[³H]SNC121: A novel high affinity ligand for rat brain delta receptors: Preliminary studies

Calderon et al. (1) recently prepared the enantiomers and their methoxy precursors of the non-peptide delta agonist (±)-BW373U86 and its benzlic epimer (2). In one of these enantiomers, SNC80, the allyl function was reduced with tritium gas to the corresponding propyl derivative to yield [³H]SNC121 (SA=26.8 Ci/mmol). Experiments with non-radioactive SNC-121 indicated that it had high affinity and selectivity (>1000-fold) for rat brain delta receptors. Incubations proceeded for 4-6 hr at 25°C (equilibrium) in 50 mM Tris-HCl, pH 7.4 with rat brain membranes. [³H]SNC121 labeled two binding sites: an opioid binding site and a non-opioid drug binding site. For the experiments reported here, DADL (10 μM) was used to define non-specific binding. Under these conditions, we obtained 70%-75% specific binding. [³H]SNC121 labeled an apparent single class of binding sites with a delta-like ligand selectivity profile. GppNHp and sodium decreased [³H]SNC121 binding by altering the K_d (P < 0.05) without changing the Bmax. GppNHp (20 μM) failed to strikingly alter the IC50 and slope factor of DPDPE, Deltorphin-II and naltrindole when displacing [³H]SNC121. Viewed collectively, [³H]SNC121 selectively labels a single class of delta opioid receptors in a manner consistent with an agonist-type interaction. Future experiments will determine the nature and/or function of the non-opioid [³H]-SNC121 binding site.