Reduction of Dendritic Inhibition in CA1 Pyramidal Neurons in Amyloidosis Models of Early Alzheimer’s Disease

Abstract

Background: In an early stage of Alzheimer’s disease (AD), before the formation of amyloid plaques, neuronal network hyperactivity has been reported in both patients and animal models. This suggests an underlying disturbance of the balance between excitation and inhibition. Several studies have highlighted the role of somatic inhibition in early AD, while less is known about dendritic inhibition. Objective: In this study we investigated how inhibitory synaptic currents are affected by elevated Aβ levels. Methods: We performed whole-cell patch clamp recordings of CA1 pyramidal neurons in organotypic hippocampal slice cultures after treatment with Aβ-oligomers and in hippocampal brain slices from AppNl - F - G mice (APP-KI). Results: We found a reduction of spontaneous inhibitory postsynaptic currents (sIPSCs) in CA1 pyramidal neurons in organotypic slices after 24 h Aβ treatment. sIPSCs with slow rise times were reduced, suggesting a specific loss of dendritic inhibitory inputs. As miniature IPSCs and synaptic density were unaffected, these results suggest a decrease in activity-dependent release after Aβ treatment. We observed a similar, although weaker reduction in sIPSCs in CA1 pyramidal neurons from APP-KI mice compared to control. When separated by sex, the strongest reduction in sIPSC frequency was found in slices from male APP-KI mice. Consistent with hyperexcitability in pyramidal cells, dendritically targeting interneurons received slightly more excitatory input. GABAergic action potentials had faster kinetics in APP-KI slices. Conclusion: Our results show that Aβ affects dendritic inhibition via impaired action potential driven release, possibly due to altered kinetics of GABAergic action potentials. Reduced dendritic inhibition may contribute to neuronal hyperactivity in early AD.

Amyloid-β Oligomers Induce Only Mild Changes to Inhibitory Bouton Dynamics

“…Both genders were used. Organotypic slice cultures were treated with synthetic Aβ oligomers (Crossbeta Biosciences, Utrecht) for 24 h at a concentration of 0.4μg/mL, as described before [ 21 ]. At this concentration, these Aβ oligomers are able to block LTP in vivo [ 40 ].…”

“…Inhibitory boutons can rapidly appear and disappear along the axon in response to changes in activity or molecular signals [ 14, 30–32 ]. Aβ toxicity has been reported to specifically induce presynaptic impairments [ 33–36 ], including at inhibitory synapses [ 21 ], but it is unknown if Aβ interferes with synaptic dynamics in inhibitory axons.…”

Axonal CB1 receptors mediate inhibitory bouton formation via cAMP increase

“…To isolate miniature inhibitory postsynaptic currents (mIPSCs) TTX, AP5 and DNQX were added to the ACSF. The mIPSCs were analyzed in pClamp and Matlab with homemade scripts (Ruiter et al, 2020). Rise time of mIPSCs were determined as the time between10% and 90% of the peak value.…”

“…Rise time of mIPSCs were determined as the time between10% and 90% of the peak value. The distribution of the rise times of mIPSCs recorded in control conditions (generated from 150 mIPSCs per cell) were fitted with two Gaussians and their crossing point determined the separation between fast and slow mIPSCs (Ruiter et al, 2020). A double Gaussian fit for the rise time distribution in 2-AG conditions gave a similar separation value (control: 0.9 ms; 2-AG: 1.1 ms) and we verified that our conclusions did not change by taking the 2-AG separation value.…”

“…Per slice, 2-6 axons per group were included in the analysis. For synapse quantification images were analyzed in Fiji using a custom macro (Ruiter et al, 2020). An average projection image was made from z-planes, images were median filtered (1 pixel radius) and individual puncta were identified using watershed segmentation.…”