BDNF and early-onset autosomal dominant Alzheimer disease: Histamine has a role in neuronal transmission,30 and elevated metabolite levels have been observed in schizophrenia,31 Alzheimer's disease, and PD patients.32 Similar to BDNF, increased histamine is a marker of neuronal damage,32 and our data suggest that 1‐methylhistamine could indeed be an additional marker for neurodegeneration in early‐stage PD.