A human neuroblastoma cell line expresses mu and delta opioid receptor sites

The delta opioid receptor tool box

In recent years, the delta opioid receptor has attracted increasing interest as a target for the treatment of chronic pain and emotional disorders. Due to their therapeutic potential, numerous tools have been developed to study the delta opioid receptor from both a molecular and a functional perspective. This review summarizes the most commonly available tools, with an emphasis on their use and limitations. Here, we describe (1) the cell-based assays used to study the delta opioid receptor. (2) The features of several delta opioid receptor ligands, including peptide and non-peptide drugs. (3) The existing approaches to detect delta opioid receptors in fixed tissue, and debates that surround these techniques. (4) Behavioral assays used to study the in vivo effects of delta opioid receptor agonists; including locomotor stimulation and convulsions that are induced by some ligands, but not others. (5) The characterization of genetically modified mice used specifically to study the delta opioid receptor. Overall, this review aims to provide a guideline for the use of these tools with the final goal of increasing our understanding of delta opioid receptor physiology.

Antibodies to probe endogenous G protein-coupled receptor heteromer expression, regulation, and function

Over the last decade an increasing number of studies have focused on the ability of G protein-coupled receptors to form heteromers and explored how receptor heteromerization modulates the binding, signaling and trafficking properties of individual receptors. Most of these studies were carried out in heterologous cells expressing epitope tagged receptors. Very little information is available about the in vivo physiological role of G protein-coupled receptor heteromers due to a lack of tools to detect their presence in endogenous tissue. Recent advances such as the generation of mouse models expressing fluorescently labeled receptors, of TAT based peptides that can disrupt a given heteromer pair, or of heteromer-selective antibodies that recognize the heteromer in endogenous tissue have begun to elucidate the physiological and pathological roles of receptor heteromers. In this review we have focused on heteromer-selective antibodies and describe how a subtractive immunization strategy can be successfully used to generate antibodies that selectively recognize a desired heteromer pair. We also describe the uses of these antibodies to detect the presence of heteromers, to study their properties in endogenous tissues, and to monitor changes in heteromer levels under pathological conditions. Together, these findings suggest that G protein-coupled receptor heteromers represent unique targets for the development of drugs with reduced side-effects.

Heteromers of μ-δ opioid receptors: new pharmacology and novel therapeutic possibilities

Several studies suggest that heteromerization between μ (MOP) and δ (DOP) opioid receptors modulates the signalling properties of the individual receptors. For example, whereas activation of MOP receptors by an agonist induces G protein-mediated signalling, the same agonist induces β-arrestin-mediated signalling in the context of the MOP-DOP receptor heteromer. Moreover, heteromer-mediated signalling is allosterically modulated by a combination of MOP and DOP receptor ligands. This has implications in analgesia given that morphine-induced antinociception can be potentiated by DOP receptor ligands. Recently reagents selectively targeting the MOP-DOP receptor heteromer such as bivalent ligands, antibodies or membrane permeable peptides have been generated; these reagents are enabling studies to elucidate the contribution of endogenously expressed heteromers to analgesia as well as to the development of side-effects associated with chronic opioid use. Recent advances in drug screening technology have led to the identification of a MOP-DOP receptor heteromer-biased agonist that activates both G protein-mediated and β-arrestin-mediated signalling. Moreover, this heteromer-biased agonist exhibits potent antinociceptive activity but with reduced side-effects, suggesting that ligands targeting the MOP-DOP receptor heteromer form a basis for the development of novel therapeutics for the treatment of pain. In this review, we summarize findings regarding the biological and functional characteristics of the MOP-DOP receptor heteromer and the in vitro and in vivo properties of heteromer-selective ligands.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Pharmacological Profiles of Oligomerized μ-Opioid Receptors

Opioids are widely prescribed pain relievers with multiple side effects and potential complications. They produce analgesia via G-protein-protein coupled receptors: μ-, δ-, κ-opioid and opioid receptor-like 1 receptors. Bivalent ligands targeted to the oligomerized opioid receptors might be the key to developing analgesics without undesired side effects and obtaining effective treatment for opioid addicts. In this review we will update the biological effects of μ-opioids on homo- or hetero-oligomerized μ-opioid receptor and discuss potential mechanisms through which bivalent ligands exert beneficial effects, including adenylate cyclase regulation and receptor-mediated signaling pathways.

Fentanyl co-administration decreases the induction dose requirement of propofol in patients with supratentorial tumors and not in patients with spinal lesions

BACKGROUND

The requirement of anesthetic drugs in a patient with an intracranial space-occupying lesion is of relevance to the neuroanesthetist. The requirement is often presumed to have reduced or at least altered. However, not much research has focused on this issue. Hence, we conducted this study to examine whether intracranial tumors reduce the induction dose of propofol in patients undergoing craniotomy based on plasma and effect site concentrations (Ce) of propofol and the effect of additional fentanyl.

METHODS

A total of 80 patients were recruited into the study. The study group included patients with supratentorial tumors undergoing craniotomy, and the control group consisted of patients undergoing spinal surgeries. Patients in each group were randomized further to receive propofol alone or propofol preceded by fentanyl for induction of anesthesia. They were divided into the following groups: patients with supratentorial tumor receiving only propofol (group T1), or fentanyl and propofol (group T2); patients who were undergoing spinal surgery and receiving only propofol (group S1) or fentanyl and propofol (group S2). Anesthesia was induced with infusion of propofol through a Target Controlled Infusion pump. At the point of loss of verbal contact, plasma concentration (Cp) and Ce of propofol, time taken for loss of consciousness, and the total dose of propofol required were noted. Hemodynamic variables were recorded before and after induction of anesthesia.

RESULTS

There were 19, 21, 19, and 21 patients in groups TI, T2, S1, and S2, respectively. In group T2 the Cp, Ce, time to loss of verbal contact, and dose required for induction were all significantly lower compared with the other groups. There were no significant differences in the study parameters between T1 and S1, whereas the differences were significant between T2 and S2 (Cp: 3.9±1.1 vs. 4.9±1.2 μg/mL; Ce: 2.6±1.0 vs. 3.7±1.2 μg/mL; P<0.05).

CONCLUSIONS

Propofol dose for induction of anesthesia was significantly reduced when administered after fentanyl in patients with supratentorial tumors. Tumors per se without fentanyl coadministration do not decrease the propofol requirement for induction of anesthesia.

Dual regulation of mu opioid receptors in SK-N-SH neuroblastoma cells by morphine and interleukin-1β: evidence for opioid-immune crosstalk

Treatment of SK-N-SH cells with morphine and interleukin-1beta (IL-1β) produced dual regulation of the mRNA for the human mu opioid receptor (MOR) protein. Morphine produced a decrease in the MOR mRNA while IL-1β increased it, as assessed by real-time quantitative PCR. These data were consistent with immunocytochemical studies of treated and untreated cells. Morphine-mediated down-regulation of MOR was blocked by naltrexone and IL-1β-induced up-regulation of MOR was blocked by interleukin-1 receptor type 1 antagonist. Immune-opioid crosstalk was examined by IL-1β and morphine co-treatment. These data are the first to show dual regulation of MOR in neuroblastoma cells.

Tracking the opioid receptors on the way of desensitization

Opioid receptors belong to the super family of G-protein coupled receptors (GPCRs) and are the targets of numerous opioid analgesic drugs. Prolonged use of these drugs results in a reduction of their effectiveness in pain relief also called tolerance, a phenomenon well known by physicians. Opioid receptor desensitization is thought to play a major role in tolerance and a lot of work has been dedicated to elucidate the molecular basis of desensitization. As described for most of GPCRs, opioid receptor desensitization involves their phosphorylation by kinases and their uncoupling from G-proteins realized by arrestins. More recently, opioid receptor trafficking was shown to contribute to desensitization. In this review, our knowledge on the molecular mechanisms of desensitization and recent progress on the role of opioid receptor internalization, recycling or degradation in desensitization will be reported. A better understanding of these regulatory mechanisms would be helpful to develop new analgesic drugs or new strategies for pain treatment by limiting opioid receptor desensitization and tolerance.

High-purity selection and maintenance of gene expression in human neuroblastoma cells stably over-expressing GFP fusion protein. Application for opioid receptors desensitization studies

Chronic use of opiates such as morphine is associated with drug tolerance, which is correlated with the desensitization of opioid receptors. This latter process involves phosphorylation of opioid receptors by G protein-coupled receptors kinases (GRKs) and subsequent uncoupling by beta-arrestins. To explore these molecular mechanisms, neuronal cell lines, endogenously expressing the opioid receptors, provide an ideal cellular model. Unfortunately, there are two major drawbacks: (1) these cells are refractory to cDNA introduction, resulting in low transfection efficiency; (2) continuous culturing of transfected cells invariably leads to phenotypic drift of the cultures even after an antibiotic selection. So, these cells were dropped in favor of heterologous expression systems, which are easier to transfect but whose relevance as adequate cellular model for studying opioid receptor regulation should be questioned, as recently demonstrated by [Haberstock-Debic, H., Kim, K.A.,Yu, Y.J., von Zastrow, M., 2005. Morphine promotes rapid, arrestin-dependent endocytosis of mu-opioid receptors in striatal neurons. J. Neurosci. 25, 7847-7857]. In this work, we describe a method, based on fluorescence-activated cell sorting (FACS), to select and maintain a high proportion of transfected SK-N-BE cells (a neuronal cell line endogenously expressing human Delta-Opioid Receptor (hDOR)), expressing the beta-arrestin1 fused to green fluorescent protein (GFP). While in functional experiments, we were not able to observe a major effect in non-sorted SK-N-BE cells expressing beta-arrestin1-GFP, the enrichment by 18-fold with FACS resulted in a robust increase of beta-arrestin1-GFP expression associated with strong hDOR desensitization. Moreover, this method also allows to counteract the phenotypic drift and to maintain a high-purity selection of SK-N-BE cells expressing beta-arrestin1-GFP. Thus, this approach provides a valuable tool for exploring opioid receptors desensitization in neuronal cells.

Comparison of opioid receptor distributions in the rat central nervous system

The opioid receptors, mu, delta and kappa, conduct the major pharmacological effects of opioid drugs, and exhibit intriguing functional relationships and interactions in the CNS. Previously established hypotheses regarding the mechanisms underlying these phenomena specify theoretical patterns of relative cellular localisation for the different receptor types. In this study, we have used double-label immunohistochemistry to compare the cellular distributions of delta and kappa receptors with those of mu receptors in the rat CNS. Regions of established significance in opioid addiction were examined. Extensive mu/delta co-localisation was observed in neuron-like cells in several regions. mu and kappa receptors were also often co-localised in neuron-like cell bodies in several regions. However, intense kappa immunoreactivity (ir) also appeared in a separate, morphologically distinct population of cells that did not express mu receptors. These small, ovoid cells were often closely apposed against the larger, mu-ir cell bodies. Such cellular appositions were seen in several regions, but were particularly common in the medial thalamus, the periaqueductal grey and brainstem regions. These findings support proposals that functional similarities, synergy and cooperativity between mu and delta receptors arise from widespread co-expression by cells and intracellular molecular interactions. Although co-expression of mu and kappa receptors was also detected, the appearance of a separate population of kappa-expressing cells supports proposals that the contrasting and functionally antagonistic properties of mu and kappa receptors are due to expression in physiologically distinct cell types. Greater understanding of opioid receptor interaction mechanisms may provide possibilities for therapeutic intervention in opioid addiction and other conditions.

A nuclear function of beta-arrestin1 in GPCR signaling: regulation of histone acetylation and gene transcription

Chromatin modification is considered to be a fundamental mechanism of regulating gene expression to generate coordinated responses to environmental changes, however, whether it could be directly regulated by signals mediated by G protein-coupled receptors (GPCRs), the largest surface receptor family, is not known. Here, we show that stimulation of delta-opioid receptor, a member of the GPCR family, induces nuclear translocation of beta-arrestin 1 (betaarr1), which was previously known as a cytosolic regulator and scaffold of GPCR signaling. In response to receptor activation, betaarr1 translocates to the nucleus and is selectively enriched at specific promoters such as that of p27 and c-fos, where it facilitates the recruitment of histone acetyltransferase p300, resulting in enhanced local histone H4 acetylation and transcription of these genes. Our results reveal a novel function of betaarr1 as a cytoplasm-nucleus messenger in GPCR signaling and elucidate an epigenetic mechanism for direct GPCR signaling from cell membrane to the nucleus through signal-dependent histone modification.

Opioids and differentiation in human cancer cells

This study was designed to examine the role of opioids on cell differentiation, with an emphasis on the mechanism of opioid growth factor (OGF, [Met5]-enkephalin)-dependent growth inhibition. Three human cancer cell lines (SK-N-SH neuroblastoma and SCC-1 and CAL-27 squamous cell carcinoma of the head and neck), along with OGF and the opioid antagonist naltrexone (NTX) at a dosage (10(-6) M) known to repress or increase, respectively, cell replication, were utilized. The effects on differentiation (neurite formation, process lengths, betaIII-tubulin, involucrin) were investigated in cells exposed to OGF or NTX for up to 6 days. In addition, the influence of a variety of other natural and synthetic opioids on differentiation was examined. OGF, NTX, naloxone, [D-Pen2,5]-enkephalin, dynorphin A1-8, beta-endorphin, endomorphin-1 and -2, [D-Ala2, MePhe4, Glycol5]-enkephalin (DAMGO), morphine, and U69,593 at concentrations of 10(-6) M did not alter cell differentiation of any cancer cell line. In NTX-treated SK-N-SH cells, cellular area was increased 23%, and nuclear area was decreased 17%, from control levels; no changes in cell or nuclear area were recorded in OGF-exposed cells. F-actin concentration was increased 40% from control values in SK-N-SH cells subjected to NTX, whereas alpha-tubulin was decreased 53% in OGF-treated cells. These results indicate that the inhibitory or stimulatory actions of OGF and NTX, respectively, on cell growth in tissue culture are not due to alterations in differentiation pathways. However, exposure to OGF and NTX modified some aspects of cell structure, but this was independent of differentiation. The absence of effects on cancer cell differentiation by a variety of other opioids supports the previously reported lack of growth effects of these compounds.

Internalization and down-regulation of mu opioid receptors by endomorphins and morphine in SH-SY5Y human neuroblastoma cells

The human neuroblastoma cell line, SH-SY5Y, was used to examine the effects of morphine and the endogenous opioid peptides, endomorphin-1 (EM-1) and endomorphin-2 (EM-2), on mu opioid receptor (MOR) internalization and down-regulation. Treatment for 24 h with EM-1, EM-2 or morphine at 100 nM, 1 microM and 10 microM resulted in a dose-dependent down-regulation of mu receptors. Exposure of cells to 10 microM EM-1 for 2.5, 5 and 24 h resulted in a time-dependent down-regulation of mu receptors. Down-regulation of mu receptors by morphine and EM-1 was blocked by treatment with hypertonic sucrose, consistent with an endocytosis-dependent mechanism. Sensitive cell-surface binding studies with a radiolabeled mu antagonist revealed that morphine was able to induce internalization of mu receptors naturally expressed in SH-SY5Y cells. EM-1 produced a more rapid internalization of mu receptors than morphine, but hypertonic sucrose blocked the internalization induced by each of these agonists. This study demonstrates that, like morphine, the endomorphins down-regulate mu opioid receptors in a dose- and time-dependent manner. This study also demonstrates that morphine, as well as EM-1, can induce rapid, endocytosis-dependent internalization of mu opioid receptors in SH-SY5Y cells. These results may help elucidate the ability of mu agonists to regulate the number and responsiveness of their receptors.

Long-term regulation of signalling components of adenylyl cyclase and mitogen-activated protein kinase in the pre-frontal cortex of human opiate addicts

Opiate addiction involves the development of chronic adaptive changes in micro -opioid receptors and associated pathways (e.g. cAMP signalling) which lead to neuronal plasticity in the brain. This study assessed the status of cAMP and mitogen-activated protein kinase (MAPK) pathways in brains (pre-frontal cortex) of chronic opiate addicts. In these subjects (n = 24), the immunodensities of adenylyl cyclase-I, PKA Calpha, total and phosphorylated CREB were not different from those in sex-, age- and PMD-matched controls. Moreover, the ratio pCREB/tCREB was similar in opiate addicts (0.74) and controls (0.76), further indicating that opiate addiction in humans is not associated with an upregulation of several key components of cAMP signalling in the pre-frontal cortex. In contrast, the components of MAPK cascade (Ras/c-Raf-1/MEK/ERK) were decreased in the same brains. Notably, pronounced downregulations of phosphorylated MEK (85%) and ERK1/2 (pERK1: 81%; pERK2: 80%) were quantitated in brains of opiate addicts. Chronic morphine treatment in rats (10-100 mg/kg for 5 days) was also associated with decreases of pERK1/2 (59-68%) in the cortex. In SH-SY5Y cells, morphine also stimulated the activity of pERK1/2 (2.5-fold) and the MEK inhibitor PD98059 blocked this effect (90%). The abnormalities of MAPK signalling might have important consequences in the long term development of various forms of neural plasticity associated with opiate addiction in humans.

Deglycosylation of Fas receptor and chronic morphine treatment up-regulate high molecular mass Fas aggregates in the rat brain

This study was designed to immunodetect and characterize Fas receptor aggregates (oligomerization) in the brain and to assess its possible modulation in opiate addiction. High molecular mass, sodium dodecyl sulfate (SDS)- and beta-mercaptoethanol-resistant Fas aggregates (approximately 110/120 and approximately 203 kDa specific peptides) were immunodetected with a cytoplasmic domain-specific antibody in brain tissue (rat, mouse and human) and SH-SY5Y cells by Western blot analysis. Preincubation of rat cortical membranes with N-ethylmaleimide (NEM; 1 mM for 1 h at 37 degrees C) reduced the immunodensity of approximately 203 kDa Fas aggregates (51%) and increased that of 35 kDa native Fas (172%) and 51/48 kDa glycosylated Fas (47%), indicating that disulfide bonds are involved in Fas dimerization. Enzymatic N-deglycosylation of Fas receptor increased the content of Fas aggregates (approximately 110/120 kDa: five- to sixfold, and approximately 203 kDa: two- to threefold), suggesting that Fas glycosylation is involved in regulating receptor dimerization. Chronic (10-100 mg/kg for 5 days), but not acute (30 mg/kg for 2 h), treatment with morphine (a micro-opioid peptide receptor agonist) induced up-regulation of Fas aggregates in the brain (approximately 110/120 kDa: 39%, and approximately 203 kDa: 89%). The acute and/or chronic treatments with delta- and kappa-opioid peptide receptor agonists and with a sigma1-receptor agonist did not readily alter the content of Fas aggregates in the rat brain. The results indicate that Fas aggregates are natively expressed in the brain and that its density is regulated by the state of Fas glycosylation. These forms of Fas (receptor homodimerization) are functionally relevant because they were up-regulated in the brain of morphine-dependent rats.

Delta opioid receptors are involved in morphine-induced inhibition of luteinizing hormone releasing hormone in SK-N-SH cells

Opioids play an important role in the regulation of lutenizing hormone releasing hormone (LHRH). In the present study, we attempted to find out the subtype of opioid receptors involved in the inhibitory effect of morphine on LHRH. Experiments were conducted on SK-N-SH neuroblastoma cells that express both micro and delta opioid receptors, LHRH mRNA, and release the LHRH peptide. Enzyme-linked immunosorbent assay (ELISA) was employed to measure the levels of LHRH. LHRH level was decreased by 1000 microM of morphine regardless of the duration of exposure or differentiation status of the SK-N-SH cells and was not reversed by naloxone. Selective antagonism of micro opioid receptors, but not delta opioid receptors, allowed lower concentrations (1-100 microM) of morphine to inhibit LHRH. The results of this study imply that (1) delta opioid receptors may mediate the inhibitory effect of lower concentrations of morphine on LHRH levels in SK-N-SH cells, and (2) inhibition of LHRH level by high concentrations of morphine may involve systems other than opioid receptors.

The mu opioid receptor: from molecular cloning to functional studies

Opioids have been used and abused by humans for centuries. The mu opioid receptor represents the high affinity binding site for opioid narcotics with high abuse liability such as morphine, codeine and fentanyl. Heroin (diacetylmorphine), a semi-synthetic derivative of morphine, crosses the blood-brain barrier more readily than morphine due to its increased hydrophobicity. Once in the brain heroin is hydrolyzed to morphine, which acts at the mu opioid receptor and results in euphoria, thus conferring the reinforcing properties of heroin. Using molecular biology techniques, the mu opioid receptors from several species have been cloned. This article reviews recent progress in this area, with respect to the two major cellular functions of the mu opioid receptor: reduction of intracellular cAMP concentration by inhibiting adenylyl cyclase activity, and inhibition of neuronal firing by modulating membrane ion channels.

Functional coupling of a mu opioid receptor to G proteins and adenylyl cyclase: modulation by chronic morphine treatment

A cloned mu opioid receptor was used to study its coupling to signal transduction pathways and its involvement in morphine-induced opioid dependence in stably transfected Chinese hamster ovary (CHO) cells. Membrane binding assays with a mu-selective agonist [(3)H]DAMGO showed that one cell line expresses a high level of mu opioid receptors with a B(max) of approximately 630 fmol/mg membrane protein and a K(d) of 0.47 nM for DAMGO. Stimulation of the transfected cells with DAMGO led to an increase in the low K(m) GTPase activity, indicative of activation of guanine nucleotide regulatory proteins (G proteins), and this effect was blocked by the opioid antagonist naloxone. In addition, binding of the mu opioid receptor to DAMGO was affected by GTP and nonhydrolizable GTP analogs, Gp(NH)pp and GTP-gamma-S. These results suggest a functional coupling between the mu opioid receptor and G proteins. Furthermore, DAMGO treatment of the cells produced a dose-dependent inhibition of the intracellular cyclic adenosine monophosphate (cAMP) level, with an EC(50) value of approximately 30 nM. Chronic treatment of the cells with morphine not only elevated the basal and forskolin-stimulated cAMP levels after morphine withdrawal, but also increased the extent of the DAMGO-induced reduction of intracellular cAMP levels. The whole cell binding assay with [(3)H]DAMGO, on the other hand, did not detect receptor down-regulation after chronic morphine treatment. These results suggest that chronic morphine treatment may trigger a compensatory mechanism in cellular signaling pathways to offset the inhibitory input from the mu receptor without down-regulation of the surface receptor number, and that withdrawal of chronic inhibition leads to elevated activities of adenylyl cyclase to provide a basis for system sensitization.

Differential effects of mu-opioid receptor ligands on Ca(2+) signaling

Activation of mu-opioid receptors (MORs) transfected into human embryonic kidney 293 cells, caused a multiphasic increase in cytosolic free Ca(2+) levels (Ca(2+)i). The first Ca(2+)i maximum (peak 1) between 5 and 7 s depended on the presence of extracellular Ca(2+) (Ca(2+)e). The second phase peaking at approximately 15 s (peak 2) was independent of Ca(2+)e and thus represents Ca(2+) release from intracellular stores. A decrease in temperature from 37 to 25 degrees C also caused reduction of peak 1 but not peak 2, suggesting that the two responses arise from mechanistically distinct pathways. A delayed Ca(2+)e-dependent third response phase is thought to represent capacitative Ca(2+)e influx evoked after release of Ca(2+) from internal stores. Agonists and antagonists of two major classes of opioid ligands, oxymorphinans (morphine and naloxone) and oripavines (etorphine and diprenorphine), had differential effects on Ca(2+) currents. Although morphine activated both phases with equal potency, etorphine was 20-fold less potent at stimulating peak 1 over peak 2. Similarly, the antagonists, naloxone and diprenorphine, blocked the Ca(2+) response to each agonist with greatly varying potencies. Specifically, concomitant injection of diprenorphine failed to affect peak 1 (thought to represent rapid Ca(2+)e influx) stimulated by morphine while fully blocking peak 2 (intracellular Ca(2+) release). However, diprenorphine potently inhibited peak 1 as well when added to the cells before morphine, indicating limited or slow access of diprenorphine to these morphine binding sites. The existence of multiple, functionally distinct binding site conformations could account for these findings. In conclusion, different opioid ligands can differentially affect Ca(2+) response patterns resulting from MOR activation.

Binding of Sp1/Sp3 to the proximal promoter of the hMOR gene is enhanced by DAMGO

The major binding site for morphine is the mu opioid receptor (MOR), which mediates morphine's analgesic and euphoric effects. The MOR gene is highly regulated at the level of transcription. The present study examined DNA-protein interactions in the human MOR (hMOR) -500 to -292 promoter region, and tested whether chronic opioid drug treatment could modulate these DNA-protein interactions. 5'-deletion and transient transfection assays in SK-N-SH cells revealed four regions that activated hMOR gene transcription. A 60 bp sequence (-351 to -292) upstream of the proximal transcription initiation site (-252) contained cis-elements required for basal promoter activity. Sp1 and Sp3 bound to this 60 bp region, which was confirmed by electromobility shift assays using a Sp1 consensus oligo as competitor and specific antibodies against Sp1 and Sp3. Methylation interference analysis localized the Sp1 binding site to the sequence CCCTCCTCCC (-310 to -301) and also suggested that additional transcription factors, other than Sp1-related proteins, contacted the -321 to -301 sequence. Moreover, the binding of Sp1/Sp3 to the hMOR promoter was significantly enhanced by chronic exposure to [D-Ala(2), N-Me-Phe(4), Gly(5)-ol] enkephalin (DAMGO), a selective MOR agonist, and this effect was attenuated specifically by pretreatment with a MOR antagonist, naloxone. Taken together, the present studies demonstrated, for the first time, that the binding of Sp1/Sp3 to the hMOR proximal promoter could be modulated by chronic DAMGO treatment. Such enhanced binding of Sp1/Sp3 to the promoter may lead to a functional change in hMOR gene transcription.

Kappa opioid agonist-induced changes in IOP: correlation with 3H-NE release and cAMP accumulation

Opioid receptors have been demonstrated to modulate various functions in the eye. This research project was designed to determine and compare the effects of kappa opioid agonists on selected parameters that influence ocular hydrodynamics. Experiments determined the effects of two relatively selective kappa opioid receptor agonists, ICI 204 448 (ICI), which has limited ability to penetrate the blood-brain barrier, and spiradoline mesylate on: (1) in vivo parameters, intraocular pressure (IOP) and pupil diameter (PD); and (2) in vitro parameters, neurotransmitter release and cAMP accumulation, in the ciliary body. Dark-adapted, reverse light cycle New Zealand white (NZW) male rabbits were used in all experiments. In in vivo experiments, intraocular pressures and pupil diameters were measured by a pneumatonometer and an optistick, respectively, before and after drug administration. Baseline readings were taken at 0.5 and 0 hr prior to agonist administration. Postdrug IOP and PD measurements were made at 0.5, 1, 2, 3, 4 and 5 hr after agonist application. In some experiments, the relatively selective kappa antagonist, norbinaltorphimine was applied 30 min prior to agonist application. In in vitro experiments, the release of tritiated norepinephrine (3H-NE) was measured from perfused electrically stimulated iris ciliary bodies and expressed as the percent change of the control. Basal and isoproterenol-stimulated cyclic AMP concentrations in iris ciliary bodies were quantified by radioimmunoassay techniques in the presence and absence of ICI and spiradoline. ICI and spiradoline decreased IOP in a dose-dependent manner in normal rabbits, but only spiradoline produced significant changes in PD. The kappa opioid receptor antagonist, norbinaltorphimine, antagonized the hypotensive effects of spiradoline and ICI in IOP experiments. Both kappa agonists inhibited the release of norepinephrine from perfused iris ciliary bodies. Isoproterenol- stimulated cAMP levels in iris ciliary bodies were suppressed by both kappa receptor agonists. The antagonism by norbinaltorphimine suggests that ICI and spiradoline lower IOP by activating kappa opioid receptors in the eye. The bilateral effects of unilaterally applied spiradoline on PD indicate that this kappa agonist activates receptors in the iris and/or the brain. The inhibition of norepinephrine release and cAMP accumulation in the iris ciliary body by ICI and spiradoline suggests that there are both pre- and postjunctional sites of action for kappa agonists.

Identification of delta- and mu-type opioid receptors on human and murine dendritic cells

The purpose of this study was to evaluate mu- and delta-opioid receptors (OR) on human and murine dendritic cells (DC). Expression of mu- and delta-OR mRNA on DC was demonstrated by RT-PCR. The immunocytochemical and Western blot analyses revealed the expression of OR protein in DC. Radioreceptor assay demonstrated the specific saturated temperature-dependent binding of [3H]-labeled opioid ligand on DC and B(max)=2.8+/-0.3 fmol/10(6) cells and K(D)=4.8+/-1.0 nM were calculated by a Scatchard analysis. Finally, OR ligands DADLE and DAGO dose-dependently modulated the capacity of DC to induce T cell proliferation in an MLR assay. Importantly, expression of functional OR on DC was significantly increased upon TNF-alpha-induced DC maturation. Thus, these data suggest a new mechanism of opioid-dependent neuroendocrine immunomodulation.

Transcriptional regulation of the human mu opioid receptor (hMOR) gene: evidence of positive and negative cis-acting elements in the proximal promoter and presence of a distal promoter

The mu opioid receptor (MOR), the primary binding site for morphine, is an important target for treating pain and drug addiction. The MOR gene is tightly regulated at the level of transcription, and potential polymorphisms in its 5' regulatory region can cause individual variation in MOR gene expression, nociception, and opiate responses. To study the 5' regulatory region of the human MOR gene (hMOR), we further investigated our previous finding of two regulatory regions and have localized a 40-bp positive cis-acting element and a 35-bp negative cis-acting element that regulate hMOR transcription in SK-N-SH cells. Electromobility shift assays and methylation interference assay with the 40-bp probe suggested that protein contacts were made with the core recognition sequence GCC (-510 to -508). The 35-bp sequence (-694 to -660) was the hMOR homolog of the mMOR negative regulatory element, and it suppressed proximal promoter activity of the hMOR gene. Additionally, the presence of an hMOR distal promoter was confirmed using RT-PCR. However, the activity of the distal promoter construct (-2325 to -777) was weak compared with the activity of the proximal promoter construct (-776 to -212).

Differential transcriptional regulation of rat vasopressin gene expression by estrogen receptor alpha and beta

Neuronal expression of vasopressin messenger RNA (mRNA) and peptide has been shown to be estrogen dependent. A 5.5-kb genomic DNA fragment, 5' of the AVP coding region, was used in luciferase reporter assays to measure transcriptional activation by either estrogen receptor alpha or beta in response to various treatments. ER alpha and ER beta displayed differential regulation of the AVP promoter. SK-N-SH cells transfected with ER alpha exhibited increased luciferase activity in response to estrogen, and the selective estrogen receptor modulators (SERMs), Tamoxifen, and ICI 182,780. Cells transfected with ER beta exhibited a high constitutive activity, which is unchanged by exposure to SERMs but can be inhibited by estrogen. Deletion of 1.5 kb from the 5' end or mutation of a single estrogen response element (ERE)-like sequence resulted in loss of estrogen-dependent induction by ER alpha and increased the ability of estrogen to inhibit the high constitutive activity of ER beta. The distal ERE-containing 1.5-kb fragment, when coupled to luciferase, is able to support both ER alpha and ER beta mediated activation of transcription by estrogen. These results suggest that a single ERE in the distal 1.5-kb portion of the 5.5-kb fragment contains the primary positive estrogen responsive sequences for ER alpha and ER beta. The data also suggest that sequences proximal to this element serve to inhibit transcription mediated by ER beta.

Effects of kappa- and mu-opioid receptor agonists on Ca2+ channels in neuroblastoma cells: involvement of the orphan opioid receptor

The effects of micro-, delta- and kappa-opioid receptor agonists, and orphanin FQ/nociceptin (Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln), on K+-induced [Ca2+]i increase were examined in SK-N-SH cells. Exposure to K+ (50 mM) resulted in a [Ca2+]i rise, which was blocked (-85%) by furaldipine (1 microM) and increased (63%) by BayK 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethyl-pyridine-5 -carboxylate) (0.5 microM), indicating the involvement of L-type Ca2+ channels. The kappa-opioid receptor agonists 3,4-dichloro-N-Methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U-50488H) (1-50 microM) and 5,7,8-N-Methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4,5]dec-8-yl]benze neacetamide (U-69593) (25 microM), and the mu-opioid receptor agonist sufentanil (100 nM-3 microM) inhibited the amplitude of K+-induced [Ca2+]i increase. The agonist of the orphan opioid receptor, orphanin FQ/nociceptin (1 microM), induced dual excitatory and inhibitory effects on the depolarisation-induced Ca2+ influx. The effects of the opioid receptor agonists were not blocked by the kappa-opioid receptor antagonist nor-binaltorphimine (1 microM), only weakly prevented by naloxone (10-100 microM) and naltrexone (100 microM), and partially prevented by pertussis toxin (100 ng/ml, 24 h). The antagonist of the orphan opioid receptor, [Phe1psi(CH2-NH)Gly2]nociceptin(1-13)NH2 (1 microM), prevented the inhibitory effect of U-50488H, sufentanil and orphanin FQ. The present study provides pharmacological evidence for the presence of L-type Ca2+ channels in SK-N-SH cells, that are modulated by opioids through orphan opioid receptor activation.

Dissociation between the inhibitory and stimulatory effects of opioid peptides on cAMP formation in SK-N-SH neuroblastoma cells

Opioid agonists either potentiate or suppress basal cAMP production in SK-N-SH cells. The inhibitory effect is mediated by PTX-sensitive GTP-binding proteins, while the stimulatory effect involves Ca++ entry and calmodulin activation. Both pathways can be activated simultaneously by opioid agonists. Low (nM) concentrations of either mu (DAMGO) or delta (DPDPE) selective opioids potentiate cAMP formation. At higher (100 nM) concentrations, however, a net suppression takes over; this suppression can be eliminated by PTX, and the underlying stimulatory effect is disclosed. Micromolar concentrations of either mu or delta selective agonists cross-activate the other (delta or mu) receptors, and augment the stimulatory pathway. The overall outcome (either stimulation or inhibition of cAMP production) is dependent on the balance between the two overlapping pathways, and can be modified by blocking either of the two opposing mechanisms.

In vivo homologous regulation of mu-opioid receptor gene expression in the mouse

Regulation of the mu-opioid receptor gene by opioid analgesic drugs has not been observed in rats and mice following in vivo treatments that produce tolerance. Although in vivo heterologous regulation of mu-opioid receptor mRNA by non-opioid compounds has been reported, the failure to observe changes in mu-opioid receptor mRNA levels in vivo after treatment with opioid agonists raised the possibility that in vivo homologous regulation by agonists may not occur. Therefore, in the present study, the effect of a high intrinsic efficacy opioid receptor agonist on opioid receptor density, gene expression and tolerance was determined. Mice were infused with etorphine for 7 days using an osmotic minipump, then the pump was removed and studies conducted 16-168 h later. Etorphine (50-250 microg/kg/day) infusion produced significant dose-dependent tolerance to the analgesic (tailflick) effects of etorphine, as well as dose-dependent mu-opioid receptor downregulation in brain at 16 h following the end of the infusion. Mu-opioid receptor density returned to control levels over a 168 h period following the end of etorphine (250 microg/kg/day) infusion. Similarly, the magnitude of tolerance decreased over the same period. Evaluation of changes in brain mu-opioid receptor mRNA 16 h following etorphine infusion indicated that there was dose-dependent increase in steady-state levels, with no significant change in GAPDH mRNA. The increase in mu-opioid receptor mRNA was approximately 55-65% over control at the highest etorphine infusion dose. Mu-opioid receptor mRNA returned to control levels over a 168 h period following the end of etorphine (250 microg/kg/day) infusion. These data suggest that the increase in mu-opioid receptor mRNA following the termination of etorphine treatment may drive the recovery of mu-opioid receptors. These data are the first demonstration of in vivo homologous regulation of mu-opioid receptor gene expression in the mouse by an opioid receptor agonist that produces tolerance and receptor downregulation.

Transcription of the mouse mu-opioid receptor gene is regulated by two promoters

Two transcription initiation sites have been identified in the mouse mu-opioid receptor (MOR) gene at approximately -793 and -268 upstream of the translation start site. To test if the MOR gene contains two functional promoters, chimeric constructs of mouse MOR deletion fragments, fused to a luciferase reporter gene, were transiently transfected into the neuroblastoma cell line SK-N-SH, a MOR-expressing cell line, and two MOR-non-expressing cell lines. Results from transient transfection assays confirmed the existence of two functional independent promoters in the mouse MOR gene, and also revealed that the region from -1337 to -93 does not contain all the elements necessary to confer tissue-specific expression of the MOR gene.

Effects of epidermal growth factor on the [3H]-thymidine uptake in the SK-N-SH and SH-SY5Y human neuroblastoma cell lines

The studies on the factors that regulate the biology of the neuroblastoma cell lines may offer important information on the development of tissues and organs that derive from the neural crest. In the present paper we study the action of epidermal growth factor (EGF) on two human neuroblastoma cell lines: SK-N-SH which is composed at least of two cellular phenotypes (neuroblastic and melanocytic/glial cells), and its pure neuroblastic subclone SH-SY5Y. The results show that EGF (10 ng/ml) significantly stimulates the incorporation of [3H]-thymidine in the SK-N-SH cells only in the presence of fetal bovine serum (FBS) (control = 58,285 +/- 9327 cpm; EGF = 75,523 +/- 4457, p < 0.05). Such effect is not observed in the presence of a chemical defined medium, that is, in the absence of FBS (control = 100,997 +/- 4375; EGF = 95,268 +/- 4683; NS) In the SH-SY5Y cells the EGF does not modify the incorporation of [3H]-thymidine either in the presence of 10% of BFS (control = 113,838 +/- 6978; EGF = 119,434 +/- 9441; NS) or in its absence (control = 46,197 +/- 3335; EGF = 44,472 +/- 3493; NS). The results here reported suggest that: a) EGF may affect the proliferation of cells derived from a primary human neuroblastoma; b) this is evident by the EGF-induced increase of [3H]-thymidine incorporation in SK-N-SH cells; c) it is required the presence of other growth factors, present in the FBS, for the mitogenic action to be accomplished; d) since the pure neuroblastic SH-SY5Y cell line are refractory to the EGF, the effects observed in SK-N-SH cells probably occur on the melanocytic/glial cell subpopulation.

Opioids potentiate transmitter release from SK-N-SH human neuroblastoma cells by modulating N-type calcium channels

Opioids induce dual (inhibitory and excitatory) regulation of depolarization-evoked [3H]dopamine release in SK-N-SH cells through either mu or delta receptors. The potentiation of dopamine release by opioid agonists is mediated by N-type voltage-dependent calcium channels and does not involve Gi/Go proteins. Removal of the excitatory opioid effect by blockade with omega-conotoxin, an N-channel antagonist, reveals the inhibitory effect of opioids on release, thus suggesting that both modulatory effects of opioids are exerted in parallel.

Gx/z is regulated by mu but not delta opioid receptors in the stimulation of the low Km GTPase activity in mouse periaqueductal grey matter

High affinity low K(m) GTPase activity was measured in membrane preparations of adult mouse mesencephalic periaqueductal grey matter (PAG). Opioids displaying selectivity towards mu- or delta-opioid receptors (OR) activated the enzyme in a concentration-dependent manner. Antibodies to mu-OR greatly impaired the potential of mu-agonists, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and morphine, to increase hydrolysis of GTP. The same antibodies had little effect on [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II, both agonists at delta-OR. Stimulation of GTPase by DPDPE and [D-Ala2]deltorphin II - but not by morphine or DAMGO - was diminished by antibodies to delta-OR. The blockade of G(i2)alpha subunits by specific antibodies impaired the activation of G alpha-related GTPase by all opioids. Antibodies in vitro, and oligodeoxynucleotides in vivo, prepared against Gx/z alpha subunits, reduced the release of Pi promoted by DAMGO and morphine. The impairment of Gx/z proteins also slightly reduced the effect of the delta2 agonist [D-Ala2]deltorphin II. At delta1 receptors, DPDPE fully expressed its activation of GTPase. These results indicate that in the PAG, mu-OR and delta-OR couple with Gi2 transducer proteins. Notably, mu-OR also regulates the pertussis toxin-insensitive G-protein Gx/z, an effect poorly exhibited by delta-OR in this tissue.

G proteins and opioid receptor-mediated signalling

Most opioid receptor-mediated functions appear to be mediated through G protein interactions, therefore an understanding of opioid signalling requires knowledge of those interactions. This review chronicles the studies examining these interactions for all the opioid receptor subtypes, both in vivo and in vitro.

Characterization of the delta-opioid receptor found in SH-SY5Y neuroblastoma cells

The delta-opioid receptor found in SH-SY5Y cells was characterized in terms of binding profile and ability to mediate the inhibition of forskolin-stimulated cAMP accumulation. Both DPDPE ([D-Pen2,D-Pen5]enkephalin) and deltorphin II, compounds reported to be selective for the delta 1- and delta 2-opioid receptor respectively, were potent agonists in these cells. Binding studies indicated that naltrindole benzofuran (NTB) had significantly higher affinity than 7-benzylidenenaltrexone (BNTX); however, both compounds have high affinity for the delta-opioid receptor found in SH-SY5Y cells. Naltrindole benzofuran was found to be a potent antagonist, with an IC50 of less than 1 nM, while 7-benzylidene naltrexone was found to be a relatively weak antagonist, requiring greater than 100 nM to inhibit 50% of agonist activity. Binding to intact SH-SY5Y cells was compared to binding to cell membranes and guinea-pig brain membranes. In each case, binding affinities were very similar. These studies suggest that the receptor found in SH-SY5Y cells could probably be classified as a delta 2-opioid receptor. However, the very similar binding characteristics of SH-SY5Y cells and guinea-pig brain membranes call into question the ability to label delta 1-opioid receptors.

Luteinizing hormone-releasing hormone (LHRH) attenuates morphine-induced inhibition of cyclic AMP (cAMP) in opioid-responsive SK-N-SH cells

SK-N-SH cells were used to assess the effects of luteinizing hormone-releasing hormone (LHRH) on opioid receptor-mediated changes in cyclic AMP (cAMP). Prostaglandin E1 (PGE1, 1 microM) caused a dramatic increase in cAMP levels. Treatment with 10 microM morphine (MOR) significantly inhibited the stimulatory effect of PGE1, LHRH (0.8 microM) caused an increase in the basal level of intracellular cAMP and potentiated the stimulatory effect of PGE1 on cAMP accumulation. In cells pretreated with LHRH the inhibitory effect of MOR on cAMP accumulation was significantly attenuated. An LHRH antagonist had no effect on cAMP. The involvement of pertussis toxin (PTX)-sensitive G proteins in the actions of LHRH was studied. PTX increased the stimulatory effect of PGE1 on cAMP and attenuated the inhibitory effect of MOR. However, PTX pretreatment prevented the effects of LHRH on the intracellular actions of PGE1 but exerted an additive effect with LHRH in blocking the MOR-induced decrease in cAMP levels. We conclude that LHRH attenuates the inhibitory, opioid receptor-mediated effect of MOR on intracellular cAMP accumulation in SK-N-SH cells, and that the G protein-independent mechanism may be involved in LHRH-induced attenuation of the inhibitory effect of MOR on neuronal cAMP.

Hormone-defined cell system for studying G-protein coupled receptor agonist-activated growth modulation: delta-opioid and serotonin-5HT2C receptor activation show opposite mitogenic effects

G-protein-coupled receptor (GPCR) agonist-activated transformation of NIH/3T3 fibroblast cells has been documented by many workers. Our present interest is in the growth control exerted by these agonists. The mechanisms involved in GPCR agonist-activated growth regulation are not known and investigations using existing cell lines are complicated by the endogenous expression of numerous different GPCRs as well as by the fact that these cell lines are cultured in serum that contains naturally occurring agonists for these receptors. To study the agonist induced growth response of cells transfected with either delta-opioid or serotonin-5HT2C neurotransmitter receptor genes, we have developed new clonal cell lines derived from NIH/3T3 mouse fibroblast cells. These new cell lines, designated with the suffix 3T3DA, can be cultured stably in serum-free, hormone-defined medium: insulin is the only exogenous growth factor added to the culture medium of proliferating 3T3DA cell lines, and their proliferation can be stopped and started by the respective removal or addition of insulin. Micromolar concentrations of agonists were used to activate the corresponding opioid and serotonin receptors over periods extending to 6 days. We observed distinct patterns of GPCR-specific, agonist-activated growth regulation in serum-free cultures, but not in serum-supplemented cultures. At concentrations > 10 microM, morphine inhibits growth of delta-opioid receptor-expressing cells by 40% with respect to normal 3T3DA cells. Opioid agonist induced inhibition of cyclic AMP (cAMP) production as well as growth down-regulation are pertussis toxin sensitive indicating that the exogenously expressed delta-opioid receptors demonstrate classical opioid receptor signaling. The presence of 1 microM serotonin stimulates growth of serotonin-5HT2C receptor- expressing cells by approximately 100% with respect to normal 3T3DA cells. Neither the untreated nor the agonist-treated cells form colonies in soft agar, indicating that they retain anchorage-dependent growth control. These cell lines provide a simple system that could be used as a tool for probing the complex molecular mechanisms associated with GPCR agonist-activated growth control.

1-(5-Isoquinolinesulfonyl)-2-methylpiperazine induces apoptosis in human neuroblastoma cells, SH-SY5Y, through a p53-dependent pathway

We have studied the effect of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), a protein kinase inhibitor, on the regulation of apoptosis in the human neuroblastoma cell line, SH-SY5Y. H-7 (20-100 microM) induced apoptosis in these cells characterized by DNA fragmentation and chromatin condensation. Immunoblot analyses were performed with specific antibody against BCL-2, BCL-XS/L, BAX, JUNB, c-JUN, ICH-1L, c-FOS, RB, CDK-2, and p53. H-7 treatment did not significantly alter the level of these proteins with the exception of p53. H-7, but not staurosporine, caused a dramatic nuclear accumulation of p53. The kinetics of nuclear accumulation of p53 correlates well with the kinetics of induction of apoptosis. The effect of H-7 was further assessed in a group of human cell lines. Only cell lines harboring the wild-type p53 gene were responsive to the stimulatory effect of H-7 on nuclear accumulation of p53. Furthermore, cell lines carrying a mutated p53 gene were resistant to the cytotoxic effect of H-7. The ability of H-7 in mediating apoptosis in the SH-SY5Y line expressing a dominant negative mutant of p53 was significantly diminished. Taken together, these data strongly suggest that a p53-dependent mechanism contributes to the cytotoxicity of H-7 in human neuroblastoma cells.

The stimulatory effect of opioids on cyclic AMP production in SK-N-SH cells is mediated by calcium ions

The present study examines the stimulatory effect of opioids on adenosine 3':5'-cyclic monophosphate (cyclic AMP) production in the human neuroblastoma cell line SK-N-SH, and its dependence on calcium. We show that, in this culture, the mu-opioid selective agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-Enkephalin stimulates cyclic AMP production by 30% in a naloxone-reversible manner. This stimulation is completely dependent on calcium and involves the activation of calcium/calmodulin since it is abolished in the presence of EGTA, calcium channel blockers or N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7). The results suggest that the activation of calcium/calmodulin dependent adenylyl cyclases by opioids in SK-N-SH cells is secondary to the induction of calcium influx and the consequent elevation of intracellular calcium level.

mu-Opioid receptor-stimulated guanosine-5'-O-(gamma-thio)-triphosphate binding in rat thalamus and cultured cell lines: signal transduction mechanisms underlying agonist efficacy

G protein activation by different mu-selective opioid agonists was examined in rat thalamus, SK-N-SH cells, and mu-opioid receptor-transfected mMOR-CHO cells using agonist-stimulated guanosine-5'-O-(gamma-thio)-triphosphate ([35S]GTP gamma S) binding to membranes in the presence of excess GDP. [D-Ala2, N-MePhe4, Gly5-ol]Enkephalin (DAMGO) was the most efficacious agonist in rat thalamus and SK-N-SH cells, followed by (in rank order) fentanyl = morphine > > buprenorphine. In mMOR-CHO cells expressing a high density of mu receptors, no differences were observed among DAMGO, morphine or fentanyl, but these agonists were more efficacious than buprenorphine, which was more efficacious than levallorphan. In all three systems, efficacy differences were magnified by increasing GDP concentrations, indicating that the activity state of G proteins can affect agonist efficacy. Scatchard analysis of net agon stimulated [35S]GTP gamma S binding revealed two major components responsible for agonist efficacy differences. First, differences in the KD values of agonist-stimulated [35S]GTP gamma S binding between high efficacy agonists (DAMGO, fentanyl, and morphine) and classic partial agonists (buprenorphine and levallorphan) were observed in all three systems. Second, differences in the Bmax value of agonist-stimulated [35S]GTP gamma S binding were observed between DAMGO and morphine or fentanyl in rat thalamus and SK-N-SH cells and between the high efficacy agonists and buprenorphine or levallorphan in all three systems. These results suggest that mu-opioid agonist efficacy is determined by the magnitude of the receptor-mediated affinity shift in the binding of GTP (or[35S]GTP gamma S) versus GDP to the G protein and by the number of G proteins activated per occupied receptor.

Expression of mu-, delta- and kappa-opioid receptors in baculovirus-infected insect cells

The mu-, delta- and kappa-opioid receptors have been expressed in Sf9 and 'High Five' insect cells using the baculovirus expression system. In both cell lines highest receptor levels (pmol/mg membrane protein) were observed 48 h after infection. Concomitant exposure to the narcotic antagonist naloxone (1 microM) enhanced the production of each receptor type. However, "High Five' cells differed from Sf9 cells in a 2-3-fold higher receptor density in the cell membrane and were therefore employed for receptor characterization. In membranes of 'High Five' cells opioid receptor levels ranged from 1.0 +/- 0.2 pmol/mg protein for the kappa-opioid receptor, 1.7 +/- 0.2 pmol/mg for the delta-opioid receptor to 2.1 +/- 0.5 pmol/mg for the mu-opioid receptor. The mu-, delta- and kappa-opioid receptor agonists [D-Ala2,N-methyl-Phe4-Gly-ol5]enkephalin ([3H]DAMGO), [D-Pen2,D-Pen5]enkephalin ([3H]DPDPE) and (5 alpha, 7 alpha, 8 beta)-(+)-N-methyl-N-(7-(1-pyrrolidinyl-1-oxaspiro(4,5)dec-8-yl) benzeneacetamide ([3H]U69,563) bound to the opioid receptors with Kd values of 3.4 +/- 0.3 nM, 4.5 +/- 0.1 nM and 1.2 +/- 0.3 nM, respectively, resembling those reported for opioid receptors expressed in mammalian cells. Testing the functionality of the receptors in 'High Five' cells, we found that high affinity agonist binding was strongly reduced in the presence of GTP gamma S/sodium, indicating their coupling to G proteins. Furthermore, activation of the three receptor types inhibited forskolin-stimulated cAMP formation. The results presented here suggest that the 'High Five' cell/baculovirus system provides a convenient method for high level expression of functionally intact opioid receptors as judged by receptor binding studies, their G-protein coupling and inhibition of adenylyl cyclase.

Stimulatory effects of opioids on transmitter release and possible cellular mechanisms: overview and original results

Opiates and opioid peptides carry out their regulatory effects mainly by inhibiting neuronal activity. At the cellular level, opioids block voltage-dependent calcium channels, activate potassium channels and inhibit adenylate cyclase, thus reducing neurotransmitter release. An increasing body of evidence indicates an additional opposite, stimulatory activity of opioids. The present review summarizes the potentiating effects of opioids on transmitter release and the possible cellular events underlying this potentiation: elevation of cytosolic calcium level (by either activating Ca2+ influx or mobilizing intracellular stores), blockage of K+ channels and stimulation of adenylate cyclase. Biochemical, pharmacological and molecular biology studies suggest several molecular mechanisms of the bimodal activity of opioids, including the coupling of opioid receptors to various GTP-binding proteins, the involvement of different subunits of these proteins, and the activation of several intracellular signal transduction pathways. Among the many experimental preparations used to study the bimodal opioid activity, the SK-N-SH neuroblastoma cell line is presented here as a suitable model for studying the complete chain of events leading from binding to receptors down to regulation of transmitter release, and for elucidating the molecular mechanism involved in the stimulatory effects of opioid agonists.

Stable expression and functional characterization of the cloned rat mu-opioid receptor in human epidermoid carcinoma (A431) cells

Regulation of intracellular cAMP levels serves as a cellular model for chronic drug action. Since the adenylate cyclase effector system is under dual control of both stimulatory as well as inhibitory receptor systems, a permanent cell line was created in order to allow evaluation of acute and chronic opioid effects on stimulatory receptor function. For this purpose, the cloned rat mu-opioid receptor was stably expressed in human epidermoid carcinoma (A431) cells, which carries high levels of endogenous beta 2-adrenoceptors. Four out of 16 cell clones were found to express considerable amounts of [3H]diprenorphine binding sites and were further characterized. Scatchard analysis of saturation binding data revealed maximal binding capacities (Bmax) between 242.2 +/- 11 and 1,271.8 +/- 221 fmol/mg of membrane protein, whereas drug affinity was found similar among all cell clones tested (Kd = 1.4 +/- 0.2 nM). The expressed mu-receptors also mediated agonist inhibition of adenylate cyclase, indicating that these receptors are functionally coupled to intracellular signalling pathways. Long-term exposure of the cells to morphine (10 microM; 2 days) produced cellular correlates of chronic opioid action as displayed by both a decrease in the maximal degree of adenylate cyclase inhibition (tolerance) as well as an increase in overall effector activity (dependence). Thus, based on these parameters, mu-opioid receptor expressing A431 cells provide a promising tool to investigate cellular mechanisms of chronic drug action.

Antiprogestins inhibit the binding of opioids to mu-opioid receptors in nervous membrane preparations

The present study showed that the glucocorticoid/progesterone antagonists, 17 beta-hydroxy-1 1 beta-(4-dimethylamino-phenyl-1)-17-(prop-1-ynyl)estra-4,9-dien+ ++-3-one (RU486) and 17 beta-hydroxy-11 beta-(4-dimethylamino-phenyl-1)-17-(propan-3-ol)estra-4,9-dien-3-o ne (ZK 98299), inhibit the binding of labeled dihydromorphine to mu-opioid receptors present on membrane preparations derived from rat and mouse brain, as well as from human neuroblastoma cells. The inhibitory effect of RU486 was dose-dependent and linked to a decrease of the affinity of labeled dihydromorphine to the mu-opioid receptors. Kinetic experiments have shown that RU486 induces a decrease of the association rate constant (k + 1) of dihydromorphine. RU486 also proved able to dissociate the dihydromorphine-mu-opioid receptor complex, although at a rate slower than that exhibited by unlabeled dihydromorphine. Finally, the addition of NaCl (100 mM) to the incubation buffer induced a 50% decrease of the inhibitory effect of RU486. A 6-day treatment of neuroblastoma cells with RU486 eliminated the inhibitory effect morphine exerts on the intracellular accumulation of cyclic AMP induced by prostaglandin E1. These results indicate that RU-486 may interact with brain mu-opioid receptors in vitro, by decreasing the affinity of opioid ligands.

Luteinizing hormone-releasing hormone in undifferentiated and differentiated SK-N-SH human neuroblastoma cells

The presence of luteinizing hormone-releasing hormone (LHRH) in SK-N-SH human neuroblastoma cells was investigated by immunocytochemistry and enzyme-linked immunosorbent assays of whole cell extracts and culture medium. In addition, ribonuclease protection assays were utilized to quantitate LHRH messenger RNA. The expression of LHRH mRNA and LHRH protein level was correlated with neuronal differentiation induced by retinoic acid (RA). In differentiated SK-N-SH cells the LHRH mRNA level as well as the amount of LHRH in cell extracts and cell medium were significantly lower than in differentiated cells. These results suggest that RA affects the expression of LHRH mRNA and the level of LHRH protein in SK-N-SH cells. These data show that altering the growth state of the human neuroblastoma SK-N-SH cells toward more neuronal phenotype results in a significant decrease in expression of LHRH mRNA and the protein. The ability of RA to induce changes in LHRH at the mRNA and at the peptide levels will allow further study of RA regulation of LHRH at the neuronal level.

Effect of low pH treatment on opioid peptides binding to their receptors and functional coupling of G-proteins to adenylyl cyclase in the rat spinal cord

Because low pH treatment is known to alter the coupling of G-proteins to brain receptors, and little is known about such an effect in the spinal cord, the present study was undertaken to examine whether preincubation of rat spinal cord membranes at low pH (pH 4.5) alters opioid receptor binding characteristics and sodium fluoride (NaF)-stimulated adenylyl cyclase (AC) activity (as a function of G, mediated). [3H][D-Ala2,MePhe4,Gly-ol]enkephalin (DAMGO) and [3H]ethylketocyclazosine (EKC) were used to label mu- and kappa-opioid receptors, respectively. AC activity was determined using ATP as substrate and cAMP formed was quantified. Low pH treatment of membranes did not affect the mu- and kappa-opioid binding characteristics in rat spinal cord. However, the low pH treatment significantly reduced the NaF-stimulated AC activity in rat spinal cord. It is concluded that low pH treatment causes selective changes in the functional coupling of Gs-proteins to AC without affecting the opioid receptor binding characteristics in the spinal cord.

Molecular biology of the opioid receptors: structures, functions and distributions

Opiates like morphine and endogenous opioid peptides exert their pharmacological and physiological effects through binding to their endogenous receptors, opioid receptors. The opioid receptors are classified into at least three types, mu-, delta- and kappa-types. Recently, cDNAs of the opioid receptors have been cloned and have greatly advanced our understanding of their structure, function and expression. This review focuses on the recent advances in the studies on opioid receptors using the cloned cDNAs. We describe the molecular cloning of the opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotide and anatomical distributions of opioid receptors.

Multiple effects of opiates on intracellular calcium level and on calcium uptake in three neuronal cell lines

The present study examines the modulation by opiates of intracellular calcium levels and calcium entry, using fura-2 imaging and 45Ca2+ uptake, in three neuronal cell lines. We show that opiates (10(-7)-10(-5) M morphine and 10(-9)-10(-7) M etorphine) exert both inhibitory and excitatory effects on KCl-induced elevation in intracellular calcium level in SK-N-SH, NG108-15 and NMB cell lines. In addition, opiates elevate basal (non KCl-stimulated) intracellular calcium level in all three cell cultures. 45Ca2+ uptake is augmented by opiates in SK-N-SH cells and this stimulatory effect is not blocked by pertussis toxin. In NMB cells, an additional inhibitory effect of opiates on basal calcium takes place: opiates reduce intracellular calcium level as measured by fura-2, and decrease calcium influx as detected by 45Ca2+ uptake. The heterogeneity in the opioid regulation of calcium could not be attributed to the type of opioid drug, neither to its concentration nor to the experimental conditions, since neighboring cells within the same culture responded differently.

Cloning and characterization of the promoter region of the mouse mu opioid receptor gene

Opioid compounds have potent analgesic and euphoric properties. They act with specific cell-membrane receptors which have been pharmacologically defined into three major classes, mu, kappa and delta. These receptors are highly regulated with respect to their gene expression, resulting in a temporally and spatially specific pattern of distribution for each receptor. To characterize the promoter sequence of the mu opioid receptor (MOR) gene, a mouse genomic DNA library was screened under high stringency with a rat MOR (MOR-1) cDNA probe and genomic sequences for the mouse MOR gene were isolated. From one genomic clone, a 2.3-kb EcoRI fragment, which hybridized to the 5'-end of the rat MOR-1 cDNA probe, was subcloned and sequenced. This fragment contains 1.3 kb of sequence upstream of the initiation codon, extends downstream through exon 1 and includes a portion of intron 1. Primer extension analysis using mouse brain poly (A)+ RNA identified a transcription initiation site 793 bp upstream from the translation start site. Chimeric constructs of mouse MOR deletion fragments fused to a luciferase reporter gene were transfected into a human neuroblastoma cell line, SK-N-SH, which constitutively expresses endogenous MOR. These transient expression studies indicated that the 0.2-kb region upstream from the transcription initiation site possesses a functional promoter, which directs the expression of the reporter gene in vitro and may possess promoter activity for the mouse MOR gene in vivo.

Molecular pharmacology of the opioid receptors

Opioid receptors are the primary sites of actions of opiates and endogenous opioid peptides, which have a wide variety of pharmacological and physiological effects. The opioid receptors are classified into at least three subtypes, mu, delta, and kappa, and their cDNAs have been cloned. In this review, we describe the molecular cloning of opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotides, and anatomical distribution of opioid receptor mRNAs.

Characterization of a triple opioid system in the human neuroblastoma NMB cell line

The human neuroblastoma NMB cell line was found to contain the three types of opioid receptors (60% delta 25% kappa and 15% mu). The opioid receptors were negatively coupled to adenylyl-cyclase. Maximal reduction in cAMP content was achieved by selectively activating single receptor types, indicating the co-presence of the various opioid receptors in the same cells. The opioid receptors in NMB cells were up-regulated following prolonged exposure to the opioid antagonist naloxone and down-regulated following chronic treatment with the opioid agonist etorphine. Down-regulation was time-, dose- and temperature-dependent and was inhibited by colchicine and sodium azide. The NMB culture is presented as an excellent experimental model for studying the selective activation and regulation of the different opioid receptor types when they are co-expressed in the same neuron, as well as for studying interactions between the various opioid receptors.