Suppression of vagal motor activities evokes laryngeal afferent-mediated inhibition of gastric motility

Oxytocin increased intragastric pressure in the forestomach of rats via the dorsal vagal complex

We previously reported that appetite-enhancing peptides facilitated phasic contractions of the distal stomach and relaxed the forestomach via the dorsal vagal complex (DVC). The present study investigated the effects of anorectic substances on gastric reservoir function. The effects of oxytocin on the motility of the forestomach were examined in rats anesthetized with urethane-chloralose. Gastric motor responses were measured using an intragastric balloon. The fourth ventricular administration of oxytocin (0.1 - 1.0 nmol) increased intragastric pressure (IGP) in the forestomach in a dose-dependent manner. Conversely, the administration of oxytocin (0.3 nmol) suppressed phasic contractions of the distal stomach. These responses were opposite to those of appetite-enhancing peptides in previous studies. The oxytocin response in the forestomach was not observed after bilateral cervical vagotomy. The effects of oxytocin on forestomach motility were examined in animals that underwent ablation of the area postrema (AP) to clarify its involvement. Although the magnitude of the response to the fourth ventricular administration of oxytocin decreased, a significant response was still observed. A microinjection of oxytocin (3 pmol) into the AP, the left medial nucleus of the nucleus tractus solitarius (mNTS), the left commissural part of the NTS, or the left dorsal motor nucleus of the vagus was performed. The oxytocin injection into the AP and/or mNTS induced a rapid and large increase in IGP in the forestomach. Prior injection of L-368,899, an oxytocin receptor antagonist, into both the AP and mNTS attenuated the oxytocin response of the forestomach induced by fourth ventricular administration of oxytocin. These results indicate that oxytocin acts on the AP and/or mNTS to increase IGP in the forestomach via vagal preganglionic neurons.

Brainstem Neuronal Circuitries Controlling Gastric Tonic and Phasic Contractions: A Review

This review is on how current knowledge of brainstem control of gastric mechanical function unfolded over nearly four decades from the perspective of our research group. It describes data from a multitude of different types of studies involving retrograde neuronal tracing, microinjection of drugs, whole-cell recordings from rodent brain slices, receptive relaxation reflex, accommodation reflex, c-Fos experiments, immunohistochemical methods, electron microscopy, transgenic mice, optogenetics, and GABAergic signaling. Data obtained indicate the following: (1) nucleus tractus solitarius (NTS)-dorsal motor nucleus of the vagus (DMV) noradrenergic connection is required for reflex control of the fundus; (2) second-order nitrergic neurons in the NTS are also required for reflex control of the fundus; (3) a NTS GABAergic connection is required for reflex control of the antrum; (4) a single DMV efferent pathway is involved in brainstem control of gastric mechanical function under most experimental conditions excluding the accommodation reflex. Dual-vagal effectors controlling cholinergic and non-adrenergic and non-cholinergic (NANC) input to the stomach may be part of the circuitry of this reflex. (5) GABAergic signaling within the NTS via Sst-GABA interneurons determine the basal (resting) state of gastric tone and phasic contractions. (6) For the vagal-vagal reflex to become operational, an endogenous opioid in the NTS is released and the activity of Sst-GABA interneurons is suppressed. From the data, we suggest that the CNS has the capacity to provide region-specific control over the proximal (fundus) and distal (antrum) stomach through engaging phenotypically different efferent inputs to the DMV.

GABAB Receptor Signaling in the Dorsal Motor Nucleus of the Vagus Stimulates Gastric Motility via a Cholinergic Pathway

Central nervous system regulation of the gastric tone and motility is primarily mediated via preganglionic neurons of the dorsal motor nucleus of the vagus (DMV). This is thought to occur by simultaneous engagement of both independent excitatory and inhibitory pathways from the DMV and has been proposed to underlie the opposing effects seen on gastric tone and motility in a number of in vivo models. Contrary to this view, we have been unable to find any evidence for this "dual effector" pathway. Since this possibility is so fundamental to how the brain-gut axis may interact in light of both peripheral and central demands, we decided to explore it further in two separate animal models previously used in conjunction with GABAB signaling to report the existence of a "dual effector" pathway. Using anesthetized rats or ferrets, we microinjected baclofen (7.5 pmol; n = 6), a GABAB agonist into the DMV of rats or intravenously administered it (0.5 mg/kg; n = 4) in ferrets. In rats, unilateral microinjection of baclofen into the DMV caused a robust dose-dependent increase in gastric tone and motility that was abolished by ipsilateral vagotomy and counteracted by pretreatment with atropine (0.1 mg/kg; IV). Similarly, as microinjection in the rats, IV administration of baclofen (0.5 mg/kg) in the ferrets induced its characteristic excitatory effects on gastric tone and motility, which were blocked by either pre- or post-treatment with atropine (0.1 mg/kg; IV). Altogether, our data provide evidence that the gastric musculature (other than the gastric sphincters) is regulated by a "single effector" DMV pathway using acetylcholine.

The role of the superior laryngeal nerve in esophageal reflexes

The aim of this study was to determine the role of the superior laryngeal nerve (SLN) in the following esophageal reflexes: esophago-upper esophageal sphincter (UES) contractile reflex (EUCR), esophago-lower esophageal sphincter (LES) relaxation reflex (ELIR), secondary peristalsis, pharyngeal swallowing, and belch. Cats (N = 43) were decerebrated and instrumented to record EMG of the cricopharyngeus, thyrohyoideus, geniohyoideus, and cricothyroideus; esophageal pressure; and motility of LES. Reflexes were activated by stimulation of the esophagus via slow balloon or rapid air distension at 1 to 16 cm distal to the UES. Slow balloon distension consistently activated EUCR and ELIR from all areas of the esophagus, but the distal esophagus was more sensitive than the proximal esophagus. Transection of SLN or proximal recurrent laryngeal nerves (RLN) blocked EUCR and ELIR generated from the cervical esophagus. Distal RLN transection blocked EUCR from the distal cervical esophagus. Slow distension of all areas of the esophagus except the most proximal few centimeters activated secondary peristalsis, and SLN transection had no effect on secondary peristalsis. Slow distension of all areas of the esophagus inconsistently activated pharyngeal swallows, and SLN transection blocked generation of pharyngeal swallows from all levels of the esophagus. Slow distension of the esophagus inconsistently activated belching, but rapid air distension consistently activated belching from all areas of the esophagus. SLN transection did not block initiation of belch but blocked one aspect of belch, i.e., inhibition of cricopharyngeus EMG. Vagotomy blocked all aspects of belch generated from all areas of esophagus and blocked all responses of all reflexes not blocked by SLN or RLN transection. In conclusion, the SLN mediates all aspects of the pharyngeal swallow, no portion of the secondary peristalsis, and the EUCR and ELIR generated from the proximal esophagus. Considering that SLN is not a motor nerve for any of these reflexes, the role of the SLN in control of these reflexes is sensory in nature only.

Fourth ventricular administration of ghrelin induces relaxation of the proximal stomach in the rat

The effects of fourth ventricular administration of ghrelin on motility of the proximal stomach were examined in anesthetized rats. Intragastric pressure (IGP) was measured using a balloon situated in the proximal part of the stomach. Administration of ghrelin into the fourth ventricle induced relaxation of the proximal stomach in a dose-dependent manner. Significant reduction of IGP was observed at doses of 3, 10, or 30 pmol. The administration of ghrelin (10 or 30 pmol) with growth hormone secretagogue receptor (GHS-R) antagonist ([d-Lys3] GHRP-6; 1 nmol) into the fourth ventricle did not induce a significant change in IGP. The sole administration of [d-Lys3] GHRP-6 also did not induce a significant change in IGP. Bilateral sectioning of the vagi at the cervical level abolished the relaxation induced by the administration of ghrelin (10 or 30 pmol) into the fourth ventricle, suggesting that relaxation induced by ghrelin is mediated by vagal preganglionic neurons. Microinjections of ghrelin (200 fmol) into the caudal part of the dorsal vagal complex (DVC) induced obvious relaxation of the proximal stomach. Similar injections into the intermediate part of the DVC did not induce significant change. Dose-response analyses revealed that the microinjection of 2 fmol of ghrelin into the caudal DVC significantly reduced IGP. These results revealed that ghrelin induced relaxation in the proximal stomach via GHS-R situated in the caudal DVC.

Dorsal motor nucleus of the vagus: a site for evoking simultaneous changes in crural diaphragm activity, lower esophageal sphincter pressure, and fundus tone

The sphincter mechanism at the esophagogastric junction includes smooth muscle of the lower esophagus and skeletal muscle of the crural diaphragm (CD). Smooth muscle is known to be under the control of the dorsal motor nucleus of the vagus (DMV), while central nervous system (CNS) control of the CD is unknown. The main purposes of our study were to determine the CNS site that controls the CD and whether simultaneous changes in lower esophageal sphincter (LES) pressure and CD activity occur when this site is activated. Experiments were performed on anesthetized male ferrets whose LES pressure, CD activity, and fundus tone were monitored. To activate DMV neurons, L-glutamate was microinjected unilaterally into the DMV at three areas: intermediate, rostral, and caudal. Stimulation of the intermediate DMV decreased CD activity (-4.8 +/- 0.1 bursts/min and -0.3 +/- 0.01 mV) and LES pressure (-13.2 +/- 2.0 mmHg; n = 9). Stimulation of this brain site also produced an increase in fundus tone. Stimulation of the rostral DMV elicited increases in the activity of all three target organs (n = 5). Stimulation of the caudal DMV had no effect on the CD but did decrease both LES pressure and fundus tone (n = 5). All changes in LES pressure, fundus tone, and some DMV-induced changes in CD activity (i.e., bursts/min) were prevented by ipsilateral vagotomy. Our data indicate that simultaneous changes in activity of esophagogastric sphincters and fundus tone occur from rostral and intermediate areas of the DMV and that these changes are largely mediated by efferent vagus nerves.

A reevaluation of the effects of stimulation of the dorsal motor nucleus of the vagus on gastric motility in the rat

Our primary purpose was to characterize vagal pathways controlling gastric motility by microinjecting l-glutamate into the dorsal motor nucleus of the vagus (DMV) in the rat. An intragastric balloon was used to monitor motility. In 39 out of 43 experiments, microinjection of l-glutamate into different areas of the DMV rostral to calamus scriptorius (CS) resulted in vagally mediated excitatory effects on motility. We observed little evidence for inhibitory effects, even with intravenous atropine or with activation of gastric muscle muscarinic receptors by intravenous bethanechol. Inhibition of nitric oxide synthase with N(omega)-nitro-l-arginine methyl ester (l-NAME) HCl did not augment DMV-evoked excitatory effects on gastric motility. Microinjection of l-glutamate into the DMV caudal to CS produced vagally mediated gastric inhibition that was resistant to l-NAME. l-Glutamate microinjected into the medial subnucleus of the tractus solitarius (mNTS) also produced vagally mediated inhibition of gastric motility. Motility responses evoked from the DMV were always blocked by ipsilateral vagotomy, while responses evoked from the mNTS required bilateral vagotomy to be blocked. Microinjection of oxytocin into the DMV inhibited gastric motility, but the effect was never blocked by ipsilateral vagotomy, suggesting that the effect may have been due to diffusion of oxytocin to the mNTS. Microinjection of substance P and N-methyl-d-aspartate into the DMV also produced inhibitory effects attributable to excitation of nearby mNTS neurons. Our results do not support previous studies indicating parallel vagal excitatory and inhibitory pathways originating in the DMV rostral to CS. Our results do support previous findings of vagal inhibitory pathways originating in the DMV caudal to CS.

Central neuropeptide Y induces proximal stomach relaxation via Y1 receptors in the dorsal vagal complex of the rat

Effects of neuropeptide Y (NPY) on motility of the proximal stomach was examined in anesthetized rats. Intragastric pressure was measured using a balloon situated in the proximal part of the stomach. Administration of NPY into the fourth ventricle induced relaxation of the proximal stomach in a dose-dependent manner. Administration of an Y1 receptor (Y1R) agonist [Leu31, Pro34]NPY induced a larger relaxation than NPY. The administration of an Y2 receptor agonist (NPY 13-36) did not induce significant changes in motility. Microinjections of [Leu31, Pro34]NPY into the caudal part of the dorsal vagal complex (DVC) induced relaxation of the proximal stomach. In contrast, similar injections into the intermediate part of the DVC increased IGP of the proximal stomach. Administration of NPY into the fourth ventricle did not induce relaxation after bilateral injections of the Y1R antagonist (1229U91) into the caudal DVC. These results indicate that NPY induces relaxation in the proximal stomach via Y1Rs situated in the DVC. Because bilateral vagotomy below the diaphragm abolished the relaxation induced by the administration of NPY into the fourth ventricle, relaxation induced by NPY is probably mediated by vagal preganglionic neurons. Intravenous injection of atropine methyl nitrate reduced relaxation induced by administration of NPY. Therefore, relaxation induced by NPY is likely mediated by peripheral cholinergic neurons.

Area postrema mediates gastric motor response induced by apomorphine in rats

The effects of apomorphine administration on the autonomic responses were investigated in rats. Distinctive gastric motor responses were observed after the intravenous administration of apomorphine (0.1 mg/kg body weight). Gastric motor responses in the distal stomach induced by apomorphine administration were classified into two types. One type involved inhibition of phasic contractions which appeared just after the administration of apomorphine. The other involved an increase in the frequency of small phasic contractions accompanied by increased gastric tone appearing with a relatively longer delay. No relaxation was observed in either the proximal or distal stomach. These gastric motor responses showed a dose-response effect to the amount of apomorphine administered (0.002-0.1 mg/kg body weight). In addition, submandibular salivary secretion was observed in response to the intravenous administration of apomorphine at a dose of 3 or 10 mg/kg body weight. Pretreatment with domperidone (1 or 2 mg/kg body weight) or the ablation of the area postrema (AP) abolished the gastric motor response and salivary secretion induced by the administration of apomorphine. In conclusion, rats showed definitive autonomic phenomena in response to the administration of apomorphine. Dopamine 2-like receptors situated in the AP mediate apomorphine-induced autonomic phenomena in rats.

Central orexin facilitates gastric relaxation and contractility in rats

The effects of the intracisternal administration of synthetic orexin-A (3 nmol) on gastric motility were examined in rats. The administration of orexin but not a vehicle induced relaxation of the proximal stomach lasting for more than 30 min. Phasic contractions in the distal stomach were facilitated in response to the administration of orexin but not a vehicle. Facilitation in the distal stomach was not observed in the animals which underwent the sectioning of the bilateral vagi at the subdiaphragmatic level. Relaxation of the proximal stomach was observed in vagotomized animals but the magnitude of relaxation was significantly smaller than that in intact animals. These results suggest that central orexin facilitates distal stomach motility and relaxation of the proximal stomach via vagi.