1. A method for producing neuronal precursor cells or glial precursor cells, comprising: (a) providing at least one neural stem cell; (b) contacting the neural stem cell with a factor selected from the group consisting of prolactin, growth hormone, estrogen, ciliary neurotrophic factor (CNTF), fibroblast growth factor (FGF), transforming growth factor alpha (TGF&agr;) and epidermal growth factor (EGF) in an amount sufficient to increase the number of neural stem cells; and (c) contacting the neural stem cells from step (b) to a factor selected from the group consisting of erythropoietin (EPO), pituitary adenylate cyclase activating polypeptide (PACAP), prolactin, serotonin, bone morphogenetic protein (BMP) and cAMP in an amount sufficient to enhance the production of neuronal precursor cells or glial precursor cells from the neural stem cells; with the proviso that when the factor in step (b) is EGF or FGF, the factor in step (c) is PACAP or prolactin.