Modulation of the striato-pallidal pathway by adenosine A2a receptors depends on dopaminergic striatal input

Adenosine A2A Receptor Modulates the Activity of Globus Pallidus Neurons in Rats

The globus pallidus is a central nucleus in the basal ganglia motor control circuit. Morphological studies have revealed the expression of adenosine A_2A receptors in the globus pallidus. To determine the modulation of adenosine A_2A receptors on the activity of pallidal neurons in both normal and parkinsonian rats, in vivo electrophysiological and behavioral tests were performed in the present study. The extracellular single unit recordings showed that micro-pressure administration of adenosine A_2A receptor agonist, CGS21680, regulated the pallidal firing activity. GABAergic neurotransmission was involved in CGS21680-induced modulation of pallidal neurons via a PKA pathway. Furthermore, application of two adenosine A_2A receptor antagonists, KW6002 or SCH442416, mainly increased the spontaneous firing of pallidal neurons, suggesting that endogenous adenosine system modulates the activity of pallidal neurons through adenosine A_2A receptors. Finally, elevated body swing test (EBST) showed that intrapallidal microinjection of adenosine A_2A receptor agonist/antagonist induced ipsilateral/contralateral-biased swing, respectively. In addition, the electrophysiological and behavioral findings also revealed that activation of dopamine D₂ receptors by quinpirole strengthened KW6002/SCH442416-induced excitation of pallidal activity. Co-application of quinpirole with KW6002 or SCH442416 alleviated biased swing in hemi-parkinsonian rats. Based on the present findings, we concluded that pallidal adenosine A_2A receptors may be potentially useful in the treatment of Parkinson's disease.

Therapeutic strategies to prevent and manage dyskinesias in Parkinson's disease

INTRODUCTION

Chronic treatment with levodopa is associated with the development of motor fluctuations and dyskinesias particularly in young Parkinson patients. In some cases, dyskinesias become so severe that they interfere with normal movement and negatively impact quality of life.

AREAS COVERED

In this review, we discuss benefits and limits of available therapeutic approaches aimed at delaying or managing dyskinesias as well as new strategies that are currently under investigation.

EXPERT OPINION

Among available treatments, monotherapy with dopamine agonists in the early phases of the disease reduces the risk for dyskinesias compared with levodopa. Nevertheless, dopamine agonists are unable to prevent dyskinesias once levodopa is added, which is always required once disease severity progresses. Convincing evidence of dyskinesia improvement has been shown only for deep brain stimulation and to some extent also for duodenal levodopa infusion and subcutaneous apomorphine. These approaches are expensive, have restrictive inclusion criteria and can cause potentially serious side effects. Alternative therapies include drugs targeting nondopaminergic neurotransmitter systems. Amantadine improves dyskinesias but its long-term effect is often unsatisfactory. Glutamatergic and gabaergic compounds have been tested in clinical trials, with promising results. By contrast, adrenergic drugs, fipamezole and idazoxan, did not show antidyskinetic effect.

Adenosine A2A receptor in the monkey basal ganglia: ultrastructural localization and colocalization with the metabotropic glutamate receptor 5 in the striatum

The adenosine A(2A) receptor (A(2A) R) is a potential drug target for the treatment of Parkinson's disease and other neurological disorders. In rodents, the therapeutic efficacy of A(2A) R modulation is improved by concomitant modulation of the metabotropic glutamate receptor 5 (mGluR5). To elucidate the anatomical substrate(s) through which these therapeutic benefits could be mediated, pre-embedding electron microscopy immunohistochemistry was used to conduct a detailed, quantitative ultrastructural analysis of A(2A) R localization in the primate basal ganglia and to assess the degree of A(2A) R/mGluR5 colocalization in the striatum. A(2A) R immunoreactivity was found at the highest levels in the striatum and external globus pallidus (GPe). However, the monkey, but not the rat, substantia nigra pars reticulata (SNr) also harbored a significant level of neuropil A(2A) R immunoreactivity. At the electron microscopic level, striatal A(2A) R labeling was most commonly localized in postsynaptic elements (58% ± 3% of labeled elements), whereas, in the GPe and SNr, the labeling was mainly presynaptic (71% ± 5%) or glial (27% ± 6%). In both striatal and pallidal structures, putative inhibitory and excitatory terminals displayed A(2A) R immunoreactivity. Striatal A(2A) R/mGluR5 colocalization was commonly found; 60-70% of A(2A) R-immunoreactive dendrites or spines in the monkey striatum coexpress mGluR5. These findings provide the first detailed account of the ultrastructural localization of A(2A) R in the primate basal ganglia and demonstrate that A(2A) R and mGluR5 are located to interact functionally in dendrites and spines of striatal neurons. Together, these data foster a deeper understanding of the substrates through which A(2A) R could regulate primate basal ganglia function and potentially mediate its therapeutic effects in parkinsonism.