HTR1A and Parkinson disease: The map of parkinsonism-related grey matter changes was significantly correlated with dopaminergic, serotonergic and opioid neurotransmitter systems (D1: rs = 0.391, P = 0.001; 5-HT1A: rs = 0.319, P = 0.009; 5-HT2A: rs = 0.380, P = 0.002; μ-opioid: rs = 0.343, P = 0.005; Fig. 2B).