Secreted and intracellular GDNF in primary ventral midbrain cultures transfected with pGDNF plasmids. Ventral midbrain cultures were established as described in the Methods section and transfected with one of the following plasmids: pGDNF_1a (1a), pGDNF_1b (1b), pGDNF_2a (2a), pGDNF_2b (2b), pGDNF_3a (3a), or pGDNF_3b (3b). Each bar represents the average (± s.e.m.; n=5) amount of GDNF protein detected in cells or secreted in the culture media. *p<0.05, 1b vs. 1a, 2a, 3a, 2b, 3b; ^p<0.05 2b vs. 3a, 2a; #p<0.05 1b vs 2a, 3a, 3b.