Highly selective photoaffinity labeling of mu and delta opioid receptors

We report the synthesis and photolabeling properties of two highly selective ligands for mu and delta opioid-binding sites: Tyr-D-Ala-Gly-MePhe (pN3)-Gly-ol (AZ-DAMGE) and Tyr-D-Thr-Gly-Phe (pN3)-Leu-Thr (AZ-DTLET). An irreversible inhibition of the electrically induced contractions of mouse vas deferens is caused by irradiation (at 254 nm) of the muscle strip in the presence of AZ-DTLET (1 nM). This phenomenon is antagonized only at large concentrations (10 microM) of naloxone, in accordance with the well-known lower selectivity of naloxone for delta sites. Competition experiments with [3H]DAMGE and [3H]DTLET on crude rat brain membranes showed that the azido photoprobes display a similar (AZ-DAMGE) and even a better (AZ-DTLET) selectivity than their respective parent compounds DAMGE and DTLET. Up to 25 nM, AZ-DTLET irreversibly and selectively photolabels the delta sites of crude rat brain homogenates. Due to its lower affinity AZ-DAMGE provides similar selective photolabeling of the mu sites but at higher concentrations (approximately equal to 0.3 microM). When [3H]DAMGE and [3H]DTLET were used as ligands for mu and delta binding subtypes, respectively, no important change in binding capacity and affinity of one receptor type was observed after photolabeling of the other.