Amyloid-Beta1-42 -Induced Increase in GABAergic Tonic Conductance in Mouse Hippocampal CA1 Pyramidal Cells

Abstract

Alzheimer's disease (AD) is a complex and chronic neurodegenerative disorder that involves a progressive and severe decline in cognition and memory. During the last few decades a considerable amount of research has been done in order to better understand tau-pathology, inflammatory activity and neuronal synapse loss in AD, all of them contributing to cognitive decline. Early hippocampal network dysfunction is one of the main factors associated with cognitive decline in AD. Much has been published about amyloid-beta_1-42 (Aβ_1-42)-mediated excitotoxicity in AD. However, increasing evidence demonstrates that the remodeling of the inhibitory gamma-aminobutyric acid (GABAergic) system contributes to the excitatory/inhibitory (E/I) disruption in the AD hippocampus, but the underlying mechanisms are not well understood. In the present study, we show that hippocampal injection of Aβ_1-42 is sufficient to induce cognitive deficits 7 days post-injection. We demonstrate using in vitro whole-cell patch-clamping an increased inhibitory GABAergic tonic conductance mediated by extrasynaptic type A GABA receptors (GABA_ARs), recorded in the CA1 region of the mouse hippocampus following Aβ_1-42 micro injection. Such alterations in GABA neurotransmission and/or inhibitory GABA_ARs could have a significant impact on both hippocampal structure and function, causing E/I balance disruption and potentially contributing to cognitive deficits in AD.

Keywords: GABA 1; GABAA receptor 2; amyloid-beta 4; hippocampus 5; tonic inhibition 3.

Figure 1

Aβ_1-42-induced increase in gamma-aminobutyric acid (GABAergic) tonic conductance in CA1 pyramidal cells. (A) Representative whole cell traces from voltage-clamp recording of CA1 pyramidal cells (PCs) (Vh = −70 mV) from naïve control (NC), artificial cerebrospinal fluid (ACSF) -injected and Aβ_1-42-injected mice in the presence of 3 mM kynurenic acid and 5 µM GABA. The lines above the traces indicate the points at which TTX (5 µM) + CdCl₂ (50 µM) and bicuculline methiodide (BMI) (100 µM) were applied. (B) Histogram plot of tonic current measured in the NC, ACSF-injected and Aβ_1-42-injected mice. Data are expressed as mean ± SD (one-way ANOVA and Tukey’s post-hoc test (* p = 0.0382, ** p = 0.0025; NC n = 4, ACSF-injected n = 4, and Aβ_1-42-injected n = 5). (C) For a given CA1 PC (in this case from a NC), tonic current was determined by generating all-points histograms for the control period (before BMI application) and during the BMI application period, corresponding to 60 s per period (600001 points each). Mean values from each histogram were used to determine the current during the control and BMI periods. Tonic current in this given CA1 PC is 410.19 − 351.95 = 58.24 pA.

Figure 2

(A–H) Representative photomicrographs of the hippocampal CA1 region showing GABA_AR α5 subunit expression (red) and α5 immunoreactivity overlaid with Hoechst (blue) labeling for representative naïve control, artificial cerebrospinal fluid- (ACSF), scrambled Aβ_1-42- (scrAβ_1-42) and Aβ₁₋₄₂-injected mice (Aβ₁₋₄₂). (I–K): Quantification of GABA_AR α5 subunit immunoreactivity in the regions and layers of the hippocampus in NC, ACSF-, scrAβ_1-42- and Aβ_1-42-injected mice. Scale bar, 50 μm. (L,M) Aβ₁₋₄₂-injected mice show long- and short-term spatial memory impairment revealed by the novel object recognition (L) and Water Morris Maze (M) tests, respectively. The Aβ_1-42-lesioned mice do not discriminate between novel and familiar objects and show longer escape latency in trial 4 of the MWM task. Data are expressed as mean ± SD (one-way ANOVA and Tukey’s post-hoc test (* p < 0.05, ** p < 0.001, *** p < 0.001; NC n = 9, ACSF-injected n = 9, scrAβ_1-42-injected n = 9 and Aβ_1-42-injected n = 9).