Synthesis and binding properties of specific photoaffinity ligands for mu and delta opioid receptor subtypes

We report the synthesis and binding properties of specific photoaffinity ligands for mu and delta opioid receptor subtypes. These ligands are derived from DAGO: Tyr-D-Ala-Gly-NMePhe-Gly-ol, a mu selective probe and DTLET: Tyr-D-Thr-Gly-Phe-Leu-Thr, a delta selective probe by modifying the Phe 4 residue. These modifications are: i) a nitro group on the para position of Phe ring as Phe(4 NO2) or Nip, ii) an azido group as Phe(4 N3) or AZ. Pharmacological responses on mouse vas deferens (delta sites) and guinea pig ileum (mu sites), as well as competition experiments with [3H] DAGO and [3H] DTLET on crude rat brain membranes have been performed. The nitro group on the phenyl ring of the Phe residue preserves the affinity and selectivity of each probe: NipDAGO for the mu sites, NipDTLET for the delta ones. However the nitro probes do not appear to be photoactivable by u.v. irradiation. Likewise, azidation of the phenyl ring of the Phe residue does not change the receptor selectivity of each probe, but AZDAGO has less affinity than its parent molecule DAGO, while AZDTLET has more affinity than DTLET. These compounds are photoactivable and provide an efficient tool to characterize and isolate the different receptor subtypes, especially the delta site.