Neuroprotection by dopamine agonists.

Research on Parkinson's disease has led to new hypotheses concerning the mechanisms of neurodegeneration and to the development of neuroprotective agents. Recent findings of impaired mitochondrial function, altered iron metabolism and increased lipid peroxidation in the substantia nigra of parkinsonian patients emphasize the significance of oxidative stress and free radical formation in the pathogenesis of Parkinson's disease. Present research is therefore focussing on improvements in neuroprotective therapy to prevent or slow the rate of progression of the disease. Possible neuroprotective strategies include free radical scavengers, monoamine oxidase-B inhibitors, iron chelators and glutamate antagonists. Recent studies point to the possibility of achieving neuroprotection in ageing and parkinsonism by the administration of dopamine agonists. In the rat, the dopamine agonist pergolide appears to preserve the integrity of nigrostriatal neurones with ageing. The prevention of age-related degeneration may be achieved as a result of a decreased dopamine turnover and reduced conversion of dopamine to toxic compounds. In our own study, bromocriptine treatment prevented the striatal dopamine reduction following MPTP administration in the mouse. These results suggest that the neurotoxic effects of MPTP can be prevented by bromocriptine. Monotherapy with the dopamine agonist lisuride in the early stages of Parkinson's disease delays the need for the initiation of levodopa treatment to a similar extent as has been reported for L-deprenyl. It remains to be shown whether this is due to neuroprotective efficacy of the dopamine agonist or to a direct symptomatic effect.