Melissa Tong

Vagally mediated effects of glucagon-like peptide 1: in vitro and in vivo gastric actions

Glucagon‐like peptide‐1 (GLP‐1) is a neuropeptide released following meal ingestion that, among other effects, decreases gastric tone and motility. The central targets and mechanism of action of GLP‐1 on gastric neurocircuits have not, however, been fully investigated. A high density of GLP‐1 containing neurones and receptors are present in brainstem vagal circuits, suggesting that the gastroinhibition may be vagally mediated. We aimed to investigate: (1) the response of identified gastric‐projecting neurones of the dorsal motor nucleus of the vagus (DMV) to GLP‐1 and its analogues; (2) the effects of brainstem application of GLP‐1 on gastric tone; and (3) the vagal pathway utilized by GLP‐1 to induce gastroinhibition. We conducted our experiments using whole‐cell recordings from identified gastric‐projecting DMV neurones and microinjection in the dorsal vagal complex (DVC) of anaesthetized rats while monitoring gastric tone. Perfusion with GLP‐1 induced a concentration‐dependent excitation of a subpopulation of gastric‐projecting DMV neurones. The GLP‐1 effects were mimicked by exendin‐4 and antagonized by exendin‐9–39. In an anaesthetized rat preparation, application of exendin‐4 to the DVC decreased gastric tone in a concentration‐dependent manner. The gastroinhibitory effects of exendin‐4 were unaffected by systemic pretreatment with the pro‐motility muscarinic agonist bethanechol, but were abolished by systemic administration of the nitric oxide synthase (NOS) inhibitor NG‐nitro‐l‐arginine methyl ester (l‐NAME), or by bilateral vagotomy. Our data indicate that GLP‐1 activates selective receptors to excite DMV neurones mainly and that the gastroinhibition observed following application of GLP‐1 in the DVC is due to the activation of an inhibitory non‐adrenergic, non‐cholinergic input to the stomach.

Effects of brain stem cholecystokinin-8s on gastric tone and esophageal-gastric reflex

The actions of cholecystokinin (CCK) on gastrointestinal functions occur mainly via paracrine effects on peripheral sensory vagal fibers, which engage vago-vagal brain stem circuits to convey effector responses back to the gastrointestinal tract. Recent evidence suggests, however, that CCK also affects brain stem structures directly. Many electrophysiological studies, including our own, have shown that brain stem vagal circuits are excited by sulfated CCK (CCK-8s) directly, and we have further demonstrated that CCK-8s induces a remarkable degree of plasticity in GABAergic brain stem synapses. In the present study, we used fasted, anesthetized Sprague-Dawley rats to investigate the effects of brain stem administration of CCK-8s on gastric tone before and after activation of the esophageal-gastric reflex. CCK-8s microinjected in the dorsal vagal complex (DVC) or applied on the floor of the fourth ventricle induced an immediate and transient decrease in gastric tone. Upon recovery of gastric tone to baseline values, the gastric relaxation induced by esophageal distension was attenuated or even reversed. The effects of CCK-8s were antagonized by vagotomy or fourth ventricular, but not intravenous, administration of the CCK-A antagonist lorglumide, suggesting a central, not peripheral, site of action. The gastric relaxation induced by DVC microinjection of CCK-8s was unaffected by pretreatment with systemic bethanecol but was completely blocked by NG-nitro-L-arginine methyl ester, suggesting a nitrergic mechanism of action. These data suggest that 1) brain stem application of CCK-8s induces a vagally mediated gastric relaxation; 2) the CCK-8s-induced gastric relaxation is mediated via activation of nonadrenergic, noncholinergic pathways; and 3) CCK-8s reverses the esophageal-gastric reflex transiently.

Time-course of recovery of gastric emptying and motility in rats with experimental spinal cord injury

Abstract  We have shown recently that spinal cord injury (SCI) decreases basal gastric contractions 3 days after injury. In the present study we used the [13C]‐octanoic acid breath test and gastric strain gauges with the aim to investigate the time‐course of recovery from postinjury gastric stasis in rats that underwent experimental SCI at the level of the third thoracic (T3) vertebra. Following verification of the [13C]‐breath test sensitivity in uninjured rats, we conducted our experiments in rats that underwent T3‐ spinal contusion injury (T3‐CI), T3‐spinal transection (T3‐TX) or laminectomy (control) surgery at 3 days, 1, 3 or 6 weeks postinjury. Our data show that compared to rats that underwent laminectomy, rats that received SCI showed a significant reduction in the cumulative per cent [13C] recovery. Although more marked in T3‐TX rats, the delayed gastric emptying in T3‐CI and T3‐TX rats was comparable in the 3 days to 3 weeks period postinjury. At 6 weeks postinjury, the gastric emptying in T3‐CI rats recovered to baseline values. Conversely animals in the T3‐TX group still show a significantly reduced gastric emptying. Interestingly, the almost complete functional recovery observed in T3‐CI rats using the [13C]‐breath test was not reflected by analysis of spontaneous gastric contractions after SCI. These data indicate that T3‐SCI produces a significant reduction in gastric emptying independent of injury severity (T3‐CI vs T3‐TX) that persists for at least 3 weeks after injury. However, 6 weeks postinjury T3‐CI, but not T3‐TX, rats begin to demonstrate functional recovery of gastric emptying.

Gastric emptying of enterally administered liquid meal in conscious rats and during sustained anaesthesia.

Background  Gastric motility studies are frequently conducted with anaesthetized animal models. Some studies on the same animal species have reported differences in vagal control of the stomach that could not be explained solely by slightly different experimental conditions. A possible limitation in the comparison between similar studies relates to the use of different anaesthetic agents. Furthermore, anaesthetic effects may also limit generalizations between mechanistic studies of gastric function and the gastric function of conscious animals. In the present study, we used the [13C]‐breath test following a liquid mixed‐nutrient test meal (Ensure®, 1 ml) with the aim to investigate the rate of gastric emptying in animals that were either conscious or anaesthetized with either Inactin® or urethane.

Gastric dysreflexia after acute experimental spinal cord injury in rats.

Abstract  Gastric reflexes are mediated mainly by vago‐vagal reflex circuits in the caudal medulla. Despite the fact that brainstem vago‐vagal circuitry remains intact after spinal cord injury (SCI), patients with SCI at the cervical level most often present gastric stasis with an increased risk of reflux and aspiration of gastric contents. Using a miniature strain gauge sutured to the gastric surface; we tested gastric motility and reflexive gastric relaxation following oesophageal distension (oesophageal‐gastric relaxation reflex) in animals 3 days after a severe spinal contusion at either the third or ninth thoracic spinal segment (acute T3‐ or T9 SCI, respectively). Both basal gastric motility and the oesophageal‐gastric relaxation reflex were significantly diminished in animals with T3 SCI. Conversely, both basal gastric motility and the oesophageal‐gastric relaxation reflex were not significantly reduced in T9 SCI animals compared to controls. The reduced gastric motility and oesophageal‐gastric reflex in T3 SCI rats was not ameliorated by celiac sympathectomy. Our results show that gastric stasis following acute SCI is independent of altered spinal sympathetic input to the stomach caudal to the lesion. Our data suggest that SCI may alter the sensitivity of vagal reflex function, perhaps by interrupting ascending spinosolitary input to brainstem vagal nuclei.

Experimental spinal cord injury in rats diminishes vagally‐mediated gastric responses to cholecystokinin‐8s

Background  We have shown recently that our model of experimental high‐thoracic spinal cord injury (T3‐SCI) mirrors the gastrointestinal clinical presentation of neurotrauma patients, whereby T3‐SCI animals show diminished gastric emptying and dysmotility. In this study we used cholecystokinin as a model peptide to test the hypothesis that the T3‐SCI induced gastroparesis is due, in part, to an impaired vagally‐mediated response to gastrointestinal peptides. Methods We measured the responses to sulfated cholecystokinin (CCK‐8s) in control and T3‐SCI (3 or 21 days after injury) rats utilizing: (i) c‐fos expression in the nucleus tractus solitarius (NTS) following peripherally administered CCK‐8s; (ii) in vivo gastric tone and motility following unilateral microinjection of CCK‐8s into the dorsal vagal complex (DVC); and (iii) whole cell recordings of glutamatergic synaptic inputs to NTS neurons. Key Results Our results show that: (i) medullary c‐fos expression in response to peripheral CCK‐8s was significantly lower in T3‐SCI rats 3 days after the injury, but recovered to control values at 3 weeks post‐SCI, (ii) Unilateral microinjection of CCK‐8s in the DVC induced a profound gastric relaxation in control animals, but did not induce any response in T3‐SCI rats at both 3 and 21 days after SCI, (iii) Perfusion with CCK‐8s increased glutamatergic currents in 55% of NTS neurons from control rats, but failed to induce any response in NTS neurons from T3‐SCI rats. Conclusions & Inferences Our data indicate alterations of vagal responses to CCK‐8s in T3‐SCI rats that may reflect a generalized impairment of gastric vagal neurocircuitry, leading to a reduction of gastric functions after SCI.