[I-125] AZIDO-DTLET AS A TOOL FOR SELECTIVE COVALENT LABELING OF DELTA-OPIOID RECEPTORS IN RAT-BRAIN SECTIONS

The binding kinetics and pharmacological selectivity of the photoaffin ity delta-opioid ligand [I-125]azido-DTLET (Tyr-D-Thr-Gly-Phe(pN3)-Leu -Thr) were investigated in serial frozen sections from rat neostriatum prior to ultraviolet irradiation (i.e., in conditions of reversibilit y). Scatchard analysis of saturation binding experiments indicated tha t [I-125]azido-DTLET binds to both a high (K(D) = 5.04 nM) and a low ( K(D) = 38 nM) affinity site. Binding to the low-affinity site was no l onger detectable in the presence of unlabeled [D-Ala2,N-MePhe4,Gly-Ol5 ]enkephalin (DAGO), suggesting that this site corresponds to mu-opioid receptors. This interpretation was further supported by the dose-depe ndent inhibition of the binding of [H-3]DAGO by non-radioactive azido- DTLET. Binding to the high-affinity site was totally inhibited, in a d ose-dependent fashion, by a variety of opioid drugs among which delta- opioid ligands showed the highest order of potency. It is concluded th at, in the nanomolar range, [I-125]azido-DTLET constitutes a highly se lective tool for covalent labeling of delta-opioid receptors in rat br ain sections.