Acute gamma-secretase inhibition improves contextual fear conditioning in the Tg2576 mouse model of Alzheimer's disease

Abstract

Transgenic mice (Tg2576) overexpressing the Swedish mutation of the human amyloid precursor protein display biochemical, pathological, and behavioral markers consistent with many aspects of Alzheimer's disease, including impaired hippocampal function. Impaired, hippocampal-dependent, contextual fear conditioning (CFC) is observed in mice as young as 20 weeks of age. This impairment can be attenuated after treatment before training with the phosphodiesterase-4 inhibitor rolipram (0.1 mg/kg, i.p.). A rolipram-associated improvement is also observed in the littermate controls, suggesting that the effect of rolipram is independent of beta-amyloid. Acute treatment before training (but not after training or before testing) with the gamma-secretase inhibitor (GSI) N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine-t-butylester (DAPT), at a dose that reduces brain concentrations of beta-amyloid (100 mg/kg), attenuates the impairment in 20- to 65-week-old Tg2576 mice. Importantly, DAPT had no effect on performance of control littermates. These data are supportive of a role of beta-amyloid in the impairment of CFC in Tg2576 mice. Furthermore, they suggest that acute treatment with GSI may provide improved cognitive functioning as well as disease-modifying effects in Alzheimer's disease.

Figure 1.

A, Schematic representation of CFC procedure. Training consisted of two cue: footshock pairings during an ∼5 min exposure to the context. Testing, unless otherwise indicated, began 18–20 h after training. B, Tg2576 mice display impaired memory for context after fear conditioning beginning as early as 20 weeks of age with a trend for an impairment observed at 16 weeks (*p < 0.05; ^#p = 0.054 vs control). C, Contextual memory was impaired at 20 h (*p < 0.05 vs control) but not at 1 or 6 h after training in 20-week-old Tg2576 mice. Error bars represent SEM.

Figure 2.

Rolipram significantly improved conditioning in 36-week-old Tg2576 mice with a trend observed in 24 weeks, such that conditioning not significantly different from vehicle-treated controls. Rolipram increased contextual memory scores in the 24-week-old controls. Brackets denote group mean comparisons (*p < 0.05; ^#p < 0.1). Error bars represent SEM.

Figure 3.

A, Oral DAPT (100 mg/kg) in 20-week-old Tg2576 mice produced a significant reduction in Aβ₄₀ and Aβ₄₂ at 1–8 and 4–8 h, respectively (*p < 0.05 vs vehicle treatment). B, Schematic representation of the effect of drug treatment (triangle) on brain levels of β-amyloid levels relative to CFC training and testing. Drug treatment before training results in β-amyloid reductions during and after the training session but not during the testing session. Drug treatment immediately after training results in β-amyloid reduction during the retention interval. Treatment before testing decreased β-amyloid at the time of testing, with the usual elevated levels during training and the retention interval.C, Acute treatment with DAPT(100mg/kg, orally) 3 h before training improved conditioning in Tg2576 mice from 20 to 65 weeks of age. DAPT-treated Tg2576 mice at 20, 36, and 54 weeks of ages had contextual memory scores significantly greater than vehicle-treated Tg2576 and not significantly different from DAPT-treated controls. No effect of DAPT was observed on the performance of controls at any age. Vehicle-treated Tg2576 mice demonstrated a significant impairment at all ages tested. Brackets denote group mean comparisons (*p < 0.05). D, Twenty-week-old Tg2576 mice treated with vehicle immediately after training displayed significant impairments in contextual memory (*p < 0.05 vs controls). DAPT (100 mg/kg, orally) administered either after training (lowering β-amyloid during the retention interval) or 2 h before testing (loweringβ-amyloid during testing) failed to attenuate the contextual memory deficit in the Tg2576 mice (*p < 0.05 vs controls). Error bars represent SEM.