Cloning and functional comparison of kappa and delta opioid receptors from mouse brain

Chronic morphine treatment inhibits opioid receptor desensitization and internalization

Chronic opioid receptor (OR) activation by morphine causes distinct cellular adaptations responsible for the development of tolerance. The present study examines the effect of chronic morphine exposure on the ability of high-efficacy agonists to mediate delta-OR (DOR) and mu-OR (MOR) uncoupling and internalization, two regulatory mechanisms contributing to rapid desensitization of OR function. Chronic morphine treatment (1 microm; 72 hr) of DOR carrying neuroblastoma x glioma (NG108-15) hybrid cells, a prototypical model system frequently used to study cellular aspects of opioid tolerance, completely blocked the capacity of [d-Ala2, d-Leu5]enkephalin (DADLE) and etorphine to desensitize opioid-stimulated [35S]GTPgammaS binding and to mediate DOR internalization. Similar findings were obtained on stably DOR- and MOR-transfected human embryonic kidney (HEK) 293 cells. Chronic morphine treatment also heterologously impaired agonist regulation of non-opioid G-protein-coupled receptors, such as the m(4)-muscarinic acetylcholine receptor and the brain-type cannabinoid receptor. As a possible underlying mechanism, we found that chronic morphine treatment completely blocked agonist-induced redistribution of beta-arrestin1 in both NG108-15 and stably MOR-transfected HEK293 cells. Moreover, attenuation of beta-arrestin1 function appears to depend on persistent stimulation of MAP kinase activity during the course of chronic morphine treatment, because coincubation of the cells together with the MAP kinase blocker PD98059 fully restored beta-arrestin1 translocation and receptor internalization. These results demonstrate that chronic morphine treatment produces adaptational changes at the beta-arrestin1 level, which in turn attenuates agonist-mediated desensitization and internalization of G-protein-coupled receptors.

Molecular cloning and tissue distribution of the ovine somatostatin receptor subtype 5: osst5

The sheep is a valuable model to study growth hormone (GH) neuroregulation since its GH secretion pattern is close to that in humans and an integrated physiological approach is possible in this species. Somatostatin receptor subtype 5 (sst5) appears to be important in GH regulation but the ovine sst5 gene (osst5) has not yet been cloned. We report here the cloning of sst5 in that species. We screened a cDNA sheep library and isolated a 1.24 kb cDNA, which includes the whole coding region of osst5. The predicted protein consists of 367 amino acids exhibiting a putative seven transmembrane domain topology typical of G protein-coupled receptors. Nucleotide sequence comparisons with that of other species sst5 showed that osst5 displays 83.8, 81 and 79.7% homology with human, rat, and mice sst5, respectively. Southern blot analysis of ovine cortex DNA demonstrated that osst5 is encoded by a single gene. Osst5 transiently expressed in Chinese Hamster ovary (CHO) cells exhibit a high affinity for somatostatin-14. Reverse transcriptase-polymerase chain reaction (RT-PCR) studies demonstrated that osst5 mRNAs are present in pituitary, cortex, hypothalamus, hippocampus, colon and adrenal gland. The cloning of osst5 should provide a useful tool to study the mechanisms through which somatostatin inhibits hormone secretion in the sheep.

Quantitative autoradiographic mapping of opioid receptors in the brain of delta-opioid receptor gene knockout mice

Using quantitative receptor autoradiography we have determined if deletion of the delta-opioid receptor gene (Oprd1) results in compensatory changes in the expression of other opioid receptors. Gene targeting was used to delete exon 1 of the mouse delta-opioid receptor gene and autoradiography was carried out on brains from wild-type, heterozygous and homozygous knockout mice. Delta-opioid receptors were labeled with [(3)H]deltorphin I (7 nM), mu- with [(3)H]DAMGO (4 nM), and kappa- with [(3)H]CI-977 (2.5 nM) or [(3)H]bremazocine (2 nM in the presence of DPDPE and DAMGO) and non-specific binding determined with naloxone. [(3)H]Deltorphin I binding was reduced by approximately 50% in heterozygous animals. In homozygous animals specific binding could only be detected after long-term film exposure (12 weeks). Regions exhibiting this residual [(3)H]deltorphin I binding correlated significantly with those demonstrating high levels of the mu-receptor and were abolished in the presence of the mu-agonist DAMGO. Autoradiographic mapping showed significant overall reductions in [(3)H]DAMGO and [(3)H]CI-977 binding throughout the brain following loss of both copies of the Oprd1 gene. In contrast, overall levels of [(3)H]bremazocine binding were higher in brains from -/- than +/+ mice. Our findings suggest that residual [(3)H]deltorphin I binding in the brain of delta-receptor gene knockout mice is the result of cross-reactivity with mu-sites and that there are no delta-receptor subtypes derived from a different gene. Changes in mu- and kappa-receptor labeling suggest compensatory changes in these subtypes in response to the absence of the delta-receptor. The differences in [(3)H]CI-977 and [(3)H]bremazocine binding indicate these ligands show differential recognition of the kappa-receptor.

Interactions between opioid and chemokine receptors: heterologous desensitization

The opioid and chemokine receptors are both members of the seven transmembrane G protein-coupled receptor (GPCR) superfamily. Desensitization is believed to be a major element of the regulation of the function of these receptors, and recent findings suggest that both agonist-dependent (homologous) desensitization and heterologous desensitization can control receptor activity. The cross-desensitization between opioid and chemokine receptors has significant implications for our understanding of both the regulation of leukocyte trafficking, as well as the regulation of chemokine receptor function in inflammatory disease states. We also review findings which suggest that pro-inflammatory chemokine receptor-induced heterologous desensitization of opioid receptors has important implications for the regulation of opioid receptor function in the nervous system.

Visualizing preference of G protein-coupled receptor kinase 3 for the process of kappa-opioid receptor sequestration

G protein-coupled receptor kinases (GRKs) phosphorylate opioid receptors, which eventually results in receptor sequestration. With respect to kappa-opioid receptors, it is known that internalization occurs in a species-specific manner. That is, the agonist-occupied human kappa-receptors will sequester whereas murine receptors fail to do so. This investigation concentrates on the internalization of kappa-opioid receptors, employing laser scanning microscopy as a major technique to examine receptor internalization in living cells. For this reason, we fused green fluorescence protein to kappa-receptors, and DsRed-fluorescent protein to GRK2 and GRK3. All fusion proteins retained their biologic activities. Permanent cell lines (HEK 293, NG 108-15) were transfected to express either green fluorescent kappa-receptors or to coexpress the tagged receptor and a specific GRK-DsRed construct. The localization of fluorescent receptors and GRKs was monitored by confocal microscopy before and after opioid exposure of transfected cells. Activation of the murine kappa-receptors triggers rapid translocation of tagged GRKs toward the cell membrane, but receptor internalization was not observed. The agonist-occupied human kappa-receptor also causes translocation of GRK2- and GRK3-DsRed, which was followed by the formation of vesicles carrying the green fluorescent kappa-receptors. Moreover, the green fluorescent vesicles consistently harbour red fluorescent GRK2 and GRK3, respectively. The phenomenon of kappa-receptor internalization as well as cointernalization of GRKs is blocked by phosducin, indicating a critical role of G protein-betagamma subunits for kappa-receptor sequestration. Comparing the effect of over-expressed GRK2 and GRK3 on sequestration of kappa-receptors, we conclude that GRK3 more strongly induces kappa-receptor internalization than GRK2.

Structure-activity relationships (SAR) of [D-Arg(2)]dermorphin(1-4) analogues, N(alpha)-amidino-Tyr-D-Arg-Phe-X

In investigating the development of compounds with potent analgesic effects after oral administration, 74 C-terminal analogues (N(alpha)-amidino-Tyr-D-Arg-Phe-X), based on the structure of N(alpha)-amidino-Tyr-D-Arg-Phe-Me beta Ala-OH (ADAMB), were synthesized. Their analgesic activity was evaluated using the mouse-tail pressure test after both subcutaneous and oral administration, and the structure-activity relationships (SAR) were examined in detail. The results clearly indicated that compounds containing beta-amino acid without a side chain at the X position are preferable for expression of potent analgesic activity, and that the free carboxyl group is superior in its analgesic activity to that of the esterified or amidated carboxy group at the C-terminal. In addition, N-methylation of the amide bond at the 4th position contributed to improved analgesic activity. These results indicated that the strong and long-lasting analgesic effect of ADAMB is expressed by the synergistic effects of N(alpha)-amidination, the N-methylation of the amide bond at the 4th position and the carbon chain length (beta-Ala) of the residue at the 4th position, and that this is the most suitable structure.

Effects of adrenoceptor agonists and antagonists on morphine-induced Straub tail in mice

Straub-tail behavior was induced by subcutaneous injection of different doses (10-60 mg/kg) of morphine to mice. The maximum response was obtained with 20-40 mg/kg of the drug. The response induced by morphine (40 mg/kg) was decreased by intraperitoneal administration of different doses of clonidine (0.05-0.1 mg/kg). Pretreatment of animals with yohimbine (1-4 mg/kg i.p.) reversed the inhibitory action of clonidine. Yohimbine did not elicit any response by itself. Administration of prazosin (0.25, 0.5, and 1 mg/kg) reduced the morphine response. The combination of prazosin with yohimbine (1 mg/kg), but not with clonidine (0.05 mg/kg), caused a potentiated inhibition of the morphine effect. Phenylephrine (2-6 mg/kg i.p.) did not elicit any effect by itself and also did not alter the response induced by morphine or morphine plus clonidine. Dobutamine (2.5-10 mg/kg i.p.), atenolol (2.5-10 mg/kg i.p.), salbutamol (2.5-10 mg/kg i.p.), and propranolol (2.5-10 mg/kg i.p.) did not alter morphine-induced Straub-tail behavior in mice. In conclusion, activation of alpha2-adrenergic pathways contributes to morphine-induced Straub tail, while alpha1- and beta2-adrenergic may not be involved in this phenomenon.

Inhibition of mu and delta opioid receptor ligand binding by the peptide aldehyde protease inhibitor, leupeptin

We reported recently that the ubiquitin-proteasome pathway is involved in agonist-induced down regulation of mu and delta opioid receptors [J. Biol. Chem. 276 (2001) 12345]. While evaluating the effects of various protease inhibitors on agonist-induced opioid receptor down regulation, we observed that while the peptide aldehyde, leupeptin (acetyl-L-Leucyl-L-Leucyl-L-Arginal), did not affect agonist-induced down regulation, leupeptin at submillimolar concentrations directly inhibited radioligand binding to opioid receptors. In this study, the inhibitory activity of leupeptin on radioligand binding was characterized utilizing human embryonic kidney (HEK) 293 cell lines expressing transfected mu, delta, or kappa opioid receptors. The rank order of potency for leupeptin inhibition of [3H]bremazocine binding to opioid receptors was mu > delta > kappa. In contrast to the effect of leupeptin, the peptide aldehyde proteasome inhibitor, MG 132 (carbobenzoxy-L-Leucyl-L-Leucyl-L-Leucinal), had significantly less effect on bremazocine binding to mu, delta, or kappa opioid receptors. We propose that leupeptin inhibits ligand binding by reacting reversibly with essential sulfhydryl groups that are necessary for high-affinity ligand/receptor interactions.

Trends in the pharmacology of opioids: implications for the pharmacotherapy of pain

Some 25 years ago, it was popular to write a figurative equation for drug treatment: Patient + drug --> effect. Clearly, this was too simplistic for the reliable pharmacotherapy of pain. The 'patient' with pain is a complex individual and the 'drug' is not the same in effect to every patient, regardless of being an opioid analgesic. Besides, the 'effect' needs specification: it really means an acceptable balance between therapeutic and side-effect--to that patient. These days, neural plasticity and neuromodulation associated with nociception are well-known. Psychological involvement in the physical domain of nociception is similarly well-known. Given these complexities, it is surprising that the pharmacological control of pain through the application of relatively simple analgesics can be so effective. Opioid analgesics have been administered by every possible route. Because the degree of invasion of the patient differs between routes, the balance between simplicity, aesthetic appeal, and efficacy of the various routes needs to be considered. The effectiveness of opioid analgesics depends in large measure on the application of the right agent, in the right dose, by the right route, at the right time for that patient. To assist in this task, researchers have produced patient-controlled analgesia and 'pharmacologically engineered' analgetic molecules to achieve receptor selectivity and pharmacokinetic predictability. Also, much relevant data concerning the time courses of analgetic drug (+/- metabolite) concentrations in the body and in the ways in which these properties are modified by different normal and pathophysiological variations have been gathered, and the philosophy of 'opioid rotation' has arisen to maintain therapeutic benefits when tolerance or metabolite-induced side-effects prevail. This essay discusses some of the trends in opioid pharmacotherapy over the past 25 years.

Nociceptin/orphanin FQ and opioid receptor-like receptor mRNA expression in dopamine systems

Although nociceptin/orphanin FQ (N/OFQ) influences dopamine (DA) neuronal activity, it is not known whether N/OFQ acts directly on DA neurons, indirectly by means of local circuitry, or both. We used two parallel approaches, dual in situ hybridization (ISH) and neurotoxic lesions of DA neurons by using 6-hydroxydopamine (6-OHDA), to ascertain whether N/OFQ and the N/OFQ receptor (NOP) mRNA are expressed in DA neurons in the ventral tegmental area (VTA) and substantia nigra compacta (SNc). In the VTA and SNc, small populations (approximately 6-10%) of N/OFQ-containing neurons coexpressed mRNA for tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis. Similarly, very few (1-2%) TH-positive neurons contained N/OFQ mRNA signal. A majority of NOP-positive neurons (approximately 75%) expressed TH mRNA and roughly half of the TH-containing neurons expressed NOP mRNA. Many N/OFQ neurons (approximately 50-60%) expressed glutamic acid decarboxylase 65 and 67 mRNAs, markers for gamma-aminobutyric acid (GABA) neurons. In the 6-OHDA lesion studies, NOP mRNA levels were nearly 80 and 85% lower in the VTA and SNc, respectively, on the lesioned side. These lesions appear to lead to compensatory changes, with N/OFQ mRNA levels approximately 60% and 300% higher in the VTA and SNc, respectively, after 6-OHDA lesions. Finally, N/OFQ-stimulated [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate levels were decreased in the VTA and SNc but not the prefrontal cortex after 6-OHDA lesions. Accordingly, it appears that N/OFQ mRNA was found largely on nondopaminergic (i.e., GABA) neurons, whereas NOP mRNA was located on DA neurons. N/OFQ is in a position to influence DA neuronal activity by means of the NOP located on DA neurons.

The diverse molecular mechanisms responsible for the actions of opioids on the cardiovascular system

The actions of opioid agonist and antagonist drugs have not been well characterized in the heart and cardiovascular system. This stems from the limited role opioid receptors have been perceived to have in the regulation of the cardiovascular system. Instead, the focus of opioid receptor research, for many years, relates to the characterization of the actions of opioid drugs in analgesia associated with receptor activation in the CNS. However, recent studies suggest that opioid receptors have a role in the heart and cardiovascular system. While some of these actions may be mediated by activation of peripheral opioid receptors, others are not, and may result from direct or receptor-independent actions on cardiac tissue and the peripheral vascular system. This review will outline some of the diverse molecular mechanisms that may be responsible for the cardiovascular actions of opioids, and will characterize the role opioid receptors have in several cardiovascular pathophysiological disease states, including hypertension, heart failure, and ischaemic arrhythmogenesis. In many instances, it would appear that the effects of opioid agonists (and antagonists) in cardiovascular disease models may be mediated by opioid receptor-independent actions of these drugs.

Immunohistochemical distribution of delta opioid receptors in the rat central nervous system: evidence for somatodendritic labeling and antigen-specific cellular compartmentalization

Many studies have reported on the distribution of delta opioid receptors (delta OR) in the mammalian central nervous system (CNS) by using a variety of techniques. However, no general consensus has emerged with regards to the localization of this receptor due to inconsistencies in the immunohistochemical literature. In the present study, we analyzed the cellular and subcellular distribution of immunoreactive delta OR in the rat CNS using two different antibodies (directed against a sequence in the C-terminus or N-terminus of the rat delta OR). By using Western blotting, these two antibodies recognized similar forms of the delta OR in COS-7 cells transfected with this receptor, but distinct forms in membranes from the rat spinal cord. By using light microscopic immunohistochemistry, both antibodies recognized identical populations of nerve cell bodies throughout the CNS; the distribution of these cell bodies conformed to that of delta OR mRNA-expressing cells detected by in situ hybridization. However, whereas the C-terminus-directed antibody recognized predominantly perikarya and proximal dendrites, the N-terminus-directed antibody also labeled extensively dendritic and terminal arbors. Furthermore, by using electron microscopy, the two antibodies were found not only to label differentially somatodendritic versus axonal compartments, but also plasma membrane versus cytoplasmic ones, suggesting that distinct immunological forms of the receptor are being targeted preferentially to different cellular and subcellular domains.

Morphine-6beta-glucuronide and morphine-3-glucuronide, opioid receptor agonists with different potencies

Using heterologous expression in Xenopus laevis oocytes, we compared the potencies of morphine, morphine-6beta-glucuronide (M6G), and morphine-3-glucuronide (M3G) for cloned human mu- (hMOR), kappa- (hKOR), and delta-opioid receptors (hDOR). Each receptor subtype was individually co-expressed with heteromultimeric G-protein coupled inwardly rectifying K(+) (GIRK) channels, consisting of GIRK1 and GIRK2 subunits, and RGS4, a regulator of G-protein signaling. The two-microelectrode voltage clamp technique was used to measure the opioid receptor-activated GIRK1/GIRK2 channel responses. Compared with morphine, M6G had higher potency at the hMOR, lower potency at the hKOR, and similar potency at the hDOR, while M3G showed a 1000-fold lower and non-selective potency via opioid receptors. In contrast to naloxone, M3G did not antagonize the effects of morphine at the hMOR. We also investigated whether Trp318 and His319 provide the molecular basis for mu/delta selectivity and mu/kappa selectivity of morphinan alkaloids by mutating these residues to their corresponding residues in kappa- and delta-opioid receptors. A single-point mutation (W318L) on hMOR completely conferred delta-like potency for morphine and M6G on the mutant mu-receptor. Double mutation at Trp318 and His319 positions (Trp318Y/His319Y) only partially conferred kappa-like potency for morphine and M6G; the decrease in potency for M6G was significantly larger than for morphine. The results of our study show that both M6G and M3G are opioid receptor agonists with different potencies and that the potency of morphinan receptor ligands can be changed by selective mutations of hMOR at the Trp318 and His319 positions.

Discriminative disulfide-bonding affinity labeling of opioid receptor subtypes

The affinity-labeling technique is an extremely important method in receptor biochemistry. The 3-nitro-2-pyridinesulfenyl (Npys) group, attached to a mercapto group, can react only with a free thiol group (the beta-mercapto group of cysteine residue) of the target receptor molecules, forming a disulfide bond. This disulfide bonding is mediated through the thiol-disulfide exchange reaction. Unlike other labeling methods, the approach utilizing such chemically activated thiol-containing ligands is able to reproduce an unlabeled protein by treatment with dithiothreitol, a reducing reagent. This provides several unique aspects for the studies elucidating the structure-function relationships between the peptide and the receptor. Based on the SNpys affinity technique, we have achieved the discriminative disulfide-bonding affinity labeling of the three different subtypes of opioid receptors: mu, delta and kappa. This article reviews our novel affinity techniques in the in vitro receptor biochemistry.

Prolonged morphine treatment targets delta opioid receptors to neuronal plasma membranes and enhances delta-mediated antinociception

Opioid receptors are known to undergo complex regulatory changes in response to ligand exposure. In the present study, we examined the effect of morphine on the in vitro and in vivo density and trafficking of delta opioid receptors (deltaORs). Prolonged exposure (48 hr) of cortical neurons in culture to morphine (10 microm) resulted in a robust increase in the internalization of Fluo-deltorphin, a highly selective fluorescent deltaOR agonist. This effect was mu-mediated because it was entirely blocked by the selective mu opioid receptor antagonist d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) and was reproduced using the selective mu agonist fentanyl citrate. Immunogold electron microscopy revealed a marked increase in the cell surface density of deltaORs in neurons exposed to morphine, indicating that the increase in Fluo-deltorphin internalization was caused by increased receptor availability. Prolonged morphine exposure had no effect on deltaOR protein levels, as assessed by immunocytochemistry and Western blotting, suggesting that the increase in bioavailable deltaORs was caused by recruitment of reserve receptors from intracellular stores and not from receptor neosynthesis. Complementary in vivo studies demonstrated that chronic treatment of adult rats with morphine (5-15 mg/kg, s.c., every 12 hr) similarly augmented targeting of deltaORs to neuronal plasma membranes in the dorsal horn of the spinal cord. Furthermore, this treatment markedly potentiated intrathecal d-[Ala(2)]deltorphin II-induced antinociception. Taken together, these results demonstrate that prolonged stimulation of neurons with morphine markedly increases recruitment of intracellular deltaORs to the cell surface, both in vitro and in vivo. We propose that this type of receptor subtype cross-mobilization may widen the transduction repertoire of G-protein-coupled receptors and offer new therapeutic strategies.

Differentiation of delta, mu, and kappa opioid receptor agonists based on pharmacophore development and computed physicochemical properties

Compounds that bind with significant affinity to the opioid receptor types, delta, mu, and kappa, with different combinations of activation and inhibition at these three receptors could be promising behaviorally selective agents. Working on this hypothesis, the chemical moieties common to three different sets of opioid receptor agonists with significant affinity for each of the three receptor types delta, mu, or kappa were identified. Using a distance analysis approach, common geometric arrangements of these chemical moieties were found for selected delta, mu, or kappa opioid agonists. The chemical and geometric commonalities among agonists at each opioid receptor type were then compared with a non-specific opioid recognition pharmacophore recently developed. The comparison provided identification of the additional requirements for activation of delta, mu, and kappa opioid receptors. The distance analysis approach was able to clearly discriminate kappa-agonists, while global molecular properties for all compounds were calculated to identify additional requirements for activation of delta and mu receptors. Comparisons of the combined geometric and physicochemical properties calculated for each of the three sets of agonists allowed the determination of unique requirements for activation of each of the three opioid receptors. These results can be used to improve the activation selectivity of known opioid agonists and as a guide for the identification of novel selective opioid ligands with potential therapeutic usefulness.

Morphine inhibits mucosal antibody responses and TGF-beta mRNA in gut-associated lymphoid tissue following oral cholera toxin in mice

In this study, we investigated the effect of morphine on the mucosal immune system using fragment cultures of ileal segments, Peyer's patches (PPs), and mesenteric lymph nodes. Mice were implanted s.c. with a morphine slow release pellet. Control groups received a naltrexone slow release pellet, a placebo pellet, or both a morphine and a naltrexone pellet. After 48 h, mice were orally immunized with cholera toxin (CT) and were boosted orally 1 wk later. Animals were sacrificed 1 wk after the booster immunization, and PPs, mesenteric lymph nodes, and ileal segments were cultured in 24-well plates for 12 days. Morphine resulted in a highly significant inhibition of CT-specific IgA and IgG production in fragment culture supernatants of all three tissues compared with placebo. Naltrexone blocked the reduction in Ab levels induced by morphine, indicating that the effect is opioid receptor mediated. Morphine did not significantly alter total IgA levels in any of the tissue culture supernatants. Morphine also inhibited CT-specific IgA and IgG levels in serum. By flow cytometry, morphine did not alter the lymphoid cell composition in PPs compared with placebo. The effect of morphine on TGF-beta, IL-5, and IL-6 mRNA expression in PPs and ileal segments was determined following oral immunization with CT. Morphine significantly decreased TGF-beta mRNA compared with that in the placebo group, and naltrexone blocked this effect. These results indicate that morphine inhibits Ag-specific IgA responses in gut-associated lymphoid tissue at least partially through the inhibition of TGF-beta, a putative IgA switch factor, in the gastrointestinal tract.

Prolonged morphine treatment targets delta opioid receptors to neuronal plasma membranes and enhances delta-mediated antinociception

Opioid receptors are known to undergo complex regulatory changes in response to ligand exposure. In the present study, we examined the effect of morphine on the in vitro and in vivo density and trafficking of delta opioid receptors (deltaORs). Prolonged exposure (48 hr) of cortical neurons in culture to morphine (10 microm) resulted in a robust increase in the internalization of Fluo-deltorphin, a highly selective fluorescent deltaOR agonist. This effect was mu-mediated because it was entirely blocked by the selective mu opioid receptor antagonist d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) and was reproduced using the selective mu agonist fentanyl citrate. Immunogold electron microscopy revealed a marked increase in the cell surface density of deltaORs in neurons exposed to morphine, indicating that the increase in Fluo-deltorphin internalization was caused by increased receptor availability. Prolonged morphine exposure had no effect on deltaOR protein levels, as assessed by immunocytochemistry and Western blotting, suggesting that the increase in bioavailable deltaORs was caused by recruitment of reserve receptors from intracellular stores and not from receptor neosynthesis. Complementary in vivo studies demonstrated that chronic treatment of adult rats with morphine (5-15 mg/kg, s.c., every 12 hr) similarly augmented targeting of deltaORs to neuronal plasma membranes in the dorsal horn of the spinal cord. Furthermore, this treatment markedly potentiated intrathecal d-[Ala(2)]deltorphin II-induced antinociception. Taken together, these results demonstrate that prolonged stimulation of neurons with morphine markedly increases recruitment of intracellular deltaORs to the cell surface, both in vitro and in vivo. We propose that this type of receptor subtype cross-mobilization may widen the transduction repertoire of G-protein-coupled receptors and offer new therapeutic strategies.

Pain management in the critically ill child

Children frequently received no treatment, or inadequate treatment, for pain and for painful procedures. The newborn and critically ill children are especially vulnerable to no treatment or under-treatment. Nerve pathways essential for the transmission and perception of pain are present and functioning by 24 weeks of gestation. The failure to provide analgesia for pain results in rewiring the nerve pathways responsible for pain transmission in the dorsal horn of the spinal cord and results in increased pain perception for future painful results. Many children would withdraw or deny their pain in an attempt to avoid yet another terrifying and painful experiences, such as the intramuscular injections. Societal fears of opioid addiction and lack of advocacy are also causal factors in the under-treatment of pediatric pain. False beliefs about addictions and proper use of acetaminophen and other analgesics resulted in the failure to provide analgesia to children. All children even the newborn and critically ill require analgesia for pain and painful procedures. Unbelieved pain interferes with sleep, leads to fatigue and a sense of helplessness, and may result in increased morbidity or mortality.

Quantitative autoradiographic mapping of the ORL1, mu-, delta- and kappa-receptors in the brains of knockout mice lacking the ORL1 receptor gene

Until recently the opioid receptor family was thought to consist of only the mu-, delta- and kappa-receptors. The cloning of opioid receptor like receptor (ORL1) and its endogenous ligand nociceptin/orphanin FQ, which displayed anti-opioid properties, has raised the issue of functional co-operativity of this system with the classical opioid system. ORL1 receptor knockout mice have been successfully developed by homologous recombination to allow the issue of potential heterogeneity of this receptor and also of compensatory changes in mu-, delta- or kappa-receptors in the absence of ORL1 to be addressed. We have carried out quantitative autoradiographic mapping of these receptors in the brains of mice that are wild-type, heterozygous and homozygous for the deletion of the ORL1 receptor. ORL1, mu-, delta- and kappa-receptors were labelled with [(3)H] leucyl-nociceptin (0.4 nM), [(3)H] DAMGO (4 nM), [(3)H] deltorphin-I (7 nM), and [(3)H] CI-977 (2.5 nM) respectively. An approximately 50% decrease in [(3)H] leucyl-nociceptin binding was seen in heterozygous ORL1 mutant mice and there was a complete absence of binding in homozygous brains indicating the single gene encodes for the ORL1 receptor and any putative subtypes. No significant gross changes in the binding to other opioid receptors were seen across genotypes in the ORL1 mutant mice demonstrating a lack of major compensation of classical opioid receptors in the absence of ORL1. There were a small number of region specific changes in the expression of classical opioid receptors that may relate to interdependent function with ORL1.

The pharmacology of mu analgesics: from patients to genes

Morphine and most clinical opioids act through mu opioid receptors. Yet, their pharmacological profiles differ. The presence of incomplete cross-tolerance among these drugs clinically was one of the first indications that these mu opioids differed in their receptor mechanisms of action. This was followed by similar studies in preclinical models, which also found genetic differences in sensitivity toward morphine and other mu opioids. This concept of mu receptor multiplicity is now supported by antisense and gene knockout models. Although all the mu opioids are sensitive to antisense probes against the mu opioid receptor gene MOR-1, the sensitivity profiles of the drugs to the antisense probes differ based on the exon being targeted. Knockout mice also reveal striking differences. In one knockout mouse, morphine analgesia is completely lost while the potent mu drugs morphine-6beta-glucuronide and heroin both retain analgesic activity. Finally, cloning studies have identified at least seven different splice variants of the MOR-1 gene, with more likely. These studies illustrate the complexity of mu opioid pharmacology.

Structure and regulation of opioid receptors

Significant advances have been made in understanding the structure, function, and regulation of opioid receptors and endogenous opioid peptides since their discovery approximately 25 years ago. This review summarizes recent studies aimed at identifying key amino acids that confer ligand selectivity to the opioid receptors and that are critical constituents of the ligand binding sites. A molecular model of the delta receptor based on the crystal structure of rhodopsin is presented. Agonist-induced down regulation of opioid receptors is discussed, highlighting recent evidence for the involvement of the ubiquitin/proteasome system in this process.

Opioids, opioid receptors, and the immune response

It is now clear that opioid receptors participate in the function of the cells of the immune system, and evidence suggests that opioids modulate both innate and acquired immune responses. We review literature here which establishes that mu-, kappa-, and delta-opioid compounds alter resistance to a variety of infectious agents, including the Human Immunodeficiency Virus (HIV). The nature of the immunomodulatory activity of the opioids has been the subject of a great deal of research over the last ten years. There is increasing evidence that effects of opioids on the immune response are mediated at several levels. Modulation of the inflammatory response appears to be a target of these compounds, including effects on phagocytic activity, as well as the response of cells to various chemoattractant molecules. Moreover, findings from several laboratories have demonstrated the impact of opioid treatment on antibody responses, and the molecular basis for this effect is likely due, at least in part, to the modulation of both cytokine and cytokine receptor expression. Future research should provide a clearer understanding of the cellular and molecular targets of opioid action within the immune system.

Investigation of the selectivity of oxymorphone- and naltrexone-derived ligands via site-directed mutagenesis of opioid receptors: exploring the "address" recognition locus

The delta-selective opioid antagonist naltrindole (NTI), as well as the kappa-selective opioid antagonists norbinaltorphimine (norBNI) and 5'-guanidinonaltrindole (GNTI), are derived from naltrexone, a universal opioid antagonist. Previous studies have indicated that extracellular loop III is the key region for discrimination by naltrexone-derived selective ligands between the delta, mu, and kappa opioid receptor types. It has been proposed that selective ligands could bind to all three receptor types if the appropriate portions of the extracellular loops were eliminated. To investigate this possibility, several single-point mutant opioid receptors have been generated with the aim of conferring enhanced affinity of selective ligands for their nonpreferred receptor types. Mutations were made in all three types of opioid receptors with the focus on two positions at the extracellular end of transmembrane regions (TM) VI and VII. It was found that the delta-selective NTI could bind both mu and kappa receptors with significantly enhanced affinity when an aromatic residue in TM VII was replaced with alanine (mu[W318A] and kappa[Y312A]). Similarly, kappa-selective antagonists, norBNI and GNTI, showed enhanced affinity for the mu[W318A] mutant and for both mu and delta receptors when a glutamate residue was incorporated into the extracellular end of TM VI (mu[K303E] and delta[W284E]). These results demonstrate that naltrexone-derived selective ligands achieve their selectivity via a combination of enhanced affinity of the address for a particular subsite along with loss of affinity due to steric interference at nonpreferred types. The results reveal key residues in the "address" recognition locus that contribute to the selectivity of opioid ligands and support the hypothesis that recognition of the naltrexone moiety is essentially the same for all three receptor types.

Regulations of opioid dependence by opioid receptor types

Three major types of opioid receptors, designated mu, delta, and kappa, are widely expressed in the CNS. Development of selective receptor ligands and recent cloning of each receptor have contributed greatly to our increasing knowledge of the neuropharmacological profile of each opioid receptor type. It is of interest to note that they include noncompetitive and allosteric interactions among their types. This review focuses on the functional interaction among these opioid receptor types that contribute to opioid dependence. Various studies provide arguments to support substantial roles for mu-opioid receptors and the possible involvement of delta-opioid receptors in the development of physical and psychological dependence on morphine. Noradrenergic transmission originating in the locus coeruleus is most likely to play the primary causal role in the expression of physical dependence on morphine. In contrast, many studies have pointed to the mesolimbic dopaminergic pathway projecting from the ventral tegmental area to the nucleus accumbens as a critical site for the initiation of psychological dependence on opioids. It is noteworthy as the broad existence of opposing interactions between mu/delta- and kappa-receptors in the brain. The activation of kappa-receptors leads to the suppression of unpleasant mu/delta-mediated side effects such as the rewarding effect. Considering the functional interaction among opioid receptor types, the co-administration of morphine-like compounds with kappa-receptor agonists may constitute a preferable and superior approach to the treatment of pain with fewer side effects.

Endogenous opioids as mediators of asthma

Changes in airway innervation are believed to play a key role in the pathophysiology of asthma. A group of regulatory peptides which act as neuroregulators is resembled by the opioids. Their localization to neurons projecting into airways suggested a possible role as regulators of neurogenic inflammation, bronchoconstriction and mucus secretion. They mainly act through modification of tachykinergic and cholinergic impulses and their ability to inhibit bronchoconstriction prompted discussion of their potential value in asthma therapy. Apart from the presence of the classical opioids and their receptors in the lung and their functional role, a new group of peptides such as nociceptin and endomorphins have been characterized in the airways. Whereas at least endomorphin-1 acts via the classical OP(3) (mu) receptor, nociceptin binds to a new receptor termed opioid receptor-like-receptor (ORL(1)) and inhibits tachykinergic constriction. Contrary to these promising modulatory effects on airway smooth muscle tone, effective therapeutic strategies have not been developed yet. In conclusion, opioids resemble a group of regulatory peptides which are present within airway-innervating nerve fibres and influence a multitude of airway functions via modification of neural transmission.

Molecular determinants of non-specific recognition of delta, mu, and kappa opioid receptors

Identification of the molecular determinants of recognition common to all three opioid receptors embedded in a single three-dimensional (3D) non-specific recognition pharmacophore has been carried out. The working hypothesis that underlies the computational study reported here is that ligands that bind with significant affinity to all three cloned opioid receptors, delta, mu, and kappa, but with different combinations of activation and inhibition properties at these receptors, could be promising behaviorally selective analgesics with diminished side effects. The study presented here represents the first step towards the rational design of such therapeutic agents. The common 3D pharmacophore developed for recognition of delta, mu, and kappa opioid receptors was based on the receptor affinities determined for 23 different opioid ligands that display no specificity for any of the receptor subtypes. The pharmacophore centers identified are a protonated amine, two hydrophobic groups, and the centroid of an aromatic group in a geometric arrangement common to all 23, non-specific, opioid ligands studied. Using this three-dimensional pharmacophore as a query for searching 3D structural databases, novel compounds potentially involved in non-specific recognition of delta, mu, and kappa opioid receptors were retrieved. These compounds can be valuable candidates for novel behaviorally selective analgesics with diminished or no side effects, and thus with potential therapeutic usefulness.

Molecular characterization and distribution of the opioid growth factor receptor (OGFr) in mouse

The native opioid growth factor (OGF), [Met(5)]-enkephalin, is a tonic inhibitory peptide that modulates cell proliferation and tissue organization during development, cancer, cellular renewal, wound healing, and angiogenesis. OGF action is mediated by a receptor mechanism. The receptor for OGF, OGFr, has been cloned and sequenced in humans and rats. Using primers based on the rat OGFr cDNA, and a mouse embryo expressed sequence tag, the full-length 2.1 kb mouse OGFr cDNA was sequenced. The open reading frame was found to encode a protein of 634 amino acids, and 14 imperfect repeats of 9 amino acids each were a prominent feature. The molecular weight of OGFr was calculated as 70679, and the isoelectric point was 4.5. Northern blot analysis revealed a 2.1 kb OGFr mRNA transcript in adult mouse brain, heart, lung, liver, kidney, and triceps surae muscle. The amino acids for mouse and rat OGFr were 93% similar and 91% identical, but the mouse and human shared only a 70% similarity and a 58% identity. These results emphasize the molecular validity of OGFr, and explain the interaction of OGF with respect to normal and abnormal growth in mouse cells and tissues.

Mu-opioid induction of monocyte chemoattractant protein-1, RANTES, and IFN-gamma-inducible protein-10 expression in human peripheral blood mononuclear cells

Strong evidence for the direct modulation of the immune system by opioids is well documented. Mu-opioids have been shown to alter the release of cytokines important for both host defense and the inflammatory response. Proinflammatory chemokines monocyte chemoattractant protein-1 (MCP-1), RANTES, and IFN-gamma-inducible protein-10 (IP-10) play crucial roles in cell-mediated immune responses, proinflammatory reactions, and viral infections. In this report, we show that [D-Ala(2),N:-Me-Phe(4),Gly-ol(5)]enkephalin (DAMGO), a mu-opioid-selective agonist, augments the expression in human PBMCs of MCP-1, RANTES, and IP-10 at both the mRNA and protein levels. Because of the proposed relationship between opioid abuse and HIV-1 infection, we also examined the impact of DAMGO on chemokine expression in HIV-infected cells. Our results show that DAMGO administration induces a significant increase in RANTES and IP-10 expression, while MCP-1 protein levels remain unaffected in PBMCs infected with the HIV-1 strain. In contrast, we show a dichotomous effect of DAMGO treatment on IP-10 protein levels expressed by T- and M-tropic HIV-infected PBMCs. The differential modulation of chemokine expression in T- and M-tropic HIV-1-infected PBMCs by opioids supports a detrimental role for opioids during HIV-1 infection. Modulation of chemokine expression may enhance trafficking of potential noninfected target cells to the site of active infection, thus directly contributing to HIV-1 replication and disease progression to AIDS.

Opioids affect acquisition of LiCl-induced conditioned taste aversion: involvement of OT and VP systems

Aversive properties of lithium chloride (LiCl) are mediated via pathways comprising neurons of the nucleus of the solitary tract (NTS) and oxytocin (OT) and vasopressin (VP) cells in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. Because opioids act on brain regions that mediate effects of LiCl, we evaluated whether administration of opioids shortly before LiCl in rats influences 1) development of conditioned taste aversion (CTA) and 2) activation of NTS neurons and OT/VP cells. Neuronal activation was assessed by applying c-Fos immunohistochemical staining. Three opioids were used: morphine (MOR), a mu-agonist, butorphanol tartrate (BT), a mixed mu/kappa-agonist, and nociceptin/orphanin FQ (N/OFQ), which binds to an ORL1 receptor. BT and N/OFQ completely blocked acquisition of CTA. MOR alleviated but did not eliminate the aversive effects. Each of the opioids decreased LiCl-induced activation of NTS neurons as well as OT and VP cells in the PVN and SON. We conclude that opioids antagonize aversive properties of LiCl, presumably by suppressing activation of pathways that encompass OT and VP cells and NTS neurons.

Naturally occurring antinociceptive substances from plants

Despite the progress that has occurred in recent years in the development of therapy, there is still a need for effective and potent analgesics, especially for the treatment of chronic pain. One of the most important analgesic drugs employed in clinical practice today continues to be the alkaloid morphine. In this review, emphasis will be given to the important contribution and the history of Papaver somniferum, Salix species, Capsicum species and Cannabis sativa in the development of new analgesics and their importance in the understanding of the complex pathways related to electrophysiological and molecular mechanisms associated with pain transmission. Recently discovered antinociceptive substances include alkaloids, terpenoids and flavonoid. Plant-derived substances have, and will certainly continue to have, a relevant place in the process of drug discovery, particularly in the development of new analgesic drugs.

Neurobehavioral damage to cholinergic systems caused by prenatal exposure to heroin or phenobarbital: cellular mechanisms and the reversal of deficits by neural grafts

Despite the basic differences in their underlying biological targets, prenatal exposure to heroin or phenobarbital produces similar syndromes of neurobehavioral deficits, involving defects in septohippocampal cholinergic innervation-related behaviors. At the cellular level, these deficits are associated with cholinergic hyperactivity, characterized by increased concentrations of muscarinic receptors and enhanced second messenger activity linked to the receptors. In the present study, we determined whether the cellular changes are mechanistically linked to altered behavior, using two different approaches: neural grafting and correlations between behavior and biochemistry within the same individual animals. Mice were exposed transplacentally to phenobarbital or heroin on gestation days 9-18 and, as adults, received fetal cholinergic grafts or were sham-operated. Prenatal drug exposure resulted in deficits in behavioral performance tested in the eight-arm radial maze, accompanied by increases in hippocampal M(1)-muscarinic receptor expression and muscarinic receptor-mediated IP formation. Neural grafting reversed both the behavioral deficits and the muscarinic hyperactivity. In the drug-exposed offspring, there was a significant correlation between maze performance and carbachol-induced inositol phosphate (IP) formation. These studies indicate that deficits of cholinergic function underlie the neurobehavioral deficits seen in the hippocampus of animals exposed prenatally to heroin or phenobarbital, and consequently that the observed cholinergic hyperactivity is an unsuccessful attempt to compensate for the loss of cholinergic function. The fact that the damage can be reversed by neural grafting opens up novel approaches to the restoration of brain function after prenatal insults.

Immunohistochemical localization of the somatostatin sst(4) receptor in rat brain

The biological actions of the neuromodulator somatostatin are mediated through a family of G-protein-coupled receptors, of which five members, sst(1-5), have been identified. Although the messenger RNA distribution of the sst(4) receptor has been reported, no information about the distribution of the receptor protein in the central nervous system is available. We have therefore raised a polyclonal peptide antibody against a rat carboxy-terminal sst(4) peptide. The selectivity of the affinity-purified antibody was demonstrated by western blotting of membrane proteins isolated from Chinese hamster ovary-K1 cells expressing the recombinant sst(4) receptor and from the rat hippocampus. This resulted in both cases in the identification of a single band of approximately 42,000 mol. wt. Furthermore, the sst(4) receptor antibody selectively labelled Chinese hamster ovary-K1 cells expressing the recombinant sst(4) receptor in immunocytochemistry. No cross-reactivity was observed with other recombinant somatostatin receptors. Immunohistochemistry on adult rat brain sections showed the sst(4) receptor to have a widespread distribution. This included labelling of cell bodies as well as processes in the cerebral cortex, hippocampus and several nuclei in the brainstem. All signals were absent following antibody preabsorption with the synthetic sst(4) peptide. This study provides the first detailed analysis of the distribution of sst(4) receptor protein in the rat brain.

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.

Nonpeptide analogues of dynorphin A(1-8): design, synthesis, and pharmacological evaluation of kappa-selective agonists

Two novel series of kappa opioid receptor agonist analogues of MPCB-GRRI and MPCB-RRI, hybrid ligands of MPCB ((-)-cis-N-(2-phenyl-2-carbomethoxy)cyclopropylmethyl-N-normetazocine ) and of the C-terminal fragments of dynorphin A(1-8), have been synthesized. The critical functional groups of the peptide fragments of hybrid compounds were maintained, and the binding affinities and selectivities for compounds 1-40 to mu, delta, and kappa opioid receptors were analyzed. Compounds 15 and 16, MPCB-Gly-Leu-NH-(CH(2))(n)()-NH-C(=NH)-C(4)H(9) (n = 5, 6), displayed high affinity and selectivity for kappa opioid receptors (K(i)(kappa) = 6.7 and 5.3 nM, K(i)(mu)/K(i)(kappa) = 375 and 408, and K(i)(delta)/K(i)(kappa) = 408 and 424, respectively). Since kappa agonists may also cause psychotomimetic effects by interaction with sigma sites, binding assays to sigma(1) sites were performed where compounds 15 and 16 showed negligible affinity (K(i) > 10 000). Compounds 15 and 16 were further characterized in vivo and showed potent antinociceptive activity in mouse abdominal constriction tests (ED(50) = 0.88 and 1.1 mg/kg, respectively), fully prevented by nor-BNI. Thus, these novel analogues open an exciting avenue for the design of peptidomimetics of dynorphin A(1-8).

Endomorphin-1 discriminates the mu-opioid receptor from the delta- and kappa-opioid receptors by recognizing the difference in multiple regions

Endomorphin-1 is a novel endogenous peptide that is highly selective for the mu-opioid receptor over the delta- and kappa-opioid receptors. The structural basis of high selectivity of endomorphin-1 to the mu-opioid receptor was examined using chimeric receptors between mu- and delta-opioid receptors and those between mu- and kappa-opioid receptors. The chimeric receptors were constructed by using restriction enzyme sites intrinsically possessed by or introduced to the mu-, delta- and kappa-opioid receptor cDNAs. The junctions for the construction were located at the first intracellular loop (Bbs I site), third transmembrane domain (Afl III site) and fifth transmembrane domain (Bgl II site). The competitive binding assay using chimeric receptors revealed that the region from the Bbs I site to the Afl III site, including the first extracellular loop, contributes to the discrimination between mu- and delta-opioid receptors by endomorphin-1 more than any other regions. However, the region from the Afl III site to the Bgl II site and that from the Bgl II site to the carboxy terminal also somewhat contribute to the discrimination between mu- and delta-opioid receptors. For the discrimination between mu- and kappa-opioid receptors, two regions, that is, the region from the Bbs I site to the Afl III site and that from the Bgl II site to the carboxy terminal, were shown to be important. The present results show that endomorphin-1 discriminates the mu-opioid receptor from the other two types of opioid receptors by recognizing the differences in several amino acid residues widely distributed through the receptor structure. We previously reported that DAMGO, a synthetic highly mu-selective peptide, discriminates between mu- and delta-opioid receptors by recognizing the difference in only one amino acid residue and discriminates between mu- and kappa-opioid receptors by recognizing the difference in four residues localized in the restricted region. Although both endomorphin-1 and DAMGO are mu-opioid receptor selective peptides, molecular mechanisms for mu-selectivity are different between these peptides.

Scanning of five chromosomes for alcohol consumption loci

In our present genetic study to map Quantitative Trait Loci (QTLs) for alcohol-related behaviors, we used 44 B6.C and 36 B6.I inbred congenic Recombinant QTL Introgression (RQI) mouse strains of the b5i7 series carrying genes of BALB/cJ (C) or CXBI (I) origin on C57BL/6ByJ (B6) genetic background. Ethyl alcohol consumption (EAC) was measured in adult males, and chromosomes 1, 2, 3, 9, and 15 were scanned with polymorphic microsatellite markers. In the B6.C set of strains, multiple regression analysis yielded a model with three microsatellite markers, which explained 32% of the genetic variance (p=0.0006). The two markers with the highest significance levels in the model, D1Mit167 and D2Mit74, have been mapped to chromosome regions close to the gene opioid receptor kappa 1 (chr. 1) and opioid receptor kappa 3 (chr. 2), respectively. The results of this gene-mapping study suggest that genetic polymorphisms in kappa opioid receptors may contribute to genetic predisposition to voluntary alcohol-drinking behavior.

Comparison of G-protein activation in the brain by mu-, delta-, and kappa-opioid receptor agonists in mu-opioid receptor knockout mice

Mice lacking the mu-opioid receptor gene have been developed by a gene knockout procedure. In this study, the activity of opioid receptor coupled G-proteins was examined to investigate whether there is a change in the extent of coupling for mu, delta-, and kappa-opioid receptors in mu-opioid receptor knockout mice. Selective agonists of mu- (DAMGO), delta- (DPDPE), and kappa- (U-69,593) opioid receptors stimulated [(35)S]GTPgammaS binding in the caudate putamen and cortex of wild-type mice. In contrast, only U-69,593 stimulated [(35)S]GTPgammaS binding in these regions of mu-opioid receptor knockout mice. These results confirmed the absence of G-protein activation by a mu-opioid receptor agonist in mu-opioid receptor knockout mice, and demonstrated that coupling of the kappa-opioid receptor to G-proteins is preserved in these mice. However, G-protein activation by the delta-opioid receptor agonist, DPDPE, was reduced in the mu-opioid receptor knockout mice, at least in the brain regions studied using autoradiography.

The hemorphins: a new class of opioid peptides derived from the blood protein hemoglobin

Hemorphins are endogenous peptides belonging to the family of "nonclassical" or "atypical" opioid peptides. They are generated by enzymatic hydrolysis of the beta-, kappa-, delta-, or epsilon-chain of the blood protein hemoglobin. Originally, the hemorphins were isolated from enzymatically treated bovine blood. In recent years hemorphin structures have been identified as naturally occurring peptides in brain, plasma, and cerebrospinal fluid. This article will review recent studies of the hemorphins regarding their structures, mechanisms for their release, and their biological actions. A particular emphasis will be directed to their role in exercising human and their clinical relevance.

Mutational analysis of the structure and function of opioid receptors

The cloning of the opioid receptors allows the investigation of receptor domains involved in the peptidic and nonpeptidic ligand interaction and activation of the opioid receptors. Receptor chimera studies and mutational analysis of the primary sequences of the opioid receptors have provided insights into the structural domains required for the ligand recognition and receptor activation. In the current review, we examine the current reports on the possible involvement of extracellular domains and transmembrane domains in the high-affinity binding of peptidic and nonpeptidic ligands to the opioid receptor. The structural requirement for the receptors' selectivity toward different ligands is discussed. The receptor domains involved in the activation and subsequent cellular regulation of the receptors' activities as determined by mutational analysis will also be discussed. Finally, the validity of the conclusions based on single amino acid mutations is examined.

New opioid peptides, peptidomimetics, and heterocyclic compounds from combinatorial libraries

Here we review the use of combinatorial libraries in opioid receptor assays. Following a brief description of the history of the combinatorial field, methods for the generation of synthetic libraries and the deconvolution of mixture-based libraries are presented. Case studies involving opioid assays used to demonstrate the viability of combinatorial libraries are described. The identification of new opioid peptides from combinatorial libraries is reviewed. The peptides found are composed of L-amino acids, D-amino acids, or L-, D-, and unnatural amino acids, and range from tetrapeptides to decapeptides. Likewise, new opioid compounds identified from peptidomimetic libraries, such as peptoids and alkylated dipeptides, and those identified from acyclic (e.g., polyamine, urea) and heterocyclic (e.g., bicyclic guanidine) libraries, are reviewed.

Distinct differences in the cannabinoid receptor binding in the brain of C57BL/6 and DBA/2 mice, selected for their differences in voluntary ethanol consumption

The two inbred strains of mice C57BL/6 and DBA/2 mice have been shown to differ significantly in their preference for alcohol (EtOH). These strains of mice have been employed to study various aspects of pharmacological and behavioral effects of EtOH. We have previously demonstrated that chronic EtOH exposure down-regulated cannabinoid receptors (CB1) in mouse synaptic plasma membranes and enhanced the synthesis of endogenous cannabimimetic compound anandamide (AnNH) in human neuroblastoma cells. The purpose of the present study was to investigate whether there were differences in the density and the affinity of CB1 receptors in the brains of the two inbred C57BL/6 (alcohol-preferring) and DBA/2 (alcohol avoiding) mice. The results indicate the presence of specific CB1 receptors in the brain membranes of both the strains. It was also found that the CB1 receptor densities (B(max)) were 25% lower in C57BL/6 (0.66 +/- 0.15 pmol/mg protein) compared with that of DBA/2 (0.88 +/- 0.08 pmol/mg protein) mice. Significant differences in the affinity were also observed between the two lines (K(d), 0.68 +/- 0.15 nM for C57BL/6 and 2.21 +/- 0.56 nM for DBA/2). The competition studies with SR141716A, a CB1 receptor antagonist, and 2-arachidonylglycerol (2-AG) and anandamide (AnNH), known CB1 receptor agonists, all showed a substantial decrease in [(3)H]CP-55,940 binding in both strains of mice with a higher K(i) values in the DBA/2 mice. These results suggest that CB1 receptor signal transduction may play an important role in controlling the voluntary EtOH consumption by these strains of mice.

Pharmacology of nociceptin and its receptor: a novel therapeutic target

Nociceptin (NC), alias Orphanin FQ, has been recently identified as the endogenous ligand of the opioid receptor-like 1 receptor (OP(4)). This new NC/OP(4) receptor system belongs to the opioid family and has been characterized pharmacologically with functional and binding assays on native (mouse, rat, guinea-pig) and recombinant (human) receptors, by using specific and selective agonists (NC, NC(1 - 13)NH(2)) and a pure and competitive antagonist, [Nphe(1)]NC(1 - 13)NH(2). The similar order of potency of agonists and affinity values of the antagonist indicate that the same receptor is present in the four species. OP(4) is expressed in neurons, where it reduces activation of adenylyl cyclase and Ca(2+) channels while activating K(+) channels in a manner similar to opioids. In this way, OP(4) mediates inhibitory effects in the autonomic nervous system, but its activities in the central nervous system can be either similar or opposite to those of opioids. In vivo experiments have demonstrated that NC modulates a variety of biological functions ranging from nociception to food intake, from memory processes to cardiovascular and renal functions, from spontaneous locomotor activity to gastrointestinal motility, from anxiety to the control of neurotransmitter release at peripheral and central sites. These actions have been demonstrated using NC and various pharmacological tools, as antisense oligonucleotides targeting OP(4) or the peptide precursor genes, antibodies against NC, an OP(4) receptor selective antagonist and with data obtained from animals in which the receptor or the peptide precursor genes were knocked out. These new advances have contributed to better understanding of the pathophysiological role of the NC/OP(4) system, and ultimately will help to identify the therapeutic potential of new OP(4) receptor ligands.

mu Opioid receptor: role for the amino terminus as a determinant of ligand binding affinity

The importance of the amino-terminal domain of the mu opioid receptor (MOR) as a component of the high affinity ligand-binding pocket was evaluated. A deletion mutant lacking 64 amino acids from the amino-terminus of MOR (DeltaN64) was constructed and expressed in HEK 293 cells. The affinities of bremazocine and cyclazocine were similar for the truncated and full-length MORs. Affinities of the mu receptor antagonist, naloxone, and the mu receptor agonist, morphine, were decreased 3.5-fold and 6-fold, respectively, for the truncated receptor relative to the wild-type MOR. Similarly, the affinities of the opioid peptide agonists, DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol), beta-endorphin, and DADL (Tyr-D-Ala-Gly-Phe-D-Leu), for the DeltaN64 receptor were decreased from 3- to 8-fold as a result of the deletion. In contrast, the affinities of the alkaloid agonists, methadone and fentanyl, and the peptide agonists, endomorphin 1 and endomorphin 2, for the truncated receptor relative to MOR were reduced dramatically by 20- to 60-fold. MOR is glycosylated when expressed in HEK 293 cells; however, analysis of N-glycosidase F-treated membranes indicated that N-glycan chains within the amino-terminal domain of MOR do not contribute significantly to ligand affinities. These results indicate that amino acid residues within the amino-terminal domain of MOR play a crucial role in the composition of the binding pocket for a select group of agonists.

Cloning, sequencing, chromosomal location, and function of cDNAs encoding an opioid growth factor receptor (OGFr) in humans

The native opioid growth factor (OGF), [Met(5)]-enkephalin, is a tonic inhibitory peptide that modulates cell proliferation and tissue organization during development, cancer, cellular renewal, wound healing, and angiogenesis. OGF action is mediated by a receptor mechanism. We have cloned and sequenced cDNAs encoding multiple spliced forms of a human OGF receptor. The open reading frame in the longest cDNA was found to encode a protein of 697 amino acids, and 8 imperfect repeats of 20 amino acids each were a prominent feature. Altogether, five alternatively spliced forms were observed. The cDNA hybridized to mRNA from a variety of normal and neoplastic cells and tissues. Functional studies using antisense oligonucleotides to OGFr demonstrated an enhancement in cell growth. Fluorescent in situ hybridization (FISH) experiments showed the chromosomal location to be 20q13.3. This OGF receptor has no homology to classical opioid receptors. These results provide molecular validity for the interaction of OGF and OGF receptor in the regulation of growth processes in humans.