[H-3] SNC121 - A NOVEL HIGH-AFFINITY LIGAND FOR RAT-BRAIN DELTA-RECEPTORS - PRELIMINARY STUDIES

Calderon et al. (1) recently prepared the enantiomers and their methox y precursors of the non-peptide delta agonist (+/-)-BW373U86 and its b enzlic epimer (2). In one of these enantiomers, SNC80, the allyl funct ion was reduced with tritium gas to the corresponding propyl derivativ e to yield [H-3]SNC121 (SA=26.8 Ci/mmol). Experiments with non-radioac tive SNC-121 indicated that it had high affinity and selectivity (>100 0-fold) for rat brain delta receptors. Incubations proceeded for 4-6 h r at 25 degrees C (equilibrium) in 50 mM Tris-HCl, pH 7.4 with rat bra in membranes. [H-3]SNC121 labeled two binding sites: an opioid binding site and a non-opioid drug binding site. For the expetiments reported here, DADL (10 mu M) was used to define non-specific binding. Under t hese conditions, we obtained 70%-75% specific binding. [H-3]SNC121 lab eled an apparent single class of binding sites with a delta-like ligan d selectivity profile. GppNHp and sodium decreased [H-3]SNC121 binding by altering the K-d (P < 0.05) without changing the Bmax. GppNHp (20 mu M) failed to strikingly alter the IC50 and slope factor of DPDPE, D eltotphin-II and naltrindofe when displacing [H-3]-SNC121. Viewed coll ectively, [H-3]SNC121 selectively labels a single class of delta opioi d receptors in a manner consistent with an agonist-type interaction. F uture experiments will determine the nature and/or function of the non -opioid [H-3]-SNC121 binding site.