Pathology of behavior in PD: What is known and what is not?

•Abnormal behaviors in PD manifest as an outcome of multi-level system dysfunction.•Many neurotransmitter pathways are disrupted in PD causing global network dysfunction.•A greater pathological understanding of PD behavior is needed to improve treatment.

Abnormal behavior in Parkinson's disease (PD) stems from a complex orchestration of impaired neural networks that result from PD-related neurodegeneration across multiple levels. Typically, cellular and tissue abnormalities generate neurochemical changes and disrupt specific regions of the brain, in turn creating impaired neural circuits and dysfunctional global networks. The objective of this chapter is to provide an overview of the array of pathological changes that have been linked to different behavioral symptoms of PD such as depression, anxiety, apathy, fatigue, impulse control disorders, psychosis, sleep disorders and dementia.