TAARs As Novel Therapeutic Targets for the Treatment of Depression: A Narrative Review of the Interconnection with Monoamines and Adult Neurogenesis

Depression is a common mental illness of great concern. Current therapy for depression is only suitable for 80% of patients and is often associated with unwanted side effects. In this regard, the search for and development of new antidepressant agents remains an urgent task. In this review, we discuss the current available evidence indicating that G protein-coupled trace amine-associated receptors (TAARs) might represent new targets for depression treatment. The most frequently studied receptor TAAR1 has already been investigated in the treatment of schizophrenia, demonstrating antidepressant and anxiolytic properties. In fact, the TAAR1 agonist Ulotaront is currently undergoing phase 2/3 clinical trials testing its safety and efficacy in the treatment of major depressive disorder and generalized anxiety disorder. Other members of the TAAR family (TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9) are not only involved in the innate olfaction of volatile amines, but are also expressed in the limbic brain areas. Furthermore, animal studies have shown that TAAR2 and TAAR5 regulate emotional behaviors and thus may hold promise as potential antidepressant targets. Of particular interest is their connection with the dopamine and serotonin systems of the brain and their involvement in the regulation of adult neurogenesis, known to be affected by the antidepressant drugs currently in use. Further non-clinical and clinical studies are necessary to validate TAAR1 (and potentially other TAARs) as novel therapeutic targets for the treatment of depression.