Aging Alters Olfactory Bulb Network Oscillations and Connectivity: Relevance for Aging-Related Neurodegeneration Studies

Abstract

The aging process eventually cause a breakdown in critical synaptic plasticity and connectivity leading to deficits in memory function. The olfactory bulb (OB) and the hippocampus, both regions of the brain considered critical for the processing of odors and spatial memory, are commonly affected by aging. Using an aged wild-type C57B/6 mouse model, we sought to define the effects of aging on hippocampal plasticity and the integrity of cortical circuits. Specifically, we measured the long-term potentiation of high-frequency stimulation (HFS-LTP) at the Shaffer-Collateral CA1 pyramidal synapses. Next, local field potential (LFP) spectra, phase-amplitude theta-gamma coupling (PAC), and connectivity through coherence were assessed in the olfactory bulb, frontal and entorhinal cortices, CA1, and amygdala circuits. The OB of aged mice showed a significant increase in the number of histone H2AX-positive neurons, a marker of DNA damage. While the input-output relationship measure of basal synaptic activity was found not to differ between young and aged mice, a pronounced decline in the slope of field excitatory postsynaptic potential (fEPSP) and the population spike amplitude (PSA) were found in aged mice. Furthermore, aging was accompanied by deficits in gamma network oscillations, a shift to slow oscillations, reduced coherence and theta-gamma PAC in the OB circuit. Thus, while the basal synaptic activity was unaltered in older mice, impairment in hippocampal synaptic transmission was observed only in response to HFS. However, age-dependent alterations in neural network appeared spontaneously in the OB circuit, suggesting the neurophysiological basis of synaptic deficits underlying olfactory processing. Taken together, the results highlight the sensitivity and therefore potential use of LFP quantitative network oscillations and connectivity at the OB level as objective electrophysiological markers that will help reveal specific dysfunctional circuits in aging-related neurodegeneration studies.

Figure 1

(a) Histological overview of the olfactory bulbs (OB) from 1 young mouse (top panel) and old mouse (bottom panel) stained for H2AX and taken at same magnification (scale bar = 25 microns). The OB of the old mouse exhibited significant increase in the numbers of H2AX-positive neurons particularly in the mitral cell layer (MCL) and the granular cell layer (GCL). H2AX-positive neurons are depicted by arrows. EPL: external plexiform layer. (b) Numbers of H2AX neurons in the olfactory bulbs of young and aged mice/mm² of olfactory bulb tissue. Data are presented as mean values ± SEM for young (black, n = 4) and aged C57BL/6 mice (green, n = 4). Two-sample t-test.

Figure 2

Relative power spectra data of wake LFP signals in low (1-7 Hz) and high (30-80 Hz left panels) frequencies for recordings performed in left hemisphere. (a) Olfactory bulb (OBL), (b) entorhinal cortex (ECL), (c) hippocampal CA1 (CA1L), and (d) basolateral amygdala (BLA) for young (black, n = 7) and aged (green, n = 8) C57BL/6 mice. Data are presented as mean values ± 95%confidence intervals for young (black, n = 7), and aged C57BL/6 mice (green, n = 8). Bars on the horizontal axis indicate clusters, which drive significant between group differences using threshold-free cluster enhancement (TFCE, α = 0.05). Right bar and whiskers panels show relative power in delta frequency 1-4 Hz (top) and gamma frequency 30-80 Hz (bottom). Lines above bar plots with asterisks indicate presence of significant between group difference, ∗p value < 0.05 (two-sample t-test), ∗p value < 0.05.

Figure 3

Heat maps showing the mean phase amplitude coupling (PAC) modulation index at the OB and CA1 recording electrodes for young (left panels in each frame) and aged (right panels in each frame) C57BL/6 mice. As shown by the color scale, “hotter” colors indicate high coupling values while “colder” colors indicate low or no coupling. Bar graphs showing the mean (across animals) theta-gamma PAC (with 95% CI) at the OB, EC, and CA1 electrodes for young (black, n = 7) and aged (green, n = 8) Bl6 mice. These means along animals' PAC values were computed as the average PAC for the large window of phase frequency: 2–12 Hz, and amplitude frequency: 10–200 Hz. Right bar charts show estimated mean PAC index phase 4-8 Hz and amplitude 40-100 Hz, and horizontal lines above bar plots with asterisks indicate presence of significant difference between genotypes (∗p value < 0.05). Data are presented as mean values (with 95% CI).

Figure 4

Coherence and imaginary part of coherency patterns between recording pairs (a) olfactory bulb left and right (OBL-OBR), (b) olfactory bulb left and CA1 left (OBL-CA1L), (c) CA1 left and basolateral amygdala (CA1L-BLA), (d) enthorinal cortex left and CA1 left (ECL-CA1L), (e) enthorinal cortex left and basolateral amygdala (ECL-BLA), in young (black, n = 7) and aged (green, n = 8) C57BL/6 mice. Graphs show mean values (±95% CI) as a function of frequency in 1-100 Hz range. Bars on the horizontal axis indicate clusters, which drive significant between group differences using threshold-free cluster enhancement (TFCE, α = 0.05). In panel (a), bar and whiskers panels show imaginary part of coherency for interval 25-35 Hz. Lines above bar plot with asterisks indicate presence of significant difference between groups, ∗p value < 0.05 (two-sample t-test).

Figure 5

(a) Placement of the stimulation-recording electrodes at the Schaffer collateral-CA1 stratum pyramidal synapses in anesthetized mice (top panel), and a typical waveform spaghetti curves where the latency to peak negative deflection of fEPSPs is within 6-10 ms and the maximum amplitude between 1500 μV and 2500 μV. (b) Collective I/O curves of stimulation voltage and fEPSP slope (top panel) or PSA (bottom panel) values relative to baseline are plotted for young (black, n = 9) and aged (green n = 8) C57BL/6 mice. There was no significant alteration in baseline synaptic response, (c) A decline in synaptic response to HFS was observed in normalized EPSP and PSA values and was maintained throughout the recording session. No difference was observed in the average wave form during the 30 min baseline interval prior tetanisation, whereas smaller waveform was observed in aged mice during 0-30 min and 60-90 min posttetanisation. Data are presented as means ± SEM (%). Line above indicates statistical significance between groups, repeated ANOVA.