Contribution of acetylcholine, vasoactive intestinal polypeptide and nitric oxide to CNS-evoked vagal gastric relaxation in the rat

Plasticity in the brainstem vagal circuits controlling gastric motor function triggered by corticotropin releasing factor

Stress impairs gastric emptying, reduces stomach compliance and induces early satiety via vagal actions. We have shown recently that the ability of the anti-stress neuropeptide oxytocin (OXT) to modulate vagal brainstem circuits undergoes short-term plasticity via alterations in cAMP levels subsequent to vagal afferent fibre-dependent activation of metabotropic glutamate receptors. The aim of the present study was to test the hypothesis that the OXT-induced gastric response undergoes plastic changes in the presence of the prototypical stress hormone, corticotropin releasing factor (CRF). Whole cell patch clamp recordings showed that CRF increased inhibitory GABAergic synaptic transmission to identified corpus-projecting dorsal motor nucleus of the vagus (DMV) neurones. In naive brainstem slices, OXT perfusion had no effect on inhibitory synaptic transmission; following exposure to CRF (and recovery from its actions), however, re-application of OXT inhibited GABAergic transmission in the majority of neurones tested. This uncovering of the OXT response was antagonized by pretreatment with protein kinase A or adenylate cyclase inhibitors, H89 and di-deoxyadenosine, respectively, indicating a cAMP-mediated mechanism. In naive animals, OXT microinjection in the dorsal vagal complex induced a NO-mediated corpus relaxation. Following CRF pretreatment, however, microinjection of OXT attenuated or, at times reversed, the gastric relaxation which was insensitive to l-NAME but was antagonized by pretreatment with a VIP antagonist. Immunohistochemical analyses of vagal motoneurones showed an increased number of oxytocin receptors present on GABAergic terminals of CRF-treated or stressed vs. naive rats. These results indicate that CRF alters vagal inhibitory circuits that uncover the ability of OXT to modulate GABAergic currents and modifies the gastric corpus motility response to OXT.

Brainstem circuits regulating gastric function

Brainstem parasympathetic circuits that modulate digestive functions of the stomach are comprised of afferent vagal fibers, neurons of the nucleus tractus solitarius (NTS), and the efferent fibers originating in the dorsal motor nucleus of the vagus (DMV). A large body of evidence has shown that neuronal communications between the NTS and the DMV are plastic and are regulated by the presence of a variety of neurotransmitters and circulating hormones as well as the presence, or absence, of afferent input to the NTS. These data suggest that descending central nervous system inputs as well as hormonal and afferent feedback resulting from the digestive process can powerfully regulate vago-vagal reflex sensitivity. This paper first reviews the essential "static" organization and function of vago-vagal gastric control neurocircuitry. We then present data on the opioidergic modulation of NTS connections with the DMV as an example of the "gating" of these reflexes, i.e., how neurotransmitters, hormones, and vagal afferent traffic can make an otherwise static autonomic reflex highly plastic.

Involvement of glutamate in gastrointestinal vago-vagal reflexes initiated by gastrointestinal distention in the rat

Vago-vagal reflexes play an integral role in the regulation of gastrointestinal function. Although there have been a number of reports describing the effects of various stimuli on the firing rates of vagal afferent fibers and vagal motor neurons, little is known regarding the neurotransmitters that mediate the vago-vagal reflexes. In the present work, we investigated the role of glutamate in the vago-vagal reflex induced by gastrointestinal distention. Using single-cell recording techniques, we determined the effects of gastric and duodenal distention on the firing rates of gut-related neurons in the dorsal vagal complex, in the absence and presence of glutamate antagonists. Kynurenic acid, a competitive glutamate receptor antagonist, injected into the dorsal vagal complex, blocked the neuronal response of neurons in the dorsal motor nucleus of the vagus and the nucleus of the solitary tract to gastrointestinal distention. Injection of glutamate into the nucleus of the solitary tract produced inhibition of dorsal motor nucleus of the vagus neurons that were also inhibited by gastric and/or duodenal distention. Thus, the distention-induced inhibition of dorsal motor nucleus of the vagus neurons may be mediated by glutamate-induced excitation of gut-related nucleus of the solitary tract neurons. To investigate the role of the various glutamate receptor subtypes in the distention-induced events, we studied the effects of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective non-NMDA receptor antagonist, and DL-2-amino-5-phosphonopentanoic acid (DL-AP5), a selective NMDA receptor antagonist. CNQX injected into the dorsal vagal complex either blocked or attenuated the inhibitory response of the neurons in the dorsal motor nucleus of the vagus and nucleus of the solitary tract neurons to gastric and duodenal distention. In contrast, DL-AP5 had less effect, especially in the vago-vagal reflex elicited by gastric distention. The results suggest (1) distention activates vagal afferents in the gastrointestinal tract; (2) the central branches of the vagal afferents from the gut terminate in the nucleus of the solitary tract and release glutamate that mainly act on non-NMDA receptors; (3) glutamate activates the inhibitory neurons in the nucleus of the solitary tract that project to the dorsal motor nucleus of the vagus; and (4) the inhibitory neurotransmitter suppresses the activity of the dorsal motor nucleus of the vagus neurons. For the excitatory neuronal responses of the dorsal motor nucleus of the vagus neurons to gastrointestinal distention, the possible circuit is that the vagal afferents containing glutamate directly activate the receptors on the dendrites of the dorsal motor nucleus of the vagus.

The innervation of the human antro-pyloric region: Organization and composition

Although the composition of the gastric innervation has been determined in animal models, relatively little known about the innervation of the human antro-pyloric region. We used immunocytochemical techniques to establish the localization and co-expression of neuropeptides and nitric oxide in the human antrum and upper duodenum. Our results demonstrate the existence of a clearly defined submucosal plexus in the antral region that is absent in rats and guinea pigs. The abundant innervation of the lamina propria contains 3 major nerve populations: VIP- and NOS-, SP- and CGRP-, and GRP-immunoreactive. For the first time, NOS-containing nerve fibers were observed throughout the length of the antral glands. Within the antrum somatostatin was confined to endocrine cells, however, at the pyloric sphincter both enteric plexi contained immunoreactive neurons and nerve fibres. Within the pyloric sphincter CGRP- and SP-immunoreactive fibres were significantly increased, correlating with the presence of large ganglia in the submucosal plexus. In conclusion, the organization and composition of the innervation of human antro-pylorus differed substantially from that reported in other mammals. The presence of an abundant mucosal innervation paralled by a well-defined submucosal plexus indicates that the functional regulation of the gastric–pyloric region will be distinct from that of smaller animal models.Key words: gastric innervation, pyloric sphincter, neuropeptides, nitric oxide, somatostatin.

Glucose effects on gastric motility and tone evoked from the rat dorsal vagal complex

1. To examine the effects of glucose on the central components of the vago-vagal reflex control of gastric function, we performed both in vivo and in vitro experiments on neurones in the medial nucleus of the tractus solitarius (mNTS) and in the dorsal motor nucleus of the vagus (DMV). 2. In the in vivo anaesthetized rat preparation, unilateral microinjection of D-glucose (10 or 50 mM (60 nl)(-1)) in mNTS produced inhibition of gastric motility and an increase in intragastric pressure. D-glucose had no effect in the DMV. 3. In the in vitro rat brainstem slice preparation, whole-cell recordings of DMV neurones showed that increasing the glucose concentration of the perfusion solution from 5 mM to 15 or 30 mM produced outward currents of 35 +/- 5 pA (n = 7) and 51 +/- 10 pA (n = 11), respectively. These were blocked by tetrodotoxin and picrotoxin, indicating that glucose was acting indirectly to cause the release of GABA. Decreasing the glucose concentration of the perfusing solution by one-half produced an inward current of 36 +/- 5 pA (n = 7). 4. Stimulation of the NTS evoked inhibitory postsynaptic currents (IPSCs) in DMV neurones. The amplitude of the evoked IPSCs was positively correlated with glucose concentration. Perfusion with the ATP-sensitive K(+) (K(ATP)) channel opener diazoxide mimicked the effect of reduced glucose, while perfusion with the K(ATP) channel blocker glibenclamide mimicked the effects of increased glucose. 5. Our data indicate that glucose had no direct excitatory effect on DMV neurones, but DMV neurones appear to be affected by an action of glucose on cell bodies of mNTS neurones via effects on an ATP-sensitive potassium channel.

Catecholaminergic neurons in rat dorsal motor nucleus of vagus project selectively to gastric corpus

Nitric oxide synthase-immunoreactive (NOS-IR) neurons in the rat caudal dorsal motor nucleus of the vagus (DMV) project selectively to the gastric fundus and may be involved in vagal reflexes controlling gastric distension. This study aimed to identify the gastric projections of tyrosine hydroxylase-immunoreactive (TH-IR) DMV neurons, whether such neurons colocalize NOS-IR, and if they are activated after esophageal distension. Gastric-projecting neurons were identified after injection of retrograde tracers into the muscle wall of the gastric fundus, corpus, or antrum/pylorus before removal and processing of the brain stems for TH- and NOS-IR. A significantly higher proportion of corpus- compared with fundus- and antrum/pylorus-projecting neurons were TH-IR (14% compared with 4% and 2%, respectively, P < 0.05). Colocalization of NOS- and TH-IR was never observed in gastric-projecting neurons. In rats tested for c-Fos activation after intermittent esophageal balloon distension, no colocalization with TH-IR was observed in DMV neurons. These findings suggest that TH-IR neurons in the caudal DMV project mainly to the gastric corpus, constitute a subpopulation distinct from that of nitrergic vagal neurons, and are not activated on esophageal distension.

Descending influences from the infralimbic cortex on vago-vagal reflex control of gastric motor activity in the rat

In experiments on urethane anaesthetised rats the influence of electrical stimulation of ventral areas of the medial prefrontal cortex (mPFC) on spontaneous and vagally-mediated gastric motility were studied. Stimulation of the mPFC resulted in gastric relaxation manifested as a fall in intragastric pressure from a baseline value of 5.0 +/- 0.5 cm H2O. These were most prominent following a short latency when the infralimbic cortex (IL) was stimulated (27.4 +/- 2.5% fall in gastric pressure). Electrical stimulation of the central end of one cervical vagus nerve caused a comparable decrease in gastric pressure (27.1 +/- 2.9%). The cortical mediated relaxation was reduced by atropine and abolished by vagotomy. The cortically induced gastric relaxation followed a shorter latency (5.9 +/- 1.0 s), time to nadir (20.1 +/- 2.7 s) and the half recovery time (21.5 +/- 4.0 s) than vagally mediated-relaxations (9.9 +/- 2.3, 56.0 +/- 5.3 and 83.4 +/- 9.5 s, respectively). Vagally mediated relaxations were inhibited by simultaneous stimulation of the infralimbic cortex. In this case the decrease of gastric pressure, the time to nadir and the half recovery time were significantly decreased in comparison with the gastric relaxatory responses to vagal stimulation alone (P < 0.05). We conclude that one way in which the mPFC influences gastric motility is via corticofugal projections from the infralimbic cortex to the brain-stem which modulate transmission of vago-vagal reflexes.

Peptidergic regulation of gastrointestinal motility in rodents

Peptides involved in the endocrine and enteric nervous systems as well as in the central nervous system exert concerted action on gastrointestinal motility. Mechanical and chemical stimuli which induce peptide release from the epithelial endocrine cells are the earliest step in the initiation of peristaltic activities. Gut peptides exert hormonal effects, but peptide-containing stimulatory (Ach/substance P/tachykinin) and inhibitory (VIP/PACAP/NO) neurons are also involved in the induction of ascending contraction and descending relaxation, respectively. The dorsal vagal complex (DVC), located in the medulla of the brainstem, constitutes the basic neural circuitry of vago-vagal reflex control of gastrointestinal motility. Several gut peptides act on the DVC to modify vagal cholinergic reflexes directly (PYY and PP) or indirectly via afferent fibers in the periphery (CCK and GLP-1). The DVC is also a primary site of action of many neuropeptides (such as TRH and NPY) in mediating gastrointestinal motor activities. The identification over the last few years of a number of neuropeptide systems has greatly changed the field of feeding and body weight regulation. By exploring the brain and gut systems that employ recently identified peptidergic molecules, it will be possible to elaborate on the central and peripheral pathways involved in the regulation of gastrointestinal motility.

Selective gastric projections of nitric oxide synthase-containing vagal brainstem neurons

Nitric oxide has been proposed to act as an intercellular messenger in central brainstem circuits controlling gastrointestinal motility. In particular, a subpopulation of preganglionic vagal neurons of the dorsal motor nucleus of the vagus have been shown to be reduced nicotinamide adenine dinucleotide phosphate(NADPH)-diaphorase positive; NADPH-diaphorase positive preganglionic fibers are also known to make contact with enteric neurons in the stomach. No studies, however, have correlated the neurochemical phenotype of preganglionic vagal neurons to their stomach target. The purpose of this study was to identify the subpopulation of nitric oxide synthase positive vagal neurons projecting to the stomach. Fluorescent retrograde tracers were injected in the fundus, corpus or antrum (Rhodamine beads) or painted on the anterior gastric branch of the vagus (DiI); five to 15 days later the brainstem was processed for nitric oxide synthase immunoreactivity. Of the 532 DiI-labeled neurons from the vagal anterior gastric branch, 25 (4.7%, n=5 rats) were co-localized with nitric oxide synthase immunoreactivity. Of the neurons labeled following injection of rhodamine beads in the antrum (N=231 neurons, n=5 rats) or corpus (N=166 neurons, n=4 rats) only three neurons showed nitric oxide synthase immunoreactivity (two in antrum and one in corpus, respectively). Conversely, 26 of 222 neurons (12%, n=7 rats) labeled following injection of rhodamine in the fundus showed nitric oxide synthase immunoreactivity. These results provide evidence for a discrete phenotypic subpopulation of vagal motoneurons that project to the gastric fundus, and suggest that these neurons may be the ones involved in the receptive relaxation reflex.

Delta9-tetrahydrocannabinol inhibits gastric motility in the rat through cannabinoid CB1 receptors

We investigated involvement of the autonomic nervous system in gastric motor and cardiovascular responses to delta9-tetrahydrocannabinol (delta9-THC) in anesthetized rats. Intravenously administered delta9-THC evoked long-lasting decreases in intragastric pressure and pyloric contractility, bradycardia, and hypotension. The changes in gastric motor function and bradycardia were abolished by vagotomy and ganglionic blockade, whereas spinal cord transection prevented the hypotensive response. Administered intravenously alone, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-met hyl-1H-pyrazole-3-carboxamide, a putative cannabinoid CB1 receptor antagonist, evoked transient decrease in intragastric pressure, and hypertension that was associated with bradycardia. However, this agent completely blocked the gastric motor and cardiovascular responses to intravenous delta9-THC. Application of delta9-THC to the dorsal surface of the medulla resulted in small and short-lasting decreases in gastric motor and cardiovascular function. We conclude that the decrease in gastric motor function and bradycardia are partially due to an action of delta9-THC in the dorsal medulla and that intact vagal nerves are required. The hypotension was mediated through sympathetic pathways. Both gastric motor and cardiovascular effects of peripherally administered delta9-THC seem to be mediated through cannabinoid CB1 receptors.