Enhancing the activity of the neurotransmitter glutamate may help with some brain disorders because it tends to excite the synapses connected to memory function. Sedatives and tranquilizers are often effective because they enhance the neurotransmitter GABA, which is a strong inhibitor of brain function. Narcotic drugs such as opiates excite the body’s dopamine system, while dysfunction in the dopamine system has been connected to Parkinson’s disease and schizophrenia. Serotonin levels affect a wide array of body functions, including mood, sleep, appetite, and learning.