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 prolactin in an amount sufficient to increase the number of neural stem cells; and (c) contacting the neural stem cells from step (b) with at least one additional factor selected from the group consisting of erythropoietin (EPO), pituitary adenylate cyclase activating polypeptide (PACAP), serotonin, bone morphogenetic protein (BMP) and cAMP in an amount sufficient to enhance the formation of neuronal precursor cells or glial precursor cells from the neural stem cells.