Potent cyclic enkephalin analogues for delta opioid receptors in the rat brain

(/sup 3/H) (D-Pen/sup 2/,D-Pen/sup 5/) enkephalin ((/sup 3/H)DPDPE) and (/sup 3/H) (D-Pen/sup 2/, L-Pen/sup 5/) enkephalin ((/sup 3/H)DPLPE) characterization studies showed high affinity binding of these radioligands to rat brain membranes with dissociation constants of 1.8 and 1.0 nM, respectively, while a similar number of receptor density was found with both radiolabeled ligands (77 fmoles/mg protein). Unlabeled DPDPE inhibited both radioligands with high affinity (IC50 = 7 nM0 while morphine (IC50 = 80 nM), DAGO (IC50 = 250 nM) and PLO17 (no inhibition at 1000 nM) were less effective in inhibiting the binding, thus, illustrating the selective action of these radiolabeled ligands at the delta opioid receptor. A series of conformationally restricted D-penicillamine containing cyclic enkephalin analogues were synthesized using standard solid phase methods and their ability to inhibit (/sup 3/H)DPDPE and (/sup 3/H)DPLPE were examined in rat brain radioreceptor assays. Substitutions in the DPDPE molecule were made in phe/sup 4/. These substitutions were pNO/sub 2/-phe/sup 4/, beta-methyl-phe/sup 4/, pNO/sub 2/-beta-methyl-phe/sub 4/, pNO/sub 2/-beta-methyl-phe/sup 4/ (three isomeric forms: A,B,D). The IC50 values for the above enkephalin analogues were 3.7, 16, 7, 7, 200 nM, respectively. Thus, these potent analogues of DPDPE should be useful in determining the structure activity relationshipsmore » of the delta opioid receptor in rat brain.« less