GDNF protects nigral dopamine neurons against 6-hydroxydopamine in vivo

Glial cell line-derived neurotrophic factor (GDNF), a novel member of the TGF-β superfamily, has been shown to promote the survival and morphological differentiation of fetal dopamine neurons in culture and increase dopamine levels and metabolism in adult rats. Since several other trophic factors are able to rescue specific populations of mature CNS neurons following injury, the present study was designed to investigate a possible neuroprotective role by GDNF for midbrain dopamine neurons in rats exposed to the neurotoxin 6-hydroxydopamine (6-OHDA). Prior to surgery, young adult male Fisher 344 rats were divided into the following groups (n = 7–8/group): (1) intranigral saline + intranigral 6-OHDA; (2) intranigral GDNF + intranigral 6-OHDA; (3) intranigral saline + intrastriatal 6-OHDA; and (4) intranigral GDNF + intrastriatal 6-OHDA. The saline treated groups received a single 2 μl intranigral injection of phosphate buffered saline (PBS) while the GDNF treated rats received 10 μ g 2 μ l GDNF in PBS. Twenty-four hours later, the animals received a unilateral 4 μg/μl 6-OHDA infusion either into the substantia nigra or striatum. The rats were sacrificed two weeks postsurgery and the brains processed for tyrosine hydroxylase (TH) immunocytochemistry. Representative TH immunoreactive (TH-IR) sections were also counterstained with hematoxylin and eosin to determine the total number of neurons remaining in the substantia nigra pars compacta and ventral tegmental area. In the nigral lesion groups, there was significantly less loss of TH-IR neurons in the substantia nigra pars compacta of GDNF (47% survival) vs. PBS (9% survival) treated animals. The same was true in the ventral tegmental area, where there was a 90% survival of TH-IR neurons in the GDNF treated animals as compared to a 68% survival in PBS treated animals. In the striatal lesion groups, there was significant sparing of TH-IR neurons in the substantia nigra pars compacta of the GDNF (40% survival) compared to the PBS (16% survival) treated animals. However, in the ventral tegmental area, the protection seen in the GDNF treated animals (69% survival) was not statistically significant when compared to the PBS treated rats (48% survival). In sections counter stained with hematoxylin and eosin, the percentage of neurons surviving in GDNF treated hosts was higher suggesting that the 6-OHDA toxicity may reduce TH expression in some dopamine neurons without inducing cell death. Therefore, in both lesion models, our results demonstrate a substantial neuroprotective effect in rats pretreated with GDNF when compared to the vehicle treated groups.