Morphological relationships of choleragenoid horseradish peroxidase-labeled spinal primary afferents with myenteric ganglia and mucosal associated lymphoid tissue in the cat esophagogastric junction

Sensory spinal interoceptive pathways and energy balance regulation

Interoception plays an important role in homeostatic regulation of energy intake and metabolism. Major interoceptive pathways include gut-to-brain and adipose tissue-to brain signaling via vagal sensory nerves and hormones, such as leptin. However, signaling via spinal sensory neurons is rapidly emerging as an additional important signaling pathway. Here we provide an in-depth review of the known anatomy and functions of spinal sensory pathways and discuss potential mechanisms relevant for energy balance homeostasis in health and disease. Because sensory innervation by dorsal root ganglia (DRG) neurons goes far beyond vagally innervated viscera and includes adipose tissue, skeletal muscle, and skin, it is in a position to provide much more complete metabolic information to the brain. Molecular and anatomical identification of function specific DRG neurons will be important steps in designing pharmacological and neuromodulation approaches to affect energy balance regulation in disease states such as obesity, diabetes, and cancer.

Spinal afferent innervation in flat-mounts of the rat stomach: anterograde tracing

The dorsal root ganglia (DRG) project spinal afferent axons to the stomach. However, the distribution and morphology of spinal afferent axons in the stomach have not been well characterized. In this study, we used a combination of state-of-the-art techniques, including anterograde tracer injection into the left DRG T7-T11, avidin-biotin and Cuprolinic Blue labeling, Zeiss M2 Imager, and Neurolucida to characterize spinal afferent axons in flat-mounts of the whole rat stomach muscular wall. We found that spinal afferent axons innervated all regions with a variety of distinct terminal structures innervating different gastric targets: (1) The ganglionic type: some axons formed varicose contacts with individual neurons within myenteric ganglia. (2) The muscle type: most axons ran in parallel with the longitudinal and circular muscles and expressed spherical varicosities. Complex terminal structures were observed within the circular muscle layer. (3) The ganglia-muscle mixed type: some individual varicose axons innervated both myenteric neurons and the circular muscle, exhibiting polymorphic terminal structures. (4) The vascular type: individual varicose axons ran along the blood vessels and occasionally traversed the vessel wall. This work provides a foundation for future topographical anatomical and functional mapping of spinal afferent axon innervation of the stomach under normal and pathophysiological conditions.

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Localization of receptors for calcitonin-gene-related peptide to intraganglionic laminar endings of the mouse esophagus: peripheral interaction between vagal and spinal afferents?

The calcitonin-gene-related peptide (CGRP) receptor is a heterodimer of calcitonin-receptor-like receptor (CLR) and receptor-activity-modifying protein 1 (RAMP1). Despite the importance of CGRP in regulating gastrointestinal functions, nothing is known about the distribution and function of CLR/RAMP1 in the esophagus, where up to 90 % of spinal afferent neurons contain CGRP. We detected CLR/RAMP1 in the mouse esophagus using immunofluorescence and confocal laser scanning microscopy and examined their relationship with neuronal elements of the myenteric plexus. Immunoreactivity for CLR and RAMP1 colocalized with VGLUT2-positive intraganglionic laminar endings (IGLEs), which were contacted by CGRP-positive varicose axons presumably of spinal afferent origin, typically at sites of CRL/RAMP1 immunoreactivity. This provides an anatomical basis for interaction between spinal afferent fibers and IGLEs. Immunoreactive CLR and RAMP1 also colocalized in myenteric neurons. Thus, CGRP-containing spinal afferents may interact with both vagal IGLEs and myenteric neurons in the mouse esophagus, possibly modulating motility reflexes and inflammatory hypersensitivity.

Identifying spinal sensory pathways activated by noxious esophageal acid

BACKGROUND

The transient receptor potential vanilloid 1 (TRPV1) channel is critical for spinal afferent signaling of burning pain throughout the body. Such pain frequently originates from the esophagus, following acid reflux. The contribution of TRPV1 to spinal nociceptor signaling from the esophagus remains unclear. We aimed to identify the spinal afferent pathways that convey nociceptive signaling from the esophagus, specifically those sensitive to acid, and the extent to which TRPV1 contributes.

METHODS

Acid/pepsin (150 mM HCl/1 mg mL(-1) pepsin) or saline/pepsin was perfused into the esophageal lumen of anesthetized wild-type and TRPV1 null mice over 20 min, followed by atraumatic perfuse fixation and removal of the cervical and thoracic spinal cord and dorsal root ganglia (DRG). To identify neurons responsive to esophageal perfusate, immunolabeling for neuronal activation marker phosphorylated extracellular receptor-regulated kinase (pERK) was used. Labeling for calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4) was then used to characterize responsive neurons.

KEY RESULTS

Esophageal acid/pepsin perfusion significantly increased the number of pERK-immunoreactive (IR) neurons in the DRG and the cervical and thoracic spinal cord dorsal horn (DH) relative to saline/pepsin (DRG P < 0.01; cervical DH P < 0.05 and thoracic DH P < 0.005). The number of pERK-IR neurons following acid perfusion was significantly attenuated in TRPV1 -/- mice (DH P < 0.05 and DRG P < 0.05).

CONCLUSIONS & INFERENCES

This study has identified populations of spinal afferent DRG neurons and DH neurons involved in signaling of noxious acid from the esophagus. There is a major contribution of TRPV1 to signaling within these pathways.

Lactobacillus reuteri ingestion and IK(Ca) channel blockade have similar effects on rat colon motility and myenteric neurones

BACKGROUND

We have previously shown that ingestion of Lactobacillus reuteri may modulate colonic enteric neuron activity but with unknown effects on colon motility. The aim of the present report was to elucidate the neuronal mechanisms of action of the probiotic by comparing the effects on motility of L. reuteri ingestion with blockade of a specific ionic current in enteric neurons.

METHODS

We have used intraluminal pressure recordings from ex vivo rat colon segments and whole cell patch clamp recordings from neurons of rat longitudinal muscle myenteric plexus preparations to investigate the effects of L. reuteri and TRAM-34 on colon motility and neurophysiology. The effects of daily feeding of 10(9) L. reuteri bacteria or acute application of TRAM-34 on threshold fluid filling pressure or pulse pressure was measured.

KEY RESULTS

Lactobacillus reuteri increased intraluminal fluid filling pressure thresholds for evoking pressure pulses by 51% from 0.47 +/- 0.17 hPa; the probiotic also decreased the pulse pressure amplitudes, but not frequency, by 18% from 3.91 +/- 0.52 hPa. The intermediate conductance calcium-dependent potassium (IK(Ca)) channel blocker TRAM-34 (3 micromol L(-1)) increased filling threshold pressure by 43% from 0.52 +/- 0.22 hPa and reduced pulse pressure amplitude by 40% from 2.63 +/- 1.11 hPa; contraction frequency was unaltered. TRAM-34 (3 micromol L(-1)) reduced membrane polarization, leak conductance and the slow afterhyperpolarization current in 16/16 myenteric rat colon AH cells but 19/19 S cells were unaffected.

CONCLUSIONS & INFERENCES

The present results are consistent with L. reuteri enhancing tonic inhibition of colon contractile activity by acting via the IK(Ca) channel current in AH cells.

Roles of gastro-oesophageal afferents in the mechanisms and symptoms of reflux disease

Oesophageal pain is one of the most common reasons for physician consultation and/or seeking medication. It is most often caused by acid reflux from the stomach, but can also result from contractions of the oesophageal muscle. Different forms of pain are evoked by oesophageal acid, including heartburn and non-cardiac chest pain, but the basic mechanisms and pathways by which these are generated remain to be elucidated. Both vagal and spinal afferent pathways are implicated by basic research. The sensitivity of afferent fibres within these pathways may become altered after acid-induced inflammation and damage, but the severity of symptoms in humans does not necessarily correlate with the degree of inflammation. Gastro-oesophageal reflux disease (GORD) is caused by transient relaxations of the lower oesophageal sphincter, which are triggered by activation of gastric vagal mechanoreceptors. Vagal afferents are therefore an emerging therapeutic target for GORD. Pain in the absence of excess acid reflux remains a major challenge for treatment.

Structure-function relationship of sensory endings in the gut and bladder

Visceral afferents play a key role in neural circuits underlying the physiological function of visceral organs. They are responsible for the detection and transmission of a variety of visceral sensations (e.g. satiety, urge, discomfort and pain) from the viscera to the central nervous system. A comprehensive account of the different functional types of visceral sensory neurons would be invaluable in understanding how sensory dysfunction occurs and how it might be diagnosed and treated. Our aim was to explore the morphology of different nerve endings of visceral afferents within the gastrointestinal tract and urinary bladder and how the morphology of these nerve endings may relate to their functional properties. Morphological studies of mechanosensitive endings of visceral afferents to the gut and bladder correlated with physiological recordings have added a new dimension to our ability to distinguish different functional classes of visceral afferents.

Innervation of the mammalian esophagus

Understanding the innervation of the esophagus is a prerequisite for successful treatment of a variety of disorders, e.g., dysphagia, achalasia, gastroesophageal reflux disease (GERD) and non-cardiac chest pain. Although, at first glance, functions of the esophagus are relatively simple, their neuronal control is considerably complex. Vagal motor neurons of the nucleus ambiguus and preganglionic neurons of the dorsal motor nucleus innervate striated and smooth muscle, respectively. Myenteric neurons represent the interface between the dorsal motor nucleus and smooth muscle but they are also involved in striated muscle innervation. Intraganglionic laminar endings (IGLEs) represent mechanosensory vagal afferent terminals. They also establish intricate connections with enteric neurons. Afferent information is implemented by the swallowing central pattern generator in the brainstem, which generates and coordinates deglutitive activity in both striated and smooth esophageal muscle and orchestrates esophageal sphincters as well as gastric adaptive relaxation. Disturbed excitation/inhibition balance in the lower esophageal sphincter results in motility disorders, e.g., achalasia and GERD. Loss of mechanosensory afferents disrupts adaptation of deglutitive motor programs to bolus variables, eventually leading to megaesophagus. Both spinal and vagal afferents appear to contribute to painful sensations, e.g., non-cardiac chest pain. Extrinsic and intrinsic neurons may be involved in intramural reflexes using acetylcholine, nitric oxide, substance P, CGRP and glutamate as main transmitters. In addition, other molecules, e.g., ATP, GABA and probably also inflammatory cytokines, may modulate these neuronal functions.

Intraganglionic laminar endings are mechano-transduction sites of vagal tension receptors in the guinea-pig stomach

1. Distension-sensitive vagal afferent fibres from the cardiac region of the guinea-pig stomach were recorded extracellularly, then filled with biotinamide, using an anterograde tracing technique. 2. Most of the stretch-sensitive units of the guinea-pig stomach (41 out of 47; number of animals N = 26) had low thresholds (less than 1 mm) to circumferential stretch and showed slow adaptation. Twenty of these units fired spontaneously under resting conditions (mean: 1.9 +/- 0.3 Hz, n = 20, N = 14). 3. Adaptation of firing during slow or maintained stretch correlated closely with accommodation of intramural tension, but tension-independent adaptation was also present. 4. Nicardipine (3 microM) with hyoscine (3 microM) reduced stretch-evoked firing of gastric vagal afferents, by inhibiting smooth muscle contraction. Gadolinium (1 mM) blocked distension-evoked firing. 5. Focal stimulation of the stomach muscle wall with a von Frey hair (0.4 mN) identified one to six punctate receptive fields in each low threshold vagal distension-sensitive afferent. These were marked on the serosal surface of the stomach wall. 6. Anterograde filling of recorded nerve trunks revealed intraganglionic laminar endings (IGLEs) within 142 +/- 34 microm (n = 38; N = 10) of marked receptive fields. The mean distance from randomly generated sites to the nearest IGLE was significantly greater (1500 +/- 48 microm, n = 380, N = 10, P < 0.0001). Viscerofugal nerve cell bodies, intramuscular arrays and varicose axons were not associated with receptive fields. The results indicate that IGLEs are the mechanotransduction sites of low threshold, slowly adapting vagal tension receptors in the guinea-pig upper stomach.

Retrograde tracing of enteric neuronal pathways

Neuroanatomical tracing techniques, and retrograde labelling in particular, are widely used tools for the analysis of neuronal pathways in the central and peripheral nervous system. Over the last 10 years, these techniques have been used extensively to identify enteric neuronal pathways. In combination with multiple-labelling immunohistochemistry, quantitative data about the projections and neurochemical profile of many functional classes of cells have been acquired. These data have revealed a high degree of organization of the neuronal plexuses, even though the different classes of nerve cell bodies appear to be randomly assorted in ganglia. Each class of neurone has a predictable target, length and polarity of axonal projection, a particular combination of neurochemicals in its cell body and distinctive morphological characteristics. The combination of retrograde labelling with targeted intracellular recording has made it possible to target small populations of cells that would rarely be sampled during random impalements. These neuroanatomical techniques have also been applied successfully to human tissue and are gradually unravelling the complexity of the human enteric nervous system.

An overview of esophageal sensory receptors

The neurophysiological basis of esophageal pain and discomfort is not well known. Functional disorder, such as noncardiac chest pain, is thought to be associated with hypersensitivity of primary afferents innervating the esophagus and/or sensitization of spinal dorsal horn cells receiving input from the organ. Although we have accumulated a large body of information about the morphologic structure and neuropeptide contents of the extrinsic primary afferents, we lack a full understanding of its integrative function in esophageal pain. The esophagus is innervated dually by vagus and spinal nerves. The majority of sensory afferents in the vagal and spinal pathway are pseudounipolar cells, with their cell bodies (soma) located in the nodose and dorsal root ganglia, respectively. These afferent fibers innervate serosa (adventitia), longitudinal and circular muscles, and mucosa of the esophagus. Afferents innervating the muscle are sensitive to intraluminal distension. In the vagus, these afferents exhibit low threshold for response, whereas the spinal afferents, including the splanchnic nerve afferents, have either low or high thresholds for response. In addition, these afferents are chemosensitive. Both vagal and spinal afferents also innervate the mucosa of the esophagus. These fibers are exquisitely sensitive to light touch of the mucosa and are sensitive to pH and chemicals. The spinal afferents, including splanchnic nerve afferents, project to the spinal cord, spanning from upper cervical (C1) to upper lumbar (L2) segments. A majority of the spinal dorsal horn neurons receiving input from the esophageal spinal afferents also receives somatic converging input. The somatic receptive fields are distributed mainly ipsilaterally over the chest and forearm area. These spinal dorsal horn neurons exhibit either excitatory, inhibitory, or biphasic (i.e., excitation followed by inhibition) responses to esophageal distension.

In vitro recordings of afferent fibres with receptive fields in the serosa, muscle and mucosa of rat colon

1. Colonic afferent fibres were recorded using a novel in vitro preparation. Fibres with endings in the colonic mucosa are described, along with those in muscle and serosa, and their responses to a range of mechanical and chemical luminal stimuli. 2. Mechanical stimuli were applied to the tissue, which included stretch, blunt probing of the mucosa and stroking of the mucosa with von Frey hairs (10-1000 mg). Chemical stimuli were applied into a ring that was placed over the mechanoreceptive field of the fibre; these were distilled water, 154 and 308 mM NaCl, 100 microM capsaicin, 50 mM HCl, and undiluted and 50% ferret bile. 3. Recordings were made from 52 fibres, 12 of which showed characteristics of having endings in the mucosa. Mucosal afferents were sensitive to a 10 mg von Frey hair and were generally chemosensitive to >= 1 chemical stimulus. 4. Ten fibres showed characteristics of having receptive fields in the muscular layer. These fibres responded readily to circumferential stretch, as well as to blunt probing. 5. Twenty-seven fibres showed characteristics of having endings in the serosal layer. They adapted rapidly to circumferential stretch and responded to blunt probing of the serosa. Fifteen of 19 serosal fibres tested also responded to luminal chemicals. 6. Three fibres were unresponsive to all mechanical stimuli but were recruited by chemical stimuli. 7. This is the first characterization of colonic afferent fibres using an in vitro method and the first documentation of afferent fibres with their endings in the mucosa of the colon. These fibres are likely to be important in aspects of colonic sensation and reflex control.

The origin and development of the vagal and spinal innervation of the external muscle of the mouse esophagus

Retrograde and anterograde tracing and immunohistochemical techniques were used to examine the origin of the extrinsic innervation, and the development of the vagal innervation to the mouse esophagus. Cholinergic nerve terminals were localised using an antiserum to the vesicular acetylcholine transporter and cholinergic cell bodies were localised using an antiserum to choline acetyltransferase. Cholinergic nerve terminals, which also contained calcitonin gene-related peptide, were present at the motor end plates in the external (striated) muscle of the esophagus. Following injection of Fast Blue into subdiaphragmatic or cervical levels of the esophagus, the only retrogradely-labelled cholinergic nerve cell bodies that also contained calcitonin gene-related peptide were found in the nucleus ambiguus. Neurons in the dorsal motor nucleus of the vagus, the nodose ganglia and dorsal root ganglia gave rise to a number of different types of nerve terminals within the myenteric plexus. Retrogradely-labelled neurons in the dorsal motor nucleus of vagus contained cholinergic markers only, nitric oxide synthase only or cholinergic markers plus nitric oxide synthase, retrogradely-labelled neurons in the dorsal root ganglia contained calcitonin gene-related peptide only, and a small number of retrogradely-labelled neurons in the nodose ganglia contained tyrosine hydroxylase. The development of the vagal innervation to the esophagus was examined following application of DiI to the vagus nerve of fixed mouse embryos. Anterogradely-labelled nerve fibres, which arose from both nodose ganglia and the medulla, were already present in the esophagus of embryonic day 12 (E12) mice. Some of the DiI-labelled vagal nerve fibres were present in among the smooth muscle cells of the external muscle layer prior to their transdifferentiation to striated muscle. We conclude that the neurons in the nucleus ambiguus that project to the esophagus differ from other extrinsic neurons in their chemistry as well as their targets within the esophagus. The development of the extrinsic innervation precedes the transdifferentiation of the external muscle to striated muscle, raising the possibility that, during development, smooth muscle of the esophagus is innervated transiently by vagal neurons.

Tyramide amplification allows anterograde tracing by horseradish peroxidase-conjugated lectins in conjunction with simultaneous immunohistochemistry

Current protocols for a combined approach of anterograde tracing with carbocyanine dyes or horseradish peroxidase (HRP) conjugates and immunohistochemistry represent a compromise between sensitive detection of the tracer and the immunohistochemical procedure. Therefore, it was investigated whether the use of tyramide amplification allows sensitive anterograde tracing with wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) in conjunction with simultaneous immunohistochemistry. Vagal afferents were anterogradely labeled by injection of WGA-HRP into the nodose ganglion of rats. By use of tyramide-biotin amplification, a dense fiber plexus of vagal afferents was visualized centrally in the nucleus of the solitary tract and in retrogradely labeled neurons in the dorsal vagal nucleus. In the esophagus and duodenum, large- and small-caliber vagal fibers and terminals could be demonstrated comparably to conventional tracing technique using carbocyanine dyes or WGA-HRP and TMB histochemistry. Combination with immunohistochemistry could easily be done, requiring only one more incubation step, and did not result in loss of sensitivity of the tracing. With this method and confocal microscopy, the presence of Ca binding proteins in vagal afferent terminals could be demonstrated. Tyramide amplification allows sensitive anterograde tracing with low background staining in conjunction with immunohistochemistry of a-axonal markers.

Lower oesophageal sphincter responses to noxious oesophageal chemical stimuli in the ferret: involvement of tachykinin receptors

Repeated oesophageal acidification is a definitive feature of gastro-oesophageal reflux disease, which in turn is caused by relaxation of the lower oesophageal sphincter (LOS). This study in anaesthetised ferrets investigates the reflex pathways involved in effects of oesophageal acidification on motor function of the LOS, with particular focus on the role of tachykinins. LOS pressure was monitored with a perfused micromanometric sleeve assembly. Oesophageal acidification reduced LOS pressure by 48 +/- 5% until washout with saline. This reduction became larger with repeated tests, and was unaffected in amplitude by acute bilateral vagotomy, although the response became slower in onset. Intra-oesophageal capsaicin (0.5% solution) caused a 68 +/- 17% decrease in LOS pressure which remained unchanged with repeated tests. The NK-1 receptor antagonist CP96,345 (1-5 mg/kg intravenous (i.v.) blocked the post-vagotomy LOS responses to both intra-luminal acid and capsaicin. Close intra-arterial (i.a.) injections of capsaicin (1-100 micrograms) gut induced LOS relaxation which was neither vagally nor NK-1 receptor-mediated. Substance P or the selective NK-1 receptor agonist [Sar9, Met(O2)11] substance P (25-500 ng close i.a.) caused a biphasic LOS response, consisting of initial brief contraction followed by prolonged, dose-dependent relaxation. Tetrodotoxin (10 micrograms/kg close i.a.) changed the biphasic response to substance P to excitation only. The neurokinin-1 (NK-1) receptor antagonist CP96,345 (0.3-10 mg/kg i.v.) dose-dependently reduced the inhibitory response to substance P. The excitatory phase of the response to substance P was larger and prolonged after guanethidine (5 mg/kg, i.v.), or propranolol (1 mg/kg, i.v.). L-NAME (100 mg/kg i.v.) reduced the inhibitory phase. The selective NK-2 receptor agonist [beta-Ala8] neurokinin A(4-10) caused LOS excitation only. These data indicate that intra-oesophageal acid causes substance P release from extrinsic afferent nerve endings which activates local inhibitory pathways to the LOS via NK-1 receptors.

Ultrastructural relationships of spinal primary afferent fibres with neuronal and non-neuronal cells in the myenteric plexus of the cat oesophago-gastric junction

Spinal primary afferent fibres innervating the myenteric area in the oesophago-gastric junction of the cat were selectively labelled by anterogradely transported cholera toxin B subunit-horseradish peroxidase conjugate injected into thoracic dorsal root ganglia. The ultrastructure of these labelled primary afferent fibres was studied in order to determine whether they display close relationships with specific cell types in the myenteric plexus. Horseradish peroxidase was revealed with tetramethylbenzidine stabilized with ammonium heptamolybdate or with the tetramethylbenzidine/tungstate reaction in order to visualize the cytoplasmic organelles and the axolemma, respectively. The labelled primary afferent fibres were unmyelinated. Two kinds of profiles of labelled fibres containing vesicles and mitochondrial accumulations were found: (i) fibres running in myenteric connectives in isolated nerve bundles, and (ii) fibres within the myenteric ganglia. The first kind had small areas of axolemma with no glial cell covering, whereas the second kind had little or no glial cell covering (termed naked primary afferent fibres). In addition, labelled fibres containing few vesicles and mitochondria and running in nerve bundles surrounded by perineurium were numerous. Within the myenteric ganglia, naked primary afferent fibres contacted myenteric neurons. The contacts were mainly axosomatic. No synaptic specializations were distinguished. In the interganglionic area, some labelled fibres terminated close to blood vessels. The intraganglionic naked primary afferent fibres are suggested to be mechanoreceptors. Their exposed axolemma might allow both mechanotransduction and release of neurotransmitters which could act on myenteric neurons. Because they are protected by their glial cell sheath and by bundles of collagen fibrils, interganglionic primary afferent fibres are likely to be less exposed to deformation.