Chronic opioid treatment may uncouple opioid receptors and G-proteins: evidence from radiation inactivation analysis

Opioid maintenance, weaning and detoxification techniques; where we have been, where we are now and what the future holds: an update

In 2017, the US Department of Health and Human Services declared a public health emergency on the opioid crisis. On average, 115 Americans die each day from an opioid overdose. The scope and breadth of this problem is continually evolving. In 2010, there was a shift in causes primarily due to the use of heroin, and currently the latest shift in opioid-related deaths involves a variety of synthetic opioids, particularly illicitly manufactured fentanyl. As the medical, sociological and political environments have drastically changed, especially in the USA, over the last 6 years with regard to opioid use and misuse, an updated review of the literature was necessary.

Opioid maintenance, weaning and detoxification techniques: where we have been, where we are now and what the future holds

SUMMARY Medically supervised opioid withdrawal is a complex and constantly evolving exercise in multimodal therapy that draws from the expertise of a variety of clinical specialties. Acute substitution and weaning has been performed utilizing opioid agonists, partial agonists (e.g., buprenorphine), mixed agonist/antagonists (e.g., Suboxone®), and α2 adrenergic agonists. While thousands of patients are being treated with these 'classic' opioid-withdrawal techniques, traditional treatment approaches are being challenged by the emergence of innovative techniques based on an understanding of the neurochemistry of addiction. Pharmacotherapy with controlled withdrawal is currently the most reliable method of opioid detoxification, but, as translational medicine continues to advance and genomic markers for opioid sensitivity and dependence are identified, the future shows great potential for growth and change.

Differential regional effects of methadone maintenance compared to heroin dependence on mu-opioid receptor desensitization in rat brain

Methadone maintenance therapy has been the mainstay of treatment for heroin addiction since the 1970s. Recent studies indicate that methadone is of greater relative intrinsic efficacy than the active metabolites of heroin at mu-opioid receptors and that the extent of mu-opioid receptor desensitization is dependent upon agonist efficacy. Regional differences have been found for mu-opioid receptor desensitization with chronic heroin self-administration, and a similar paradigm was employed to compare regional differences between the effects of heroin and methadone. Rats were trained to self-administer heroin i.v., and the dose available was increased incrementally to a terminal value of 6 mg/kg for each infusion. Half of these rats were allowed to continue to self-administer heroin, while dependence was maintained in the others by hourly infusions of 3 mg/kg of methadone. A separate group of animals was kept on a low dose of heroin. Activation of G-proteins by the high efficacy agonist DAMGO was decreased to a greater extent in animals treated chronically with methadone compared with those allowed to self-administer heroin in amygdala, periaqueductal gray, and subicular nucleus. Activation of G-proteins by the partial agonist endomorphin was decreased in striatum, thalamus, and amygdala in rats from all drug treatment groups, but to a greater extent in the striatum in methadone treated rats compared with the heroin groups. Elucidating the mechanisms by which methadone induces differential desensitization of mu-opioid receptors across brain regions compared with heroin could provide insights to improve the pharmacotherapy of heroin addiction.

Effects of long-term biogenic amine transporter blockade on receptor/G-protein coupling in rat brain

This study examines the effect of long-term elevation of brain monoamine levels on receptor/G-protein coupling by chronic administration of a highly potent tropane analog, WF-23 (2beta-propanoyl-3beta-(2-naphthyl) tropane). WF-23 blocks dopamine, serotonin and norepinephrine transporters with high affinity in vitro, and blocks transporters for at least two days following a single in vivo administration. Rats were chronically treated for 15 days with 1mg/kg WF-23, injected i.p. every two days. Receptor activation of G-proteins was determined by [35S]GTPgammaS autoradiography in brain sections for D2, 5-HT1A and alpha2-adrenergic receptors, as well as mu opioid receptors as a non-monoamine receptor control. Chronic treatment with WF-23 produced significant reductions in D2, 5-HT1A, and alpha2-adrenergic receptor-stimulated [35S]GTPgammaS binding in caudate/putamen, hippocampus and amygdala, respectively. There were no effects of WF-23 treatment on mu opioid-stimulated [35S]GTPgammaS binding. Additionally, there was no effect of WF-23 treatment on D2 receptor binding, as determined by [3H]spiperone autoradiography. These data show that chronic blockade of monoamine transporters produces specific uncoupling of receptors and G-proteins in specific brain regions in the absence of receptor downregulation.

Chronic agonist treatment converts antagonists into inverse agonists at delta-opioid receptors

In cellular models, chronic exposure to mu-opioid agonists converts antagonists into inverse agonists at mu-receptors. Such adaptations could contribute to the development of tolerance and/or dependence. To determine whether delta-receptors respond similarly, or whether this adaptation is unique for mu-receptors, this study examined the effects of prolonged agonist exposure on the intrinsic activity of several delta-opioid ligands in GH(3) cells expressing delta-receptors. In opioid naive cells, delta-receptors were constitutively active, and a series of delta-ligands displayed a range of intrinsic activities for G protein activation. Chronic treatment with the full delta-agonist [D-Pen(2,5)]-enkephalin reduced the acute ability of [D-Pen(2,5)]-enkephalin to stimulate and the full inverse agonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI-174864) to inhibit G protein activation. In contrast, although naloxone and naltriben exhibited weak partial agonism in opioid naive cells, both ligands acted as full inverse agonists to produce concentration-dependent inhibition of guanosine 5'-O-(3-[(35)S]thio)triphosphate binding after prolonged exposure to [D-Pen(2,5)]-enkephalin or to the partial agonist morphine. This effect was reversed by a neutral delta-antagonist (N,N-bisallyl)-Tyr-Gly-Gly-psi-(CH(2)S)-Phe-Leu-OH (ICI-154129). Finally, as is also characteristic of inverse agonists, naloxone and naltriben demonstrated higher affinities for uncoupled delta-receptors in cells chronically treated with [D-Pen(2,5)]-enkephalin, relative to opioid naive cells. Therefore, this relatively novel adaptation is shared by both mu- and delta-opioid receptors and therefore may serve as an important common mechanism involved the development of tolerance and/or dependence.

Threonine 180 is required for G-protein-coupled receptor kinase 3- and beta-arrestin 2-mediated desensitization of the mu-opioid receptor in Xenopus oocytes

To determine the sites in the mu-opioid receptor (MOR) critical for agonist-dependent desensitization, we constructed and coexpressed MORs lacking potential phosphorylation sites along with G-protein activated inwardly rectifying potassium channels composed of K(ir)3.1 and K(ir)3.4 subunits in Xenopus oocytes. Activation of MOR by the stable enkephalin analogue, [d-Ala(2),MePhe(4),Glyol(5)]enkephalin, led to homologous MOR desensitization in oocytes coexpressing both G-protein-coupled receptor kinase 3 (GRK3) and beta-arrestin 2 (arr3). Coexpression with either GRK3 or arr3 individually did not significantly enhance desensitization of responses evoked by wild type MOR activation. Mutation of serine or threonine residues to alanines in the putative third cytoplasmic loop and truncation of the C-terminal tail did not block GRK/arr3-mediated desensitization of MOR. Instead, alanine substitution of a single threonine in the second cytoplasmic loop to produce MOR(T180A) was sufficient to block homologous desensitization. The insensitivity of MOR(T180A) might have resulted either from a block of arrestin activation or arrestin binding to MOR. To distinguish between these alternatives, we expressed a dominant positive arrestin, arr2(R169E), that desensitizes G protein-coupled receptors in an agonist-dependent but phosphorylation-independent manner. arr2(R169E) produced robust desensitization of MOR and MOR(T180A) in the absence of GRK3 coexpression. These results demonstrate that the T180A mutation probably blocks GRK3- and arr3-mediated desensitization of MOR by preventing a critical agonist-dependent receptor phosphorylation and suggest a novel GRK3 site of regulation not yet described for other G-protein-coupled receptors.

Association of a lower molecular weight protein to the mu-opioid receptor demonstrated by (125)I-beta-endorphin cross-linking studies

Cross-linking experiments using the (125)I-beta-endorphin revealed the presence of several receptor-related species in cell lines expressing endogenous opioid receptors, including a small molecular mass protein (approximately 22 kDa). Previous reports have suggested that this 22-kDa (125)I-beta-endorphin cross-linked protein could be the degradative product from a higher molecular mass species, i.e., a fragment of the receptor. To determine if this protein is indeed a degraded receptor fragment, (125)I-beta-endorphin was cross-linked to the (His)(6) epitope-tagged mu-opioid receptor (His-mu) stably expressed in the murine neuroblastoma Neuro(2A) cells. Similar to earlier reports with cell lines expressing endogenous receptors, two major bands of 72- and 25-kDa proteins were specifically cross-linked. Initial cross-linking experiments indicated the absolute requirement of the high-affinity (125)I-beta-endorphin binding to the mu-opioid receptor prior to the appearance of the low molecular weight species, suggesting that the 22-kDa protein could be a degraded fragment of the receptor. However, variations in the ratios of these protein bands being cross-linked by several homo- or heterobifunctional cross-linking agents were observed. Although neither the carboxyl terminus mu-opioid receptor-specific antibodies nor the antibodies against the epitope at the amino terminus of the receptor could recognize the 22-kDa protein, this (125)I-beta-endorphin cross-linked species could be coimmunoprecipitated with the receptor antibodies or could be isolated with a nickel resin affinity chromatography. The direct physical association of the 22-kDa protein with the receptor was demonstrated also by the observation that the 22-kDa protein could not bind to the nickel resin alone, but that its binding to the nickel resin was restored in the presence of the His-mu. Taken together, these results suggest that the 22-kDa protein cross-linked by (125)I-beta-endorphin is not a degradative product, but a protein located within the proximity of the mu-opioid receptor, and that it is tightly associated with the receptor.

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.

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.

Tolerance to morphine-induced antinociception is decreased by chronic sucrose or polycose intake

Chronic intake of palatable fluids alters morphine-induced antinociception. Two experiments were conducted to evaluate how long-term access to palatable fluids alters the development of tolerance to morphine-induced antinociception. In Experiment 1, 40 adult male Long-Evans rats were used. In addition to ad lib chow and water, 10 rats were given a 0.15% saccharin solution, 10 were given a 32% sucrose solution, and 10 were given a 32% Polycose solution to drink for 3 weeks. Ten rats were given chow and water alone, and served as dietary controls. Morphine-induced antinociception was assessed using the radiant-heat tail-flick method (TF). Half of the animals in each dietary condition were given preexposure to 7.5 mg/kg morphine; the other half received saline. All rats were given a TF 30-min postinjection. To determine whether tolerance developed, a cumulative dose paradigm (0.625, 1.25, 2.5, 5.0, 10.0 mg/kg) was employed 1 week after initial morphine injections, and was repeated at weekly intervals for 3 weeks. Antinociception was significantly lower in rats preexposed to morphine relative to rats preexposed to saline. Although all rats displayed decreased antinociception relative to the first morphine injection, rats that drank saccharin showed greater reductions in morphine-induced antinociception relative to rats that drank sucrose or Polycose. Experiment 2 was conducted to determine whether initial pairing of the TF with morphine preexposure produced differences in the development of opioid tolerance. All conditions and procedures were identical to Experiment 1, except that the initial morphine and saline injections were not followed by TF. As in Experiment 1, rats that drank saccharin showed less antinociception than rats that drank sucrose or Polycose. The present results suggest that long-term intake of palatable nutritive solutions curbs tolerance to morphine-induced antinociception, whereas long-term intake of a nonnutritive, sweet saccharin solution does not.

Opioids disrupt Ca2+ homeostasis and induce carbonyl oxyradical production in mouse astrocytes in vitro: transient increases and adaptation to sustained exposure

Pharmacologically distinct subpopulations of astroglia express mu, delta, and/or kappa opioid receptors. Activation of mu, delta, or kappa opioid receptors can destabilize intracellular calcium ([Ca2+]i) in astrocytes leading to cellular hypertrophy and reactive injury. To assess whether acute or sustained opioid exposure might adversely affect astroglial function by disrupting Ca2+ homeostasis or by producing reactive oxygen species, fura-2 and a novel fluorescent-tagged biotin-4-amidobenzoic hydrazide reagent, respectively, were used to detect [Ca2+]i and carbonyl oxidation products within individual murine astrocytes. Acute (3 h) exposure to mu; (H-Tyr-Pro-Phe (N-Me) -D-Pro-NH2; PLO17), delta ([D-Pen2, D-Pen5]-enkephalin), and kappa (trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrr olidinyl) cyclohexyl] benzeneacetamide methanesulfonate; U50,488H) opioid agonists caused significant mean increases in [Ca2+]i and in the levels of oxidative products in astrocytes. In contrast, following 72 h of continuous opioid exposure, [Ca2+]i and carbonyl levels returned to normal, irrespective of opioid treatment. These preliminary findings indicate that opioids initially destabilize [Ca2+]i and increase reactive oxygen species in astrocytes; however, astrocytes later recover and adapt to sustained opioid exposure.

Mu and delta opioid receptors are differentially desensitized by the coexpression of beta-adrenergic receptor kinase 2 and beta-arrestin 2 in xenopus oocytes

The Xenopus oocyte expression system was used to test the hypothesis that homologous opioid receptor desensitization results from receptor phosphorylation by G protein-coupled receptor kinases. Activation of delta (DOR), mu (MOR) opioid, or beta2-adrenergic receptors increased K+ conductance in oocytes coexpressing the G protein-gated inwardly rectifying K+ channel subunits GIRK1 and GIRK4, and the intrinsic rate of desensitization was small. Coexpression of beta-adrenergic receptor kinase 2 (beta-ARK2) and beta-arrestin 2 (beta-arr2) synergistically produced a rapid desensitization of both DOR and beta2-adrenergic receptor signaling with a t1/2 < 4 min. beta-ARK2 and beta-arr2 more slowly desensitized MOR responses; a similar synergistic effect on MOR required 2-3 h of agonist treatment. DOR mutants lacking serine and threonine residues at the end of the cytoplasmic tail coupled effectively to GIRK channels but were insensitive to beta-ARK2 and beta-arr2. However, a DOR mutant having serine residues mutated to alanine in the third cytoplasmic loop was indistinguishable in coupling and desensitization from the wild type DOR. These studies establish that opioid receptors can be regulated by beta-ARK2 and beta-arr2 and that a portion of the COOH terminus of DOR enhances sensitivity to this modulation.

Chronic U50,488H abolishes inositol 1,4,5-trisphosphate and intracellular Ca2+ elevations evoked by kappa-opioid receptor in rat myocytes

The inositol 1,4,5-trisphosphate (IP3) content and intracellular free Ca2+ ([Ca2+]i) level in response to kappa-opioid receptor stimulation with selective kappa-opioid receptor agonists, dynorphin-(1-13) and trans-3,4-dichloro-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeacetamidel (U50,488H) were determined in ventricular myocytes. Both IP3 and [Ca2+]i were increased following kappa-opioid receptor stimulation. The responses of IP3 and [Ca2+]i to kappa-opioid receptor stimulation were abolished in myocytes of rats that had received chronic injection of U50,488H for 4 days. kappa-Opioid receptor stimulation with U50,488H also reduced the [Ca2+]i transient, induced by electrical stimulation and caffeine, both known to mobilize [Ca2+]i. The effect was abolished after the myocytes had been incubated with U50,488H at a subthreshold concentration for its effect on [Ca2+]i for 24 h. The present study showed for the first time that, upon the development of tolerance to a kappa-opioid receptor agonist, the responses of IP3 and [Ca2+]i to kappa-opioid receptor stimulation were abolished. The lack of response in [Ca2+]i was due to a failure of mobilization of Ca2+ from its intracellular pool. Further study is needed to determine the events that occur after the kappa-opioid receptor stimulation to production of IP3 upon the development of tolerance to a kappa-opioid.

Chronic intracerebroventricular administration of morphine down-regulates spinal adenosine A1 receptors in rats

Previous studies from our laboratory have shown that systemic chronic morphine treatment causes down-regulation of spinal adenosine A1 receptors in rats. In this study, we further investigated whether supraspinal morphine treatment causes this effect. Adult male Sprague-Dawley rats were rendered tolerant to morphine by multiple intracerebroventricular (i.c.v.) injections for 2 or 4 days. Adenosine A1 receptor binding activities were measured with [3H]cyclohexyladenosine in the spinal cord and midbrain. A significant decrease in [3H]cyclohexyladenosine binding was found in the spinal cord but not in the midbrain region after 2 or 4 days of chronic i.c.v. morphine treatment. A decrease in the number of binding sites (Bmax) with no change in the affinity (Kd) of the ligand for the adenosine A1 receptor was observed. These results suggest that supraspinal morphine administration could cause the down-regulation of spinal adenosine A1 receptors and this may play a role in the mechanism of morphine tolerance.