Ultrastructural confirmation of neuronal protection by melatonin against the neurotoxin 6-hydroxydopamine cell damage.

6-Hydroxydopamine (6-OHDA) is a neurotoxin used in the induction of experimental Parkinson's disease in both animals and cultured neuronal cells. Biochemical and molecular approaches showed previously that low doses of 6-OHDA induced apoptosis in PC12 cells, while high doses of this neurotoxin induced necrosis. Melatonin has been shown to protect against the neuronal programmed cell death induced by 6-OHDA, although it was not able to prevent the massive necrotic cellular death occurring after the addition of high doses of the neurotoxin. In the present work, we demonstrate by ultrastructural analysis that although low doses of 6-OHDA induced apoptosis in PC12 cells, it also damaged the non-apoptotic cells, morphologically corresponding this damage to incipient and reversible necrotic lesions. When the doses of the neurotoxin increase, there are still apoptotic cells, although most of the cells show necrotic irreversible lesions. We also found that melatonin partially prevents the incipient necrotic lesions caused by low doses of 6-OHDA. The fact that melatonin was shown in previous work to prevent apoptosis caused by low doses of 6-OHDA, but not necrosis induced by high doses of the neurotoxin, seemed to indicate that this agent is only able to protect against apoptosis. However, our present results, melatonin preventing also the incipient necrotic neuronal lesions, suggest that this hormone may provide a general protection against cell death, suggesting that higher doses should be tried in order to prevent the necrotic cell death induced by high doses of the neurotoxin.