Osthole or Imperatorin-Mediated Facilitation of Glutamate Release Is Associated With a Synaptic Vesicle Mobilization in Rat Hippocampal Glutamatergic Nerve Endings

Osthole and imperatorin, two active compounds of Cnidium monnieri (L.) Cusson, have previously been shown to facilitate depolarization-evoked glutamate release from rat hippocampal nerve terminals by increasing voltage-dependent Ca2+ entry. In this study, we further investigated whether osthole and imperatorin possess an action at the exocytotic machinery itself, downstream of a Ca2+ influx. Our data showed that ionomycin-induced glutamate release and KCI-evoked FMI-43 release were facilitated by osthole and imperatorin, suggesting that some steps after Ca2+ entry are regulated by these two compounds. Consistent with this, osthole or imperatorin-mediated facilitation of ionomycin-induced glutamate release was occluded by cytochalasin D that inhibits actin polymerization, implying that the disassembly of cytoskeleton is involved. In addition, the facilitatory action of osthole or imperatorin on ionomycin-induced glutamate release was attenuated by the Ca2+/calmodulin-dependent kinase II (CaMKII) inhibitor KN62. Furthermore, Western blotting analysis further showed that osthole or imperatorin significantly increased ionomycin-induced phosphorylation of CaMKII and synapsin I, the main presynaptic target of CaMKII. These results suggest, therefore, that osthole or imperatorin-mediated facilitation of glutamate release involves modulation of downstream events controlling synaptic vesicle recruitment and exocytosis, possibly through an increase of CaMKII activation and synapsin I phosphorylation, thereby increasing synaptic vesicle availability for exocytosis. Synapse 64:390-396, 2010. (C) 2009 Wiley-Liss, Inc.