Blackwell Publishing Journal Backfiles 1879-2005
Adenosine, by acting on adenosine A1 and A2A receptors, exerts opposite modulatory roles on striatal extracellular levels of glutamate and dopamine, with activation of A1 inhibiting and activation of A2A receptors stimulating glutamate and dopamine release. Adenosine-mediated modulation of striatal dopaminergic neurotransmission could be secondary to changes in glutamate neurotransmission, in view of evidence for a preferential colocalization of A1 and A2A receptors in glutamatergic nerve terminals. By using in vivo microdialysis techniques, local perfusion of NMDA (3, 10 µm), the selective A2A receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5′-N-ethylcarboxamidoadenosine (CGS 21680; 3, 10 µm), the selective A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT; 300, 1000 µm), or the non-selective A1-A2A receptor antagonist in vitro caffeine (300, 1000 µm) elicited significant increases in extracellular levels of dopamine in the shell of the nucleus accumbens (NAc). Significant glutamate release was also observed with local perfusion of CGS 21680, CPT and caffeine, but not NMDA. Co-perfusion with the competitive NMDA receptor antagonist dl-2-amino-5-phosphonovaleric acid (APV; 100 µm) counteracted dopamine release induced by NMDA, CGS 21680, CPT and caffeine. Co-perfusion with the selective A2A receptor antagonist MSX-3 (1 µm) counteracted dopamine and glutamate release induced by CGS 21680, CPT and caffeine and did not modify dopamine release induced by NMDA. These results indicate that modulation of dopamine release in the shell of the NAc by A1 and A2A receptors is mostly secondary to their opposite modulatory role on glutamatergic neurotransmission and depends on stimulation of NMDA receptors. Furthermore, these results underscore the role of A1 vs. A2A receptor antagonism in the central effects of caffeine.
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