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  Vagus Nerve Stimulation Therapy: 
A New Tool for Suppressing Visceral Pain

  On the Architecture of Synapses

  Unlocking the Causes of Stroke in Asia: The Importance of Intracranial Atherosclerosis

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It is well known that a hot drink or a nourishing meal is relaxing and helps to calm anxiety suggesting that enhanced sensory vagal inputs originating from the gut modulate attitude and behavior. Although vagal afferents are activated by noxious gastrointestinal stimuli, the contribution of the vagal nerve to visceral pain remains unresolved. Rodents do not have the forebrain structures to generate the cognitive feelings of humans, the use of behavioral paradigms visceromotor responses (VMR) to assess spinal pain reflexes in the conscious rat may help to identify the regulatory role of the vagal nerve in visceral pain sensation. We demonstrated that chronic subdiaphragmatic vagotomy decreases the threshold and enhances the VMR to all grades of colorectal distension (CRD) suggesting vagal nerves are involved in the inhibitory modulation of visceral pain responses.

Vagus nerve stimulation with the neuro-cybernetic prosthesis generator has been used clinically as a treatment for refractory epilepsy, major depression, and gastric dysrhythmia. To date, the influences of vagal nerve stimulation in visceral pain evoked by viscera nociceptive stimuli have not been investigated. Vagal electrical stimulation by different intensities may activate different types of nerve fibers. In this study, vagal afferent neuronal responses to low or high intensity electrical vagal stimulation (EVS) of afferent A or C fibers were distinguished by calculating their conduction velocity. Here, we showed that CRD produced contractions of the lateral abdominal musculature. High intensity EVS (400 μA,) which activated C-type fibers had no effect on CRD-induced abdominal pain. In contrast, low intensity electrical vagal stimulation (40 μA) which activated vagal A-type fibers reduced CRD-induced abdominal muscle contractions. This response was not affected by perivagal capsaicin-treatment. These observations suggest that vagal afferent nerves modulate visceral pain. Low intensity EVS which activates vagal afferent A- fibers reduced visceral pain.

From left to right: Prof Ying Li, Miss Chun Hao, Miss Ni Yan, Miss Jiahe Xu




Figure 1. Vagal afferent C-type neuron 
Upper panel A, the cell was marked by Neurobiotin. B, the ganglion contains several neurons expressing VR1 recepor. C, the overlay of images shows that the Neurobiotin-marked cell contains VR1 receptor. Lower panel A’, the same neuron marked by Neurobiotin B’, incubated with P2X receptor antiserum C’, this Neurobiotin-labeled neuron does not express P2X receptor.

Figure 2. Vagal afferent A-type neuron
Upper panel A, the neuron labeled by Neurobioti B, the neuron contains VR1 receptors. C, overlay of images showing the Neurobiotin-marked cell does not express VR1 receptor. Lower panel shows the same neurons marked by Neurobiotin contain P2X receptors.

To identify the neurotransmitters and receptors of vagal afferent neurons activated by EVS single neuronal activities of nodose neurons were recorded in vivo in rats, followed by juxtacellular neurobiotin labeling. In consistent with our published data intestinal perfusion of serotonin (5-HT) activates subpopulations of both A- and C-type vagal afferent neurons. Double labeling immunocytochemistry showed that all of the C-type neurons labeled by neurobiotin contained VR1 receptors (Fig. 1). In contrast, while A-type neurons contained P2X receptors (Fig. 2), none of them expressed VR1 receptors suggesting P2X and VR1 receptors on vagal afferent neurons are involved in mediating distinct 5-HT activated autonomic functions.

In summary, we explore the visceral analgesic properties of subdiaphragmatic vagus nerve in rats and show that acute low intensity electrical vagal stimulation reduces visceral pain suggesting that a group of vagal afferents innervating viscera may have remarkable functions that are related to visceral pain inhibition. Thus, vagal nerve stimulation may have therapeutic potential in visceral pain treatment. 

Prof Ying LI
Department of Biology and Chemistry
City University of Hong Kong