Purpose: The ocular administration of the neurotrophin nerve growth factor (NGF) has been successfully used in humans to recover damaged ocular tissues. Studies on animal models have demonstrated the ability of ocular applied NGF to reach the retina and the optic nerve and affect brain visual areas. The aim of this study was to examine whether the ocular application of NGF as eye drops might affect brain areas other than the primary visual centers.
Methods: Two drops (10 μL) of NGF solution (200 μg/mL) or saline were applied as collyrium to both eyes of adult male Sprague-Dawley rats. The animals were sacrificed at 4, 8, or 24 h after treatment and the brains were fixed through intracardiac perfusion. Coronal brain sections were cut with a cryostat and used for immunohistochemical time series and double immunofluorescence studies using c-fos and NeuN as markers for neuronal activation.
Results: The immunohistochemical studies show a time-dependent effect of NGF eye drop treatment. At 4 h after NGF ocular administration, the increase in c-fos immunoreactivity is mainly observed in areas belonging to the central visual system, such as the lateral geniculate nucleus and visual cortex. At 8 h posttreatment, c-fos expression is enhanced in several limbic structures, including the frontal cortex, hippocampus, amygdala, and hypothalamus. The effects of NGF on c-fos distribution persist at 24 h postadministration. Specificity of NGF-induced c-fos in brain was confirmed using inactivated NGF. The neuronal nature of the NGF-activated cells was demonstrated by confocal microscopy observation of c-fos and NeuN colocalization.
Conclusion: This study demonstrates that NGF, when applied on ocular surface, is able to activate c-fos in several areas of the limbic system in a time-dependent manner. These findings suggest that the effects of NGF eye drops are not restricted to the primary visual areas, but are extended to all the retinal central targets, including the forebrain structure. Based on these data, the use of NGF eye drops as a strategy to produce NGF-mediated protective and reparative actions in brain is hypothesizable.