The porcine mu opioid receptor: molecular cloning and mRNA distribution in lymphoid tissues

Up-regulation of μ-, δ- and κ-opioid receptors in concanavalin A-stimulated rat spleen lymphocytes

Regulation of μ-, δ- and κ-opioid receptor protein level in spleen lymphocytes when stimulated by mitogen is not known. To answer the question whether these cells do express opioid receptor (OR) proteins, primary, fresh rat spleen lymphocytes were prepared and stimulated for 48 h with mitogenic dose of Con A. The unstimulated lymphocytes did not express μ- and δ-OR proteins in detectable amounts, however, stimulation with Con A resulted in appearance of clearly detectable immunoblot signals of both μ-OR and δ-OR. κ-OR were detected already in primary cells and increased 2.4-fold in Con A-stimulated cells. These results were supported by data obtained by flow cytometry analysis indicating a dramatic increase in number of μ-, δ- and κ-OR expressing cells after mitogen stimulation. The newly synthesized μ-, δ- and κ-OR in Con A-stimulated spleen lymphocytes were present in the cells interior and not functionally mature, at least in terms of their ability to enhance activity of trimeric G proteins determined by three different protocols of agonist-stimulated, high-affinity [³⁵S]GTPγS binding assay. The up-regulation of μ-, δ- and κ-OR was associated with specific decrease of their cognate trimeric G proteins, G_i1α/G_i2α; the other Gα and Gβ subunits were unchanged. The level of β-arrestin-1/2 was also decreased in Con A-stimulated splenocytes. We conclude that up-regulation of OR expression level in spleen lymphocytes by Con A proceeds in conjunction with down-regulation of their intracellular signaling partners, G_i1α/G_i2α proteins and β-arrestin-1/2. These regulatory proteins are expressed in high amounts already in unstimulated cells and decreased by mitogen stimulation.

Opioid receptors: methods for detection and their modes of actions in the eye

This study provides evidence for the presence of opioid-receptors in the retina, optic nerve, and optic nerve head astrocytes. These receptors were measured by more than one technique including Western blotting, immunohistochemistry, and functional assays such as scotopic electroretinogram (ERG) and Pattern ERG. I also have provided evidence in recently published work from my laboratory that opioid receptors, more specifically δ-opioid receptors, play crucial roles in retina neuroprotection against ischemic and glaucomatous injuries. This chapter provides detailed procedures to measure opioid receptor activation and their roles in retina neuroprotection using functional assays such as scotopic ERG and pattern ERG.

Pharmacogenomics of the human µ-opioid receptor

The µ-opioid receptor is a primary target for clinically important opioid analgesics, including morphine, fentanyl and methadone. Many genetic variations have been identified in the human µ-opioid receptor MOP gene (OPRM1), and their implications have been reported in the effects of opioid drugs and susceptibility to drug dependence. Interestingly, agonistic and antagonistic opioid effects are inversely associated with the A118G polymorphism genotype. The A118G polymorphism may also be associated with substance dependence and susceptibility to other disorders, including epilepsy and schizophrenia. The IVS1+A21573G, IVS1-T17286C, and TAA+A5359G polymorphisms in the OPRM1 gene may be associated with alcohol, opioid and tobacco dependence, respectively. However, some studies have failed to confirm the correlations between the polymorphisms and opioid effects and substance dependence. Further studies are needed to elucidate the molecular mechanisms underlying the effects of OPRM1 polymorphisms.

Human peripheral blood mononuclear cells express nociceptin/orphanin FQ, but not mu, delta, or kappa opioid receptors

BACKGROUND

Expression of opioid receptors on peripheral blood mononuclear cells (PBMC) is controversial. These receptors are currently classified as classical (MOP/mu/mu, DOP/delta/delta and KOP/kappa/kappa) and nonclassical NOP (nociceptin/orphanin FQ; N/OFQ).

METHODS

In this volunteer study we probed for the expression of both classical and nonclassical opioid receptors using 1) radioligand binding, 2) specific antibody binding, and 3) polymerase chain reaction-based experimental paradigms.

RESULTS

Membranes prepared from PBMC from healthy volunteers did not bind either [3H]diprenorphine (a nonselective radioligand for classical opioid receptors) or [3H]N/OFQ. There was significant concentration-dependent binding of each radioligand to control tissues expressing recombinant MOP and NOP. In addition, using fluorescence-activated cell sorting paradigms, there was no binding of fluorescent naloxone or either of two MOP antibodies to whole PBMC, though fluorescent naloxone did bind to recombinant MOP (as a positive control). Using primers specific for classical and nonclassical opioid receptors, and RNA extracted from the PBMC of 10 healthy volunteers, we were also unable to detect MOP, DOP, and KOP transcripts. In contrast, NOP was detected in all samples.

CONCLUSIONS

Despite using several complementary experimental strategies, we failed to demonstrate protein for classical or nonclassical opioid receptors on PBMC from healthy volunteers. We detected NOP mRNA, suggesting low-density NOP expression on these immunocytes. It is possible that N/OFQ, produced by the PBMC itself, may be involved in the control of immune function.

Delta and mu opioid receptors from the brain of a urodele amphibian, the rough-skinned newt Taricha granulosa: cloning, heterologous expression, and pharmacological characterization

Two full-length cDNAs, encoding delta (delta) and mu (mu) opioid receptors, were cloned from the brain of the rough-skinned newt Taricha granulosa, complementing previous work from our laboratory describing the cloning of newt brain kappa (kappa) and ORL1 opioid receptors. The newt delta receptor shares 82% amino acid sequence identity with a frog delta receptor and lower (68-70%) identity with orthologous receptors cloned from mammals and zebrafish. The newt mu receptor shares 79% sequence identity with a frog mu receptor, 72% identity with mammalian mu receptors, and 66-69% identity with mu receptors cloned from teleost fishes. Membranes isolated from COS-7 cells transiently expressing the newt delta receptor possessed a single, high-affinity (Kd = 2.4 nM) binding site for the nonselective opioid antagonist [3H]naloxone. In competition binding assays, the newt delta receptor displayed highest affinity for Met-enkephalin, relatively low affinity for Leu-enkephalin, beta-endorphin, and [D-penicillamine, D-penicillamine] enkephalin (DPDPE) (a delta-selective agonist in mammals), and very low affinity for mu-, kappa-, or ORL1-selective agonists. COS-7 cells expressing the newt mu receptor also possessed a high-affinity (Kd = 0.44 nM) naloxone-binding site that showed highest affinity for beta-endorphin, moderate-to-low affinity for Met-enkephalin and Leu-enkephalin and DAMGO (a mu-selective agonist in mammals), and very low affinity for DPDPE and kappa- or ORL1-selective agonists. COS-7 cells expressing either receptor type (delta or mu) showed very high affinity (Kd = 0.1-0.3 nM) for the nonselective opioid antagonist diprenorphine. Taricha granulosa expresses the same four subtypes (delta, mu, kappa, and ORL1) of opioid receptors found in other vertebrate classes, but ligand selectivity appears less stringent in the newt than has been documented in mammals.

Characterisation of the in vitro modulation of splenocyte proliferation by non-4,5-epoxymorphinan opioids

Opioids, such as morphine, can directly alter immune function via receptors expressed on immunocompetent cells. However, several studies have questioned the classical opioid nature of this change in immune response. Therefore, it is unclear how opioids that are not from the same structural class as morphine (4,5-epoxymorphinan), will modulate the immune system, if they do not behave in a classical opioid manner. Therefore, the aim of this study was to investigate the in vitro modulatory effects of a range of non-4,5-epoxymorphinan opioids on splenocyte proliferation and compare the response characteristics to their central opioid characteristics. The modulation of concanavalin A stimulated mouse splenocyte proliferation by non-4,5-epoxymorphinan opioids resulted in three types of responses: an inhibitory concentration-response curve (e.g. methadone, inhibitory EC(50)=79.4 microM), an inverted bell shaped curve (e.g. fentanyl, inhibitory EC(50)=0.06 microM) and an induction concentration response curve (e.g. nor-binaltorphimine, induction EC(50)=0.16 microM). Non-stereoselectivity, naloxone-insensitivity, naloxone-sensitivity and non-classical opioid rank order of effect were all observed. These data support the non-classical opioid nature of direct opioid modulation of splenocyte proliferation.

The potential promoter regions on the 5′ flank sequence of the mu opioid receptor gene in lymphocytes

The human mu opioid receptor is known to mediate a variety of physiological and pharmacological effects of morphine in many tissues. However, the molecular processes that regulate the expression of the mu opioid receptor gene in immune cells are not well understood. To study regulatory elements that affect the expression of the mu opioid receptor gene in human lymphocytes (LMOR), a 2,278 bp fragment of the 5' regulatory region of the mu opioid receptor gene was cloned and sequenced from CEM x174 cells. The transcriptional initiation site was mapped through a primer extension assay. A series of 5'-deleted plasmids were constructed and transiently transfected into cultured CEM x174 cells. The data indicated that morphine up-regulated the mRNA level of LMOR in a dose-dependent manner, which could be blocked by the opioid receptor antagonist naloxone. Only one transcription initiation site (TIS) about 110 bp upstream of the translation start codon was identified. The regions from -372 to -253 and -2279 to -1371 located in the 5' regulatory sequence of the mu opioid receptor gene contained enhancer elements, while the regions from -1371 to -968 and -650 to -370 possessed repressor elements. Those promoter elements were involved in the transcriptional regulation of the mu opioid receptor gene. Collectively, this data strongly indicates that the expression of the mu opioid receptor gene in lymphocytes is subject to the regulation of cis-elements upstream from the TIS.

Pharmacological characterization of a 7-benzylidenenaltrexone-preferring opioid receptor in porcine ileal submucosa

In the intestine, opioids produce antidiarrhoeal and constipating actions that are mediated by enteric neurones. Through interactions with opioid receptors (ORs) on submucosal neurones, opioids suppress active ion transport evoked by transmural electrical stimulation (TES) in mucosa-submucosa sheets from the porcine ileum. In this study, we examined the pharmacological characteristics of the previously described OR, which is sensitive to the delta1-OR antagonist 7-benzylidenenaltrexone and modulates neurogenic transepithelial ion transport in this tissue preparation. Increases in short-circuit current (Isc, a measure of active anion transport) evoked by TES in ileal mucosa-submucosa sheets were inhibited by opioid agonists possessing high selectivity for either delta- or micro-ORs including [d-Pen2,5]enkephalin (DPDPE), [d-Ala2, Glu4]deltorphin II, and [d-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO). As determined by the Schild analysis, the actions of these agonists were competitively inhibited by 7-benzylidenenaltrexone. The nonequilibrium micro-OR antagonist beta-funaltrexamine inhibited the actions of DAMGO only at a high concentration (1 microm) but did not alter DPDPE or deltorphin II action. At concentrations up to 10 microm, the nonequilibrium delta-OR antagonist naltrindole 5'-isothiocyanate did not alter the actions of delta- or micro-OR agonists. Radioligand binding analyses of neuronal homogenates from the ileal submucosa revealed that the nonselective OR ligand [3H]diprenorphine bound to two populations of specific binding sites. One of these sites possessed binding characteristics similar to the delta-OR. In summary, neurogenic ion transport in the porcine intestine is modulated by an OR which shares pharmacological characteristics of both micro- and delta-ORs and may represent a novel receptor entity.

Opioid receptors

Opioid receptors belong to the large superfamily of seven transmembrane-spanning (7TM) G protein-coupled receptors (GPCRs). As a class, GPCRs are of fundamental physiological importance mediating the actions of the majority of known neurotransmitters and hormones. Opioid receptors are particularly intriguing members of this receptor family. They are activated both by endogenously produced opioid peptides and by exogenously administered opiate compounds, some of which are not only among the most effective analgesics known but also highly addictive drugs of abuse. A fundamental question in addiction biology is why exogenous opioid drugs, such as morphine and heroin, have a high liability for inducing tolerance, dependence, and addiction. This review focuses on many aspects of opioid receptors with the aim of gaining a greater insight into mechanisms of opioid tolerance and dependence.

In vitro opioid induced proliferation of peripheral blood immune cells correlates with in vivo cold pressor pain tolerance in humans: a biological marker of pain tolerance

There is substantial evidence for bidirectional communication between the immune system and the central nervous system, as the cells and signalling molecules of the immune system influence many central nervous system functions, for instance nociception. Opioids, such as morphine, produce analgesia and numerous other central and peripheral effects including sedation and euphoria, while their effects on the immune system are wide-ranging. There is considerable interindividual variability in basal nociception and response to opioids, however, the physiological and biological mechanisms underlying this are unclear. Therefore, we investigated the relationship between the immune system and basal nociceptive thresholds, using the proliferative response of isolated peripheral blood mononuclear cells and cold pressor pain tolerance. Here we show that the percent increase in proliferation of peripheral immune cells from 13 healthy subjects incubated with morphine ex vivo is highly correlated with the subjects' tolerance to noxious cold stimuli (Pearson r = 0.92, P < 0.0001). These pilot data provide evidence of a novel objective biological marker of pain tolerance in humans, which also links the immune and opioid systems with basal pain tolerance.

The morphine-binding site on human activated T-cells is not related to the mu opioid receptor

Mitogen activation of human T-lymphocytes induces a morphine-binding site. Morphine binding is displaceable by beta-endorphin (1--31) and (--)-naloxone but not DAMGO. This site is not stereoselective for (--)-morphine. T-lymphocytes, expressing this binding site, were assayed by reverse-transcription polymerase chain reaction (RT-PCR) for expression of hMOR-1 mRNA. Several primer sets were used and each assay compared with cells known to express human or mouse MOR-1 mRNA. Neither hMOR-1 nor any homologous receptor was detected in human T-lymphocytes. Therefore, the morphine-binding site on mitogen-activated T-lymphocytes is unlikely to be closely related to hMOR-1.

Morphine upregulates mu opioid receptors of human and monkey lymphocytes

Opioid receptors of subtypes delta, kappa, and mu similar to those found in brain cells have been identified in immune cells. The current study demonstrates by competitive polymerase chain reaction the treatment of human lymphocytic cells with morphine resulting in an increased amount of gene expression of mu opioid receptors. Antibodies against the MOR-1, the neuronal mu opioid receptor, were used in Western blot analysis of mu proteins and the results revealed a single band of approximately 50 kDa, the intensity of which was increased by morphine treatment. Similar results of mu opioid receptor activation were observed when monkey lymphocytes were treated with morphine. These studies suggest that in addition to causing an immune effect through communication with the neuroendocrine system, the psychoactive drug morphine may modulate immune functions by acting directly on the mu opioid receptors expressed on lymphocytes.

Expression of nociceptin/OFQ receptor and prepro-nociceptin/OFQ in lymphoid tissues

This study was undertaken to examine the presence of functional nociceptin/orphanin FQ (N/OFQ) receptors in the immune system. Receptor mRNA signals were detected by RT-PCR in porcine thymus, lymph nodes, spleen and freshly-isolated splenocytes; the distribution of prepro-nociceptin/-orphanin FQ (PP-N/-OFQ) mRNA was similar, with the exception of lymph nodes. However, specific [(3)H]nociceptin binding sites were not detected in rat or porcine lymphoid tissues, and 0.1-100 nM nociceptin had no effect on forskolin-stimulated cyclic AMP concentrations in porcine splenocytes. Thus, it appears that nociceptin/orphanin FQ receptor mRNA, but not a functional receptor protein is expressed in the immune system.

The bovine mu-opioid receptor: cloning of cDNA and pharmacological characterization of the receptor expressed in mammalian cells

The cDNA coding for the bovine mu-opioid receptor has been cloned and sequenced. Conserved sequences from murine delta-receptor cDNA were used as primers in polymerase chain reaction (PCR) to amplify cDNA, prepared by reverse transcription of bovine brain mRNA. This cDNA was used to probe a bovine brain library. The partial sequence obtained was extended to provide the full length clone by PCR. The cDNA has an open reading frame of 1203 base pairs (bp) with a 3'-untranslated region of 1900 bp and a 5'-untranslated region of 265 bp. The protein contains 401 amino acids and has 94% amino acid identity with the human and 91% with the rat mu-opioid receptor. It has the putative seven transmembrane domains, characteristic of G protein-coupled receptors and contains 5 potential N-linked glycosylation sites near the N-terminus. Several potential phosphorylation sites and a putative palmitoylation site are also present. The receptor was stably expressed in HEK293 cells. The binding profile was found to be that of a typical mu receptor, i. e., mu agonists and antagonists, but not delta and kappa ligands, bound with high affinity. Functional assays, namely, opioid stimulation of [35S]GTPgammaS binding and inhibition of forskolin-activated adenylyl cyclase, were also found to be highly specific for mu-opioid agonists. The receptor was downregulated by chronic exposure to mu agonists but not delta or kappa agonists. Evidence is presented indicating that the cloned receptor is the same as the bovine mu receptor previously purified to homogeneity in our laboratory. No evidence was found for genes for multiple mu-type opioid receptors.