Role of Sleep Homeostasis in the Development of the L-Dopa Induced Dyskinesias in Patients With Parkinson's Disease

Wakefulness in PD is frequently interrupted by sleep episodes, even rapid eye movement sleep (REM), whilst nocturnal sleep is disturbed by abnormal motor activity interference, such as REM sleep behaviour disorder (RBD) or periodic limb movements (PLMS). Several lines of evidence have suggested a close relation between sleep-wake changes and dopaminergic degeneration in PD(1-7) while sleep disorders commonly precede the clinical motor onset by many years (8,9). On the other hand, morning motor performance improvement is frequently mentioned by patients, mainly in those with long disease duration and motor fluctuation (10,11). Of note, the same state of vigilance can profoundly influence some clinical PD features (for instance rigidity) as well as basal ganglia neuronal activity(12).

Few reports have shown changes in sleep architecture: a reduction of the amount of SWS in parallel with disease duration (13-15) has been observed, but no study, so far, has addressed how sleep may impact on LID. Once these abnormal movements develop, they are difficult to treat and negatively affect the quality of life and the treatment costs of PD patients (16,17). Although levodopa represents, so far, the more effective treatment for PD patients (18,19) to ameliorate the cardinal signs such as bradykinesia/akinesia and rigidity (20), as the disease progresses, these benefits are in some measure abolished by the emergence of dyskinesia (21). During the early stages PD patients experience a rather satisfying quality of life that is impeded in the advanced stages by the emerging of these involuntary movements frequently at the peak of the levodopa effect (16,22,23). In other words, when the patients experience these motor complications (shortening motor response and development of dyskinesia) the delivery of levodopa without inducing dyskinesia becomes increasingly difficult (21,24-27).

Several efforts have been made in order to find "pure" anti-dyskinetic drugs that are able to uncouple the anti-akinetic effect from the dyskinetic response. Serotoninergic receptor agonists were claimed to be drugs against dyskinesia (28), however, it was observed that they may also impair levodopa efficacy (29-31). As of today, no clinical effective therapies are able to alleviate dyskinesia without worsening parkinsonism.

Great efforts have been made to clarify LID pathogenesis, emphasizing the role of pulsative stimulation of striatal receptors by dopaminergic treatment (25) and more recently, on molecular changes of postsynaptic (32,33) or presynaptic mechanisms (34). Although no conclusive results on LID pathogenesis have been achieved, indeed disease duration (i.e. the degree of dopaminergic degeneration) rather than long-term use of levodopa, seems to play a crucial role (22,23). This notion can be inferred from clinical practice, but is also evident in MPTP-induced parkinsonism in humans in which the extended dopaminergic lesion caused within a few days the development of dyskinesias undistinguishable from those in the idiopathic form (35).

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