Implication of NK1 and NK2 receptors in rat colonic hypersecretion induced by interleukin 1 beta: role of nitric oxide

Aloe vera extract activity on human corneal cells

CONTEXT

Ocular diseases are currently an important problem in modern societies. Patients suffer from various ophthalmologic ailments namely, conjunctivitis, dry eye, dacryocystitis or degenerative diseases. Therefore, there is a need to introduce new treatment methods, including medicinal plants usage. Aloe vera [Aloe barbadensis Miller (Liliaceae)] possesses wound-healing properties and shows immunomodulatory, anti-inflammatory or antioxidant activities.

MATERIALS AND METHODS

NR uptake, MTT, DPPH• reduction, Griess reaction, ELISA and rhodamine-phalloidin staining were used to test toxicity, antiproliferative activity, reactive oxygen species (ROS) reduction, nitric oxide (NO) and cytokine level, and distribution of F-actin in cells, respectively.

AIM

The present study analyzes the effect of Aloe vera extracts obtained with different solvents on in vitro culture of human 10.014 pRSV-T corneal cells.

RESULTS

We found no toxicity of ethanol, ethyl acetate and heptane extracts of Aloe vera on human corneal cells. No ROS reducing activity by heptane extract and trace action by ethanol (only at high concentration 125 µg/ml) extract of Aloe vera was observed. Only ethyl acetate extract expressed distinct free radical scavenging effect. Plant extracts decreased NO production by human corneal cells as compared to untreated controls. The cytokine (IL-1β, IL-6, TNF-α and IL-10) production decreased after the addition of Aloe vera extracts to the culture media.

DISCUSSION AND CONCLUSIONS

Aloe vera contains multiple pharmacologically active substances which are capable of modulating cellular phenotypes and functions. Aloe vera ethanol and ethyl acetate extracts may be used in eye drops to treat inflammations and other ailments of external parts of the eye such as the cornea.

Transforming growth factor-beta1 (TGF-beta1) and acetylcholine (ACh) alter nitric oxide (NO) and interleukin-1beta (IL-1beta) secretion in human colon adenocarcinoma cells

Colon adenocarcinoma is one of the most common fatal malignancies in Western countries. Progression of this cancer is dependent on tumor microenvironmental signaling molecules such as transforming growth factor-beta (TGF-beta) or acetylcholine (ACh). The present study was conducted to assess the influence of recombinant human transforming growth factor (rhTGF)-beta1 or ACh on nitric oxide (NO) and interleukin-1beta (IL-1beta) secretion by three human colon adenocarcinoma cell lines: HT29, LS180, and SW948, derived from different grade tumors (Duke's stage). The cells were cultured in 2D and 3D (spheroids) conditions. Colon carcinoma cells exhibited different sensitivities to rhTGF-beta1 or ACh dependent on the tumor grade and the culture model. ACh exhibited significant inhibitory effects towards NO, endothelial nitric oxide synthase (eNOS), and IL-1beta secretion especially by tumor cells derived form Duke's C stage of colon carcinoma. rhTGF-beta1 also decreased NO, IL-1beta, and eNOS expression, but its effect was lower than that observed after the administration of ACh. The inhibition of NO and IL-1beta production was more striking in 3D tumor spheroids than in 2D culture monolayers. Taken together, the TGF-beta1-ACh axis may regulate colon carcinoma progression and metastasis by altering NO secretion and influence inflammatory responses by modulating IL-1beta production.

Prolonged elevation of galanin and tachykinin expression in mucosal and myenteric enteric nerves in trinitrobenzene sulphonic acid colitis

Diverticulitis causes recurrent abdominal pain associated with increased mucosal expression of mucosal galanin and substance P (SP). We studied changes in mucosal and myenteric plexus neuropeptides in adult rats using a model of colonic inflammation, trinitrobenzenesulphonic acid colitis. We assessed the effects on the pan-neuronal markers protein gene product 9.5 (PGP9.5) and neurofilament protein, as well as specific neuropeptides at 1, 2, 3, 4, 6, 8, 10 and 14 weeks. Following the acute injury there was macroscopic resolution of inflammation but minor microscopic abnormalities persisted. Percent area stained of mucosal PGP9.5 fell initially but average levels on days 21 and 28 levels were significantly elevated (P < 0.001), returning to normal by day 42. Percent area staining of PGP9.5 in the muscle rose immediately and remained significantly elevated at 70 days (P < 0.001). SP, neuropeptide K and galanin followed a similar overall pattern. SP to PGP9.5 ratio was significantly increased in the muscle both acutely (days 1-28) and in the long term (days 70 and 98), whereas the galanin to PGP9.5 ratio was significantly increased in the mucosa throughout the study. Low-grade chronic inflammation after an acute initial insult causes a persistent increase in the expression of galanin in the mucosa and SP in muscle layer.

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 potential therapeutic targets in visceral diseases

More than 10 years of intensive preclinical investigation of selective tachykinin (TK) receptor antagonists has provided a rationale to the speculation that peripheral neurokinin (NK)-1, -2 and -3 receptors may be involved in the pathophysiology of various human diseases at the visceral level. In the airways, despite promising effects in animal models of asthma, pilot clinical trials with selective NK-1 or -2 receptor antagonists in asthmatics have been ambiguous, whereas the potential antitussive effects of NK-1, -2 or -3 antagonists have not yet been verified in humans. In the gastrointestinal (GI) tract, irritable bowel syndrome (IBS) and pancreatitis are appealing targets for peripherally-acting NK-1 and -2 antagonists, respectively. In the genito-urinary tract, NK-1 receptor antagonists could offer some protection against nephrotoxicity and cytotoxicity induced by chemotherapeutic agents, whereas NK-2 receptor antagonists appear to be promising new agents for the treatment of neurogenic bladder hyperreflexia. Finally, there is preclinical evidence for hypothesising an effect of NK-3 receptor antagonists on the cardiovascular disturbance that characterises pre-eclampsia. Other more speculative applications are also mentioned.

Potent NK1 antagonism by SR-140333 reduces rat colonic secretory response to immunocyte activation

The potent neurokinin receptor 1 (NK1) antagonist SR-140333 has previously been shown to reduce castor oil-induced secretion in animal models. The importance of tachykinins in neuroimmune control of secretion and the effect of SR-140333 on key points in this pathway were elucidated in the present study to determine the type of intestinal dysfunction best targeted by this antagonist. Rat colonic secretion and substance P (SP) release were determined in vitro with the use of Ussing chamber and enzyme immunoassay techniques. NK1 receptors played a secretory role as receptor agonists stimulated secretion and SR-140333 antagonized the response to SP response (pK(b) = 9.2). Sensory fiber stimulation released SP and evoked a large secretion that was reduced by 69% in the presence of SR-140333 (10 nM). Likewise, mastocytes also released SP. The subsequent secretory response was reduced by 43% in the presence of SR-140333 (50 nM). SP was also released from granulocytes; however, this did not cause secretion. Functional NK3 receptors were present in the colon as senktide stimulated secretion, an effect that was increased during stress. We conclude that NK3 receptors may play a role in stress-related disorders, whereas NK1 receptors are more important in mast cell/afferent-mediated secretion.

Tachykinin NK(2) receptor mediates contraction and ion transport in rat colon by different mechanisms

We have characterized the tachykinin NK(2) receptor-mediated contraction and vectorial ion transport responses in the muscularis mucosae and mucosa of the rat isolated distal colon, respectively. The tachykinin NK(2) receptor-selective antagonist nepadutant (c([(beta-D-GlcNAc)Asn-Asp-Trp-Phe-Dpr-Leu]c(2beta-5beta))) produced competitive antagonism of [betaAla(8)]neurokinin A-(4-10)-induced contraction (pK(B) = 9.3) in the muscularis mucosae, and insurmountable blockade of increases in short-circuit current (I(sc)) responses (pK(B) = 8.6) in the mucosa. However, this latter effect was completely reversed by washout of the antagonist. [betaAla(8)]Neurokinin A-(4-10)-induced contractions were unaffected by indomethacin (3 microM). In sharp contrast, I(sc) responses induced by [betaAla(8)]neurokinin A-(4-10) (100 nM) were inhibited (>70%) by indomethacin (3 microM), while I(sc) responses to substance P (3 microM) were unchanged. Our study provides the first evidence that in the same organ stimulation of tachykinin NK(2) receptors leads to two independent responses mediated by different effector mechanisms both of which are blocked (albeit with different kinetics) by the potent and selective tachykinin NK(2) receptor antagonist, nepadutant.

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.

Neurokinin A inhibits oxytocin and GABA release from the posterior pituitary by stimulating nitric oxide synthase

Neurokinin A (NKA) is a tachykinin that participates in the control of neuroendocrine functions. The posterior pituitary lobe (PP) contains abundant nitric oxide synthase (NOS), suggesting that nitric oxide (NO) may play a role in controlling the release of neuropeptides and neurotransmitters. In the present project, we investigated the in vitro effect of NKA on oxytocin release from hypothalamic explants and PP of male rats and the possible involvement of NO in the action of NKA. Since NKA inhibits gamma-aminobutyric acid (GABA) release from PP, we also examined the role of NO in the effect of NKA on basal and K(+)-evoked GABA release. NKA (10(-7)-10(-5) M) significantly decreased oxytocin release from PP, whereas it did not affect its release from hypothalamic explants. The inhibitory effect of NKA on oxytocin release from PP was completely blocked by the NOS inhibitors N(G)-monomethyl-L-arginine (L-NMMA, 0.5 mM) or N(G)-nitro-L-arginine-methyl-ester (L-NAME, 1 mM). Sodium nitroprusside (0.5 mM), an NO releaser, had no effect on basal GABA release but significantly decreased K(+)-evoked GABA release. L-NMMA (0.3 mM) and L-NAME (0.5 mM) increased K(+)-evoked GABA release, indicating that NO plays an inhibitory role in GABA release from PP. The inhibition in both basal and K(+)-evoked GABA release induced by NKA (10(-7) M) was reduced by L-NAME (1 mM). Also, NKA (10(-7) M) increased NO synthesis as measured by [(14)C] citrulline production. Considered all together, our data indicate that NO may mediate the inhibitory effect of NKA on the release of both oxytocin and GABA from PP.

Intraepithelial lymphocytes coinduce nitric oxide synthase in intestinal epithelial cells

The study of mucosal immunity has revealed that complex reciprocal interactions occur between intestinal intraepithelial lymphocytes (IEL) and intestinal epithelial cells (IEC). The present study focuses on the induction of inducible nitric oxide (NO) synthase in cocultures of freshly isolated rat IEL and the rat epithelial cell line IEC-18 after the addition of interleukin-1beta (IL-1beta), tumor necrosis factor-alpha, or lipopolysaccharide. When IEL and IEC were separated using Transwell chambers, NO synthesis was not induced, indicating that cell-cell contact was required. Culture of IEC-18 with IEL, even in the absence of inflammatory stimuli such as IL-1beta, resulted in upregulation of class I and II antigens on IEC-18, due to the interferon-gamma (IFN-gamma) that is constitutively produced by IEL. Addition of anti-IFN-gamma antibody to the NO-producing cocultures resulted in inhibition of NO synthesis as well as the upregulation of class I and II antigen expression. These data indicate that IFN-gamma production by IEL conditions IEC for the expression of other components of the inflammatory process.

Neural mechanism of the antisecretory effect of peptide YY in the rat colon in vivo

The purpose of this work was to determine the mechanism of the antisecretory effect of peptide YY in the rat colon and whether this effect is physiological. In this prospect, doses of exogenous peptide YY producing physiological and supraphysiological plasma levels were intravenously infused in rats provided with colonic and jejunal ligated loops in vivo, under secretory stimulation by vasoactive intestinal peptide. Peptide YY decreased the secretory effect of VIP in a dose-related fashion. The effect of peptide YY was blocked or strongly decreased by tetrodotoxin, hexamethonium, idazoxan, haloperidol, and the sigma antagonist BMY 14, 802 in both the colon and jejunum. We conclude that peptide YY decreases water and electrolyte secretion in the colonic mucosa by a complex neural mechanism involving at least two neurons connected through a nicotinic synapse, alpha-2 adrenoceptors and sigma receptors, and that this effect can occur with physiological doses of peptide YY.

The role of nitric oxide in aloe-induced diarrhoea in the rat

The role of nitric oxide (NO) on aloe-induced diarrhoea was studied in the rat. Nine hours after oral administration, aloe produced diarrhoea at doses of 5 g kg(-1)(20% rats with diarrhoea) and 20 g kg(-1) (100% of rats with diarrhoea). Lower doses of aloe (0.1 and 1 g kg(-1) did not produce a diarrhoeal response. Pre-treatment (i.p.) of rats with the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME 2.5-25 mg kg(-1) reduced the diarrhoea induced by aloe (20 g kg(-1) 9 h after its oral administration. L-NAME (25 mg kg(-1)) also reduced the increase in faecal water excretion produced by aloe (20 g kg(-1). L-arginine (1500 mg kg(-1), i.p.), administered to rats pre-treated with L-NAME (25 mg kg(-1), drastically reduced the effect of L-NAME on diarrhoea and increase in faecal water excretion induced by aloe (20 g kg(-1). Given alone, L-arginine did not modify aloe-induced diarrhoea. Basal Ca2+ -dependent NO synthase activity in the rat colon was dose-dependently inhibited by aloe (0.1-20 g kg(-1)) and by aloin (0.1-1 g kg(-1)), the active ingredient of aloe. These results suggest that endogenous NO modulates the diarrhoeal effect of aloe.

Antisecretory effect of peptide YY through neural receptors in the rat jejunum in vitro

Basal short circuit current (Isc) was measured in stripped rat jejunum after addition of neural antagonists and of peptide YY (PYY). Basal Isc was slightly (by 10-21%) but significantly inhibited by tetrodotoxin, hexamethonium, idazoxan, and the sigma antagonist BMY 14,802. PYY (10(-7) M) reduced basal Isc by approximately 54%. This inhibition was unchanged by hexamethonium but reduced by 44-68% in the presence of tetrodotoxin, idazoxan, haloperidol, BMY 14,802, and atropine. The Y2 agonist pYY(3-36) was more potent than the Y1 agonist (Leu31,Pro34)PYY. In conclusion, PYY reduces basal Isc in rat jejunum in part through a neural mechanism involving muscarinic receptors, alpha2 adrenoceptors, and sigma receptors and, in part, through a direct effect on enterocytes. The PYY effect seems mainly carried out through Y2-receptor activation.

Low-level exposure to diquat induces a neurally mediated intestinal hypersecretion in rats: involvement of nitric oxide and mast cells

Diquat, a nonselective desiccant herbicide, induces a significant secretion of fluid into the lumen of the gastrointestinal tract of rats at sublethal doses (from 0.5 to 50 mg/kg). This study investigated the effect of an acute low-level exposure to diquat (0. 1, 0.5, and 1 mg/kg) on intestinal net water flux and the mechanisms involved. In anesthetized rats, an intestinal loop (7 cm) was infused with Ringer's buffer containing [14C]-polyethylene glycol 4000. After equilibration, diquat (0.1, 0.5, and 1 mg/kg) was added to Ringer's buffer during 60 min. Net water flux was calculated according to [14C] activity determined in the effluent collected at 15-min intervals. Infused in the intestinal loop for 60 min at doses of 0.5 and 1 mg/kg but not at 0.1 mg/kg, diquat induced an intestinal net water secretion during 180 min with a maximal effect at the highest dose used and during the first hour following the end of diquat infusion. Diquat-induced (1 mg/kg) intestinal net water secretion was blocked by a neurotoxin, tetrodotoxin (5 micrograms/kg iv), doxantrazole (5 mg/kg ip), a mast cell stabilizer, and two inhibitors of NO synthases: l-NAME (25 mg/kg ip) and aminoguanidine (2 mg/kg ip). It is concluded that a single low-level (0.5 and 1 mg/kg) intrajejunal administration of diquat induces a net water intestinal secretion and that this secretory effect is nerve-mediated, implying mast cell degranulation and NO release.

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.

Involvement of interleukin-1, prostaglandins and mast cells in rectal distension-induced colonic water secretion in rats

1. In vivo rectal distension (RD) induces a neurally mediated colonic net water hypersecretion in rats. Interleukin-1 beta (IL-1 beta) also induces neural colonic water hypersecretion involving the release of prostaglandins (PGs) and a mast cell degranulation in rats. This study investigated in vivo the role of IL-1, PGs and mast cells in RD-induced colonic hypersecretion. 2. Proximal colonic net water flux was determined using [14C]polyethylene glycol (PEG) 4000 (mol. wt. 4000) in anaesthetized rats. On strips taken from the distal colon: (i) a histological analysis was performed to determine the number of mucosal mast cells (MMC); and (ii) histamine levels were measured by radioimmunoassay after stimulation with compound 48/80. 3. RD induced a net colonic water secretion that was blocked by i.c.v. administration of IL-1ra (an IL-1 receptor antagonist) and indomethacin, and by systemic treatment with doxantrazole and indomethacin. RD decreased the number of resident mast cells and the release of histamine from the distal colonic strips. Moreover, using SDS-PAGE immunoblotting the expression of IL-1 beta was detected in the brain. 4. These results suggest that, in rats, RD induces colonic net water hypersecretion by the activation of a neuro-immunological reflex pathway, involving IL-1 beta, PG release and peripheral mast cell degranulation.

Hyperdynamic circulation of cirrhotic rats: role of substance P and its relationship to nitric oxide

BACKGROUND

It has been suggested that excessive formation of nitric oxide (NO) is responsible for the hyperdynamic circulation observed in portal hypertension. Substance P is a neuropeptide partly cleared by the liver and causes vasodilatation through the activation of the endothelial NO pathway. However, there are no previously published data concerning the plasma level of substance P in cirrhotic rats and its relationship to NO.

METHODS

Plasma concentrations of substance P and nitrate/nitrite (an index of NO production) were determined in control rats and cirrhotic rats with or without ascites using an enzyme-linked immununosorbent assay and a colorimetric assay, respectively. In addition, systemic and portal hemodynamics were evaluated by a thermodilution technique and catheterization.

RESULTS

Cirrhotic rats with and without ascites had a lower systemic vascular resistance (2.6 +/- 0.2 and 3.9 +/- 0.4 mmHg ml(-1) x min x 100 g body weight, respectively) and higher portal pressure (14.6 +/- 0.6 and 11.3 +/- 1.8 mmHg) than control rats (6.5 +/- 0.3 mmHg x ml(-1) x min x 100 g BW and 6.8 +/- 0.2 mmHg, respectively, P < 0.05), and cirrhotic rats with ascites had the lowest systemic vascular resistance. Plasma levels of nitrate/nitrite progressively increased in relation to the severity of liver dysfunction (control rats, 2.7 +/- 0.5 nmol/ml; cirrhotic rats without ascites, 5.6 +/- 1.3 nmol/ml; cirrhotic rats with ascites, 8.3 +/- 2.2 nmol/ml; P < 0.05). Cirrhotic rats with ascites displayed higher plasma values of substance P (57.7 +/- 5.9 pg/ml) than cirrhotic rats without ascites (37.9 +/- 3.1 pg/ml, P < 0.05) and control rats (30.1 +/- 1.0 pg/ml, P < 0.05). There was no significant difference in plasma substance P values between control rats and cirrhotic rats without ascites (P > 0.05). No correlation was found between plasma levels of substance P and nitrate/nitrite (r = 0.318, P > 0.05).

CONCLUSIONS

Excessive formation of NO may be responsible, at least partly, for the hemodynamic derangements in cirrhosis. Although substance P may not participate in the initiation of a hyperdynamic circulation in cirrhosis, it may contribute to the maintenance of the hyperdynamic circulation observed in cirrhotic rats with ascites.

Role of tachykinins in castor oil diarrhoea in rats

1. We set out to ascertain the role of tachykinins, neurokinin A and substance P, in castor oil-induced diarrhoea in rats as disclosed by the inhibitory effect of the non-peptide NK1- and NK2-receptor antagonists. SR 140333 and SR 48968, respectively. 2. SR 48968 (0.02 to 20 micrograms kg-1, s.c. or p.o.), and the opioid receptor agonist loperamide (1-10 mg kg-1, p.o.), dose-dependently prevented castor oil effects: % inhibition vs castor oil, diarrhoea 0 to 100, increase in faecal mass 7 to 90 and water content 16 to 90. SR 140333 (0.02 to 20 micrograms kg-1, s.c.) and the platelet activating factor antagonist SR 27417 (5 to 500 micrograms kg-1, p.o.) did not prevent the increase in faecal water content, but reduced faecal mass (35 to 66%) and diarrhoea (0 to 57%). 3. The R-enantiomers of tachykinin NK1 and NK2 receptor antagonists, SR 140603 and SR 48605 (both at 2 or 20 micrograms kg-1, s.c.) had no effect other than reducing faecal mass at the highest dose tested. 4. SR 48968 (20 micrograms kg-1, p.o.) but not loperamide (10 mg kg-1, p.o.) given 24 h before castor oil, still slightly but significantly reduced by 30% the increase of faecal mass output; both treatments significantly reduced (30 to 70%) the effect of castor oil on faecal water content, although the incidence of diarrhoea was only slightly less than in controls. 5. In castor oil-treated rats, naloxone (2 mg kg-1, s.c.) completely blocked the antidiarrhoeal action of loperamide (10 mg kg-1, p.o.) but not of SR 48968 (20 micrograms kg-1, p.o.): a similar result was obtained on faecal mass and water content. 6. Castor oil strongly increased the occurrence of manometrically recorded propulsive giant contractions (500 to 1000% over control values) of transverse and distal colon, this effect being significantly prevented (80 to 100%) by SR 48968 and loperamide and partially by SR 140333 (35% distal colon, 70% transverse colon). 7. In castor oil free rats, loperamide but not SR 48968 or SR 140333 significantly reduced by 50% the gastrointestinal transit of a charcoal test meal, as well as 24 h faecal mass output. Consistently, loperamide, unlike the tachykinin receptor antagonists, had a dramatic effect on manometric recordings of intestinal motility, reducing all kinds of colonic contractions. 8. Our findings suggest that castor oil diarrhoea in rats entails activation of NK1 and NK2 receptors by endogenous tachykinins, whose antagonists may have a potential as antidiarrhoeal agents free from the constipating action of opioids.

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

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

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.