Modulation of vascular endothelial growth factor (VEGF) expression in motor neurons and its electrophysiological effects.

Previous studies have shown that VEGF expression in forebrain increases after experimental manipulations that increase neuronal activity. One question is whether this also occurs in motor neurons. If so, it could be potentially advantageous from a therapeutic perspective, because VEGF prevents motor neuron degeneration. Therefore, we asked whether endogenous VEGF expression in motor neurons could be modulated. We also asked how VEGF exposure would influence motor neurons using electrophysiology. Immunocytochemistry showed that motor neuron VEGF expression increased after a stimulus that increases neuronal and motor activity, i.e., convulsive seizures. The increase in VEGF immunoreactivity occurred in all motor neuron populations that were examined 24h later. This effect was unlikely to be due to seizure-induced toxicity, because silver degeneration stain did not show the typical appearance of a dying or dead neuron. To address the effects of VEGF on motor neuron function, VEGF was applied directly to motor neurons while recording intracellularly, using a brainstem slice preparation. Exposure to exogenous VEGF (200 ng/ml) in normal conditions depressed stimulus-evoked depolarization of hypoglossal motor neurons. There was no detectable effect of VEGF on membrane properties or firing behavior. We suggest that VEGF is upregulated in neurons when they are strongly activated, and VEGF depresses neuronal excitation as a compensatory mechanism. Failure of this mechanism may contribute to diseases that involve a dysregulation of VEGF, excessive excitation of motor neurons, and motor neuron loss, such as amyotrophic lateral sclerosis (ALS).