title: Failure of intravenous morphine to serve as an effective instrumental reinforcer in dopamine D2 receptor knock-out mice.
abstract: The rewarding effects of opiates are thought to be mediated through dopaminergic mechanisms in the ventral tegmental area, dopamine-independent mechanisms in the nucleus accumbens, or both. The purpose of the present study was to explore the contribution of dopamine to opiate-reinforced behavior using D2 receptor knock-out mice. Wild-type, heterozygous, and D2 knock-out mice were first trained to lever press for water reinforcement and then implanted with intravenous catheters. The ability of intravenously delivered morphine to maintain lever pressing in these mice was studied under two schedules of reinforcement: a fixed ratio 4 (FR4) schedule (saline, 0.1, 0.3, or 1.0 mg/kg, per injection) and a progressive ratio (PR) schedule (1.0 mg/kg, per injection). In the wild-type and heterozygous mice, FR4 behavior maintained by morphine injections was significantly greater than behavior maintained by vehicle injections. Response rate was inversely related to injection dose and increased significantly in the wild-type and heterozygous mice when the animals were placed on the PR schedule. In contrast, the knock-out mice did not respond more for morphine than for saline and did not respond more when increased ratios were required by the PR schedule. Thus, morphine served as a positive reinforcer in the wild-type and heterozygous mice but failed to do so in the knock-out mice. Under this range of doses and response requirements, the rewarding effects of morphine appear to depend critically on an intact D2 receptor system.
authors: Neuroscience Program, Maryland Psychiatric Research Center, Department of Psychiatry, University of Maryland School of Medicine, Baltimore, Maryland 21228, USA. gelmer@mprc.umaryland.edu
authors: Greg I
authors: GI
authors: Elmer
authors: Jeanne O
authors: JO
authors: Pieper
authors: Marcelo
authors: M
authors: Rubinstein
authors: Malcolm J
authors: MJ
authors: Low
authors: David K
authors: DK
authors: Grandy
authors: Roy A
authors: RA
authors: Wise
mesh_terms: D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D003216:Conditioning, Operant / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004326:Drinking / Q000235:genetics; D006579:Heterozygote; D006720:Homozygote; D007275:Injections, Intravenous; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009020:Morphine / Q000008:administration & dosage*; D017448:Receptors, Dopamine D2 / Q000172:deficiency* / Q000235:genetics; D012054:Reinforcement, Psychology; D012201:Reward; D012646:Self Administration
chemical_list: D017448:Receptors, Dopamine D2; D009020:Morphine