Ultimately, arginine is formed, and its cleavage yields urea and regenerates ornithine. See pages 84-85 and page 93 of textbook for more information on the above. Tyrosine is a precursor to a class of neurotransmitters called catecholamines. Low levels of L-DOPA are associated with Parkinson's disease. Tryptophan is a precursor to serotonin, which has a sedative effect, giving a pleasant feeling.
Milk proteins have high levels of tryptophan.
Very low levels of seratonin are associated with depression, while extremely high levels actually produce a manic state. Manic-depressive schizophrenia can actually be controlled by monitoring the levels of serotonin. Histidine is a precursor to histamine - a small molecule, released by mast cells, that participates in inflammation and other responses.