Whereas these three phases of CF synapse elimination are severely impaired in PC-selective P/Q-type VDCC knockout mice (Hashimoto et al., 2011), Arc does not seem to be a downstream

mediator of P/Q-type VDCCs for these events during the first 10 days of postnatal cerebellar development. Because endogenous Arc mRNA expression exhibits more than a 2-fold increase from P9 to P16, Arc is considered to play an important role in the late phase of CF synapse elimination. We found that Arc knockdown in PCs in vivo at P2-P3 did not affect CF innervation when examined at P11–P12 but significantly impaired CF synapse elimination thereafter, particularly in the removal of redundant CF synapses from PC somata. The effect of Arc knockdown on CF synapse elimination was completely occluded by simultaneous R428 P/Q knockdown,

indicating that Arc mediates CF synapse elimination downstream of P/Q-type VDCCs. In contrast, Arc overexpression in PCs did not rescue the impaired CF synapse elimination caused by P/Q knockdown. Therefore, Arc is considered to require other factors induced by P/Q-type VDCC-mediated Ca2+ elevation in PCs to remove redundant CF synapses from PC somata during the late phase of CF elimination. Previous studies have clarified that mGluR1 to protein kinase Cγ (PKCγ) cascade in PCs is crucial for the late phase of CF synapse elimination (Ichise et al., 2000, Obeticholic Acid clinical trial Kano et al., 1995, Kano et al., 1997, Kano et al., 1998 and Offermanns et al., 1997). Besides this pathway involving mGluR1, the present study demonstrates that P/Q-type VDCC-mediated Ca2+ elevation and Arc activation is another activity-dependent pathway for the late phase of CF synapse elimination. It remains to be investigated whether and how these two pathways interact in PCs to eliminate redundant CF synapses on the PC soma. It has been demonstrated that both long-term potentiation (LTP) and LTD

occur at CF-PC synapses in rats (Bosman et al., 2008) and mice (Ohtsuki and Hirano, 2008) during the first postnatal week. Importantly, LTP also has been reported to occur exclusively at strong CF inputs that can produce spikes and significant Ca2+ transients, whereas LTD has been shown to be induced at weak CF inputs that are not associated with Ca2+ transients (Bosman et al., 2008). The LTP and LTD during the first postnatal week may contribute to selective strengthening of single CF inputs and the prevention of other CF inputs from potentiation in individual PCs (Bosman et al., 2008 and Ohtsuki and Hirano, 2008). These processes are not considered to involve Arc. In contrast, only LTD has been reported at CF-PC synapses during the second and third postnatal weeks (Hansel and Linden, 2000) when Arc seems to contribute to CF synapse elimination. Because the loss of Arc is reported to impair LTD in hippocampal neurons (Plath et al., 2006) and cultured cerebellar PCs (Smith-Hicks et al.