DIFFERENTIAL BINDING OF OPIOID-PEPTIDES AND OTHER DRUGS TO 2 SUBTYPESOF OPIOID DELTA-NCX BINDING-SITES IN MOUSE-BRAIN - FURTHER EVIDENCE FOR DELTA-RECEPTOR HETEROGENEITY

Research into the functional role of the opioid delta receptor has int ensified with the recent in vivo identification of delta receptor subt ypes, termed delta1 and delta2, which mediate antinociception in the m ouse. A variety of data also support the hypothesis of an opioid recep tor complex composed of distinct, yet interacting, mu, delta, and perh aps kappa binding sites. This model postulates two classes of delta bi nding sites: a delta binding site not associated with the opioid recep tor complex, termed the delta(ncx) site, and a delta site associated w ith the receptor complex, termed the delta(cx) site. A major purpose o f this study was to clarify the relationship between the delta(ncx) bi nding sites and the delta1 and delta2 receptors. Mouse brain membranes were depleted of mu sites by pretreatment with the site-directed acyl ating agent, BIT, and the delta(ncx) binding sites were labeled with [ H-3][D-Ala2,D-Leu5]enkephalin. Binding surface analysis readily resolv ed two binding sites (delta(ncx-1) and delta(ncx-2)) in the absence an d presence of 100 mM NaCl. Control experiments with guanine nucleotide s and the ligand-selectivity analysis indicated that the two sites wer e not two states of a single receptor. Pretreatment of membranes with DALCE, but not [Cys4]deltorphin, decreased [H-3][D-Ala2,D-Leu5]enkepha lin and [H-3][D-Ser2,Thr6]enkephalin binding. Ligand-selectivity analy sis of the two binding sites suggested that neither delta(ncx) binding site had the characteristics expected of the delta2 receptor, and tha t the delta(ncx-1) site, but not the delta(ncx-2) site, was synonymous with the delta1 receptor. Moreover, our finding that the racemic nonp eptide delta agonist, BW373U86, had high affinity at and selectivity f or the delta(ncx-2) site suggests that this site may be a novel delta receptor that mediates some of the effects of BW373U86.