Dopamine is a neurotransmitter that helps us feel pleasure and stay motivated. A lack of dopamine is also central to the development of Parkinson's Disease and restless leg syndrome. Though oral dosing of dopamine is ineffective---it cannot cross the blood-brain barrier---dopamine building blocks can enter the central nervous system and be converted into dopamine in the brain. In addition, certain pharmaceutical medicines can create a functional increase in dopamine---though this strategy may deplete this important neurotransmitter over the long term.
Levodopa is a compound that the human body is able to convert into dopamine. This process does not just happen in the brain, however. If levodopa is taken by itself, much of it is converted to dopamine outside of the central nervous system, which blunts its therapeutic effect. This dopamine is not able to cross the blood-brain barrier and tends to cause side effects like nausea and vomiting. Some levodopa will cross into the brain and be converted to dopamine, but the amount is unpredictable. Therefore, levodopa is combined with carbidopa---a compound that prevents conversion to dopamine in the periphery. Carbidopa cannot cross the blood brain barrier, so it acts as a kind of escort---or bodyguard---for the levodopa through the body and up to the blood brain barrier, allowing it to cross undisturbed into the brain in order to treat conditions like Parkinson's, restless leg syndrome and depression.
Mucuna pruriens is an herb that happens to contain levodopa in its seeds. In Ayurvedic medicine, it is a traditional treatment for depression. Ingesting a powder of crushed Mucuna seeds does not seem to result in the conversion problems encountered with a pharmaceutical preparation of levodopa. In a report on a study published in the December 2004 "Journal of Neurology Neurosurgery and Psychiatry," mucuna pruriens went head-to-head with levodopa/carbidopa and demonstrated a greater ability to quiet the involuntary tremors of Parkinson's Disease. A comparison was made of 30mg of mucuna with a 100mg dose of levodopa/carbidopa. However, this was only one study, and more research is needed.
Tyrosine is an amino acid that is the foundational building block of dopamine. L-tyrosine is converted into levodopa which is, in turn, converted into dopamine. When tyrosine is taken at levels that are greater than what is possible from diet alone, central nervous system amounts of dopamine increase.
Pharmaceuticals to Increase Dopamine Action
Depression drugs like bupropion and MAO inhibitors---a category of drugs that isn't used very much anymore---act within the brain to reduce the destruction and recycling of dopamine. This strategy gives each dopamine molecule a longer lifespan and increases dopamine-derived nerve transmission. However, according to "Food And Nutrients In Disease Management," there is some concern that by preventing the natural recycling of dopamine without providing building blocks like tyrosine or levodopa, the long-term effect will be a dopamine depletion.