Chronic administration of morphine and naltrexone up-regulate[3H][D-Ala2,D-leu5]enkephalin binding sites by different mechanisms

DARK Classics in Chemical Neuroscience: Etonitazene and Related Benzimidazoles

Etonitazene and related 2-benzylbenzimidazoles are potent analgetics invented in the research laboratories of the Swiss pharmaceutical giant CIBA in the late 1950s. Though the unprecedented structure distinguishes this class of compounds from poppy-derived and other synthetic analgetics, a range of studies indicate that these drugs are selective μ opioid receptor agonists possessing morphine-like pharmacotoxicological properties in animals as well as humans. Several unscheduled members of this synthetically readily accessible class of opioids that are not controlled under the international and national drug control systems have recently emerged on the illicit drug market. Among them, isotonitazene has been implicated in at least 200 fatalities in Europe and North America. None of the 2-benzylbenzimidazole derivatives have been developed into medicines, but etonitazene and some of its derivatives have been used as receptor probes and in addiction behavior studies in animals. The unique structure has inspired research on such benzimidazoles and related benzimidazolones of which "brorphine" made its debut as one of the newest psychoactive substance to emerge on the illicit opioid drug market in mid-2019. This in-depth review provides a historical introduction, an overview on the chemistry, pharmacological profiles, adverse effects, addiction liability, regulatory status, and the impact on chemical neuroscience of the 2-benzylbenzimidazoles. Structurally related benzimidazoles with opioid and/or analgesic properties are also discussed briefly.

Recent advances on the δ opioid receptor: from trafficking to function

Within the opioid family of receptors, δ (DOPrs) and μ opioid receptors (MOPrs) are typical GPCRs that activate canonical second-messenger signalling cascades to influence diverse cellular functions in neuronal and non-neuronal cell types. These receptors activate well-known pathways to influence ion channel function and pathways such as the map kinase cascade, AC and PI3K. In addition new information regarding opioid receptor-interacting proteins, downstream signalling pathways and resultant functional effects has recently come to light. In this review, we will examine these novel findings focusing on the DOPr and, in doing so, will contrast and compare DOPrs with MOPrs in terms of differences and similarities in function, signalling pathways, distribution and interactions. We will also discuss and clarify issues that have recently surfaced regarding the expression and function of DOPrs in different cell types and analgesia.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Dopamine D₄ receptor counteracts morphine-induced changes in µ opioid receptor signaling in the striosomes of the rat caudate putamen

The mu opioid receptor (MOR) is critical in mediating morphine analgesia. However, prolonged exposure to morphine induces adaptive changes in this receptor leading to the development of tolerance and addiction. In the present work we have studied whether the continuous administration of morphine induces changes in MOR protein levels, its pharmacological profile, and MOR-mediated G-protein activation in the striosomal compartment of the rat CPu, by using immunohistochemistry and receptor and DAMGO-stimulated [³⁵S]GTPγS autoradiography. MOR immunoreactivity, agonist binding density and its coupling to G proteins are up-regulated in the striosomes by continuous morphine treatment in the absence of changes in enkephalin and dynorphin mRNA levels. In addition, co-treatment of morphine with the dopamine D₄ receptor (D₄R) agonist PD168,077 fully counteracts these adaptive changes in MOR, in spite of the fact that continuous PD168,077 treatment increases the [³H]DAMGO B_max values to the same degree as seen after continuous morphine treatment. Thus, in spite of the fact that both receptors can be coupled to G_i/0 protein, the present results give support for the existence of antagonistic functional D₄R-MOR receptor-receptor interactions in the adaptive changes occurring in MOR of striosomes on continuous administration of morphine.

A microdialysis study of extracellular levels of acamprosate and naltrexone in the rat brain following acute and repeated administration

Acamprosate and naltrexone are widely used in the treatment of alcoholism. However, numerous studies in rodents have shown differential effects of these compounds on alcohol consumption and/or relapse-like behavior following acute versus repeated administration. In order to determine if these differential behavioral effects could be attributable to changes in extracellular levels of these compounds, we used in vivo microdialysis to monitor extracellular levels of acamprosate and naltrexone in the rat medial prefrontal cortex following acute and repeated intraperitoneal administration. For acute treatment, animals received a single administration of acamprosate (100 or 300 mg/kg) or naltrexone (1 or 3 mg/kg). For repeated treatment, animals received once daily treatment with saline, acamprosate (300 mg/kg) or naltrexone (3 mg/kg) for 10 days before a subsequent challenge with the compound according to their respective pretreatment group. Dialysate levels of acamprosate and naltrexone were analyzed by liquid chromatography-tandem mass spectrometry and high performance liquid chromatography, respectively. Following acute administration, peak dialysate concentrations of each compound were dose-dependent, observed within 1 hour of administration, and were found to be in the low micromolar range for acamprosate and in the low to mid-nanomolar range for naltrexone. Pretreatment with acamprosate, but not naltrexone, for 10 days resulted in higher dialysate concentrations of the compound relative to saline-pretreated controls. Thus, repeated administration of acamprosate, but not naltrexone, results in augmented extracellular levels of the compound in the brain relative to saline-pretreated controls, which may explain the need for repeated administration of acamprosate in order to observe effects on alcohol consumption and/or relapse.

Quantitative autoradiography of adenosine receptors in brains of chronic naltrexone-treated mice

1. Manipulation of micro opioid receptor expression either by chronic morphine treatment or by deletion of the gene encoding micro opioid receptors leads to changes in adenosine receptor expression. Chronic administration of the opioid receptor antagonist naltrexone leads to upregulation of micro receptor binding in the brain. 2. To investigate if there are any compensatory alterations in adenosine systems in the brains of chronic naltrexone-treated mice, we carried out quantitative autoradiographic mapping of A(1) and A(2A) adenosine receptors in the brains of mice treated for 1 week with naltrexone (8 mg(-1) kg(-1) day(-1)), administered subcutaneously via osmotic minipump. 3. Adjacent coronal brain sections were cut from chronic saline- and naltrexone-treated mice for the determination of binding of [(3)H] D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin ([(3)H] DAMGO), [(3)H]1,3-dipropyl-8-cyclopentylxanthine ([(3)H] DPCPX) or [(3)H] 2-[p-(2-carbonylethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine ([(3)H] CGS21680) to micro, A(1) and A(2A) receptors, respectively. 4. A significant increase in micro and A(1) receptor binding was detected in chronic naltrexone-treated brains. The changes in micro receptors were significant in several regions, but changes in A(1) were relatively smaller but showed significant upregulation collectively. No significant change in A(2A) receptor binding was detected in chronic naltrexone-treated brains. 5. The results show that blockade of opioid receptors causes upregulation of A(1) receptors, but not A(2A) receptors, by as yet undefined mechanisms.

Receptor-selective changes in mu-, delta- and kappa-opioid receptors after chronic naltrexone treatment in mice

Chronic treatment with the opioid antagonist naltrexone induces functional supersensitivity to opioid agonists, which may be explained by receptor up-regulation induced by opioid receptor blockade. In the present study, the levels of opioid receptor subtypes through the brain of mice were determined after chronic naltrexone treatment using quantitative in vitro autoradiography. This is the first complete mapping study in mice for micro-, delta- and kappa-opioid receptors after chronic naltrexone exposure. Treatment with naltrexone clearly induced up-regulation of micro- (mean 80%) and, to a lesser extent, delta-opioid receptors (mean 39%). The up-regulation of micro- and delta-opioid receptors was evident throughout the brain, although there was variation in the percentage change across brain regions. In contrast, consistent up-regulation of kappa-opioid receptors was observed in cortical structures only and was not so marked as for micro- and delta-opioid receptors. In noncortical regions kappa-opioid receptor expression was unchanged. Taken together, the present findings suggest opioid receptor subtype-selective regulation by chronic naltrexone treatment in mice.

Endogenous CART peptide regulates mu opioid and serotonin 5-HT(2A) receptors

Previous experiments conducted in this laboratory showed that intracerebroventricular (i.c.v.) administration of IgG antibodies directed against selected neuropeptides changed the density of CNS receptors, suggesting that neuropeptides in the cerebrospinal fluid can perform a regulatory role. To further test this hypothesis, we administered anti-CART peptide (the peptide product of cocaine amphetamine related transcript) IgG to rats via the i.c.v. route, and measured the density of opioid mu and delta receptors, beta-adrenergic and alpha(2)-adrenergic receptors and serotonin 5-HT(2A) receptors using ligand binding methods. We also used Western blots to determine the expression level of the mu, delta and 5-HT(2A) receptors. The results demonstrated that anti-CART peptide IgG up-regulates mu and 5-HT(2A) receptor in the hippocampus and caudate We conclude that CART peptides in the cerebrospinal fluid may exert regulatory effects in the brain.

Endogenous corticotropin releasing factor regulates adrenergic and opioid receptors

Previous work from this laboratory demonstrated that intracerebroventricular (i.c.v.) administration of IgG antibodies directed against selected neuropeptides changed the density of opioid receptors, suggesting that neuropeptides in the CNS can perform a regulatory role. To further test this hypothesis, we administered anticorticotropin (CRF) IgG to rats via the i.c.v. route and measured the density of opioid mu and delta receptors and also beta- and alpha(2)-adrenergic receptors. The results demonstrated that anti-CRF IgG upregulates mu and beta-adrenergic receptors. We conclude that CRF in the cerebrospinal fluid may exert regulatory effects throughout the brain.

Prenatal exposure to morphine differentially alters gonadal hormone regulation of delta-opioid receptor binding in male and female rats

The present study tested the hypothesis that exposure to morphine on gestation days 11-18 differentially alters delta-opioid receptors in the brain of adult male and female rats. In Experiment 1, the binding characteristics of delta-opioid receptors were examined in membrane homogenates from six brain regions, including the hypothalamus (HYP), preoptic area, frontal cortex (CX), ventral tegmental area, striatum (STR) and cerebellum of adult male and female rats. In Experiment 2, the density of delta-opioid receptors was assessed in the CX and STR using receptor autoradiography. Prenatal morphine exposure has no effects on delta-opioid receptors in the brain of gonadally intact, adult male rats regardless of methodology. However, when male rats were gonadectomized in Experiment 2, morphine-exposed males have fewer delta-opioid receptors than controls in the CX but not in the STR. These reductions in cortical delta-opioid receptors are restored by testosterone replacement, demonstrating that prenatal morphine exposure alters testosterone regulation in the CX of male rats. In ovariectomized (OVX) female rats, prenatal morphine exposure increases the density of delta-opioid receptors in the frontal CX. Interestingly, this up-regulation of delta-opioid receptors is not present when the CX is investigated by autoradiography. Moreover, progesterone given alone or in combination with estrogen reduces the density of delta-opioid receptors in the CX and STR of both saline- and morphine-exposed, OVX females. Thus, mid to late gestational morphine exposure differentially alters the influence of adult gonadal hormones on delta-opioid receptors in the CX, decreasing the sensitivity in females and increasing it in males. This is also the first report to demonstrate that gonadal hormones regulate delta receptor densities in brain regions other than the HYP of OVX females.

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

In animal experiment and in cell culture, chronic morphine treatment has been followed by a reduction as well as an increase of the delta-opioid receptor (OR) number. The present postmortem morphometric study of morphine-related fatalities of drug addicts (n=12, 22-35 years old, with blood unconjugated morphine levels from 27.1 to 407 ng/ml, m.v. 176.9 ng/ml) versus a non-addicted control group (n=13, 10-44 years old) is intended to examine whether chronic opiate exposure also affects the numerical density of deltaOR expressing neurons in the human neocortex (area 10 according to Brodmann (Vergleichende Lokalisationslehre der Grosshirnrinde (1909) Johann Ambrosius Barth, Leipzig)). For the immunohistochemical procedure, vibratome sections (100 microm) were incubated with a monoclonal antibody against the deltaOR diluted 1:100, and immunoreactive sites were visualized using an immunoperoxidase protocol. The numerical densities of OR expressing and Nissl-stained neurons were assessed morphometrically (camera lucida drawings). In both collectives, the anti deltaOR immunoreactivity was predominantly localized in pyramidal neurons of layers (L) II/III and V as well as in round and ovoid neurons of L VI. In the drug-related fatalities, the density of neurons expressing deltaOR protein amounted for 2515+/-240/mm(3), in the control group for 2616+/-204/mm(3), thus displaying no statistically significant difference. These findings go along with the binding behavior of opioid ligands in postmortem brains of heroin addicts revealing similar receptor densities and affinities in the control subjects and addicts.

Morphine tolerance in mice changes response of heroin from mu to delta opioid receptors

Heroin produced antinociception in the tail flick test through mu receptors in the brain of ICR and CD-1 mice, a response inhibited by 3-O-methylnaltrexone. Tolerance to morphine was produced by subcutaneous morphine pellet implantation. By the third day, the heroin response was produced through delta opioid receptors. The response was inhibited by simultaneous intracerebroventricular (i.c. v.) administration of naltrindole, a delta opioid receptor antagonist. More specifically, delta1 rather than delta2 receptors were involved because 7-benzylidenenaltrexone, a delta1 receptor antagonist, inhibited but naltriben, a delta2 antagonist, did not. Also, antinociception produced by i.c.v. heroin was inhibited by intrathecal administration of bicuculline and picrotoxin consistent with the concept that delta1 receptors in the brain mediated the antinociceptive response through descending neuronal pathways to the spinal cord to activate GABAA and GABAB receptors rather than spinal alpha2-adrenergic and serotonergic receptors activated originally by the mu agonist action in naive mice. The mu response of 6-monoacetylmorphine, a metabolite of heroin, was changed by morphine pellet implantation to a delta2 response (inhibited by naltriben but not 7-benzylidenenaltrexone). The agonist action of morphine in these morphine-tolerant mice remained mu. Thus, the opioid receptor selectivity of heroin and 6-monoacetylmorphine in the brain is changed by production of tolerance to morphine. Such a change explains how morphine tolerant mice are not cross-tolerant to heroin.

Regulation of opioid receptors by opioid antagonists: implications for rapid opioid detoxification

Opioid receptor antagonists have long been used in the diagnosis of opioid dependence and in the treatment of both opioid overdose and addiction. More recently they have been used in rapid opioid detoxification, a technique which has generated much ethical and scientific controversy. Because of this, the present review aims to integrate and summarize the current state of knowledge on adaptational changes to opioid systems as a result of antagonist administration. It is generally accepted that chronic treatment with an opioid antagonist results in opioid receptor upregulation. However, the mechanism(s) underlying this resultant opioid supersensitivity remain unresolved. In addition, there is not yet consensus regarding whether changes in opioid receptor number are directly responsible for the functional changes observed after chronic opioid antagonist treatment. Moreover, changes in opioid receptor number and sensitivity to opioid agonists and antagonists after chronic opioid antagonist treatment are dependent on dosing regimes as well as the kinetic properties of the antagonist itself. The role of these variables is appraised critically given the implication that an opioid antagonist can enhance functional responses. For example, such responses are an important consideration in the use of opioids because of possible adverse outcomes, such as overdose, after cessation of administration. Based on the literature discussed in this review it is concluded that caution is essential in the use of opioid antagonists for rapid opioid detoxification.

Quantitative immunolocalization of mu opioid receptors: regulation by naltrexone

The present study utilized a newly developed quantitative immunohistochemical assay to measure changes in mu opioid receptor abundance following chronic administration of the opioid receptor antagonist naltrexone. These data were compared with those obtained from mu receptor radioligand binding on adjacent tissue sections, in order to determine whether the characteristic antagonist-induced increase in radioligand binding is due to an increase in the total number of mu receptors and/or to an increase in the proportion of receptors that are in an active binding conformation in the absence of a change in the total number of receptors. Adult male Sprague-Dawley rats were administered naltrexone, 7-8 mg/kg per day, or saline continuously for seven days by osmotic minipumps, after which time their brains were processed for immunohistochemistry and receptor autoradiography on adjacent fresh frozen tissue sections. Semiquantitative immunohistochemistry was performed using a radiolabelled secondary antibody for autoradiographic determination and a set of radioactive standards. Results demonstrate an overall concordance between the distribution of mu opioid receptors as measured by the two different methods with a few exceptions. Following naltrexone administration, mu receptor immunoreactivity was significantly higher in the amygdala, thalamus, hippocampus, and interpeduncular nucleus as compared with the saline-treated control animals. [3H]D-Ala2,N-Me-Phe4,Gly-ol5-enkephalin binding to mu opioid receptors was significantly higher in the globus pallidus, amygdala, thalamus, hypothalamus, hippocampus, substantia nigra, ventral tegmental area, central gray, and interpeduncular nucleus of the naltrexone-treated rats. These findings indicate that in some brain regions chronic naltrexone exposure increases the total number of mu opioid receptors, while in other regions there is an increase in the percent of active receptors without an observable change in the total number of receptors. Quantitative receptor immunodetection together with ligand autoradiography provides a new approach for investigating the regulation of mu opioid receptors on tissue sections.

Opiate tolerance and dependence: receptors, G-proteins, and antiopiates

Despite the existence of a large body of information on the subject, the mechanisms of opiate tolerance and dependence are not yet fully understood. Although the traditional mechanisms of receptor down-regulation and desensitization seem to play a role, they cannot entirely explain the phenomena of tolerance and dependence. Therefore, other mechanisms, such as the presence of antiopiate systems and the coupling of opiate receptors to alternative G-proteins, should be considered. A further complication of studies of opiate tolerance and dependence is the multiplicity of endogenous opiate receptors and peptides. This review will focus on the endogenous opioid system--peptides, receptors, and coupling of receptors to intracellular signaling via G-proteins--in the context of their roles in tolerance and dependence. Opioid peptides include the recently discovered endomorphins and those encoded by three known genes--pro-opiomelanocortin, pro-enkephalin, and pro-dynorphin. They bind to three types of receptors--mu, delta, and kappa. Each of the receptor types is further divided into multiple subtypes. These receptors are widely known to be coupled to G-proteins of the Gi and Go subtypes, but an increasing body of results suggests coupling to other G-proteins, such as Gs. The coupling of opiate receptors to Gs, in particular, has implications for tolerance and dependence. Alterations at the receptor and transduction level have been the focus of many studies of opiate tolerance and dependence. In these studies, both receptor down-regulation and desensitization have been demonstrated in vivo and in vitro. Receptor down-regulation has been more easily observed in vitro, especially in response to morphine, a phenomenon which suggests that some factor which is missing in vitro prevents receptors from down-regulating in vivo and may play a critical role in tolerance and dependence. We suggest that antiopiate peptides may operate in vivo in this capacity, and we outline the evidence for the antiopiate properties of three peptides: neuropeptide FF, orphanin FQ/nociceptin, and Tyr-W-MIF-1. In addition, we provide new results suggesting that Tyr-W-MIF-1 may act as an antiopiate at the cellular level by inhibiting basal G-protein activation, in contrast to the activation of G-proteins by opiate agonists.

Chronic morphine and naltrexone fail to modify mu-opioid receptor mRNA levels in the rat brain

Previous radioligand-binding studies have reported conflicting results concerning the effect of chronic morphine administration on the regulation of mu-opioid receptor (MOR) density. On the other hand, chronic administration of an opioid antagonist, such as naltrexone, has been shown to increase the density of the MOR. In order to determine if the changes in the MOR are associated with alterations in receptor mRNA levels, we investigated MOR gene expression following chronic treatment with morphine and/or naltrexone. MOR mRNA levels, determined by the ribonuclease protection assay (RPA), were unchanged with respect to control during chronic morphine treatment and morphine withdrawal in each of the analysed brain areas. Furthermore, chronic administration of naltrexone did not result in changes of MOR mRNA levels in rat striatum of naive and morphine-dependent rats, suggesting that the up-regulation of the MOR density, at least in this tissue, is not regulated at transcriptional level.

Effects of chronic morphine pretreatment on amygdaloid kindling development, postictal seizure and suppression and benzodiazepine receptor binding in rats

Effects of chronic morphine pretreatment on the development of amygdaloid kindling, seizure suppression and benzodiazepine (BDZ) receptor binding in rats were evaluated. The morphine-pretreated animals showed faster acquisition of seizure activity. Further evaluation of the postictal seizure suppression immediately after a fully kindled seizure demonstrated that morphine-pretreated rats had a decreased sensitivity to subsequent kindling stimulations. Twenty-four hours after the last electrical stimulation, saline-pretreated fully kindled rats showed enhanced BDZ receptor binding in dentate gyrus, and decreased binding in cingulate cortex ipsilateral to the stimulation site, compared to saline controls. Morphine-pretreated amygdala-kindled rats had significantly higher BDZ binding in piriform, entorhinal and sensorimotor cortices, basolateral and cortical amygdaloid nuclei, dentate gyrus, CAI-3 areas, substantia nigra pars reticulata and periaqueductal gray. The present study indicates that the previous experience with chronic morphine modifies the kindling process and that the enhanced BDZ receptor binding detected in our experiments may be involved in the enhanced postictal seizure suppression observed in these animals.

Downregulation of mu-opioid binding sites following chronic administration of neuropeptide FF (NPFF) and morphine

The effect of continuous ICV infusion of NPFF (10 micrograms/microliter) and morphine (40 micrograms/microliter) on mu-opioid binding sites was examined in rats using the in vitro radiolabeled techniques of whole-brain homogenate receptor binding and quantitative autoradiography. Mu receptors were labeled with [3H][D-Ala2-MePhe4,Glyol5] enkephalin in the homogenate binding experiments and with [125I][D-Ala2-MePhe4,Gly-ol5]enkephalin in autoradiographic studies. In homogenate binding studies, chronic administration of NPFF or morphine significantly downregulated mu receptors by 20% and 44%, respectively. Quantitative autoradiographic experiments demonstrated downregulation of mu opioid receptors in specific brain nuclei for both NPFF- and morphine-treated animals. Within the striatum and several nuclei of the thalamus, the mu receptors of the NPFF- and morphine-treated animals were decreased by 20-30% and 38-73%, respectively. These results suggest that NPFF may modulate opioid-mediated responses in part by altering the density of mu-opioid receptors.

Chronic opioid antagonist administration upregulates mu opioid receptor binding without altering mu opioid receptor mRNA levels

Chronic administration of opioid antagonists has been shown to increase radioligand binding to brain opioid receptors. The present study was conducted to determine whether chronic exposure to the opioid antagonist naltrexone would similarly increase mu opioid receptor gene expression as measured by mRNA levels. Male Sprague-Dawley rats were administered naltrexone, 7-8 mg/kg/day, or saline by osmotic minipumps for 7 days. As expected, the density of mu opioid receptor binding sites was significantly higher in the brains of animals treated chronically with naltrexone as compared with saline-treated control animals. However, mu opioid receptor mRNA content determined by a solution hybridization RNase protection assay was not significantly altered in any brain region investigated. These results indicate that the upregulation of mu opioid receptors as measured by radioligand binding is not accompanied by increased levels of mu receptor mRNA.

Prenatal morphine exposure differentially alters expression of opioid peptides in striatum of newborns

The biochemical and cellular mechanisms involved in the development and/or maintenance of morphine tolerance remain unclear. In the adult central nervous system (CNS) results are contradictory. For the neonate, a variety of drug induced deficits have been observed following prenatal addiction to opioids, although very little work on the biochemical and molecular level has been done. Therefore, the present study was carried out to investigate the effects of prenatal morphine treatment on the levels and expression of endogenous opioid peptides in brain regions of newborns. Dams were implanted with one morphine pellet (75 mg each) 1 week prior to the birth of pups. Changes in mRNA levels for the opioid peptides were determined by Northern blot analysis. Alterations in opioid peptide levels were determined by radioimmunoassays. Prenatal morphine treatment significantly increased proenkephalin mRNA levels and decreased met-enkephalin levels in striatum of newborns. These data are in contrast to what is observed in the adult CNS. These data indicate that prenatal morphine treatment may increase met-enkephalin release and/or cause inhibition at the level of translation. In addition, increased transcription may be necessary to maintain equilibrium in the system when there is an increase in met-enkephalin release.

Opioid peptide receptor studies. 1. Identification of a novel delta-opioid receptor binding site in rat brain membranes

Our laboratory was among the first to propose the existence of delta receptor subtypes: a delta site thought to be associated with a mu-delta-opioid receptor complex termed the delta cx binding site and delta site not associated with the mu-delta-opioid receptor complex, termed the delta ncx site. In previous studies, we assayed the delta cx site with [3H][D-Ala2,D-Leu5]enkephalin using rat brain membranes depleted of delta ncx sites by pretreatment with the site-directed acylating agent, (+)-trans-SUPERFIT. In the present study, we investigated, using (+)-trans-SUPERFIT-pretreated membranes, the possibility of heterogeneity of the delta cx binding site. Two sites were resolved: the delta cx-1 site at which mu ligands are potent noncompetitive inhibitors and delta ligands are weak competitive inhibitors, and the delta cx-2 site where delta ligands are potent and mu ligands are weak, mixed competitive-noncompetitive inhibitors. Although the delta cx-2 site has a delta-like ligand-selectivity profile, several experiments distinguished it from the delta ncx site. Two lines of evidence suggest that the delta ncx site corresponds to the cloned delta receptor. One, the delta receptor was cloned from the NG108-15 cell line, and this receptor, like the delta ncx binding site, irreversibly binds SUPERFIT and (+)-trans-SUPERFIT. Secondly, administration of delta-antisense DNA selectively decreases delta ncx binding. Viewed collectively, the major finding of this study is the discovery of a novel SUPERFIT-insensitive and delta-antisense-insensitive delta cx-2 binding site.

Opiate withdrawal intensity correlates with the presence of DSLET high-affinity binding

The goal of this study was to compare the characteristics of mu- and delta-opioid receptors in the cortex of DBA/2 and C57BL/6 mice, which differ in sensitivity to the long- and short-term effects of morphine. The characteristics of mu-opiate receptors were not different in the cortex of both strains. Both high- and low-affinity binding sites of DSLET, a specific ligand of delta-opiate receptors, were present in the cortex of C57BL/6 mice, whereas the high-affinity binding sites were not found in the cortex of DBA/2 mice. The absence of high-affinity DSLET binding sites, which are similar to the delta 2 type of opioid receptors, may explain the less intensive naloxone-precipitated withdrawal reaction of DBA/2 as compared with C57BL/6 mice.

Granulocyte defects and opioid receptors in chronic exposure to heroin or methadone in humans

In order to elucidate better the immunological effect of opioid abuse in the absence of HIV infection as a confounding factor, granulocyte function was investigated in three groups of HIV-negative subjects, including 20 active parenteral heroin abusers (H), 20 long-term methadone-maintained former opiate abusers (M) and 20 healthy controls (C). Chemotaxis to N-formyl methionyl-leucyl-phenylalanine (fMLP), casein and activated plasma were markedly and similarly reduced (approx. 50%) in both H and M groups, as was true for superoxide production after fMLP and PMA stimulation, 47% decrease of C values. Polymorphonuclear (PMN) of H and M subjects also exhibited a very marked and similar reduction in the expression of CD11b/CD18 integrin receptors after fMLP treatment, with values that were less than 10% of those in controls, as observed by flow cytometry. In parallel, PMN of H and M individuals presented an approximately four-fold increase in opioid receptors numbers compared to controls, a significant inverse correlation existing between the increase in opiate receptors and defective chemotaxis. The possible mechanism underlying the observed changes in PMN of H and M individuals is discussed.

Inhibition of morphine tolerance and dependence by diazepam and its relation to the CNS Met-enkephalin levels

The effect of diazepam on the development of morphine tolerance and dependence was investigated. Male Sprague-Dawley rats were rendered tolerant and dependent by subcutaneous implantation of six morphine pellets. Diazepam (0.025, 0.25 or 2.5 mg/kg body weight) was once daily injected intraperitoneally into rats starting on the first day of implantation. Antinociception was measured by tail-flick (TF) and hot plate (HP) tests, and the extent of sedation determined by a rotarod test before and one hour after diazepam injections everyday for 5 days. Physical dependence on morphine was assessed by an antagonist-precipitated abstinence syndrome on the fifth day of treatment by injecting naloxone 10 mg/kg subcutaneously. Diazepam (0.025-2.5 mg/kg body weight) did not produce significant antinociception or sedation (sensorimotor impairment) in rats implanted with placebo pellets. Diazepam (0.25 and 2.5 mg/kg) inhibited tolerance to TF antinociception in rats implanted with morphine pellets. Sedation as evidenced by sensorimotor impairment induced by morphine pellet implantation was not influenced by diazepam (0.025-2.5 mg/kg). Diazepam administration (0.25 mg/kg) also decreased the degree of jumping behavior observed following naloxone injection in morphine pellet implanted rats. Serum morphine concentration in morphine-diazepam treated rats was not significantly different from that in morphine-saline treated rats. Finally, a decrease in the Met-enkephalin levels observed in the hypothalamus, hippocampus, cortex and spinal cord of morphine dependent rats was reversed by injecting diazepam along with morphine pellet implantation. These results suggest that diazepam inhibits morphine tolerance and dependence, and also prevents morphine-induced decrease in the CNS Met-enkephalin levels in morphine dependent rats.

Morphine-induced reciprocal alterations in G alpha s and opioid peptide mRNA levels in discrete brain regions

The mechanisms involved in the development of morphine tolerance and dependence are still unknown. Recently much attention has been directed toward the changes in post receptor events. Opiate receptors, like other hormone and neurotransmitter receptors, have been shown to mediate their effects through guanine nucleotide binding proteins (G-proteins). This, in turn, may cause alterations in intracellular events, one of which is transcription of specific genes. We investigated the changes in the levels of mRNA of proenkephalin (PPE) and prodynorphin (DYN) and the stimulatory G protein alpha subunit (G alpha s) in adult morphine tolerant rats. Chronic morphine treatment induced reciprocal alterations in the levels of opioid peptide mRNA and G alpha s mRNA in discrete brain regions. In striatum, PPE mRNA decreased by 49% (P < .01) and in hypothalamus, DYN mRNA showed a decrease of 21% (P < .01). In contrast, G alpha s mRNA increased 20% (P < .01) in striatum and 97% (P < .01), in hypothalamus. In hippocampus the changes were reversed: PPE mRNA increased (55%, P < .05) and G alpha s mRNA decreased (33%, P < .01). Frontal cortex exhibited a small decrease in PPE (11.5%, P < .05) without any change on G alpha s or DYN mRNA levels. These reciprocal alterations suggest an opposing mode of regulation of G alpha s and PPE/DYN gene expression in morphine tolerant animals.

Blockade of tolerance to morphine but not to kappa opioids by a nitric oxide synthase inhibitor

The nitric oxide synthase inhibitor NG-nitro-L-arginine (NO2Arg) blocks morphine tolerance in mice. After implantation of morphine pellets the analgesic response decreases from 100% on the first day to 0% on the third. Coadministration of NO2Arg along with the pellets markedly retards the development of tolerance; 60% of mice are analgesic after 3 days, and 50% of mice are analgesic after 5 days. In a daily injection paradigm the analgesic response to morphine is reduced from 60% to 0% by 5 days. Concomitant administration of morphine along with NO2Arg at doses of 2 mg/kg per day prevents tolerance for 4 weeks. A single NO2Arg dose retards morphine tolerance for several days, and dosing every 4 days is almost as effective as daily NO2Arg. NO2Arg slowly reverses preexisting tolerance over 5 days despite the continued administration of morphine along with NO2Arg. NO2Arg also reduces dependence and reverses previously established dependence. NO2Arg does not prevent tolerance to analgesia mediated by the kappa 1 agonist trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolindinyl)cyclohexyl]- benzene-acetamide (U50,488H) or the kappa 3 agent naloxone benzoylhydrazone, indicating a selective action of NO in the mechanisms of mu tolerance and dependence.

Opioid and substance P receptor adaptations in the rat spinal cord following sub-chronic intrathecal treatment with morphine and naloxone

The effect of continuous intrathecal infusion with morphine (5 mu/h) or naloxone (2 micrograms/h) was investigated with regard to analgesia and the apparent density of mu- and delta-opioid and neurokinin-I/substance P receptors in the rat spinal cord. Morphine infusion increased tail-flick and paw-pressure responses until day 4 after the mini-osmotic pump implant. A decline in antinociception, reflecting tolerance to morphine, was then apparent in both tests. Quantitative in vitro receptor autoradiography of [125I]FK-33824, [125I][D.Ala2]deltorphin-I and [125I] Bolton-Hunter substance P binding sites, as ligands of mu, delta and neurokinin-I/substance P receptors, respectively, was performed on lumbosacral spinal cord sections of seven-days tolerant animals. Treatments with morphine and naloxone induced a similar increase (37%) in the number of delta binding sites in the superficial laminae of the dorsal horn. In contrast, the density of mu-opioid receptors was only affected by naloxone (50% increase). Neurokinin-I/substance P binding parameters were not altered by these treatments. Thus, it appears that delta-opioid binding sites may be of special relevance with regard to the development of tolerance to opiates in the spinal cord.

A review of the role of anti-opioid peptides in morphine tolerance and dependence

Studies on the mechanisms of tolerance and dependence have mostly focused on changes at the receptor level. These experiments, conducted with model systems ranging from clonal cell lines to whole animals, have identified a number of important adaptive mechanisms which occur at the receptor level. However, none of these adaptive mechanisms can completely account for the phenomena which serve to define the state of morphine tolerance and dependence, especially the observation that as an animal becomes more tolerant to morphine, less naloxone is required to trigger withdrawal. The data reviewed in this paper provide strong support for the hypothesis that the brain synthesizes and secretes neuropeptides which act as part of a homeostatic system to attenuate the effects of morphine and endogenous opioid peptides. According to this model, administration of morphine releases anti-opioid peptides (AOP), which then attenuate the effects of morphine. As more morphine is given, more AOP are released, thereby producing tolerance to the effects of morphine. Cessation of morphine administration, or administration of naloxone, produces a relative excess of anti-opioid, which is in part responsible for the withdrawal syndrome. Since endogenous and exogenous antagonists might together produce synergistic effects, less naloxone might be required to trigger withdrawal in the presence of higher levels of AOPs. Although the study of AOP is in its infancy, a deeper understanding of the central nervous system (CNS) anti-opioid systems may lead to new treatments for chronic pain, substance abuse, and psychiatric disorders.

Delta-opioid-receptor activation by [D-Pen2,D-Pen5]enkephalin and morphine inhibits substance P release from trigeminal nucleus slices

The release of substance P (SP) from spinal dorsal horn slices is partially inhibited by micromolar concentrations of selective delta-opioid receptor agonists. In the present study, we have examined the effect of nanomolar concentrations of [D-Pen2,D-Pen5]enkephalin (DPDPE, delta-opioid receptor agonist) and low micromolar of concentrations morphine on K(+)-evoked SP release from rat trigeminal nucleus caudalis (TNC) slices. DPDPE and morphine inhibited SP release with an apparent maximal effect at 3 nM and at 3 microM, respectively. DPDPE and morphine produced U-shaped concentration-response curves that were completely autoinhibited at 100 nM DPDPE and 1 microM morphine. The inhibition of SP release produced by 3 nM DPDPE and 3 microM morphine was blocked by the opioid receptor antagonists naloxone (30 nM; non-selective) and ICI 174,864 (0.3 microM; delta-selective) but not by nor-binaltorphimine (3 nM n-BNI; kappa-selective), naloxonazine (1 nM; micro 1-selective) or beta-funaltrexamine (20 nM beta-FNA; mu-selective). These findings indicate that delta-opioid receptor-mediated inhibition of SP release from TNC can be achieved by nanomolar concentrations of selective delta-opioid receptor agonists. Activation of delta-opioid receptors by morphine might be involved in the residual analgesia observed after mu 1-opioid receptor blockade and in the analgesia produced by high doses of morphine.

Ontogeny of zeta (zeta), the opioid growth factor receptor, in the rat brain

Opioid growth factor (OGF), [Met5]enkephalin, serves as an inhibitory influence on the developing nervous system and is especially targeted to cell proliferative events. OGF interacts with the zeta (zeta) opioid receptor to perform its function. Using [3H]-[Met5]enkephalin, the ontogeny of the zeta receptor in the whole brain and cerebellum of rats was explored. Specific and saturable binding was recorded at the earliest time sampled, prenatal day 15 (E15). In the whole brain, binding capacity (Bmax) was two-fold greater at E15 than at E18 and E20. The quantity of zeta receptor appeared to increase in the first postnatal week, reaching a maximum on postnatal day 8. Binding decreased the remainder of the 2nd week and between postnatal days 15 and 25 binding was no longer recorded. In the cerebellum, binding capacity increased from E20 to the 2nd postnatal week, reaching a maximum on postnatal days 8-10. The Bmax of the zeta receptor decreased precipitously on postnatal day 11, being 5.4-fold lower than on postnatal day 10. Between postnatal days 21 and 30, no binding was observed. The binding affinities of the whole brain and cerebellum were 2.3 and 2.7 nM, respectively, and no differences between ages could be detected. Continuous opioid receptor blockade from birth to postnatal day 6 increased body weight, the Bmax of the zeta receptor in the whole brain and cerebellum (but not the Kd), and increased the number of layers of germinal cells in the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opioids accumulate in plasma in a rat model of acute cholestasis

To obtain data on the degree to which the opioid system is changed in cholestasis, endogenous opioid activity in plasma of rats with acute cholestasis was determined 5 days after bile duct resection. Total plasma opioid activity was determined using a radioreceptor technique that measured the displacement of the opiate receptor ligand [3H]-DAMGO from lysed synaptosomal fractions of normal rat brain. Plasma total opioid activity was threefold greater in bile duct-resected rats than in sham-operated and unoperated controls (P less than or equal to 0.05). Plasma levels of the individual endogenous opioid, methionine-enkephalin, were determined using a sensitive radioimmunoassay, and the specificity of the assay was confirmed using high-performance liquid chromatography. In cholestatic rats, plasma methionine-enkephalin levels were more than six-fold greater than in sham-operated controls (P less than or equal to 0.001) and more than 17-fold greater than in unoperated controls (P less than or equal to 0.001). However, plasma methionine-enkephalin levels accounted for less than 5% of total plasma opioid activity after bile duct resection. Plasma methionine-enkephalin levels in both cholestatic plasma and plasma from sham-operated animals were stable when incubated in vitro despite the presence of undiminished activity of the major enkephalin-degrading enzymes. Thus, protection of methionine-enkephalin from degradation may be a factor contributing to the elevated plasma levels of methionine-enkephalin found in cholestasis. The magnitude of the increase in plasma endogenous opioid activity in bile duct-resected rats provides support for the hypothesis that endogenous opioids contribute to the pathophysiology of cholestasis.

A study of the effect of the irreversible delta receptor antagonist [D-Ala2,Leu5,Cys6]-enkephalin on delta cx and delta ncx opioid binding sites in vitro and in vivo

Several lines of data support the existence of two classes of delta receptors: the delta cx binding site, which is the delta binding site of the mu-delta opioid receptor complex, and the delta ncx, which is the noncomplexed delta receptor. [D-Ala2,Leu5,Cys6]Enkephalin (DALCE) is an extended analog of [Leu5]enkephalin, which has been shown to bind irreversibly to delta receptors via the terminal cysteine by formation of a disulfide bond with the receptor. In vivo studies have shown that DALCE produces short-lived antinociceptive actions, followed by long-term antagonism of delta receptor-mediated antinociception. The major goal of the present study was to examine the effect of DALCE on the delta cx and delta ncx binding sites in vitro and in vivo. Intracerebroventricular administration of 40 micrograms DALCE failed to decrease [3H][D-Ala2,D-Leu5]enkephalin binding to the delta cx and delta ncx binding sites. Pretreatment of membranes with DALCE in vitro greatly reduced the Bmax of the delta ncx binding site, without significantly altering the Bmax of the delta cx binding site. These findings suggest that when administered in vivo, DALCE fails to distribute uniformly throughout the brain, and that it therefore binds covalently to opioid receptors mostly in the periventricular regions. Viewed collectively, these data support the hypothesis that DALCE acts as a selective delta ncx antagonist, and that the delta ncx binding site, which is sensitive to DALCE, is most likely synonymous with the recently described delta 1 receptor.

Up-regulation of [3H]DAMGO and [3H]DTLET opioid binding sites in laminae I-II of the spinal cord in intact and deafferented morphine-tolerant rats

Using quantitative autoradiography and selective opioid ligands, we have measured the effects of morphine-induced tolerance on [3H]DAMGO and [3H]DTLET binding sites in the superficial spinal dorsal horn (laminae I-II) of intact and deafferented rats (unilateral C4-T2 dorsal rhizotomy). In intact rats, the treatment induced an up-regulation of 26% and 39% for [3H]DAMGO and [3H]DTLET binding sites, respectively, without modification of receptor affinity. In deafferented rats, the treatment similarly induced an up-regulation of 31% and 29% for [3H]DAMGO and [3H]DTLET binding sites, respectively, on the contralateral side, and of 21% and 25%, respectively, on the ipsilateral side. These data demonstrate that the up-regulation induced by morphine tolerance is of similar magnitude for both presynaptic (on primary afferent fibers) and postsynaptic (on spinal neurons) opioid binding sites in the rat dorsal horn.

Chronic morphine treatment increases the number, but decreases the affinity of [3H]-U69,593 binding sites in the rat heart

In the present study, the effects of chronic morphine treatment on the binding properties of tritiated U69,593, a specific kappa-ligand, in the rat heart were determined. Adult rats were given morphine through osmotic pumps at a rate of 80 micrograms/hour supplemented by daily injection of morphine at increasing doses for 9 days. The increase in colonic temperature to morphine, used as an indicator of development of tolerance in the rat, was measured daily. It was shown that, on day 7 following morphine treatment, the rats developed tolerance to morphine as indicated by an attenuated hyperthermic response to morphine. The [3H]-U69,593 specific binding properties were determined by direct receptor binding assay. The binding sites increased gradually and reached a significantly higher level at day 10. Scatchard analysis showed that both Bmax and Kd increased at day 10 following morphine treatment, indicating an increase in number of sites and a reduction in affinity to the kappa-ligand. Acute morphine injection at a dose of 10 mg/kg did not cause any significant alteration of [3H]-U69,593 binding sites. Two days after withdrawal of morphine, the [3H]-U69,593 binding sites returned to the original level. The finding of the present study indicates that there is an up-regulation of number, but a reduction in affinity of kappa-binding sites following chronic morphine treatment.

Differences in the binding of [3H][D-Ser2,Thr6]leucine-enkephalin and [3H][D-Pen2,D-Pen5]enkephalin to brain membranes of morphine tolerant-dependent rats

The effect of morphine tolerance-dependence and abstinence on the characteristics of delta-opiate receptors was determined in male Sprague-Dawley rats. Two ligands used for characterizing the receptors were [3H][D-Ser2,Thr6]leucine-enkephalin ([3H]DSTLE) and [3H][D-Pen2,D-Pen5]enkephalin ([3H]DPDPE). Rats were implanted s.c. under light ether anesthesia with six morphine pellets (each containing 75 mg of morphine free base). Rats which served as controls were implanted similarly with placebo pellets. Two sets of rats were used. In one group of rats, the pellets were left intact (tolerant-dependent) at the time of sacrificing and in the other the pellets had been removed 18 h earlier (abstinent). The spinal cord and brain regions (amygdala, hippocampus, hypothalamus, corpus striatum, mid-brain, pons and medulla and cortex) were dissected for binding studies. The binding of [3H]DSTLE to membranes of cerebral cortex of morphine-tolerant-dependent rats was decreased in comparison to control rats, and was due to a decrease in Bmax rather than Kd value. The binding of [3H]DSTLE to other brain regions or spinal cord of morphine-tolerant-dependent and abstinent rats did not differ from their respective controls. On the other hand, the binding of [3H]DPDPE was unaffected in any brain region or the spinal cord of morphine-tolerant-dependent and abstinent rats when compared to their controls. The decrease in binding of [3H]DSTLE to cortical membranes of morphine-tolerant-dependent rats amounted to 15%. Since DSTLE also binds to mu-opiate receptors, which have earlier been shown to be decreased in cortex of morphine-tolerant-dependent rats, and the binding of a more selective delta-opiate ligand [3H]DPDPE was unaffected, it is concluded that central delta-opiate receptors do not play a role in the development of morphine-induced tolerance-dependence or abstinence processes in the rat.

Chronic effect of [D-Pen2,D-Pen5]enkephalin on rat brain opioid receptors

In previous studies, we have demonstrated that chronic etorphine or [D-Ala2,D-Leu5]enkephalin (DADLE) treatment of rats results in the reduction of mu- and delta-opioid receptor binding activities as tolerance develops. As both etorphine and DADLE are relatively non-specific opioid ligands, interacting with both mu- and delta-receptors, these studies could not determine whether down-regulation of a specific receptor type occurs. Therefore, in the present studies, animals were rendered tolerant to the delta-opioid receptor-selective agonist [D-Pen2,D-Pen5]enkephalin (DPDPE), and receptor binding activities were measured. Treating Sprague-Dawley rats with increasing doses of DPDPE (80-160-240-320 micrograms/kg) i.c.v. for 1 to 4 days resulted in a time-dependent increase in the AD50 of DPDPE to elicit an antinociceptive response. When delta-receptor binding was determined by using [3H]DPDPE, a 40-50% decrease in binding in the midbrain and cortex, and 25-35% decrease in binding in the striatum were observed after 3 or 4 days of DPDPE treatment. Scatchard analysis of the [3H]DPDPE saturation binding data revealed a decrease in Bmax values and no significant change in Kd values. To our surprise, when mu-receptor binding was determined by using [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO), a 10-15% decrease in binding was also observed in the midbrain and cortex after 4 days of DPDPE treatment. Our conclusion is that chronic DPDPE treatment preferentially reduces delta-opioid receptor binding activity. Its minor effect on the mu-opioid receptor maybe due to an interaction between delta cx and mu cx binding sites.

Interaction of beta-funaltrexamine with [3H]cycloFOXY binding in rat brain: further evidence that beta-FNA alkylates the opioid receptor complex

beta-Funaltrexamine (beta-FNA) is an alkylating derivative of naltrexone. In addition to acting as an irreversible inhibitor of mu-receptor-mediated physiological effects, intracerebroventricular (i.c.v.) administration of beta-FNA to rat attenuates the ability of selective delta receptor antagonists and naloxone to reverse delta receptor-mediated effects. Moreover, recent work demonstrated that i.c.v. administration of beta-FNA alters the conformation of the opioid receptor complex, as inferred by a decrease in the Bmax of the lower affinity [3H][D-ala2,D-leu5]enkephalin binding site. Consistent with the decreased potency of naloxone as an inhibitor of delta receptor mediated effects, beta-FNA doubled the naloxone IC50 for displacing [3H][D-ala2,D-leu5]enkephalin from its lower affinity binding site. These data collectively support the hypothesis that the opioid receptor complex postulated to mediate mu-delta interactions in vivo is identical to the opioid receptor complex as defined by vitro ligand binding studies. A direct prediction of this hypothesis is that beta-FNA should increase the Kd of antagonists for the mu binding site (mu cx) of the receptor complex. The data reported in this paper demonstrate that beta-FNA doubled the IC50 of the potent narcotic antagonist, 6-desoxy-6 beta-fluoronaltrexone (cycloFOXY) for displacing [3H][D-ala2,D-leu5]enkephalin from its lower affinity binding site, and doubled the Kd of [3H]cycloFOXY for its mu binding site, providing additional data that the mu binding site labeled by [3H]cycloFOXY is the mu binding site of the opioid receptor complex. beta-FNA also altered the kappa binding site labeled by [3H]cycloFOXY, and when administered intrathecally to mice, beta-FNA produced a longlasting antinociception in the acetic acid writhing test.

Probing the opioid receptor complex with (+)-trans-superfit. I. Evidence that [D-Pen2,D-Pen5]enkephalin interacts with high affinity at the delta cx binding site

A variety of data support the existence of an opioid receptor complex composed of distinct but interacting mu cx and delta cx binding sites, where "cx" indicates "in the complex." The ability of subantinociceptive doses of [Leu5]enkephalin and [Met5]enkephalin to potentiate and attenuate morphine-induced antinociception, respectively, is thought to be mediated via their binding to the delta cx binding site. [D-Pen2,D-Pen5]Enkephalin also modulates morphine-induced antinociception, but has very low affinity for the delta cx binding site in vitro. In the present study, membranes were depleted of their delta ncx binding sites by pretreatment with the site-directed acylating agent, (3S,4S)-(+)-trans-N-[1-[2-(4-isothiocyanato)phenyl)-ethyl]-3-methy l-4- piperidyl]-N-phenylpropaneamide hydrochloride, which permits selective labeling of the delta cx binding site with [3H][D-Ala2,D-Leu5]enkephalin. The major findings of this study are that with this preparation of rat brain membranes: a) there are striking differences between the delta cx and mu binding sites; and b) both [D-Pen2,D-Pen5]enkephalin and [D-Pen2,L-Pen5]enkephalin exhibit high affinity for the delta cx binding site.

The neutral endopeptidase-24.11 inhibitor SCH 34826 does not change opioid binding but reduces D1 dopamine receptors in rat brain

The effect of repeated administration of the neutral endopeptidase-24.11 (NEP) inhibitor SCH 34826 on the kinetic properties of opioid and dopamine binding in the rat cerebral cortex and striatum was investigated. SCH 34826, given at 100 and 300 mg/kg orally twice a day for 14 days, did not alter either Bmax or Kd for the mu, delta, or kappa opioid receptor type in the cortex, as measured by studying binding parameters for the mu-selective ligand [3H][D-Ala2, Me-Phe4,Gly(ol)5]enkephalin (DAGO), the delta-selective ligand [3H][D-Pen2,D-Pen5]enkephalin (DPDPE) and the kappa ligand [3H]ethylketazocine (EKC). SCH 34826 reduced significantly the number of D1 dopamine receptors labeled with [3H]SCH 23390 in the striatum (Bmax was 90 and 84% of controls at 100 and 300 mg/kg, respectively). The number of D2 receptors, measured by [3H]spiperone binding was unaltered. The Kd values for both receptor types were not affected. The data demonstrate that chronic inhibition of enkephalin degradation by SCH 34826 does not alter opioid receptors, whereas it reduces the number of D1 receptors. These findings provide further support for the role of opioids in modulating central dopaminergic systems. As a reduction in the number of D1 receptors is an effect common to antidepressant treatments, the antidepressant potential of NEP inhibitors should be investigated.

Multiple opiate receptors of brain and spinal cord in opiate addiction

1. Chronic administration of opiates to rodents results in the development of tolerance to their pharmacological effects. Physical dependence also develops and is shown by the appearance of abstinence syndrome. 2. Opiates produce their effects by acting on three types of opiate receptors, namely mu, delta and kappa. The qualitative and quantitative aspects of the tolerance-dependence and abstinence symptoms observed after chronic administration of agonists acting at mu-, delta- and kappa-opiate receptors appear to differ. 3. Tolerance-dependence on mu-opiate agonists, such as morphine, is associated with down-regulation of mu-opiate receptors in spinal cord and specific areas of the brain but delta- and kappa-opiate receptors are unchanged. During abstinence from mu-opiate agonists, brain and spinal cord mu-, delta- and kappa-opiate receptors are unaffected. 4. Chronic administration of kappa-opiate agonists, such as U-50,488H, results in the development of tolerance to its pharmacological effects and a mild degree of physical dependence. Such changes are associated not only with alterations of delta and kappa opiate receptors in brain and spinal cord, but also primarily with a down-regulation of kappa-opiate receptors in spinal cord and specific brain regions. mu-Opiate receptors are unaffected. 5. Chronic administration of delta-opiate agonists results in down-regulation of brain delta-opiate receptors. 6. It is concluded that tolerance-dependence on mu-, delta- and kappa-opiate receptors is associated with down-regulation of their own type of receptors in the spinal and supraspinal areas. Abstinence, on the other hand, does not alter brain and spinal cord opiate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Upregulation of the opioid receptor complex by the chronic administration of morphine: a biochemical marker related to the development of tolerance and dependence

Studies conducted after the development of the rapid filtration assay for opiate receptors, and before the recognition of multiple opioid receptors, failed to detect changes in opioid receptors induced by chronic morphine. Recent experiments conducted in our laboratories were designed to examine the hypothesis that only one of several opioid receptor types might be altered by chronic morphine. Using binding surface analysis and irreversible ligands to increase the "resolving power" of the ligand binding assay, the results indicated that chronic morphine increased both the Bmax and Kd of the opioid receptor complex, labeled with either [3H][D-Ala2,D-Leu5]enkephalin, [3H][D-Ala2-MePhe4,Gly-ol5]enkephalin or [3H]6-desoxy-6 beta-fluoronaltreone. In the present study rats were pretreated with drugs known to attenuate the development of tolerance and dependence [the irreversible mu-receptor antagonist, beta-funaltrexamine (beta-FNA), and the inhibitor of tryptophan hydroxylase, para-chlorophenylalanine], prior to subcutaneous implantation of morphine pellets. The results demonstrated that 1) unlike chronic naltrexone, beta-FNA failed to upregulate opioid receptors and 2) both beta-funaltrexamine and PCPA pretreatment attenuated the chronic morphine-induced increase in the Bmax, but not the Kd, of the opioid receptor complex. These results provide evidence that naltrex-one-induced upregulation of the opioid receptor complex might occur indirectly as a consequence of interactions at beta-funaltrexamine-insensitive opioid receptors and that morphine-induced upregulation (increased Bmax) of the opioid receptor complex is a relevant in vitro marker related to the development of tolerance and dependence. These data collectively support the hypothesis that endogenous antiopiate peptides play an important role in the development of tolerance and dependence to morphine.

Chronic pretreatment with naltrexone facilitates acquisition of intravenous cocaine self-administration in rats

Opioid systems may be involved in the reinforcing effects of drugs of abuse. It has been shown that the opioid antagonist naltrexone attenuates acquisition of intravenous cocaine self-administration behaviour in rats. Using a similar experimental set-up the effect of chronic blockade of opioid systems prior to cocaine exposure was examined. Rats were tested for acquisition of self-administration of one of 3 graded unit doses of cocaine (0.08, 0.16 and 0.32 mg.kg-1 per infusion) or saline. Chronic pretreatment with naltrexone (10 mg.kg-1 per day for 12 days) enhanced acquisition of intravenous cocaine self-administration but only in rats tested with the medium cocaine unit dose. It is concluded that chronic blockade of opioid systems facilitates acquisition of cocaine self-administration, probably by enhancing the reinforcing effect of cocaine.

Pretreatment of rats with the irreversible mu-receptor antagonist, beta-FNA, fails to prevent naltrexone-induced upregulation of mu-opioid receptors

This study examined the effect of beta-funaltrexamine (beta-FNA), an irreversible mu-receptor antagonist, on naltrexone-induced upregulation of mu-(mu cx + mu nex) and delta nex-opioid receptors. [The subscripts 'cx' and 'nex' denote binding sites 'in' (cx) and 'not in' (nex) the opioid receptor complex.] Rats were treated according to the following protocol. Two naltrexone or two placebo pellets were implanted subcutaneously in a nylon mesh on day 1. and were removed intact on day 8. Rats were given either saline or 20 nmol of beta-FNA in 10 microliters of saline (i.c.v.) on days 1, 3, 5 and 6, 60 min prior to implantation of the pellet. On day 9 frozen lysed-P2 membranes were prepared for assay of mu binding sites. In other experiments, membranes were depleted of mu-receptors by pretreatment with the site-directed acylating agent 2-(4-ethoxybenzyl)-l-diethylaminoethyl-5-isothiocyanatobenzimid azole.HCl (BIT) for assay of delta nex binding sites, using [3H] [D-ala2, D-leu5]enkephalin. The results demonstrated that beta-FNA did not upregulate the mu binding sites and also did not prevent naltrexone-induced upregulation of mu binding sites. Both beta-FNA and naltrexone increased the Bmax of delta nex binding sites and their effects were additive. These data suggest that the mechanism(s) responsible for antagonist-induced upregulation of opioid receptors are more complex than previously appreciated.

Characterization of kappa opiate receptors in rat spinal cord-dorsal root ganglion cocultures and their regulation by chronic opiate treatment

We have investigated the expression and regulation of kappa opiate receptors in rat spinal cord-dorsal root ganglion primary cocultures. The density of opiate receptors increased markedly during the differentiation of the cultures; after 10 days in vitro the number of [3H]diprenorphine binding sites reached 244 +/- 47 fmol/mg protein. Most of the binding sites were of the kappa type, representing about 65-80% of total opiate receptors, while mu sites were expressed at a lower density (ca. 20% of total opiate sites). Following this period of development, the number of kappa and mu receptors did not change significantly. No detectable delta sites were observed at any time of culture (up to 4 weeks in vitro). Chronic opiate agonist treatment (24 h) of the cultured cells with either 10 microM U50488 (a selective kappa agonist), or 1 microM etorphine (a nonselective opiate agonist), did not change the number of kappa receptors and their binding affinity to [3H]diprenorphine. On the other hand, 50% of the mu receptor sites down-regulated following 24 h treatment with 1 microM etorphine. Chronic antagonist exposure (5 days) with 10 microM naloxone, markedly up-regulated the mu receptors (261% of control), whereas kappa sites exhibited a much weaker upregulation (164% of control). These data demonstrate that kappa opiate receptors are expressed at high concentration in spinal cord-dorsal root ganglion cocultures and that contrary to mu sites, kappa receptor density is less susceptible to modulation by chronic opiate treatment. The results also suggest that postreceptor components are important in regulating the kappa receptor function following prolonged opiate exposure.

Interaction of endogenous opioid peptides and other drugs with four kappa opioid binding sites in guinea pig brain

Guinea pig brain membranes depleted of mu and delta receptors by pretreatment with the site-directed acylating agents, 2-(4-ethoxybenzyl)-1- diethylaminoethyl-5-isothiocyanatobenzimidazole.HCl (BIT) and N-phenyl-N-[1-(2-(4-isothiocyanato)phenethyl)-4- piperidinyl]-propanamide.HCl (FIT), were used in this study to test the hypothesis that guinea pig brain possesses subtypes of kappa receptors. Pretreatment of membranes with either (-)-(1S,2S)-U50,488 or the kappa selective acylating agent, (1S,2S)-trans-2-isothiocyanato-N-methyl-N-[2-(1- pyrrolidinyl)cyclohexyl]benzeneacetamide, caused a wash-resistant inhibition of kappa 1 binding sites labeled by [3H]U69,593 binding, but not kappa 2 binding sites labeled by [3H]bremazocine. Binding surface analysis of [3H]bremazocine binding resolved two binding sites, termed kappa 2 and kappa 2b, present at densities of 212 and 225 fmol/mg protein, which had low affinity for (-)-(1S,2S)-U50,488 and U69,593. The kappa 2b site had high affinity for beta-endorphin(1-31) (Kd = 5.5 nM) and [D-Ala2,D-Leu5]enkephalin (Kd = 14 nM), and lower affinity for [D-Ala2-MePhe4,Gly-ol5]enkephalin (Kd = 147 nM) and [Leu5]enkephalin (Kd = 46.0 nM). Binding surface analysis of [3H]U69,593 binding also resolved two binding sites, termed kappa 1a and kappa 1b, present at densities of 6.0 and 40.0 fmol/mg protein. The kappa 1a binding site was characterized by very high affinity for alpha-neoendorphin. Quantitative autoradiographic studies demonstrated that kappa 2a and kappa 2b binding sites are heterogeneously distributed in guinea pig brain, and that the anatomical distribution of kappa 1 binding sites reported in the literature is different from that observed in this study for the kappa 2 binding sites. Viewed collectively, these data provide evidence for four kappa receptor subtypes in guinea pig brain.

Chronic administration of morphine and naltrexone up-regulate mu-opioid binding sites labeled by [3H][D-Ala2,MePhe4,Gly-ol5]enkephalin: further evidence for two mu-binding sites

A variety of data support the hypothesis of an opiate receptor complex composed of distinct, yet interacting mu and delta binding sites (termed mu cx and delta cx to indicate binding sites 'in the complex'), in addition to independent mu and delta binding sites, termed mu ncx and delta ncx, to indicate binding sites 'not in the complex'. Ligand binding studies using membranes and slide-mounted sections of rat brain support the hypothesis that the irreversible mu-antagonist beta-funaltrexamine (FNA) selectively alkylates the opiate receptor complex, altering the binding of mu agonists to the mu cx binding site and the binding of [3H][D-Ala2,D-Leu5]enkephalin to the delta cx site. Previous studies demonstrated that the chronic administration of morphine to rats selectively 'upregulates' the opiate receptor complex. In contrast, the chronic administration of naltrexone upregulates several types of opioid receptors, including kappa, the delta ncx binding site, and multiple binding sites labeled by mu agonists. A prediction based upon these observations is that, using [3H][D-Ala2,MePhe4,Gly-ol5]enkephalin to label mu binding sites, chronic morphine should upregulate only the mu cx binding site, whereas chronic naltrexone should additionally up-regulate the mu ncx binding site. In this study we test and confirm this hypothesis, using sensitivity to FNA to define the mu cx binding site. The implications of these data for models of the opioid receptors and the mechanism(s) of tolerance and dependence are discussed.

Chronic morphine increases mu-opiate receptor binding in rat brain: a quantitative autoradiographic study

Quantitative autoradiography was used to show the locations of mu-opiate receptor binding sites which are upregulated following chronic morphine treatment in rats. A saturating concentration of the mu-specific ligand [3H]D-ala2-N-methyl-Phe4,Gly-ol5-enkephalin was used to label sites in slide-mounted sections through one level of the thalamus in rats implanted subcutaneously with morphine pellets for 5 days. In vitro binding and autoradiography showed the largest increase in binding in the hypothalamus, especially the ventromedial nucleus (155%), with smaller increases in the basolateral and medial amygdaloid nuclei and the striatum. The set of structures showing the upregulation appears to be a subset of those upregulated by opiate antagonists, but there appears to be no correlation of the mu-sites showing upregulation with other anatomical features of the brain opiate system. The physiological significance of the upregulation is not known at present.