Reconsideration of 5-hydroxytryptamine (5-HT)7 receptor distribution using [3H]5-carboxamidotryptamine and [3H]8-hydroxy-2-(di-n-propylamino)tetraline:: Analysis in brain of 5-HT1A knockout and 5-HT1A/1B double-knockout mice

The characterization and anatomical distribution of 5-hydroxytryptamine (5-HT)(7) receptor binding sites in brain tissue has been hampered by the lack of a specific radioligand. In the present autoradiographic study, we took advantage of 5-HT1A knockout and 5-HT1A/1B double-knockout mice to revisit the pharmacological characterization and anatomical localization of 5-HT7 binding sites in mouse brain using [H-3]5-carboxamidotryptamine (5-CT) and [H-3]8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT). The distribution pattern of [H-3]5-CT binding sites (2 nM) in the brain of mice lacking the 5-HT1A/1B receptor was scarce and confined to the septum, globus pallidus, thalamus, hypothalamus, amygdala, cortex, and substantia nigra. The low densities of [H-3]5-CT binding sites detected in septum, thalamus, hypothalamus, amygdala, and cortex were displaced by 10 muM of the selective 5-HT7 receptor antagonist (R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)pyrrolidine-1-sulfonyl) phenol (SB-269970). The SB-269970-insensitive [H-3]5-CT binding sites detected in globus pallidus and substantia nigra of 5-HT1A/1B knockout mice were displaced by N-[3-(2-dimethylamino)ethoxy-4-methoxy-phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide hydrochloride (SB-216641) (1 muM), demonstrating the 5-HT1D nature of these binding sites. In contrast to the low densities of [H-3]5-CT binding sites, high-to-moderate densities of [H-3]8-OH-DPAT binding sites (10 nM) were found throughout the brain of 5-HT1A and 5-HT1A/1B knockout mice (olfactory system, septum, thalamus, hypothalamus, amygdala, CA3 field of the hippocampus, cortical mantle, and central gray). These [H-3]8-OH-DPAT binding sites were displaced by 10 muM SB-269970, risperidone, and methiothepin but not by pindolol, N-tert-butyl-3[4-(2-methoxyphenyl)piperazin-1-yl]-2-phenylpropanamide (WAY-100135), or citalopram. We conclude that despite its high affinity for the 5-HT7 receptor in tissue homogenates, [H-3]5-CT is not a good tracer for measuring 5-HT7 receptor binding sites autoradiographically. Also, the lower affinity ligand [H-3]8-OH-DPAT is a much better tracer for autoradiographic studies at the 5-HT7 receptor binding sites.