Neurogenesis along the septo-temporal axis of the hippocampus: are depression and the action of antidepressants region-specific?

In recent years, both major depression and antidepressant therapy have been linked to adult hippocampal neurogenesis. The hippocampus is not a homogeneous brain area, and a converging body of evidence indicates a functional dissociation along its septo-temporal axis, the dorsal part being involved more in learning/memory and spatial navigation, while the ventral sub-region is linked more to emotional behavior and regulation of the neuroendocrine stress axis. Research has therefore been conducted in an attempt to relate effects of models of depression and of antidepressant therapies to adult neurogenesis along the septo-temporal axis of the hippocampus. The present paper reviews the current literature addressing this question and discusses the possible mechanisms involved and the functional significance of such regional effects. This review shows that animal models of depression elicit an effect restricted to the ventral hippocampus more frequently than a dorsal-specific effect. However, this is also stage specific, and concerns neurogenesis, rather than cell proliferation or survival. Surprisingly, the same does not apply regarding the effects of selective serotonin re-uptake inhibitors that act in a more uniform way on dorsal and ventral adult neurogenesis in most studies. Some recently introduced clinical compounds (e.g., agomelatine) or putative antidepressants have a specific action on the ventral sub-region, indicating that an action restricted to this part of the brain may be sufficient to achieve remission. Finally, non-pharmacological manipulations that are also endowed with antidepressant effects, such as environmental enrichment or physical exercise, also act on both subdivisions, although some studies pointed to specificity of dorsal neurogenesis. The different treatments, acting either on the dorsal or on the ventral sub-regions, could promote recovery by improving either ventral- or dorsal-related functions, both contributing in a different way to treatment efficacy.