Development of vagal afferent projections circumflex to the obex in the embryonic chick brainstem visualized with voltage-sensitive dye recording

Vagus nerve stimulation for primary headache disorders: An anatomical review to explain a clinical phenomenon

BACKGROUND

Non-invasive stimulation of the vagus nerve has been proposed as a new neuromodulation therapy to treat primary headache disorders, as the vagus nerve is hypothesized to modulate the headache pain pathways in the brain. Vagus nerve stimulation can be performed by placing an electrode on the ear to stimulate the tragus nerve, which contains about 1% of the vagus fibers. Non-invasive vagus nerve stimulation (nVNS) conventionally refers to stimulation of the cervical branch of the vagus nerve, which is made up entirely of vagal nerve fibers. While used interchangeably, most of the research to date has been performed with nVNS or an implanted vagus nerve stimulation device. However, the exact mechanism of action of nVNS remains hypothetical and no clear overview of the effectiveness of nVNS in primary headache disorders is available.

METHODS

In the present study, the clinical trials that investigated the effectiveness, tolerability and safety of nVNS in primary headache disorders were systematically reviewed. The second part of this study reviewed the central connections of the vagus nerve. Papers on the clinical use of nVNS and the anatomical investigations were included based on predefined criteria, evaluated, and results were reported in a narrative way.

RESULTS

The first part of this review shows that nVNS in primary headache disorders is moderately effective, safe and well-tolerated. Regarding the anatomical review, it was reported that fibers from the vagus nerve intertwine with fibers from the trigeminal, facial, glossopharyngeal and hypoglossal nerves, mostly in the trigeminal spinal tract. Second, the four nuclei of the vagus nerve (nuclei of the solitary tract, nucleus ambiguus, spinal nucleus of the trigeminal nerve and dorsal motor nucleus (DMX)) show extensive interconnections. Third, the efferents from the vagal nuclei that receive sensory and visceral input (i.e. nuclei of the solitary tract and spinal nucleus of the trigeminal nerve) mainly course towards the main parts of the neural pain matrix directly or indirectly via other vagal nuclei.

CONCLUSION

The moderate effectiveness of nVNS in treating primary headache disorders can possibly be linked to the connections between the trigeminal and vagal systems as described in animals.

Dialysis membrane-enforced microelectrode array measurement of diverse gut electrical activity

A variety of electrical activities occur depending on the functional state in each section of the gut, but the application of microelectrode array (MEA) is rather limited. We thus developed a dialysis membranes-enforced technique to investigate diverse and complex spatio-temporal electrical activity in the gut. Muscle sheets isolated from the gastrointestinal (GI) tract of mice along with a piece of dialysis membrane were woven over and under the strings to fix them to the anchor rig, and mounted on an 8×8 MEA (inter-electrode distance=150µm). Small molecules (molecular weight <12,000) were exchanged through the membrane, maintaining a physiological environment. Low impedance MEA was used to measure electrical signals in a wide frequency range. We demonstrated the following examples: 1) pacemaker activity-like potentials accompanied by bursting spike-like potentials in the ileum; 2) electrotonic potentials reflecting local neurotransmission in the ileum; 3) myoelectric complex-like potentials consisting of slow and rapid oscillations accompanied by spike potentials in the colon. Despite their limited spatial resolution, these recordings detected transient electric activities that optical probes followed with difficulty. In Addition, propagation of pacemaker-like potential was visualized in the stomach and ileum. These results indicate that the dialysis membrane-enforced technique largely extends the application of MEA, probably due to stabilisation of the access resistance between each sensing electrode and a reference electrode and improvement of electric separation between sensing electrodes. We anticipate that this technique will be utilized to characterise spatio-temporal electrical activities in the gut in health and disease.

Functiogenesis of the embryonic central nervous system revealed by optical recording with a voltage-sensitive dye

Clarification of the functiogenesis of the embryonic central nervous system (CNS) has long been problematic, because conventional electrophysiological techniques have several limitations. First, early embryonic neurons are small and fragile, and the application of microelectrodes is challenging. Second, the simultaneous monitoring of electrical activity from multiple sites is limited, and as a consequence, spatiotemporal response patterns of neural networks cannot be assessed. We have applied multiple-site optical recording with a voltage-sensitive dye to the embryonic CNS and paved a new way to analyze the functiogenesis of the CNS. In this review, we discuss key points of optical recording in the embryonic CNS and introduce recent progress in optical investigations on the embryonic CNS with special emphasis on the development of the chick olfactory system. The studies clearly demonstrate the usefulness of voltage-sensitive dye recording as a powerful tool for elucidating the functional organization of the vertebrate embryonic CNS.

Development of synaptic networks in the mouse vagal pathway revealed by optical mapping with a voltage-sensitive dye

The central issue in developmental neuroscience is when and how neural synaptic networks are established and become functional within the central nervous system (CNS). Investigations of the neural network organization have been hampered because conventional electrophysiological means have some technical limitations. In this study, the multiple-site optical recording technique with a voltage-sensitive dye was employed to survey the developmental organization of the vagal system in the mouse embryo. Stimulation of the vagus nerve in E11-E14 mouse embryos elicited optical responses in areas corresponding to the vagal sensory and motor nuclei. Postsynaptic responses in the first-order sensory nucleus, the nucleus of the tractus solitarius (NTS), were identified from E11, suggesting that sensory information becomes transferred to the brain at this stage. In addition to the NTS, optical responses were identified in the rostral and contralateral brainstem regions, which corresponded to second/higher order nuclei of the vagus nerve including the parabrachial nucleus (PBN). Postsynaptic responses in the second/higher-order nuclei were detected from E12, suggesting that polysynaptic networks were functional at this stage. We discuss the results of our optical mapping, comparing them with previous findings obtained in the chick and rat embryos, and suggest some fundamental principles in the functional organization of synaptic networks in the embryonic brain.

The embryonic brain and development of vagal pathways

To regulate the autonomic function, the vagus nerve transfers various sensory information from peripheral organs, and appropriate motor reflexes are produced in the neural circuit. The functional development of the vagal pathway during the early phase of embryonic development has long been unclear. Optical recording with voltage-sensitive dyes has provided a new approach to the analysis of the functional development of the embryonic central nervous system. In this review, we present recent progress in optical studies on the vagal pathway in the embryonic chick and rat brainstems. The topics include how neural excitability is initially expressed in the motor and sensory nuclei [e.g. the dorsal motor nucleus of the vagus nerve (DMNV) and the nucleus of the tractus solitarius (NTS)] and how synapse networks are formed in the primary and higher-ordered sensory nuclei [e.g. the parabrachial nucleus (PBN)]. We also refer to the functional development of the glossopharyngeal nuclei and compare the developmental steps with those of the vagal nuclei.