Recent developments in the research of opioid receptor subtype molecular characterization

Role of µ, κ, and δ opioid receptors in tibial inhibition of bladder overactivity in cats

In α-chloralose anesthetized cats, we examined the role of opioid receptor (OR) subtypes (µ, κ, and δ) in tibial nerve stimulation (TNS)-induced inhibition of bladder overactivity elicited by intravesical infusion of 0.25% acetic acid (AA). The sensitivity of TNS inhibition to cumulative i.v. doses of selective OR antagonists (cyprodime for µ, nor-binaltorphimine for κ, or naltrindole for δ ORs) was tested. Naloxone (1 mg/kg, i.v., an antagonist for µ, κ, and δ ORs) was administered at the end of each experiment. AA caused bladder overactivity and significantly (P < 0.01) reduced bladder capacity to 21.1% ± 2.6% of the saline control. TNS at 2 or 4 times threshold (T) intensity for inducing toe movement significantly (P < 0.01) restored bladder capacity to 52.9% ± 3.6% or 57.4% ± 4.6% of control, respectively. Cyprodime (0.3-1.0 mg/kg) completely removed TNS inhibition without changing AA control capacity. Nor-binaltorphimine (3-10 mg/kg) also completely reversed TNS inhibition and significantly (P < 0.05) increased AA control capacity. Naltrindole (1-10 mg/kg) reduced (P < 0.05) TNS inhibition but significantly (P < 0.05) increased AA control capacity. Naloxone (1 mg/kg) had no effect in cyprodime pretreated cats, but it reversed the nor-binaltorphimine-induced increase in bladder capacity and eliminated the TNS inhibition remaining in naltrindole pretreated cats. These results indicate a major role of µ and κ ORs in TNS inhibition, whereas δ ORs play a minor role. Meanwhile, κ and δ ORs also have an excitatory role in irritation-induced bladder overactivity.

Discovery of N-methyltetrahydroprotoberberines with κ-opioid receptor agonists-opioid receptor agonist activities from corydalis yanhusuo W. T. Wang by using two-dimensional liquid chromatography

ETHNOPHARMACOLOGICAL RELEVANCE

The need for an efficacious analgesic without unwanted side effects is urgent. κ-opioid receptor agonists are known to exhibit potent analgesic effects and elicited fewer side effects than other opioid agonists. Thus in this study we chose the κ-opioid receptor as the target to identify the active components from traditional Chinese medicines (TCMs).

MATERIALS AND METHODS

The κ-opioid receptor was expressed in human embryonic kidney-293 T cells (HEK293T). Fluorometric Imaging Plate Reader (FLIPR) assay was used for the determination of Ca(2+) response when κ-opioid receptor was activated. A novel 2D separation system employing C18HCE as the first dimension and a strong cation exchange column (SCX) as the second dimension was conducted for the purification of the active principles.

RESULTS

With the aid of HPLC-based activity profiling, activities could be linked to two peaks from Corydalis yanhusuo W. T. Wang (C. yanhusuo) extract. Two N-methyltetrahydroprotoberberines with κ-opioid receptor agonist activities were isolated for the first time from C. yanhusuo by using 2D-LC.

CONCLUSIONS

Our study suggests that N-methyltetrahydroprotoberberines may serve as a new scaffold for κ-opioid receptor ligands. The strategy that we adopted can be applied to other naturally-occurring active alkaloids acting at different receptors.

BmK-YA, an enkephalin-like peptide in scorpion venom

By screening extracts of venom from the Asian scorpion Buthus martensii Karsch (BmK) for their abilities to activate opioid receptors, we have identified BmK-YA, an amidated peptide containing an enkephalin-like sequence. BmK-YA is encoded by a precursor that displays a signal sequence and contains four copies of BmK-YA sequences and four of His⁴-BmK-YA, all flanked by single amino acid residues. BmK-YA and His⁴-BmK-YA are amidated and thus fulfill the characteristics expected of bioactive peptides. BmK-YA can activate mammalian opioid receptors with selectivity for the δ subtype while His⁴-BmK-YA is inactive at opioid receptors. The discovery of BmK-YA suggests that scorpion venom may represent a novel source of bioactive molecules targeting G protein-coupled receptors (GPCRs) and reveal additional insights on the evolution of the opioid precursors.

Josef Rudinger Memorial Lecture 2002. The chemistry of the opioid receptor binding sites

Since the discovery of the opioid receptors and their endogenous ligands an immense research work has been devoted to the exploration of their specificity, the mechanism of ligand binding and ligand-receptor interactions. One of the main goals has been the location and characterization of the binding sites. The present review compiles the results achieved in this field in the last quarter of a century, and puts some questions concerning the success of these efforts.

Presence of a reduced opioid response in interleukin-6 knock out mice

Cytokines are known to influence neuronal functions. The purpose of this study was to investigate the putative role of the cytokine interleukin-6 (IL-6) in the pathways involved in opioid-mediated responses, by using IL-6-deficient mice. We reported that with a thermal stimulus IL-6-knock-out (IL-6KO) mice presented nociceptive thresholds similar to those measured in their controls. However, they showed a reduced analgesic response both to the restraint stress and to the administration of low doses of morphine. Hypothalamic levels of the opioid peptide beta-endorphin were significantly higher in IL-6KO mice than they were in their controls. The development of tolerance to the analgesic effect of morphine was more rapid in IL-6-deficient mice than in wild-type controls. Binding experiments showed that the number of opioid receptors in the midbrain, but not in the hypothalamus, decreased in IL-6KO mice. Autoradiographic binding analysis revealed that the density of mu receptors diminished while the delta-opioid receptors did not. These results suggest that IL-6 is necessary for a correct development of neuronal mechanisms involved in the response to both endogenous and exogenous opiates.

Functional analysis of cloned opioid receptors in transfected cell lines

Opioids modulate numerous central and peripheral processes including pain perception neuroendocrine secretion and the immune response. The opioid signal is transduced from receptors through G proteins to various different effectors. Heterogeneity exists at all levels of the transduction process. There are numerous endogenous ligands with differing selectivities for at least three distinct opioid receptors (mu, delta, kappa). G proteins activated by opioid receptors are generally of the pertussis toxin-sensitive Gi/Go class, but there are also opioid actions that are thought to involve Gq and cholera toxin-sensitive G proteins. To further complicate the issue, the actions of opioid receptors may be mediated by G-protein alpha subunits and/or beta gamma subunits. Subsequent to G protein activation several effectors are known to orchestrate the opioid signal. For example activation of opioid receptors increases phosphatidyl inositol turnover, activates K+ channels and reduces adenylyl cyclase and Ca2+ channel activities. Each of these effectors shows considerable heterogeneity. In this review we examine the opioid signal transduction mechanism. Several important questions arise: Why do opioid ligands with similar binding affinities have different potencies in functional assays? To which Ca2+ channel subtypes do opioid receptors couple? Do opioid receptors couple to Ca2+ channels through direct G protein interactions? Does the opioid-induced inhibition of vesicular release occur through modulation of multiple effectors? We are attempting to answer these questions by expressing cloned opioid receptors in GH3 cells. Using this well characterized system we can study the entire opioid signal transduction process from ligand-receptor interaction to G protein-effector coupling and subsequent inhibition of vesicular release.

Characterization of the murine mu opioid receptor gene

The analgesic and addictive properties of morphine and other opioid drugs are thought to result from their interaction with mu opioid receptors. Using a delta opioid receptor cDNA as a probe, we have isolated a murine mu opioid receptor cDNA clone (mMOR). Stable expression of mMOR in Chinese hamster ovary cells conferred high binding affinity for mu receptor ligands including morphine and [D-Ala2,N-methyl-Phe4,Gly5-ol]-enkephalin and low affinity for delta and kappa preferring ligands. Treatment of these cell lines with morphine and other mu agonists inhibited forskolin-induced cAMP accumulation, demonstrating a functional coupling of mMOR to the inhibition of adenylate cyclase. The predicted amino acid sequence of mMOR shares approximately 55% overall amino acid identity with the delta receptor and approximately 97% identity with the recently reported rat mu opioid receptor. Expression of the mu receptor in mouse brain as revealed by in situ hybridization parallels the reported pattern of distribution of mu-selective ligand binding sites. Chromosomal localization (to mouse chromosome 10) and Southern analysis are consistent with a single mu opioid receptor gene in the mouse genome, suggesting that the various pharmacologically distinct forms of the mu receptor arise from alternative splicing, post-translational events, or from a highly divergent gene(s).

Assessment of endogenous enkephalins efficacy in the hot plate test in mice: comparative study with morphine

The in vivo spare receptor population and the relative efficacies of morphine and the endogenous enkephalins to alleviate thermal nociceptive inputs were compared by using the mu irreversible antagonist beta-funaltrexamine (beta-FNA). Twenty-four hours after i.c.v. administration of beta-FNA at increasing concentrations (0.005-2.5 micrograms), parallel rightward shifts of both morphine and RB 101 (mixed enkephalin-degrading-enzyme inhibitor) dose-response curves, were observed, but the concentration of beta-FNA required to reduce the analgesic responses was about 10 times higher for RB 101 (0.1 microgram) than for morphine (0.01 microgram). The preferential involvement of mu receptors in the analgesic responses obtained after beta-FNA pretreatment, was supported by the inability of the delta-selective antagonist naltrindole to block these effects. In conclusion, it seems that to elicit the same antinociceptive responses, enkephalins could occupy a smaller proportion of mu opioid receptors than morphine, suggesting that the endogenous peptides have a higher efficacy.

Acute tolerance to the excitatory effects of opioids in the rat hippocampus

Prolonged iontophoretic administrations of delta- and mu-selective opioid receptor agonists were conducted in the hippocampus of rats, in order to study the possible development of acute tolerance to the excitatory effects of the opioids. Acute tolerance (AT) to the excitatory effects of the delta-selective opioid receptor agonist Tyr-D-Ser-Gly-Phe-Leu-Thr (DSLET) was observed when the drug was applied locally for 3-5 min in the CA1 hippocampal pyramidal neurons. The acute tolerance was expressed as a decrease in the commissurally evoked spike responsiveness during peptide's administration and led to a long-lasting potentiation of the population spike (PS) upon its withdrawal. In all cases, where AT and spike potentiation were evident, the population excitatory postsynaptic potential (pEPSP) remained unaltered. Pharmacological studies of AT and long-lasting spike potentiation showed the following: (1) the nonselective opioid receptor antagonist, naloxone, while effective in blocking the excitatory effects of DSLET when applied prior and during the application of the latter, failed to exhibit any effect on the long-lasting potentiating effect of the opioid; and (2) during the spike potentiation phase, administration of DSLET exhibited a depressant effect towards baseline values. This depressant effect of the opioid was evident 2-3 min from the beginning of the application and was completely antagonized by naloxone. The above results show that the development of acute tolerance to the excitatory effects of the DSLET led to long-lasting spike potentiation, which manifests a withdrawal phenomenon.

Purification to homogeneity of an active opioid receptor from rat brain by affinity chromatography

Active opioid binding proteins were solubilized from rat brain membranes in high yield with sodium deoxycholate in the presence of NaCl. Purification of opioid binding proteins was accomplished by opioid antagonist affinity chromatography. Chromatography using the delta-opioid antagonist N,N-diallyl-Tyr-D-Leu-Gly-Tyr-Leu attached to omega-aminododecyl-agarose (Affi-G) (procedure A) yielded a partially purified protein that binds selectively the delta-opioid agonist [3H]Tyr-D-Ser-Gly-Phe-Leu-Thr ([3H]DSLET), with a Kd of 19 +/- 3 nM and a Bmax of 5.1 +/- 0.4 nmol/mg of protein. Subsequently, Lens culinaris agglutinin-Sepharose 4B chromatography of the Affi-G eluate resulted in isolation of an electrophoretically homogeneous protein of 58 kDa that binds selectively [3H]DSLET with a Kd of 21 +/- 3 nM and a Bmax of 16.5 +/- 1.0 nmol/mg of protein. Chromatography using the nonselective antagonist 6-aminonaloxone coupled to 6-aminohexanoic acid-Sepharose 4B (Affi-NAL) (procedure B) resulted in isolation of a protein that binds selectively [3H]DSLET with a Kd of 32 +/- 2 nM and a Bmax of 12.4 +/- 0.5 nmol/mg of protein, and NaDodSO4/PAGE revealed a major band of apparent molecular mass 58 kDa. Polyclonal antibodies (Anti-R IgG) raised against the Affi-NAL protein inhibit the specific [3H]DSLET binding to the Affi-NAL eluate and to the solubilized membranes. Moreover, the Anti-R IgG inhibits the specific binding of radiolabeled Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol (DAMGO; mu-agonist), DSLET (delta-agonist), and naloxone to homogenates of rat brain membranes with equal potency. Furthermore, immunoaffinity chromatography of solubilized membranes resulted in the retention of a major protein of apparent molecular mass 58 kDa. In addition, immunoblotting of solubilized membranes and purified proteins from the Affi-G and Affi-NAL matrices revealed that the Anti-R IgG interacts with a protein of 58 kDa.

Morphine: new aspects in the study of an ancient compound

Morphine is the most widely used compound among narcotic analgesics and remains the gold standard when the effects of other analgetic drugs are compared. Apart from its presence in the poppy plant Papaver somniferum, morphine has been shown to be present in milk, cerebrospinal fluid and also in nervous tissue extracts. Recent evidence suggests that biosynthetic pathways for morphine exist in animal and even human tissues such as liver, blood and brain. The most characteristic effect of morphine is the modulation of pain perception resulting in an increase in the threshold of noxious stimuli. Antinociception induced by morphine is mediated via opioid receptors and therefore can be inhibited by opioid antagonists, e.g., naloxone. Nevertheless, consideration of morphine as endogenous ligand for opioid receptors seems to be speculative. Recently, the primary receptor for morphine-type drugs called the mu-opioid receptor has been cloned from rat brain. There is accumulating evidence that morphine actions are, at least partly, due to one of its major metabolite morphine-6-glucuronide in man. It is concluded that further investigations are necessary to elucidate the mechanisms, whereby multiple actions of morphine are expressed in the nervous system.

Pharmacological characterization of an opioid receptor in the ciliate Tetrahymena

A pharmacological characterization has been performed of the opioid receptor involved in modulation of phagocytosis in the protozoan ciliate Tetrahymena. Studies on inhibition of phagocytosis by mammalian prototypic opioid agonists revealed that morphine and beta-endorphin have the highest intrinsic activity, whereas all the other opioids tested can only be considered partial agonists. However, morphine (a mu-receptor agonist) is twice as potent as beta-endorphin (a delta-receptor agonist). Furthermore, the sensitivity for the opioid antagonist naloxone, determined in the presence of morphine and beta-endorphin, is very similar to the sensitivity exhibited by mammalian tissues rich in mu-opioid receptors. We suggest that the opioid receptor coupled to phagocytosis in Tetrahymena is mu-like in some of its pharmacological characteristics and may serve as a model system for studies on opioid receptor function and evolution.

The delta-opioid signal transduction on the gonadotropin-releasing hormone release is eicosanoid dependent

In static incubations, the K+ induced release of gonadotropin-releasing hormone (GnRH) and of prostaglandin (PG) E2, was 2-3 times higher in the isolated median eminence (ME) compared to the hypothalamic area containing the arcuate nucleus (ARN) plus the ME. The delta-opioid agonist DTLET, induced a parallel, dose-dependent reduction of GnRH and PGE2 release in the ARN plus ME. Both effects of DTLET were blocked by the delta-opioid antagonist Diallyl-G. In the isolated ME, DTLET reduced the secretion of PGE2 but enhanced the release of GnRH. In this area Diallyl-G had no effect on the PGE2 release but blocked the GnRH secretion. When the PGE2 production was blocked by indomethacin in the ARN plus ME preparation, DTLET increased the release of GnRH and induced the production of leukotrienes (LTs). On the other hand, DTLET decreased the release of both GnRH and PGE2 in the presence of nordihydroguaiaretic acid (NDGA), an inhibitor of the production of LTs. The above results suggest that: (a) the delta-opioid agonist DTLET modulates GnRH release differentially in the hypothalamic areas examined; and (b) the arachidonic acid metabolites are involved in the mode of action of DTLET on the release of GnRH in the ARN plus ME hypothalamic fragment.

Zeta (zeta), the opioid growth factor receptor: identification and characterization of binding subunits

The zeta (zeta) opioid receptor mediates the activity of the opioid growth factor, [Met5]-enkephalin, a peptide that regulates developmental events in a variety of normal and tumorigenic tissues and cells, including the nervous system. To identify the binding subunit(s) of the zeta receptor, protein blots of rat cerebellar proteins from 6-day-old animals were separated by sodium dodecyl sulfate 10% polyacrylamide gel electrophoresis (SDS-PAGE) and electrophoretically transferred onto nitrocellulose. Ligand blotting of these blots with 1.5 nM [125I]-[Met5]-enkephalin revealed four binding polypeptides of 32, 30, 17, and 16 kDa. Binding occurred at concentrations relevant to the Kd of the receptor, was blocked by cold ligand and opioid antagonists, and exhibited a stereospecific response. No binding was recorded in the adult rat cerebellum. Subcellular fractionation studies using ligand blotting and receptor-binding analysis indicated that these binding subunits were associated with the nucleus. Two-dimensional protein analysis using non-equilibrium pH gradient electrophoresis (NEPHGE) SDS-PAGE of 6-day-old cerebellum and ligand blotting showed that the 32-, 30-, 17-, and 16-kDa subunits have basic isoelectric points. Two-dimensional chymotryptic peptide mapping showed a strong homology between the 32- and 30-kDa subunits, but not with the 17- and 16-kDa polypeptides. The 17- and 16-kDa subunits had only a partial homology to each other. These results are consistent with the biology, biochemistry, and pharmacology of the zeta receptor, and are the first to identify and characterize the binding polypeptides of an opioid receptor that has important growth-related properties.

The kappa-opioid receptor: evidence for the different subtypes

Classification of drugs acting on the kappa-opioid receptors seems to be difficult, since some of these ligands are also sigma agonists and/or display non-opioid actions as well. Furthermore, certain benzomorphans having kappa-agonistic character, are shown to be mu-antagonists too. Therefore the classification of the kappa-opioid receptor has to be presently restricted to two subclasses that also have physiological meaning. Dynorphin and Met-enkephalin-Arg6-Phe7 are proposed as endogenous peptide ligands for kappa-receptors. Nonpeptide agonists are benzeneacetamides interacting with the kappa1 receptor. Benzomorphans bind to both subtypes of kappa-receptors. No selective nonpeptide ligand for the kappa2 receptor exists as yet. Nor-binaltorphimine, a specific kappa-antagonist also inhibits both kappa-subtypes. Further research for kappa2 selective drugs is necessary for clear distinction between the two kappa-opioid binding sites. Molecular cloning of opioid receptors including their subtypes are expected to provide direct proof of their existence.

The modulatory effects of mu and kappa opioid agonists on 5-HT release from hippocampal and hypothalamic slices of euthermic and hibernating ground squirrels

To elucidate the role of opioids in regulating hibernation, the modulatory effects of different opioids on 35 mM K(+)-stimulated [3H]-5-HT release from brain slices were examined in the Richardson's ground squirrels. DAGO ([D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin), a specific mu agonist, evoked a significant dose-dependent (10(-7)-10(-5) M) inhibition of K(+)-stimulated 5-HT release from hippocampal slices of the non-hibernating squirrels. The inhibitory effect of DAGO was attenuated by either the opioid antagonist naloxone (10(-6) M) or the voltage dependent sodium channel blocker tetrodotoxin (TTX, 10(-6) M). The inhibitory effect of DAGO persisted in the hibernating squirrels; however, a ten fold higher concentration of DAGO (10(-6)-10(-5) M) was required to elicit a significant inhibition. In contrast, kappa agonist U50488 (10(-5) M) exerted a significant enhancement of K(+)-stimulated 5-HT release from hippocampal slices of the non-hibernating squirrels. This enhancement was blocked by either the specific kappa antagonist nor-binaltorphimine (10(-6) M) or TTX (10(-6) M). However, in the hibernating squirrels, the stimulatory effect of U50488 (10(-5) M) on 5-HT release was absent. DAGO and U50488 had no modulatory effects on K(+)-stimulated 5-HT release from the hypothalamic slices of either the non-hibernating or hibernating squirrels. These results demonstrate that the modulatory effects of opioids on 5-HT release are receptor-specific and state-dependent, indicating the complex nature of the roles of different opioids in regulating hibernation.

Repeated systemic administration of the mixed inhibitor of enkephalin-degrading enzymes, RB101, does not induce either antinociceptive tolerance or cross-tolerance with morphine

The potent analgesic responses elicited by systemic administration of RB101, N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyldithio]-1-oxopro pyl]- 1-oxopropyl]-L-phenylalanine benzyl ester, a prodrug able to inhibit enkephalin-degrading enzymes completely after in vivo bioactivation, has made it possible to investigate the development of antinociceptive tolerance after chronic potentiation of endogenous enkephalins. The ED50 values of RB101 obtained 10 min after i.v. injection were not significantly different in mice treated for 4 days with i.p. administered vehicle (ED50 = 9.50 (6.37-14.15) mg/kg), or with 80 mg/kg of RB101 twice daily (ED50 = 9.50 (5.86-15.39) mg/kg). In contrast, a parallel rightwards shift of the dose-response curves, corresponding to a significant 1.92 (1.49-2.52)-fold decrease in analgesic potency, was observed after i.v. administration of morphine in mice chronically treated with morphine (3 mg/kg, twice daily for 4 days) (ED50 = 3.10 (2.52-3.81) mg/kg) vs. saline (ED50 = 1.60 (1.22-2.09) mg/kg). No tolerance to RB101 was observed even after a longer period (8 days) of chronic treatment with the prodrug. Moreover, no cross-tolerance between morphine and RB101 appeared to occur since the ED50 values obtained after i.v. administration of RB101 were not significantly different in mice chronically pretreated with vehicle (ED50 = 9.50 (6.37-14.15) mg/kg) or with morphine (ED50 = 10.00 (6.62-15.10) mg/kg). The analgesic effect of RB101 observed in morphine-tolerant mice was antagonized by prior injection of naloxone, but not naltrindole (delta-selective antagonist), supporting a preferential involvement of mu-opioid receptors in the antinociceptive effect of RB101, at least in mice in the hot-plate test.(ABSTRACT TRUNCATED AT 250 WORDS)

Microinjection of opioid antagonists into the substantia nigra reduces stress-induced eating in rats

Stress produced by pinching the tail has been shown to cause satiated animals to eat and to display oral stereotypies. Endogenous opioids and central dopamine systems have been implicated in the mediation of these effects. In order to test the possibility that the substantia nigra (SN) might be involved, the amount of food intake and gnawing produced by mild tail pinch were assessed following bilateral microinjections of opioid antagonists into the SN. Evaluations of nociceptive thresholds were also conducted using tail flick and hot plate tests. Eating induced by tail pinch was reduced by microinjections of the non-selective opioid antagonist naloxone (3, 10, 20 and 30 nmol) and by the mu-selective antagonist Cys2, Tyr3, Orn5, Pen7 Amide (CTOP) (1, 3 and 10 nmol). These effects on eating occurred in the absence of effects on gnawing. kappa- and delta-antagonists (10 nmol) had no effect on eating or gnawing. Naloxone did not alter either tail flick or hot-plate response latencies. The highest dose of CTOP increased response latency on the hot-plate test only. The results are interpreted as suggesting that the SN may be an important central site of action for opioid antagonists in reducing stress-induced eating. The possibility that the SN may be a central site mediating the effects of dopamine on this phenomenon is also discussed.

Purification and reconstitution of mu-opioid receptors in liposome

Opioid receptors solubilized from rat brain membranes with digitonin were partially purified with a newly prepared affinity resin, AF-Amino Toyopearl, coupled with a mu-antagonist Tyr-Pro-Tyr-Tyr at the C-terminus of the peptide. The purified materials were reconstituted with an inhibitory GTP-binding protein (Gi) in liposome. From displacement analyses, two binding states, with a high and a low affinities for the mu-agonist [D-Ala2,Me-Phe4,Gly-ol5]enkephalin, were observed in the reconstituted system with Gi, only a low-affinity state was observed in the reconstituted system without Gi. The results suggested that the purified materials contained the mu-opioid receptors and could functionally couple with Gi as observed in the cell membranes.

Effects of nor-binaltorphimine on the development of analgesic tolerance to and physical dependence on morphine

The effects of a highly selective kappa antagonist, nor-binaltorphimine (nor-BNI), on the development of tolerance to morphine analgesia and physical dependence on morphine were examined. Pretreatment with nor-BNI (5 mg/kg s.c.) 2 h prior to injection of morphine or a selective kappa agonist, U-50,488H, significantly antagonized the analgesic effect of U-50,488H, but not morphine analgesia in mice. The development of tolerance to morphine analgesia was significantly potentiated by pretreatment of mice with nor-BNI 2 h prior to morphine treatment during chronic morphine treatment for 5 days. Additionally, the pretreatment with nor-BNI during chronic treatment with the high dose of morphine for 5 days significantly potentiated the naloxone-induced body weight loss in morphine-dependent mice and rats. These findings suggest that inactivation of the kappa opioid system may potentiate the development of tolerance to morphine analgesia in mice and may aggravate the naloxone-precipitated body weight loss in morphine-dependent mice and rats.

Preparation and biodistribution in mice of [11C]carfentanil: a radiopharmaceutical for studying brain mu-opioid receptors by positron emission tomography

A potent mu-opioid agonist, [11C]carfentanil, was prepared by the methylation of carfentanil carboxylic acid with [11C]methyl iodide in order to study brain mu-opioid receptors by positron emission tomography. Synthesis (including purification) was completed within 25 min and the radiochemical yield was approximately 40%. The radiochemical purity of the product was more than 99% and its specific activity was 3.7-7.4 GBq/mumol. Biodistribution studies performed in mice after intravenous injection showed a high brain uptake and rapid blood clearance, so a high brain/blood ratio of 1.5-1.8 was found from 5 to 30 min. Regional cerebral distribution studies in the mouse showed a significantly higher uptake of [11C]carfentanil by the thalamus and striatum than by the cerebellum, with the radioactivity in the striatum disappearing more rapidly than that in the thalamus. Treatment with naloxone significantly reduced the uptake of [11C]carfentanil by the thalamus and striatum. These results indicate that [11C]carfentanil binds specifically to brain mu-opioid receptors.

Opioid agonists binding and responses in SH-SY5Y cells

SH-SY5Y (human neuroblastoma) cultured cells, known to have mu-opioid receptors, have been used to assess and compare the ability of eight representative mu-selective compounds from diverse opioid families to recognize and activate these receptors. A wide range of receptor affinities spanning a factor of 10,000 was found between the highest affinity fentanyl analogs (Ki = 0.1nM) and the lowest affinity analog, meperidine (Ki = 1 microM). A similar range was found for inhibition of PGE1-stimulated cAMP accumulation with a rank order of activities that closely paralleled binding affinities. Maximum inhibition of cAMP accumulation by each compound was about 80%. Maximum stimulation of GTPase activity (approximately 50%) was also similar for all compounds except the lowest affinity meperidine. Both effects were naloxone reversible. These results provide further evidence that mu-receptors are coupled to inhibition of adenylate cyclase and that the SH-SY5Y cell line is a good system for assessment of mu-agonists functional responses.

Effect of opioid receptor antagonists on vasodilator nerve actions in the perfused rat mesentery

Our previous work suggests that opioid peptides modulate sensory nerves in the perfused rat mesentery. Therefore we tested the hypothesis that opioids are involved in the ongoing regulation of sensory nerve activity using selective opioid receptor antagonists. In the presence of guanethidine and methoxamine, transmural nerve stimulation caused a vasodilator response which was potentiated significantly by naloxone (3 x 10(-7) M). However, naloxone did not affect vasodilator responses to exogenous calcitonin gene related peptide. IC1 174.864 (3 x 10(-7) M), a selective delta receptor antagonist, had no effect on vasodilator responses to transmural nerve stimulation. In contrast CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH2) (3 x 10(-7) M), a selective mu receptor antagonist, significantly inhibited vasodilator responses to transmural nerve stimulation, effects which were abolished by naloxone treatment. In preparations pretreated with beta-FNA (beta-funaltrexamine HCl), an irreversible mu receptor antagonist, naloxone no longer potentiated vasodilator responses to transmural nerve stimulation. These results suggest that potentiation of vasodilator responses to transmural nerve stimulation by naloxone may be due to blockade of mu receptors, resulting in a reduced inhibitory modulation by endogenous opioids. These findings support the contention that prejunctional opioid receptors on sensory nerves may play a role in modulating activity of the cardiovascular system.

Glial growth is regulated by agonists selective for multiple opioid receptor types in vitro

To determine whether one or more opioid receptor types might be preferentially involved in gliogenesis, primary mixed glial cultures derived from mouse cerebra were continuously treated with varying concentrations of opioid agonists selective for mu (mu), i.e., DAGO ([D-Ala2, MePhe4, Gly(ol)5]enkephalin), delta (delta), i.e., DPDPE ([D-PEN2,D-PEN5]enkephalin), or kappa (kappa), i.e., U69,593, opioid receptor types. In addition, a group of cultures was treated with [Met5]-enkephalin, an agonist for delta opioid receptors as well as putative zeta (zeta) opioid receptors. Opioid-dependent changes in growth were assessed by examining alterations in (1) the number of cells in mixed glial cultures at 3, 6, and 8 days in vitro (DIV), (2) [3H]thymidine incorporation by glial fibrillary acidic protein (GFAP) immunoreactive, flat (type 1) astrocytes at 6 DIV, and (3) the area and form factor of GFAP-immunoreactive, flat (type 1) astrocytes. DPDPE at 10(-8) or 10(-10) M, as well as [Met5]-enkephalin at 10(-6), 10(-8), or 10(-10) M, significantly reduced the total number of glial cells in culture; but this effect was not observed with DAGO or U69,593 (both at 10(-6), 10(-8), or 10(-10) M). Equimolar concentrations (i.e., 10(-6) M) of [Met5]enkephalin or U69,593, but not DPDPE or DAGO, suppressed the rate of [3H]thymidine incorporation by GFAP-immunoreactive, flat (type 1) astrocytes. DAGO had no effect on growth, although in previous studies morphine was found to inhibit glial numbers and astrocyte DNA synthesis. [Met5]enkephalin (10(-6) M) was the only agonist to significantly influence astrocyte area. Collectively, these results indicate that delta (and perhaps mu) opioid receptor agonists reduce the total number of cells in mixed glial cultures; while [Met5]enkephalin-responsive (and perhaps kappa-responsive) opioid receptors mediate DNA synthesis in astrocytes. This implies that delta opioid receptors, as well as [Met5]enkephalin-sensitive, non-delta opioid receptors, mediate opioid-dependent regulation of astrocyte and astrocyte progenitor growth. These data support the concept that opioid-dependent changes in central nervous system growth are the result of endogenous opioid peptides acting through multiple opioid receptor types.

A general role for adaptations in G-proteins and the cyclic AMP system in mediating the chronic actions of morphine and cocaine on neuronal function

Previous studies have shown that chronic morphine increases levels of the G-protein subunits Gia and Goa, adenylate cyclase, cyclic AMP-dependent protein kinase, and certain phosphoproteins in the rat locus coeruleus, but not in several other brain regions studied, and that chronic morphine decreases levels of Gia and increases levels of adenylate cyclase in dorsal root ganglion/spinal cord (DRG-SC) co-cultures. These findings led us to survey the effects of chronic morphine on the G-protein/cyclic AMP system in a large number of brain regions to determine how widespread such regulation might be. We found that while most regions showed no regulation in response to chronic morphine, nucleus accumbens (NAc) and amygdala did show increases in adenylate cyclase and cyclic AMP-dependent protein kinase activity, and thalamus showed an increase in cyclic AMP-dependent protein kinase activity only. An increase in cyclic AMP-dependent protein kinase activity was also observed in DRG-SC co-cultures. Morphine regulation of G-proteins was variable, with decreased levels of Gia seen in the NAc, increased levels of Gia and Goa in amygdala, and no change in thalamus or the other brain regions studied. Interestingly, chronic treatment of rats with cocaine, but not with several non-abused drugs, produced similar changes compared to morphine in G-proteins, adenylate cyclase, and cyclic AMP-dependent protein kinase in the NAc, but not in the other brain regions studied. These results indicate that regulation of the G-protein/cyclic AMP system represents a mechanism by which a number of opiate-sensitive neurons adapt to chronic morphine and thereby develop aspects of opiate tolerance and/or dependence. The findings that chronic morphine and cocaine produce similar adaptations in the NAc, a brain region important for the reinforcing actions of many types of abused substances, suggest further that common mechanisms may underlie psychological aspects of drug addiction mediated by this brain region.

Opioid receptor labeling with the chloromethyl ketone derivative of [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol (DAMGO) II: Covalent labeling of mu opioid binding site by 3H-Tyr-D-Ala-Gly-(Me)Phe chloromethyl ketone

Opioid receptors of rat brain membranes were prelabeled with 3H-Tyr-D-Ala2-(Phe4)-Gly-CH2Cl, a chloromethyl ketone derivative of enkephalin, and solubilized in 1% digitonin. Hydrodynamic parameters of the receptor detergent complex derived from gel filtration and sucrose density gradient ultracentrifugation were found to be 51 A and 8.7 S, respectively, and the size was estimated to be about 200 kDa. Sodium dodecyl sulfate gel electrophoresis followed by fluorography revealed specific alkylation of a major protein at 58 kDa.

Evidence for phosphatidylinositol anchorage of opioid binding proteins in rat brain

Treatment of rat brain sections and of thalamic and striatal membranes with phosphatidylinositol phospholipase C (PIPLC), an enzyme known to hydrolyse only phosphatidylinositol and its derivatives, significantly alters the specific binding of mu- and delta-opioid ligands on their receptors. These results suggest that some opioid binding proteins are membrane-anchored by a glycosylphosphatidylinositol (GPI) linkage.