Evidence for the involvement of substance P in the atropine-resistant peristalsis of the guinea-pig ileum

Enteric nervous system: sensory transduction, neural circuits and gastrointestinal motility

The gastrointestinal tract is the only internal organ to have evolved with its own independent nervous system, known as the enteric nervous system (ENS). This Review provides an update on advances that have been made in our understanding of how neurons within the ENS coordinate sensory and motor functions. Understanding this function is critical for determining how deficits in neurogenic motor patterns arise. Knowledge of how distension or chemical stimulation of the bowel evokes sensory responses in the ENS and central nervous system have progressed, including critical elements that underlie the mechanotransduction of distension-evoked colonic peristalsis. Contrary to original thought, evidence suggests that mucosal serotonin is not required for peristalsis or colonic migrating motor complexes, although it can modulate their characteristics. Chemosensory stimuli applied to the lumen can release substances from enteroendocrine cells, which could subsequently modulate ENS activity. Advances have been made in optogenetic technologies, such that specific neurochemical classes of enteric neurons can be stimulated. A major focus of this Review will be the latest advances in our understanding of how intrinsic sensory neurons in the ENS detect and respond to sensory stimuli and how these mechanisms differ from extrinsic sensory nerve endings in the gut that underlie the gut-brain axis.

Substance P is essential for maintaining gut muscle contractility: a novel role for coneurotransmission revealed by botulinum toxin

Substance P (SP) is commonly coexpressed with ACh in enteric motor neurons, and, according to the classical paradigm, both these neurotransmitters excite smooth muscle via parallel pathways. We hypothesized that, in addition, SP was responsible for maintaining the muscular responsiveness to ACh. We tested this hypothesis by using botulinum toxin (BoNT/A), a known blocker of vesicular release of neurotransmitters including ACh and neuropeptides. BoNT/A was injected into rat pyloric sphincter in different doses; as control we used boiled BoNT/A. At the desired time point, pylorus was dissected out and pyloric contractility was measured ex vivo in an organ bath and by measuring phosphorylation of myosin light chain 20 (MLC20). BoNT/A (10 IU) significantly reduced the response of pyloric muscle to exogenous ACh, an effect that was accompanied by reduced MLC20 phosphorylation in the muscle. Both effects were reversed by exogenous SP. CP-96345, a NK1 receptor antagonist, blocked the ability of exogenous SP to reverse the cholinergic hyporesponsiveness as well as the reduction in MLC20 phosphorylation induced by BoNT/A. In conclusion, we have identified a novel role for SP as a coneurotransmitter that appears to be important for the maintenance of muscular responsiveness to the principal excitatory neurotransmitter, ACh. These results also provide new insight into the effects of botulinum toxin on the enteric nervous system and gastrointestinal smooth muscle.

Selective depletion of substance P-immunoreactive neurones in the transitional zone of the colon in piebald lethal mice

The distribution of substance P in the colon of piebald lethal (s(1)/s(1)) mice was studied by radioimmunoassay and immunohistochemistry. These animals inherit as a Mendelian recessive trait an aganglionic distal colon. In the region proximal to the aganglionic segment, there is an extensive transitional or hypoganglionic zone, in which the total number of nerve cells in the myenteric plexus is reduced, while those in the submucous plexus tend to be normal. Immunohistochemical studies indicate that substance P-immunoreactive neurones accounted for approx. 10% of the total number of normal myenteric neurones, but in the hypoganglionic region they accounted for about 5%, and this difference was statistically significant. By radioimmunoassay, the concentrations of substance P in both the aganglionic and the hypoganglionic regions of the colon were reduced compared with the corresponding segments in normal mice. However calculation of the mean substance P content per neurone revealed similar quantities (about 1 fmol) in both normal and s(1)/s(1) mice. Substance P-immunoreactivity in the tissue extracts eluted in the same position as the synthetic peptide on ion exchange and gel filtration chromatography. It is suggested that a sub-population of substance P-immunoreactive neurones in the hypoganglionic zone is selectively depleted compared with other myenteric neurones. The factors involved remain to be elucidated, but this strain of mice could prove useful for studies of the mechanisms involved in differentiation and development of enteric peptidergic neurones.

Modulation of motor and sensory pathways of the peristaltic reflex by cannabinoids

Cannabinoids have long been known to be potent inhibitors of intestinal and colonic propulsion. This effect has generally been attributed to their ability to prejunctionally inhibit release of acetylcholine from excitatory motor neurons that mediate, in part, the ascending contraction phase of the peristaltic reflex. In the present study we examined the effect of cannabinoids on the other transmitters known to participate in the peristaltic reflex using a three-compartment preparation of rat colon that allows separation of ascending contraction, descending relaxation, and the sensory components of the reflex. On addition to the orad motor compartment, anandamide decreased and AM-251, a CB-1 antagonist, increased ascending contraction and the concomitant substance P (SP) release. Similarly, on addition to the caudad motor compartment, anandamide decreased and AM-251 increased descending relaxation and the concomitant vasoactive intestinal peptide (VIP) release. On addition to the central sensory compartment, anandamide decreased and AM-251 increased both ascending contraction and SP release orad, and descending relaxation and VIP release caudad. This suggested a role for CB-1 receptors in modulation of sensory transmission that was confirmed by the demonstration that central addition of anandamide decreased and AM-251 increased release of the sensory transmitter, calcitonin gene-related peptide (CGRP). We conclude that the potent antipropulsive effect of cannabinoids is the result of inhibition of both excitatory cholinergic/tachykininergic and inhibitory VIPergic motor neurons that mediate ascending contraction and descending relaxation, respectively, as well as inhibition of the intrinsic sensory CGRP-containing neurons that initiate the peristaltic reflex underlying propulsive motility.

Role of NK1 and NK2 receptors in mouse gastric mechanical activity

1. The aim of the present study was to examine the role of NK1 and NK2 receptors in the control of mechanical activity of mouse stomach. In this view, the motor effects induced by NK1 and NK2 receptor agonists and antagonists were analyzed, measuring motility as intraluminal pressure changes in mouse-isolated stomach preparations. In parallel, immunohistochemical studies were performed to identify the location of NK1 and NK2 receptors on myenteric neurons and smooth muscle cells. 2. Substance P (SP) induced biphasic effects: a contraction followed by relaxation; neurokinin A (NKA) and [beta-Ala8]-NKA(4-10), selective agonist of NK2 receptors, evoked concentration-dependent contractions, whereas [Sar9, Met(O2)11]-SP, selective agonist of NK1 receptors, induced concentration-dependent relaxation. 3. SR48968, NK2 receptor antagonist, did not modify the spontaneous activity and reduced the contractile effects induced by tachykinins without affecting the relaxation. SR140333, NK1 receptor antagonist, did not modify the spontaneous activity and antagonized the relaxant response to tachykinins, failing to affect the contractile effects. 4. The relaxation to SP or to [Sar9, Met(O2)11]-SP was abolished by tetrodotoxin (TTX) and significantly reduced by N(omega)-nitro-L-arginine methyl ester (L-NAME). 5. NK2-immunoreactivity (NK2-IR) was seen at the level of the smooth muscle cells of both circular and longitudinal muscle layers. NK1-immunoreactive (NK1-IR) neurons were seen in the myenteric ganglia and NK1/nNOS double labeling revealed that some neurons were both NK1-IR and nNOS-IR. 6. These results suggest that, in mouse stomach, NK1 receptors, causing relaxant responses, are present on nitrergic inhibitory myenteric neurons, whereas NK2 receptors, mediating contractile responses, are present at muscular level.

Idiopathic chronic constipation: tachykinins as cotransmitters in colonic contraction

BACKGROUND

Tachykinins (TKs) have been shown to be involved in the excitatory enteric motor pathway. This study aimed to examine the direct and nerve-mediated effect of specific NK1, NK2 and NK3 receptor agonists and antagonists in colonic preparations from control subjects and patients with idiopathic chronic constipation (ICC).

MATERIALS AND METHODS

Cumulative concentrations of Sar9Met(O2)11 substance P (selective NK1 receptor agonist), [Ala5,beta-Ala8]-neurokinin A (4-10) (selective NK2 receptor agonist) and [MePhe7]-neurokinin B (selective NK3 receptor agonist) were tested on colonic circular muscle strips to evaluate the direct drug effects. In addition, in the presence of atropine, the role of TKs in the off-contraction that follows the typical inhibitory response evoked by low frequencies of electrical field stimulation (0.5--10 Hz, 20 V, 1 ms pulse trains lasting 1 min) was investigated.

RESULTS

In control preparations, the rank order of potency was: NK2 receptor-selective agonist > NK3 receptor-selective agonist > NK1 receptor-selective agonist. The off-contraction was found to be reduced by about 30--40% in colonic circular muscle from ICC patients with respect to controls. Incubation with the NK1 receptor agonist did not modify the off-contraction measurements in either control or ICC preparations. Conversely, both NK2 and NK3 receptor agonists significantly (P < 0.01) increased the off-contraction in ICC preparations only. This increased response was fully antagonized by MEN-10627, a NK2 and NK3 receptor antagonist depending on the dose.

CONCLUSIONS

We may conclude that ICC is hyporesponsive to TKs and that the contractile reflex to distension is greatly reduced in ICC disease, but can be restored by incubation with NK2 and NK3 receptor agonists.

Evaluation of substance P as a neurotransmitter in equine jejunum

OBJECTIVE

To determine whether substance P (SP) functions as a neurotransmitter in equine jejunum.

SAMPLE POPULATION

Samples of jejunum obtained from horses that did not have lesions in the gastrointestinal tract.

PROCEDURE

Jejunal smooth muscle strips, oriented in the plane of the circular or longitudinal muscle, were suspended isometrically in muscle baths. Neurotransmitter release was induced by electrical field stimulation (EFS) delivered at 2 intensities (30 and 70 V) and various frequencies on muscle strips that were maintained at low tension or were under contraction. A neurokinin-1 receptor blocker (CP-96,345) was added to baths prior to EFS to interrupt SP neurotransmission. Additionally, direct effects of SP on muscle strips were evaluated, and SP-like immunoreactivity was localized in intestinal tissues, using indirect immunofluorescence testing.

RESULTS

Substance P contracted circularly and longitudinally oriented muscle strips. Prior treatment with CP-96,345 altered muscle responses to SP and EFS, suggesting that SP was released from depolarized myenteric neurons. Depending on orientation of muscle strips and stimulation variables used, CP-96,345 increased or decreased the contractile response to EFS. Substance P-like immunoreactivity was detected in the myenteric plexus and circular muscle layers.

CONCLUSIONS AND CLINICAL RELEVANCE

Substance P appears to function as a neurotransmitter in equine jejunum. It apparently modulates smooth muscle contractility, depending on preexisting conditions. Effects of SP may be altered in some forms of intestinal dysfunction. Altering SP neurotransmission in the jejunum may provide a therapeutic option for motility disorders of horses that are unresponsive to adrenergic and cholinergic drugs.

Regulation of guinea pig intestinal peristalsis by endogenous endothelin acting at ET(B) receptors

BACKGROUND & AIMS

Endothelins are expressed in many enteric neurons of the gut. Because activation of endothelin ET(A) and ET(B) receptors is known to alter intestinal muscle activity, the effect of ET(A) and ET(B) receptor agonists and antagonists on propulsive peristalsis was examined.

METHODS

Repetitive peristalsis in fluid-perfused segments of the guinea pig isolated small intestine was elicited by a rise of the intraluminal pressure and recorded via the pressure changes generated by the peristaltic waves.

RESULTS

Endothelin 1 (0.3-10 nmol/L added to the organ bath) stimulated peristalsis as shown by a decrease in the pressure threshold at which peristaltic waves were triggered, whereas the endothelin analog sarafotoxin 6c (0.3-10 nmol/L) inhibited peristalsis as reflected by an increase in the pressure threshold. The ET(A) receptor antagonist BQ-123 (3 micromol/L) converted the properistaltic action of endothelin 1 to an antiperistaltic action, whereas the ET(B) receptor antagonist BQ-788 (3 micromol/L) prevented the antiperistaltic action of sarafotoxin 6c. BQ-788, but not BQ-123, facilitated peristalsis on its own. Additional experiments indicated that the properistaltic action of endothelin 1 is mediated by enteric neurons, whereas the peristaltic motor effects of sarafotoxin 6c and BQ-788 are caused by a direct action on the muscle.

CONCLUSIONS

ET(A) receptor activation stimulates, whereas ET(B) receptor activation inhibits, intestinal peristalsis. The ability of BQ-788 to facilitate peristalsis per se points to a physiologic role of ET(B) receptors in peristaltic motor regulation.

Differences in circular muscle contraction and peristaltic motor inhibition caused by tachykinin NK1 receptor agonists in the guinea-pig small intestine

The tachykinin NK1 receptor agonist substance P methyl ester (SPOME) impedes intestinal peristalsis by releasing nitric oxide (NO) from inhibitory motor neurones. Since NK1 receptor agonists differ in their receptor interaction, we set out to compare a range of NK1 receptor agonists including SPOME, septide and GR-73 632 in their effects on propulsive peristalsis and circular muscle activity in the guinea-pig isolated small intestine. SPOME (100-300 nM) inhibited peristalsis by a rise of the pressure threshold at which peristaltic waves were triggered, whereas septide and GR-73 632 (30-300 nM) interrupted peristalsis by causing circular muscle spasms. Separate experiments showed that all three NK1 receptor agonists caused contraction of the circular muscle, which was enhanced by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (300 mM) and the P2X purinoceptor antagonist suramin (300 mM). In contrast, tetrodotoxin (300 nM) augmented the contractile effect of septide and GR-73 632 but not that of SPOME. It is concluded that the motor response to NK1 receptor agonists involves release of NO and adenosine triphosphate from inhibitory motor neurones. However, the NK1 receptor agonists differ in the mechanism by which they cause inhibitory transmitter release, which corresponds to differences in their antiperistaltic action.

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.

Tachykinin receptors mediate atropine-resistant rat duodenal reflex contractions in vivo

The study aimed to establish the possible role of tachykinins as mediators of atropine-resistant reflex contractions evoked by balloon distension in the proximal duodenum of urethane-anesthetized, guanethidine (34 mumol/kg s.c.)-pretreated rats. Distension of the balloon with a small amount (0.2-0.3 ml) of saline induced the appearance of phasic rhythmic contractions (about 11 mmHg in amplitude) which were promptly suppressed by either atropine (3 mumol/kg i.v.) or hexamethonium (28 mumol/kg i.v.). Despite the continuous i.v. infusion of atropine (2 mumol/h), low-amplitude rhythmic phasic contractions recovered, which were promptly suppressed by hexamethonium, to indicate the involvement of an atropine-resistant excitatory reflex. The amplitude of these atropine-resistant contractions was increased to about 4-5 mmHg by further distension of the balloon (0.4-0.6 ml) : under these conditions, the atropine-resistant contractions undergo a progressive fading. The fading was prevented by i.v. administration of the nitric oxide (NO) synthase inhibitor, L-nitroarginine methyl ester (L-NAME, 55 mumol/h), to provide a suitable baseline (amplitude of contractions was 7-8 mmHg) for studying the effect of tachykinin receptor antagonists. I.v. administration of the selective tachykinin NK2 receptor antagonists, MEN 10,627 (10-100 nmol/kg) and SR 48968 (100-300 nmol/kg) or of the selective NK1 antagonist SR 140333 (100 nmol/kg), at doses which do not affect the duodenal contractions induced by acetylcholine (5.5 mumol/kg i.v.), produced a prompt and long lasting suppression of the atropine-resistant reflex duodenal contractions produced by balloon distension in urethane-anesthetized rats, whilst SR-48965 (300 nmol/kg), the enantiomer of SR-48968 devoid, of NK2 receptor blocking activity, was without effect. I.v. administration of the selective NK1 receptor agonists [Sar9] substance P sulfone and septide or of the NK2 receptor selective agonist, [beta Ala8] neurokinin A(4-10) produced dose-dependent contractions of the duodenum. SR 140333 (100 nmol/kg i.v.) selectively antagonized the duodenal contractions produced by [Sar9] substance P sulfone and septide without affecting those produced by [beta Ala8] neurokinin A(4-10). On the other hand, MEN 10,627 (30-100 nmol/kg i.v.) and SR 48968 (100-300 nmol/kg i.v.) but not SR 48965 (300 nmol/kg i.v.) antagonized, at a comparable extent, duodenal contractions induced by both the selective NK2 and NK1 receptor agonists. We conclude that endogenous tachykinins are involved in mediating atropine-resistant reflex contractions evoked by distension of the rat duodenum in vivo: both NK1 and NK2 receptors are activated by endogenous ligands to produce NANC contractions of rat duodenum in vivo. However, the contractile response to i.v. administered NK1 receptor agonists, [Sar9] substance P sulfone and septide, may involve the release of mediators producing smooth muscle contraction via NK2 receptors.

Localisation of NK1 receptor immunoreactivity to neurons and interstitial cells of the guinea-pig gastrointestinal tract

Tachykinins, including substance P, neurokinin A, and neuropeptides K and gama, are expressed widely in the peripheral nervous system where they affect smooth muscle contraction, exocrine gland secretion, vascular permeability, and neurotransmission. Substance P, the preferred ligand for the NK1 receptor, is found in high concentrations in the enteric nervous system. In the present study, the localisation and distribution of the NK1 receptor was studied throughout the gastrointestinal tract of the guinea-pig by using a polyclonal antiserum raised against the C-terminal 15 amino acids of the NK1 receptor. Co-localisation with other neuronal markers was examined in the ileum. Nerve cell bodies reactive for the NK1 receptor were found in the myenteric plexus of all regions and the submucous plexus of the small and large intestines. In the small intestine, the interstitial cells of Cajal were also immunoreactive. Immunoreactivity was largely confined to cell surfaces. Almost all immunoreactive myenteric nerve cells had Dogiel type I morphology, and most of these were immunoreactive for nitric oxide synthase, a transmitter of inhibitory neurons to the muscle and of descending interneurons. Neuropeptide Y-containing secretomotor neurons in the submucous and myenteric plexuses also exhibited NK1 receptor immunoreactivity. NK1 receptors were present on a minority of tachykinin immunoreactive neurons of submucous ganglia. The results suggest that receptors on the longitudinal muscle might not be conventional NK1 receptors, that excitation of the circular muscle of the ileum is indirect, perhaps via the interstitial cells of Cajal, and that enteric inhibitory neurons may be excited via NK1 receptors.

Neuronal tachykinin NK2 receptors mediate release of non-adrenergic non-cholinergic inhibitory transmitters in the circular muscle of guinea-pig colon

The aims of this study were: (i) verify the usefulness of the recently described non-peptide antagonist, SR 142801, for blocking tachykinin NK3 receptors in the circular muscle of the guinea-pig colon and (ii) after occlusion of NK3 receptors by SR 142801, test the hypothesis that tachykinins may activate non-adrenergic non-cholinergic inhibitory neurons via non-NK3 receptors. In sucrose gap, we found that SR 142801 (0.1 microM) time-dependently inhibited the senktide-induced atropine (1 microM)-sensitive depolarization, action potentials and contractions of circular muscle of guinea-pig colon without affecting the cholinergic excitatory junction potential and contraction produced by single pulse electrical field stimulation. Likewise, SR 142801 (0.1 microM) time-dependently inhibited the senktide-induced non-adrenergic non-cholinergic hyperpolarization and relaxation of the circular muscle, without affecting the non-adrenergic non-cholinergic inhibitory junction potentials and relaxation produced by single pulse electrical field stimulation. Therefore, SR 142801 is a suitable tool to occlude neuronal NK3 receptors in guinea-pig colon. In the presence of SR 142801 (0.1 microM), atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM) and nifedipine (1 microM) superfusion with neurokinin A (0.3 microM) produced depolarization on which a series of inhibitory junction potentials were superimposed. The incidence, number and amplitude of the inhibitory junction potentials evoked by neurokinin A was partly reduced by pretreatment with either apamin (0.1 microM) or L-nitroarginine (30 microM) and was totally blocked by pretreatment with apamin plus L-nitroarginine or by tetrodotoxin (1 microM). None of these treatments affected the depolarization and contraction produced by neurokinin A. The NK1 receptor selective antagonist, GR 82,334 (3 microM), did not affect the responses to neurokinin A, which were abolished by the NK2 receptor-selective antagonist GR 94,800 (0.1 microM). Substance P (0.3 microM) produced a large depolarization of the membrane but was poorly effective in producing superimposed inhibitory junction potentials. The NK1 receptor-selective agonist [Sar9]substance P sulfone (0.3 microM) produced large depolarization without inducing superimposed inhibitory junction potentials, while the NK2 receptor-selective synthetic agonist [beta-Ala8]neurokinin A(4-10) (0.3 microM) produced depolarization and superimposed inhibitory junction potentials. We conclude that neurokinin A, in addition to direct excitation and contraction of circular muscle activates, via neuronal NK2 receptors, inhibitory non-adrenergic non-cholinergic motorneurons. Thus, neuronal NK2 receptors should be considered as targets for endogenous tachykinins in enteric circuitries leading to descending relaxation in guinea-pig colon.

Role of tachykinins as excitatory mediators of NANC contraction in the circular muscle of rat small intestine

1. The aim of this study was to assess the role of tachykinins, acting via NK1 and NK2 receptors, in mediating nonadrenergic noncholinergic (NANC) contractions produced by electrical field stimulation (EFS) in the circular muscle of the rat small intestine. 2. In the presence of atropine (1 microM), guanethidine (3 microM), indomethacin (10 microM), apamin (0.3 microM) and L-nitroarginine (L-NOARG, 100 microM) and after in vitro capsaicin (10 microM for 15 min) pretreatment, EFS (0.25 ms pulse width, 100 V, 1-30 Hz for 5 s) produced a frequency-dependent NANC contraction of mucosa-free circular muscle strips from the rat proximal duodenum and terminal ileum. In the duodenum, the NANC contraction was preceded by a transient NANC relaxation. All responses to EFS were abolished by 1 microM tetrodotoxin. 3. The NK1 receptor selective antagonist, SR 140,333 (0.1 microM for 60 min) and the NK2 receptor selective antagonist, MEN 10,627 (0.1 microM for 60 min), both produced a partial inhibition of the contractile response to EFS. The co-administration of SR 140,333 and MEN 10,627 produced a profound inhibition of the response to EFS in the duodenum, larger than that produced by each antagonist alone; a fraction (about 25% of the response at 30 Hz) of the NANC contraction of the duodenum persisted in the presence of the two antagonists. This residual response was however abolished after co-administration of the NK1 and NK2 receptor antagonists, GR 94,800 (1 microM) and GR 82,334 (10 microM). The co-administration of SR 140,333 and MEN 10,627 nearly abolished the NANC contraction to EFS in the ileum. 4. Nifedipine (1 microM) induced a profound depression of the NANC contraction to EFS in both duodenal and ileal strips. A fraction of the response to EFS (about 25 and 5-10% of the response at 30 Hz in the duodenum and ileum, respectively) was nifedipine-resistant. SR 140,333 (0.1 microM) had little effect on the nifedipine-resistant response to EFS in the duodenum although it reduced by about 50% the response in the ileum. MEN 10,627 (0.1 microM) produced a partial inhibitory effect of the nifedipine-resistant response in both regions. The co-administration of SR 140,333 and MEN 10,627 nearly abolished the nifedipine-resistant response in the ileum while a small fraction (about 20% of control) of the response persisted in the duodenum.

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.

Specialization of tachykinin NK1 and NK2 receptors in producing fast and slow atropine-resistant neurotransmission to the circular muscle of the guinea-pig colon

We studied the relative contribution of tachykinin NK1 and NK2 receptors in producing nonadrenergic noncholinergic excitation of the circular muscle of the guinea-pig proximal colon in response to electrical field stimulation. All experiments were performed in the presence of atropine, guanethidine, indomethacin, apamin and L-nitroarginine. In organ bath experiments, electrical stimulation produced a tetrodotoxin-sensitive frequency-dependent contraction. The NK1 receptor antagonists, FK 888 (1-10 microM) and GR 82,334 (0.3-3 microM) markedly reduced but did not abolish the nonadrenergic noncholinergic response. The NK2 receptor antagonist, GR 94,800 (0.3-3 microM) was partly effective at 3 microM. The combined administration of FK 888 (10 microM) and GR 94,800 (3 microM) or GR 82,334 and GR 94,800 abolished the nonadrenergic noncholinergic contraction. The response to a prolonged period of stimulation (3 Hz for 5 min) was evenly depressed by FK 888 or GR 82,334, while GR 94,800 was more effective in inhibiting the late (87% inhibition) than the peak response (25% inhibition). In the presence of nifedipine (1 microM) a marked inhibition of the nonadrenergic noncholinergic contraction was observed and a time lag was evident between stimulus application and onset of contraction, which showed slow onset and offset kinetics. The nifedipine-resistant nonadrenergic noncholinergic contraction was unaffected by FK 888 or GR 82,334 but was suppressed by GR 94,800. Submaximally effective (1-3 nM) concentrations of substance P and neurokinin A produced distinct patterns of contraction: the response to substance P was fast and declined rapidly toward baseline; the response to neurokinin A was slow and sustained. In the presence of nifedipine, the response to substance P was greatly depressed and became slower in onset; nifedipine did not affect the contraction to neurokinin A but slowed its time-course. In sucrose gap experiments, either a short (10 Hz for 1 s) or a prolonged period of electrical stimulation (3 Hz for 3 min) evoked membrane depolarization, action potentials and contraction: in response to the "prolonged" stimulation, distinct phasic and tonic component of contraction were observed. Nifedipine abolished action potentials and the phasic contraction produced by a short period of stimulation, reduced by about 50% the maximal contraction developed during the prolonged stimulation without affecting the amplitude of the tonic response. In the presence of nifedipine, GR 82,334 (3 microM) blocked the membrane depolarization but did not affect contraction; GR 94,800 (0.1 microM) did not affect depolarization but abolished contraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Tachykinin NK1 and NK2 receptors mediate atropine-resistant ileal circular muscle contractions evoked by capsaicin

The atropine-resistant contractile action of the sensory stimulant drug capsaicin was examined on guinea-pig ileum circular muscle in vitro, with special regard to the involvement of endogenous tachykinins acting through tachykinin NK1 and NK2 receptors. A protocol, using ruthenium red was developed for overcoming desensitization to capsaicin so that two reproducible responses to this drug were obtained. Capsaicin (10(-6) M) caused tonic and phasic contractions of the tissue. This effect was significantly inhibited by the tachykinin NK1 receptor blocking drug FK 888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-proly]-N- methyl-N-phenylmethyl-3-(-2-naphthyl)-L-alaninamide) or the tachykinin NK2 receptor inhibitor GR 94,800 (PhCO-Ala-Ala-D.Trp-Phe-D.Pro-Pro-NleNH2) (10(-6) M each) and was practically abolished by the combined administration of the two tachykinin receptor blockers. Likewise, the neuronal Na+ channel inhibitor tetrodotoxin abolished the response to capsaicin. It is concluded that the contractile effect of capsaicin in the circular muscle is predominantly mediated by tachykinin release and both subtypes of tachykinin receptor (NK1 and NK2) play an important role in this process. The source of tachykinins, however, is probably intrinsic neurons of the myenteric plexus, indirectly activated by capsaicin-sensitive nerves, as shown by the sensitivity of the response to tetrodotoxin.

Tachykinin NK1 and NK2 receptor antagonists and atropine-resistant ascending excitatory reflex to the circular muscle of the guinea-pig ileum

1. The aim of this study was to investigate the effect of various antagonists, selective for the tachykinin NK1 or NK2 receptor, on the atropine-resistant ascending excitatory reflex (AER) to the circular muscle of the guinea-pig ileum elicited by radial stretch (balloon distension) or electrical field stimulation. 2. Submaximal and maximal atropine- (1 microM) resistant AER elicited by balloon distension averaged about 40-50% and 70-90% of maximal circular spasm to 80 mM KCl, respectively. The NK1 receptor antagonist, (+/)-CP 96,345 (1 microM) inhibited both maximal and submaximal AER. FK 888 (1-3 microM) inhibited submaximal AER only. RP 67,580 (1 microM) was ineffective. The NK2 receptor antagonist, GR 94,800, inhibited both maximal and submaximal AER at all concentrations tested (0.1-3.0 microM), while SR 48,968 was effective only at 1.0 microM. The NK2 receptor antagonists, MEN 10,376 and MEN 10,573 inhibited both submaximal and maximal AER at 10 and 1.0 microM, respectively. 3. In other experiments, an NK1 receptor antagonist, (+/-)-CP 96,345 or FK 888 (1.0 microM in each case) was administered first and the effect of GR 94,800 (1.0 microM) on the residual AER response was determined; or GR 94,800 was administered first and the effect of (+/-)-CP 96,345 or FK 888 was determined. The results of these experiments indicated an additive effect produced by the combined treatment with NK1 and NK2 receptor antagonists. 4. Electrical field stimulation (10 Hz for 0.5 s, 10-20 V, 0.15-0.3 ms pulse width) with electrodes placed at 1.4-1.8 cm anal to the recording site, produced ascending contractions which were almost abolished by 10 MicroM hexamethonium (electrically-evoked AER). In the presence of apamin (0.1 MicroM) and N0-nitro-L-arginine (30 MicroM) these contractions were reproducible over 10 consecutive stimulation cycles.GR 94,800 (1 MicroM) and FK 888 (1 MicroM) both produced a partial inhibition of the electrically-evoked AER and their combined administration produced an inhibitory effect which was larger than that induced by each antagonist alone.5. FK 888 (1-3 MicroM), GR 94,800 (1-3 MicroM), MEN 10,573 (1 MicroM) and MEN 10,376 (10 MicroM) did not significantly affect the atropine-sensitive twitch contractions produced by electrical field stimulation of the guinea-pig ileum longitudinal muscle-myenteric plexus preparation, which were abolished by 10-30 MicroM procaine, 1 MicroM tetrodotoxin or 1 MicroM atropine. (+/-)-CP 96,345 (1 MicroM) and SR 48,968 (1 ILM)produced 12% and 27% inhibition of cholinergic twitches in the longitudinal muscle of the ileum,respectively.6. We conclude that both NK1 and NK2 receptors mediate the atropine-resistant AER to the circular muscle of the ileum. NK2 receptor activation plays a more important role than NK1 receptor activation in the AER evoked by radial stretch. Since a consistent fraction of the distension- and electrically evoked atropine-resistant AER persists in the presence of combined NK1 and NK2 receptor blockade,the existence of a third excitatory transmitter to the circular muscle of the ileum, in addition to acetylcholine and tachykinins, is suggested.

Different Ca2+ influx pathways mediate tachykinin receptor-induced contraction in circular muscle of guinea-pig colon

We used an electrophysiological approach (single sucrose gap) to compare the mechanism of action of selective tachykinin NK1 and NK2 receptor agonists ([Sar9]substance P sulfone and [beta ala8]neurokinin A-(4-10), respectively) in producing contraction of the circular muscle of the guinea-pig proximal colon. [Sar9]Substance P sulfone produced a marked depolarization, action potentials and increase in membrane conductance. On the other hand, [beta Ala8]neurokinin A-(4-10) produced less depolarization of the cell membrane and did not change membrane resistance. Nifedipine (1 microM) greatly reduced (80% inhibition) the contraction due to [Sar9]substance P sulfone while that due to [beta Ala8]neurokinin A-(4-10) was slightly affected (13% inhibition). Action potentials induced by either agonist were suppressed by nifedipine, while depolarization was reduced only to a minor extent. When tested in a Ca(2+)-free medium, the contraction produced by either agonist was greatly reduced (84-89%) as compared to the control. In organ bath experiments [Sar9]substance P sulfone and [beta Ala8]neurokinin A-(4-10) produced concentration-dependent contraction of the circular muscle of the colon (EC50 8 and 12 nM, respectively). Nifedipine (1 microM) markedly suppressed the response to [Sar9]substance P sulfone while that to [beta Ala8]neurokinin A-(4-10) was only slightly depressed. These findings demonstrate that NK1 receptor-mediated contraction is strictly linked to membrane depolarization and action potentials generation through nifedipine-sensitive Ca2+ channels (electromechanical coupling) while the NK2 receptor-mediated contraction is substantially unrelated to depolarization and, while being largely dependent upon extracellular Ca2+, is nifedipine-resistant, possibly linked to the opening of non-selective (Ca(2+)-permeable) receptor-gated cation channels (pharmacomechanical coupling).

Accommodation mediated by enteric inhibitory reflexes in the isolated guinea-pig small intestine

1. The aim of the present study was to investigate whether the guinea-pig small intestine shows accommodation to infused fluid, similarly to other regions of the gastrointestinal tract. Tetrodotoxin, papaverine and transmitter antagonists were used to establish the existence of reflex pathways and the nature of the neurotransmitters involved. 2. Compliance, measured as the change in volume of infused fluid divided by the intraluminal pressure change, was reduced by tetrodotoxin (0.6 microM), indicating that there is an overall neurally mediated relaxation of the circular muscle in response to low rates of distension. Papaverine (10 microM) did not have any significant effect on compliance at the low rates of distension, suggesting that the circular muscle is fully relaxed. 3. At each rate of distension, 400 microM N omega-nitro-L-arginine methyl ester (L-NAME, a nitric oxide synthase inhibitor) significantly decreased the compliance of the intestinal wall, indicating that the circular muscle was relaxed by a nitric oxide-mediated mechanism. Apamin (0.5 microM), which blocks a component of inhibitory transmission, did not have a significant effect. 4. In control preparations, the intestinal wall was less compliant when distended by fluid at a fast rate, compared with the lower rates of distension. This was not due to changes in passive components of the intestinal wall or a myogenic response to rapid stretch. 5. When the intestine was distended rapidly, 1 microM hyoscine and 100 microM hexamethonium increased intestinal compliance. However, they had no detectable effect on compliance with low rates of distension.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of longitudinal muscle-myenteric plexus removal and indomethacin on the response to tachykinin NK-2 and NK-3 receptor agonists in the circular muscle of the guinea-pig ileum

1. The effect of removal of the longitudinal muscle-myenteric plexus (LM-MP) and/or indomethacin (10 microM) on the response to the tachykinin NK-2 receptor selective agonist, [beta Ala8]NKA(4-10), or to the NK-3 receptor selective agonist, senktide, was investigated by measuring mechanical activity (isotonic recording) of circular muscle (ring preparation) of the guinea-pig ileum. 2. Indomethacin (10 microM) increased the percentage of ileal rings displaying spontaneous activity, either intact or LM-MP-free. The response to senktide (10 nM and 1 microM) was lower in LM-MP-free than in intact ileal rings, either in the absence or presence of indomethacin. The response to a low concentration (10 nM) of [beta Ala8] NKA (4-10) was enhanced in LM-MP-free rings and by indomethacin. 3. In intact ileal rings, the response to senktide was unaffected by atropine (3 microM) alone or by the tachykinin NK-2 receptor antagonist MEN 10,376 (10 microM) alone while it was reduced by the combined administration of the two antagonists. The response to senktide was greatly reduced by tetrodotoxin (TTX, 1 microM). Senktide-induced contractions (10 nM) were also reduced by the blocker of N-type voltage-sensitive calcium channels, omega-contoxin (CTX, 0.1 microM). 4. In about 30% of preparations tested, an inhibitory response (decrease in spontaneous activity) to 10 nM senktide, was disclosed in CTX-treated intact ileal rings. This inhibitory effect was TTX-sensitive. 5. In LM-MP-free ileal rings, the response to senktide was abolished or reduced by atropine and MEN 10,376, alone or in combination, and was also reduced or abolished by TTX and CTX. 6. The response to [beta Ala8]NKA (4-10) was inhibited by MEN 10,376, in both intact and LM-MP-free ileal rings while it was unaffected by atropine, TTX or CTX. 7. These results indicate that indomethacin pretreatment induces a regular background activity for studying the motor response to tachykinins in the circular muscle of the ileum, probably by blocking the formation of relaxant prostanoids. A further increase in sensitivity to direct smooth muscle stimulation (NK-2 receptor agonist) can be obtained by removal of the LM-MP. The response to NK-3 receptor stimulation is diminished but not abolished by removal of the LM-MP, suggesting that NK-3 receptors are located on neuronal bodies of myenteric neurons, but possibly also at other sites (possibly, nerve terminals).(ABSTRACT TRUNCATED AT 400 WORDS)

Gastrointestinal neurotransmitters

The enteric nervous system contains neurones that are intrinsic to the gastrointestinal tract and the axons of extrinsic neurones. More than 30 functional types of neurone are present and about 25 different possible neurotransmitters have been identified in enteric neurones. Most neurones utilize several transmitters; amongst the transmitters of an individual neurone, one is usually a primary transmitter and other substances are subsidiary transmitters or neuromodulators. The primary transmitter is the substance that has the major role in acutely changing the excitability of the innervated cell. Current evidence indicates that primary transmitters are strongly conserved; that is, the same substance will be the neurotransmitter in functionally equivalent neurones in different regions of the gastrointestinal tract and in different species. In contrast, subsidiary transmitters and neuromodulators of equivalent neurones in different regions are not necessarily the same. Only about seven of the approximately 25 enteric neurotransmitters are known to be primary transmitters. Acetylcholine is the primary transmitter of vagal and pelvic preganglionic neurones, of enteric interneurones, of one class of secretomotor neurone in the intestine and of motor neurones controlling gastric acid secretion. Acetylcholine and tachykinins are co-primary transmitters of muscle motor neurones, with acetylcholine appearing to have the greater role. Tachykinins are probably primary transmitters of enteric sensory neurones at neuroneuronal synapses. Serotonin may also be a transmitter to neurones in the enteric ganglia. Nitric oxide appears to be the usual primary transmitter of enteric inhibitory motor neurones to the muscle. ATP and vasoactive intestinal peptide are subsidiary transmitters of these neurones, although in some regions they may have a primary transmitter role. Vasoactive intestinal peptide is the primary transmitter of non-cholinergic secretomotor neurones. Gastrin releasing peptide is the primary transmitter of motor neurones to gastrin cells. Noradrenaline is the primary transmitter of sympathetic neurones that supply the intestine.

Tachykinin NK1 but not NK2 receptors mediate non-cholinergic excitatory junction potentials in the circular muscle of guinea-pig colon

1. The effect of tachykinin NK1 and NK2 receptor antagonists on noncholinergic excitatory junction potentials (e.j.ps) evoked by electric field stimulation (EFS) in the circular muscle of the guinea-pig proximal colon was investigated by means of a sucrose-gap technique. 2. In the presence of 1 microM atropine, submaximal EFS (10 Hz, 20-30 V, 0.5 ms pulse width, 1 s train duration) evoked an inhibitory junction potential (i.j.p.) followed by e.j.p. with superimposed action potentials (APs) and contraction. Addition of either NG-nitro-L-arginine (L-NOARG, 0.1 mM) or apamin (0.1 microM) inhibited the evoked i.j.p. and the combined administration of the two agents almost abolished it. In the presence of both L-NOARG and apamin, an atropine-resistant e.j.p. was the only electrical response evoked by EFS in 50% of cases and a small i.j.p. (10% of original amplitude) followed by e.j.p. was evident in the remainder. 3. In the presence of L-NOARG and apamin, the tachykinin NK1 receptor antagonists, (+/-)-CP 96,345 and GR 82,334 (10 nM-3 microM) concentration-dependently inhibited the atropine-resistant e.j.p. and accompanying contraction evoked by EFS. EC50 values were: 0.77 microM (e.j.p. inhibition) and 0.22 microM (inhibition of contraction) for (+/-)-CP 96,345; 0.61 microM (e.j.p. inhibition) and 0.20 microM (inhibition of contraction) for GR 82,334. The tachykinin NK2 receptor antagonists, MEN 10,376 (up to 3 microM) and SR 48,968 (up to 1 microM) had no effect on the atropine-resistant e.j.p. MEN 10,376 (3 microM) but not SR 48,968 produced a slight inhibition of the evoked contraction. 4. (+/- )-CP 96,345 (3 microM) and GR 82,334 (3 microM) markedly reduced (81 and 89% inhibition, respectively)the atropine-resistant ej.p. in the absence of L-NOARG and apamin, without affecting the ij.p. MEN 10,376 (3 microM) and SR 48,968 (1 microM) had no significant effect on noncholinergic ij.p. and ej.p. evoked in the absence of apamin and L-NOARG.5. The electrical and mechanical responses to the NK, receptor agonist [Sar9]substance P (SP) sulfone were blocked by (+/-)-CP 96,345 (3 1M) or GR 82,334 (3 microM) which, at the same concentration, failed to affect the responses to the NK2 receptor agonist [PAla8] neurokinin A (NKA) (4-10). In contrast, MEN10,376 (3 microM) or SR 48,968 (1 microM) blocked the response to [beta Ala8]NKA(4-10) without affecting the response to [Sar9]SP sulfone.6. In the presence of L-NOARG and apamin, and in the absence of atropine, EFS of low pulse width(0.02-0.03 ms, other parameters as above) produced cholinergic ej.ps and contraction which were unaffected by GR 82,334 (3 microM). (+/-)-CP 96,345 (3 JAM) produced 24% reduction in the area of the atropine-sensitive ej.p. without affecting the peak amplitude of ej.p. or contraction.7. These findings demonstrate that the noncholinergic ej.ps and accompanying contraction of the circular muscle of the guinea-pig colon are produced through activation of intramural tachykininergic nerves and that the resultant smooth muscle response is almost entirely mediated through NK1 receptors.

Multiple inhibitory mechanisms mediate non-adrenergic non-cholinergic relaxation in the circular muscle of the guinea-pig colon

The mechanisms responsible for nerve-mediated, non-adrenergic, non-cholinergic (NANC) relaxation in mucosa-free circular muscle strips from the proximal colon of the guinea-pig were investigated. Electrical field stimulation (EFS, 1-20 Hz, trains of 5 s duration, 100 V, 0.25 ms pulse width) in the presence of atropine (1 mumol/l) and guanethidine (3 mumol/l) evoked a triphasic motor response consisting of: (a) a primary relaxation, (b) a rebound contraction and (c) a secondary relaxation. These three responses were abolished by tetrodotoxin (1 mumol/l). B oth apamin (0.01-0.3 mumol/l), a known blocker of low conductance, calcium-activated potassium channels in smooth muscles, and L-nitro-arginine (L-NOARG) (1-100 mumol/l), a known blocker of nitric oxide (NO) synthase, increased the tone of the strips. Maximum effects on tone were observed with 0.1 mumol/l apamin (21 +/- 3% of KCl-induced contraction) and 30 mumol/l L-NOARG (26 +/- 4% of KCl response). The combined administration of 0.1 mumol/l apamin and 30 mumol/l L-NOARG produced an increase in tone (47 +/- 5% of KCl response) that was larger than that produced by either compound alone. Neither apamin (0.1 mumol/l) nor L-NOARG (30 mumol/l) affected the isoprenaline-induced relaxation. Apamin (0.1 mumol/l) depressed, but did not abolish, the primary relaxation to EFS at all frequencies without affecting the secondary relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Ascending neural pathways in the rat ileum in vitro — Effect of capsaicin and involvement of nitric oxide

The aim of the present study was to develop and characterize an in vitro model of the rat ileum in which activation of the orally projecting neural excitatory pathway of the myenteric reflex is produced by electrical field stimulation anally to the recording site. The motility of a 10-cm segment of rat ileum was recorded using a perfused manometric assembly with side holes 2 and 4 cm orally to the stimulation site. Electrical field stimulation caused a contractile response in the oral but not in the aboral direction of the stimulation site. The contractile response, which was maximal using low stimulus frequencies (3 or 5 pulses per second (pps)) and decreased with higher frequencies (10 or 20 pps), was blocked by atropine (10(-6) M) at all frequencies tested after acute and after prolonged (greater than 30 min) treatment. The maximal contractile response at 3 pps was abolished by hexamethonium (10(-4) M), tetrodotoxin (5 x 10(-7) M) and by complete transection of the muscular wall between the stimulation and the recording site. Acute administration of capsaicin (8 x 10(-7) M) to the bath reduced the lag between the start of the electrical stimulation and the onset of the contractile response. Higher concentrations of capsaicin (10(-5) M) reduced the contractile response, but this was partly due to an unspecific effect of capsaicin. Blockade of nitric oxide (NO) synthesis by L-NG-nitro-arginine-methyl ester (L-NAME) (3 x 10(-4) M) augmented the contractile response to anal stimulation by 222.4% and reduced the lag period by 54.5%, whereas the stereoisomer D-NAME had no significant effect. The potentiating effects of L-NAME were reversed in the presence of L-arginine (3 x 10(-3) M) but not in the presence of the stereoisomer D-arginine (3 x 10(-3) M). This model can be used to study ascending neural pathways in the rat small intestine. The ascending excitatory response is abolished by atropine and hexamethonium and is modulated by capsicin-sensitive fibers. The ascending pathway is under tonic inhibition of metabolites of the L-arginine-NO pathway.

Tachykininergic transmission to the circular muscle of the guinea-pig ileum: evidence for the involvement of NK2 receptors

1. The effect of newly developed, receptor-selective tachykinin antagonists (GR 71,251 for NK1 receptors, MEN 10,376 and L 659,877 for NK2 receptors) on noncholinergic transmission to the circular muscle of the guinea-pig ileum has been investigated. 2. In circular muscle strips of the ileum, electrical field stimulation in the presence of atropine (2 microM) and apamin (0.1 microM) evoked a complex motor response. The tonic primary contraction in this response was reduced by GR 71,251 (10 microM) and MEN 10,376 (3-10 microM) but not by L 659,877 (up to 10 microM). The presence of apamin was necessary in this experimental arrangement to unmask an atropine-resistant primary contraction, sensitive to tachykinin antagonists. The motor response was abolished by tetrodotoxin. 3. In circular strips of the ileum GR 71,251 (10 microM) inhibited the tonic contraction produced by [Sar9] substance P sulphone, a selective NK1 receptor agonist but not that produced by [beta Ala8] neurokinin A (4-10), a selective NK2 receptor agonist. By contrast, MEN 10,376 antagonized the effect of the NK2 agonist while leaving the response to the NK1 agonist unaffected. 4. In whole segments of the ileum, distension of the gut wall by an intraluminal balloon placed at about 1 cm from the point of recording of mechanical activity of the circular muscle produced atropine-sensitive phasic contractions (ascending enteric reflex). In the presence of atropine (2 microM), a noncholinergic response was elicited, which required larger volumes of distension that the cholinergic one. The atropine-resistant ascending enteric reflex was enhanced by apamin (0.1 microM) and abolished by tetrodotoxin, either in the presence or absence of apamin.5. MEN 10,376 (3-lOmicroM) inhibited the atropine-resistant ascending enteric reflex in the presence of apamin while GR 71,251 or L 659,877 (10 microM each) were ineffective. MEN 10,376 inhibited the atropine-resistant ascending enteric reflex to a larger extent in the absence than in the presence of apamin and also slightly inhibited the ascending enteric reflex in the absence of atropine.6. These findings provide evidence for an involvement of NK2 tachykinin receptors in excitatory transmission to the circular muscle of the guinea-pig ileum. NK2 receptors are also involved in the physiological-like circular muscle activation produced by stimulation of intramural neuronal pathways which subserve the atropine-resistant ascending enteric reflex.

[Role of substance P in the nervous system control of digestive motility]

Substance P is a 11 amino-acids peptide which belongs to the tachykinins, a family of peptide which induces a rapid contraction of the smooth muscle of the digestive tract. The occurrence of substance P has been demonstrated by immunohistochemical and radioimmunological techniques in most parts of the central and peripheral nervous system. Substance P exerts on the smooth muscle of all the areas of the digestive tract a strong excitatory effect which is either direct or relayed by the cholinergic intramural neurones. Numerous electrophysiological, pharmacological and immunohistochemical data lead to the conclusion that substance P is released by intrinsic neurones of the digestive tract or by extrinsic nerves (vagus and splanchnic nerves, etc...). This release is enhanced by acetylcholine, cholecystokinin, serotonin and neurotensin, it is reduced by opioid peptides and noradrenaline. Substance P participates in the intestinal peristaltic reflex by the activation of the smooth muscle cells of the intestine, either directly or through the activation of the cholinergic intrinsic neurones. Substance P is also involved in the genesis of a non-cholinergic ascending excitatory activity likely occurring during vomiting. Lastly, substance P participates in the reflex contraction of the lower oesophageal sphincter following acidification of the distal part of the oesophagus.

Identification and immunohistochemistry of cholinergic and non-cholinergic circular muscle motor neurons in the guinea-pig small intestine

Motor neurons which innervate the circular muscle layer of the guinea-pig small intestine were retrogradely labelled, in vitro, with the carbocyanine dye, DiI, applied to the deep muscular plexus. By combining retrograde tracing and immunohistochemistry, the chemical coding of motor neurons was investigated. Five classes of neuron could be distinguished on the basis of the co-localization of immunoreactivity for the different antigens; the five classes were also characterized by different lengths and polarities of their axonal projections and by their cell body shapes. Two classes with local or orally directed axons were immunoreactive for choline acetyltransferase and substance P and are likely to be cholinergic excitatory motor neurons. Two other classes had anally directed axons; they were immunoreactive for vasoactive intestinal polypeptide and are likely to be inhibitory motor neurons. A small proportion of neurons with short projections to the circular muscle were immunoreactive for neither substance P nor for vasoactive intestinal polypeptide, but are likely to be cholinergic. The morphological and histochemical identification of excitatory and inhibitory motor neurons provides a neuroanatomical basis for the final motor pathways involved in the polarized reflex motor activity of the gut.

Neuropeptide release from isolated myenteric nerve endings derived from the guinea pig myenteric plexus

Isolated myenteric nerve varicosities prepared from the myenteric plexus of the guinea pig ileum were investigated as a suitable model system with which to study the release of several neuropeptide-like immunoreactivities (-LI). Basal release of substance P-LI, neurokinin A-LI, Leu-enkephalin-LI and Met-enkephalin-LI was determined, and clear depolarization-induced release of the enkephalin-LI's and neurokinin A-LI was obtained using this preparation, providing further support for their roles as putative mediators in the enteric nervous system. Evoked-release of these peptides was dependent on the presence in the incubation mixture of certain antagonists to known endogenous neuronal mediators. In the absence of such antagonists, no unequivocal evidence of release was seen. Clear evoked release of Leu-enkephalin-LI occurred only in the presence of the adenosine receptor antagonist 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX), atropine and naloxone. Release of Met-enkephalin-LI occurred in the presence of either atropine or naloxone. The release of neurokinin A-LI was evident in the presence of DPSPX. These findings suggest the existence of either distinct subpopulations of nerve varicosities or distinct neuronal pools containing each peptide and that these peptides may be under differential regulation by endogenous inhibitory mediators. It is concluded that, under suitable conditions, isolated myenteric nerve varicosities provide a useful model system for the study of release, and the modulation of release, of endogenous neuropeptides.

Distension-evoked ascending and descending reflexes in the circular muscle of guinea-pig ileum: an intracellular study

Reflex responses evoked by distension of the guinea-pig small intestine were recorded from the circular muscle with intracellular microelectrodes. For this purpose a mechanically stable preparation that allowed the intestinal wall to be distended within 9 mm of the recording site was developed. A segment of intestine was opened along the mesenteric border and pinned mucosa uppermost over a balloon set in the base of an organ bath, so that inflation of the balloon could distend the intestinal wall without simultaneously pushing against the mucosa. Compound excitatory junction potentials (EJPs) and compound inhibitory junction potentials (IJPs) were recorded at sites up to 40 mm oral and anal to the distending stimulus, respectively. The compound EJPs recorded orally had amplitudes of up to 24 mV and declined to baseline during distensions that exceeded 10-15 s. Distensions at intervals of less than 20 s evoked successively smaller oral compound EJPs; after four distensions in 30 the amplitude of the compound EJP had fallen to less than 10%. The amplitude of the oral compound EJP was reduced by hyoscine (1 microM), but the extent of the reduction depended on the degree of distension; responses to mild stimuli were blocked, whereas those to strong stimuli were only slightly reduced. The amplitude of the hyoscine-resistant component of the compound EJP was markedly reduced by antagonists of substance P receptors in the muscle. In the presence of muscarinic and substance P receptor antagonists, a transient compound IJP could be detected on the oral side of the stimulus. The compound IJPs recorded anal to the distension had amplitudes up to 22 mV but the potential returned to baseline during prolonged distension. In the presence of hyoscine (1 microM) some inhibitory activity continued throughout prolonged stimuli. Compound IJP amplitudes were not significantly reduced by repeated distensions separated by more than 6 s. At anal sites a transient depolarization (off-response) was recorded immediately following the termination of a distension in some preparations. The off-response was unaffected by hyoscine and was more readily observed after the further addition of substance P antagonists. The compound IJPs were almost completely blocked by apamin (0.2 microM). The compound EJPs and IJPs recorded orally were blocked by hexamethonium (100 microM), but the amplitudes of compound IJPs recorded anally were significantly reduced by hexamethonium (100-200 microM) only at recording sites greater than 15 mm from the centre of the balloon. The off-response was reduced by hexamethonium at all sites.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical analysis of potential neurotransmitters present in the myenteric plexus and muscular layers of the corpus of the guinea pig stomach

Recent electrophysiological studies of neurons of the myenteric plexus of the corpus of the guinea pig stomach have revealed that slow synaptic events are extremely rare. In contrast, they are commonly encountered in similar investigations of myenteric ganglia of the guinea pig small intestine. The current immunocytochemical analysis of the myenteric plexus and innervation of the muscularis externa of the corpus of the guinea pig stomach was undertaken in order to determine whether putative neurotransmitters capable of mediating slow synaptic events are present in gastric ganglia. A major difference between the small intestine and the stomach was found in the innervation of the musculature. Whereas the longitudinal muscle layer of the small intestine contains very few nerve fibers and is innervated mainly at its interface with the myenteric plexus, the longitudinal muscle of the corpus of the stomach contained as many varicose substance P (SP)-, vasocative intestinal polypeptide (VIP)-, and neuropeptide Y (NPY)-immunoreactive axons as the circular muscle layer. These putative neurotransmitters were also present in the ganglia of the myenteric plexus, where varicose SP-, VIP-, and NPY-immunoreactive fibers encircled nonimmunoreactive neurons. Varicose 5-hydroxytryptamine (5-HT)-immunoreactive terminal axons were essentially limited to the myenteric plexus and were found both in ganglia and in interganglionic connectives, where they were particularly numerous; 5-HT-immunoreactive neurons appeared to be more abundant in the stomach than in the small intestine. Tyrosine hydroxylase (TH)- and calcitonin-gene-related-peptide (CGRP)-immunoreactive axons were also more common in the myenteric plexus than in the musculature, but of these, only the TH-immunoreactive neurites tended, like those of the other putative transmitters, to encircle neurons in myenteric ganglia. Evidence was obtained that, as in the small intestine, at least some of the SP-, VIP-, NPY-, and 5-HT-immunoreactive fibers in the stomach are derived from intrinsic gastric myenteric neurons. In contrast, unlike the small intestine, gastric myenteric ganglia appeared to lack intrinsic CGRP-immunoreactive neurons; therefore, the CGRP-immunoreactive gastric axons are probably of extrinsic origin.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for an involvement of substance P, but not cholecystokinin-like peptides, in hexamethonium-resistant intestinal peristalsis

It has previously been found that, in the presence of naloxone, the ganglionic blocking drug hexamethonium fails to completely block peristaltic motility in the isolated ileum of the guinea-pig. This hexamethonium-resistant peristaltic activity is coordinated by enteric nerves since it is abolished by tetrodotoxin. In the present study the neurotransmitter circuitry of this type of peristalsis was studied by means of specific antagonists. Atropine totally suppressed hexamethonium-resistant peristalsis. This type of peristalsis was also strongly inhibited by the tachykinin antagonist, spantide, if a concentration sufficient to antagonize neuronally located substance P receptors was employed. In contrast, the cholecystokinin antagonist, lorglumide, caused only a slight inhibition of hexamethonium-resistant peristalsis. Both substance P and the cholecystokinin-related peptide, ceruletide, potently stimulated the hexamethonium-resistant type of peristaltic activity. These data indicate that, after blockade of nicotinic acetylcholine receptors, tachykinins mediate neuroneuronal coordination of peristalsis whereas acetylcholine acting via muscarinic receptors may be primarily responsible for neuromuscular transmission. Cholecystokinin-like peptides appear to play a modulator rather than a mediator role in hexamethonium-resistant peristalsis.

Reflex changes in circular muscle activity elicited by stroking the mucosa: an electrophysiological analysis in the isolated guinea-pig ileum

A preparation of isolated small intestine from the guinea-pig was studied in which reflex responses of the circular muscle were recorded intracellularly when sensory receptors in the mucosa were stimulated mechanically. This preparation was used to examine the properties of mucosa to muscle reflexes that involve non-cholinergic motor neurons innervating the circular muscle. Reproducible stimulation of the mucosa was achieved by stroking with a motor-driven brush. Gentle brush-strokes applied to the mucosa typically evoked inhibitory junction potentials anal to the stimulus and excitatory junction potentials at recording sites oral to the stimulus. Both events were rapid in onset and up to 25 mV in amplitude. The reflexes were blocked by tetrodotoxin (0.5 microM). Junction potentials declined in amplitude with distance from the stimulus, the amplitude of the excitation 15 mm oral to the stimulus was half that at 5 mm from the stimulus, whereas the amplitude of the inhibitory potential at 40-45 mm was about 60% of that at 5-10 mm anal to the stimulus. Hexamethonium (100-200 microM) blocked the ascending excitation but only slightly reduced the descending inhibition. Ascending excitation was blocked by antagonists for substance P receptors in the muscle, and inhibition was substantially reduced by apamin (0.2 microM), both before and after hexamethonium. Both responses were abolished by removal of the mucosa from the stimulus site and when lesions were made through the myenteric plexus between the stimulation and recording sites, but persisted when similar lesions were made through the submucous plexus. It is concluded that there are neurons with mechanoreceptive nerve endings in the mucosa. Stimulation of such sensory neurons leads to activation of pathways in the myenteric plexus that excite motor neurons to the muscle both oral and anal to the stimulation site. The demonstration that mucosa to muscle reflexes can be consistently evoked in the small intestine in vitro provides an opportunity for close analysis of the reflex pathways. Such analysis is not so readily achieved when reflexes are initiated by distension that, by moving the intestine, can dislodge the recording electrode.

Non-cholinergic ascending excitatory response in the cat small intestine: possible involvement of substance P

Using an electromyographic technique, an ascending excitatory response was recorded "in vitro" in the presence of atropine in the cat small intestine up to 70 mm orally with respect to the site of repetitive transmural nerve stimulation. This non-cholinergic ascending excitatory response was characterized by an increase in the slow wave amplitude and spiking activity. This response was reversibly abolished by Tetrodotoxin (3,1 X 10(-6) M) but remained unchanged after exposure of the intestine to: Hexamethonium (4,9 X 10(-6) M) plus Tubocurarine (1,4 X 10(-5) M), Guanethidine (5 X 10(-7) to 5 X 10(-5) M), Domperidone (2,3 X 10(-7) to 2,3 X 10(-5) M), Naloxone (3 X 10(-7) to 3 X 10(-5) M), Methysergide (2,8 X 10(-7) to 2,8 X 10(-5) M), Metergoline (2,4 X 10(-5) M), Methiotepin (2,1 X 10(-5) M) and Mepyramine (2,3 X 10(-5) M). This response was unaffected by the substance P analogues, D-Pro2, D-Phe7, D-Trp9-Substance P (10(-5) M) or D-Pro2, D-Trp7-9-Substance P (10(-5) M) but was reversibly abolished after exposure of the intestine to substance P (10(-6) M). Moreover substance P still effectively abolished this response in the presence of any two of the above analogues. The results of the present study show that the non-cholinergic excitatory response elicited in the cat small intestine due to the activity of long ascending pathways probably involved substance P. The functional significance of this response is discussed.

An electrophysiological study of the projections of motor neurones that mediate non-cholinergic excitation in the circular muscle of the guinea-pig small intestine

The projections of neurones that produce the fast non-cholinergic excitatory junction potentials (e.j.p.s) in the circular muscle were analysed in the isolated ileum of the guinea-pig. Standard intracellular microelectrode techniques were used to record the amplitudes of such e.j.p.s in response to short trains of stimuli from transmural electrodes. Projections of the neurones around the circumference of the intestine were determined by plotting the change in e.j.p. amplitude with distance from longitudinally placed electrodes. Projections in the oral and anal directions were examined by recording at varying distances from transversely placed electrodes, and also by recording responses elicited close to longitudinal electrodes at various distances from lesions made 3-5 days earlier to interrupt orally and anally directed pathways. Experiments were performed in the presence of hexamethonium to determine the projections of the final motor neurones and in the absence of the drug to examine the projections of excitatory inputs to these neurones. With hexamethonium present, there was a decline in e.j.p. amplitude to 7.5% of maximum at 12 mm (a half circumference) from longitudinal stimulating electrodes. The decline was much less if hexamethonium was not present, and slightly greater if the myenteric plexus was removed. Thus, excitatory motor neurones and cholinergic neurones that impinge upon them both project circumferentially. When the longitudinal muscle and myenteric plexus were removed, and 3-5 days allowed for terminals to degenerate, no e.j.p.s could be recorded in the circular muscle, indicating that the fibres reach the circular muscle from the myenteric plexus. Following transverse lesions, substantial deficits in excitatory transmission only occurred within 1 mm oral or anal to the lesions indicating that the majority of neurones have only short projections along the intestine. A slight deficit in e.j.p. amplitude, up to 20%, was observed extending to about 5 mm oral; but normal transmission was restored by about 10 mm for the lesions. Thus there is a minority of excitatory motor neurones with oral projections up to about 10 mm in length. Results with transmural stimulation showed that these nerve fibres can cause excitation both when orthodromically and when antidromically stimulated, indicating that they provide collaterals along their lengths. With no hexamethonium present, e.j.p.s exhibited little decrement in amplitude over distances of over 30 mm oral or anal indicating that there are both ascending and descending cholinergic pathways that impinge on the final motor neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Propagated motor activity in the small intestine of urethane-anaesthetized rats: inhibitory action of sympathetic and capsaicin-sensitive nerves

1. In the duodenum/small intestine of urethane-anesthetized rats, a series of atropine- and hexamethonium-sensitive phasic contractions activity were recorded by using a balloon. With two balloons, one in the proximal duodenum (B1) and the other at a distal site (B2) a co-ordinated motor activity was observed which migrated from B1 to B2. 2. Spontaneous activity of the duodenum was greater in either adrenalectomized, guanethidine- or capsaicin-pretreated than in control rats. In guanethidine-pretreated animals a series of atropine-resistant but hexamethonium-resistant rhythmic contractions could be demonstrated. 3. In control rats, distension of the balloon produced an inhibitory effect on rhythmic contractions which were ultimately suppressed. Distension had little inhibitory effect on duodenal motor activity of adrenalectomized, guanethidine- or capsaicin-pretreated rats. 4. These findings indicate that the propagated motor activity in the duodenum/small intestine of urethane anesthetized rats can be modulated by the adrenals as well as sympathetic and capsaicin-sensitive nerves.

The in vitro release of immunoreactive dynorphin and alpha-neoendorphin from the perfused rat duodenum

The release of immunoreactive (ir) dynorphin (DYN) and alpha-neoendorphin (ir-ANEO) from the isolated perfused rat duodenum was demonstrated using specific radioimmunoassays (RIAs). Depolarization of the tissue by increasing the potassium (K+) concentration up to 108 mM enhanced the release of ir-DYN and ir-ANEO in Ca2+-dependent manner. Administration of the serotonin-releasing agent fenfluramine (10(-6) M) and the serotonin receptor agonist m-chlorophenylpiperazine (m-CPP, 10(-6) M) stimulated the release of ir-DYN and ir-ANEO from the duodenum. A subsequent study revealed that serotonin (5-HT, 10(-6)-10(-4) M) induced a dose-dependent increase in the release of ir-DYN and ir-ANEO from the duodenum. The effect of 5-HT on the release of ir-DYN and ir-ANEO from the duodenum was antagonized by 5-HT antagonist cyproheptadine (10(-6) M). The presence of dynorphin and the related peptides in the gastrointestinal tract (GIT) and their release from the duodenum in vitro indicate that these peptides may act as transmitters involved in some GIT functions. Furthermore, our results suggest that at least part of 5-HT effects on the GIT may be mediated by the release of dynorphin and the related peptides.

Cholecystokinin octapeptide activates an opioid mechanism in the guinea-pig ileum: a possible role for substance P

To test the hypothesis that excitatory peptides release endogenous opioids from the myenteric plexus longitudinal muscle (MPLM) preparation of the guinea-pig ileum (GPI), the effect of cholecystokinin (CCK8) was studied in the absence and presence of the opioid antagonist naloxone. The maximum height of the contracture induced by CCK8 was not altered by the presence of naloxone in the incubation medium, however, the subsequent sustained excitation was clearly increased. This effect is interpreted as being a result of the release of endogenous opioids during the first moments of the CCK8-evoked excitation of the plexus. CCK8 still induced neurogenic contractures in the presence of atropine; these contractures were probably mediated by the release of substance P. Naloxone was used to evidence the opioid control of the CCK8-induced release of substance P. Desensitization to the effect of substance P reduced the action of CCK8 and also abolished the non-cholinergic contractures evoked by CCK8 and the subsequent effect of naloxone. These facts suggest the release of endogenous opioids within the plexus in response to the neurally mediated excitatory action of CCK8.

Motor activity of the rat duodenum in vivo: evidence for the existence of an atropine-resistant peristalsis

The motor activity of the proximal duodenum has been investigated by means of a balloon method in urethane-anaesthetized rats. A series of atropine- (1 mg/kg i.v.) or hexamethonium- (20 mg/kg i.v.) sensitive rhythmic contractions can be demonstrated in the rat duodenum which represents a peristaltic reflex mediated by intramural cholinergic neurons. The atropine-sensitive peristaltic reflex is transiently suppressed by intravenous DMPP (0.1 mg/kg) GABA (3 mg/kg) or noradrenaline (10 micrograms/kg). In rats pretreated with guanethidine (20 mg/kg s.c.) plus naloxone (2 mg/kg i.v.) and theophylline (2 mg/kg i.v.) a second type of peristaltic reflex which is atropine-resistant but hexamethonium-sensitive can be demonstrated. These findings indicate that in the rat small intestine a peristaltic activity can be sustained by both atropine-sensitive and atropine-resistant mechanism, which may explain failure of atropine to affect intestinal motility in conscious rats.

A tachykinin antagonist inhibits gastric emptying and gastrointestinal transit in the rat

The effect of a substance P antagonist, [D-Pro2, D-Trp7,9]-substance P (SPA), on gastric emptying and gastrointestinal transit in the rat was studied in order to elucidate a possible physiological role of endogenous substance P and other tachykinins in gastrointestinal motility. SPA was given by intraperitoneal injection concurrently with the intragastric administration of a test meal containing charcoal and 51Cr. Examination 15 min after the test meal showed that SPA (0.13-1.3 mumol kg-1) inhibited gastric emptying and gastrointestinal transit in a dose-dependent manner. The inhibitory effect of SPA on gastric emptying and gastrointestinal transit remained unchanged after pretreatment of rats with mepyramine (8.7 mumol kg-1) plus cimetidine (19.8 mumol kg-1) or with guanethidine (67 mumol kg-1). Since a full examination of SPA as a specific tachykinin antagonist was not possible in vivo, SPA was also tested on circular muscle strips from the rat gastric corpus in vitro. Submaximal contractions in response to bombesin or bethanechol were not reduced by SPA (50 microM), whereas those in response to substance P were inhibited. The results suggest that SPA inhibits gastric emptying and gastrointestinal transit by interfering with the action of tachykinins released from enteric nerves and that endogenous tachykinins are involved in the regulation of gastrointestinal motility.

Studies on effects of the substance P analogues [D-Pro2, D-Trp7,9]-substance P and [D-Arg1, D-Trp7,9, L-Leu11]-substance P not related to their antagonist action

Two tachykinin antagonists, [D-Pro2, D-Trp7,9]-substance P (AP-2) and [D-Arg1, D-Trp7,9, L-Leu11]-substance P (spantide) were injected or infused intraarterially into the isolated perfused rabbit ear connected to the body via the nerve only. The effects of these antagonists on venous outflow, release of histamine, and on acetylcholine-induced reflex fall in blood pressure were recorded. The effect of spantide was also investigated on cholinergic "twitch" responses to the isolated field stimulated ileum of the guinea-pig. Bolus injections of AP-2 (6.6 nmol and 20 nmol) and spantide 20 nmol and 66 nmol i.a. caused a dose-dependent reduction in venous outflow, which could mainly be explained by the release of histamine since the histamine H1 receptor blocker mepyramine inhibited this effect; release of histamine was also directly demonstrated. Injections of AP-2 (20 nmol) and spantide (66 nmol) caused nociceptor stimulation which might in part result from the histamine release. The reflex fall in blood pressure due to nociceptor stimulation by acetylcholine was reduced by less than 30% by infusion of the tachykinin antagonists in a concentration of 12 mumol l-1 but not at 2.4 mumol l-1. Spantide (up to 100 mumol l-1) did not inhibit electrically evoked "twitch" responses of the guinea-pig ileum. The local anaesthetic drug procaine (4.2-42 mumol l-1) inhibited these contractions in a concentration-dependent manner. It is concluded that the tachykinin antagonists might show effects which are not related to their specific tachykinin antagonistic action as indicated by the findings in the rabbit ear.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation and inhibition of gastrointestinal propulsion induced by substance P and substance K in the rat

Substance P and substance K (neurokinin A) (dose range: 0.08-80 nmol kg-1) were tested for their effects on gastrointestinal propulsion in the rat. The peptides were given by intraperitoneal injection concurrently with the intragastric administration of a test meal containing charcoal and 51Cr. Examination 3 min after the test meal showed that high doses of substance P (greater than 0.74 nmol kg-1) and substance K (greater than 8.8 nmol kg-1) inhibited gastric emptying and gastrointestinal transit. This inhibitory effect was changed to a stimulant effect by pretreatment of the rats with atropine (3.5 mumol kg-1). Guanethidine pretreatment (67 mumol kg-1) revealed a facilitatory effect of low doses of the two tachykinins (about 1 nmol kg-1) on gastrointestinal propulsion. Examination 15 min after the test meal demonstrated that substance P (greater than 0.74 nmol kg-1) dose-dependently enhanced gastrointestinal propulsion, an effect that was also seen after atropine pretreatment. Low doses of substance K (about 1 nmol kg-1) also stimulated gastrointestinal propulsion but this effect was abolished by atropine. In addition, atropine pretreatment revealed a stimulant effect of high doses of substance K (88 nmol kg-1) on gastric emptying. These results show that the effects of substance P and substance K on gastrointestinal propulsion vary with dose and time and involve, at least partly, activation of the autonomic nervous system.

Atropine- and tetrodotoxin-resistant motor activity, responsive to 4-aminopyridine, present in isolated rabbit jejunum

The effects of 4-aminopyridine on the spontaneous contractile activity of rabbit jejunum segments were studied. Apart from its effects on the cholinergic component of the spontaneous activity, the drug was found to be effective to increase an atropine-, tetrodotoxin-resistant activity of the preparation, suggesting either a direct effect on intestinal smooth muscle cells or an indirect effect through an unidentified excitatory innervation.