Abstract
The binding of [3H]paroxetine and [3H]citalopram to the brain serotonin (5-HT) uptake site was characterized and compared in human hypothalamus, frontal cortex and caudate nucleus. Our results showed that the binding exclusively related to the 5-HT uptake site (specific or 5-HT displaceable binding) was identical for both [3H]ligands. The selective 5-HT uptake inhibitor citalopram displayed the highest potency (Ki) in inhibiting the [3H]paroxetine and [3H]citalopram binding, as compared with the values obtained for desipramine and norzimelidine. This fact is in accordance with the reported blockage of these compounds of the 5-HT uptake. Furthermore, similar Bmax values were obtained for both radioligands in the brain regions under study, indicating their binding to the same presynaptic membrane protein. These findings give evidence of the suitability of both [3H]paroxetine and [3H]citalopram as markers of the 5-HT transporter. However, the higher affinity of [3H] paroxetine (10 to 30-fold with respect to [3H]citalopram) confirms that the radiolabelled form of this compound is the best tool for the study of the 5-HT uptake site available today.
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