METH treatment resulted in a significant reduction in striatal tissue levels of DA and 5-HT that did not differ between genotypes at 3 months of age (*p < 0.05). A, B, D, Saline (SAL)-treated GDNF+/− mice had higher tissue levels of DA (A) and DOPAC (B) than WT mice (p < 0.05), but there was no difference in 5-HT (D) tissue levels. METH treatment caused a significant reduction in DA, DOPAC, and 5-HT tissue levels in WT and GDNF+/− mice (p < 0.05). C, The DA/DOPAC ratio was not different between the genotypes treated with saline, and the ratio was reduced similarly in both genotypes by METH treatment (p < 0.05). D, 5-HT tissue levels were similar in saline-treated GDNF+/− and WT mice and were decreased to the same extent by METH (*p = 0.05; ***p = 0.001). n = 5 per group. Error bars indicate SEM.