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

A novel rat mu 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 mu 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 mu opioid agonist DAMGO indicated that rMOR1B is much more resistant to agonist-induced desensitization than rMOR1.