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Although type 1 cannabinoid receptors (CB1Rs) are expressed abundantly throughout the brain, the presence of type 2 cannabinoid receptors (CB2Rs) in neurons is still somewhat controversial. Taking advantage of newly designed CB1R and CB2R mRNA riboprobes, we demonstrate by PCR and in situ hybridization that transcripts for both cannabinoid receptors are present within labeled pallidothalamic-projecting neurons of control and MPTP-treated macaques, whereas the expression is markedly reduced in dyskinetic animals. Moreover, an in situ proximity ligation assay was used to qualitatively assess the presence of CB1Rs and CB2Rs, as well as CB1R-CB2R heteromers within basal ganglia output neurons in all animal groups (control, parkinsonian and dyskinetic macaques). A marked reduction in the number of CB1Rs, CB2Rs and CB1R-CB2R heteromers was found in dyskinetic animals, mimicking the observed reduction in CB1R and CB2R mRNA expression levels. The fact that chronic levodopa treatment disrupted CB1R-CB2R heteromeric complexes should be taken into consideration when designing new drugs acting on cannabinoid receptor heteromers.

Heteromerization of G-protein-coupled receptors is an important event as they integrate the actions of extracellular signals to give heteromer-selective ligand binding and signaling, opening new avenues in the development of potential drug targets in pharmacotherapy. The aim of the present paper was to check for cannabinoid CB1-GPR55 receptor heteromers in the central nervous system (CNS), specifically in striatum. First, a direct interaction was demonstrated in cells transfected with the cDNA for the human version of the receptors, using bioluminescence resonance energy transfer and in situ proximity ligation assays (PLA). In the heterologous system, a biochemical fingerprint consisting on cross-antagonism in ERK1/2 phosphorylation was detected. The cross-antagonism was also observed on GPR55-mediated NFAT activation. Direct identification of GPR55 receptors in striatum is here demonstrated in rat brain slices using a specific agonist. Moreover, the heteromer fingerprint was identified in these rat slices by ERK1/2 phosphorylation assays whereas PLA assays showed results consistent with receptor-receptor interactions in both caudate and putamen nuclei of a non-human primate. The results indicate not only that GPR55 is expressed in striatum but also that CB1 and GPR55 receptors form heteromers in this specific CNS region., (Copyright © 2014 Elsevier Inc. All rights reserved.)