title: Ethanol reversal of cellular tolerance to morphine in rat locus coeruleus neurons.
abstract: Consumption of ethanol is a considerable risk factor for death in heroin overdose. We sought to determine whether a mildly intoxicating concentration of ethanol could alter morphine tolerance at the cellular level. In rat locus coeruleus (LC) neurons, tolerance to morphine was reversed by acute exposure of the brain slice to ethanol (20 mM). Tolerance to the opioid peptide [d-Ala(2),N-MePhe(4),Gly-ol]-enkephalin was not reversed by ethanol. Previous studies in LC neurons have revealed a role for protein kinase C (PKC)α in μ-opioid receptor (MOPr) desensitization by morphine and in the induction and maintenance of morphine tolerance, but we have been unable to demonstrate that 20 mM ethanol produces significant inhibition of PKCα. The ability of ethanol to reverse cellular tolerance to morphine in LC neurons was absent in the presence of the phosphatase inhibitor okadaic acid, indicating that dephosphorylation is involved. In human embryonic kidney 293 cells expressing the MOPr, ethanol reduced the level of MOPr phosphorylation induced by morphine. Ethanol reversal of tolerance did not appear to result from a direct effect on MOPr since acute exposure to ethanol (20 mM) did not modify the affinity of binding of morphine to the MOPr or the efficacy of morphine for G-protein activation as measured by guanosine 5'-O-(3-[(35)S]thio)triphosphate binding. Similarly, ethanol did not affect MOPr trafficking. We conclude that acute exposure to ethanol enhances the effects of morphine by reversing the processes underlying morphine cellular tolerance.
authors.affiliation: School of Physiology and Pharmacology, University of Bristol, Bristol, United Kingdom.
authors.forename: Javier
authors.initials: J
authors.lastname: Llorente
authors.forename: Sarah
authors.initials: S
authors.lastname: Withey
authors.forename: Guadalupe
authors.initials: G
authors.lastname: Rivero
authors.forename: Margaret
authors.initials: M
authors.lastname: Cunningham
authors.forename: Alex
authors.initials: A
authors.lastname: Cooke
authors.forename: Kunal
authors.initials: K
authors.lastname: Saxena
authors.forename: Jamie
authors.initials: J
authors.lastname: McPherson
authors.forename: Sue
authors.initials: S
authors.lastname: Oldfield
authors.forename: William L
authors.initials: WL
authors.lastname: Dewey
authors.forename: Chris P
authors.initials: CP
authors.lastname: Bailey
authors.forename: Eamonn
authors.initials: E
authors.lastname: Kelly
authors.forename: Graeme
authors.initials: G
authors.lastname: Henderson
mesh_terms: D000818:Animals; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D002460:Cell Line; D004347:Drug Interactions; D004361:Drug Tolerance; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D000431:Ethanol / Q000494:pharmacology*; D019204:GTP-Binding Proteins / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D008125:Locus Coeruleus / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism; D018847:Opioid Peptides / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D051571:Protein Kinase C-alpha / Q000037:antagonists & inhibitors / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D017450:Receptors, Opioid, mu / Q000378:metabolism
chemical_list: D018847:Opioid Peptides; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D000431:Ethanol; D009020:Morphine; D051571:Protein Kinase C-alpha; D010744:Phosphoric Monoester Hydrolases; D019204:GTP-Binding Proteins