Cloning and expression of an isoform of the rat μ opioid receptor (rMOR1B) which differs in agonist induced desensitization from rMOR1

Abstract
A novel rat μ opioid receptor (rMOR1B) has been isolated. It shows identity to the recently published sequence of rMOR1 [Chen, et al., Mol. Pharmacol., 44 (1993) 8–12] up to amino acid 386 and differs only in length and amino acid composition at the very carboxy‐terminal tail. Both μ opioid receptor isoforms, when stably expressed in CHO‐K1 cells, show similar affinities to opioid compounds and are equally effective in the inhibition of forskolin‐induced cAMP formation. Reverse transcription polymerase chain reaction (RT‐PCR) revealed that rMOR1B displays a similar distribution as rMOR1 in various rat brain areas. Studies measuring the inhibition of adenylate cyclase in cells that had been pre‐exposed to the μ opioid agonist DAMGO indicated that rMOR1B is much more resistant to agonist‐induced desensitization than rMOR1.