QUANTITATIVE AUTORADIOGRAPHIC CHARACTERIZATION OF THE BINDING OF [H-3] WAY-100635, A SELECTIVE 5-HT1A RECEPTOR ANTAGONIST

The binding characteristics of [H-3]WAY-100635 ([O-methyl H-3]-N-(2-(4-(2-methopyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexane carboxamide trihydrochloride), a potent and selective 5-HT1A antagonist radioligand, were examined in the rat brain using in vitro quantitative receptor autoradiography. The regional distribution of specific [H-3]WAY-100635 binding sites was heterogeneous and demonstrated a strong correlation with that of [H-3]8-OH-DPAT binding. The highest concentrations of [H-3]WAY-100635-labelled sites were found in the lateral septal areas, dorsal raphe n., entorhinal cortex and the hippocampal formation (CA1, CA3 and dentate gyrus). Scatchard transformation of saturation isotherms revealed saturable [H-3]WAY-100635 binding sites of high-affinity: in the hippocampal formation, K-d was similar to 1 nM and B-max ranged between 187 and 243 fmol/mg tissue wet weight, in the entorhinal cortex, K-d = 0.44 nM and B-max = 194 fmol/mg tissue wet weight, and in the rostral portion of the dorsal raphe n., K-d = 0.52 nM and B-max = 157 fmol/mg tissue wet weight. The affinity of [H-3]WAY-100535 for the 5-HT1A binding site tended to be higher in the dorsal raphe n, and entorhinal cortex compared with that of the hippocampal formation. In contrast, the binding affinity of [H-3]8-OH-DPAT in the hippocampal formation was between 1.1 and 2.3 nM and the B-max was 137 to 183 fmoles/mg tissue wet weight; in the entorhinal cortex, K-d = 3.2 nM and B-max = 141 fmoles/mg tissue wet weight, and in the rostral portion of the dorsal raphe n., K-d = 3.4 nM and B-max = 163 fmol/mg tissue wet weight. Our findings support the use of [H-3]WAY-100635 as a novel antagonist radioligand for studying the binding characteristics of the 5-HT1A receptor by quantitative receptor autoradiography.