In the forced swim test, mutant animals during the acute phase spend less time immobile on day 2 (Figure 8B), suggesting that they

are hyper-reactive and more anxious about the water-swim stress. These results suggest that shortly after cell ablation causes mossy cell degeneration, granule-cell excitability increases, eliciting anxiety-like behaviors. To MS-275 order address whether mossy cell degeneration leads to impaired hippocampus-dependent learning, we subjected DT-treated mutants and control littermates to two contextual discrimination paradigms. Mice in acute and chronic DT treatment phases were subjected to a one-trial contextual fear-conditioning test to assess whether, 3 hr and 24 hr after conditioning, mutants can discriminate shocking context A from a modality-different context B (Figure S5D). Whether in acute (Figure 8C) or chronic (Figure 8D) phases of DT-treatment, mutants and controls show similar freezing levels before and immediately after shock during conditioning, indicating mossy cell ablation has no impact on contextual fear learning. On the recall test, however, mutant animals in the acute phase (but not chronic-phase mutants or controls) are unable to distinguish context A from context

B 3 hr (genotype-context interaction F(1,38) = 3.1, p < 0.05; Newman-Keuls post hoc test, p < 0.05 for control, p = 0.42 for mutant) and 24 hr (Figure 8C) after conditioning. Notably, when mice are tested in the chronic phase of DT exposure, this impairment disappears both at 3 hr (context effect, F(1,30) = 24.1, p < 0.01; Newman-Keuls post hoc test, Selleck FG-4592 p < 0.005 for control, p < 0.01 for mutant) and 24 hr (Figure 8D) after conditioning. Contextual discrimination impairment in mutants therefore appears to occur only in the acute phase of DT exposure, when granule cell excitability is highest. To investigate whether mutants’ inability to discriminate contexts is consistent across tasks, we subjected naive animals with found acute DT exposure to a contextual step-through

active avoidance task. In the initial latency test crossing from light to dark compartments, no difference was detected among genotypes before conditioning (42.4 ± 11.3 s for control, 58.3 ± 24.0 s for mutant, t test, p = 0.54). In context X, mice entering the dark compartment received a single foot shock (0.12 mA, 2 s). Twenty-four hours after conditioning, the mice were placed back in the dark compartment, either in the non-shock context Y or in the US-associated context X (Figure S5E). Reverse latency to escape from the dark compartment (X or Y) was measured for each context. Control mice had longer escape latency from safe context Y, while this was not seen in the mutants (Figure 8E). These results confirm that in the acute phase of mossy cell degeneration, mutants’ recall for a fear memory in a specific context is impaired.