Heterogeneity of [3H]neurotensin binding: studies with dynorphin, L-156,903 and levocabastine

Abstract The binding of [ 3 H]neurotensin (NT) to membranes from rat forebrain was complex, exhibiting ‘high’ affinity (K d ∼ 0.5nM) and ‘low’ affinity (K d ∼ 5.0nM) binding components. Dynorphin A(1–13) (DYN A(1–13)) and L-156,903 (N-oxo-3-(10H-phenothiazine-10-yl)propyl-1-arginyl-1-propyl-1-phenylalanine) potently inhibited [ 3 H]NT binding to brain with shallow biphasic competition curves. Saturation binding studies conducted in the presence or absence of DYN A(1–13) or L-156,903 indicated that these compounds, like levocabastine, exhibited substantial selectivity for the ‘low’ affinity NT site. Structure—activity studies indicated rigid structural requirements for the NT binding activity of DYN A(1–13) and L-156,903. In contrast to the results using brain tissue, DYN A(1–13), L-156,903 and levocabastine were very weak or inactive to inhibit [ 3 H]NT binding to rat uterus. These studies further characterize the heterogeneity of [ 3 H]NT binding in vitro and demonstrate clear tissue differences in binding within a given species.