Neurogenesis is the differentiation of neural stem cells into neurones, astrocytes and oligodendrocytes. There is evidence for postnatal neurogenesis in the hippocampus, brainstem and central canal area of the spinal cord (Bauer et al., 2005, Barnabé-Heider et al., 2010, Lois and Alvarez-Buylla, 1994). We therefore sought to determine the relationship between 5-HTergic fibres and neural stem cells using nestin-GFP mice. Then in vivo and in vitro experiments were conducted. Administration of 10mg/kg of 5-ethynyl-2′-deoxyuridine (EdU), a thymidine analogue, to allow detection of newly proliferated cells followed by immunofluorescent detection of differentiation markers for astrocytes (s100β), oligodendrocytes (PanQKI) and neurones (HUC/D) were performed. In nestin-GFP mice, 5-HTergic fibres formed close appositions with nestin-positive neural stem cells in brain stem and spinal cord suggesting that 5-HT may modulate the stem cells. Intraperitoneal administration of fluoxetine hydrochloride (10mg/kg) for 10 days and EdU for 5 days in C57/Bl6 (n=8) mice promotes greater numbers of proliferating cells in dentate gyrus of hippocampus (50.0±2.2 vs 30.7±3.0, p≤0.05), DVC (32.71±3.45 vs 19.87±1.64, p ≤0.05), raphe (3.8±1.2 vs 0.63±0.4, p≤0.05), inferior olives of brainstem (17±1.9 vs 9.5±1.5, p≤0.05) and dorsal horn of gray matter in thoracic spinal cord (15.3±1.4 vs 10.0±0.8, p<0.05). Furthermore, the proportion of new cells that became oligodendrocytes was significantly higher in both brainstem (26±4.4 vs 15.4±2.7, p≤0.05) and thoracolumbar spinal cord (24.2±2.2 vs 14.6±1.7, p≤0.05). To determine which 5-HTR is involved, in vitro experiments revealed that 4 hours incubation of 10µM of cisapride (5-HT4 agonist) with 1µM EdU elicited higher numbers of proliferating cells in central canal of spinal cord (3.6±0.3 vs 2.6±0.2, p≤0.05). In vivo experiments (n=8) by intraperitoneal administration of 1mg/kg of tegaserod maleate (5-HT4 partial agonist) and EdU for 7 days elicited significantly fewer proliferating cells in dentate gyrus of hippocampus (32.6±0.9 vs 40.3±2.0, p≤0.05) and selected areas in spinal cord, compared to control. None of the differentiation markers show any significant results in brainstem and spinal cord. However in hippocampus, activation of 5-HTR4 significantly supressed the differentiation of proliferating cells into neurones (8.8%±0.8 vs13.7%±1.2, p<0.05). Further in vitro experiments using 1µM of tegaserod maleate revealed significantly lower numbers of proliferating cells in central canal of spinal cord (9.21±0.8 vs 12±1.2, p≤0.05). In conclusion, this indicated the influence of endogenous 5-HT in promoting high levels of cell proliferation since fluoxetine blocks the 5-HT transporters, increasing 5-HT levels in the synaptic cleft. However, this positive effects were not mediated by 5-HTR4. <!–![endif]—->