Binding characteristics of [D-Ala2-MePhe4-Glyol5] enkephalin and [D-Pen2,D-Pen5] enkephalin in rat brain: inhibition studies with fentanyl-like drugs.

We have investigated the binding characteristics of two tritiated enkephalin analogues [D-Ala2-MePhe4-Glyol5] enkephalin (DAGO) and [D-Pen2, D-Pen5] enkephalin (DPDPE) which are highly selective for mu and delta opioid sites respectively, in homogenates of whole rat brain and of the pons/medulla region. In addition, we have explored the binding profile of four 4-anilinopiperidine drugs (fentanyls) by means of inhibition studies. Our studies show that the delta involvement in pons/medulla of rat is small compared with that in whole brain and with mu binding in both whole brain and pons/medulla. In addition the rank order of affinities of the fentanyl series is the same at the mu and delta site, but the affinities are between 1 and 600-fold greater at the former site.