, 2000) and brain injury (Lowenstein et al., 1992; Santhakumar et al., 2001; Johansen

check details et al., 1987; Hsu and Buzsáki, 1993), and extensive loss of these cells following seizures or head trauma is associated with immediate granule cell hyperexcitability (Sloviter, 1983; Lowenstein et al., 1992; Toth et al., 1997). Yet whether mossy cell loss is responsible for this observed granule cell hyperexcitability is not known. According to the “dormant basket cell” hypothesis, because mossy cells normally excite inhibitory basket cells to inhibit granule cells, their loss should lead to granule cell hyperexcitability and spontaneous granule cell epileptiform behavior (Sloviter, 1991; Sloviter et al., 2003). The “irritable mossy cell” hypothesis (Santhakumar et al., 2000; Ratzliff et al.,

2002), by contrast, holds that following injury, surviving mossy cells hyperexcite granule cells by sending uncontrolled excitatory feedback. Because it was not possible to eliminate mossy cells selectively until now, researchers were unable to test these hypotheses in vivo. To determine how selective and extensive loss of mossy cells affects the excitability and behavior of dentate granule cells, we developed CT99021 molecular weight a toxin-mediated, mossy-cell-specific ablation mouse line in which mossy cells selectively express the diphtheria toxin (DT) receptor. In these mutants, following DT treatment ∼75% of mossy cells degenerate rapidly. To evaluate granule cell excitability after degeneration, we recorded local field potential (LFP) activity in vivo and assessed dentate gyrus hippocampal slices for synaptic reorganization believed Levetiracetam to be triggered by mossy cell loss (Jiao and Nadler, 2007).

Context-discrimination tasks were used to assess pattern separation. To generate hilar mossy cell-specific transgenic mice, we coinjected Cre recombinase cDNA with a minimal promoter element and a DNA fragment containing 5′-transcriptional regulatory region of calcitonin receptor-like receptor (Crlr) gene (see Figure S1A available online) into fertilized eggs from the C57BL/6N strain. After crossing with a loxP-flanked Rosa26lacZ reporter mouse, a transgenic line Cre #4688 (mossy cell/CA3-Cre) at 8 weeks old shows lacZ-positive somata exclusively in the dentate hilus and area CA3 pyramidal cells, with almost none in the dentate granule cell layer, area CA1, or neocortex ( Figures 1A and S1A). Homogenous staining of the IML where mossy cell axons terminate ( Blackstad, 1956; Amaral and Witter, 1989) reveals intense Cre-immunoreactivity (-IR) throughout the dentate hilus but not in the CA3 pyramidal cell layer ( Figure 1B). LacZ-positive cells appear in hilus/CA3c by postnatal day 9 and remain stable to at least 25 weeks, while ∼20% cells in the CA3b pyramidal cell layer are Cre-positive ( Figure S1A).