Cellular mechanisms of the acute increase of glutamate release induced by nerve growth factor in rat cerebral cortex

Abstract

Nerve growth factor (NGF) was found to increase glutamate release in the developing visual cortex. We investigated the cellular mechanisms of this effect and its dependence on extracellular and intracellular Ca2+. The NGF-induced enhancement of glutamate release from superfused rat visual cortex synaptosomes required mild depolarization. Removal of external Ca2+ during depolarization with 15 mM K+ only halved the effect of NGF on glutamate release. NGF increased [Ca2+]i in K+-depolarized synaptosomes preloaded with fura-2AM both in the presence and in the absence of external Ca2+. The effects of NGF on glutamate release and [Ca2+]i elevation were prevented by an anti-TrkA receptor monoclonal antibody. NGF increased synaptosomal inositol (1,4,5)-triphosphate (InsP3) during depolarization and the InsP3 receptor antagonist heparin abolished the effect of NGF on evoked glutamate release both in the presence and in the absence of external Ca2+. The effect of NGF on the evoked glutamate release in Ca2+-free medium was abolished by dantrolene, a ryanodine receptor blocker, by CGP 37157, a blocker of the mitochondrial Na+/Ca2+ exchanger and by pretreatment of synaptosomes with caffeine. NGF significantly increased the depolarization-induced activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and the subsequent phosphorylation of synapsin I in the absence of external Ca2+ and the NGF effect on evoked glutamate release was inhibited by the CaMKII inhibitors KN-93 and CaMKII 281-309 peptide but not by the MAP kinase inhibitor PD 98059. Thus, the effect of NGF on evoked glutamate release is linked to an increase in [Ca2+]i contributed by both Ca2+ entry and mobilization from InsP3-sensitive, ryanodine-sensitive and mitochondrial stores and to the subsequent activation of CaMKII.