The molecular mechanisms underlying Arc function in LTP could include Arc regulation of cytoskeleton dynamics and spine morphology or Arc function in other signaling cascades. LTP results in actin polymerization and stabilization and increases in spine number [59,62]. Arc localizes to the actin cytoskeleton, and Arc overexpression increases spine number [16]. Interestingly, LTP causes an increase in synapse number on thin spines [63], and Arc specifically promotes thin spine formation [16], as discussed above. In addition, the loss of LTP maintenance in response to Arc knockdown can be prevented by adding an actin-stabilizing drug, Jasplakinolide [61]. Yet, Arc has not yet been definitively shown to regulate actin polymerization in response to HFS, and Arc may affect LTP through other signaling cascades. Arc co-immunoprecipitates with, and activates, Notch1 [15], a receptor that is important in regulating morphology and synaptic plasticity in mature neurons, in addition to its roles in development. Arc appears to promote the cleavage and activation of Notch in response to activity [15] (Figure 2). How this occurs is not yet known, but disruption of Notch1 signaling impairs both LTP and LTD in hippocampal slices [15].