title: Inhibition of morphine analgesia by lithium: role of peripheral and central opioid receptors.
abstract: Intraperitoneal (i.p.) administration of lithium chloride (LiCl) has two effects on pain sensation: (1) it induces a transient hyperalgesia that is reversed by intracerebroventricular (i.c.v.) or intrathecal (i.t.) administration of the opioid receptor antagonist naloxone or by peripheral administration of the quaternary compound naloxone methiodide [Behav. Neurosci. 114 (2000) 1183]; (2) it produces a long-lasting (24 h) reduction in morphine analgesia and does so in the absence of hyperalgesia [Behav. Brain Res. 142 (2003) 89]. We confirmed that rats administered with LiCl showed a reduction in analgesia when administered morphine 24 h later. We also found that morphine analgesia was restored if LiCl had been preceded by i.p. or i.c.v. administration of naloxone or by i.p. administration of naloxone methiodide. However, i.p. administration of naloxone methiodide prior to testing 24 h after an injection with LiCl did not restore morphine analgesia. Thus, activity at peripheral and central opioid receptors is necessary for the inhibition of morphine analgesia by LiCl, but peripheral opioid receptors are not critical for the expression of this inhibition.
authors.affiliation: Department of Psychology, University of Colorado, Campus Box 0345, Boulder, CO 80309, USA. johnstoi@psych.colorado.edu
authors.forename: Ian N
authors.initials: IN
authors.lastname: Johnston
authors.forename: R Frederick
authors.initials: RF
authors.lastname: Westbrook
mesh_terms: D000698:Analgesia; D000701:Analgesics, Opioid / Q000037:antagonists & inhibitors*; D000704:Analysis of Variance; D000818:Animals; D018692:Antimanic Agents / Q000494:pharmacology; D004333:Drug Administration Routes; D004347:Drug Interactions; D007276:Injections, Intraventricular; D007278:Injections, Spinal / Q000379:methods; D018021:Lithium Chloride / Q000494:pharmacology*; D008297:Male; D009020:Morphine / Q000037:antagonists & inhibitors*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects; D017288:Pain Threshold / Q000187:drug effects; D051381:Rats; D017208:Rats, Wistar; D011957:Receptors, Opioid / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors
chemical_list: D000701:Analgesics, Opioid; D018692:Antimanic Agents; D009292:Narcotic Antagonists; D011957:Receptors, Opioid; D009270:Naloxone; D009020:Morphine; D018021:Lithium Chloride