Autoradiographic localization of 5-HT 1A receptors in the post-mortem human brain using [ 3H]WAY-100635 and [ 11C]WAY-100635

Abstract

The distribution of 5-HT 1A receptors was examined in the post-mortem human brain using whole hemisphere autoradiography and the selective 5-HT 1A receptor antagonist [ 3H]WAY-100635 ([ O-methyl- 3H]- N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)- N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride). The autoradiograms showed very dense binding to hippocampus, raphe nuclei and neocortex. The labeling in neocortex was slightly lower than in the hippocampus and was mainly at superficial layers, although a faintly labeled band could be seen in deeper neocortical layers. Other regions, such as the amygdala, septum and claustrum, showed low densities of [ 3H]WAY-100635 binding, reflecting low densities of 5-HT 1A receptors. The labeling was very low in basal ganglia, such as nucleus caudatus and putamen, in cerebellum or in structures of the brain stem except in the raphe nuclei. The labeling of human 5-HT 1A receptors with [ 3H]WAY-100635 was antagonized by the addition of the 5-HT 1A receptor ligands, 5-HT, buspirone, pindolol or 8-OH-DPAT (10 μM), leaving a very low background of non-specific binding. Saturation analysis of semiquantitative data from several human regions indicated that [ 3H]WAY-100635 has a K d of approximately 2.5 nM. The selective labeling of 5-HT 1A receptors with [ 3H]WAY-100635 clearly show that this compound is useful for further studies of the human 5-HT 1A receptor subtype in vitro. [ 11C]WAY-100635 is used for the characterization of 5-HT 1A receptors with positron emission tomography (PET). WAY-100635 was also radiolabeled with the short-lived positron-emitting radionuclide carbon-11 ( t 1/2=20 min) and used for in vitro autoradiography on human whole hemisphere cryosections. [ 11C]WAY-100635 gave images qualitatively similar to those of [ 3H]WAY-100635, although with a lower resolution. Thus, the hippocampal formation was densely labeled, with lower density in the neocortex. Buspirone, pindolol or 8-OH-DPAT (10 μM), blocked all binding of [ 11C]WAY-100635. The in vitro autoradiography of the distribution of 5-HT 1A receptors obtained with radiolabeled WAY-100635 provide detailed qualitative and quantitative information on the distribution of 5-HT 1A-receptors in the human brain. Moreover, the studies give reference information for the interpretation of previous initial results at much lower resolution in humans with PET and [ 11C]WAY-100635. These data provide a strong basis for expecting [ 11C]WAY-100635 to behave as a highly selective radioligand in vivo.