Basic Fibroblast Growth Factor Increases Functional L-Type Ca²⁺Channels in Fetal Rat Hippocampal Neurons: Implications for Neurite Morphogenesis<i>In Vitro</i>

Abstract

Basic fibroblast growth factor (bFGF) is a potent neurotrophic factor that regulates cell proliferation and differentiation during neuronal development. Here we report that fetal hippocampal neurons chronically treated with bFGF displayed larger [Ca2+]i increases than nontreated neurons in response to high K(+)-induced depolarization. This [Ca2+]i response was abolished by nicardipine and was little affected by treatments that depleted intracellular Ca2+ stores, thus reflecting the activities of L-type voltage-dependent Ca2+ channels. Whole-cell recordings also demonstrated increased high-voltage-activated Ca2+ currents in bFGF-treated neurons, whereas low-voltage-activated Ca2+ currents remained unchanged. bFGF-stimulated increase in Ca2+ response was not observed in neurons treated with cycloheximide or actinomycin D, indicating that protein and RNA synthesis were required for this effect. Visualization using a fluorescent dihydropyridine analog revealed that bFGF-treated neurons expressed increased amounts of L-type Ca2+ channels on the cell body. In addition, bFGF-treated neurons acquired distinctive morphology of neurites that was characterized by markedly increased neuritic branching. The branching points in neurites were associated with clusters of L-type Ca2+ channels and resultant "Ca2+ hotspots" that showed large [Ca2+]i increases in response to membrane depolarization. Concurrent application of nicardipine completely blocked the bFGF-stimulated increase in neuritic branching. Therefore, bFGF enhances the expression of functional L-type Ca2+ channels on the cell body and neurites of fetal hippocampal neurons, which may play an important role in the regulation of their differentiation and the establishment of their neurite morphology.