Research ReportThoracic cross-over pathways of the rat vagal trunks
Introduction
The vagus is a mixed nerve containing afferent and efferent components that connect the brain to almost all of the internal organs (for review of the vagal afferent system see [2]). Considerable focus has been directed toward understanding the sensory functions of the subdiaphragmatic vagus because it is likely to play important roles in providing information to the brain for nutrition, immune signaling, and detection of toxins. Total subdiaphragmatic vagotomy is commonly used to investigate the role of the subdiaphragmatic vagus in physiology and behavior in the rat. However, results from studies using this lesion method are difficult to interpret because both the vagal afferent and efferent nerve fibers are sectioned. A primary side effect of total subdiaphragmatic vagotomy is gastrointestinal stasis and dysfunction that results from cutting vagal efferent fibers [23]. Total subdiaphragmatic vagotomized animals typically have bloated stomachs and must be fed a liquid diet to return body weight and food intake from reduced to normal levels [17].
Three techniques have been developed to selectively lesion the afferent fibers of the rat vagus. First, kainic acid injections into the nodose ganglia result in destruction of sensory cell bodies and appear not to damage efferent fibers of passage [20]; but, perhaps because of the relative difficulty of surgically isolating and injecting the nodose ganglia, this technique has not been widely used. Second, the neurotoxin capsaicin injected systemically or applied directly to the vagus results in destruction of vagal afferent fibers (e.g., [5], [31]). Unfortunately, many vagal afferent fibers are capsaicin-resistant and are not destroyed by this treatment [3]. A third technique uses a unilateral rhizotomy (cutting the afferent vagal rootlets as they exit the cranium before entering the nodose ganglion) combined with a contralateral subdiaphragmatic vagotomy [19], [24], [26], [28], [34], [35], [36], [37], [39]. The result of this method is a bilateral ablation of vagal afferent fibers and a unilateral lesion of vagal efferent fibers. This method can also be used to investigate the efferent system by selectively cutting the efferent vagal rootlets [39]. Unilateral rhizotomy plus contralateral subdiaphragmatic vagotomy is an improvement over the total subdiaphragmatic vagotomy because it does not produce noticeable gastrointestinal motor impairments and animals have normal levels of food intake and body weight shortly after surgery [26], [37], [39]. This methodology for selective ablation of the afferent (or efferent) vagal fibers relies on the assumption that there are no cross-over pathways between vagal trunks within the thoracic cavity of the rat. However, vagal cross-over pathways are well established for a another species, i.e., the thoracic communicating branch in the ferret [8], [9].
Although it is frequently stated that unilateral afferent rootlet rhizotomy combined with contralateral subdiaphragmatic vagotomy is a “complete” or “total” deafferentation of the subdiaphragmatic vagal system, there is some evidence from electrophysiology and tract-tracing studies for vagal cross-over pathways in the rat. Sauter and colleagues reported that electrical stimulation of either the left or right cervical vagus evoked compound action potentials from both the dorsal and ventral subdiaphragmatic vagal trunks [33]. Of the six rats tested, five were reported to show a minor left cross-over pathway and four a minor right cross-over pathway [33] (see Fig. 1 for a schematic of the minor and major pathways of the vagal system). Sauter et al. indicate that the minor vagal pathways accounted for approximately 20% of the total responses in those animals that showed minor pathway responses [33].
Anatomical tracing also suggests that there might be thoracic cross-over pathways between the rat vagal trunks. In contrast to the heavy staining occurring in the ipsilateral nodose ganglion, there were a few cells observed in the contralateral nodose ganglion when HRP (horseradish peroxidase) was applied to a specific subdiaphragmatic vagal trunk [27]. However, evidence suggests that HRP is a less sensitive method for tract tracing and could potentially underestimate the magnitude of a neural pathway (e.g., [11], [38]). More recently, a viral tract-tracing study from the stomach to the brainstem suggests that not all of the vagal afferent fibers are destroyed when rats receive a unilateral afferent rhizotomy combined with contralateral subdiaphragmatic vagotomy [32].
The current experiments were conducted to make a more complete assessment of the vagal cross-over pathways in the rat using electrophysiological techniques. Our experiments were designed to address five issues: (1) use a large group of animals to show descriptive statistics for any cross-over pathways, (2) eliminate sources of signal artifacts, potentially produced by animal movements and neural feedback, by paralyzing animals with pancuronium bromide and cutting the central and distal connections of the vagi, (3) greatly reduce the stimulator produced artifact, probably caused by capacitance discharge through electrode cables, by using biphasic current pulses (see [25]) and offline data processing to remove a slow artifact signal, using a 0.05 Hz high pass filter, (4) use a local anesthetic, bupivacaine, to determine if the electrically evoked compound action potential is neurally mediated, and (5) demonstrate cross-over pathways using a different stimulus, namely direct application of capsaicin to the vagus. This last issue is particularly important since electrical stimulation can produce electromyographic artifacts that can confound interpretations of compound action potential recordings from the vagus nerve (see [33]), but this should not be a problem when using a chemical stimulus, such as capsaicin. Capsaicin was used as a stimulus because it is known to activate a large portion of afferent fiber population of the vagus.
Section snippets
Subjects
Experiments were conducted on male Sprague–Dawley rats (350–500 g; Charles River, Kingston, NY, USA). Surgery and anesthesia is based on established methodology [13], [14]. Animals were euthanized at the conclusion of an experiment by jugular vein infusion of sodium pentobarbital (60 mg). All experiments conformed to established standards of the NIH and the Monell Center IACUC.
Surgery and electrophysiological recordings
Rats were initially anesthetized with sodium pentobarbital (i.p.; 50 mg/kg) followed by a continuous infusion of sodium
Experiment 1
Fig. 2 shows the raw data from a representative recording. A stimulus of 1 mA was used for all analyses because this produced the most reliable responses. Electrical stimulation of the cervical vagi evoked responses from the ventral and dorsal trunks that were dominated by two distinct phases of activity. A statistical analysis of the signals from 1 to 150 ms revealed that electrical stimulation produced statistically significant differences in responses from the major pathways compared to the
Discussion
The current electrophysiology experiments provide strong evidence for vagal cross-over pathways in the rat. It is likely that these cross-over pathways are within the thoracic cavity like they are in the ferret [8], [9]. Electrical stimulation of the left or right cervical vagi revealed C-fiber cross-over pathways that account for an average of 9% of the total response, but there was a great amount of variability (range = 0 to 29%) (see Fig. 6). A neural origin for these responses is indicated
Acknowledgments
This work was supported by grants from the National Institutes of Health (DK065971 and DK36339). The authors are grateful for the initial instruction in recording compound action potentials from the vagi of the rat and for the excellent comments on the manuscript by Dr. Ralph Norgren, Pennsylvania State College of Medicine. The authors also acknowledge the excellent technical assistance of Mr. Marc Ciucci.
References (39)
- et al.
Functional and chemical anatomy of the afferent vagal system
Auton. Neurosci.
(2000) - et al.
Capsaicin-resistant vagal afferent fibers in the rat gastrointestinal tract: anatomical identification and functional integrity
Brain Res.
(1997) - et al.
Truncal and hepatic vagotomy reduce suppression of feeding by jejunal lipid infusions
Physiol. Behav.
(2004) - et al.
Connections of a vagal communicating branch in the ferret: I. Pathways and cell body location
Brain Res. Bull.
(1988) - et al.
Connections of a vagal communicating branch in the ferret: II. Central projections
Brain Res. Bull.
(1988) - et al.
Longitudinal columnar organization within the dorsal motor nucleus represents separate branches of the abdominal vagus
Brain Res.
(1985) - et al.
Plasticity in the mesenteric afferent response to cisplatin following vagotomy in the rat
J. Auton. Nerv. Syst.
(1999) - et al.
Detection of single unit activity from the rat vagus using cluster analysis of principal components
J. Neurosci. Methods
(2003) - et al.
Differential effects on gastrointestinal and hepatic vagal afferent fibers in the rat by the anti-cancer agent cisplatin
Auton. Neurosci.
(2004) - et al.
Specific postoperative syndromes after total and selective vagotomies in the rat
Appetite
(1986)
Reflex suppression and initiation of gastric contractions by electrical stimulation of the hepatic vagus nerve
Neurosci. Lett.
Subdiaphragmatic vagal deafferentation fails to block the anorectic effect of hydroxycitrate
Physiol. Behav.
Excitotoxin-induced degeneration of rat vagal afferent neurons
Neuroscience
Anatomical considerations for surgery of the rat abdominal vagus: distribution, paraganglia and regeneration
Vagal neurons and pathways to the rat's lower viscera: an electrophysiological study
Brain Res. Bull.
Selective effects of vagal deafferentation and celiac-superior mesenteric ganglionectomy on the reinforcing and satiating action of intestinal nutrients
Physiol. Behav.
Conjugates of horseradish peroxidase (HRP) with cholera toxin and wheat germ agglutinin are superior to free HRP as orthogradely transported markers
Brain Res.
The abdominal visceral innervation and the emetic reflex: pathways, pharmacology, and plasticity
Can. J. Physiol. Pharmacol.
Vagal and spinal mechanosensors in the rat stomach and colon have multiple receptive fields
Am. J. Physiol., Regul. Integr. Comp. Physiol.
Cited by (11)
Functional anatomy of the vagus system – Emphasis on the somato-visceral interface
2021, Autonomic Neuroscience: Basic and ClinicalCitation Excerpt :Here, the right and left vagus form a plexus suggestive of fiber exchange between both sides (Fig. 2). Tracing and functional studies in rat indicated that this occurs for efferent and afferent fibers to different though small extents (Fox and Powley, 1985; Norgren and Smith, 1988; Horn and Friedman, 2005). Both left and right vagi enter the abdominal cavity as the anterior and posterior, respectively, trunk, together with the esophagus through its hiatus in the diaphragm.
Protective effect of microinjection of glutamate into hypothalamus paraventricular nucleus on chronic visceral hypersensitivity in rats
2020, Brain ResearchCitation Excerpt :The data from the rats whose target sites did not correspond with the histological criteria were removed from the statistical analysis. The discharge frequency of the vagus was measured according to the method described by Horn and Friedman (2005). The rats were anesthetized and then fixed onto a stereotaxic apparatus.
Implication of the vagus nerve in breathing pattern during sequential swallowing in rats
2017, Physiology and BehaviorCitation Excerpt :The differences concern the length, the route to the larynx and the histological structure [26–27]. In the rat, a cross innervation assured by the vagus nerves and concerning the lungs and abdominal viscera was demonstrated [24,28]. However, no data were reported about the possibility of vagal cross innervation for upper airways.
Effects of chronic aspirations on breathing pattern and ventilatory drive in vagatomized rats
2011, Respiratory Physiology and NeurobiologyCitation Excerpt :This may be due to the fact that the left vagal nerve, the right superior laryngeal nerve, and the two glosso-pharyngeal nerves were left intact. In addition, the thoracic cross innervation of the vagus nerve could explain the absence of modifications following a vagotomy (Horn and Friedman, 2005). Since ventilatory control was not affected, and had no repercussions at rest, our results can only explained by the unilateral laryngeal paralysis and not the suppression of ventilatory afference.
Vagal afferent input alters the discharge of osmotic and ANG II-responsive median preoptic neurons projecting to the hypothalamic paraventricular nucleus
2007, Brain ResearchCitation Excerpt :Alternatively, the different responses of these two neurons may reflect the complexity of vagal afferent activation by electrical stimuli. Since electrical stimulation of the vagus nerve in the present study was likely to activate both A- and C- type vagal afferents (Fan and Andresen, 1998; Horn and Friedman, 2005) and because the cervical vagus nerve contains afferents originating from a variety of sources including the cardiopulmonary circulation and gastrointestinal tract (Berthoud and Neuhuber, 2000), additional studies will be needed to identify the specific type and origin of vagal afferent input that alters the discharge of MnPO-PVH neurons. This not withstanding, Potts and colleagues (Potts et al., 2000) have reported that both volume expansion and hypovolemia increase Fos immunoreactivity in MnPO neurons.
Acute Visceral Pain in Rats: Vagal Nerve Block Compared to Bupivacaine Administered Intramuscularly
2021, Anesthesia and Analgesia