Basic fibroblast growth factor selectively increases AMPA-receptor subunit G1uR1 protein level and differentially modulates Ca2+ responses to AMPA and NMDA in hippocampal neurons

The excitatory neurotransmitter glutamate is believed to play important roles in development, synaptic plasticity, and neurodegenerative conditions. Recent studies have shown that neurotrophic factors can modulate neuronal excitability and survival and neurite outgrowth responses to glutamate, but the mechanisms are unknown. The present study tested the hypothesis that neurotrophic factors modulate responses to glutamate by affecting the expression of specific glutamate-receptor proteins. Exposure of cultured embryonic rat hippocampal cells to basic fibroblast growth factor (bFGF) resulted in a concentration-dependent increase in levels of α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor subunit GluR1 protein as determined by western blot, dot-blot, and immunocytochemical analyses. In contrast, bFGF did not alter levels of GluR2/3, GluR4, or the NMDA-receptor subunit NR1. Nerve growth factor did not affect GluR1 levels. Calcium-imaging studies revealed that elevation of [Ca²⁺]_i, resulting from selective AMPA-receptor activation, was enhanced in bFGF-pretreated neurons. On the other hand, [Ca²⁺]_i responses to NMDA-receptor activation were suppressed in bFGF-treated neurons, consistent with previous studies showing that bFGF can protect neurons against NMDA toxicity. Moreover, neurons pretreated with bFGF were relatively resistant to the toxicities of glutamate and AMPA, both of which were shown to be mediated by NMDA receptors. These data suggest that differential regulation of the expression of specific glutamate-receptor subunits may be an important mechanism whereby neurotrophic factors modulate activity-dependent neuronal plasticity and vulnerability to excitotoxicity.