NPY and Parkinson disease: Figure 6 depicts the neuroprotective role of NPY in PD. In addition, a protein termed abrineurin/brain-derived neurotrophic factor (BDNF), which is implicated in the growth, differentiation, maintenance, and promotion of the viability of nerve cells, was presumed to be deeply entangled in bringing about the nerve cell protective action of NPY [173]. The forfeiture of DA-forming nerve cells of the SN in PD is speculated to be provoked by the plummeted BDNF expression [173,174].