The input-output curves showed that the fEPSP slope of the EC-DG pathway was decreased in the EC::TeTxLC-tau-lacZ mice relative to control littermates by ∼35% at postnatal day 11 (P11) (Figures 1D and 1E), while the fiber volley amplitude, which represents the number of axons, was similar between them (Figure 1D and data not shown). This indicates that synaptic transmission

by tTA-expressing neurons in the EC::TeTxLC-tau-lacZ line is effectively inactivated by TeTxLC. We then used these mice to examine the developmental projection selleck chemicals llc of active and inactive EC axons in the EC transgenic lines. Both active (EC::tau-lacZ) and inactive (EC::TeTxLC-tau-lacZ) EC axons reached the DG between P6 and P9, as visualized by lacZ staining, without any aberrant projections (Figure 1F). This result suggests www.selleckchem.com/products/lgk-974.html that initial axon projections from the EC to the DG are largely independent of synaptic neurotransmitter release. In contrast, inactivation of EC axons resulted in their elimination from the DG after they developed.

Active EC axons (EC::tau-lacZ) increased in the DG from P12 to P16 (Figures 1F and 1G). However, inactive axons (EC::TeTxLC-tau-lacZ) were quickly eliminated from the DG after P12, and very few remained by P18 (Figures 1F and 1G). This axon elimination was likely initiated by axon retraction (also see Figure 4), because (1) no apparent cell death was detected in the EC at P18 (Figure S1B available online), and (2) EC axons were still detected in the presubiculum at P21 (Figure S1C). These results

suggest that EC axons are refined after P12 in an activity-dependent manner. In EC::tau-lacZ mice, lacZ intensity decreased between P16 and P21 by ∼25% (Figure 1G), suggesting that EC axons are refined during normal development and that the elimination of inactive axons in EC::TeTxLC-tau-lacZ mice reflects an exaggerated instance of normal physiological refinement. Since only 43% of the superficial layer neurons of the medial EC express TeTxLC in EC::TeTxLC-tau-lacZ mice (Yasuda and Mayford, 2006), there are two possible mechanisms for the elimination of inactive axons: (1) inactivity per se drives Resminostat axon elimination, or (2) they are eliminated via activity-dependent competition with other active EC neurons. To distinguish between these possibilities, we globally suppressed neural activity of EC axons by injecting the sodium channel blocker tetrodotoxin (TTX) (Burrone et al., 2002 and Echegoyen et al., 2007) into the DG of EC::TeTxLC-tau-lacZ mice once a day from P9 and analyzed the elimination of TeTxLC-expressing EC axons at P12, P14, and P16 (4, 6, or 8 days total of TTX injections) (Figure 2A). TTX injections markedly suppressed the elimination of TeTxLC-expressing EC axons between P12 and P16 (Figures 2B and 2C). Therefore, the refinement of EC axons in the DG is mostly achieved by an activity-dependent competition between EC neurons.