Radioligand binding analysis of knockout mice reveals 5-hydroxytryptamine7 receptor distribution and uncovers 8-hydroxy-2-(di-N-propylamino)tetralin interaction with α2 adrenergic receptors

in the present autoradiographic study, we took advantage of 5-hydroxytryptamine7 (5-HT7) receptor knockout mice to analyze the brain distribution of 5-HT7 receptor binding sites using [H-3]5-carboxamidotryptamine (5-CT; a 5-1-1A/1B/1D/5/7 receptor ligand) and [H-3]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; a 5-HT1A/7 receptor ligand). Low to moderate densities of [3 H]5-CT (2 nM) binding sites insensitive to pindolol (10 muM, for 5-HT1A/1B receptor blockade) and GR-127935 (1 muM; for 5-HT1D receptor blockade) were observed in wild-type mice (mainly in thalamus and hypothalamus) but not in 5-HT7 receptor knockout mice. Surprisingly, moderate to high densities of [H-3]8-OH-DPAT (10 nM) binding sites insensitive to pindolol (10 muM) remained in 5-HT7 receptor knockout mouse brain. These non-5-HT1A, non-5-HT7 binding sites were found to be adrenergic alpha(2A) receptor binding sites. In alpha(2A) receptor knockout mice low to moderate densities of [H-3]8-OH-DPAT binding sites insensitive to pindolol but sensitive to the selective 5-HT7 receptor antagonist SB-269970 (300 nM) were observed mainly in thalamus and hypothalamus. Therefore, in addition to 5-HT1A and 5-HT7 binding sites, [H-3]8-OH-DPAT also binds to alpha(2A) receptor binding sites in wild-type mouse brain. [H-3]8-OH-DPAT (in the presence of pindolol and 1 muM RX-821002 for alpha(2) receptor blockade) and [H-3]5-CT (in the presence of pindolol and GR127935) bind to a similar receptor binding population corresponding to 5-HT7 binding sites. Detailed anatomical mapping of 5-HT7 receptor binding sites in wild-type mouse brain was then performed using both radioligands in the presence of suitable pharmacological agents for non-5-HT7 receptor binding sites blockade. The mapping revealed binding sites consistent with the mRNA distribution with the highest densities found in anterior thalamic nuclei. (C) 2004 IBRO. Published by Elsevier Ltd. All rights reserved.