Utility of organotypic raphe slice cultures to investigate the effects of sustained exposure to selective 5‐HT reuptake inhibitors on 5‐HT release

Abstract

Selective 5-hydroxytryptamine (5-HT, serotonin) reuptake inhibitors (SSRIs) are widely used antidepressants and their therapeutic effect requires several weeks of drug administration. The delayed onset of SSRI efficacy is due to the slow neuroadaptive changes of the 5-hydroxytryptaminergic (5-HTergic) system. In this study, we examined the acute and chronic effects of SSRIs on the 5-HTergic system using rat raphe slice cultures. For organotypic raphe slice cultures, mesencephalic coronal sections containing dorsal and median raphe nuclei were prepared from neonatal Wistar rats and cultured for 14-16 days. Acute treatment with citalopram, paroxetine or fluoxetine (0.1-10 µM) in the slice cultures slightly increased extracellular 5-HT levels, while sustained exposure for 4 days augmented the elevation of 5-HT level in a time-dependent manner. Sustained exposure to citalopram had no effect on tissue contents of 5-HT and its metabolite, expression of tryptophan hydroxylase or the membrane expression of 5-HT transporters. The augmented 5-HT release was attenuated by Ca(2+) -free incubation medium or treatment with tetrodotoxin. Experiments with 5-HT(1A/B) receptor agonists and antagonists revealed that desensitization of 5-HT(1) autoreceptors was not involved in the augmentation of 5-HT release. Finally, co-treatment with an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, but not an N-methyl-d-aspartate, receptor antagonist, suppressed this augmentation. These results suggest that sustained exposure to SSRIs induces the augmentation of exocytotic 5-HT release, which is caused, at least in part, by the activation of AMPA/kainate receptors in the raphe slice cultures.