We routinely culture (embryonic) primary rat/mouse motor neurons using our protocol described in "Ivermectin inhibits AMPA receptor-mediated excitotoxicity in cultured motor neurons and extends the life span of a transgenic mouse model of amyotrophic lateral sclerosis." Andries M, Van Damme P, Robberecht W, Van Den Bosch L. Neurobiol Dis. 2007 Jan;25(1):8-16. Not easy, but results (after (5-7 days) in nicely differentiated primary motor neurons. Also works without the astrocytic feeder layer. If interested, I can send you a detailed protocol.