Since its introduction in the late 1960s l-dopa or 'levodopa' (see Figure 1) has been generally accepted as the most effective treatment for the alleviation of the symptoms associated with PD. The effective chemical species required is dopamine, but being charged it cannot cross the membrane barrier: dopa is uncharged and can cross into the cells where it is decarboxylated to yield therequired dopamine. However, not only is it unpleasant to take the large doses required, but it also gives rise to serious side effects (although these can be minimized by adding a blocker for decarboxylase and using the l-form of dopa exclusively).
With reference to the cycle of events that may lead to the progression of PD (Figure 9), it can be shown that a continual increase in the dopamine content of the cells will result in ever increasing yield of the neurotoxin 6-OHDA. This has been confirmed by an examination of the urine of patients . Thus, it is not surprising that levodopa treatment usually becomes inefficient after a few years and indeed assists in the advancement of the disease and accelerates the death of the patient. But it must not be forgotten that this therapy gives an extremely important improvement to the quality of life.
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