GDNF and Parkinson disease: Although there is undisputed about its significance of the potential neuroprotective effects of both BDNF and GDNF in preclinical studies and GDNF has been shown in some PD patients to have a positive effect (Gill et al., 2003; Love et al., 2005; Lu et al., 2013), unfortunately owing to large polypeptide structure and consequent poor oral bioavailability of exogenous BDNF, in clinical studies using BDNF as a therapeutic agent have been inconclusive (Thoenen and Sendtner, 2002).