Sample preparation

At autopsy, the brain weight and post‐mortem delay were recorded. The right hemisphere was fixed in 10% formalin. Following fixation, the hemisphere was cut into 7mm coronal slices then dissected into blocks and embedded in paraffin wax for neuropathological assessment. Paraffin embedded blocks selected for analysis corresponded to striatal and midbrain subregions, including the NAcc, caudate nucleus, anterior and posterior putamen, globus pallidus internus and externus, insula cortex, as well as SN and VTA at the level of the red nucleus. Six micrometer sections were immunostained with primary antibodies to α‐syn, HPT and Aβ following antigen retrieval according to optimized protocols (supplementary Table S3) 99. The midbrain sections were stained with Cresyl fast violet to assess cell density, with the images captured using 63x oil immersion. A modified stereological method was employed to determine the neuronal cell density 67, with values calculated as cells per mm².