Distraction of olfactory bulb-medial prefrontal cortex circuit may induce anxiety-like behavior in allergic rhinitis

Abstract

Allergic rhinitis is a chronic inflammatory disease of the upper respiratory tract, which is associated with high incidence of anxiety symptom. There is evidence that medial prefrontal cortex modulates anxiety-related behaviors and receives projections from olfactory bulb. Since olfactory dysfunction has been reported in allergic rhinitis, we aimed to evaluate anxiety-like behavior and oscillations of olfactory bulb-medial prefrontal cortex circuit in an animal model of allergic rhinitis. The number of open arm entries in elevated zero maze was significantly reduced in sensitized rats exposed to intranasal ovalbumin compared to the control group, which was indicating the enhancement of anxiety-like behavior in allergic rhinitis animals. Analysis of local field potentials in olfactory bulb and medial prefrontal cortex during immobility and exploration state showed that anxiety-like behavior induced by allergic rhinitis was in association with increased activity of medial prefrontal cortex and enhancement of olfactory bulb-medial prefrontal cortex coupling in delta and theta bands. Moreover, in allergic rhinitis animals, theta strongly coordinates local gamma activity in olfactory bulb and medial prefrontal cortex, which means to have a strong local theta/gamma coupling. We suggested that disruption of olfactory bulb-medial prefrontal cortex circuit due to allergic reactions might have a governing role for inducing anxiety-like behavior in the allergic rhinitis experimental model.

Fig 1. Schematic representation of the experimental design for AR rat model induced by OVA and electrode implantation.

(A) Timeline of the study design. The rats received 7 intraperitoneal injections of saline or OVA-Al(OH)₃ every 2 days from day 0 to day 14, and 7 intra-nasal injections every day from day 15 to day 21 with 10 μL of 10% OVA. (B) Schematic representation of electrode implantation sites on rat skull. (C) Histological confirmation of a recording site in OB and mPFC. (D) Representative traces of simultaneous LFP from OB and mPFC. AR: allergic rhinitis; OB: olfactory bulb; mPFC: medical prefrontal cortex; LFP: local field potential.

Fig 2. The number of nose rubbing of control and AR groups.

Values express as the mean ± SEM. There was a significant increase in the nose rubbing in AR animals compared to the control group on day 21. ** p < 0.01; AR: allergic rhinitis.

Fig 3. AR induces anxiety-like behavior.

(A) Representative tracking areas and heat maps by animals in the EZM for a control (upper panels) and an AR animal (lower panels). Warmer colors represent the animals which spent more time on that sector. (B, C) Spent time and number of entries to the open arena in EZM. (Control: n = 8, OVA: n = 8). (D) Total distance traveled, as an indicator of locomotor activity, in open field test (Control: n = 8, OVA: n = 8). The bar graphs represent mean values ± SEM. Data were analyzed by t-test. * p < 0.05. EZM: elevated zero maze; AR: allergic rhinitis; OB: olfactory bulb; mPFC: medical prefrontal cortex.

Fig 4. AR increases delta and theta power in OB and mPFC.

(A-H) Immobility state. (A, E) Examples of OB and mPFC LFP traces filtered at delta and theta frequencies (< 12 Hz). (B, F) Averaged PSD of recordings in OB and mPFC. Shaded regions denote SEM. (G) AR increases power spectral density of mPFC in delta (< 4 Hz). I-P: Exploration state. (N-P) AR increases delta (< 4 Hz) and theta (4–12 Hz) PSD of mPFC. Inserted panel shows significant differences between AR and control rats in mentioned frequencies. The gray areas indicate significant differences between AR and control rats. Bar graphs represent mean values. Data were analyzed by t-test, n = 8 per group. * p < 0.05, ** p < 0.01 compared to control group. PSD, power spectral density; AR: allergic rhinitis; OB: olfactory bulb; mPFC: medical prefrontal cortex; LFP: local field potential.

Fig 5. Coherence between OB and mPFC is enhanced at delta and theta frequency in AR rats.

(A-C) Immobility state. (A, D) Coherence spectra between OB and mPFC in delta and theta frequencies (< 12 Hz). Shaded area indicates SEM. The gray areas indicate significant differences between AR and control rats. D-F: Exploration state. (B, C, E, F) The bar graphs represent mean values of coherence at delta and theta frequencies. Mean data were analyzed by t-test, n = 8 per group. ** p < 0.01, *** p < 0.05 compared to control group. AR: allergic rhinitis; OB: olfactory bulb; mPFC: medical prefrontal cortex; LFP: local field potential.

Fig 6. Synchrony of delta and theta activity in OB-mPFC circuit.

(A, B) The bar graphs represents cross-correlation coefficients between OB and mPFC in immobility state in delta (< 4 Hz) and theta (4–12 Hz) filtered signals. (C, D) indicates cross-correlation coefficients in delta and theta during exploration state. Data were analyzed by t-test, n = 8 per group. * p < 0.05, ** p < 0.01 compared to control. AR: allergic rhinitis; OB: olfactory bulb; mPFC: medical prefrontal cortex; LFP: local field potential.

Fig 7. AR increases phase-amplitude coupling of theta and fast gamma oscillations in OB of immobile rats.

(A) Peak values of modulation index across amplitude frequencies of theta phase (4–12 Hz) in OB during immobility state. Shaded area indicates standard errors. The gray areas indicate significant differences in fast gamma (60–120 Hz) between AR and control animals. (B) Peak values of modulation index across theta phase frequencies computed for gamma amplitude (60–120 Hz) in OB. (C) The representative comodulogram of modulation index computed for theta phase (4–12 Hz) and gamma (30–120 Hz) in OB. (D) The bar graphs represent mean values of modulation index. (E) The polar plot shows distribution of the gamma (60–120) for the theta cycle phase. Green arrow indicates length of resultant vector. Data were analyzed by t-test, n = 8 per group. **** p < 0.0001 compared to control group. AR: allergic rhinitis; OB: olfactory bulb; LFP: local field potential; PAC: phase amplitude coupling.

Fig 8. AR increases phase-amplitude coupling of theta and fast gamma oscillations in mPFC of exploring rats.

(A) Peak values of modulation index across amplitude frequencies of theta phase (4–12 Hz) in mPFC during exploration state. Shaded area indicates standard errors. The gray area indicates significant differences in fast gamma (60–120 Hz) between AR and control animals. (B) Peak values of modulation index across theta phase frequencies computed for gamma amplitude (60–120 Hz) in mPFC. The gray areas indicate significant differences in 4–7 (Hz). (C) The representative comodulogram of modulation index computed for theta phase (4–12 Hz) and gamma (60–120 Hz) in mPFC. (D) The bar graphs represent mean values of modulation index. (E) The polar plot shows distribution of gamma (60–120) for the theta cycle phase. Green arrow indicates length of resultant vector. Data were analyzed by t-test, n = 8 per group. * p < 0.05, **** p < 0.0001 compared to control group. AR: allergic rhinitis; mPFC: medical prefrontal cortex; LFP: local field potential; PAC: phase amplitude coupling.