, 2006 and Soulières et al., 2009;
Figure 3). As in the DG, environmental chronic stress impairs neurogenesis and reduces the population of newborn neurons in the olfactory bulb granule cell layer (Hitoshi et al., 2007). These findings suggest that chronic stress may also impair olfactory bulb pattern separation and odor acuity for highly similar odors. Olfactory impairments are associated with a wide range of disorders including mild cognitive impairment, Alzheimer’s disease, Parkinson’s disease, and schizophrenia. Normal aging can also both reduce OB neurogenesis and impair fine odor discrimination (Enwere et al., 2004). Although the level of olfactory bulb neurogenesis in humans is still debated, it is unclear why olfactory BGB324 dysfunction would be comorbid with disorders having such diverse etiologies.
Thus, investigation of olfactory pattern separation in these disorders is warranted. Here, we propose a common role in pattern separation for adult neurogenesis in the olfactory bulb and hippocampus. Specifically, in both regions, new granule cells may modulate inhibition of principal cells either directly (OB) or via interneurons (DG) and this inhibition may contribute to pattern separation. We also propose that different levels of neurogenesis represent an adaptation to environmental changes in cognitive demands such as those that take place with changing seasons, exposure to enriched environment, or in response to stress and adversity. When
exaggerated, these adaptive changes may lead to pathologies associated with dysregulated Rigosertib mouse pattern separation. For example, the excessive generalization observed in anxiety disorders may stem from impaired pattern separation while the excessive attention to details seen in individuals with autism spectrum disorders Avelestat (AZD9668) may result from excessive pattern separation. Major questions remain unanswered. For example, if adult neurogenesis is such an effective strategy for promoting pattern separation, why is it not more widespread in the brain? Is neurogenesis the privilege of neural circuits devoted to encoding but not storage? Are there costs (such as erosion of memories) that preclude its inclusion in other circuits, or is adult neurogenesis in the OB and DG simply an evolutionary holdover not available to other regions (Kaslin et al., 2008)? Is the potential for neurogenesis latent in other parts of the brain? Addressing these questions will undoubtedly continue to transform our ideas regarding the regenerative potential of the adult mammalian brain. We thank Susanne Ahmari and Mazen Kheirbek for comments on the manuscript. The work was supported by NIMH Grant 5K99MH086615-02 (A.S.), NIDCD Grant R01-DC003906 (D.A.W.) and NARSAD, the New York Stem Cell Initiative (NYSTEM), NIH R01 MH068542 Grants (R.H.).