GDNF and sleep disorder: PD patients with sleep disorders were found to have higher 24-item Hamilton Depression Scale (HAMD), 14-item Hamilton Anxiety Scale (HAMA), Epworth Sleepiness Scale (ESS), Movement Disorder Society-Unified Parkinson’s Disease Rating Scale III (MDS-UPDRS III) and Hoehn-Yahr (H&Y) staging scores, accompanied by lower levels of adenosine, GDNF, and GABA (Wang et al., 2023).