Morphine-induced desensitization and down-regulation at mu-receptors in 7315C pituitary tumor cells

Enhancing fentanyl antinociception and preventing tolerance with α-2 adrenoceptor agonists in rats

Fentanyl (FEN) is a potent opioid analgesic used for pain management. Opioid analgesic tolerance poses a significant challenge to the clinical utility of opioid agonists. Preventing the development of tolerance to opioid analgesia is crucial for improving its efficacy and safety. The noradrenergic system is involved in pain regulation. This study examined the effects of α-2 adrenoceptor (AR) agonists, dexmedetomidine (DEX), and xylazine (XYL) on FEN tolerance and antinociception, and their impact on μ-opioid receptor (MOR) expression in the posterior horn of the spinal cord (SC). Male rats were divided into six groups and treated with different drug combinations for three consecutive days. Analgesia tests and motor performance assessments were conducted, followed by SC analysis using immunohistochemistry (IHC). Analgesia tests revealed the development of FEN tolerance on the second day, but the groups receiving combined drugs did not develop tolerance. Instead, FEN antinociception was enhanced, with a prolonged duration of its effects. None of the drugs caused sedation or motor impairment, and SC morphology appeared normal. MOR expression levels did not differ significantly between the groups based on IHC analysis. These findings suggest that changes in the secondary messenger system may play a role in the early development of FEN tolerance. Combining drugs can prevent tolerance, while enhancing FEN's antinociceptive effects. These results have promising implications for chronic pain management; however, further research is needed to explore the molecular effects of α-2 AR agonists on FEN tolerance. Overall, this study sheds light on the mechanism of FEN tolerance and identifies potential avenues for future research.

Key differences in regulation of opioid receptors localized to presynaptic terminals compared to somas: Relevance for novel therapeutics

Opioid receptors are G protein-coupled receptors (GPCRs) that regulate activity within peripheral, subcortical and cortical circuits involved in pain, reward, and aversion processing. Opioid receptors are expressed in both presynaptic terminals where they inhibit neurotransmitter release and postsynaptic locations where they act to hyperpolarize neurons and reduce activity. Agonist activation of postsynaptic receptors at the plasma membrane signal via ion channels or cytoplasmic second messengers. Agonist binding initiates regulatory processes that include phosphorylation by G protein receptor kinases (GRKs) and recruitment of beta-arrestins that desensitize and internalize the receptors. Opioid receptors also couple to effectors from endosomes activating intracellular enzymes and kinases. In contrast to postsynaptic opioid receptors, receptors localized to presynaptic terminals are resistant to desensitization such that there is no loss of signaling in the continuous presence of opioids over the same time scale. Thus, the balance of opioid signaling in circuits expressing pre- and postsynaptic opioid receptors is shifted toward inhibition of presynaptic neurotransmitter release during continuous opioid exposure. The functional implication of this shift is not often acknowledged in behavioral studies. This review covers what is currently understood about regulation of opioid/nociceptin receptors, with an emphasis on opioid receptor signaling in pain and reward circuits. Importantly, the review covers regulation of presynaptic receptors and the critical gaps in understanding this area, as well as the opportunities to further understand opioid signaling in brain circuits. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Evaluating the expression pattern of the opioid receptor in pituitary neuroendocrine tumors (PitNET) and the role of morphine and naloxone in the regulation of pituitary cell line growth and apoptosis

PURPOSE

The expression pattern of the opioid receptor (MOR) in pituitary neuroendocrine tumors (PitNET) and the possible effect of morphine and naloxone on GH3 cell growth and apoptosis were evaluated.

METHODS

The 114 pituitary tissues including non-functioning, GH-producing and ACTH-producing PitNET and healthy cadaver pituitary tissues were included. The expression level of the MOR gene and protein was assessed using real-time PCR and Western blot. The association with patient demographic characteristics was assessed. Morphine and naloxone were applied to assess their possible pharmacological role in GH3 pituitary adenoma cell death. The cytotoxic effect, the apoptosis rate, the cell cycle distribution, the content of reactive oxygen species and the caspase 3 activity were measured.

RESULTS

MOR gene levels increased significantly in pituitary neuroendocrine tumors (PitNET) compared to the healthy pituitary samples. The increased level of MOR gene expression was prominent in invasive functional and non-functional pituitary tumors. A consistent expression pattern was demonstrated for MOR protein levels in PitNET samples. A dose- and time-dependent reduction in the rate of GH3 pituitary cells was observed after morphine treatment with an IC50 of 483 µM after 24 h of incubation. Morphine induced early apoptosis, accumulation of cells in sub-G1 phase, increase in cellular ROS levels and caspase-3 activity. The observed effects of morphine were reversed after MOR blockade using 10 and 25 µM naloxone.

CONCLUSION

The possible contributing role of the MOR in pituitary tumor cell growth and the putative pharmaceutical effect of morphine in pituitary neuroendocrine tumor cell death (PitNET) is illustrated.

Brain volume changes after long-term injectable opioid treatment: A longitudinal voxel-based morphometry study

Clinical research has demonstrated the efficacy of injectable opioid treatment for long-term, treatment-refractory opioid-dependent patients. It has been hypothesized that compulsive drug use is particularly associated with neuroplasticity changes in the networks corresponding to withdrawal/negative affect and preoccupation/anticipation rather than binge/intoxication. However, as yet, no study has investigated the effect of long-term opioid treatment on key regions within these networks. Magnetic resonance imaging (MRI) was used to assess brain volumes changes during long-term (approximately 9 years) injectable opioid agonist treatment with diacetylmorphine (DAM) in 22 patients with opioid use disorder. Voxel-based morphometry was applied to detect volumetric changes within the networks of binge/intoxication (ventral/dorsal striatum, globus pallidus and thalamus), withdrawal/negative affect (amygdala and ventral striatum) and preoccupation/anticipation (hippocampus, orbitofrontal and anterior cingulate cortex). The relationships between significant volume changes and features of opioid use disorder were tested using Pearson correlation. Long-term opioid agonist treatment was associated with the enlargement of the right caudate nucleus, which was related to the duration of opioid use disorder. In contrast, reduced volume in the right amygdala, anterior cingulate cortex and orbitofrontal cortex were found that were related to opioid dose, onset of opioid consumption and state anxiety. These findings suggest that long-term opioid agonist treatment is related to structural changes in key brain regions underlying binge/intoxication, withdrawal/negative affect and preoccupation/anticipation, suggesting sustained interaction between these systems.

Mechanisms of rapid opioid receptor desensitization, resensitization and tolerance in brain neurons

Agonists acting on µ-opioid receptors (MOR) are very effective analgesics but cause tolerance during long-term or repeated exposure. Intensive efforts have been made to find novel opioid agonists that are efficacious analgesics but can elude the signalling events that cause tolerance. µ-Opioid agonists differentially couple to downstream signalling mechanisms. Some agonists, such as enkephalins, D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), methadone and sufentanyl are efficacious at mediating G-protein and effector coupling, as well as triggering MOR regulatory events that include MOR phosphorylation, β-arrestin binding, receptor endocytosis and recycling. By contrast, morphine and closely related alkaloids can mediate efficacious MOR-effector coupling but poorly trigger receptor regulation. Several models have been proposed to relate differential MOR regulation by different opioids with their propensity to cause tolerance. Most are based on dogma that β-arrestin-2 (βarr-2) binding causes MOR desensitization and/or that MOR endocytosis and recycling are required for receptor resensitization. This review will examine some of these notions in light of recent evidence establishing that MOR dephosphorylation and resensitization do not require endocytosis. Recent evidence from opioid-treated animals also suggests that impaired MOR-effector coupling is driven, at least in part, by enhanced desensitization, as well as impaired resensitization that appears to be βarr-2 dependent. Better understanding of how chronic exposure to opioids alters receptor regulatory mechanisms may facilitate the development of effective analgesics that produce limited tolerance.

Opioid-induced regulation of µ-opioid receptor gene expression in the MCF-7 breast cancer cell line

The aim of the study was to investigate the presence of opioid receptor types in human breast adenocarcinoma MCF-7 cells and to characterize the changes in MOR expression induced by opioid agonist and antagonist treatment. We have shown that all three types of opioid receptors, but predominantly MOR, are expressed in MCF-7 cells. Selective MOR agonists, morphine, endomorphin-1, and endomorphin-2 downregulated MOR mRNA levels in a concentration- and time-dependent manner, but the effect produced by endomorphins was much stronger. Downregulation was blocked by the opioid antagonist naloxone. Naloxone alone produced a slight increase in MOR gene expression. Immunoblotting with antiserum against MOR-1 confirmed these results at the protein level. The results of our study indicate that, in MCF-7 cells, MOR gene expression is downregulated by opioid agonists and upregulated by opioid antagonists. We propose that the opioid-induced regulation of MOR mRNA expression is mediated by reduced binding of the transcription factors NFκB and AP-1 to the promoter region on the MOR gene.

Role of receptor internalization in opioid tolerance and dependence

Agonist-induced mu-opioid receptor (MOPr) internalization has long been suggested to contribute directly to functional receptor desensitization and opioid tolerance. In contrast, recent evidence suggests that opioid receptor internalization could in fact reduce opioid tolerance in vivo, but the mechanisms that are responsible for the internalization-mediated protection against opioid tolerance are controversely discussed. One prevailing hypothesis is, that receptor internalization leads to decreased receptor signaling and therefore to reduced associated compensatory changes in downstream signaling systems that are involved in the development of opioid tolerance. However, numerous studies have demonstrated that desensitized and internalized mu-opioid receptors are rapidly recycled to the cell surface in a reactivated state, thus counteracting receptor desensitization and opioid tolerance. Further studies revealed agonist-selective differences in the ability to induce opioid receptor internalization. Recently it has been demonstrated that the endocytotic efficacies of opioids are negatively correlated to the induced opioid tolerance. Thus, clearer understanding of the role of opioid receptor trafficking in the regulation of opioid tolerance and dependence will help in the treatment of patients suffering from chronic pain or drug dependence.

Chronic morphine treatment up-regulates mu opioid receptor binding in cells lacking filamin A

We investigated the effects of morphine and other agonists on the human mu opioid receptor (MOP) expressed in M2 melanoma cells, lacking the actin cytoskeleton protein filamin A and in A7, a subclone of the M2 melanoma cells, stably transfected with filamin A cDNA. The results of binding experiments showed that after chronic morphine treatment (24 h) of A7 cells, MOP-binding sites were down-regulated to 63% of control, whereas, unexpectedly, in M2 cells, MOP binding was up-regulated to 188% of control naive cells. Similar up-regulation was observed with the agonists methadone and levorphanol. The presence of antagonists (naloxone or CTAP) during chronic morphine treatment inhibited MOP down-regulation in A7 cells. In contrast, morphine-induced up-regulation of MOP in M2 cells was further increased by these antagonists. Chronic morphine desensitized MOP in A7 cells, i.e., it decreased DAMGO-induced stimulation of GTPgammaS binding. In M2 cells DAMGO stimulation of GTPgammaS binding was significantly greater than in A7 cells and was not desensitized by chronic morphine. Pertussis toxin treatment abolished morphine-induced receptor up-regulation in M2 cells, whereas it had no effect on morphine-induced down-regulation in A7 cells. These results indicate that, in the absence of filamin A, chronic treatment with morphine, methadone or levorphanol leads to up-regulation of MOP, to our knowledge, the first instance of opioid receptor up-regulation by agonists in cell culture.

Region-dependent attenuation of mu opioid receptor-mediated G-protein activation in mouse CNS as a function of morphine tolerance

BACKGROUND AND PURPOSE

Chronic morphine administration produces tolerance in vivo and attenuation of mu opioid receptor (MOR)-mediated G-protein activation measured in vitro, but the relationship between these adaptations is not clear. The present study examined MOR-mediated G-protein activation in the CNS of mice with different levels of morphine tolerance.

EXPERIMENTAL APPROACH

Mice were implanted with morphine pellets, with or without supplemental morphine injections, to induce differing levels of tolerance as determined by a range of MOR-mediated behaviours. MOR function was measured using agonist-stimulated [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) and receptor binding throughout the CNS.

KEY RESULTS

Morphine pellet implantation produced 6-12-fold tolerance in antinociceptive assays, hypothermia and Straub tail, as measured by the ratio of morphine ED(50) values between morphine-treated and control groups. Pellet implantation plus supplemental injections produced 25-50-fold tolerance in these tests. In morphine pellet-implanted mice, MOR-stimulated [(35)S]GTPgammaS binding was significantly reduced only in the nucleus tractus solitarius (NTS) and spinal cord dorsal horn in tissue sections from morphine pellet-implanted mice. In contrast, MOR-stimulated [(35)S]GTPgammaS binding was significantly decreased in most regions examined in morphine pellet+morphine injected mice, including nucleus accumbens, caudate-putamen, periaqueductal gray, parabrachial nucleus, NTS and spinal cord.

CONCLUSIONS AND IMPLICATIONS

Tolerance and the regional pattern of apparent MOR desensitization were influenced positively by the level of morphine exposure. These results indicate that desensitization of MOR-mediated G-protein activity is more regionally widespread upon induction of high levels of tolerance, suggesting that this response contributes more to high than low levels of tolerance to CNS-mediated effects of morphine.

Internalization and down-regulation of mu opioid receptors by endomorphins and morphine in SH-SY5Y human neuroblastoma cells

The human neuroblastoma cell line, SH-SY5Y, was used to examine the effects of morphine and the endogenous opioid peptides, endomorphin-1 (EM-1) and endomorphin-2 (EM-2), on mu opioid receptor (MOR) internalization and down-regulation. Treatment for 24 h with EM-1, EM-2 or morphine at 100 nM, 1 microM and 10 microM resulted in a dose-dependent down-regulation of mu receptors. Exposure of cells to 10 microM EM-1 for 2.5, 5 and 24 h resulted in a time-dependent down-regulation of mu receptors. Down-regulation of mu receptors by morphine and EM-1 was blocked by treatment with hypertonic sucrose, consistent with an endocytosis-dependent mechanism. Sensitive cell-surface binding studies with a radiolabeled mu antagonist revealed that morphine was able to induce internalization of mu receptors naturally expressed in SH-SY5Y cells. EM-1 produced a more rapid internalization of mu receptors than morphine, but hypertonic sucrose blocked the internalization induced by each of these agonists. This study demonstrates that, like morphine, the endomorphins down-regulate mu opioid receptors in a dose- and time-dependent manner. This study also demonstrates that morphine, as well as EM-1, can induce rapid, endocytosis-dependent internalization of mu opioid receptors in SH-SY5Y cells. These results may help elucidate the ability of mu agonists to regulate the number and responsiveness of their receptors.

Opioid receptors

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

MOR1 receptor mRNA expression in human brains of drug-related fatalities—a real-time PCR quantification

The expression of the human micro-opiate receptor (MOR1) in post mortem human brain tissue was examined using real-time PCR technology. Tissue samples from 11 fatalities due to opiate overdose and five normal subjects with different causes of death were analysed in order to elucidate whether chronic opiate abuse is followed by a regulation of MOR1 expression. In each case nine selected brain regions (thalamus, caudate nucleus, hypothalamus, ventral tegmentum, hippocampus, amygdala, frontal cortex, nucleus accumbens, putamen) were evaluated. The MOR1-mRNA level was determined relative to the housekeeping gene beta2-microglobulin. While in most regions the MOR mRNA levels in the brain of addicts were not different from the control group-with varying levels between 0 and 15% of housekeeping gene level-in the brains of three drug-related fatalities an enormous increase was encountered in the thalamus where the MOR-mRNA level amounted for up to 10,000% of the measured housekeeping gene level. The results obtained by toxicological hair analysis in the group of drug-related fatalities indicate that the enormous thalamic MOR1-expression is primarily found in individuals who died from acute heroin overdose but did not show signs of a substantial chronic administration of the drug. Further studies have to be performed to evaluate if the observed MOR1-mRNA up-regulation in the thalamus in a subpopulation of acute lethal intoxications mirrors a state of functional hypersensitivity associated with the occurrence of death.

The mu opioid receptor: from molecular cloning to functional studies

Opioids have been used and abused by humans for centuries. The mu opioid receptor represents the high affinity binding site for opioid narcotics with high abuse liability such as morphine, codeine and fentanyl. Heroin (diacetylmorphine), a semi-synthetic derivative of morphine, crosses the blood-brain barrier more readily than morphine due to its increased hydrophobicity. Once in the brain heroin is hydrolyzed to morphine, which acts at the mu opioid receptor and results in euphoria, thus conferring the reinforcing properties of heroin. Using molecular biology techniques, the mu opioid receptors from several species have been cloned. This article reviews recent progress in this area, with respect to the two major cellular functions of the mu opioid receptor: reduction of intracellular cAMP concentration by inhibiting adenylyl cyclase activity, and inhibition of neuronal firing by modulating membrane ion channels.

Functional coupling of a mu opioid receptor to G proteins and adenylyl cyclase: modulation by chronic morphine treatment

A cloned mu opioid receptor was used to study its coupling to signal transduction pathways and its involvement in morphine-induced opioid dependence in stably transfected Chinese hamster ovary (CHO) cells. Membrane binding assays with a mu-selective agonist [(3)H]DAMGO showed that one cell line expresses a high level of mu opioid receptors with a B(max) of approximately 630 fmol/mg membrane protein and a K(d) of 0.47 nM for DAMGO. Stimulation of the transfected cells with DAMGO led to an increase in the low K(m) GTPase activity, indicative of activation of guanine nucleotide regulatory proteins (G proteins), and this effect was blocked by the opioid antagonist naloxone. In addition, binding of the mu opioid receptor to DAMGO was affected by GTP and nonhydrolizable GTP analogs, Gp(NH)pp and GTP-gamma-S. These results suggest a functional coupling between the mu opioid receptor and G proteins. Furthermore, DAMGO treatment of the cells produced a dose-dependent inhibition of the intracellular cyclic adenosine monophosphate (cAMP) level, with an EC(50) value of approximately 30 nM. Chronic treatment of the cells with morphine not only elevated the basal and forskolin-stimulated cAMP levels after morphine withdrawal, but also increased the extent of the DAMGO-induced reduction of intracellular cAMP levels. The whole cell binding assay with [(3)H]DAMGO, on the other hand, did not detect receptor down-regulation after chronic morphine treatment. These results suggest that chronic morphine treatment may trigger a compensatory mechanism in cellular signaling pathways to offset the inhibitory input from the mu receptor without down-regulation of the surface receptor number, and that withdrawal of chronic inhibition leads to elevated activities of adenylyl cyclase to provide a basis for system sensitization.

Area-specific increased density of mu-opioid receptor immunoreactive neurons in the cerebral cortex of drug-related fatalities

In animal experiments and in cell culture, chronic morphine treatment has been followed by "up-regulation" as well as "down-regulation" of the mu-opioid receptor (OR) number. The present postmortem morphometric study of morphine-related fatalities of drug-addicts (n=13, 20-35 years old, with blood unconjugated morphine levels from 27.1 ng/ml to 458 ng/ml, m.v. 198.5 ng/ml) versus a non-addicted control group (n=13, 10-44 years old) was intended to examine, whether chronic opiate exposure affects the numerical density of mu-OR expressing neurons in the human neocortex (areas 11, 24 and 25 according to Brodmann). For the immunohistochemical procedure, vibratome sections (100 microm) were incubated with a monoclonal antibody against the mu-OR, diluted 1:100, and immunolabelled sites were visualized using an immunoperoxidase protocol. The numerical densities of OR immunoreactive neuronal profiles and Nissl-stained central profiles were assessed morphometrically (camera lucida-drawings). In both groups, the anti-mu-OR-immunoreactivity was mainly localized in pyramidal neurons of layers (L) II/III and V and in multiform neurons of L VI. In the areas 24 and 25, the density of the immunoreactive neuronal profiles did not display a significant difference between the two examined groups. In the area 11, however, the number of immunolabelled neuronal profiles amounted to 2777+/-206 mm(3) in the drug-related fatalities and to 2320+/-124 mm(3) in the control group and thus was significantly increased.

Resensitization of blood pressure response to mu-opioid peptide agonists after acute desensitization

IV administration of mu-opioid peptide agonists (DAMGO, DALDA, and [Dmt(1)]DALDA) results in a transient, naloxone-sensitive, increase in blood pressure in awake sheep. Despite significant differences in pharmacokinetics, these blood pressure responses all last < 15 min. The lack of correlation between half-life and duration of action suggested rapid desensitization. When a second dose of the same agonist was repeated 30 min later, the response was completely abolished. An increase in blood pressure and rapid desensitization was also observed with the kappa-opioid agonist (U50488H), whereas delta-agonists (DPDPE and DELT) had no effect on blood pressure. The response to DAMGO was abolished after prior exposure to DAMGO or DALDA, but there was no evidence of cross-desensitization between mu and delta, or mu and kappa, opioid agonists. Full resensitization of the blood pressure response occurred by 4 h for DAMGO (t(1/2) = 15 min) and by 48 h for [Dmt(1)]DALDA (t(1/2) = 1.8 h). These data support our hypothesis that the transient nature of the blood pressure response to mu-opioid agonists is caused by rapid desensitization and suggest that the rate of resensitization is dependent on the pharmacokinetics of the agonist.

mu-Opioid receptor downregulation contributes to opioid tolerance in vivo

The present study examined the contribution of downregulation of mu-opioid receptors to opioid tolerance in an intact animal model. Mice were implanted subcutaneously with osmotic minipumps that infused etorphine (50-250 microg/kg/day) for 7 days. Other mice were implanted subcutaneously with a morphine pellet (25 mg) or a morphine pellet plus an osmotic minipump that infused morphine (5-40 mg/kg/day) for 7 days. Controls were implanted with an inert placebo pellet. At the end of treatment, pumps and pellets were removed, and saturation binding studies were conducted in whole brain ([3H]DAMGO) or morphine and etorphine analgesic ED(50)s were determined (tail-flick). Morphine tolerance increased linearly with the infusion dose of morphine (ED(50) shift at highest infusion dose, 4.76). No significant downregulation of mu-receptors in whole brain was observed at the highest morphine treatment dose. Etorphine produced dose-dependent downregulation of mu-opioid receptor density and tolerance (ED(50) shift at highest infusion dose, 6.97). Downregulation of mu-receptors only occurred at the higher etorphine infusion doses (> or =150 microg/kg/day). Unlike morphine tolerance, the magnitude of etorphine tolerance was a nonlinear function of the dose and increased markedly at infusion doses that produced downregulation. These results suggest that mu-opioid receptor downregulation contributes to opioid tolerance in vivo. Therefore, opioid tolerance appears to rely upon both "receptor density-dependent" and " receptor density-independent" mechanisms.

Mu opioid receptor efficacy and potency of morphine-6-glucuronide in neonatal guinea pig brainstem membranes: comparison with transfected CHO cells

The major side effect of morphine and its active metabolite, morphine-6-glucuronide (M6G), is respiratory depression, which is mediated by mu opioid receptors in the medulla and pons. Although the effect of morphine on coupling between mu opioid receptors and G proteins has been studied, the effect of M6G on this coupling has not. Therefore, stimulation of guanylyl-5'-O-([gamma(35)S]-thio)-triphosphate ([(35)S]-GTPgammaS) binding by these two narcotic analgesic drugs was compared to the mu-specific synthetic opioid peptide [D-Ala(2), N-MePhe(4), Gly-ol(5)]enkephalin in Chinese hamster ovarian cells stably transfected with the murine mu opioid receptor and in brainstem membranes prepared from 3-, 7-, and 14-day-old guinea pigs. All three agonists stimulated [(35)S]-GTPgammaS binding in transfected cells and neural tissue, and the stimulation was antagonized by naloxone. In brainstem membranes, but not transfected cells, M6G was less efficacious but more potent than morphine, which may be due to differences between murine and guinea pig mu opioid receptors or in the G proteins in these two tissues. Efficacy of the agonists did not change during development, but overall potency decreased between 3 and 14 days after birth. In vivo potency differences for respiratory depression between morphine and M6G are qualitatively similar to in vitro potency differences of these drugs to stimulate [(35)S]-GTPgammaS binding in neonatal guinea pig brainstem membranes. Tolerance to opioid effects on [(35)S]-GTPgammaS binding developed in transfected cells incubated with morphine with the maximum decrease in potency occurring 18 h later than the maximum decline in efficacy.

Numerical density of mu opioid receptor expressing neurons in the frontal cortex of drug related fatalities

In animal and cell culture experiments, chronic morphine treatment has been followed by 'up'- as well as 'down-regulation' of the mu opioid receptor (mu OR) number. The present postmortem morphometric study of morphine-related fatalities of drug addicts (n=12, and 22-35 years old, with blood unconjugated morphine levels from 27.1 to 458 ng/ml, m.v. 198.5 ng/ml) versus a non-addicted control group (n=13 and 10-44 years old) was intended to examine whether chronic opiate exposure affects the numerical density of mu OR expressing neurons in the human neocortex (area 10 according to Brodmann). For the immunohistochemical procedure, thick (100 microm) vibratome sections were incubated with a monoclonal antibody against the mu OR [Arvidsson et al., J. Neurosci. 15 (1995) 3328] and immunoreactive sites were visualized using an immunoperoxidase protocol. The numerical densities of mu OR-expressing and Nissl-stained neurons were assessed morphometrically (camera lucida-drawings). In both collectives, the anti-mu OR immunoreactivity was mainly found in pyramidal neurons of layers (L) II/III and V and in multiform neurons of L VI. In the drug-related fatalities and the control group, the density of neurons expressing mu OR protein was similar, amounting for 2698 +/- 153 and 2688 +/- 172/mm(3), respectively. These findings extend the binding studies of opioid ligands in postmortem brains of heroin addicts [Gabilondo et al., Psychopharmacology 115 (1994) 135] revealing similar receptor densities and affinities by showing no difference in the density of mu OR-positive neurons.

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.

Chronic heroin self-administration desensitizes mu opioid receptor-activated G-proteins in specific regions of rat brain

In previous studies from our laboratory, chronic noncontingent morphine administration decreased mu opioid receptor-activated G-proteins in specific brainstem nuclei. In the present study, mu opioid receptor binding and receptor-activated G-proteins were examined after chronic heroin self-administration. Rats were trained to self-administer intravenous heroin for up to 39 d, achieving heroin intake up to 366 mg. kg(-1). d(-1). mu opioid-stimulated [(35)S]GTPgammaS and [(3)H]naloxone autoradiography were performed in adjacent brain sections. Agonist-stimulated [(35)S]GTPgammaS autoradiography also examined other G-protein-coupled receptors, including delta opioid, ORL-1, GABA(B), adenosine A(1), cannabinoid, and 5-HT(1A). In brains from heroin self-administering rats, decreased mu opioid-stimulated [(35)S]GTPgammaS binding was observed in periaqueductal gray, locus coeruleus, lateral parabrachial nucleus, and commissural nucleus tractus solitarius, as previously observed in chronic morphine-treated animals. In addition, decreased mu opioid-stimulated [(35)S]GTPgammaS binding was found in thalamus and amygdala after heroin self-administration. Despite this decrease in mu-activated G-proteins, [(3)H]naloxone binding demonstrated increased mu opioid receptor binding in several brain regions after heroin self-administration, and there was a significant decrease in mu receptor G-protein efficiency as expressed as a ratio between agonist-activated G-proteins and mu receptor binding. No effects on agonist-stimulated [(35)S]GTPgammaS binding were found for any other receptor examined. The effect of chronic heroin self-administration to decrease mu-stimulated [(35)S]GTPgammaS binding varied between regions and was highest in brainstem and lowest in the cortex and striatum. These results not only provide potential neuronal mechanisms that may contribute to opioid tolerance and dependence, but also may explain why various chronic effects of opioids develop to different degrees.

The effect of nimodipine on opioid antagonist-induced upregulation and supersensitivity

Regulation of calcium flux has been suggested to play a role in acute and chronic effects of opioids. Previous studies have shown calcium channel blockers can inhibit opioid agonist-induced downregulation of mu-opioid receptors and may reduce the magnitude of tolerance. In the present study, we determined if calcium channel blockade would affect increases in opioid receptor density and functional supersensitivity produced by chronic opioid antagonist treatment in the mouse. Mice were implanted subcutaneously with a 15-mg naltrexone (NTX) or placebo pellet. Mice also were implanted with an osmotic minipump that infused nimodipine (100 microg/kg/day) or a second placebo pellet. This protocol yielded four groups: nimodipine-NTX; nimodipine-placebo; placebo-NTX; placebo-placebo. On the seventh day, pumps and pellets were removed. Twenty-four hours later, a morphine dose-response study was conducted (tail flick); or mice were sacrificed and saturation binding studies ([3H]DAMGO) were performed in whole brain. NTX treatment significantly increased the analgesic potency of morphine by approximately 60%. Nimodipine increased the potency of morphine by approximately 50%. For mice treated with both nimodipine and NTX, there was an additive effect on morphine potency ( approximately 120% increase). In binding studies, NTX increased the density of mu-opioid receptors similarly ( approximately 60-70%) in the presence and absence of nimodipine treatment, with no change in affinity. No effect of chronic nimodipine alone on mu-opioid receptor binding was observed. These data indicate that NTX-induced upregulation and supersensitivity are independent of calcium channel blockade by nimodipine. These results contrast with those from tolerance and downregulation studies, and confirm suggestions that different substrates mediate chronic opioid agonist and antagonist-induced effects in vivo. Finally, in a separate study, morphine potency was unaffected by acute nimodiopine (100 microg/kg; SC), suggesting that prolonged exposure to this calcium channel blocker is required to increase morphine potency.

Resolution of two [(35)S]GTP-gamma-S binding sites and their response to chronic morphine treatment: a binding surface analysis

The mechanisms by which prolonged exposure to morphine leads to tolerance are not fully understood. We investigated the effects of etorphine (ET) on [(35)S]guanosine 5'-(-thio)-triphosphate ([(35)S]GTP-gamma-S) binding in brains of rats made tolerant to morphine via the implantation of morphine (or placebo) pellets. Binding surface analysis was used to characterize the interactions of ET, Gpp(Np)H and GTP-gamma-S with sites labeled by [(35)S]GTP-gamma-S. Data sets were fitted to one- and two-site binding models using the nonlinear least squares curve fitting program MLAB-PC (Civilized Software, Bethesda, MD, USA). Two binding sites were readily resolved. Chronic morphine significantly increased the B(max) and K(d) of the high affinity binding site. ET stimulated [(35)S]GTP-gamma-S binding in placebo membranes via an increase in the B(max) of the high affinity binding site. In contrast, ET stimulated [(35)S]GTP-gamma-S in chronic morphine membranes via a large decrease in the K(d) of the high affinity site. These results suggest that chronic morphine treatment alters the mechanism by which ET stimulates [(35)S]GTP-gamma-S binding to G-proteins. Since proper G-protein/receptor coupling increases [(35)S]GTP-gamma-S binding via an increase in B(max) values, these results suggest that opioid receptors in chronic morphine membranes are not normally coupled to G-proteins. These findings corroborate earlier studies that reported changes in G-protein function in morphine tolerant animals.

Regulation of adenylyl cyclase isozymes on acute and chronic activation of inhibitory receptors

Adenylyl cyclase superactivation, a phenomenon by which chronic activation of inhibitory Gi/o-coupled receptors leads to an increase in cAMP accumulation, is believed to play an important role as a compensatory response of the cAMP signaling system in the cell. However, to date, the mechanism by which adenylyl cyclase activity is regulated by chronic exposure to inhibitory agonists and the nature of the adenylyl cyclase isozymes participating in this process remain largely unknown. Here we show, using COS-7 cells transfected with the various AC isozymes, that acute activation of the D2 dopaminergic and m4 muscarinic receptors inhibited the activity of adenylyl cyclase isozymes I, V, VI, and VIII, whereas types II, IV, and VII were stimulated and type III was not affected. Conversely, chronic receptor activation led to superactivation of adenylyl cyclase types I, V, VI, and VIII and to a reduction in the activities of types II, IV, and VII. The activity of AC-III also was reduced. This pattern of inhibition/stimulation of the various adenylyl cyclase isozymes is similar to that we recently observed on acute and chronic activation of the mu-opioid receptor, suggesting that isozyme-specific adenylyl cyclase superactivation may represent a general means of cellular adaptation to the activation of inhibitory receptors and that the presence/absence and intensity of the adenylyl cyclase response in different brain areas (or cell types) could be explained by the expression of different adenylyl cyclase isozyme types in these areas.

Regulation of kappa-opioid receptor mRNA level by cyclic AMP and growth factors in cultured rat glial cells

The mRNA of the kappa-opioid receptor (KOR) has been found recently in cultured astrocytes and in microglia. By using RT-PCR and Southern hybridization, we confirmed these observations and, in addition, we observed that KOR mRNA was expressed in oligodendrocytes and in the precursors of astrocytes and oligodendrocytes. KOR mRNA level was the highest in the immature astrocytes and decreased with their maturation. Very few data are available on the regulation of KOR level by extracellular signals. Therefore, we examined the effect of three growth factors known to be present in the adult brain, basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF-BB) and leukemia inhibitory factor (LIF) and of two cyclic AMP (cAMP) generating systems, the cAMP analog, 8-(4-chlorophenylthio)-cAMP and forskolin, on this level. It was found that in astrocytes, KOR mRNA level decreased dramatically under the effect of cAMP and less under the effect of bFGF while it did not change significantly after LIF treatment. In oligodendrocytes, it also decreased with cAMP, but increased under the effect of bFGF and PDGF-BB. In microglia, a decrease was observed with cAMP and lipopolysaccharides (LPS), the most used activators of macrophages. These results shed new evidence on the expression of opioid receptor mRNA in the glial cells of the rat CNS. The regulation of KOR mRNA level under the effect of extracellular signals suggests that opioids take part in dynamic processes in glial cells, possibly related to glial-neuron communication.

Adaptations to chronic agonist exposure of mu-opioid receptor-expressing Chinese hamster ovary cells

To investigate cellular adaptation responses induced by chronic agonist treatment of the mu-opioid receptor, Chinese hamster ovary (CHO) cells were stably transfected with the rat mu-opioid receptor cDNA. Chronic treatment with agonists selective for the mu-opioid receptor, [D-Ala2, N-MePhe4, Gy-ol5]enkephalin (DAMGO), morphine and fentanyl, time- and dose-dependently induced down-regulation of the mu-opioid receptor. The down-regulation was not significantly affected by pretreatment with pertussis toxin, but was completely blocked by treatment with hypertonic sucrose, suggesting that receptor internalization mediated by clathrin-coated vesicles is an essential step in the mu-opioid receptor down-regulation. On the other hand, forskolin-stimulated cyclic AMP formation was increased by chronic DAMGO treatment, which was inhibited by pertussis toxin pretreatment. These results indicate that two adaptation responses induced by chronic agonist treatment of the mu-opioid receptor-expressing CHO cells, down-regulation of the mu-opioid receptor and supersensitization of adenylate cyclase, are mediated by distinct mechanisms.

The effects of repeated morphine exposure on mu opioid receptor number and affinity in C57BL/6J and DBA/2J mice

C57BL/6J (B6) mice self-administer substantial quantities of morphine compared to DBA/2J (D2) mice, and most of the genetic component of this strain difference has been attributed to a locus on chromosome 10 in the vicinity of the mu opioid receptor gene. To compare binding characteristics of mu opioid receptor populations between the two strains, mice were given single daily injections of a long-acting preparation of morphine sulfate (80 mg/kg, s.c.) or saline for a period of seven days, and euthanatized six hours after the last injection. Brains were removed and dissected into specific regions. Receptor binding studies were performed on frontal cortex and striatum. Data were analyzed using non-linear regression, and Kd and Bmax comparisons made between strains and treatments. Specific [3H]DAMGO binding in striatum indicates that the density of mu opioid receptors in saline-treated B6 mice and saline-treated D2 mice does not differ significantly. After repeated morphine injection, B6 mice exhibited a decrease in striatal [3H]DAMGO binding, indicating a downregulation of receptor density by approximately 45% (p=.0003 vs saline-treated B6), a phenomenon not observed in D2 mice. In frontal cortex, no differences in [3H]DAMGO binding were observed between strains or treatment groups. These results demonstrate a significant difference between mu opioid receptor regulation in B6 and D2 mice, and may underlie well documented strain differences in specific opioid-related behaviors.

Down-regulation of mu-opioid receptor by full but not partial agonists is independent of G protein coupling

In C6 glial cells stably expressing rat mu-opioid receptor, opioid agonist activation is negatively coupled to adenylyl cyclase through pertussis toxin-sensitive G proteins. In membranes, [D-Ala2, N-MePhe4,Gly-ol5]enkephalin (DAMGO) increases guanosine-5'-O-(3-[35S]thio)triphosphate (GTP[gamma-35S]) binding by 367% with an EC50 value of 28 nM. Prolonged exposure to agonists induced desensitization of the receptor as estimated by a reduction in the maximal stimulation of GTP[gamma-35S] binding by DAMGO and rightward shifts in the dose-response curves. In cells treated with 10 microM concentrations of etorphine, DAMGO, beta-endorphin, morphine, and butorphanol, DAMGO-stimulated GTP[gamma-35S] binding was 58%, 149%, 205%, 286%, and 325%, respectively. Guanine nucleotide regulation of agonist binding was correspondingly lower in membranes from tolerant cells. Furthermore, chronic opioid treatment increased forskolin-stimulated adenylyl cyclase activity, and potency of DAMGO to inhibit cAMP accumulation was lower in morphine- and DAMGO-tolerant cells (EC50 = 55 and 170 nM versus 18 nM for control). Chronic treatment with agonists reduced [3H]DAMGO binding in membranes with the rank order of etorphine > DAMGO = beta-endorphin > morphine > butorphanol, and the affinity of DAMGO in alkaloid- but not peptide-treated membranes was significantly lower in comparison with control. Pertussis toxin treatment of the cells before agonist treatment did not prevent the down-regulation by full agonists; DAMGO and etorphine exhibited approximately 80% internalization, whereas the ability of partial agonists was greatly impaired. In addition to establishing this cell line as a good model for further studies on the mechanisms of opioid tolerance, these results indicate important differences in the inactivation pathways of receptor triggered by full and partial agonists.

G proteins and opioid receptor-mediated signalling

Most opioid receptor-mediated functions appear to be mediated through G protein interactions, therefore an understanding of opioid signalling requires knowledge of those interactions. This review chronicles the studies examining these interactions for all the opioid receptor subtypes, both in vivo and in vitro.

Mechanisms of actions of opioids and non-steroidal anti-inflammatory drugs

Opioids and non-steroidal anti-inflammatory drugs (NSAIDs) are the commonest drugs used to treat pain. Opioids mimic the actions of endogenous opioid peptides by interacting with mu, delta or kappa opioid receptors. The opioid receptors are coupled to G1 proteins and the actions of the opioids are mainly inhibitory. They close N-type voltage-operated calcium channels and open calcium-dependent inwardly-rectifying potassium channels. This results in hyperpolarization and a reduction in neuronal excitability. They also decrease intracellular cAMP which modulates the release of nociceptive neurotransmitters (e.g. substance P). Inhibition of prostaglandin synthesis by cyclooxygenase is the principal mode of the analgesic and anti-inflammatory actions of NSAIDs. Cyclo-oxygenase is inhibited irreversibly by aspirin and reversibly by other NSAIDs. The widespread inhibition of cyclo-oxygenase is responsible for many of the adverse effects of these drugs. NSAIDs also reduce prostaglandin production within the CNS. This is the main action of paracetamol.

Opiate-induced adenylyl cyclase superactivation is isozyme-specific

While acute activation of inhibitory Gi/o-coupled receptors leads to inhibition of adenylyl cyclase, chronic activation of such receptors leads to an increase in cAMP accumulation. This phenomenon, observed in many cell types, has been referred to as adenylyl cyclase superactivation. At this stage, the mechanism leading to adenylyl cyclase superactivation and the nature of the isozyme(s) responsible for this phenomenon are largely unknown. Here we show that transfection of adenylyl cyclase isozymes into COS-7 cells results in an isozyme-specific increase in AC activity upon stimulation (e.g. with forskolin, ionomycin, or stimulatory receptor ligands). However, independently of the method used to activate specific adenylyl cyclase isozymes, acute activation of the mu-opioid receptor inhibited the activity of adenylyl cyclases I, V, VI, and VIII, while types II, IV, and VII were stimulated and type III was not affected. Chronic mu-opioid receptor activation followed by removal of the agonist was previously shown, in transfected COS-7 cells, to induce superactivation of adenylyl cyclase type V. Here we show that it also leads to superactivation of adenylyl cyclase types I, VI, and VIII, but not of type II, III, IV, or VII, demonstrating that the superactivation is isozyme-specific. Not only were isozymes II, IV, and VII not superactivated, but a reduction in the activities of these isozymes was actually observed upon chronic opiate exposure. These results suggest that the phenomena of tolerance and withdrawal involve specific adenylyl cyclase isozymes.

Receptor-induced internalization of selective peptidic mu and delta opioid ligands

The binding and internalization of radioiodinated and fluorescent mu and delta opioid peptides in mammalian cells were quantitatively studied by biochemical techniques and directly visualized by confocal microscopy. The labeled peptides were prepared by inserting either a 125I-Bolton-Hunter group or a fluorescent probe into the C-terminal part of 5-aminopentylamide derivatives of deltorphin-I and [Lys7]dermorphin. The purified derivatives kept most of their specificity and selectivity toward delta and mu opioid receptors, respectively. Biochemical and confocal microscopy data showed that both mu and delta opioid peptides were internalized in mammalian cells transfected with the corresponding opioid receptor according to a receptor-mediated mechanism. The internalization process was time- and temperature-dependent and was completely blocked by the endocytosis inhibitor phenylarsine oxyde. Internalization of both delta and mu ligands occurred from a single large cap at one pole of the cell, indicating that polymerization of ligand-receptor complexes preceeded internalization. Finally, green and red fluorescent analogues of deltorphin-I and [Lys7]dermorphin, respectively, were found to internalize through partly distinct endocytic pathways in cells co-transfected with mu and delta receptors, suggesting that each of these receptors interacts with distinct proteins mediating intracellular sorting and trafficking.

Magnitude of tolerance to fentanyl is independent of mu-opioid receptor density

The effect of a mu-opioid receptor irreversible antagonist on the development of tolerance to fentanyl was determined in mice. Mice were injected with saline or clocinnamox (3.2 mg/kg, i.p.) and 4 h later mice implanted s.c. with a placebo pellet or an osmotic minipump that infused fentanyl (0.165 mg/kg per day) for 3 days. Fentanyl pumps and placebo pellets were removed on the third day following implantation and 4 h later mu-opioid receptor saturation binding studies in whole brain ([3H][D-Ala2,MePhe4,Gly-ol5]enkephalin: DAMGO) or fentanyl analgesic dose-response studies (tailflick assay) were conducted. Fentanyl infusions and clocinnamox both significantly reduced the potency of fentanyl by 2.8- and 2.4-fold, respectively. When fentanyl and clocinnamox were administered together, a significant 5.0-fold reduction in fentanyl potency relative to the saline-placebo group was observed, which represents an additive effect of clocinnamox and fentanyl. The ED50 of fentanyl in clocinnamox-treated mice was shifted 2.1-fold by fentanyl infusion relative to the clocinnamox-placebo group. This is comparable to the 2.8-fold shift in the ED50 produced by fentanyl infusion in saline-treated mice. In binding studies, fentanyl produced a small (-9%) reduction in Bmax, while clocinnamox significantly reduced (-41%) mu-opioid receptor density without altering affinity (Kd). In the clocinnamox-fentanyl group, there was a 50% reduction in Bmax, which is similar to the additive effect observed in analgesia studies. These data indicate that changes in mu-opioid receptor density prior to the development of tolerance to fentanyl do not impact on the magnitude of tolerance.

Differential opioid agonist regulation of the mouse mu opioid receptor

Mu opioid receptors mediate the analgesia induced by morphine. Prolonged use of morphine causes tolerance development and dependence. To investigate the molecular basis of tolerance and dependence, the cloned mouse mu opioid receptor with an amino-terminal epitope tag was stably expressed in human embryonic kidney (HEK) 293 cells, and the effects of prolonged opioid agonist treatment on receptor regulation were examined. In HEK 293 cells the expressed mu receptor showed high affinity, specific, saturable binding of radioligands and a pertussis toxin-sensitive inhibition of adenylyl cyclase. Pretreatment (1 h, 3 h, or overnight) of cells with 1 microM morphine or [D-Ala2MePhe4,Gly(ol)5]enkephalin (DAMGO) resulted in no apparent receptor desensitization, as assessed by opioid inhibition of forskolin-stimulated cAMP levels. In contrast, the morphine and DAMGO pretreatments (3 h) resulted in a 3-4-fold compensatory increase in forskolin-stimulated cAMP accumulation. The opioid agonists methadone and buprenorphine are used in the treatment of addiction because of a markedly lower abuse potential. Pretreatment of mu receptor-expressing HEK 293 cells with methadone or buprenorphine abolished the ability of opioids to inhibit adenylyl cyclase. No compensatory increase in forskolin-stimulated cAMP accumulation was found with methadone or buprenorphine; these opioids blocked the compensatory effects observed with morphine and DAMGO. Taken together, these results indicate that methadone and buprenorphine interact differently with the mouse mu receptor than either morphine or DAMGO. The ability of methadone and buprenorphine to desensitize the mu receptor and block the compensatory rise in forskolin-stimulated cAMP accumulation may be an underlying mechanism by which these agents are effective in the treatment of morphine addiction.

Differential desensitization of mu- and delta- opioid receptors in selected neural pathways following chronic morphine treatment

1. Morphine produces a plethora of pharmacological effects and its chronic administration induces several side-effects. The cellular mechanisms by which opiates induce these side-effects are not fully understood. Several studies suggest that regulation of adenylyl cyclase activity by opioids and other transmitters plays an important role in the control of neural function. 2. The aim of this study was to evaluate desensitization of mu- and delta- opioid receptors, defined as a reduced ability of opioid agonists to inhibit adenylyl cyclase activity, in four different brain structures known to be involved in opiate drug actions: caudate putamen, nucleus accumbens, thalamus and periaqueductal gray (PAG). Opiate regulation of adenylyl cyclase in these regions has been studied in control and morphine-dependent rats. 3. The chronic morphine treatment used in the present study (subcutaneous administration of 15.4 mg morphine/rat/day for 6 days via osmotic pump) induced significant physical dependence as indicated by naloxone-precipitated withdrawal symptoms. 4. Basal adenylyl cyclase in the four brain regions was not modified by this chronic morphine treatment. In the PAG and the thalamus, a desensitization of mu- and delta-opioid receptors was observed, characterized by a reduced ability of Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO; mu), Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE; delta) and [D-Ala2]-deltorphin-II (DT-II; delta) to inhibit adenylyl cyclase, activity following chronic morphine treatment. 5. The opioid receptor desensitization in PAG and thalamus appeared to be heterologous since the metabotropic glutamate receptor agonists, L-AP4 and glutamate, and the 5-hydroxytryptamine (5-HT)1A receptor agonist, R(+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), also showed reduced inhibition of adenylyl cyclase activity following chronic morphine treatment. 6. In the nucleus accumbens and the caudate putamen, desensitization of delta-opioid receptor-mediated inhibition without modification of mu-opioid receptor-mediated inhibition was observed. An indirect mechanism probably involving dopaminergic systems is proposed to explain the desensitization of delta-mediated responses and the lack of mu-opioid receptor desensitization after chronic morphine treatment in caudate putamen and nucleus accumbens. 7. These results suggest that adaptive responses occurring during chronic morphine administration are not identical in all opiate-sensitive neural populations.

Adenylylcyclase supersensitization in mu-opioid receptor-transfected Chinese hamster ovary cells following chronic opioid treatment

Using CHO cells stably transfected with rat mu-opioid receptor cDNA, we show that the mu-agonists morphine and [D-Ala2,N-methyl-Phe4,Gly-ol5]enkephalin are negatively coupled to adenylylcyclase and inhibit forskolin-stimulated cAMP accumulation. Chronic exposure of cells to morphine leads to the rapid development of tolerance. Withdrawal of morphine or [D-Ala2,N-methyl-Phe4,Gly-ol5]enkephalin following chronic treatment (by wash or addition of the antagonist naloxone) leads to an immediate increase in cyclase activity (supersensitization or overshoot), which is gradually reversed upon further incubation with naloxone. Phosphodiesterase inhibitors do not affect the overshoot, indicating that it results from cyclase stimulation rather than phosphodiesterase regulation. Morphine's potency to inhibit cAMP accumulation is the same before and after chronic treatment, suggesting that the apparent tolerance results from cyclase activation, rather than from receptor desensitization. The similar kinetics of induction of tolerance and overshoot support this idea. Both the overshoot and acute opioid-induced cyclase inhibition are blocked by naloxone and are pertussis toxin-sensitive, indicating that both phenomena are mediated by the mu-receptor and Gi/G(o) proteins. The supersensitization is cycloheximide-insensitive, indicating that it does not require newly synthesized proteins. This is supported by the rapid development of supersensitization. Taken together, these results show that mu-transfected cells can serve as a model for investigating molecular and cellular mechanisms underlying opiate drug addiction.

Effects of morphine on forskolin-stimulated pro-enkephalin mRNA levels in rat striatum: a model for acute and chronic opioid actions in brain

Opioid agonists inhibit adenylyl cyclase in brain through Gi-coupled receptors. One potential biological role for this reaction would be to decrease pro-enkephalin mRNA synthesis by decreasing intracellular cAMP levels and preventing stimulation of gene expression via the cAMP regulatory element (CRE). To determine whether such effects occur in vivo, rats were injected i.c.v. with a water-soluble analog of forskolin, 7-beta-deacetyl-7 beta[gamma-(morpholino-butyryl] butyryl] forskolin (DMB-forskolin), to stimulate the CRE. Pro-enkephalin mRNA levels were assayed in striatum by Northern blot analysis. The treatment with the forskolin analog increased striatal pro-enkephalin mRNA levels approximately 2-fold in 4 h. When rats were injected with morphine (20 mg/kg i.p.) 1 h before DMB-forskolin administration, the stimulation of pro-enkephalin mRNA levels was eliminated. This acute effect of morphine was blocked by co-administration with 10 mg/kg naloxone. When rats were chronically treated with morphine for 8 days, then injected with morphine (20 mg/kg i.p.) 1 h before DMB-forskolin administration, the inhibitory effect of morphine was lost (i.e. DMB-forskolin increased pro-enkephalin mRNA levels by 2 fold in either control and morphine-treated rats). These data not only demonstrate the in vivo relevance of opioid-inhibited adenylyl cyclase in the control of pro-enkephalin mRNA levels, but also show that this model is useful for studying how this signal transduction system is attenuated during the development of tolerance.

Neuroblastoma Neuro2A cells stably expressing a cloned mu-opioid receptor: a specific cellular model to study acute and chronic effects of morphine

Several cellular systems display desensitization and downregulation of opioid receptors upon chronic treatment, suggesting that they could be used as a model system to understand opioid tolerance/dependence. However, a model system containing a homogeneous population of mu-opioid receptors, the receptors at which morphine and related opioids act, has been lacking. To approach this problem, the mu-opioid receptor (MOR-1) was stably expressed in murine neuroblastoma Neuro2A cells after transfection. The expressed receptor was negatively coupled to adenylyl cyclase through Gi/Go proteins, displayed high affinity ligand binding, and was expressed in high number (2.06 pmol/mg of [3H]diprenorphine binding sites). In addition, loss of ability of mu-opioids to acutely inhibit forskolin-stimulated cAMP formation was observed after 4-24 h of chronic exposure to these agonists with concentrations as low as 300-500 nM. The effects of chronic morphine or [D-Ala2,N-MePhe4,Gly-ol]enkephalin (DAMGO) administration were found to be time- and concentration-dependent. Cross 'tolerance' was also observed. Thus the IC50 value of DAMGO to inhibit adenylyl cyclase was increased by 27-fold from 4.3 nM in control cells to 117 nM in cells pretreated with 300 nM morphine; there was no effect on the inhibition of adenylyl cyclase mediated by muscarinic receptors. Further, receptor downregulation accompanied the desensitization process. However, different time-dependence for these two processes suggests, in line with other studies, that these are entirely different cellular adaptation processes. In addition, the opioid antagonist naloxone induced an acute increase in intracellular cAMP level (2-3 times above the control level) following chronic agonist exposure. This process was also concentration-dependent. Overall, these results suggest that the cell line utilized in this study has a homogeneous population of mu-opioid receptors, providing an ideal cellular model to study the molecular mechanisms underlying chronic morphine treatment.

Inhibition of morphine tolerance and dependence by diazepam and its relation to cyclic AMP levels in discrete rat brain regions and spinal cord

Diazepam inhibits morphine tolerance and dependence and reverses a decrease in the met-enkephalin level in brain induced by morphine. In this study, we investigated whether inhibition of morphine-induced tolerance and dependence by diazepam involved a change in cyclic AMP levels in discrete rat brain regions and spinal cord. Male Sprague-Dawley rats were made tolerant and dependent by subcutaneous (s.c.) implantation of six morphine pellets (two pellets on the first day, and four on the second day). Diazepam (0.25 mg/kg b. wt) was injected once daily intraperitoneally (i.p.) for 5 days. Control rats were implanted with placebo pellets and injected once daily with saline or diazepam (i.p.). Tail-flick antinociception was measured 1 h after injections everyday. Animals were administered s.c. naloxone (10 mg/kg) to induce naloxone-precipitated withdrawal syndrome on the final day of the experiment (day 5), and the jumping behavior was observed for 30 min. Concomitant treatment with diazepam (0.25 mg/kg) significantly decreased the development of morphine tolerance and dependence. Diazepam (0.25 mg/kg) treated rats also showed a significant decrease in the jumping behavior compared to animals treated with morphine alone. Rats were sacrificed 2 h after the injection of saline or diazepam (0.25 mg/kg) on the fifth day. Cyclic AMP was estimated by RIA. In the control rats, the concentration of cyclic AMP in cortex was > hippocampus > cerebellum > hypothalamus > striatum > midbrain > pituitary > pons/medulla > spinal cord. There was no change in the concentration of cyclic AMP in any of the brain regions examined from morphine tolerant animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term exposure to opioid antagonists up-regulates prodynorphin gene expression in rat brain

We investigated the effect of long-term administration of opioid antagonists on the regulation of prodynorphin gene expression in rat brain. Intracerebroventricular (i.c.v.) injections for seven days of nor-binaltorphimine (nor-BNI), the highly selective kappa opioid antagonist, naloxone and its longer acting analog naltrexone, both relatively selective antagonists for the mu opioid receptor, markedly raised prodynorphin mRNA levels in rat hypothalamus, hippocampus and striatum. Peptides, namely immunoreactive-dynorphin A (ir-dyn A), were unaffected after chronic treatment with all antagonists, in the same tissues. These results, taken together with our previous observations, suggest that chronic opioid antagonists, acting on kappa and mu opioid receptors, clearly up-regulate prodynorphin gene expression in discrete rat brain regions, activating its biosynthesis. Moreover, our data support the hypothesis that the endogenous opioid system plays a role in the mechanisms underlying the development of opiate tolerance.

Changes in CNS neuropeptide FF-like material, pain sensitivity, and opiate dependence following chronic morphine treatment

Tolerance and dependence to opiates may be an adaptive process that limits excessive effects of morphine on the CNS. Because no consistent opiate receptor reduction in chronically treated rats seems to underlie the hyposensitivity to morphine, an alternative hypothesis has postulated a role of "antiopioid" peptides. It is possible to speculate that the administration of morphine stimulates antiopioid systems such as neuropeptide FF (NPFF), as part of an homeostatic mechanism contributing to the development of tolerance. To test this hypothesis, pain sensitivity, opiate dependence, and CNS NPFF-IR levels were estimated at different times after implantation of morphine pellets (2 x 75 mg; NIDA). Three hours after morphine pellet treatment the analgesic effect was maximum and it decreased rapidly during the following 12 h. Naloxone-precipitated withdrawal syndrome was detected as soon as 3 h after morphine pellet implantation and was maximal after 24 h. NPFF-IR levels were measured in the spinal cord, brain stem, and hypothalamus. A significant decrease of NPFF-IR was observed 1 h after morphine pellet implantation (-25% to -45% depending on the structures) followed by a drastic increase of NPFF-IR levels (+60 to +140%) between 3 and 6 h. NPFF-IR levels rapidly returned to baseline after 24-36 h. It is suggested that the activity of these NPFF-IR neurones may increase gradually as a consequence of the continuous stimulation of opiate receptors and be part of an adaptive process that is able to counteract morphine effects and to induce dependence and tolerance to the analgesic effects of opiates.

mu-Opioid receptor and alpha 2-adrenoceptor agonist binding sites in the postmortem brain of heroin addicts

The biochemical status of human brain mu-opioid receptors and alpha 2-adrenoceptors during opiate dependence was studied by means of the binding of [3H] [D-Ala2, MePhe4, Gly-ol5] enkephalin (DAGO) and [3H]clonidine, respectively, in postmortem brains of heroin addicts who had died by opiate overdose or other causes. In the frontal cortex, thalamus and caudate of heroin addicts the density (Bmax) and affinity (KD) of mu-opioid receptors were similar to those in controls. In contrast, the density of alpha 2-adrenoceptors in heroin addicts was found to be significantly decreased in frontal cortex (Bmax 31% lower), hypothalamus (Bmax 40% lower) and caudate (Bmax 32% lower) without changes in KD values. When heroin addicts were divided into two subgroups according to the presence or absence of morphine in body fluids, only the group with positive screening for morphine showed relevant decreases in brain alpha 2-adrenoceptor density (Bmax 36-48% lower), whereas the decreases in receptor density observed in the subgroup with negative screening for morphine did not reach statistical significance. The results suggest that desensitization of brain alpha 2A-adrenceptors is a relevant adaptative receptor mechanism during opiate addiction in humans.

SK-N-BE: a human neuroblastoma cell line containing two subtypes of delta-opioid receptors

A human neuroblastoma cell line, SK-N-BE, was shown to express a substantial amount of opioid receptors (200-300 fmol/mg of protein). A ligand binding profile of these receptors revealed that they could belong to two distinct subtypes of delta-opioid receptors. Results from sucrose-gradient sedimentation experiments were compared with similar data obtained with the mu-opioid receptor of the rabbit cerebellum and the delta-opioid receptor of the hybrid NG108-15 cell line and have shown that the opioid receptor of the SK-N-BE cell line behaved hydrodynamically as an intermediate between mu- and delta-opioid receptors. Taken together, pharmacological and hydrodynamic studies suggest that the opioid receptors present in the SK-N-BE cell membranes could belong to two delta-opioid receptor subtypes interacting allosterically. Functional experiments suggest that at least one of these subtypes of delta-opioid receptor is negatively coupled to the adenylate cyclase via a Gi protein and that the opiate receptors of the SK-N-BE neuroblastoma cell line undergo a rapid down-regulation when preincubated in the presence of the high-affinity opioid, etorphine.

Chronic morphine induces tolerance and desensitization of mu-opioid receptor but not down-regulation in rabbit

Tolerance to chronic morphine treatment was studied in adult rabbits and modifications in the number and the state of coupling of the mu-opioid receptors were investigated in the cerebellum. Tolerance was induced by the subcutaneous injection of progressively increasing doses of morphine (5-100 mg/kg/injection) over 6 days and its occurrence was controlled by a nociceptive test: electrical stimulation of the dental pulp. At the end of the treatment, the rabbits were tolerant to the analgesic effects of morphine and the tolerance phenomenon correlated well with a significant decrease in the adenylate cyclase inhibition (approximately 60%). The functional uncoupling between the enzyme and the mu-opioid receptor was accompanied neither by a decrease in the number of high affinity receptors measured by equilibrium binding techniques (Kd = 0.19 +/- 0.03 in control vs. 0.11 +/- 0.04 nM in tolerant animals; Bmax = 322 +/- 62 vs. 362 +/- 58 fmol/mg of protein), nor by a modification of the physical coupling between the receptor and its G-protein. It can be concluded that desensitization, under our experimental conditions, can be clearly distinguished from down-regulation.

Adaptive changes in the number of Gs- and Gi-proteins underlie adenylyl cyclase sensitization in morphine-treated rat striatal neurons

In the present study we investigated the possible role of changes in the number of membrane-bound G-proteins in the sensitization of dopamine D1 receptor-stimulated adenylyl cyclase, observed in primary cultures of rat striatal neurons chronically exposed to morphine. Whereas exposure of these neurons to 10 microM morphine for 7 days caused a profound increase in cyclic AMP production, induced by the dopamine D1 receptor agonist SKF 38393 (1 microM), Scatchard analysis of [125I]SCH 23982 binding to membrane preparations revealed that neither the Bmax nor the Kd values of dopamine D1 receptor binding sites were affected. Interestingly, immunoblotting experiments revealed an increase (of more than 50%) in the number of stimulatory G-proteins (G alpha s) in neurons displaying an enhanced adenylyl cyclase activity. In morphine-treated neurons, the number of inhibitory G-proteins (G alpha i) appeared to be slightly reduced (by about 16%). Moreover, the observation that cholera toxin (0.1 nM)-stimulated cyclic AMP production, unlike that induced by forskolin (1 microM), was enhanced in morphine-treated neurons, indicates a causal relationship between the reciprocal changes in G-protein number and the increase of dopamine D1 receptor-stimulated adenylyl cyclase activity. The possible role of these changes in G-protein number in the development of morphine tolerance and dependence is discussed.

Characterization of mu opioid receptor binding during amygdala kindling in rats and effects of chronic naloxone pretreatment: an autoradiographic study

Using in vitro autoradiography, mu receptor binding in rat brain was characterized at different amygdala kindling stages and in amygdaloid kindled animals pretreated chronically with naloxone. Male Sprague-Dawley rats implanted with bipolar electrodes in the right amygdala received one of the following pretreatments s.c. for 14 days via osmotic minipumps: normal saline solution, 0.5 microliters/h, or naloxone HCl, 75 micrograms/h. Two days after treatments were accomplished animals were stimulated daily. Our data showed different patterns of mu receptor binding during the normal kindling process: during stage II-III, pronounced binding increase was detected in cingulate, temporal and entorhinal cortices, anterior amygdala, caudate putamen, thalamic nuclei, ventrolateral and dorsolateral portions of central gray, substantia nigra pars compacta and pars reticulata. Twenty-four hours after the last stage V kindled seizure, enhanced binding was observed in cingulate and frontoparietal cortices, anterior amygdala, caudate putamen and ventromedial thalamic nucleus. Twenty-eight days after the last stage V kindled seizure, binding augmentation was noticed in cingulate and frontoparietal cortices, whereas decreased binding was detected in amygdala complex, substantia nigra pars reticulata, piriform, perirhinal, parietal, temporal and entorhinal cortices. Mu receptor binding in kindled rats chronically pretreated with naloxone was significantly higher in several structures when compared with control and normal kindled groups. Our data indicate different regional selective patterns of mu receptor binding during amygdala kindling which may depend on epileptogenesis and long-term changes induced by this process. In addition, even higher mu receptor binding results from chronic naloxone administration prior to kindling.

Opioid-inhibited adenylyl cyclase in rat brain membranes: lack of correlation with high-affinity opioid receptor binding sites

Opioid agonists bind to GTP-binding (G-protein)-coupled receptors to inhibit adenylyl cyclase. To explore the relationship between opioid receptor binding sites and opioid-inhibited adenylyl cyclase, membranes from rat striatum were incubated with agents that block opioid receptor binding. These agents included irreversible opioid agonists (oxymorphone-p-nitrophenylhydrazone), irreversible antagonists [naloxonazine, beta-funaltrexamine, and beta-chlornaltrexamine (beta-CNA)], and phospholipase A2. After preincubation with these agents, the same membranes were assayed for high-affinity opioid receptor binding [3H-labeled D-alanine-4-N-methylphenylalanine-5-glycine-ol-enkephalin (mu), 3H-labeled 2-D-serine-5-L-leucine-6-L-threonine enkephalin (delta), and [3H]ethylketocylazocine (EKC) sites] and opioid-inhibited adenylyl cyclase. Although most agents produced persistent blockade in binding of ligands to high-affinity mu, delta, and EKC sites, no change in opioid-inhibited adenylyl cyclase was detected. In most treated membranes, both the IC50 and the maximal inhibition of adenylyl cyclase by opioid agonists were identical to values in untreated membranes. Only beta-CNA blocked opioid-inhibited adenylyl cyclase by decreasing maximal inhibition and increasing the IC50 of opioid agonists. This effect of beta-CNA was not due to nonspecific interactions with G(i), Gs, or the catalytic unit of adenylyl cyclase, as neither guanylylimidodiphosphate-inhibited, NaF-stimulated, nor forskolin-stimulated activity was altered by beta-CNA pretreatment. Phospholipase A2 decreased opioid-inhibited adenylyl cyclase only when the enzyme was incubated with brain membranes in the presence of NaCl and GTP. These results confirm that the receptors that inhibit adenylyl cyclase in brain do not correspond to the high-affinity mu, delta, or EKC sites identified in brain by traditional binding studies.