Life-Course Contribution of Prenatal Stress in Regulating the Neural Modulation Network Underlying the Prepulse Inhibition of the Acoustic Startle Reflex in Male Alzheimer's Disease Mice

Abstract

The prepulse inhibition (PPI) of the acoustic startle reflex (ASR), as an index of sensorimotor gating, is one of the most extensively used paradigms in the field of neuropsychiatric disorders. Few studies have examined how prenatal stress (PS) regulates the sensorimotor gating during the lifespan and how PS modifies the development of amyloid-beta (Aβ) pathology in brain areas underlying the PPI formation. We followed alternations in corticosterone levels, learning and memory, and the PPI of the ASR measures in APPNL-G-F/NL-G-F offspring of dams exposed to gestational noise stress. In-depth quantifications of the Aβ plaque accumulation were also performed at 6 months. The results indicated an age-dependent deterioration of sensorimotor gating, long-lasting PS-induced abnormalities in PPI magnitudes, as well as deficits in spatial memory. The PS also resulted in a higher Aβ aggregation predominantly in brain areas associated with the PPI modulation network. The findings suggest the contribution of a PS-induced hypothalamic-pituitary-adrenal (HPA) axis hyperactivity in regulating the PPI modulation substrates leading to the abnormal development of the neural protection system in response to disruptive stimuli. The long-lasting HPA axis dysregulation appears to be the major underlying mechanism in precipitating the Aβ deposition, especially in brain areas contributed to the PPI modulation network.

Keywords: Acoustic startle reflex; Alzheimer's disease; HPA axis; amyloid-beta plaque; corticosterone; noise; prenatal stress; prepulse inhibition; sensorimotor gating.

Figure 1

(A) Time of each procedure across age in APP^{NL-G-F/NL-G-F} offspring. (B) The number of animals per procedure. The PPI of the ASR was performed 8 times at ages 1, 2, 4, 6, 9, 12, 15, 18 months for APP^{NL-G-F/NL-G-F} mice. To keep track long-lasting effects of PS on corticosterone levels and spatial learning and memory, blood was collected 4 times at ages 2, 6, 12, and 18 months, and the MWT was performed 3 times at 2, 6, and 15 months. Half of the animals were sacrificed at 6 months, and digital pathology by Nanozoomer was performed to quantify the total number of Aβ plaques, total plaque area (%), plaque area in specific brain regions, and the plaque size (μm). Aβ, amyloid beta; ASR, acoustic startle reflex; Cort, corticosterone; MWT, Morris water task; PPI, pre-pulse inhibition, PS, prenatal stress.

Figure 2

Results of corticosterone assays and behavioral tests. (A) The corticosterone levels (ng/ml): the corticosterone level was significantly higher in the PS group compared with the control group over age. The PPI of the ASR test: (B1) The ASR amplitude (%) significantly decreased by age in both groups. (B2) The PS animals exhibited a significantly lower PPI amplitude (%) than the controls across age. (B3,B4) The ASR and PPI latencies (msec) significantly increased by age in both groups, and the difference between the 2 groups in PPI latency was significant within age. The MWT: The PS animals demonstrated impairments in swim latency (C1) and swim distance (C2) than the control animals at 6 and 15 months. (C3) The probe test: The PS animals displayed a significant impairment in probe time at older ages, 6 and 15 months, than the controls. Results reported as mean ± S.E.M. ^*P < 0.05, ^**P < 0.01. ASR, acoustic startle reflex; MWT, Morris water task; PPI, pre-pulse inhibition; PS, prenatal stress.

Figure 3

The Aβ plaque quantifications at 6 months. (A) Six coronal sections (A1–A6: Bregma ~2.96, 0.98, −2.06, −2.92, −2.92, −4.48, and −5.34 mm) were selected for quantifying the Aβ plaques. (B) Samples of brain sections used in a control (B1) and a PS (B2) animal. (C) Their corresponding distributions of plaque size (μm) by the total number of plaques (C1–C6). Scale bar, 0.5 ml. Aβ, amyloid beta; Au1, primary auditory cortex; BLA, basolateral amygdala; Cg1&2, anterior cingulate area; CN, cochlear nuclei; FrA, frontal cortex area; Hipp, hippocampal region; IC, inferior colliculus; Ncb, nucleus accumbens; PaS, parasubiculum; PnC, caudal pontine reticular nucleus; PPTg, pedunculopontine tegmental nucleus; PrL, prelimbic cortex; PS, prenatal stress; SC, superior colliculus; VLTg, ventrolateral tegmental nucleus; VTA, ventral tegmental area.

Figure 4

The PS effect on Aβ aggregation at 6 months. The PS group showed a significant increase in total plaque area (A), plaque area in specific brain regions (B), total number of plaques (C), old plaque ratio (D), and largest plaque size (E) in 6 brain sections (A1–A6) selected for statistical analyses. Among 15 specific brain regions chosen for detailed comparisons in brain areas associated with the PPI of the ASR formation, a significantly larger plaque area was observed in all brain areas associated with the PPI modulation network, except for Au1. No significant difference was, however, shown in brain regions linked to both PPI mediation network and startle pathway, except for IC from the PPI mediation network. Results reported as mean ± S.E.M. ^*P < 0.05, ^**P < 0.01. Aβ, amyloid beta; Au1, primary auditory cortex; BLA, basolateral amygdala; Cg1&2, anterior cingulate area; CN, cochlear nuclei; FrA, frontal cortex area; Hipp, hippocampal region; IC, inferior colliculus; Ncb, nucleus accumbens; PaS, parasubiculum; PnC, caudal pontine reticular nucleus; PPTg, pedunculopontine tegmental nucleus; PrL, prelimbic cortex; PS, prenatal stress; SC, superior colliculus; VLTg, ventrolateral tegmental nucleus; VTA, ventral tegmental area.

Figure 5

Bivariate correlations between behavioral findings (the latency in MWT and the PPI magnitude) and brain measures (plaque area in Hipp [Bregma ~−2.06], BLA [Bregma ~−2.0], and IC [Bregma ~−5.34]). Relationships were observed between (A) the latency in MWT and the PPI amplitude (B) the latency in MWT and the Hipp plaque area, (C) the PPI amplitude and the Hipp plaque area, (D) the PPI amplitude and the BLA plaque area, (E) the PPI amplitude and the IC plaque area, and (F) the BLA plaque area and the IC plaque area in both PS (⋄) and control () groups. BLA, basolateral amygdala; Hipp, hippocampal region; IC, inferior colliculus; MWT, Morris water task; PS, prenatal stress; PPI, prepulse inhibition.