Acetylcholine is an indirect inhibitory transmitter in the canine ileocolonic junction

The effects of cholinergic agents, electrical stimulation and vasoactive intestinal polypeptide (VIP) were studied on transverse muscle strips of the canine ileum, ileocolonic junction and colon. Acetylcholine, methacholine and carbachol caused concentration-dependent contractions in the three gut tissues. Only acetylcholine (> 10⁻⁵ M) evoked transient relaxations in the ileum and the ileocolonic junction before the onset of contractions. During contractions by noradrenaline, acetylcholine induced relaxations, which were enhanced by atropine; electrical stimulation also caused frequency-dependent relaxations. Propranolol or naloxone did not affect the relaxations. Hexamethonium, cocaine or lidocaine inhibited the relaxations induced by acetylcholine but not those evoked by electrical stimuli. Tetrodotoxin inhibited all relaxations. VIP did not evoke relaxation in the ileocolonic junction. These data indicate that acetylcholine stimulates nicotinic receptors on non-adrenergic non-cholinergic neurons, which do not release VIP or opioids. It is thus suggested that there is a nicotinic inhibitory innervation at the canine ileum and ileocolonic junction.     

Alpha 2-adrenoceptor-mediated modulation of the nitrergic innervation of the canine isolated ileocolonic junction.

1. The effects of specific alpha-adrenoceptor agonists and antagonists on electrically-evoked non-adrenergic non-cholinergic (NANC) relaxations, previously demonstrated as nitrergic, were investigated in isolated circular muscle strips of the canine ileocolonic junction. 2. During a substance P-induced contraction and in the presence of atropine and guanethidine, the specific alpha 1-adrenoceptor agonist, phenylephrine and antagonist, prazosin, as well as the specific alpha 2-adrenoceptor antagonist, yohimbine, had no effect on the NANC relaxations evoked by electrical field stimulation. In contrast, clonidine and the more specific alpha 2-adrenoceptor agonist, UK-14,304, significantly reduced the electrically-induced relaxations, preferentially those in response to low frequency stimulation. The inhibitory effect of UK-14,304 on these relaxations was antagonized by yohimbine. 3. During a noradrenaline-induced contraction, clonidine, but not UK-14,304 significantly augmented the relaxations to electrical stimulation. 4. The adrenoceptor agonists and antagonists used had no effect on concentration-response curves to NO or on the relaxation induced by nitroglycerin. 5. These results indicate that stimulation of prejunctional alpha 2-adrenoceptors inhibits the nitrergic NANC relaxations induced by field stimulation and thus suggest prejunctional regulation of nitric oxide release via alpha 2-adrenoceptors in the canine ileocolonic junction.     

Antagonist discrimination between subtypes of tachykinin receptors in the guinea-pig ileum

The effects of substance P and eledoisin on spontaneous and electrically-evoked release of [3H]acetylcholine, and on smooth muscle were studied in the guinea-pig myenteric plexus-longitudinal muscle preparation preloaded with [3H]choline. Substance P and eledoisin caused transient increases in spontaneous release of [3H]-acetylcholine and in longitudinal muscle tone. Both tachykinins were equipotent in contracting the muscle, but eledoisin was more potent than substance P in eliciting [3H]acetylcholine release. The release caused by substance P was enhanced in the presence of naloxone and scopolamine which suggests that the release is modulated through opioid and muscarinic receptors. Substance P and eledoisin inhibited the release of [3H]acetylcholine evoked by electrical stimulation at 0.1 Hz. The inhibition was not due to an activation of alpha-adrenoceptors, histamine or opioid receptors. The substance P antagonists (D-Pro2, D-Trp7,9)SP (10 and 30 microM) and (Arg5, D-Trp7,9, Nle11)SP5-11 (1 and 10 microM) competitively antagonized both the contractile effects of substance P and eledoisin, and the inhibition by the tachykinins of the electrically-evoked release of [3H]acetylcholine. The increase in spontaneous [3H]acetylcholine release elicited by substance P and eledoisin was not prevented by the substance P antagonists. The results suggest that the neuronal receptor whose activation causes inhibition of acetylcholine release and the smooth muscle receptor correspond to the SP-P type, whereas the neuronal receptor mediating an increase in spontaneous acetylcholine release is of the SP-E type. The two antagonists, (D-Pro2, D-Trp7,9)SP and (Arg5, D-Trp7,9, Nle11)SP5-11, selectively block only the SP-P receptor.     

Further evidence for the presence of cannabinoid CB1 receptors in guinea-pig small intestine.

1. CP 50,556, CP 55,940, nabilone, CP 56,667, delta 9 -tetrahydrocannabinol (THC) and cannabinol each inhibited electrically-evoked contractions of the myenteric plexus-longitudinal muscle preparation of guinea-pig small intestine in a concentration-related manner. The IC50 values of these cannabinoids, respectively 3.45, 3.46, 30.61, 162.94, 214.63, and 3913.5 nM, correlate well with previously obtained potency values for displacement of [3H]-CP 55,940 from cannabinoid binding sites. 2. Electrically-evoked contractions of the myenteric plexus-longitudinal muscle preparation were also inhibited by AM 630 (6-iodo-pravadoline) and by WIN 55,212-2 (IC50 = 1923.0 and 5.54 nM, respectively). The present finding that AM 630 is an agonist, contrasts with a previous observation that it behaves as a cannabinoid receptor antagonist in the mouse isolated vas deferens. 3. SR141716A produced dose-related parallel rightward shifts in the log concentration-response curves of CP 55,940, WIN 55,212-2, THC and AM 630 for inhibition of electrically-evoked contractions of the myenteric plexus-longitudinal muscle preparation. SR141716A (1 microM) did not reverse the inhibitory effects of normorphine and clonidine on electrically-evoked contractions or potentiate the contractile response to acetylcholine. 4. Doses of naloxone and yohimbine that reversed the inhibitory effects of normorphine or clonidine on electrically-evoked contractions of the myenteric plexus-longitudinal muscle preparation did not affect the inhibitory response to WIN 55,212-2. 5. Electrically-evoked release of acetylcholine from strips of myenteric plexus-longitudinal muscle was decreased by 200 nM CP 55,940 and this inhibitory effect was almost completely reversed by 1 microM SR141716A. Acetylcholine-induced contractions were not affected by 200 nM CP 55,940. 6. These results support the hypothesis that guinea-pig small intestine contains prejunctional cannabinoid CB1 receptors through which cannabinoids act to inhibit electrically-evoked contractions by reducing release of the contractile transmitter, acetylcholine. 7. THC was found to be more susceptible to antagonism by SR141716A than CP 55,940 or AM 630, raising the possibility that guinea-pig small intestine contains more than one type of cannabinoid receptor. 8. At concentrations of 10 nM and above, SR141716A produced small but significant increases in the amplitude of electrically-evoked contractions of the myenteric plexus-longitudinal muscle preparation suggesting that this tissue may release an endogenous cannabinoid receptor agonist or that some cannabinoid receptors in this tissue are precoupled and that SR141716A can reduce the number of receptors in this state.     

Blockade of adrenergic and cholinergic transmissions by tetramethylpyrazine.

The pharmacological actions of an amide alkaloid, tetramethylpyrazine (TMPZ), isolated from the stem of JATROPHA PODAGRICA H OOK (Euphorbiaceae) have been investigated on the electrically-evoked contractions, relaxations and twitches of some cholinergically- and adrenergically-innervated muscle preparations IN VITRO and IN VIVO. The amide alkaloid depressed or abolished the electrically-evoked contractions of the chick oesophagus, rabbit duodenum and guinea-pig vas deferens IN VITRO. Tetramethylpyrazine also inhibited or abolished the indirect electrically-induced twitches of the rat isolated hemidiaphragm, or the contractions of the cat nictitating membrane evoked by pre-ganglionic cervial sympathetic nerve electrical stimulation IN VIVO. Moreover, TMPZ prevented or reversed the high frequency electrical stimulation-induced relaxations of the rabbit isolated duodenum. These results were taken to imply blocking actions of the amide alkaloid at the cholinergic and adrenergic neuro-effector junctions, the neuro-muscular junction, and at the ganglia (presumably indicating blockade of peripheral cholinergic and adrenergic transmissions). The inhibitory effects of TMPZ on the electrically-provoked contractions (or relaxations) of the cholinergically-innervated and adrenergically-innervated muscle preparations set up are thought to be possibly linked with the non-specific spasmolytic action of the amide alkaloid, or, probably mediated through its local anaesthetic (membrane-stabilizing) activity; since its inhibitory effects were not modified by any specific antagonist. The pharmacological implications of the above findings are discussed.     

Regionally selective cholinergic stimulation by BRL 24924 in the human isolated gut.

1. The effects of BRL 24924 on cholinergic activity was studied in longitudinal muscle strips obtained from human stomach and colon (taenia). Contractions were evoked by exogenous acetylcholine (ACh) or by electrical field stimulation (EFS) of cholinergic neurones. Inhibitory nerve activity (predominantly non-adrenergic, non-cholinergic; NANC) was detected by measuring the relaxations evoked by EFS in the presence of atropine 1.4 microM. 2. In the stomach, BRL 24924 0.28-28 microM consistently increased the EFS-evoked contractions; lower concentrations (0.0028 and 0.028 microM) increased the contractions in most, but not all of the specimens studied. Since BRL 24924 28 microM did not affect NANC-mediated, EFS-evoked relaxations, and since high concentrations of BRL 24924 (28 and 282 microM) were required to increase ACh-evoked contractions, the increase in EFS-evoked contractions caused by BRL 24924 may be due to a facilitation of ACh release. 3. In the colon, BRL 24924 0.0028-282 microM did not affect EFS-evoked contractions. 4. BRL 24924 may therefore be regionally selective in its ability to stimulate human gastrointestinal cholinergic activity.     

Effects of pentazocine on the intestine and biliary tract of the rabbit in vitro

Summary Pentazocine impairs peristaltic activity and relaxes longitudinal muscle in the colon and in the ileum. The circular coat is excited in the colon, while in the ileum pentazocine exhibits both excitatory and inhibitory effects depending on the concentration employed. Pentazocine does not exert a spasmogenic effect in the smooth muscle of terminal bile duct but instead reduces the electrically-induced contraction. The effect of pentazocine does not seem to involve endogenous acetylcholine or catecholamine release.     

Effects of omega-conotoxin GVIA on autonomic neuroeffector transmission in various tissues.

1. The effects of omega-conotoxin GVIA (conotoxin), a potent inhibitor of neuronal N-type Ca2+ channels, have been examined on responses to stimulation of noradrenergic, cholinergic and non-adrenergic, non-cholinergic (NANC) nerves in a range of isolated tissues to investigate the role of conotoxin-sensitive Ca2+ channels in neurotransmission. 2. Contractions elicited by field stimulation of noradrenergic nerves in rat and mouse anococcygeus muscles, rabbit ear artery and rat vas deferens (epididymal portion) were inhibited by conotoxin. Responses to noradrenaline, and to adenosine triphosphate in the vas deferens, were not affected. 3. Positive chronotropic responses to field stimulation of noradrenergic nerves were inhibited by conotoxin in rat and mouse atria, but responses to noradrenaline and tyramine were not affected. 4. The stimulation-induced release of noradrenaline was inhibited by conotoxin in the rabbit ear artery and in rat and mouse atria. 5. Relaxations in response to stimulation of the noradrenergic perivascular mesenteric nerves were reduced or abolished by conotoxin in rat and rabbit jejunum. The response to noradrenaline in rat jejunum was not affected. 6. Contractions elicited by stimulation of cholinergic nerves were inhibited by conotoxin in rat jejunum and mouse ileum (perivascular mesenteric nerves), and in guinea-pig taenia caeci (field stimulation). Responses to acetylcholine in rat jejunum and mouse ileum were not affected. 7. Contractions elicited by stimulation of the cholinergic plus NANC pelvic nerves were inhibited by conotoxin in rabbit colon, and to a lesser extent in guinea-pig colon. The stimulation-induced contraction of the guinea-pig colon was inhibited by conotoxin by a greater proportion in the presence than in the absence of atropine. Responses to acetylcholine were not affected in the rabbit colon but were slightly reduced in the guinea-pig colon. 8. Relaxations in response to field stimulation of NANC nerves were inhibited by conotoxin in guinea-pig taenia caeci and rat gastric fundus strips, and in rat anococcygeus muscle when the tone was raised by guanethidine but not when it was raised by carbachol. The relaxations produced by sodium nitroprusside in the rat gastric fundus and anococcygeus were not affected. 9. Contractions of the rat bladder elicited by stimulation of the peri-urethral nerves, which are NANC- and cholinergically mediated, were relatively insensitive to inhibition by conotoxin. The response were almost completely abolished by tetrodotoxin. 10. The conotoxin-induced inhibitions of responses to nerve stimulation developed slowly and persisted after removal of conotoxin.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of K(ATP) channel modulators on acetylcholine release from guinea-pig isolated atria and small intestine

The effects of K(ATP) channel blockers (glibenclamide, HMR 1883, HMR 1372) and openers (cromakalim, pinacidil, diazoxide) on the electrically-evoked (5 Hz) release of [(3)H]acetylcholine were studied in isolated guinea-pig atria and myenteric plexus-longitudinal muscle preparations which had been preincubated with [(3)H]choline. Atria: Cromakalim (0.3 microM and 1 microM), pinacidil (10 microM) and diazoxide (30 microM) significantly reduced the stimulation-evoked release of [(3)H]acetylcholine. The inhibition produced by cromakalim and pinacidil was prevented by 1 microM of either HMR 1883, HMR 1372 or glibenclamide. The blockers alone significantly increased the release at concentrations of 30 microM, whereas 1 microM and 10 microM had no effect. Myenteric plexus-longitudinal muscle preparation: The electrically-evoked release of [(3)H]acetylcholine was not affected by K(ATP) channel blockers or openers. In contrast, the contractions of the longitudinal muscle caused by electrical stimulation or by carbachol were strongly inhibited by 1 microM cromakalim which suggests that the relaxant effect of the K(ATP) channel openers is exclusively a direct effect on intestinal smooth muscle.The findings suggest that blockade of activated K(ATP) channels in vagal nerves of guinea-pig atria stimulates acetylcholine release, and that this effect may contribute to the antiarrhythmic actions of K(ATP) channel blockers. By contrast, release of acetylcholine from guinea-pig myenteric plexus is not modulated by K(ATP) channels which suggests heterogeneity of K(ATP) channel distribution in peripheral autonomic nerves.     

Mechanical responses to catecholamines in isolated strips of the guinea-pig stomach muscle

Effects of catecholamines were investigated on isolated strips of the guinea-pig stomach. In the circular muscles, the relaxation and biphasic response caused by adrenaline and noradrenaline were suppressed partly by phentolamine and also partly by propranolol. The relaxation by catecholamines in the longitudinal muscles was inhibited mainly by propranolol. Thus, there are probably two types of relaxation. Alpha relaxation was more dominant in the circular muscles and beta relaxation in the longitudinal muscles. Contractions in both muscles were mediated through alpha-effects. The circular muscle is more sensitive to the alpha effect and the longitudinal muscle to the beta effect. In the presence of propranolol, isoprenaline. Field stimulation which excited intramural nerves resulted in contraction followed by relaxation in both circular and longitudinal muscles. The inhibition of the electrically evoked contractions by adrenaline and noradrenaline in both circular and longitudinal muscles was abolished by phentolamine but the electrically evoked relaxations were affected little.     

Regional differences of reactivity to stimulants in the dog portal tree.

We studied the regional differences both of reactivity to various stimulants and of neurogenic responses elicited by transmural stimulation in the longitudinal and circular muscles of the truncal portal vein, mesenteric vein, splenic vein and gastric vein of the dog portal tree. Strong spontaneous activity appeared in the longitudinal muscle of the truncal portal vein (96% of preparations tested). Weak spontaneous activity sometimes appeared in the circular muscle of the truncal portal vein (41%) and rarely in the longitudinal muscle of the mesenteric vein (12%). It did not appear in other segments. The splenic vein and the gastric vein showed similar patterns in the relationship between resting tension and response to noradrenaline; that is, the responsiveness of either longitudinal or circular muscle of these two veins increased and then decreased almost parallel as resting tension increased and reached a maximum under the same resting tension. The longitudinal muscle of both the truncal portal vein and the mesenteric vein was more responsive to noradrenaline, acetylcholine, histamine and KCl than the circular muscle; for example 2.02 and 1.44 times more responsive to noradrenaline, respectively. On the other hand, the longitudinal muscle of the splenic vein and the gastric vein responded less well than the circular muscle; for example 0.36 and 0.16 times as responsive to noradrenaline, respectively. Acetylcholine and histamine caused marked contractions which were comparable to those elicited by noradrenaline in the longitudinal muscle of the truncal portal vein. Acetylcholine also elicited similar contractions in the longitudinal muscle of the mesenteric vein but the responses induced in preparations of other segments were small.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of Mercuric Chloride and Methyl Mercury on Cholinergic Neuromuscular Transmission in the Guinea-Pig Ileum

Abstract: The effects of mercuric chloride (HgCl₂) and methyl mercury (MeHg) were examined on basal mechanical activity and electrically-induced neurogenic cholinergic contractions (twitch contractions) in longitudinal muscle-myenteric plexus strips from guinea-pig distal ileum. Both compounds at 0.3-3 μM slightly enhanced the amplitude of twitch contractions in ~50% preparations. This effect was probably due to facilitation of acetylcholine (ACh) release since 0.1 and 1 μM mercurials increased electrically-evoked tritium outflow from [³H]choline preloaded muscle layer with attached myenteric plexus. Conversely, higher mercury concentrations inhibited twitch contractions (HgCl₂ IC₅₀=21.3±6.4 μM; MeHg IC₅₀=45.1±5.5 μM), as well as contractions to exogenous ACh (0.1 μM) in resting preparations, and concomitantly increased the basal tone. The former effects possibly reflected an antimuscarinic activity of mercury, while the latter was related to alterations of calcium homeostasis in the effector cells. Indeed, the effect of HgCl₂ on basal tone was antagonized by the Ca²⁺ entry blocker nifedipine (3, 10, 30 nM), indicating Hg-induced facilitation of Ca²⁺ influx through voltage-dependent channels. On the whole, our results suggest that cholinergic neuromuscular transmission and Ca²⁺-dependent mechanisms underlying smooth muscle contractility are targets for mercury toxicity in the intestine.     

Spontaneous and evoked release of neurotransmitter substances in the longitudinal muscle of the anterior mesenteric artery of the domestic fowl

1. Changes in length of the longitudinal muscle of the anterior mesenteric artery of the domestic fowl (LMAMA) were recorded isotonically.2. The actions of physostigmine and hyoscine suggest that the tone of the LMAMA is dependent on a resting release of acetylcholine within the tissue.3. Catecholamines inhibited the contractions obtained in response to stimulation of cholinergic nerves; this was a beta-receptor effect. After beta-receptor blockade, noradrenaline, but not isoprenaline, weakly facilitated the effects of stimulating cholinergic nerves.4. In the presence of hyoscine, nicotine relaxed preparations of the LMAMA in which the tone had been raised with barium chloride. These relaxations were considered to be due to a release of catecholamine probably from sympathetic nerves.     

Modulation by endogenous opioid peptides of non-adrenergic neurotransmission in the guinea-pig taenia coli (author's transl)

The effects of opioid peptides on th response to stimulation of adrenergic or non-adrenergic inhibitory nerve were investigated in the guinea-pig taenia coli. Leu-enkephalin, met-enkephalin, (D-ala2) met-enkephalin and beta-endorphin inhibited non-adrenergic inhibitory response, in a dose-dependent manner. (D-ala2) metenkephalin and beta-endorphin were the most potent compounds in inhibiting the elicited relaxation, having ID50 values of 1 micro M. The inhibitory actions of these opioid peptides were reversed by naloxone (1 micro M). Naloxone itself potentiated the non-adrenergic inhibitory response. On the contrary, the adrenergic inhibitory response was not affected by application of leu-enkephalin, beta-endorphin or naloxone. Using an immunohistochemical method, met-enkephalin immunoreactive nerve fibers were observed in the longitudinal muscle layer, Auerbach plexus and circular muscle layer of the taenia. From these observations, it is concluded that endogenous opioid peptides may play a regulatory role in non-adrenergic inhibitory neurotransmission but endogenous opioid peptides may play a regulatory role in non-adrenergic inhibitory neurotransmission but not in adrenergic neurotransmission.     

Functional coordination of motor activity in colonic and recto-anal smooth muscles in rat experimental model

Spontaneous or electrically elicited contraction and/or relaxation of the longitudinal and circular muscles of the colon and rectum and the anal canal in rat segment preparations were recorded simultaneously to display contractile potency and functional coordination of muscles in the large intestine. Spontaneous high-amplitude contractions, but not relaxations, appeared synchronously in the longitudinal and circular muscles of the colon and rectum. The anal canal showed contractions following the activity of rectal muscles. The colonic and rectal longitudinal muscle responded to electrical stimulation with frequency-dependent contraction. The response of the circular muscle of the colon initially consisted of frequency-independent relaxation followed by frequency-dependent contraction, which was more pronounced in the distal colon, while the rectal circular muscle responded with contraction. The responses appeared synchronously, demonstrating the coactivation of the nervous pathways that supply both muscles. The contractions of longitudinal muscles were more pronounced suggesting a dominant role of this layer in the coordinated motor activity. The local response of the internal anal sphincter was biphasic, comprising short contractions followed by relaxation, while the response of the anal canal was contraction. The contractile local responses increased from the colon to the rectum, while differences in the relaxations were not observed, indicating rather higher contractile potency than the relaxation ability of muscles in the distal part of the gut.     

Contractile effects of cysteamine on the guinea-pig ileum

Cysteamine (beta-mercaptoethylamine HCl) (1.0-40.0 mM) induced a concentration-dependent increase in tonic and phasic contractions of segments of guinea-pig ileum in vitro. Myenteric plexus-longitudinal muscle (MPLM) preparations also responded with an increase in tonic contractions but phasic contractions were either greatly reduced or absent, indicating that these were a response of the circular muscle. Atropine (5 microM) inhibited the cysteamine-induced contractions, whereas hexamethonium and guanethidine had no effect, suggesting that cysteamine was acting at least partly via a cholinergic mechanism involving muscarinic receptors. Tetrodotoxin increased the phasic contractions of ileal segments, but had no effect on the tonic component. Treatment of MPLM preparations with morphine (1 microM) resulted in a small reduction in responsiveness to cysteamine, and blocked electrically-induced contractions by at least 90%. Since morphine acts by inhibiting acetylcholine release via hyperpolarization of intrinsic neurones, a small but significant part of the cysteamine-induced contractions probably resulted from stimulation of acetylcholine release from intrinsic neurones. Following a response to high cysteamine concentrations (greater than 15 mM) tissues were refractory to subsequent cysteamine administration. Cross-desensitization between cysteamine and acetylcholine also occurred, as short-term (1-3 min) incubation of MPLM preparations with high concentrations of either compound (1-10 microM acetylcholine or 20 mM cysteamine) resulted in a reduced responsiveness to both. A reduced sensitivity to acetylcholine or cysteamine was obtained following long-term (45 min) incubation with acetylcholine (1 microM). Removal of Na+ from the incubation medium negated this effect. In contrast, the refractoriness to acetylcholine or cysteamine following long-term (45 min) incubation with cysteamine (20 mM) was accentuated in low Na+ medium. It is suggested that cysteamine induces a contraction of both the circular and longitudinal muscle of the guinea-pig ileum by stimulating the release of acetylcholine from intrinsic neurones, by an action at the level of the smooth muscle muscarinic receptor, and possibly by a non-cholinergic mechanism. However, the mechanisms by which acetylcholine and cysteamine induce tissue refractoriness probably differ.     

Opioid mu and kappa receptors on axons of cholinergic excitatory motor neurons supplying the circular muscle of guinea-pig ileum

Summary In preparations of guinea-pig ileum comprising the circular muscle and the axonal processes of myenteric neurons, electrical stimulation evoked contractions of the circular muscle which were abolished by tetrodotoxin and by hyoscine, indicating that they resulted from action potential-mediated release of acetylcholine. The selective mu opioid agonist, (d-Ala²-N-Me-Phe⁴-Gly⁵-ol)-enkephalin (DAGO), and the selective kappa opioid agonist, trans-(±)-3,4-dichloro-N-(2-(I-pyrrolidinyl) cyclohexyl) benzeneacetamide, U-50488H, caused concentration-dependent and naloxone-reversible inhibitions of nerve-mediated contractions. The experiments indicate that opioid mu and kappa receptors are present on the axonal processes of cholinergic excitatory motor neurons supplying the circular muscle of the guinea-pig ileum.Send offprint requests to S. Johnson at the above address     

The effects of metoclopramide on acetylcholine release and on smooth muscle response in the isolated guinea-pig ileum

Summary The effects of metoclopramide on smooth muscle contraction and on release of acetylcholine were studied in the guinea-pig myenteric plexus longitudinal muscle preparation. Acetylcholine was determined either as endogenous acetylcholine, or as labelled transmitter from strips preloaded with ³H-choline.Metoclopramide caused an increase in resting tension of longitudinal muscle as well as an increase in resting output of either endogenous or labelled acetylcholine. Tetrodotoxin abolished the metoclopramide-evoked increase in transmitter release. The increase in smooth muscle tension was clearly related to the increase in resting output. The effects of metoclopramide on both longitudinal muscle contraction and resting release of labelled acetylcholine were prevented in the presence of a concentration of 5-hydroxytryptamine (5-HT) that desensitized 5-HT receptors. This suggests that metoclopramide stimulates neuronal 5-HT receptors and, thereby, facilitates acetylcholine release.Metoclopramide augmented the twitch-like contractions induced by field stimulation at 0.1 Hz. Contractions elicited at 1 Hz were only slightly enhanced. Similarly, metoclopramide facilitated only the release of labelled acetylcholine evoked by electrical stimulation at 0.1 Hz, but not that at 1 Hz. The facilitatory effetts of metoclopramide on twitch height and evoked release could not be attributed to a blockade of presynaptic inhibitory -adrenoceptors, dopamine or muscarine receptors.     

The effect of naloxone on opioid-induced inhibition and facilitation of acetylcholine release in brain slices.

1 The effect of morphine, methionine-enkephalin (Met-enkephalin) and D-Ala2-D-Leu5-enkephalin (DADLE) were tested on the spontaneous and electrically-evoked release of acetylcholine (ACh) from superfused slices of guinea-pig thalamus, caudate nucleus and cerebral cortex. 2 At no concentration did morphine, Met-enkephalin or DADLE modify the outflow of ACh at rest but Met-enkephalin in the presence of naloxone, reduced the resting ACh release. 3 Morphine, at a low dose (3 microM) had no effect in slices of cerebral cortex, but it enhanced the evoked release of ACh in thalamic and caudate, slices. At higher doses of morphine (10-30 microM), the ACh release evoked by electrical pulses was significantly inhibited in every area. 4 Met-enkephalin behaved like morphine in thalamic slices, whereas DADLE, a specific delta agonist, produced a slight inhibition of ACh outflow only at 10 microM. 5 Naloxone antagonized the inhibitory effect of morphine in the cerebral cortex and caudate nucleus slices. Naloxone and also spiroperidol blocked the releasing effect of morphine in caudate slices. In contrast naloxone did not affect the increase of ACh release caused by morphine and Met-enkephalin in thalamic slices. The inhibitory effect of both opioids at high doses was reversed by naloxone so that they then enhanced ACh release. 6 A two fold increase of calcium concentration in the Krebs solution prevented the inhibitory effects of morphine 10 microM. 7 It is suggested that two receptors are present in thalamic slices, one of which inhibits and the other facilitates ACh release.     

Three different ways in which 5-hydroxytryptamine can affect cholinergic activity in guinea-pig isolated ileum

The effects of 5-hydroxytryptamine (5-HT) have been studied on electrically-evoked contractions mediated by cholinergic nerves in guinea-pig isolated ileum. Low concentrations of 5-HT (0.0001-0.01 microgram ml-1) caused a sustained increase in submaximal, electrically-evoked contractions. Higher concentrations of 5-HT (0.1-10 micrograms ml-1) initially evoked a fast, rapidly-fading contraction of the muscle. Subsequently, 5-HT 0.1-10 micrograms ml-1 caused a sustained reduction in the height of the electrically-evoked contractions. The effects of 5-HT 0.01 and 0.1 microgram ml-1 on the electrically-evoked contractions were not blocked by methysergide 0.1 microgram ml-1 or by hexamethonium 10 micrograms ml-1, and may be due to changes in neuronal acetylcholine (ACh) release, since contractions evoked by exogenous ACh were unaffected by 5-HT. The results therefore imply that 5-HT can affect gut cholinergic activity in at least three different ways, two of which may modulate evoked ACh release by mechanisms which may be insensitive to tachyphylaxis.