Figure 2.

Cognitive efficacy of allopregnanolone (APα) prior to extraneuronal beta-amyloid plaque. Triple-transgenic Alzheimer’s disease (3xTgAD) mice were exposed to a single dose of APα. Data were plotted as percent change relative to age-matched vehicle control to assess the age-related differences in response to APα administered at 3-, 6-, 9-, or 12-months of age. Mice were treated with either APα (subcutaneous, 10 mg/kg) or vehicle and 1 h later with bromodeoxyuridine (BrdU) (intraperitoneal, 100 mg/kg). Learning and memory performances were measured by trace eyeblink conditioning, a hippocampal-dependent task. Mice were trained by pairing delivery of a tone (conditioned stimulus; CS, 250 ms, 2 kHz, 85 dB) as the conditioned stimulus followed by a 250-ms period of no stimuli, followed by a mild periorbital shock (unconditioned stimulus; US, 100 ms) to elicit an eyeblink response. Mice received two blocks of 30 trials per day (30–60 s intertrial intervals, 3–4 h interblock intervals). This behavioral paradigm is subthreshold for inducing neurogenesis (Wang et al., 2010; Singh et al., 2011). One week following a single dose of APα, mice were subjected to trace eyeblink conditioning, with 1 day of habituation and 5 days of paired training. Following paired training, mice were left undisturbed in their home cages for 9 days and subsequently were tested for memory. Following the final learning trial, BrdU+ cell survival/hippocampus was measured at the end of the study, 3-weeks following a single dose of APα and the thymidine analog DNA-synthesis marker BrdU. Bars represent percent change ± sem in response to a single exposure to APα compared to age-matched vehicle at 3-, 6-, 9-, 12-months of age in 3xTgAD mice (n ≥ 7; Wang et al., 2010; Singh et al., 2011). Within 3 weeks following a single exposure to APα, neurogenesis, maximal learning, and memory indicators were increased ∼100% relative to age-matched vehicle control in adult male 3xTgAD mice when administered at ages prior to overt AD pathology. The 3xTgAD mouse model displays age-associated decrements in endogenous neurogenic cell survival in the subgranular zone (SGZ) as compared with the non-transgenic mice in addition to age-associated increments in Aβ pathology burden (depicted supra to bar graph). At 12-months of age, intra- and extraneuronal Aβ 6E10 antibody staining is apparent and plaque structures are developed in subiculum (Wang et al., 2010; Singh et al., 2011). The therapeutic response to APα was specific to the transgenic AD phenotype, as the age-matched non-transgenic mice did not benefit from a single exposure to APα. Remarkably, a single exposure to APα increased neurogenesis and subsequent cell survival in aged non-transgenic mice when administered at 15-months of age (non-Tg data not in figure; Singh et al., 2011). At 12-months of age, the point when extraneuronal plaques are known to be present in this AD mouse model, a single exposure to APα was ineffective. At ages prior to extraneuronal Aβ plaques, APα significantly (P < 0.05) increased BrdU+ cell survival, maximal learning, and memory function relative to age-matched vehicle control.