Section III: Mediation: muscle receptors, neurotransmitters and drugs

Moderation: T.W. Burks H E.E. Daniel

Digestive Diseases and Sciences, Vol. 39, No. 12 (December 1994 Supplement), pp. 63S-68S

R E V I E W A R T I C L E

Mediators and Enteric Nerve Pathways Controlling Gastric Emptying

EDWIN DANIEL, PhD, GERVAIS TOUGAS, MD, CM, HANS-DIETER ALLESCHER, MD, PhD, PATROCINIO VERGARA, PhD, and JO-ANN FOX-THRELKELD, PhD

KEY WORDS: gastric emptying; CCK; nitric oxide; VIP; 5-HT, receptors; motilin; cisapride.

Our studies have revealed a complex series of en- teric controls of motor activity in the pyloric region by various neurotransmitters. They provide a list of possibilities for mechanistic control but do not define how these controls relate to gastric emptying in vivo.

ENTERIC NEURAL MECHANISMS OPERATING ACROSS THE PYLORUS

Studies in chloralose-urethane-anesthetized dogs have shown that there are ascending excitatory and descending enteric reflexes leading to and some- times crossing the pylorus to the duodenum (1, 2). Electrical field stimulation (EFS) of enteric nerves of the duodenum (Figure 1A) activates pyloric sphincter closure and contractions of the duode- num. Vagal stimulation (Figure 1B) or local electri- cal field stimulation (EFS) of antral enteric nerves (Figure 1C, D) or contraction of the antrum (1, 2) leads to pyloric sphincteric relaxation and relax- ation along the upper duodenum.

Descending inhibitory pathways to the pylorus involve primarily nonadrenergic inhibition, while those reaching the duodenum contain a larger ad- renergic component (1, 2). Ascending excitation involves a chain of cholinergic neurons (2). These excitatory responses were blocked by hexametho- nium or atropine.

Manuscript received May 27, 1994; accepted August 1, 1994. From the Department of Biomedical Sciences, McMaster Uni-

versity, Hamilton, Ontario, Canada. This work was supported by the Medical Research Council of

Canada. Dr. Tougas is an MRC Clinical Scientist. Dr. Allescher was supported by a research grant of Deutsche Forschungsge- meinschaft DFG A1/245 2-1. Dr. Vergara was supported by Personal Investigator Visiting Grant from the Direocion General de Investigation Cientifica y Tecniea (Spain).

Address for reprint requests: Edwin Daniel, Department of Biomedical Sciences, McMaster University, 1200 Main St. W., Hamilton, Ontario, L8N 3Z5, Canada.

Vagal motor influences also reach the pylorus and, depending on the parameters of vagal stimulation applied, lead to inhibition or excitation of the pyloric sphincter (2). Analysis of vagal stimulation with higher parameters of EFS (frequency or pulse dura- tion) on the pylorus is complicated by the fact that distal antral contractions are also initiated and affect sleeve pressure and may set up descending inhibi- tory reflexes or propagating excitatory contractions.

MEDIATORS MODULATING OR INFLUENCING ANTROPYLORODUODENAL

ACTIVITY

Many neuropeptide mediators/modulators were found to affect this region in ex vivo or in vivo studies. Table 1 summarizes these actions and their sites of action. Particularly worthy of comment are the following observations: (1) CCK, known to de- lay gastric emptying, excites and closes the pylorus. (2) Motilin and substance P or neurokinin A also excite the pylorus, but there is no information about their effect on gastric emptying in dogs. (3) Opioids inhibit the cholinergic excitatory nerves, but can initiate duodenal excitation (and ascending excita- tion) by turning off tonic NO-mediated inhibition (NO acts to inhibit both ACh release and muscle activity) (see ref. 3). This may explain some claims in the literature that exogenous opioids excite the pylorus. In general, opioids are thought to delay gastric emptying. (4) VIP inhibits the pylorus by a direct smooth muscle action. (5) Galanin inhibits it by activating inhibitory nerves. There is no infor- mation about whether either galanin or VIP plays a role to control gastric emptying. So far there is no conclusive evidence about the role of NO in gastric emptying in dogs; our data suggest that inhibition of NO synthesis delays emptying by closing the py-

Digestive Diseases and Sciences, Vol. 39, No. 12 (December 1994 Supplement) 0163.2116/94/1200-063S$07.00/1) o 1994 Plenum Publishing Corporation

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DANIEL ET AL

TABLE 1. EFFECTS OF NEUROPEPTIDES AND NEUROTRANSMITIERS ON PYLORIC MUSCLE OF CANINE (m Vwo STUDIES)*

Mediator/peptide Method Action Site o f action Reference

ACh In vivo, IA Contraction SM (Musc. R) 1, 2 N E In vivo, IA Excitation N (ACh release) I3-AR 5

Relaxation SM (13-AR) Inhibition N (ACh release) a2-AR

SP In vivo, IA Contraction SM N (ACh release) 17 Excitation NK~ R

N K A In vivo, IA Contraction SM (NKI and NKzR) 17 5-HT In vivo, IA Contraction SM (5-HTz) 12, 13

Excitation N NO In vivo, IA and IV Relaxation SM (N?) 7 VIP In vivo, IA Relaxation SM 18 Opioids In vivo, IA Inhibition N (ACh release) 6

Excitation Activation of duodenum Motilin In vivo, IA Contraction N (ACh release) 15 CCK-8 In vivo, IA Excitation N (ACh release) 4

Contraction SM CGRP In vivo, IA Excitation N (ACh release) 15

Relaxation SM Galanin In vivo, IA Inhibition N (ACh release) 18

*Relaxation was studied during stimulation of ascending cholinergic excitation from the duodenum. Contraction was initiated by the mediator/modulator. Inhibition refers to inhibition of ACh release. Excitation refers to activation of cholinergie nerves. N -- neural site; SM = smooth muscle site; AR = adrenergic receptor; IA = intraarterial; IV = intravenous.

lorus and preventing the neurally mediated relax- ation of the pylorus. It may also affect duodenal contractions, making them nonperistaltic and in- creasing resistance to gastric outflow (7).

Which of these modulations/mediators may play a physiological role is not dear . Neither CCK an- tagonists (4), adrenergic antagonists (5), nor nalox- one (6) affected responses to enteric nerve stimula- tion at low frequencies . However , the main nonadrenergic, noncholinergic mediator of pyloric inhibitors has recently been shown to be NO or a related compound (7).

MECHANISMS A C T I V A ~ D BY ACID IN THE DUODENUM

Some years ago we showed that acid in the hu- man (8, 9) or alkali in the canine (9) duodenum initiated a MMC-like response. Motilin was re- leased by acid in humans and alkali in dogs (8-10), and this led to the suggestion that acid movement into the human duodenum might be a normal com- ponent of spread of the MMC from the antrum to the duodenum. Current evidence does not support this concept. However, recent work has elaborated the enteric mechanisms involved in responses to acid in the canine duodenal lumen (11). Acid (0.1 N HCI) in the duodenal lumen closes the pylorus and initiates clearing contractions of the duodenum (Figure 1E).

64S

Acid can be shown to release 5-HT both into the lumen and into the duodenal venous blood (12). Acid, 5-HT, or phenylbiguanide (a 5HT3-agonist ) in the lumen all initiate pyloric closure and duodenal clearing contractions (11, 12). A local anesthetic in the lumen inhibits the pyloroduodenal responses to acid (10-12). 5-HT 3 antagonists given locally (in- traarterially, or intraluminally) or systematically in- hibit responses to acid, 5-HT, or phenylbiguanide (12). Hexamethonium (intravenously) or atropine (intravenously or intraarterially) inhibit responses to acid (Figure 1E) or 5-HT in the lumen (11-13)just as they do responses to electrical field stimulation of duodenal enteric nerves (11). Naloxone (6) and a CCK antagonist (4) had no effect on the response to luminal acid infusion (11).

These findings lead to the suggestion (Figure IF) that acid releases 5-HT from endocrine cells to act on sensory nerve endings near the lumen by way of 5-HT 3 receptors. These sensory nerves activate the chain of ascending excitatory cholinergic nerves, which mediates pyloric closure and duodenal con- traction. How or whether they would activate vagal afferents in unanesthetized animals is unknown.

MODE OF ACHON OF PUTATIVE PROKINETIC AGENTS

The same model has recently been used to study the actions of a number of agents thought to acti-

Digestive Diseases and Sciences, Vol. 39, No. 12 (December 1994 Supplement)

ENTERIC NERVES AND PYLORUS

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Fig IA. Dynograph recording of the wlor ic motor response due to duodenal field stimulation 3 to 4 cm aborad from the pylorus (0.5 Hz, 0.5 ms, 40 V). Note the instantaneous onset of the pyloric motor response and the ability of the contraction to follow each stimulus. The contractile frequency under these conditions was 30/rain (from 2).

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vate antropyloroduodenal contractions. Table 2 shows a summary of our findings (13, 14). Intraar- terial and intravenous motilin and erythromycin ac- tivate antral, pyloric, and duodenal contractions. However, when metoclopramide or cisapride were given intraarterially by the gastroepiploic artery to the antroduodenal region, neither had any effect until amounts also effective when given by intrave- nous administration were applied. Acetylcholine (ACh) infusions showed that the infusion sites in- cluded the appropriate regions. There was no evi- dence of enhanced responses to Ach given intraar- terially. Intravenous infusion of the prokinetics causes activation of antrum, pylorus, and duode- num at doses known to be active in vivo. These effects were markedly reduced by naloxone, even when a form of naloxone unable to cross the blood- brain barrier was used. Domperidone was ineffec- tive by all routes we tested. We concluded that only motilin and erythromycin had local actions in this region and that metoclopramide and cisapride acted at some site outside the brain (possibly prevertebral

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Digestive Diseases and Sciences, Vol. 39, No. 12 (December 1994 Supplement) 6 5 5

DANIEL ET AL

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66S Digestive Diseases and Sciences, Vol. 39, No. 12 (December 1994 Supplement)

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l~g IE. Dynograph recording showing the effect of intraduodenal acid infusion (0.92 ml/min, 0.1 N HCL, pH 1.0) on pyloric motor activity before (left panel) and after hexamethonium (10 mg/kg i.v. and 1 mg i.a.). Note the reduction of antral activity during acid infusion (from 1t).

Pig IF. Diagram of proposed enteric neural control of antropy- F Ioroduodenal region. Adrenergic nerves (NE, norepinephrine) with 1 ~ [ cell bodies located in cefiac ganglia, are depicted as supplying the ANTRUM pylorns and proximal duodenum. Field stimulation (FS) of antral enteric nerves initiates distal inhibition of pylorus and duodenum; the duodenal response is partly blocked by phentolamine or pyloric transection, suggesting a more complex path than shown. FS of antral enteric nerves also activates nonadrenergJc, nonchofnerglc descend- ing nerves, which release NO or VIP as the primary mediator. These ~ ......... nerves also reach the duodenum and inhibit it. FS of duodenal nerves activates a chain of ascending cholinergic (ACh) enteric nerves, leading to the pylorus and proximal duodenum. Descending inhibitory [--] nerves (releasing NO) are also activated (not shown). Hexametho- nium, at ganglia, and atropine, at circular muscle sites of innervation, inhibit the excitatory responses to duodenal FS. No evidence that ascending excitation reaches the antrum is available. Acid in the duodenum acts to release serotonin (5-HT) from endocrine cells (EC) [~ N E to act on 5-HT 3 receptors. 5-HT3 antagonists block the response. The sensory nerve (SENS. N) activates the ascending cholinergic chain of [---] N O nerves. The mediator released by the sensory nerve is unknown. V I P ? Activation of this reflex closes the pylorus and initiates cleating contractions of the duodenum. In vivo 5-HT release may also activate vagal reflexes. Vagal control mechanisms are omitted from the dia- gram.

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Digestive Diseases and Sciences, Vol. 39, No. 12 (December 1994 Supplement) 67S

D A N I E L ET A L

TABLE 2. MODES OF ACTION OF So-CALLED "PROKINETIC AGENTS" ON CANINE PYLORUS IN VIVO*

Method o f Agent administration Action Mode o f action

Motilin In vivo, IA or IV Excitation Erythromycin In vivo, IA or IV Excitation Metoclopramide In vivo, IA No effect

IV Excitation Cisapride In vivo, IA No effect

IV Excitation Domperidone In vivo, IA No effect

IV No effect

Cholinergic N and via opioid receptors Cholinergic N and via opioid receptors

Naloxone-sensitive, atropine-sensitive

Naloxone-sensitive, atropine-sensitive (releases motilin)

*N = neuron; IA = intraarterial; IV = intravenous.

ganglia) to ac t iva te a p a t h w a y con ta in ing opioid r eceo to r s . S ince moti l in re lease might have been invo lved in the r e s p o n s e (at least to c isapr ide) , it is poss ib le tha t the op io id r e c e p t o r w a s ac t iva ted b y moti l in re lease ra ther than d i rec t ly (13).

C O N C L U S I O N S

It is clear that cont ro l o f the pylor ic region is c o m p l e x even w h e n centra l cont ro l inputs are elim- inated. Thus , in addi t ion to the impor tan t role o f vagal reflexes, there are powerfu l enter ic ne rvous cont ro ls o v e r this region in the dog and p robab ly similar ones exist in o the r spec ies (15, 16). H o w e v e r , it is c lear that the h u m a n pylorus , in possible con- trast to the dog, does no t have significant basal act ive tension, but is capable o f isolated phas ic con t rac - t ions that can modula te gas t r ic emp ty ing (16). T h e s tudies summar i zed here p rov ide no p r o o f that the m e c h a n i s m s descr ibed p lay a role in physiological cont ro l o f gast r ic empty ing . T h e y do s h o w s o m e of the neural m e c h a n i s m s that m a y be opera t ing and begin to define tools that need to be applied.

R E F E R E N C E S

1. Reddy SN, Daniel EE: Neural control of antroduodenal inhibition in the dog. Am J Physiol 258:G24-G31, 1990

2. Allescher H-D, Daniel EE, Dent J, Fox JET, Kostolanska F: Extrinsic and intrinsic neural control of pyloric sphincter pressure in the dog. J Physiol 401:17-38, 1988

3. Fox-Threlkeld JET, Daniel EE, Christinck F, Hruby VJ, Cip- ris S, Woskowska Z: Identification of mechanisms and sites of actions of Mu and Delta opioid receptor activation in the canine intestine. J Pharmacol Exp Ther 268:779-784, 1994

4. AUescher H-D, Daniel EE, Fox JET, Kostolanska F, Rovati LA: LA: Effect of the novel CCK-receptor antagonist CR- 1392 on CCK induced antro-duodenal and pyloric motor activity in vivo. J Pharmacol Exp Ther 251:1134-1141, 1989

5. Allescher H-D, Ahmad S, Kostolanska F, Kwan CY, Daniel EE: Modulation of pyloric motor activity via adrenergic receptors. J Pharmacol Exp Ther 249:652-659, 1989

6. Allescher H-D, Ahmad S, Daniel EE, Dent J, Kostolanska F, Fox JET: Inhibitory opioid receptors in canine pylorus. Am J Physiol 255:G352-G360, 1988

7. Allescher H-D, Tougas G, Vergara P, Lu S, Daniel EE: Nitric oxide as a putative non-adrenergic non-cholinergic inhibitory transmitter in the canine pylorus in vivo. Am J Physiol 262:G695-G702, 1992

8. Lewis TD, Collins SM, Fox JET, Daniel EE: Initiation of duodenal acid-induced motor complexes. Gastroenterology 77:1217-1224, 1979

9. Daniel EE, Fox JET, Collins SM, Lewis TD, Meghji M, Track NS: Initiation of migrating myoelectric complexes in human subjects: role of duodenal acidification and plasma motilin. Can J Physiol Pharmacol 59:173-179, 1981

10. Fox JET, Track NS, Daniel EE: Relationship of plasma motilin concentration to fat ingestion, duodenal acidification and alkalinization, and migrating motor complexes in dogs. Can J Physiol Pharmacol 59:180-187, 1981

11. Allescher H-D, Daniel EE, Dent J, Fox JET, Kostolanska F: Neural reflex of the canine pylorus to intraduodenal acid infusion. Gastroenterology 96:18-28, 1989

12. Tougas G, Allescher H-D, Dent J, Daniel EE: Sensory nerves of the intestines: Role in control of pyloric region in dogs. Adv Exp Med Biol 298:199-211, 1991

13. Tougas G, Fox-Threlkeld JET, Diamant NE, Greenberg G, Daniel EE: The effect of cisapride may be due to motilin release. Clin Invest Med 13:B145, 1990

14. Tougas G, Woskowska Z, Fox-Threlkeld JET, Daniel EE: Effects of prokinetic drugs (metoclopramide, domperidone, cisapride and erythromycin) on antropyloroduodenal motor activity in the dog. J Pharmacol Exp Ther 1993 (submitted)

15. AUescher H-D: Pylorus: Myogenic acid and neural control of the pyloric sphincter. In Sphincters: Normal Function-- Changes in Diseases. EE Daniel, T Tomita, S Tsuchida, M Watanabe (eds). Boca Raton, Florida, CRC Press, 1992, pp 83-115

16. Heddle R, Dent J: Pylorus: The human pylorus. In Sphinc- ters: Normal Function---Changes in Diseases. EE Daniel, T Tomita, S Tsuchida, M Watanabe (eds). Boca Raton, Flor- ida, CRC Press, 1992, pp 131-154

17. Allescher H-D, Kostolanska F, Tougas G, Fox JET, Regoli D, Drapeau G, Daniel EE: The actions of neurokinins and substance P in canine pylorus, antrum and duodenum. Pep- ~ides 10:671-679, 1989

18. Allescher H-D, Daniel EE, Dent J, Fox JET: Inhibitory function of VIP/PHI and galanin in the canine pylorus. Am J Physiol 256(Gastrointest Liver Physiol 19):G789-G797, 1989

6 8 S Digestive Diseases and Sciences, Vol. 39, No. 12 (December 1994 SupplemenO