Finally, to examine whether this role of BDNF is local or more global, we locally scavenged BDNF (via restricted perfusion of TrkB-Fc) during AMPAR blockade (120 min CNQX) and found that the increase in syt-lum uptake was disrupted at presynaptic terminals in the treated area; in CH5424802 the absence of AMPAR blockade (bath vehicle), local

TrkB-Fc had no effect (Figure S7). Conversely, direct local application of BDNF (250 ng/ml, 60 min) induced a selective increase in syt-lum uptake at terminals in the treated area, relative to untreated terminals terminating on the same dendrite (Figure S7). Taken together, these results suggest a model whereby AMPAR blockade triggers dendritic BDNF release, which drives retrograde enhancement of presynaptic function selectively at active presynaptic terminals.

Previous studies have demonstrated that rapid postsynaptic compensation at synapses induced by blocking miniature transmission is protein synthesis dependent (Sutton et al., 2006 and Aoto et al., 2008; see also, Ju et al., 2004), so we next examined whether the rapid presynaptic or postsynaptic changes associated with AMPAR blockade require new protein synthesis. As suggested by these earlier studies, we found that the rapid increase in surface 3-Methyladenine GluA1 expression at synapses induced either by AMPAR blockade alone (3 hr CNQX) or AMPAR and AP blockade (CNQX + TTX) is prevented by the protein synthesis inhibitor anisomycin (40 μM, 30 min prior) (Figure 5A); a different translation inhibitor emetine (25 μM, 30 min prior) similarly blocked changes in sGluA1 induced by 3 hr CNQX treatment (data not shown). We also found (Figure 5B) that the state-dependent during increase in syt-uptake induced by AMPAR blockade was prevented by pretreatment (30 min prior to CNQX) with either anisomycin (40 μM) or emetine (25 μM). To verify that these changes in surface GluA1 expression and syt-lum uptake are indicative of changes in postsynaptic and presynaptic function, respectively, we examined the effects of anisomycin on mEPSCs

(Figures 5C and 5D). In addition to preventing the enhancement of mEPSC amplitude, blocking protein synthesis prevented the state-dependent increase in mEPSC frequency induced by AMPAR blockade, suggesting that rapid homeostatic control of presynaptic function also requires new protein synthesis. We next examined whether BDNF acts upstream or downstream of translation to persistently alter presynaptic function. BDNF has a well-recognized role in enduring forms of synaptic plasticity via its ability to potently regulate protein synthesis in neurons (Kang and Schuman, 1996, Takei et al., 2001, Messaoudi et al., 2002 and Tanaka et al., 2008), suggesting that BDNF release might engage the translation machinery to induce sustained changes in presynaptic function.