Research paperGastric vagal afferent neuropathy following experimental spinal cord injury
Introduction
Widespread loss of autonomic function is a common consequence of cervical and high thoracic spinal cord injury (SCI). Clinical reports indicate reduced gastric emptying after T3-SCI (Kirshblum et al., 2002;Williams et al., 2011;Wolf and Meiners, 2003) and early satiety that may persist for years after the initial injury (Berlly and Wilmot, 1984; Cosman et al., 1991; Fealey et al., 1984; Kao et al., 1999; Kewalramani, 1979; Nino-Murcia and Friedland, 1991; Rajendran et al., 1992; Segal et al., 1995; Stinneford et al., 1993). We have demonstrated in a rat model of experimental T3-SCI that ad libitum feeding and gastric motility are diminished beginning immediately after injury (Tong and Holmes, 2009) and that chronic derangements in feeding, gastric motility and emptying persist outward to 6–18 weeks (Primeaux et al., 2007; Qualls-Creekmore et al., 2010a; Tong and Holmes, 2009).
Gastric motility is dominated by parasympathetic vagal reflex control (reviewed in Holmes et al., 2017). The vagal afferents originating from the gastrointestinal (GI) tract are composed of two distinct sensory mechanoreceptors to detect both the mechanical distension of the stomach and luminal stimuli. Mechanoreceptors with morphological specializations known as intraganglionic laminar endings (IGLEs; Powley and Phillips, 2002) and intramuscular arrays (IMAs; Berthoud and Powley, 1992) line the gastrointestinal mucosa and smooth muscle and detect both tension (stretch) exerted upon the gut wall as well as stimuli regarding composition and density of the chyme (Page et al., 2002).
One other component of the homeostatic regulation of energy intake is the reflex transmission of chemical feeding-related signals from the GI tract to the central nervous system (CNS). Essentially, afferent fibers lining the gastric wall detect chemical mediators released by specialized cells lining the lumen of the GI tract. For example, the chemical composition of macronutrients as well as other chemicals within the GI lumen triggers the release of a wide range of peptides and classical neurotransmitters from enteroendocrine cells as part of both a paracrine and endocrine signaling cascade (Douglas et al., 1988; Liddle, 1997). These gut-derived factors (e.g., CCK, GLP-1 and ghrelin) are released during feeding or fasting and act peripherally upon receptors expressed on vagal sensory terminals in the gut wall as well as on the vagal afferent cell bodies within the NG (Broberger et al., 2001; Dockray, 2014; Helke and Hill, 1988). Specifically, the regulatory effect of the GI peptide cholecystokinin (CCK) has received the greatest attention and has been shown to modulate the firing rate of vagal afferent fibers (Okano-Matsumoto et al., 2011).
Derangements in these mechano- and chemosensory afferent pathways have been implicated in a wide variety of pathophysiological states (Dyavanapalli et al., 2016; Kentish and Page, 2014; Mazzone and Undem, 2016). For example, the molecular mechanism behind the GI response to pungent foods became evident with the identification of the transient receptor potential vanilloid type 1 receptor (TRPV1; Yu et al., 2016). The role of TRPV1 has frequently received attention in association with inflammatory conditions and nociceptive sensation in somatic and visceral tissues (Caterina and Julius, 2001; Jia and Lee, 2007; Lee and Gu, 2009; Zhao and Simasko, 2010; Zhao et al., 2010). In addition to activation by the ingestion of capsaicin, the active compound in pungent foods, TRPV1 is activated by noxious stimuli (Furuta et al., 2012; Nilius et al., 2007a; Nilius et al., 2007b) and TRPV1 upregulation may occur as a consequence of inflammation, including GI inflammation that accompanies SCI (Besecker et al., 2017). Physiological roles of the TRPV channel family, including TRPV1 involvement in mechano- and osmosensitivity are emerging (Liedtke and Kim, 2005). Furthermore, a recent report has demonstrated a functional role for TRPV1 in the GI tract. Mice deficient in the TRPV1 receptor also demonstrated attenuated response of mechanoreceptors to stretch (Kentish et al., 2015). Therefore, TRPV1 channels are in a position to modulate the satiety-inducing effects of stretch and anecdotal reports of early satiety by many individuals with SCI suggesting a potential role of TRPV1-mediated sensitivity of mechanoreceptors after injury.
The apparent reduction of GI chemosensory signaling to medullary neural circuits (Besecker et al., 2018; Tong et al., 2011) raises the question of pathophysiological remodeling of vagal afferent fibers following SCI. Using molecular techniques and in vivo electrophysiological recordings of the vagal afferent fibers innervating the stomach, we tested the hypothesis that acute and chronic high-thoracic (spinal level T3) SCI attenuate gastric mechanosensitivity. In order to establish if any previously observed reduction in sensitivity to CCK following SCI could be due to changes in receptor expression, we utilized qRT-PCR to determine the relative expression of CCK-A receptor as well as TRPV1 receptor mRNA in the gastric vagal afferent cell bodies of the NG. Finally, we recorded the afferent neural responses to systemic administration of pharmacological doses of CCK and the TRPV1 agonist, capsaicin to evaluate the functional chemosensitivity of vagal afferent fibers.
Section snippets
Animals
All studies followed National Institutes of Health guidelines and were approved by the Institutional Animal Care and Use Committee at the Penn State Hershey College of Medicine. Male Wistar rats ≥8 weeks of age, upon entrance into the experiment, and initially weighing 175–200 g (Harlan, Indianapolis, IN, USA) were used. Animals were initially double housed in a room maintained at 21–24 °C and a 12:12-h light-dark cycle with food and water provided ad libitum. After surgery animals were single
Characterization of 300 kdyne injury level
The extent of the lesion epicenter was determined based upon the reduction of LFB-stained white matter at the T3 spinal cord segment. Our 300 kdyne lesion, with a 15 s dwell time, historically produces very little white mater sparing at the lesion epicenter which is necessary to affect the diffuse distribution of descending supraspinal inputs to autonomic preganglionic centers (Besecker et al., 2017; Swartz and Holmes, 2014). The white matter calculated as percent area of total spinal cord
Discussion
Data from this study demonstrated that vagal afferent signaling in response to gastric mechano- and chemosensory stimuli is reduced following high-thoracic SCI. Specifically, these experimental data indicate that: 1) T3-SCI provoked a prolonged desensitization of mechanoceptors of gastric vagal afferents; 2) T3-SCI contributed to an acute chemoceptor desensitization of systemic administration of CCK-8 s with differential recovery long-term; 3) T3-SCI provoked a significant amplification in the
Conclusion and translational perspective
We have previously shown that the efferent limb of the gastric vago-vagal reflex remained functionally capable of modulating gastric contractility (Swartz and Holmes, 2014) and our current data support the suggestion that diminished signaling within the afferent limb may result in a net inhibition of the efferent vagus (Holmes, 2012). In a broader sense, there is considerable attention to an anti-inflammatory circuit which involves the afferent and efferent vagus nerve and α7 nicotinic
Funding sources
This work was supported by National Institute of Neurological Disorders and Stroke (NINDS) Grants R01-NS-049177 and F31-NS-087834.
Declaration of Competing Interest
The authors have declared that no conflicts of interest exist.
Acknowledgements
The authors wish to thank Dr. Amanda Troy for providing initial guidance for nodose excision. Dr. Victor Ruiz-Velasco provided assistance in nodose ganglion dissociations. Margaret McLean provided assistance with animal care, tissue harvest, data entry and analysis. Dr. Salvatore Stella, Jr. graciously assisted with all aspects of confocal microscopy.
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