The mechanism of action responsible for the motor depressant effects of cannabinoids, which operate through centrally expressed cannabinoid CB 1 receptors, is still a matter of debate. In the present study, we report that CB 1 and adenosine A 2A receptors form heteromeric complexes in co-transfected HEK-293T cells and rat striatum, where they colocalize in fibrilar structures. In a human neuroblastoma cell line, CB 1 receptor signaling was found to be completely dependent on A 2A receptor activation. Accordingly, blockade of A 2A receptors counteracted the motor depressant effects produced by the intrastriatal administration of a cannabinoid CB 1 receptor agonist. These biochemical and behavioral findings demonstrate that the profound motor effects of cannabinoids depend on physical and functional interactions between striatal A 2A and CB 1 receptors.

Adenosine A2A-dopamine D2 receptor interactions play a very important role in striatal function. A2A-D2 receptor interactions provide an example of the capabilities of information processing by just two different G protein-coupled receptors. Thus, there is evidence for the coexistence of two reciprocal antagonistic interactions between A2A and D2 receptors in the same neurons, the GABAergic enkephalinergic neurons. An antagonistic A2A-D2 intramembrane receptor interaction, which depends on A2A-D2 receptor heteromerization and Gq/11-PLC signaling, modulates neuronal excitability and neurotransmitter release. On the other hand, an antagonistic A2A-D2 receptor interaction at the adenylyl-cyclase level, which depends on Gs/olf-and Gi/o-type V adenylyl-cyclase signaling, modulates protein phosphorylation and gene expression. Finally, under conditions of upregulation of an activator of G protein signaling (AGS3), such as during chronic treatment with addictive drugs, a synergistic A2A-D2 receptor interaction can also be demonstrated. AGS3 facilitates a synergistic interaction between Gs/olf -and Gi/o-coupled receptors on the activation of types II/IV adenylyl cyclase, leading to a paradoxical increase in protein phosphorylation and gene expression upon co-activation of A2A and D2 receptors. The analysis of A2-D2 receptor interactions will have implications for the pathophysiology and treatment of basal ganglia disorders and drug addiction.Key Words: Adenosine A 2A Receptor, Dopamine D 2 Receptor, G Protein-Coupled Receptors, Receptor Heteromers, Striatum, Basal Ganglia Disorders, Drug Addiction. LOCALIZATION OF THE A 2A -D 2 RECEPTOR HETERO-MERApplying a broad definition of "neurotransmitter" [1], adenosine can be considered as an important neurotransmitter in the CNS, which acts through different subtypes of G protein-coupled receptors (GPCRs). From the four cloned adenosine receptors (adenosine A 1 , A 2A , A 2B and A 3 receptors ) , A 1 and A 2A receptors are the main targets for the physiological effects of adenosine in the brain [2]. A 1 receptor is widely distributed in the brain, including the striatum, while A 2A receptor is mostly concentrated in the striatum [2,3]. It is becoming increasingly obvious that the modulatory role of adenosine in the striatum is related to the ability of A 1 and A 2A receptors to heteromerize with themselves and with other GPCRs, such as dopamine, glutamate, cannabinoid and ATP receptors [4][5][6][7][8][9][10][11][12][13][14]. The present review focuses on the role of one particular adenosine receptor heteromer, the one constituted by the A 2A and the dopamine D 2 receptor, which is already having important implications for the treatment of neuropathologies involving the striatum (see below).Striatal medium spiny neurons are GABAergic efferent neurons which constitute more that 95% of the striatal neuronal population. They receive two main afferents, cortical-limbic-thalamic glutamatergic inputs and dopaminergic mesencephalic inputs, from the substantia nigra pars compac...