Reconsideration of 5-Hydroxytryptamine (5-HT)₇Receptor Distribution Using [³H]5-Carboxamidotryptamine and [³H]8-Hydroxy-2-(di-n-propylamino)tetraline: Analysis in Brain of 5-HT_1A Knockout and 5-HT_1A/1B Double-Knockout Mice

Abstract

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-HT_1Aknockout and 5-HT_1A/1B double-knockout mice to revisit the pharmacological characterization and anatomical localization of 5-HT₇ binding sites in mouse brain using [³H]5-carboxamidotryptamine (5-CT) and [³H]8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT). The distribution pattern of [³H]5-CT binding sites (2 nM) in the brain of mice lacking the 5-HT_1A/1Breceptor was scarce and confined to the septum, globus pallidus, thalamus, hypothalamus, amygdala, cortex, and substantia nigra. The low densities of [³H]5-CT binding sites detected in septum, thalamus, hypothalamus, amygdala, and cortex were displaced by 10 μM 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 [³H]5-CT binding sites detected in globus pallidus and substantia nigra of 5-HT_1A/1B knockout mice were displaced byN-[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 μM), demonstrating the 5-HT_1D nature of these binding sites. In contrast to the low densities of [³H]5-CT binding sites, high-to-moderate densities of [³H]8-OH-DPAT binding sites (10 nM) were found throughout the brain of 5-HT_1A and 5-HT_1A/1B knockout mice (olfactory system, septum, thalamus, hypothalamus, amygdala, CA3 field of the hippocampus, cortical mantle, and central gray). These [³H]8-OH-DPAT binding sites were displaced by 10 μM 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, [³H]5-CT is not a good tracer for measuring 5-HT₇ receptor binding sites autoradiographically. Also, the lower affinity ligand [³H]8-OH-DPAT is a much better tracer for autoradiographic studies at the 5-HT₇ receptor binding sites.