Type-2 Diabetes Alters Hippocampal Neural Oscillations and Disrupts Synchrony between the Hippocampus and Cortex

Abstract

Type 2 diabetes mellitus (T2DM) increases the risk of neurological diseases, yet how brain oscillations change as age and T2DM interact is not well characterized. To delineate the age and diabetic effect on neurophysiology, we recorded local field potentials with multichannel electrodes spanning the somatosensory cortex and hippocampus (HPC) under urethane anesthesia in diabetic and normoglycemic control mice, at 200 and 400 days of age. We analyzed the signal power of brain oscillations, brain state, sharp wave associate ripples (SPW-Rs), and functional connectivity between the cortex and HPC. We found that while both age and T2DM were correlated with a breakdown in long-range functional connectivity and reduced neurogenesis in the dentate gyrus and subventricular zone, T2DM further slowed brain oscillations and reduced theta-gamma coupling. Age and T2DM also prolonged the duration of SPW-Rs and increased gamma power during SPW-R phase. Our results have identified potential electrophysiological substrates of hippocampal changes associated with T2DM and age. The perturbed brain oscillation features and diminished neurogenesis may underlie T2DM-accelerated cognitive impairment.

Figure 1.

T2DM increased slowing score in the hippocampus (HPC) and age altered brain state. Slowing score of the pyramidal (A), slm (B) layers of the hippocampus and cortex (C). T2DM mice shifted the power towards lower frequency oscillations in comparison to the control mice in the pyramidal and slm layers of the HPC. (D) Age altered brain state by reducing theta/delta (T/D) ratio. Two-way ANOVA test followed by Tukey’s post-hoc test. 200 d: 200 days old, 400 d: 400 days old. *p < 0.05, ***p < 0.001.

Figure 2.

T2DM reduced spectral exponent in the slm layer of the HPC. (A) Average power spectral density of recordings and best-fit trendline for each group. (B) Comparison of the spectral exponent showing the older T2DM mice had the most reduced spectral exponent. Two-way ANOVA test followed by Tukey’s post-hoc test. 200 d: 200 days old, 400 d: 400 days old. *p < 0.05, ***p < 0.001.

Figure 3.

Age and T2DM reduced coherence at a number of frequency bands between cortex and HPC. (A) Cortex- pyramidal coherence: Age decreased coherency between the cortex and pyramidal layer in all but delta frequency. (B) Cortex-slm coherence: T2DM effect in reducing coherency was observed in alpha, beta, and high gamma frequency bands. Two-way ANOVA test followed by Tukey’s post-hoc test. 200 d: 200 days old, 400 d: 400 days old. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 4.

Phase locking index (PLI) quantification for individual frequency bands between (A) cortex and pyramidal and (B) cortex and slm layers. Age significantly decreases PLI between the cortex and pyramidal layer in all frequency bands except delta frequency. T2DM decreased PLI between cortex and pyramidal layer in delta and between cortex and slm layer in theta, beta, and gamma bands. Two-way ANOVA test followed by Tukey’s post-hoc test. 200 d: 200 days old, 400 d: 400 days old. *p < 0.05.

Figure 5.

Cross regional phase-amplitude coupling between cortex and pyramidal or slm layer of the HPC. (A) Comodulograms of cortex-pyramidal areas demonstrating delta-high gamma (upper left), theta-high gamma (upper right), delta-low gamma (lower left), and theta low-gamma (lower right) coupling area marked by white boxes. (B) Quantified average log modulation index (MI) within areas of interest. Age or T2DM increased MI between delta and gamma, while T2DM reduced MI between theta and gamma. Two-way ANOVA test followed by Tukey’s post-hoc test. 200 d: 200 days old, 400 d: 400 days old. *p < 0.05, **p < 0.01.

Figure 6.

Characteristics of sharp wave associate ripples (SPW-Rs). (A) Average ripple waveform in 200 msec clips. (B-D) box-whisker plots of the 10-90% quantile comparing gamma signal power during SPW-Rs (B) and duration of SPW-Rs (C). Age and T2DM increased ripple duration and gamma power during ripples. (D) Comparison of inter-ripple intervals (IRI). Age significantly reduced IRI and this reduction was especially prominent in T2DM mice. Two-way ANOVA test followed by Tukey’s post-hoc test. 200 d: 200 days old, 400 d: 400 days old. An average of 1190 ripples were analyzed per group. **p < 0.01, ***p < 0.001, ****p < 0.0001.

Figure 7.

Age and diabetes reduced cell proliferation and immature neurons in the dentate gyrus subgranular zone (SGZ). The numbers of proliferating neural progenitor cells and neuroblasts in the SGZ of the HPC were quantified using BrdU (A) and double cortin (DCX) staining (B). Two-way ANOVA test followed by Tukey’s post-hoc test showed significant effect of age and T2DM on hippocampal neurogenesis. 200 d: 200 days old, 400 d: 400 days old. ***p < 0.001, and ****p < 0.0001. (C) Representative double immuno-fluorescent staining of the dentate gyrus in lower magnification (20X) view from 200-day old db/+ (upper panel) and db/db (lower panel) for BrdU (green) and DCX, NeuN or GFAP (red). gcl: granule cell layer. h: hilus. Scale bar, 50 µm. (D) Orthogonal reconstructions of confocal microscope merged images from db/+ (upper panel) and db/db (lower panel) with BrdU as green and cell markers (DCX, NeuN or GFAP) as red as viewed in the x-z (top) and y-z (right) planes. Scale bar, 10 µm. Group sizes for 200d: 10/db/+, 11/dbdb; 400d: 14/db+, 7/dbdb.

Figure 8.

Age reduced progenitor cell activity in the subventricular zone (SVZ). The number of proliferating neural progenitor cells and neuroblasts in the SVZ was quantified using Cresyl violet (CV) staining (A, D) DCX staining (B, E) and BrdU staining (C, F). Representative images (A-C) from each group showed the regions in the SVZ and lateral ventricle where neuroprogenitor cells and neuroblasts reside. Scale bar, 100 µm. BrdU counts were not shown for the 400 d groups due to very low numbers were found (F). There was an overall effect of age on neuroprogenitor cells and immature neurons, while T2DM reduced neuroprogenitor cells and cell proliferation in the 200 d group. Two-way ANOVA test followed by Tukey’s post-hoc test. *p < 0.05, **p < 0.01, and ***p < 0.001. 200 d: 200 days old, 400 d: 400 days old. Group sizes for 200 d: 10-13/db/+, 10-13/dbdb; 400 d: 9/db+, 7/dbdb.