The effects of endomorphin-1 and endomorphin-2 in CHO cells expressing recombinant mu-opioid receptors and SH-SY5Y cells

Design of κ-Opioid Receptor Agonists for the Development of Potential Treatments of Pain with Reduced Side Effects

The κ-opioid receptor (KOR) has recently emerged as an alternative therapeutic target for the development of pain medications, without deleterious side effects associated with the μ-opioid receptor (MOR). However, modulation of KOR is currently under investigation for the treatment of depression, mood disorders, psychiatric comorbidity, and specific drug addictions. However, KOR agonists also trigger adverse effects including sedation, dysphoria, and hallucinations. In this respect, there is currently much debate on alternative paradigms. Recent effort has been devoted in search of biased ligands capable of selectively activating favorable signaling over signaling associated with unwanted side effects. On the other hand, the use of partial agonists is expected to allow the analgesia to be produced at dosages lower than those required to produce the adverse effects. More empirically, the unwanted central effects can be also avoided by using peripherally restricted agonists. In this review, we discuss the more recent trends in the design of KOR-selective, biased or partial, and finally, peripherally acting agonists. Special emphasis is given on the discussion of the most recent approaches for controlling functional selectivity of KOR-specific ligands.

Effect of Endomorphin-1 on Contractile Activity of Rat Mesenteric Lymphatic Vessels

We studied the effect of endomorphin-1 on isolated mesenteric lymphatic vessels in rats. It was found that endomorphin-1 caused a dose-dependent increase in the contractile activity of lymphangions, which was associated with stimulation of intracellular calcium depots. The observed effect of endomorphin-1 in isolated lymphatic vessels has a complex mechanism; it depends on the concentration of the applied peptide and is probably determined by its interaction with non-opioid receptors.

Constraining Endomorphin-1 by β,α-Hybrid Dipeptide/Heterocycle Scaffolds: Identification of a Novel κ-Opioid Receptor Selective Partial Agonist

Herein we present the expedient synthesis of endomorphin-1 analogues containing stereoisomeric β²-homo-Freidinger lactam-like scaffolds ([Amo²]EM), and we discuss opioid receptor (OR) affinity, enzymatic stability, functional activity, in vivo antinociceptive effects, and conformational and molecular docking analysis. Hence, H-Tyr-Amo-Trp-PheNH₂ resulted to be a new chemotype of highly stable, selective, partial KOR agonist inducing analgesia, therefore displaying great potential interest as a painkiller possibly with reduced harmful side effects.

Food-Derived Hemorphins Cross Intestinal and Blood-Brain Barriers In Vitro

A qualitative study is presented, where the main question was whether food-derived hemorphins, i.e., originating from digested alimentary hemoglobin, could pass the intestinal barrier and/or the blood-brain barrier (BBB). Once absorbed, hemorphins are opioid receptor (OR) ligands that may interact with peripheral and central OR and have effects on food intake and energy balance regulation. LLVV-YPWT (LLVV-H4), LVV-H4, VV-H4, VV-YPWTQRF (VV-H7), and VV-H7 hemorphins that were previously identified in the 120 min digest resulting from the simulated gastrointestinal digestion of hemoglobin have been synthesized to be tested in in vitro models of passage of IB and BBB. LC-MS/MS analyses yielded that all hemorphins, except the LLVV-H4 sequence, were able to cross intact the human intestinal epithelium model with Caco-2 cells within 5-60 min when applied at 5 mM. Moreover, all hemorphins crossed intact the human BBB model with brain-like endothelial cells (BLEC) within 30 min when applied at 100 µM. Fragments of these hemorphins were also detected, especially the YPWT common tetrapeptide that retains OR-binding capacity. A cAMP assay performed in Caco-2 cells indicates that tested hemorphins behave as OR agonists in these cells by reducing cAMP production. We further provide preliminary results regarding the effects of hemorphins on tight junction proteins, specifically here the claudin-4 that is involved in paracellular permeability. All hemorphins at 100 µM, except the LLVV-H4 peptide, significantly decreased claudin-4 mRNA levels in the Caco-2 intestinal model. This in vitro study is a first step toward demonstrating food-derived hemorphins bioavailability which is in line with the growing body of evidence supporting physiological functions for food-derived peptides.

MEL-N16: A Series of Novel Endomorphin Analogs with Good Analgesic Activity and a Favorable Side Effect Profile

Opioid peptides are neuromodulators that bind to opioid receptors and reduce pain sensitivity. Endomorphins are among the most active endogenous opioid peptides, and they have good affinity and selectivity toward the μ opioid receptor. However, their clinical usage is hindered by their inability to cross the blood-brain barrier and their poor in vivo activity after peripheral injection. In order to overcome these defects, we have designed and synthesized a series of novel endomorphin analogs with multiple site modifications. Radioligand binding, cAMP accumulation, and β-arrestin-2 recruitment assays were employed to determine the activity of synthesized endomorphin analogs toward opioid receptors. The blood-brain barrier permeability and antinociceptive effect of these analogs were determined in several rodent models of acute and persistent pain. In addition, the side effects of the analogs were examined. The radioligand binding assay and functional activity examination indicated that the MEL-N16 series of compounds were more active agonists against μ opioid receptor than were the parent peptides. Notably, the analogs displayed biased downstream signaling toward G-protein pathways over β-arrestin-2 recruitment. The analogs showed highly potent antinociceptive effects in the tested nociceptive models. In comparison with endomorphins, the synthesized analogs were better able to penetrate the blood-brain barrier and exerted their pain regulatory activity in the central nervous system after peripheral injection. These analogs also have lower tendency to cause side effects than morphine does at similar or equal antinociceptive doses. The MEL-N16 compounds have highly potent and efficacious analgesic effects in various pain models with a favorable side effect profile.

Opioid activity profiles of oversimplified peptides lacking in the protonable N-terminus

Recently, we described cyclopeptide opioid agonists containing the d-Trp-Phe sequence. To expand the scope of this atypical pharmacophore, we tested the activity profiles of the linear peptides Ac-Xaa-Phe-Yaa (Xaa = l/d-Trp, d-His/Lys/Arg; Yaa = H, GlyNH(2)). Ac-d-Trp-PheNH(2) appeared to be the minimal binding sequence, while Ac-d-Trp-Phe-GlyNH(2) emerged as the first noncationizable short peptide (partial) agonist with high μ-opioid receptor affinity and selectivity. Conformational analysis suggested that 5 adopts in solution a β-turn conformation.

Lipo-endomorphin-1 derivatives with systemic activity against neuropathic pain without producing constipation

To enhance the drug-like properties of the endogenous opioid peptide endomorphin-1 (1 = Tyr-Pro-Trp-Phe-NH(2)), the N-terminus of the peptide was modified with 2-aminodecanoic acid, resulting in compound 3. Tyr in compound 1 was replaced with 2,6-dimethyltyrosine yielding compound 2. Derivative 2 was also substituted with 2-aminodecanoic acid producing compound, 4. Lipoamino acid-modified derivatives showed improved metabolic stability and membrane permeability while maintaining high μ-opioid (MOP) receptor binding affinity and acting as a potent agonist. In vivo studies showed dose-dependent antinociceptive activity following intravenous (i.v.) administration of compounds 3 and 4 in a chronic constriction injury (CCI)-rat model of neuropathic pain with ED(50) values of 1.22 (± 0.93) and 0.99 (± 0.89) µmol/kg, respectively. Pre-treatment of animals with naloxone hydrochloride significantly attenuated the anti-neuropathic effects of compound 3, confirming the key role of opioid receptors in mediating antinociception. In contrast to morphine, no significant constipation was produced following i.v. administration of compound 3 at 16 µmol/kg. Furthermore, following chronic administration of equi-potent doses of compound 3 and morphine to rats, there was less antinociceptive tolerance for compound 3 compared with morphine.

G protein-coupled receptor heteromerization: a role in allosteric modulation of ligand binding

It is becoming increasingly recognized that G protein-coupled receptors physically interact. These interactions may provide a mechanism for allosteric modulation of receptor function. In this study, we examined this possibility by using an established model system of a receptor heteromer consisting of μ and δ opioid receptors. We examined the effect of a number of μ receptor ligands on the binding equilibrium and association and dissociation kinetics of a radiolabeled δ receptor agonist, [(3)H]deltorphin II. We also examined the effect of δ receptor ligands on the binding equilibrium and association and dissociation kinetics of a radiolabeled μ receptor agonist, [(3)H][d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin ([(3)H]DAMGO). We show that μ receptor ligands are capable of allosterically enhancing δ receptor radioligand binding and vice versa. Thus, there is strong positive cooperativity between the two receptor units with remarkable consequences for ligand pharmacology. We find that the data can be simulated by adapting an allosteric receptor model previously developed for small molecules, suggesting that the ligand-occupied protomers function as allosteric modulators of the partner receptor's activity.

Synthesis and in vitro evaluation of a library of modified endomorphin 1 peptides

Endomorphin 1 (Endo-1=Tyr-Pro-Trp-Phe-NH(2)), an endogenous opioid with high affinity and selectivity for mu-opioid receptors, mediates acute and neuropathic pain in rodents. To overcome metabolic instability and poor membrane permeability, the N- and C-termini of Endo-1 were modified by lipoamino acids (Laa) and/or sugars, and 2',6'-dimethyltyrosine (Dmt) replacement of Tyr. Analogues were assessed for mu-opioid receptor affinity, inhibition of cAMP accumulation, enzymatic stability, and permeability across Caco-2 cell monolayers. C-terminus modification decreased receptor affinity, while N-terminus C8-Laa improved stability and permeability with slight change in receptor affinity. Dmt provided a promising lead compound: [C8Laa-Dmt[1]]-Endo-1 is nine times more stable (t(1/2)=43.5min), >8-fold more permeable in Caco-2 cell monolayers, and exhibits 140-fold greater mu-opioid receptor affinity (K(imu)=0.08nM).

From MIF-1 to endomorphin: the Tyr-MIF-1 family of peptides

The Tyr-MIF-1 family of small peptides has served a prototypic role in the introduction of several novel concepts into the peptide field of research. MIF-1 (Pro-Leu-Gly-NH(2)) was the first hypothalamic peptide shown to act "up" on the brain, not just "down" on the pituitary. In several situations, including clinical depression, MIF-1 exhibits an inverted U-shaped dose-response relationship in which increasing doses can result in decreasing effects. This tripeptide also can antagonize opiate actions, and the first report of such activity also correctly predicted the discovery of other endogenous antiopiate peptides. The tetrapeptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH(2)) not only shows antiopiate activity, but also considerable selectivity for the mu-opiate binding site. Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH(2)) is an even more selective ligand for the mu receptor, leading to the discovery of two more Tyr-Pro tetrapeptides that have the highest specificity and affinity for this site. These are the endomorphins: endomorphin-1 is Tyr-Pro-Trp-Phe-NH(2) and endomorphin-2 is Tyr-Pro-Phe-Phe-NH(2). Tyr-MIF-1 proved, contrary to the then prevailing dogma, that peptides can be saturably transported across the blood-brain barrier by a quantifiable transport system. Unexpectedly, the Tyr-MIF-1 transporter is shared with Met-enkephalin. In the era in which it was doubtful whether a peripheral peptide could exert CNS effects, the Tyr-MIF-1 family of peptides also explicitly showed that they can exert more than one central action that persists longer than their half-lives in blood. These peptides clearly illustrate that the name of a peptide restricts neither its actions nor its conceptual implications.

Effects of chronic opioid exposure on guinea pig mu opioid receptor in Chinese hamster ovary cells: comparison with human and rat receptor

Chronic opioid treatment leads to agonist-specific effects at the mu opioid receptor. The molecular mechanisms resulting from chronic opioid exposure include desensitization, internalization and down-regulation of membrane-bound mu opioid receptors (MOP). The purpose of this study was to compare the cellular regulation of guinea pig, human and rat MOP expressed in Chinese hamster ovary (CHO) cells, following exposure to two clinically important opioids, morphine and methadone. MOP expressing CHO cells were treated in culture with methadone or morphine for up to 48 h. Radioligand diprenorphine and [D-AIa(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO)-stimulated GTP gamma S binding assays were carried out using paired control and opioid-exposed CHO cells. Methadone induced downregulation of the mu opioid receptor, while morphine induced desensitization of the receptor for all three species. Furthermore, morphine predominantly decreased the potency of DAMGO to stimulate GTP gamma S binding, whereas methadone primarily reduced its efficacy. Changes in DAMGO potency and efficacy differed among species and depended on the opioid used to treat the cells. Our results showed similarities between guinea pig and human MOP for morphine-induced desensitization, but identified differences between the two for methadone-induced desensitization. In contrast, human and rat MOP differed in response to morphine treatment, but were not distinct in their response to methadone treatment. The guinea pig is an excellent and established animal model to study opioid effects, but its molecular opioid pharmacology has not been investigated thus far. These results can assist in understanding species differences in the effects of opioid ligands activating the mu opioid receptor.

Opioid-induced regulation of gene expression in PC12 cells stably transfected with mu-opioid receptor

It has been postulated that opiates induce addictive behaviour via changes in gene expression. PC12 cells were stably transfected with the recombinant human mu-opioid receptor (MOR) to study opioid-induced gene expression. Expression was verified by binding assay, immunocytochemistry, and immunblotting experiments. Forskolin-induced cAMP formation was inhibited by [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO 1 microM), a specific MOR agonist. This effect was completely antagonized by naloxone. By using cDNA arrays, including approximately 1,200 well-defined genes normally expressed in neural tissue, we monitored semi-quantitative changes in gene expression after 3 h short-term exposure to DAMGO. Incubation with DAMGO increased mRNA levels for 13 genes and down-regulated 13 other genes. Annexin V, RGS4 and CREB genes showed pronounced increase in expression after stimulation with DAMGO. Quantitative RT-PCR confirmed that DAMGO increased mRNA levels of Annexin V, an apoptosis-induced gene. We suggest that the PC12 cell transfected with the recombinant human MOR is a useful tool for identification of opioid-induced genes that may provide information on opiate effects of relevance for dependence.

Opioid elevation of intracellular free calcium: possible mechanisms and physiological relevance

Opioid receptors are seven transmembrane domain Gi/G0 protein-coupled receptors, the activation of which stimulates a variety of intracellular signalling mechanisms including activation of inwardly rectifying potassium channels, and inhibition of both voltage-operated N-type Ca2+ channels and adenylyl cyclase activity. It is now apparent that like many other Gi/G0-coupled receptors, opioid receptor activation can significantly elevate intracellular free Ca2+ ([Ca2+]i), although the mechanism underlying this phenomenon is not well understood. In some cases opioid receptor activation alone appears to elevate [Ca2+]i, but in many cases it requires concomitant activation of Gq-coupled receptors, which themselves stimulate Ca2+ release from intracellular stores via the inositol phosphate pathway. Given the number of Ca2+-sensitive processes known to occur in cells, there are therefore a myriad of situations in which opioid receptor-mediated elevations of [Ca2+](i) may be important. Here, we review the literature documenting opioid receptor-mediated elevations of [Ca2+]i, discussing both the possible mechanisms underlying this phenomenon and its potential physiological relevance.

Synthesis and evaluation of the affinity toward mu-opioid receptors of atypical, lipophilic ligands based on the sequence c[-Tyr-Pro-Trp-Phe-Gly-]

An ultimate and general model describing the interaction between opioid ligands and mu-opioid receptors is not available yet, so the mode of action of atypical peptide analogues or peptidomimetics is worthy of investigation. In this context, the peptide c[-Tyr-d-Pro-d-Trp-Phe-Gly-] was observed to act as an agonist toward mu-opioid receptors with appreciable potency, albeit deprived of a protonable nitrogen. This compound was synthesized as a member of a library of diastereo- or enantiomeric cyclic peptides based on the sequence of endomorphin-1, aiming to obtain lipophilic peptide ligands active at the mu-opioid receptors, having good performances in terms of resistance to enzymatic degradation and permeation of biological barriers.

Structure−Activity Study on the Phe Side Chain Arrangement of Endomorphins Using Conformationally Constrained Analogues

Endomorphins-1 and -2 were substituted with all the beta-MePhe stereoisomers in their Phe residues to generate a conformationally constrained peptide set. This series of molecules was subjected to biological assays, and for beta-MePhe(4)-endomorphins-2, a conformational analysis was performed. Incorporation of (2S,3S)-beta-MePhe(4) resulted in the most potent analogues of both endomorphins with enhanced enzymatic stability. Their micro opioid affinities were 4-times higher than the parent peptides, they stimulated [(35)S]GTPgammaS binding, and they were found to be full agonists. NMR experiments revealed that C-terminal (2S,3S)-beta-MePhe in endomorphin-2 strongly favored the gauche (-) spatial orientation which implies the presence of the chi(1) = -60 degrees rotamer of Phe(4) in the binding conformer of endomorphins. Our results emphasize that the appropriate orientation of the C-terminal aromatic side chain of endomorphins is substantial for binding to the micro opioid receptor.

Visualizing activation of opioid circuits by internalization of G protein-coupled receptors

Mu-opioid receptor (MOR) and opioid receptor-like receptor (ORL-1) circuits in the limbic hypothalamic system are important for the regulation of sexual receptivity in the female rat. Sexual receptivity is tightly regulated by the sequential release of estrogen and progesterone from the ovary suggesting ovarian steroids regulate the activity of these neuropeptide systems. Both MOR and ORL-1 distributions overlap with the distribution of estrogen and progesterone receptors in the hypothalamus and limbic system providing a morphological substrate for interaction between steroids and the opioid circuits in the brain. Both MOR and ORL-1 are receptors that respond to activation by endogenous ligands with internalization into early endosomes. This internalization is part of the mechanism of receptor desensitization or down regulation. Although receptor activation and internalization are separate events, internalization can be used as a temporal measure of circuit activation by endogenous ligands. This review focuses on the estrogen and progesterone regulation of MOR and ORL-1 circuits in the medial preoptic nucleus and ventromedial nucleus of the hypothalamus that are central to modulating sexual receptivity.

Conformational analysis and μ-opioid receptor affinity of short peptides, endomorphin models in a low polarity solvent

Peptide carbamates containing the sequence H-Pro-Trp-PheNH2 showed in CDCl3 restricted conformations stabilized by the presence of a gamma-turn. To test the reliability of the peptides as endomorphin conformational models, we measured the affinities for mu-receptors labelled with [3H]-DAMGO. In particular, Cbz-Pro-Trp-PheNH2 displayed a nanomolar affinity.

In vitro quantitative study of the degradation of endomorphins

The catabolism of the endomorphins was investigated in detail. The endomorphins were degraded relatively slowly in the rat brain homogenate (t1/2(endomorphin-1)=4.94 min; t1/2(endomorphin-2)=3.81 min). The inhibition of metalloproteases and aminopeptidases stabilised the endomorphins to the greatest extent. The digestion of endomorphins tritiated specifically on Tyr(1), Pro(2) or Phe(3) established also that only the aminopeptidase pathways were essential for inactivation of the endomorphins, and that the tetrapeptides were degraded by cleavage of the Pro(2)-Trp(3) or Pro(2)-Phe(3) bond. The end-products of the catabolism were amino acids; the fragments Tyr-Pro-OH and Pro-Trp-Phe-NH2 were present as intermediates. Metabolites produced by brain carboxypeptidases were not detected.

Morphological studies of the endomorphinergic neurons in the central nervous system

Endomorphins (EMs) are newly found endogenous opioid peptides. Both endomorphin-1 (EM-1) and -2 (EM-2) are composed of four amino acids. Their high affinity and specificity for mu-opioid receptors have been confirmed by many physiological and pharmacological studies. In the present minireview, we discuss the distribution and localization of these peptides. While EM-2 is more prevalent in the spinal cord and lower brainstem, EM-1 is more widely and densely distributed throughout the brain than EM-2. We also discuss the possible coexistence of EM with other neurotransmitters. Finally, we introduce some new results regarding the ultrastructure and synaptic relationships of EM-2 obtained by the immunoelectron microscopic method.

Endomorphin-1 analogues containing beta-proline are mu-opioid receptor agonists and display enhanced enzymatic hydrolysis resistance

In this paper we describe the synthesis and affinity toward the mu-opioid receptor of some tetrapeptides obtained from endomorphin-1, H-Tyr-Pro-Trp-Phe-NH(2) (1), by substituting each amino acid in turn with its homologue. The ability to bind mu-opioid receptors depends on the beta-amino acid, and in particular 4, which contains beta-L-Pro, has a K(I) in the nanomolar range. The peptides 4 and 5 are significantly more resistant to enzymatic hydrolysis than 1. The same compounds, as well as the mu-opioid receptor agonist DAMGO, produced a concentration-dependent inhibition of forskolin-stimulated cyclic AMP formation, thus behaving as mu-opioid agonists. These features suggest that this novel class of endomorphin-1 analogues may represent suitable candidates for the in vivo investigation as potential mu-opioid receptor agonists.

Activation of mu-opioid receptors inhibits lordosis behavior in estrogen and progesterone-primed female rats

The present study investigated the effect of highly selective mu-opioid receptor (OR) agonists on lordosis behavior in ovariectomized rats treated with 3 microg of estradiol benzoate followed 48 h later by 200 microg of progesterone. Ventricular infusion of the endogenous mu-OR agonists endomorphin-1 and -2 suppressed receptive behavior in a time- and dose-dependent fashion. At 6 microg, both endomorphin-1 and -2 inhibited lordosis behavior within 30 min. However, while the effect of endomorphin-1 lasted 60 min, endomorphin-2 inhibition lasted up to 120 min after infusion. Pretreatment with naloxone (5 mg/kg sc) was able to block both endomorphin-1 and endomorphin-2 effects on lordosis. Site-specific infusions of endomorphin-1 or endomorphin-2 into the medial preoptic area (mPOA), the ventromedial nucleus of the hypothalamus (VMH), or into the mesencephalic central gray did not affect receptivity. In contrast, infusion of 1 mug of either compound into the medial septum/horizontal diagonal band of Broca inhibited lordosis in a pattern very similar to that seen after intraventricular infusions. Infusion of the potent synthetic mu-OR agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (0.08 microg) into the VMH and mPOA inhibited lordosis behavior for at least 60 min after infusion. The nonspecific opioid receptor antagonist naloxone was able to facilitate lordosis in partially receptive female rats when infused into the mPOA but not when infused into the VMH. The behavioral effects of the agonists and antagonist used in this study suggest that the endogenous mu-opioid system modulates estrogen and progesterone-induced lordosis behavior.

Endomorphin-1 and endomorphin-2: pharmacology of the selective endogenous mu-opioid receptor agonists

The recently discovered endogenous opioid peptides, endomorphins-1 and -2, appear to have properties consistent with neurotransmitter/neuromodulator actions in mammals. This review surveys the information gained so far from studies of different aspects of the endomorphins. Thus, the endomorphins have been found unequally in the brain; they are stored in neurons and axon terminals, with a heterogeneous distribution; they are released from synaptosomes by depolarization; they are enzymatically converted by endopeptidases; and they interact specifically and with high affinity with mu-opioid receptors. The most outstanding effect of the endomorphins is their antinociceptive action. This depends on both central and peripheral neurons. Additionally, the endomorphins cause vasodilatation by stimulating nitric oxide release from the endothelium. Their roles in different central and peripheral functions, however, have not been fully clarified yet. From a therapeutic perspective, therefore, they may be conceived at present as potent antinociceptive and vasodilator agents.

Endomorphin-1 induced desensitization and down-regulation of the recombinant mu-opioid receptor

1. Endomorphin-1 (E1) is a peptide with high affinity and selectivity for the mu-opioid receptor. The aim of this study was to determine if endomorphin-1 caused desensitization and down-regulation of the mu-opioid receptor expressed in Chinese hamster ovary cells. 2. Following 10 microM E1 pre-treatment, desensitization was assessed by measuring cyclic AMP inhibition, down-regulation was assessed by [(3)H]-diprenorphine ([(3)H]-DPN) binding and immuno-blotting. 3. Pre-treatment of CHO mu cells with 10 microM E1 for 11 and 18 h caused significant reduction in cyclic AMP inhibition. (11 h=39.0+/-16.7%, 18 h 47.0+/-11.1% reduction). 4. At 18 h E1 pre-treatment there was an enhancement (4.5 fold) of cyclic AMP production under forskolin stimulated conditions accompanied by a small rightward shift in the concentration-response curve (pEC(50) control=7.8+/-0.3, pEC(50) E1=7.3+/-0.2) when cells were re-challenged with E1. 5. In membranes prepared from untreated and 0.5 h E1 pre-treated cells, addition of GTP gamma S produced a significant rightward shift in the concentration response curves for E1 displacement of [(3)H]-DPN (0 h K(i) control=7.86+/-0.11, GTP gamma S=7.37+/-0.15; 0.5 h K(i) control=7.92+/-0.12, GTP gamma S=7.36+/-0.08) This was not observed in membranes prepared from cells that had been treated with E1 for 18 h (18 h K(i) control=7.69+/-0. 11, GTP gamma S=7.75+/-0.08). 6. In whole cells E1 treatment caused a rapid loss of cell surface receptors such that at 0.5 h there was a 30.5+/-1.5 reduction (this was unchanged for 18 h). In crude membranes a loss of receptors was also observed using radioligand binding or immuno-blotting protocols. 7. These data show that E1 causes desensitization and down-regulation of the rat mu-opioid receptor expressed in CHO cells. However, these two responses appear temporally distinct.