Absence of G-protein activation by μ-opioid receptor agonists in the spinal cord of μ-opioid receptor knockout mice
Article first published online: 29 JAN 2009
1999 Nature Publishing Group
British Journal of Pharmacology
Volume 126, Issue 2, pages 451–456, January 1999
How to Cite
Narita, M., Mizoguchi, H., Narita, M., Sora, I., Uhl, G. R. and Tseng, L. F. (1999), Absence of G-protein activation by μ-opioid receptor agonists in the spinal cord of μ-opioid receptor knockout mice. British Journal of Pharmacology, 126: 451–456. doi: 10.1038/sj.bjp.0702330
- Issue published online: 29 JAN 2009
- Article first published online: 29 JAN 2009
- (Received September 28, 1998, Revised October 27, 1998, Accepted November 3, 1998)
- knockout mice;
- homologous recombination;
- opioid peptides;
- μ-opioid receptors;
- signal transduction;
- spinal cord
The ability of μ-opioid receptor agonists to activate G-proteins in the spinal cord of μ-opioid receptor knockout mice was examined by monitoring the binding to membranes of the non-hydrolyzable analogue of GTP, guanosine-5′-O-(3-[35S]thio)triphosphate ([35S]GTPγS).
In the receptor binding study, Scatchard analysis of [3H][D-Ala2,NHPhe4,Gly-ol]enkephalin ([3H]DAMGO; μ-opioid receptor ligand) binding revealed that the heterozygous μ-knockout mice displayed approximately 40% reduction in the number of μ-receptors as compared to the wild-type mice. The homozygous μ-knockout mice showed no detectable μ-binding sites.
The newly isolated μ-opioid peptides endomorphin-1 and -2, the synthetic selective μ-opioid receptor agonist DAMGO and the prototype of μ-opioid receptor agonist morphine each produced concentration-dependent increases in [35S]GTPγS binding in wild-type mice. This stimulation was reduced by 55–70% of the wild-type level in heterozygous, and virtually eliminated in homozygous knockout mice.
No differences in the [35S]GTPγS binding stimulated by specific δ1- ([D-Pen2,5]enkephalin), δ2- ([D-Ala2]deltorphin II) or κ1- (U50,488H) opioid receptor agonists were noted in mice of any of the three genotypes.
The data clearly indicate that μ-opioid receptor gene products play a key role in G-protein activation by endomorphins, DAMGO and morphine in the mouse spinal cord. They support the idea that μ-opioid receptor densities could be rate-limiting steps in the G-protein activation by μ-opioid receptor agonists in the spinal cord. These thus indicate a limited physiological μ-receptor reserve. Furthermore, little change in δ1-, δ2- or κ1-opioid receptor-G-protein complex appears to accompany μ-opioid receptor gene deletions in this region.
British Journal of Pharmacology (1999) 126, 451–456; doi:10.1038/sj.bjp.0702330