How schizophrenia and depression disrupt reward circuitry

Altered reward behavior in Parkinson's disease (PD) is supported by observations of a placebo effect, prevalence of addiction to dopamine agonists, incidence of compulsive reward-seeking behaviors, and disturbed affective symptoms in PD patients. However, it is not clear how dopamine neuron loss causes or supports these aberrant reward behaviors and alterations in affect. For example, striatal dopamine transporter loss has a small, significant relationship with depression and anxiety in mild/moderate PD, but not in severe PD. Also, dopamine loss itself does not appear to predict depression or anhedonia, the diminished capacity to experience pleasure. Other neuropsychiatric disorders such as schizophrenia and depression may provide models of disturbed reward biology that may prove useful when thinking about altered reward circuitry and behavior in PD and other neurological disorders.