Quantitative autoradiographic analysis of the distribution of binding sites for [125I]Bolton Hunter derivatives of eledoisin and substance P in the rat brain.

[125I]Bolton and Hunter eledoisin binds to a single class of non-interacting sites in rat cerebral cortex tissue sections with an apparent Kd of 9.9 nM and a Bmax of 244 fmol/mg protein. When concentrations of up to 23 nM [125I]Bolton and Hunter eledoisin were used, [125I]Bolton and Hunter eledoisin binding was specific, saturable and reversible. Kassinin, eledoisin and neurokinin B were more potent than substance P and neurokinin A in inhibiting the specific binding of [125I]Bolton and Hunter eledoisin to cerebral cortex tissue sections. These kinetic and pharmacological characteristics are consistent with results obtained from binding studies on cortical synaptosomes. When the localization of [125I]Bolton and Hunter substance P and [125I]Bolton and Hunter eledoisin binding sites were compared, differences in many areas of the brain were noted. Large differences were seen in the paraventricular and supraoptic hypothalamic nuclei, and in layers IV and V of the cerebral cortex, which were densely labeled by [125I]Bolton and Hunter eledoisin, but not by [125I]Bolton and Hunter substance P. In contrast, nuclei of the septum (diagonal band of Broca, septohippocampal nucleus, dorsal part of the lateral septal nucleus), the rostrodorsal part of the hippocampus and other discrete nuclei [endopyriform nucleus, anterior cortical amygdaloid nucleus, the vermis columns (9-10), the dorsal tegmental nucleus, the hypoglossal and ambiguus nucleus] had high levels of [125I]Bolton and Hunter substance P binding but were only labeled weakly by [125I]Bolton and Hunter eledoisin. Thus, the two ligands seem to label different sites, since these binding sites have different biochemical and pharmacological properties, and are localized in different anatomical structures.