Neural Dynamics of Olfactory Perception: Low- and High-Frequency Modulations of Local Field Potential Spectra in Mice Revealed by an Oddball Stimulus

Abstract

Recent brain connectome studies have evidenced distinct and overlapping brain regions involved in processing olfactory perception. However, neural correlates of hypo- or anosmia in olfactory disorder patients are poorly known. Furthermore, the bottom-up and top-down processing of olfactory perception have not been well-documented, resulting in difficulty in locating the disease foci of olfactory disorder patients. The primary aim of this study is to characterize the bottom-up process of the neural dynamics across peripheral and central brain regions in anesthetized mice. We particularly focused on the neural oscillations of local field potential (LFP) in olfactory epithelium (OE), olfactory blub (OB), prefrontal cortex (PFC), and hippocampus (HC) during an olfactory oddball paradigm in urethane anesthetized mice. Odorant presentations evoked neural oscillations across slow and fast frequency bands including delta (1-4 Hz), theta (6-10 Hz), beta (15-30 Hz), low gamma (30-50 Hz), and high gamma (70-100 Hz) in both peripheral and central nervous systems, and the increases were more prominent in the infrequently presented odorant. During 5 s odorant exposures, the oscillatory responses in power were persistent in OE, OB, and PFC, whereas neural oscillations of HC increased only for short time at stimulus onset. These oscillatory responses in power were insignificant in both peripheral and central regions of the ZnSO₄-treated anosmia model. These results suggest that olfactory stimulation induce LFP oscillations both in the peripheral and central nervous systems and suggest the possibility of linkage of LFP oscillations in the brain to the oscillations in the peripheral olfactory system.

Keywords: anterior cingulate cortex; anterior olfactory; attention; local field potential; neural oscillations; oddball paradigm; olfaction; primary olfactory cortex.

FIGURE 1

Experimental setup. (A) Schematic diagrams of the olfactometer that was used to deliver the two odor stimuli. (B) Schematic overview of the two-odor oddball paradigm. The standard, deviant, and vacuum channels were paired with separate solenoid valves, which are numbered in (A).

FIGURE 2

Simultaneous recording of local field potentials (LFPs) during odor stimulation. (A) Example of stimulus-locked LFPs and the odorant concentration. Single-trial LFPs are displayed after being bandpass filtered with a cut-off frequency from 1 to 100 Hz. Averaged photo ionization detector (PID) signals averaged over 70 trials (one session) are depicted for the odorant concentration. The vertical lines indicate the onset and offset moments of odor release. Methyl salicylate was released in this trial. The rising time of the tested odorants in this experiment was ∼100 ms. (B) Examples of the frequency components of the LFPs in the early responses (∼2.5 s) of the olfactory epithelium. Specifically, the δ (1–4 Hz), θ (6–10 Hz), β (15–30 Hz), low γ (30–50 Hz), and high γ (70–100 Hz) bands.

FIGURE 3

Desensitization patterns of the local field potential (LFP) signals at each channel. The mean z-scores of each trial of the standard stimulation condition are aligned by trial number. The x-axis indicates the time course of each trial from the 2-second baseline period to the post-stimulation period. The y-axis indicates the first 60 standard trials during each session. (A) Mean z-score of the total power (1–100 Hz). (B) Mean z-score of δ (1–4 Hz). (C) Mean z-score of θ (6–10 Hz). (D) Mean z-score of β (15–30 Hz). (E) Mean z-score of low γ (30–50 Hz). (F) Mean z-score of high γ (70–100 Hz).

FIGURE 4

Spectrograms of the μ values of the lognormal fit of the z-score power from two different stimulation conditions in control mice. Methyl salicylate was used for the standard stimulation condition and ethyl acetate was used for the deviant stimulation condition or vice versa. The bars underneath the spectrograms indicate the statistical significance at each frequency band (Wilcoxon rank-sum tests). The red or blue bar indicates p < 0.001 and pink or sky blue bars indicate 0.001 ≤ p < 0.05. δ (1–4 Hz), θ (6–10 Hz), β (15–30 Hz), low γ (30–50 Hz), and high γ (70–100 Hz). (A) Standard trial μ value spectrogram. (B) Deviant trial μ value spectrogram. (C) Deviant – standard trial μ value differential spectrogram.

FIGURE 5

Histology of the olfactory epithelium in control and ZnSO₄-treated mice. Olfactory epithelial tissues were stained using hematoxylin and eosin (H & E) to determine the pathology (A,B) and immunostained with the anti-olfactory marker protein (OMP) antibody to identify mature olfactory response neurons (C,D). (A) Olfactory epithelial tissue from control mice showing an intact olfactory epithelium, which is composed of olfactory sensory neurons, supporting cells, and basal cells. (B) Olfactory epithelial tissue from ZnSO₄-treated mice showing an olfactory epithelium that is disrupted and detached from the turbinate bone. (C) Olfactory epithelial tissue from control mice showing strong expression of OMP. (D) Olfactory epithelial tissue from ZnSO₄-treated mice showing the absence of OMP-stained cells.

FIGURE 6

Stimulus-locked local field potentials (LFPs) in ZnSO₄-treated mice. (A) Examples of stimulus-locked LFPs from ZnSO₄-treated mice do not show differences from before to after odor stimulation in the raw time traces of single-trial LFPs. Averaged photo ionization detector (PID) signals averaged over 70 trials (one session) are depicted for the odorant concentration. The vertical lines indicate the onset and offset moments of odor release. Methyl salicylate was released in this trial. The rising time of the tested odorants in this experiment was ∼100 ms. (B) Examples of the frequency components of the LFPs from before stimulation to the early responses (–1.5∼1.5 s) in the olfactory epithelium. Specifically, the δ (1–4 Hz), θ (6–10 Hz), β (15–30 Hz), low γ (30–50 Hz), and high γ (70–100 Hz) bands.

FIGURE 7

Spectrograms of the μ values of the lognormal fit of the z-score power from two different stimulation conditions in ZnSO₄-treated mice. Methyl salicylate was used for the standard stimulation condition and ethyl acetate was used for the deviant stimulation condition or vice versa. The bars underneath the spectrograms indicate the statistical significance at each frequency band (Wilcoxon rank-sum tests). The red or blue bar indicates p < 0.001 and pink or sky blue bars indicate 0.001 ≤p < 0.05. δ (1–4 Hz), θ (6–10 Hz), β (15–30 Hz), low γ (30–50 Hz), and high γ (70–100 Hz). (A) Standard trial μ value spectrogram. (B) Deviant trial μ value spectrogram. (C) Deviant – standard trial μ value differential spectrogram.