Cross-linking of human 125I-beta-endorphin to a mu-delta opioid receptor complex in rat striatum

Contributions of the International Narcotics Research Conference to Opioid Research Over the Past 50 years

The International Narcotics Research Conference (INRC), founded in 1969, has been a successful forum for research into the actions of opiates, with an annual conference since 1971. Every year, scientists from around the world have congregated to present the latest data on novel opiates, opiate receptors and endogenous ligands, mechanisms of analgesic activity and unwanted side effects, etc. All the important discoveries in the opiate field were discussed, often first, at the annual INRC meeting. With an apology to important events and participants not discussed, this review presents a short history of INRC with a discussion of groundbreaking discoveries in the opiate field and the researchers who presented from the first meeting up to the present.

Evidence for a mu-delta opioid receptor complex in CHO cells co-expressing mu and delta opioid peptide receptors

Based on non-competitive binding interactions we suggested that mu and delta receptors associate as a mu/delta receptor complex in rat brain. We hypothesized that the same non-competitive binding interactions observed in rat brain will be seen in CHO cells that co-express mu and delta receptors, but not in cells that express just mu or delta receptors. We used CHO cells expressing the cloned human mu receptor, cloned human delta receptor, or cloned mouse delta/human mu ("dimer cell"). Cell membranes were prepared from intact cells pretreated with 100nM SUPERFIT. [(3)H][d-Ala(2),d-Leu(5)]enkephalin binding assays followed published procedures. SUPERFIT, a delta-selective irreversible ligand, decreased [(3)H][d-Ala(2),d-Leu(5)]enkephalin binding to delta receptors by approximately 75% and to mu receptors by approximately 50% in dimer cells. SUPERFIT treatment did not decrease [(3)H][d-Ala(2),d-Leu(5)]enkephalin binding to mu cells. The IC(50) values observed in SUPERFIT-treated dimer cells were: [d-Pen(2),d-Pen(5)]enkephalin (1820nM) and morphine (171nM). Saturation binding experiments with SUPERFIT-treated dimer cells showed that [d-Pen(2),d-Pen(5)]enkephalin (5000nM) was a competitive inhibitor. In contrast, morphine (1000nM) lowered the B(max) from 1944fmol/mg to 1276fmol/mg protein (35% decrease). Both [d-Pen(2),d-Pen(5)]enkephalin and morphine competitively inhibited [(3)H][d-Ala(2),d-Leu(5)]enkephalin binding to SUPERFIT-treated mu cells. The results indicate that the mu-delta opioid receptor complex defined on the basis of non-competitive binding interactions in rat brain over 20 years ago likely occurs as a consequence of the formation of mu-delta heterodimers. SUPERFIT-treated dimer cells may provide a useful model to study the properties of mu-delta heterodimers.

Inflammation enhances mu-opioid receptor transcription and expression in mice intestine

Opioid receptors (ORs) and their mRNA are present in the central and peripheral nervous systems of mammals and in different peripheral tissues, including the gut. Using a model of croton oil-induced (CO) intestinal inflammation in mice, we have shown a 6-fold increase in the potency of the antitransit and antisecretory effects of mu-OR agonists, mediated by peripheral ORs. We postulate that the enhanced effects are mediated by an increase in the expression of intestinal OR. We used jejunum (stripped of the mucosal layer) from mice with CO-induced intestinal inflammation and, as control subjects, saline-treated animals (SS). We evaluated the quantity of mu-OR mRNA determined by a competitive reverse-transcriptase polymerase chain reaction; the levels of mu-OR protein by Western blot immunoassay, and the localization and number of cells expressing mu-OR using immunohistochemistry. The results show a significant increase of mu-OR mRNA (7.7-fold) and receptor protein (3-fold) during intestinal inflammation. Inflammation also induced a 64.3% increase in the number of neurons expressing mu-OR immunoreactivity in the myenteric plexus but not in the submucosal plexus. Our results show that intestinal inflammation enhances the transcription and translation of mu-OR mRNA, thus explaining the increased potency of mu-opioids during inflammation.

Quantitative evaluation of the lengths of homobifunctional protein cross-linking reagents used as molecular rulers

Homobifunctional chemical cross-linking reagents are important tools for functional and structural characterization of proteins. Accurate measures of the lengths of these molecules currently are not available, despite their widespread use. Stochastic dynamics calculations now provide quantitative measures of the lengths, and length dispersions, of 32 widely used molecular rulers. Significant differences from published data have been found.

SUPPLEMENTAL MATERIAL

See www.proteinscience.org

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.

Identification of the opioid receptor types mediating beta-endorphin-induced alterations in dopamine release in the nucleus accumbens

In the present study we used in vivo microdialysis to examine the influence of beta-endorphin on dopamine (DA) release in the nucleus accumbens of anesthetized rats and to identify the opioid receptor types mediating its effects. Microdialysis probes were inserted into the nucleus accumbens and perfusates were analysed for DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), using a reversed phase HPLC system with electrochemical detection for separation and quantification. Intracerebroventricular (i.c.v.) administration of beta-endorphin resulted in a dose-dependent increase in DA and its metabolites. Pretreatment with the selective delta-antagonist ICI 174,864 significantly attenuated the beta-endorphin-induced increase in DA release and metabolism whereas pretreatment with the selective mu-antagonist CTOP resulted abolition of the beta-endorphin effect. These data demonstrate that the blockade of either mu- or delta-opioid receptors is sufficient to antagonize the stimulatory effects of beta-endorphin on DA release and metabolism. As such, these findings suggest that the concomitant activation of both mu- and delta-receptors underlies the effects of beta-endorphin on DA release in the nucleus accumbens.