The opioid system modulates several physiological processes, including analgesia, the stress response, the immune response and neuroendocrine function. Pharmacological and molecular cloning studies have identified three opioid-receptor types, delta, kappa and mu, that mediate these diverse effects. Little is known about the ability of the receptors to interact to form new functional structures, the simplest of which would be a dimer. Structural and biochemical studies show that other G-protein-coupled receptors (GPCRs) interact to form homodimers. Moreover, two non-functional receptors heterodimerize to form a functional receptor, suggesting that dimerization is crucial for receptor function. However, heterodimerization between two fully functional receptors has not been documented. Here we provide biochemical and pharmacological evidence for the heterodimerization of two fully functional opioid receptors, kappa and delta. This results in a new receptor that exhibits ligand binding and functional properties that are distinct from those of either receptor. Furthermore, the kappa-delta heterodimer synergistically binds highly selective agonists and potentiates signal transduction. Thus, heterodimerization of these GPCRs represents a novel mechanism that modulates their function.

No abstract

Opiates such as morphine are the choice analgesic in the treatment of chronic pain. However their long-term use is limited because of the development of tolerance and dependence. Due to its importance in therapy, different strategies have been considered for making opiates such as morphine more effective, while curbing its liability to be abused. One such strategy has been to use a combination of drugs to improve the effectiveness of morphine. In particular, ␦ opioid receptor ligands have been useful in enhancing morphine's potency. The underlying molecular basis for these observations is not understood. We propose the modulation of receptor function by physical association between and ␦ opioid receptors as a potential mechanism. In support of this hypothesis, we show that -␦ interacting complexes exist in live cells and native membranes and that the occupancy of ␦ receptors (by antagonists) is sufficient to enhance opioid receptor binding and signaling activity. Furthermore, ␦ receptor antagonists enhance morphine-mediated intrathecal analgesia. Thus, heterodimeric associations between -␦ opioid receptors can be used as a model for the development of novel combination therapies for the treatment of chronic pain and other pathologies. Opioid receptors belong to the rhodopsin family of G proteincoupled receptors (GPCRs). Like many GPCRs, these receptors were thought to function as single units. This notion has been revised in recent years by a number of studies showing that GPCRs associate with each other to form dimers and͞or oligomers (1-3). Of particular significance are the studies with rhodopsin, a prototypical member of the GPCR family, where infrared-laser atomic-force microscopy of native mouse disk membranes showed the receptors to be arranged in crystalline arrays of dimeric units (4, 5). Also, data from x-ray crystallographic studies with rhodopsin (6, 7) and the N terminus of metabotropic glutamate receptors (8), support the notion that dimerization is an integral feature of these receptors and could play a key role in modulating their function.The three types of opioid receptors (, ␦, and ) have been shown to associate with each other in a homotypic or heterotypic fashion when expressed in heterologous cells (9-11). Furthermore, heterotypic interactions appear to alter the ligand-binding and signaling properties of these receptors (12). However, until now, it was not clear whether these interactions occurred in live cells and in endogenous tissues and whether they were physiologically relevant. In this study, we addressed these questions by using multiple approaches. We used the bioluminescence resonance energy transfer (BRET) assay to show that and ␦ receptors interact in living cells. In addition, we show that signaling by clinically relevant drugs, such as morphine, fentanyl, and methadone can be enhanced by ␦ receptor ligands. This potentiation of receptor signaling by the ␦ receptor antagonist is seen in membranes from WT mice and not in membranes from ␦ receptor lacking mice (␦ k͞o). Finally, w...

No abstract

Five novel peptides were identified in the brains of mice lacking active carboxypeptidase E, a neuropeptide-processing enzyme. These peptides are produced from a single precursor, termed proSAAS, which is present in human, mouse, and rat. ProSAAS mRNA is expressed primarily in brain and other neuroendocrine tissues (pituitary, adrenal, pancreas); within brain, the mRNA is broadly distributed among neurons. When expressed in AtT-20 cells, proSAAS is secreted via the regulated pathway and is also processed at paired-basic cleavage sites into smaller peptides. Overexpression of proSAAS in the AtT-20 cells substantially reduces the rate of processing of the endogenous prohormone proopiomelanocortin. Purified proSAAS inhibits prohormone convertase 1 activity with an IC 50 of 590 nM but does not inhibit prohormone convertase 2. Taken together, proSAAS may represent an endogenous inhibitor of prohormone convertase 1.

Receptor heteromers constitute a new area of research that is reshaping our thinking about biochemistry, cell biology, pharmacology and drug discovery. In this commentary, we recommend clear definitions that should facilitate both information exchange and research on this growing class of transmembrane signal transduction units and their complex properties. We also consider research questions underlying the proposed nomenclature, with recommendations for receptor heteromer identification in native tissues and their use as targets for drug development.

No abstract

The μ and δ types of opioid receptors form heteromers that exhibit pharmacological and functional properties distinct from those of homomeric receptors. To characterize these complexes in the brain, we generated antibodies that selectively recognize the μ-δ heteromer and blocked its in vitro signaling. With these antibodies, we showed that chronic, but not acute, morphine treatment caused an increase in the abundance of μ-δ heteromers in key areas of the central nervous system that are implicated in pain processing. Because of its distinct signaling properties, the μ-δ heteromer could be a therapeutic target in the treatment of chronic pain and addiction.