Characterization of the binding of a morphine (mu) receptor-specific ligand: Tyr-Pro-NMePhe-D-Pro-NH2, [3H]-PL17

The highly mu-receptor-selective ligand Tyr-Pro-NMePhe-D-Pro-NH2 (PL17) was custom tritiated and its binding to rat brain membranes was directly measured. The data were well fit by a model assuming the existence of a single homogeneous population of binding sites. Scatchard analysis yielded values for Kd and binding sites of 6 nM and 0.16 pmol/mg, respectively. As expected for binding to an opiate receptor, addition of sodium, magnesium, and guanyl nucleotides to the assays resulted in a modulation of ligand binding. In all cases tested, however, no significant deviations of the Scatchard plots from linearity were observed. Furthermore, displacement of [3H]-PL17 by all opioid ligands tested was monophasic, consistent with simple competitive inhibition at a single binding site. The IC50 values thus obtained showed good agreement with previously published values determined using less selective radiolabeled ligands. The regional distribution of [3H]-PL17 binding to rat brain was examined by in vitro autoradiography. The labeling pattern was similar to that seen for other mu-type ligands. Because of its high selectivity, the binding of [3H]-PL17 to the mu-type receptor can be measured directly without the need to suppress binding to other sites. As such, [3H]-PL17 should be a useful ligand for dissecting the actions of multiple opiate receptors.