Features and a possible role of Mash1-immunoreactive cells in the dentate gyrus of the hippocampus in the adult rat

Neurogenesis occurs throughout life in both the subventricular zone (SVZ) and subgranular zone (SGZ) of the dentate gyrus (DG) in the hippocampus in the adult brain. In the SVZ, it has been demonstrated that transit-amplifying neural progenitor cells, which appear between neural stem/progenitor cells (NSPCs) and neuroblasts during the neuronal differentiation process, express mammalian achaete–scute homolog 1 (Mash1), which regulates differentiation during neurogenesis. Although Mash1-positive cells (Mash1+ cells) are observed in the SGZ, the importance of Mash1 in hippocampal neurogenesis is not sufficiently understood. In the present study, using immunohistochemical techniques, we examined whether Mash1+ cells in the SGZ act as transit-amplifying neural progenitor cells, and whether chronic treadmill running can induce alterations of the Mash1+ cells in the SGZ of the DG. The present results indicated that Mash1 immunoreactivity is detected in proliferative cells, and that astrocytes or NSPCs and neuroblasts express Mash1. A quantitative analysis of Mash1-positive astrocytes or NSPCs and Mash1-positive neuroblasts indicated that approximately 90% of Mash1+ cells did not belong to astrocytic and neuronal cells. Furthermore, chronic treadmill running induced an increase in the number of proliferating Mash1+ cells. The present study suggests that the majority of the Mash1+ cells in the SGZ may be transit-amplifying neural progenitor cells. In addition, the proliferation of Mash1-positive transit-amplifying neural progenitor cells may contribute to the exercise-induced neurogenesis that is associated with the improvement of learning and memory function.