This same phenomenon was detected in WT neurons 20–24 hr after the addition of bicuculline (Figure 7C and 7D), which parallels the time course of Homer1a protein induction (Figure 5A). Acute Bay and MPEP did not alter mEPSCs in WT neurons after 48 hr treatment of bicuculline (Figures S6A and S6B). These data suggest that Homer1a contributes to the induction of homeostatic scaling by enhancing mGluR activity, and this mechanism makes relatively less contribution to maintenance of scaling.

In further support of this model, Bay and MPEP treatment, Kinase Inhibitor Library molecular weight which blocks the scaling effect of bicuculline (Figure 1), does not reverse bicuculline scaling even if applied for an additional 48 hr after bicuculline (Figures S6C and S6D). To examine Homer 1a scaling in vivo, we monitored responses of layer II-III pyramidal neurons in the acute cortical slices. As in culture, Homer1a KO pyramidal neurons had larger amplitude mEPSCs than

WT neurons (Figures S7A and S7B: Homer1a KO 13.2 ± 0.8 pA; n = 7 cells; WT 10.4 ± 0.7 pA; n = 8 cells; ∗p < 0.05). There was no difference in mEPSC frequency between WT (14.9 ± 1.7 Hz; n = 8 cells) and Homer1a KO neurons (16.7 ± 3.8 Hz; n = 7 cells; Figures S7A and S7B). Acute application of Bay (50 μM) and MPEP (10 μM) to slices from naive WT or Homer1a KO mice did not change the amplitude of mEPSCs (not shown). To assess the contribution of activity-inducible Homer1a, we treated WT and Homer1a KO mice with MECS and prepared cortical slices 2 hr later. Slices were

used for recordings 1–2 hr after preparation to correspond to the point of maximal expression of Homer1a protein after MECS (Brakeman et al., selleck inhibitor 1997). Homer1a mRNA was detected by in situ hybridization in ∼13% of layer II-III cortical neurons in naive mice, and in ∼35% of neurons in MECS treated mice (Figures S7C and S7D). Accordingly, we anticipated that effects of Homer1a might be evident in approximately one-third of randomly selected neurons. A comparison of mEPSC amplitudes in neurons from naive WT mice versus those treated with MECS showed a significant decrease in mEPSC amplitudes, whereas a similar comparison in Homer1a KO mice did not (Figure S7E). Bath application of Bay and MPEP increased the amplitude of mEPSCs in a subset of WT neurons (4/15) after MECS (Figures 7E and 7F). By contrast, bath application of Bay and MPEP Resveratrol did not result in an increase in mEPSC amplitude of neurons from Homer1a KO mice (0 of 16 neurons; different from WT neurons p < 0.05 using Fisher’s exact test) (Figures 7E and 7F). To confirm the hypothesis that activity evokes constitutive mGluR signaling that reduces synaptic strength in WT mice, we employed a fos-GFP reporter mouse to identify living neurons that were activated by MECS (Barth et al., 2004). C-fos and Homer1a are coordinately induced in neurons by MECS ( Barnes et al., 1994), and GFP was detected in approximately one-third of layer II-III pyramidal neurons after MECS.