Non-motor symptoms in Parkinson’s disease : modeling and mechanisms

Abstract

Parkinson’s disease (PD) is a common neurodegenerative disorder, typically characterized by the progressive death of midbrain dopaminergic neurons projecting from the substantia nigra to other areas within the basal ganglia. Historically, PD has been diagnosed as a purely motor disorder dominated by bradykinesia (slowness of movement), rigidity, resting tremor, and postural instability. While there are no approved disease-modifying therapies, these symptoms can be counteracted by dopamine replacement therapies based on the use of LDopa and dopamine receptor agonists.

In the past decades, the classic view of PD as an exclusively motor disease has been challenged by increasing evidence showing that patients display a wide range of non-motor symptoms (NMS), including hyposmia, sleep disturbances, cognitive impairment, depression and anxiety. These ailments often appear in the early, pre-motor stage of the disease and progressively worsen, significantly affecting the patient’s quality of life. The neuropathological mechanisms underlying NMS are still poorly understood, but clinical findings have consistently shown the involvement of both the dopaminergic and nondopaminergic systems. Interestingly, these symptoms only partially respond to dopaminergic treatments used to handle motor deficits, complicating the pharmacological management of patients. The increasing demand for more effective therapies for NMS indicates the importance of developing translational approaches based on the use of appropriate animal models.

In this thesis, we developed and validated a mouse model of PD for the study of NMS. This model is characterized by a bilateral partial degeneration of the dopaminergic system achieved through intra-striatal injection of 6-hydroxydopamine (6-OHDA). We found that this model presents only subtle gait modifications reminiscent of early stage PD. Most importantly, we showed that the 6-OHDA lesion impairs olfactory discrimination, disrupts circadian rhythm, causes long-term memory deficits, as well as depression- and anxiety-like behaviors. Using a combination of anatomical and pharmacological approaches, we validated this model for the study of NMS and described the effects produced on these deficits by administration of dopaminergic and non-dopaminergic drugs.

Altogether, these studies provide a well-characterized tool for the study of NMS in PD, as well as information on molecular and neural targets implicated, thereby opening new vistas for the design of broader therapeutic interventions.