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l-DOPA, also known as l-3,4-dihydroxyphenylalanine and used medically as levodopa, is made and used as part of the normal biology of some plants [2] and animals, including humans. Humans, as well as a portion of the other animals that utilize l -DOPA, make it via biosynthesis from the amino acid l -tyrosine .
Because levodopa bypasses the enzyme tyrosine hydroxylase, the rate-limiting step in dopamine synthesis, it is much more readily converted to dopamine than tyrosine, which is normally the natural precursor for dopamine production. In humans, conversion of levodopa to dopamine does not only occur within the central nervous system.
In normal dopamine and serotonin (5-HT) neurotransmitter synthesis, AADC is not the rate-limiting step in either reaction. However, AADC becomes the rate-limiting step of dopamine synthesis in patients treated with L -DOPA (such as in Parkinson's disease ), and the rate-limiting step of serotonin synthesis in people treated with 5-HTP (such as ...
L-DOPA (Levodopa), another precursor, is used in the treatment of Parkinson's disease. Prodrugs of levodopa, including melevodopa, etilevodopa, foslevodopa, and XP-21279 also exist. They are inactive themselves but are converted into dopamine and hence act as non-selective dopamine receptor agonists.
Peripherally selective DDCIs incapable of crossing the protective blood–brain barrier (BBB) are used in augmentation of L-DOPA (levodopa) in the treatment of Parkinson's disease (PD) to block the conversion of L-DOPA into dopamine outside the brain, for the purpose of reducing adverse side effects. [3]
In animals, tolcapone by itself does not increase dopamine levels in the striatum, nucleus accumbens, or frontal cortex, but does augment brain L-DOPA levels when combined with levodopa and benserazide. [11] There may be compensatory activation of the monoamine oxidase dopamine-metabolizing pathway with brain COMT inhibition. [11]
If methamphetamine is administered while cytoplasmic dopamine is depleted to about 50% of the control levels, its neurotoxic effects are averted (Thomas et al., 2008). The recovery of dopamine to normal levels after AMPT administration takes about 2 to 7 days, and this repletion of dopamine is not changed by methamphetamine. [ 18 ]
Levodopa is a precursor to the neurotransmitter dopamine, which is administered to increase its levels in the central nervous system.However, most levodopa is decarboxylated to dopamine before it reaches the brain, and since dopamine is unable to cross the blood–brain barrier, this translates to little therapeutic gain with strong peripheral side effects.