The involvement of nitric oxide (NO) and cyclic GMP (cGMP) in neurogenesis has been progressively unmasked over the last decade. Phosphodiesterase 5 (PDE5) specifically degrades cGMP and is highly abundant in the mammalian brain. Inhibition of cGMP hydrolysis by blocking PDE5 is a possible strategy to enhance the first step of neurogenesis, proliferation of neural stem cells (NSC). In this work, we have studied the effect on cell proliferation of 3 inhibitors with different selectivity and potency for PDE5, T0156, sildenafil, and zaprinast, using subventricular zone-(SVZ-) derived NSC cultures. We observed that a short- (6？h) or a long-term (24？h) treatment with PDE5 inhibitors increased SVZ-derived NSC proliferation. Cell proliferation induced by PDE5 inhibitors was dependent on the activation of the mitogen-activated protein kinase (MAPK) and was abolished by inhibitors of MAPK signaling, soluble guanylyl cyclase, and protein kinase G. Moreover, sildenafil neither activated ERK1/2 nor altered levels, suggesting the involvement of pathways different from those activated by T0156 or zaprinast. In agreement with the present results, PDE5 inhibitors may be an interesting therapeutic approach for enhancing the proliferation stage of adult neurogenesis. 1. Introduction Neurogenesis is the biological process of generating new neurons from progenitor cells or neural stem cells (NSC). NSC proliferate in two main regions of the adult mammalian brain: the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of dentate gyrus of the hippocampus. Following brain injury, such as stroke, NSC in the endogenous niches proliferate and migrate to the affected brain areas where they may differentiate into neurons, but survival is limited [1–3]. There is still a lack of knowledge concerning the use of effective therapeutic strategies in order to overcome the limited ability of brain self-repair following an insult. Understanding the signaling pathways involved in the regulation of neurogenesis is paramount in order to enhance brain repair. Neurogenesis is affected by several factors, including nitric oxide (NO). NO is a free radical of particular interest due to its cellular function as a second messenger, which includes the regulation of NSC proliferation. Several studies recently reported the effect of NO on the stimulation of adult neurogenesis in the dentate gyrus and in the SVZ [4–8]. Thus, the increase of NO levels following brain injury, such as seizures or ischemia, has been shown to promote proliferation of NSC and the formation of
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