| Abstract: |
A series of mu opioid receptor gene (Oprm) splice variants have been reported. These variants all contain exons 1, 2 and 3, the exons responsible for encoding all seven transmembrane domains. Whereas MOR-1 (MOP-1) additionally contains exon 4 that encodes 12 amino acids at the tip of the C-terminus, the other variants encode unique amino acid sequences distinct from the MOR-1 C-terminus due to their alternative splicing. All of these variants are mu selective in ligand binding assays. The current study explored the ability of these variants to stimulate (35S)-GTPgammaS binding, a measure of receptor activation. In CHO cells stably expressing each variant, only mu opioids stimulated (35S)-GTPgammaS binding. Among the mu opioids, we noted marked differences in the maximal stimulation of the clones. This was most prominent with beta-endorphin, which stimulated (35S)-GTPgammaS binding in the MOR-1E expressing cells to a greater degree than (D-Ala2,MePhe4,Gly(ol5)enkephalin (DAMGO; 130%) and was far less effective than DAMGO in MOR-1C expressing cells (44%). The rank order of maximal stimulation of the drugs varied among the clones as well. Dynorphin A, beta-endorphin and morphine were most effective in stimulating (35S)-GTPgammaS binding in MOR-1E expressing cells, while M6G and fentanyl were most effective in cells expressing MOR-1. The potency (EC50) of some of the drugs also varied extensively among the clones. However, there was a poor correlation between the potency of the drugs to stimulate (35S)-GTPgammaS binding and their binding affinity. Together, these findings reveal marked functional differences among the variants that can only be explained by structural differences at their C-termini. |
