Effect of apomorphine on cortical inhibition in Parkinson's disease patients: a transcranial magnetic stimulation study

In this study, transcranial magnetic stimulation (TMS) of the primary motor hand area was used to test cortical excitability in Parkinson's disease (P D) patients. Motor evoked potentials (MEPs) to TMS were studied at rest by utilising distinct paired-pulse TMS protocols. Out of 29 untreated PD patie nts and 29 healthy subjects, early cortical inhibition (1-6 ms) was studied in a first subgroup of 17 PD patients and 15 healthy subjects, whereas lat e cortical inhibition (20-200 ms) was studied in a second subgroup of 21 PD patients and 19 healthy subjects. In all PD patients the same TMS protocol s were performed before and after 3 h of apomorphine infusion. In compariso n to healthy subjects, untreated PD patients showed a significant reduction of both early and late cortical inhibition, which was maximal at 2-3 ms, a nd at 80-100 ms, respectively. Apomorphine administration consistently reve rsed all the MEP abnormalities found in PD patients. The lack of TMS effect s on the Hoffman's reflex (HR), at those intervals revealing the reduced in hibition in PD patients, is compatible with a supraspinal origin of the obs erved MEP abnormalities. Our data suggest that the cortical and/or subcorti cal loss of dopaminergic transmission in PD patients is associated with imp aired motor cortical inhibitory mechanisms, as tested by a decreased early and late MEP inhibition.