For many G-protein-coupled receptors (GPCRs), including cannabinoid receptor 1 (CB 1 R), desensitization has been proposed as a principal mechanism driving initial tolerance to agonists. GPCR desensitization typically requires phosphorylation by a G-proteincoupled receptor kinase (GRK) and interaction of the phosphorylated receptor with an arrestin. In simple model systems, CB 1 R is desensitized by GRK phosphorylation at two serine residues (S426 and S430). However, the role of these serine residues in tolerance and dependence for cannabinoids in vivo was unclear. Therefore, we generated mice where S426 and S430 were mutated to nonphosphorylatable alanines (S426A/S430A). S426A/S430A mutant mice were more sensitive to acutely administered delta-9-tetrahydrocannabinol (⌬ 9 -THC), have delayed tolerance to ⌬ 9 -THC, and showed increased dependence for ⌬ 9-THC. S426A/S430A mutants also showed increased responses to elevated levels of endogenous cannabinoids. CB 1 R desensitization in the periaqueductal gray and spinal cord following 7 d of treatment with ⌬ 9 -THC was absent in S426A/S430A mutants. ⌬ 9 -THC-induced downregulation of CB 1 R in the spinal cord was also absent in S426A/S430A mutants. Cultured autaptic hippocampal neurons from S426A/S430A mice showed enhanced endocannabinoid-mediated depolarization-induced suppression of excitation (DSE) and reduced agonist-mediated desensitization of DSE. These results indicate that S426 and S430 play major roles in the acute response to, tolerance to, and dependence on cannabinoids. Additionally, S426A/S430A mice are a novel model for studying pathophysiological processes thought to involve excessive endocannabinoid signaling such as drug addiction and metabolic disease. These mice also validate the approach of mutating GRK phosphorylation sites involved in desensitization as a general means to confer exaggerated signaling to GPCRs in vivo.

RationaleThe nucleus accumbens (Acb) shell and caudate-putamen nucleus (CPu) are respectively implicated in the motivational and motor effects of dopamine, which are mediated in part through dopamine D2-like receptors (D2Rs) and modulated by activation of the cannabinoid-1 receptor (CB1R). The dopamine D2/D3 receptor agonist, quinpirole elicits internalization of D2Rs in isolated cells; however, dendritic and axonal targeting of D2Rs may be highly influenced by circuit-dependent changes in vivo and potentially influenced by endogenous CB1R activation.ObjectiveWe sought to determine whether quinpirole alters the surface/cytoplasmic partitioning of D2Rs in striatal neurons in vivo.MethodsTo address this question, we examined the electron microscopic immunolabeling of D2 and CB1 receptors in the Acb shell and CPu of male mice at 1 h following a single subcutaneous injection of quinpirole (0.5 mg/kg) or saline, a time point when quinpirole reduced locomotor activity.ResultsMany neuronal profiles throughout the striatum of both treatment groups expressed the D2R and/or CB1R. As compared with saline, quinpirole-injected mice showed a significant region-specific decrease in the plasmalemmal and increase in the cytoplasmic density of D2R-immunogold particles in postsynaptic dendrites without CB1R-immunolabeling in the Acb shell. However, quinpirole produced a significant increase in the plasmalemmal density of D2R immunogold in CB1R negative axons in both the Acb shell and CPu.ConclusionsOur results provide in vivo evidence for agonist-induced D2R trafficking that is inversely related to CB1R distribution in postsynaptic neurons of Acb shell and in presynaptic axons in this region and in the CPu.