Characterization of the receptor binding profile of (3H)-dermorphin in the rat brain

Amphibian skin synthesizes a variety of biologically active peptides. Of these, dermorphin (Tyr-d -Ala-Phe-Gly-Tyr-Pro-Ser-NH₂) is an extraordinarily potent opioid peptide up to 1000 times more active than morphine in inducing analgesia after intracerebroventricular administration. Dermorphin has little in common with the sequence of all hitherto known mammalian opioid peptides and is unique in having a d -amino acid residue in position 2. Specific binding properties of tritium labeled dermorphin were characterized in the rat brain. Scatchard or Hill analysis of equilibrium measurements performed over a large range of concentrations revealed a single population of dermorphin binding sites with a Kd value of 0.46 nM. Dermorphin and the selective μ-receptor ligand (d -Ala², MePhe⁴, Gly⁵-ol)- enkephalin (DAGO) had similar high potencies in competing with (³H)-dermorphin binding, whereas the inverse holds for the prototypical delta receptor ligand (d -Pen²,d -Pen⁵)-enkephalin (DPDPE), which exhibited a potency three orders of magnitude lower. Dermorphin was tested for its relative affinity to μ and delta binding sites by determining its potency in displacing (³H)-DAGO and (³H)-DPDPE from rat brain membrane preparations. Based on these comparisons, dermorphin exhibited a selectivity ratio Ki(DPDPE)/Ki(DA-GO) = 100, a value almost identical to that of DAGO, this ligand being considered as the protypical μ-receptor probe. The high affinity and selectivity of (³H)-dermorphin together with its very low nonspecific binding make this peptide a useful tool for dissecting the role(s) of the μ-receptor(s).