In Vivo Labeling of Serotonin Uptake Sites with [3H]Paroxetine

Previous work has shown that 3H]paroxetine is a potent and selective in vitro label for serotonin uptake sites in the mammalian brain. In the present study, 3H]paroxetine was tested in mice as an in vivo label for serotonin uptake sites. Maximum tritium concentration in the whole brain (1.4% of the intravenous dose) was reached 1 h after injection into a tail vein. Distribution of the tracer at 3 h after injection followed the distribution of serotonin uptake sites known from previous in vitro binding studies (r = 0.85). The areas of highest 3H]paroxetine concentration, in decreasing order, were: hypothalamus greater than frontal cortex greater than olfactory tubercles greater than thalamus greater than upper colliculi greater than brainstem greater than hippocampus greater than striatum greater than cerebellum. Preinjection of carrier paroxetine (1 mg/kg) significantly decreased 3H]paroxetine concentration in all areas except in the cerebellum, which is known to contain a relatively low number of specific binding sites. Kinetic studies showed highest specific 3H]paroxetine binding (tissue minus cerebellum) at 2 h after injection and slow clearance of activity thereafter (half-time of dissociation from the hypothalamus, 215 min). The specificity of in vivo 3H]paroxetine binding was studied by preinjecting monoamine uptake blockers or receptor antagonists 5 min before administration of 3H]paroxetine. Serotonergic or muscarinic cholinergic receptor antagonists and dopamine or norepinephrine uptake blockers did not reduce the in vivo binding of 3H]paroxetine. In contrast, there was an excellent correlation (r = 0.99) between the in vivo inhibitory potencies of serotonin uptake blockers in this study and previously published in vitro data on inhibition of 3H] serotonin uptake in brain synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)