Analysis of innervation of human mesenteric vessels in non-inflamed and inflamed bowel--a confocal and functional study

Role of perivascular nerve and sensory neurotransmitter dysfunction in inflammatory bowel disease

Inflammatory bowel disease (IBD) is associated with both impaired intestinal blood flow and increased risk of cardiovascular disease, but the functional role of perivascular nerves that control vasomotor function of mesenteric arteries (MAs) perfusing the intestine during IBD is unknown. Because perivascular sensory nerves and their transmitters calcitonin gene-related peptide (CGRP) and substance P (SP) are important mediators of both vasodilation and inflammatory responses, our objective was to identify IBD-related deficits in perivascular sensory nerve function and vascular neurotransmitter signaling. In MAs from an interleukin-10 knockout (IL-10-/-) mouse model, IBD significantly impairs electrical field stimulation (EFS)-mediated sensory vasodilation and inhibition of sympathetic vasoconstriction, despite decreased sympathetic nerve density and vasoconstriction. The MA content and EFS-mediated release of both CGRP and SP are decreased with IBD, but IBD has unique effects on each transmitter. CGRP nerve density, receptor expression, hyperpolarization, and vasodilation are preserved with IBD. In contrast, SP nerve density and receptor expression are increased, and SP hyperpolarization and vasodilation are impaired with IBD. A key finding is that blockade of SP receptors restores EFS-mediated sensory vasodilation and enhanced CGRP-mediated vasodilation in MAs from IBD but not Control mice. Together, these data suggest that an aberrant role for the perivascular sensory neurotransmitter SP and its downstream signaling in MAs underlies vascular dysfunction with IBD. We propose that with IBD, SP signaling impedes CGRP-mediated sensory vasodilation, contributing to impaired blood flow. Thus, substance P and NK1 receptors may represent an important target for treating vascular dysfunction in IBD.NEW & NOTEWORTHY Our study is the first to show that IBD causes profound impairment of sensory vasodilation and inhibition of sympathetic vasoconstriction in mesenteric arteries. This occurs alongside decreased SP-containing nerve density and increased expression of NK1 receptors for SP. In contrast, CGRP dilation, nerve density, and receptor expression are unchanged. Blocking NK1 receptors restores sensory vasodilation in MAs and increases CGRP-mediated vasodilation, indicating that SP interference with CGRP signaling may underlie impaired sensory vasodilation with IBD.

Comparison of Adrenergic and Purinergic Receptor Contributions to Vasomotor Responses in Mesenteric Arteries of C57BL/6J Mice and Wistar Rats

INTRODUCTION

The sympathetic nervous system can modulate arteriolar tone through release of adenosine triphosphate and norepinephrine, which bind to purinergic and adrenergic receptors (ARs), respectively. The expression pattern of these receptors, as well as the composition of neurotransmitters released from perivascular nerves (PVNs), can vary both in organ systems within and across species, such as mice and rats.

OBJECTIVE

This study explores the function of α1A subtypes in mouse and rat third-order mesenteric arteries and investigates PVN-mediated vasoconstriction to identify which neurotransmitters are released from sympathetic PVNs.

METHODS

Third-order mesenteric arteries from male C57BL/6J mice and Wistar rats were isolated and mounted on a wire myograph for functional assessment. Arteries were exposed to phenylephrine (PE) and then incubated with either α1A antagonist RS100329 (RS) or α1D antagonist BMY7378, before reexposure to PE. Electrical field stimulation was performed by passing current through platinum electrodes positioned adjacent to arteries in the absence and presence of a nonspecific alpha AR blocker phentolamine and/or P2X1-specific purinergic receptor blocker NF449.

RESULTS

Inhibition of α1 ARs by RS revealed that PE-induced vasoconstriction is primarily mediated through α1A and that the contribution of the α1A AR is greater in rats than in mice. In the mouse model, sympathetic nerve-mediated vasoconstriction is mediated by both ARs and purinergic receptors, whereas in rats, vasoconstriction appeared to only be mediated by ARs and a nonpurinergic neurotransmitter. Further, neither model demonstrated that α1D ARs play a significant role in PE-mediated vasoconstriction.

CONCLUSIONS

The mesenteric arteries of male C57BL/6J mice and Wistar rats have subtle differences in the signaling mechanisms used to mediate vasoconstriction. As signaling pathways in humans under physiological and pathophysiological conditions become better defined, the current study may inform animal model selection for preclinical studies.

Sympathetic and Sensory-Motor Nerves in Peripheral Small Arteries

Small arteries, which play important roles in controlling blood flow, blood pressure, and capillary pressure, are under nervous influence. Their innervation is predominantly sympathetic and sensory motor in nature, and while some arteries are densely innervated, others are only sparsely so. Innervation of small arteries is a key mechanism in regulating vascular resistance. In the second half of the previous century, the physiology and pharmacology of this innervation were very actively investigated. In the past 10-20 yr, the activity in this field was more limited. With this review we highlight what has been learned during recent years with respect to development of small arteries and their innervation, some aspects of excitation-release coupling, interaction between sympathetic and sensory-motor nerves, cross talk between endothelium and vascular nerves, and some aspects of their role in vascular inflammation and hypertension. We also highlight what remains to be investigated to further increase our understanding of this fundamental aspect of vascular physiology.

Neuroimmune Communication in Health and Disease

The immune and nervous systems are tightly integrated, with each system capable of influencing the other to respond to infectious or inflammatory perturbations of homeostasis. Recent studies demonstrating the ability of neural stimulation to significantly reduce the severity of immunopathology and consequently reduce mortality have led to a resurgence in the field of neuroimmunology. Highlighting the tight integration of the nervous and immune systems, afferent neurons can be activated by a diverse range of substances from bacterial-derived products to cytokines released by host cells. While activation of vagal afferents by these substances dominates the literature, additional sensory neurons are responsive as well. It is becoming increasingly clear that although the cholinergic anti-inflammatory pathway has become the predominant model, a multitude of functional circuits exist through which neuronal messengers can influence immunological outcomes. These include pathways whereby efferent signaling occurs independent of the vagus nerve through sympathetic neurons. To receive input from the nervous system, immune cells including B and T cells, macrophages, and professional antigen presenting cells express specific neurotransmitter receptors that affect immune cell function. Specialized immune cell populations not only express neurotransmitter receptors, but express the enzymatic machinery required to produce neurotransmitters, such as acetylcholine, allowing them to act as signaling intermediaries. Although elegant experiments have begun to decipher some of these interactions, integration of these molecules, cells, and anatomy into defined neuroimmune circuits in health and disease is in its infancy. This review describes these circuits and highlights continued challenges and opportunities for the field.

Characterization of Perivascular Nerve Distribution in Rat Mesenteric Small Arteries

The distribution pattern of perivascular nerves in some branches of rat mesenteric arteries was studied. Mesenteric arteries isolated from 8-week-old Wistar rats were divided into the 1st-, 2nd-, and 3rd-order branches. The distribution of perivascular nerves in each branch was immunohistochemically evaluated using antibodies against neuropeptide Y (NPY), tyrosine hydroxylase (TH), calcitonin gene-related peptide (CGRP), substance P (SP), and neuronal nitric oxide synthase (nNOS). The density of NPY-, TH-, CGRP-, and SP-like immunoreactive (LI) nerves in the 2nd and 3rd branches was significantly greater than that in the 1st branch, and a negative relationship was found between nerve density and arterial diameter, except for TH-LI nerves. The density of NPY- and TH-LI nerves in all branches, which was similar, was greater than that of CGRP- (except for NPY-LI nerves in the 1st branch), SP-, or nNOS-LI nerves. Double immunostaining revealed that TH-LI nerves made contact with nNOS-LI, CGRP-LI, and SP-LI nerves and that CGRP-LI nerves made contact with TH-, NPY-, or nNOS-LI nerves, while TH-LI and CGRP-LI nerves nearly merged with NPY-LI and SP-LI nerves, respectively. These results suggest that the each branch of mesenteric arteries is densely innervated by vasoconstrictor nerves containing NPY, TH, and vasodilator CGRP nerves. They also suggest that the intense density of perivascular nerves in the 2nd and 3rd branches may contribute to maintaining vascular tone.

Splicing Regulation of Pro-Inflammatory Cytokines and Chemokines: At the Interface of the Neuroendocrine and Immune Systems

Alternative splicing plays a key role in posttranscriptional regulation of gene expression, allowing a single gene to encode multiple protein isoforms. As such, alternative splicing amplifies the coding capacity of the genome enormously, generates protein diversity, and alters protein function. More than 90% of human genes undergo alternative splicing, and alternative splicing is especially prevalent in the nervous and immune systems, tissues where cells need to react swiftly and adapt to changes in the environment through carefully regulated mechanisms of cell differentiation, migration, targeting, and activation. Given its prevalence and complexity, this highly regulated mode of gene expression is prone to be affected by disease. In the following review, we look at how alternative splicing of signaling molecules—cytokines and their receptors—changes in different pathological conditions, from chronic inflammation to neurologic disorders, providing means of functional interaction between the immune and neuroendocrine systems. Switches in alternative splicing patterns can be very dynamic and can produce signaling molecules with distinct or antagonistic functions and localization to different subcellular compartments. This newly discovered link expands our understanding of the biology of immune and neuroendocrine cells, and has the potential to open new windows of opportunity for treatment of neurodegenerative disorders.

Alterations in the distal colon innervation in Winnie mouse model of spontaneous chronic colitis

The gastrointestinal tract is innervated by extrinsic sympathetic, parasympathetic and sensory nerve fibers as well as by intrinsic fibers from the neurons in myenteric and submucosal ganglia embedded into the gastrointestinal wall. Morphological and functional studies of intestinal innervation in animal models are important for understanding the pathophysiology of inflammatory bowel disease (IBD). The recently established Winnie mouse model of spontaneous chronic colitis caused by a point mutation in the Muc2 mucin gene develops inflammation due to a primary epithelial defect. Winnie mice display symptoms of diarrhea, ulcerations and rectal bleeding similar to those in IBD. In this study, we investigated myenteric neurons, noradrenergic, cholinergic and sensory nerve fibers in the distal colon of Winnie (Win/Win) mice compared to C57/BL6 and heterozygote littermates (Win/Wt) using histological and immunohistochemical methods. All Win/Win mice used in this study had inflammation with signs of mucosal damage, goblet cell loss, thickening of muscle and mucosal layers, and increased CD45-immunoreactivity in the distal colon. The density of sensory, cholinergic and noradrenergic fibers innervating the myenteric plexus, muscle and mucosa significantly decreased in the distal colon of Win/Win mice compared to C57/BL6 and Win/Wt mice, while the total number of myenteric neurons as well as subpopulations of cholinergic and nitrergic neurons remained unchanged. In conclusion, changes in the colon morphology and innervation found in Winnie mice have multiple similarities with changes observed in patients with ulcerative colitis.

Participation of interleukin 17A in neuroimmune interactions

Inflammation involving the helper T cell 17 (Th17) subset of lymphocytes has been implicated in a number of diseases that affect the nervous system. As the canonical cytokine of Th17 cells, interleukin 17A (IL-17A) is thought to contribute to these neuroimmune interactions. The main receptor for IL-17A is expressed in many neural tissues. IL-17A has direct effects on neurons but can also impact neural function via signaling to satellite cells and immune cells. In the central nervous system, IL-17A has been associated with neuropathology in multiple sclerosis, epilepsy syndromes and ischemic brain injury. Effects of IL-17A at the level of dorsal root ganglia and the spinal cord may contribute to enhanced nociception during neuropathic and inflammatory pain. Finally, IL-17A plays a role in sympathetic axon growth and regeneration of damaged axons that innervate the cornea. Given the widespread effects of IL-17A on neural tissues, it will be important to determine whether selectively mitigating the damaging effects of this cytokine while augmenting its beneficial effects is a possible strategy to treat inflammatory damage to the nervous system.

Neuroplasticity and dysfunction after gastrointestinal inflammation

The gastrointestinal tract is innervated by several distinct populations of neurons, whose cell bodies either reside within (intrinsic) or outside (extrinsic) the gastrointestinal wall. Normally, most individuals are unaware of the continuous, complicated functions of these neurons. However, for patients with gastrointestinal disorders, such as IBD and IBS, altered gastrointestinal motility, discomfort and pain are common, debilitating symptoms. Although bouts of intestinal inflammation underlie the symptoms associated with IBD, increasing preclinical and clinical evidence indicates that infection and inflammation are also key risk factors for the development of other gastrointestinal disorders. Notably, a strong correlation exists between prior exposure to gut infection and symptom occurrence in IBS. This Review discusses the evidence for neuroplasticity (structural, synaptic or intrinsic changes that alter neuronal function) affecting gastrointestinal function. Such changes are evident during inflammation and, in many cases, long after healing of the damaged tissues, when the nervous system fails to reset back to normal. Neuroplasticity within distinct populations of neurons has a fundamental role in the aberrant motility, secretion and sensation associated with common clinical gastrointestinal disorders. To find appropriate therapeutic treatments for these disorders, the extent and time course of neuroplasticity must be fully appreciated.

Evidence against a systemic arterial defect in patients with inflammatory bowel disease

BACKGROUND

Despite increasing interest in local microvascular alterations associated with inflammatory bowel disease (IBD), the potential contribution of a primary systemic vascular defect in the etiology of IBD is unknown. We compared reactivity of large diameter mesenteric arteries from segments affected by Crohn disease (CD) or ulcerative colitis (UC) to an uninvolved vascular bed in both IBD and control patients.

METHODS

Mesenteric and omental arteries were obtained from UC, CD, and non-IBD patients. Isometric arterial contractions were recorded in response to extracellular potassium (K(+)) and cumulative additions of norepinephrine (NE). In addition, relaxation in response to pinacidil, an activator of adenosine triphosphate-sensitive K(+) channels was examined.

RESULTS

Contraction to K(+) and sensitivity to NE were not significantly different in arteries from CD, UC, and controls. Relaxation to pinacidil was also similar between groups.

CONCLUSIONS

Potassium-induced contractions and sensitivity to NE and pinacidil were not significantly different in large diameter mesenteric and omental arteries obtained from IBD patients. Furthermore, there was no significant difference in the sensitivity to K(+), NE, and pinacidil between mesenteric and omental arteries of CD and UC patients and those from non-IBD patients. Our results suggest an underlying vascular defect systemic to CD or UC patients is unlikely to contribute to the etiology of IBD.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

Neural regulation of gastrointestinal inflammation: role of the sympathetic nervous system

The sympathetic innervation of the gastrointestinal (GI) tract regulates motility, secretion and blood flow by inhibiting the activity of the enteric nervous system (ENS) and direct vasoconstrictor innervation of the gut microvasculature. In addition to these well-established roles, there is evidence that the sympathetic nervous system (SNS) can modulate GI inflammation. Postganglionic sympathetic neurons innervate lymphoid tissues and immune cells within the GI tract. Furthermore, innate and adaptive immune cells express receptors for sympathetic neurotransmitters. Activation of these receptors can affect a variety of important immune cell functions, including cytokine release and differentiation of helper T lymphocyte subsets. This review will consider the neuroanatomical evidence of GI immune cell innervation by sympathetic axons, the effects of blocking or enhancing SNS activity on GI inflammation, and the converse modulation of sympathetic neuroanatomy and function by GI inflammation.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Effects of inflammation on the innervation of the colon

Inflammatory bowel diseases (IBD) such as ulcerative colitis and Crohn's disease lead to altered gastrointestinal (GI) function as a consequence of the effects of inflammation on the tissues that comprise the GI tract. Among these tissues are several types of neurons that detect the state of the GI tract, transmit pain, and regulate functions such as motility, secretion, and blood flow. This review article describes the structure and function of the enteric nervous system, which is embedded within the gut wall, the sympathetic motor innervation of the colon and the extrinsic afferent innervation of the colon, and considers the evidence that colitis alters these important sensory and motor systems. These alterations may contribute to the pain and altered bowel habits that accompany IBD.

Emerging neuropeptide targets in inflammation: NPY and VIP

The enteric nervous system (ENS), referred to as the "second brain," comprises a vast number of neurons that form an elegant network throughout the gastrointestinal tract. Neuropeptides produced by the ENS play a crucial role in the regulation of inflammatory processes via cross talk with the enteric immune system. In addition, neuropeptides have paracrine effects on epithelial secretion, thus regulating epithelial barrier functions and thereby susceptibility to inflammation. Ultimately the inflammatory response damages the enteric neurons themselves, resulting in deregulations in circuitry and gut motility. In this review, we have emphasized the concept of neurogenic inflammation and the interaction between the enteric immune system and enteric nervous system, focusing on neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP). The alterations in the expression of NPY and VIP in inflammation and their significant roles in immunomodulation are discussed. We highlight the mechanism of action of these neuropeptides on immune cells, focusing on the key receptors as well as the intracellular signaling pathways that are activated to regulate the release of cytokines. In addition, we also examine the direct and indirect mechanisms of neuropeptide regulation of epithelial tight junctions and permeability, which are a crucial determinant of susceptibility to inflammation. Finally, we also discuss the potential of emerging neuropeptide-based therapies that utilize peptide agonists, antagonists, siRNA, oligonucleotides, and lentiviral vectors.

3-D imaging and illustration of mouse intestinal neurovascular complex

Because of the dispersed nature of nerves and blood vessels, standard histology cannot provide a global and associated observation of the enteric nervous system (ENS) and vascular network. We prepared transparent mouse intestine and combined vessel painting and three-dimensional (3-D) neurohistology for joint visualization of the ENS and vasculature. Cardiac perfusion of the fluorescent wheat germ agglutinin (vessel painting) was used to label the ileal blood vessels. The pan-neuronal marker PGP9.5, sympathetic neuronal marker tyrosine hydroxylase (TH), serotonin, and glial markers S100B and GFAP were used as the immunostaining targets of neural tissues. The fluorescently labeled specimens were immersed in the optical clearing solution to improve photon penetration for 3-D confocal microscopy. Notably, we simultaneously revealed the ileal microstructure, vasculature, and innervation with micrometer-level resolution. Four examples are given: 1) the morphology of the TH-labeled sympathetic nerves: sparse in epithelium, perivascular at the submucosa, and intraganglionic at myenteric plexus; 2) distinct patterns of the extrinsic perivascular and intrinsic pericryptic innervation at the submucosal-mucosal interface; 3) different associations of serotonin cells with the mucosal neurovascular elements in the villi and crypts; and 4) the periganglionic capillary network at the myenteric plexus and its contact with glial fibers. Our 3-D imaging approach provides a useful tool to simultaneously reveal the nerves and blood vessels in a space continuum for panoramic illustration and analysis of the neurovascular complex to better understand the intestinal physiology and diseases.

The roles of purinergic signaling during gastrointestinal inflammation

Extracellular purines play important roles as neurotransmitters and paracrine mediators in the gastrointestinal (GI) tract. Inflammation of the GI tract causes marked changes in the release and extracellular catabolism of purines, and can modulate purinoceptor expression and/or signaling. The functional consequences of this include suppression of the purinergic component of inhibitory neuromuscular and neurovascular transmission, increased release of purines from immune and epithelial cells, loss of enteric neurons to damage through P2X(7) purinoceptors, and enhanced activation of pain fibres. The purinergic system represents an important target for drug therapies that may improve GI inflammation and its consequences.

Interleukin-17A increases neurite outgrowth from adult postganglionic sympathetic neurons

Inflammation can profoundly alter the structure and function of the nervous system. Interleukin (IL)-17 has been implicated in the pathogenesis of several inflammatory diseases associated with nervous system plasticity. However, the effects of IL-17 on the nervous system remain unexplored. Cell and explant culture techniques, immunohistochemistry, electrophysiology, and Ca2+ imaging were used to examine the impact of IL-17 on adult mouse sympathetic neurons. Receptors for IL-17 were present on postganglionic neurons from superior mesenteric ganglia (SMG). Supernatant from activated splenic T lymphocytes, which was abundant in IL-17, dramatically enhanced axonal length of SMG neurons. Importantly, IL-17-neutralizing antiserum abrogated the neurotrophic effect of splenocyte supernatant, and incubation of SMG neurons in IL-17 (1 ng/ml) significantly potentiated neurite outgrowth. The neurotrophic effect of IL-17 was accompanied by inhibition of voltage-dependent Ca2+ influx and was recapitulated by incubation of neurons in a blocker of N-type Ca2+ channels (ω-conotoxin GVIA; 30 nM). IL-17-induced neurite outgrowth in vitro appeared to be independent of glia, as treatment with a glial toxin (AraC; 5 μM) did not affect the outgrowth response to IL-17. Moreover, application of the cytokine to distal axons devoid of glial processes enhanced neurite extension. An inhibitor of the NF-κB pathway (SC-514; 20 μM) blocked the effects of IL-17. These data represent the first evidence that IL-17 can act on sympathetic somata and distal neurites to enhance neurite outgrowth, and identify a novel potential role for IL-17 in the neuroanatomical plasticity that accompanies inflammation.

Tissue concentrations of vasoactive intestinal peptide are affected by peritonitis-induced sepsis and hemofiltration in pigs

Vasoactive intestinal peptide (VIP) is a neuropeptide released from the autonomic nerves exerting multiple antiinflammatory effects. The aim of the present study was to investigate the impact of severe sepsis and hemofiltration in two settings on plasma and tissue concentrations of VIP in a porcine model of sepsis. Thirty-two pigs were divided into 5 groups: 1) control group; 2) control group with conventional hemofiltration; 3) septic group; 4) septic group with conventional hemofiltration; 5) septic group with high-volume hemofiltration. Sepsis induced by faecal peritonitis continued for 22 hours. Hemofiltration was applied for the last 10 hours. Hemodynamic, inflammatory and oxidative stress parameters (heart rate, mean arterial pressure, cardiac output, systemic vascular resistance, plasma concentrations of tumor necrosis factor-alpha, interleukin-6, thiobarbituric acid reactive species, nitrate + nitrite, asymmetric dimethylarginine) and the systemic VIP concentrations were measured before faeces inoculation and at 12 and 22 hours of peritonitis. VIP tissue levels were determined in the left ventricle, mesenteric and coronary arteries. Sepsis induced significant increases in VIP concentrations in the plasma and mesenteric artery, but it decreased peptide levels in the coronary artery. Hemofiltration in both settings reduced concentrations of VIP in the mesenteric artery. In severe sepsis, VIP seems to be rapidly depleted from the coronary artery and, on the other hand, upregulated in the mesenteric artery. Hemofiltration in both settings has a tendency to drain away these upregulated tissue stores which could result in the limited secretory capacity of the peptide.

Presynaptic inhibition of neural vasodilator pathways to submucosal arterioles by release of purines from sympathetic nerves

Capsaicin-sensitive extrinsic sensory nerves and submucosal vasodilator neurons provide important vasodilator input to submucosal arterioles, but relatively little is known about the signaling between these populations and the sympathetic vasoconstrictor innervation. This study examined whether release of sympathetic purines can modulate dilator nerves. In vitro submucosal preparations from guinea pig ileum were modified to leave the parent mesenteric artery intact so that perivascular sympathetic and extrinsic afferent nerves could be activated by a bipolar stimulating electrode placed on the parent artery, and submucosal vasodilator neurons were activated using focal electrodes placed on submucosal ganglia. The outside diameter of submucosal arterioles was monitored using videomicroscopy, and dilator responses were examined after preconstricting vessels 80-95% with prostaglandin F(2alpha) (400 nM). Mesenteric nerve stimulation evoked a frequency-dependent dilation, with suramin (100 microM) present throughout to inhibit P(2X) receptor-mediated vasoconstrictions. In the presence of guanethidine (10 microM) to inhibit sympathetic purine release, superfusion of ATP (200 nM-6 microM) caused a concentration-dependent inhibition of nerve-evoked dilations. Vasodilations to substance P (10 nM) were not inhibited by ATP in the presence of guanethidine, implicating a presynaptic effect of ATP on neurotransmitter release. The inhibitory effect of ATP was blocked by the adenosine receptor antagonist 8-phenyltheophylline (8-PT; 10 microM). In addition, 8-PT increased the amplitude of nerve-evoked dilations, suggesting a tonic inhibitory effect of adenosine receptors on vasodilator release. Dilations evoked by electrical stimulation of submucosal ganglia were also inhibited almost 50% by ATP (2 microM) and its nonhydrolyzable analog, alpha,beta-methylene-ATP (10 microM). These data suggest that sympathetic varicosities release ATP or a related purine that can act at presynaptic adenosine receptors on extrinsic sensory and submucosal vasodilator neurons to inhibit neurotransmitter release.

The participation of the sympathetic innervation of the gastrointestinal tract in disease states

Knowledge of neural circuits, neurotransmitters and receptors involved in the sympathetic regulation of gastrointestinal (GI) function is well established. However, it is only recently that the interaction of sympathetic neurons, and of sympathetic transmitters, with the GI immune system and with gut flora has begun to be explored. Changes in the behaviour of sympathetic nerves when gut function is compromised, for example in ileus and in inflammation, have been observed, but the roles of the sympathetic innervation in these and other pathologies are not adequately understood. In this article, we first review the principal roles of the sympathetic innervation of the GI tract in controlling motility, fluid exchange and gut blood flow in healthy individuals. We then discuss the evidence that there are important interactions of sympathetic transmitters with the gut immune system and enteric glia, and evidence that inflammation has substantial effects on sympathetic neurons. These reciprocal interactions contribute to pathological changes in ways that are not yet clarified. Finally, we focus on inflammation, diabetes and postoperative ileus as conditions in which there is sympathetic involvement in compromised gut function.

Autonomic denervation and the origins of chronic Western diseases

Many chronic Western diseases result from lifestyles that include refined diets, poor bowel habits, limited physical exercise and suboptimal patterns of childbirth. Western diets result in reduced stool weights, increased bowel transit times and persistent physical efforts during defaecation. Prolonged physical efforts during defaecation and childbirth cause latent, or direct, injuries to branches of the cardiac (thorax), coeliac (abdomen) and hypogastric (pelvis) plexi. Injuries to autonomic nerves result in impaired visceral function including visceral dysmotility, tissue hypoplasia and hyperplasia, increased susceptibility to infection, and, aberrant reinnervation with sensitisation of the central nervous system (CNS). These unrecognised injuries are vulnerable to the long list of causes of autonomic Dysfunction, e.g. stress, alcohol, drugs, infection, trauma, cancer, etc. Specific injuries at different anatomical locations in midline autonomic pathways give rise to a wide range of Western diseases from infancy to old age, through diverse and cumulative mechanisms.

Clinical and experimental evidence of sympathetic neural dysfunction during inflammatory bowel disease

1. Inflammatory bowel diseases (IBD) alter the function of the enteric nervous system and the sensory innervation of the gastrointestinal (GI) tract. Less is known about whether IBD also affects the sympathetic nervous system (SNS). Given the importance of the SNS in regulating GI function and possibly immune system activation, the present review examines the evidence of sympathetic dysfunction during IBD and its possible consequences. 2. Sympathetic axons within the GI tract innervate several cell types, including vascular myocytes, enteric neurons and immune cells. The major neurotransmitters released from sympathetic varicosities are noradrenaline, neuropeptide Y and ATP or a related purine. 3. Clinical studies of IBD patients have provided evidence of an association between IBD and axonal or demyelinating neuropathy. Assays of autonomic function suggest that ulcerative colitis and Crohn's disease, the two major forms of IBD, have contrasting effects on sympathetic neural activity. 4. Animal models of IBD have been used to examine the effects of these diseases on sympathetic neurophysiology. A decrease in the release of noradrenaline from sympathetic varicosities in inflamed and uninflamed regions of the GI tract has consistently been reported. Recent findings suggest that the decrease in neurotransmitter release may be due to inhibition of N-type voltage-gated Ca(2+) current in post-ganglionic sympathetic neurons. 5. Interest in the role of the SNS in IBD is rapidly increasing. However, much work needs to be done to enhance understanding of how SNS function is altered during IBD and what contribution, if any, these changes make to pathogenesis.

Inhibition of sympathetic N-type voltage-gated Ca2+ current underlies the reduction in norepinephrine release during colitis

Inflammatory bowel diseases (IBD) are associated with altered neuronal regulation of the gastrointestinal (GI) tract and impairment of norepinephrine release from sympathetic varicosities. The sympathetic innervation of the GI tract modulates motility, blood flow, and secretion, and therefore defective norepinephrine release may contribute to symptom generation in IBD. Accordingly, our aim here was to utilize the mouse model of dextran sulfate sodium (DSS; 5% wt/vol) of IBD to determine how norepinephrine release is reduced during GI inflammation. We hypothesized that the inflammation-induced reduction in norepinephrine release was due to inhibition of voltage-gated Ca(2+) current (I(Ca)) in prevertebral sympathetic neurons. We compared [(3)H]norepinephrine release in the colon and jejunum and I(Ca) amplitude in superior mesenteric ganglion (SMG) neurons from control mice and mice with DSS-induced colitis. Changes to voltage-gated Ca(2+) channels were investigated by fura 2-AM Ca(2+) imaging, perforated patch-clamp electrophysiology, and real-time PCR. Depolarization-induced norepinephrine release from the inflamed colon and uninflamed jejunum was significantly impaired in mice treated with DSS, as was depolarization-induced Ca(2+) influx in SMG neurons. Colitis reduced I(Ca) in SMG neurons by inhibiting omega-conotoxin GVIA (300 nM)-sensitive N-type Ca(2+) channels. The omega-conotoxin GVIA-sensitive component of norepinephrine release was significantly smaller in the colon during colitis. The inhibition of I(Ca) was accompanied by a decrease in mRNA encoding the Ca(2+) channel alpha subunit (CaV 2.2) and a rightward shift in the voltage dependence of activation of I(Ca). These findings suggest that DSS-induced colitis attenuates norepinephrine release in the colon and jejunum due to inhibition of N-type voltage-gated Ca(2+) channels. This may contribute to functional alterations in both inflamed and uninflamed regions of the GI tract during inflammation.

Loss of purinergic vascular regulation in the colon during colitis is associated with upregulation of CD39

Evidence from patients with inflammatory bowel disease (IBD) and animal models suggests that inflammation alters blood flow to the mucosa, which precipitates mucosal barrier dysfunction. Impaired purinergic sympathetic regulation of submucosal arterioles, the resistance vessels of the splanchnic vasculature, is one of the defects identified during IBD and in mouse models of IBD. We hypothesized that this may be a consequence of upregulated catabolism of ATP during colitis. In vivo and in vitro video microscopy techniques were employed to measure the effects of purinergic agonists and inhibitors of CD39, an enzyme responsible for extracellular ATP catabolism, on the diameter of colonic submucosal arterioles from control mice and mice with dextran sodium sulfate [DSS, 5% (wt/vol)] colitis. Using a luciferase-based ATP assay, we examined the degradation of ATP and utilized real-time PCR, Western blotting, and immunohistochemistry to examine the expression and localization of CD39 during colitis. Arterioles from mice with DSS colitis did not constrict in response to ATP (10 microM) but did constrict in the presence of its nonhydrolyzable analog alpha,beta-methylene ATP (1 microM). alpha,beta-Methylene ADP (100 microM), an inhibitor of CD39, restored ATP-induced vasoconstriction in arterioles from mice with DSS-induced colitis. CD39 protein and mRNA expression was markedly increased during colitis. Immunohistochemical analysis demonstrated that, in addition to vascular CD39, F4/80-immunoreactive macrophages accounted for a large proportion of submucosal CD39 staining during colitis. These data implicate upregulation of CD39 in impaired sympathetic regulation of gastrointestinal blood flow during colitis.

Immunohistochemical characterization of the innervation of human colonic mesenteric and submucosal blood vessels

The aim was to characterize quantitatively the classes of nerves innervating human mesenteric and submucosal vessels. Specimens of uninvolved normal human mesentery and colon were obtained with prior informed consent from patients undergoing elective surgery for bowel carcinoma. Mesenteric and submucosal vessels were processed for double-labelling immunohistochemical localization of tyrosine hydroxylase (TH), neuropeptide Y (NPY), calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal polypeptide (VIP), nitric oxide synthase (NOS), somatostatin (SOM), vesicular acetylcholine transporter (VAChT) and enkephelin (ENK), each compared to the pan-neuronal marker protein gene product 9.5. Branching patterns of individual nerve fibres were investigated using in vitro anterograde tracing. Sympathetic neurons containing TH and NPY were the largest population, accounting for more than 85% on all vessels. Extrinsic sensory axons, containing SP but not CGRP comprised a second major population on mesenteric vessels: these axons generally lacked TH, NPY and VAChT. On submucosal, but not mesenteric vessels, an additional population of SOM-immunoreactive fibres was present: these axons did not co-localize with TH. Major similarities and differences with enteric vessel innervation in laboratory animals were identified. Sympathetic neurons comprise the largest input. Extrinsic sensory neurons in humans largely lack CGRP but contain SP. Submucosal vessels receive an additional source of innervation not present in mesenteric vessels, which contain SOM, but are rarely cholinergic. These results have significant implications for understanding the control of blood flow to the human gut.