Evidence that tachykinins are the main NANC excitatory neurotransmitters in the guinea-pig common bile duct

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

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

Knockout of the neurokinin-1 receptor reduces cholangiocyte proliferation in bile duct-ligated mice

In bile duct-ligated (BDL) rats, cholangiocyte proliferation is regulated by neuroendocrine factors such as α-calcitonin gene-related peptide (α-CGRP). There is no evidence that the sensory neuropeptide substance P (SP) regulates cholangiocyte hyperplasia. Wild-type (WT, (+/+)) and NK-1 receptor (NK-1R) knockout (NK-1R(-/-)) mice underwent sham or BDL for 1 wk. Then we evaluated 1) NK-1R expression, transaminases, and bilirubin serum levels; 2) necrosis, hepatocyte apoptosis and steatosis, and the number of cholangiocytes positive by CK-19 and terminal deoxynucleotidyl transferase biotin-dUTP nick-end labeling in liver sections; 3) mRNA expression for collagen 1α and α-smooth muscle (α-SMA) actin in total liver samples; and 4) PCNA expression and PKA phosphorylation in cholangiocytes. In cholangiocyte lines, we determined the effects of SP on cAMP and D-myo-inositol 1,4,5-trisphosphate levels, proliferation, and PKA phosphorylation. Cholangiocytes express NK-1R with expression being upregulated following BDL. In normal NK-1R(-/-) mice, there was higher hepatocyte apoptosis and scattered hepatocyte steatosis compared with controls. In NK-1R (-)/(-) BDL mice, there was a decrease in serum transaminases and bilirubin levels and the number of CK-19-positive cholangiocytes and enhanced biliary apoptosis compared with controls. In total liver samples, the expression of collagen 1α and α-SMA increased in BDL compared with normal mice and decreased in BDL NK-1R(-/-) compared with BDL mice. In cholangiocytes from BDL NK-1R (-)/(-) mice there was decreased PCNA expression and PKA phosphorylation. In vitro, SP increased cAMP levels, proliferation, and PKA phosphorylation of cholangiocytes. Targeting of NK-1R may be important in the inhibition of biliary hyperplasia in cholangiopathies.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Exogenous purines induce differential responses in the proximal and distal regions of the possum sphincter of Oddi

1. The aim of this study was to compare the effect of exogenous ATP and adenosine on spontaneous motility of the proximal and distal regions of the possum sphincter of Oddi (SO). 2. ATP or adenosine (1 microm-1 mm) was applied to distal-SO or proximal-SO muscle rings in organ baths in the absence or presence of tetrodotoxin (TTX) or P1/P2 antagonists. 3. Both ATP and adenosine altered spontaneous activity, predominantly in proximal-SO rings. 4. Exogenous ATP induced a bi-phasic response consisting of a brief TTX-sensitive excitatory component, and a longer-lasting TTX-insensitive inhibitory component. 5. The excitatory ATP response likely involves P2X receptors, whereas the late inhibitory response likely involves P2Y receptors. 6. Exogenous adenosine decreased spontaneous SO activity, via a TTX-insensitive mechanism. 7. Exogenous purines modulate SO motility, acting primarily in the proximal region of the SO, via neural and non-neural mechanisms and multiple purine receptor subtypes.

Effects of neonatal capsaicin treatment in the neutrophil production, and expression of preprotachykinin-I and tachykinin receptors in the rat bone marrow

Bone marrow is richly innervated with both myelinated and non-myelinated nerve fibers, but the role of this innervation on hemopoiesis is poorly understood. Therefore, the aim of this study was to investigate the role of C-fibers on hematopoiesis. Wistar rats were neonatally injected with either capsaicin or its vehicle, and used at adult ages (8-10 weeks). In capsaicin-pretreated rats, the levels of substance P (SP) in bone marrow fluid were markedly reduced in comparison with the vehicle group (13.1+/-4.5 pg/ml versus 47.3+/-5.5 pg/ml, p<0.05). In bone marrow, the number of total leukocytes was 28% higher (p<0.05) in capsaicin-pretreated group, and this accompanied by a higher number of neutrophils, particularly of the immature forms. The mononuclear cell and eosinophils counts did not differ significantly among vehicle and capsaicin groups. In peripheral blood, the number of circulating neutrophils in the capsaicin group increased by 53.8% (p<0.05), whereas the number of mononuclear cells did not change significantly among groups. Eosinophils were virtually absent in the circulating blood in both groups. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed that both preprotachykinin (PPT)-I mRNA and the tachykinin neurokinin (NK)-1 mRNA expression in bone marrow cells significantly increased in capsaicin group, whereas the NK-2 mRNA expression was unchanged after capsaicin pretreatment. In conclusion, our data show that chronic neuropeptide depletion enhance the neutrophil proliferation and differentiation in the rat bone marrow by mechanisms involving upregulation of PPT-I gene and NK-1 receptors.

Exogenous adenosine triphosphate and adenosine stimulate proximal sphincter of oddi motility via neural mechanisms in the anesthetized Australian possum

We aimed to determine if exogenous adenosine triphosphate or adenosine modulated sphincter of Oddi motility and involved neural mechanisms. Sphincter of Oddi motility was recorded in anesthetized possums by manometry. Adenosine triphosphate or adenosine (1 microM-10 mM) was applied topically to the sphincter before and after pretreatment with tetrodotoxin, hexamethonium, atropine, or Nomega-nitro-L-arginine methyl ester. Sphincter contraction amplitude and frequency were quantified. Adenosine triphosphate induced a concentration-dependent increase in proximal sphincter contraction amplitude and frequency (P < 0.05). This response was reduced by tetrodotoxin and atropine but enhanced by hexamethonium and Nomega-nitro-L-arginine methyl ester. Adenosine concentration dependently increased proximal sphincter contraction amplitude (P < 0.05) only. This response was reduced by tetrodotoxin, atropine, and Nomega-nitro-L-arginine methyl ester, whereas hexamethonium had no effect. We conclude that exogenous adenosine triphosphate and adenosine stimulate proximal sphincter of Oddi motility via neural mechanisms, involving cholinergic motor neurons. Adenosine triphosphate may further modulate sphincter motility via nicotinic and nitrergic pathways.

Effects of capsaicin on visceral smooth muscle: a valuable tool for sensory neurotransmitter identification

Studying the visceral effects of the sensory stimulant capsaicin is a useful and relatively simple tool of neurotransmitter identification and has been used for this purpose for approximately 25 years in the authors' and other laboratories. We believe that conclusions drawn from experiments on visceral preparations may have an impact on studies dealing with the central endings of primary afferent neurons, i.e. research on nociception at the spinal level. The present review concentrates on the effects of capsaicin--through the transient receptor potential vanilloid receptor type 1 (TRPV1) receptor--on innervated gastrointestinal, respiratory and genitourinary smooth muscle preparations. Tachykinins and calcitonin gene-related peptide (CGRP) are the most widely accepted transmitters to mediate "local efferent" effects of capsaicin-sensitive nerves in tissues taken from animals. Studies more and more frequently indicate a supra-additive interaction of various types of tachykinin receptors (tachykinin NK(1), NK(2), NK(3) receptors) in the excitatory effects of capsaicin. There is also evidence for a mediating role of ATP, acting on P(2) purinoceptors. Non-specific inhibitory actions of capsaicin-like drugs have to be taken into consideration while designing experiments with these drugs. Results obtained on human tissues may be sharply different from those of animal preparations. Capsaicin potently inhibits tone and movements of human intestinal preparations, an effect mediated by nitric oxide (NO) and/or vasoactive intestinal polypeptide.

Electrical activation of common bile duct nerves modulates sphincter of Oddi motility in the Australian possum

BACKGROUND

Sphincter of Oddi (SO) motility is regulated by extrinsic and intrinsic nerves. The existence of neural circuits between the SO and the proximal extrahepatic biliary tree has been reported, but they are poorly understood. Using electrical field stimulation (EFS), we determined if a neural circuit exists between the common bile duct (CBD) and the SO in anaesthetized Australian brush-tailed possums.

METHODS

The gallbladder, cystic duct or CBD were subjected to EFS with a stimulating electrode. Spontaneous SO phasic waves were measured by manometry.

RESULTS

EFS at sites on the distal CBD (12-20 mm proximal to the SO), but less commonly at more proximal CBD, evoked a variety of responses consisting of an excitatory and/or inhibitory phase. Bi-phasic responses consisting of an excitation followed by inhibition were the most common. Tri-phasic responses were also observed as well as excitation or inhibition only. These evoked responses were blocked by topical application of local anaesthetic to the distal CBD or transection of the CBD. EFS at sites on the gallbladder body, neck or cystic duct did not consistently evoke an SO response. Pretreatment with atropine or guanethidine reduced the magnitude of the evoked response by about 50% (p<0.05), pretreatment with hexamethonium had no consistent effect and pretreatment with a nitric oxide synthase inhibitor increased the response.

DISCUSSION

A neural circuit(s) between the SO and the distal CBD modulates SO motility. Damage to this area of the CBD during bile duct exploration surgery could adversely affect SO motility.

Tachykinins and tachykinin receptors: a growing family

The peptides of the tachykinin family are widely distributed within the mammalian peripheral and central nervous systems and play a well-recognized role as excitatory neurotransmitters. Currently, the concept that tachykinins act exclusively as neuropeptides is being challenged, since the best known members of the family, substance P, neurokinin A and neurokinin B, are also present in non-neuronal cells and in non-innervated tissues. Moreover, the recently cloned mammalian tachykinins hemokinin-1 and endokinins are primarily expressed in non-neuronal cells, suggesting a widespread distribution and important role for these peptides as intercellular signaling molecules. The biological actions of tachykinins are mediated through three types of receptors denoted NK(1), NK(2) and NK(3) that belong to the family of G protein-coupled receptors. The identification of additional tachykinins has reopened the debate of whether more tachykinin receptors exist. In this review, we summarize the current knowledge of tachykinins and their receptors.

Bradykinin B2 receptors mediate contraction in the normal and inflamed human gallbladder in vitro

BACKGROUND & AIMS

The components of the kinin system, including kinongens, kininogenases, and B(2) and B(1) receptors, are expressed and activated during inflammation. Here, we investigated the expression of the kinin B(2) receptor messenger RNA, kininogen and kallikrein immunoreactivity, and the ability of kinins to contract control and inflamed gallbladders in vitro.

METHODS

Human gallbladders, obtained from patients undergoing cholecystectomy either for acute cholecystitis secondary to gallstone disease or during elective gastro-entero-pancreatic surgery (controls), were processed for reverse-transcription polymerase chain reaction analysis, kallikrein and kininogen immunohistochemistry, binding studies, and in vitro contractility studies.

RESULTS

Tissue expression of B(2) receptor messenger RNA and specific binding of [(3)H]-bradykinin increased significantly in acute cholecystitis compared to controls. Kallikrein immunoreactivity was detected in the epithelium and infiltrating leukocytes, whereas kininogen immunoreactivity in the lumen of blood vessels and interstitial space. Bradykinin contracted isolated strips of control and acute cholecystitis gallbladders. In acute cholecystitis tissue, efficacy of bradykinin was higher than that of control gallbladders and similar to that of cholecystokinin. The contraction induced by bradykinin was significantly attenuated by B(2) receptor antagonism but not by cyclooxygenase inhibition and B(1), muscarinic, or tachykinin receptor antagonism.

CONCLUSIONS

All the components of the kinin system are expressed in the human gallbladder. Bradykinin is a powerful spasmogen via B(2) receptor activation in the normal and, especially, in the inflamed human gallbladder.

Tachykinins contribute to nerve-mediated contractions in the human esophagus

BACKGROUND & AIMS

Tachykinins mediate nonadrenergic, noncholinergic excitation in the gastrointestinal tract, but their role in esophageal peristalsis remains unclear.

METHODS

We used muscle strips from the distal third of human esophagus, obtained from patients undergoing esophagectomy for cancer, to investigate the contribution of tachykinins to nerve-mediated contractions. Isometric tension responses to agonists or electrical field stimulation were recorded in circular and longitudinal muscle strips.

RESULTS

Tachykinins produced concentration-dependent increases in tension in circular and longitudinal muscle strips, with the following order of potency: beta-Ala(8)-neurokinin (NK) A (4-10) > NKB > substance P, suggesting NK(2) receptor involvement. The NK(2) receptor antagonist, SR48968 (1 micromol/L), inhibited responses to tachykinins in both muscles. Nerve activation produced on- and off-contractions in circular muscle and a duration-contraction in longitudinal muscle. Atropine (10 micromol/L)-insensitive nerve-evoked contractions were identified for the 3 types of responses. SR48968 produced concentration-dependent inhibition of atropine-insensitive on- and off-contractions but had no effect on the duration-contraction. At low stimulus frequency (1 Hz), on-contractions showed greater sensitivity to SR48968 than off-contractions.

CONCLUSIONS

Nerve-mediated contractions in the human esophagus have a significant atropine-insensitive component. Tachykinins acting on NK(2) receptors can account for some, but not all, of this response, suggesting that other excitatory mechanisms also contribute.

Principles of tachykininergic co-transmission in the peripheral and enteric nervous system

The tachykinins substance P (SP) and neurokinin A (NKA) are synthesized and released from nerves in the peripheral and enteric nervous system (PNS and ENS). They act as nonadrenergic noncholinergic (NANC) excitatory transmitters in mammalian airways, and the genitourinary and gastrointestinal tract. At the postjunctional level, both NK(1) (SP-preferring) and NK(2) (NKA-preferring) receptors are often co-expressed by target cells innervated by TKergic nerves. Thus an issue of duplication seems to exists with regard to peripheral tachykininergic co-transmission, the duplication involving both messengers (the peptides) and effectors (the receptors). By using receptor selective antagonists it has been possible to dissect the relative contribution of different receptors to TKergic co-transmission: the available results indicate that multiple arrangements exist involving both summation, cooperation and specialization of different messengers/effectors in producing the overall response.

Connections between P2 purinoceptors and capsaicin-sensitive afferents in the intestine and other tissues

Relations between P2 purinoceptors and capsaicin-sensitive sensory neurons include an excitatory action of P2 purinoceptor agonists on spinal afferent neurons, as well as release of ATP from afferents at their central and peripheral endings, and a possible participation of ATP in nociception and/or in 'local efferent' responses mediated by sensory nerves at the periphery. The present paper briefly summarizes available evidence on these interrelations. Ample evidence shows that ATP and other P2 purinoceptor agonists can activate primary afferent neurons, through P2X3 receptors and probably other purinoceptors as well, but evidence for an involvement of P2 purinoceptors in nociception or in 'local efferent' responses due to activation of primary afferents is, at best, circumstantial. The possibility is also dealt with that P2 purinoceptor activation may cause small intestinal contraction with the mediation of capsaicin-sensitive sensory neurons and that the motor response to capsaicin in this tissue may involve the release of a P2 purinoceptor stimulant from sensory nerves. Our data show that cholinergic contractions of the guinea-pig ileum in response to the P2 purinoceptor agonist alpha,beta-methylene ATP (alpha,beta-meATP) are blocked by atropine, but not by in vitro capsaicin pretreatment (which completely blocks the contractile action of capsaicin). Cholinergic ileum contractions due to capsaicin (2 microM) are insensitive to suramin (a P2 purinoceptor antagonist; 100 microM). In the presence of antagonists acting at tachykinin NK1 and NK2 receptors, however, suramin (100 microM) causes a significant inhibition of the capsaicin-evoked contraction. These data indicate that capsaicin-sensitive nerves are not involved in the excitatory effect of alpha,beta-methylene ATP on myenteric neurons. On the other hand, ATP is probably involved in the 'non-tachykininergic' component of the capsaicin-induced excitatory response of the small intestine. ATP may originate from sensory neurons and probably acts as activator of myenteric nerves.