Stimulation of high-affinity adenosine A2 receptors decreases the affinity of dopamine D2 receptors in rat striatal membranes.

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Abstract
Since high-affinity adenosine A2 receptors (A2a) are localized exclusively in dopamine-rich regions in the central nervous system and mediate inhibition of locomotor activity, we have examined the effect of A2a receptor activation on D1 and D2 receptor binding in membrane preparations of the rat striatum. The A2a agonist 2-[p-(2-carboxyethyl)phenethylamino]-59- N-ethylcarboxamidoadenosine (CGS 21680) increased the Kd of the dopamine D2 agonist L-(-)-N-[3H]propylnorapomorphine without affecting the Bmax. The increase in Kd was maximal (40%) at 30 nM CGS 21680. CGS 21680 (30 nM) decreased the dopamine-induced inhibition of [3H]raclopride (a D2 antagonist) binding due to an increase (about 3-fold) in KH and KL, the dissociation constants of high- and low-affinity binding sites. The effects of CGS 21680 were antagonized by the adenosine antagonist 8-phenyltheophylline (10 microM). (-)-N6-(2-Phenylisopropyl)adenosine produced an effect similar to that of CGS 21680, provided the concentration used was high enough to stimulate A2a receptors (300 nM). GTP (50 microM) also decreased the dopamine-induced inhibition of [3H]raclopride binding but, in contrast to CGS 21680, GTP decreased the proportion of D2 receptors in the high-affinity state. CGS 21680 (30 nM) did not affect the Kd or Bmax of [3H]raclopride and failed to affect ligand binding to D1 receptors. Thus, stimulation of A2a receptors potently reduces the affinity of D2 agonist binding sites within the plasma membrane of striatal neurons. This A2a-D2 interaction may underlie the neuroleptic-like actions of adenosine agonists and the enhancing effects of adenosine antagonists, such as caffeine, on locomotor activity.