title: Stabilization of morphine tolerance with long-term dosing: association with selective upregulation of mu-opioid receptor splice variant mRNAs.
abstract: Chronic morphine administration is associated with the development of tolerance, both clinically and in animal models. Many assume that tolerance is a continually progressive response to chronic opioid dosing. However, clinicians have long appreciated the ability to manage cancer pain in patients for months on stable opioid doses, implying that extended dosing may eventually result in a steady state in which the degree of tolerance remains constant despite the continued administration of a fixed morphine dose. Preclinical animal studies have used short-term paradigms, typically a week or less, whereas the clinical experience is based upon months of treatment. Chronic administration of different fixed morphine doses produced a progressive increase in the ED50 that peaked at 3 wk in mice, consistent with prior results at shorter times. Continued morphine dosing beyond 3 wk revealed stabilization of the level of tolerance for up to 6 wk with no further increase in the ED50. The degree of tolerance at all time points was dependent upon the dose of morphine. The mRNA levels for the various mu opioid receptor splice variants were assessed to determine whether stabilization of morphine tolerance was associated with changes in their levels. After 6 wk of treatment, mRNA levels of the variants increased as much as 300-fold for selected variants in specific brain regions. These findings reconcile preclinical and clinical observations regarding the development of morphine tolerance.
authors.affiliation: Department of Neurology and the Molecular Pharmacology and Chemistry Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065; and.
authors.forename: Jin
authors.initials: J
authors.lastname: Xu
authors.affiliation: Department of Neurology and the Molecular Pharmacology and Chemistry Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065; and.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Faskowitz
authors.affiliation: Department of Psychology, Long Island University, C. W. Post, Brookville, NY 11548.
authors.forename: Grace C
authors.initials: GC
authors.lastname: Rossi
authors.affiliation: Department of Neurology and the Molecular Pharmacology and Chemistry Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065; and.
authors.forename: Mingming
authors.initials: M
authors.lastname: Xu
authors.affiliation: Department of Neurology and the Molecular Pharmacology and Chemistry Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065; and.
authors.forename: Zhigang
authors.initials: Z
authors.lastname: Lu
authors.affiliation: Department of Neurology and the Molecular Pharmacology and Chemistry Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065; and pany@mskcc.org pasterng@mskcc.org.
authors.forename: Ying-Xian
authors.initials: YX
authors.lastname: Pan
authors.affiliation: Department of Neurology and the Molecular Pharmacology and Chemistry Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065; and pany@mskcc.org pasterng@mskcc.org.
authors.forename: Gavril W
authors.initials: GW
authors.lastname: Pasternak
mesh_terms: D017398:Alternative Splicing / Q000187:drug effects / Q000235:genetics*; D000818:Animals; D001921:Brain / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D008297:Male; D051379:Mice; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000235:genetics* / Q000378:metabolism; D015854:Up-Regulation / Q000187:drug effects* / Q000235:genetics
chemical_list: D012333:RNA, Messenger; D017450:Receptors, Opioid, mu; D009020:Morphine
keywords: MOR-1; analgesia; opiate receptor; opioid; splice variant