<qa>  What is the biological explanation for depression?However, the disease affects also other, non motor functions and may cause other disorders, such as depression. Depression may be a reaction to the disability caused by the disease, but many studies show that depression is more common in PD than in other chronic debilitating illnesses. Moreover, there is also a biological explanation for the phenomenon: dopamine is also used in brain circuits involved in the experience of pleasure, and loss of pleasure in daily physical or social activity is one of the key manifestations of depression. 