TH and Parkinson disease: BDNF signaling increases DA release, TH synthesis, DA turnover, and DA neuron activity, all of which are decreased in PD (Knusel et al., 1991; Beck et al., 1993; Altar et al., 1994; Hyman et al., 1994; Shen et al., 1994; Zhou et al., 1994; Blöchl and Sirrenberg, 1996; Benskey et al., 2016).