Effects of chronic treatment with fluoxetine and citalopram on 5-HT uptake, 5-HT1B autoreceptors, 5-HT3 and 5-HT4 receptors in rats

Abstract

The effect in rats of chronic treatment with two specific 5-HT reuptake inhibitors (SSRI) with antidepressant properties, citalopram (10 mg/kg, i.p. twice a day for 14 days, one day washout) and fluoxetine (15 mg/kg, p.o. twice a day for 21 days, 7 days washout), was evaluated on some mechanisms involved in central 5-HT neurotransmission. No adaptive modifications of brain 5-HT uptake (sites) were found by measuring functional [³H]5-HT uptake and [³H]citalopram binding in cortical and hippocampal synaptosomes, and by [³H]citalopram binding autoradiography in the raphe nuclei (5-HT cell bodies) and the ventral tegmental area (5-HT axonal pathway). Chronic treatments had no effect on presynaptic 5-HT_1B autoreceptors, functionally evaluated by measuring 5-HT_1B-mediated inhibition of depolarization-induced [³H]5-HT release from cortical and hippocampal synaptosomes. Chronic citalopram or fluoxetine did not significantly affect the binding of [³H]BRL-43694 to 5-HT₃ receptors in the rat brain cortex. Citalopram had no effect on [¹²⁵I]SB-207710 binding to 5-HT₄ receptors, measured by autoradiography in the substantia nigra. Negative results, such as those reported in the present study, could be due to a number of variables including the animal species, the treatment schedule or the brain areas considered, thus explaining the differences from some previous reports of significant effects of SSRI. However, our negative data are in agreement with many other published studies, suggesting that adaptive modifications of brain 5-HT transporters, terminal 5-HT_1B receptors, 5-HT₃ and 5-HT₄ receptors may not be a general effect induced by all SSRI.