Adult Neurogenesis and Hippocampal Memory Function: New Cells, More Plasticity, New Memories?
Section snippets
Basic processes of adult neurogenesis
Adult neurogenesis is an evolutionary conserved process in various species, including birds [20], rodents [21], primates [22], [23], and human beings [24]. In mammals, under normal conditions, active adult neurogenesis is primarily restricted to two brain regions, the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) and the subventricular zone (SVZ) of the lateral ventricles [16]. The SGZ is located at the interface between the granule cell layer (GCL) and the hilus of the DG, deep
Regulation of adult neurogenesis by learning
The basic rate of neurogenesis in the DG is thought to be genetically determined [28] but can be dramatically regulated under various conditions, including aging [32]; gender [33]; steroids [34], [35]; stress [36]; enriched environment [37]; voluntary exercise [38]; and pathologic conditions, such as seizures [39] or cerebral ischemia [40].
It is widely accepted that medial temporal lobe structures, including the hippocampus and surrounding cortical areas, are critical to the processes of
Potential involvement of adult neurogenesis in learning and memory
The functional relevance of adult neurogenesis in memory processes was first suggested in studies looking at the neural basis of song learning in birds [19]. In adult song birds, the volume of song-related nuclei showed seasonal and hormonal changes, with thousands of new neurons being added daily. These putative neurons responded to sound with action potentials, and neurogenesis in the avian hippocampus was modulated by the environmental complexity and learning experience [19], [49]. Since
Potential mechanisms underlying the contribution of adult neurogenesis on learning and memory
Despite some unresolved issues, if adult hippocampal neurogenesis can be assumed to play a critical role in learning and memory, how do these new cells contribute to the process? It takes approximately 2 to 4 weeks before the newly generated neurons are functionally integrated and start modifying active hippocampal circuits. Thus, it seems that mere replacement of the old neuronal population with newly generated cells is too slow and cannot explain the mechanism of plasticity alone. In addition
Summary and future directions
Although profound progress has been made in understanding and characterizing the mechanisms of adult neurogenesis, current studies are still insufficient to establish the true functional relevance of newborn neurons in adult mammalian brain. Learning and memory consist of extremely complicated and tightly orchestrated functions causing complex higher order behavior. It seems that the classic assumption in learning-induced structural plasticity—“more neurons are better”—may not address this
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Supported by a Postdoctoral Fellowship Award from the Sankyo Foundation of Life Science in Japan (Y. Kitabatake), the National Institute of Health (H. Song and G-l. Ming), Klingenstein Fellowship Award in the Neurosciences (G-l. Ming and H. Song), the Whitehall Foundation (G-l. Ming), McKnight Scholar Award (H. Song).