It has been recognized that estrogen promotes synaptogenesis and synaptic plasticity [8,36], as well as increases LTP, via enhancement of glutamate receptor activation [26]. In addition, previous studies have reported that estrogen induced synaptogenesis via the phosphoinositide-3 kinase (PI-3K)–[1,40] and mitogen-activated protein kinase (MAPK)-dependent pathways [17]. However, the cellular mechanisms underlying these effects of estrogen are still unclear. Since the enhancement of Arc expression in synaptic plasticity requires glutamate receptor activation [3], and estrogen acts through the glutamate receptor to increase synaptic plasticity [35], estrogen-enhanced synaptic plasticity probably involves expression of Arc.