Use the chemically defined culture medium R12 of Romijn, you can get it now at most farmaceutical firms. Serum based cultures have the disadventage that you never know what kind of stuff has been added during live and what as a consequence is in your serum: impurities! Romijn 1964, Neurosci Biobehav Rev 8:301-334. Don't make it yourself, as we had to do earlier: hell of a job.