Projections of peptide-containing neurons in rat colon

Multiple Regulatory Signals and Components in the Modulation of Bicarbonate Transporters

Bicarbonate transporters are responsible for the appropriate flux of bicarbonate across the plasma membrane to perform various fundamental cellular functions. The functions of bicarbonate transporters, including pH regulation, cell migration, and inflammation, are highlighted in various cellular systems, encompassing their participation in both physiological and pathological processes. In this review, we focused on recently identified modulatory signaling components that regulate the expression and activity of bicarbonate transporters. Moreover, we addressed recent advances in our understanding of cooperative systems of bicarbonate transporters and channelopathies. This current review aims to provide a new, in-depth understanding of numerous human diseases associated with the dysfunction of bicarbonate transporters.

Somatostatin as an Active Substance in the Mammalian Enteric Nervous System

Somatostatin (SOM) is an active substance which most commonly occurs in endocrine cells, as well as in the central and peripheral nervous system. One of the parts of the nervous system where the presence of SOM has been confirmed is the enteric nervous system (ENS), located in the wall of the gastrointestinal (GI) tract. It regulates most of the functions of the stomach and intestine and it is characterized by complex organization and a high degree of independence from the central nervous system. SOM has been described in the ENS of numerous mammal species and its main functions in the GI tract are connected with the inhibition of the intestinal motility and secretory activity. Moreover, SOM participates in sensory and pain stimuli conduction, modulation of the release of other neuronal factors, and regulation of blood flow in the intestinal vessels. This peptide is also involved in the pathological processes in the GI tract and is known as an anti-inflammatory agent. This paper, which focuses primarily on the distribution of SOM in the ENS and extrinsic intestinal innervation in various mammalian species, is a review of studies concerning this issue published from 1973 to the present.

Colonic levels of vasoactive intestinal peptide decrease during infection and exogenous VIP protects epithelial mitochondria against the negative effects of IFNγ and TNFα induced during Citrobacter rodentium infection

Citrobacter rodentium infection is a model for infection with attaching and effacing pathogens, such as enteropathogenic Escherichia coli. The vasoactive intestinal peptide (VIP) has emerged as an anti-inflammatory agent, documented to inhibit Th1 immune responses and successfully treat animal models of inflammation. VIP is also a mucus secretagogue. Here, we found that colonic levels of VIP decrease during murine C. rodentium infection with a similar time dependency as measurements reflecting mitochondrial function and epithelial integrity. The decrease in VIP appears mainly driven by changes in the cytokine environment, as no changes in VIP levels were detected in infected mice lacking interferon gamma (IFNγ). VIP supplementation alleviated the reduction of activity and levels of mitochondrial respiratory complexes I and IV, mitochondrial phosphorylation capacity, transmembrane potential and ATP generation caused by IFNγ, TNFα and C. rodentium infection, in an in vitro mucosal surface. Similarly, VIP treatment regimens that included the day 5–10 post infection period alleviated decreases in enzyme complexes I and IV, phosphorylation capacity, mitochondrial transmembrane potential and ATP generation as well as increased apoptosis levels during murine infection with C. rodentium. However, VIP treatment failed to alleviate colitis, although there was a tendency to decreased pathogen density in contact with the epithelium and in the spleen. Both in vivo and in vitro, NO generation increased during C. rodentium infection, which was alleviated by VIP. Thus, therapeutic VIP administration to restore the decreased levels during infection had beneficial effects on epithelial cells and their mitochondria, but not on the overall infection outcome.

Sensory neuron regulation of gastrointestinal inflammation and bacterial host defence

Sensory neurons in the gastrointestinal tract have multifaceted roles in maintaining homeostasis, detecting danger and initiating protective responses. The gastrointestinal tract is innervated by three types of sensory neurons: dorsal root ganglia, nodose/jugular ganglia and intrinsic primary afferent neurons. Here, we examine how these distinct sensory neurons and their signal transducers participate in regulating gastrointestinal inflammation and host defence. Sensory neurons are equipped with molecular sensors that enable neuronal detection of diverse environmental signals including thermal and mechanical stimuli, inflammatory mediators and tissue damage. Emerging evidence shows that sensory neurons participate in host-microbe interactions. Sensory neurons are able to detect pathogenic and commensal bacteria through specific metabolites, cell-wall components, and toxins. Here, we review recent work on the mechanisms of bacterial detection by distinct subtypes of gut-innervating sensory neurons. Upon activation, sensory neurons communicate to the immune system to modulate tissue inflammation through antidromic signalling and efferent neural circuits. We discuss how this neuro-immune regulation is orchestrated through transient receptor potential ion channels and sensory neuropeptides including substance P, calcitonin gene-related peptide, vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide. Recent studies also highlight a role for sensory neurons in regulating host defence against enteric bacterial pathogens including Salmonella typhimurium, Citrobacter rodentium and enterotoxigenic Escherichia coli. Understanding how sensory neurons respond to gastrointestinal flora and communicate with immune cells to regulate host defence enhances our knowledge of host physiology and may form the basis for new approaches to treat gastrointestinal diseases.

Structural and functional consequences of buserelin-induced enteric neuropathy in rat

BACKGROUND

Women treated with gonadotropin-releasing hormone (GnRH) analogs may develop enteric neuropathy and dysmotility. Administration of a GnRH analog to rats leads to similar degenerative neuropathy and ganglioneuritis. The aim of this study on rat was to evaluate the early GnRH-induced enteric neuropathy in terms of distribution of neuronal subpopulations and gastrointestinal (GI) function.

METHODS

Forty rats were given the GnRH analog buserelin (20 μg, 1 mg/ml) or saline subcutaneously, once daily for 5 days, followed by 3 weeks of recovery, representing one treatment session. Two weeks after the fourth treatment session, the animals were tested for GI transit time and galactose absorption, and fecal weight and fat content was analyzed. After sacrifice, enteric neuronal subpopulations were analyzed. Blood samples were analyzed for zonulin and antibodies against GnRH and luteinizing hormone, and their receptors.

RESULTS

Buserelin treatment transiently increased the body weight after 5 and 9 weeks (p < 0.001). Increased estradiol in plasma and thickened uterine muscle layers indicate high estrogen activity. The numbers of both submucous and myenteric neurons were reduced by 27%-61% in ileum and colon. The relative numbers of neurons containing calcitonin gene-related peptide (CGRP), cocaine- and amphetamine-related transcript (CART), galanin, gastrin-releasing peptide (GRP), neuropeptide Y (NPY), nitric oxide synthase (NOS), serotonin, substance P (SP), vasoactive intestinal peptide (VIP) or vesicular acetylcholine transporter (VAchT), and their nerve fiber density, were unchanged after buserelin treatment, but the relative number of submucous neurons containing somatostatin tended to be increased (p = 0.062). The feces weight decreased in buserelin-treated rats (p < 0.01), whereas feces fat content increased (p < 0.05), compared to control rats. Total GI transit time, galactose absorption, zonulin levels in plasma, and antibody titers in serum were unaffected by buserelin treatment.

CONCLUSIONS

A marked enteric neuronal loss with modest effects on GI function is found after buserelin treatment. Increased feces fat content is suggested an early sign of dysfunction.

Lipopolysaccharide-induced loss of cultured rat myenteric neurons - role of AMP-activated protein kinase

OBJECTIVE

Intestinal barrier function is vital for homeostasis. Conditions where the mucosal barrier is compromised lead to increased plasma content of lipopolysaccharide (LPS). LPS acts on Toll-like receptor 4 (TLR4) and initiates cellular inflammatory responses. TLR4 receptors have been identified on enteric neurons and LPS exposure causes neuronal loss, counteracted by vasoactive intestinal peptide (VIP), by unknown mechanisms. In addition AMP activated protein kinase (AMPK) stimulation causes loss of enteric neurons. This study investigated a possible role of AMPK activation in LPS-induced neuronal loss.

DESIGN

Primary cultures of myenteric neurons isolated from rat small intestine were used. Cultures were treated with LPS (0.2-20 µg/mL) with and without TAK1-inhibitor (5Z)-7-Oxozeaenol (10-6 M) or AMPK inhibitor compound C (10-5 M). AMPK-induced neuronal loss was verified treating cultures with three different AMPK activators, AICAR (10-4-3×10-3 M), metformin (0.2-20 µg/mL) and A-769662 (10-5-3×10-4 M) with or without the presence of compound C (10-5 M). Upstream activation of AMPK-induced neuronal loss was tested by treating cultures with AICAR (10-3 M) in the presence of TAK1 inhibitor (5Z)-7-Oxozeaenol (10-6 M). Neuronal survival and relative numbers of neurons immunoreactive (IR) for VIP were evaluated using immunocytochemistry.

RESULTS

LPS caused a concentration dependent loss of neurons. All AMPK activators induced loss of myenteric neurons in a concentration dependent manner. LPS-, AICAR- and metformin-,but not A-769662-, induced neuronal losses were inhibited by presence of compound C. LPS, AICAR or metformin exposure increased the relative number of VIP-IR neurons; co-treatment with (5Z)-7-Oxozeaenol or compound C reversed the relative increase in VIP-IR neurons induced by LPS. (5Z)-7-Oxozeaenol, compound C or A-769662 did not per se change neuronal survival or relative numbers of VIP-IR neurons.

CONCLUSION

AMPK activation mimics LPS-induced loss of cultured myenteric neurons and LPS-induced neuronal loss is counteracted by TAK1 and AMPK inhibition. This suggests enteric neuroimmune interactions involving AMPK regulation.

Localisation and activation of the neurokinin 1 receptor in the enteric nervous system of the mouse distal colon

The substance P neurokinin 1 receptor (NK1R) regulates motility, secretion, inflammation and pain in the intestine. The distribution of the NK1R is a key determinant of the functional effects of substance P in the gut. Information regarding the distribution of NK1R in subtypes of mouse enteric neurons is lacking and is the focus of the present study. NK1R immunoreactivity (NK1R-IR) is examined in whole-mount preparations of the mouse distal colon by indirect immunofluorescence and confocal microscopy. The distribution of NK1R-IR within key functional neuronal subclasses was determined by using established neurochemical markers. NK1R-IR was expressed by a subpopulation of myenteric and submucosal neurons; it was mainly detected in large multipolar myenteric neurons and was colocalized with calcitonin gene-related peptide, neurofilament M, choline acetyltransferase and calretinin. The remaining NK1R-immunoreactive neurons were positive for nitric oxide synthase. NK1R was expressed by most of the submucosal neurons and was exclusively co-expressed with vasoactive intestinal peptide, with no overlap with choline acetyltransferase. Treatment with substance P resulted in the concentration-dependent internalisation of NK1R from the cell surface into endosome-like structures. Myenteric NK1R was mainly expressed by intrinsic primary afferent neurons, with minor expression by descending interneurons and inhibitory motor neurons. Submucosal NK1R was restricted to non-cholinergic secretomotor neurons. These findings highlight key differences in the neuronal distribution of NK1R-IR between the mouse, rat and guinea-pig, with important implications for the functional role of NK1R in regulating intestinal motility and secretion.

The enteric nervous system of P2Y13 receptor null mice is resistant against high-fat-diet- and palmitic-acid-induced neuronal loss

Gastrointestinal symptoms have a major impact on the quality of life and are becoming more prevalent in the western population. The enteric nervous system (ENS) is pivotal in regulating gastrointestinal functions. Purinergic neurotransmission conveys a range of short and long-term cellular effects. This study investigated the role of the ADP-sensitive P2Y13 receptor in lipid-induced enteric neuropathy. Littermate P2Y13 (+/+) and P2Y13 (-/-) mice were fed with either a normal diet (ND) or high-fat diet (HFD) for 6 months. The intestines were analysed for morphological changes as well as neuronal numbers and relative numbers of vasoactive intestinal peptide (VIP)- and neuronal nitric oxide synthase (nNOS)-containing neurons. Primary cultures of myenteric neurons from the small intestine of P2Y13 (+/+) or P2Y13 (-/-) mice were exposed to palmitic acid (PA), the P2Y13 receptor agonist 2meSADP and the antagonist MRS2211. Neuronal survival and relative number of VIP-containing neurons were analysed. In P2Y13 (+/+), but not in P2Y13 (-/-) mice, HFD caused a significant loss of myenteric neurons in both ileum and colon. In colon, the relative numbers of VIP-containing submucous neurons were significantly lower in the P2Y13 (-/-) mice compared with P2Y13 (+/+) mice. The relative numbers of nNOS-containing submucous colonic neurons increased in P2Y13 (+/+) HFD mice. HFD also caused ileal mucosal thinning in P2Y13 (+/+) and P2Y13 (-/-) mice, compared to ND fed mice. In vitro PA exposure caused loss of myenteric neurons from P2Y13 (+/+) mice while neurons from P2Y13 (-/-) mice were unaffected. Presence of MRS2211 prevented PA-induced neuronal loss in cultures from P2Y13 (+/+) mice. 2meSADP caused no change in survival of cultured neurons. P2Y13 receptor activation is of crucial importance in mediating the HFD- and PA-induced myenteric neuronal loss in mice. In addition, the results indicate a constitutive activation of enteric neuronal apoptosis by way of P2Y13 receptor stimulation.

Gonadotropin-releasing hormone analog buserelin causes neuronal loss in rat gastrointestinal tract

Gonadotropin-releasing hormone (GnRH) analogs are given to women undergoing in vitro fertilization. Case reports describing the development of chronic intestinal pseudo-obstruction and auto-antibodies against GnRH after such treatment suggest a strong association between intestinal dysfunction and GnRH analogs. No experimental model for studying such a relationship is currently at hand. Our main goal was to investigate possible enteric neurodegeneration and titers of GnRH antibodies in response to repeated administration of the GnRH analog buserelin in rat. Rats were treated for 1-4 sessions with daily subcutaneous injections of buserelin or saline for 5 days, followed by 3 weeks of recovery. Buserelin treatment caused significant loss of submucous and myenteric neurons in the fundus, ileum, and colon. The loss of enteric neurons can, at least partly, be explained by increased apoptosis. No GnRH- or GnRH-receptor-immunoreactive (IR) enteric neurons but numerous luteinizing hormone (LH)-receptor-IR neurons were detected. After buserelin treatment, the relative number of enteric LH-receptor-IR neurons decreased, whereas that of nitric-oxide-synthase-IR neurons increased. No intestinal inflammation or increased levels of circulating interleukins/cytokines were noted in response to buserelin treatment. Serum GnRH antibody titers were undetectable or extremely low in all rats. Thus, repeated administrations of buserelin induce neurodegeneration in rat gastrointestinal tract, possibly by way of LH-receptor hyperactivation. The present findings suggest that enteric neurodegenerative effects of GnRH analog treatment in man can be mimicked in rat. However, in contrast to man, no production of GnRH auto-antibodies has been noted in rat.

Neurochemical phenotypes of myenteric neurons in the rhesus monkey

Understanding the neurochemical composition of the enteric nervous system (ENS) is critical for elucidating neurological function in the gastrointestinal (GI) tract in health and disease. Despite their status as the closest models of human neurological systems, relatively little is known about enteric neurochemistry in nonhuman primates. We describe neurochemical coding of the enteric nervous system, specifically the myenteric plexus, of the rhesus monkey (Macaca mulatta) by immunohistochemistry and directly compare it to human tissues. There are considerable differences in the myenteric plexus along different segments of the monkey GI tract. While acetylcholine neurons make up the majority of myenteric neurons in the stomach (70%), they are a minority in the rectum (47%). Conversely, only 22% of gastric myenteric neurons express nitric oxide synthase (NOS) compared to 52% in the rectum. Vasoactive intestinal peptide (VIP) is more prominent in the stomach (37%) versus the rest of the GI tract (≈10%), and catecholamine neurons are rare (≈1%). There is significant coexpression of NOS and VIP in myenteric neurons that is more prominent in the proximal GI tract. Taken as a whole, these data provide insight into the neurochemical anatomy underlying GI motility. While overall similarity to other mammalian species is clear, there are some notable differences between the ENS of rhesus monkeys, humans, and other species that will be important to take into account when evaluating models of human diseases in animals.

Somatostatin, substance P and calcitonin gene-related peptide-positive intramural nerve structures of the human large intestine affected by carcinoma

The aim of this study was to investigate the arrangement and chemical coding of enteric nerve structures in the human large intestine affected by cancer. Tissue samples comprising all layers of the intestinal wall were collected during surgery form both morphologically unchanged and pathologically altered segments of the intestine (n=15), and fixed by immersion in buffered paraformaldehyde solution. The cryostat sections were processed for double-labelling immunofluorescence to study the distribution of the intramural nerve structures (visualized with antibodies against protein gene-product 9.5) and their chemical coding using antibodies against somatostatin (SOM), substance P (SP) and calcitonin gene-related peptide (CGRP). The microscopic observations revealed distinct morphological differences in the enteric nerve system structure between the region adjacent to the cancer invaded area and the intact part of the intestine. In general, infiltration of the cancer tissue resulted in the gradual (depending on the grade of invasion) first decomposition and reduction to final partial or complete destruction and absence of the neuronal elements. A comparative analysis of immunohistochemically labeled sections (from the unchanged and pathologically altered areas) revealed a statistically significant decrease in the number of CGRP-positive neurons and nerve fibres in both submucous and myenteric plexuses in the transitional zone between morphologically unchanged and cancer-invaded areas. In this zone, a decrease was also observed in the density of SP-positive nerve fibres in all intramural plexuses. Conversely, the investigations demonstrated statistically insignificant differences in number of SP- and SOM-positive neurons and a similar density of SOM-positive nerve fibres in the plexuses of the intact and pathologically changed areas. The differentiation between the potential adaptive changes in ENS or destruction of its elements by cancer invasion should be a subject of further investigations.

Stimulation of apical Cl⁻/HCO₃⁻(OH⁻) exchanger, SLC26A3 by neuropeptide Y is lipid raft dependent

Neuropeptide Y (NPY), an important proabsorptive hormone of the gastrointestinal tract has been shown to inhibit chloride secretion and stimulate NaCl absorption. However, mechanisms underlying the proabsorptive effects of NPY are not fully understood. The present studies were designed to examine the direct effects of NPY on apical Cl⁻/HCO₃⁻(OH⁻) exchange activity and the underlying mechanisms involved utilizing Caco2 cells. Our results showed that NPY (100 nM, 30 min) significantly increased Cl⁻/HCO₃⁻(OH⁻) exchange activity (∼2-fold). Selective NPY/Y1 or Y2 receptor agonists mimicked the effects of NPY. NPY-mediated stimulation of Cl⁻/HCO₃⁻(OH⁻) exchange activity involved the ERK1/2 MAP kinase-dependent pathway. Cell surface biotinylation studies showed that NPY does not alter DRA (apical Cl⁻/HCO₃⁻(OH⁻) exchanger) surface expression, ruling out the involvement of membrane trafficking events. Interestingly, DRA was found to be predominantly expressed in the detergent-insoluble (DI) and low-density fractions (LDF) of human colonic apical membrane vesicles (AMVs) representing lipid rafts. Depletion of membrane cholesterol by methyl-β-cyclodextrin (MβCD, 10 mM, 1 h) remarkably decreased DRA expression in the DI fractions. Similar results were obtained in Triton-X 100-treated Caco2 plasma membranes. DRA association with lipid rafts in the DI and LDF fractions of Caco2 cells was significantly enhanced (∼45%) by NPY compared with control. MβCD significantly decreased Cl⁻/HCO₃⁻(OH⁻) exchange activity in Caco2 cells as measured by DIDS- or niflumic acid-sensitive ³⁶Cl⁻ uptake (∼50%). Our results demonstrate that NPY modulates Cl⁻/HCO₃⁻(OH⁻) exchange activity by enhancing the association of DRA with lipid rafts, thereby resulting in an increase in Cl⁻/HCO₃⁻(OH⁻) exchange activity. Our findings suggest that the alteration in the association of DRA with lipid rafts may contribute to the proabsorptive effects of NPY in the human intestine.

Immunohistochemical characteristics of submucosal Dogiel type II neurons in rat colon

Secretory and motility reflexes are evoked by physiological stimuli in the isolated rat distal colon, which is therefore expected to contain intrinsic primary afferent (sensory) neurons. Dogiel type II neurons (putative intrinsic primary afferent neurons) exhibit several long processes emerging from large oval or round cell bodies. This study has examined the immunohistochemical characteristics of type II neurons in the submucosal plexus of rat distal colons by using whole-mount preparations. Neuronal cell bodies positive for both substance P (SP) and calretinin have been observed in colchicine-treated rats. Neurofilament 200 immunostaining has confirmed the type II morphology of SP-positive neurons. Moreover, all submucosal type II neurons identified by neurofilament 200 immunoreactivity are positive for calretinin. Calcitonin gene-related peptide (CGRP)-positive neurons in the submucosal plexus are distinct from type II neurons because they are negative for calretinin and have smaller cell bodies than the SP-positive submucosal type II neurons. Most (73%) of the submucosal neurons including type II neurons exhibit immunoreactivity for the neurokinin-1 receptor (NK1R), a receptor for SP, on the surface of cell bodies. Immunoreactivity for the EP3 receptor (EP3R), a receptor for prostaglandin E2, has been detected in 51% of submucosal neurons including type II neurons. Thus, submucosal type II neurons in the rat distal colon are immunopositive for SP/calretinin but immunonegative for CGRP. SP released from submucosal type II neurons probably acts via NK1Rs on type II and non-type II submucosal neurons to mediate intrinsic reflexes. EP3R-positive submucosal type II neurons may be potential targets of prostaglandin E2.

Characterisation of calcitonin gene-related peptide-immunoreactive neurons in the myenteric plexus of rat colon

A mechanical or chemical stimulus applied to the intestinal mucosa induces motility reflexes in the rat colon. Enteric neurons containing calcitonin gene-related peptide (CGRP) have been suggested as intrinsic primary afferent neurons responsible for mediating such reflexes. In the present study, immunohistochemistry was performed on whole-mount stretch preparations to investigate chemical profiles, morphological characteristics and projections of CGRP-containing neurons in the myenteric plexus of the rat colon. CGRP-positive neuronal cell bodies were detected in preparations incubated with colchicine-containing medium, whereas CGRP-positive nerve fibres were found in colchicine-untreated preparations. These neurons had large oval or round cell bodies that were also immunoreactive for the calcium-binding protein calretinin and neurofilament 200. Myenteric neurons positive for both calretinin and neurofilament 200 had several long processes that emerged from the cell body, consistent with Dogiel type II morphology. Application of the neural tracer DiI to the intestinal mucosa revealed that DiI-labelled myenteric neurons each had an oval or round cell body immunoreactive for calretinin. Thus, CGRP-containing myenteric neurons are Dogiel type II neurons and are immunoreactive for calretinin and neurofilament 200 in the rat colon. These neurons probably project to the intestinal mucosa.

Mast cells reduce survival of myenteric neurons in culture

Mast cell-nerve interactions play a key role in intestinal inflammation and irritable bowel disease. Loss of enteric neurons has been reported in inflammatory conditions but the contribution of mast cells in this event is unknown. To study neuronal survival and plasticity of myenteric neurons in contact with mast cells a co-culture system using myenteric neurons from rat small intestine and peritoneal mast cells was set up. Dissociated myenteric neurons were cultured for 4 days before addition of mast cells isolated by peritoneal lavage. Neuronal survival and expression of vasoactive intestinal peptide (VIP) and nitric oxide synthase (NOS) were studied by immunocytochemistry and neuronal cell counting. Myenteric neurons cultured without mast cells were used to study the rate of neuronal survival after the addition of various mast cell mediators, proteinase-activated receptor(2) (PAR(2)) agonist, VIP or corticosteroid. A striking mast cell-induced neuronal cell death was found after co-culturing. It was counteracted by the addition of mast cell stabiliser doxantrazole, protease inhibitors, PAR(2) antagonist FSLLRY-amide, corticosteroid or VIP. In myenteric neurons cultured without mast cells the PAR(2) agonist SLIGRL-amide, prostaglandin D(2) and interleukin (IL) 6 reduced neuronal survival while histamine, serotonin, heparin, IL1beta and tumour necrosis factor alpha had no effect; corticosteroid and VIP enhanced neuronal survival. The relative numbers of VIP-, but not NOS-expressing myenteric neurons increased after co-culturing. Mast cell-induced neuronal cell death is suggested to be mediated via PAR(2) activation, IL6 and prostaglandin D(2). Corticosteroid and VIP are neuroprotective and able to prevent cell death of myenteric neurons in co-culture.

Endogenous PYY and NPY mediate tonic Y1- and Y2-mediated absorption in human and mouse colon

Objective

To establish the functional significance of endogenous peptide YY (PYY) and neuropeptide Y (NPY) as mediators of Y₁ and Y₂ absorptive tone in colonic mucosa.

Methods

Functional studies utilized descending colon from adult mice (wild type [WT] and peptide nulls) and ex vivo human colonic tissue (from patients undergoing bowel resections) measuring changes in basal ion transport. Peak increases in ion transport to Y₁ or Y₂ antagonists (BIBO3304 300 nM; BIIE0246 1 μM) were pooled (mean ± SEM) and compared using Student's unpaired t test (P ≤ 0.05); some tissues received tetrodotoxin (TTX; 100 nM). PYY-positive L-cell numbers and NPY innervation were also compared.

Results

Y₁ and Y₂ tones were present in human and WT mouse colon mucosa and only the latter was TTX sensitive. Y₁ tone was unchanged in NPY^−/− but was ∼90% inhibited in PYY^−/− and abolished in PYYNPY^−/− colon mucosa. Y₂ tone was reduced ∼50% in NPY^−/− and PYY^−/− tissues and was absent from PYYNPY^−/− colon. Residual Y₂ and Y₁ tones present in PYY^−/− mucosa were abolished by TTX. PYY ablation had no apparent effect on NPY innervation and PYY-positive cells were observed at the same frequency in NPY^−/− (56.7 ± 6.8 cells/section) and WT (55.0 ± 4.6 cells/section) colons. Double knockouts lacked PYY and NPY expression, but endocrine cells and enteric nerves were present with similar frequencies to those of WT mice.

Conclusion

Endogenous PYY mediates Y₁ absorptive tone that is epithelial in origin, whereas Y₂ tone is a combination of PYY and NPY mediation.

Neuromuscular changes in a rat model of colitis

Intracolonic administration of Trichinella spiralis larvae in rats causes colitis with features similar to ulcerative colitis, notably with inflammation predominantly limited to the colonic mucosa. Our aim was to characterize the functional and neurochemical changes occurring within the myenteric (MP) and submucosal plexuses (SMP) during T. spiralis-induced colitis. Infected rats had decreased body weight, altered stool consistency and elevated myeloperoxidase activity, 6 and 14 days post-infection (PI). Responses to acetylcholine and KCl in circular muscle strips were reduced in infected tissues, demonstrating an impairment of contractility. In addition, there was a decrease in spontaneous motor activity and reduced sensitivity to the nitric oxide synthase (NOS) inhibitor L-NOArg, corresponding with a significant reduction in NOS immunoreactive neurons in the MP of infected animals. T. spiralis did not alter the total number of myenteric or submucosal neurons. Substance P innervation of submucosal blood vessels was reduced after infection, as were submucosal calretinin and calbindin immunoreactive neurons. No changes in choline acetyltransferase and calcitonin gene-related peptide immunoreactivity were observed. T. spiralis-induced colitis causes profound neuromuscular adaptations. The reduction in NOS neurons appears to underlie changes in motility.

Colonic vascular conductance increased by Daikenchuto via calcitonin gene-related peptide and receptor-activity modifying protein 1

BACKGROUND

Daikencyuto (DKT) is a traditional Japanese medicine (Kampo) and is a mixture of extract powders from dried Japanese pepper, processed ginger, ginseng radix, and maltose powder and has been used as the treatment of paralytic ileus. DKT may increase gastrointestinal motility by an up-regulation of the calcitonin gene-related peptide (CGRP). CGRP is also the most powerful vasoactive substance. In the present study, we investigated whether DKT has any effect on the colonic blood flow in rats.

MATERIALS AND METHODS

Experiments were performed on fasted anesthetized and artificially ventilated Wistar rats. Systemic mean arterial blood pressure and heart rate were recorded. Red blood cell flux in colonic blood flow was measured using noncontact laser tissue blood flowmetry, and colonic vascular conductance (CVC) was calculated as the ratio of flux to mean arterial blood pressure. We examined four key physiological mechanisms underlying the response using blocker drugs: CGRP1 receptor blocker (CGRP(8-37)), nitric oxide synthase inhibitor, vasoactive intestinal polypeptide (VIP) receptor blocker ([4-Cl-DPhe6, Leu17]-VIP), and substance P receptor blocker (spantide). Reverse transcription-polymerase chain reaction was used for the detection of mRNA of calcitonin receptor-like receptor, receptor-activity modifying protein 1, the component of CGRP 1 receptor and CGRP. After laparotomy, a cannula was inserted into the proximal colon to administer the DKT and to measure CVC at the distal colon.

RESULTS

Intracolonal administration of DKT (10, 100, and 300 mg/kg) increased CVC (basal CVC, 0.10 mL/mmHg) from the first 15-min observation period (0.14, 0.17, and 0.17 mL/mmHg, respectively) and with peak response at either 45 min (0.17 mL/mmHg by 10 mg/kg), or 75 and 60 min (0.23 and 0.21 mL/mmHg by 100 and 300 mg/kg, respectively). CGRP(8-37) completely abolished the DKT-induced hyperemia, whereas nitric oxide synthase inhibitor partially attenuated the DKT-induced hyperemia. [4-Cl-DPhe6, Leu17]-VIP and spantide did not affect the hyperemia. Japanese pepper significantly increased CVC at 45 min or later, whereas ginseng radix only showed a significant increase at 15 min. Reverse transcription-polymerase chain reaction showed that mRNA for calcitonin receptor-like receptor, receptor-activity modifying protein 1, and CGRP were expressed in rat colon and up-regulated by DKT.

CONCLUSIONS

The present study demonstrated that DKT increased CVC, which was mainly mediated by CGRP and its receptor components.

Prolonged elevation of galanin and tachykinin expression in mucosal and myenteric enteric nerves in trinitrobenzene sulphonic acid colitis

Diverticulitis causes recurrent abdominal pain associated with increased mucosal expression of mucosal galanin and substance P (SP). We studied changes in mucosal and myenteric plexus neuropeptides in adult rats using a model of colonic inflammation, trinitrobenzenesulphonic acid colitis. We assessed the effects on the pan-neuronal markers protein gene product 9.5 (PGP9.5) and neurofilament protein, as well as specific neuropeptides at 1, 2, 3, 4, 6, 8, 10 and 14 weeks. Following the acute injury there was macroscopic resolution of inflammation but minor microscopic abnormalities persisted. Percent area stained of mucosal PGP9.5 fell initially but average levels on days 21 and 28 levels were significantly elevated (P < 0.001), returning to normal by day 42. Percent area staining of PGP9.5 in the muscle rose immediately and remained significantly elevated at 70 days (P < 0.001). SP, neuropeptide K and galanin followed a similar overall pattern. SP to PGP9.5 ratio was significantly increased in the muscle both acutely (days 1-28) and in the long term (days 70 and 98), whereas the galanin to PGP9.5 ratio was significantly increased in the mucosa throughout the study. Low-grade chronic inflammation after an acute initial insult causes a persistent increase in the expression of galanin in the mucosa and SP in muscle layer.

Vasoactive intestinal peptide rescues cultured rat myenteric neurons from lipopolysaccharide induced cell death

The role of the enteric nervous system in intestinal inflammation is not fully understood and the plethora of cellular activities concurrently ongoing in vivo renders intelligible studies difficult. In order to explore possible effects of bacterial lipopolysaccharide (LPS) on enteric neurons we utilised cultured myenteric neurons from rat small intestine. Exposure to LPS caused markedly reduced neuronal survival and increased neuronal expression of vasoactive intestinal peptide (VIP), while the expression of Toll-like receptor 4 (TLR4) was unchanged. TLR4 was expressed in approximately 35% of all myenteric neurons irrespective of if they were cultured in the presence or absence of LPS. In neurons cultured in medium, without LPS, 50% of all TLR4-immunoreactive neurons contained also VIP. Addition of LPS to the neuronal cultures markedly increased the proportion of TLR4-immunoreactive neurons also expressing VIP, while the proportion of TLR4 neurons devoid of VIP decreased. Simultaneous addition of LPS and VIP to the neuronal cultures resulted in a neuronal survival comparable to controls.

CONCLUSIONS

LPS recognition by myenteric neurons is mediated via TLR4 and causes neuronal cell death. Presence of VIP rescues the neurons from LPS-induced neurodegeneration.

A functional comparison of recombinant and native somatostatin sst2 receptor variants in epithelia

BACKGROUND AND PURPOSE

Somatostatin (SRIF-14) exerts broad spectrum antisecretory effects by activating the somatostatin 2 (sst(2)) receptor. The rat (r) sst(2) receptor exists in 'long' (sst(2a)) and 'short' (sst(2b)) forms that differ in their C termini, while a single human (h) sst(2a) exists. This study compares the characteristics of recombinant rsst(2a), rsst(2b) and hsst(2a) activation in human epithelia, and with native sst(2) responses in rat colon.

EXPERIMENTAL APPROACH

Epithelial layers of each clone or rat colon were placed in Ussing chambers and short-circuit current (I (SC)) measured in response to SRIF-14 and chosen analogues. The relative potencies and ability to cause desensitization to SRIF-14 were assessed, and the affinities of the sst(2) antagonist, D-Tyr(8) CYN154806 for hsst(2a), rsst(2a) and native rat colon sst(2) receptors were established.

KEY RESULTS

Basolateral SRIF-14 responses were transient in hsst(2a) and rsst(2a) epithelia, but prolonged in rsst(2b)-expressing cells. Activation of rsst(2a) resulted in significant desensitization to SRIF-14 and receptor phosphorylation, whereas the rsst(2b) receptor did neither. Sst(2)-preferred agonists (BIM23190C and BIM23027) reduced I (sc) with similar potency and both caused complete desensitization to SRIF-14. CYN154806 antagonized hsst(2a) and rsst(2a) receptors with pK (B) values of 7.9 and 7.8, respectively. In rat colon mucosa, CYN154806 blocked SRIF-14 responses with a pA (2) value of 8.2, and BIM23190C responses with a pK (B) of 8.4.

CONCLUSIONS AND IMPLICATIONS

SRIF-14 caused rapid rsst(2a) receptor phosphorylation and desensitization of epithelial antisecretory responses, neither of which occurred with the rsst(2b) receptor. These mechanisms are most likely to be a prerequisite for sensitivity to sst(2)-analogues with radiotherapeutic potential.

Survival and neurotransmitter plasticity in cultured rat colonic myenteric neurons

The enteric nervous system is of great importance for maintenance and proper function of the gastrointestinal tract. The aim of this study was to quantify myenteric neuronal subpopulations expressing calcitonin gene-related peptide (CGRP), galanin, neuropeptide Y (NPY), somatostatin, vasoactive intestinal peptide (VIP) and nitric oxide synthase (NOS) in rat colon in vivo and after culturing. Further we investigated if culturing in the presence of CGRP, galanin, VIP, S-nitroso-N-acetyl-D,L-penicillamine (SNAP, a NO donor) or N-nitro-L-arginine methyl ester (L-NAME, a NOS inhibitor) affect neuronal survival. After 4 days of culturing the proportions of neurons expressing CGRP, NPY, somatostatin or VIP increased as compared to in vivo, while the proportions of neurons expressing galanin or NOS did not change. Neuronal survival was unaffected after culturing in media enriched with CGRP, galanin, VIP, SNAP or L-NAME. Neither did addition of CGRP, galanin nor VIP to the cultures affect the relative numbers of neurons expressing CGRP, galanin or VIP respectively. Addition of SNAP or L-NAME did not change the percentage of neurons expressing NOS. In conclusion, cultured rat colonic myenteric neurons increase their expression of CGRP, NPY, somatostatin and VIP, suggesting that these neuropeptides are of importance for neuronal survival.

Peptide YY: a neuroendocrine neighbor of note

Endocrine cells, enteric neurons and enterocytes provide an integrated functional defense against luminal factors, including nutrients, microbes and toxins. Prominent among intrinsic mediators is peptide YY (PYY) which is present in approximately 50% of colorectal endocrine cells and neuropeptide Y (NPY), a neurotransmitter expressed in submucous and myenteric nerves. Both peptides and their long fragments (PYY(3-36) and NPY(3-36)) are potent, long-lasting anti-secretory agents in vitro and in vivo and, they provide significant Y receptor-mediated absorptive tone in human and mouse colon mucosa. The main function of the colon is to absorb 90% of approximately 2l of daily ileal effluent (in adult humans) and Y-absorptive tone can contribute significantly to this electrolyte absorption. Blockade or loss of this mucosal Y-absorptive tone (i.e. with Y(1) or Y(2) antagonists) leads to hypersecretion and potentially to diarrhea, so Y agonists are predicted to rescue absorption by mimicking endogenous neuroendocrine PYY or neuronal NPY.

Quantification of subclasses of human colonic myenteric neurons by immunoreactivity to Hu, choline acetyltransferase and nitric oxide synthase

An accurate method to count human enteric neurons is essential to develop a comprehensive account of the classes of nerve cells responsible for gut function and dysfunction. The majority of cells in the enteric nervous system utilize acetyl choline, or nitric oxide, or a combination of these, as neurotransmitters. Antisera raised against the RNA-binding protein Hu, were used to identify nerve cell bodies in whole mounts of the myenteric plexus of human colon, and then were utilized to analyse cells immunoreactive for combinations of choline acetyltransferase and nitric oxide synthase. Antisera to Hu provided a reliable means to count apparently all enteric nerve cell bodies, revealing 10% more cell bodies than labelling with neuron specific enolase, and no labelling of glial cells as revealed by S100. ChAT+/NOS- neurons accounted for 48% (+/-3%) of myenteric neurons and ChAT-/NOS+ neurons accounted for 43% (+/-2.5%). ChAT+/NOS+ neurons comprised 4% (+/-0.5) of the total number of neurons, and a novel class of small ChAT-/NOS- neurons, making up 5% (+/-0.9%) of all cells, was described for the first time.

Neuropeptide Y receptors; antisecretory control of intestinal epithelial function

This paper reviews the cellular localisation, mechanisms of release and intestinal absorptive actions of neuropeptide Y and its related peptides, peptide YY, pancreatic polypeptide and major fragments NPY(3-36) and PYY(3-36). While NPY is commonly found in inhibitory enteric neurons that can be interneurons, motor neurons or secretomotor-nonvasodilator in nature, its analogue, peptide YY in contrast, is located in neuroendocrine L-cells that predominate in the colorectal mucosa. Peptide YY is released from these cells when nutrients arrive in the small or large bowel, exerting paracrine as well as hormonal actions. Pancreatic polypeptide is found in relatively few, scattered intestinal endocrine cells, the majority of this peptide being produced by, and released from pancreatic islet F-cells in response to food intake. An introduction to the current pharmacology of this family of peptides is provided and the different types of neuropeptide Y (termed Y) receptors, their agonist preferences, antagonism, and preferred signalling pathways, are described. Our current understanding of specific Y receptor localisation within the intestine as determined by immunohistochemistry, is presented as a prelude to an assessment of functional studies that have monitored ion transport across isolated mucosal preparations. It is becoming clear that three Y receptor types are significant functionally in human colon, as well as particular rodent models (e.g. mouse) and these, namely the Y(1), Y(2) and Y(4) receptors, are discussed in detail. Their presence within the basolateral aspect of the epithelial layer (Y(1) and Y(4) receptors) or on enteric neurons (Y(1) and Y(2) receptors) and their activation by endogenous neuropeptide Y, peptide YY (Y(1) and Y(2) receptors) or pancreatic polypeptide (which prefers Y(4) receptors) results consistently in antisecretory/absorptive responses. The recent use of novel mouse knockouts has helped establish loss of specific intestinal functions including Y(1) and Y(2) receptor-mediated absorptive tone in colon mucosa. Progress in this field has been rapid recently, aided by the availability of selective antagonists and mutant mice lacking either one (e.g. Y(4)-/-, for which no antagonists exist at present) or more Y receptor types. It is therefore timely to review this work and present a rational basis for developing stable synthetic Y receptor agonists as novel anti-diarrhoeals.

Distribution of adrenergic receptors in the enteric nervous system of the guinea pig, mouse, and rat

Adrenergic receptors in the enteric nervous system (ENS) are important in control of the gastrointestinal tract. Here we describe the distribution of adrenergic receptors in the ENS of the ileum and colon of the guinea pig, rat, and mouse by using single- and double-labelling immunohistochemistry. In the myenteric plexus (MP) of the rat and mouse, alpha2a-adrenergic receptors (alpha2a-AR) were widely distributed on neurons and enteric glial cells. alpha2a-AR mainly colocalized with calretinin in the MP, whereas submucosal alpha2a-AR neurons colocalized with vasoactive intestinal polypeptide (VIP), neuropeptide Y, and calretinin in both species. In the guinea pig ileum, we observed widespread alpha2a-AR immunoreactivity on nerve fibers in the MP and on VIP neurons in the submucosal plexus (SMP). We observed extensive beta1-adrenergic receptor (beta1-AR) expression on neurons and nerve fibers in both the MP and the SMP of all species. Similarly, the beta2-adrenergic receptor (beta2-AR) was expressed on neurons and nerve fibers in the SMP of all species, as well as in the MP of the mouse. In the MP, beta1- and beta2-AR immunoreactivity was localized to several neuronal populations, including calretinin and nitrergic neurons. In the SMP of the guinea pig, beta1- and beta2-AR mainly colocalized with VIP, whereas, in the rat and mouse, beta1- and beta2-AR were distributed among the VIP and calretinin populations. Adrenergic receptors were widely localized on specific neuronal populations in all species studied. The role of glial alpha2a-AR is unknown. These results suggest that sympathetic innervation of the ENS is directed toward both enteric neurons and enteric glia.

Gastrin-releasing peptide is a modulatory neurotransmitter of the descending phase of the peristaltic reflex

The physiological role of gastrin-releasing peptide (GRP) and of its cognate receptors in regulating the intestinal peristaltic reflex was examined in a three-compartment flat-sheet preparation of rat colon. Mucosal stimulation applied to the central compartment at high, but not low levels of intensity, induced GRP release in the caudad compartment where descending relaxation was measured, but not into the ascending compartment where ascending contraction was measured or into the central compartment where the stimuli were applied. The selective GRP (BB(2)) receptor antagonist, [D-Phe(6),des-Met(14)]bombesin(6-14), inhibited descending relaxation and VIP release in the caudad compartment induced by high but not by low levels of stimulation applied to the mucosa in the central compartment. The selective neuromedin B (BB(1)) receptor antagonist, BIM-23127, had no effect on descending relaxation or VIP release. Neither the BB(1) nor the BB(2) antagonist had any effect on ascending contraction or substance P release in the orad compartment. Consistent with the effects of the antagonists on the peristaltic reflex, the BB(2) antagonist but not the BB(1) antagonist decreased the velocity of propulsion of artificial fecal pellets through isolated segments of guinea pig distal colon. The results indicate that GRP is selectively released from myenteric neurons in descending pathways during the peristaltic reflex and that it acts via BB(2) receptors to augment the descending phase of the peristaltic reflex and propulsion.

Increased expression of nitric oxide synthase in cultured neurons from adult rat colonic submucous ganglia

Neuronal plasticity in the enteric nervous system (ENS) is probably a key step in intestinal adaptation during growth, maturation and ageing as well as in several pathophysiological situations. Studies on cultured myenteric neurons have revealed an increased vasoactive intestinal peptide (VIP) expression in neuronal nitric oxide synthase (NOS)-expressing neurons. In addition, both VIP and nitric oxide (NO) promote survival of cultured myenteric neurons. The aim of the present study was to investigate possible changes in the expression of VIP and NOS in cultured submucous neurons from adult rat large intestine. Submucous neurons were cultured as explants or as dissociated neurons for 3 and 8 days. Immunocytochemistry was used to determine the proportions of neurons containing VIP or NOS in preparations of uncultured controls (reflects the conditions in vivo) and in cultured explants of submucosa and dissociated submucous neurons. In situ hybridization was used to determine changes in the expressions of NOS and VIP mRNA. The relative number of NOS-expressing neurons increased significantly during culturing. The percentage of all neurons expressing NOS was 22% in controls, while approximately 50% of the cultured submucous neurons expressed NOS. VIP-expressing neurons constituted approximately 80% of all submucous neurons in controls as well as in cultured explants or dissociated neurons. Studies on coexistence revealed that the VIP-containing neurons were the ones that started to express NOS during culture. The induced expression of NOS in cultured adult submucous neurons indicates that nitric oxide, possibly in cooperation with VIP, is important for neuronal adaptation, maintenance and survival.

Mechanically evoked reflex electrogenic chloride secretion in rat distal colon is triggered by endogenous nucleotides acting at P2Y1, P2Y2, and P2Y4 receptors

Mechanical activation of the mucosal lining of the colon by brush stroking elicits an intestinal neural reflex and an increase in short circuit current (Isc) indicative of electrogenic chloride ion transport. We tested whether endogenous nucleotides are physiologic regulators of mucosal reflexes that control ion transport. The brush stroking-evoked Isc response in mucosa and submucosa preparations (M-SMP) of rat colon was reduced by the P2Y1 receptor (R) antagonist 2'deoxy-N6-methyl adenosine 3',5'-diphosphate diammonium salt (MRS 2179) and further blocked by tetrodotoxin (TTX). M-SMP Isc responses to serosal application of the P2Y1 R agonist 2-methylthioadenosine-diphosphate (2MeSADP) or the P2Y2/P2Y4 R agonist 5'uridine-triphosphate (UTP) were reduced but not abolished by TTX. The potency profile of nucleotides for increasing Isc was 5'adenosine-triphosphate (ATP; effective concentration at half maximal response [EC50] 0.65 x 10(4) M) congruent with UTP (EC50 1.0 x 10(-4) M) congruent with 2MeSADP (EC50 = 1.60 x 10(-4) M). Mucosal touch and distention-induced Ca2+ transients in submucous neurons were reduced by apyrase and prevented by blocking the P2Y1 R with MRS 2179 and TTX; denervation of the mucosa. It did not occur by touching a ganglion directly. 2MeSADP Ca2+ responses occurred in subsets of neurons with or without substance P (SP) responses. The potency profile of nucleotides on the neural Ca2+ response was 2MeSADP (5 x 10(-7) M) > UTP (6 x 10(-6) M) > ATP (9 x 10(-5) M). The expression of P2Y R immunoreactivity (ir) in nerve cell bodies was in the order of P2Y1 R > P2Y4 R >> P2Y2 R. P2Y1R ir occurred in the cell somas of more than 90% of neuronal nitric oxide synthase, vasoactive intestinal peptide (VIP), calretinin, or neuropeptide Y (NPY)-ir neurons, 78% of somatostatin neurons, but not in calbindin or SP neurons. P2Y2 R ir was expressed in a minority of SP, VIP, NPY, vesicular acetylcholine transporter, and calcitonin gene-related peptide-ir varicose fibers (5-20%) and those surrounding calbindin (5-20%) neurons. P2Y4 ir occurred mainly in the cell somas of 93% of NPY neurons. Reverse transcriptase polymerase chain reaction of the submucosa demonstrated mRNA for P2Y1R, P2Y2, P2Y4, P2Y6, and P2Y12 Rs. Expression of P2Y1, P2Y2, and P2Y4 protein was confirmed by western blots. In conclusion, endogenous nucleotides acting at P2YRs transduce mechanically evoked reflex chloride ion transport in rat distal colon. Nucleotides evoke reflexes by acting primarily at postsynaptic P2Y1 Rs and P2Y4 R on VIP+/NPY+ secretomotor neurons, at P2Y2 Rs on no more than 2% of VIP+ secretomotor neurons, and 2Y2 Rs mainly of extrinsic varicose fibers surrounding putative intrinsic primary afferent and secretomotor neurons. During mucosal mechanical reflexes, it is postulated that P2Y1 R, P2Y2 R, and P2Y4 R are activated by endogenous ATP, UTP, and 5'uridine-diphosphate.

Cocaine- and amphetamine-regulated transcript: distribution and function in rat gastrointestinal tract

Cocaine- and amphetamine-regulated transcript (CART) peptide, originally isolated from brain, is also expressed in the peripheral nervous system. The distribution, origin and projections of CART-expressing enteric neurones by immunocytochemistry and in situ hybridization in rat gastrointestinal (GI) tract were studied. Possible motor functions of CART were studied in vitro using longitudinal muscle strips from stomach, ileum and colon. Cocaine- and amphetamine-regulated transcript peptide was found in numerous myenteric neurones throughout the GI tract while CART-expressing submucous neurones were scarce. Cocaine- and amphetamine-regulated transcript was also expressed in the antral gastrin cells. Myenteric CART-expressing neurones in both small and large intestine issued short descending projections. In atrophic ileum, CART mRNA-expressing neurones increased in number while neurones containing CART peptide decreased. In hypertrophied ileum, no change in CART peptide or CART mRNA containing myenteric neurones was detected. Cocaine- and amphetamine-regulated transcript 55-102 (10(-9)-10(-7) mol L-1) did not induce any contractile or relaxatory responses in the muscle strips, neither did it affect responses induced by vasoactive intestinal peptide, pituitary adenylate cyclase-activating peptide or neuronal stimulation. In colonic, but not in ileal, strips addition of CART attenuated nitric oxide (NO) donor-induced relaxations. Although CART does not seem to play a pivotal role in classic neurotransmission to the longitudinal muscle, it may serve a modulatory role in NO transmission. It may, moreover, be involved in intestinal adaptation, and an additional hormonal role is also possible.

Functional consequences of neuropeptide Y Y 2 receptor knockout and Y2 antagonism in mouse and human colonic tissues

1 Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) differentially activate three Y receptors (Y(1), Y(2) and Y(4)) in mouse and human isolated colon. 2 The aim of this study was to characterise Y(2) receptor-mediated responses in colon mucosa and longitudinal smooth muscle preparations from wild type (Y(2)+/+) and knockout (Y(2)-/-) mice and to compare the former with human mucosal Y agonist responses. Inhibition of mucosal short-circuit current and increases in muscle tone were monitored in colonic tissues from Y(2)+/+ and Y(2)-/- mice+/-Y(1) ((R)-N-[[4-(aminocarbonylaminomethyl)phenyl)methyl]-N(2)-(diphenylacetyl)-argininamide-trifluoroacetate (BIBO3304) or Y(2) (S)-N(2)-[[1-[2-[4-[(R,S)-5,11-dihydro-6(6H)-oxodibenz[b,e]azepin-11-yl]-1-piperazinyl]-2-oxoethyl]cyclopentyl]acetyl]-N-[2-[1,2-dihydro-3,5(4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-argininamide (BIIE0246) antagonists. 3 Predictably, Y(2)-/- tissues were insensitive to Y(2)-preferred agonist PYY(3-36) (</=100 nM), but unexpectedly Y(4)-preferred PP responses were right-shifted probably as a consequence of elevated circulating PP levels, particularly in male Y(2)-/- mice (Sainsbury et al., 2002). 4 BIBO3304 and BIIE0246 elevated mucosal ion transport, indicating blockade of inhibitory mucosal tone in Y(2)+/+ tissue. While BIBO3304 effects were unchanged, those to BIIE0246 were absent in Y(2)-/- mucosae. Neither antagonist altered muscle tone; however, BIIE0246 blocked NPY and PYY(3-36) increases in Y(2)+/+ basal tone. BIBO3304 abolished residual Y(1)-mediated NPY responses in Y(2)-/- smooth muscle. 5 Tetrodotoxin significantly reduced BIIE0246 and PYY(3-36) effects in Y(2)+/+ mouse and human mucosae, but had no effect upon Y-agonist contractile responses, indicating that Y(2) receptors are located on submucosal, but not myenteric neurones. 6 Tonic activation of submucosal Y(2) receptors by endogenous NPY, PYY or PYY(3-36) could indirectly reduce mucosal ion transport in murine and human colon, while direct activation of Y(2) receptors on longitudinal muscle results in contraction.

Reciprocal activity of longitudinal and circular muscle during intestinal peristaltic reflex

A two-compartment, flat-sheet preparation of rat colon was devised, which enabled exclusive measurement of longitudinal muscle activity during the ascending and descending phases of the peristaltic reflex. A previous study using longitudinal muscle strips revealed the operation of an integrated neuronal circuit consisting of somatostatin, opioid, and VIP/pituitary adenylate cyclase-activating peptide (PACAP)/nitric oxide synthase (NOS) interneurons coupled to cholinergic/tachykinin motor neurons innervating longitudinal muscle strips that could lead to descending contraction and ascending relaxation of this muscle layer. Previous studies in peristaltic preparations have also shown that an increase in somatostatin release during the descending phase causes a decrease in Met-enkephalin release and suppression of the inhibitory effect of Met-enkephalin on VIP/PACAP/NOS motor neurons innervating circular muscle and a distinct set of VIP/PACAP/NOS interneurons. The present study showed that in contrast to circular muscle, longitudinal muscle contracted during the descending phase and relaxed during the ascending phase. Somatostatin antiserum inhibited descending contraction and augmented ascending relaxation of longitudinal muscle, whereas naloxone had the opposite effect. VIP and PACAP antagonists inhibited descending contraction of longitudinal muscle and augmented ascending relaxation. Atropine and tachykinin antagonists inhibited descending contraction of longitudinal muscle. As shown in earlier studies, the same antagonists and antisera produced opposite effects on circular muscle. We conclude that longitudinal muscle contracts and relaxes in reverse fashion to circular muscle during the peristaltic reflex. Longitudinal muscle activity is regulated by excitatory VIP/PACAP/NOS interneurons coupled to cholinergic/tachykinin motor neurons innervating longitudinal muscle.

Nicotinic transmission at functionally distinct synapses in descending reflex pathways of the rat colon

We examined descending reflex pathways in the rat colon using intracellular recording techniques. Inhibitory junction potentials (IJPs) were recorded from circular smooth muscle when descending pathways were excited by combined mucosal compression and distension. IJPs were reduced to 71% of control when synaptic transmission was blocked in the oral stimulation chamber of a divided organ bath suggesting that two reflex pathways exist, the one involving descending sensory neurones and the other involving descending interneurones. Hexamethonium (200 micromol L(-1)) in the recording chamber abolished reflexly evoked IJPs, while in the stimulation chamber, it was as effective as synaptic blockade. When hexamethonium was added to a chamber lying between the stimulation and recording chambers, it again sharply depressed IJPs to 27% of control; an extent similar to synaptic blockade. A P2 receptor antagonist did not reveal any purinergic neurotransmission. Either granisetron (5-HT3 receptor antagonist, 1 micromol L(-1)) or SB204070 (5-HT4 receptor antagonist, 1 micromol L(-1)) in the stimulation chamber significantly decreased IJPs; these decreases were not additive. We conclude that some sensory neurones and interneurones in rat colon have long anally projecting axons and that acetylcholine, acting via nicotinic receptors, is the primary neurotransmitter from sensory neurones, to inhibitory motor neurones and between interneurones.

Identification of cells expressing somatostatin receptor 2 in the gastrointestinal tract of Sstr2 knockout/lacZ knockin mice

Somatostatin is found in neurons and endocrine cells in the gastrointestinal tract. The actions of somatostatin are mediated by a family of G-protein-coupled receptors that compose five subtypes (SSTR1-5), each of which is encoded by a separate gene. lacZ "knockin" mice, in which the reporter gene lacZ was engineered into the genomic locus of Sstr2 by gene targeting, were used to examine the expression pattern of Sstr2 and identify potential targets for neurally released and hormonal somatostatin in the gastrointestinal tract. In the body of the stomach, a large proportion of epithelial cells and subpopulations of myenteric neurons expressed Sstr2. Double- or triple-labeling with antisera to H(+)K(+)ATPase (to identify parietal cells) and/or histidine decarboxylase (to identify enterochromaffin-like [ECL] cells) combined with beta-galactosidase staining revealed that both parietal cells and ECL cells expressed Sstr2, and these two cell types accounted for almost all of the Sstr2-expressing epithelial cells. Somatostatin inhibits gastric acid secretion. The presence of SSTR2 on both parietal and ECL cells suggests that somatostatin acting on SSTR2 may reduce acid secretion by both acting directly on parietal cells and by reducing histamine release from ECL cells. In the small and large intestine, subpopulations of neurons in the myenteric and submucosal plexuses expressed Sstr2, and many of the Sstr2-expressing myenteric neurons also showed SSTR2(a) immunostaining. Most of Sstr2-expressing neurons in the myenteric plexus showed nitric oxide synthase (NOS) immunoreactivity. Previous studies have shown that NOS neurons are descending interneurons and anally projecting, inhibitory motor neurons. Thus, somatostatin acting at SSTR2 receptors on NOS neurons might modulate descending relaxation.

Origins and projections of nerve fibres in rat pyloric sphincter

Nerve fibres play an important role in the regulation of gastric emptying. The aims of this study were to clarify the distribution, projections and origin of neuronal type nitric oxide synthase (NOS)-, tyrosine hydroxylase (TH)-, vesicular acetylcholine transporter (VAchT)- and peptide-containing nerve fibres of the rat pyloric sphincter. Extrinsic and local denervations of the sphincter were performed in order to reveal the origin and projections of the various nerve fibre populations. Pylorus from control and denervated animals were processed for the immunocytochemical demonstration of cholecystokinin (CCK), enkephalin, gastrin-releasing peptide (GRP), somatostatin, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), pituitary adenylate cyclase-activating peptide (PACAP), substance P (SP), vasoactive intestinal peptide (VIP), galanin, NOS, VAchT and TH. VAchT, TH, nNOS, and all of the peptides investigated were found in nerve fibres innervating the pyloric sphincter, and coexistence of several putative neurotransmitters were revealed. Extrinsic denervation caused a total loss of NPY/TH-, SP/CGRP- and SP/CGRP/VIP/NOS/PACAP-containing nerve fibres. Local denervation immediately proximal to the sphincter markedly reduced the numbers of VIP/NOS/galanin- and VIP/NOS/galanin/PACAP +/- NPY-containing fibres within the sphincter suggesting an origin of these fibres in myenteric ganglia in the antral region; denervation at the level of the oxyntic-pyloric border had no effect. Local denervation immediately distal to the sphincter caused a marked decrease in VAchT-, SP/enkephalin-, enkephalin-, somatostatin-, CCK- and GRP-containing fibres within the sphincter suggesting that these emanate from the duodenum. The latter procedure also reduced the number of SP/CGRP-containing fibres of extrinsic origin within the pyloric sphincter.

Neuropeptide Y, Y1, Y2 and Y4 receptors mediate Y agonist responses in isolated human colon mucosa

1. The aim of this study was to provide a pharmacological characterization of the Y receptor types responsible for neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) effects upon electrogenic ion transport in isolated human colonic mucosa. 2. Preparations of descending colon were voltage-clamped at 0 mV in Ussing chambers and changes in short-circuit current (I(sc)) continuously recorded. Basolateral PYY, NPY, human PP (hPP), PYY(3 - 36), [Leu(31), Pro(34)]PYY (Pro(34)PYY) and [Leu(31), Pro(34)]-NPY (Pro(34)NPY) all reduced basal I(sc) in untreated colon. Of all the Y agonists tested PYY(3 - 36) responses were most sensitive to tetrodotoxin (TTX) pretreatment, indicating that Y(2)-receptors are located on intrinsic neurones as well as epithelia in this tissue. 3. The EC(50) values for Pro(34)PYY, PYY(3 - 36) and hPP were 9.7 nM (4.0 - 23.5), 11.4 nM (7.6 - 17.0) and 14.5 nM (10.2 - 20.5) and response curves exhibited similar efficacies. The novel Y(5) agonist [Ala(31), Aib(32)]-NPY had no effect at 100 nM. 4. Y(1) receptor antagonists, BIBP3226 and BIBO3304 both increased basal I(sc) levels per se and inhibited subsequent PYY and Pro(34)PYY but not hPP or PYY(3 - 36) responses. The Y(2) antagonist, BIIE0246 also raised basal I(sc) levels and attenuated subsequent PYY(3 - 36) but not Pro(34)PYY or hPP responses. 5. We conclude that Y(1) and Y(2) receptor-mediated inhibitory tone exists in human colon mucosa. PYY and NPY exert their effects via both Y(1) and Y(2) receptors, but the insensitivity of hPP responses to either Y(1) or Y(2) antagonism, or to TTX, indicates that Y(4) receptors are involved and that they are predominantly post-junctional in human colon.

Morphology, electrophysiology, and calbindin immunoreactivity of myenteric neurons in the guinea pig distal colon

The morphological and physiological characteristics of myenteric neurons in the guinea pig distal colon were determined using Lucifer yellow- or N-(2-aminoethyl) biotinamide-containing microelectrodes and intracellular recording and staining methods. The neurons in this study (n = 204) were classified on the basis of the shapes of their cell bodies and short processes or dendrites and the number of long processes or axons as Dogiel type I (n = 75 neurons; 36.8%), filamentous (n = 31 neurons; 15.2%), Dogiel type II (n = 38 neurons; 18.6%), and unclassified (n = 60 neurons; 29.4%). All Dogiel type II neurons had action potentials followed by an after-spike hyperpolarization (AH), and most of them (84%) had large, smooth somata and filamentous, short processes in addition to multiple, long processes or axons. Most of Dogiel type I, filamentous, and unclassified neurons (98%) had a single, long process, but four Dogiel type I neurons and one unclassified neuron had two long processes terminating as varicosities within other ganglia or on the surface of longitudinal muscle. The projections of monoaxonal neurons were distributed equally between oral and aboral directions, and most of them received fast excitatory postsynaptic potentials (EPSPs). All of the Dogiel type II neurons and seven Dogiel type I neurons were positive for calbindin immunoreactivity, but three filamentous neurons received fEPSPs, had spikes followed by AH, and were negative for calbindin. The presence of calbindin-immunoreactive(-IR) neurons was quite variable among the ganglia. These results confirm that neither the presence of calbindin immunoreactivity nor the absence of fEPSPs can be used as a predictor of cellular morphology or electrophysiological properties of myenteric neurons in the distal colon.

Multiple Y receptors mediate pancreatic polypeptide responses in mouse colon mucosa

A functional study has been performed to characterise the Y receptors responsible for NPY, PYY and PP-stimulated responses in mouse colonic mucosal preparations. Electrogenic ion secretion was stimulated with VIP following which NPY, PYY and PP analogues were, to varying degrees, inhibitory. PYY(3-36), hPP, Gln(23)hPP and rPP were effective but less potent than full length PYY, NPY or their Pro(34)-substituted analogues, while the Y(5) agonist Ala(31), Aib(32)hNPY was the least active peptide tested. The Y(1) antagonists, BIBP3226 and BIBO3304 virtually abolished Pro(34)PYY and PYY responses while PYY(3-36) responses were selectively inhibited by the Y(2) antagonist, BIIE0246. A combination of BIBO3304 and BIIE0246 also partially attenuated hPP responses, leaving residual effects that were most probably Y(4)-mediated. Thus we conclude that Y(1), Y(2) and Y(4) receptors attenuate ion secretion in mouse colon.

Neuropeptides in drug research

Neuropeptides have been a subject of considerable interest in the pharmaceutical industry over the last 20 years or more. Many drug discovery teams have contributed to our understanding of neuropeptide biology but no significant drugs that act selectively upon neuropeptide receptors have yet emerged from the clinic. There are, however, a plethora of clinically useful drugs that act at other classes of neurotransmitter and neuromodulator receptors, many of them discovered over the last 20 years. Nevertheless, we think that the future for the discovery of novel drugs acting at neuropeptide receptors looks bright for two reasons: (1) there has been a substantial increase in our understanding of the function of neuropeptides; and (2) high-throughput screening (HTS) against neuropeptide receptors has now begun to yield many interesting drug-like molecules, rather than peptides, that have the potential to become clinically useful drugs. The objective of this review is to summarise our current understanding of specific areas of neuropeptide biology and pharmacology in the CNS as well as the PNS. We will also speculate on where we think the new generation of neuropeptide agonists and antagonists could emerge from the clinic.

Neurochemical coding and projection patterns of gastrin-releasing peptide-immunoreactive myenteric neurone subpopulations in the guinea-pig gastric fundus

The aim of this study was to characterise the projection and neurochemical coding patterns of gastrin-releasing peptide (GRP)-containing subpopulations of myenteric neurones in the guinea-pig gastric fundus. For this purpose, we used retrograde tracing with the dye DiI and immunohistochemistry against GRP, choline acetyltransferase (ChAT), enkephalin (ENK), substance P (SP) and neuropeptide Y (NPY). Cell counts revealed that 44% of the myenteric neurones were GRP-positive. Of the GRP-positive neurones, 92% were ChAT-positive and, hence, 8% were presumptively nitric oxide synthase positive (NOS). The GRP-positive subpopulations were ChAT/GRP (40% of all GRP neurones), ChAT/NPY/GRP (25%), ChAT/SP/GRP/+/-ENK (20%), ChAT/ENK/GRP (8%), NOS/NPY/GRP/+/-ENK (5%) and NOS/GRP (3%). The tracing experiments revealed the relative contributions of the various GRP-positive subpopulations to the innervation of the circular muscle and the mucosa. GRP immunoreactivity was detected in 46 and 38% of the DiI-labelled muscle and mucosa neurones, respectively. GRP was almost exclusively found in ascending ChAT-positive mucosa and muscle neurones. The populations encoded ChAT/SP/GRP/+/-ENK and ChAT/ENK/GRP projected predominantly to the circular muscle, whereas the ChAT/NPY/GRP and ChAT/GRP populations had primarily projections to the mucosa. GRP was colocalised with ChAT, ENK and/or SP in varicose nerve fibres innervating the circular muscle and the muscularis mucosae, whereas in the mucosal epithelium GRP was mainly present in nerve fibres containing ChAT and NPY. The data suggest that in the guinea-pig gastric fundus, the ChAT/SP/GRP/+/-ENK and ChAT/ENK/GRP neurones are ascending excitatory muscle motor neurones, whereas the ChAT/NPY/GRP and ChAT/GRP neurones are very likely involved in the regulation of mucosal functions.

Receptors in interstitial cells of Cajal: identification and possible physiological roles

Interstitial cells of Cajal (ICCs) are specialized cells of the gastrointestinal tract forming distinct populations depending on their location in the gut wall. Morphological observations and functional data have led to the hypothesis of two functions for the ICCs: (1) as pacemakers of the rhythmic activity; (2) as intermediaries in neural inputs to the muscle. The identification of specific receptors on the ICCs has represented an important step in the knowledge of these cells. Immunohistochemical labeling of these receptors provided information on both ICC morphology and contacts (particularly those with nerve endings) and on the functions of these cells. All ICC possess the Kit receptor, which represents the best tool to identify these cells under the light microscope. It has been demonstrated that this receptor is essential for ICC differentiation, and, by using mutant mice lacking the Kit-related gene, it has been possible to discriminate among all the ICC those with a primary role as pacemakers. The ileal ICC, in particular those at the deep muscular plexus, express the tachykinin receptor NK1 and a subtype of somatostatin receptors and contain nitric oxide synthase. All these data support a primary role of these ICC in neural transmission.

Number and size of myenteric neurons of the duodenum of adult rats with acute diabetes

This study had as its purpose to assess the effects of acute diabetes induced by streptozotocin (35 mg/kg body weight) on the number and size of the myenteric neurons of the duodenum of adult rats considering equally the antimesenteric and intermediate regions of the intestinal circumference. Experimental period extended for a week. Neuronal counts were carried out on the same number of fields of both regions of the duodenal circumference and measurements of neuronal and nuclear areas on equal numbers of cells. Number and size of the myenteric neurons stained with Giemsa were not significantly different between groups. On the other hand, the proportion of NADH-positive neurons increased from 18.54% on the controls to 39.33% on the diabetics. The authors discuss that this increased reactivity probably results from a greater NADH/NAD+ ratio, described in many tissues of diabetic animals, which has consequences on the modulation of the enzymes that use these cofactors and whose activity is detected by the NADH-diaphorase technique.

Regulation of excitatory neural input to longitudinal intestinal muscle by myenteric interneurons

The circuit of myenteric interneurons that regulate excitatory input to longitudinal colonic muscle was identified using dispersed ganglia and longitudinal muscle strips with adherent myenteric plexus from rat distal colon. The preparations enabled measurement of neurotransmitter release from interneurons and/or excitatory motoneurons innervating longitudinal muscle. 1, 1-Dimethyl-4-phenylpiperizinium (DMPP) and somatostatin were used to activate myenteric neurons in dispersed ganglia and muscle strips, respectively. DMPP-stimulated vasoactive intestinal peptide (VIP) release in dispersed ganglia was inhibited by [Met]enkephalin and bicuculline and augmented by naloxone and GABA, implying that inhibitory opioid and stimulatory GABA neurons regulate the activity of VIP interneurons. In muscle strips, VIP stimulated basal and augmented somatostatin-induced substance P (SP) release; the somatostatin-induced increase in SP release was inhibited by VIP-(10-28) and NG-nitro-L-arginine, implying that excitatory VIP neurons regulate tachykinin motoneurons innervating longitudinal muscle. Somatostatin inhibited [Met]enkephalin and stimulated VIP release; basal and somatostatin-stimulated VIP release were inhibited by [Met]enkephalin and bicuculline and augmented by naloxone and GABA, implying that inhibitory pathways linking somatostatin, opioid, and GABA neurons regulate VIP interneurons, which in turn regulate tachykinin and probably cholinergic motoneurons.

Distribution of calcitonin gene-related peptide, somatostatin, substance P and vasoactive intestinal polypeptide in experimental colitis in rats

Immunohistochemistry was used to examine the distribution of calcitonin gene-related peptide (CGRP), substance P, somatostatin and vasoactive intestinal polypeptide (VIP) in experimental colitis induced with trinitrobenzene sulphonic acid (TNBS) in rats. CGRP immunoreactivity was observed throughout the colonic wall. A significant reduction of CGRP-immunoreactive (IR) nerve fibres was observed in the mucosa after the induction of colitis. After TNBS treatment substance P immunoreactivity was reduced throughout the colon; however, after 7 days there was a marked re-innervation of the circular muscle. Somatostatin immunoreactivity was distributed sparsely within the colonic wall, and was comparatively less affected by colitis. VIP immunoreactivity was abundantly distributed in the colonic wall and underwent an immediate reduction in the mucosa after TNBS treatment. After 2 days, there was a consistent and progressive increase in the number and density of VIP-IR nerve fibres in the inflamed colon, particularly the circular muscle. This change was associated with a proliferation of nerve fibres within the muscle layers. It was concluded that the early decrease in these neuropeptides was consistent with release from peripheral nerve terminals or the loss of nerves during the initial stages of colonic inflammation, which may be an essential condition for the development of colitis in this model. The observation that the intensity and density of substance P and VIP-IR nerve fibres increased in the circular muscle 7 days after the induction of colitis suggests their possible involvement in tissue repair.

Projections of pelvic autonomic neurons within the lower bowel of the male rat: an anterograde labelling study

The tissues of the large intestine which receive an innervation by neurons of the major pelvic ganglia were identified following in vivo and in vitro anterograde labelling with the lipophilic tracer 1,1'didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate in the male rat. The primary target in the gut of major pelvic ganglion neurons is the myenteric plexus of the distal colon and the rectum. The serosal ganglia, on the surface of the most distal region of the rectum and the circular muscle of the distal colon and rectum were less densely innervated. The pelvic ganglia do not innervate the longitudinal muscle, submucosal blood vessels, submucosal plexus, or mucosa. The pelvic supply reaches the bowel via two groups of rectal nerves and branches of the penile nerves. All of these connections also carry the axons of viscerofugal neurons from the bowel, some of which have terminal axons in the major pelvic ganglia. Finally, the different nerves supplied different targets. In particular, while the rectal nerves carried pelvic axons supplying the myenteric plexus, circular muscle, and serosal ganglia, the penile nerves only innervated the serosal ganglia. In addition, the two groups of rectal nerves innervated slightly different regions of the bowel and provided different projection patterns. However, successful in vivo labelling was achieved in only 6/12 animals and while all in vitro experiments resulted in successful labelling, it was clear that only a proportion of pelvic projections in any given nerve were labelled. These studies have shown that the major pelvic ganglia are primarily involved in the control of motility, but not of vascular and secretomotor functions. Thus pelvic neurons do not innervate the same range of target tissues within the bowel as the prevertebral ganglia. This study has also shown that the different pathways to the gut from the major pelvic ganglia innervate different tissues, suggesting that the autonomic innervation of the gut is not homogeneous along its length.

Somatostatin 2A receptor is expressed by enteric neurons, and by interstitial cells of Cajal and enterochromaffin-like cells of the gastrointestinal tract

Somatostatin exerts multiple effects by activating distinct G protein-coupled receptors. Here we report the cellular sites of expression of the somatostatin subtype 2A (sst2A) receptor in the rat enteric nervous system by using a C-terminus-specific, affinity-purified antiserum and immunohistochemistry. Antibody specificity was confirmed by the cell surface staining of human embryonic kidney 293 cells expressing the sst2A receptor, the lack of staining of cells expressing the somatostatin subtype 2B receptor, and the abolition of staining by preincubating the antiserum with the C-terminus peptide used for immunization, SSt2A(361-369). The SSt2A receptor antibody recognized a broad 80 kDa band on Western blots of membranes prepared from cells transfected with sst2A receptor cDNA; following receptor membrane deglycosylation, the antibody detected an additional 40 kDa band. In the enteric nervous system, the sst2A antibody primarily stained neurons of the myenteric and submucosal plexuses, and abundant fibers distributed to the muscle, mucosa, and vasculature. Immunoreactive staining was also observed in non-neuronal cells, including presumed interstitial cells of Cajal of the intestine and enterochromaffin-like cells of the stomach. Fibers expressing sst2A receptor immunoreactivity were often in close proximity to D cells of the gastric and intestinal mucosa. Colocalization of somatostatin and sst2A receptor immunoreactivities was not observed in endocrine cells nor in enteric neurons. Double-label immunohistochemistry revealed colocalization of sst2A and vasoactive intestinal peptide immunoreactivities in enteric neurons. The multiple types of cells expressing the sst2A receptor, including enteric neurons and non-neuronal structures, in addition to the relationship between somatostatin and sst2A receptor elements, provide evidence that the sst2A receptor mediates somatostatin effects in the gastrointestinal tract via neuronal and paracrine pathways.