Despite tremendous efforts in the search for safe, efficacious and non-addictive opioids for pain treatment, morphine remains the most valuable painkiller in contemporary medicine. Opioids exert their pharmacological actions through three opioid-receptor classes, mu, delta and kappa, whose genes have been cloned. Genetic approaches are now available to delineate the contribution of each receptor in opioid function in vivo. Here we disrupt the mu-opioid-receptor gene in mice by homologous recombination and find that there are no overt behavioural abnormalities or major compensatory changes within the opioid system in these animals. Investigation of the behavioural effects of morphine reveals that a lack of mu receptors abolishes the analgesic effect of morphine, as well as place-preference activity and physical dependence. We observed no behavioural responses related to delta- or kappa-receptor activation with morphine, although these receptors are present and bind opioid ligands. We conclude that the mu-opioid-receptor gene product is the molecular target of morphine in vivo and that it is a mandatory component of the opioid system for morphine action.

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

The role of the opioid system in controlling pain, reward and addiction is well established, but its role in regulating other emotional responses is poorly documented in pharmacology. The mu-, delta- and kappa- opioid receptors (encoded by Oprm, Oprd1 and Oprk1, respectively) mediate the biological activity of opioids. We have generated Oprd1-deficient mice and compared the behavioural responses of mice lacking Oprd1, Oprm (ref. 6) and Oprk1 (ref. 7) in several models of anxiety and depression. Our data show no detectable phenotype in Oprk1-/- mutants, suggesting that kappa-receptors do not have a role in this aspect of opioid function; opposing phenotypes in Oprm-/- and Oprd1-/- mutants which contrasts with the classical notion of similar activities of mu- and delta-receptors; and consistent anxiogenic- and depressive-like responses in Oprd1-/- mice, indicating that delta-receptor activity contributes to improvement of mood states. We conclude that the Oprd1-encoded receptor, which has been proposed to be a promising target for the clinical management of pain, should also be considered in the treatment of drug addiction and other mood-related disorders.

Activation of ERK (extracellular signal-regulated kinase) MAP (mitogen-activated protein) kinase in dorsal horn neurons of the spinal cord by peripheral noxious stimulation contributes to short-term pain hypersensitivity. We investigated ERK activation by peripheral inflammation and its involvement in regulating gene expression in the spinal cord and in contributing to inflammatory pain hypersensitivity. Injection of complete Freund's adjuvant (CFA) into a hindpaw produced a persistent inflammation and a sustained ERK activation in neurons in the superficial layers (laminae I-IIo) of the dorsal horn. CFA also induced an upregulation of prodynorphin and neurokinin-1 (NK-1) in dorsal horn neurons, which was suppressed by intrathecal delivery of the MEK (MAP kinase kinase) inhibitor U0126. CFA-induced phospho-ERK primarily colocalized with prodynorphin and NK-1 in superficial dorsal horn neurons. Although intrathecal injection of U0126 did not affect basal pain sensitivity, it did attenuate both the establishment and maintenance of persistent inflammatory heat and mechanical hypersensitivity. Activation of the ERK pathway in a subset of nociceptive spinal neurons contributes, therefore, to persistent pain hypersensitivity, possibly via transcriptional regulation of genes, such as prodynorphin and NK-1.

Molecular mechanisms underlying C-fiber stimulation-induced ERK (extracellular signal-regulated kinase) activation in dorsal hornneurons and its contribution to central sensitization have been investigated. In adult rat spinal slice preparations, activation of C-fiber primary afferents by a brief exposure of capsaicin produces an eightfold to 10-fold increase in ERK phosphorylation (pERK) in superficial dorsal horn neurons. The pERK induction is reduced by blockade of NMDA, AMPA/kainate, group I metabotropic glutamate receptor, neurokinin-1, and tyrosine receptor kinase receptors. The ERK activation produced by capsaicin is totally suppressed by inhibition of either protein kinase A (PKA) or PKC. PKA or PKC activators either alone or more effectively together induce pERK in superficial dorsal horn neurons. Inhibition of calcium calmodulin-dependent kinase (CaMK) has no effect, but pERK is reduced by inhibition of the tyrosine kinase Src. The induction of cAMP response element binding protein phosphorylation (pCREB) in spinal cord slices in response to C-fiber stimulation is suppressed by preventing ERK activation with the MAP kinase kinase inhibitor 2-(2-diamino-3-methoxyphenyl-4H-1-benzopyran-4-one (PD98059) and by PKA, PKC, and CaMK inhibitors. Similar signaling contributes to pERK induction after electrical stimulation of dorsal root C-fibers. Intraplantar injection of capsaicin in an intact animal increases expression of pCREB, c-Fos, and prodynorphin in the superficial dorsal horn, changes that are prevented by intrathecal injection of PD98059. Intrathecal PD98059 also attenuates capsaicin-induced secondary mechanical allodynia, a pain behavior reflecting hypersensitivity of dorsal horn neurons (central sensitization). We postulate that activation of ionotropic and metabotropic receptors by C-fiber nociceptor afferents activates ERK via both PKA and PKC, and that this contributes to central sensitization through post-translational and CREB-mediated transcriptional regulation in dorsal horn neurons.

A random primed expression cDNA library was constructed from the RNA of NG 108-15 cells. Pools of plasmid DNA were transfected into COS cells, which were screened for their ability to bind 'H-labeled Tyr-D-Thr-GlyPhe-Leu-Thr, a tritiated agonist for the -opioid receptor. A cDNA was isolated that encodes a 371-amino acid-residue protein presenting all the structural characteristics of receptors that interact with guanine nucleotide-binding proteins. Opioid receptors have long been described as membrane receptors of the nervous system mediating the analgesic effects of opium-derived alkaloids. Endogeneous ligands and their precursors have been characterized, and their role in response to pain and stress has been widely studied (1). Pharmacological studies have shown the existence of three subtypes of receptors, ,u (morphine), 8 (enkephalin), and K (dynorphin), the cellular inhibitory actions of which are linked to G protein activation (2). Opioid receptors are, therefore, believed to be part of the G protein-coupled receptor (GPR) family, a class of membrane-bound receptors that exhibit a seven-transmembrane-spanning domain topology and represent 80% of all known receptors (3).Several attempts to clone cDNAs encoding opioid receptors have been reported. A cDNA encoding an opioid-binding protein (OBCAM) with g selectivity was isolated (4), but the predicted protein lacks transmembrane domains, presumed necessary for signal transduction. More recently, the isolation of another cDNA was reported, which was obtained by expression cloning (5). The deduced protein sequence displays seven putative transmembrane domains and is very similar to the human neuromedin K receptor. However, the affinity of opioid ligands for this receptor expressed in COS cells is two orders of magnitude below the expected value, and no subtype selectivity can be shown. Finally, attempts to clone opioid receptors via approaches relying on sequence similarities in the GPR family, using the PCR technology, were unsuccessful. There is, therefore, no convincing evidence that a cDNA encoding an opioid receptor has been cloned.We report here the isolation of a cDNA encoding a 8-opioid receptorA We used a transient expression strategy where a cDNA library derived from NG 108-15 cells was screened using 3H-labeled Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET), a tritiated agonist for the 8-opioid receptor. Our pharmacological study of the cloned cDNA expressed in COS cells shows that it possesses all expected properties of a 8 receptor. A comparative analysis of the deduced protein sequence with other members of the GPR family is also presented.MATERIALS AND METHODS Library Construction and Screening. NG 108-15 cells were provided by B. Foucaud (URA 1836, Facultd de Pharmacie, Strasbourg, France). Cells were harvested at 50% confluency, RNA was prepared by LiCl/urea precipitation (6), and polyadenylylated RNA was purified with an oligo(dT) column (Pharmacia). The cDNA library construction in the mammalian expression vector pCDM8 has been described (7). Plas...

Compared to the better-known mu opioid receptor, delta opioid receptors have been relatively understudied. However, the development of highly selective delta opioid agonists, and the availability of genetic mouse models have extended our knowledge of delta opioid receptors in vivo. Here we review recent developments in the characterization of delta opioid receptor biology, and aspects of delta opioid receptor function that have potential for therapeutic targeting. Preclinical data have confirmed that delta opioid receptor activation reduces persistent pain and improves negative emotional states; clinical trials have been initiated to assess the effectiveness of delta opioid agonists in chronic pain and depression. Further, a possible role for these receptors in neuroprotection is being investigated. The usefulness of targeting delta opioid receptors in drug abuse remains open, and there is an emerging role of these receptors in impulse control disorders. Finally, the recent demonstration of biased agonism at the delta opioid receptor in vivo opens novel perspectives towards targeting specific therapeutic effects through drug design.

Large scale sequencing of the human -opioid receptor (hMOR) gene has revealed polymorphic mutations that occur within the coding region. We have investigated whether the mutations N40D in the extracellular N-terminal region, N152D in the third transmembrane domain, and R265H and S268P in the third intracellular loop alter functional properties of the receptor expressed in mammalian cells. The N152D receptor was produced at low densities. Binding affinities of structurally diverse opioids (morphine, diprenorphine, DAMGO and CTOP) and the main endogenous opioid peptides (␤-endorphin, [Met]enkephalin, and dynorphin A) were not markedly changed in mutant receptors (<3-fold). Receptor signaling was strongly impaired in the S268P mutant, with a reduction of efficacy and potency of several agonists (DAMGO, ␤-endorphin, and morphine) in two distinct functional assays. Signaling at N40D and R265H mutants was highly similar to wild type, and none of the mutations induced detectable constitutive activity. DAMGO-induced down-regulation of receptor-binding sites, following 20 h of treatment, was identical in wild-type and mutant receptors. Our data show that natural sequence variations in hMOR gene have little influence on ligand binding or receptor down-regulation but could otherwise modify receptor density and signaling. Importantly, the S268P mutation represents a loss-of-function mutation for the human -opioid receptor, which may have an incidence on opioid-regulated behaviors or drug addiction in vivo.The opioid system controls pain perception and mood and is generally implicated in a wide variety of behaviors that are essential in facing threatening situations (1, 2). Opioid receptors also mediate the strong analgesic and addictive actions of opiate drugs. Pharmacological studies indicate that the prototypic opiate morphine, the main clinically useful opiates such as fentanyl or methadone, and the closely related drug of abuse heroin preferably act by activating the -opioid receptor (MOR) 1 rather than ␦-or -opioid receptors (3-5). In support of this, gene targeting experiments have shown the absence of morphine-induced analgesia (6 -10), reward and physical dependence (6), immunosuppression (11, 12), respiratory depression (13), or constipation (14) in MOR-deficient mice, demonstrating unambiguously that the -opioid receptor is a main molecular target for morphine action in vivo. The finding of genetic mutations altering the expression or functional activity of MOR is therefore important to understand inter-individual variable responses to the major opioid drugs, both in the clinical management of pain or heroin addiction. In addition to the direct mediation of opiate-induced euphoria, tolerance, and dependence, -opioid receptors have been shown to regulate the effects of other substances with high addictive potential such as cocaine or alcohol (15). As an example, in humans, the -receptor antagonists naloxone and naltrexone have been shown not only to reverse heroin overdose but also to alter alcohol consumption (16,...