Mu-opioid receptor ligands lack receptor subtype selectivity in the aequorin luminescence-based calcium assay.
The aim of the present study was to characterize the binding selectivity of the mu-opioid receptor ligands, endomorphin-1, endomorphin-2, and DAMGO, in the in vitro functional assay, based on the changes in intracellular calcium levels. For the experiments Chinese hamster ovary cells, stably expressing human mu-receptor, were used. The mu-agonist-induced calcium responses were significantly inhibited by naloxone, an opioid antagonist with high preference for the mu-opioid receptors. Naloxonazine, a mu1-non-peptide antagonist, inhibited the effect of all tested mu-agonists. However, there was no significant difference in the antagonist effect of naloxonazine on the calcium response induced by mu1- (endomorphin-2) and mu2-agonists (endomorphin-1, DAMGO). [D-Pro2]endomorphin-1 and [D-Pro2]endomorphin-2, putative peptide mu2- and mu1-antagonists, respectively, which had been shown in vivo to inhibit the antinociception induced by mu-agonists, produced no inhibitory effect in our in vitro experiments. Our results demonstrated that there is only one population of the mu-opioid receptors expressed in the Chinese hamster ovary cells. We suggest that the mu-opioid receptors form a homogenous population in the in vitro systems. However, the existence of mu-receptor subtypes in vivo is still pharmacologically possible.
Fichna
Jakub
J
Laboratory of Biomolecular Chemistry, Institute of Biomedicinal Chemistry, Medical University, Mazowiecka 6/8, 92-215 Lodz, Poland.
Staniszewska
Renata
R
Poels
Jeroen
J
Vanden Broeck
Jozef
J
Janecka
Anna
A
D000331:Aequorin / Q000032:analysis* / Q000737:chemistry; D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D006224:Cricetinae; D003412:Cricetulus; D008024:Ligands; D008163:Luminescent Measurements; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000037:antagonists & inhibitors / Q000145:classification* / Q000378:metabolism*; D012680:Sensitivity and Specificity; D013379:Substrate Specificity
D008024:Ligands; D017450:Receptors, Opioid, mu; D000331:Aequorin; D002118:Calcium