ORPHANIN FQ - RECEPTOR-BINDING AND ANALOG STRUCTURE-ACTIVITY-RELATIONSHIPS IN RAT-BRAIN

A tritiated form of orphanin FQ (a heptadecapeptide also known as Noci ceptin) has been prepared. This radioligand (33 Ci/mmole) was used to develop a radioreceptor assay using rat brain homogenates. Binding was observed to be saturable, and analyses of the binding data indicate t he presence of a single binding site with a dissociation constant of 5 +/- 1.1 nM and Bmax of 535 +/- 85 fmoles/mg protein. Thirty-four anal ogues of orphanin FQ, including a complete alanine ''scan'' of orphani n FQ, and truncation analogues from both the N- and C- terminals were synthesized and tested. The data obtained indicate that the N-terminus plays a more critical role in binding than the C-terminus and that re sidues 1, 2, 4, and 8 are essential for binding.