Autoradiographic localization of 3H‐paroxetine‐labeled serotonin uptake sites in rat brain

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Abstract
Paroxetine is a potent and selective inhibitor of serotonin uptake into neurons. Serotonin uptake sites have been identified, localized, and quantified in rat brain by autoradiography with 3H‐paroxetine; 3H‐paroxetine binding in slide‐mounted sections of rat forebrain was of high affinity (KD = 10 pM) and the inhibition affinity constant (Ki) values of various drugs in competing 3H‐paroxetine binding significantly correlated with their reported potencies in inhibiting synaptosomal serotonin uptake. Serotonin uptake sites labeled by 3H‐paroxetine were highly concentrated in the dorsal and median raphe nuclei, central gray, superficial layer of the superior colliculus, lateral septal nucleus, paraventricular nucleus of the thalamus, and the islands of Calleja. High concentrations of 3H‐paroxetine binding sites were found in brainstem areas containing dopamine (substantia nigra and ventral tegmental area) and norepinephrine (locus coeruleus) cell bodies. Moderate concentrations of 3H‐paroxetine binding sites were present in laminae I and IV of the frontal parietal cortex, primary olfactory cortex, olfactory tubercle, regions of the basal ganglia, septum, amygdala, thalamus, hypothalamus, hippocampus, and some brainstem areas including the interpeduncular, trigeminal, and parabrachial nuclei. Lower densities of 3H‐paroxetine binding sites were found in other regions of the neocortex and very low to nonsignificant levels of binding were present in white matter tracts and in the cerebellum. Lesioning of serotonin neurons with 3,4‐methylenedioxyamphetamine caused large decreases in 3H‐paroxetine binding. The autoradiographic distribution of 3H‐paroxetine binding sites in rat brain corresponds extremely well to the distribution of serotonin terminals and cell bodies as well as with the pharmacological sites of action of serotonin.