Transcutaneous auricular vagus nerve stimulation inhibits mental stress‐induced cortisol release—Potential implications for inflammatory conditions

Figures

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  FIGURE 1. Study participants reported for two study sessions at least 1 week apart. Throughout the study sessions the electrocardiogram (ECG) was recorded continuously. Each study session consisted of a 30‐min baseline recording, followed by 30 min of either transcutaneous auricular vagus nerve stimulation (taVNS, experimental study day) or sham‐taVNS (control study day). Electrode placement is shown in the inset. The order of the experimental versus control study day was randomized. Ten minutes into the taVNS or sham taVNS intervention, a mental arithmetic stress test (MAST) was conducted for 15 min. The 30 min of taVNS or sham‐taVNS were followed by a 40‐min recovery period. B1, B2, V1, VS, V2, R1, R2, R3, and R4 represent segments within the experimental protocol that were used for heart rate variability analysis. Throughout the experimental protocol saliva samples were collected at 15, 30, 40, 55, 60, 70, 80, 90, and 100 min (black arrows on bottom of figure).

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  FIGURE 2. Heart rate (HR, top row) and heart rate variability expressed as low frequency heart rate variability (LFHR, 2nd row), high frequency RR‐interval variability (HFRR, 3rd row), and LFHR to HFRR ratio (LFHR/HFRR, bottom row). The left column shows absolute data, the right column shows changes (Δ) from time interval B2 (15–30 min, see Figure 1). Error bars represent standard errors of the mean. #: Sham‐taVNS vs. taVNS; *: Respective time interval vs. time interval B2.

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  FIGURE 3. Top row: Salivary cortisol concentrations in absolute units (ng/mL, left) and in percent from time point B1 (15 min, right). The data from all 12 study participants are included. Error bars represent standard errors of the mean. Bottom row: In 7 study participants, the experiments were performed in the morning (AM) and in 5 study participants, the experiments were performed in the afternoon (PM). For individual study participants, the sham‐taVNS and taVNS experiments were always performed at the same time of the day (either AM or PM). Salivary cortisol concentrations expressed as percent from time point B1 (15 min) are shown for the experiments performed in the AM (left) and PM (right). The absolute cortisol concentrations at time point B1 (in ng/mL) are provided for the sham‐taVNS (S) and taVNS (V) conditions. #: Sham‐taVNS versus taVNS; *: Respective time interval versus time interval B1. Error bars represent standard errors of the mean.

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  FIGURE 4. Saliva flow rate did not change significantly throughout the sham‐taVNS and taVNS protocols. Error bars represent standard errors of the mean.

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  FIGURE 5. Left: Diurnal salivary cortisol concentrations in a single patient with plaque psoriasis before (days −9, −8, −2, and −1, white circles) initiation of a chronic taVNS protocol and during the early (days 11 and 12, gray circles) and chronic (days 35, 66, 90, and 91, black circles) phases of the taVNS protocol. Error bars represent the variability between the respective study days expressed as standard errors of the mean. Right: Cumulative salivary cortisol concentrations in a single patient with plaque psoriasis during the 15 h between 6:00 am and 9:00 pm expressed as the area under the curve (AUC) for the baseline before taVNS (days −9, −8, −2, and −1, white bar) and for the early (days 11 and 12, gray bar) and chronic (days 35, 66, 90, and 91, black bar) phases of the taVNS protocol. Data are shown as Box‐Whisker plots with the “whiskers” representing the upper and lower extreme values, the “box” representing the upper and lower quartiles and the horizontal line representing the median.