Characterization of somatodendritic neuronal nicotinic receptors located on the myenteric plexus

5-HT4 receptors facilitate cholinergic neurotransmission throughout the murine gastrointestinal tract

BACKGROUND

In the gastrointestinal tract of several species, facilitating 5-HT₄ receptors were proposed on myenteric cholinergic neurons innervating smooth muscle by in vitro study of the effect of the selective 5-HT₄ receptor agonist prucalopride on submaximal cholinergic contractions. This was not yet established in the murine gastrointestinal tract.

METHODS

In circular smooth muscle strips from murine fundus, jejunum and colon, contractions were induced by electrical field stimulation in the presence of guanethidine, L-NAME and for colon also MRS 2500. Submaximal contractions were induced to study the influence of prucalopride.

KEY RESULTS

Electrical field stimulation at reduced voltage induced reproducible submaximal neurogenic and cholinergic contractions as the contractions were abolished by tetrodotoxin and atropine. Hexamethonium had no systematic inhibitory effect but mecamylamine reduced the responses, suggesting that part of the cholinergic response is due to activation of preganglionic neurons. Prucalopride concentration-dependently increased the submaximal cholinergic contractions in the three tissue types, reaching maximum from 0.03 μmol/L onwards. The facilitation in the different series with 0.03 μmol/L prucalopride ranged from 41% to 104%, 30% to 76% and 24% to 74% in fundus, jejunum, and colon, respectively. The effect of 0.03 μmol/L prucalopride was concentration-dependently inhibited by GR 113808.

CONCLUSIONS & INFERENCES

In the murine gastrointestinal tract, activation of 5-HT₄ receptors with prucalopride enhances cholinergic contractions, illustrating facilitation of myenteric cholinergic neurotransmission. The degree of enhancement with prucalopride is of similar magnitude as previously reported in other species, but the effective concentrations are lower than those needed in the gastrointestinal tract of other species.

The comparative effects of aminoglycoside antibiotics and muscle relaxants on electrical field stimulation response in rat bladder smooth muscle

It has been reported that several aminoglycoside antibiotics have a potential of prolonging the action of non-depolarizing muscle relaxants by drug interactions acting pre-synaptically to inhibit acetylcholine release, but antibiotics itself also have a strong effect on relaxing the smooth muscle. In this study, four antibiotics of aminoglycosides such as gentamicin, streptomycin, kanamycin and neomycin were compared with skeletal muscle relaxants baclofen, tubocurarine, pancuronium and succinylcholine, and a smooth muscle relaxant, papaverine. The muscle strips isolated from the rat bladder were stimulated with pulse trains of 40 V in amplitude and 10 s in duration, with pulse duration of 1 ms at the frequency of 1-8 Hz, at 1, 2, 4, 6, 8 Hz respectively. To test the effect of four antibiotics on bladder smooth muscle relaxation, each of them was treated cumulatively from 1 μM to 0.1 mM with an interval of 5 min. Among the four antibiotics, gentamicin and neomycin inhibited the EFS response. The skeletal muscle relaxants (baclofen, tubocurarine, pancuronium and succinylcholine) and inhibitory neurotransmitters (GABA and glycine) did not show any significant effect. However, papaverine, had a significant effect in the relaxation of the smooth muscle. It was suggested that the aminoglycoside antibiotics have inhibitory effect on the bladder smooth muscle.

The Inhibitory Mechanism of Gentamicin on Electrical Field Stimulation Response in Rat Bladder Smooth Muscle

To see the inhibitory mechanism of gentamicin in response to electrical field stimulation (EFS) using the rat bladder smooth muscle, atropine or guanethidine was treated but had no effect. Methylsergide, a non-selective 5-HT1, 5-HT2 receptor antagonist was also treated but had on effect. Kinase inhibitors, such as chelerythrine (PKC inhibitor), ML-9 (MLCK inhibitor), or Y27632 (rho kinase inhibitor) were pretreated before gentamicin treatment, but did not have effect. For U73122, a phospholipase C (PLC) inhibitor however, the inhibitory effect to gentamicin was significantly attenuated in all frequencies given by the EFS. Therefore gentamicin induced inhibitory effect on EFS response in rat bladder smooth muscle was not mediated by the activation of adrenergic, cholinergic, or serotonergic receptor. The inhibition of gentamicin might be mediated through the PLC dependent pathway, but not through the PKC, MLCK or rho kinase dependent pathway.

Differential release of β-NAD(+) and ATP upon activation of enteric motor neurons in primate and murine colons

BACKGROUND

The purinergic component of enteric inhibitory neurotransmission is important for normal motility in the gastrointestinal (GI) tract. Controversies exist about the purine(s) responsible for inhibitory responses in GI muscles: ATP has been assumed to be the purinergic neurotransmitter released from enteric inhibitory motor neurons; however, recent studies demonstrate that β-nicotinamide adenine dinucleotide (β-NAD(+)) and ADP-ribose mimic the inhibitory neurotransmitter better than ATP in primate and murine colons. The study was designed to clarify the sources of purines in colons of Cynomolgus monkeys and C57BL/6 mice.

METHODS

High-performance liquid chromatography with fluorescence detection was used to analyze purines released by stimulation of nicotinic acetylcholine receptors (nAChR) and serotonergic 5-HT(3) receptors (5-HT(3)R), known to be present on cell bodies and dendrites of neurons within the myenteric plexus.

KEY RESULTS

Nicotinic acetylcholine receptor or 5-HT(3)R agonists increased overflow of ATP and β-NAD(+) from tunica muscularis of monkey and murine colon. The agonists did not release purines from circular muscles of monkey colon lacking myenteric ganglia. Agonist-evoked overflow of β-NAD(+), but not ATP, was inhibited by tetrodotoxin (0.5 μmol L(-1)) or ω-conotoxin GVIA (50 nmol L(-1)), suggesting that β-NAD(+) release requires nerve action potentials and junctional mechanisms known to be critical for neurotransmission. ATP was likely released from nerve cell bodies in myenteric ganglia and not from nerve terminals of motor neurons.

CONCLUSIONS & INFERENCES

These results support the conclusion that ATP is not a motor neurotransmitter in the colon and are consistent with the hypothesis that β-NAD(+), or its metabolites, serve as the purinergic inhibitory neurotransmitter.

Functional interactions between the SK2 channel and the nicotinic acetylcholine receptor in enteric neurons of the guinea pig ileum

The neurotransmitter acetylcholine (ACh) plays a critical role in gastrointestinal function. The role of the small conductance Ca2+-activated K+ (SK) channel in ACh release was examined using myenteric plexus preparations of guinea pig ileum. Apamin, an inhibitor of the SK channel, significantly enhanced nicotine-induced ACh release, but neither electrical field stimulation- nor 5-hydroxytryptamine-induced ACh release, suggesting that SK channels might be selectively involved in the regulation of nicotine-induced ACh release. Therefore, we investigated the distribution of SK2 and SK3 subunits and the interaction between SK2 channels and nicotinic ACh receptors (nAChRs) in the guinea pig ileum. The immunoreactivity of SK2 subunits was located in enteric neuronal cells. Furthermore, SK2-immunoreactive cells stained with an antibody for choline acetyltransferase, a marker for cholinergic neurons, and with an antibody for the alpha3/5 subunits of nAChR. In contrast, immunoreactivity of SK3 subunits was not found in enteric neurons. A co-immunoprecipitation assay with Triton X-100-soluble membrane fractions prepared from the ileum revealed an association of the SK2 subunit with the alpha3/5 subunits of nAChR. These results suggest that SK2 channels negatively regulate the excitation of enteric neurons via functional interactions with nAChRs.

Role of presynaptic nicotinic acetylcholine receptors in the regulation of gastrointestinal motility

Presynaptic nicotinic acetylcholine receptors (nAChRs) located on cholinergic terminals facilitate the release of acetylcholine (ACh), thereby constituting a fail-safe mechanism at strategic locations, such as the neuromuscular junction, where reliable transmission is vital. Accumulating data indicate that myenteric neurons in the enteric nervous system possess not only somatodendritic nAChRs, which mediate cholinergic transmission between neurons, but also presynaptic nAChRs. Functional evidence shows that these receptors mediate a positive feedback with respect to ACh release from myenteric motoneurons, and might therefore play an important role in the regulation of gastrointestinal motility. These presynaptic nAChRs were found to be more sensitive to nicotinic ligands than somatodendritic nAChRs and could therefore be primary targets of exogenous compounds, such as nicotine. This interaction might provide a neurochemical basis for the effect of smoking on gastrointestinal motility. Another important human pharmacological implication is based on our recent observation that monoamine uptake inhibitor-type antidepressant drugs are able to inhibit presynaptic nAChRs in the enteric nervous system. The disruption of the nAChR-mediated positive feedback modulation by antidepressants might explain the frequent occurrence of constipation, a common side effect, attributed to these drugs. Clarification of the role of presynaptic nAChRs in feedback mechanisms in the enteric nervous system might be instrumental in the development of new drugs affecting gastrointestinal motility.

Nicotine potentiates the neurogenic contractile response of rabbit bladder tissue via nicotinic acetylcholine receptors: Nitric oxide and prostaglandins have no role in this process

Nicotine, a nicotinic acetylcholine receptors (nAChRs) agonist, has a role in modulation of the neurotransmitter release following nerve stimulation in both the central and peripheral nervous systems. The aim of this study was to determine whether electrical field stimulation (EFS)-evoked contractions are altered in rabbit bladder in the presence of nicotine and, if an alteration occurs, to investigate the effects of nitric oxide and prostaglandins on nicotine-induced alternation in isolated rabbit bladder. EFS-evoked contractile responses from rabbit bladder obtained were recorded with isometric force displacement transducers. Nicotine was added to preparations at various concentrations. The effects of hexamethonium, cadmium (Cd(2+)), indomethacin and N-nitro-L-arginine methyl ester (L-NAME) were tested on the EFS-evoked contractions in the presence of nicotine. Nicotine led to a dose-dependent increase in the amplitude of the EFS-evoked contractile responses. Cd(2+) and hexamethonium inhibited the nicotine-induced increase in EFS-evoked responses, whereas indomethacin and L-NAME had no effect. In conclusion, nicotine increased the EFS-evoked contractile responses possibly by facilitating release of neurotransmitters from nerve terminals by a mechanism dependent on the influx of Ca(2+) from voltage-gated Ca(2+) channels (VGCCs) via activation of nAChRs in isolated rabbit bladder. Nitric oxide and prostaglandins do not have a physiological role in the regulation of neurotransmitter release.

Nicotine Potentiates the Electrical Field Stimulation-Evoked Contraction of Non-Pregnant Rabbit Myometrium

The women who smoke have lower fertility rates which might be due to harmful effects of nicotine on tubal function and menstrual cycle. Although the uterine contractility of the non-pregnant uterus plays an important role in the human reproduction process, the influence of nicotine on the contractile responses in uterus is not known. Nicotine increases the release of neurotransmitters following nerve stimulation both in the central and peripheral nervous system through acting on nicotinic acetylcholine receptors (nAchRs). The aim of this study was to examine whether the electrical field stimulation (EFS)-evoked contraction is altered in rabbit myometrium strips in the presence of nicotine to evaluate the changes in contractility. EFS-evoked contractile responses were recorded from myometrium strips obtained from non-pregnant rabbits in the absence and presence of nicotine. Nicotine led to the increase in the amplitudes of the EFS-evoked contractile responses in a dose-dependent manner. Therefore, the effects of hexamethonium, cadmium, indomethacin, atropine, and N(omega)-Nitro-L-arginine methyl ester hydrochloride were tested on the EFS-evoked contractions in the absence or presence of nicotine to clarify the mechanisms of nicotine-induced potentiation in EFS-evoked contractile responses. Indomethacin, a non-selective cyclooxygenase inhibitor, and hexamethonium, a ganglionic blocker, inhibited nicotine-induced increase in EFS-evoked responses, whereas other chemicals produced no effect. These results suggest that nicotine-induced potentiation may be mediated by nAchRs and prostaglandins. In conclusion, failure of quiescence in the uterus due to increased contractility by nicotine might be one of the factors contributing to infertility in female smokers.

Inhibitory effect of hemicholinium-3 on presynaptic nicotinic acetylcholine receptors located on the terminal region of myenteric motoneurons

Previously we have demonstrated the presence of presynaptic nicotinic acetylcholine receptors on the terminals of myenteric neurons in Auerbach's plexus of guinea-pig ileum. During these studies we observed, that the presence of hemicholinium-3, an inhibitor of the high affinity choline uptake significantly influences the contraction of the longitudinal muscle strip preparation. Our aim was to investigate the neurochemical background of this effect and quantitatively characterize the action of HC-3. We studied the effect of HC-3 on epibatidine- and electrical stimulation-evoked contraction and release of [3H]acetylcholine from the guinea-pig longitudinal muscle strip preparation. We found that in the presence of tetrodotoxin, when the contribution of somatodendritic nicotinic acetylcholine receptors to the response was prevented due to the inhibition of axonal conduction, HC-3 inhibited the epibatidine-evoked contraction and [3H]acetylcholine release in the submicromolar range (IC50 = 897 nM and IC50 = 693 nM, respectively), whereas the electrical stimulation-evoked contraction was not affected by HC-3, and the release of [3H]acetylcholine was apparently enhanced. Our data indicate that HC-3 inhibits the presynaptic nicotinic acetylcholine receptors of myenteric neurons. Since these receptors play an important role in the regulation of cholinergic neurotransmission in the enteric nervous system, the use of HC-3 in [3H]acetylcholine release experiments might bias the interpretation of data.

Pharmacology and function of nicotinic acetylcholine and P2X receptors in the enteric nervous system

There are many cell surface receptors expressed by neurones in the enteric nervous system (ENS). Ligand-gated ion channels are an important class of receptors expressed by enteric neurones. This review will focus on nicotinic acetylcholine receptors (nAChRs) and P2X receptors for ATP, as these receptors contribute to fast synaptic transmission in identified pathways in the ENS. There are multiple subunit proteins that compose nAChRs and P2X receptors in the nervous system. Functional and pharmacological studies indicate that the predominant class of nAChR mediating fast synaptic transmission in enteric neurones is composed of alpha3 and beta4 subunits. P2X receptors mediating fast synaptic excitation are predominately P2X2 homomeric receptors.

Functional neurochemical evidence for the presence of presynaptic nicotinic acetylcholine receptors at the terminal region of myenteric motoneurons: a study with epibatidine

The aim of this study was to verify the presence of presynaptic nicotinic acetylcholine receptors (nAChRs) at the terminals of myenteric motoneurons using a potent and highly selective nicotinic agonist, epibatidine. We examined contraction, and release of [3H]ACh on a guinea-pig longitudinal muscle strip preparation. First, we compared the ability of epibatidine and nicotine to induce isometric contraction and found epibatidine (EC50 = 23.1 nM) to be 300-fold more potent than nicotine (EC50 = 7.09 microM). The release and contraction induced by 30 nM epibatidine were inhibited by the nicotinic antagonist mecamylamine (3 microM) and the Na(+)-channel blocker TTX (1 microM), indicating that the effects are mediated via nAChRs and are fully dependent on the propagation of action potentials. Atropine (0.1 microM) significantly increased the [3H]ACh release but could not block contraction suggesting that a substantial part of the response develops via a noncholinergic mechanism. Epibatidine at a higher concentration (300 nM) induced contraction, which was only partly (45%) inhibited by TTX (1 microM). The TTX-resistant contraction, however, was completely blocked by mecamylamine (3 microM). Our data provide functional neurochemical evidence for the existence of presynaptic nAChRs at myenteric motoneuron terminals and suggest that these receptors can be activated only/by a higher concentration of agonists.

Ligand-gated ion channels in the enteric nervous system

There are many cell surface receptors expressed by neurones in the enteric nervous system (ENS). These receptors respond to synaptically released neurotransmitters, circulating hormones and locally released substances. Cell surface receptors are also targets for many therapeutically used drugs. This review will focus on ligand-gated ion channels, i.e. receptors in which the ligand binding site and the ion channel are parts of a single multimeric receptor. Ligand-gated ion channels expressed by enteric nerves are: nicotinic acetylcholine receptors (nAChRs), P2X receptors, 5-hydroxytryptamine3 (5-HT3) receptors, gamma-aminobutyric acid (GABAA) receptors, N-methyl-d-aspartate (NMDA) receptors,alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and glycine receptors. P2X, 5-HT3 and nAChRs participate in fast synaptic transmission in S-type neurones in the ENS. Fast synaptic transmission occurs in some AH-type neurones, and AH neurones express all the ligand-gated ion channels listed above. Ligand-gated ion channels may be localized at extra-synaptic sites in some AH neurones and these extra-synaptic receptors may be useful targets for drugs that can be used to treat disorders of gastrointestinal function.

Pharmacological properties of nicotinic acetylcholine receptors expressed by guinea pig small intestinal myenteric neurons

The electrophysiological and pharmacological properties of nicotinic acetylcholine receptors (nAChRs) were studied in guinea pig small intestinal myenteric neurons maintained in culture or in acutely isolated preparations. Acetylcholine and nicotine caused inward currents that desensitized in approximately 4 s. The current-voltage (I-V) relationship rectified inwardly with a reversal potential near 0 mV. The agonist rank order potency was 1,1-dimethyl-4-phenyl-piperazinium > acetylcholine = nicotine >> cytisine. Agonist-induced currents were blocked by nAChR antagonists with a rank order potency of mecamylamine > hexamethonium > dihydro-beta-erythroidine (DHbetaE); mecamylamine and DHbetaE exhibit high potency at beta4 and beta2 subunit-containing nAChRs, respectively. alpha-Bungarotoxin (0.1 microM) or alpha-methyllycaconitine (0.1 microM), antagonists that block nAChRs containing alpha7 subunits, did not affect acetylcholine-induced responses. Immunohistochemical studies revealed that nearly every neuron in culture was labeled by an antibody (mAb35) that recognizes nAChR alpha3 and alpha5 subunits. Antibodies selective for alpha3, alpha5, or beta2 subunits also stained most neurons, whereas an alpha7 subunit antibody revealed very few neurons. In neurons in the intact myenteric plexus from newborn and adult guinea pigs, local application of acetylcholine (1 mM) and cytisine (1 mM) caused similar amplitude depolarizations, and these responses were blocked by nAChR antagonists with a rank order potency of mecamylamine > hexamethonium > DHbetaE. These data indicate that myenteric neurons maintained in culture predominantly express nAChRs composed of alpha3, alpha5, beta2, and beta4 subunits. These subunits may be in a homogeneous population of receptors with unique pharmacological properties, or multiple receptors of different subunit composition may be expressed by individual neurons.

Somatostatin and intestinal schistosomiasis: therapeutic and neuropathological implications in host-parasite interactions

A better insight into the mechanisms regulating the human body can lead to improved knowledge of the patho-physiological processes of many diseases. New therapeutic possibilities can be devised at the level of these regulatory mechanisms. Somatostatin is one of the major regulatory hormones in the central nervous system (CNS) and digestive system. Its wide variety of activities means it is implicated in a broad range of conditions. One symptom common to both the acute and chronic stages of schistosomiasis is intestinal pathology characterized by abdominal pain, diarrhoea that is bloody in more chronic stages, nausea and fever. Some chronic patients develop severe hepatosplenic fibrosis, leading to fatal oesophageal variceal bleeding. In this review we assess the therapeutic potential of somatostatin in the treatment of intestinal pathology associated with schistosomiasis. The activity of somatostatin is mediated via binding to specific cell surface receptors. While we are making progress in studies of the expression and regulation of the different somatostatin receptors, the true role and distribution of each receptor subtype is far from fully understood. Animal models will help to define the specific role of individual receptors in physiological and pathological conditions. The regulation of receptor expression as well as receptor internalization can give us insight into the effect of exogenous somatostatin on schistosomiasis-mediated intestinal pathology, as well as its modulation by intrinsically produced somatostatin levels.

Amantadine increases gait steadiness in frontal gait disorder due to subcortical vascular encephalopathy: a double-blind randomized placebo-controlled trial based on quantitative gait analysis

In a randomized, double-blind placebo-controlled trial, 40 patients diagnosed as subcortical vascular encephalopathy (SVE) were given a daily dose of 500 ml i.v. amantadine vs. placebo for 5 days. Both groups were treated with physiotherapy on a daily basis. Quantitative gait analyses were performed at days 1 and 6 to evaluate gait steadiness from cadence, length of heel-to-toe movements, variability of centre of gravity (COG) and double support time. Both placebo- and amantadine-receiving patient groups showed mild improvement in gait parameters after study, which failed to show the superiority of amantadine, when comparing drug-induced changes between both groups. However, analysing the treatment effects from day 0 to day 6 in both groups separately, statistically significant changes could be found in the amantadine group for cadence, length of heel-to-toe movements in single support phase as well as for variability in double support phase and double support time (two-tailed paired t-test, p < 0.05), whereas in the placebo group, a statistically significant effect could only be seen for double support time (p < 0.05). In this small pilot study, amantadine tends to improve gait steadiness as evaluated by cadence, length of heel-to-toe movements in single support phase, variability in double support phase and double support time, in patients with moderate frontal gait disorder due to SVE. Improvements in the placebo group can be interpreted as physiotherapy effect, which improved gait steadiness slightly, however, this was statistically significant only for double support time.

Disturbance of the prejunctional modulation of cholinergic neurotransmission during chronic granulomatous inflammation of the mouse ileum

The effect of chronic granulomatous inflammation of the intestine was studied on the prejunctional modulation of cholinergic nerve activity in the mouse ileum. Contractions to carbachol (0.01 - 0.3 microM) and to electrical field stimulation (EFS, 0.25 - 8 Hz) of enteric neurons were higher in inflamed ileum as compared to control ileum. However, when the neurally-mediated contractions to EFS were expressed as percentage of the direct smooth muscle contraction to carbachol, the responses to EFS were similar in control and inflamed ileum. Atropine (1 microM) abolished all contractions to EFS and carbachol in control and inflamed ileum. DMPP (3 - 30 microM), a nicotinic receptor agonist, induced concentration-dependent contractions that were more pronounced in inflamed ileum as compared to control ileum. Hexamethonium (100 microM), a nicotinic receptor blocker, significantly inhibited the contractions to EFS in inflamed ileum but not in control ileum. In control ileum, histamine (10 - 100 microM) and the histamine H(1) receptor agonist HTMT (3 - 10 microM) inhibited the contractions to EFS concentration-dependently without affecting the contractions to carbachol. The inhibitory effect of histamine and HTMT was prevented by the histamine H(1) antagonist mepyramine (5 - 10 microM) but not by the H(2)- and H(3)-receptor antagonists cimetidine and thioperamide (both 10 microM). In chronically inflamed ileum however, histamine (10 - 100 microM) and HTMT (3 - 10 microM) failed to inhibit the contractions to EFS. The histamine H(2) and H(3) receptor agonists dimaprit and R(-)-alpha-methylhistamine did not affect the contractions to EFS in control and inflamed ileum. The alpha(2)-receptor agonist UK 14.304 (0.01 - 0.1 microM) inhibited the contractions to EFS in control and inflamed ileum without affecting the contractions to carbachol. The effect of UK 14.304 was reversed by the alpha(2)-receptor antagonist yohimbine (1 microM). The inhibitory effect of UK 14.304 on contractions to EFS was of similar potency in control and inflamed ileum. Our results suggest that the prejunctional modulation of cholinergic nerve activity by nicotinic and histaminic H(1) receptors is disturbed during chronic intestinal inflammation whereas the modulation by alpha(2)-receptors is preserved. Such a disturbance of cholinergic nerve activity may contribute to the motility disturbances that are often observed during chronic intestinal diseases in humans.

Presynaptic nicotinic acetylcholine receptors in the myenteric plexus of guinea pig intestine

Presynaptic nicotinic acetylcholine receptors (nAChRs) were studied in myenteric plexus preparations from guinea pig ileum using intracellular electrophysiological methods. Microapplication of nicotine (1 mM) caused a biphasic depolarization in all AH neurons (n = 30) and in 36 of 49 S neurons. Cytisine (1 mM) caused fast depolarizations in S neurons and no response in AH neurons. Mecamylamine (10 microM) blocked all responses caused by nicotine and cytisine. TTX (0.3 microM) blocked slow excitatory synaptic potentials in S and AH neurons but had no effect on fast depolarizations caused by nicotine. Nicotine-induced slow depolarizations were reduced by TTX in two of twelve AH neurons (79% inhibition) and four of nine S neurons (90+/-12% inhibition). Slow nicotine-induced depolarizations in the remaining neurons were TTX resistant. TTX-resistant slow depolarizations were inhibited after neurokinin receptor 3 desensitization caused by senktide (0.1 microM); senktide desensitization inhibited the slow nicotine-induced depolarization by 81+/-5% and 63+/-15% in AH and S neurons, respectively. A low-calcium and high-magnesium solution blocked nicotine-induced slow depolarizations in AH neurons. In conclusion, presynaptic nAChRs mediate the release of substance P and/or neurokinin A to cause slow depolarizations of myenteric neurons.

Characterization of nicotinic acetylcholine receptor-mediated noradrenaline release from the isolated rat stomach

We characterized nicotinic acetylcholine receptor-mediated noradrenaline release from the isolated, vascularly perfused rat stomach. The stomach was perfused via the coeliac artery with Krebs-Ringer solution at a constant flow rate of 4 ml per minute. Endogenous noradrenaline released into the perfusate was electrochemically measured using high-performance liquid chromatography. Nicotinic receptor agonists were applied once into the perfusion medium for 2 min and nicotinic receptor antagonists were administered throughout the experiments. The (-)-nicotine (3x10(-5) M)-induced noradrenaline release was abolished by tetrodotoxin and hexamethonium and partially blocked by dihydro-beta-erythroidine (up to 10(-5) M) (a relatively selective antagonist of alpha4beta2 nicotinic receptors) and abolished by mecamylamine (10(-5) M) (a relatively selective antagonist of alpha3beta4 nicotinic receptors), but not influenced by alpha-bungarotoxin (3x10(-7) M) or alpha-conotoxin ImI (10(-6) M) (antagonists of alpha7 nicotinic receptors). (+/-)-Epibatidine (3x10(-7) M) (a very potent, but non-selective agonist) and (-)-cytisine (3x10(-4) M) (an agonist of beta4 nicotinic receptors) effectively evoked the release of noradrenaline, while (E)-N-methyl-4-(3-pyridinyl)-3-butene-1-amine (RJR-2403) (up to 10(-4) M) (an agonist of alpha4beta2 nicotinic receptors) had no effect. The potency of these agonists was as followed; (+/-)-epibatidine>>(-)-nicotine>(-)-cytisine>>>RJR -2403. These results are compatible with the published view that alpha3beta4 nicotinic receptors are predominant in other parts of the autonomic nervous system. These receptors (probably located on the gastric sympathetic ganglia) are involved in the release of noradrenaline from the rat stomach.

Nitric oxide modulates release of noradrenaline in guinea-pig gastric fundus

The interaction between nitric oxide (NO) and the release of [(3)H]noradrenaline ([(3)H]NA) in conditions of non-activated and activated nicotinic receptors in guinea-pig gastric fundus preincubated with [(3)H]NA was studied. Nicotinic receptor agonist, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) (100 microM) significantly increased the resting release of [(3)H]NA. NO-synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA) (100 microM) significantly decreased DMPP-induced release of [(3)H]NA. Field electrical stimulation (FES) (2Hz; 1 ms; 360 st) significantly increased the release of [(3)H]NA above the basal levels. L-NNA significantly decreased the stimulation-evoked release of [(3)H]NA. DMPP increased the stimulation-evoked release of [(3)H]NA, effect which was significantly decreased by L-NNA. The data suggests that endogenous NO increases the release of [(3)H]NA, evoked either by activation of the nicotinic receptors or by electrical stimulation in guinea-pig gastric fundus.

Modulatory role of presynaptic nicotinic receptors in synaptic and non-synaptic chemical communication in the central nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a family of ligand-gated channels closely related to but distinct from the muscle nAChRs. Recent progress in neurochemical and pharmacological methods supports the hypothesis of presynaptically located nAChRs on axon terminals and indicates that the major effect of nAChR is the modulation rather than processing of fast synaptic transmission. Strong neurochemical evidence indicate that the most important function of presynaptic nAChRs in either synaptic or non-synaptic localization is to increase transmitter release initiated by axonal firing, or directly induce Na(+) and Ca(2+) influx followed by a depolarization sufficient to activate local voltage-sensitive Ca(2+) channels, as a result transmitter of vesicular origin will be released. Therefore, it is somewhat expected that nicotine-induced transmitter release of different monoamines including norepinephrine (NE), dopamine (DA), serotonin (5-HT) can be tetrodotoxin (TTX)- and [Ca(2+)](o)-sensitive. However, some of the nAChR agonists at higher concentrations (1, 1-dimethyl-4-phenylpiperazinium (DMPP) and lobeline), besides their effects on presynaptic nAChRs, are able to inhibit the uptake of NE and 5-HT into nerve terminals, thereby their transmitter releasing effects are extended in time and space. The effect on the uptake process is different from classical nicotinic actions, not being sensitive to nAChR antagonism, but can be prevented by selective uptake blockers or reduced temperature. Considering neurochemical, pharmacological and electrophysiological evidence it seems likely that presynaptic nAChRs on monoaminergic fibers are composed of alpha3 or alpha4 subunits in combination with the beta2 subunit. This is supported by the observation that nicotinic agonists have no presynaptic effect on transmitter release in knockout mice lacking the beta2 nAChR subunit gene. The essential brain function lies not only in impulse transmission within a hard-wired neuronal circuitry but also within synaptic and non-synaptic communication subjected to presynaptic modulation. Since the varicose noradrenergic, dopaminergic, serotonergic, glutamatergic and cholinergic axon terminals mainly do not make synaptic contact, but their varicosities are equipped with nAChRs and these non-synaptically localized receptors are of high affinity, it is suggested that nicotine inhaled during smoking might exert its behavioral, psychological, neurological and neuroendocrinological effects via these receptors.

Nitric oxide (NO) increases acetylcholine release from and inhibits smooth muscle contraction of guinea-pig gastric fundus

Experiments were carried out to investigate the interaction between nitric oxide (NO) and cholinergic neurotransmission in smooth muscle strips of guinea-pig gastric fundus. Electrical field stimulation (2 Hz, 1 ms, 360 shocks) evoked atropine-sensitive contractions. Dimethylphenylpiperazinium (DMPP) (100 microM), a nicotinic receptor agonist, reversed the stimulation-evoked contraction and resulted in relaxation. Nomega-nitro-L-arginine (L-NNA) (100 microM), an NO synthase inhibitor, significantly increased the amplitude of stimulation-evoked contraction and abolished the effect of DMPP. Electrical stimulation increased the release of [3H]acetylcholine ([3H]ACh) from the tissue strips above the basal levels. Neither L-NNA (100 microM) nor DMPP (100 microM) alone influenced the basal release of [3H]ACh. Nomega-nitro-L-arginine (100 microM) decreased the electrical stimulation-evoked release of [3H]ACh. Dimethylphenylpiperazinium increased the stimulation-evoked release of [3H]ACh but had no effect in the presence of L-NNA. It is suggested that in guinea-pig gastric fundus, endogenous NO released in response to field stimulation has an opposite effect at the pre- and postsynaptic sites: it increases the release of ACh from cholinergic nerve terminals but reduces smooth muscle responses to ACh.

Modulation of electrically evoked responses in rat duodenum by activation of nicotinic cholinoceptors

1. The effect of activation of nicotinic cholinoceptors in rat duodenal segments following electrical field stimulation (EFS) was investigated. 2. Electrical field stimulation elicited a two-component response: transient relaxation followed by contraction. The EFS-evoked response was tetrodotoxin (TTX; 1 mumol/L) sensitive. The relaxation component was NG-nitro-L-arginine (L-NNA; 100 mumol/L) sensitive, while the contractile response was atropine (1 mumol/L) sensitive. 3. 1,1-Dimethyl-4-phenyl-piperazinium iodide (DMPP; 20 mumol/L) induced relaxation of spontaneously active preparations that was L-NNA sensitive. L-Arginine (1 mmol/L) reversed the effects of L-NNA on DMPP-induced relaxation. 4. When EFS was applied, DMPP increased the amplitude of the relaxation component of the response and reduced the contractile component. 5. In the presence of L-NNA, the effect of DMPP on the relaxation component of the response to EFS was reduced, but the contractile response was not affected. L-Arginine partly reduced this effect of L-NNA. 6. Neither propranolol (1 mumol/L) nor yohimbine (1 mumol/L) had any effect on the actions of DMPP on EFS-evoked responses, but prazosin (1 mumol/L) strongly reduced the effect of DMPP on the contractile component of the response to EFS and slightly reduced the effect of DMPP on the relaxation response. 7. Histochemical studies demonstrated that, in the myenteric plexus of the rat duodenum, there are many reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-positive neurons and that their number decreased after treatment with L-NNA. In the presence of L-arginine and L-NNA, the number of NADPH-d-positive neurons was similar to that found in control samples. 8. The data suggest that activation of nicotinic cholinoceptors modulates EFS-evoked responses in the rat duodenum as a result of the potentiation of nitrergic and adrenergic neurotransmission.

Side effects of nondepolarizing muscle relaxants: relationship to their antinicotinic and antimuscarinic actions

Since acetylcholine (ACh) is the 'master key' to different subtypes of nicotinic and muscarinic receptors, and muscle relaxants (MRs) available in clinical practice are structurally related to it, MRs may exert their unwanted effects through inhibition of these receptors. Since the subunit composition of nicotinic ACh receptors (nAChRs) of pre- and/or postsynaptic location and the binding potency of MRs to these and muscarinic receptors are different, a search for selective muscle nAChR antagonists without or with less side effects seems to be promising and important for clinical practice.

Role of nitric oxide in mediating non-adrenergic non-cholinergic relaxation of the cat ileocecal sphincter

The participation of nitric oxide (NO) in field stimulation- or nicotine-evoked non-adrenergic non-cholinergic (NANC) relaxation of cat ileocecal sphincter was studied in vitro. During a 30 microM noradrenaline-induced contraction, both the application of electrical field stimulation (2-20 Hz, 0.2 ms, supramaximal current intensity, 10 s duration) and (-)-nicotine (10-500 microM) produced a tetrodotoxin-sensitive relaxation. The maximal relaxation was observed at 10 Hz or 100 microM (-)-nicotine. In 12 out of 19 strips the pretreatment with N omega-nitro-L-arginine (100 microM) decreased the amplitude of the field stimulation-evoked relaxation, while in the remaining strips the relaxation was transformed into a contraction. By increasing the concentration of N omega-nitro-L-arginine up to 1 mM all strips responded to field stimulation with a frequency-dependent tetrodotoxin-resistant contraction. N omega-Nitro-L-arginine (100 microM) completely inhibited the nicotine-induced relaxation. L-Arginine (1 mM) restored the amplitude of both field stimulation- and nicotine-evoked relaxations. These data indicate that NO appears to be involved in both field stimulation- and nicotine-evoked NANC relaxations. Evidence has been obtained for the existence of tetrodotoxin-resistant NANC contraction in cat ileocecal sphincter.

Possible mechanisms of the effect of physostigmine on the facilitation of acetylcholine release in the guinea pig myenteric plexus

The automodulation of acetylcholine (ACh) release in the guinea pig myenteric plexus-longitudinal muscle preparation was investigated by studying the electric stimulation-evoked release of radiolabeled ACh. When the release associated with neuronal activity was challenged by the muscarinic antagonist atropine, the release was not significantly enhanced. When the acetylcholinesterase (AChE) blocker physostigmine was present, the well-established muscarinic receptor-mediated autoinhibition was operative, i.e., the release was significantly reduced. However, when both drugs were added together, the release was much higher than under control conditions. Therefore, it seems likely that there is also a facilitatory system. We made an effort to clear up the mechanism of this facilitation by blocking possible nicotinic presynaptic receptors, by excluding the alpha 2-adrenoceptor-mediated masking effect of noradrenergic heteromodulation, by preventing a possible ATP-mediated mechanism, and by attempting to prevent the direct effect of physostigmine. None of these manipulations resulted in a decrease of the surplus release. It is concluded, that when the negative feedback modulation of ACh is inhibited and AChE activity is reduced, high levels of ACh facilitates additional release of ACh from the nerve terminals, possibly through a not yet verified class of receptors.

Mediators of nicotine-induced relaxations of the rat gastric fundus

1. Relaxations of strips of rat gastric fundus were elicited with nicotine (100 mumol/L), nitric oxide (NO; 30 mumol/L), sodium nitroprusside (SNP; 100 nmol/L) and vasoactive intestinal polypeptide (VIP; 1 nmol/L). 2. Methylene blue (30 mumol/L), an inhibitor of soluble guanylate cyclase, reduced relaxations elicited by NO and nicotine, but not those elicited by VIP. 3. Chymotrypsin (1 U/mL) abolished VIP-induced relaxations and reduced nicotine-induced relaxations, but had no effect on SNP-induced relaxations. 4. NG-nitro-L-arginine methyl ester (L-NAME; 100 mumol/L), an inhibitor of NO synthase, reduced relaxations elicited by nicotine, but not those elicited by SNP or VIP. 5. When nicotine-induced relaxations had been reduced by either L-NAME or chymotrypsin, the addition of the other agent produced a greater reduction. However, the relaxations were not abolished. 6. Nicotine-induced relaxations were abolished by tetrodotoxin (1 mumol/L) or hexamethonium (100 mumol/L), indicating that they were due to activation of neuronal nicotinic receptors. Their reduction by methylene blue and L-NAME indicates that an NO-like mediator was involved. Their reduction by chymotrypsin indicates that a VIP-like peptide was involved. However, since they were not abolished by a combination of L-NAME and chymotrypsin, it appears that at least one more as yet unidentified mediator may be involved.