Localization and characterization of neuropeptide receptors in human colon

Cholecystokinin (CCK): a neuromodulator with therapeutic potential in Alzheimer's and Parkinson's disease

Cholecystokinin (CCK) is a neuropeptide modulating digestion, glucose levels, neurotransmitters and memory. Recent studies suggest that CCK exhibits neuroprotective effects in Alzheimer's disease (AD) and Parkinson's disease (PD). Thus, we review the physiological function and therapeutic potential of CCK. The neuropeptide facilitates hippocampal glutamate release and gates GABAergic basket cell activity, which improves declarative memory acquisition, but inhibits consolidation. Cortical CCK alters recognition memory and enhances audio-visual processing. By stimulating CCK-1 receptors (CCK-1Rs), sulphated CCK-8 elicits dopamine release in the substantia nigra and striatum. In the mesolimbic pathway, CCK release is triggered by dopamine and terminates reward responses via CCK-2Rs. Importantly, activation of hippocampal and nigral CCK-2Rs is neuroprotective by evoking AMPK activation, expression of mitochondrial fusion modulators and autophagy. Other benefits include vagus nerve/CCK-1R-mediated expression of brain-derived neurotrophic factor, intestinal protection and suppression of inflammation. We also discuss caveats and the therapeutic combination of CCK with other peptide hormones.

Neuropeptide Y, a paracrine factor secreted by cancer cells, is an independent regulator of angiogenesis in colon cancer

BACKGROUND

Resistance to anti-angiogenic therapies targeting vascular endothelial growth factor-A (VEGF-A) stems from VEGF-A independent angiogenesis mediated by other proangiogenic factors. Therefore identifying these factors in colon adenocarcinoma (CA) will reveal new therapeutic targets.

METHODS

Neuropeptide Y (NPY) and Y2 receptor (Y2R) expressions in CA were studied by immunohistochemical analysis. Orthotopic HT29 with intact VEGF-A gene and VEGF-A knockdown (by CRISPR/Cas9 gene-editing technique) HT29 colon cancer-bearing mice were treated with specific Y2R antagonists, and the effects on angiogenesis and tumour growth were studied. The direct effect of NPY on angiogenesis and the underlying molecular mechanism was elucidated by the modulation of Y2R receptors expressed on colonic endothelial cells (CEC).

RESULTS

The results demonstrated that NPY and Y2R are overexpressed in human CA, orthotopic HT29, and most interestingly in VEGF-A-depleted orthotopic HT29 tumours. Treatment with Y2R antagonists inhibited angiogenesis and thereby HT29 tumour growth. Blocking /silencing Y2R abrogated NPY-induced angiogenic potential of CEC. Mechanistically, NPY regulated the activation of the ERK/MAPK signalling pathway in CEC.

CONCLUSIONS

NPY derived from cancer cells independently regulates angiogenesis in CA by acting through Y2R present on CEC. Targeting NPY/Y2R thus emerges as a novel potential therapeutic strategy in CA.

PDGFRα+ Interstitial Cells are Effector Cells of PACAP Signaling in Mouse and Human Colon

BACKGROUND & AIMS

Platelet-derived growth factor receptor α (PDGFRα)-positive interstitial cells (PIC) are interposed between enteric nerve fibers and smooth muscle cells (SMCs) in the tunica muscularis of the gastrointestinal tract. PIC have robust expression of small conductance Ca²⁺ activated K⁺ channels 3 (SK3 channels) and transduce inhibitory inputs from purinergic and sympathetic nerves in mouse and human colon. We investigated whether PIC also express pituitary adenylate cyclase-activating polypeptide (PACAP) receptors, PAC1 (PAC₁R), and are involved in mediating inhibitory regulation of colonic contractions by PACAP in mouse and human colons.

METHODS

Gene expression analysis, Ca²⁺ imaging, and contractile experiments were performed on mouse colonic muscles. Ca²⁺ imaging, intracellular electrical recordings, and contractile experiments were performed on human colonic muscles.

RESULTS

Adcyap1r1 (encoding PAC₁R) is highly expressed in mouse PIC. Interstitial cells of Cajal (ICC) and SMCs expressed far lower levels of Adcyap1r. Vipr1 and Vipr2 were expressed at low levels in PIC, ICC, and SMCs. PACAP elicited Ca²⁺ transients in mouse PIC and inhibited spontaneous phasic contractions via SK channels. In human colonic muscles, PAC₁R agonists elicited Ca²⁺ transients in PIC, hyperpolarized SMCs through SK channels and inhibited spontaneous phasic contractions.

CONCLUSIONS

PIC of mouse and human colon utilize PAC₁R-SK channel signal pathway to inhibit colonic contractions in response to PACAP. Effects of PACAP are in addition to the previously described purinergic and sympathetic inputs to PIC. Thus, PIC integrate inhibitory inputs from at least 3 neurotransmitters and utilize several types of receptors to activate SK channels and regulate colonic contractile behaviors.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

NPY and Gene Therapy for Epilepsy: How, When,... and Y

Neuropeptide Y (NPY) is a neuropeptide abundantly expressed in the mammalian central and peripheral nervous system. NPY is a pleiotropic molecule, which influences cell proliferation, cardiovascular and metabolic function, pain and neuronal excitability. In the central nervous system, NPY acts as a neuromodulator, affecting pathways that range from cellular (excitability, neurogenesis) to circuit level (food intake, stress response, pain perception). NPY has a broad repertoire of receptor subtypes, each activating specific signaling pathways in different tissues and cellular sub-regions. In the context of epilepsy, NPY is thought to act as an endogenous anticonvulsant that performs its action through Y2 and Y5 receptors. In fact, its overexpression in the brain with the aid of viral vectors can suppress seizures in animal models of epilepsy. Therefore, NPY-based gene therapy may represent a novel approach for the treatment of epilepsy patients, particularly for pharmaco-resistant and genetic forms of the disease. Nonetheless, considering all the aforementioned aspects of NPY signaling, the study of possible NPY applications as a therapeutic molecule is not devoid of critical aspects. The present review will summarize data related to NPY biology, focusing on its anti-epileptic effects, with a critical appraisal of key elements that could be exploited to improve the already existing NPY-based gene therapy approaches for epilepsy.

Effect of Monoclonal Antibody Blockade of Long Fragment Neurotensin on Weight Loss, Behavior, and Metabolic Traits After High-Fat Diet Induced Obesity

BACKGROUND

Obesity is a major public health problem of our time as a risk factor for cardiometabolic disease and the available pharmacological tools needed to tackle the obesity pandemic are insufficient. Neurotensin (NTS) is a 13 amino acid peptide, which is derived from a larger precursor hormone called proneurotensin or Long Form NTS (LF NTS). NTS modulates neuro-transmitter release in the central system nervous, and facilitates intestinal fat absorption in the gastrointestinal tract. Mice lacking LF NTS are protected from high fat diet (HFD) induced obesity, hepatic steatosis and glucose intolerance. In humans, increased levels of LF NTS strongly and independently predict the development of obesity, diabetes mellitus, cardiovascular disease and mortality. With the perspective to develop therapeutic tools to neutralize LF NTS, we developed a monoclonal antibody, specifically inhibiting the function of the LF NTS (LF NTS mAb). This antibody was tested for the effects on body weight, metabolic parameters and behavior in mice made obese by high-fat diet.

METHODS

C57bl/6j mice were subjected to high-fat diet (HFD) until they reached an obesity state, then food was switched to chow. Mice were treated with either PBS (control therapy) or LF NTS mAb at the dose of 5 mg/kg once a week (i.v.). Mice weight, plasma biochemical analysis, fat and muscle size and distribution and behavioral tests were performed during the losing weight period and the stabilization period.

RESULTS

Obese mice treated with the LF NTS mAb lost weight significantly faster than the control treated group. LF NTS mAb treatment also resulted in smaller fat depots, increased fecal cholesterol excretion, reduced liver fat and larger muscle fiber size. Moreover, mice on active therapy were also less stressed, more curious and more active, providing a possible explanation to their weight loss.

CONCLUSION

Our results demonstrate that in mice subjected to HFD-induced obesity, a blockade of LF NTS with a monoclonal antibody results in reduced body weight, adipocyte volume and increased muscle fiber size, possibly explained by beneficial effects on behavior. The underlying mechanisms as well as any future role of LF NTS mAb as an anti-obesity agent warrants further studies.

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.

Xenin-25 induces anion secretion by activating noncholinergic secretomotor neurons in the rat ileum

Xenin-25 is a neurotensin-like peptide that is secreted by enteroendocrine cells in the small intestine. Xenin-8 is reported to augment duodenal anion secretion by activating afferent neural pathways. The intrinsic neuronal circuits mediating the xenin-25-induced anion secretion were characterized using the Ussing-chambered, mucosa-submucosa preparation from the rat ileum. Serosal application of xenin-25 increased the short-circuit current in a concentration-dependent manner. The responses were abolished by the combination of Cl^--free and HCO3- -free solutions. The responses were almost completely blocked by TTX (10^-6 M) but not by atropine (10^-5 M) or hexamethonium (10^-4 M). The selective antagonists for neurotensin receptor 1 (NTSR1), neurokinin 1 (NK1), vasoactive intestinal polypeptide (VIP) receptors 1 and 2 (VPAC1 and VPAC2, respectively), and capsaicin, but not 5-hydroxyltryptamine receptors 3 and 4 (5-HT₃ and 5-HT₄), NTSR2, and A803467, inhibited the responses to xenin-25. The expression of VIP receptors (Vipr) in rat ileum was examined using RT-PCR. The Vipr1 PCR products were detected in the submucosal plexus and mucosa. Immunohistochemical staining showed the colocalization of NTSR1 and NK1 with substance P (SP)- and calbindin-immunoreactive neurons in the submucosal plexus, respectively. In addition, NK1 was colocalized with noncholinergic VIP secretomotor neurons. Based on the results from the present study, xenin-25-induced Cl^-/ HCO3- secretion is involved in NTSR1 activation on intrinsic and extrinsic afferent neurons, followed by the release of SP and subsequent activation of NK1 expressed on noncholinergic VIP secretomotor neurons. Finally, the secreted VIP may activate VPAC1 on epithelial cells to induce Cl^-/ HCO3- secretion in the rat ileum. Activation of noncholinergic VIP secretomotor neurons by intrinsic primary afferent neurons and extrinsic afferent neurons by postprandially released xenin-25 may account for most of the neurogenic secretory response induced by xenin-25. NEW & NOTEWORTHY This study is the first to investigate the intrinsic neuronal circuit responsible for xenin-25-induced anion secretion in the rat small intestine. We have found that nutrient-stimulated xenin-25 release may activate noncholinergic vasoactive intestinal polypeptide (VIP) secretomotor neurons to promote Cl^-/ HCO3- secretion through the activation of VIP receptor 1 on epithelial cells. Moreover, the xenin-25-induced secretory responses are mainly linked with intrinsic primary afferent neurons, which are involved in the activation of neurotensin receptor 1 and neurokinin 1 receptor.

What doesn't kill you makes you stranger: Dipeptidyl peptidase-4 (CD26) proteolysis differentially modulates the activity of many peptide hormones and cytokines generating novel cryptic bioactive ligands

Dipeptidyl peptidase 4 (DPP4) is an exopeptidase found either on cell surfaces where it is highly regulated in terms of its expression and surface availability (CD26) or in a free/circulating soluble constitutively available and intrinsically active form. It is responsible for proteolytic cleavage of many peptide substrates. In this review we discuss the idea that DPP4-cleaved peptides are not necessarily inactivated, but rather can possess either a modified receptor selectivity, modified bioactivity, new antagonistic activity, or even a novel activity relative to the intact parent ligand. We examine in detail five different major DPP4 substrates: glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), peptide tyrosine-tyrosine (PYY), and neuropeptide Y (NPY), and stromal derived factor 1 (SDF-1 aka CXCL12). We note that discussion of the cleaved forms of these five peptides are underrepresented in the research literature, and are both poorly investigated and poorly understood, representing a serious research literature gap. We believe they are understudied and misinterpreted as inactive due to several factors. This includes lack of accurate and specific quantification methods, sample collection techniques that are inherently inaccurate and inappropriate, and a general perception that DPP4 cleavage inactivates its ligand substrates. Increasing evidence points towards many DPP4-cleaved ligands having their own bioactivity. For example, GLP-1 can work through a different receptor than GLP-1R, DPP4-cleaved GIP can function as a GIP receptor antagonist at high doses, and DPP4-cleaved PYY, NPY, and CXCL12 can have different receptor selectivity, or can bind novel, previously unrecognized receptors to their intact ligands, resulting in altered signaling and functionality. We believe that more rigorous research in this area could lead to a better understanding of DPP4's role and the biological importance of the generation of novel cryptic ligands. This will also significantly impact our understanding of the clinical effects and side effects of DPP4-inhibitors as a class of anti-diabetic drugs that potentially have an expanding clinical relevance. This will be specifically relevant in targeting DPP4 substrate ligands involved in a variety of other major clinical acute and chronic injury/disease areas including inflammation, immunology, cardiology, stroke, musculoskeletal disease and injury, as well as cancer biology and tissue maintenance in aging.

The potential role of thyrotropin-releasing hormone in colonic dysmotility induced by water avoidance stress in rats

OBJECTIVE

This study sought to investigate the effect and underlying mechanism of thyrotrophin releasing hormone (TRH) on colonic contractile disorders induced by chronic water avoidance stress (WAS).

METHODS

Male SD rats were exposed to daily 1-h WAS or sham WAS for 10 consecutive days. The presence of TRH in the serum and colonic mucosa were determined using enzyme immunoassay kits. Immunohistochemistry and western blotting were performed to detect the expression of TRH receptor 1 (TRH-R1). The contractions of proximal colonic smooth muscle were studied in an organ bath system. The whole-cell patch-clamp technique was used to record the currents of both L-type calcium currents (I_Ca,L) and large conductance Ca²⁺-activated K⁺ (BK_Ca) channels in colonic smooth muscle cells (SMCs) isolated from adult rats.

RESULTS

Enzyme immunoassay revealed that TRH was present in both serum and colonic mucosa and that this expression increased in the WAS group. Immunohistochemistry revealed that the TRH-R1 level increased in colons devoid of mucosa and submucosa from the stressed rats as compared with the control group. TRH increased the spontaneous contractions of the longitudinal muscle and circular muscle strips in a dose-dependent manner in vitro. The effect was also confirmed in an vivo experiment, where an intraperitoneal injection of TRH in rats significantly increased fecal pellet output during a 24-h period as compared with the control group. Furthermore, intraperitoneal injection of a non-specific TRH receptor antagonist, chlordiazepoxide and a TRH-R1 antibody, partially decreased the fecal pellets of WAS rats during the 10-day stress period. Furthermore, TRH increased the peak current of L-type channels in colonic smooth muscle cells (SMCs) at a membrane potential of 0 mV, while the current of large conductance Ca²⁺-activated K⁺ (BK_Ca) channels was not changed following the addition of TRH.

CONCLUSION

TRH may be involved in the dysmotility induced by chronic stress and may have some potential clinical therapeutic use in regulating gut motility.

Exploiting cancer's phenotypic guise against itself: targeting ectopically expressed peptide G-protein coupled receptors for lung cancer therapy

Lung cancer, claiming millions of lives annually, has the highest mortality rate worldwide. This advocates the development of novel cancer therapies that are highly toxic for cancer cells but negligibly toxic for healthy cells. One of the effective treatments is targeting overexpressed surface receptors of cancer cells with receptor-specific drugs. The receptors-in-focus in the current review are the G-protein coupled receptors (GPCRs), which are often overexpressed in various types of tumors. The peptide subfamily of GPCRs is the pivot of the current article owing to the high affinity and specificity to and of their cognate peptide ligands, and the proven efficacy of peptide-based therapeutics. The article summarizes various ectopically expressed peptide GPCRs in lung cancer, namely, Cholecystokinin-B/Gastrin receptor, the Bombesin receptor family, Bradykinin B1 and B2 receptors, Arginine vasopressin receptors 1a, 1b and 2, and the Somatostatin receptor type 2. The autocrine growth and pro-proliferative pathways they mediate, and the distinct tumor-inhibitory effects of somatostatin receptors are then discussed. The next section covers how these pathways may be influenced or 'corrected' through therapeutics (involving agonists and antagonists) targeting the overexpressed peptide GPCRs. The review proceeds on to Nano-scaled delivery platforms, which enclose chemotherapeutic agents and are decorated with peptide ligands on their external surface, as an effective means of targeting cancer cells. We conclude that targeting these overexpressed peptide GPCRs is potentially evolving as a highly promising form of lung cancer therapy.

Genetics of enteric neuropathies

Abnormal development or disturbed functioning of the enteric nervous system (ENS), the intrinsic innervation of the gastrointestinal tract, is associated with the development of neuropathic gastrointestinal motility disorders. Here, we review the underlying molecular basis of these disorders and hypothesize that many of them have a common defective biological mechanism. Genetic burden and environmental components affecting this common mechanism are ultimately responsible for disease severity and symptom heterogeneity. We believe that they act together as the fulcrum in a seesaw balanced with harmful and protective factors, and are responsible for a continuum of symptoms ranging from neuronal hyperplasia to absence of neurons.

Neurotensin Changes Propulsive Activity into a Segmental Motor Pattern in the Rat Colon

BACKGROUND/AIMS

Neurotensin is a gut-brain peptide with both inhibitory and excitatory actions on the colonic musculature; our objective was to understand the implications of this for motor patterns occurring in the intact colon of the rat.

METHODS

The effects of neurotensin with concentrations ranging from 0.1-100 nM were studied in the intact rat colon in vitro, by investigating spatio-temporal maps created from video recordings of colonic motility before and after neurotensin.

RESULTS

Low concentration of neurotensin (0.1-1 nM) inhibited propagating long distance contractions and rhythmic propagating motor complexes; in its place a slow propagating rhythmic segmental motor pattern developed. The neurotensin receptor 1 antagonist SR-48692 prevented the development of the segmental motor pattern. Higher concentrations of neurotensin (10 nM and 100 nM) were capable of restoring long distance contraction activity and inhibiting the segmental activity. The slow propagating segmental contraction showed a rhythmic contraction-- relaxation cycle at the slow wave frequency originating from the interstitial cells of Cajal associated with the myenteric plexus pacemaker. High concentrations given without prior additions of low concentrations did not evoke the segmental motor pattern. These actions occurred when neurotensin was given in the bath solution or intraluminally. The segmental motor pattern evoked by neurotensin was inhibited by the neural conduction blocker lidocaine.

CONCLUSIONS

Neurotensin (0.1-1 nM) inhibits the dominant propulsive motor patterns of the colon and a distinct motor pattern of rhythmic slow propagating segmental contractions develops. This motor pattern has the hallmarks of haustral boundary contractions.

Mucosal layers and related nerve fibres in non-chagasic and chagasic human colon--a quantitative immunohistochemical study

Chagasic megacolon is accompanied by extensive myenteric and, simultaneously, moderate submucosal neuron loss. Here, we examined changes of the innervation pattern of the lamina propria (LP) and muscularis mucosae (MM). Two alternating sets of cryosections were taken from seven non-chagasic colonic and seven chagasic megacolonic specimens (the latter included both the dilated megacolonic and the non-dilated transitional oral and anal zones) and were immunohistochemically triple-stained for smooth-muscle actin (SMA), synaptophysin (SYN) and glial acid protein S100 and, alternatively, for SMA, vasoactive intestinal peptide (VIP) and somatostatin (SOM). Subsequent image analysis and statistical evaluation of nervous tissue profile areas revealed that, in LP, the most extreme differences (i.e. increase in thickness or decrease in nerve, glia and muscle tissue profile area, respectively) compared with control values occurred in the dilated megacolonic zone itself. In contrast, the most extreme differences in the MM were in the anal-to-megacolonic zone (except the profile area of muscle tissue, which was lowest in the megacolonic zone). This parallels our previous results in the external muscle coat. A partial and selective survival of VIP-immunoreactive in contrast to SOM-immunoreactive nerve fibres was observed in both mucosal layers investigated. Thus, VIPergic nerve elements might be crucial for the maintenance of the mucosal barrier. The differential changes of neural tissue parameters in LP and MM might reflect a multifactorial rather than a pure neurogenic development of megacolon in chronic Chagas' disease.

Mutations in arrestin-3 differentially affect binding to neuropeptide Y receptor subtypes

Based on the identification of residues that determine receptor selectivity in arrestins and the phylogenetic analysis of the arrestin (arr) family, we introduced fifteen mutations of receptor-discriminator residues in arr-3, which were identified previously using mutagenesis, in vitro binding, and BRET-based recruitment assay in intact cells. The effects of these mutations were tested using neuropeptide Y receptors Y1R and Y2R. NPY-elicited arr-3 recruitment to Y1R was not affected by these mutations, or even alanine substitution of all ten residues (arr-3-NCA), which prevented arr-3 binding to other receptors tested so far. However, NCA and two other mutations prevented agonist-independent arr-3 pre-docking to Y1R. In contrast, eight out of 15 mutations significantly reduced agonist-dependent arr-3 recruitment to Y2R. NCA eliminated arr-3 binding to active Y2R, whereas Tyr239Thr reduced it ~7-fold. Thus, manipulation of key residues on the receptor-binding surface generates arr-3 with high preference for Y1R over Y2R. Several mutations differentially affect arr-3 pre-docking and agonist-induced recruitment. Thus, arr-3 recruitment to the receptor involves several mechanistically distinct steps. Targeted mutagenesis can fine-tune arrestins directing them to specific receptors and particular activation states of the same receptor.

177Lu-labeled RGD-BBN heterodimeric peptide for targeting prostate carcinoma

INTRODUCTION

Radiolabeled Arg-Gly-Asp (RGD) and bombesin (BBN) heterodimers have been investigated for dual targeting of tumor integrin αvβ3 receptors and gastrin-releasing peptide receptors. The goal of this study was to evaluate the potential use of a Lu-labeled RGD-BBN heterodimer for targeted prostate cancer therapy.

MATERIALS AND METHODS

A 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-conjugated RGD-BBN peptide (DO3A-RGD-BBN) was radiolabeled with Lu and purified by high-performance liquid chromatography. The in-vivo biodistribution study of Lu-DO3A-RGD-BBN was carried out in mice bearing human prostate cancer PC3 xenografts. The receptor-targeting specificity of the radiolabeled peptide was assayed by injecting the tracer with the unlabeled RGD-BBN peptide. Radiation absorbed doses in adult male patients, based on biodistribution data from mice, were also calculated.

RESULTS

DO3A-RGD-BBN peptides were successfully labeled with Lu, and high radiochemical purity (>95%) could be achieved after high-performance liquid chromatography purification. In human PC3 xenograft-bearing mice, the tumor accumulation of Lu-DO3A-RGD-BBN was 5.88±1.12, 2.77±0.30, 2.04±0.19, and 1.18±0.19%ID/g at 0.5, 2, 24, and 48 h, respectively. With rapid clearance from normal tissues, the radiolabeled probe displayed high tumor-to-blood and tumor-to-muscle ratios. On calculating the radiation absorbed doses for Lu-DO3A-RGD-BBN, we found that the prostate tumor and the pancreas were the organs receiving the highest radiation absorbed doses.

CONCLUSION

Dual integrin αvβ3 and GPRP-targeted agent Lu-DO3A-RGD-BBN shows excellent prostate cancer-targeting ability, and it is worthy of further evaluation for prostate cancer-targeted therapy.

Myogenic regional responsiveness to cholinergic and vipergic stimulation in human colon

BACKGROUND

Differences in the actions of enteric neurotransmitters on colonic circular and longitudinal muscle layers have not been clearly determined, nor the possible existence of intrinsic myogenic phenotypes that might contribute to regional differences in human colon motor activity. The aim of this study was to analyze the direct pharmaco-mechanical coupling of carbachol (CCh) and vasoactive intestinal polypeptide (VIP) on human colonic smooth muscle strips and cells.

METHODS

Circular and longitudinal muscle strips and cells were obtained from 15 human specimens of ascending and sigmoid colon. Both isometric tension on muscle strips and contraction and relaxation on cells were measured in response to increasing CCh and VIP concentrations.

KEY RESULTS

Circular muscle strips of ascending colon were more sensitive to the effect of CCh than that of sigmoid colon, EC(50) values being, respectively, 4.15μmolL(-1) and 8.47μmolL(-1) (P<0.05), although there were no differences in maximal responses. No regional differences were observed in longitudinal muscle strips or in smooth muscle cells. Maximal responses to CCh were higher on circular than longitudinal muscle strips and cells throughout the colon. A greater sensitivity to VIP was observed in ascending colon compared with sigmoid colon, both in circular (EC(50:) 0.041 and 0.15μmolL(-1) , respectively, P<0.01) and longitudinal (EC(50:) 0.043 and 0.09μmolL(-1) , respectively, P<0.05) strips, and similar differences were observed in longitudinal smooth muscle cells (EC(50:) 44.85 and 75.24nmolL(-1) , respectively, P<0.05).

CONCLUSIONS & INFERENCES

Regional myogenic differences in pharmaco-mechanical coupling between the enteric neurotransmitters and smooth muscle contribute to the complex regional motor patterns of human colon.

Plasma hormones facilitated the hypermotility of the colon in a chronic stress rat model. PLoS One. 2012;7:e31774. [PMID: 22363728 PMCID: PMC3282740 DOI: 10.1371/journal.pone.0031774]

Objective

To study the relationship between brain-gut peptides, gastrointestinal hormones and altered motility in a rat model of repetitive water avoidance stress (WAS), which mimics the irritable bowel syndrome (IBS).

Methods

Male Wistar rats were submitted daily to 1-h of water avoidance stress (WAS) or sham WAS (SWAS) for 10 consecutive days. Plasma hormones were determined using Enzyme Immunoassay Kits. Proximal colonic smooth muscle (PCSM) contractions were studied in an organ bath system. PCSM cells were isolated by enzymatic digestion and IKv and IBKca were recorded by the patch-clamp technique.

Results

The number of fecal pellets during 1 h of acute restraint stress and the plasma hormones levels of substance P (SP), thyrotropin-releasing hormone (TRH), motilin (MTL), and cholecystokinin (CCK) in WAS rats were significantly increased compared with SWAS rats, whereas vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP) and corticotropin releasing hormone (CRH) in WAS rats were not significantly changed and peptide YY (PYY) in WAS rats was significantly decreased. Likewise, the amplitudes of spontaneous contractions of PCSM in WAS rats were significantly increased comparing with SWAS rats. The plasma of WAS rats (100 µl) decreased the amplitude of spontaneous contractions of controls. The IKv and IBKCa of PCSMs were significantly decreased in WAS rats compared with SWAS rats and the plasma of WAS rats (100 µl) increased the amplitude of IKv and IBKCa in normal rats.

Conclusion

These results suggest that WAS leads to changes of plasma hormones levels and to disordered myogenic colonic motility in the short term, but that the colon rapidly establishes a new equilibrium to maintain the normal baseline functioning.

Neurotensin regulates cortical glutamate transmission by modulating N-methyl-D-aspartate receptor functional activity: an in vivo microdialysis study

The aim of the present in vivo microdialysis study was to investigate whether the tridecapeptide neurotensin (NT) influences the N-methyl-D-aspartate (NMDA) receptor-mediated increase of cortical glutamate transmission in freely moving rats. Intracortical perfusion with NT influenced local extracellular glutamate levels in a bell-shaped, concentration-dependent manner. One hundred and three hundred nanomolar NT concentrations increased glutamate levels (151% ± 7% and 124% ± 3% of basal values, respectively). Higher (1,000 nM) and lower (10 nM) NT concentrations did not alter extracellular glutamate levels. The NT receptor antagonist SR48692 (100 nM) prevented the NT (100 nM)-induced increase in glutamate levels. NMDA (100 and 500 μM) perfusion induced a concentration-dependent increase in extracellular glutamate levels, the lower 10 μM NMDA concentration being ineffective. When NT (10 nM, a concentration by itself ineffective) was added in combination with NMDA (100 μM) to the perfusion medium, a significant greater increase in extracellular glutamate levels (169% ± 7%) was observed with respect to the increase induced by NMDA (100 μM) alone (139% ± 4%). SR48692 (100 nM) counteracted the increase in glutamate levels induced by the treatment with NT (10 nM) plus NMDA (100 μM). The enhancement of cortical glutamate levels induced by NMDA (100 and 500 μM) was partially antagonized by the presence of SR48692, at a concentration (100 nM) that by itself was ineffective in modulating glutamate release. These findings indicate that NT plays a relevant role in the regulation of cortical glutamatergic transmission, especially by modulating the functional activity of cortical NMDA receptors. A possible role in glutamate-mediated neurotoxicity is suggested.

Role of tachykinin receptors in the modulation of colonic peristaltic activity in mice

Tachykinins are important mediators of neuroneuronal and neuromuscular transmission in the gastrointestinal tract, however their contribution to colonic peristalsis in mice remains unclear. Therefore, our aim was to characterise the functional role of tachykinins in mediating peristalsis by evaluating the effect of selective tachykinin NK(1), NK(2) and NK(3) receptor agonists and antagonists on in vitro colonic peristaltic activity in mice. Using a modified Trendelenburg set-up, gradual distension of proximal and distal colonic segments evoked rhythmic, aborally migrating contractions. Peristaltic activity was assessed by quantifying the amplitude and interval of the corresponding pressure waves. Stimulation of NK(1) receptors showed regional differences as both the pressure amplitude and interval were enhanced in the distal colon without affecting peristalsis proximally. Blockade of NK(1) receptors reduced the peristaltic pressure amplitude in the proximal and distal colon while the interval was not significantly altered. NK(2) receptor stimulation resulted in a modest enhancement of the amplitude in proximal and distal segments and a slightly prolonged interval distally. Blockade of NK(2) receptors reduced the peristaltic pressure amplitude and interval in the distal colon. NK(3) receptor stimulation significantly augmented the amplitude in both segments and prolonged the interval distally. However, NK(3) receptor blockade had no effect on peristaltic activity. In conclusion, tachykinins contribute to colonic peristalsis in mice by acting mainly on NK(1) and NK(2) receptors and their effects show a proximal-to-distal gradient. NK(3) receptors might play a role in conditions of excess tachykinin release but appear not to be involved under the conditions of the present study.

Mechanism of action of cholecystokinin on colonic motility in isolated, vascularly perfused rat colon

BACKGROUND/AIMS

It is generally believed that cholecystokinin (CCK) stimulates colonic motility, although there are controversial reports. It has also been suggested that postprandial peptide YY (PYY) release is CCK-dependent. Using a totally isolated, vascularly perfused rat colon, we investigated: (1) the roles of CCK and PYY on colonic motility, (2) to determine if CCK modulates PYY release from the colon to influence the motility and (3) to clarify whether the action of CCK and PYY on colonic motility is mediated via the influence of cholinergic input.

METHODS

An isolated whole rat colon was used. Luminal pressure was monitored via microtip catheter pressure transducers from proximal and distal colon. After a control period, CCK-8 or PYY was administerd intraarterially with or without an anti-PYY serum, loxiglumide or atropine at 12, 60 and 240 pM. Each dose was given for a period of 15-minute and the contractile response was expressed as % changes over basal. PYY concentration in the portal effluent was determined by radioimmunoassay.

RESULTS

Exogenous CCK-8 increased colonic motility which paralleled the increase in PYY release in the portal effluent. Exogenous PYY also significantly increased colonic motility although it was less potent than CCK. The stimulating effect of CCK-8 was significantly inhibited by an anti-PYY serum, and was completely abolished by loxiglumide, and almost completely abolished by atropine.

CONCLUSIONS

CCK increases colonic motility via CCK(1) receptor and it is mediated partly by PYY. Cholinergic input is required for the increased motility by either PYY or CCK.

The role(s) of somatostatin, structurally related peptides and somatostatin receptors in the gastrointestinal tract: a review

Extensive functional and morphological research has demonstrated the pivotal role of somatostatin (SOM) in the regulation of a wide variety of gastrointestinal activities. In addition to its profound inhibitory effects on gastrointestinal motility and exocrine and endocrine secretion processes along the entire gastrointestinal tract, SOM modulates several organ-specific activities. In contrast to these well-known SOM-dependent effects, knowledge on the SOM receptors (SSTR) involved in these effects is much less conclusive. Experimental data on the identities of the SSTRs, although species- and tissue-dependent, point towards the involvement of multiple receptor subtypes in the vast majority of gastrointestinal SOM-mediated effects. Recent evidence demonstrating the role of SOM in intestinal pathologies has extended the interest of gastrointestinal research in this peptide even further. More specifically, SOM is supposed to suppress intestinal inflammatory responses by interfering with the extensive bidirectional communication between mucosal mast cells and neurons. This way, SOM not only acts as a powerful inhibitor of the inflammatory cascade at the site of inflammation, but exerts a profound antinociceptive effect through the modulation of extrinsic afferent nerve fibres. The combination of these physiological and pathological activities opens up new opportunities to explore the potential of stable SOM analogues in the treatment of GI inflammatory pathologies.

In vitro binding evaluation of 177Lu-AMBA, a novel 177Lu-labeled GRP-R agonist for systemic radiotherapy in human tissues

Members of the gastrin-releasing peptide (GRP) family and its analogs bombesin (BBN) have been implicated in the biology of several human cancers including prostate, breast, colon and lung. To date, three mammalian GRP/BBN receptor subtypes have been cloned and characterized: the neuromedin B receptor (NMBR), the GRP receptor (GRPR) and the BBN-receptor subtype 3 (BB(3)). The fourth BBN receptor subtype, BB(4), has only been identified in amphibian and at present no mammalian equivalent of this receptor has been described. GRPR analogs have been used as carriers to deliver drugs, radionuclides and cytotoxins to target various cancer types that are GRPR positive. We investigated the in vitro binding properties of (177)Lu-AMBA, a novel radiolabelled BBN analog currently undergoing clinical trial as systemic radiotherapy for hormone refractory prostate cancer (HRPC) patients. Pharmacological analyses of the (177)Lu-AMBA was determined using in vitro binding studies using membrane target system containing specific receptor subtypes. We investigated the distribution of binding sites for (177)Lu-AMBA by receptor autoradiography on human neoplastic and non-neoplastic tissues. Pharmacological characterizations of (177)Lu-AMBA shows, high affinity towards NMB and GRP receptors, while little or no affinity towards BB(3) receptor. Among the 40 different types of non-neoplastic tissues tested seven of them showed limited but specific binding of (177)Lu-AMBA. Fourteen of 17 primary prostate cancers, six of 13 primary breast cancers expressed binding sites for (177)Lu-AMBA. Furthermore, no apparent differences in (177)Lu-AMBA-binding sites expression were observed between matched pairs (primary vs. secondary) of prostate and breast cancer tissues. These data represent the molecular basis for clinical applications of (177)Lu-AMBA for diagnosis and treatment of GRP-R and NMB-R positive tumors.

Five Stabilized 111In-labeled neurotensin analogs in nude mice bearing HT29 tumors

Neurotensin (NT) receptors are overexpressed in different human tumors, such as human ductal pancreatic adenocarcinoma. New stable neurotensin analogs with high receptor affinity have been synthesized by replacing arginine residues with lysine and arginine derivatives. The aim of this study was to explore the biodistribution, tumor uptake, kidney localization, and stability characteristics of these new analogs in order to develop new diagnostic tools for exocrine pancreatic cancer. Four (111)In-labeled DTPA-chelated NT analogs and one (111)In-labeled DOTA-chelated NT analog were evaluated in NMRI nude mice bearing NT receptor-positive HT29 tumors. Experiments with a coinjection of unlabeled NT or lysine were performed to investigate receptor-mediated uptake and kidney protection, respectively. In addition, the in vivo serum stability of the most promising analog was analyzed. In the biodistribution study in mice, at 4 hours postinjection, a low percentage of the injected dose per gram (%ID/g) of tissue for all compounds was found in NT receptor-negative organs, such as the blood, spleen, pancreas, liver, muscle, and femur. A high uptake was found in the colon, intestine, kidneys, and in implanted HT29 tumors. The coinjection of excess unlabeled neurotensin significantly reduced tumor uptake, showing tumor uptake to be receptor-mediated. To a lesser extent, this was also observed for the colon, but not for other tissues. We concluded that DTPA-(Pip)Gly-Pro-(PipAm)Gly-Arg-Pro-Tyr-tBuGly-Leu-OH and the DOTA-linked counterpart have the most favorable biodistribution properties regarding tumor uptake.

Decrease in binding for the neuropeptide VIP in response to marked inflammation of the mucosa in ulcerative colitis

The neuropeptide vasoactive intestinal peptide (VIP) is involved in the neuroimmunomodulation of the intestine. In the present study, specimens from the sigmoid colon of ulcerative colitis (UC) and non-UC patients were examined for immunohistochemistry and in vitro receptor autoradiography. Marked occurrence of VIP binding was observed in the mucosa. However, there were very low levels of binding in areas showing pronounced inflammation/derangement. The study shows that marked derangement of the mucosa leads to a distinct decrease in VIP binding. Thus, it is possible that a decrease in trophic and anti-inflammatory VIP effects occurs in areas exhibiting a very marked inflammation.

Novel 111In-labelled bombesin analogues for molecular imaging of prostate tumours

PURPOSE

It has been shown that some primary human tumours and their metastases, including prostate and breast tumours, overexpress gastrin-releasing peptide (GRP) receptors. Bombesin (BN) is a neuropeptide with a high affinity for these GRP receptors. We demonstrated successful scintigraphic visualisation of BN receptor-positive tumours in preclinical studies using the radiolabelled BN analogue [(111)In-DTPA-Pro(1),Tyr(4)]BN. However, the receptor affinity as well as the serum stability of this analogue leave room for improvement. Therefore new (111)In-labelled BN analogues were synthesised and evaluated in vitro and in vivo.

METHODS AND RESULTS

The receptor affinity of the new BN analogues was tested on human GRP receptor-expressing prostate tumour xenografts and rat colon sections. Analogues with high receptor affinity (low nM range) were selected for further evaluation. Incubation in vitro of GRP receptor-expressing rat CA20948 and human PC3 tumour cells with the (111)In-labelled analogues resulted in rapid receptor-mediated uptake and internalisation. The BN analogue with the best receptor affinity and in vitro internalisation characteristics, Cmp 3 ([(111)In-DTPA-ACMpip(5),Tha(6),betaAla(11),Tha(13),Nle(14)]BN(5-14)), was tested in vivo in biodistribution studies using rats bearing GRP receptor-expressing CA20948 tumours, and nude mice bearing human PC3 xenografts. Injection of (111)In-labelled Cmp 3 in these animals showed high, receptor-mediated uptake in receptor-positive organs and tumours which could be visualised using planar gamma camera and microSPECT/CT imaging.

CONCLUSION

With their enhanced receptor affinity and their rapid receptor-mediated internalisation in vitro and in vivo, the new BN analogues, and especially Cmp 3, are promising candidates for use in diagnostic molecular imaging and targeted radionuclide therapy of GRP receptor-expressing cancers.

Effect of intestinal inflammation on the cell-specific expression of somatostatin receptor subtypes in the murine ileum

Despite our knowledge of somatostatin (SOM) in gastrointestinal functions, little information is available on the SOM receptors (SSTRs) mediating these effects. This study focussed on the expression of SSTRs in non-inflamed and Schistosoma mansoni-infected murine ileum using immunocytochemistry, reverse transcriptase (RT)-PCR and quantitative real time RT-PCR (qPCR). In the non-inflamed ileum, SSTRs showed a widespread, cell-type specific expression pattern. For instance, SSTR2A immunoreactivity was detected in a minor population of submucous but not myenteric glial cells. In the inflamed ileum, significant changes in the expression pattern of SSTRs occurred, with SSTR1 and SSTR3 expression on mucosal mast cells (MMCs) and mucosal nerve fibres. SSTR4-immunoreactive nerve fibres were detected in granulomas and the lamina propria. qPCR experiments indicated significantly increased mRNA levels for SOM, SSTR1 and SSTR3 in inflamed ileum. This study reveals that SSTRs are expressed in specific cell types in murine ileum. Expression of SSTR1 and SSTR3 on MMCs and increased density of SOM-expressing nerve fibres in the lamina propria during inflammation, support the hypothesis that SOM is implicated in the physiological control of MMCs during intestinal inflammation. Evidence is provided that in mouse mainly SSTR1, SSTR3 and SSTR4 are involved in the somatostatinergic inflammatory effects during intestinal schistosomiasis.

[99mTc]Demotensin 5 and 6 in the NTS1-R-targeted imaging of tumours: synthesis and preclinical results

PURPOSE

The aim of this study was to evaluate the applicability of [(99m)Tc]Demotensin 5 and 6 [Formula: see text] in the targeted diagnostic imaging of neurotensin subtype 1 receptor (NTS1-R)-expressing tumours.

METHODS

Labelling of Demotensin 5 and 6 with (99m)Tc was conducted by brief incubation with (99m)TcO(4) (-), SnCl(2) and citrate anions in alkaline medium at ambient temperature. Affinities of conjugates for the NTS1-R were determined by competition binding experiments in WiDr cell membranes using [(125)I-Tyr(3)]NT as the radioligand. Saturation binding assays were conducted for [(99m)Tc/(99g)Tc]Demotensin 6 in WiDr cell membranes. Internalisation of [(99m)Tc]Demotensin 5 and 6 was studied at 37 degrees C in WiDr cells. Biodistribution of [(99m)Tc]Demotensin 5 and 6 was performed in female Swiss nu/nu mice bearing human WiDr xenografts.

RESULTS

Unlabelled conjugates showed a high affinity for the human NTS1-R (Demotensin 5 IC(50)=0.03+/-0.01 nM; Demotensin 6 IC(50)=0.08+/-0.02 nM), while high affinity was also exhibited by (radio)metallated [(99m)Tc/(99g)Tc]Demotensin 6 (K (d)=0.13+/-0.01 nM). [(99m)Tc]Demotensin 5 and 6 internalised rapidly and specifically in WiDr cells. After injection in WiDr tumour-bearing mice, radiopeptides, and especially the doubly stabilised [(99m)Tc]Demotensin 6, showed NTS1-R-mediated uptake in the intestines and in the implanted tumour (4.30+/-0.45%ID/g at 1 h post injection) and rapid renal excretion from non-target tissues into the urine.

CONCLUSION

[(99m)Tc]Demotensin 6 shows a favourable preclinical profile and further testing in patients is warranted to monitor its eventual applicability as a radiotracer in the diagnostic imaging of NTS1-R-positive tumours.

Cholecystokinin CCK2 receptors mediate the peptide's inhibitory actions on the contractile activity of human distal colon via the nitric oxide pathway

BACKGROUND AND PURPOSE

Cholecystokinin is known to exert stimulant actions on intestinal motility via activation of type 1 cholecystokinin receptors (CCK(1)). However, the role played by cholecystokinin 2 (CCK(2)) receptors in the regulation of gut motility remains undetermined. This study was designed to examine the influence of CCK(2) receptors on the contractile activity of human distal colon.

EXPERIMENTAL APPROACH

The effects of compounds acting on CCK(2) receptors were assessed in vitro on motor activity of longitudinal smooth muscle, under basal conditions as well as in the presence of KCl-induced contractions or transmural electrical stimulation.

KEY RESULTS

Cholecystokinin octapeptide sulphate induced concentration-dependent contractions which were enhanced by GV150013 (CCK(2) receptor antagonist; +57% at 0.01 microM). These effects were unaffected by tetrodotoxin. The enhancing actions of GV150013 on contractions evoked by cholecystokinin octapeptide sulphate were unaffected by N(omega)-propyl-L-arginine (NPA, neuronal nitric oxide synthase inhibitor), while they were prevented by N(omega)-nitro-L-arginine methylester (L-NAME, non-selective nitric oxide synthase inhibitor). In the presence of KCl-induced contractions, cholecystokinin octapeptide sulphate elicited concentration-dependent relaxations (-36%), which were unaffected by NPA, but were counteracted by GV150013 or L-NAME. The application of electrical stimuli evoked phasic contractions which were enhanced by GV150013 (+41 % at 0.01 microM).

CONCLUSIONS AND IMPLICATIONS

CCK(2) receptors mediate inhibitory actions of cholecystokinin on motor activity of human distal colon. It is suggested that CCK(2) receptors exert their modulating actions through a nitric oxide pathway, independent of the activity of the neuronal nitric oxide synthase isoform.

Role of CCK/gastrin receptors in gastrointestinal/metabolic diseases and results of human studies using gastrin/CCK receptor agonists/antagonists in these diseases

In this paper, the established and possible roles of CCK1 and CCK2 receptors in gastrointestinal (GI) and metabolic diseases are reviewed and available results from human agonist/antagonist studies are discussed. While there is evidence for the involvement of CCK1R in numerous diseases including pancreatic disorders, motility disorders, tumor growth, regulation of satiety and a number of CCK-deficient states, the role of CCK1R in these conditions is not clearly defined. There are encouraging data from several clinical studies of CCK1R antagonists in some of these conditions, but their role as therapeutic agents remains unclear. The role of CCK2R in physiological (atrophic gastritis, pernicious anemia) and pathological (Zollinger-Ellison syndrome) hypergastrinemic states, its effects on the gastric mucosa (ECL cell hyperplasia, carcinoids, parietal cell mass) and its role in acid-peptic disorders are clearly defined. Furthermore, recent studies point to a possible role for CCK2R in a number of GI malignancies. Current data from human studies of CCK2R antagonists are presented and their potential role in the treatment of these conditions reviewed. Furthermore, the role of CCK2 receptors as targets for medical imaging is discussed.

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.

CCK2 receptors mediate inhibitory effects of cholecystokinin on the motor activity of guinea-pig distal colon

Cholecystokinin and related peptides are involved in the control of intestinal motility and cholecystokinin receptor ligands might represent new pharmacological tools for the treatment of symptoms associated with functional bowel disorders. However, the respective roles played by cholecystokinin receptor subtypes and the mechanisms underlying these regulatory actions remain undetermined. This study was designed to examine the influence of cholecystokinin receptor subtypes on the motor activity of guinea-pig distal colon. The effects of drugs acting on CCK1 and CCK2 receptors were assessed in vitro on the contractile activity of longitudinal smooth muscle, both under basal conditions and in the presence of transmural electrical stimulation or KCl-induced contractions. The application of cholecystokinin octapeptide sulphate (cholecystokinin-8S) to colonic preparations induced concentration-dependent contractions which were prevented by devazepide (CCK1 receptor antagonist), enhanced by GV150013 (CCK2 receptor antagonist) or N(omega)-nitro-L-arginine methylester (L-NAME, nitric oxide synthase inhibitor), and unaffected by tetrodotoxin. The application of gastrin-17 to colonic preparations resulted in relaxant responses which were insensitive to devazepide, and prevented by GV150013, L-NAME or tetrodotoxin. L-NAME, N(omega)-propyl-L-arginine (NPA, neuronal nitric oxide synthase inhibitor) or GV150013 enhanced electrically evoked contractile responses, whereas devazepide did not. When tested in the presence of L-NAME or NPA the enhancing effect of GV150013 on electrically induced contractions no longer occurred. In the presence of KCl-induced pre-contractions, cholecystokinin-8S or gastrin-17 evoked concentration-dependent relaxations, which were unaffected by devazepide and were counteracted by GV150013, L-NAME, NPA or tetrodotoxin. In conclusion, the present results indicate that, at level of distal colon, CCK1 receptors mediate direct contractile effects on smooth muscle, whereas CCK2 receptors on enteric neurons mediate relaxant responses via nitric oxide release.

Toward stable N4-modified neurotensins for NTS1-receptor-targeted tumor imaging with 99mTc

A series of Gly-neurotensin(8-13) analogues modified at the N-terminus by acyclic tetraamines (Demotensin 1-4) were obtained by solid-phase peptide synthesis techniques. Strategic replacement of amino acids and/or reduction of sensitive peptide bonds were performed to enhance conjugate resistance against proteolytic enzymes. During 99mTc-labeling, single species radiopeptides, [99mTc]Demotensin 1-4, were easily obtained in high yields and typical specific activities of 1 Ci/micromol. Peptide conjugates displayed a high affinity binding to the human neurotensin subtype 1 receptor (NTS1-R) expressed in colon adenocarcinoma HT-29 or WiDr cells and/or in human tumor sections. [99mTc]Demotensin 1-4 internalized very rapidly in HT-29 or WiDr cells by a NTS1-R-mediated process. [99mTc]Demotensin 3 and 4, which remained stable during 1 h incubation in murine plasma, were selectively studied in nude mice bearing human HT-29 and WiDr xenografts. After injection, [99mTc]Demotensin 3 and 4 effectively and specifically localized in the experimental tumors and were rapidly excreted via the kidneys into the urine, exhibiting overall biodistribution patterns favorable for NTS1-R-targeted tumor imaging in man.

Selective in vitro targeting of GRP and NMB receptors in human tumours with the new bombesin tracer 177Lu-AMBA

PURPOSE

To investigate the in vitro binding properties of a novel radiolabelled bombesin analogue, (177)Lu-AMBA, in human neoplastic and non-neoplastic tissues selected for their expression of the bombesin receptor subtypes GRP-R, NMB-R and BRS-3.

METHODS

In vitro receptor autoradiography was performed in cancers expressing the various bombesin receptor subtypes. The novel radioligand (177)Lu-AMBA was used and compared with established bombesin radioligands such as (125)I-Tyr(4)-bombesin and (125)I-[D-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]-bombesin(6-14). In vitro incidence of detection of each of the three bombesin receptor subtypes was evaluated in each tumour.

RESULTS

(177)Lu-AMBA identified all GRP-R-expressing tumours, such as prostatic, mammary and renal cell carcinomas as well as gastrointestinal stromal tumours. (177)Lu-AMBA also identified all NMB-expressing tumours, but did not detect BRS-3-expressing tumours or BRS-3-expressing pancreatic islets. GRP-R-expressing peritumoral vessels were heavily labelled with (177)Lu-AMBA. In contrast to the strongly GRP-R-positive mouse pancreas, the human pancreas was not labelled with (177)Lu-AMBA unless chronic pancreatitis was diagnosed. In general, the sensitivity was slightly better with (177)Lu-AMBA than with the conventional bombesin radioligands.

CONCLUSION

The present in vitro study suggests that (177)Lu-AMBA may be a very useful in vivo targeting agent for GRP-R-expressing tumours, NMB-R-expressing tumours and GRP-R-expressing neoangiogenic vessels.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Enzyme-based visualization of receptor-ligand binding in tissues

New methods of elucidating the ligand-binding activity of receptors could improve our understanding of receptor function, key events they control, and their presence in normal and pathological states. We describe a method for visualizing receptor-ligand binding in cells and tissues that substitutes fluorescein for radioactive labels, and detects receptor bound, fluoresceinated ligand with an antifluorescein/horseradish peroxidase amplification system. Receptor-bound ligand is then visualized by light microscopy against a standard hemotoxylin-stained background of cell structure. Quantitative versions of the assay provide an apparent dissociation constant and number of receptors per cell at saturation in cell or tissue specimens. Receptors examined include the folate receptor, bombesin peptide-binding receptors, the epidermal growth factor receptor, the neuropeptide Y receptor, the asialoglycoprotein receptor, and RGD peptide-binding integrins. Using fluoresceinated versions of molecules, we show the method can visualize and quantitate receptor-bound ligands in cell culture monolayers and animal tissue specimens. Ligand binding to receptors present in tissues was visualized in normal and pathological samples of human tissue microarrays. The enzyme-amplified detection of receptor-bound fluoresceinated ligand is a simple and nonradioactive-based method that provides information on the receptor activity in tissue specimens.

Progastrin-releasing peptide and gastrin-releasing peptide receptor mRNA expression in non-tumor tissues of the human gastrointestinal tract

AIM: To investigate the expression of gastrin-releasing peptide (GRP) and GRP-receptor mRNA in non-tumor tissues of the human esophagus, gastrointestinal tract, pancreas and gallbladder using molecular biology techniques.

METHODS: Poly A⁺ mRNA was isolated from total RNA extracts using an automated nucleic acid extractor and, subsequently, converted into single-stranded cDNA (ss-cDNA). PCR amplifications were carried out using gene-specific GRP and GRP-receptor primers. The specificity of the PCR amplicons was further confirmed by Southern blot analyses using gene-specific GRP and GRP-receptor hybridization probes.

RESULTS: Expression of GRP and GRP-receptor mRNA was detected at various levels in nearly all segments of the non-tumor specimens analysed, except the gallbladder. In most of the biopsy specimens, co-expression of both GRP and GRP-receptor mRNA appeared to take place. However, expression of GRP mRNA was more prominent than was GRP-receptor mRNA.

CONCLUSION: GRP and GRP-receptor mRNAs are expressed throughout the gastrointestinal tract and provides information for the future mapping and determination of its physiological importance in normal and tumor cells.

Tachykinins and tachykinin receptors in the gut, with special reference to NK2 receptors in human

Tachykinins (TKs), substance P (SP), neurokinin A (NKA) and B (NKB) are important peptide modulators of intestinal motility in animal species studied so far, including humans. Modulation of motility by TKs can occur at various levels, since these peptides are expressed in cholinergic excitatory motor neurons projecting to both circular and longitudinal muscle, interneurons, and intramural and extramural sensory neurons. The effects of SP, NKA and NKB are preferentially mediated through the stimulation of NK1, NK2 and NK3 receptors, respectively; however, the selectivity of natural TKs for their preferred receptors is relative. In addition, SP and NKA are expressed in similar quantities in the human intestine and adequate stimuli can release similar amount of these TKs from enteric nerves. Furthermore, a single anatomical substrate can express more than one TK receptor type, so that the blockade of a single receptor type may not reveal functional effects in integrated models of motility. In isolated human small intestine and colon circular muscle strips, both NK1 and NK2 receptors mediate contractile effects. Indeed, in the human small intestine, smooth muscle electrical and motor events induced by electrical field stimulation (EFS) can involve either or both NK1 and NK2 receptors or these latter receptors predominantly, depending on the experimental conditions. In contrast, in the human colonic smooth muscle, only the NK2 receptor-mediated component of the response to EFS is prominent and some evidence would suggest that this component is the main excitatory motor mechanism at this level. Furthermore, a NK2 receptor-mediated secretory component in the human colonic mucosa has been recently demonstrated. Thus, it could be speculated that the blockade of both NK1 and NK2 receptors will be necessary to antagonise motor effects induced by exogenous administration or endogenous release of TKs in the small intestine, whereas the blockade of the NK2 receptors would be sufficient to disrupt physiological motor and, possibly, secretory activity at the colonic level. Available evidence indicates that, in healthy volunteers, the infusion of NKA (25 pmol/kg/min i.v.) stimulated small intestine motility and precipitated a series of intestinal and non-intestinal adverse events. Nepadutant (8 mg i.v.), a selective NK2 receptor antagonist, antagonised small intestine motility induced by NKA and prevented associated intestinal adverse events. In another study, the same dose of nepadutant increased colo-rectal compliance during isobaric balloon distension in healthy volunteers pretreated with a glycerol enema, disclosing a NK2 receptor-mediated component in the regulation of colonic smooth muscle tone. However, the prolonged blockade of NK2 receptors by nepadutant (16 mg i.v. b.i.d. for 8 days) did not affect bowel habits, neither in term of movements nor of stool consistency. Altogether, these results indicate that, even when there is a significant redundance in the effects of TKs and in the role of their receptors, the selective blockade of tachykinin NK2 receptors can have functional consequences on human intestinal motility and perception, but this can occur without the disruption of the physiological functions.

Expression of the neurokinin type 1 receptor in the human colon

The distribution of the neurokinin type 1 receptor (NK1r) in human intestine, mapped in a few immunohistochemical investigations in the antrum and the duodenum, is comparable to that widely studied in rodents. Importantly, despite pharmacological evidence of their presence in mammalian intestinal muscle, their immunohistochemical visualization in smooth muscle cells remains to be determined in human digestive tract. In the present work, we studied the distribution of NK1r in the human colon, with a particular view to visualize their expression in muscle cells. With this aim, part of colonic segments were incubated with nicardipine and TTX in order to induce accumulation of the NK1r on cell membrane. NK1r were visualized by using immunohistochemistry combined with fluorescence and confocal microscopy. Without incubation, NK1r-IR was clearly observed on the membrane and the cytoplasm of myenteric and submucous neurons and interstitial cells of Cajal, but could not be clearly determined in the longitudinal and circular muscle. NK1r-IR-expressing neurons and interstitial cells were closely surrounded by substance P (SP) immunoreactive nerves. Incubation of colonic segments with nicardipine and TTX at 4 degrees C for 1 h with SP allowed to reveal a strong NK1r-IR at the surface of muscle cells. Incubation with SP (10(-6) M) at 37 degrees C for 1 min induced a relocation of NK1r-IR into the cytoplasm of muscle. This is interpreted as an internalization of NK1r induced by the binding of SP on muscular NK1r. The present data contribute to emphasize the role of NK1r in tachykinin-mediated neuronal processes regulating intestinal motility.

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.

Gastrin releasing peptide receptor expression is decreased in patients with Crohn's disease but not in ulcerative colitis

BACKGROUND

Gastrin releasing peptide (GRP) and neuromedin B are bombesin (BN)-like peptides involved in regulating motility and inflammation in the gastrointestinal tract, which may be useful in treating inflammatory bowel disease (IBD). Three bombesin-like peptide receptors have been reported, but no studies have investigated their localisation in normal and inflamed human intestine.

AIM

To localise and characterise BN receptors in normal intestine and to see whether this is modified in IBD.

METHODS

Full thickness intestinal tissue samples were collected from 13 patients with Crohn's disease (CD), 11 with ulcerative colitis (UC), and 19 controls. BN receptor expression was characterised and quantified with storage phosphor autoradiography using BN, GRP, neuromedin B, and the synthetic analogue BN(6-14) as ligands.

RESULTS

Only BN receptor type 2 (high affinity for GRP) was present in intestinal tissue. Minimal BN binding was detected in the mucosa. In normal colonic smooth muscle, mean BN binding was 336 fmol/g tissue in longitudinal muscle, including the myenteric plexus, and 71 fmol/g in circular muscle. In CD, colonic smooth muscle BN binding was significantly decreased (longitudinal muscle, 106; circular muscle, 19 fmol/g), in contrast to UC (377 and 62 fmol/g, respectively). In CD, a small (not significant) decrease was seen in ileal muscle compared with controls (111 v 169 and 18 v 32 fmol/g tissue for longitudinal and circular muscle, respectively).

CONCLUSIONS

Only the GRP receptor is expressed in human intestine; expression is highest in longitudinal muscle and myenteric plexus of the colon. Expression is decreased in inflamed and non-inflamed colon of CD, but not in UC.