- In Alzheimer's disease, the primary neurotransmitter that is decreased and leads to cognitive dysfunction is acetylcholine.
- This decrease occurs because of the degeneration of cholinergic neurons in the basal forebrain, particularly in the nucleus basalis of Meynert.
- Acetylcholine plays a critical role in learning, memory, and attention, which are commonly impaired in Alzheimer's patients.

- While other neurotransmitters like dopamine, norepinephrine, and serotonin may also be affected to some extent, the hallmark neurotransmitter deficit in Alzheimer's disease is the loss of acetylcholine.
- This understanding has led to the development of therapeutic agents, such as acetylcholinesterase inhibitors, aimed at increasing acetylcholine levels in the brain to improve cognitive symptoms.

Reference: Adams and Victor's Principles of Neurology, 11th Edition, Chapter 32 – Alzheimer's Disease / Page 482