Effects of selective serotonin and serotonin/noradrenaline reuptake inhibitors on extracellular serotonin in rat diencephalon and frontal cortex

Some clinical reports suggest that tricyclic antidepressants which block both noradrenaline and serotonin (5-HT) reuptake (SNRIs) are more effective than selective 5-HT reuptake inhibitors (SSRIs) in treating severe depression. Moreover, one neurochemical study reported larger increases in extracellular 5-HT in rat frontal cortex in response to the tricyclic antidepressant imipramine compared to the SSRI fluoxetine. However, imipramine, which blocks both 5-HT and noradrenaline reuptake, also binds with relatively high affinity to receptors for noradrenaline, histamine and acetylcholine. Thus, to test the hypothesis that compounds that inhibit both 5-HT and noradrenaline reuptake produce larger increases in 5-HT efflux, we compared the effects of acute systemic administration of several SNRIs and SSRIs. Extracellular 5-HT was measured using microdialysis probes implanted in the diencephalon and frontal cortex of unanesthetized rats. We tested the SSRIs paroxetine (0.3–10 mg/kg), citalopram (10–20 mg/kg) and fluoxetine (10 mg/kg), the nonselective tricyclic antidepressant imipramine (20 mg/kg) and the more selective SNRIs duloxetine (3–30 mg/kg) and venlafaxine (30–50 mg/kg). During the lights-off period, paroxetine and duloxetine increased 5-HT in the diencephalon ~300 and ~200%, respectively. During the lights-on period, paroxetine and duloxetine each increased 5-HT ~400% in the diencephalon. In the frontal cortex, both paroxetine and duloxetine increased 5-HT ~200%. Citalopram and venlafaxine each increased 5-HT in the diencephalon ~300%. Fluoxetine and imipramine increased 5-HT in the diencephalon by ~125 and ~80%, respectively. Thus, these results do not support the hypothesis that compared to SSRIs, compounds which inhibit both 5-HT and noradrenaline reuptake have a larger acute effect on extracellular 5-HT.