-6002 (3mg/kg, p.o.) caused a marked and sustained decrease of extracellular GABA levels in the globus pallidus of the 6-hydroxydopamine-lesioned rats, whereas no changes in GABA levels were observed in the globus pallidus of the non-lesioned rats. Microinjection of the A(2A) receptor agonist CGS21680 (0.005-0.5 microg) into the striatum of non-lesioned animals increased GABA concentrations in the globus pallidus, which was abolished by the voltage-dependent Na(+) channel blocker tetrodotoxin (1 micromol/l) delivered locally to the globus pallidus via the dialysis membrane. Furthermore, intrapallidal infusion of CGS21680 (10 micromol/l) also increased GABA levels in the globus pallidus. These data indicate that GABA release from striatopallidal neurons is regulated through A(2A) receptors in both the striatum and globus pallidus. The reversal of the 6-hydroxydopamine-induced increase in pallidal GABA levels by KW-6002 suggests that the antiparkinsonian effects of A(2A) receptor antagonists occur on the striatopallidal neurons.

The role of the D(2) dopamine receptor (D(2)R) in A(2A) adenosine receptor (A(2A)R)-mediated behavioral and cellular responses as revealed by A(2A) and D(2) receptor knockout mice

The A(2A)R is largely coexpressed with D(2)Rs and enkephalin mRNA in the striatum where it modulates dopaminergic activity. Activation of the A(2A)R antagonizes D(2)R-mediated behavioral and neurochemical effects in the basal ganglia through a mechanism that may involve direct A(2A)R-D(2)R interaction. However, whether the D(2)R is required for the A(2A)R to exert its neural function is an open question. In this study, we examined the role of D(2)Rs in A(2A)R-induced behavioral and cellular responses, by using genetic knockout (KO) models (mice deficient in A(2A)Rs or D(2)Rs or both). Behavioral analysis shows that the A(2A)R agonist 2-4-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine reduced spontaneous as well as amphetamine-induced locomotion in both D(2) KO and wild-type mice. Conversely, the nonselective adenosine antagonist caffeine and the A(2A)R antagonist 8-(3-chlorostyryl)caffeine produced motor stimulation in mice lacking the D(2)R, although the stimulation was significantly attenuated. At the cellular level, A(2A)R inactivation counteracted the increase in enkephalin expression in striatopallidal neurons caused by D(2)R deficiency. Consistent with the D(2) KO phenotype, A(2A)R inactivation partially reversed both acute D(2)R antagonist (haloperidol)-induced catalepsy and chronic haloperidol-induced enkephalin mRNA expression. Together, these results demonstrate that A(2A)Rs elicit behavioral and cellular responses despite either the genetic deficiency or pharmacological blockade of D(2)Rs. Thus, A(2A)R-mediated neural functions are partially independent of D(2)Rs. Moreover, endogenous adenosine acting at striatal A(2A)Rs may be most accurately viewed as a facilitative modulator of striatal neuronal activity rather than simply as an inhibitory modulator of D(2)R neurotransmission.

Adenosine A(2A) receptor antagonists KF17837 and KW-6002 potentiate rotation induced by dopaminergic drugs in hemi-Parkinsonian rats

The effects of novel adenosine A(2A) receptor antagonists KF17837 ((E)-1,3-dipropyl-8-(3,4-dimethoxystyryl)-7-methyl-3, 7-dihydro-1H-purine-2,6-dione) and KW-6002 ((E)-1,3-diethyl-8-(3, 4-dimethoxystyryl)-7-methyl-3,7-dihydro-1H-purine-2,6-dione), on rotational behavior induced by apomorphine or L-DOPA (L-3, 4-dihydroxyphenylalanine) were investigated in rats with unilateral 6-hydroxydopamine lesions. Both KF17837 and KW-6002 slightly induced rotational behavior per se. However, KF17837 and KW-6002 significantly increased the total counts of turning induced by apomorphine at doses of 3 mg/kg, p.o. and 10 mg/kg, p.o., and at doses of 1 mg/kg, p.o. and higher, respectively. KF17837 and KW-6002 also potentiated the rotational behavior induced by L-DOPA at a dose of 3 mg/kg, p.o. Furthermore, i.c.v. injection (10 microg/20 microl) of a selective adenosine A(2) receptor agonist CGS 21680 [2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoaden osine] partially prevented the rotational behavior induced by apomorphine and this inhibition was reversed by KW-6002 (1 mg/kg, p.o.). The increase in total counts of apomorphine-induced turning by the adenosine A(2A) receptor antagonists seems to be mainly attributable to prolongation of turning duration rather than enhancement of intensity. These results suggest that these adenosine A(2A) receptor antagonists may be useful to ameliorate shortening in the duration of dopaminergic drug response in patients with advanced Parkinson's disease.

Rescue of locomotor impairment in dopamine D2 receptor-deficient mice by an adenosine A2A receptor antagonist

In Parkinson's disease a degeneration of dopaminergic neurons of the nigrostriatal pathway is observed. Loss of dopaminergic regulation of striatal neuron activity results in altered motor functions. Adenosine A2A (A2AR) and dopamine D2 (D2R) receptors are colocalized in striatal medium spiny neurons. It has been proposed that adenosine binding to A2AR lowers the affinity of dopamine for D2R, thus modulating the function of this receptor. Absence of D2R in knockout mice (D2R-/-) results in impaired locomotion and coordinated movements. This indicates that absence of dopamine in Parkinson's disease might principally affect D2R-mediated effects with regard to locomotor functions. A2AR-selective antagonists have been demonstrated to have anti- parkinsonian activities in various models of Parkinson's disease in rodents and nonhuman primates. In this article, D2R-/- mice were used to explore the possibility that an A2AR antagonist might reestablish their motor impairment. Interestingly, blockade of A2AR rescues the behavioral parameters altered in D2R-/- mice. In addition, the level of expression of enkephalin and substance P, which were altered in D2R-/-, were also reestablished to normal levels after A2AR antagonist treatment. These results show that A2AR and D2R have antagonistic and independent activities in controlling neuronal and motor functions in the basal ganglia. They also provide evidence that selective A2AR antagonists can exhibit their anti-parkinsonian activities through a nondopaminergic mechanism.