<P> L - DOPA, a precursor of dopamine that crosses the blood--brain barrier, is used in the treatment of Parkinson's disease . For depressed patients where low activity of the neurotransmitter norepinephrine is implicated, there is only little evidence for benefit of neurotransmitter precursor administration . L - phenylalanine and L - tyrosine are both precursors for dopamine, norepinephrine, and epinephrine . These conversions require vitamin B6, vitamin C, and S - adenosylmethionine . A few studies suggest potential antidepressant effects of L - phenylalanine and L - tyrosine, but there is much room for further research in this area . </P> <P> Administration of L - tryptophan, a precursor for serotonin, is seen to double the production of serotonin in the brain . It is significantly more effective than a placebo in the treatment of mild and moderate depression . This conversion requires vitamin C. 5 - hydroxytryptophan (5 - HTP), also a precursor for serotonin, is more effective than a placebo . </P> <P> Diseases and disorders may also affect specific neurotransmitter systems . For example, problems in producing dopamine can result in Parkinson's disease, a disorder that affects a person's ability to move as they want to, resulting in stiffness, tremors or shaking, and other symptoms . Some studies suggest that having too little or too much dopamine or problems using dopamine in the thinking and feeling regions of the brain may play a role in disorders like schizophrenia or attention deficit hyperactivity disorder (ADHD). Similarly, after some research suggested that drugs that block the recycling, or reuptake, of serotonin seemed to help some people diagnosed with depression, it was theorized that people with depression might have lower - than - normal serotonin levels . Though widely popularized, this theory was not borne out in subsequent research . Furthermore, problems with producing or using glutamate have been suggestively and tentatively linked to many mental disorders, including autism, obsessive compulsive disorder (OCD), schizophrenia, and depression . </P> <P> Generally, there are no scientifically established "norms" for appropriate levels or "balances" of different neurotransmitters . It is in most cases pragmatically impossible to even measure levels of neurotransmitters in a brain or body at any distinct moments in time . Neurotransmitters regulate each other's release, and weak consistent imbalances in this mutual regulation were linked to temperament in healthy people . Strong imbalances or disruptions to neurotransmitter systems have been associated with many diseases and mental disorders . These include Parkinson's, depression, insomnia, Attention Deficit Hyperactivity Disorder (ADHD), anxiety, memory loss, dramatic changes in weight and addictions . Chronic physical or emotional stress can be a contributor to neurotransmitter system changes . Genetics also plays a role in neurotransmitter activities . Apart from recreational use, medications that directly and indirectly interact one or more transmitter or its receptor are commonly prescribed for psychiatric and psychological issues . Notably, drugs interacting with serotonin and norepinephrine are prescribed to patients with problems such as depression and anxiety--though the notion that there is much solid medical evidence to support such interventions has been widely criticized . </P>

Which of the following is an inhibitory neurotransmitter in the cns