Agonist activation of delta-opioid receptor but not mu-opioid receptor potentiates fetal calf serum or tyrosine kinase receptor-mediated cell proliferation in a cell-line-specific manner

Regulation of PI3K/Akt signaling by N-desmethylclozapine through activation of δ-opioid receptor

We have previously reported that N-desmethylclozapine (NDMC), a major clozapine metabolite, acts as a δ-opioid receptor agonist. Here, we show that in different cellular systems NDMC regulates protein kinase B/Akt (Akt) signaling through the activation of δ-opioid receptors. In Chinese hamster ovary cells transfected with the human δ-opioid receptor (CHO/DOR), NDMC induced a time- and concentration-dependent phosphorylation of Akt at Thr308 and glycogen synthase kinase-3β (GSK-3β) at Ser9 and these effects were fully blocked by the δ-opioid receptor antagonist naltrindole. NDMC-induced Akt and GSK-3β phosphorylations were completely prevented by pertussis toxin, the Src tyrosine kinase inhibitor PP2 and the selective insulin-like growth factor-I (IGF-I) receptor tyrosine kinase inhibitor tyrphostin AG 1024. NDMC stimulated IGF-I receptor β subunit tyrosine phosphorylation and this effect was prevented by either naltrindole or PP2. Blockade of phosphatidylinositol 3-kinase (PI3K) α, but not PI3Kγ, suppressed NDMC-induced Akt and GSK-3β phosphorylation, whereas inhibition of Akt curtailed the stimulation of GSK-3β phosphorylation. In rat nucleus accumbens, NDMC induced Akt and GSK-3β phosphorylation either in vitro or in vivo and these effects were prevented by naltrindole. NDMC also regulated Akt and GSK-3β phosphorylation through δ-opioid receptors in NG108-15 cells. In these cells NDMC counteracted oxidative stress-induced apoptosis and the effect was lost following PI3K inhibition. These data demonstrate that in different cell systems NDMC can stimulate Akt signaling by activating Gi/Go-coupled δ-opioid receptors, which, at least in CHO/DOR cells, regulate PI3Kα through Src-dependent transactivation of the IGF-I receptor, and indicate that through this mechanism NDMC can exert neuroprotective effects.

Signaling pathways mediating phosphorylation and inactivation of glycogen synthase kinase-3β by the recombinant human δ-opioid receptor stably expressed in Chinese hamster ovary cells

Besides being involved in analgesia, δ-opioid receptors have recently been shown to exert antidepressant-like and neuroprotective effects. Glycogen synthase kinase-3β (GSK-3β), a key enzyme involved in cellular apoptosis and in mood disorders, may constitute a molecular target of δ-opioid receptors. However, relatively little is known on how δ-opioid receptors affect the multiple signaling pathways regulating GSK-3β. In the present study, we show that activation of human δ-opioid receptors stably expressed in Chinese hamster ovary (CHO) cells induced a rapid GSK-3β phosphorylation on Ser9 and a significant inhibition of the kinase activity. This effect was dependent on G proteins Gi/Go, unaffected by cell transfection with the Gβγ scavenger transducin, required the Src non-receptor tyrosine kinase and the specific involvement of the α isoform of phosphatidylinositol 3-kinase. δ-Opioid agonists activated the protein kinase Akt in a Src-dependent manner and chemical inhibition of Akt or stable expression of a dominant negative Akt1 mutant reduced the stimulation of GSK-3β phosphorylation. Moreover, δ-opioid receptor regulation of Akt and GSK-3β was dependent on transphosphorylation and transactivation of platelet-derived growth factor and insulin-like growth factor-1 receptor tyrosine kinases. AMP-activated protein kinase (AMPK) activity was also required, as δ-opioid effects on Akt and GSK-3β were mimicked by the AMPK activator A-769662 and reduced by the AMPK inhibitor Compound C. Conversely, inhibition of protein kinase C isoforms, extracellular signal-regulated protein kinases 1/2 and mammalian target of rapamycin was without effect, although the latter two kinases were activated by δ-opioid agonists. The results identify Src-dependent transactivation of receptor tyrosine kinases as a key process in δ-opioid receptor inhibitory control of GSK-3β and reveal a novel δ-opioid regulatory mechanism mediated by AMPK. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Opioid modulation of cell proliferation in the ventricular zone of adult zebra finches (Taenopygia guttata)

Besides modulating pain, stress, physiological functions, motivation, and reward, the opioid system has been implicated in developmental and adult mammalian neurogenesis and gliogenesis. In adult male songbirds including zebra finches, neurons generated from the ventricular zone (VZ) of the lateral ventricles are incorporated throughout the telencephalon, including the song control nuclei, HVC, and area X. Although the endogenous opioid met-enkephalin is present in neurons adjacent to the VZ and is upregulated in song control regions during singing, it is not known whether the opioid system can modulate adult neurogenesis/gliogenesis in zebra finches. We used quantitative RT-PCR and in situ hybridization to demonstrate that μ- and δ-opioid receptors are expressed by the VZ of adult male zebra finches. Treating cultured VZ cells from male birds with the opioid antagonist naloxone led to an increase in cell proliferation measured by 5-bromo-2-deoxyuridine incorporation, whereas administering met-enkephalin had the opposite effect, compared with saline-treated cultures. Systemically administering naloxone (2.5 mg/kg body wt) to adult male zebra finches for 4 d also led to a significant increase in cell proliferation in the ventral VZ of these birds, compared with saline-treated controls. Our results show that cell proliferation is augmented by naloxone in the VZ adjacent to the anterior commissure, suggesting that the endogenous opioids modulate adult neurogenesis/gliogenesis by inhibiting cell proliferation in songbirds.

Opioids as modulators of cell death and survival--unraveling mechanisms and revealing new indications

Opioids are powerful analgesics but also drugs of abuse. Because opioid addicts are susceptible to certain infections, opioids have been suspected to suppress the immune response. This was supported by the finding that various immune-competent cells express opioid receptors and undergo apoptosis when treated with opioid alkaloids. Recent evidence suggests that opioids may also effect neuronal survival and proliferation or migrating properties of tumor cells. A multitude of signaling pathways has been suggested to be involved in these extra-analgesic effects of opioids. Growth-promoting effects were found to be mediated through Akt and Erk signaling cascades. Death-promoting effects have been ascribed to inhibition of nuclear factor-kappaB, increase of Fas expression, p53 stabilization, cytokine and chemokine release, and activation of nitric oxide synthase, p38, and c-Jun-N-terminal kinase. Some of the observed effects were inhibited with opioid receptor antagonists or pertussis toxin; others were unaffected. It is still unclear whether these properties are mediated through typical opioid receptor activation and inhibitory G-protein-signaling. The present review tries to unravel controversial findings and provides a hypothesis that may help to integrate diverse results.

Antiapoptotic and cytotoxic properties of delta opioid peptide [D-Ala(2),D-Leu(5)]enkephalin in PC12 cells

The delta opioid peptide [D-Ala(2),D-Leu(5)]enkephalin (DADLE) has been shown to promote organ survival and to protect against methamphetamine-induced neurodegeneration. However, the cellular mechanisms of these actions of DADLE are not totally clear. We examined the action of DADLE in serum-deprived pheochromocytoma cells (PC12) and found that DADLE protected against cell death in those cells. However, the dose-response curves of the protective effects of DADLE are U-shaped as judged by three biochemical or morphological assays: the LDH release, the DNA laddering, and the apoptotic nuclei. It was found that femtomolar to picomolar concentrations of DADLE are antiapoptotic, whereas micormolar concentrations of DADLE are cytotoxic in PC12 cells. The protective effect of DADLE could be attenuated by a selective delta2 opioid antagonist and the cytotoxic action of DADLE was reduced by a selective mu opioid receptor antagonist. The treatment of cells with PD98059, a selective inhibitor of ERK kinase (MEK), or the transfection of cells with a dominant interfering form of MEK (MEK-KA97) blocked both the protective effect of DADLE and the ERK phosphorylation induced by DADLE. Cytotoxic concentrations of DADLE, on the other hand, caused an increase of Fas-ligand (FasL) in PC12 cells that was attenuated by a selective mu antagonist. Our results suggest, therefore, that endogenous opioid peptides may, at low concentrations, promote cell survival via the MEK-ERK pathway perhaps through delta2 opioid receptors, whereas they may kill cells at high concentrations via the activation of FasL through an as-yet unknown mechanism involving mu opioid receptors.

Transplacental transfer of the opioid growth factor, [Met(5)]-enkephalin, in rats

Placental transfer of the pentapeptide [Met5]-enkephalin, known to function as a growth regulating factor and neuromodulatory agent, was studied in pregnant Sprague-Dawley rats. Using separation by reversed phase high-performance liquid chromatography, and analysis by derivative spectroscopy, [Met5]-enkephalin was detected in 20-day-old fetal tissue including brain, heart, lung, and kidney. Fetal tissues from pregnant rats given an injection of 40 mg/kg [Met5]-enkephalin on gestation day 20 had markedly elevated levels of peptide within 1 h, indicating the transplacental transfer of this opioid. [Met5]-enkephalin levels were increased from control samples at 1, 2, 4, and 14 h post-injection of peptide, but not at 24 h. Evaluation of breakdown products of [Met5]-enkephalin, along with the related peptide [Leu5]-enkephalin, revealed that elution times differed substantially from [Met5]-enkephalin. These data indicate that [Met5]-enkephalin is present in fetal organs, crosses the placenta, does not appear to be restrictive in organ specificity, and is sustained in fetal tissues at detectable levels for at least 14 h. Given that [Met5]-enkephalin tonically inhibits DNA synthesis in the fetus, these results raise the question of whether an elevated level of this peptide (either maternally or from the fetus) may be detrimental to cellular ontogeny in the fetus, and perhaps have long-term implications for postnatal development.

P2Y(2) receptor of MDCK cells: cloning, expression, and cell-specific signaling

Madin-Darby canine kidney (MDCK)-D1 cells, a canine renal epithelial cell line, co-express at least three different P2Y receptor subtypes: P2Y(1), P2Y(2), and P2Y(11) (24). Stimulation of P2Y receptors in these cells results in the release of arachidonic acid (AA) and metabolites and the elevation of intracellular cAMP. To define in more precise terms the signaling contributed by the MDCK-D1 P2Y(2) (cP2Y(2)) receptor, we have cloned and heterologously expressed it in CF2Th (canine thymocyte) cells, a P2Y(2)-null cell. Analysis by RT-PCR indicated that canine P2Y(2) receptors are expressed in skeletal muscle, spleen, kidney, lung, and liver. When expressed in CF2Th cells, cP2Y(2) receptors promoted phospholipase C-mediated phosphatidylinositol (PI) hydrolysis [uridine 5'-triphosphate > or = ATP > adenosine 5'-diphosphate > 2MT-ATP] and mobilization of intracellular Ca(2+). In contrast to their actions in MDCK-D1 cells, cP2Y(2) receptors did not stimulate formation of cAMP or AA release when expressed in CF2Th cells. The data indicate that cell setting plays an essential role in the ability of P2Y receptors to regulate AA release and cAMP formation. In particular, renal epithelial cells preferentially express components critical for cP2Y(2)-induced cAMP formation, including the expression of enzymes involved in the generation and metabolism of AA and receptors that respond to PGE(2).

Mechanisms of proliferation synergy by receptor tyrosine kinase and G protein-coupled receptor activation in human airway smooth muscle

Despite recent studies depicting the capacity of G protein-coupled receptors (GPCRs) to activate mitogenic signaling pathways more commonly associated with receptor tyrosine kinases (RTKs), little is known regarding the interactive effects of GPCR and RTK activation on cell growth and signal transduction. Such interactions likely mediate the physiologic growth in most cells in vivo as well as the aberrant, non-neoplastic growth that occurs in diseases such as asthma, where disruptions of the local hormonal or inflammatory state can contribute to significant GPCR activation. In this study, we show that numerous inflammatory or contractile agents, including thrombin, histamine, and carbachol, potentiate epidermal growth factor (EGF)-stimulated proliferation of human airway smooth muscle (ASM), thus demonstrating a clear synergy between RTK and GPCR activation. Alterations in promitogenic nuclear signaling were evidenced by additive or synergistic increases in Elk-1 and activator protein-1 activation, and by increases in cyclin D1 expression. Interestingly, GPCR activation did not cause EGF receptor tyrosine phosphorylation nor did it increase EGF-stimulated autophosphorylation. In the presence of EGF, histamine or carbachol did not alter the time-dependent phosphorylation of p42/p44, whereas thrombin was capable of increasing phospho-p42/p44 levels at selected time points in some, but not all, cultures. In contrast to their relative inability to alter EGF receptor-linked p42/p44 activation, thrombin, histamine, and carbachol consistently increased the late phase (> 1 h) activity of p70 S6 kinase. Collectively, these findings suggest that inflammatory and contractile agents that activate GPCRs can significantly modulate RTK-mediated ASM growth through a p70 S6 kinase-dependent, p42/p44-independent mechanism.

Molecular mechanisms and regulation of opioid receptor signaling

Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.

Association of a lower molecular weight protein to the mu-opioid receptor demonstrated by (125)I-beta-endorphin cross-linking studies

Cross-linking experiments using the (125)I-beta-endorphin revealed the presence of several receptor-related species in cell lines expressing endogenous opioid receptors, including a small molecular mass protein (approximately 22 kDa). Previous reports have suggested that this 22-kDa (125)I-beta-endorphin cross-linked protein could be the degradative product from a higher molecular mass species, i.e., a fragment of the receptor. To determine if this protein is indeed a degraded receptor fragment, (125)I-beta-endorphin was cross-linked to the (His)(6) epitope-tagged mu-opioid receptor (His-mu) stably expressed in the murine neuroblastoma Neuro(2A) cells. Similar to earlier reports with cell lines expressing endogenous receptors, two major bands of 72- and 25-kDa proteins were specifically cross-linked. Initial cross-linking experiments indicated the absolute requirement of the high-affinity (125)I-beta-endorphin binding to the mu-opioid receptor prior to the appearance of the low molecular weight species, suggesting that the 22-kDa protein could be a degraded fragment of the receptor. However, variations in the ratios of these protein bands being cross-linked by several homo- or heterobifunctional cross-linking agents were observed. Although neither the carboxyl terminus mu-opioid receptor-specific antibodies nor the antibodies against the epitope at the amino terminus of the receptor could recognize the 22-kDa protein, this (125)I-beta-endorphin cross-linked species could be coimmunoprecipitated with the receptor antibodies or could be isolated with a nickel resin affinity chromatography. The direct physical association of the 22-kDa protein with the receptor was demonstrated also by the observation that the 22-kDa protein could not bind to the nickel resin alone, but that its binding to the nickel resin was restored in the presence of the His-mu. Taken together, these results suggest that the 22-kDa protein cross-linked by (125)I-beta-endorphin is not a degradative product, but a protein located within the proximity of the mu-opioid receptor, and that it is tightly associated with the receptor.

Distinct roles for Galpha(i)2 and Gbetagamma in signaling to DNA synthesis and Galpha(i)3 in cellular transformation by dopamine D2S receptor activation in BALB/c 3T3 cells

Control of cell proliferation depends on intracellular mediators that determine the cellular response to external cues. In neuroendocrine cells, the dopamine D2 receptor short form (D2S receptor) inhibits cell proliferation, whereas in mesenchymal cells the same receptor enhances cell proliferation. Nontransformed BALB/c 3T3 fibroblast cells were stably transfected with the D2S receptor cDNA to study the G proteins that direct D2S signaling to stimulate cell proliferation. Pertussis toxin inactivates G(i) and G(o) proteins and blocks signaling of the D2S receptor in these cells. D2S receptor signaling was reconstituted by individually transfecting pertussis toxin-resistant Galpha(i/o) subunit mutants and measuring D2-induced responses in pertussis toxin-treated cells. This approach identified Galpha(i)2 and Galpha(i)3 as mediators of the D2S receptor-mediated inhibition of forskolin-stimulated adenylyl cyclase activity; Galpha(i)2-mediated D2S-induced stimulation of p42 and p44 mitogen-activated kinase (MAPK) and DNA synthesis, whereas Galpha(i)3 was required for formation of transformed foci. Transfection of toxin-resistant Galpha(i)1 cDNA induced abnormal cell growth independent of D2S receptor activation, while Galpha(o) inhibited dopamine-induced transformation. The role of Gbetagamma subunits was assessed by ectopic expression of the carboxyl-terminal domain of G protein receptor kinase to selectively antagonize Gbetagamma activity. Mobilization of Gbetagamma subunits was required for D2S-induced calcium mobilization, MAPK activation, and DNA synthesis. These findings reveal a remarkable and distinct G protein specificity for D2S receptor-mediated signaling to initiate DNA synthesis (Galpha(i)2 and Gbetagamma) and oncogenic transformation (Galpha(i)3), and they indicate that acute activation of MAPK correlates with enhanced DNA synthesis but not with transformation.

Opioid growth factor and organ development in rat and human embryos

In addition to neurotransmission, the native opioid peptide, [Met5]enkephalin, is a tonically active inhibitory growth molecule that is termed opioid growth factor (OGF). OGF interacts with the zeta (zeta) opioid receptor to influence cell proliferation and tissue organization. We now identify OGF and the zeta receptor in embryonic derivatives including ectoderm, mesoderm, and endoderm of the rat on gestation day 20. Messenger RNA for preproenkephalin (PPE), the precursor of OGF, was detected in the developing cells, suggesting an autocrine production of this peptide. Acute exposure of the pregnant female to OGF resulted in a decrease in DNA synthesis in cells of organs representing all three germ layers, and did so in a receptor-mediated fashion. The influence of OGF was direct, as evidenced in organ culture studies. Blockade of endogenous opioid interaction using naltrexone (NTX) produced an increase in DNA synthesis, indicating the constitutive and functional nature of opioid activity on growth during prenatal life. Human fetal cells contained OGF and the zeta receptor. These data support the hypothesis that endogenous opioid modulation of organ development is a fundamental principle of mammalian embryogenesis, and that OGF has a profound influence on ontogeny. Irregularities in the role of opioids as growth regulators in relationship to the more than 500,000 newborns suffering from birth defects each year in the US needs to be examined.

Opioids modulate cell division in the germinal zone of the late embryonic neocortex

Opioid effects on cell division in the embryonic cerebral cortex were examined using two experimental approaches: (i) the presence of opioid receptors in the embryonic day 16 mouse neocortex was tested using immunohistochemical techniques; (ii) the values of the indices of [3H]thymidine pulse labelled cells and mitotic indices were estimated in the ventricular zone of the embryonic day 16 mouse neocortex 2.5, 4.5 and 8.5 h after administration to pregnant females of selected opioid receptor agonists or the opioid antagonist naloxone. The immunohistochemical study demonstrated that distinct subpopulations of the ventricular zone cells express mu, delta or kappa opioid receptors. Acute exposure of mouse embryos to mu, delta and kappa opioid receptor agonists or naloxone differentially affects the indices of [3H] thymidine pulse labelled cells and mitotic indices indicating changes in the cell cycle composition. Treatment with the mu opioid receptor agonist D-Ala2-MePhe4, Gly-ol5-enkephalin (DAGO), or the partially selective kappa opioid receptor agonist bremazocine, increased the [3H]thymidine labelling and mitotic indices. In contrast, the delta receptor agonist (D-Ser8)-leucine enkephalin-Thr (DSLET) produced a decrease in the labelled cell indices and mitotic indices. Naloxone provided a biphasic effect: a decrease in the values of labelled cell indices 2.5 h after naloxone administration, followed by an increase in the values of the indices at 4.5 and 8.5 h. These results suggest that the endogenous embryonic/maternal opioid systems are involved in the regulation of cell division in the ventricular zone of the late embryonic cortex.

Opioid agonists modify breast cancer cell proliferation by blocking cells to the G2/M phase of the cycle: involvement of cytoskeletal elements

Opioids decrease cell proliferation in different systems including breast, prostate, lung, kidney, and intestine, through an interaction with opioid as well as other membrane-receptor systems (somatostatin, cholinergic), through an unidentified mechanism. Recently, we have reported an interaction of taxol with opioid membrane sites (BBRC 235, 201-204, 1997), and an involvement of opioids to the modification of actin cytoskeleton in renal OK cells (J Cell Biochem. [19981 70:60-69), indicating a possible action of the opioid effect. In the present work, we have examined the effect of two general opioid agonists (ethylketocyclazocine and etorphine) on the cell cycle, in human breast cancer T47D cells, as well as a possible modification of the cellular cytoskeleton under their action, in order to explain the antiproliferative effect of these agents. These two opioids produce a dose-dependent and reversible decrease of the proliferation of T47D cells, with a maximum attained at 10(-8) M. The addition of 10(-8) M of either opioid produced a significant increase of the number of cells arrested in the G2/M phase. Confocal laser microscopy revealed a modification of the actin and tubulin microfilaments, with a clear redistribution at the periphery of the cell, reversed by the addition of the general opioid antagonist diprenorphine. Furthermore, differences between the two opioids were obvious, attributed to the different receptor affinity of each agent. The observed redistribution of actin and tubulin cytoskeletal elements gives therefore a possible answer of the antiproliferative action of opioids. The modification of the cytoskeleton, directly involved to cell division, might provoke a "mechanical" obstacle, which could be the reason of the antiproliferative effect of these agonists. Furthermore, the observed tubulin-opioid interaction by opioids provides a possible explanation of the arrest at the G2/M phase of T47D cells under opioid treatment. Nevertheless, although the observed interaction of opioids with cytoskeletal elements gives a plausible answer of the antiproliferative effects of the agents, this might not be the only action of these agents in cell proliferation. Other, direct or indirect, genomic actions, which which remains to be elucidated, might be taken into consideration.

Early alterations of actin cytoskeleton in OK cells by opioids

Recently we identified and characterized opioid binding sites in OK (opossum kidney) cells and observed decreased proliferation of these cells in response to opioids. In the present study we investigated the effects of opioids on the actin cytoskeleton and explored whether their antiproliferative action may relate to alterations in the distribution or the dynamics of actin microfilaments. Exposure of OK cells to the opioids alphaS1 casomorphin and ethylketocyclazocine resulted in a rapid and substantial actin microfilament reorganization. This was documented by a significant dose-dependent decrease in the amounts of F-actin, determined by measurements of quantitative fluorescence, by immunoblot analysis and by a concomitant increase of the G/total-actin ratio measured by the DNase I inhibition assay. These changes were verified by confocal laser scanning microscopy, which showed marked redistribution of the microfilamentous structures in the presence of the opioids without affecting the organization of microtubules or vimentin intermediate filaments. The effect of opioids on actin polymerization dynamics occurred within 15 min and persisted for at least 2 h, while their restoration to control levels was accomplished 6 h later, indicating a reversible phenomenon. Northern blot analysis showed that the concentration of the actin transcript was unaffected. The addition of diprenorphine, a general opioid antagonist, prevented the effects of opioids on the actin cytoskeleton. The inhibition of OK cell proliferation, induced by ethylketocyclazocine and alphaS1 casomorphin was partially prevented in the presence of phallacidin, which stabilizes microfilaments. Our findings demonstrate that opioids, acting via kappa 1 binding sites, induce rapidly modifications in the dynamics of actin polymerization, and in the organization of microfilaments in OK cells, which may relate to their antiproliferative effect on these cells.

G protein-coupled-receptor cross-talk: the fine-tuning of multiple receptor-signalling pathways

Signalling via the large family of G protein-coupled receptors (GPCRs) can lead to many cellular responses, ranging from regulation of intracellular levels of cAMP to stimulation of gene transcription. Members of this receptor family have been grouped into different categories dependent on the particular G protein subtypes that they predominantly interact with. Thus, receptors that couple to GS proteins will stimulate adenylate cyclase in many cells, while Gq/11-coupled receptors can mobilize intracellular Ca2+ via activation of phospholipase C. There is accumulating evidence, however, that activation of one particular signalling pathway by a GPCR can amplify intracellular signalling within a parallel but separate pathway. In this article Lisa Selbie and Stephen Hill review some of the evidence for these synergistic interactions and suggest that they may have an important role in finetuning signals from multiple receptor signalling pathways.

TAN-67, a delta 1-opioid receptor agonist, reduces infarct size via activation of Gi/o proteins and KATP channels

We have previously shown that delta (delta)-opioid receptors, most notably delta 1, are involved in the cardioprotective effect of ischemic preconditioning (PC) in rats; however, the mechanism by which delta-opioid receptor-induced cardioprotection is mediated remains unknown. Therefore, we hypothesized that several of the known mediators of ischemic PC such as the ATP-sensitive potassium (KATP) channel and Gi/o proteins are involved in the cardioprotective effect produced by delta 1-opioid receptor activation. To address these possibilities, anesthetized, open-chest Wistar rats were randomly assigned to five groups. Control animals were subjected to 30 min of coronary artery occlusion and 2 h of reperfusion. To demonstrate that stimulating delta 1-opioid receptors produces cardioprotection, TAN-67, a new selective delta 1-agonist, was infused for 15 min before the long occlusion and reperfusion periods. In addition, one group received 7-benzylidenenaltrexone (BNTX), a selective delta 1-antagonist, before TAN-67. To study the involvement of KATP channels or Gi/o proteins in delta 1-opioid receptor-induced cardioprotection, glibenclamide (Glib), a KATP channel antagonist, or pertussis toxin (PTX), an inhibitor of Gi/o proteins, was administered before TAN-67. Infarct size (IS) as a percentage of the area at risk (IS/AAR) was determined by tetrazolium stain. TAN-67 significantly reduced IS/AAR as compared with control (56 +/- 2 to 27 +/- 5%, n = 5, P < 0.05). The cardioprotective effect of TAN-67 was completely abolished by BNTX, Glib, and PTX (51 +/- 3, 53 +/- 5, and 61 +/- 4%, n = 6 for each group, respectively). These results are the first to suggest that stimulating the delta 1-opioid receptor elicits a cardioprotective effect that is mediated via Gi/o proteins and KATP channels in the intact rat heart.

Delta and kappa opioid receptors are differentially regulated by dynamin-dependent endocytosis when activated by the same alkaloid agonist

Many alkaloid drugs used as analgesics activate multiple opioid receptors. Mechanisms that distinguish the actions of these drugs on the regulation of individual micro, delta, and kappa receptors are not understood. We have observed that individual cloned opioid receptors differ significantly in their regulation by rapid endocytosis in the presence of alkaloid drug etorphine, a potent agonist of mu, delta, and kappa opioid receptors. Internalization of epitope-tagged delta opioid receptors from the plasma membrane is detectable within 10 min in the presence of etorphine. In contrast, kappa receptors expressed in the same cells remain in the plasma membrane and are not internalized for >/=60 min, even when cells are exposed to saturating concentrations of etorphine. The rapid internalization of delta receptors is specifically inhibited in cells expressing K44E mutant dynamin I, suggesting that type-specific internalization of opioid receptors is mediated by clathrin-coated pits. Examination of a series of chimeric mutant kappa/delta receptors indicates that at least two receptor domains, including the highly divergent carboxyl-terminal cytoplasmic tail, determine the type specificity of this endocytic mechanism. We conclude that structurally homologous opioid receptors are differentially sorted by clathrin-mediated endocytosis following activation by the same agonist ligand. These studies identify a fundamental mechanism of receptor regulation mediating type-specific effects of analgesic drugs that activate more than one type of opioid receptor.