Activity Patterns Elicited by Airflow in the Olfactory Bulb and Their Possible Functions

Abstract

Olfactory sensory neurons (OSNs) can sense both odorants and airflows. In the olfactory bulb (OB), the coding of odor information has been well studied, but the coding of mechanical stimulation is rarely investigated. Unlike odor-sensing functions of OSNs, the airflow-sensing functions of OSNs are also largely unknown. Here, the activity patterns elicited by mechanical airflow in male rat OBs were mapped using fMRI and correlated with local field potential recordings. In an attempt to reveal possible functions of airflow sensing, the relationship between airflow patterns and physiological parameters was also examined. We found the following: (1) the activity pattern in the OB evoked by airflow in the nasal cavity was more broadly distributed than patterns evoked by odors; (2) the pattern intensity increases with total airflow, while the pattern topography with total airflow remains almost unchanged; and (3) the heart rate, spontaneous respiratory rate, and electroencephalograph power in the β band decreased with regular mechanical airflow in the nasal cavity. The mapping results provide evidence that the signals elicited by mechanical airflow in OSNs are transmitted to the OB, and that the OB has the potential to code and process mechanical information. Our functional data indicate that airflow rhythm in the olfactory system can regulate the physiological and brain states, providing an explanation for the effects of breath control in meditation, yoga, and Taoism practices.SIGNIFICANCE STATEMENT Presentation of odor information in the olfactory bulb has been well studied, but studies about breathing features are rare. Here, using blood oxygen level-dependent functional MRI for the first time in such an investigation, we explored the global activity patterns in the rat olfactory bulb elicited by airflow in the nasal cavity. We found that the activity pattern elicited by airflow is broadly distributed, with increasing pattern intensity and similar topography under increasing total airflow. Further, heart rate, spontaneous respiratory rate in the lung, and electroencephalograph power in the β band decreased with regular airflow in the nasal cavity. Our study provides further understanding of the airflow map in the olfactory bulb in vivo, and evidence for the possible mechanosensitivity functions of olfactory sensory neurons.

Keywords: airflow map; airflow rhythm; functional MRI; mechanosensitivity; olfactory bulb.

Figure 1.

Diagram of the mechanical stimulus models. The tube connected to the opened trachea for transporting pure air to the nasal cavity was linked to a ventilator, which controls the flow rate of the mechanical stimulation. Another tube connected to the lungs was kept opened for free breathing. Three stimulus models were used: Model I: fixed tidal volume with varying frequency (1.0 Hz/1.0 ml, 1.5 Hz/1.0 ml, and 2.5 Hz/1.0 ml); Model II, fixed frequency and varying tidal volume (1.5 Hz/0.5 ml, 1.5 Hz/1.0 ml, and 1.5 Hz/2.0 ml); and Model III, fixed total airflow strength while tidal volume and frequency were varied simultaneously (1.0 Hz/2.5 ml, 1.5 Hz/1.67 ml, and 2.5 Hz/1.0 ml). Each model included three stimulation conditions, and a total of seven conditions were used (1.0 Hz/1.0 ml, 1.5 Hz/1.0 ml, 2.5 Hz/1.0 ml, 1.5 Hz/0.5 ml, 1.5 Hz/2.0 ml, 1.0 Hz/2.5 ml, and 1.5 Hz/1.67 ml).

Figure 2.

The activity patterns in the OB evaluated by BOLD-fMRI. A, B, Patterns from individual rat evoked by (A) different odorants (Odor 1: amyl acetate; Odor 2: hexanal) in the OB and (B) by airflow mechanical stimulation. Activation voxels displayed with warmer colors indicate stronger activations. C, Activation rate in the OB to three different stimuli. D, The SCCs between different maps, including between two odor maps (Odor 1 | Odor 2), between each of the two mechanical maps within Model I (1 Hz/1 ml | 1.5 Hz/1 ml, 1 Hz/1 ml | 2.5 Hz/1 ml, and 1.5 Hz/1 ml | 2.5 Hz/1 ml), Model II (1.5 Hz/0.5 ml | 1.5 Hz/1 ml, 1.5 Hz/0.5 ml | 1.5 Hz/2 ml, and 1.5 Hz/1 ml | 1.5 Hz/2 ml), and Model III (1 Hz/2.5 ml | 1.5 Hz/1.67 ml, 1 Hz/2.5 ml | 2.5 Hz/1 ml, and 1.5 Hz/1.67 ml | 2.5 Hz/1 ml). Unpaired t test, *p < 0.05.

Figure 3.

Similarity analysis among odor maps and mechanical maps in the OB. A, One example of how the OB slice was divided into 16 parts. B–D, Averaged recombined pattern of two odor maps and mechanical map, created by dividing each OB slice into 8 parts (B), 16 parts (C), and 32 parts (D). Warm colors indicated high activation. D, Dorsal; L, lateral; V, ventral; M, medial. E–G, Correlation coefficient between patterns reconstructed from 8 parts (E), 16 parts (F), and 32 parts (G). The green lines indicated statistical cutoffs at p = 0.001.

Figure 4.

BOLD responses to different mechanical stimuli in the OB. A–C, Model I: The elicited BOLD patterns (A), corresponding time courses of BOLD signals (B), and statistical analysis (C). Data were normalized to the response of the 2.5 Hz/1.0 ml stimulation. Paired t test, *p < 0.05; n = 5/one-way ANOVA, F_(2,12) = 17.78, *p < 0.05. D–F, Model II: the elicited BOLD patterns (D), corresponding time courses of BOLD signals (E), and statistical analysis (F). Data were normalized to the response of 1.5 Hz/2.0 ml stimulation. Paired t test, *p < 0.05; n = 5/one-way ANOVA, F_(2,12) = 14.15, *p < 0.05. G–I, Model III: the elicited BOLD patterns (G), corresponding time courses of BOLD signals (H), and statistical analysis (I). Data were normalized to the response of 2.5 Hz/1.0 ml stimulation. Paired t test, N.S, p > 0.5; n = 6/one-way ANOVA, F_(2,15) = 0.042, N.S, p > 0.5. Error bar, SE.

Figure 5.

LFP in the OB under different mechanical stimulation conditions. A, An example of raw LFP signals and filtered signals for three different frequency bands (θ band: 1–12 Hz; β band: 12–35 Hz; γ band: 35–100 Hz) elicited by mechanical stimulus. B, The time courses of the LFP signals induced by mechanical stimulation in A. C, The γ-band signal change in the OB to Model I conditions. Data were normalized to the response of 2.5 Hz/1.0 ml. Paired t test, *p < 0.05, n = 10/one-way ANOVA, F_(2,27) = 7.26, *p < 0.05. D, The γ-band response to the Model II conditions. Data were normalized to the response of 1.5 Hz/2.0 ml. Paired t test, *p < 0.05, n = 10/one-way ANOVA, F_(2,27) = 50.5, *p < 0.05. E, The γ-band signal to the Model III conditions. Data were normalized to the response of 2.5 Hz/1.0 ml. Paired t test, N.S, p > 0.5, n = 10/one-way ANOVA, F_(2,27) = 0.52, N.S, p > 0.1. Error bar, SE.

Figure 6.

The main frequency of the LFP θ band was coupled with the mechanical airflow frequency in the endotracheal intubation animal. A, Frequency spectra of the θ band at different stimulus frequencies (top, 1.0 Hz/1.0 ml; middle, 1.5 Hz/1.0 ml; bottom, 2.5 Hz/1.0 ml). B, Frequency spectra of the θ-band signal at different stimulus strengths with the same frequency (top, 1.5 Hz/0.5 ml; middle, 1.5 Hz/1.0 ml; bottom, 1.5 Hz/2.0 ml). The frequency spectra during and before stimulation (resting) were presented with red and blue lines respectively. The main frequencies were indicated by green arrows.

Figure 7.

Respiratory rate, heart rate, and EEG were influenced by mechanical rhythm. A, B, Respiratory rates at resting state and mechanical state as a group (A) and as individuals (B). C, D, Heart rates at resting state and mechanical state as a group (C) and as individuals (D). E, β Band of EEG powers at resting state and mechanical state as a group. F, Powers of δ band, θ band, α band, and γ band of EEG at resting state and mechanical state. Unpaired t test, *p < 0.05; N.S, p > 0.05. Error bar: SE. The mechanical stimulation was 1.0 Hz/2.5 ml.