Is serotonin an upper or a downer? The evolution of the serotonergic system and its role in depression and the antidepressant response

The role of serotonin in depression and antidepressant treatment remains unresolved despite decades of research. In this paper, we make three major claims. First, serotonin transmission is elevated in multiple depressive phenotypes, including melancholia, a subtype associated with sustained cognition. The primary challenge to this first claim is that the direct pharmacological effect of most symptom-reducing medications, such as the selective …