The adenosine A2a receptor (A2aR) is co-expressed with the dopamine D2 receptor (D2R) in striatopallidal neurons and exerts an antagonistic influence on postsynaptic D2R function in brain. Our understanding of A2aR-dopamine interaction, however, is based primarily on pharmacological evidence that is intrinsically limited by the lack of specificity of adenosinergic agents. To elucidate the cellular mechanisms underlying the interaction between A2aRs and the dopaminergic system in vivo, we have generated A2aR knock-out (KO) mice. Our initial characterization of A2aR KO mice reveals a presynaptic role for the A2aR by demonstrating its facilitative effect on striatal dopamine release and, in turn, on dopamine-mediated locomotor activity. Thus, we propose a modified model for the interaction between the A2aR and the dopaminergic system: A2aR-mediated presynaptic facilitation of dopamine release may counterbalance the A2aR-mediated postsynaptic inhibition of D2R function. Hence A2a adenosinergic regulation of dopaminergic activity may depend upon a fine balance between pre- and post-synaptic functions of A2aRs. To pursue this hypothesis, we will study A2aR-dopamine interactions at presynaptic sites using a synaptosomal preparation and in vivo microdialysis (SA #1), and at postsynaptic sites using reserpinized mice and primary cultures of striatal neurons (SA #2). Furthermore, using D2R KO and A2aR-D2R double KO mice we will address the central question whether or not the D2R mediates A2aR functions in vivo (SA #3). We will explore A2aR-dopamine interactions at the behavioral (locomotor activity), neurochemical (dopamine release) and cellular (cAMP formation and c-Fos expression) levels to gain insight into the integrated role of A2aR in vivo. By complementing classical pharmacological studies with these transgenic approaches receptor function, we seek more refined answers to several fundamental questions of adenosine physiology: (1) Do basal levels of endogenous adenosine acting at the A2aR exert tonic physiological effects (inhibitory or excitatory) on the brain dopaminergic system? (2) What are the cellular mechanisms underlying A2aR-dopamine interactions at pre- and post-synaptic sites? (3) Does A2aR specifically require the D2R to exert its neuronal functions in vivo? The answers to these questions will foster the rational development of A2aR agents as an alternative or adjunctive treatment for Parkinson's disease and related disorders.
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