enkephalin--ala(2)-mephe(4)-gly(5)- and Heroin-Dependence

The etiology and pathophysiology underlying opioid tolerance and dependence are still unknown. Because mu opioid receptor (MOR) plays an essential role in opioid action, many vulnerability-related studies have focused on single nucleotide polymorphisms of MOR, particularly on A118G. In this study, we found that a single-point mutation at the MOR T394 phosphorylation site could be another important susceptive factor in the development of opioid tolerance and dependence in mice. T394A mutation, in which a threonine at 394 was replaced by an alanine, did not alter agonist binding to MOR and opioid analgesia, but resulted in loss of etorphine-induced MOR internalization in spinal dorsal horn neurons and opioid analgesic tolerance induced by either morphine or etorphine. In addition, this mutation also caused an increase in intravenous heroin self-administration and in nucleus accumbens dopamine response to heroin. These findings suggest that T394 phosphorylation following MOR activation causes MOR internalization and desensitization, which subsequently contributes to the development of tolerance in both opioid analgesia and opioid reward. Accordingly, T394A mutation blocks opioid tolerance and leads to an increase in brain dopamine response to opioids and in opioid-taking behavior. Thus, the T394 may serve as a new drug target for modulating opioid tolerance and the development of opioid abuse and addiction.. The mechanisms underlying opioid tolerance and susceptibility to opioid addiction remain unclear. The present studies demonstrate that a single-point mutation at the T394 phosphorylation site in the C-terminal of mu opioid receptor (MOR) results in loss of opioid tolerance and enhanced vulnerability to heroin self-administration. These findings suggest that modulation of the MOR-T394 phosphorylation or dephosphorylation status may have therapeutic potential in management of pain, opioid tolerance, and opioid abuse and addiction. Accordingly, MOR-T394 mutation or polymorphisms could be a risk factor in developing opioid abuse and addiction and therefore be used as a new biomarker in prediction and prevention of opioid abuse and addiction.

Topics: Analgesia; Analgesics, Opioid; Animals; Drug Tolerance; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Etorphine; Heroin Dependence; Male; Mice; Mice, Inbred C57BL; Morphine; Motor Activity; Mutation; Pain Measurement; Phosphorylation; Point Mutation; Receptors, Opioid, mu; Reward; Self Administration

Relapse is the most serious limitation of effective medical treatment of opiate addiction. Opiate-related behaviors appear to be modulated by cannabinoid CB1 receptors (CB1) through poorly understood cross-talk mechanisms. Opiate and CB1 receptors are coexpressed in the nucleus accumbens (NAc) and dorsal striatum. These regions also have the highest density of adenosine A2a receptors (A2a) in the brain. We have been investigating the postsynaptic signaling mechanisms of mu-opiate receptors (MORs) and CB1 receptors in primary NAc/striatal neurons. In this article, we present evidence that MOR and CB1 act synergistically on cAMP/PKA signaling in NAc/striatal neurons. In addition, we find that synergy requires adenosine and A2a. Importantly, an A2a antagonist administered either directly into the NAc or indirectly by i.p. injection eliminates heroin-induced reinstatement in rats trained to self-administer heroin, a model of human craving and relapse. These findings suggest that A2a antagonists might be effective therapeutic agents in the management of abstinent heroin addicts.

Topics: Adenosine A2 Receptor Antagonists; Analgesics, Opioid; Animals; Arachidonic Acids; Behavior, Animal; Corpus Striatum; Cyclic AMP; Cyclic AMP-Dependent Protein Kinases; Drug Synergism; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enzyme Activation; Gene Expression Regulation; Heroin Dependence; Humans; Male; Neurons; Nucleus Accumbens; Rats; Rats, Sprague-Dawley; Receptor, Adenosine A2A; Receptor, Cannabinoid, CB1; Receptors, Opioid, mu; Self Administration; Signal Transduction; Theobromine; Xanthines

3-O-Methylnaltrexone (3-MNTX), a putative antagonist of morphine-6-beta-d-glucuronide (M6G) receptors, has been reported to block the behavioral effects of heroin at doses that do not block those of morphine, suggesting that M6G receptors may play a unique role in the addictive properties of heroin. This study investigated the effects of 3-MNTX in monkeys trained to discriminate i.v. heroin from vehicle or to self-administer i.v. heroin under a progressive-ratio schedule. Additional in vitro studies determined the effects of 3-MNTX and reference drugs on adenylyl cyclase activity in caudate-putamen membranes of monkeys and rats. In drug discrimination experiments, heroin, morphine, and M6G substituted for heroin in all subjects, whereas 3-MNTX substituted for heroin in one-half the monkeys tested. In these latter monkeys, the effects of 3-MNTX were antagonized by naltrexone, and pretreatment with 3-MNTX enhanced the effects of heroin, M6G, and morphine, indicative of micro-agonist activity. In monkeys showing no substitution of 3-MNTX for heroin, 3-MNTX antagonized the effects of heroin, M6G, and morphine. In self-administration experiments, heroin and 3-MNTX maintained injections per session significantly above those maintained by vehicle when the initial response requirement (IRR) was low; only heroin maintained significant self-administration when the IRR was high. In vitro, 3-MNTX inhibited adenylyl cyclase activity in both monkey and rat brain membranes. The degree of inhibition produced by 3-MNTX was less than that produced by the full agonist [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO). The results suggest that 3-MNTX functions primarily as a partial agonist at micro-receptors in monkeys and do not support a singular role for M6G receptors in the abuse-related effects of heroin.

Topics: Adenylyl Cyclases; Animals; Cocaine; Discrimination Learning; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Heroin; Heroin Dependence; Macaca mulatta; Male; Morphine; Morphine Derivatives; Naltrexone; Narcotics; Quaternary Ammonium Compounds; Receptors, Opioid, mu; Self Administration

These studies were undertaken to determine the effects of mu-opioid receptor depletion through irreversible alkylation on the dose-effect curve for heroin self-administration. Heroin maintained responding in rats with an inverted U-shaped dose-effect curve and administration of 10 nmol of beta-funaltrexamine i.c.v. (beta-FNA) significantly increased the ED50 on the ascending limb from 1.9 to 5.3 micrograms/infusion, and from 24.3 to 211.8 micrograms/infusion on the descending limb. Administration of saline i.c.v. produced no effect on heroin self-administration. Administration of 40 nmol of beta-FNA increased the ED50S from 5.1 to 33.9 and from 14.4 to 502.8 micrograms/infusion on the ascending and descending portions of heroin's dose-effect curve, respectively. beta-FNA (40 nmol, i.c.v.) had no effect on cocaine self-administration. [3H]DAMGO binding density was decreased in the caudate and nucleus accumbens by 29 or 54% 24 h after administration of 10 or 40 nmol of beta-FNA i.c.v., respectively. The effects of beta-FNA on heroin self-administration were completely overcome by increasing the dose of heroin however, as the shape and slope of the self-administration dose-effect curve was not different when higher doses of heroin were made available for self-administration compared to control data or saline administration. Therefore, there appear to be spare mu-opioid receptors for heroin for the production of its reinforcing effects in rats. Furthermore, the self-administration dose-effect curves returned to control values prior to the return of [3H]DAMGO binding, further suggesting that the full complement of mu-opioid receptors is not necessary for heroin to produce its reinforcing effects. These findings support the existence of spare mu-opioid receptors for heroin in maintaining self-administration in rats.

Topics: Analgesics, Opioid; Animals; Brain; Caudate Nucleus; Cocaine; Culture Techniques; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Heroin; Heroin Dependence; Motivation; Naltrexone; Narcotic Antagonists; Nucleus Accumbens; Radioligand Assay; Rats; Receptors, Opioid, mu; Self Administration

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

Topics: Adrenergic alpha-2 Receptor Agonists; Adult; Amino Acid Sequence; Analgesics; Brain Chemistry; Chronic Disease; Clonidine; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Female; Heroin Dependence; Humans; Male; Molecular Sequence Data; Radioligand Assay; Receptors, Opioid, mu