AMPAR blockade induces two rapid and dissociable forms of synaptic compensation: (1) a postsynaptic increase in expression of GluA2-lacking AMPARs and a corresponding enhancement of mEPSC amplitude that is independent of background spiking activity and (2) a retrograde enhancement of presynaptic function that is driven by the convergence of BDNF-TrkB signaling with AP-triggered Ca2+-influx through P/Q/N-type channels. Whereas the postsynaptic changes are sensitive to activity at either AMPARs or NMDARs, the enhancement in presynaptic function is unique to loss of AMPAR activity. Both pre- and postsynaptic

changes RNA Synthesis inhibitor require new protein synthesis, but appear to depend on distinct dendritically synthesized protein products—GluA1 synthesis is likely to be critical for rapid postsynaptic compensation (Ju et al., 2004, Thiagarajan et al., 2005, Sutton et al., 2006 and Aoto et al., 2008), whereas BDNF synthesis is critical

for orchestrating retrograde compensatory changes in presynaptic function. Many studies have demonstrated postsynaptic forms of homeostatic compensation associated with enhanced expression of AMPARs at synapses (e.g., O’Brien et al., 1998, Wierenga et al., 2005 and Sutton et al., 2006). However, clear evidence for homeostatic regulation of presynaptic neurotransmitter release has also been documented (e.g., Bacci et al., 2001, Murthy et al., 2001, Burrone find more et al., 2002, Bumetanide Thiagarajan et al., 2005, Wierenga

et al., 2006 and Branco et al., 2008). Although methodological factors can contribute to this heterogeneity in expression (Wierenga et al., 2006), previous examples of retrograde effects of postsynaptic manipulations on presynaptic structure (e.g., Pratt et al., 2003) and function (e.g., Paradis et al., 2001, Burrone et al., 2002 and Frank et al., 2006) suggest that intrinsic synaptic properties might also play a role. Indeed, we find that in addition to rapid postsynaptic effects, AMPAR blockade induces rapid (<3 hr) functional compensation in the presynaptic compartment, an effect that is not observed with either acute (3 hr) or chronic (24 hr) AP blockade (see also Bacci et al., 2001). Not only was AP blockade insufficient to produce changes in presynaptic function on its own, it also prevented AMPAR blockade from producing those changes. Hence, the compensatory increase in release probability induced by AMPAR blockade is state dependent, requiring presynaptic spiking and P/Q/N-type Ca2+-channel function during the period of AMPAR blockade for its induction. Our findings complement recent studies regarding retrograde homeostatic regulation of presynaptic neurotransmitter release.