Effect of a selective 5-hydroxytryptamine reuptake inhibitor on brain extracellular noradrenaline: microdialysis studies using paroxetine

The clinical efficacy of selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors (SSRIs) is normally attributed to their ability to increase brain 5-HT function although recent preclinical findings indicate that their selectivity for 5-HT over noradrenaline may be less evident in vivo. The present study investigated the effects of the SSRI, paroxetine, on extracellular levels of noradrenaline. Microdialysis was carried out in the hippocampus of the awake rat. In rats treated twice daily for 14 days with paroxetine (5 mg/kg s.c.), dialysate levels of noradrenaline showed a maintained two-fold increase compared to saline-injected controls. Paroxetine (5 mg/kg s.c.) administered once daily for 14 days did not cause a sustained increase in noradrenaline but levels showed a moderate (+58%) increase in response to a paroxetine challenge. Acute injection of paroxetine (5 mg/kg s.c.) did not elevate noradrenaline levels. Paroxetine (5 mg/kg s.c.) elevated dialysate 5-HT after both acute and repeated (twice daily for 14 days) treatment. The paroxetine-induced increase in noradrenaline (and 5-HT) was positively correlated with plasma concentrations of the drug, which were around the therapeutic range. In comparison to paroxetine, desipramine (10 mg/kg s.c.) caused a four-fold increase in dialysate noradrenaline (but did not change 5-HT) following repeated (once daily for 14 days) treatment and a two-fold increase at for acute treatment. In summary, despite its selectivity as a 5-HT reuptake inhibitor, paroxetine increased extracellular levels of noradrenaline in rat hippocampus following repeated administration. We discuss the possibility that a facilitation of noradrenaline function might be involved in the antidepressant effect of paroxetine, and possibly other SSRIs.