Progression of monoaminergic dysfunction in Parkinson's disease: A longitudinal 18F-dopa PET study

Post-mortem and neuroimaging studies in Parkinson's disease (PD) have shown involvement of the brain serotoninergic, noradrenergic and cholinergic pathways alongside the characteristic degeneration of nigrostriatal dopamine neurons. The rate of progression of the degenerative process in these extrastriatal areas is still unclear. We used (18)F-dopa PET, a marker of aromatic aminoacid decarboxylase activity in monoaminergic neurons, to assess longitudinal changes in tracer uptake in brain noradrenergic, serotoninergic and extrastriatal dopaminergic structures over a 3-year period in a group of early PD patients. Ten PD patients had (18)F-dopa PET twice: at baseline and again after 37.1±21.5 months follow up. A standard object map was used to extract tracer influx constants (Ki) in 11 striatal and extrastriatal regions. Progressive decreases in (18)F-dopa Ki occurred over the follow-up period in the majority of the investigated areas, the fastest annual declines occurring in putamen (8.1%), locus coeruleus (7.8%), and globus pallidus interna (7.7%). Caudate and hypothalamus showed 6.3% and 6.1% annual Ki declines, respectively. At baseline, some structures showed increased levels of (18)F-dopa uptake in PD compared to controls (internal pallidum, locus coeruleus), indicating possible compensatory upregulation of monoamine turnover. These increased levels had normalised (globus pallidus interna) or become subnormal (locus coeruleus) at follow-up suggesting exhaustion of these mechanisms within the first years of disease. Loss of monoaminergic function in extrastriatal regions, as reflected by(18)F-dopa PET, is delayed and occurs independently from nigrostriatal degeneration. When assessing the efficacy of novel neuroprotective agents on nigrostriatal dysfunction in PD, (18)F-dopa PET could provide supplementary information concerning function of extrastriatal monoaminergic structures.