Inhibitory effects of opioids in a circular muscle-myenteric plexus preparation of guinea-pig ileum

Biological redundancy of endogenous GPCR ligands in the gut and the potential for endogenous functional selectivity

This review focuses on the existence and function of multiple endogenous agonists of the somatostatin and opioid receptors with an emphasis on their expression in the gastrointestinal tract. These agonists generally arise from the proteolytic cleavage of prepropeptides during peptide maturation or from degradation of peptides by extracellular or intracellular endopeptidases. In other examples, endogenous peptide agonists for the same G protein-coupled receptors can be products of distinct genes but contain high sequence homology. This apparent biological redundancy has recently been challenged by the realization that different ligands may engender distinct receptor conformations linked to different intracellular signaling profiles and, as such the existence of distinct ligands may underlie mechanisms to finely tune physiological responses. We propose that further characterization of signaling pathways activated by these endogenous ligands will provide invaluable insight into the mechanisms governing biased agonism. Moreover, these ligands may prove useful in the design of novel therapeutic tools to target distinct signaling pathways, thereby favoring desirable effects and limiting detrimental on-target effects. Finally we will discuss the limitations of this area of research and we will highlight the difficulties that need to be addressed when examining endogenous bias in tissues and in animals.

An unusual kynurenine-containing opioid tetrapeptide from the skin gland secretion of the Australian red tree frog Litoria rubella. Sequence determination by electrospray mass spectrometry

The Kyn-containing peptide FP-Kyn-L(NH(2)) is an unusual minor component of the skin peptide profile of the Australian red tree frog Litoria rubella collected from an area within a 20 kilometre radius of Alice Springs in central Australia. The structure was determined by electrospray mass spectrometry and synthesis. The major component of the skin secretion is the analogous tryptophyllin peptide FPWL(NH(2)). Both peptides show opioid activity at 10(-7) M, and are likely to act via the μ opioid receptor.

Skin peptides from anurans of the Litoria rubella Group: sequence determination using electrospray mass spectrometry. Opioid activity of two major peptides

Many species of frogs of the genus Litoria secrete bioactive peptides from their skin glands. These peptides are normally host-defence compounds and may have one, or more of the following activities; smooth muscle contraction, analgesic, antimicrobial, antiviral, lymphocyte proliferator (immunomodulator) and neuronal nitric oxide synthase (nNOS) inactivation. Two frog species of the Litoria rubella Group that have been studied before, namely, Litoria electrica and Litoria rubella, are different from other species of the genus Litoria in that they produce small peptides that show neither membrane, lymphocyte nor nNOS activity. In this study we have used electrospray mass spectrometry together with Edman sequencing to identify eight skin peptides of the third member of this Group, Litoria dentata: surprisingly, none of these peptides show activity in our biological screening program. However, two major peptides (FPWL-NH(2) and FPWP-NH(2)) from L. electrica and L. rubella are opioids at the micromolar concentration.

The opioid system in the gastrointestinal tract

Mu-, delta- and kappa-opioid receptors (ORs) mediate the effects of endogenous opioids and opiate drugs. Here we report (1) the distribution of muOR in the guinea-pig and human gastrointestinal tract in relation to endogenous ligands, to functionally distinct structures in the gut and to deltaOR and kappaOR; and (2) the ligand-induced muOR endocytosis in enteric neurones using in vitro and in vivo models. In the guinea pig, muOR immunoreactivity is confined mainly to the myenteric plexus. MuOR myenteric neurones are most numerous in the small intestine, followed by the stomach and the proximal colon. MuOR immunoreactive fibres are dense in the muscle layer and the deep muscular plexus, where they are in close association with interstitial cells of Cajal. This distribution closely matches the pattern of enkephalin. MuOR enteric neurones comprise functionally distinct populations of neurones of the ascending and descending pathways of the peristaltic reflex. In human gut, muOR immunoreactivity is localized to myenteric and submucosal neurones and to immune cells of the lamina propria. DeltaOR immunoreactivity is located in both plexuses where it is predominantly in varicose fibres in the plexuses, muscle and mucosa, whereas kappaOR immunoreactivity appears to be confined to the myenteric plexus and to bundles of fibres in the muscle. MuOR undergoes endocytosis in a concentration-dependent manner, in vitro and in vivo. Pronounced muOR endocytosis is observed in neurones from animals that underwent abdominal surgery that has been shown to induce delay in gastrointestinal transit. We can conclude that all three ORs are localized to the enteric nervous system with differences among species, and that muOR endocytosis can be utilized as a means to visualize enteric neurones activated by opioids and sites of opioid release.

Abdominal surgery induces μ opioid receptor endocytosis in enteric neurons of the guinea-pig ileum

Immunohistochemistry and confocal microscopy were used to investigate mu opioid receptor (muOR) internalization in enteric neurons of the guinea-pig ileum following abdominal surgery. The following surgical procedures were performed under halothane or isofluorane anesthesia: a) midline abdominal skin incision, b) laparotomy or c) laparotomy with intestinal manipulation. Gastrointestinal transit was evaluated by using a non-absorbable marker and measuring fecal pellet output. In neurons from normal and control (anesthesia alone) animals, muOR was predominantly at the cell surface. muOR endocytosis following skin incision was not significantly different from controls (21.2+/-3.5% vs. 13.7+/-2.1%, mean+/-S.E.M.), whereas it was significantly increased by laparotomy (46.5+/-6.1%; P<0.01 vs. controls) or laparotomy plus intestinal manipulation (40.5+/-6.1%; P<0.01 vs. controls) 30 min following surgery compared with controls. muOR endocytosis remained elevated at 4 h (38.6+/-1.2%; P<0.01 vs. controls), whereas it was similar to controls at 6 and 12 h (17.5+/-5.8% and 11.2+/-3.0%). muOR endocytosis occurred in cholinergic and nitrergic neurons. Gastrointestinal transit was significantly delayed by laparotomy or laparotomy plus intestinal manipulation (12.8+/-1.2 and 13.8+/-0.6 h vs. 7.0+/-0.5 in controls; P<0.01), but was not significantly changed by skin incision (8.2+/-0.6 h). The findings of the present study support the concept that the noxious stimulation caused by abdominal surgery induces release of endogenous opioids thus resulting in muOR endocytosis in neurochemically distinct enteric neurons. muOR internalization can serve as indirect evidence of opioid release and as a means to visualize neuronal pathways activated by opioids.

Neurochemically distinct classes of myenteric neurons express the mu-opioid receptor in the guinea pig ileum

The mu-opioid receptor (muOR), which mediates many of the opioid effects in the nervous system, is expressed by enteric neurons. The aims of this study were to determine whether 1) different classes of myenteric neurons in the guinea pig ileum contain muOR immunoreactivity by using double- and triple-labeling immunofluorescence and confocal microscopy, 2) muOR immunoreactivity is localized to enteric neurons immunoreactive for the endogenous opioid enkephalin, and 3) muOR immunoreactivity is localized to interstitial cells of Cajal visualized by c-kit. In the myenteric plexus, 50% of muOR-immunoreactive neurons contained choline acetyltransferase (ChAT) immunoreactivity, whereas about 43% of ChAT-immunoreactive neurons were muOR immunoreactive. Approximately 46% of muOR myenteric neurons were immunoreactive for vasoactive intestinal polypeptide (VIP), and about 31% were immunoreactive for nitric oxide synthase (NOS). MuOR immunoreactivity was found in about 68% of VIP-containing neurons and 60% of NOS-immunoreactive neurons. Triple labeling showed that about 32% of muOR neurons contained VIP and ChAT immunoreactivities. The endogenous opioid enkephalin (ENK) was observed in about 30% of muOR neurons; conversely, 48% of ENK neurons contained muOR immunoreactivity. MuOR was not detected in neurons containing calbindin, nor in interstitial cells of Cajal. MuOR-immunoreactive fibers formed a dense network around interstitial cells of Cajal in the deep muscular plexus. This study demonstrates that muOR is expressed by neurochemically distinct classes of myenteric neurons that are likely to differ functionally, is colocalized with the endogenous opioid ENK, and is not expressed by interstitial cells of Cajal.

Involvement of mu- and kappa-, but not delta-, opioid receptors in the peristaltic motor depression caused by endogenous and exogenous opioids in the guinea-pig intestine

Opiates inhibit gastrointestinal propulsion, but it is not clear which opioid receptor types are involved in this action. For this reason, the effect of opioid receptor - selective agonists and antagonists on intestinal peristalsis was studied. Peristalsis in isolated segments of the guinea-pig small intestine was triggered by a rise of the intraluminal pressure and recorded via the intraluminal pressure changes associated with the peristaltic waves. Mu-opioid receptor agonists (DAMGO, morphine), kappa-opioid receptor agonists (ICI-204,448 and BRL-52,537) and a delta-opioid receptor agonist (SNC-80) inhibited peristalsis in a concentration-related manner as deduced from a rise of the peristaltic pressure threshold (PPT) and a diminution of peristaltic effectiveness. Experiments with the delta-opioid receptor antagonists naltrindole (30 nM) and HS-378 (1 microM), the kappa-opioid receptor antagonist nor-binaltorphimine (30 nM) and the mu-opioid receptor antagonist cyprodime (10 microM) revealed that the antiperistaltic effect of ICI-204,448 and BRL-52,537 was mediated by kappa-opioid receptors and that of morphine and DAMGO by mu-opioid receptors. In contrast, the peristaltic motor inhibition caused by SNC-80 was unrelated to delta-opioid receptor activation. Cyprodime and nor-binaltorphimine, but not naltrindole and HS-378, were per se able to stimulate intestinal peristalsis as deduced from a decrease in PPT. The results show that the neural circuits controlling peristalsis in the guinea-pig small intestine are inhibited by endogenous and exogenous opioids acting via mu- and kappa-, but not delta-, opioid receptors.

Receptors and transmission in the brain-gut axis: potential for novel therapies. III. Mu-opioid receptors in the enteric nervous system

G protein-coupled receptors are cell surface signal-transducing proteins, which elicit a variety of biological functions by the activation of different intracellular effector systems. Many of these receptors, including the mu-opioid receptor (mu OR), have been localized in the gastrointestinal tract. mu OR is the target of opioids and alkaloids, potent analgesic drugs with high potential for abuse. mu OR is expressed by enteric neurons, and it undergoes ligand-selective endocytosis. It is of clinical importance because it mediates tolerance and other major side effects of opiate analgesics, including impairment of gastrointestinal propulsion. An important observation of mu OR is its differential trafficking and desensitization properties in response to individual agonists, which might have long-term physiological consequences and be involved in the development of opiate side effects. Receptor activation by agonists is the basis for signaling, and alterations of the mechanisms controlling cellular responses of G protein-coupled receptors to agonists might be the basis of several diseases, including gastrointestinal diseases. Therefore, understanding these basic cellular mechanisms is important for developing appropriate therapeutic agents.

Role of kappa opioid receptors in modulating cholinergic twitches in the circular muscle of guinea-pig colon

1. Single pulse electrical field stimulation (EFS, 0.5 ms pulse width, 60 V at a frequency of 0.05 Hz) induced twitch contractions of mucosa-free circular muscle strips from the guinea-pig proximal colon which were abolished by atropine (0.3 microM), tetrodotoxin (0.3 microM) or omega-conotoxin GVIA (0.1 microM). 2. Various opioid receptor agonist concentration-dependently inhibited twitches with the following rank order of potency (EC50 values in brackets): U 50488 (0.31 nM) > dermorphin (4.3 nM) = dynorphin A (1-13) (6.2 nM) > [D-Ala2, N-MePhe4, Gly5-ol]-enkephalin (DAMGO, 33.5 nM) = [D-Ala2, D-Leu5]-enkephalin (DADLE, 60 nM) > [D-Pen2, D-Pen2, D-Pen5]-enkepahlin (DPDPE, 1144 nM). 3. Peptidase inhibitors (captopril, thiorphan and bestatin, 1 microM each) did not modify the amplitude of twitches. In the presence of peptidase inhibitors the concentration-response curve to dynorphin A (1-13) was displaced to the left to yield an EC50 of 0.35 nM, comparable to that of the selective kappa receptor agonist, U50488. The curves to the other opioid receptor agonist were unaffected by peptidase inhibitors. 4. DPDPE, DADLE, dermorphin and DAMGO consistently induced a concentration-unrelated transient increase in basal tone and a small and transient facilitation of twitches before development of their inhibitory effect. These transient excitatory effects were not observed upon application of dynorphin A (1-13) or U 50488. The contraction produced by DPDPE (30 nM) was largely inhibited (> 80%) by 1 microM atropine. 5. Twitches suppression induced by dynorphin A (1-13) (30 nM) was partly reversed (46 +/- 8%, n = 6) by naloxone (0.3 microM). The potent and selective kappa opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 3-100 nM)) did not affect the amplitude of twitches and potently antagonized (pKB 9.83 +/- 0.09, n = 10) the inhibitory effect of dynorphin. 6. Naloxone (1-300 nM) concentration-dependently depressed the cholinergic twitches: this depressant effect was largely counteracted in the presence of apamin (0.1 microM) and NG-nitro-L-arginine (30 microM) which potentiated cholinergic twitches on their own. 7. Dynorphin A (1-13) (10 nM, n = 6) did not affect the contractile response to exogenous acetylcholine (1 microM), indicating that depression of evoked twitches occurs prejunctionally. 8. We conclude that multiple opioid receptors modulate cholinergic twitches in the circular muscle of guinea-pig proximal colon. While mu and delta opioid receptor agonists produced mixed excitatory and inhibitory effects, kappa opioid receptors, activated by sub-nanomolar concentrations of dynorphin A (1-13), mediate a powerful and pure prejunctional inhibition of acetylcholine release.

Tonic modulation of neurotransmitter release in the guinea-pig myenteric plexus: effect of mu and kappa opioid receptor blockade and of chronic sympathetic denervation

We have studied the effects of mu- and kappa-opioid receptor blockade on endogenous acetylcholine and noradrenaline overflow from the myenteric plexus of the guinea-pig isolated colon. Cyprodime (putative mu-selective antagonist) and nor-binaltorphimine (kappa-selective antagonist) had a concentration-dependent facilitatory effect on both acetylcholine and noradrenaline overflow. Moreover, in colonic specimens obtained from sympathetically denervated animals, the effect of opioid antagonists on acetylcholine overflow was significantly higher with respect to normal preparations. Evidence is thus given in favour of an involvement of mu- and kappa-opioid receptor pathways in the tonic modulation of neurotransmitter release at the colonic level. Enhanced sensitivity to the effect of mu and kappa antagonists after chronic sympathetic denervation is strongly suggestive for the existence of a functional link between opioid and adrenergic pathways in this model.

Modulation of peristalsis in the guinea-pig isolated small intestine by exogenous and endogenous opioids

1. A recording method was developed to measure physiological parameters of the preparatory and emptying phases of peristalsis in vitro. This method enabled measurement of: the compliance of the intestinal wall during the preparatory phase (a reflection of the resistance of the wall to distension); longitudinal muscle contraction during the preparatory phase; the threshold volume required to trigger the emptying phase; the maximal ejection pressure and the average power generated during the emptying phase, which reflects the rate at which the intestine performs work. Modulation of these parameters by exogenous and endogenous opioids acting at mu, kappa and delta opioid receptors was investigated. 2. The compliance of the intestinal wall during the preparatory phase was reduced by the mu opioid receptor agonist, [D-Ala2, N-methyl-Phe4, Gly5-ol] enkephalin (DAMGO) but not by the kappa agonist, dynorphin, or the delta agonist, [D-penicillamine2, D-penicillamine5] enkephalin (DPDPE). Reflex contraction of the longitudinal muscle during the preparatory phase was inhibited by DAMGO, dynorphin and DPDPE. The threshold volume required to trigger the emptying phase of peristalsis was increased by DAMGO, dynorphin and DPDPE. 3. The maximal ejection pressure generated during the emptying phase was reduced by dynorphin and DPDPE, but not by DAMGO. The average power generated by the intestine when emptying was not altered by any of the agonists. 4. Electrically stimulated contractions of longitudinal muscle in strips of longitudinal muscle-myenteric plexus were not inhibited by DPDPE. Similarly, DPDPE did not significantly inhibit electrically induced contraction of circular muscle in strips of circular muscle-myenteric plexus.5. Each of the agonist effects on peristaltic parameters was antagonized by the appropriate antagonist:D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) (mu), norbinaltorphimine (nor-BNI) (kappa), naltrindole(delta).6. It is concluded that mu and kappa agonists act primarily on excitatory circular and longitudinal muscle motor neurones. The delta agonist probably acts on enteric neurones presynaptic to excitatory circular and longitudinal muscle motor neurones.7. Antagonists for mu, delta and kappa receptors did not affect any parameters of peristalsis when the intestine emptied against a low resistance. However, when emptying against a high outflow resistance, the average power generated by the intestine was increased by the kappa antagonist, nor-BNI, but not by CTOP or naltrindole.8. It is concluded that endogenous opioids appear to have little role in peristalsis when the intestine is working against a low outflow resistance. However endogenous opioids, acting primarily at kappa receptors,provide a braking mechanism by inhibiting the emptying phase of peristalsis in conditions in which the intestine empties against a higher resistance.

Sites of action of morphine on the ascending excitatory reflex in the guinea-pig small intestine

The effect of morphine on the ascending excitatory reflex of the circular muscle elicited by radial distension of the gut wall was studied in the isolated guinea-pig small intestine. A three compartment bath, in which an intermediate compartment divided the site of intraluminal stimulation (caudal compartment) from the site of reflex contraction recording (oral compartment), was used. Morphine (0.01-10 microM) applied independently to each compartment, caused a concentration-dependent depression (up to 90%) of the amplitude of distension-evoked reflex contractions. Concentration-response curves to morphine were shifted to the right by naloxone (30 nM) with an apparent pA2 value of about 8.5, which suggests an interaction with opioid mu-receptor subtypes. Our results indicate that morphine not only depressed transmission from excitatory motor neurons to the circular muscle but also neuro-neuronal transmission along the ascending excitatory reflex pathway.

The electrophysiological effects of [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin on guinea-pig myenteric neurons

The electrophysiological effects of a highly selective mu opioid agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO) were investigated on S and AH myenteric neurons in the guinea-pig ileum. Administration of DAGO (500 nM) caused a mean membrane hyperpolarization (+/- S.E.M.) of 7.8 +/- 0.5 mV in 58 of 138 S neurons, associated with a decrease in input resistance from 158 +/- 18 to 132 +/- 13 m omega. DAGO also produced a significant decrease in the mean amplitude of the cholinergic fast excitatory postsynaptic potentials (EPSPs), from 12.0 +/- 1.1 to 6.6 +/- 1.4 mV in 13 of 27 S neurons. On the other hand, in AH neurons, DAGO did not significantly affect the membrane potential, input resistance, action potential configuration, slow after-hyperpolarization or the antidromic action potential. The experiments indicate that the acute effects of mu opioids in the guinea-pig ileum are confined to a subpopulation of a single electrophysiological class of myenteric neurons.

Activation of delta-type opioid receptors modulates the responses of cat terminal ileum to field electrical stimulation

1. The effects of (D-Ala2, D-Leu5) enkephalin amide (DADLE) on the responses of the cat terminal ileum to field electrical stimulation (pulse duration of 0.5 msec, train duration of 10 sec, 30 V) were evaluated by the changes in the contractile or the relaxatory responses of longitudinal and circular strips to electrical stimuli with a frequency of 2, 10 or 30 Hz. 2. Stimulation with a frequency of 2, 10 or 30 Hz elicited contractile responses from the longitudinal strips while in the circular strips 2 Hz stimulation induced contractions and 10 or 30 Hz stimulation caused relaxation. Tetrodotoxin (TTX) (0.1 mumol/l) abolished the electrically-induced responses in both longitudinal and circular strips. 3. DADLE (1 nmol/l) significantly inhibited the cholinergic contractile responses of the longitudinal strips to 2, 10 or 30 Hz stimulation and the contractile responses of the circular strips to 2 Hz stimulation. The relaxatory responses of the circular strips to 10 or 30 Hz stimulation were insignificantly increased by DADLE. 4. On the background of guanetidine (10 mumol/l) and atropine (3 mumol/l) DADLE significantly decreased the nonadrenergic, noncholinergic relaxatory responses of the circular strips to 2, 10 or 30 Hz stimulation. 5. DADLE did not change the maximum effects and the EC50 values of acetylcholine and noradrenaline in both longitudinal and circular strips. 6. It is suggested that in the cat terminal ileum activation of delta-type opioid receptors modulates the mechanical activity suppressing the cholinergic responses in the longitudinal and circular layers as well as the adrenergic and nonadrenergic, noncholinergic responses in the circular layer.

Opioid dependence in myenteric neurons innervating the circular muscle of guinea-pig ileum

Guinea-pigs were treated with morphine for 6-8 days by subcutaneous implantation of pellets, each containing a mixture of morphine base (120 mg) and morphine hydrochloride (35 mg). Each guinea-pig received a single pellet. Mechanical activity of the circular muscle was recorded in vitro in preparations comprising the circular muscle and myenteric plexus. Exposure to morphine was maintained by addition of 1 microM morphine to the organ baths. After 90 min, morphine was withdrawn, either by repeatedly washing tissues in morphine-free Krebs' solution, or by addition of naloxone to reduce the occupancy of the opioid receptors by morphine. Withdrawal of morphine resulted in markedly enhanced contractile activity compared with that in circular muscle-myenteric plexus preparations from untreated control guinea-pigs. The withdrawal contractions were abolished by tetrodotoxin (600 nM) and greatly reduced by hyoscine (1 microM), indicating that they resulted from action potential discharge in myenteric neurons that release acetylcholine onto the circular muscle. Activation of the cholinergic excitatory motor neurons was not secondary to synaptic activation by cholinergic interneurons, because hexamethonium (100 microM) did not affect withdrawal contractions. The withdrawal response may therefore arise in the cholinergic excitatory motor neurons themselves, or in neurons that activate them via noncholinergic mechanisms.

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

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-(1-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.

Quantitative ultrastructural analysis of enkephalin-, substance P-, and VIP-immunoreactive nerve fibers in the circular muscle of the guinea pig small intestine

The present work was undertaken to determine what proportion of all nerve fibers in the circular muscle of the guinea pig small intestine contain the neuropeptides enkephalin, substance P, and vasoactive intestinal peptide and in which combinations these peptides occur in the fibers. It was envisaged that such an analysis would provide insights into the chemical identity of excitatory and inhibitory nerve fibers that innervate the muscle. Whole-mount preparations from normal and extrinsically denervated gut were labelled with antiserum to the individual peptides or with combinations of antipeptide antisera and processed for electron microscopy. Reactive and nonreactive vesicle-containing nerve fiber profiles were examined and counted in ultrathin sections. Vesicle-containing nerve fiber profiles immunoreactive for enkephalin, substance P, or vasoactive intestinal peptide had similar morphologies in that they all contained variable proportions of small clear and large granular vesicles. In all samples stained for single peptides or combinations of peptides, a small proportion of immunoreactive profiles approached smooth muscle cells to within 15-20 nm with no intervening basal lamina. A total of 14,694 vesiculated nerve fiber profiles from three control and three extrinsically denervated animals were scored for the presence of immunoreactivity to enkephalin, substance P, vasoactive intestinal peptide, or combinations of these peptides. Analysis of variance showed that the number of profiles labelled for substance P was not different from the number of profiles labelled for vasoactive intestinal peptide and that the number labelled with the substance P and vasoactive intestinal peptide antisera simultaneously were not different from the sum of the numbers obtained with each alone. The number of profiles labelled for substance P plus enkephalin was greater than the number labelled for substance P alone and the number labelled with vasoactive intestinal peptide plus enkephalin was greater than that with vasoactive intestinal peptide alone. Simultaneous labelling for substance P and vasoactive intestinal peptide resulted in immunoreactivity in the same number of profiles as did reaction for all three peptides at the same time. In both cases, about 95% of the profiles were labelled. The results from extrinsically denervated muscle were not different from control circular muscle. These results indicate that nearly all the intrinsic nerve fibers supplying the circular muscle of the guinea pig small intestine contain either substance P or vasoactive intestinal peptide but not both.(ABSTRACT TRUNCATED AT 400 WORDS)