© 1990 S. Kargcr AG, Basel 0257-2753/90/0086-036 IS2.75/0
Dig Dis 1990;8:361-373
Endogenous Opioids, the Enteric Nervous System and Gut Motility Wolfgang Kromer Department of Pharmacology, Byk Gulden Pharmaceuticals, Konstanz, FRG
Key Words. Opioids • Intestinal motility • Enteric nervous system
Introduction Opiates display potent and stereospecific effects in vivo and in vitro and show satura ble and reversible binding of high affinity to neuronal membranes. Properties like these prompted the search for specific opiate re ceptors. This search was eventually success ful in 1973 [1-3]. A surge of publications was seen during the following decade, un raveling the biochemistry and physiological role of endogenous opioid peptides as hor mones and neuromodulators activating opioid receptors in a variety of organs [see ref. 4 and related articles of the same vol ume; 5-9]. Radioimmunological determina
tions in extracts of the intestinal wall as well as immunohistochemical studies in various species demonstrated the occurrence of all 3 classes of opioid peptides in neurons and nerve fibers of the myenteric and submuco sal plexus and in endocrine cells of the mu cosa of the gastrointestinal (GI) tract [for review, see ref. 7], This implies a variety of potential physiological and pathophysiologi cal functions (table 1). The tissue concentra tions of enkephalins were, on average, high est, those of dynorphin and (3-endorphin were intermediate and lowest, respectively (table 2). Enkephalin-degrading enzymes have also been detected in the gut [10], and both biosynthesis in [11] and electrically in-
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
Abstract. Opium alkaloids have been used for centuries as potent antidiarrheals and anal gesics, their constipating action in the latter instance taken as an unwanted effect. It was only during the last decade that the physiological role of opioid peptides present in both neurons and endocrine cells of the gastrointestinal (GI) tract has been defined. The recognition of distinct opioid receptor types which may be differentially involved in the control of motility, acid and electrolyte secretion in the GI tract presently focuses the attention of researchers in this field on the identification of receptor-type-selective opioid agonists in order to free these clinically extremely useful drugs from side effects. The present review provides a survey of mostly physiological data on the functional role of intestinal opioids.
362
Kromer
Table 1. Biological role of GI opioids Physiology
Pathophysiology
'Normal' life conditions Inhibition of peristalsis Induction of segmentations Increase in circular muscle tone Electrolyte absorption Duodenal bicarbonate secretion Dual effect on acid secretion Hormonal effects inside and outside the gut? (unknown)
Motility (largely speculative) Opioid hyper- or dysfunction in Achalasia? Chronic idiopathic constipation? Crohn’s disease? Intestinal inflammation? GI tumors? Loss of opioid function in Hirschsprung’s disease?
Particular life conditions Inhibition of gut motility During fetal life During hibernation? During stress?
Secretion (largely speculative) Opioid dysfunction (dual effects) in GI ulcer development? Opioid counterregulation of chloride secretion in diarrhea?
duced release of opioids from [ 12] the guinea pig ileum in vitro have been reported. Although Waterfield and Kosterlitz [13] had already demonstrated enhancement of electrically induced acetylcholine (ACh) re lease from the guinea pig myenteric plexus by the competitive opioid antagonist (-)naloxone, it was a challenge to researchers in the field when van Nueten et al. [14] claimed that naloxone was able to reverse ‘fatigued’ reflex peristalsis in the intact guinea pig ileum in vitro. The reason for the initial sur prise was that, in many of the laboratories, the electrically stimulated longitudinal mus cle myenteric plexus preparation of the guinea pig ileum was used (and still is used) as a biological test system for opioids. Nalox one was considered to have no effect on its own in this test system. It was regarded as a specific tool for proving opioid functions by blocking opioid receptors and, thus, un masking endogenous opioid actions. During the following years, however, it was recog
nized that opioid peptides intrinsic to the GI tract physiologically modulate reflex peri stalsis in an inhibitory fashion [15] and acid secretion by isolated gastric parietal cells in an excitatory fashion [16], thus explaining the opposite effects of naloxone alone.
Both the efficiency and frequency of in testinal peristaltic contractions can be easily quantified in the intact intestinal segment in vitro when rhythmic, expulsive circular mus cle contractions are elicited by sustained dis tension of the intestinal wall due to elevated intraluminal pressure. The resulting reflex peristalsis in various species is shown in fig ure 1. It is easily distinguished from segmen tations in this in vitro model and is charac-
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
Functional Role of Intestinal Opioids in the Control of Peristalsis in Different Species in vitro and in vivo
363
Intestinal Opioids
Table 2. Neuronal opioid receptors in the guinea pig ileum Receptor type K-opioid
p-opioid
8-opioid
Receptor location [35, 36]
myenteric plexus
myenteric plexus
myenteric and submucosal plexus
Selective antagonists [37]
binaltorphimine
c t p -n h 2
ICI 174,864
Endogenous agonists [38-40]
dynorphin
P-endorphin? (nonselective)
met- and leuenkephalin
Tissue concentration (whole intestinal wall) of endogenous agonists [26, 41, 42]
— 10-11 mol/g tissue
— 10"12 mol/g tissue
~ 10"10 mol/g tissue
Agonist potency (peristalsis) [43]
high
low
intermediate
Receptor mechanism [35, 36]
inhibition of voltage-dependent calcium channels
activation of calcium-dependent potassium channels
Receptor-effector coupling [35, 36]
unknown
direct coupling to potassium channel via G protein
Modulation of ACh release [13, 33, 34] and action (peristalsis) [44]
opioid receptor type unknown
Physiological receptor function [45, 46]
inhibition of reflex peristalsis
unknown; inhibition of longitudinal muscle contraction?
probably activation of electrolyte absorption
Cause of inhibition of agonist release [26, 30]
reflex peristalsis
unknown
reflex peristalsis
Potential interaction between receptor systems (speculative)
primary inhibition of peristalsis: secondary activation of electrolyte absorption?
unknown
primary activation of electrolyte absorption: secondary inhibition of peristalsis?
terized by circular muscle contractions prop agated from oral to aboral, thereby expelling the luminal contents of the intestinal seg ment. It is evident from figure 1 that the inhibitory action of opioid agonists is the
same in the guinea pig, rat and rabbit. By contrast, naloxone is excitatory in all 3 spe cies. Both the agonists and the antagonist act in a stereospecific fashion (fig. 1A-C). The situation is more complex in the cat. Actual-
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
Smooth muscle opioid receptors, which exert contractile effects, have not been considered in this table (see text).
364
Kromer
ly, both morphine and naloxone displayed sometimes inhibitory, sometimes excitatory actions in this species, but worked in an opposite fashion in each single ileal segment (fig. ID, E) [17 and unpubl. data]. Although Wood [18] reported unequivocal inhibition of intracellularly recorded, evoked discharge of cat myenteric neurons by morphine, this primary inhibition by opioids may well re sult in either functional inhibition or disinhibition dependent on whether excitatory or inhibitory neurons were inhibited by the opioid. This hypothesis would be consistent with the dual opioid effect on both in vitro
peristalsis (fig. 1D, E) and extracellularly re corded activity of cat myenteric neurons [19]. Even more difficult to explain is the opioid effect on propulsive peristalsis in the isolated dog intestine. Early observations [17] suggested exclusive inhibition by nalox one and excitation by opioid agonists of ca nine peristalsis in vitro. However, later ex periments (fig. IF) [Kromer, unpubl. data] consistently showed that what initially ap peared to be suppression of high-frequency but low-amplitude peristaltic contractions by naloxone turned out to be transformation
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
Fig. 1. Representative examples of the influence of opioid agonists [levorphanol (•) and morphine (♦)] and the antagonist (-)naloxone ( a ) on peristalsis in the isolated ileum taken from the guinea pig (A), rat (B). rabbit (C), cat (D. E) and dog (F). The biologically inactive enan tiomers dextrorphan (o) and (+)naloxone (A) were used to test for stereospecificity. All experiments were performed under comparable conditions; A and B were adult ani mals, C -F were animals a few weeks old. Reflex peristalsis was elicited by sustained elevated intra luminal pressure (2-10 cm HjO, de pending on the species) and the aboral luminal volume displace ment measured. See text.
Intestinal Opioids
365
Ti ml
1£Tin
A jlíujuuuuuuu ^ A A (♦)
bT
^
(-) NALOXONE 2x10'7M
m î 'n r
T
11 g
A (-) NALOXONE 2 x10-7 m
h
3 «n
Imin
>9 ♦
t
ON
OFF STIMULUS
A
A
O
•
(♦) (-) DEXTROR- LEVORNALOXONE PHAN PHANOL 2x 10"7 M 1x 10'6M
igl
f STIMULUS ON
mm* i I OFF
At») A (-) NALOXONE 2x1(T7M
Fig. 2. Examples of the naloxone effect on in vitro reflex peristalsis in the guinea pig large intestine (A) and adult guinea pig ileum (B), and on longitudinal contractions in the fetal guinea pig ileum just prior to parturition (C) and ileum of the hibernating garden dormouse (D). The large intestine (A) occasionally showed a transient spasm but otherwise responded in the same way as the small intestine (see fig. 1). An extremely rare event in the guinea pig ileum, i.e. a transient stimulation with subsequent inhibition of peristalsis by naloxone, is seen in B. The example demonstrates that endogenous opioids exert a dual effect even in the guinea pig. Note that traces A and B
represent peristaltic circular muscle contractions travelling over the segment from oral to aboral. Trace C first shows electrically elicited longitudinal contrac tions of the fetal intestinal segment for reference. Sub sequent application of (-)naloxone, but not (+)naloxone, caused a tonic opiate withdrawal-like contrac ture. The intestinal segment of the hibernating garden dormouse (D) responded to (-)naloxone with rhyth mic, longitudinal contractions, again reminiscent of opioid withdrawal contractions. 5 out of 6 intestines taken from hibernating, but only 1 intestine out of 6 taken from conscious animals displayed the phenom enon [Kromer, unpubl. data].
into low-frequency but high-amplitude con tractions. Hence, opioid receptors modulate the patterns of peristalsis. These observa tions are congruent with dual opioid effects on in vivo gut motility [for review, see ref. 7] and strongly support the concept of func
tionally contrasting opioid systems operative in the gut. Actually, Crain and Shen [20] reported very recently electrophysiological data on contrasting excitatory-inhibitory ac tions of opioids in cultured spinal cord neu rons (see below). Apparent species differ-
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
» *»>«»■*»**A*V
ences may just be of a quantitative nature, since even the guinea pig ileum, though ex tremely seldom, may respond to naloxone with a decrease instead of the usual increase in peristaltic activity (fig. 2B). Although opioids enhance and naloxone delays the appearance of the migrating mo tor complex (MMC) in the dog in vivo [21], thus supporting the assumption of an endog enous opioid system, opioids inhibit intesti nal transit in all species [for review, see ref. 7], Including man, the final outcome is clearly constipation which underlines that MMC phase III activity transiently induced by opioids is not necessarily related to intes tinal transit [Wienbeck, pers. commun.]. Overall, however, opioids seem to switch the motility pattern of the gut from the peristal tic to the segmenting mode. This can be eas ily observed in the isolated rabbit ileum (fig. 1C, right example), when morphine in hibits peristaltic contractions but elicits high-frequency segmentations, while nalox one acts in an opposite fashion. Only recent ly, this mode of action has been described in the human colon in vivo [22]. It may be rec ognized in this context that naloxone also increases the frequency of peristaltic con tractions in the large intestine in vitro [23] (fig. 2A). Although the dissociation constant (K
Dig Dis 1990;8:361-373
Endogenous Opioids, the Enteric Nervous System and Gut Motility Wolfgang Kromer Department of Pharmacology, Byk Gulden Pharmaceuticals, Konstanz, FRG
Key Words. Opioids • Intestinal motility • Enteric nervous system
Introduction Opiates display potent and stereospecific effects in vivo and in vitro and show satura ble and reversible binding of high affinity to neuronal membranes. Properties like these prompted the search for specific opiate re ceptors. This search was eventually success ful in 1973 [1-3]. A surge of publications was seen during the following decade, un raveling the biochemistry and physiological role of endogenous opioid peptides as hor mones and neuromodulators activating opioid receptors in a variety of organs [see ref. 4 and related articles of the same vol ume; 5-9]. Radioimmunological determina
tions in extracts of the intestinal wall as well as immunohistochemical studies in various species demonstrated the occurrence of all 3 classes of opioid peptides in neurons and nerve fibers of the myenteric and submuco sal plexus and in endocrine cells of the mu cosa of the gastrointestinal (GI) tract [for review, see ref. 7], This implies a variety of potential physiological and pathophysiologi cal functions (table 1). The tissue concentra tions of enkephalins were, on average, high est, those of dynorphin and (3-endorphin were intermediate and lowest, respectively (table 2). Enkephalin-degrading enzymes have also been detected in the gut [10], and both biosynthesis in [11] and electrically in-
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
Abstract. Opium alkaloids have been used for centuries as potent antidiarrheals and anal gesics, their constipating action in the latter instance taken as an unwanted effect. It was only during the last decade that the physiological role of opioid peptides present in both neurons and endocrine cells of the gastrointestinal (GI) tract has been defined. The recognition of distinct opioid receptor types which may be differentially involved in the control of motility, acid and electrolyte secretion in the GI tract presently focuses the attention of researchers in this field on the identification of receptor-type-selective opioid agonists in order to free these clinically extremely useful drugs from side effects. The present review provides a survey of mostly physiological data on the functional role of intestinal opioids.
362
Kromer
Table 1. Biological role of GI opioids Physiology
Pathophysiology
'Normal' life conditions Inhibition of peristalsis Induction of segmentations Increase in circular muscle tone Electrolyte absorption Duodenal bicarbonate secretion Dual effect on acid secretion Hormonal effects inside and outside the gut? (unknown)
Motility (largely speculative) Opioid hyper- or dysfunction in Achalasia? Chronic idiopathic constipation? Crohn’s disease? Intestinal inflammation? GI tumors? Loss of opioid function in Hirschsprung’s disease?
Particular life conditions Inhibition of gut motility During fetal life During hibernation? During stress?
Secretion (largely speculative) Opioid dysfunction (dual effects) in GI ulcer development? Opioid counterregulation of chloride secretion in diarrhea?
duced release of opioids from [ 12] the guinea pig ileum in vitro have been reported. Although Waterfield and Kosterlitz [13] had already demonstrated enhancement of electrically induced acetylcholine (ACh) re lease from the guinea pig myenteric plexus by the competitive opioid antagonist (-)naloxone, it was a challenge to researchers in the field when van Nueten et al. [14] claimed that naloxone was able to reverse ‘fatigued’ reflex peristalsis in the intact guinea pig ileum in vitro. The reason for the initial sur prise was that, in many of the laboratories, the electrically stimulated longitudinal mus cle myenteric plexus preparation of the guinea pig ileum was used (and still is used) as a biological test system for opioids. Nalox one was considered to have no effect on its own in this test system. It was regarded as a specific tool for proving opioid functions by blocking opioid receptors and, thus, un masking endogenous opioid actions. During the following years, however, it was recog
nized that opioid peptides intrinsic to the GI tract physiologically modulate reflex peri stalsis in an inhibitory fashion [15] and acid secretion by isolated gastric parietal cells in an excitatory fashion [16], thus explaining the opposite effects of naloxone alone.
Both the efficiency and frequency of in testinal peristaltic contractions can be easily quantified in the intact intestinal segment in vitro when rhythmic, expulsive circular mus cle contractions are elicited by sustained dis tension of the intestinal wall due to elevated intraluminal pressure. The resulting reflex peristalsis in various species is shown in fig ure 1. It is easily distinguished from segmen tations in this in vitro model and is charac-
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
Functional Role of Intestinal Opioids in the Control of Peristalsis in Different Species in vitro and in vivo
363
Intestinal Opioids
Table 2. Neuronal opioid receptors in the guinea pig ileum Receptor type K-opioid
p-opioid
8-opioid
Receptor location [35, 36]
myenteric plexus
myenteric plexus
myenteric and submucosal plexus
Selective antagonists [37]
binaltorphimine
c t p -n h 2
ICI 174,864
Endogenous agonists [38-40]
dynorphin
P-endorphin? (nonselective)
met- and leuenkephalin
Tissue concentration (whole intestinal wall) of endogenous agonists [26, 41, 42]
— 10-11 mol/g tissue
— 10"12 mol/g tissue
~ 10"10 mol/g tissue
Agonist potency (peristalsis) [43]
high
low
intermediate
Receptor mechanism [35, 36]
inhibition of voltage-dependent calcium channels
activation of calcium-dependent potassium channels
Receptor-effector coupling [35, 36]
unknown
direct coupling to potassium channel via G protein
Modulation of ACh release [13, 33, 34] and action (peristalsis) [44]
opioid receptor type unknown
Physiological receptor function [45, 46]
inhibition of reflex peristalsis
unknown; inhibition of longitudinal muscle contraction?
probably activation of electrolyte absorption
Cause of inhibition of agonist release [26, 30]
reflex peristalsis
unknown
reflex peristalsis
Potential interaction between receptor systems (speculative)
primary inhibition of peristalsis: secondary activation of electrolyte absorption?
unknown
primary activation of electrolyte absorption: secondary inhibition of peristalsis?
terized by circular muscle contractions prop agated from oral to aboral, thereby expelling the luminal contents of the intestinal seg ment. It is evident from figure 1 that the inhibitory action of opioid agonists is the
same in the guinea pig, rat and rabbit. By contrast, naloxone is excitatory in all 3 spe cies. Both the agonists and the antagonist act in a stereospecific fashion (fig. 1A-C). The situation is more complex in the cat. Actual-
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
Smooth muscle opioid receptors, which exert contractile effects, have not been considered in this table (see text).
364
Kromer
ly, both morphine and naloxone displayed sometimes inhibitory, sometimes excitatory actions in this species, but worked in an opposite fashion in each single ileal segment (fig. ID, E) [17 and unpubl. data]. Although Wood [18] reported unequivocal inhibition of intracellularly recorded, evoked discharge of cat myenteric neurons by morphine, this primary inhibition by opioids may well re sult in either functional inhibition or disinhibition dependent on whether excitatory or inhibitory neurons were inhibited by the opioid. This hypothesis would be consistent with the dual opioid effect on both in vitro
peristalsis (fig. 1D, E) and extracellularly re corded activity of cat myenteric neurons [19]. Even more difficult to explain is the opioid effect on propulsive peristalsis in the isolated dog intestine. Early observations [17] suggested exclusive inhibition by nalox one and excitation by opioid agonists of ca nine peristalsis in vitro. However, later ex periments (fig. IF) [Kromer, unpubl. data] consistently showed that what initially ap peared to be suppression of high-frequency but low-amplitude peristaltic contractions by naloxone turned out to be transformation
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
Fig. 1. Representative examples of the influence of opioid agonists [levorphanol (•) and morphine (♦)] and the antagonist (-)naloxone ( a ) on peristalsis in the isolated ileum taken from the guinea pig (A), rat (B). rabbit (C), cat (D. E) and dog (F). The biologically inactive enan tiomers dextrorphan (o) and (+)naloxone (A) were used to test for stereospecificity. All experiments were performed under comparable conditions; A and B were adult ani mals, C -F were animals a few weeks old. Reflex peristalsis was elicited by sustained elevated intra luminal pressure (2-10 cm HjO, de pending on the species) and the aboral luminal volume displace ment measured. See text.
Intestinal Opioids
365
Ti ml
1£Tin
A jlíujuuuuuuu ^ A A (♦)
bT
^
(-) NALOXONE 2x10'7M
m î 'n r
T
11 g
A (-) NALOXONE 2 x10-7 m
h
3 «n
Imin
>9 ♦
t
ON
OFF STIMULUS
A
A
O
•
(♦) (-) DEXTROR- LEVORNALOXONE PHAN PHANOL 2x 10"7 M 1x 10'6M
igl
f STIMULUS ON
mm* i I OFF
At») A (-) NALOXONE 2x1(T7M
Fig. 2. Examples of the naloxone effect on in vitro reflex peristalsis in the guinea pig large intestine (A) and adult guinea pig ileum (B), and on longitudinal contractions in the fetal guinea pig ileum just prior to parturition (C) and ileum of the hibernating garden dormouse (D). The large intestine (A) occasionally showed a transient spasm but otherwise responded in the same way as the small intestine (see fig. 1). An extremely rare event in the guinea pig ileum, i.e. a transient stimulation with subsequent inhibition of peristalsis by naloxone, is seen in B. The example demonstrates that endogenous opioids exert a dual effect even in the guinea pig. Note that traces A and B
represent peristaltic circular muscle contractions travelling over the segment from oral to aboral. Trace C first shows electrically elicited longitudinal contrac tions of the fetal intestinal segment for reference. Sub sequent application of (-)naloxone, but not (+)naloxone, caused a tonic opiate withdrawal-like contrac ture. The intestinal segment of the hibernating garden dormouse (D) responded to (-)naloxone with rhyth mic, longitudinal contractions, again reminiscent of opioid withdrawal contractions. 5 out of 6 intestines taken from hibernating, but only 1 intestine out of 6 taken from conscious animals displayed the phenom enon [Kromer, unpubl. data].
into low-frequency but high-amplitude con tractions. Hence, opioid receptors modulate the patterns of peristalsis. These observa tions are congruent with dual opioid effects on in vivo gut motility [for review, see ref. 7] and strongly support the concept of func
tionally contrasting opioid systems operative in the gut. Actually, Crain and Shen [20] reported very recently electrophysiological data on contrasting excitatory-inhibitory ac tions of opioids in cultured spinal cord neu rons (see below). Apparent species differ-
Downloaded by: Washington University 128.252.67.66 - 1/8/2018 4:07:23 PM
» *»>«»■*»**A*V
ences may just be of a quantitative nature, since even the guinea pig ileum, though ex tremely seldom, may respond to naloxone with a decrease instead of the usual increase in peristaltic activity (fig. 2B). Although opioids enhance and naloxone delays the appearance of the migrating mo tor complex (MMC) in the dog in vivo [21], thus supporting the assumption of an endog enous opioid system, opioids inhibit intesti nal transit in all species [for review, see ref. 7], Including man, the final outcome is clearly constipation which underlines that MMC phase III activity transiently induced by opioids is not necessarily related to intes tinal transit [Wienbeck, pers. commun.]. Overall, however, opioids seem to switch the motility pattern of the gut from the peristal tic to the segmenting mode. This can be eas ily observed in the isolated rabbit ileum (fig. 1C, right example), when morphine in hibits peristaltic contractions but elicits high-frequency segmentations, while nalox one acts in an opposite fashion. Only recent ly, this mode of action has been described in the human colon in vivo [22]. It may be rec ognized in this context that naloxone also increases the frequency of peristaltic con tractions in the large intestine in vitro [23] (fig. 2A). Although the dissociation constant (K
