The selectivity of the opioid antagonist, naltrindole, for δ-opioid receptors

Abstract
In the mouse vas deferens, naltrindole gave pKB values of 9.7, 8.3, and 7.5 at the δ-, μ-, and K-sites and in binding assays, pIC50 values of 9.6, 7.8 and 7.2 at the same sites. The affinity of naltrindole for the δ binding site was increased in the presence of sodium ions and 5′-guanylylimidophosphate. Naltrindole is, thus, a potent opioid antagonist with marked selectivity for the δ-opioid receptor.