HNMT and Parkinson disease: In addition, although no direct evidence indicates any changes in M1 histaminergic system in PD, the decrease of HNMT enzymatic activity observed in some PD patients suggest that a decreased HA degradation might occur leading to alterations of M1 modulation by HA in PD patients (Agúndez et al., 2008; Palada et al., 2012).