Effects of tachykinins on myenteric neurones of the guinea-pig gastric corpus: involvement of NK-3 receptors

Mechanosensitive enteric neurons in the guinea pig gastric corpus

For long it was believed that a particular population of enteric neurons, referred to as intrinsic primary afferent neuron (IPAN)s, encodes mechanical stimulation. We recently proposed a new concept suggesting that there are in addition mechanosensitive enteric neurons (MEN) that are multifunctional. Based on firing pattern MEN behaved as rapidly, slowly, or ultra-slowly adapting RAMEN, SAMEN, or USAMEN, respectively. We aimed to validate this concept in the myenteric plexus of the gastric corpus, a region where IPANs were not identified and existence of enteric sensory neurons was even questioned. The gastric corpus is characterized by a particularly dense extrinsic sensory innervation. Neuronal activity was recorded with voltage sensitive dye imaging after deformation of ganglia by compression (intraganglionic volume injection or von Fry hair) or tension (ganglionic stretch). We demonstrated that 27% of the gastric neurons were MEN and responded to intraganglionic volume injection. Of these 73% were RAMEN, 25% SAMEN, and 2% USAMEN with a firing frequency of 1.7 (1.1/2.2), 5.1 (2.2/7.7), and of 5.4 (5.0/15.5) Hz, respectively. The responses were reproducible and stronger with increased stimulus strength. Even after adaptation another deformation evoked spike discharge again suggesting a resetting mode of the mechanoreceptors. All MEN received fast synaptic input. Fifty five percent of all MEN were cholinergic and 45% nitrergic. Responses in some MEN significantly decreased after perfusion of TTX, low Ca(++)/high Mg(++) Krebs solution, capsaicin induced nerve defunctionalization and capsazepine indicating the involvement of TRPV1 expressing extrinsic mechanosensitive nerves. Half of gastric MEN responded to intraganglionic volume injection as well as to ganglionic stretch and 23% responded to stretch only. Tension-sensitive MEN were to a large proportion USAMEN (44%). In summary, we demonstrated for the first time compression and tension-sensitive MEN in the stomach; many of them responded to one stimulus modality only. Their proportions and the basic properties were similar to MEN previously identified by us in other intestinal region and species. Unlike in the intestine, the responsiveness of some gastric MEN is enhanced by extrinsic TRPV1 expressing visceral afferents.

Differential role of tachykinin NK3 receptors on cholinergic excitatory neurotransmission in the mouse stomach and small intestine

BACKGROUND AND PURPOSE

Tachykinin NK(3) receptors are widely expressed in the mouse gastrointestinal tract but their functional role in enteric neuromuscular transmission remains unstudied in this species. We investigated the involvement of NK(3) receptors in cholinergic neurotransmission in the mouse stomach and small intestine.

EXPERIMENTAL APPROACH

Muscle strips of the mouse gastric fundus and ileum were mounted in organ baths for tension recordings. Effects of NK(3) agonists and antagonists were studied on contractions to EFS of enteric nerves and to carbachol.

KEY RESULTS

EFS induced frequency-dependent tetrodotoxin-sensitive contractions, which were abolished by atropine. The cholinergic contractions to EFS in the stomach were enhanced by the NK(3) antagonist SR142801, but not affected by the NK(3) agonist senktide or neurokinin B. The cholinergic contractions to EFS in the small intestine were not affected by SR142801, but dose-dependently inhibited by senktide and neurokinin B. This inhibitory effect was prevented by SR142801 but not by hexamethonium. SR142801, senktide or neurokinin B did not induce any response per se in the stomach and small intestine and did not affect contractions to carbachol.

CONCLUSIONS AND IMPLICATIONS

NK(3) receptors modulate cholinergic neurotransmission differently in the mouse stomach and small intestine. Blockade of NK(3) receptors enhanced cholinergic transmission in the stomach but not in the intestine. Activation of NK(3) receptors inhibited cholinergic transmission in the small intestine but not in the stomach. This indicates a physiological role for NK(3) receptors in mouse stomach contractility and a pathophysiological role in mouse intestinal contractility.

Stimulation of the neurokinin 3 receptor activates protein kinase C epsilon and protein kinase D in enteric neurons

Tachykinins, acting through NK(3) receptors (NK(3)R), contribute to excitatory transmission to intrinsic primary afferent neurons (IPANs) of the small intestine. Although this transmission is dependent on protein kinase C (PKC), its maintenance could depend on protein kinase D (PKD), a downstream target of PKC. Here we show that PKD1/2-immunoreactivity occurred exclusively in IPANs of the guinea pig ileum, demonstrated by double staining with the IPAN marker NeuN. PKCepsilon was also colocalized with PKD1/2 in IPANs. PKCepsilon and PKD1/2 trafficking was studied in enteric neurons within whole mounts of the ileal wall. In untreated preparations, PKCepsilon and PKD1/2 were cytosolic and no signal for activated (phosphorylated) PKD was detected. The NK(3)R agonist senktide evoked a transient translocation of PKCepsilon and PKD1/2 from the cytosol to the plasma membrane and induced PKD1/2 phosphorylation at the plasma membrane. PKCepsilon translocation was maximal at 10 s and returned to the cytosol within 2 min. Phosphorylated-PKD1/2 was detected at the plasma membrane within 15 s and translocated to the cytosol by 2 min, where it remained active up to 30 min after NK(3)R stimulation. PKD1/2 activation was reduced by a PKCepsilon inhibitor and prevented by NK(3)R inhibition. NK(3)R-mediated PKCepsilon and PKD activation was confirmed in HEK293 cells transiently expressing NK(3)R and green fluorescent protein-tagged PKCepsilon, PKD1, PKD2, or PKD3. Senktide caused membrane translocation and activation of kinases within 30 s. After 15 min, phosphorylated PKD had returned to the cytosol. PKD activation was confirmed through Western blotting. Thus stimulation of NK(3)R activates PKCepsilon and PKD in sequence, and sequential activation of these kinases may account for rapid and prolonged modulation of IPAN function.

Sensory transmission in the gastrointestinal tract

The gastrointestinal (GI) tract must balance ostensibly opposite functions. On the one hand, it must undertake the process of digestion and absorption of nutrients. At the same time, the GI tract must protect itself from potential harmful antigenic and pathogenic material. Central to these processes is the ability to 'sense' the mechanical and chemical environment in the gut wall and lumen in order to orchestrate the appropriate response that facilitates nutrient assimilation or the rapid expulsion through diarrhoea and/or vomiting. In this respect, the GI tract is richly endowed with sensory elements that monitor the gut environment. Enteric neurones provide one source of such sensory innervation and are responsible for the ability of the decentralized gut to perform complex reflex functions. Extrinsic afferents not only contribute to this reflex control, but also contribute to homeostatic mechanisms and can give rise to sensations, under certain circumstances. The enteric and extrinsic sensory mechanisms share a number of common features but also some remarkably different properties. The purpose of this review is to summarize current views on sensory processing within both the enteric and extrinsic innervation and to specifically address the pharmacology of nociceptive extrinsic sensory pathways.

Defensive and pathological functions of the gastrointestinal NK3 receptor

In general, normal gut functions are unaffected by selective NK(3) receptor antagonists such as talnetant (SB-223412), osanetant (SR 142901) or SB-235375. However, NK(3) receptors may mediate certain defensive or pathological intestinal processes. The precise mechanisms, by which this role is achieved, are not fully understood. In summary, intense stimulation of the intrinsic primary afferent neurones (IPANs) of the enteric nervous system is thought to release tachykinins from these neurones, to induce slow excitation (slow EPSPs) of connecting IPANs. This is hypothesised to cause hypersensitivity and disrupt intestinal motility, at least partly explaining why NK(3) receptor antagonism can reduce the level of disruption caused by supramaximal distension pressures in vitro. Tachykinin release from IPANs may also increase C-fibre sensitivity, directly or indirectly. Thus, NK(3) receptor antagonists can inhibit nociception associated with intestinal distension, in normal animals or after pre-sensitisation by restraint stress. Importantly, such inhibition has been found with SB-235375, a peripherally restricted antagonist. SB-235375 can also reduce a visceromotor response to brief colorectal distension without affecting similar responses to skin pinch, providing additional evidence for intestinal-specific activity. NK(3) receptor biology is, therefore, revealing a novel pathway by which disruptions in intestinal motility and nociception can be induced.

Slow excitatory metabotropic signal transmission in the enteric nervous system

Metabotropic mechanisms of excitatory signalling in enteric neurones underlie both slow synaptic transmission and paracrine transmission from enteric non-neuronal cells. The type of neurone in which signalling occurs determines the characteristics of synaptic- and paracrine-mediated slow excitatory responses. Slow excitatory responses in neurones with AH-type electrophysiological behaviour and multipolar Dogiel type II morphology are characterized by membrane depolarization associated with closure of Ca2+ -gated K+ channels that is reflected by increased neuronal input resistance. Slow excitatory responses in neurones with S-type electrophysiological behaviour and uniaxonal morphology are characterized by membrane depolarization associated with opening of cationic channels and decreased neuronal input resistance. Postreceptor signalling that involves activation of adenylate cyclase, stimulation of cAMP formation and activation protein kinase A generates excitatory responses characterized by increased neuronal input resistance in AH neurones. Postreceptor signalling that involves activation of phospholipase C, release of IP3 and diacylglycerol and activation of protein kinase C and calmodulin kinases generates excitatory responses characterized by decreased neuronal input resistance in S neurones. Slow excitatory responses that are characterized by increased neuronal input resistance are a property of AH-type neurones that function as interneurones in the neural networks of the ENS. Slow excitatory responses that are characterized by decreased neuronal input resistance are a property of S-type neurones that function either as interneurones or as musculomotor and secretomotor neurones in the neural networks of the ENS.

Neurokinin NK1 and NK3 receptors as targets for drugs to treat gastrointestinal motility disorders and pain

NK1 and NK3 receptors do not appear to play significant roles in normal GI functions, but both may be involved in defensive or pathological processes. NK1 receptor antagonists are antiemetic, operating via vagal sensory and motor systems, so there is a need to study their effects on other gastro-vagal functions thought to play roles in functional bowel disorders. Interactions between NK1 receptors and enteric nonadrenergic, noncholinergic motorneurones suggest a need to explore the role of this receptor in disrupted colonic motility. NK1 receptor antagonism does not exert consistent analgesic activity in humans, but similar studies have not been carried out against pain of GI origin, where NK1 receptors may have additional influences on mucosal inflammatory or "irritant" processes. NK3 receptors mediate certain disruptions of intestinal motility. The activity may be driven by tachykinins released from intrinsic primary afferent neurones (IPANs), which induce slow EPSP activity in connecting IPANs and hence, a degree of hypersensitivity within the enteric nervous system. The same process is also proposed to increase C-fibre sensitivity, either indirectly or directly. Thus, NK3 receptor antagonists inhibit intestinal nociception via a "peripheral" mechanism that may be intestine-specific. Studies with talnetant and other selective NK3 receptor antagonists are, therefore, revealing an exciting and novel pathway by which pathological changes in intestinal motility and nociception can be induced, suggesting a role for NK3 receptor antagonism in irritable bowel syndrome.

Neurokinin-1 and -3 receptor blockade inhibits slow excitatory synaptic transmission in myenteric neurons and reveals slow inhibitory input

Recent studies have shown that tachykinins mediate slow synaptic transmission to myenteric AH (afterhyperpolarising) neurons via neurokinin-3 receptors (NK(3)R). This study investigated a similar role for neurokinin-1 receptors (NK(1)R) and compared the effect of selective receptor antagonists on non-cholinergic slow excitatory post-synaptic potentials (EPSPs) recorded in myenteric AH neurons of the guinea-pig ileum. Slow EPSPs evoked by electrical stimulation of circumferentially oriented presynaptic nerves were mimicked by application of senktide, an NK(3)R agonist. [Sar(9),Met(O(2))(11)]-substance P, an NK(1)R agonist, depolarised a smaller number of neurons. SR142801, a selective NK(3)R antagonist (100 nM), inhibited slow EPSPs and responses to senktide, but had no effect on depolarisations evoked by forskolin, an activator of adenylate cyclase. SR140333, a selective NK(1)R antagonist, inhibited slow EPSPs in a subset of neurons and blocked responses to [Sar(9),Met(O(2))(11)]-substance P, but not to senktide or forskolin. Slow EPSPs that were predominantly mediated by NK(1)R had significantly shorter latencies than those due to activation of NK(3)R. After blockade of slow EPSPs, slow hyperpolarizing responses to presynaptic nerve stimulation were revealed in one-third of neurons. These events, which were associated with a decrease in input resistance and blocked by tetrodotoxin, were equated with slow inhibitory postsynaptic potentials. They were abolished by the 5-hydroxytryptamine(1A) receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), but unaffected by phentolamine, an alpha-adrenoceptor antagonist. In conclusion, these results provide the first direct evidence that NK(1)R mediate some slow excitatory synaptic input to myenteric AH neurons, and suggest that NK(1)R and NK(3)R activate distinct signal transduction pathways. These results also demonstrate that slow inhibitory synaptic transmission, which may be mediated by 5-hydroxytryptamine, is more prevalent in the myenteric plexus than previously indicated.

Virokinin, a bioactive peptide of the tachykinin family, is released from the fusion protein of bovine respiratory syncytial virus

Tachykinins, an evolutionary conserved family of peptide hormones in both invertebrates and vertebrates, are produced by neuronal cells as inactive preprotachykinins that are post-translationally processed into different neuropeptides such as substance P, neurokinin A, and neurokinin B. We show here that furin-mediated cleavage of the bovine respiratory syncytial virus fusion protein results in the release of a peptide that is converted into a biologically active tachykinin (virokinin) by additional post-translational modifications. An antibody directed to substance P cross-reacted with the C terminus of mature virokinin that contains a classical tachykinin motif. The cellular enzymes involved in the C-terminal maturation of virokinin were found to be present in many established cell lines. Virokinin is secreted by virus-infected cells and was found to act on the tachykinin receptor 1 (TACR1), leading to rapid desensitization of this G protein-coupled receptor as shown by TACR1-green fluorescent protein conjugate translocation from the cell surface to endosomes and by co-internalization of the receptor with beta-arrestin 1-green fluorescent protein conjugates. In vitro experiments with isolated circular muscle from guinea pig stomach indicated that virokinin is capable of inducing smooth muscle contraction by acting on the tachykinin receptor 3. Tachykinins and their cognate receptors are present in the mammalian respiratory tract, where they have potent effects on local inflammatory and immune processes. The viral tachykinin-like peptide represents a novel form of molecular mimicry, which may benefit the virus by affecting the host immune response.

Nonpeptide tachykinin receptor antagonists. III. SB 235375, a low central nervous system-penetrant, potent and selective neurokinin-3 receptor antagonist, inhibits citric acid-induced cough and airways hyper-reactivity in guinea pigs

In this report the in vitro and in vivo pharmacological and pharmacokinetic profile of (-)-(S)-N-(alpha-ethylbenzyl)-3-(carboxymethoxy)-2-phenylquinoline-4-carboxamide (SB 235375), a low central nervous system (CNS)-penetrant, human neurokinin-3 (NK-3) receptor (hNK-3R) antagonist, is described. SB 235375 inhibited (125)I-[MePhe(7)]-neurokinin B (NKB) binding to membranes of Chinese hamster ovary (CHO) cells expressing the hNK-3R (CHO-hNK-3R) with a K(i) = 2.2 nM and antagonized competitively NKB-induced Ca(2+) mobilization in human embryonic kidney (HEK) 293 cells expressing the hNK-3R (HEK 293-hNK-3R) with a K(b) = 12 nM. SB 235375 antagonized senktide (NK-3R)-induced contractions in rabbit isolated iris sphincter (pA(2) = 8.1) and guinea pig ileal circular smooth muscles (pA(2) = 8.3). SB 235375 was selective for the hNK-3R compared with hNK-1 (K(i) > 100,000 nM) and hNK-2 receptors (K(i) = 209 nM), and was without effect, at 1 microM, in 68 other receptor, enzyme, and ion channel assays. Intravenous SB 235375 produced a dose-related inhibition of miosis induced by i.v. senktide in the rabbit (ED(50) of 0.56 mg/kg). Intraperitoneal SB 235375 (10-30 mg/kg) inhibited citric acid-induced cough and airways hyper-reactivity in guinea pigs. In mice oral SB 235375 (3-30 mg/kg) was without significant effect on the behavioral responses induced by intracerebral ventricular administration of senktide. Pharmacokinetic evaluation in the mouse and rat revealed that oral SB 235375 was well absorbed systemically but did not effectively cross the blood-brain barrier. The preclinical profile of SB 235375, encompassing high affinity, selectivity, oral activity, and low CNS penetration, suggests that it is an appropriate tool compound to define the pathophysiological roles of the NK-3Rs in the peripheral nervous system.

Tachykinins mediate slow excitatory postsynaptic transmission in guinea pig sphincter of Oddi ganglia

Intracellular recording techniques were used to test whether tachykinins could be mediators of slow excitatory postsynaptic potentials (EPSPs) in guinea pig sphincter of Oddi (SO) ganglia. Application of the tachykinin substance P (SP) onto SO neurons caused a prolonged membrane depolarization that was reminiscent of the slow EPSP in these cells. Pressure ejection of the neurokinin 3 (NK3) receptor-specific agonist senktide caused a similar depolarization; however, no responses were detected on application of NK1 or NK2 receptor agonists. The NK3 receptor antagonist SR-142801 (100 nM) significantly inhibited both SP-induced depolarization and the stimulation-evoked slow EPSP, as did NK3 receptor desensitization with senktide. Capsaicin, which causes the release of SP from small-diameter afferent fibers, induced a depolarization that was similar to the evoked slow EPSP in both amplitude and duration. The capsaicin-induced depolarization was significantly attenuated in the presence of SR-142801. These data indicate that tachykinins, released from extrinsic afferent fibers, act via NK3 receptors to provide slow excitatory synaptic input to SO neurons.

Receptor-induced Ca(2+) signaling in cultured myenteric neurons

We studied the effect of excitatory neurotransmitters (10(-5) M) on the intracellular Ca(2+) concentration ([Ca(2+)](i)) of cultured myenteric neurons. ACh evoked a response in 48.6% of the neurons. This response consisted of a fast and a slow component, respectively mediated by nicotinic and muscarinic receptors, as revealed by specific agonists and antagonists. Substance P evoked a [Ca(2+)](i) rise in 68.2% of the neurons, which was highly dependent on Ca(2+) release from intracellular stores, since after thapsigargin (5 microM) pretreatment only 8% responded. The responses to serotonin, present in 90.7%, were completely blocked by ondansetron (10(-5) M), a 5-HT(3) receptor antagonist. Specific agonists of other serotonin receptors were not able to induce a [Ca(2+)](i) rise. Removing extracellular Ca(2+) abolished all serotonin and fast ACh responses, whereas substance P and slow ACh responses were more persistent. We conclude that ACh-induced signaling involves both nicotinic and muscarinic receptors responsible for a fast and a more delayed component, respectively. Substance P-induced signaling requires functional intracellular Ca(2+) stores, and the 5-HT(3) receptor mediates the serotonin-induced Ca(2+) signaling in cultured myenteric neurons.

Characteristics of mucosally projecting myenteric neurones in the guinea-pig proximal colon

1. Using retrograde tracing with 1,1'-didodecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI) in combination with electrophysiological and immunohistochemical techniques we determined the properties of the putative intrinsic primary afferent myenteric neurones with mucosal projections in the guinea-pig proximal colon. 2. Eighty-four out of eighty-five DiI-labelled myenteric neurones were AH neurones with a late after-hyperpolarization. Thirty-three per cent of them exhibited atropine- and tetrodotoxin-resistant spontaneously occurring hyperpolarizing potentials (SHPs) during which the membrane resistance and excitability decreased. 3. DiI-labelled AH neurones had multipolar Dogiel type II morphology, primarily of the dendritic type. Sixty-one per cent of the neurones were immunoreactive for choline acetyltransferase (ChAT) and calbindin (Calb) and 23 % were ChAT positive but Calb negative. 4. DiI-labelled neurones did not receive fast excitatory postsynaptic potentials but 94 % (34/36) received slow excitatory postsynaptic potentials (sEPSPs). The neurokinin-3 (NK-3) agonist (MePhe7)-NKB but not the NK-1 agonist [(SAR9,Met(O2)11]-SP mimicked this response. The NK-3 receptor antagonist SR 142801 (1 microM) significantly decreased the amplitude and duration of the sEPSPs; the NK-1 receptor antagonist CP-99,994 (1 microM) was ineffective. Atropine (0.5 microM) increased the duration but not the amplitude of the sEPSPs. 5. Microejection of 100 mM sodium butyrate onto the neurones induced in 90 % of the DiI-labelled neurones a transient depolarization associated with an increased excitability. In neurones with SHPs sodium butyrate evoked, additionally, a late onset hyperpolarization. Perfusion of 0.1-10 mM sodium butyrate induced a dose-dependent increase in neuronal excitability. Sodium butyrate was ineffective when applied directly onto the mucosa. 6. Mucosally projecting myenteric neurones of the colon are multipolar AH neurones with NK-3-mediated slow EPSPs and somal butyrate sensitivity.

Neurokinin-1 (NK-1) receptor is required in Clostridium difficile- induced enteritis

Toxin A, a 308,000-Mr enterotoxin from Clostridium difficile, mediates antibiotic-associated diarrhea and colitis in humans. Injection of toxin A into animal intestine triggers an acute inflammatory response characterized by activation of sensory neurons and immune cells of the intestinal lamina propria, including mast cells and macrophages, and migration of circulating neutrophils in the involved intestinal segment. In this study we show that mice genetically deficient in the neurokinin-1 receptor are protected from the secretory and inflammatory changes as well as from epithelial cell damage induced by toxin A. The protective effect of neurokinin-1R deletion correlates with diminished intestinal levels of the cytokine TNF-alpha and its mRNA and the leukocyte enzyme myeloperoxidase. These results demonstrate a major requirement for substance P receptors in the pathogenesis of acute inflammatory diarrhea.

Cellular pathways mediating tachykinin-evoked secretomotor responses in guinea pig ileum

This study characterized tachykinin-evoked secretomotor responses in in vitro submucosal and mucosal-submucosal preparations of the guinea pig ileum using combined intracellular and Ussing chamber recording techniques. Superfusion of endogenous tachykinins substance P (SP), neurokinin A (NKA), and neurokinin B depolarized single submucosal neurons and evoked increased short-circuit current (Isc) responses in Ussing chamber preparations. The NK1-receptor agonist [Sar9,Met(O2)11]SP [50% effective concentration (EC50) = 2 nM] depolarized all submucosal neurons examined. The NK3-receptor agonist senktide (EC50 = 20 nM) depolarized approximately 50% of neurons examined, whereas the NK2-receptor agonist [Ala5,beta-Ala8]NKA-(4-10) had no effect on membrane potential. [Sar9,Met(O2)11]SP and senktide evoked similar increases in Isc that were tetrodotoxin sensitive (91 and 100%, respectively) and were selectively blocked by the NK1 antagonist CP-99,994 and the NK3 antagonist SR-142,801, respectively. Capsaicin-evoked increases in Isc were significantly inhibited (54%, P < 0.05) by CP-99,994 but not by SR-142,801. Neither antagonist inhibited slow excitatory postsynaptic potentials. These findings suggest that tachykinin-evoked secretion in guinea pig ileum is mediated by NK1 and NK3 receptors on submucosal secretomotor neurons and that capsaicin-sensitive nerves release tachykinin(s) that activate the NK1 receptors.

Neurochemical coding in the myenteric plexus of the upper gastrointestinal tract of hibernating hamsters

As part of our investigation of the plasticity of autonomic nerves in physiological and pathological conditions, we have examined the effect of hibernation on the neurochemical content of myenteric nerves and nerve cell bodies of the upper gastrointestinal tract of the non-seasonal hibernator, the golden hamster. Age matched hamsters kept at room temperature and those kept at 5 degrees C but which failed to hibernate, were used as controls. Possible changes in nerve fibres and nerve cell bodies containing the general neuronal marker, protein gene product 9.5, the peptides, vasoactive intestinal polypeptide, substance P (SP) and calcitonin gene-related peptide (CGRP), the catecholamine synthesizing enzyme tyrosine hydroxylase and the enzyme responsible for synthesizing nitric oxide, nitric oxide synthase, were examined in the oesophagus, proventriculus and proximal and distal stomach of the golden hamsters using immunohistochemical techniques. The results of the present study revealed a significant increase in the number of nerve cell bodies and density of nerve fibres containing SP-immunoreactivity and increased number of CGRP-immunoreactive cell bodies but not the other markers examined in the proximal stomach and proventriculus. In contrast, there was no change in the distribution of any of the neuroactive substances examined in the myenteric plexus of the oesophagus and distal stomach. It is suggested that the change in the environment of the hibernating hamsters perturbs the normal digestive physiology in the proximal stomach and proventriculus that is reflected by the selective changes in SP- and CGRP-containing enteric nerves; these changes may be part of protective reflex mechanisms to the environmental changes resulting from hibernation, where upgrading of nerve cell bodies expressing CGRP and SP has occurred.

Tachykinins in the gut. Part II. Roles in neural excitation, secretion and inflammation

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

Tachykinins as mediators of slow EPSPs in guinea-pig gall-bladder ganglia: involvement of neurokinin-3 receptors

1. The effects of endogenous tachykinins and related peptides on intact guinea-pig gall-bladder neurones were investigated with single-electrode voltage- and current-clamp recording techniques. 2. Pressure ejection of substance P (100 microM) caused a long lasting membrane depolarization that was associated with a decrease in input resistance. In cells that were voltage-clamped to their resting membrane potential, substance P activated an inward current. 3. The reversal potentials of the substance P-induced depolarization and inward current were congruent to 0 mV. In a low-Na+ solution, the substance P-induced depolarization and inward current were reduced in amplitude. 4. Substance P increased the excitability of neurones, as evidenced by a greater anodal break activity and an increase in the number of action potentials generated during a depolarizing current pulse. 5. Substance P, neurokinin A (NKA) and neurokinin B (NKB) were applied by superfusion to determine the relative potencies of these tachykinins. NKB was the most potent, with an EC50 of 24 nM. The EC50 values for NKA and substance P were 47.8 and 281 nM, respectively. 6. The neurokinin-3 (NK-3) receptor agonist senktide depolarized neurones with an EC50 of 6.3 nM. Neither the NK-1 receptor agonist [Sar9,Met(O2)11]-substance P nor the NK-2 receptor agonist [beta-Ala8]-NKA(4-10) caused a measurable depolarization. 7. The NK-3 antagonist [Trp7,beta-Ala8]-NKA (4-10) inhibited the responsiveness of gall-bladder neurones to substance P with a KB (dissociation constant of receptor antagonist) of 49 nM, and depressed both capsaicin-induced depolarizations and stimulus-evoked slow EPSPs. 8. These data indicate that tachykinins mediate slow EPSPs in guinea-pig gall-bladder ganglia by activating NK-3 receptors on gall-bladder neurones. It is proposed that in response to inflammation or high intraluminal pressure, tachykinins may be released within ganglia by sensory fibres and act directly on intrinsic neurones to facilitate ganglionic transmission.

Tachykinin receptors in gastrointestinal motility

For a long time research on the action of TKs on gastrointestinal tissue has been demonstrating the importance of the TKs as non-cholinergic stimulators of motility in most parts of the mammalian gastrointestinal tract. The past years witnessed the development of TK agonists and antagonists selective for the various receptor types, which prompted a wealth of new insight into the pharmacology and molecular biology of the TK receptors. This knowledge now allows a more specific elucidation of the role of TKs and their receptors in the various aspects of gastrointestinal motility, not only in normal tissue but also under pathological conditions.

Neurochemical coding of enteric neurons in the guinea pig stomach

The aim of this study was to investigate the neurochemical coding of myenteric neurons in the guinea pig gastric corpus by using immunohistochemical methods. Antibodies and antisera against calbindin (CALB), calretinin (CALRET), choline acetyltransferase (ChAT), calcitonin gene-related peptide (CGRP), dopamine beta-hydroxylase (DBH), beta-endorphin (ENK), neuropeptide Y (NPY), neuron-specific enolase (NSE), nitric oxide synthase (NOS), protein gene product 9.5 (PGP), parvalbumin (PARV), serotonin (5-HT), somatostatin (SOM), substance P (SP), tyrosine hydroxylase (TH), and vasoactive intestinal peptide (VIP) were used. Double- and triple-labeling studies revealed colocalization of certain transmitters and enabled the identification of distinct subpopulations of gastric enteric neurons. NPY/VIP/NOS/ENK were present in 28% of all neurons, whereas 11% had NPY/VIP/DBH/ChAT; NOS-only neurons made up 2% of the population. The combination SP/ChAT/ENK occurred in 21% of the population, whereas SP/ChAT/ENK/CALRET and SP/CHAT/SOM/ +/- CALRET was identified in 5% and 6% of all cells, respectively. 5-HT-containing neurons comprised 2% of all cells and could be further classified by the presence of additional antigens as 5-HT/SP/(ChAT) or 5-HT/VIP/(ChAT). Approximately 21% of all neurons contained only ChAT with no additional antigen present and are referred to as ChAT/-. Gastric myenteric ganglion cells were not immunoreactive for CALB, PARV, CGRP, or TH. The results of this study indicate that gastric myenteric neurons can be characterized on the basis of different chemical coding. Neurochemical coding of corpus myenteric neurons revealed some similarities and significant differences in comparison with other regions of the gut. These differences might reflect adaptation of enteric nerves according to regional specialization and the distinct functions of the proximal stomach as a gastric reservoir.

Electrophysiological effects of tachykinins and capsaicin on guinea-pig bronchial parasympathetic ganglion neurones

1. We evaluated the effects of neurokinins, tachykinin analogues, or capsaicin on passive membrane properties of guinea-pig bronchial parasympathetic neurones using intracellular recording techniques. 2. Substance P (SP) and the tachykinin analogue, acetyl-[Arg6,Sar9,Met(O2)11]-SP(6-11) (ASMSP), at concentrations selective for the neurokinin (NK)-1 receptor subtype, depolarized the resting potential (3 and 5 mV, respectively) with no change in input resistance. Neurokinin A and beta Ala8NKA(4-10), at concentrations selective for the NK-2 receptor subtype (0.1 microM), were without effect. 3. Neurokinin B (NKB) and [Asp5,6,methyl-Phe8]SP(5-11) (senktide analogue), at concentrations selective for NK-3 receptor subtype, elicited maximum depolarizations of 16 +/- 2 mV for both agonists. The peak of the depolarization was associated with an decrease in membrane resistance (35 +/- 4 and 50 +/- 7%, respectively). 4. Capsaicin (1 microM) elicited a 3-24 mV depolarization of the resting potential of thirteen of eighteen bronchial ganglion neurones and decreased the input resistance of seven of thirteen of these neurones. The effects of capsaicin were reduced by desensitization with senktide analogue at a concentration selective for the NK-3 receptor subtype, whereas a non-peptide NK-1 receptor antagonist had no effect. 5. Using voltage clamp analysis, capsaicin and senktide analogue evoked an inward current and an increase in membrane conductance at the resting membrane potential. The reversal potential for senktide analogue was estimated to be + 4 mV. 6. These data support the hypothesis that neurokinin-containing nerve terminals are localized within guinea-pig bronchial parasympathetic ganglia and, when released, the predominant effect of the neurokinins is by activation of NK-3 receptors.

Analysis of a nonpeptide antagonist for substance P on myenteric neurons of guinea-pig small intestine

CP-96,345 [(2S,3S)-cis-2-(diphenylmethyl)-N-[(2-methoxyphenyl)-methyl]- 1-azabicyclo[2.2.2.]octan-3-amine], a novel nonpeptide antagonist of the substance P receptor, was evaluated for blocking action at substance P receptors on myenteric neurons of guinea-pig small intestine. Intracellular electrophysiological recording was used to determine actions of the drug on excitatory responses to substance P, on slow and fast excitatory postsynaptic potentials and action potential initiation and propagation in the neurons. CP-96,345 suppressed responses to substance P. It also suppressed spike initiation and propagation in the neuronal processes, as well as in the somal membranes. The effects of the drug on substance P responses could not be attributed to an action at substance P receptors. The mechanism of action appeared to be a nonselective local anesthetic effect on initiation and propagation of action potentials.