title: Endomorphin-1 induced desensitization and down-regulation of the recombinant mu-opioid receptor.
abstract: 1. Endomorphin-1 (E1) is a peptide with high affinity and selectivity for the mu-opioid receptor. The aim of this study was to determine if endomorphin-1 caused desensitization and down-regulation of the mu-opioid receptor expressed in Chinese hamster ovary cells. 2. Following 10 microM E1 pre-treatment, desensitization was assessed by measuring cyclic AMP inhibition, down-regulation was assessed by [(3)H]-diprenorphine ([(3)H]-DPN) binding and immuno-blotting. 3. Pre-treatment of CHO mu cells with 10 microM E1 for 11 and 18 h caused significant reduction in cyclic AMP inhibition. (11 h=39.0+/-16.7%, 18 h 47.0+/-11.1% reduction). 4. At 18 h E1 pre-treatment there was an enhancement (4.5 fold) of cyclic AMP production under forskolin stimulated conditions accompanied by a small rightward shift in the concentration-response curve (pEC(50) control=7.8+/-0.3, pEC(50) E1=7.3+/-0.2) when cells were re-challenged with E1. 5. In membranes prepared from untreated and 0.5 h E1 pre-treated cells, addition of GTP gamma S produced a significant rightward shift in the concentration response curves for E1 displacement of [(3)H]-DPN (0 h K(i) control=7.86+/-0.11, GTP gamma S=7.37+/-0.15; 0.5 h K(i) control=7.92+/-0.12, GTP gamma S=7.36+/-0.08) This was not observed in membranes prepared from cells that had been treated with E1 for 18 h (18 h K(i) control=7.69+/-0. 11, GTP gamma S=7.75+/-0.08). 6. In whole cells E1 treatment caused a rapid loss of cell surface receptors such that at 0.5 h there was a 30.5+/-1.5 reduction (this was unchanged for 18 h). In crude membranes a loss of receptors was also observed using radioligand binding or immuno-blotting protocols. 7. These data show that E1 causes desensitization and down-regulation of the rat mu-opioid receptor expressed in CHO cells. However, these two responses appear temporally distinct.
authors.affiliation: University Department of Anaesthesia and Pain Management, Leicester Royal Infirmary, Leicester LE1 5WW.
authors.forename: C
authors.initials: C
authors.lastname: Harrison
authors.forename: D J
authors.initials: DJ
authors.lastname: Rowbotham
authors.forename: D K
authors.initials: DK
authors.lastname: Grandy
authors.forename: D G
authors.initials: DG
authors.lastname: Lambert
mesh_terms: D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism*; D004174:Diprenorphine / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects* / Q000502:physiology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D009292:Narcotic Antagonists / Q000378:metabolism; D009842:Oligopeptides / Q000494:pharmacology*; D051381:Rats; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000378:metabolism
chemical_list: D009292:Narcotic Antagonists; D009842:Oligopeptides; D017450:Receptors, Opioid, mu; C105293:endomorphin 1; D004174:Diprenorphine; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000242:Cyclic AMP