Two actions are better than one: Avoiding self-inhibition of serotonergic neurones enhances the effects of serotonin uptake inhibitors

The serotonin (5-HT)-increasing action of 5-HT uptake or monoamine oxidase inhibitors is limited by a negative feedback at somatodendritic level. The excess 5-HT produced by these antidepressant drugs in the interstitial space of the midbrain raphe activates somatodendritic 5-HT1A autoreceptors, thereby attenuating terminal 5-HT release. This effect is maximal in forebrain areas innervated by the dorsal raphe nucleus and can be prevented by the administration of non-selective [(-)pindolol, (-)tertatolol] and selective (WAY-100635) 5-HT1A antagonists. In keeping with these observations, the combined administration of selective serotonin reuptake inhibitors (SSRIs) and 5-HT1A antagonists increase the cortical and striatal extracellular 5-HT concentration more than the former alone. Also, concurrent inhibition of the 5-HT and noradrenaline transporters with 20 mg/kg imipramine increases cortical extracellular 5-HT concentration more than SSRI doses which maximally block the 5-HT transporter. Moreover, the effects of fluoxetine on frontal cortex 5-HT are potentiated by a dose of desipramine that does not modify extracellular 5-HT by itself. Given the relevance of increased serotonergic transmission in the treatment of depression, these experimental data indicate that dual-action antidepressant treatments may be more effective than those which selectively inhibit the 5-HT transporter.