Is adult hippocampal neurogenesis really relevant for the treatment of psychiatric disorders?

Adult neurogenesis consists in the generation of newborn neurons from neural stem cells taking place in the adult brain. In mammals, this process is limited to very few areas of the brain, and one of these neurogenic niches is the subgranular layer of the dentate gyrus (DG) of the hippocampus. Adult newborn neurons are generated from quiescent neural progenitors (QNPs), which differentiate through different steps into mature granule cells (GCs), to be finally integrated into the existing hippocampal circuitry. In animal models, adult hippocampal neurogenesis (AHN) is relevant for pattern discrimination, cognitive flexibility, emotional processing and resilience to stressful situations. Imaging techniques allow to visualize newborn neurons within the hippocampus through all their stages of development and differentiation. In humans, the evidence of AHN is more challenging, and, based on recent findings, it persists through the adulthood, even if it declines with age. Whether this process has an important role in human brain function and how it integrates into the existing hippocampal circuitry is still a matter of exciting debate. Importantly, AHN deficiency has been proposed to be relevant in many psychiatric disorders, including mood disorders, anxiety, post-traumatic stress disorder and schizophrenia. This review aims to investigate how AHN is altered in different psychiatric conditions and how pharmacological treatments can rescue this process. In fact, many psychoactive drugs, such as antidepressants, mood stabilizers and atypical antipsychotics (AAPs), can boost AHN with different results. In addition, some non-pharmacological approaches are discussed as well.