There has been much speculation regarding the functional relevance of G protein-coupled receptor heterodimers, primarily because demonstrating their existence in vivo has proven to be a considerable challenge. Here we show that the opioid agonist ligand 6-guanidinonaltrindole (6-GNTI) has the unique property of selectively activating only opioid receptor heterodimers but not homomers. Importantly, 6-GNTI is an analgesic, thereby demonstrating that opioid receptor heterodimers are indeed functionally relevant in vivo. However, 6-GNTI induces analgesia only when it is administered in the spinal cord but not in the brain, suggesting that the organization of heterodimers is tissue-specific. This study demonstrates a proof of concept for tissue-selective drug targeting based on G protein-coupled receptor heterodimerization. Importantly, targeting opioid heterodimers could provide an approach toward the design of analgesic drugs with reduced side effects.opioid

The low effectiveness of morphine and related mu opioid analgesics for the treatment of chronic inflammatory pain is a result of opioid-induced release of proinflammatory cytokines and glutamate that lower the pain threshold. In this regard, the use of opioids with metabotropic glutamate-5 receptor (mGluR 5 ) antagonist has been reported to increase the efficacy of morphine and prevent the establishment of adverse effects during chronic use. Given the presence of opioid receptors (MORs) and mGluR 5 in glia and neurons, together with reports that suggest coexpressed MOR/mGluR 5 receptors in cultured cells associate as a heteromer, the possibility that such a heteromer could be a target in vivo was addressed by the design and synthesis of a series of bivalent ligands that contain mu opioid agonist and mGluR 5 antagonist pharmacophores linked through spacers of varying length (10-24 atoms). The series was evaluated for antinociception using the tail-flick and von Frey assays in mice pretreated with lipopolysaccharide (LPS) or in mice with bone cancer. In LPS-pretreated mice, MMG22 (4c, 22-atom spacer) was the most potent member of the series (intrathecal ED 50 ∼9 fmol per mouse), whereas in untreated mice its ED 50 was more than three orders of magnitude higher. As members of the series with shorter or longer spacers have ≥500-fold higher ED 50s in LPS-treated mice, the exceptional potency of MMG22 may be a result of the optimal bridging of protomers in a putative MOR-mGluR 5 heteromer. The finding that MMG22 possesses a >10 6 therapeutic ratio suggests that it may be an excellent candidate for treatment of chronic, intractable pain via spinal administration.MPEP | tolerance | respiratory depression T he excitatory neurotransmitter, glutamate, in the central nervous system (CNS) is an important mediator of opioid nociception, dependence, and withdrawal (1). Glutamate exerts its effect via two different classes of glutamate receptors: ionotropic and metabotropic. Among the metabotropic receptors (mGluRs), the receptor-5 subtype (mGluR 5 ) is widely distributed in the CNS (2), where it modulates synaptic transmission, neuronal excitability, and plasticity. The mGluR 5 is a class C G protein-coupled receptor (GPCR), the activation of which is mediated by binding of glutamate to its extracellular Venus flytrap domain. It is noteworthy that the selective mGluR 5 antagonist, 2-methyl-6-(phenylethynyl) pyridine (MPEP), acts allosterically by binding to the seven transmembrane (7TM) domain of the receptor (3).Reports of presence of the mu opioid receptor (MOR) and mGluR 5 in the spinal cord and the ability of coadministered MPEP and morphine to enhance morphine antinociception and suppress morphine-induced tolerance and dependence suggest a design strategy for the development of potent analgesics based on the targeting of both MOR and mGluR 5 (4-6). Given evidence (7) that supports the physical association of coexpressed MOR and mGluR 5 as heteromer (MOR-mGluR 5 ) in cultured cells, and the presence of MOR and mGluR 5 in t...