HISTAMINE H-3 BINDING-SITES IN RAT-BRAIN - LOCALIZATION IN THE NUCLEUS OF THE SOLITARY TRACT

The distribution of high-affinity binding sites for the histamine H-3 agonist [H-3]-N(alpha)-methylhistamine ([H-3]NAMH, 4 nM) in the rat brain was examined by autoradiography. Binding in all brain regions was displaceable with the H-3 antagonist thioperamide (2 muM). The pattern of H-3 binding was heterogeneous: highest levels were in the striatum and substantia nigra, which receives sparse histaminergic innervations. Intermediate H-3 binding was present in neocortex and diencephalon, while low binding was evident in the medial septum and dorsal hippocampus. Hitherto unreported H-3 binding sites were noted in the dorsolateral tegmentum and in the nucleus of the solitary tract (NTS). Because the NTS receives a dense histaminergic innervation, [H-3]NAMH binding in the NTS was further studied by Scatchard analysis. The presence of saturable binding (B(max) = 10 +/- 4 fmol/mg, K(d) = 0.6 +/- 0.3 nM, n = 3) suggested that histamine may act via H-3 receptors in the NTS.