Singular Location and Signaling Profile of Adenosine A 2A -Cannabinoid CB 1 Receptor Heteromers in the Dorsal Striatum.

The dorsal striatum is a key node for many neurobiological processes such as motor activity, cognitive functions, and affective processes. The proper functioning of striatal neurons relies critically on metabotropic receptors. Specifically, the main adenosine and endocannabinoid receptors present in the striatum, ie, adenosine A _2A receptor (A _2A R) and cannabinoid CB ₁ receptor (CB ₁ R), are of pivotal importance in the control of neuronal excitability. Facilitatory and inhibitory functional interactions between striatal A _2A R and CB ₁ R have been reported, and evidence supports that this cross-talk may rely, at least in part, on the formation of A _2A R-CB ₁ R heteromeric complexes. However, the specific location and properties of these heteromers have remained largely unknown. Here, by using techniques that allowed a precise visualization of the heteromers in situ in combination with sophisticated genetically modified animal models, together with biochemical and pharmacological approaches, we provide a high-resolution expression map and a detailed functional characterization of A _2A R-CB ₁ R heteromers in the dorsal striatum. Specifically, our data unveil that the A _2A R-CB ₁ R heteromer (i) is essentially absent from corticostriatal projections and striatonigral neurons, and, instead, is largely present in striatopallidal neurons, (ii) displays a striking G protein-coupled signaling profile, where co-stimulation of both receptors leads to strongly reduced downstream signaling, and (iii) undergoes an unprecedented dysfunction in Huntington's disease, an archetypal disease that affects striatal neurons. Altogether, our findings may open a new conceptual framework to understand the role of coordinated adenosine-endocannabinoid signaling in the indirect striatal pathway, which may be relevant in motor function and neurodegenerative diseases.