Dopamine autoreceptor function is lost in advanced Parkinson's disease

Area of Science:

• Neuroscience
• Radiochemistry
• Pharmacology

Background:

• Parkinson's disease (PD) involves progressive dopaminergic system changes.
• Motor complications arise with disease progression and dopamine agonist treatment.
• Presynaptic and postsynaptic alterations influence dopaminergic efficacy.

Purpose of the Study:

• To investigate the effect of therapeutic apomorphine on L-[11C]dopa uptake in early and advanced Parkinson's disease (PD).
• To assess regional presynaptic dopaminergic function and its regulation by dopamine agonists using L-[11C]dopa and PET.

Main Methods:

• Positron Emission Tomography (PET) with L-[11C]dopa was used to study patients with early and advanced PD.
• Patients underwent two PET scans: one drug-free and one during a standardized apomorphine infusion.

Main Results:

• Apomorphine infusion decreased L-[11C]dopa influx rate in early PD patients, most notably in the dorsal putamen.
• In advanced PD patients, apomorphine did not alter striatal L-[11C]dopa influx rate.
• Subregional striatal analysis revealed differential effects of apomorphine based on PD stage.

Conclusions:

• Mild, stable PD exhibits upregulated presynaptic inhibitory feedback, particularly in the dorsal putamen, maintaining dopaminergic system balance during antiparkinsonian medication.
• This inhibitory feedback mechanism diminishes with nigrostriatal degeneration and chronic dopamine agonist therapy in advanced PD.