A study of peripheral input to and its control by post-ganglionic neurones of the inferior mesenteric ganglion

Enteric Control of the Sympathetic Nervous System

The autonomic nervous system that regulates the gut is divided into sympathetic (SNS), parasympathetic (PNS), and enteric nervous systems (ENS). They inhibit, permit, and coordinate gastrointestinal motility, respectively. A fourth pathway, "extrinsic sensory neurons," connect gut to the central nervous system, mediating sensation. The ENS resides within the gut wall and its activities are critical for life; ENS failure to populate the gut in development is lethal without intervention."Viscerofugal neurons" are a distinctive class of enteric neurons, being the only type that escapes the gut wall. They form a unique circuit: their axons project out of the gut wall and activate sympathetic neurons, which then project back to the gut, and inhibit gut movements.For 80 years viscerofugal/sympathetic circuits were thought to have a restricted role, mediating simple sensory-motor reflexes. New data shows viscerofugal and sympathetic neurons behaving unexpectedly, compelling a re-evaluation of these circuits: both viscerofugal and sympathetic neurons transmit higher order, synchronized firing patterns that originate within the ENS. This identifies them as driving long-range motility control between different gut regions.There is need for gut motor control over distances beyond the range of ENS circuits, yet no mechanism has been identified to date. The entero-sympathetic circuits are ideally suited to meet this need. Here we provide an overview of the structure and functions of these peripheral sympathetic circuits, including new data showing the firing patterns generated by enteric networks can transmit through sympathetic neurons.

A Novel Mode of Sympathetic Reflex Activation Mediated by the Enteric Nervous System

Enteric viscerofugal neurons provide a pathway by which the enteric nervous system (ENS), otherwise confined to the gut wall, can activate sympathetic neurons in prevertebral ganglia. Firing transmitted through these pathways is currently considered fundamentally mechanosensory. The mouse colon generates a cyclical pattern of neurogenic contractile activity, called the colonic motor complex (CMC). Motor complexes involve a highly coordinated firing pattern in myenteric neurons with a frequency of ∼2 Hz. However, it remains unknown how viscerofugal neurons are activated and communicate with the sympathetic nervous system during this naturally-occurring motor pattern. Here, viscerofugal neurons were recorded extracellularly from rectal nerve trunks in isolated tube and flat-sheet preparations of mouse colon held at fixed circumferential length. In freshly dissected preparations, motor complexes were associated with bursts of viscerofugal firing at 2 Hz that aligned with 2-Hz smooth muscle voltage oscillations. This behavior persisted during muscle paralysis with nicardipine. Identical recordings were made after a 4- to 5-d organotypic culture during which extrinsic nerves degenerated, confirming that recordings were from viscerofugal neurons. Single unit analysis revealed the burst firing pattern emerging from assemblies of viscerofugal neurons differed from individual neurons, which typically made partial contributions, highlighting the importance and extent of ENS-mediated synchronization. Finally, sympathetic neuron firing was recorded from the central nerve trunks emerging from the inferior mesenteric ganglion. Increased sympathetic neuron firing accompanied all motor complexes with a 2-Hz burst pattern similar to viscerofugal neurons. These data provide evidence for a novel mechanism of sympathetic reflex activation derived from synchronized firing output generated by the ENS.

Afferent hypersensitivity in a mouse model of post-inflammatory gut dysfunction: role of altered serotonin metabolism

Visceral hypersensitivity is an important clinical feature associated with irritable bowel syndrome which in some patients has been linked to prior infection. Here we employ an animal model in which transient infection leads to persistent gut dysfunction to investigate the role of altered 5-HT metabolism upon afferent mechanosensensitivity in the post-infected gut. Jejunal segments isolated from Trichinella spiralis-infected mice were used to assess 5-HT metabolism whilst afferent activity in T. spiralis-infected mice was studied by extracellular recordings from jejunal mesenteric afferent bundles and patch clamp recordings of isolated nodose ganglion neurons (NGNs). During acute infection, intestinal 5-HT content and release increased, 5-HT turnover decreased and afferent discharge in response to mechanical stimulation was attenuated. By day 28 post infection (PI), 5-HT turnover had normalized, but 5-HT content and release were still elevated. This was associated with afferent mechano-hypersensitivity, which persisted for 8 weeks PI and was susceptible to 5-HT(3) receptor blockade. NGNs from post-infected animals were more excitable than controls but their current densities in response to 2-methyl-5-HT were lower. T. spiralis infection increased mucosal 5-HT bioavailability and affected the spontaneous activity and mechanosensitivity of gastrointestinal sensory nerves. This involved an initial hyposensitivity occurring during acute infection followed by long-term hypersensitivity in the post-infectious period that was in part mediated by 5-HT acting via 5-HT(3) receptors. Functional down-regulation of 5-HT(3) receptors also occurs in the post-infected animals, which may represent an adaptive response to increased mucosal 5-HT bioavailability.

Development of enteric neurons from non-recognizable precursor cells

Precursors of the neurons that populate enteric ganglia cannot be recognized morphologically when they first enter the gut; therefore embryonic gut in culture, explanted before neurons appear, develops a myenteric plexus that contains cholinergic and serotonergic neurons. The evidence indicates that the developing gut maintains an immature proliferating pool of neuronal precursors that may tentatively and transiently express a given neuronal phenotype. Catecholaminergic expression is an example of such a transient phenotype. It is possible that sequential changes, occurring as a function of gestational age in the enteric neuronal microenvironment and interacting with this persistent pool of neuronal precursors, are responsible for the generation of enteric neuronal diversity. The sequential appearance of the various types of enteric neuron is consistent with this hypothesis. The persistence of a dividing cell population may also be linked to the generation of the large number of enteric neurons.

Influence of the pattern of jejunal distension on mesenteric afferent sensitivity in the anaesthetized rat

Vagal, spinal and intestino-fugal fibres all potentially transmit mechanosensory afferent information from the gastrointestinal tract. We aimed to characterize the relative mechanosensitivity of these three different afferent populations supplying the rat jejunum. Afferent nerve discharge was recorded from pentobarbitone-anaesthetized rats during different distension protocols. Saline ramp distension (1 mL min(-1)) and barostat ramp distension (2 mmHg 4 s(-1)) each evoked biphasic responses but with the latter significantly attenuated especially at low distending pressures. Barostat controlled phasic distensions (10-50 mmHg, 25 s) evoked an afferent response with a peak at the onset of distension adapting to a plateau level that was maintained and comparable to the barostat ramp responses at the corresponding pressures. Chronic subdiaphragmatic vagotomy significantly attenuated the low pressure component of the response to balloon ramp distension and both peak and plateau responses to phasic distension. Single unit analysis showed an absence of low threshold afferent activity after vagotomy while the response to fibres with wide-dynamic range and high threshold sensitivity were preserved hexamethonium had no effect on the responses to either ramp or phasic distension. These findings suggest that the nature of the distension stimulus is critical in determining the pattern of response observed from the various subpopulations of afferents supplying the bowel wall.

Ileal brake: neuropeptidergic control of intestinal transit

Digestion and absorption of a meal are time-intensive processes. To optimize digestion and absorption, transit of the meal through the gastrointestinal tract is regulated by a complex integration of neuropeptidergic signals generated as the jejunal brake and ileal brake response to nutrients. Mediators involved in the slowing of transit responses include peptide YY (PYY), chemosensitive afferent neurons, intestinofugal nerves, noradrenergic nerves, myenteric serotonergic neurons, and opioid neurons. The activation of this circuitry modifies the peristaltic reflex to convert the intestinal motility pattern from propagative to segmenting. Fat is the most potent trigger of these transit control mechanisms. The integrated circuitry of gut peptides and neurons involved in transit control in response to nutrients is described in this review.

Characterization of the primary spinal afferent innervation of the mouse colon using retrograde labelling

Visceral pain is the most common form of pain produced by disease and is thus of interest in the study of gastrointestinal (GI) complaints such as irritable bowel syndrome, in which sensory signals perceived as GI pain travel in extrinsic afferent neurones with cell bodies in the dorsal root ganglia (DRG). The DRG from which the primary spinal afferent innervation of the mouse descending colon arises are not well defined. This study has combined retrograde labelling and immunohistochemistry to identify and characterize these neurones. Small to medium-sized retrogradely labelled cell bodies were found in the DRG at levels T8-L1 and L6-S1. Calcitonin gene-related peptide (CGRP)- and P2X3-like immunoreactivity (LI) was seen in 81 and 32%, respectively, of retrogradely labelled cells, and 20% bound the Griffonia simplicifolia-derived isolectin IB4. CGRP-LI and IB4 were co-localized in 22% of retrogradely labelled cells, whilst P2X3-LI and IB4 were co-localized in 7% (vs 34% seen in the whole DRG population). Eighty-two per cent of retrogradely labelled cells exhibited vanilloid receptor 1-like immunoreactivity (VR1-LI). These data suggest that mouse colonic spinal primary afferent neurones are mostly peptidergic CGRP-containing, VR1-LI, C fibre afferents. In contrast to the general DRG population, a subset of neurones exist that are P2X3 receptor-LI but do not bind IB4.

Circumferential, not longitudinal, colonic stretch increases synaptic input to mouse prevertebral ganglion neurons

The relationship between longitudinal and circular muscle tension in the mouse colon and mechanosensory excitatory synaptic input to neurons in the superior mesenteric ganglion (SMG) was investigated in vitro. Electrical activity was recorded intracellularly from SMG neurons, and muscle tension was simultaneously monitored in the longitudinal, circumferential, or both axes. Colonic intraluminal pressure and volume changes were also monitored simultaneously with muscle tension changes. The results showed that the frequency of fast excitatory postsynaptic potentials (fEPSPs) in SMG neurons increased when colonic muscle tension decreased, when the colon relaxed and refilled with fluid after contraction, and during receptive relaxation preceding spontaneous colonic contractions. In contrast, fEPSP frequency decreased when colonic muscle tension increased during spontaneous colonic contraction and emptying. Manual stretch of the colon wall to 10-15% beyond resting length in the circumferential axis of flat sheet preparations increased fEPSP frequency in SMG neurons, but stretch in the longitudinal axis to 15% beyond resting length in the same preparations did not. There was no increase in synaptic input when tubular colon segments were stretched in their long axes up to 20% beyond their resting length. The circumferential stretch-sensitive increase in the frequency of synaptic input to SMG neurons persisted when the colonic muscles were relaxed pharmacologically by nifedipine (2 microM) or nicardipine (3 microM). These results suggest that colonic mechanosensory afferent nerves projecting to the SMG function as length or stretch detectors in parallel to the circular muscle layer.

Slowing of intestinal transit by fat or peptide YY depends on beta-adrenergic pathway

Although the enteric reflex pathway triggered by volume distension is known to depend on an adrenergic nerve, it is not known whether the slowing of intestinal transit by fat or peptide YY (PYY) also depends on an adrenergic pathway. The aim of this study was to test the hypotheses that the slowing of transit by fat or PYY may depend on a beta-adrenergic pathway, and this adrenergic pathway may act via the serotonergic and opioid pathways previously observed for the slowing of transit by fat. Eighteen dogs were equipped with duodenal and midgut fistulas. The small intestine was compartmentalized into the proximal and distal half of gut. The role of adrenergic, serotonergic, and opioid pathways was then tested in the slowing of intestinal transit by fat, PYY, and norepinephrine. Intestinal transit results were compared as the cumulative percent marker of recovery over 30 min. We found that the slowing of transit by fat, PYY, or norepinephrine was reversed by propranolol. In addition, the slowing effect of fat was reversed by metoprolol (beta1-adrenoreceptor antagonist) but not phentolamine (alpha-adrenoreceptor antagonist). Furthermore, norepinephrine-induced slowing of transit was reversed by ondansetron (5-HT3 receptor antagonist) or naloxone (opioid receptor antagonist). Extending these physiological results, we also found by immunohistochemistry that beta1-adrenoreceptors are expressed by neurons of the intrinsic plexuses of the small intestine. We conclude that the slowing of intestinal transit by fat or PYY depends on a beta-adrenergic pathway and that this adrenergic pathway acts on serotonergic and opioid pathways.

Effects of extrinsic autonomic inputs on expression of c-Fos immunoreactivity in myenteric neurons of the guinea pig distal colon

c-Fos protein is a nuclear protein coded by c-fos proto-oncogene subsequent to synaptic activation of the neurons. We used immunohistochemical methods to visualize the expression of c-Fos protein in myenteric neurons of the guinea pig distal colon and examined the effects of the extrinsic autonomic inputs on the enteric circuits. No c-Fos immunoreactivity was observed in the colonic segments fixed immediately after removal from the animal body. A number of c-Fos-immunoreactive nuclei of myenteric neurons, however, appeared in all preparations that were incubated in Krebs solution in vitro (n=10). Application of tetrodotoxin (0.2 microM) abolished the expression of c-Fos-immunoreactivity (n=6), but hexamethonium (100 microM) failed to decrease the number of c-Fos-positive neurons despite a complete suppression of spontaneous peristaltic movements (n=5). Neither the electrical stimulation (n=8) nor the severing of the pelvic nerves (n=5) changed the number of c-Fos-positive neurons. Application of clonidine, an alpha(2)-agonist, (0.1 microM) abolished the expression of c-Fos protein in all preparations (n=5), while denervation of the sympathetic fibers in the lumbar colonic and hypogastric nerves in vivo increased the number of c-Fos-positive neurons (n=5). The results indicate that the enteric circuit in the distal part of the gastrointestinal tract is under tonic inhibition by the sympathetic nervous system from the lumbar spinal cord. c-Fos immunoreactivity expressed in the colonic preparations in vivo might be the results of enhanced activation of non-nicotinic receptors after removal of the sympathetic inhibition.

Relationship between colonic motility and cholinergic mechanosensory afferent synaptic input to mouse superior mesenteric ganglion

Abstract Abdominal prevertebral ganglion neurones receive excitatory synaptic input from intestinofugal neurones. To better understand the physiological significance of this input, we examined the relationship between synaptic input to mouse superior mesenteric ganglion (SMG) neurones and intracolonic pressure and volume changes that accompany spontaneous colonic contractions in vitro. Electrical activity was recorded intracellularly from SMG neurones in ganglia attached to a segment of distal colon. The majority of neurones examined received ongoing fast excitatory potentials (F-EPSPs). F-EPSP frequency increased when the colon was distended with fluid and during spontaneous increases in colonic volume that accompanied colonic relaxation. In contrast, F-EPSP frequency in SMG neurones decreased when the colon emptied, and remained at a reduced frequency until the colon refilled and volume increased. Nicotinic blockade of the colon abolished spontaneous colonic contractions and reduced or abolished synaptic input to SMG neurones, suggesting that most of the synaptic input arose from second or higher order neurones. Retrograde labelling identified cell bodies of intestinofugal neurones in myenteric ganglia. Most had short, club-like dendritic processes and appeared uni-axonal. These results show that mechanosensory intestinofugal afferent nerves monitor intracolonic volume changes.

Pituitary adenylate cyclase activating peptide (PACAP) in the enteropancreatic innervation

Pancreatic ganglia are innervated by neurons in the gut and are formed by precursor cells that migrate into the pancreas from the bowel. The innervation of the pancreas, therefore, may be considered an extension of the enteric nervous system. Pituitary adenylate cyclase-activating polypeptide (PACAP) is present in a subset of enteric neurons. We investigated the presence of PACAP in the enteropancreatic innervation in guinea pigs, and the response of pancreatic neurons to PACAP-related peptides. PACAP immunoreactivity was found in nerve fibers in both enteric and pancreatic ganglia and in nerve bundles that travelled between the duodenum and pancreas. PACAP-immunoreactive nerve fibers were densely distributed in the pancreatic ganglia, where they surrounded a subset of cholinergic cell bodies. Pancreatic ganglia did not contain PACAP-immunoreactive cell bodies; however, neuronal perikarya with PACAP immunoreactivity were found in the myenteric plexus of the duodenum. These cells co-stored vasoactive intestinal peptide (VIP). PACAP depolarized pancreatic neurons. Pancreatic neurons were also depolarized by VIP; however, PACAP was more efficacious at depolarizing pancreatic cells than VIP. These findings are consistent with the view that the PACAP effects were mediated through PACAP-selective (PAC1) receptors. PACAP-responsive neurons displayed PAC1 receptor immunoreactivity, which was also found in islet cells and enteric neurons. These results provide support for the hypothesis that PACAP modulates reflex activity between the gut and pancreas. The excitatory effect of PACAP would be expected to potentiate pancreatic secretion.

Origins of cholinergic inputs to the cell bodies of intestinofugal neurons in the guinea pig distal colon

Integration of function between gut regions is mediated by means of hormones and long neuronal reflex pathways. Intestinofugal neurons, which participate in one of these pathways, have cell bodies within the myenteric plexus and project their axons from the gut with the mesenteric nerves. They form excitatory synapses on neurons in prevertebral ganglia that in turn innervate other gut regions. The aim of the present study was to characterise immunohistochemically the synaptic input to intestinofugal neurons. The cell bodies of intestinofugal neurons that project from the distal colon were labelled with Fast Blue that was injected into the inferior mesenteric ganglia. Varicosities surrounding Fast Blue-labelled neurons were analysed for immunoreactivity for the vesicular acetylcholine transporter, vasoactive intestinal peptide, and bombesin. Most intestinofugal neurons were surrounded by nerve terminals immunoreactive for the vesicular acetylcholine transporter; many of these terminals also contained vasoactive intestinal peptide and bombesin immunoreactivity. This combination of markers occurs in axons of descending interneurons. Extrinsic denervation had no effect on the distribution of cholinergic terminals around intestinofugal neurons. A decrease in the number of vesicular acetylcholine transporter and vasoactive intestinal peptide immunoreactive terminals occurred around nerve cells immediately anal, but not oral, to myotomy operations. Consistent with previous physiological studies, it is concluded that intestinofugal neurons receive cholinergic synaptic input from other myenteric neurons, including cholinergic descending interneurons. Thus, intestinofugal neurons are second, or higher, order neurons in reflex pathways, although physiological data indicate that they also respond directly to distension of the gut wall.

PACAP27 and other neuropeptides in the inferior mesenteric ganglion

The presence and location of PACAP27-like immunoreactivity (PACAP27-LI) in the colon-inferior mesenteric ganglion (IMG) reflex pathway and the effect of exogenously administered PACAP27 on the excitability of IMG are reported. The results provide morphological and electrophysiological support for the hypothesis that PACAP modulates reflex activity between the large intestine and IMG. The intense excitatory effect would be expected to increase the rate of action potential discharge in IMG neurons, increasing sympathetic drive to the colon thereby decreasing of colonic activity.

Electrophysiology, shape, and chemistry of neurons that project from guinea pig colon to inferior mesenteric ganglia

BACKGROUND & AIMS

Prevertebral sympathetic ganglia receive inputs from intestinofugal neurons, with cell bodies located in the wall of the bowel. Intestinofugal neurons are part of the afferent limbs of intestino-intestinal reflexes. The aim of this study was to define the properties of intestinofugal neurons using intracellular recordings.

METHODS

Intestinofugal neurons of the distal colon were retrogradely labeled from the inferior mesenteric ganglia. In whole mounts of the myenteric plexus/longitudinal muscle of the distal colon, labeled neurons were identified by their fluorescence and recordings were made using biocytin-filled electrodes. Labeled nerves were characterized immunohistochemically and morphologically.

RESULTS

Intestinofugal neurons were uniaxonal neurons with multiple dendrites that had lamellar expansions. They were immunoreactive for choline acetyltransferase. Stimulation of nerve fiber tracts elicited large-amplitude excitatory postsynaptic potentials in all labeled neurons. Some received spontaneous fast excitatory postsynaptic potentials. Those cells that fired action potentials fired only one or two at the start of a depolarizing current pulse. No intestinofugal neurons had Dogiel type II morphology or a late afterhyperpolarizing potential.

CONCLUSIONS

Intestinofugal neurons are likely to be activated by other neurons in the gut wall. They are not AH or Dogiel type II neurons. Thus they seem to be second order neurons in afferent pathways of intestino-intestinal reflexes.

In situ identification and visualization of neurons that mediate enteric and enteropancreatic reflexes

To identify neurons participating in enteric and enteropancreatic reflexes, we validated the use of the activity-dependent markers FM1-43 and FM2-10 as "on-line" probes for the visualization of activated guinea pig enteric and pancreatic neurons. FM1-43 or FM2-10 labeling of neuronal perikarya and processes was induced by KCl (70 mM), veratridine (1.0 microM), intracellular injection of depolarizing current pulses, stimulation of afferent inputs, evoking reflexes (by inflating an intraluminal balloon, blowing puffs of N2 at, or applying glucose to, the villous surface of the duodenum), or injury; labeling was prevented by tetrodotoxin (0.5 microM). Intracellular recording and injection of Neurobiotin confirmed that FM1-43 labeled neurons that spike, but not those that exhibit only fast excitatory postsynaptic potentials. Perikarya did not label if axonal transport was blocked by colchicine. When pulses of N2 or glucose were directed at duodenal villi in vitro, labeling by FM1-43 or FM2-10 was observed in myenteric and pancreatic neurons, as well as in subsets of cells in pancreatic islets and intestinal crypts. Hexamethonium blocked the spread of label via nicotinic synapses and thus enabled primary afferent neurons to be located. Balloon distension elicited hexamethonium-resistant labeling of epithelial cells, interstitial cells, and Dogiel type II neurons in each plexus; however, in preparations stimulated with pulses of N2 or glucose, hexamethonium-resistant labeling of neurons occurred only in the submucosal plexus and not in myenteric ganglia. These observations suggest that primary afferent neurons responsible for mucosal pressure- or glucose-induced enteric and enteropancreatic reflexes are submucosal, whereas myenteric afferent neurons become activated only when the wall of the bowel is distended. The data are compatible with the possibility that primary afferent neurons are activated by a signaling molecule released from intestinal epithelial cells.

The mammalian sympathetic prevertebral ganglia: integrative properties and role in the nervous control of digestive tract motility

The prevertebral ganglia which are a constitutive part of the sympathetic system have long been considered as a simple relay on this efferent pathway. In fact, these ganglia must be considered as true peripheral nervous centres. They possess various integrative properties, such as projections of central and peripheral inputs onto the ganglionic neurones, gating of these projections and pacemaker activity of the ganglionic neurones. These properties explain the ability of these ganglia to participate in the regulation of various visceral functions, including digestive tract motility.

Chemical coding of neurons that project from different regions of intestine to the coeliac ganglion of the guinea pig

The chemical codings of neurons that project from the small intestine, caecum, proximal colon, distal colon and rectum to the coeliac ganglion of the guinea pig were investigated. The coeliac ganglion was injected with the retrogradely transported dye Fast Blue, and each of the regions was examined 6 days later in wholemounts that had been prepared for immunohistochemical localisation of pairs of antigens. In both the small and large intestines, all intestinofugal neurons were immunoreactive (IR) for choline acetyltransferase (ChAT). In each region of the large intestine, the largest population, representing 50-60% of retrogradely labelled neurons in each region, was immunoreactive for ChAT, bombesin (BN), calbindin (Calb) and nitric oxide synthase (NOS). Most intestinofugal neurons in the small intestine contain bombesin and VIP-IR along with ChAT-IR but none contain either Calb or NOS. Thus, nerve endings of enteric origin in the coeliac ganglion that contain NOS-IR or Calb-IR come from the large intestine and those with bombesin-IR but not NOS-IR are from the small intestine. The gastric wall was injected with Fast Blue in order to label noradrenergic (NA) neurons in the coeliac ganglion and to determine, by localisation of NOS and bombesin-IR, whether they receive inputs from the small and large intestine. Some NA neurons received inputs from the large intestine (and perhaps also from the small intestine) and some received inputs exclusively from the small intestine. Most NA neurons that received intestinofugal inputs had the chemical code NA/-; some were immunoreactive for somatostatin (NA/SOM neurons), but those with IR for neuropeptide Y (NA/NPY) rarely received intestinofugal inputs.

Neuronal regulation of cochlear blood flow in the guinea-pig

1. Previous studies have shown that electrical stimulation (ES) of the guinea-pig cochlea causes a neurally mediated increase in cochlear blood flow (CBF). It is known that the centrifugal neuronal input to the cochlea comes through the perivascular sympathetic plexus from the cervical sympathetic chain and along the vestibular nerve (VN) from the periolivary area of the brainstem. Both of these neuronal systems are distributed topographically in the cochlea. 2. In order to study the neural origins of ES-evoked CBF increase, laser Doppler flowmetry was used to test the following hypotheses. (a) The response is regional, that is, limited to the area of the cochlea stimulated. To test this we performed differential ES of the cochlear turns. CBF was measured from either the third or the first turn. (b) The response is mediated via autonomic receptors within the cochlea. To study this, we applied atropine, succinylcholine and idazoxan locally to the cochlea. (c) The response is influenced by neuronal input via the sympathetic cervical chain (SC) and components of the VN. We stimulated and sectioned the SC, and sectioned the VN, to test this hypothesis. 3. We observed that the CBF response was topographically restricted to the stimulated region. Locally applied muscarinic or nicotinic antagonists (atropine and succinylcholine respectively) did not affect the response. However, local idazoxan (an alpha 2-blocker) eliminated the response. Locally applied adrenaline and SC stimulation modified the dynamic range of the response. SC sectioning enhanced the responsiveness of the cochlear vasculature to ES. The VN section caused a temporary decrease in CBF and elimination of the ES-evoked CBF response. 4. We conclude that the release of dilating agents is topographical with respect to ES current flow, the ES-evoked CBF increase is peripherally mediated via alpha 2-receptors, and the response is influenced by input via the SC. The elimination of the response by VN sectioning proximal to the brainstem indicated that fibres of the VN mediate the CBF increase during direct cochlear ES. The data suggest that these fibres may be the efferent limb of a neural loop involved with the regulation of CBF. Such a system could provide a mechanism for the rapid increase in CBF with organ stress.

NADPH diaphorase (nitric oxide synthase)-containing nerves in the enteropancreatic innervation: sources, co-stored neuropeptides, and pancreatic function

Pancreatic ganglia are innervated by neurons in the gut and are formed by precursor cells that migrate into the pancreas from the bowel. The innervation of the pancreas, therefore, may be considered an extension of the enteric nervous system. NADPH-diaphorase is present in a subset of enteric neurons. We investigated the presence of NADPH-diaphorase in the enteropancreatic innervation, the contribution of extrinsic nerves to the NADPH-diaphorase-containing fibers of the gut and pancreas, and the coincident expression of NADPH-diaphorase NADPH-diaphorase in intrinsic neurons of these organs with neuropeptides. The possible role of nitric oxide in the neural regulation of pancreatic secretion was studied in isolated pancreatic lobules. Neuronal perikarya with NADPH-diaphorase activity were found in both Dogiel type I and type II neurons of the myenteric plexus of the stomach and duodenum. All galanin (GAL)-immunoreactive neurons and a small subset of vasoactive intestinal polypeptide (VIP)- and neuropeptide Y (NPY)-immunoreactive neurons contained NADPH-diaphorase activity. NADPH-diaphorase activity was also found in a subset of VIP and NPY-immunoreactive pancreatic neurons. Retrograde tracing with FluoroGold established that NADPH-diaphorase-containing neurons in the bowel project to the pancreas. NADPH-diaphorase-containing fibers were also found to be provided to both organs by neurons in dorsal root ganglia. Secretion of amylase was evoked by L-arginine. This effect was prevented by N(G)-nitro-L-arginine (L-NNA), which also inhibited VIP-stimulated secretion of amylase; however, L-NNA had no effect on amylase secretion stimulated by carbachol. These results provide support for the hypothesis that nitric oxide plays a role in the neural regulation of pancreatic secretion.

Ileal distention relaxes the canine colon: a model of megacolon?

BACKGROUND/AIMS

Reflex relaxation is a mechanism whereby the colon can show short-term dilatation in the absence of mechanical obstruction. This study investigated the tonic response of the canine colon to ileal distention and its pharmacological control.

METHODS

In four dogs, the tone of the proximal colon was recorded by a barostat during distention of the terminal ileum.

RESULTS

Ileal distention inhibited ileal motility and relaxed the colon. Adrenergic blockade by propranolol plus phentolamine, nicotinic blockade by hexamethonium, and the nitric oxide synthase inhibitor N omega-nitro-L-arginine methylester significantly increased resting tone of the colon but did not inhibit relaxation induced by ileal distention. Muscarinic blockade by atropine completely relaxed the colon, and no further decrease in tone was observed after ileal distention. The neurokinin 2 antagonist SR48968 did not alter colonic tone.

CONCLUSIONS

The barostat was able to monitor resting tone of the canine colon, which was shown to be under inhibitory control by adrenergic, cholinergic-nicotinic, and nitric oxide-like transmitters. Inhibition of colonic tone by ileal distention was not mediated solely by adrenergic, cholinergic-nicotinic, or nitric oxide mechanisms. Reflex relaxation, possibly predisposing to acute colonic dilatation, may be activated by multiple mechanisms that may differ from those controlling resting tone.

Electrophysiological analysis of the convergence of peripheral inputs onto neurons of the coeliac ganglion in the guinea pig

The convergence of intestinofugal axons from different intestinal regions onto individual neurons in the coeliac ganglion of the guinea pig was investigated using intracellular recording methods in vitro. Peripheral nerve trunks from the distal ileum, the most proximal colon and the colon near the colonic flexure were electrically stimulated along with preganglionic fibres running in the splanchnic nerve. Fast cholinergic excitatory synaptic potentials (EPSPs) were seen in ganglion cells in response to stimulation of each nerve trunk. Roughly half of 78 neurons impaled received inputs from stimulation of peripheral nerves, and almost all of these received input from the proximal colon. Most cells responded to stimulation of more than one peripheral nerve indicating that coeliac neurons receive converging inputs from intestinofugal neurons located in more than one intestinal region. In a second series of experiments, segments of intestine were left attached to the ganglion and distended with saline to stimulate peripheral mechanosensory input to the coeliac ganglion. In each experiment, two segments were stimulated. A subgroup of ganglion cells exhibited spontaneous fast EPSPs and the frequency of these potentials was increased by distension of one or other of the attached intestinal segments. However, few neurons responded to distension of both of the attached intestinal segments suggesting that some of the intestinofugal inputs to the coeliac ganglion identified by electrical stimulation may be sensitive to sensory modalities other than distension. Hexamethonium (0.5 mM) applied to the intestine, and not to the coeliac ganglion, reduced the frequency of the spontaneous synaptic potentials seen in coeliac ganglion cells, but did not abolish the response to distension of the colon (n = 8). When the Ca2+ concentration of the solution bathing the proximal colon was reduced to block all synaptic transmission in the enteric plexuses the background synaptic input was further depressed, but again the response to distension was little changed (n = 4). This suggests that at least some of the neurons projecting from the colon to the coeliac ganglion are first order mechanosensory neurons.

Activity following colonic distension in enteric sensory fibres projecting to the inferior mesenteric ganglion in the guinea pig

In this investigation the characteristics of the response to colonic distension of afferent fibres projecting to the inferior mesenteric ganglion (IMG) of the guinea pig were studied in vitro. Intracellular membrane potential recordings were made from neurons in the IMG. Post-synaptic potentials arising from activity in afferent fibres were recorded in these cells following distensions of a small segment of colon placed in a separate and independently perfused organ bath. These afferent fibres showed both transient and sustained responses to distension. Application of a solution containing 0.25 mM Ca2+ and 10 mM2+ Mg2+ to the colon (but not to the IMG) reduced the overall response but did not abolish the activity in these fibres. It is concluded that some afferent fibres which respond to colonic distension project directly to the IMG.

New insights into the organization of a gastroduodenal inhibitory reflex by the coeliac plexus

The mechanisms involved at the prevertebral ganglionic level in a gastroduodenal inhibitory reflex were investigated in the rabbit on an in vitro preparation of the coeliac plexus connected to the stomach and duodenum. Intraluminal gastric and duodenal pressures were measured using water-filled balloons. Gastric distension inhibited duodenal motility via a nerve reflex which was abolished by section of the nerves connecting the coeliac plexus to the viscera. Superfusion of the coeliac plexus with a low Ca(2+)-high Mg2+ solution abolished the gastroduodenal inhibitory reflex, indicating a synaptic link at the ganglion level. The reflex was unaffected by superfusion of the coeliac plexus with hexamethonium and tubocurarine, ruling out a nicotinic mechanism. The reflex persisted when the coeliac plexus was superfused with tetrodotoxin or when the nerves connecting the coeliac plexus to the viscera were superfused with a Na(+)-free solution; these results indicate that the reflex does not involve sodium-dependent action potentials. Moreover, superfusion of the nerves connecting the coeliac plexus to the viscera with a calcium blocker or with a Ca(2+)-free solution also failed to abolish the reflex, suggesting that calcium-dependent action potentials are not involved. Our study demonstrates that a gastrointestinal inhibitory reflex via the coeliac ganglion is not based on fast synaptic inputs or action potentials. These results provide new insights concerning the physiology of the sympathetic prevertebral ganglia.

Rectospinal neurons: cell bodies, pathways, immunocytochemistry and ultrastructure

A novel class of enteric neurons projecting directly from the rectal wall to the spinal cord, "rectospinal neurons", was investigated in rats by combined retrograde neuronal tracing, immunocytochemistry and electron microscopy. Rectospinal neurons were almost confined to myenteric ganglia of the distal rectum below the pelvic diaphragm and were labeled preferentially by injections into spinal cord segments L6/S1. Injections into more rostral spinal cord segments resulted in hardly any labeled enteric neurons. Dorsal and ventral rhizotomy experiments indicated an almost exclusive projection of rectospinal neurons through dorsal roots L6/S1 to the respective spinal cord segments. Among various peptides immunostained, vasoactive intestinal polypeptide and calcitonin gene-related peptide were selectively found in rectospinal neurons, which were also shown to contain calbindin, neurofilament protein- and peripherin-immunoreactivity. Vasoactive intestinal polypeptide- and calbindin-immunostaining were frequently co-localized in the same perikarya, while calcitonin gene-related peptide-immunoreactive rectospinal neurons probably represented a separate population. Neonatal capsaicin treatment did not significantly reduce the number of rectospinal neurons. Electron microscopy revealed synaptic contacts on the surface of rectospinal neurons. Taken together, these results establish rectospinal neurons as an anatomically and neurochemically distinct class of enteric neurons. Synaptic contacts on rectospinal neurons suggest that these neurons may function as a direct link from the enteric to the central nervous system, thus indicating that connections between these two networks are reciprocal.

Topographical distribution and immunocytochemical features of colonic neurons that project to the cranial mesenteric ganglion in the pig

Using the retrograde neuronal tracers Fast blue and Fluorogold, the topographical distribution and morphological features of porcine colonic neurons projecting to the cranial (superior) mesenteric ganglion have been investigated. Two to four weeks after injection of the tracer into the cranial mesenteric ganglion of immature pigs, labelled neurons were found throughout the colon. In the myenteric and outer submucous plexuses, they were present in ganglia situated to the side of the mesenteric attachment. The highest density of labelled neurons was observed at the end of the ascending colon, which in the pig represents 78-80% of the total colon length. The viscerofugal neurons had a multidendritic appearance and part of them were immunoreactive for calcitonin gene-related peptide or serotonin. This study has revealed similarities but also significant differences in the colono-sympathico-colonic pathways between the pig and small laboratory animals such as the guinea-pig.

Distribution of NADPH-diaphorase activity in rat paravertebral, prevertebral and pelvic sympathetic ganglia

Paravertebral (superior cervical and stellate), prevertebral (coeliac-superior mesenteric, inferior mesenteric) and pelvic (hypogastric) sympathetic ganglia of the rat were investigated by enzyme histochemistry to ascertain the distribution of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-diaphorase) activity. In the paravertebral ganglia the majority of the sympathetic neuronal perikarya contained lightly and homogeneously distributed formazan reaction product but there was a range of staining intensities amongst the neuron population. In contrast, in the prevertebral ganglia, intense NADPH-diaphorase staining was present in certain neurons. Firstly, a population of neurons of the coeliac-superior mesenteric ganglion complex were surrounded by densely NADPH-diaphorase-positive 'baskets' of fibres and other stained fibres were seen in interstitial nerve bundles and in nerve trunks connected to the ganglion complex. Secondly, in both the inferior mesenteric ganglion and hypogastric ganglion there were many very intensely NADPH-diaphorase positive neurons. Stained dendritic and axonal processes emerged from these cell bodies. In both ganglia this population of neurons was smaller in size than the lightly stained ganglionic neurons and commonly had only one long (presumably axonal) process. The similarity of these highly NADPH-diaphorase-positive neurons with previously described postganglionic parasympathetic neurons in the hypogastric ganglion is discussed.

Distribution of enteric nerve cells that project to the coeliac ganglion of the guinea-pig

The digestive tract of the guinea-pig, from the esophagus to the rectum, was examined in detail to determine the distribution and relative abundances of neurons in these organs that project to the coeliac ganglion and the routes by which their axons reach the ganglion. A retrogradely transported neuronal marker, Fast Blue, was injected into the coeliac ganglion. The esophagus, stomach, gallbladder, pancreas, duodenum, small intestine, caecum, proximal colon, distal colon and rectum were analysed for labelled neurons. Retrogradely labelled neurons were found only in the myenteric plexus of these organs, and in the pancreas. No labelled neurons were found in the gallbladder or the fundus of the stomach, or in the submucous plexus of any region. A small number of labelled neurons was found in the gastric antrum. An increasing density of labelled neurons was found along the duodenum. Similarly, an increasing density of labelled neurons was found from proximal to distal along the jejuno-ileum. However, the greatest densities of labelled neurons were in the large intestine. Many labelled neurons were found in the caecum, including a high density underneath its taeniae. An increasing density of labelled neurons was found along the length of the proximal colon, and labelled neurons were found in the distal colon and rectum. In total, more labelled cell bodies occurred in the large intestine than in the small intestine. The routes taken by the axons of viscerofugal neurons were ascertained by lesioning the nerve bundles which accompany vessels supplying regions of the digestive tract. Viscerofugal neurons of the caecum project to the coeliac ganglion via the ileocaeco-colic nerves; neurons in the proximal colon project to the ganglion via the right colic nerves, and neurons in the distal colon project to the ganglion via the mid colic and intermesenteric nerves. Neurons in the rectum project to the coeliac ganglion via the intermesenteric nerves. These nerves (except for the intermesenterics) all join nerve bundles from the small intestine that follow the superior mesenteric artery. All viscerofugal neurons of the caecum were calbindin-immunoreactive (calb-IR) and 94% were immunoreactive for vasoactive intestinal peptide (VIP-IR). In the proximal colon, 49% of labelled neurons were calb-IR and 85% were VIP-IR. In the distal colon, 80% of labelled neurons were calb-IR and 71% were VIP-IR.

Calbindin-immunoreactive nerve terminals in the guinea pig coeliac ganglion originate from colonic nerve cells

Previous work has shown that calbindin-immunoreactive (calbindin-IR) nerve terminals are numerous in guinea pig prevertebral ganglia. A high proportion of those colonic nerve cells that project to the inferior mesenteric ganglia are calbindin-IR, but none of the neurons that project from the small intestine to the coeliac ganglion are immunoreactive for calbindin. The present work was designed to determine the source of the calbindin-IR fibres and the pathways by which they reach the coeliac ganglion. Sections through the major nerve trunks that connect with the coeliac ganglion revealed numerous calbindin-IR fibres in the inferior coeliac nerves and in the intermesenteric nerves, while there were very few fibres in the splanchnic or superior coeliac nerves. When all peripheral nerve connections to a lobe of the coeliac ganglion were cut, all calbindin-IR terminals degenerated. Cutting the ileo-caeco-colic nerves caused a substantial reduction in the density of nerve fibres in the coeliac ganglion, whereas no significant reduction could be detected when the intermesenteric nerves were cut. However, lesion of both the ileo-caeco-colic and intermesenteric nerves caused all the calbindin-IR nerve fibres in the coeliac ganglion to degenerate. It is concluded that most or all of the calbindin-reactive nerve terminals in the coeliac ganglion originate from the large intestine and that most reach the ganglion via the ileo-caeco-colic nerves. Thus many colonic intestinofugal neurons, supplying both the coeliac and inferior mesenteric ganglia, are immunoreactive for calbindin, whereas small intestinal intestinofugal neurons are not immunoreactive for this protein.

Morphological and chemical identification of neurons that project from the colon to the inferior mesenteric ganglia in the guinea-pig

Labelled nerve cells were located in the distal colon of the guinea-pig 4-5 days after the retrograde tracing agent, Fast blue, was injected into the inferior mesenteric ganglia. Labelled neurons were only found in the myenteric plexus. Their frequency increased from oral to anal and was greater towards the mesenteric border, compared with the anti-mesenteric aspect, of the colon. Many retrogradely labelled neurons were immunoreactive for vasoactive intestinal peptide or calbindin. In the inferior mesenteric ganglia, vasoactive intestinal peptide and calbindin immunoreactive nerve fibres surrounded the same clumps of nerve cell bodies. Almost all calbindin and vasoactive intestinal peptide immunoreactive terminals degenerated after the nerves running from the large intestine to the inferior mesenteric ganglia were cut. It is concluded that the great majority of calbindin and vasoactive intestinal peptide immunoreactive terminals in the inferior mesenteric ganglia arise from nerve cell bodies in the myenteric plexus of the large intestine.

Evaluation of the activity of chemically identified enteric neurons through the histochemical demonstration of cytochrome oxidase

The measurement of the density of the reaction product produced by the histochemical demonstration of cytochrome oxidase activity provides a method for the visual identification of physiologically active enteric neurons. The current study utilized the cytochrome oxidase technique in order to evaluate the metabolic history of neurons in different regions of the bowel and in chemically identified types of neuron. In addition, the effect of drugs or neurotoxins commonly used in the immunocytochemical identification of enteric neuronal phenotypes was also analyzed. Cytochrome oxidase activity was visualized with a blue-black reaction product resulting from the cobalt-intensified oxidation of 3,3'-diaminobenzidine. Peptides or 5-hydroxytryptamine (5-HT) were localized with biotinylated secondary antibodies and alkaline phosphatase-labeled avidin. Bound avidin or endogenous alkaline phosphatase was visualized with a red reaction product in the presence or absence, respectively, of levamisole. Use of measured without interference from a simultaneously demonstrated histo- or immunochemical marker. A multi-peptidergic class of cholinergic submucosal secretomotor neuron containing neuropeptide Y (NPY) and calcitonin gene related peptide (CGRP) immunoreactivities was found to be less metabolically active than the average of all submucosal neurons. In contrast, a non-cholinergic submucosal secretomotor neuron containing dynorphin (which is also known to contain vasoactive intestinal peptide) immunoreactivity was more metabolically active than submucosal neurons that do not contain this peptide. On average, submucosal neurons were more metabolically active than those of the myenteric plexus, and levels of metabolic activity in the myenteric plexus were found to be higher in the duodenum and the cecum than in the jejunum-ileum or colon. Myenteric neurons characterized by CGRP or NPY immunoreactivities or by endogenous alkaline phosphatase activity, were all less metabolically active than the average of all neurons in myenteric ganglia. Colchicine, which stimulates intestinal motility, was observed to increase cytochrome oxidase activity in enteric neurons, suggesting that an effect on the enteric nervous system contributes to its action on the bowel. The neurotoxins, 6-hydroxydopamine and 5,7-dihydroxytryptamine (5,7-DHT) were each found to stimulate neuronal metabolic activity. 5,7-DHT appeared to activate excitatory subtypes of 5-HT receptor since its effects were blocked or mimicked by compounds that act as antagonists or agonists, respectively, at these receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Peripheral nerve pathways to neurons in the guinea pig inferior mesenteric ganglion determined electrophysiologically after chronic nerve section

Peripheral synaptic pathways to neurons in the guinea pig inferior mesenteric ganglion (IMG) were studied. Nerve trunks innervating neurons in the ganglion were surgically sectioned and intracellular electrical responses to nerve stimulation were measured 6-8 days after surgery. In all animals ganglia were decentralized by removal of the lumbar sympathetic chain ganglia L2 through L4 and in addition two peripheral nerves were sectioned leaving the ganglion innervated by only one peripheral nerve. Fast and slow excitatory postsynaptic potential (EPSP) were evoked with electrical stimulation of each of the nerve trunks and with distension of the colon. The thresholds to evoke fast EPSPs and the amplitude of slow EPSPs were compared for each nerve trunk among the different surgical groups including sham-operated controls and completely denervated ganglia. Both fast and slow EPSPs could be evoked electrically from each intact peripheral nerve trunk after the other three nerve trunks had been sectioned, which demonstrates that nerve fibers with cell bodies in the regions innervated by the peripheral nerves make functional synaptic connections with neurons in the inferior mesenteric ganglion. In general, nerve sections increased the threshold for evoking fast EPSPs and decreased the amplitude of electrically-evoked slow EPSPs compared to control ganglia. Synaptic potentials could also be evoked with stimulation of cut nerve trunks, demonstrating that branches of nerve fibers from peripheral nerves enter other nerve trunks. The hypogastric nerve was unique in that branches of axons eliciting fast but not slow synaptic potentials in the ganglion entered this nerve trunk. Distension-induced fast and slow EPSPs were present only if the lumbar colonic nerve was intact and they were not altered by section of the other nerve trunks. In contrast, the slow EPSPs evoked with electrical stimulation of the lumbar colonic nerve were significantly smaller when at least one other nerve trunk was sectioned suggesting that the axon branches from other nerve trunks which enter the lumbar colonic nerve are not activated by distension. These studies demonstrate that neurons eliciting either fast or slow synaptic potentials with cell bodies in regions innervated by the peripheral nerve trunks make functional synaptic connections with neurons of the inferior mesenteric ganglion. The results also suggest that the majority of mechanosensory neurons mediating excitatory synaptic responses to colon distension are neurons with a peripheral cell body.

Comparison of central versus peripheral nerve pathways to the guinea pig inferior mesenteric ganglion determined electrophysiologically after chronic nerve section

The contributions of central versus peripheral nerve pathways to neurons of the inferior mesenteric ganglion of guinea pigs were studied. Nerve trunks innervating neurons in the ganglion were surgically sectioned and intracellular electrical responses to nerve stimulation were measured 6-8 days after surgery. Guinea pigs were divided into two experimental groups: (1) those that had the lumbar sympathetic chain ganglia (LSG) L2 through L4 removed and (2) those that had the intermesenteric, lumbar colonic and hypogastric nerves sectioned leaving central connections intact. After 6-8 days fast excitatory postsynaptic potentials (EPSPs) and slow EPSPs were recorded intracellularly in randomly selected principal ganglionic neurons. The threshold stimulus voltage to elicit a fast EPSP, the amplitude of the slow EPSP and the number of neurons in which each type of synaptic potential occurred in response to stimulation of each of the nerve trunks was compared between surgically-sectioned animals and sham-operated controls. Neither section of preganglionic nerve trunks nor of postganglionic nerve trunks eliminated all synaptic input to neurons in the ganglion, indicating that neurons with cell bodies located central to the ganglion as well as in visceral target organs made synaptic connections in the ganglion. Both fast and slow synaptic potentials could be evoked by stimulation of postganglionic nerve trunks even after they were sectioned provided that preganglionic nerves were intact, indicating that axons of central origin which synapse in the ganglion may continue out into postganglionic nerve trunks. In like manner, evidence was obtained indicating that fibers from peripheral nerve trunks which initiate either fast or slow synaptic potentials in ganglionic neurons may continue out into the lumbar splanchnic nerves. These studies demonstrate that the synaptic potentials recorded in the inferior mesenteric ganglion arise not only from neurons with cell bodies central to the ganglion but also from neurons with cell bodies located in the visceral organs which this ganglion subserves.

Distribution of enteric nerve cells that project from the small intestine to the coeliac ganglion in the guinea-pig

The retrograde tracing agent, Fast blue, was injected into the coeliac ganglia of guinea-pigs and 4-7 days later, nerve cell bodies containing this dye were examined in the small intestine. The cell bodies were found in the ganglia of the myenteric plexus but not in submucous ganglia. The labeled cell bodies were large, on average 42 by 19 microns when viewed in whole mounts, with 4-9 fine processes. The cells increased in frequency anally along the small intestine; the number of neurons per unit length of gut in the distal ileum was more than double that near the duodeno-jejunal flexure. At all points along the intestine the nerve cells were more numerous near the mesenteric attachment than opposite this attachment. About half of the neurons showed immunoreactivity for VIP. It is deduced that the neurons that project from the intestine to the coeliac ganglion are likely to be second-order neurons in the afferent limbs of intestino-visceral reflex pathways.

Structure, afferent innervation, and transmitter content of ganglia of the guinea pig gallbladder: relationship to the enteric nervous system

Although a well-developed plexus of nerves and ganglia is known to be present in the wall of the gallbladder, little has previously been learned about the function or organization of this innervation. The current study was undertaken in order to evaluate the hypothesis that the ganglionated plexus of the gallbladder is analogous to elements of the enteric nervous system (ENS). The ganglionated plexus of the gallbladder was found to resemble closely the submucosal plexus of the small intestine in its organization into two irregular anastomosing and interwoven networks of ganglia, in the numbers of neurons per ganglion, and in the manifestation of histochemically demonstrable acetylcholinesterase activity in virtually all ganglion cells. In common with enteric ganglia, laminin immunoreactivity was observed to be excluded from the interiors of gallbladder ganglia, which were surrounded by a periganglionic laminin-immunoreactive sheath. As in the submucosal plexus, intrinsic substance P-, vasoactive intestinal polypeptide (VIP)-, and neuropeptide Y (NPY)-immunoreactive neurons were seen in the ganglionated plexus of the gallbladder. Extrinsic nerves in the gallbladder that degenerated following chemical sympathectomy with 6-hydroxydopamine (6-OHDA), and which contained NPY, tyrosine hydroxylase (TH), and dopamine-beta-hydroxylase (DBH) immunoreactivities, formed a perivascular plexus closely associated with blood vessels. Endogenous catecholamines could also be demonstrated in these perivascular nerves by aldehyde-induced histofluorescence. In addition to perivascular nerves, paravascular nerve bundles were observed that were loosely associated with vessels, did not degenerate following administration of 6-OHDA, and contained NPY immunoreactivity. Other paravascular nerves, probably visceral sensory axons, coexpressed substance P and calcitonin-gene-related peptide (CGRP) immunoreactivities. The ganglionated plexus of the gallbladder resembled enteric ganglia in having intrinsic 5-hydroxytryptamine (5-HT)-immunoreactive cells and highly varicose nerve fibers. The 5-HT-immunoreactive gallbladder axons were, like those of the gut, resistant to 6-OHDA, and separate from fibers that expressed TH immunoreactivity. Differences between the ganglionated plexus of the gallbladder and enteric ganglia of the small intestine included in the gallbladder are 1) the presence of TH-immunoreactive cells that contain an endogenous catecholamine, but not DBH; 2) DBH-immunoreactive neurons, some of which coexpress substance P immunoreactivity, but which contain neither a catecholamine nor TH immunoreactivity; 3) an apparent absence of CGRP-immunoreactive cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Central neurotensin nerves modulate colo-colonic reflex activity in the guinea-pig inferior mesenteric ganglion

1. The effects of neurotensin and of stimulation of preganglionic nerves on peripheral afferent synaptic input from segments of distal colon to neurones in the inferior mesenteric ganglia of guinea-pigs were studied using intracellular recording techniques in vitro. 2. Electrical stimulation of colonic afferent nerve fibres evoked fast, nicotinic synaptic responses (fast EPSPs or action potentials) followed by a slow depolarizing response (slow EPSP). 3. Neurotensin (1 microM) increased the amplitude and duration of slow EPSPs evoked by stimulation of colonic afferents. 4. Distention of a segment of distal colon left attached to an inferior mesenteric ganglion evoked a slow depolarization. Neurotensin (1 microM) increased the amplitude and duration of distention-induced depolarizations. 5. Electrical stimulation of central preganglionic nerve fibres present in the third and fourth lumbar ventral roots increased the amplitude and duration of slow EPSPs evoked by electrical stimulation of colonic afferent nerves. This facilitatory effect was abolished after desensitization to neurotensin. 6. Slow depolarizations evoked by neurotensin and by stimulation of central preganglionic nerves converted subthreshold fast EPSPs due to mechanosensory synaptic input from an attached segment of distal colon to action potentials. This increase in firing rate of sympathetic ganglion cells led to a decrease in colonic intraluminal pressure. 7. Taken together these data support the hypothesis that neurotensin or a closely related substance contained in central preganglionic nerves facilitated release of a non-cholinergic excitatory transmitter from colonic mechanosensory nerves. The slow depolarization evoked by the non-cholinergic transmitter converted on-going subthreshold fast EPSPs to action potentials thereby increasing sympathetic output to the colon. 8. It is suggested that under normal in vivo conditions, central preganglionic fibres containing neurotensin or a closely related peptide modulate peripheral reflex activity through prevertebral ganglia in guinea-pigs.

Short-chain fatty acids stimulate ileal motility in humans

We tested the hypothesis that short-chain fatty acids (SCFAs) and distention would stimulate ileal motility in humans. Intraluminal pressures in the ileocolonic region were recorded in 18 healthy human volunteers after instillation of boluses of SCFAs, air, and saline. Ileal motility was stimulated more often by SCFAs than by similar volumes of air or saline. Although increasing volumes of distention evoked greater numbers of contractions, this phenomenon was not apparent after repeated stimulation, suggesting that the "mechanoreceptor" in the human ileum has a refractory period. Symptoms of abdominal pain, cramps, and an urge to defecate may have resulted from instillation of SCFAs, even at small volumes. The motility stimulated in the ileum by SCFAs was not associated with systemic release of gastrointestinal regulatory peptides and was not affected by naloxone or indomethacin. Short-chain fatty acids, which can be considered as "markers" of colonic contents, might be associated with the motor response to coloileal reflux in humans.

Rectospinal neurons: evidence for a direct projection from the enteric to the central nervous system in the rat

Wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP), horseradish peroxidase (HRP), and Fluorogold injections were made into the spinal cord segments L4-S2, and HRP was applied to cut L6 and S1 dorsal roots in the rat. These procedures resulted in retrograde labeling of neuronal cell bodies in the rectal wall. Labeled neurons were found both inside and outside myenteric ganglia. Their occurrence was restricted to 5 mm proximal of the external anal sphincter. These cell bodies might represent an additional type of afferent neuron, furnishing a direct information pathway from the rectal wall to the spinal cord.

Effects of potassium channel-blocking agents on neurons in the inferior mesenteric ganglion in guinea-pig

Intracellular recordings were made from neurons (n = 121) in the inferior mesenteric ganglion (IMG) in guinea-pig. The pharmacological actions of 4-aminopyridine (4-AP), barium ions (Ba2+) and tetraethylammonium ions (TEA) were studied on IMG cells which received an excitatory, cholinergic input from mechanosensory nerves in the gastrointestinal tract. 4-AP mediated an excitatory action which involved two separate effects. Firstly, 4-AP increased the incidence of spontaneously occurring, fast EPSPs which gave rise to a spontaneous discharge of action potentials. This indirect, excitatory effect was attributed to an increase in the spontaneous release of acetylcholine from excitatory nerves to the IMG. Secondly, 4-AP altered the excitability of IMG cells and brought about burst discharges and continuous discharges of action potentials. This direct, excitatory effect was not dependent on the spontaneous release of acetylcholine; instead, it was attributed to the blockade of a potassium current similar to the A-current (IA). The excitatory action of Ba2+ also involved two separate effects. Firstly, Ba2+ increased the incidence of spontaneously occurring, fast EPSPs which gave rise to a spontaneous discharge of action potentials. This indirect, excitatory effect was interpreted as Ba2+ mimicking the actions of Ca2+ to facilitate the spontaneous release of acetylcholine. Secondly, Ba2+ altered the excitability of IMG cells and brought about a continuous discharge of action potentials. This excitatory effect was attributed to the blockade of a potassium current similar to the M-current (IM). TEA exerted an excitatory, then inhibitory, action on IMG cells. Initially, TEA brought about the continuous discharge of action potentials; firing gradually arrested as IMG cells depolarized slowly and a depolarizing block of excitation (i.e. inhibition) developed. The block on excitation was relieved by first restoring the resting membrane potential of IMG cells with hyperpolarizing current-clamp. Thereafter, action potentials were elicited by anode-break excitation by temporarily removing the hyperpolarizing current-clamp. The durations of action potentials and afterspike hyperpolarizations were prolonged in the presence of TEA. The effect of TEA on the action potential of IMG cells was attributed to the blockade of the delayed rectifier (IK). The effect on the afterspike hyperpolarization was considered the indirect consequence of a blockade of IK; it allowed the development of an inward calcium current which enhanced the calcium-activated, potassium current (IKCa) mediating afterhyperpolarizations.

Afterspike-hyperpolarization of neurons in the inferior mesenteric ganglion in guinea-pig

Intracellular recordings were made from neurons (n = 121) in the inferior mesenteric ganglion (IMG) in guinea-pig. The afterspike hyperpolarization (ASH) following a single action potential was studied in IMG cells which received an excitatory, cholinergic innervation from mechanosensory nerves in the gastrointestinal tract. The amplitude of ASH was dependent on the membrane potential of IMG cells and the concentration of K+ in the bathing solution. The reversal potential of ASH (-80- -90 mV, in normal Krebs solution) appeared to follow the equilibrium potential for K+, as [K+]o was changed, suggesting that ASH was the product of K+-efflux. Further evidence suggested that a major component of the K+-efflux was dependent on the concentration of Ca2+ in the bathing medium. Elevation and reduction of [Ca2+]o increased and decreased, respectively, the amplitude and duration of ASH. In the presence of tetrodotoxin, depolarizing current pulses elicited spike-like events which (1) were dependent on [Ca2+]o and the degree of depolarization by current-clamp and (2) were followed by afterhyperpolarizations that were also dependent on [Ca2+]o and degree of depolarization by current-clamp. In the combined presence of tetrodotoxin and tetraethylammonium, depolarizing current pulses elicited prolonged action potentials (up to 100 ms in duration) followed by prolonged ASH (up to 3 s in duration). Spike-like events, prolonged action potentials and their afterhyperpolarizations were reduced in amplitude and duration when the calcium-channel blocking ion, Co2+, or blocking drug, verapamil, was present in the bathing medium. In normal Krebs solution, the ASH of action potentials produced by nerve stimulation was reduced but not abolished in the presence of Co2+. These results suggested that Ca2+ entered IMG cells during depolarization and activated the K+-conductance mechanisms responsible for the ASH. However, an initial component of the ASH may have involved other voltage-dependent K+-currents known to be activated during the excitation of sympathetic neurons. The amplitude and duration of ASH differed during non-synaptic and synaptic excitation of IMG cells, and differed when action potentials resulted from fast and slow EPSPs. In addition, the amplitude and duration of ASH were altered by noradrenaline, by the cholinomimetic, carbachol, and by 3 neuropeptides present in the IMG, namely leucine-enkephalin, substance P and vasoactive intestinal polypeptide.(ABSTRACT TRUNCATED AT 400 WORDS)

Projection pathways, co-existence of peptides and synaptic organization of nerve fibers in the inferior mesenteric ganglion of the guinea-pig

The presence of immunoreactive enkephalin, dynorphin, vasoactive intestinal polypeptide, cholecystokinin, substance P and neuropeptide Y in nerve fibers that project to the guinea-pig inferior mesenteric ganglion was analysed, after different denervation and ligation procedures. A quantitative analysis demonstrates that enkephalin- and substance P fibers reach the ganglion mainly via lumbar splanchnic and partly via intermesenteric nerves. Dynorphin-, vasoactive intestinal polypeptide- and cholecystokinin fibers reach the ganglion mainly via colonic and partly via hypogastric or intermesenteric nerves. Neuropeptide Y fibers enter via intermesenteric, lumbar splanchnic and hypogastric nerves and pass through the ganglion. Analysis of serial 0.5 micron sections tends to confirm co-existence: of dynorphin, vasoactive intestinal polypeptide and cholecystokinin in fibers projecting from the colon; of dynorphin with substance P in the lumbar splanchnic nerves; and of neuropeptide Y with substance P in the hypogastric and colonic fibers. Synaptic contacts, predominantly axodendritic, onto the ganglion cells from enkephalin-, vasoactive intestinal polypeptide-, and substance P-containing terminals were revealed by electron microscopy. Enkephalin-immunoreactive axon varicosities are filled with small, clear vesicles with a few large, cored vesicles and form asymmetric synapses; dynorphin-, vasoactive intestinal polypeptide- and cholecystokinin-immunoreactive axon varicosities are rich in large, dense-cored vesicles and form symmetric synapses.

Immunohistochemical localisation and electrophysiological actions of GABA in prevertebral ganglia in guinea-pig

Immunohistochemical techniques were used to detect the presence of a GABA-like material in prevertebral sympathetic ganglia in guinea-pigs. Varicose, immunolabelled nerve fibres were observed in close proximity to sympathetic neurones in inferior mesenteric ganglia and coeliac ganglia. Non-varicose, immunolabelled nerve fibres were observed in lumbar colonic nerves and superior coeliac nerves, i.e. in nerve bundles peripheral to prevertebral ganglia. Immunolabelling was also present in neurones in the myenteric plexus and in nerve fibres in the circular muscle of the colon, as shown previously (Hills et al., Neuroscience, 22 (1987) 301-312). However, GABA-like immunoreactivity was not observed in the cell bodies of prevertebral ganglia nor in splanchnic nerves central to prevertebral ganglia. It was concluded from these results that prevertebral ganglia in guinea-pig receive a GABAergic innervation from neurones peripheral to the ganglia, possibly from GABA-containing neurones in the myenteric plexus of the gastrointestinal tract. Intracellular recordings were made from sympathetic neurones in the inferior mesenteric ganglion (IMG). Application of GABA onto the IMG caused a slow depolarisation of sympathetic neurones, during which there was a marked decrease in the input resistance of IMG cells. Application of GABA also depressed excitatory postsynaptic potentials (EPSPs) and action potentials in sympathetic neurones excited by cholinergic nerve fibres in the lumbar colonic nerves. The reversal potential of the GABA-induced slow depolarisation was -37 mV, a value close to the chloride equilibrium potential for sympathetic neurones. The actions of GABA were reversibly reduced by the GABAA antagonist, bicuculline, and were modulated in a predictable manner by substituting chloride ions with methane-sulphonate ions. These results indicated that GABA, and presumably GABAergic nerves in prevertebral ganglia, modulate the excitability of sympathetic neurones by acting on GABAA receptors linked to a chloride ionophore.

The sensory-efferent function of capsaicin-sensitive sensory neurons

Capsaicin-sensitive sensory neurons convey to the central nervous system signals (chemical and physical) arising from viscera and the skin which activate a variety of visceromotor and neuroendocrine reflexes integrated at various levels (intramurally in peripheral organs, at level of prevertebral ganglia, spinal and supraspinal level). Much evidence is now available that peripheral terminals of certain sensory neurons, widely distributed in skin and viscera have the ability to release, upon adequate stimulation, their transmitter content. In addition to the well-known "axon reflex" arrangement, the capsaicin-sensitive sensory neurons have the ability to release the stored transmitter also from the same terminal which is excited by the environmental stimulus. The efferent function of these sensory neurons is realized through the direct and indirect (i.e. mediated by activation of other cells) effects of released mediators. The action of released transmitters on postjunctional elements covers a wide range of effects which may have a physiological or pathological relevance. Development of drugs capable of controlling the sensory-efferent functions of the capsaicin-sensitive sensory neurons represent a new and very promising area of research for pharmacological treatment of various human diseases.