Vagus Nerve Stimulation: Integration of Behavior and Cardiac Modulation

Major depressive disorder (MDD) has been associated with alterations of the stress response circuitry, including the hypothalamus, amygdala, hippocampus, anterior cingulate cortex, ventromedial, dorsolateral and orbital prefrontal cortices. Many of these regions are morphologically and functionally sexually dimorphic and associated with vulnerability for sex differences in MDD. A major role for the stress response circuitry is to assess potentially stressful stimuli and respond with a neuroendocrine signal that coordinates homeostatic responses throughout the body. Neuroimaging studies have suggested that alterations in this circuitry are implicated in mood dysregulation, increased activation of the hypothalamic-pituitary-adrenal (HPA) axis, and imbalance between the sympathetic and parasympathetic nervous system in depressed persons. A better understanding of the mechanisms underlying alterations in physiological response to stress in major depression may contribute to the development of novel interventions that regulate this system with a significant impact on the improvement of clinical and physiological alterations of MDD. It has been previously suggested that modulation of vagus nerve activity may have a significant effect on the modulation of the brain circuitry involved in the regulation of mood and stress response. Recently, a non-invasive approach for modulation of vagus nerve activity, transcutaneous auricular vagus nerve stimulation (tVNS), which targets the auricular branch of the vagus nerve (ABVN) has been proposed. Moreover, previous studies have shown that vagal afference is highly regulated by respiration and that modulation of vagus nerve activity may be optimized by gating ABVN stimulation to the exhalatory phase of the respiratory cycle. Thus, this study proposed to characterize the impact of respiratory-gated tVNS on the modulation of the stress response circuitry, vagal tone and depressed mood in patients with recurrent major depression (MDD). This is not a clinical trial aimed to test a medical device, but a basic experimental study oriented to understand the effects of vagal afference modulation on brain and cardiovagal physiological response to stress in major depression. Twenty premenopausal women with recurrent MDD in an active episode were recruited into a single-blind cross-over study that included two functional MRI visits, within a one week period, with simultaneous mood and physiological assessments. Randomization to exhalatory- or inhalatory-gated tVNS was performed to control for order effects. Subjects were exposed to a mild visual stress challenge that preceded and followed 30 minutes of exhalatory- or inhalatory-gated tVNS. The study hypothesis was that exhalatory-gated tVNS would have a significantly greater impact on the regulation of brain activity in stress response circuitry, vagal tone and depressed mood in MDD patients compared to inhalation-gated tVNS