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L-DOPA kills dopamine neurones in culture but is the most effective drug for the treatment of Parkinson's disease, where it exhibits no clear toxicity. While glial cells surround and protect neurones in vivo, neurones are usually cultured in vitro in the absence of glia. We treated fetal midbrain rat neurones with L-DOPA, mesencephalic glia conditioned medium (CM) and L-DOPA + CM. L-DOPA reduced the number of tyrosine hydroxylase-positive (TH+) cells and [3H]DA uptake, and increased quinone levels. L-DOPA + CM restored [3H]DA uptake and quinone levels to normal, and increased the number of TH+ cells and terminals to 170% of control. CM greatly increased the number of TH+ cells and [3H]DA uptake. Mesencephalic glia therefore produced soluble factors which are neurotrophic for dopamine neurones, and which protect these neurones from the toxic effects of l.-DOPA.