In this study we investigated the characteristics of binding sites with which δ opioid receptor agonists interact in homogenates of mouse brain using Krebs-HEPES medium. [3H][D-Ser2, Leu5, Thr6]enkephalin (DSLET), [3H][D-Ala2, D-Leu5]enkephalin (DADLE) and [3H][D-Pen2, D-Pen5]enkephalin (DPDPE) were used to label δ opioid binding sites. The analyses of the saturation binding data of these ligands (Scatchard plots) gave best fits to single rather than multiple site models. The binding capacity (Bmax) labelled [3H]DSLET was found to be significantly greater than those of [3H]DADLE and [3H]DPDPE in brains of mice. Naltriben (the benzofuran analogue of natrindole) was equally effective in competing for [3H]DSLET, [3H]DPDPE and [3H]DADLE binding sites. On the other hand, DADLE was significantly more potent in competing for [3H]DADLE and [3H]DPDPE binding sites than for [3H]DSLET binding sites. Also DPDPE was more potent in competing for the binding sites of DADLE and [3H]DPDPE than for those of [3DSLET. DSLET was found to be equipotent in competing for [3H]DSLET, [3H]DPDPE and [3H]DADLE binding sites. These results suggest a heterogeneity of δ opioid receptors which may be explained possibly by the existence of δ opioid receptor subtypes.
- DADLE (D-Ala, D-Leu]enkephalin)
- DPDPE ([D-Pen, D-Pen]enkephalin)
- DSLET ([D-Ser, Leu, Thr]enkephalin)
- δ-Opioid receptor subtypes