Expression of cannabinoid CB ₁ R-GPR55 heteromers in neuronal subtypes of the Macaca fascicularis striatum

The cannabinoid CB₁ receptor (CB₁ R) is the most abundant G protein-coupled receptor in the central nervous system, consistent with the important role of endocannabinoids as neuromodulators. Cannabinoids also modulate the function of G protein-coupled receptor 55 (GPR55), which forms heteroreceptor complexes with the CB₁ R in the striatum. The aim was to characterize cannabinoid CB₁ R-GPR55 heteromers (CB₁ R/GPR55Hets) in the basal ganglia input nuclei of nonhuman primates, Macaca fascicularis, both in projection neurons and interneurons, by the in situ proximity ligation assay. Striatal projecting neurons were identified by the retrograde neuroanatomical tracer, biotinylated dextran amine (BDA), injected into external or internal subdivisions of the globus pallidus. Triple immunofluorescent stains were carried out to visualize (1) BDA-labeled neurons, (2) CB₁ R/GPR55Hets, and (3) striatal interneurons positive for choline acetyltransferase, parvalbumin, calretinin, or nitric oxide synthase. CB₁ R/GPR55Hets were identified within both types of projection neurons as well as all interneurons except those that are cholinergic. Moreover, CB₁ R/GPR55Hets were found specifically in the neuronal cell surface, and also in intracellular membranes. Further research efforts will be needed to confirm the intracellular occurrence of heteromers and their potential as therapeutic targets in diseases related to motor control imbalances, particularly within a parkinsonian context (with or without levodopa-induced dyskinesia).

Keywords: CB1; G protein-coupled receptor (GPCR); biotinylated dextran amine; cannabinoid receptor; heteromer; interneurons; projection neurons.