Common factors among Alzheimer's disease, Parkinson's disease, and epilepsy: possible role of the noradrenergic nervous system.

The neurodegenerative disorders Alzheimer's disease (AD) and Parkinson's disease (PD) share in common the neuropathologic loss of locus coeruleus (LC) noradrenergic neurons. In addition, these two neurodegenerative disorders share two symptoms that define these disorders: cognitive impairment and depression. The hippocampus is a region that is known to play a role in cognition and depression, and the hippocampus receives sole noradrenergic innervation from LC neurons. However, it is unclear how the loss of LC noradrenergic neurons contributes to these common symptoms in these two disorders. Epilepsy is not considered a neurodegenerative disorder, but the hippocampus is severely affected in temporal lobe epilepsy. Of interest, cognitive impairment and depression are also common comorbid disorders in temporal lobe epilepsy. This article describes common symptoms among these three neurologic disorders and a possible role of the noradrenergic nervous system.