What causes depleted neurotransmitters in the brain?

Neurotransmitters are protein based agents with a temporary lifespan (seconds) before they age out/are broken down. Depletion implies that they are not being created by the system at normal rates or being used up.

Changes in neurotransmitter levels are characteristic of several diseases. Both Alzheimer's and Parkinson's diseases are considered Multi-neurotransmitter deficiency disorders, with the main problem with the former involving acetylcholine, and the latter, dopamine. Both show striking loss of norepinephrine in the locus ceruleus.