A possible role of the L-arginine-nitric oxide pathway in the modulation of cholinergic transmission in the guinea-pig taenia coli

Origin and modulation of circular smooth muscle layer contractions in the porcine esophagus

BACKGROUND

The origin and modulation mechanisms controlling timing and amplitude of esophageal body peristalsis are not fully understood. We aimed to characterize the neurotransmitters involved in the origin and modulation of circular smooth muscle esophageal body (EB) contractions.

METHODS

Responses of porcine EB strips to electrical stimulation of motor neurons (MNs) were assessed in organ baths and with microelectrodes. The effect of antagonists of inhibitory (L-NAME 1 mmol L(-1) , MRS2179 10 μmol L(-1) ) and excitatory neurotransmitters (atropine 1 μmol L(-1) ; SR140333 1 μmol L(-1) -NK(1) ra-, GR94800 1 μmol L(-1) -NK(2) ra-) and of ganglionic neurotransmitters (hexamethonium 100 μmol L(-1) , ondansetron 1 μmol L(-1) , NF279 10 μmol L(-1) ) were characterized.

KEY RESULTS

Electrical field stimulation (EFS) induced a frequency-dependent off-contraction (16.8 ± 0.8 g) following a latency period. Latency was significantly reduced by L-NAME (-66.1 ± 4.1%) and MRS2179 (-25.9 ± 5.6%), and strongly increased by atropine (+36.8 ± 5.8%). Amplitude was reduced by L-NAME (-69.9 ± 10.4%), MRS2179 (-34.1 ± 6.0%), atropine (-42.3 ± 4.7%), hexamethonium (-18.9 ± 3.3%), NF279 (-20.7 ± 3.5%), ondansetron (-16.3 ± 3.2%), GR94800 (-28.0 ± 4.8%) SR140333 (-20.9 ± 7.1%), and α-chymotrypsin (-31.3 ± 7.0%). The EFS induced a monophasic nitrergic inhibitory junction potential.

CONCLUSIONS & INFERENCES

Our results suggest that timing (latency) and amplitude of esophageal contractions are determined by a balance of complex interactions between excitatory and inhibitory MNs. Latency depends on the activation of inhibitory MNs releasing NO and a minor purinergic contribution through P2Y(1) receptors, and excitatory MNs releasing ACh. Amplitude depends on a major contribution of excitatory MNs releasing ACh and tachykinins, and also on inhibitory MNs releasing NO, ATP or related purines, and peptidergic neurotransmitters acting as strong modulators of the excitatory neuroeffector transmission.

Rhythmic changes in colonic motility are regulated by period genes

Human bowel movements usually occur during the day and seldom during the night, suggesting a role for a biological clock in the regulation of colonic motility. Research has unveiled molecular and physiological mechanisms for biological clock function in the brain; less is known about peripheral rhythmicity. This study aimed to determine whether clock genes such as period 1 (per1) and period2 (per2) modulate rhythmic changes in colonic motility. Organ bath studies, intracolonic pressure measurements, and stool studies were used to examine measures of colonic motility in wild-type and per1per2 double-knockout mice. To further examine the mechanism underlying rhythmic changes in circular muscle contractility, additional studies were completed in neuronal nitric oxide synthase (nNOS) knockout mice. Intracolonic pressure changes and stool output in vivo, and colonic circular muscle contractility ex vivo, are rhythmic with greatest activity at the start of night in nocturnal wild-type mice. In contrast, rhythmicity in these measures was absent in per1per2 double-knockout mice. Rhythmicity was also abolished in colonic circular muscle contractility of wild-type mice in the presence of N(omega)-nitro-L-arginine methyl ester and in nNOS knockout mice. These findings suggest that rhythms in colonic motility are regulated by both clock genes and a nNOS-mediated inhibitory process and suggest a connection between these two mechanisms.

Detection of NADPH-diaphorase activity in Paramecium primaurelia

Recently, we showed that Paramecium primaurelia synthesizes molecules functionally related to the cholinergic system and involved in modulating cell-cell interactions leading to the sexual process of conjugation. It is known that nitric oxide (NO) plays a role in regulating the release of transmitter molecules, such as acetylcholine, and that the NO biosynthetic enzyme, nitric oxide synthase (NOS), shows nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity. In this work, we detected the presence of NADPH-d activity in P. primaurelia. We characterized this activity histochemically by examining its specificity for beta-NADPH and alpha-NADH co-substrates, and sensitivity both to variations in chemico-physical parameters and to inhibitors of enzymes showing NADPH-d activity. Molecules immunologically related to NOS were recognized by the anti-rat brain NOS (bNOS) antibody. Moreover, bNOS immunoreactivity and NADPH-d activity sites were found to be co-localized. The non-denaturing electrophoresis, followed by exposure to beta-NADPH or alpha-NADH co-substrates, revealed the presence of a band of apparent molecular mass of about 124 kDa or a band of apparent molecular mass of about 175 kDa, respectively. In immunoblot experiments, the bNOS antibody recognized a single band of apparent molecular mass of about 123 kDa.

Sources and implications of basal nitric oxide in spontaneous contractions of guinea pig taenia caeci

We examined in vitro the source and role of basal nitric oxide (NO) in proximal segments of guinea pig taenia caeci in nonadrenergic, noncholinergic (NANC) conditions. Using electron paramagnetic resonance (EPR), we measured the effect of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), the neuronal blocker tetrodotoxin (TTX, 10(-6) M), or both on spontaneous contractions and on the production of basal NO. Both L-NAME and TTX, when tested alone, increased the amplitude and frequency of contractions. NO production was abolished by L-NAME and was inhibited by 38% by TTX. When tested together, L-NAME in the presence of TTX or TTX in the presence of L-NAME had no further effect on the amplitude or frequency of spontaneous contractions, and the NO production was inhibited. These findings suggest that basal NO consists of TTX-sensitive and TTX-resistant components. The TTX-sensitive NO has an inhibitory effect on spontaneous contractions; the role of TTX-resistant NO is unknown.

Potentiation of motilin-induced contraction by nitric oxide synthase inhibition in the isolated chicken gastrointestinal tract

The present experiments were designed to determine whether or not endogenous nitric oxide (NO) modifies the contractile response to chicken motilin (ch-MT) in the gastrointestinal (GI) tract (proventriculus and small intestine) of the chicken. ch-MT (1 nmol L(-1)-1 micromol L(-1)) caused contractions of longitudinal muscle strips of the proventriculus through both myogenic and neurogenic (mostly cholinergic) mechanisms. On the other hand, ch-MT (0.1 nmol L(-1)-100 nmol L(-1)) contracted the small intestine (duodenum, jejunum and ileum) only through a myogenic mechanism. L-Nitroarginine methylester (L-NAME) potentiated, and L-arginine inhibited, the ch-MT- induced contraction without affecting the responsiveness of acetylcholine (ACh) or 5-hydroxytryptamine in the proventriculus. Electrical field stimulation (EFS)- and 1,1-dimethyl-4-phenylpiperazinium (DMPP)- induced contractions were also potentiated by L-NAME. The potentiation by L-NAME was prevented by L-arginine but not by D-arginine. However, in the presence of atropine or tetrodotoxin, neither L-NAME nor L-arginine modified the responses to ch-MT and DMPP. In contrast to the proventriculus, L-NAME and L-arginine were both ineffective in modifying the ch-MT-induced contraction in the small intestine. These results indicate that NO synthase inhibition potentiates the contractile response of ch-MT, EFS and DMPP in the chicken proventriculus through reduction of endogenous NO-mediated presynaptic inhibition on neural ACh release. However, NOS inhibition did not modify the myogenic (direct) action of ch-MT in gastric and intestinal smooth muscles of the chicken.

The effect of nitric oxide on acetylcholine release in the rabbit bladder

We evaluated the effects of nitric oxide (NO) on acetylcholine release and the contractile response induced by electrical field stimulation in rabbit bladder smooth muscles using a muscle bath and high performance liquid chromatography coupled with microdialysis. Electrical field stimulation (supramaximum voltage, pulse duration 0.5 ms, frequency 5 and 20 Hz) was applied to a smooth muscle strip isolated from rabbit bladder. With low-frequency (5 Hz) stimulation, pretreatment with Nomega-nitro-L-arginine (L-NNA) (100 microM) significantly increased electrical field stimulation-induced acetylcholine release and contractile response, which were reduced by the addition of L-arginine. Pretreatment with sodium nitroprusside in the absence or presence of L-NNA significantly decreased electrical field stimulation-induced acetylcholine release and contractile response. In contrast, with high frequency (20 Hz) stimulation, pretreatment with L-NNA and sodium nitroprusside had no significant effect on either contractile response or acetylcholine release. Pretreatment with sodium nitroprusside caused no significant changes in carbachol and ATP-induced contractile responses. Sodium nitroprusside and L-NNA had no significant effects on the atropine-resistant part of the contraction induced by electrical field stimulation in rabbit bladder smooth muscles. The results suggest that there is a NO-mediated mechanism inhibiting acetylcholine release from cholinergic nerve endings in rabbit bladder, which may contribute to bladder function.

Innervation of NADPH diaphorase-containing neurons correlated with acetylcholinesterase, tyrosine hydroxylase, and neuropeptides in the pigeon cloaca

The motility of the avian cloaca is under neural control, but little is known about the neural network that accomplishes this function. This present study was designed to determine the distribution of nitric oxide-synthesising neurons in the pigeon cloaca by enzyme histochemistry for reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d). NADPH-d-positive staining was seen in the neurons and fibres in the cloaca. The highest density of nerve fibres was noted in the coprodeum and the lowest in the proctodeum. In the coprodeum, NADPH-d neurons were found singly, formed small groups of 2-10 neurons, or were seen in plexuses in the muscle layer, lamina propria, or around the arterioles. Several NADPH-d-positive neurons were also observed in the ganglia of the cloaca. NADPH-d fibres ran in the muscle layer, lamina muscularis mucosae and lamina propria, or surrounded blood vessels. The distribution pattern of acetylcholinesterase (AChE)-stained neurons and fibres in the cloaca was similar to that of NADPH-d. Double staining for NADPH-d and AChE showed colocalisation of the 2 enzymes in many neurons of the cloaca. Tyrosine hydroxylase (TH)-immunoreactive nerve fibres originating outside the cloaca were also noted. In the urodeum and proctodeum, neurons or fibres positive for NADPH-d, AChE or TH were scattered in the lamina propria. Nerve fibres immunoreactive for calcitonin-gene related peptide, galanin, methionine-enkephalin, substance P, and vasoactive intestinal peptide were found sparsely in the cloaca. Our results demonstrate that nitrergic neurons constitute a subpopulation which is closely associated with the cholinergic system in the pigeon cloaca.

Neural mechanisms underlying migrating motor complex formation in mouse isolated colon

1. Little is known about the intrinsic enteric reflex pathways associated with migrating motor complex (MMC) formation. Acetylcholine (ACh) mediates the rapid component of the MMC, however a non-cholinergic component also exists. The present study investigated the possible role of endogenous tachykinins (TKs) in the formation of colonic MMCs and the relative roles of excitatory and inhibitory pathways. 2. MMCs were recorded from the circular muscle at four sites (proximal, proximal-mid, mid-distal and distal) along the mouse colon using force transducers. 3. The tachykinin (NK(1) and NK(2)) receptor antagonists SR-140 333 (250 nM) and SR-48 968 (250 nM) reduced the amplitude of MMCs at all recording sites, preferentially abolishing the long duration contraction. Residual MMCs were abolished by the subsequent addition of atropine (1 microM). 4. The neuronal nitric oxide synthase inhibitor, N(omega)nitro-L-arginine (L-NOARG, 100 microM), increased MMC amplitude in the distal region, whilst reducing the amplitude in the proximal region. In preparations where MMCs did not migrate to the distal colon, addition of L-NOARG resulted in the formation of MMCs. Subsequent addition of apamin (250 nM) or suramin (100 microM) further increased MMC amplitude in the distal region, whilst suramin increased MMC amplitude in the mid-distal region. Apamin but not suramin reduced MMC amplitude in the proximal region. Subsequent addition of SR-140 333 and SR-48 968 reduced MMC amplitude at all sites. Residual MMCs were abolished by atropine (1 microM). 5. In conclusion, TKs, ACh, nitric oxide (NO) and ATP are involved in the neural mechanisms underlying the formation of MMCs in the mouse colon. Tachykinins mediate the long duration component of the MMC via NK(1) and NK(2) receptors. Inhibitory pathways may be involved in determining whether MMCs are formed.

Innervation of the pigeon oviduct: correlation of NADPH diaphorase with acetylcholinesterase, tyrosine hydroxylase, and neuropeptides

The motility of the avian oviduct is controlled by hormones and neurons, but little is microscopically known about a neural network in the oviduct. The present study was investigated to determine the distribution of nitric oxide-synthesizing neurons in the oviduct of the pigeon by histochemistry for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d). The NADPH-d reaction was seen in the neurons and fibers. NADPH-d neurons were mainly distributed around the arterioles of the intermuscular tissue in the upper oviduct (infundibulum, magnum, and isthmus); in addition, NADPH-d neurons were also seen in the smooth muscle layers and lamina propria in the lower oviduct (uterus and vagina). NADPH-d neurons were found singly or in small groups of two-eight cell bodies. The number of NADPH-d neurons was smallest in the infundibulum, gradually increased toward the vagina. NADPH-d was also shown to be strongly positive in many neurons in the ganglia of the vaginal adventitia. Bundles of NADPH-d fibers ran in the smooth muscle layer, surrounded blood vessels, or connected with small groups of NADPH-d neurons by forming strands. Thin fibers branched from these bundles and constituted a finer network in the smooth muscle layer and lamina propria. Acetylcholinesterase staining in neurons and fibers showed a similar pattern of NADPH-d distribution in the oviduct. By double staining, 70 approximately 77% of neurons showed colocalization of NADPH-d and acetylcholinesterase in the uterus and vagina. Tyrosine hydroxylase immunoreactivity stained only nerve fibers and were distributed largely around blood vessels in the oviduct. Nerve fibers immunoreactive for calcitonin-gene related peptide, galanin, methionine-enkephalin, substance P, or vasoactive intestinal peptide were found sparsely in the oviduct. These results demonstrate that nitrergic neurons make up a large subpopulation of intrinsic neurons that are closely associated with a cholinergic system in the pigeon oviduct, thus suggesting that nitric oxide and acetylcholine could be used to modify the relaxation of the avian oviduct.

Role of nitric oxide- and vasoactive intestinal polypeptide-containing neurones in human gastric fundus strip relaxations

The morphological pattern and motor correlates of nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) innervation in the human isolated gastric fundus was explored. By using the nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-diaphorase and specific rabbit polyclonal NO-synthase (NOS) and VIP antisera, NOS- and VIP-containing varicose nerve fibres were identified throughout the muscle layer or wrapping ganglion cell bodies of the myenteric plexus. NOS-immunoreactive (IR) neural cell bodies were more abundant than those positive for VIP-IR. The majority of myenteric neurones containing VIP coexpressed NADPH-diaphorase. Electrical stimulation of fundus strips caused frequency-dependent NANC relaxations. N(G)-nitro-L-arginine (L-NOARG: 300 microM) enhanced the basal tone, abolished relaxations to 0.3 - 3 Hz (5 s) and those to 1 Hz (5 min), markedly reduced ( approximately 50%) those elicited by 10 - 50 Hz, and unmasked or potentiated excitatory cholinergic responses at frequencies > or =1 Hz. L-NOARG-resistant relaxations were virtually abolished by VIP (100 nM) desensitization at all frequencies. Relaxations to graded low mechanical distension (< or =1 g) were insensitive to tetrodotoxin (TTX: 1 microM) and L-NOARG (300 microM), while those to higher distensions (2 g) were slightly inhibited by both agents to the same extent ( approximately 25%). In the human gastric fundus, NOS- and VIP immunoreactivities are colocalized in the majority of myenteric neurones. NO and VIP mediate electrically evoked relaxations: low frequency stimulation, irrespective of the duration, caused NO release only, whereas shortlasting stimulation at high frequencies induced NO and VIP release. Relaxations to graded mechanical distension were mostly due to passive viscoelastic properties, with a slight NO-mediated neurogenic component at 2 g distension. The difference between NO and VIP release suggests that in human fundus accommodation is initiated by NO. British Journal of Pharmacology (2000) 129, 12 - 20

Evidence that nitric oxide acts as an inhibitory neurotransmitter supplying taenia from the guinea-pig caecum

Nitric oxide synthase-containing nerve fibres are abundant within taenia of the guinea-pig caecum, but there is little previous evidence supporting a direct role for nitric oxide (NO) in responses to enteric inhibitory nerve stimulation. In this study we have attempted to identify an NO-dependent component of inhibitory transmission in isolated taenia coli. Isometric tension was recorded in the presence of atropine and guanethidine (both 1 microM). Tone was raised with histamine (1 microM), and intrinsic inhibitory neurons stimulated using either a nicotinic agonist (1,1-dimethyl-4-phenylpiperazinium iodide; DMPP) or electrical field stimulation (EFS). DMPP (1-100 microM) produced concentration-dependent biphasic relaxations, comprising an initial peak relaxation followed by a sustained relaxation. Responses to DMPP were antagonized by tetrodotoxin (1 microM) or apamin (0.3 microM) and abolished by hexamethonium (300 microM). L-nitro-arginine (L-NOARG; 100 microM) and oxyhaemoglobin (2%) both significantly reduced sustained relaxations produced by DMPP. EFS (5 Hz, 30 s) also produced biphasic relaxations. Both L-NOARG and an inhibitor of soluble guanylate cyclase (ODQ, 1-10 microM) reduced the sustained component of EFS responses. Two NO donors, sodium nitroprusside (SNP) and diethylenetriamine-nitric oxide adduct (DENO), produced concentration-dependent relaxations. Responses to SNP and DENO were antagonized by ODQ (1 microM) and by apamin (0.3 mM). These results suggest that NO contributes directly to a component of inhibitory transmission in guinea-pig taenia coli. The actions of NO appear to be mediated via cyclic GMP synthesis, and may involve activation of small conductance calcium activated K+ channels. A role for NO is most evident during sustained relaxations evoked by longer stimulus trains or chemical stimulation of intrinsic neurons.

Evidence that inhibitory neurotransmission differs between the proximal and distal segments of guinea-pig taenia caeci

The effect of atropine (1 microM) and N(G)-nitro-L-arginine (L-NOARG, 10 microM) on electrical field stimulation induced relaxation in proximal and distal segments of guinea-pig taenia caeci in the presence of guanethidine (4 microM) was studied. The frequency-dependent relaxations were lower in proximal than in distal segments both in the presence and in the absence of atropine. The effect of L-NOARG (an inhibitor of nitric oxide (NO) synthase) on relaxation in the presence of atropine depended on the frequency of electrical stimulation and the segment used; the effect of L-NOARG was greater in proximal segments than in distal segments. In the absence of atropine, the inhibitory effect of L-NOARG was the same in both segments at all frequencies tested. This study demonstrates differences between the opposite extremes of guinea-pig taenia caeci in relaxations induced by electrical stimulation. Our data also show a role of NO that is dependent on the integrity of cholinergic transmission.

Effect of sodium rhein on electrically-evoked and agonist-induced contractions of the guinea-pig isolated ileal circular muscle

1. This study examined the effects of sodium rhein (0.03-30 microM) on the contractions of the isolated circular muscle of guinea-pig ileum induced by acetylcholine (100 nM), substance P (3 nM) and electrical stimulation (10 Hz for 0.3 s, 100 mA, 0.5 ms pulse duration). The effect of sodium rhein was also evaluated on the ascending excitatory reflex using a partitioned bath (oral and anal compartments). Ascending excitatory enteric nerve pathways were activated by electrical field stimulation (10 Hz for 2 s, 20 mA, 0.5 pulse duration) in the anal compartment and the resulting contraction of the guinea-pig intestinal circular muscle in the oral compartment was recorded. 2. Sodium rhein (0.3, 3 and 30 microM) significantly potentiated (52+/-11% at 30 microM) acetylcholine-induced contractions. In the presence of tetrodotoxin (0.6 microM) or omega-conotoxin GVIA (10 nM) sodium rhein (3 and 30 microM) did not enhance, but significantly reduced (49+/-10% and 44+/-8%, respectively, at 30 microM) acetylcholine-induced contractions. 3. Sodium rhein (0.3, 3 and 30 microM) significantly increased (65+/-11% at 30 microM) substance P-induced contractions. In the presence of tetrodotoxin (0.6 microM), omega-conotoxin GVIA (10 nM) or atropine (0.1 microM), sodium rhein (3 and 30 microM) significantly reduced (50+/-10%, 55+/-8% and 46+/-10%, respectively, at 30 microM) substance P-induced contractions. 4. NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) abolished the potentiating effect of sodium rhein on acetylcholine and substance P-induced contractions. At the highest concentration (30 microM), sodium rhein, in presence of L-NAME, reduced the acetylcholine (30+/-6%)- or substance P (36+/-6%)-induced contractions. 5. Sodium rhein (30 microM) significantly potentiated (29+/-9%) the electrically-evoked contractions. L-NAME (100 microM), but not phentolamine, enhanced the effect of sodium rhein. Sodium rhein (30 microM) significantly increased (32+/-9%) the ascending excitatory reflex when applied in the oral, but not in the anal compartment. 6. These results indicate that sodium rhein (i) activates excitatory cholinergic nerves on circular smooth muscle presumably through a facilitation of Ca2+ entry through the N-type Ca2+ channel, (ii) has a direct inhibitory effect on circular smooth muscle and (iii) does not affect enteric ascending neuroneural transmission. Nitric oxide could have a modulatory excitatory role on sodium rhein-induced changes of agonist-induced contractions and an inhibitory modulator role on sodium rhein-induced changes of electrically-induced contractions.

Involvement of the L-arginine/nitric oxide neural pathway in non-adrenergic, non-cholinergic relaxation of the bovine oesophageal groove

1. The distribution and localization of nitric oxide synthase (NOS) immunoreactivity and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) activity in the bovine oesophageal groove were investigated by immnunohistochemical and histochemical staining techniques. Functional in vitro studies were performed to correlate the presence of NOS-immunoreactivity (IR) and NADPH-d staining with smooth muscle relaxations involving the L-arginine/nitric oxide neural pathway in the bovine oesophageal groove activity. 2. NOS-IR and NADPH-d were expressed in nerve cell bodies of the myenteric, submucosal and intramuscular ganglia, and in nerve fibres distributed around blood vessels and throughout the different muscular layers of the bovine oesophageal groove. 3. In oesophageal groove strips treated with guanethidine (10(-5) M) and atropine (10(-7) M) to block noradrenergic neurotransmission and muscarinic receptors, respectively, electrical field stimulation (EFS, 0.5-32 Hz, 1 ms duration, 20-s trains) induced relaxations which were practically abolished by tetrodotoxin (TTX, 10(-6) M). 4. Incubation with an inhibitor of nitric oxide synthesis, NG-nitro-L-arginine (L-NOARG, 3 x 10(-5) M), significantly inhibited relaxations induced by EFS. This inhibition was partially reversed by L-arginine (L-arg, 5 x 10(-3) M). D-NOARG (3 x 10(-5) M) had no effect on EFS-induced relaxations. 5. NO added as an acidified solution of NaNO2 (10(-6) - 10(-3) M) and S-nitroso-L-cysteine (10(-7) - 10(-4) M) caused concentration-dependent relaxations of the bovine oesophageal groove. These relaxations were unaffected by L-NOARG (3 x 10(-5) M). 6. The presence of NO-synthesizing enzyme in nerves and ganglia, and the pharmacological evidence for NO-mediated smooth muscle relaxation suggested that the L-arg/NO neuronal pathway is involved in the inhibitory neurotransmission of the bovine oesophageal groove.

Nitric oxide-sensitive guanylyl cyclase inhibits acetylcholine release and excitatory motor transmission in the guinea-pig ileum

This study examined the mechanism through which nitric oxide inhibits the release of acetylcholine and excitatory motor neurotransmission in the guinea-pig ileum. The selective inhibitor of nitric oxide-sensitive guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), concentration-dependently enhanced both basal release (-log EC50: 6.8) and electrically (10 Hz)-evoked release (-log EC50: 6.0) of [3H]acetylcholine from longitudinal muscle-myenteric plexus preparations preincubated with [3H]choline. The increase by ODQ of basal release appeared to be exocytotic since it was prevented by tetrodotoxin (300 nM) and absence of calcium from the superfusion medium. In addition, ODQ (1 microM) increased the electrically-evoked tachykininergic and cholinergic muscle contractions as measured in the presence of scopolamine (100 nM) or of the neurokinin-1 receptor antagonist CP 99994 (100 nM), respectively. The nitric oxide synthase inhibitor L-N(G)-nitro-arginine (100 microM) behaved similar to ODQ and increased cholinergic and tachykininergic motor neurotransmission. The nitric oxide-independent activator of soluble guanylyl cyclase, 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole, concentration-dependently inhibited the electrically evoked acetylcholine release (-log EC50: 6.0) and longitudinal muscle contractions (-log EC50: 5.7). ODQ (10 microM) antagonized the effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole. The results suggest that endogenous nitric oxide tonically activates soluble guanylyl cyclase in myenteric neurons which leads to inhibition of the release of the excitatory transmitters acetylcholine and substance P. ODQ prevents the effects of nitric oxide and thus facilitates cholinergic and tachykininergic motor neurotransmission in the guinea-pig ileum.

Partial co-existence of NADPH-diaphorase and acetylcholinesterase in the hypothalamic magnocellular secretory nuclei of the rat

Co-localization of NADPH-diaphorase (ND) and acetylcholinesterase (AChE) activities were explored in the magnocellular secretory nuclei of the rat hypothalamus by means of a double histochemical staining of the same sections. Partial co-existence was found in all the nuclei studied (paraventricular, supraoptic, fornicals and circular nuclei). No particular location of the neurons expressing both markers was found, although in the paraventricular nucleus all of them (ND +, AChE + and neurons expressing both markers) were preferentially located in the magnocellular subdivisions whereas in the parvicellular ones only some neurons belonging to all three types were detected, mainly located in the periventricular and medial subdivisions. The lowest degree of co-existence was found at the level of the main magnocellular nuclei (supraoptic and paraventricular) when compared with the accessory magnocellular nuclei, especially the posterior fornical and the circular nuclei. These results extend previous data on the chemical nature of the neurons producing nitric oxide in the neurosecretory nuclei and the possible functional role of this atypical messenger in the hypothalamus.

Nitrergic modulation of cholinergic responses in the opossum lower oesophageal sphincter

1. Electrical field stimulation (EFS) of the superfused lower oesophageal sphincter from opossum (Monodelphis domestica) elicited biphasic responses. The first phase (relaxation) was strictly dependent on the duration of the EFS. The second phase (contraction) started following termination of the EFS (< or = 15 Hz). EFS at frequencies above 15 Hz led only to contraction, which started immediately upon initiation of the stimulation. 2. In the presence of NG-nitro-L-arginine (L-NOARG; 0.1-300 microM), the relaxation phase was abolished and the contractile response started with the initiation of EFS (at all frequencies) and was greater in magnitude. The contractile response to EFS was completely blocked with scopolamine (10 microM). 3. Exogenous acetylcholine (1-100 microM) elicited concentration-dependent contractions of the sphincter in the presence of botulinum toxin. These contractions were abolished when EFS was applied during administration of acetylcholine. This inhibitory effect of EFS was completely reversed when the tissue was treated with L-NOARG (100 microM). 4. These results suggest that the cholinergic response in the opossum lower oesophageal sphincter is under nitrergic control.

Involvement of nitric oxide in the substance P-induced inhibition of intestinal peristalsis

Although considered as an intestinal motor stimulant, substance P can inhibit intestinal peristalsis via stimulation of tachykinin NK1 receptors. Since NK1 receptors are present on enteric nitrergic neurones, the contribution of nitric oxide (NO) to the peristaltic motor inhibition caused by tachykinins was examined in luminally perfused segments of isolated guinea-pig ileum. Substance P (100 nM) and the NK1 receptor agonist substance P methyl ester (100 nM) increased the intraluminal pressure threshold at which peristaltic contractions were elicited. This inhibitory influence on peristalsis was prevented by the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (300 microM) in an enantiomer-selective manner. It is concluded that the substance P/NK1 receptor-mediated depression of intestinal peristalsis involves inhibitory motor pathways utilizing NO as a transmitter.

Evidence that NO acts as a redundant NANC inhibitory neurotransmitter in the guinea-pig isolated taenia coli

1. The relative contribution of the putative transmitters, nitric oxide (NO) and an apamin-sensitive factor, possibly ATP, to inhibitory responses evoked by electrical field stimulation (EFS; 0.2-5 Hz, 0.2 ms duration, supra-maximal voltage for 10 s) of non-adrenergic, non-cholinergic (NANC) nerves was investigated in the guinea-pig isolated taenia coli contracted with histamine (1 microM). 2. Peak relaxations to EFS (0.2-5 Hz) were tetrodotoxin (1 microM)-sensitive, maximal at 0.2 Hz and completely resistant to the nitric oxide synthase inhibitor, NG-nitro-L-arginine (L-NOARG; 100 microM) in either the presence or absence of atropine (1 microM). Furthermore, the specific inhibitor of soluble guanylyl cyclase, 1H-[1,2,4] oxadiazolo [4,3-a] quinoxaline-1-one (ODQ; 10 microM), the cytochrome P450 inhibitor and free radical generator, 7-ethoxyresorufin (7-ER; 10 microM) and the NO scavenger, oxyhaemoglobin (HbO; 30 microM) had no effect on EFS-induced relaxations alone and in combination with L-NOARG (100 microM). 3. Maximum relaxation to the NO donor, sodium nitroprusside (SNP; 1 microM) was significantly reduced by HbO (30 microM), abolished by 7-ER (10 microM) and ODQ (10 microM) but was unaffected by apamin (0.1 microM), an inhibitor of small conductance Ca(2+)-activated K+ channels. 4. The relaxation to EFS at 0.2 Hz was resistant to apamin but those to 0.5 and 5 Hz were significantly reduced. EFS (0.2-5 Hz)-evoked relaxations that persisted in the presence of apamin were further significantly inhibited by L-NOARG (100 microM) or ODQ (10 microM), but not by HbO (30 microM) or 7-ER (10 microM). 5. ATP (1-30 microM) produced concentration-dependent relaxations that were abolished by apamin (0.1 microM), unaffected by ODQ (10 microM) but only significantly reduced by L-NOARG (100 microM) at the lowest concentration of ATP (1 microM) used. 6. Nifedipine (0.3 microM), abolished contractions to 67 mM KCl, histamine (10 microM), endothelin-1 (0.03 microM), 5-hydroxytryptamine (5-HT; 10 microM) and the thromboxane-mimetic, 9-11-dideoxy-9 alpha, 11 alpha-methano-epoxy-prostaglandin F2 alpha (U46619; 0.1 microM). 7. The findings of the present study suggest that NO is released during NANC nerve stimulation, but plays no role in NANC relaxations in the guinea-pig taenia coli unless the effects of another apamin-sensitive, nerve-derived hyperpolarizing factor (NDHF) are blocked. Thus, we propose that in this tissue, NO acts as a 'backup' or redundant NANC nerve inhibitory transmitter and like NDHF mediates relaxation via hyperpolarization.

Inhibition of acetylcholine induced intestinal motility by interleukin 1 beta in the rat

BACKGROUND/AIMS

The fact that raised interleukin 1 beta (IL 1 beta) concentrations have been found in the colonic mucosa of rats with experimentally induced colitis and of patients with inflammatory bowel disease indicates that this cytokine may participate in the disturbed intestinal motility seen during inflammatory bowel disease. This study investigated whether IL 1 beta could change the contractility of (a) a longitudinal muscle-myenteric plexus preparation from rat jejunum, ileum, and colon and (b) isolated jejunal smooth muscle cells.

METHODS

Isometric mechanical activity of intestinal segments was recorded using a force transducer. Moreover, smooth muscle cell length was measured by image analysis.

RESULTS

Although IL 1 beta did not affect jejunal, ileal, and colonic basal contractility, it significantly reduced contractile response to acetylcholine (ACh). This significant inhibition was seen only after 90 or 150 minutes of incubation with IL 1 beta. Pretreatment with cycloheximide blocked IL 1 beta induced inhibition of ACh stimulated jejunal contraction, suggesting that a newly synthesised protein was involved in the effect. NW-nitro-L-arginine (a nitric oxide synthase inhibitor) did not prevent the inhibition induced by IL 1 beta. Blocking neural transmission with tetrodotoxin abolished the IL 1 beta effect on jejunal contractile activity, whereas IL 1 beta had no effect on isolated and dispersed smooth muscle cells.

CONCLUSIONS

IL 1 beta inhibits ACh induced intestinal contraction and this inhibitory effect involves protein synthesis but is independent of nitric oxide synthesis. This effect does not involve a myogenic mechanism but is mediated through the myenteric plexus.

Differential effects of nitric oxide donors on basal and electrically evoked release of acetylcholine from guinea-pig myenteric neurones

1. The effects of the nitric oxide (NO) donors, 3-morpholino-sydnonimine (SIN-1), S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside on basal and electrically evoked release of [3H]-acetylcholine were studied in myenteric plexus longitudinal muscle preparations of the guinea-pig small intestine preincubated with [3H]-choline. 2. The NO donors concentration-dependently increased basal release of [3H]-acetylcholine. The increase in release was calcium-dependent and was prevented in the presence of tetrodotoxin. Superoxide dismutase (150 u ml-1) potentiated the effect of SIN-1. The selective inhibitor of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ, 0.01-1 microM), antagonized the facilitatory effect of SNAP. 8-Bromo cyclic GMP and the cyclic GMP-specific phosphodiesterase inhibitor, zaprinast (both 0.1-1 mM), also enhanced basal [3H]-acetylcholine release. The effect of 10 microM SNAP was significantly enhanced in the presence of zaprinast. 3. The NO donors concentration-dependently inhibited the electrically evoked release of [3H]-acetylcholine, whereas 8-bromo cyclic GMP and zaprinast enhanced the evoked release. The inhibition of acetylcholine release by SNAP was not affected by ODQ (0.01-1 microM). 4. It is concluded that NO stimulates basal acetylcholine release from myenteric neurones through activation of guanylyl cyclase. In addition, NO inhibits the depolarization evoked release of acetylcholine by a presynaptic mechanism unrelated to cyclic GMP. The data imply that NO is not only an inhibitory transmitter to intestinal smooth muscles but also a modulator of cholinergic neurotransmission in the myenteric plexus.

Hyperpolarization and inhibition of contraction mediated by nitric oxide released from enteric inhibitory neurones in guinea-pig taenia coli

1. Inhibition of nitric oxide synthase by NG-nitro-L-arginine (L-NNA) reduced the neurogenic relaxation of precontracted taenia coli only in the absence of atropine. The membrane hyperpolarization associated with the neurogenic relaxation was also reduced by inhibition of NOS only when atropine was absent. 2. The membrane hyperpolarization associated with the neurogenic relaxation of the taenia coli was inhibited by oxyhaemoglobin only in the absence of atropine. In the presence of atropine, oxyhaemoglobin did not reduce the i.j.p. or nerve evoked relaxation. 3. Inhibition of NOS by L-NNA did not affect the overflow of [3H]-ACh in response to electrical field stimulation (EFS), suggesting that, under the conditions of our experiments, endogenous NO did not modulate release of ACh. Sodium nitroprusside also had no effect on the neurogenic overflow of [3H]-ACh; however, noradrenaline significantly reduced [3H]-ACh overflow. 4. In summary, the postjunctional effects of neurally-released NO are not apparent in guinea-pig taenia coli when atropine is present. This implies muscarinic regulation of NO release or muscarinic regulation of another excitatory substance, such as tachykinin(s), that, when blocked, masks the postjunctional effects of NO. These data, together with previous studies, suggest a possible regulatory role for NO in enteric neurotransmission that may be more prominent in some species or tissues than others.

Nitric oxide modulates cholinergic neurotransmission in cat duodenum

1. Longitudinal muscle strips isolated from cat proximal duodenum were characterized by spontaneous phasic contractions. 2. Electrical field stimulation (EFS) (0.5 ms, 1-20 Hz, supramaximal voltage intensity for 40 sec) produced frequency-dependent contractions, and maximal amplitude was achieved at 10 Hz. The EFS-induced contractions were abolished either by atropine (10(-6) M) or by tetrodotoxin (3 x 10(-7) M). 3. The nitric oxide (NO) synthase blocker N infinity-nitro-L-arginine (L-NNA, 10(-4) M) or the inhibitor of the soluble guanylyl cyclase methylene blue (MB, 3 x 10(-5) M) increased the amplitude of the electrically evoked contractions. 4. L-Arginine (10(-3) M) or sodium nitroprusside (SNP, 10(-4) M) significantly decreased the amplitude of the EFS-induced, L-NNA- or MB-potentiated contractions as the effect of SNP was much more pronounced. 5. Neither L-NNA nor MB affected the contraction evoked by exogenous acetylcholine. 6. The L-NNA or MB-induced interruption of the L-arginine-NO pathway potentiated the electrically evoked cholinergic contractions, suggesting the inhibitory role of NO in the cholinergic neurotransmission realized probably at the pre-synaptic level in cat duodenum.

Colocalization of NADPH-diaphorase and acetylcholinesterase in the rat olfactory bulb

The colocalization of NADPH-diaphorase and acetylcholinesterase activities in the rat main and accessory olfactory bulbs has been studied by successive double histochemical staining of the same sections. In the main olfactory bulb, three patterns of glomerular labeling were found: typical/NADPH-diaphorase-positive, typical/NADPH-diaphorase-negative, and atypical/NADPH-diaphorase-negative glomeruli. Although both enzymatic activities were present in periglomerular cells and superficial short-axon cells, colocalization of NADPH-diaphorase and acetylcholinesterase was not observed in these neuronal types. By contrast, both enzymes were colocalized in a small subpopulation (less than 3% of NADPH-diaphorase- or acetylcholinesterase-positive cells) of short-axon cells located in the external plexiform layer, internal plexiform layer, granule cell layer, and white matter. In the accessory olfactory bulb, deep short-axon cells were the only neurons where both enzymes were present, and colocalization of both markers was observed in some of these cells located in the granule cell layer.

Inhibitory action of PPADS on relaxant responses to adenine nucleotides or electrical field stimulation in guinea-pig taenia coli and rat duodenum

1. The effect of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on the relaxant response to adenine nucleotides was examined in the carbachol-contracted guinea-pig taenia coli and rat duodenum, two tissues possessing P2y-purinoceptors. In addition, in the taenia coli PPADS was investigated for its effect on relaxations evoked by adenosine, noradrenaline and electrical field stimulation. In order to assess the selectivity of PPADS between P2-purinoceptor blockade and ectonucleotidase activity, its influence on ATP degradation was studied in guinea-pig taenia coli. 2. The resulting rank order of potency for the adenine nucleotides in guinea-pig taenia coli was: 2-methylthio ATP >> ATP > alpha,beta-methylene ATP with the respective pD2-values 7.96 +/- 0.08 (n = 23), 6.27 +/- 0.12 (n = 21) and 5.88 +/- 0.04 (n = 24). 3. In guinea-pig taenia coli, PPADS (10-100 microM) caused a consistent dextral shift of the concentration-response curve (CRC) of 2-methylthio ATP and ATP resulting in a biphasic Schild plot. A substantial shift was only observed at 100 microM PPADS, the respective pA2-values at this particular concentration were 5.26 +/- 0.16 (n = 5) and 5.15 +/- 0.13 (n = 6). Lower concentrations of PPADS (3-30 microM) antagonized the relaxant effects to alpha,beta-methylene ATP in a surmountable manner. An extensive shift of the CRC was produced only by 30 microM PPADS (pA2 = 5.97 +/- 0.08, n = 6), and the Schild plot was again biphasic. 4. The relaxant responses to electrical field stimulation (80 V, 0.3 ms, 5 s, 0.5-16 Hz) in guinea-pigtaenia coli were concentration-dependently inhibited by PPADS (10-100 microM).5. In guinea-pig taenia coli, the potency of ATP in inducing relaxation appeared to be independent of its rate of degradation by ecto-nucleotidases, since the Km-value (366 microM) obtained in the enzyme assay was much higher than the functional EC50-value (0.45 microM) of ATP. PPADS (3-100 microM) was only weakly active in inhibiting ecto-nucleotidase activity leaving a residual activity of 81.8 +/- 5.1% at 100 microM.Enzyme inhibition by PPADS was concentration-independent and non-competitive.6. In rat duodenum, the rank order of potency was: 2-methylthio ATP >ATP> >alpha,beta-methylene ATP,the respective pD2-values being 6.98 +/- 0.04 (n = 76), 6.26 +/- 0.02 (n = 6) and 4.83 +/- 0.02 (n = 6). Among these agonists, 2-methylthio ATP displayed the lowest apparent efficacy.7. The CRC of 2-methylthio ATP in rat duodenum was shifted to the right by PPADS (10-100 microM) ina concentration-dependent manner, and Schild analysis gave a pA2-value of 5.09 +/- 0.06 (slope = 1.02,n=14).8 PPADS was without any effect on the carbachol-induced contraction in guinea-pig taenia coli or rat duodenum and on the relaxation to noradrenaline or adenosine in guinea-pig taenia coli.9 In conclusion, the antagonistic properties of PPADS at the taenia coli and rat duodenum P2y-purinoceptors were different from those recently described at the P2x-subtype: inhibition of P2y-purinoceptor-mediated responses was observed at higher concentrations (3-100 microM vs. 1-10 (30) microM).Furthermore, we conclude that in addition to the classical P2y-subtype, which is largely PPADS-resistant,the guinea-pig taenia coli may be endowed with a distinct relaxation-mediating P2-purinoceptor subtype which is sensitive to PPADS.

Characteristic features of inhibitory junction potentials evoked by single stimuli in the guinea-pig isolated taenia caeci

1. Changes in membrane potential of the guinea-pig isolated taenia caeci evoked by single stimuli have been investigated using intracellular recording techniques. Nifedipine (10 microM) was used to arrest spontaneous muscle action potentials. Single stimuli elicited complex junction potentials which consisted of both excitatory and inhibitory components. 2. The excitatory component of the compound junction potential was unaffected by hexamethonium (100 microM) but abolished by atropine (1 microM) and omega-conotoxin GVIA (10-100 nM). 3. In the presence of atropine, single stimuli elicited fast inhibitory junction potentials (IJPs). IJPs were sometimes biphasic during repolarization with a noticeable 'slow tail'. Apamin (30-100 nM) potently inhibited the fast IJP and revealed an underlying slow IJP. 4. The fast IJP was also abolished by omega-conotoxin GVIA (100 nM). However, the slow IJP was insensitive to omega-conotoxin GVIA but was abolished by cadmium (30 microM). 5. Guanethidine (3 microM) and N omega-nitro-L-arginine (10-100 microM) had no detectable effects on either of the IJPs. The dye Reactive Blue 2 reduced the amplitude of the fast IJP but this reduction was associated with a membrane hyperpolarization. 6. The existence of two distinct IJPs in the guinea-pig taenia caeci has been demonstrated. The ability of omega-conotoxin GVIA to selectively abolish the fast IJP leaving the slow IJP intact suggests that separate nerves are involved in mediating these responses.

Pharmacological identification of different inhibitory mediators involved in the innervation of the internal anal sphincter

1. Inhibitory non-adrenergic, non-cholinergic (NANC) responses were studied in isolated strips from the rabbit internal anal sphincter. 2. In the presence of atropine and guanethidine, transmural field stimulation induced frequency-dependent relaxations that reached a plateau at frequencies > or = 4 Hz. 3. These relaxations were inhibited by apamin (10(-6) M) and by N omega-nitro-L-arginine (L-NOARG, 10(-4) M). With these two substances in combination, relaxations were still seen in response to field stimulation, but only at frequencies > 2 Hz. 4. In the presence of both apamin (10(-6) M) and L-NOARG (10(-4) M), responses at high frequencies consisted of a fast relaxation followed by a slow return to prestimulus tension level. alpha-Chrymotrypsin hastens the return of tension to prestimulus level after high frequency stimulation. 5. Zinc-protoporphyrin IX, an inhibitor of haeme oxygenase, had a significant inhibitory effect on relaxations induced by transmural field stimulation. It was found, however, that responses to sodium nitroprusside and to isoprenaline (both 10(-9)-10(-4) M) were reduced comparably, indicating that the effect of zinc-protoporphyrin IX was unspecific. 6. It is concluded that pharmacological analysis allows identification of at least three distinguishable components of the inhibitory NANC innervation of the rabbit internal anal sphincter. The study does not allow conclusions about the role of carbon monoxide, a recently proposed mediator of NANC responses in opossum internal anal sphincter.

Co-existence of nitrergic, peptidergic and acetylcholine esterase-positive nerves in the pig lower urinary tract

The distribution of NO synthase (NOS) immunoreactive nerves and the possible co-existence with other neurotransmitters were investigated in the pig lower urinary tract. NOS immunoreactive nerves were found in the muscle layer, in the lamina propria and around blood vessels. The density of NOS immunoreactive nerves was more prominent in the trigone and urethra than in the detrusor. All parts of the lower urinary tract were supplied by numerous acetylcholine esterase (AChE) positive nerves. The number of adrenergic nerves in the trigone and urethra was moderate to rich, whereas only very few adrenergic nerves were demonstrated in the detrusor. A low to moderate number of nerve fibres containing neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP) were observed in the trigone and urethra, while very few were found in the detrusor. A small number of nerves, confined to the trigone and urethra, were stained for calcitonin-gene-related peptide, somatostatin and leu-enkephalin. Nerve fibres exhibiting immunoreactivity to bombesin/gastrin releasing peptide, gastrin/cholecystokinin, substance P or neurokinin A were virtually absent. Co-localization studies revealed that some NOS-immunoreactive nerves also stained for NPY, VIP or AChE. The present study shows that nitrergic nerves are present in the pig lower urinary tract in a density lower than the cholinergic, but higher than any of the studied peptidergic nerves. Coinciding localization of NOS-positive nerves with nerves expressing AChE, VIP and NPY suggests that NO may have a role as a messenger in the lower urinary tract directly and by interaction with other transmitters.

Participation of nitric oxide in the nicotine-induced relaxation of the cat lower esophageal sphincter

The participation of nitric oxide in the relaxation of the cat lower esophageal sphincter muscle strip in response to electrical field stimulation or administration of nicotine was studied. The nicotine-induced relaxation was mediated via a neuronal pathway, since it was inhibited by administration of hexamethonium or tetrodotoxin. Inhibition of nitric oxide biosynthesis by N-nitro-L-arginine decreased the relaxation induced by nicotine (50 microM) or field stimulation. With the maximal concentration of N-nitro-L-arginine (1 mM) electrical field stimulation-induced relaxation was abolished, while nicotine-induced relaxation decreased by 70%. L-Arginine (1 mM) partly restored this relaxation. Desensitization of P2x receptors by alpha, beta methylene-adenosine 5-triphosphate (alpha, beta-m-ATP) did not change the relaxation induced by either electrical field stimulation or administration of nicotine. It is therefore suggested that the field stimulation-induced relaxation is mediated by the release of nitric oxide, but in the nicotine-produced relaxation is only partly due to nitric oxide, other factor(s) might be also be involved.

Effects of transmural field stimulation in isolated muscle strips from rabbit sphincter of Oddi and duodenum

The purpose of the study was to compare the effect of transmural field stimulation (TMS) on isolated smooth muscle strips from rabbit sphincter of Oddi (SO), duodenal circular layer (Dc) and duodenal longitudinal layer (D1). The strips were suspended in thermostatically controlled 5-ml organ baths containing Krebs solution constantly bubbled with 5% CO2 in O2. TMS was delivered through platinum electrodes (140 V, 0.4 ms, 5 s trains, 40 Hz). The TMS responses could be divided in two main responses: (1) contraction initiated after cessation of the stimulus train, preceded by an inhibitory phase during TMS ('off'); and (2) contraction initiated during TMS ('duration'). The 'duration' response was observed in one out of 20 strips in the SO and Dc compartments, whereas 11 D1 strips (55%) showed 'duration' responses (P < 0.001). Atropine (10(-6)) converted all 'duration' responses to an 'off' response preceded by an inhibitory phase during TMS and reduced the contractile amplitudes with 40-65%. L-NNA significantly increased the number of 'duration' responses in all types of muscle, and caused a 40% increase in D1 contractile amplitude. Inhibitory responses could not be removed by atropine, propranolol and phentolamine. The results suggest that the intrinsic innervation of SO and duodenal muscle consists of a mixture of excitatory, cholinergic and inhibitory NANC pathways. The latter may utilize, wholly or partly, NO or a related compound as transmitter. A relative dominance of excitatory, cholinergic responses was present in the D1 strips, whereas inhibitory responses were dominating in the SO and Dc strips.

Increase by NO synthase inhibitors of acetylcholine release from guinea-pig myenteric plexus

The effects of nitric oxide (NO) synthase inhibitors on the electrically evoked release of [3H]acetylcholine were studied in guinea-pig myenteric plexus preparations preincubated with [3H]choline. NG-monomethyl-L-arginine (EC50 5.3 mumol l-1) and NG-nitro-L-arginine (EC50 1.3 mumol l-1) concentration-dependently increased the evoked release of [3H]acetylcholine without affecting the basal outflow. The facilitatory effect of NG-mono-methyl-L-arginine was prevented by L-arginine but not by D-arginine. The results suggest that endogenous NO inhibits the depolarisation-evoked release of acetylcholine.

The role of the L-arginine/nitric oxide pathway for relaxation of the human lower oesophageal sphincter

Smooth muscle specimens were taken from the oesophagogastric junction (OGJ) in patients operated on for gastrointestinal malignancies not involving the OGJ. The smooth muscle bundles of the inner, circular layer of the OGJ were richly innervated by fine nerve fibres staining positively for NADPH diaphorase. The outer longitudinal layer had a markedly lower number of NADPH-diaphorase positive nerve fibres. When the preparations were suspended in organ baths for recording of isometric tension, they developed active tension. Transmural field stimulation (TMS) induced frequency-dependent relaxations, which were abolished by NG-nitro-L-arginine (L-NNA; 10(-4) M), and were often converted to atropine-sensitive contractions. The effect of L-NNA was concentration-dependent, and the concentration-response curve for L-NNA was shifted to the right by L-arginine pre-incubation. The enantiomer NG-nitro-D-arginine (10(-4) M) also showed inhibitory actions on the responses to TMS, but significantly less than L-NNA. Relaxant responses to vasoactive intestinal polypeptide (VIP), forskolin, and sodium nitroprusside were unaffected by L-NNA pre-incubation. Exposure to a 124 mM K+ solution resulted in a biphasic relaxation of the preparations. This relaxation was not seen in preparations treated with scorpion venom (20 micrograms ml-1) or L-NNA (10(-4) M). Instead, a contractile response to 124 mM K+ solution was found. The results suggest that NANC responses to electrical stimulation of nerves in the human OGJ are mediated by a product generated from L-arginine.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal nitric oxide in the gut

Motility of the gastrointestinal tract is directly controlled by enteric inhibitory and excitatory motor neurons that innervate the layers of smooth muscle. Inhibitory motor neurons mediate receptive and accommodative relaxations and control the opening of sphincters, thus playing an important role in normal gut motility. Recent studies have demonstrated that nitric oxide (NO) is an important neurotransmitter released by inhibitory motor neurons in animal and human gut. Antagonists of nitric oxide synthase (NOS), the synthetic enzyme for NO, reduce the effectiveness of transmission from inhibitory motor neurons. Exogenous NO mimics inhibitory nerve activation, and a variety of compounds that affect the availability of endogenously produced NO modulate relaxations of gastrointestinal smooth muscle. It is clear, however, that NO is unlikely to be the only transmitter released by enteric inhibitory motor neurons: several other substances such as vasoactive intestinal polypeptide (VIP), or related peptides, and adenosine triphosphate (ATP) are also likely to contribute to nerve-mediated inhibition. The identification of NO as a major inhibitory neurotransmitter to gastrointestinal smooth muscle fills an important gap in our understanding of the physiological control of motility and opens up a wide range of new experimental possibilities. It may eventually lead to the development of new drugs for motility disorders. It should be noted, however, that NO is important in the brain, in cardiovascular control, in blood cell function and in many other organ systems, suggesting that it may be difficult to achieve specific pharmacological intervention targeted on inhibitory neurotransmission in the gut, without undesirable side effects.

Inhibition of nitric oxide synthesis reveals non-cholinergic excitatory neurotransmission in the canine proximal colon

1. Neuromuscular transmission in the circular muscle of the canine proximal colon was examined, in the presence and absence of nitric oxide synthase inhibitors, by use of mechanical and intracellular microelectrode recording techniques. 2. Electrical field stimulation (EFS; 0.1-20 HZ) produced frequency-dependent contractions of circular muscle strips which reached a maximum at 15 Hz. These responses were enhanced by NG-monomethyl-L-arginine (L-NMMA; 300 microM) and reduced by atropine (1 microM). The effects of L-NMMA were reversed by L-arginine (3 mM). All responses to EFS were abolished by tetrodotoxin (1 microM). 3. In the presence of atropine, phentolamine and propranolol (all at 1 microM; 'non-adrenergic, non-cholingergic (NANC) conditions'), EFS evoked frequency-dependent inhibition of phasic contractions which reached a maximum at 5 Hz. At higher frequencies of EFS, inhibition diminished, and these responses were followed by post-stimulus excitation. 4. Under NANC conditions and in the presence of L-NG-nitroarginine methyl ester (L-NAME; 200 microM), EFS evoked contractions at frequencies of 5 Hz or greater. These contractions were reduced by co-incubation with L-arginine (2 mM) and abolished by tetrodotoxin (1 microM). 5. In the presence of atropine (1 microM), EFS (5-20 Hz) caused frequency-dependent inhibition of electrical slow waves. In the presence of L-NAME (100 microM) and atropine, the inhibitory response to EFS was abolished and an increase in slow wave duration was seen at stimulation frequencies greater than 5 Hz. The effects of EFS on slow wave duration were abolished by tetrodotoxin (1 microM). 6. Atropine-resistant contractions to EFS were enhanced by indomethacin (10 microM) and reduced or abolished by the non-selective NK1/NK2 tachykinin receptor antagonist D-Pro2, D-Trp7,9 SP, and by the selective NK2 receptor antagonist MEN 10,376 (10 microM).7. Exogenous tachykinins mimicked non-cholinergic excitatory electrical and mechanical responses. The rank order of potency for contraction was neurokinin A>neurokinin B>substance P, suggesting a predominance of the NK2 sub-type of tachykinin receptors on colonic smooth muscle cells. Low concentrations of neurokinin A also increased the amplitude and duration of electrical slow waves.8. These results suggest that: (i) in previous studies, non-cholinergic excitatory responses were masked by the simultaneous release of NO; (ii) non-cholinergic excitatory responses occur throughout the period of stimulation and are not manifest only as 'rebound' excitation; (iii) one or more tachykinins, possibly,acting via NK2 receptors, may mediate non-cholinergic excitatory responses.