Midbrain SERT in degenerative parkinsonisms: a 123I-FP-CIT SPECT study

SPECT imaging is widely used for the differential diagnosis of degenerative parkinsonisms by exploiting the high affinitiy of the radiotracer (123)I-FP-CIT for the dopamine transporter. Reduced levels of DAT are found in Parkinson Disease (PD), Dementia with Lewy Bodies (DLB), and Progressive Supranuclear Palsy (PSP) compared to in Essential Tremor (ET) and Healthy Controls (HC). However, the extent of the neurodegenerative process may extend beyond nigrostriatal system. We have exploited the affinity of the same radiotracer (123)I-FP-CIT for the serotonin transporter to investigate SERT levels in the midbrain of patients with PD, DLB, PSP, and ET compared to HC. Using MRI images as anatomical templates for midbrain uptake quantification, we found a mild decrease in SERT levels in PD compared to ET and HC, with marked inter-individual variability; on the other side, PSP and DLB patients displayed markedly reduced to undetectable levels of SERT, respectively. These findings show that the neurodegenerative process affects serotoninergic neurons in parkinsonisms, with much more severe involvement in DLB than in PD patients, despite the comparable loss of striatal DAT. SERT-dependent (123)I-FP-CIT uptake may allow a more comprehensive assessment of neurochemical disturbances in degenerative parkinsonisms and may have a value for differential diagnosis.