L-dopa is an amino acid that is the precursor of dopamine, (it also helps enhance norepinephrine), the powerful brain neurotransmitters that are linked with drive, libido, memory and attention, and of course Dopamine is the neurotransmitter most affected in Parkinson's disease, which is why Sinemet is a front line treatment for the same.
L-dopa is the only way that "Dopamine" can be delivered through the blood-brain-barrier, and it was developed in the 1970s and was even made famous in the Hollywood film, The Sleepers, that starred Robin Williams in an early acting role.
Apart from the treatment of Parkinson's disease, and the ability of L-dopa to increase brain Dopamine levels, perhaps the most common, "off-label" use of L-dopa is to increase Growth Hormone (GH) levels. To this end, L-dopa is a powerful agonist, one study was noted in the book; Life Extension, when L-dopa at 500mg a day in healthy men aged 60 years, increased their GH levels to the levels of men nearer the biological age of 30 years.
L-Dopa is often combined with dopa-decarboxylase inhibitors such as carbidopa or benserazide, with these formulations known as 'Co-Careldopa' and 'Co-Beneldopa' respectively. This reduces peripheral side effects arising from dopamine formation in the periphery (since dopa-decarboxylase is stored in all adrenergic nerve endings) Dopa-decarboxylase inhibitors cannot cross the BBB and thus allow for reduced doses of L-dopa and lessen the side effects resulting from excess peripheral dopamine such as nausea, vomiting and arrhythmias. There is also a quicker onset of action and a more stable clinical response.
L-Dopa effectively replenishes striatal dopamine levels and is the most effective treatment for alleviating the symptoms of PD. However, response among patients varies and it is more effective at alleviating bradykinesia and rigidity than tremor. Treatment is usually initiated with low doses and gradual small increments result in a compromise between the most effective dose and minimal side effects for each patient. A gradual improvement is usually seen for six to eighteen months of early PD treatment and this optimum control of symptoms is maintained for up to two years, followed by a gradual decline. L-dopa is able to cross into the brain, and there is transformed to dopamine (decarboxylation), which is an active neurotransmitter. L-dopa is often combined with a decarboxylase inhibitor as in Sinemet or Madopar. This inhibitor (which cannot cross into the brain) causes higher levels of L-dopa to be available to cross into the brain, and also reduces some of the side effects outside the brain of dopamine.