An in vivo binding assay to determine central α 1-adrenoceptor occupancy using [ 3H]prazosin

Abstract

An alpha 1 adrenoceptor (α 1-AdR) assay using [ 3H]prazosin binding in mouse brain is described which allows in vivo determination of central α 1-AdR occupancy for ligands with α 1-AdR affinity. Binding of [ 3H]prazosin in rat and mouse brain membranes in vitro was used to characterise the pharmacological profile of α 1-AdRs in order to determine any potential species variations. Saturation and displacement studies yielded comparable affinity and pharmacological profile for [ 3H]prazosin binding in mouse and rat brain homogenates. These studies confirmed the absence of species variation for ligands in central α 1-AdR pharmacology which is in good agreement with previous studies in rat brain. Subsequently, in vivo binding of [ 3H]prazosin in mouse whole brain was used to measure the occupancy of a number of AdR ligands. Timecourse studies revealed that a [ 3H]prazosin (5 μCi/mouse) pretreatment time of at least 20 min following intravenous (i.v.) administration was required for optimal specific binding. Ligands were administered systemically 40 min prior to i.v. administration of radiolabel. The α 1-adrenoceptor ligands prazosin (ED 50=0.15 mg/kg i.p.), benoxathian (0.52 mg/kg i.p.) and phentolamine (51 mg/kg i.p.) were all able to block in vivo [ 3H]prazosin binding from mouse brain. In addition, receptor occupancy values for a number of compounds including haloperidol (ED 50=0.83 mg/kg s.c.), clozapine (2.2 mg/kg s.c.) and MDL-100 907 [ R(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol], (10 mg/kg s.c.)], which possess high to moderate affinity at α 1-adrenoceptors, were also determined. These results suggest that in the mouse, [ 3H]prazosin binding can be used to measure in vivo receptor occupancy of ligands with affinity at central α 1-adrenoceptors.