Final answer:
Dopamine is the neurotransmitter most closely associated with both Parkinson's disease and schizophrenia. In Parkinson's disease, there is a decrease in dopamine levels, while in schizophrenia, there is an overactivity of dopamine neurotransmission.
Step-by-step explanation:
The neurotransmitter most closely associated with both Parkinson's disease and schizophrenia is dopamine. In Parkinson's disease, there is a loss of dopaminergic neurons in the substantia nigra pars compacta, resulting in a decrease in dopamine levels. This leads to the motor symptoms seen in Parkinson's disease. In schizophrenia, there is an overactivity of dopamine neurotransmission, which is thought to contribute to the positive symptoms of the disorder.
Learn more about Neurotransmitter