Abstract
The μ-opioid receptor gene-dose dependent reductions in G-protein activation in the pons/medulla and antinociception induced by endomorphins in μ-opioid receptor knockout mice
H Mizoguchi (1), M Narita (1), DE Oji(1), C Suganuma(1), H Nagase (2), I Sora (3), GR Uhl (3,4), EY Cheng(1) and LF Tseng (1)
(1) Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, WI 53226, USA
(
2)Basic Research Laboratory, Toray Industries Inc., Kamakura, Kanagawa 248, Japan
(3) Molecular Neurobiology Branch, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Baltimore, MD 21224, USA
(4) Department of Neurology and Neuroscience, Johns Hopkins University School of Medicine, Baltimore, MD 21224, USA
ABSTRACT: There appear to be different relationships between μ-opioid receptor densities and the acute and neuroadaptive μ-opioid agonist-induced responses of the multiple opioid neuronal systems, including important pons/medulla circuits. The recent success in creating μ-opioid receptor knockout mice allows studies of μ-opioid agonist-induced pharmacological and physiological effects in animals that express no, one or two copies of the μ-opioid receptor gene. We now report that the binding of μ-opioid receptor ligand, [3H][D-Ala2,NHPhe4,Gly-ol]enkephalin to membrane preparations of the pons/medulla was reduced by half in heterozygous μ-opioid receptor knockout mice and eliminated in homozygous μ-opioid receptor knockout mice. The endogenous μ-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 μ-opioid receptor knockout mice, and virtually abolished in homozygous knockout mice. The μ-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 μ-opioid agonists
These data support the notion that there are limited physiological μ-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.
Abbreviations: DAMGO, [D-Ala2,NHPhe4,Gly-ol]enkephalin; EGTA, ethyleneglycolbis(aminoethyl ether)tetra-acetate; GDP, guanosine-5′-diphosphate; GTPγS, guanosine-5′-o-(3-thio)triphosphate
Complete article published in: Neuroscience (1999) 94, 203-207