The neurochemistry and innervation patterns of extrinsic sensory and sympathetic nerves in the myenteric plexus of the C57Bl6 mouse jejunum

Expression of the relaxin family peptide 4 receptor by enterochromaffin cells of the mouse large intestine

The gastrointestinal hormone, insulin-like peptide 5 (INSL5), is found in large intestinal enteroendocrine cells (EEC). One of its functions is to stimulate nerve circuits that increase propulsive activity of the colon through its receptor, the relaxin family peptide 4 receptor (RXFP4). To investigate the mechanisms that link INSL5 to stimulation of propulsion, we have determined the localisation of cells expressing Rxfp4 in the mouse colon, using a reporter mouse to locate cells expressing the gene. The fluorescent signal indicating the location of Rxfp4 expression was in EEC, the greatest overlap of Rxfp4-dependent labelling being with cells containing 5-HT. In fact, > 90% of 5-HT cells were positive for Rxfp4 labelling. A small proportion of cells with Rxfp4-dependent labelling was 5-HT-negative, 11-15% in the distal colon and rectum, and 35% in the proximal colon. Of these, some were identified as L-cells by immunoreactivity for oxyntomodulin. Rxfp4-dependent fluorescence was also found in a sparse population of nerve endings, where it was colocalised with CGRP. We used the RXFP4 agonist, INSL5-A13, to activate the receptor and probe the role of the 5-HT cells in which it is expressed. INSL5-A13 administered by i.p. injection to conscious mice caused an increase in colorectal propulsion that was antagonised by the 5-HT₃ receptor blocker, alosetron, also given i.p. We conclude that stimuli that excite INSL5-containing colonic L-cells release INSL5 that, through RXFP4, excites 5-HT release from neighbouring endocrine cells, which in turn acts on 5-HT₃ receptors of enteric sensory neurons to elicit propulsive reflexes.

Mapping of Extrinsic Innervation of the Gastrointestinal Tract in the Mouse Embryo

Precise extrinsic afferent (visceral sensory) and efferent (sympathetic and parasympathetic) innervation of the gut is fundamental for gut-brain cross talk. Owing to the limitation of intrinsic markers to distinctively visualize the three classes of extrinsic axons, which intimately associate within the gut mesentery, detailed information on the development of extrinsic gut-innervating axons remains relatively sparse. Here, we mapped extrinsic innervation of the gut and explored the relationships among various types of extrinsic axons during embryonic development in mice. Visualization with characterized intrinsic markers revealed that visceral sensory, sympathetic, and parasympathetic axons arise from different anatomic locations, project in close association via the gut mesentery, and form distinctive innervation patterns within the gut from embryonic day (E)10.5 to E16.5. Genetic ablation of visceral sensory trajectories results in the erratic extension of both sympathetic and parasympathetic axons, implicating that afferent axons provide an axonal scaffold to route efferent axons. Coculture assay further confirmed the attractive effect of sensory axons on sympathetic axons. Taken together, our study provides key information regarding the development of extrinsic gut-innervating axons occurring through heterotypic axonal interactions and provides an anatomic basis to uncover neural circuit assembly in the gut-brain axis (GBA).SIGNIFICANCE STATEMENT Understanding the development of extrinsic innervation of the gut is essential to unravel the bidirectional neural communication between the brain and the gut. Here, with characterized intrinsic markers targeting vagal sensory, spinal sensory, sympathetic, and parasympathetic axons, respectively, we comprehensively traced the spatiotemporal development of extrinsic axons to the gut during embryonic development in mice. Moreover, in line with the somatic nervous system, pretarget sorting via heterotypic axonal interactions is revealed to play critical roles in patterning extrinsic efferent trajectories to the gut. These findings provide basic anatomic information to explore the mechanisms underlying the process of assembling neural circuitry in the gut-brain axis (GBA).

Glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide stimulate release of substance P from TRPV1- and TRPA1-expressing sensory nerves

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are released from enteroendocrine cells (EECs) in response to nutrient ingestion and lower blood glucose levels by stimulation of insulin secretion and thus are defined as incretins. GLP-1 receptor (GLP-1R) expression has been identified on enteric neurons that include intrinsic afferent neurons, extrinsic spinal, and vagal sensory afferents but has not been shown to have an incretin effect through these nerves. GLP-1 and GIP enter the mesenteric lymphatic fluid (MLF) after a meal via the interstitial fluid (IF) from local tissue secretion and/or blood capillaries. We tested if MLF could induce diet-dependent intransient increases in intracellular calcium ([Ca²⁺]_i) in cultured sensory neurons. Postprandial rat MLF, collected from the superior mesenteric lymphatic duct, induced a significant twofold higher intransient increase in [Ca²⁺]_i in primary-cultured sensory neurons than MLF from fasted rats. Inhibition of transient receptor potential vanilloid 1 (TRPV1) and TRPV1 and ankyrin 1 cation channels (TRPA1) with ruthenium red eliminated the difference. Substance P (SP) (a peptide that stimulates insulin secretion) sensor cells cocultured with sensory neurons showed both the GLP-1R agonist exendin-4 (Ex-4) and GIP induced transient increases in [Ca²⁺]_i directly coupled to SP secretion in the sensory nerves. Ex-4-induced release of SP required expression of either TRPA1 or TRPV1. These data identify unrecognized actions of GLP-1 and GIP as incretins by acting as neurolymphocrines and suggest a mechanism for sensory nerves to respond to the postprandial state through MLF.NEW & NOTEWORTHY Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted upon eating to lower blood sugar. GLP-1 and GIP were found to induce the secretion of substance P (SP) from cultured sensory nerves. SP enhances insulin secretion. Mesenteric lymphatic fluid (MLF) also stimulates sensory neurons in a diet-dependent manner. These studies identify new actions of GLP-1 and GIP as incretins and suggest a mechanism for sensory nerves to respond to diet through MLF.

Prokinetic effects of spinal cord stimulation and its autonomic mechanisms in dogs

BACKGROUND

Spinal cord stimulation (SCS) is widely used to treat chronic pain by inhibiting sympathetic activity; however, it is unknown whether it exerts a prokinetic effect on gastric motility. Our aim was to explore effects and possible mechanisms of SCS on glucagon-induced gastric dysmotility and dysrhythmia.

METHODS

Seven female dogs with electrodes chronically placed on the dorsal column of the spinal cord between T10 and T12 segments were studied in 2 randomized sessions (glucagon + sham-SCS, glucagon + SCS). SCS at T10 using a set of optimized stimulation parameters was performed for 30 minute immediately after glucagon injection. The antral manometry, electrogastrogram, and electrocardiogram were recorded to assess gastric contractions, gastric slow waves (GSW), and autonomic functions, respectively.

KEY RESULTS

(a) Compared to baseline, glucagon decreased antral motility index (MI) (6315 ± 565 vs 3243 ± 775, P < 0.001), reduced the percentage of normal GSW (89 ± 3% vs 58 ± 3%, P < 0.01), and increased sympathetic activity (0.25 ± 0 0.06 vs 0.60 ± 0.07, P < 0.01). (b) The sympathetic activity was negatively correlated with antral MI (r = -0.558; P < 0.01) and the percentage of gastric normal slow wave (r = -0.616; P < 0.01). (c) SCS prevented the glucagon-induced impairment in antral hypomotility (MI: 5770 ± 927 vs 5521 ± 1238, P > 0.05) and GSW abnormalities (% of normal waves: 84 ± 4% vs 79 ± 6%, P > 0.05) and sympathetic activity (0.27 ± 0.03 vs 0.33 ± 0.07, P > 0.05).

CONCLUSION

Spinal cord stimulation dramatically improves glucagon-induced impairment in gastric contractions and slow waves by inhibiting sympathetic activity.

Effects of Oxaliplatin Treatment on the Myenteric Plexus Innervation and Glia in the Murine Distal Colon

Oxaliplatin (platinum-based chemotherapeutic agent) is a first-line treatment of colorectal malignancies; its use associates with peripheral neuropathies and gastrointestinal side effects. These gastrointestinal dysfunctions might be due to toxic effects of oxaliplatin on the intestinal innervation and glia. Male Balb/c mice received intraperitoneal injections of sterile water or oxaliplatin (3 mg/kg/d) triweekly for 2 weeks. Colon tissues were collected for immunohistochemical assessment at day 14. The density of sensory, adrenergic, and cholinergic nerve fibers labeled with calcitonin gene-related peptide (CGRP), tyrosine hydroxylase (TH), and vesicular acetylcholine transporter (VAChT), respectively, was assessed within the myenteric plexus of the distal colon. The number and proportion of excitatory neurons immunoreactive (IR) against choline acetyltransferase (ChAT) were counted, and the density of glial subpopulations was determined by using antibodies specific for glial fibrillary acidic protein (GFAP) and s100β protein. Oxaliplatin treatment induced significant reduction of sensory and adrenergic innervations, as well as the total number and proportion of ChAT-IR neurons, and GFAP-IR glia, but increased s100β expression within the myenteric plexus of the distal colon. Treatment with oxaliplatin significantly alters nerve fibers and glial cells in the colonic myenteric plexus, which could contribute to long-term gastrointestinal side effects following chemotherapeutic treatment.

Pathways of CGRP Release from Primary Sensory Neurons

The benefit reported in a variety of clinical trials by a series of small molecule antagonists for the calcitonin gene-related peptide (CGRP) receptor, or four monoclonal antibodies against the neuropeptide or its receptor, has underscored the release of CGRP from terminals of primary sensory neurons, including trigeminal neurons, as one of the major mechanisms of migraine headaches. A large variety of excitatory ion channels and receptors have been reported to elicit CGRP release, thus proposing these agonists as migraine-provoking agents. On the other side, activators of inhibitory channels and receptors may be regarded as potential antimigraine agents. The knowledge of the intracellular pathways underlying the exocytotic process that results in CGRP secretion or its inhibition is, therefore, of importance for understanding how migraine pain originates and how to treat the disease.

Development of the intrinsic and extrinsic innervation of the gut

The gastrointestinal (GI) tract is innervated by intrinsic enteric neurons and by extrinsic efferent and afferent nerves. The enteric (intrinsic) nervous system (ENS) in most regions of the gut consists of two main ganglionated layers; myenteric and submucosal ganglia, containing numerous types of enteric neurons and glial cells. Axons arising from the ENS and from extrinsic neurons innervate most layers of the gut wall and regulate many gut functions. The majority of ENS cells are derived from vagal neural crest cells (NCCs), which proliferate, colonize the entire gut, and first populate the myenteric region. After gut colonization by vagal NCCs, the extrinsic nerve fibers reach the GI tract, and Schwann cell precursors (SCPs) enter the gut along the extrinsic nerves. Furthermore, a subpopulation of cells in myenteric ganglia undergoes a radial (inward) migration to form the submucosal plexus, and the intrinsic and extrinsic innervation to the mucosal region develops. Here, we focus on recent progress in understanding the developmental processes that occur after the gut is colonized by vagal ENS precursors, and provide an up-to-date overview of molecular mechanisms regulating the development of the intrinsic and extrinsic innervation of the GI tract.

Individual sympathetic postganglionic neurons coinnervate myenteric ganglia and smooth muscle layers in the gastrointestinal tract of the rat

A full description of the terminal architecture of sympathetic axons innervating the gastrointestinal (GI) tract has not been available. To label sympathetic fibers projecting to the gut muscle wall, dextran biotin was injected into the celiac and superior mesenteric ganglia (CSMG) of rats. Nine days postinjection, animals were euthanized and stomachs and small intestines were processed as whole mounts (submucosa and mucosa removed) to examine CSMG efferent terminals. Myenteric neurons were counterstained with Cuprolinic Blue; catecholaminergic axons were stained immunohistochemically for tyrosine hydroxylase. Essentially all dextran-labeled axons (135 of 136 sampled) were tyrosine hydroxylase-positive. Complete postganglionic arbors (n = 154) in the muscle wall were digitized and analyzed morphometrically. Individual sympathetic axons formed complex arbors of varicose neurites within myenteric ganglia/primary plexus and, concomitantly, long rectilinear arrays of neurites within circular muscle/secondary plexus or longitudinal muscle/tertiary plexus. Very few CSMG neurons projected exclusively (i.e., ∼100% of an arbor's varicose branches) to myenteric plexus (∼2%) or smooth muscle (∼14%). With less stringent inclusion criteria (i.e., ≥85% of an axon's varicose branches), larger minorities of neurons projected predominantly to either myenteric plexus (∼13%) or smooth muscle (∼27%). The majority (i.e., ∼60%) of all individual CSMG postganglionics formed mixed, heterotypic arbors that coinnervated extensively (>15% of their varicose branches per target) both myenteric ganglia and smooth muscle. The fact that ∼87% of all sympathetics projected either extensively or even predominantly to smooth muscle, while simultaneously contacting myenteric plexus, is consistent with the view that these neurons control GI muscle directly, if not exclusively. J. Comp. Neurol. 524:2577-2603, 2016. © 2016 Wiley Periodicals, Inc.

Involvement of catecholaminergic neurons in motor innervation of striated muscle in the mouse esophagus

Enteric co-innervation is a peculiar innervation pattern of striated esophageal musculature. Both anatomical and functional data on enteric co-innervation related to various transmitters have been collected in different species, although its function remains enigmatic. However, it is unclear whether catecholaminergic components are involved in such a co-innervation. Thus, we examined to identify catecholaminergic neuronal elements and clarify their relationship to other innervation components in the esophagus, using immunohistochemistry with antibodies against tyrosine hydroxylase (TH), vesicular acetylcholine transporter (VAChT), choline acetyltransferase (ChAT) and protein gene product 9.5 (PGP 9.5), α-bungarotoxin (α-BT) and PCR with primers for amplification of cDNA encoding TH and dopamine-β-hydroxylase (DBH). TH-positive nerve fibers were abundant throughout the myenteric plexus and localized on about 14% of α-BT-labelled motor endplates differing from VAChT-positive vagal nerve terminals. TH-positive perikarya represented a subpopulation of only about 2.8% of all PGP 9.5-positive myenteric neurons. Analysis of mRNA showed both TH and DBH transcripts in the mouse esophagus. As ChAT-positive neurons in the compact formation of the nucleus ambiguus were negative for TH, the TH-positive nerve varicosities on motor endplates are presumably of enteric origin, although a sympathetic origin cannot be excluded. In the medulla oblongata, the cholinergic ambiguus neurons were densely supplied with TH-positive varicosities. Thus, catecholamines may modulate vagal motor innervation of esophageal-striated muscles not only at the peripheral level via enteric co-innervation but also at the central level via projections to the nucleus ambiguus. As Parkinson's disease, with a loss of central dopaminergic neurons, also affects the enteric nervous system and dysphagia is prevalent in patients with this disease, investigation of intrinsic catecholamines in the esophagus may be worthwhile to understand such a symptom.

The TRPV1 channel in rodents is a major target for antinociceptive effect of the probiotic Lactobacillus reuteri DSM 17938

Certain probiotic bacteria have been shown to reduce distension-dependent gut pain, but the mechanisms involved remain obscure. Live luminal Lactobacillus reuteri (DSM 17938) and its conditioned medium dose dependently reduced jejunal spinal nerve firing evoked by distension or capsaicin, and 80% of this response was blocked by a specific TRPV1 channel antagonist or in TRPV1 knockout mice. The specificity of DSM action on TRPV1 was further confirmed by its inhibition of capsaicin-induced intracellular calcium increases in dorsal root ganglion neurons. Another lactobacillus with ability to reduce gut pain did not modify this response. Prior feeding of rats with DSM inhibited the bradycardia induced by painful gastric distension. These results offer a system for the screening of new and improved candidate bacteria that may be useful as novel therapeutic adjuncts in gut pain. Certain bacteria exert visceral antinociceptive activity, but the mechanisms involved are not determined. Lactobacillus reuteri DSM 17938 was examined since it may be antinociceptive in children. Since transient receptor potential vanilloid 1 (TRPV1) channel activity may mediate nociceptive signals, we hypothesized that TRPV1 current is inhibited by DSM. We tested this by examining the effect of DSM on the firing frequency of spinal nerve fibres in murine jejunal mesenteric nerve bundles following serosal application of capsaicin. We also measured the effects of DSM on capsaicin-evoked increase in intracellular Ca(2+) or ionic current in dorsal root ganglion (DRG) neurons. Furthermore, we tested the in vivo antinociceptive effects of oral DSM on gastric distension in rats. Live DSM reduced the response of capsaicin- and distension-evoked firing of spinal nerve action potentials (238 ± 27.5% vs. 129 ± 17%). DSM also reduced the capsaicin-evoked TRPV1 ionic current in DRG neuronal primary culture from 83 ± 11% to 41 ± 8% of the initial response to capsaicin only. Another lactobacillus (Lactobacillus rhamnosus JB-1) with known visceral anti-nociceptive activity did not have these effects. DSM also inhibited capsaicin-evoked Ca(2+) increase in DRG neurons; an increase in Ca(2+) fluorescence intensity ratio of 2.36 ± 0.31 evoked by capsaicin was reduced to 1.25 ± 0.04. DSM releasable products (conditioned medium) mimicked DSM inhibition of capsaicin-evoked excitability. The TRPV1 antagonist 6-iodonordihydrocapsaicin or the use of TRPV1 knock-out mice revealed that TRPV1 channels mediate about 80% of the inhibitory effect of DSM on mesenteric nerve response to high intensity gut distension. Finally, feeding with DSM inhibited perception in rats of painful gastric distension. Our results identify a specific target channel for a probiotic with potential therapeutic properties.

The novel CGRP receptor antagonist BIBN4096BS alleviates a postoperative intestinal inflammation and prevents postoperative ileus

BACKGROUND

Abdominal surgery results in neuronal mediator release and subsequent acute intestinal hypomotility. This phase is followed by a longer lasting inflammatory phase resulting in postoperative ileus (POI). Calcitonin gene-related peptide (CGRP) has been shown to induce motility disturbances and in addition may be a candidate mediator to elicit neurogenic inflammation. We hypothesized that CGRP contributes to intestinal inflammation and POI.

METHODS

The effect of CGRP in POI was tested in mice treated with the highly specific CGRP receptor antagonist BIBN4096BS and in CGRP receptor-deficient (RAMP-1(-/-) ) mice. POI severity was analyzed by cytokine expression, muscular inflammation and gastrointestinal (GI) transit. Peritoneal and muscularis macrophages and mast cells were analyzed for CGRP receptor expression and functional response to CGRP stimulation.

KEY RESULTS

Intestinal manipulation (IM) resulted in CGRP release from myenteric nerves, and a concurrent increased interleukin (IL)-6 and IL-1β transcription and leukocyte infiltration in the muscularis externa and increased GI transit time. CGRP potentiates IM-induced cytokine transcription within the muscularis externa and peritoneal macrophages. BIBN4096BS reduced cytokine levels and leukocyte infiltration and normalized GI transit. RAMP1(-/-) mice showed a significantly reduced leukocyte influx. CGRP receptor was expressed in muscularis and peritoneal macrophages but not mast cells. CGRP mediated macrophage activation but failed to induce mast cell degranulation and cytokine expression.

CONCLUSIONS & INFERENCES

CGRP is immediately released during abdominal surgery and induces a neurogenic inflammation via activation of abdominal macrophages. BIBN4096BS prevented IM-induced inflammation and restored GI motility. These findings suggest that CGRP receptor antagonism could be instrumental in the prevention of POI.

Prolonged high fat diet ingestion, obesity, and type 2 diabetes symptoms correlate with phenotypic plasticity in myenteric neurons and nerve damage in the mouse duodenum

Symptoms of diabetic gastrointestinal dysmotility indicate neuropathy of the enteric nervous system. Long-standing diabetic enteric neuropathy has not been fully characterized, however. We used prolonged high fat diet ingestion (20 weeks) in a mouse model to mimic human obese and type 2 diabetic conditions, and analyzed changes seen in neurons of the duodenal myenteric plexus. Ganglionic and neuronal size, number of neurons per ganglionic area, density indices of neuronal phenotypes (immunoreactive nerve cell bodies and varicosities per ganglion or tissue area) and nerve injury were measured. Findings were compared with results previously seen in mice fed the same diet for 8 weeks. Compared to mice fed standard chow, those on a prolonged high fat diet had smaller ganglionic and cell soma areas. Myenteric VIP- and ChAT-immunoreactive density indices were also reduced. Myenteric nerve fibers were markedly swollen and cytoskeletal protein networks were disrupted. The number of nNOS nerve cell bodies per ganglia was increased, contrary to the reduction previously seen after 8 weeks, but the density index of nNOS varicosities was reduced. Mice fed high fat and standard chow diets experienced an age-related reduction in total neurons, with bias towards neurons of sensory phenotype. Meanwhile, ageing was associated with an increase in excitatory neuronal markers. Collectively, these results support a notion that nerve damage underlies diabetic symptoms of dysmotility, and reveals adaptive ENS responses to the prolonged ingestion of a high fat diet. This highlights a need to mechanistically study long-term diet-induced nerve damage and age-related impacts on the ENS.

Identification of different types of spinal afferent nerve endings that encode noxious and innocuous stimuli in the large intestine using a novel anterograde tracing technique

In mammals, sensory stimuli in visceral organs, including those that underlie pain perception, are detected by spinal afferent neurons, whose cell bodies lie in dorsal root ganglia (DRG). One of the major challenges in visceral organs has been how to identify the different types of nerve endings of spinal afferents that transduce sensory stimuli into action potentials. The reason why spinal afferent nerve endings have been so challenging to identify is because no techniques have been available, until now, that can selectively label only spinal afferents, in high resolution. We have utilized an anterograde tracing technique, recently developed in our laboratory, which facilitates selective labeling of only spinal afferent axons and their nerve endings in visceral organs. Mice were anesthetized, lumbosacral DRGs surgically exposed, then injected with dextran-amine. Seven days post-surgery, the large intestine was removed. The characteristics of thirteen types of spinal afferent nerve endings were identified in detail. The greatest proportion of nerve endings was in submucosa (32%), circular muscle (25%) and myenteric ganglia (22%). Two morphologically distinct classes innervated myenteric ganglia. These were most commonly a novel class of intraganglionic varicose endings (IGVEs) and occasionally rectal intraganglionic laminar endings (rIGLEs). Three distinct classes of varicose nerve endings were found to innervate the submucosa and circular muscle, while one class innervated internodal strands, blood vessels, crypts of lieberkuhn, the mucosa and the longitudinal muscle. Distinct populations of sensory endings were CGRP-positive. We present the first complete characterization of the different types of spinal afferent nerve endings in a mammalian visceral organ. The findings reveal an unexpectedly complex array of different types of primary afferent endings that innervate specific layers of the large intestine. Some of the novel classes of nerve endings identified must underlie the transduction of noxious and/or innocuous stimuli from the large intestine.

Innervation of enteric mast cells by primary spinal afferents in guinea pig and human small intestine

Mast cells express the substance P (SP) neurokinin 1 receptor and the calcitonin gene-related peptide (CGRP) receptor in guinea pig and human small intestine. Enzyme-linked immunoassay showed that activation of intramural afferents by antidromic electrical stimulation or by capsaicin released SP and CGRP from human and guinea pig intestinal segments. Electrical stimulation of the afferents evoked slow excitatory postsynaptic potentials (EPSPs) in the enteric nervous system. The slow EPSPs were mediated by tachykinin neurokinin 1 and CGRP receptors. Capsaicin evoked slow EPSP-like responses that were suppressed by antagonists for protease-activated receptor 2. Afferent stimulation evoked slow EPSP-like excitation that was suppressed by mast cell-stabilizing drugs. Histamine and mast cell protease II were released by 1) exposure to SP or CGRP, 2) capsaicin, 3) compound 48/80, 4) elevation of mast cell Ca²⁺ by ionophore A23187, and 5) antidromic electrical stimulation of afferents. The mast cell stabilizers cromolyn and doxantrazole suppressed release of protease II and histamine when evoked by SP, CGRP, capsaicin, A23187, electrical stimulation of afferents, or compound 48/80. Neural blockade by tetrodotoxin prevented mast cell protease II release in response to antidromic electrical stimulation of mesenteric afferents. The results support a hypothesis that afferent innervation of enteric mast cells releases histamine and mast cell protease II, both of which are known to act in a diffuse paracrine manner to influence the behavior of enteric nervous system neurons and to elevate the sensitivity of spinal afferent terminals.

The gut-brain axis rewired: adding a functional vagal nicotinic "sensory synapse"

It is generally accepted that intestinal sensory vagal fibers are primary afferent, responding nonsynaptically to luminal stimuli. The gut also contains intrinsic primary afferent neurons (IPANs) that respond to luminal stimuli. A psychoactive Lactobacillus rhamnosus (JB-1) that affects brain function excites both vagal fibers and IPANs. We wondered whether, contrary to its primary afferent designation, the sensory vagus response to JB-1 might depend on IPAN to vagal fiber synaptic transmission. We recorded ex vivo single- and multiunit afferent action potentials from mesenteric nerves supplying mouse jejunal segments. Intramural synaptic blockade with Ca(2+) channel blockers reduced constitutive or JB-1-evoked vagal sensory discharge. Firing of 60% of spontaneously active units was reduced by synaptic blockade. Synaptic or nicotinic receptor blockade reduced firing in 60% of vagal sensory units that were stimulated by luminal JB-1. In control experiments, increasing or decreasing IPAN excitability, respectively increased or decreased nerve firing that was abolished by synaptic blockade or vagotomy. We conclude that >50% of vagal afferents function as interneurons for stimulation by JB-1, receiving input from an intramural functional "sensory synapse." This was supported by myenteric plexus nicotinic receptor immunohistochemistry. These data offer a novel therapeutic target to modify pathological gut-brain axis activity.-Perez-Burgos, A., Mao, Y.-K., Bienenstock, J., Kunze, W. A. The gut-brain axis rewired: adding a functional vagal nicotinic "sensory synapse."

A novel anterograde neuronal tracing technique to selectively label spinal afferent nerve endings that encode noxious and innocuous stimuli in visceral organs

BACKGROUND

One major weakness in our understanding of pain perception from visceral organs is the lack of knowledge of the location, morphology and neurochemistry of all the different types of spinal afferent nerve endings, which detect noxious and innocuous stimuli. This is because we lack techniques to selectively label only spinal afferents. Our aim was to develop an anterograde tracing technique that labels only spinal afferent nerve endings in visceral organs, without also labeling all other classes of extrinsic afferent and efferent nerves.

METHODS

Mice were anesthetized with isoflurane and dextran-biotin injected, via glass micropipettes (diameter 5 μm), into L6 and S1 dorsal root ganglia. Mice recovered for 7 days, were then euthanized and the colon removed.

KEY RESULTS

Anterograde labeling revealed multiple unique classes of afferent endings that terminated within distinct anatomical layers of the colon and rectum. We characterized a particular class of intramuscular ending in the circular muscle (CM) layer of the colon that consists of multiple varicose axons that project circumferentially.

CONCLUSIONS & INFERENCES

We demonstrate a technique for selective anterograde labeling of spinal afferent nerve endings in visceral organs. This approach facilitates selective visualization of the precise morphology and location of the different classes of spinal afferent endings, without visual interference caused by indiscriminant labeling of other classes of afferent and efferent nerve axons which also innervate internal organs. We have used this new technique to identify and describe the details of a particular class of intramuscular spinal afferent ending in the CM layer of mouse large intestine.

Sympathetic axonopathies and hyperinnervation in the small intestine smooth muscle of aged Fischer 344 rats

It is well documented that the intrinsic enteric nervous system of the gastrointestinal (GI) tract sustains neuronal losses and reorganizes as it ages. In contrast, age-related remodeling of the extrinsic sympathetic projections to the wall of the gut is poorly characterized. The present experiment, therefore, surveyed the sympathetic projections to the aged small intestine for axonopathies. Furthermore, the experiment evaluated the specific prediction that catecholaminergic inputs undergo hyperplastic changes. Jejunal tissue was collected from 3-, 8-, 16-, and 24-month-old male Fischer 344 rats, prepared as whole mounts consisting of the muscularis, and processed immunohistochemically for tyrosine hydroxylase, the enzymatic marker for norepinephrine, and either the protein CD163 or the protein MHCII, both phenotypical markers for macrophages. Four distinctive sympathetic axonopathy profiles occurred in the small intestine of the aged rat: (1) swollen and dystrophic terminals, (2) tangled axons, (3) discrete hyperinnervated loci in the smooth muscle wall, including at the bases of Peyer's patches, and (4) ectopic hyperplastic or hyperinnervating axons in the serosa/subserosal layers. In many cases, the axonopathies occurred at localized and limited foci, involving only a few axon terminals, in a pattern consistent with incidences of focal ischemic, vascular, or traumatic insult. The present observations underscore the complexity of the processes of aging on the neural circuitry of the gut, with age-related GI functional impairments likely reflecting a constellation of adjustments that range from selective neuronal losses, through accumulation of cellular debris, to hyperplasias and hyperinnervation of sympathetic inputs.

Myenteric neuron numbers are maintained in aging mouse distal colon

BACKGROUND Age-associated myenteric neuronal loss has been described in several species. In some studies,cholinergic neurons have been reported to be selectively vulnerable, whereas nitrergic neurons are spared. Aging of the mouse enteric nervous system(ENS) and the subtypes of mouse myenteric neurons that may be lost have been little studied. We therefore investigated changes in the numbers of total neurons and two neuronal subpopulations in the mouse distal colon during aging. METHODS Wholemount preparations from 3–4-, 12–13-, 18–19-, and 24–25-month-old C57BL/6 mice were double immunolabeled with HuC/D antibody to identify the total neuronal population and antisera to either calbindin or neuronal nitric oxide synthase (nNOS) to identify myenteric neuronal subpopulations. Samples were analyzed by confocal microscopy. New procedures were employed to ensure unbiased counting and to correct for changes in gut dimensions with age and stretch during sample preparation. The density of nerve fibers in the tertiary plexus was also studied. KEY RESULTS No significant change in numbers of total neurons or of either subpopulation with age was measured, but because of gut growth, the density of myenteric neurons decreased between 3–4 and 12–13 months. The density of nNOS-immunoreactive nerve fibers in the tertiary plexus increased significantly with age, up to 18–19 months. Numerous swollen processes of CB and nNOS-immunoreactive neurons were observed in 18–19- and 24–25-month-old animals. Conclusions &Inferences These results indicate that aging does not result in a loss of myenteric neurons in mouse distal colon at the ages studied, although neurodegenerative changes, which may impact on neuronal function, do occur.

Visualization of spinal afferent innervation in the mouse colon by AAV8-mediated GFP expression

BACKGROUND

Primary afferent neurons whose cell bodies reside in thoracolumbar and lumbosacral dorsal root ganglia (DRG) innervate colon and transmit sensory signals from colon to spinal cord under normal conditions and conditions of visceral hypersensitivity. Histologically, these extrinsic afferents cannot be differentiated from intrinsic fibers of enteric neurons because all known markers label neurons of both populations. Adeno-associated virus (AAV) vectors are capable of transducing DRG neurons after intrathecal administration. We hypothesized that AAV-driven overexpression of green fluorescent protein (GFP) in DRG would enable visualization of extrinsic spinal afferents in colon separately from enteric neurons.

METHODS

Recombinant AAV serotype 8 (rAAV8) vector carrying the GFP gene was delivered via direct lumbar puncture. Green fluorescent protein labeling in DRG and colon was examined using immunohistochemistry.

KEY RESULTS

Analysis of colon from rAAV8-GFP-treated mice demonstrated GFP-immunoreactivity (GFP-ir) within mesenteric nerves, smooth muscle layers, myenteric plexus, submucosa, and mucosa, but not in cell bodies of enteric neurons. Notably, GFP-ir colocalized with CGRP and TRPV1 in mucosa, myenteric plexus, and globular-like clusters surrounding nuclei within myenteric ganglia. In addition, GFP-positive fibers were observed in close association with blood vessels of mucosa and submucosa. Analysis of GFP-ir in thoracolumbar and lumbosacral DRG revealed that levels of expression in colon and L6 DRG appeared to be related.

CONCLUSIONS & INFERENCES

These results demonstrate the feasibility of gene transfer to mouse colonic spinal sensory neurons using intrathecal delivery of AAV vectors and the utility of this approach for histological analysis of spinal afferent nerve fibers within colon.

Dorsal root ganglion neurons innervating pelvic organs in the mouse express tyrosine hydroxylase

Previous studies in rat and mouse documented that a subpopulation of dorsal root ganglion (DRG) neurons innervating non-visceral tissues express tyrosine hydroxylase (TH). Here we studied whether or not mouse DRG neurons retrogradely traced with Fast Blue (FB) from colorectum or urinary bladder also express immunohistochemically detectable TH. The lumbar sympathetic chain (LSC) and major pelvic ganglion (MPG) were included in the analysis. Previously characterized antibodies against TH, norepinephrine transporter type 1 (NET-1) and calcitonin gene-related peptide (CGRP) were used. On average, ∼14% of colorectal and ∼17% of urinary bladder DRG neurons expressed TH and spanned virtually all neuronal sizes, although more often in the medium-sized to small ranges. Also, they were more abundant in lumbosacral than thoracolumbar DRGs, and often coexpressed CGRP. We also detected several TH-immunoreactive (IR) colorectal and urinary bladder neurons in the LSC and the MPG, more frequently in the former. No NET-1-IR neurons were detected in DRGs, whereas the majority of FB-labeled, TH-IR neurons in the LSC and MPG coexpressed this marker (as did most other TH-IR neurons not labeled from the target organs). TH-IR nerve fibers were detected in all layers of the colorectum and the urinary bladder, with some also reaching the basal mucosal cells. Most TH-IR fibers in these organs lacked CGRP. Taken together, we show: (1) that a previously undescribed population of colorectal and urinary bladder DRG neurons expresses TH, often CGRP but not NET-1, suggesting the absence of a noradrenergic phenotype; and (2) that TH-IR axons/terminals in the colon or urinary bladder, naturally expected to derive from autonomic sources, could also originate from sensory neurons.