Adenosine A2A antagonists: potential preventive and palliative treatment for Parkinson's disease

Caffeine protects against disruptions of the blood-brain barrier in animal models of Alzheimer's and Parkinson's diseases

Sporadic Alzheimer's disease (AD) and Parkinson's disease (PD) are two of the most common neurodegenerative diseases and as such they represent major public health problems. Finding effective treatments for AD and PD represents an unmet and elusive goal largely because these diseases are chronic and progressive, and have a complicated and ill-understood pathogenesis. Although the underlying mechanisms are not fully understood, caffeine, the most commonly ingested psychoactive drug in the world, has been shown in human and animal studies to be protective against AD and PD. One mechanism implicated in the pathogenesis of AD and PD is blood-brain barrier (BBB) dysfunction and we reported recently that caffeine exerts protective effects against AD and PD at least in part by keeping the BBB intact. The present review focuses on the role of BBB dysfunction in the pathogenesis of AD and PD, caffeine's protective effects against AD and PD, and potential mechanisms whereby caffeine protects against BBB leakage.

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.

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.

Modulation of the motor response to dopaminergic drugs in a parkinsonian model of combined dopaminergic and noradrenergic degeneration

Besides dopaminergic deficiency, other neurotransmitter systems such as noradrenergic nuclei are affected in Parkinson's disease. Locus coeruleus degeneration might influence the response to dopamine replacement and the presence of long-term complications such as dyskinesias. In this scenario of noradrenergic and dopaminergic neurodegeneration, behavioural effects induced by dopaminergic-interacting drugs are incompletely known. We investigated whether noradrenergic lesion modulates the levodopa (l-DOPA) response and modifies the response to adenosine antagonists and its interaction with l-DOPA. We examined the motor behaviour induced by: 1) subthreshold doses of l-DOPA (2mg/kg, i.p.), 2) the adenosine-receptor antagonist caffeine (10mg/kg), and 3) the combination of l-DOPA (2mg/kg) and caffeine (10mg/kg). Each study was done in two experimental conditions: a) rats with unilateral 6-OHDA lesion and b) rats with a lesion of the nigrostriatal pathway (6-OHDA) combined with selective denervation of locus coeruleus-noradrenergic terminal fields by N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4). While only 28% of the 6-OHDA-lesioned animals presented circling behaviour after l-DOPA challenge, all (100%) double-denervated animals rotated after the same l-DOPA dose (p<0.05). No statistical differences in the percentage of rotating animals were observed between single- and double-denervated rats after caffeine challenge. Combined l-DOPA-caffeine challenge produced rotational behaviour in all (100%) single- and double-denervated rats. No differences in total turns were observed between single- and double-denervated animals in each treatment condition. These findings suggest that additional noradrenergic denervation selectively decreases the motor threshold to l-DOPA treatment without modifying the magnitude or the pattern of the motor response to adenosinergic antagonism.

Characterization of the antiparkinsonian effects of the new adenosine A2A receptor antagonist ST1535: Acute and subchronic studies in rats

Antagonism of adenosine A2A receptor function has been proposed as an effective therapy in the treatment of Parkinson's disease. Thus, the study of new adenosine receptor antagonists is of great importance for the potential use of these drugs in clinical practice. The present study evaluated effects of the new preferential adenosine A2A receptor antagonist 2-butyl-9-methyl-8-(2H-1,2,3-triazol-2-yl)-9H-purin-6-ylamine (ST1535) in unilaterally 6-hydroxydopamine lesioned rats. Acute ST1535 dose-dependently potentiated contralateral turning behaviour induced by a threshold dose of l-3,4-dihydroxyphenylalanine (L-DOPA) (3 mg/kg i.p.), a classical test for antiparkinson drug screening. Subchronic (18 days, twice a day) ST1535 (20 mg/kg i.p.)+L-DOPA (3 mg/kg i.p.) did not induce sensitization to turning behaviour or abnormal involuntary movements during the course of treatment, indicating a low dyskinetic potential of the drug. Moreover, while subchronic administration of a fully effective dose of L-DOPA (6 mg/kg i.p.) significantly increased GABA synthesizing enzyme glutamic acid decardoxylase (GAD67), dynorphin and enkephalin mRNA levels in the lesioned striatum, subchronic ST1535 (20 mg/kg i.p.)+L-DOPA (3 mg/kg i.p.) did not modify any of these markers, although it induced a similar number of contralateral rotations at the beginning of treatment. Finally, acute administration of ST1535 (20 mg/kg i.p.) proved capable of reducing jaw tremors in tacrine model of Parkinson's disease tremor. Results showed that ST1535, in association with a low dose of L-DOPA, displayed antiparkinsonian activity similar to that produced by a full dose of L-DOPA without exacerbating abnormal motor side effects. Moreover, in agreement to other well characterized adenosine A2A receptor antagonists, ST1535 features antitremorigenic effects.

Adenosine A2A receptor antagonists improve deficits in initiation of movement and sensory motor integration in the unilateral 6-hydroxydopamine rat model of Parkinson's disease

Evidence obtained in rodent and primate models of Parkinson's disease (PD) and preliminary clinical trials, indicates that adenosine A(2A) receptor antagonists might represent a promising nondopaminergic therapeutic tool for the treatment of PD. Those studies demonstrated the ability of adenosine A(2A) receptor antagonists to potentiate l-dopa-mediated motor improvement, whereas very little is known about counteraction of specific motor deficits and on the effects of these compounds when administered alone. To this aim we evaluated the effects of different adenosine A(2A) receptor antagonists on initiation of movement deficits, gait impairment and sensory-motor deficits, induced in rats by a unilateral 6-hydroxydopamine lesion of dopaminergic nigrostriatal neurons. The following tests were used: (1) initiation time of stepping; (2) adjusting step (stepping with forelimb was measured as the forelimb was dragged laterally); (3) vibrissae-elicited forelimb placing (as index of sensory-motor integration deficits). Acute administration of the A(2A) receptor antagonists SCH 58261 (5 mg/kg i.p.) and ST 1535 (20 mg/kg i.p.) similarly to l-dopa (6 mg/kg i.p.) counteracted the impairments in the initiation time of stepping test, in the adjusting step and in the vibrissae-elicited forelimb placing induced by the lesion. The intensity of the effect was l-dopa > SCH 58261 > ST 1535. The results provide the first evidence that blockade of A(2A) receptors is effective in antagonizing specific motor deficit induced by DA neuron degeneration, such as initiation of movement and sensory-motor integration deficits, even without l-dopa combined administration.

Dopamine and adenosine receptor interaction as basis for the treatment of Parkinson's disease

Preclinical evidence strongly indicate that adenosine A(2A) receptor antagonists represent a promising class of drugs for the treatment of motor deficits associated to Parkinson's disease. The effects of adenosine A(2A) receptor antagonists were here assessed in a rat model of parkinsonian tremor induced by cholinomimetic drugs by evaluating the counteraction of tremulous jaw movements. Systemic administration of the A(2A) antagonist SCH 58261 dose-dependently reduced the magnitude of perioral tremor induced by the acetylcholinesterase inhibitor tacrine (2.5 mg/kg). Furthermore, intrastriatal infusion of SCH BT2 (5 microg/microl), a water-soluble analogue of SCH 58261, antagonized tacrine-induced jaw movements with a maximal effect in the ventrolateral striatum. On the other hand, SCH 58261 (5 mg/kg) was ineffective in blocking tremulous jaw movements stimulated by the direct muscarinic agonist pilocarpine (1 mg/kg). Taken together, these results indicate that A(2A) antagonists reduce parkinsonian tremor stimulated in rats by tacrine and that the striatum is deeply involved in the observed effect. Moreover, the ineffectiveness of SCH 58261 in blocking pilocarpine-stimulated perioral tremor suggests that the antitremorigenic effects of A(2A) antagonists described here are not related to a direct action on muscarinic receptor. The prospective of providing additional antitremor benefits considerably enhances the therapeutic potential of A(2A) antagonists.

Neuroprotective effects of caffeine in the model of 6-hydroxydopamine lesion in rats

The work shows the effects of caffeine after the intrastriatal injection of 6-OHDA in rats, considered as a model of Parkinson disease (PD). Two weeks after the 6-OHDA lesion, rats exhibit a characteristic rotation behavior as a response to the apomorphine challenge. Our results showed significant increases in the number of apomorphine-induced rotations in 6-OHDA-lesioned rats, as compared to sham-operated animals. A partial recovery was observed in 6-OHDA-lesioned rats, after caffeine (10 and 20 mg/kg, i.p., daily for 14 days) treatment. The stereotaxic injection of 6-OHDA produced loss of striatal neurons, as indicated by the decrease in monoamines levels, in the ipsilateral side (75-85%) when compared to the contralateral side. Significant decreases in noradrenaline levels were seen in the ipsilateral side of 6-OHDA group (62%), and this effect was not significantly reversed in caffeine-treated groups. While significant decreases in dopamine levels were seen in the ipsilateral side of 6-OHDA group (78%), in the caffeine-treated group (10 and 20 mg/kg, i.p.) the decreases were only 53 and 18%, indicating significant recoveries. In conclusion, our data demonstrated beneficial effects of caffeine in this model of PD, suggesting the potential use of A2A antagonists as a novel treatment for this neurodegenerative disease.

Novel gene therapeutic strategies for neurodegenerative diseases

The convergent pathobiologic model of Parkinson's disease stipulates that disparate insults initiate a disease process that obligately share a common pathway leading to cell death. A combinatorial treatment which targets various steps in this pathway is likely to be the most successful therapeutic strategy. As advances are made in the field of neuroimaging and pharmacogenomics, early detection of sporadic PD will become a reality. Early intervention will likely spare more dopaminergic neurons and extend the quality of life for the patient. Continued advancements in the fields of pharmacology, neurosurgery, and gene therapy will strengthen the armamentarium available for the treatment of PD patients.

Subchronic intermittent caffeine administration to unilaterally 6-hydroxydopamine-lesioned rats sensitizes turning behaviour in response to dopamine D1 but not D2 receptor agonists

The effects of caffeine, an antagonist of adenosine A(1) and A(2A) receptors, are significantly influenced by modifications in dopamine transmission. Administration of caffeine to unilaterally 6-hydroxydopamine-lesioned rats induces ipsilateral turning behaviour in rats never exposed to a dopamine receptor agonist, whereas contralateral turning is elicited if rats are repeatedly primed with a dopamine receptor agonist. In this study, rats unilaterally lesioned with 6-hydroxydopamine and subchronically treated with an intermittent administration of caffeine (15 mg/kg) or vehicle, were administered, 3 days after discontinuations of the treatment, with the dopamine D(1) receptor agonist 1-phenyl 1,2,3,4,5-tetrahydro(1H)-3-benzazepine-7,8-diolhydrochloride (SKF 38393), the D(2)/D(3) receptor agonist quinpirole, the D(2) receptor agonist R(-)-propylnorapomorphine or the dopamine precursor L-3,4-dihydroxyphenyl-alanine. Administration of SKF 38393 (1.5 mg/kg) or L-3,4-dihydroxyphenyl-alanine (6 mg/kg), but not quinpirole (0.15 mg/kg) or R(-)-propylnorapomorphine (0.01 mg/kg), induced a significantly higher contralateral turning behaviour in rats subchronically treated with caffeine than in vehicle-pretreated rats. The results show that repeated intermittent caffeine exposure enhances the motor stimulant effects elicited by dopamine agonists by a preferential sensitization of dopamine D(1) receptors.

New adenosine A2A receptor antagonists: Actions on Parkinson's disease models

The 8-substituted 9-ethyladenine derivatives: 8-bromo-9-ethyladenine (ANR 82), 8-ethoxy- 9-ethyladenine (ANR 94), and 8-furyl-9-ethyladenine (ANR 152) have been characterized in vitro as adenosine receptor antagonists. Adenosine is deeply involved in the control of motor behaviour and substantial evidences indicate that adenosine A(2A) receptor antagonists improve motor deficits in animal models of Parkinson's disease. On this basis, the efficacy of ANR 82, ANR 94, and ANR 152 in rat models of Parkinson's disease was evaluated. All compounds tested reversed the catalepsy induced by haloperidol. However, in unilaterally 6-hydroxydopamine-lesioned rats, only ANR 94 and ANR 152 potentiated l-dihydroxy-phenylalanine (l-DOPA) effect on turning behaviour and induced contralateral turning behaviour in rats sensitised to l-DOPA. Taken together the results of this study indicate that some 8-substituted 9-ethyladenine derivatives ameliorate motor deficits in rat models of Parkinson's disease, suggesting a potential therapeutic role of these compounds.

Blockade of adenosine A2A receptors antagonizes parkinsonian tremor in the rat tacrine model by an action on specific striatal regions

Acute administration of the acetylcholinesterase inhibitor tacrine to rats induces tremulous jaw movements which can be used as a valuable model of parkinsonian tremor. In the present study, the number of tremor episodes and jaw movements were evaluated to assess the effects of the selective A2A antagonists SCH 58261 and SCH BT2 on tremorgenesis. SCH 58261 dose-dependently, and maximally at 5 mg/kg, reduced the number of both tremor episodes (-35%) and jaw movements (-50%), induced in rats by tacrine (2.5 mg/kg ip). Since adenosine A2A receptors are largely expressed throughout the striatum, chronic cannulae were implanted in the rat dorsomedial (DMS) and ventrolateral striatum (VLS) to investigate whether A2A antagonists could act at this level. Infusion of SCH BT2 (5 microg/microl), a water-soluble analogue of SCH 58261, in VLS antagonized both tremor episodes (-68%) and jaw movements (-76%) elicited by tacrine (2.5 mg/kg ip), whereas SCH BT2 infusion in DMS was less effective in blocking jaw movements (-50%) and did not significantly affect the number of tremor episodes. Taken together, the results of this study indicate that A2A antagonists effectively reduce the magnitude of tremulous jaw movements induced in rats by acute tacrine, mainly by an action in VLS and suggest that A2A antagonists might be used as specific agents against parkinsonian tremor.