The presence of a serotonin uptake inhibitor alters pharmacological manipulations of serotonin release

The present study investigated the effects of the presence of the serotonin uptake inhibitor citalopram in the perfusion medium on pharmacological manipulations which increased and decreased striatal serotonin release using in vivo microdialysis. A high performance liquid chromatography detection system equipped with a microbore column was used which reduced the detection limit to 0.5 fmol serotonin/5 μl sample and enabled basal striatal serotonin release to be measured without the addition of a serotonin uptake inhibitor to the perfusion medium. Although serotonin uptake inhibitors have frequently been used to enhance the serotonin content of dialysate samples, the effects of the presence of serotonin uptake inhibitors on pharmacological manipulations which increased and decreased the release of serotonin have not yet been characterized. Serotonin release was reduced by the systemic administration of the 5-HT_IA receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Although 5-HT release was reduced by 8-OH-DPAT after the addition of citalopram, the 5-HT_IA receptor agonist did not reduce absolute levels of extracellular serotonin below basal values of serotonin measured in the absence of citalopram. In addition, citalopram dramatically prevented the four-fold increase in the release of serotonin produced by the systemic administration of the serotonin-releasing agent fenfluramine. The blockade of fenfluramine's effects by citalopram supports the hypothesis that transport of fenfluramine into serotonergic neurons is necessary to increase serotonin release. This study demonstrates that the use of an HPLC detection system equipped with a microbore column can reliably measure basal serotonin release using in vivo microdialysis. Evaluation of the effects of the addition of citalopram to the perfusion medium suggests that the response to pharmacological manipulations of serotonin release may be substantially altered by the presence of an uptake inhibitor.