, we found that blockade of CB1Rs with AM251 (2 μM) inhibited the induction of ITDP (Figure 9A). However, we also found that the block of ITDP was incomplete, with a residual 1.36-fold ± 0.31-fold (p < 0.005, n = 8) potentiation of the PSP, which matches the residual ITDP observed in the presence of GABAR blockers (or following PSEM-mediated silencing of CCK INs). This suggests that the activation of CB1Rs by eCBs may be selectively required for the iLTD, but not eLTP component of ITDP. To test this idea, we examined the extent of inhibition remaining after ITDP was

induced in the continuous presence of AM251 (2 μM). We first applied GABAR antagonists to slices exposed to AM251 (no ITDP pairing). GABAR blockade produced a large increase in the SC-evoked FK228 in vitro DNA Damage inhibitor PSP in CA1 PNs (110.8% ± 14.6%, p < 0.001, n = 6; Figures 9B1 and 9C1) similar to the increase seen in the absence of AM251 (Figures 2C and S1E), indicating that CB1R blockade did not alter basal FFI under the conditions of our experiments (cf. Losonczy et al., 2004). Next, we applied GABAR antagonists 30–40 min after the induction of ITDP in slices continuously exposed to AM251 to assess the residual IPSP. The CB1 antagonist effectively blocked the suppression of inhibition that normally accompanies ITDP (Figures 9B2 and 9C2). After induction

of ITDP with CB1Rs blocked, the GABAR antagonists produced a large increase in the SC PSP (112.4% ± 24.2%, p < 0.003, n = 5), similar to that seen in slices where ITDP was not induced (p = 0.194, unpaired t test). These results indicate that the eCB pathway is necessary for the iLTD component of ITDP. Previous studies report that hippocampal ITDP is sensitive to antagonists of group I mGluRs (mGluR1 and mGluR5) (Dudman et al., 2007 and Xu et al., 2012) and that the mGluR1 subtype mediates eCB release during 100 Hz iLTD (Chevaleyre and Castillo, 2003). We extended the characterization of the mGluR subtypes required for ITDP Substrate-level phosphorylation and found that selective blockade of mGluR1a using LY367385 (100 μM) eliminated the iLTD component of ITDP but left intact a residual potentiation most likely resulting from eLTP (Figure S6). As eCBs are diffusible lipid

molecules, we asked whether iLTD during ITDP represents a global depression of inhibition by CCK INs or is limited to those CCK IN terminals that contact CA1 PNs activated during the pairing protocol. We addressed this by obtaining whole-cell recordings from two neighboring CA1 PNs, with one cell voltage clamped at −85 mV to prevent its depolarization during the pairing protocol and the other cell current clamped to allow for depolarization (Figures 9D1–9E2). ITDP was almost fully blocked in the voltage-clamped cell (1.17-fold ± 0.12-fold potentiation, p = 0.1849, paired t test, n = 14), whereas it was expressed normally in the adjacent current-clamped cell (2.67-fold ± 0.4-fold potentiation, p < 0.0001, paired t test, n = 11) (Figures 9E1–9F).