Furthermore, amphetamine, a drug that increases synaptic dopamine levels and is associated with increased susceptibility to developing schizophrenia [21–28], increases endogenous GDNF expression in the nigrostriatal tract [21, 24], and a two-fold increase in endogenous GDNF levels is sufficient to increase striatal dopamine synthesis and release in mice [29]. Here, GDNF is linked to schizophrenia.