title: Involvement of intracellular calcium in morphine tolerance in mice.
abstract: Opioid analgesic tolerance is associated with a disruption in Ca++ homeostasis. Drugs reducing Ca++ influx can prevent and reverse tolerance. The hypothesis was tested that both Ca++ influx and mobilization from intracellular pools maintains the expression of morphine tolerance. Ca++ modulating drugs were injected ICV at doses not affecting morphine's potency in placebo pellet-implanted mice, in order to determine whether tolerance would be reversed in morphine pellet-implanted mice. The Ca++ chelator EGTA significantly reversed tolerance. The Ca++ channel antagonists nifedipine and omega-conotoxin GVIA also reversed tolerance. The role of intracellular Ca++ was investigated using the membrane permeable intracellular Ca++ chelator EGTA-AM. EGTA-AM reversed tolerance at lower morphine doses, but not at higher morphine doses. Thus, mobilization of intracellular Ca++ contributes to the expression of tolerance. Finally, 1,4-dihydropyridine-sensitive Ca++ channels are known to stimulate Ca++-induced Ca++ release (CICR) from Ca++/caffeine-sensitive microsomal pools possessing ryanodine receptors. We examined whether blocking Ca++ mobilization from these pools with ryanodine would reverse morphine tolerance. Ryanodine's effects were similar to EGTA-AM. Tolerance was reversed at lower morphine doses, but not at higher doses. Thus, morphine tolerance appears to be associated with increases in Ca++ influx and mobilization from Ca++/caffeine-sensitive pools.
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Virginia of Virginia Commonwealth University, Richmond 23298-0613, USA.
authors.forename: F L
authors.initials: FL
authors.lastname: Smith
authors.forename: D S
authors.initials: DS
authors.lastname: Dombrowski
authors.forename: W L
authors.initials: WL
authors.lastname: Dewey
mesh_terms: D000698:Analgesia; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D001692:Biological Transport; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004361:Drug Tolerance / Q000502:physiology*; D008297:Male; D051379:Mice; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism; D009020:Morphine / Q000494:pharmacology*