σ-1 Receptor Agonist SKF10047 Inhibits Glutamate Release in Rat Cerebral Cortex Nerve Endings

sigma-1 Receptors are expressed in the brain, and their activation has been shown to prevent neuronal death associated with glutamate toxicity. This study investigates the possible mechanism and effect of [2S-(2 alpha,6 alpha,11R*]-1,2,3,4,5,6-hexahydro6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol (SKF10047), a sigma-1 receptor agonist, on endogenous glutamate release in the nerve terminals of rat cerebral cortex. Results show that SKF10047 inhibited the release of glutamate evoked by the K+ channel blocker 4-aminopyridine (4-AP), and the sigma-1 receptor antagonist N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl2-(dimethylamino) ethylamine (BD1047) blocked this phenomenon. The effects of SKF10047 on the evoked glutamate release were prevented by the chelating extracellular Ca2+ ions and the vesicular transporter inhibitor bafilomycin A1. However, the glutamate transporter inhibitor DL-threo-beta-benzyl-oxyaspartate did not have any effect on the action of SKF10047. SKF10047 decreased the depolarization-induced increase in the cytosolic free Ca2+ concentration ([Ca2+](C)), but did not alter 4-AP-mediated depolarization. Furthermore, the effects of SKF10047 on evoked glutamate release were prevented by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking the ryanodine receptors or the mitochondrial Na+/Ca2+ exchange. In addition, conventional protein kinase C (PKC) inhibitors abolished the SKF10047 effect on 4-AP-evoked glutamate release. Western blot analyses showed that SKF10047 decreased the 4-AP-induced phosphorylation of PKC and PKC alpha. These results show that sigma-1 receptor activation inhibits glutamate release from rat cortical nerve terminals. This effect is linked to a decrease in [Ca2+](C) caused by Ca2+ entry through presynaptic voltage-dependent Ca2+ channels and the suppression of the PKC signaling cascade.