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

The binding of [3H]neurotensin (NT) to membranes from rat forebrain was complex, exhibiting 'high' affinity (Kd approximately 0.5 nM) and 'low' affinity (Kd approximately 5.0 nM) 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-prolyl-1-phenylalanine) potently inhibited [3H]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 '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 [3H]NT binding to rat uterus. These studies further characterize the heterogeneity of [3H]NT binding in vitro and demonstrate clear tissue differences in binding within a given species.