Sex differences in heart rate and heart rate variability responses to transcutaneous auricular vagal nerve stimulation in rats

The identification of reliable biomarkers of transcutaneous auricular vagus nerve stimulation (taVNS) responsiveness is a key challenge both at the clinical and preclinical level. Vagally-mediated heart rate variability (vmHRV), a surrogate measure of cardiac vagal efferent activity, is an ideal candidate. Yet, the effects of taVNS on vmHRV remain inconclusive, likely due to the high degree of heterogeneity in stimulation protocols (e.g., taVNS parameters and side of the ear target), and little consideration of contributing factors such as sex differences. This study investigates sex differences in heart rate and vmHRV responses to different protocols of taVNS in adult rats. Male and female wild-type Groningen rats received sham or active stimulation (6 Hz or 20 Hz, 1 ms, 6 V) on the left or right auricular concha region. ECG signals were recorded before (10 min), during (20 min) and after (10 min) each session in a between-subject design. We found differential side-, frequency- and sex-specific chronotropic responses to taVNS, whereby heart rate decreased and vmHRV indexes increased at 6 Hz in males and at 20 Hz in females. Also, increases in vmHRV were only observed for right-side taVNS. The current findings suggest that biological sex should be considered for fine-tuning regulation of taVNS-induced cardiac responses and provide information regarding the side-specific effects of taVNS on vmHRV. These results will likely guide future rodent research to the choice of the most appropriate stimulation protocol in both sexes for generating information that can be translated into taVNS-related strategies in humans.