Neurotrophic and neurotoxic effects of nitric oxide on fetal midbrain cultures

There is evidence suggesting that nitric oxide (NO) may play an important role in dopamine (DA) cell death. Thus, the aim of this study was to investigate the effects of NO on apoptosis and functionality of DA neurones and glial cells. The experiments were carried out in neuronal‐enriched midbrain cultures treated with the NO donor diethylamine–nitric oxide complexed sodium (DEA–NO). DEA–NO, at doses of 25 and 50 µm, exerted neurotrophic effects on dopamine cells, increasing the number of tyrosine hydroxylase positive (TH+) cells, TH+ neurite processes, DA levels and [3H]DA uptake. A dose of 25 µm DEA–NO protected DA cells from apoptosis. In addition, it induced de novo TH synthesis and increased intracellular reduced glutathione (GSH) levels, indicating a possible neuroprotective role for GSH. However, in doses ranging from 200 to 400 µm, DEA–NO decreased TH+ cells, DA levels, [3H]DA uptake and the number of mature oligodendrocytes (O1+ cells). No changes in either the amount or morphology of astrocytes and glial progenitors were detected. A dose‐ and time‐dependent increase in apoptotic cells in the DEA–NO‐treated culture was also observed, with a concomitant increase in the proapoptotic Bax protein levels and a reduction in the ratio between Bcl‐xL and Bcl‐xS proteins. In addition, DEA–NO induced a dose‐ and time‐dependent increase in necrotic cells. 1H‐[1,2,4]oxadiazolo[4,3a]quinoxaline‐1‐one (ODQ, 0.5 µm), a selective guanylate cyclase inhibitor, did not revert the NO‐induced effect on [3H]DA uptake. Glia‐conditioned medium, obtained from fetal midbrain astrocyte cultures, totally protected neuronal‐enriched midbrain cultures from NO‐induced apoptosis and rescued [3H]DA uptake and TH+ cell number. In conclusion, our results show that low NO concentrations have neurotrophic effects on DA cells via a cGMP‐independent mechanism that may implicate up‐regulation of GSH. On the other hand, higher levels of NO induce cell death in both dopamine neurones and mature oligodendrocytes that is totally reverted by soluble factors released from glia.