Allosteric Interactions between Adenosine A 2A and Dopamine D 2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging.

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A _2A and dopamine D ₂ receptors (R). Stimulation of A _2A R inhibits and blockade of A _2A R enhances D ₂ R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D ₂ R via its interaction with A _2A R. Reciprocal A _2A R/D ₂ R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A _2A R and D ₂ R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor-receptor interactions within A _2A R/D ₂ R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson's disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A _2A R/D ₂ R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.