Introduction

Parkinson's disease (PD) is an age-related progressive neurodegenerative disorder that affects approximately 1–2% of the population (see Volume II, Chapter 35). It is characterized by bradykinesia, rigidity, postural instability and tremor (Lang and Lozano, 1998; Olanow et al., 2001). Pathologically the hallmark of PD is degeneration of the substantia nigra pars compacta (SNc) with the loss of midbrain dopaminergic neurons combined with the presence of intraneuronal inclusion (Lewy) bodies. Importantly, degeneration also occurs in nondopaminergic regions including epinephrine neurons of the locus coeruleus, serotonin neurons of the dorsal raphe, cholinergic neurons of the nucleus basalis of Meynert, and nerve cells in the dorsal motor nucleus of the vagus, the pedunculopontine nucleus, and peripheral autonomic system. Despite the involvement of multiple brain regions and multiple transmitter systems, treatment of PD is primarily based on a dopamine replacement strategy. Levodopa is the most widely employed and most effective symptomatic agent. It is converted to dopamine within the brain by an aromatic acid decarboxylase (AADC). Treatment with levodopa is extremely effective in the early stages of the disease, however, chronic levodopa treatment is associated with the development of motor complications (motor fluctuations and dyskinesias) which affect as many as 80% of patients after 5–10 years of treatment (Marsden and Parkes, 1976; Ahlskog and Muenter, 2001; Olanow, 2004). Motor complications can be an important source of disability for many patients who cycle between “on” periods in which they respond to levodopa but have complicating dyskinesia, and “off” periods in which they do not respond to the drug and suffer features of parkinsonism.