title: The mu-opioid receptor gene-dose dependent reductions in G-protein activation in the pons/medulla and antinociception induced by endomorphins in mu-opioid receptor knockout mice.
abstract: There appear to be different relationships between mu-opioid receptor densities and the acute and neuroadaptive mu-opioid agonist-induced responses of the multiple opioid neuronal systems, including important pons/medulla circuits. The recent success in creating mu-opioid receptor knockout mice allows studies of mu-opioid agonist-induced pharmacological and physiological effects in animals that express no, one or two copies of the mu-opioid receptor gene. We now report that the binding of mu-opioid receptor ligand, [3H][D-Ala2,NHPhe4,Gly-ol]enkephalin to membrane preparations of the pons/medulla was reduced by half in heterozygous mu-opioid receptor knockout mice and eliminated in homozygous mu-opioid receptor knockout mice. The endogenous mu-opioid agonist peptides endomorphin-1 and -2 activate G-proteins in the pons/medulla from wild-type mice in a concentration-dependent fashion, as assessed using [35S]guanosine-5'-o-(3-thio)triphosphate binding. This stimulation was reduced to half of the wild-type levels in heterozygous mice and eliminated in homozygous knockout mice. The intracerebroventricular injection of either endomorphin-1 or endomorphin-2 produced marked antinociception in the hot-plate and tail-flick tests in wild-type mice. These antinociceptive actions were significantly reduced in heterozygous mu-opioid receptor knockout mice, and virtually abolished in homozygous knockout mice. The mu-opioid receptors are the principal molecular targets for endomorphin-induced G-protein activation in the pons/medulla and the antinociception caused by the intracerebroventricular administration of mu-opioid agonists. These data support the notion that there are limited physiological mu-opioid receptor reserves for inducing G-protein activation in the pons/medulla and for the nociceptive modulation induced by the central administration of endomorphin-1 and -2.
authors.affiliation: Department of Anesthesiology, Medical College of Wisconsin, Milwaukee 53226, USA.
authors.forename: H
authors.initials: H
authors.lastname: Mizoguchi
authors.forename: M
authors.initials: M
authors.lastname: Narita
authors.forename: D E
authors.initials: DE
authors.lastname: Oji
authors.forename: C
authors.initials: C
authors.lastname: Suganuma
authors.forename: H
authors.initials: H
authors.lastname: Nagase
authors.forename: I
authors.initials: I
authors.lastname: Sora
authors.forename: G R
authors.initials: GR
authors.lastname: Uhl
authors.forename: E Y
authors.initials: EY
authors.lastname: Cheng
authors.forename: L F
authors.initials: LF
authors.lastname: Tseng
mesh_terms: D000701:Analgesics, Opioid / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000378:metabolism / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism*; D018628:Gene Dosage; D005838:Genotype; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism / Q000494:pharmacology; D007276:Injections, Intraventricular; D008526:Medulla Oblongata / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009619:Nociceptors / Q000187:drug effects / Q000502:physiology; D009842:Oligopeptides / Q000378:metabolism / Q000494:pharmacology*; D011149:Pons / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D011869:Radioligand Assay; D017450:Receptors, Opioid, mu / Q000235:genetics*; D013462:Sulfur Radioisotopes; D014316:Tritium
chemical_list: D000701:Analgesics, Opioid; D009842:Oligopeptides; D017450:Receptors, Opioid, mu; D013462:Sulfur Radioisotopes; C105293:endomorphin 1; D014316:Tritium; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C105294:endomorphin 2; D019204:GTP-Binding Proteins