c-fos and tyrosine hydroxylase expression after an excitotoxic lesion on the nigrostriatal system: a study on the effects of hypoxia used as a preconditioning stimulus

Hypoxia-ischemia during the perinatal period causes excitotoxic lesions in sensitive brain areas, such as the striatum. The impact of hypoxia-ischemia on nigral neurons is less well known. Hypoxia alone, a less traumatic event without overt histological sequelae, has neuroprotective properties when used as a preconditioning stimulus. In some pathologies, injured neurons of the nigrostriatal system in the adult may be the result of neurodegenerative processes that originated at early stages of life. The effects of hypoxia on the immunoreactivity to tyrosine hydroxylase of the dopaminergic neurons of the substantia nigra pars compacta and the effects of a period of hypoxia previous to an excitotoxic lesion were examined by means of histological and Western blot methods, at immediate and late periods of the episode. By counting the number of tyrosine hydroxylase-stained neurons and c-fos-positive nuclei a short period after injection of quinolinic acid into the striatum, we observed that hypoxia induced a more marked decrease in the number of tyrosine hydroxylase-stained neurons. On the contrary, c-fos-positive profiles decreased in the substantia nigra pars reticulata of the quinolinic acid-injected animals after the preconditioning hypoxia. Hypoxia alone did not affect the number of tyrosine hydroxylase-positive neurons in the pars compacta nor did hypoxia induce c-fos expression in the pars reticulata. More sensitive Western blot analysis of tissue blocks that included the whole substantia nigra demonstrated the same trend as the immunohistochemical results. We conclude that the responses of the substantia nigra neurons to hypoxia are regionalized and potential neuroprotective effects may depend on the vulnerability of each neuronal type.