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

Chronic inflammation alters the contribution of neurokinin receptor subtypes to epithelial function in rat colon

We have previously shown that neurokinin-1 (NK1) receptors predominantly mediate substance P-induced secretion of the non-inflamed rat colonic mucosa in vitro with a gradient in the magnitude of these responses. The aim of this study was to examine the effects of chronic inflammation on the contributions of different neurokinin receptor subtypes to colonic mucosal secretion. Colitis was induced by the intracolonic administration of 2,4,6-trinitrobenzene sulfonic acid in rats, reactivated 6 weeks later. Segments of proximal, mid- and distal colon were stripped of muscularis propria and mounted in Ussing chambers for measurement of short-circuit current. Use of selective agonists suggests that in the chronically inflamed rat colon NK1 receptors play a greater role in neurokinin-mediated mucosal secretion than do either NK2 or NK3. Selective antagonism implies that this is region-specific, with the inflammatory process altering the relative contribution of the neurokinin receptor subtypes within each region of the rat colon.

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

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

Regional variations in neurokinin receptor subtype contributions to muscularis mucosae and epithelial function in rat colon

It is known that the muscularis mucosae and mucosa are not pharmacologically homogeneous throughout the rat colon. The aim of this study was to simultaneously characterize all three neurokinin (NK) receptors in the muscularis mucosae and mucosa along the length of the rat colon. Strips of proximal, mid, and distal colonic muscularis mucosae were prepared for isometric recording or sheets of muscle-free mucosa were mounted in Ussing chambers for measurement of short-circuit current. In both muscularis mucosae and mucosa the greatest responses to substance P were found in the proximal region. Use of selective agonists revealed the presence of all three NK receptors in both structures, however, selective antagonism suggests that only NK2 receptors in the muscularis mucosae and NK1 receptors in the mucosa are physiologically relevant. In conclusion, substance P-induced responses in the rat colon are region-specific and not mediated by a single NK receptor subtype common to both structures.

Role of nitric oxide in the impairment of circular muscle contractility of distended, uninflamed mid-colon in TNBS-induced acute distal colitis in rats

AIM: To evaluate the role of nitric oxide (NO) in the motor disorders of the dilated uninflamed mid-colon (DUMC) from trinitrobenzene sulfonic acid (TNBS)-induced acute distal colitis in rats.

METHODS: Colitis was induced in male Sprague-Dawley rats by a single intracolonic administration of TNBS. Control rats received an enema of 0.9% saline. The rats were killed 48 h after TNBS or saline administration. Macroscopic and histologic lesions of the colon were evaluated. Myeloperoxidase (MPO) and nitric oxide synthase (NOS) activity were measured on the colonic tissue. In TNBS rats, we evaluated spontaneous and evoked contractile activity in circular muscle strips derived from DUMC in comparison to the same colonic segment of control rats, both in the presence and in the absence of a non-selective NOS isoforms inhibitor N-nitro-L-arginine (L-NNA). Pharmacological characterization of electric field stimulation (EFS)-evoked contractile responses was also performed.

RESULTS: In TNBS rats, the distal colon showed severe histological lesions and a high MPO activity, while the DUMC exhibited normal histology and MPO activity. Constitutive NOS activity was similar in TNBS and control rats, whereas inducible NOS activity was significantly increased only in the injured distal colon of TNBS rats. Isometrically recorded mechanical activity of circular muscle strips from DUMC of TNBS rats showed a marked reduction of the force and frequency of spontaneous contractions compared to controls, as well as of the contractile responses to a contracting stimulus. In the presence of L-NNA, the contractile activity and responses displayed a significantly greater enhancement compared to controls. The pharmacological characterization of EFS contractile responses showed that a cooperative-like interaction between cholinergic muscarinic and tachykinergic neurokinin 1 and 2 receptors mediated transmission in DUMC of TNBS rats vs a simple additive interaction in controls.

CONCLUSION: The results of this study show that, during TNBS-induced acute distal colitis, circular muscle intrinsic contractile mechanisms and possible enteric neural excitatory activity are inhibited in the distended uninflamed mid-colon. Suppression of NO synthesis markedly improves spontaneous and evokes muscle contractions, in spite of any evident change in local NO activity.

Tachykinin NK2 receptor antagonists for the treatment of irritable bowel syndrome

Tachykinin NK2 receptors are expressed in the gastrointestinal tract of both laboratory animals and humans. Experimental data indicate a role for these receptors in the regulation of intestinal motor functions (both excitatory and inhibitory), secretions, inflammation and visceral sensitivity. In particular, NK2 receptor stimulation inhibits intestinal motility by activating sympathetic extrinsic pathways or NANC intramural inhibitory components, whereas a modulatory effect on cholinergic nerves or a direct effect on smooth muscle account for the NK2 receptor-mediated increase in intestinal motility. Accordingly, selective NK2 receptor antagonists can reactivate inhibited motility or decrease inflammation- or stress-associated hypermotility. Intraluminal secretion of water is increased by NK2 receptor agonists via a direct effect on epithelial cells, and this mechanism is active in models of diarrhoea since selective antagonists reverse the increase in faecal water content in these models. Hyperalgesia in response to intraluminal volume signals is possibly mediated through the stimulation of NK2 receptors located on peripheral branches of primary afferent neurones. NK2 receptor antagonists reduce the hyper-responsiveness that occurs following intestinal inflammation or application of stressful stimuli to animals. Likewise, NK2 receptor antagonists reduce intestinal tissue damage induced by chemical irritation of the intestinal wall or lumen. In healthy volunteers, the selective NK2 antagonist nepadutant reduced the motility-stimulating effects and irritable bowel syndrome-like symptoms triggered by intravenous infusion of neurokinin A, and displayed other characteristics that could support its use in patients. It is concluded that blockade of peripheral tachykinin NK2 receptors should be considered as a viable mechanism for decreasing the painful symptoms and altered bowel habits of irritable bowel syndrome patients.

Peripheral tachykinin receptors as targets for new drugs

Tachykinins are widely distributed in the peripheral nervous system of the respiratory, urinary and gastrointestinal tract, stored in enteric neurons and in peripheral nerve endings of capsaicin-sensitive primary afferent neurons from which are released by stimuli having both pathological and physiological relevance. The most studied effects produced by tachykinins in these systems are smooth muscle contraction, plasma protein extravasation, mucus secretion and recruitment/activation of immune cells. The use of tachykinin receptor-selective antagonists and knockout animals has enabled to identify the involvement of tachykinin NK(1), NK(2) and NK(3) receptors as mediators of peripheral effects of tachykinins in different systems/species. The bulk of data obtained in experimental animal models suggests that tachykinins could contribute to the genesis of symptoms accompanying various human diseases including asthma/bronchial hyperreactivity, cystitis of various aetiology, inflammatory bowel diseases and irritable bowel syndrome. Tachykinin receptor antagonists are expected to afford therapeutically relevant effects.

Tachykinin NK(2) receptors and enhancement of cholinergic transmission in the inflamed rat colon: an in vivo motility study

In the gastrointestinal tract, tachykinin NK(2) receptors are localized both on smooth muscle and nerve fibres. NK(2) receptor antagonists reduce exaggerated intestinal motility in various diarrhoea models but the site of action contributing to this effect is unknown. In this study we investigated the effects of atropine (1.4 micromol kg(-1), i.v.), hexamethonium (13.5 micromol kg(-1), i.v.), and nepadutant (0.1 micromol kg(-1), i.v.), a selective tachykinin NK(2) receptor antagonist, on distension (0.5 and 1 ml)-, or irritation (acetic acid, 0.5 ml of 7.5% v v(-1))-induced motility in the rat distal colon in vivo. The effects of atropine, hexamethonium or N(omega)-nitro-L-argininemethylester (L-NAME, 1.85 micromol kg(-1), i.v.) on [betaAla(8)]NKA(4-10) (10 nmol kg(-1), i.v.)-induced colonic contractions were also investigated. When the colonic balloon was filled with a subthreshold volume (0.5 ml), the intraluminal instillation of acetic acid triggered a high-amplitude phasic colonic motility which was partially reduced by nepadutant and suppressed by either hexamethonium or atropine. Filling of the balloon with 1 ml evoked reflex (hexamethonium-sensitive), atropine-sensitive phasic colonic motility: nepadutant had no significant effect on the distension-evoked motility. Neither hexamethonium nor atropine significantly reduced [betaAla(8)]NKA(4-10)-induced colonic contractions, whereas nepadutant suppressed them. Following L-NAME pretreatment, [betaAla(8)]NKA(4-10)-induced colonic contractions were inhibited by both atropine and hexamethonium. In hexamethonium-pretreated animals, an atropine-sensitive component of [betaAla(8)]NKA(4-10)-induced colonic contractions was also evident. These results indicate that the application of irritants onto the colonic mucosa induces the release of endogenous tachykinins which enhance excitatory cholinergic mechanisms through the stimulation of NK(2) receptors.

Effect of a tachykinin NK(2) receptor antagonist, nepadutant, on cardiovascular and gastrointestinal function in rats and dogs

The effect of the tachykinin NK(2) receptor antagonist, nepadutant (MEN 11420 or (c[[(beta-D-GlcNAc)Asn-Asp-Trp-Phe-Dpr-Leu]c(2beta-5beta)])) was assessed on cardiovascular function (unanaesthetized rats and anaesthetized dogs) and gastrointestinal motor activity (fasted unanaesthetized dogs). The selective tachykinin NK(2) receptor agonist, [betaAla(8)]neurokinin A (4-10), up to 100 nmol/kg, i.v., did not produce changes on mean blood pressure or heart rate in unanaesthetized rats. Nepadutant did not affect blood pressure and heart rate up to 10 micromol/kg, whereas saredutant (SR 48968 or ((S)-N-methyl-N[4-(4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl] benzamide), a nonpeptide antagonist, produced a transient reduction of mean blood pressure and heart rate. Nepadutant up to 20 micromol/kg, i.v. neither caused changes of cardiovascular and respiratory parameters in anaesthetized dogs nor induced any changes in left ventricular systolic pressure, left ventricular dP/dt or of electrocardiogram (lead II) waveforms. Intravenous administration of neurokinin A (9 nmol/kg) in unanaesthetized dogs stimulated gastrointestinal motility for 20-25 min. Nepadutant at 0.1 micromol/kg suppressed the stimulant effects of neurokinin A but, up to a dose of 10 micromol/kg, did not produce significant changes in the basal migrating motor complexes. We conclude that tachykinin NK(2) receptors do not participate in the physiologic regulation of resting cardiovascular and respiratory functions and that they do not regulate the fasted pattern of gastrointestinal motility. The cardiovascular changes induced by the nonpeptide tachykinin NK(2) receptor antagonist, saredutant, likely arise from nonspecific effects unrelated to tachykinin NK(2) receptor blockade.

Tachykinins contribute to nerve-mediated contractions in the human esophagus

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Peripheral actions of tachykinins

Tachykinins mediate a variety of physiological processes in the gastrointestinal, pulmonary and genito-urinary tract mainly through the stimulation of NK1 and NK2 receptors. Preclinical evidence obtained through the use of selective tachykinin receptor antagonists indicates that endogenous tachykinins are involved in augmented smooth muscle contraction, vasodilatation, chemotaxis and activation of immune cells, mucus secretion, water absorption/secretion. Recent evidence also suggests that endogenous tachykinins released at the peripheral level may play a role in visceral inflammation, hyperreflexia and hyperalgesia. Possible mechanisms underlying the stimulation of primary afferent neurons by tachykinins may involve a direct excitation of these neurons and the release of mediators which sensitise or stimulate sensory nerves. Tachykinin receptor antagonists could have a clinical utility in several human diseases such as irritable bowel syndrome, asthma, and in micturition disturbances characterized by a hyperactive bladder.

The tachykinin NK1 receptor. Part II: Distribution and pathophysiological roles

The tachykinin NK1 receptor is widely distributed in both the central and peripheral nervous system. In the CNS, NK1 receptors have been implicated in various behavioural responses and in regulating neuronal survival and degeneration. Moreover, central NK1 receptors regulate cardiovascular and respiratory function and are involved in activating the emetic reflex. At the spinal cord level, NK1 receptors are activated during the synaptic transmission, especially in response to noxious stimuli applied at the receptive field of primary afferent neurons. Both neurophysiological and behavioural evidences support a role of spinal NK1 receptors in pain transmission. Spinal NK1 receptors also modulate autonomic reflexes, including the micturition reflex. In the peripheral nervous system, tachykinin NK1 receptors are widely expressed in the respiratory, genitourinary and gastrointestinal tracts and are also expressed by several types of inflammatory and immune cells. In the cardiovascular system, NK1 receptors mediate endothelium-dependent vasodilation and plasma protein extravasation. At respiratory level, NK1 receptors mediate neurogenic inflammation which is especially evident upon exposure of the airways to irritants. In the carotid body, NK1 receptors mediate the ventilatory response to hypoxia. In the gastrointestinal system, NK1 receptors mediate smooth muscle contraction, regulate water and ion secretion and mediate neuro-neuronal communication. In the genitourinary tract, NK1 receptors are widely distributed in the renal pelvis, ureter, urinary bladder and urethra and mediate smooth muscle contraction and inflammation in response to noxious stimuli. Based on the knowledge of distribution and pathophysiological roles of NK1 receptors, it has been anticipated that NK1 receptor antagonists may have several therapeutic applications at central and peripheral level. At central level, it is speculated that NK1 receptor antagonists could be used to produce analgesia, as antiemetics and for treatment of certain forms of urinary incontinence due to detrusor hyperreflexia. In the peripheral nervous system, tachykinin NK1 receptor antagonists could be used in several inflammatory diseases including arthritis, inflammatory bowel diseases and cystitis. Several potent tachykinin NK1 receptor antagonists are now under evaluation in the clinical setting, and more information on their usefulness in treatment of human diseases will be available in the next few years.