Differential antagonism of opioid delta antinociception by [D-Ala2,Leu5,Cys6]enkephalin and naltrindole 5'-isothiocyanate: evidence for delta receptor subtypes

Abstract

The present study has investigated the direct opioid delta receptor-mediated antinociception produced by i.c.v. administration of the highly selective delta agonists, [D-Pen2,D-Pen5]enkephalin (DPDPE) and [D-Ala2]deltorphin II, as well as that of the less delta-selective [D-Ser2,Leu5,Thr6]enkephalin (DSLET), by using two novel nonequilibrium opioid antagonists, [D-Ala2,Leu5,Cys6] enkephalin (DALCE) and naltrindole 5'-isothiocyanate (5'-NTII). At times ranging from 8 to 48 hr after a single i.c.v. pretreatment of mice with 5'-NTII, the antinociceptive effects of [D-Ala2] deltorphin II were significantly antagonized. In contrast, 5'-NTII pretreatment at times between 10 min and 24 hr failed to antagonize the antinociceptive effects of DPDPE. Previous studies have shown that pretreatment with i.c.v. DALCE produces a dose- and time-related antagonism of DPDPE, but not morphine, antinociception. However, pretreatment with i.c.v. DALCE failed to antagonize the antinociceptive effects of [D-Ala2]deltorphin II. Similarly, i.c.v. administration of DSLET produced time- and dose-related antinociception which was partially antagonized by either beta-funaltrexamine (beta-FNA) or by ICI 174,864 (N,N-dialyl-Tyr-Aib-Aib-Phe-Leu-OH), suggesting mixed activity at mu and delta receptors. ICI 174,864 produced essentially complete antagonism of DSLET antinociception in beta-FNA-pretreated mice. Pretreatment with 5'-NTII (at -8 to -48 hr), blocked the antinociception produced by DSLET in control or in beta-FNA-pretreated mice. In contrast, pretreatment with DALCE failed to antagonize the antinociception produced by i.c.v. DSLET in either control or in beta-FNA-pretreated mice.(ABSTRACT TRUNCATED AT 250 WORDS)