Neurogenic Brain Regions

New neurons are continuously generated in two neurogenic regions in the adult brain: the subgranular zone (SGZ) of dentate gyrus (DG) in the hippocampal formation and the subventricular zone (SVZ) of lateral ventricles, as illustrated in Fig. 9.1 (Ming and Song 2005; Lledo et al. 2006; Abrous et al. 2005; Christie and Cameron 2006; Ehninger and Kempermann 2008). In both regions, the glial origin of adult neural stem cells and their multipotentiality, giving rise to neurons, astro-cytes, and oligodendrocytes, have been well established (Doetsch et al. 1999; Gage 2000; Alvarez-Buylla et al. 2001). In the SVZ, newborn cells migrate over a long distance via the rostral migratory stream (RMS) and differentiate mainly into inhibitory g-aminobutyric acid (GABA)-ergic interneurons in the granular layer of

Neurogenic zones and Serotonergic innervation

BrdU ceils 5-HT fibers

Granule cell * Layer

Hilus

Fig. 9.1 Schematic representation of the neurogenic zones in adult rat brain and visualization of the corresponding serotonergic innervation. The parasagittal view shows the location of the hippocampus (Hp), the lateral ventricle (Lv), and the striatum (Str) limiting the subventricular zone (SVZ) that lines the wall of the lateral ventricle. From there, neuroblasts migrate along the rostral migratory stream (RMS) towards the olfactory bulb (OB). Frontal sections illustrate the precise location of the SVZ and the subgranular zone (SGZ) of the granule cell layer (GCL) in the dentate gyrus (DG) of the hippocampal formation. Immunostaining for serotonergic fibers (5-HT fibers) shows them in close proximity to dividing cells labeled with bromodeoxyuridine (BrdU), a marker of cell division, in the SVZ and the SGZ. Notably, a dense plexus of 5-HT fibers is seen crossing the SVZ and the ependymal layer (Ctx cortex, CC corpus callosum)

the OB and into a small proportion of peri-glomerular cells expressing tyrosine hydroxylase (Alvarez-Buylla and Garcia-Verdugo 2002). By contrast, in the DG, neural stem cells and progenitors are aligned on the hilar border and can easily integrate the granule cell layer. These new glutamatergic cells develop dendritic arborization into the molecular layer and axonal projections (mossy fibers) to area CA3 and finally integrate the hippocampal three-synaptic circuitry (Kempermann et al. 2004a). Another difference between the two neurogenic zones is the level of cell production, which is much higher in the SVZ (about 30,000 new cells per day) compared with the SGZ (about 9,000 new cells per day), as quantified in young rodent (Alvarez-Buylla and Garcia-Verdugo 2002; Cameron and McKay 2001). Although a large number of progenitors are created through proliferation, only about 50% of newborn cells are selected for long-term survival (Winner et al. 2002; Kempermann et al. 2003). These cells are continuously added to the dentate gyrus (Cameron and McKay 2001), while they replace old cells in the olfactory bulb in phase with the turnover of sensory inputs (Lledo et al. 2006).

In spite of the low rate of adult neuronal production, the constant integration of newborn cells into the functional circuitry and their unique properties compared with more mature neurons suggest that these neurons may have a very specific function (Kempermann et al. 2004b; Doetsch and Hen 2005; Lledo and Saghatelyan 2005; Aimone et al. 2006). This function has not been fully elucidated and remains a topic of intense investigation and discussion (Scharfman and Hen 2007; Kuhn et al. 2007; Ortega-Perez et al. 2007; Kempermann 2008; Aimone et al. 2009). However, a number of studies have suggested the implication of adult neurogenesis in learning and memory process (Abrous et al. 2005). More specifically, newborn bulbar neurons are presumed to contribute to essential aspects of olfactory function, notably in odor discrimination (Lledo et al. 2006), whereas new granule cells in the DG have been involved in hippocampus-dependent behaviors (Leuner et al. 2006; Bruel-Jungerman et al. 2007; Shors 2008) and are presumed to be particularly important in episodic memory (Aimone et al. 2006). Until recently, a major difficulty in testing whether bulbar or hippocampal newborn neurons are required for specific tasks was the lack of selective method to block neurogenesis, without the use of irradiation or neuro-toxic approaches. Transgenic strategies aimed to ablate neural precursors provided new information indicating that hippocampal newborn neurons contribute to contextual and spatial memory formation and that bulbar neurogenesis is required not for acquisition but for short-term retention of odor-associated memory (Dupret et al. 2008; Imayoshi et al. 2008; Jessberger et al. 2009).

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