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...

Chemokine release promotes crosstalk between opioid and chemokine receptors that in part leads to reduced efficacy of morphine in the treatment of chronic pain. Based on the possibility that a MOR-CCR5 heteromer is involved in such crosstalk, we have synthesized bivalent ligands (MCC series) that contain mu opioid agonist and CCR5 antagonist pharmacophores linked through homologous spacers (14–24 atoms). When tested on lipopolysaccharide-inflamed mice, a member of the series (MCC22; 3e) with a 22-atom spacer exhibited profound antinociception (i.t. ED50 = 0.0146 pmol/mouse) that was >2000× greater than morphine. Moreover, MCC22 was ~3500× more potent than a mixture of mu agonist and CCR5 antagonist monovalent ligands. These data strongly suggest that MCC22 acts by bridging the protomers of a MOR-CCR5 heteromer having a TM5,6 interface. Molecular simulation studies are consistent with such bridging. This study supports the MOR-CCR5 heteromer as a novel target for treatment of chronic pain.

Background: Allosteric ligands targeting cholecystokinin receptors are needed. Results: Stereochemically distinct iodinated 1,4-benzodiazepine antagonists of type 1 and 2 cholecystokinin receptors dock to analogous intramembranous pockets that have distinct shape and molecular determinants. Conclusion: The geometry of the binding pockets and specific residue interactions are unique for each receptor. Significance: The predictive power of these insights should be useful in the discovery of lead compounds and in their refinement.

Both mu opioid (MOP) † and type 2 cholecystokinin (CCK 2 ) receptors are present in areas of the central nervous system that are involved in modulation of pain processing. We conducted bioluminescence resonance energy transfer (BRET) studies on COS cells coexpressing MOP and CCK 2 receptors to determine whether receptor heterodimerization is involved in such modulation. These studies revealed the absence of constitutive or monovalent ligand-induced heterodimerization. Heterodimerization of MOP and CCK 2 receptors therefore is unlikely to be responsible for the opposing effects between morphine and CCK in the CNS. However, association was induced, as indicated by a positive BRET signal, on exposure of the cells to bivalent ligands containing muopioid agonist and CCK 2 receptor antagonist pharmacophores linked through spacers containing 16 to 22 atoms, but not with a shorter (9-atom) spacer. These studies demonstrate for the first time that an appropriately designed bivalent ligand is capable of inducing association of G protein-coupled receptors. The finding that opioid tolerance studies with these ligands in mice showed no correlation with the BRET data is consistent with the absence of association of MOP and CCK 2 receptors in vivo.