Ionic conditions differentially affect 3H-DADL binding to type-I and type-II opiate delta receptors in vitro

It is widely accepted that the prototypic delta agonist DADL (D-ala2-D-leu5-enkephalin) labels two binding sites in vitro. Using the site directed, receptor selective alkylating agents, BIT and FIT (Rice et al.. Science 220:314-316, 1983), we recently described (Rothman et al, Neuropeptides, in press) the preparation of membranes possessing only lower affinity 3H-DADL binding sites (FIT-treated membranes, type-I delta sites) as well as membranes greatly enriched with higher affinity binding sites (BIT-treated membranes, type-II delta sites). In this paper we report that ionic conditions differentially affect the binding of 3H-DADL to FIT- and BIT-treated membranes, supporting the notion that 3-H-DADL labels two distinct delta binding sites.