Common Noradrenergic Mechanisms in Parkinson´s Disease and L-DOPA Induced Dyskinesia

Introduction: The major source of NE in the central nervous system (CNS) is the locus coeruleus (LC), which sends projections to virtually all parts of the CNS, integrating cognitive and autonomic functions with the state of arousal (Sara SJ 2009).

With the onset of pathogenesis of PD in the lower brainstem, early symptoms of PD that include sleep disorder, cognitive deficits, and autonomic dysfunction, may appear prior to the motor symptoms of PD, and these symptoms have been linked to the degeneration of the LC and subsequent loss of noradrenergic innervations in the peripheral and central nervous systems (Rascol et al., 2009, Hawkes et al., 2010, Braak et al., 2003).

The loss of neurons in the LC exceeds that of the substantia nigra in some studies (Zarow et al. 2003), in which it is also argued that the loss may be related to the symptoms of endogenous depression in 40% of patients with PD. In these patients, the severity of motor symptoms also may be aggravated by loss of NE as release of dopamine (DA) and firing of dopaminergic neurons normally are both facilitated by activation of noradrenergic neurons.

Lesion of the LC and the subsequent loss of this facilitation reduces nigrostriatal DA release. Similarly, lesions to both dopaminergic and noradrenergic neurons induce more severe motor deficits, compared to lesions of dopaminergic neurons alone (Mavridis M et al., 1999). This relation suggests direct and indirect roles for NE in the emergence and severity of both motor and non-motor symptoms in PD, including depression.

The effects of NE are mediated by stimulation of the three receptor subtypes alpha-1, alpha-2, and ß. In the proposed studies, the investigators focus on alpha-2 and ß-receptors. Of the alpha-2 receptors, the alpha2C subtype is densely expressed in structures of the basal ganglia and therefore may mediate the direct effects of alpha-2 receptors on motor behavior.

However, in contrast to the possible beneficial effects of NE receptor activation in some brain regions, non-physiological increases of NE, after L- DOPA administration, may elicit dyskinesia (Buck et al., 2010) while fluctuations of NE concentration in other regions may contribute to generation of symptoms of depression (Zarow et al. 2003). Thus, several lines of evidence suggest that loss of NE may have direct and indirect roles in the appearance and severity of both motor and non-motor symptoms of PD and in LIDs.

Background:

Noradrenergic treatment of PD: The initial relief from symptoms of PD offered by DA agonist therapy is complicated by the manifestation of motor fluctuations, dyskinesia and psychiatric side effects following chronic treatment (Ahlskog et al., 2001, Fox et al., 2008).

The efficacy of conventional DA agonist therapy to reduce motor symptoms in PD is related to its ability to restore lost dopaminergic innervations. However, recent evidence suggests that activation of non- dopaminergic transmitter systems, including the noradrenergic system may play an important role in mediating the anti-parkinsonian effects of L-DOPA. L-DOPA and DA are sequential precursors of NE, and excitation of noradrenergic receptors following L-DOPA administration may contribute to the anti-parkinsonian effects of L-DOPA ( Schapira et al., 2008). Non-physiologically high release of NE derived from exogenous L-DOPA derived NE may contribute to both co-morbid depression and LID.

It further suggests that therapies that maintain L-DOPA- induced activation of NE receptors at physiological levels would reduce the severity of LID in patients. However, the underlying mechanisms of possible anti-parkinsonian and dyskinetogenic roles of NE remain unresolved. In this project, the investigators propose to identify the mechanisms through which NE conveys both beneficial and adverse effects of L-DOPA in a concerted attempt to help improve current treatment of PD by suggesting therapies that target the non-physiological L- DOPA-induced activation of the NE receptors as a potential contributor to LID.

The investigators developed and validated a novel PET tracer to be used in this project. Carbon-11 labelled yohimbine is an alpha-2 adrenoceptor antagonist and have been validated in studies with PET in pigs (Jakobsen et al., 2006, Landau et al. 2012) and approved for human use. To show that binding of [11C]yohimbine is sensitive to endogenously released NE, the investigators determined the binding before and after Vagus Nerve Stimulation in minipigs in vivo.

This group of researchers also developed the selective NET ligand, [11C]MeNER, for clinical PET studies in Denmark. Patients with PD and age-matched healthy controls will undergo PET-scans with the above-mentioned tracers to map pathological changes in noradrenergic transporters and receptors in-vivo.