Peptide neurons in the canine small intestine

Localization of substance P (SP)-immunoreactivity in the myenteric plexus of the rat esophagus

The present immunohistochemical study was performed to examine the number, distribution, and chemical coding of intrinsic substance P (SP) neurons and nerve fibers within the esophagus and discuss their functional roles. Many SP neurons and nerve fibers were found in the myenteric plexus, and the SP neurons gradually decreased from the oral side toward the aboral side of the esophagus. Double-immunolabeling showed that most SP neurons were cholinergic (positive for choline acetyltransferase), and few were nitrergic (positive for nitric oxide synthase). Some cholinergic SP nerve terminals surrounded cell bodies of several myenteric neurons. In the muscularis mucosa and lower esophageal sphincter, and around blood vessels, numerous SP nerve endings were present, and many of them were cholinergic. Also, SP nerve endings were found on only a few motor endplates of the striated muscles, and most of them were calcitonin gene-related peptide (CGRP)-positive. Retrograde tracing using Fast Blue (FB) showed that numerous sensory neurons in the dorsal root ganglia (DRGs) and nodose ganglion (NG) projected to the esophagus, and most FB-labeled SP neurons were CGRP-positive. These results suggest that the intrinsic SP neurons in the rat esophagus may play roles as, at least, motor neurons, interneurons, and vasomotor neurons, which are involved in local regulation of smooth muscle motility, neuronal transmission, and blood circulation, respectively. Moreover, SP nerve endings on only a minority of motor endplates may be extrinsic, derived from DRGs or NG, and possibly detect chemical circumstances within motor endplates to modulate esophageal motility.

Vasoactive Intestinal Polypeptide (VIP) in the Intestinal Mucosal Nerve Fibers in Dogs with Inflammatory Bowel Disease

Simple Summary

Canine inflammatory bowel disease (IBD)—a group of gastrointestinal disorders—is a serious problem in veterinary medicine. The etiology of IBD remains unknown, and its diagnosis and effective treatment are difficult. One of the less-known aspects of IBD pathology is the influence of this disease on the enteric nervous system, which is located in the intestinal wall and regulates most of the gastrointestinal functions. Therefore, the aim of the present study was to evaluate the influence of IBD on the intramucosal nerve fibers containing vasoactive intestinal polypeptide (VIP). VIP is one of the most important substances produced by the enteric nervous structures that is involved in many regulatory processes in the gastrointestinal tract. The obtained results show that IBD induces changes in the density of intramucosal VIP-positive nerve fibers in the canine gastrointestinal tract. It suggests that VIP is involved in the pathological processes occurring during this disease. Observed changes may be a result of neuroprotective and/or adaptive processes regulated by VIP, aimed at the homeostasis maintenance in the inflamed gastrointestinal (GI) tract and induced by proinflammatory factors.

Abstract

Canine inflammatory bowel disease (IBD) is a group of enteropathies with nonspecific chronic symptoms and poorly understood etiology. Many aspects connected with IBD are not understood. One of them is the participation of the intestinal nervous system in the development of pathological processes. Thus, this study aimed to demonstrate changes in the density of intramucosal nerve fibers containing vasoactive intestinal polypeptide (VIP)—one of the most important intestinal nervous factors caused by the various stages of IBD development. Mucosal biopsy specimens collected from the duodenum, jejunum and descending colon of healthy dogs and dogs with varied severity of IBD were included in the experiment. The density of VIP-like immunoreactive (VIP-LI) nerves was determined by a single immunofluorescence technique and a semi-quantitative method consisting in VIP-LI fiber counts in the field of view (0.1 mm²). The obtained results indicate that IBD induces changes in the density of mucosal VIP-LI nerve fibers in the canine gastrointestinal tract. The initial decrease is followed by an increase in VIP-like immunoreactivity in successive stages of the disease. These observations show that VIP is a neuronal factor that participates in the pathological processes connected with canine IBD. The observed changes probably result from the neuroprotective and/or adaptive properties of VIP. Protective and adaptive reactions induced by inflammation aim to protect the GI tract against damage by proinflammatory factors and ensure the homeostasis in the enteric nervous system (ENS) under the conditions changed by the disease process.

Somatostatin as an Active Substance in the Mammalian Enteric Nervous System

Somatostatin (SOM) is an active substance which most commonly occurs in endocrine cells, as well as in the central and peripheral nervous system. One of the parts of the nervous system where the presence of SOM has been confirmed is the enteric nervous system (ENS), located in the wall of the gastrointestinal (GI) tract. It regulates most of the functions of the stomach and intestine and it is characterized by complex organization and a high degree of independence from the central nervous system. SOM has been described in the ENS of numerous mammal species and its main functions in the GI tract are connected with the inhibition of the intestinal motility and secretory activity. Moreover, SOM participates in sensory and pain stimuli conduction, modulation of the release of other neuronal factors, and regulation of blood flow in the intestinal vessels. This peptide is also involved in the pathological processes in the GI tract and is known as an anti-inflammatory agent. This paper, which focuses primarily on the distribution of SOM in the ENS and extrinsic intestinal innervation in various mammalian species, is a review of studies concerning this issue published from 1973 to the present.

Substance P and the neurokinin-1 receptor expression in dog ileum with and without inflammation

In the gastrointestinal tract, the tachykinin Substance P (SP) is involved in motility, fluid and electrolyte secretion, and blood flow and regulation of immunoinflammatory response. SP exerts its biological activity on target cells by interacting mainly with the neurokinin-1 receptor (NK₁R). The present study aims to quantify the percentage of SP-immunoreactive (SP-IR) enteric neurons and the density of SP-IR nerve fibers in the ileum of control dogs (CTRL-dogs; n=7) vs dogs with spontaneous ileal inflammation (INF-dogs; n=8). In addition, the percentage of enteric neurons bearing NK₁R, and nitrergic neurons (nNOS-IR) expressing NK₁R immunoreactivity were evaluated in both groups. The percentages of SP-IR neurons were similar in CTRL- and INF-dogs, in either the myenteric (MP) (15±8% vs. 16±7%, respectively) and submucosal plexus (SMP) (26±7% vs. 24±14%, respectively). In INF-dogs, the density of SP-IR mucosal nerve fibers showed a trend to decrease (P=0.07). Myenteric neurons of CTRL- and INF-dogs expressed similar percentages of NK₁R-immunoreactivity (39±5% vs. 38±20%, respectively). Submucosal NK₁R-IR neurons were occasionally observed in a CTRL-dog. MP nitrergic neurons bearing NK₁R showed a trend to decrease in INF-dogs vs. CTRL- dogs (41±22% vs. 65±10%, respectively; P=0.11). In INF-dogs, muscle cells and immune cells overexpressed NK₁R immunoreactivity. These findings should be taken as a warning for possible intestinal motility disorders, which might occur during administration of NK₁R-antagonist drugs. Conversely, the strong expression of NK₁R immunoreactivity observed in muscle and mucosal immune cells of inflamed tissues may provide a rationale for the use of NK₁R antagonist drugs in the treatment of intestinal inflammation.

The innervation of the human antro-pyloric region: Organization and composition

Although the composition of the gastric innervation has been determined in animal models, relatively little known about the innervation of the human antro-pyloric region. We used immunocytochemical techniques to establish the localization and co-expression of neuropeptides and nitric oxide in the human antrum and upper duodenum. Our results demonstrate the existence of a clearly defined submucosal plexus in the antral region that is absent in rats and guinea pigs. The abundant innervation of the lamina propria contains 3 major nerve populations: VIP- and NOS-, SP- and CGRP-, and GRP-immunoreactive. For the first time, NOS-containing nerve fibers were observed throughout the length of the antral glands. Within the antrum somatostatin was confined to endocrine cells, however, at the pyloric sphincter both enteric plexi contained immunoreactive neurons and nerve fibres. Within the pyloric sphincter CGRP- and SP-immunoreactive fibres were significantly increased, correlating with the presence of large ganglia in the submucosal plexus. In conclusion, the organization and composition of the innervation of human antro-pylorus differed substantially from that reported in other mammals. The presence of an abundant mucosal innervation paralled by a well-defined submucosal plexus indicates that the functional regulation of the gastric–pyloric region will be distinct from that of smaller animal models.Key words: gastric innervation, pyloric sphincter, neuropeptides, nitric oxide, somatostatin.

Presynaptic modulation by VIP, secretin and isoproterenol of somatostatin release from enriched enteric synaptosomes: role of cAMP

The release of somatostatin-like immunoreactivity was studied in isolated synaptosomes. A significant release of somatostatin-like immunoreactivity was observed in the presence of vasoactive intestinal polypeptide (VIP) (10(-6) M: 53.0 +/- 12.4 pg/mg, basal: 14.3 +/- 1.7 pg/mg, n = 5, P < 0.05), secretin (10(-6) M: 56.1 +/- 3.8 pg/mg, basal: 25.8 +/- 1.6 pg/mg, n = 6, P < 0.01) and isoproterenol (10(-5) M: 54.0 +/- 13.4 pg/mg, basal: 20.0 +/- 3.4 pg/mg, n = 8, P < 0.05). Forskolin, an unspecified activator of the adenylate cyclase, caused a significant release of somatostatin-like immunoreactivity (10(-6) M: 57.3 +/- 13.2 pg/mg, basal: 30.0 +/- 5.8 pg/mg, n = 13, P < 0.01) which was further augmented in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX 10(-4) M) (77.0 +/- 17.8 pg/mg, n = 13, P < 0.01). 3-Isobutyl-l-methylxanthine and N6, 2'-O-dibutyryladenosine-3',5'-cyclic monophosphate mimicked at effect of forskolin and VIP. The release of somatostatin was paralleled by an increase of cAMP immunoreactivity in the presence of VIP (10(-6) M: 37.1 +/- 9.4 pmol/mg, basal: 19.8 +/- 4.2 pmol/mg, n = 10, P < 0.05), isoproterenol (10(-5) M: 42.4 +/- 9.8 pmol/mg basal: 16.7 +/- 2.4 pmol/mg, n = 12, P < 0.01) and forskolin (10(-6) M: 47.1 +/- 12.4 pmol/mg, basal: 19.8 +/- 4.2 pmol/mg, n = 10, P < 0.01). The effect of nitric oxide (NO) which acts as an inhibitory neurotransmitter in the enteric nervous system was studied. NO is known to activate guanylate cyclase to induce transmitter release. The NO-generating compound sodium nitroprusside and bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) had no effect on the release of somatostatin-like immunoreactivity. These data demonstrate the stimulatory effect of VIP, secretin and isoproterenol on release of somatostatin-like immunoreactivity from enteric synaptosomes, which is presumably mediated by cAMP-dependent mechanisms. cGMP-dependent mechanisms seem to be of minor relevance.

Motility pattern of isolated rat proximal colon and excitatory action of neurotensin

The investigation concerned the effects of neurotensin on mechanical activity of isolated rat proximal colon. An isometric-isovolumic preparation was used. Colonic segments showed spontaneous contractile activity, consisting of regular changes in both endoluminal and isometric tension. Neurotensin (1 pM to 0.1 microM) induced a concentration-dependent tonic contraction of both circular and longitudinal muscle accompanied by high frequency oscillatory activity. Desensitization of the neurotensin receptors antagonized the contractile activity of neurotensin. The excitatory effects of neurotensin were partially blocked to the same degree by tetrodotoxin and atropine, indicating that a component of the neurotensin-mediated contraction involves the release of endogenous acetylcholine. The tetrodotoxin-resistant component of the neurotensin-induced effect seems to be due to a direct action on the smooth muscle cells.

Neural somatostatin, vasoactive intestinal polypeptide and substance P in canine and human jejunum

The distribution and co-localization of substance P immunoreactivity (SP-IR), somatostatin-IR (SS-IR) and vasoactive intestinal peptide-IR (VIP-IR) have been determined in canine and human jejunum by immunocytochemistry (ICC). Immunostaining with an antibody to protein gene product 9.5 revealed fewer neuronal cell bodies in the submucosal ganglion of human than in canine intestine. Double immunostaining demonstrated that SP-IR and SS-IR were always co-localized in human but never in canine submucosal neurons. In both species, VIP-IR was present in a separate population of neurons. In canine submucosal ganglia, each peptide represented approximately one-third of the neurons while the proportions of VIP and SP/SS-containing neurons in human were 40% and 42%, respectively. These results provide neuroanatomical evidence for different functions of neural SP and SS in canine and human jejunum.

Distribution of galanin-immunoreactive nerves in the canine gastrointestinal tract

The distribution of nerves containing galanin immunoreactivity (GAL-IR) in the canine gastrointestinal tract was determined by immunohistochemistry. GAL-IR was observed in nerve cell bodies and nerve fibers in all layers of the lower esophagus, gastric antrum, pylorus, ileum, and colon, and in the sphincters of lower esophagus and pylorus. There were numerous GAL-IR-containing nerve cell bodies in the myenteric plexus of gastric antrum. GAL-IR nerve somata were also common in the myenteric plexus of lower esophagus, ileum, pylorus, and colon, and in the submucous plexus of stomach, ileum, and colon. GAL-IR-positive nerve fibers were also observed around submucous blood vessels of the stomach. GAL-IR nerves are distributed more widely in the canine enteric nervous system than previously recognized.

Distribution of extrinsic enkephalin-containing nerve fibers in the rat rectum and their origin in the major pelvic ganglion

The distribution of nerve fibers containing enkephalin (ENK)-like immunoreactivity was examined in the rectum of aganglionosis rats (AGRs) which completely lack the intramural ganglion cells in the large intestine, and was compared with that of their normal littermates. Furthermore, Met5-enkephalin-Arg6-Gly7-Leu8 (MEAGL)-like immunoreactive neurons projecting to the rectum were examined using retrograde tracing combined with immunohistochemistry in the major pelvic ganglion of normal male rats. In the intermuscular space of the aganglionic rectum of AGRs, unlike the pattern of the normal intermuscular plexus, moderate numbers of ENK-like-immunoreactive fibers were arranged in an irregular, coarse network; greatly diminished numbers of immunoreactive fibers were found in the submucosa. No ENK-like-immunoreactive fibers were seen in the circular muscle layer and mucosa. In the normal rat rectum, ENK-like-immunoreactive fibers were seen throughout all layers, and immunoreactive nerve cells were found predominantly in the myenteric plexus of colchicine-treated animals. Fluoro-Gold injected into the upper rectum labelled numerous principal ganglion neurons in the major pelvic and inferior mesenteric ganglia. Less than 10% of tracer-labelled neurons were positive for fluorescein immunolabelling of MEAGL in the major pelvic ganglion; no immunoreactive neurons were found in the inferior mesenteric ganglion. In the major pelvic ganglion of the colchicine-treated normal rats, about 5% of principal ganglion neurons were immunoreactive for MEAGL. Comparison of serial paraffin sections of the major pelvic ganglion stained for tyrosine hydroxylase (TH), MEAGL and vasoactive intestinal polypeptide (VIP), respectively, revealed that more than half of MEAGL-like immunoreactive neurons were also positive for TH; there was no case showing co-existence of MEAGL with VIP in the principal neurons. These results indicate that a small number of enkephalin-containing neurons in the major pelvic ganglion project to the rectum, and that more than half of these neurons are postganglionic sympathetic. They may terminate mainly in the myenteric ganglia in the rectum.

Distribution and characterization of vasoactive intestinal polypeptide binding in canine lower esophageal sphincter

BACKGROUND

Vasoactive intestinal polypeptide (VIP) may be a nonadrenergic, noncholinergic inhibitory transmitter in the lower esophageal sphincter (LES). There is no biochemical evidence of VIP receptors in the LES.

METHODS

Using membranes from canine LES, VIP receptor distribution and characterization were analyzed by radioligand binding and cross-linking experiments.

RESULTS

High densities of saturable VIP receptors were found (maximum bound [Bmax], 539.2 fmol/mg in the synaptosome-enriched fraction [P2] and 732.7 fmol/mg in the smooth muscle, plasma membrane-enriched fraction [Mic II]), with high affinity for 125I-VIP (dissociation constant [Kd], 1.38 nmol/L in P2 and 1.40 nmol/L in Mic II). Competition binding studies suggested the presence of two binding sites, a high-affinity (inhibitor constant [Ki1], 0.064 nmol/L) and a low-affinity (Ki2, 2.68 nmol/L) binding site in P2 membranes, but only one binding site (Ki, 1.18 nmol/L) in Mic II membranes. Guanosine triphosphate-gamma-s pretreatment eliminated high-affinity binding in P2 membranes by conversion to binding sites of lower affinity (Ki, 2.82 nmol/L). Studies with a cross-linking agent identified VIP receptors in synaptosomal and smooth muscle plasma membrane fractions; a single polypeptide of approximately 60 kilodaltons was found in each membrane.

CONCLUSIONS

Specific VIP receptors exist in both synaptosomal and smooth muscle plasma membrane of canine LES.

Peptide YY: a neuropeptide in the gut. Immunocytochemical and immunochemical evidence

Peptide YY immunoreactivity was detected in neuronal elements in the upper digestive tract of the rat, cat, ferret and pig by immunocytochemistry and radioimmunoassay combined with high-performance liquid chromatography. The two peptide YY antisera used do not recognize the related peptides neuropeptide Y and pancreatic polypeptide. In the rat peptide YY-immunoreactive nerve fibres were virtually restricted to the stomach smooth muscle of the minor curvature where they were numerous. Peptide YY-immunoreactive nerve cell bodies occurred in small ganglia on the serosal surface of the minor curvature. In the cat and ferret peptide YY-immunoreactive nerve fibres occurred in the circular smooth muscle layer of both the minor and major curvatures of the stomach and also in the upper small intestine; such fibres were numerous also in myenteric ganglia in these regions. In the pig, they were few but had roughly the same distribution as in the cat and ferret, except that they were quite numerous in thick muscle bundles close to the oesophagogastric junction. The presence of peptide YY-immunoreactive nerve cell bodies within the myenteric ganglia along the upper digestive tract of cat, ferret and pig, and in serosal ganglia of the rat stomach, indicates that at least some of the peptide YY-immunoreactive fibres demonstrated originate in or close to the gut wall. Double-staining experiments revealed that virtually all peptide YY-containing neurons and nerve fibres were distinct from those storing neuropeptide Y. Peptide YY-immunoreactive endocrine cells were encountered not only in the lower intestines but also in the stomach of the four species studied. In the antrum such cells were numerous and constituted a subpopulation of the gastrin-containing cells. In the oxyntic mucosa they were few and contained somatostatin. Radioimmunoassay revealed peptide YY-like peptides in gastric mucosa and smooth muscle from the upper digestive tract of all four species examined. The results of high-performance liquid chromatography suggest that the peptide YY-like material in the upper digestive tract is distinct from neuropeptide Y and pancreatic polypeptide and identical with authentic peptide YY except in the antral mucosa where only a small proportion of the peptide YY-immunoreactive material eluted like authentic peptide YY.

In vivo modulation of gastrointestinal motor activity by Met-enkephalin, morphine and enkephalin analogs in chickens

The main objectives of this work have been to study (1) whether or not opioid effects on gastrointestinal motility in chicken are mediated through central or peripheral pathways; (2) the receptors involved; (3) the correlation of the motor response to the distribution of Met-enkephalin in the gastrointestinal tract and (4) to evaluate the physiological role of endogenous opioids in spontaneous MMCs. Intravenous infusion of Met-enkephalin and morphine (5 x 10(-7) mol/kg) induced gastric inhibition and a migrating intestinal hyperactivity. Induced intestinal activity was faster in vagotomized chickens. In the stomach there was a correlation between the duration of the inhibitory response and the affinity of the agonists for mu-receptors ([D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAGO) > morphine > Met-enkephalin > Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE)). DPDPE induced duodenal hyperactivity which was not propagated. Immunohistochemistry showed that Met-ENK like material is mainly located at the myenteric plexus and the outer circular muscle in stomach. In the intestine, it was found in the myenteric and the deep muscular plexuses. When endogenous MMCs were studied, i.v. infusion of naloxone lengthened significantly their duration. In conclusion, the fact that Met-enkephalin and their analogs induced a migrating activity in the intestine and the lengthening of MMC by naloxone, suggest a physiological role for opioids on induction of MMC in birds, acting at peripheral level.

Regulatory role of enteric kappa opioid receptors in human colonic motility

The effects of different kappa opioid agonists and antagonists on spontaneous mechanical activities and responses to electrical transmural nerve stimulation of both longitudinal and circular muscle strips from the human sigmoid colon were studied. A superfusion apparatus was used to record isometric contractions. Exogenously added kappa agonists did not modify spontaneous contractile activities on either type of strip. Nerve stimulation induced a triphasic response composed of a first contraction (C1) followed by a relaxation (C2) and an off-contraction (C3); this response was mediated by cholinergic excitatory nerves and non-adrenergic, non-cholinergic excitatory and inhibitory nerves. Dynorphin 1-13 and the synthetic kappa agonist trans-3,4-dichloro-N-methyl-N-(2- [1pyrolidinyl]-cyclohexyl) dramatically decreased the amplitude of the excitatory components C1 and C3. The effects of both kappa agonists were blocked in presence of the kappa antagonist Nor-Binaltorphimine. The delta antagonist ICI 174864 did not prevent the inhibition of the contractions C1 and C3 induced by dynorphin. Therefore, these data suggest that kappa receptors are involved in the neuroregulation of smooth muscle of human colon and mediate inhibition of cholinergic and non-cholinergic excitatory transmission within myenteric plexus.

Immunohistochemistry and nerve lesion experiments on the methionine-enkephalin immunopositive neurons in the small intestine of the bullfrog (Rana catesbeiana)

Nerve elements in the small intestine of the bullfrog. Rana catesbeiana, were studied by immunohistochemistry with anti-methionine enkephalin antisera and by nerve lesion experiments, using laser irradiation. Methionine-enkephalin immunopositive nerve fibers occur in the myenteric plexus, circular muscle layer, submucosa, and mucosa. Immunopositive nerve cell bodies in the myenteric plexus have dendrite-like and a long axon-like processes. In the froglet (3 months after metamorphosis), these axon-like processes lead posteriorly in the nerve strand of the myenteric plexus. Some bifurcate, one branch continuing posteriorly, the other doubling back to lead anteriorly; both form terminal varicose fibers in the circular muscle layer. Nerve lesion experiments, in the adult bullfrog, resulted in accumulations of methionine-enkephalin immunoreactivity at the oral and hinder edges of the laser-irradiated necrotic area; there were sprouting and nonsprouting immunopositive stumps. It is suggested that bidirectional flow of methionine-enkephalin in the myenteric plexus is mediated via the anterior and posterior branches of the axon-like process. The difference in sprouting behavior of immunopositive nerve fiber stumps, after nerve lesion, is discussed with reference to regional differences of the axon-like process.

Distribution and origin of extrinsic nerve fibers containing calcitonin gene-related peptide, substance P and galanin in the rat upper rectum

The distributions of nerve fibers containing calcitonin gene-related peptide (CGRP), substance P (SP) and galanin (GAL) were examined in the rat rectum of mutants rats, aganglionic rats (AGRs), which completely lack the intramural nerve cells in the large intestine, and of their normal littermates. The origin of extrinsic peptide-containing nerve fibers was examined using retrograde tracing combined with immunohistochemistry in normal rats. In the rectum of normal rats, CGRP-, SP- and GAL-immunoreactive varicose fibers were observed throughout all layers of the rectal wall, and immunoreactive nerve cells were present in the enteric ganglia of colchicine-treated rats. In the aganglionic rectum of AGR, a rich supply of CGRP-immunoreactive fibers was observed in the mucosa, around the blood vessels, and in the submucous and intermuscular spaces. SP- and GAL-immunoreactive fibers in the aganglionic rectum showed a similar distribution to CGRP-immunoreactive fibers but were less dense. These results suggest that most of CGRP-positive fibers in the rectum are extrinsic whereas a large part of SP- or GAL-positive fibers are intrinsic. Fluoro-gold injected into the upper rectum of normal rat labelled nerve cells (less than 10% of total ganglion cells) in the lumbar (L1 and L2) and lumbosacral (L6 and S1) dorsal root ganglia. More than half of nerve cells in the dorsal root ganglia (L6 and S1) projecting to the rectum were immunoreactive for CGRP, and less than 10% were immunoreactive for SP or GAL. Comparison of serial sections of the dorsal root ganglion revealed that about half of the CGRP-immunoreactive cells were also positive for SP or GAL. These results indicate that SP- or GAL-positive neurons projecting to the rectum are scarce in the dorsal root ganglia. The present investigation suggests that CGRP-containing nerves are visceral afferents forming a major component of the sensory innervation of the rat rectum, and SP- and GAL-containing nerves which share their extrinsic origins appear to form a lesser proportion of the sensory innervation.

Responses of porcine gastric and duodenal smooth muscle to VIP

1. Mechanical activity was recorded in isolated muscle preparations from circular and longitudinal layers of gastric fundus, corpus and antrum and from the duodenum of pigs, using conventional organ bath technique. Rectangular current pulses were applied to the muscle strips for electrical field stimulation (EFS). 2. Fundic and circular corpus preparations developed a marked spontaneous tonic activity. Vasoactive intestinal polypeptide (VIP, 10(-9)-10(-7) mol l-1) inhibited this spontaneous activity. This inhibitory effect was not affected by application of tetrodotoxin (TTX) showing its myogenic nature. 3. Pretreatment of fundic and circular corpus preparations with VIP reduced the excitatory responses to substance P, bombesin, serotonin and histamine, but it had no effect on the acetylcholine (ACh)-induced tonic and phasic activity. 4. Longitudinal duodenal preparations showed purely phasic activity which was almost insensitive to VIP. In circular duodenal preparations particularly strong spontaneous tonic contractions were observed which could be inhibited by VIP. 5. Circular duodenal preparations excised 3-5 cm postpyloric had a spontaneous tone which could reach up to 80% of the maximum contractions induced by 10(-4) mol l-1 ACh. These preparations were chosen for further pharmacological studies and for experiments with EFS. VIP was the most powerful substance for the inhibition of spontaneous tone, followed by serotonin, PGE2 and bradykinin. This type of preparation exhibited particularly strong inhibitory effects to EFS; even single stimuli could induce near maximum relaxation. The inhibition induced by EFS was unaffected by treatment with ATP, guanethidine, atropine, methysergide and apamin. TTX completely abolished the EFS-induced relaxation, showing its neurogenic nature. 6. Porcine circular duodenum is a good model for studying the transmitter system of the non-adrenergic, non-cholinergic (NANC) innervation. The results are consistent with the assumption that VIP is the transmitter in this system, although the very slow time-course of the VIP-induced inhibition in comparison with the EFS-induced inhibition is not consistent with this notion.

Substance P and neurokinin A are codistributed and colocalized in the porcine gastrointestinal tract

Immunoreactive substance P and neurokinin A were measured with radioimmunoassay in extracts of different segments of porcine gastrointestinal tract using C-terminally directed antisera. In all segments, the concentrations of substance P and neurokinin A were similar. The largest concentrations of both peptides were found in the mid-colon. By gel chromatography and reversed-phase high pressure liquid chromatography the immunoreactivity in extracts from ileum eluted as homogenous peptides at the positions of synthetic substance P and neurokinin A, respectively. No neurokinin B was found. By immunohistochemistry of porcine duodenum, jejunum, ileum and mid-colon, identical localization patterns were found for substance P and neurokinin A, and the two peptides demonstrated by double immunofluorescence to be colocalized in the enteric nervous system of the ileum. We conclude that the tachykinins substance P and neurokinin A are codistributed and colocalized in the procine gastrointestinal tract and suggest that the two peptides are produced from a common precursor, beta- and/or gamma-preprotachykinin, in the same neurons.

Stimulation of circular muscle motility of the isolated perfused canine ileum: relationship to VIP output

Perfusion of ileal segments with tetrodotoxin; opioids, Met-enkephalin and dynorphin; and alpha 2 adrenoceptor agonist, BHT920, increased motility concomitant with the decreased VIP output into the venous effluent reported previously. This suggested that increased motility resulted from release of the muscle from tonic inhibition when VIP output was reduced sufficiently. However, blockade of nicotinic receptors also reduced VIP output but did not induce motility. Thus release of myogenic activity from inhibition is not a sufficient explanation for increased motility and a further excitatory mediator is required. Field stimulation of nerves increased VIP output and delayed distal contractions, suggesting that VIP does participate in the canine ileal distal inhibition reflex.

Receptors for neurotensin in canine small intestine

We have previously characterized the neurotensin receptors on the circular smooth muscle (CM) of the canine small intestine (1). In the present studies, using radioligand binding technique, neurotensin receptors were localized on the membranes from deep muscular (DMP) and the submucous plexus while no binding was observed on either the longitudinal smooth muscle or myenteric plexus membranes. The high affinity binding sites (Kd 0.1-0.2 nM) on DMP membranes were similar to those on CM; the low affinity component was of much lower affinity (Kd approximately 40 nM). DMP had 4-6 times higher density of binding sites than the CM. The recognition properties of DMP receptors were similar to those on the CM and reduced sulfhydryl groups were required for the binding activity. The action of neurotensin on the contractility of the canine small intestine, therefore, appears to be through a direct action on the circular smooth muscle and through the prejunctional action on the DMP neurons through distinct receptors. Thiol groups in the neurotensin receptors may be important for the receptor function.

Effect of opioid peptides on circular muscle of canine duodenum

1. The effects of opioid peptides on inhibitory transmission in the circular muscle layer of canine duodenum were investigated in vitro using simultaneous mechanical and intracellular electrical recording techniques. 2. Exogenously added [Met5]enkephalin, [Leu5]enkephalin and dynorphin (1-13) decreased the amplitude of non-adrenergic, non-cholinergic inhibitory junction potentials (IJPs) evoked by transmural nerve stimulation. 3. A selective delta-receptor agonist, DPDPE ([D-Pen2, D-Pen5]enkephalin), and a selective mu-receptor agonist, PL017 (Try-Pro-NMePhe-D-Pro-NH2), decreased the amplitude of IJPs whereas a selective kappa-receptor agonist, U-50,488H ([trans-3,4-dichloro-N-methyl-N-(2-91-pyrolidinyl)-cyclohexyl]- benzeneacetamide methanesulphonate), in large doses (1 microM) produced only a small reduction. 4. A selective delta-receptor antagonist, ICI-174,864, blocked the effect of DPDPE but not that of PL017 suggesting the presence of distinct delta- and mu-opioid receptors on inhibitory motor nerves. 5. Exogenously added dynorphin (1-13) decreased the amplitude of IJPs. delta-Opioid receptors appeared to be involved because ICI-174,864, a selective delta-antagonist, blocked the inhibitory effect of exogenously added dynorphin (1-13). 6. The inhibitory effect of the opioid peptides was still observed in preparations of circular muscle devoid of myenteric and submucosal plexuses, indicating that the site of action was on inhibitory motor nerve fibres located within the circular muscle layer and not on neuronal cell bodies in the enteric plexuses. 7. It was concluded that in the canine small intestine, opioid peptides could modulate release of inhibitory transmitter(s) at or near nerve terminals of inhibitory motor nerves innervating circular muscle cells.

The innervation of the gastrointestinal tract of a chelonian reptile, Pseudemys scripta elegans. II. Distribution of neuropeptides in the myenteric plexus

The myenteric plexus of the stomach, midgut and hindgut of the red-eared turtle, Pseudemys scripta elegans, has been investigated for the occurrence of immunoreactivity to nine neuropeptides. Neuropeptide Y (NPY)-, calcitonin gene-related peptide (CGRP)-, bombesin (BOM)- as well as substance P (SP)-like immunoreactivity (LI) were found in nerve fibres of all investigated gut regions. From all peptides investigated immunoreactivity for NPY was more pronounced. In the stomach NPY-LI was mainly found in the perikarya, while in the midgut region both NPY-immunoreactive (IR) somata and nerve fibres were revealed. The hindgut harboured few NPY-IR nerve cells and nerve fibres. A few SP-IR nerve cell bodies were observed in the stomach and midgut region. In the hindgut BOM-IR neuronal cell bodies were found. Neuromedin U (NMU)-LI was mainly observed in the stomach region, revealing both immunoreactive perikarya and nerve fibres. Immunoreactivity for vasoactive intestinal polypeptide, somatostatin, galanin and enkephalin could not be detected so far. Double labelling experiments revealed the coexistence of CGRP and SP in some nerve fibres in all three gut regions examined. Some SP-IR fibres in the midgut were immunoreactive for NMU.

Immunohistochemical study of peptide-containing nerves in the gastrointestinal tract of the Japanese field vole, Microtus montebelli

The distribution and relative frequency of nerves containing immunoreactivity for substance P (SP), vasoactive intestinal polypeptide (VIP), gastrin-releasing polypeptide (GRP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY) and menthionine-enkephalin (MENK) were studied by immunohistochemistry in the gastrointestinal tract of the herbivorous Japanese field vole, Microtus montebelli.

Distinct distribution of CGRP-, enkephalin-, galanin-, neuromedin U-, neuropeptide Y-, somatostatin-, substance P-, VIP- and serotonin-containing neurons in the two submucosal ganglionic neural networks of the porcine small intestine

In addition to differences between the two submucosal ganglionic neural networks, i.e., the plexus submucosus externus (Schabadasch) and the plexus submucosus internus (Meissner), with respect to the occurrence and distribution of serotonin as neurotransmitter, immunocytochemistry also revealed a distinct distribution for various neuropeptides in these two plexuses. Immunoreactivity for galanin, vasoactive intestinal polypeptide, calcitonin gene-related peptide, substance P, neuromedin U, enkephalin, somatostatin and neuropeptide Y was found in varicose and non-varicose nerve fibres of both submucosal ganglionic plexuses, albeit with a distinct distributional pattern. The difference in neurotransmitter and/or neuromodulator content between both neural networks became even more obvious when attention was focussed on the immunoreactivity of the nerve cell bodies for these substances. Indeed, neuropeptide Y, enkephalin- and somatostatin-immunoreactive neuronal perikarya as well as serotonergic neuronal cell bodies appear solely in the plexus submucosus externus. Neuromedin U-immunoreactive perikarya, mostly coexisting with substance P, are observed in large numbers in the plexus submucosus internus, whilst they are rare in the plexus submucosus externus. Double-labelling immunostaining for substance P with CGRP and galanin revealed a different coexistence pattern for the two submucosal ganglionic plexuses. The differing chemical content of the neuronal populations supports the hypothesis that the existence of the two submucosal ganglionic plexuses, present in most large mammals including man, not only reflects a morphological difference but also points to differentiated functions.

Canine myenteric, deep muscular, and submucosal plexus preparations of purified nerve varicosities: content and chromatographic forms of certain neuropeptides

Partially purified nerve varicosities prepared from canine small intestinal myenteric, deep muscular and submucosal plexuses were found to contain, by radioimmunoassay, gastrin-releasing polypeptide (GRP), substance P, Leu-enkephalin, Met-enkephalin, vasoactive intestinal polypeptide (VIP) and neurokinin A, but did not contain detectable amounts of neurokinin B. In all three plexus preparations, VIP was present in the highest concentration. In contrast to other species, GRP and the enkephalins were found to be present in relatively high concentrations in the submucosal plexus and GRP was present in low concentrations in the deep muscular plexus. Equal concentrations of substance P and neurokinin A were found in the myenteric and deep muscular plexus preparations but greater concentrations of substance P relative to neurokinin A were found in the submucosal plexus preparations. On reverse phase HPLC, a major peak of immunoreactivity occurred at the retention times of standard preparations for all six neuropeptides measured. Significant heterogeneity was found for GRP- and VIP-like immunoreactivity, especially in the submucosal plexus preparations. These partially purified canine small intestine nerve varicosity preparations may prove of value in studying release mechanisms for, and the posttranslational processing of, neuropeptides.

Neurotensin binding sites in porcine jejunum: biochemical characterization and intramural localization

Neurotensin is present in high concentrations in the mammalian gut, especially in enteroendocrine cells of the mucosa. Exogenous neurotensin has been shown to alter ion transport by the mucosa and contractile activity of intestinal smooth muscle. The purpose of this study was to determine the distribution of neurotensin binding sites within the intestinal wall. Initially, biochemical characteristics of [125I]neurotensin binding sites were determined within two preparations of the distal porcine jejunum: (1) the mucosa and submucosa, and (2) the circular and longitudinal muscle with their intramural plexuses. Ligand binding data for the preparation including the mucosa and submucosa indicated that [125I]neurotensin bound specifically to two sites having apparent equilibrium dissociation constants of approximately 0.046 and 0.37 nM. A binding site with a dissociation constant of approximately 0.38 nM was confirmed for the preparation of muscle and associated intramural plexuses. Xenopsin and neurotensin were equipotent to neurotensin in competing for these binding sites; neuromedin N was approximately 40 times less potent in the preparation of mucosa and submucosa. Receptor autoradiography was used to determine the distribution of [125I]neurotensin binding sites within the wall of the jejunum. Autoradiograms of [125I]neurotensin bound to cross sections of the proximal and distal jejunum showed that the highest densities of silver grains were associated with the internal submucosal ganglia, external submucosal plexus and myenteric ganglia. A moderate density of silver grains was associated with the circular muscle. The localization of neurotensin binding sites to submucosal ganglia is consistent with observations that neurotensin effects on active anion secretion by the mucosa are blocked by tetrodotoxin. Immunohistochemical localization of neurotensin in the porcine jejunum demonstrated a limited population of neurotensin immunoreactive cells within the mucosal epithelium. It is possible that neurotensin released from these cells in the mucosa as well as neurotensin-related peptides released from enteric neurons may be the endogenous ligands for the binding sites visualized in this study.

Neurotensin receptors on circular smooth muscle of canine small intestine: role of disulfide bridges

We have studied the effect of sulfhydryl agents on the binding of 125I-tyr3-neurotensin to the purified plasma membranes from circular smooth muscle and on the in vitro response of circular muscle strips of canine small intestine to neurotensin. Dithiothreitol (DTT) enhanced the binding by about 80%. Cysteine (a reductant) also enhanced the binding while cystine (an oxidant) reduced the binding to the similar extent. DTT stimulated the tissue in the organ bath and abolished the stimulatory response to low concentrations of neurotensin. The stimulatory response to acetylcholine was not altered by DTT. The implications of the role of disulfide bridges in the neurotensin response is discussed.

Autoradiographic localization of sigma opioid receptors in the gastrointestinal tract of the guinea pig

The distribution of sigma and phencyclindine binding sites in the gastrointestinal tract of the guinea pig was studied by autoradiography after in vitro incubation of tissue slices with (+)3H-SKF 10,047 and 3H-1-1-[(2-thienyl)cyclohexyl] piperidine to locate sigma and phencyclidine receptors, respectively. A dense distribution of sigma binding sites was observed in the mucosa and in the submucosal plexus, particularly at the level of the fundus and duodenum. Muscular layers were only marginally labeled. No phencyclidine binding site could be demonstrated in any area. This selective distribution suggests a functional role of sigma receptors mainly in the control of endocrine or exocrine secretions, or both.

Involvement of substance P neurons in contractions of canine small intestine produced by mesenteric nerve stimulation

Pathways for contractions of in vivo canine small intestine produced by mesenteric nerve stimulation (MNS) were studied. In intact and chronically sympathectomized dogs, contractions of jejunal and ileal segments were largely reduced by intra-arterial infusion of capsaicin (10-100 microM, 0.07 ml/min), substance P (SP) antagonist, (D-Pro4, D-Trp7.9) SP (4-11) (100 microM, 0.14 ml/min), hexamethonium (100-1000 microM, 0.07 ml/min) or atropine (100 microM, 0.07 ml/min). In chronically vagotomized dogs, capsaicin, SP-antagonist or atropine significantly reduced MNS-induced contractions, but hexamethonium did not. In dogs in which the coeliac and superior mesenteric ganglia had previously been removed, MNS caused no response although intra-arterial injection of 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP, 0.1 mumol) caused marked contractions. It may therefore be suggested that extrinsic SP neurons probably originating in spinal ganglia and intrinsic SP neurons receiving input from vagal preganglionic cholinergic neurons are involved in the excitatory pathways to MNS-induced contractions and that activation of these neurons excites myenteric cholinergic neurons, thereby causing contractions of the small intestine.

Actions of gastrin-releasing peptide and related mammalian and amphibian peptides on ion transport in the porcine proximal jejunum

The 27-amino acid peptide gastrin-releasing peptide (GRP) and the decapeptide neuromedin B (NMB) are structurally related to bombesin (BB) and exist within the mammalian small intestine. We examined the actions of porcine GRP and NMB on ion transport in the porcine proximal jejunum in vitro and compared their activities to those of their respective C-terminal amphibian homologs BB and ranatensin (RT). The 4 peptides transiently increased potential difference and short-circuit current (Isc) in jejunal mucosal sheets after their serosal administration in subnanomolar concentrations with an order of potency: GRP approximately RT greater than or equal to NMB greater than BB. BB and RT were more effective in elevating Isc than GRP and NMB; all peptides had variable effects on tissue conductance. Mucosal Isc responses to GRP (1 nM) were due in part to a stimulation of net Cl- secretion. GRP-induced Isc increases were halved by serosal furosemide (0.3 mM) and reduced by 65% and 90% in tissue bathing solutions lacking Cl- or Cl- and HCO3-, respectively. Tetrodotoxin reduced Isc responses to the peptide by 40%; GRP activity remained unaffected after blockade of gut muscarinic or nicotinic cholinergic receptors by atropine or hexamethonium, respectively. These results suggest that GRP and its natural homologs stimulate active electrogenic Cl- secretion in the porcine jejunum through interactions with GRP receptors located in the intestinal mucosa and submucosa.

Quantification and characterization of enkephalins in the upper part of the cat digestive tract and the coeliac ganglia

The [Met]enkephalin, [Leu]enkephalin and [Met]enkephalin-arg-gly-leu contents of the upper part of the digestive tract (lower oesophageal sphincter, fundus, antrum, pylorus, duodenum, ileum) and coeliac ganglia of the cat were determined and identified. The enkephalin content of all the structures studied, expressed in femtomole/mg of wet tissue, was found to range from 83 to 446 with [Met]enkephalin; 19 to 63 with [Leu]enkephalin; 2.5 to 13 with [Met]enkephalin-arg-gly-leu. In the muscular and plexus layers the [Met]- and [Leu]enkephalin contents increase gradually from the lower oesophageal sphincter to the pylorus and then decrease from the duodenum to the ileum. The [Met]enkephalin versus [Leu]enkephalin ratio is 2.7 in the coeliac ganglia and ranges from 4.3 to 8.1 in the areas of the digestive tract investigated. In addition, the presence of authentic [Met]- and [Leu]enkephalin was confirmed in all the structures assayed by high pressure liquid chromatography. Owing to the low amounts of [Met]enkephalin-arg-gly-leu detected in individual samples of the coeliac ganglia and in the areas of the digestive tract investigated, it was not possible to characterize this peptide using high pressure liquid chromatography and therefore to confirm the presence of authentic [Met]enkephalin-arg-gly-leu in these structures. The differences in the enkephalin concentrations observed among these various areas of the digestive tract suggest that these peptides may act differently from one area to another, thus playing a complex integrative role in the nervous control of gastrointestinal tract motility.

Meal-stimulated release of methionine-enkephalin into the canine jejunal lumen

Application of enkephalins to the luminal surface of the bowel augments intestinal absorption. However, to date, endogenous enkephalins have not been demonstrated within intestinal luminal fluid. To determine whether enkephalins are present in the intestinal lumen, five adult dogs had 25-cm chronic jejunal Thiry-Vella loops constructed. Dogs were studied in the awake, fasted state. Jejunal loops were perfused with isoosmotic, neutral Krebs buffer containing protease inhibitors. After basal sampling, the dogs received a high fat meat meal. Collections were made during the meal and for 60 min postprandially. Luminal met-enkephalin levels were determined by radioimmunoassay and confirmed by HPLC. HPLC separation of luminal samples demonstrated two immunoreactive peaks which co-eluted with pure met-enkephalin and met-enkephalin-sulfoxide. Basal met-enkephalin outputs averaged 52 +/- 13 ng/min. The meal significantly increased mean luminal met-enkephalin output to 137 +/- 71 ng/min. During the initial 20-min postprandial period, output remained elevated (180 +/- 73 ng/min), after which it returned to basal levels. We conclude that met-enkephalin is present in the jejunal lumen, and that luminal release of this opioid is augmented by a meal.

The role of vasoactive intestinal polypeptide in the inhibition of antral and pyloric electrical activity in rabbits

The nature of the neurotransmitter released by intramural non-cholinergic non-adrenergic inhibitory neurones and the type of vagal afferents involved in the inhibition of antral and pyloric electrical activity induced by vagal afferent stimulation were investigated in conscious rabbits in which both splanchnic nerves had previously been cut. The inhibitory effect of duodenal distension was reversed by bilateral thoracic vagotomy. Either intravenous or intra-aortic infusions of vasoactive intestinal polypeptide (VIP) inhibited the electrical activity of both antrum and pylorus for several minutes. Electrical stimulation of afferent vagal fibres and duodenal distension both inhibited antral and pyloric activity and produced a significant increase of portal plasma VIP concentration. Numerous VIP-immunoreactive fibres were found to be present in the muscular layers of the pylorus. The possibility that this form of inhibition is mediated by VIPergic fibres is discussed as is the likely involvement of vagal afferent fibres in the case of the response to duodenal distension.

Structural and functional consequence of neonatal sympathectomy on the blood vessels of spontaneously hypertensive rats

Neonatal sympathectomy of spontaneously hypertensive rats (SHR) and control Wistar-Kyoto rats (WKY) was performed by a combined treatment with antiserum to nerve growth factor and guanethidine during the first 4 weeks after birth. The development of hypertension was completely prevented in the treated SHR: at 28 to 30 weeks of age, systolic blood pressure of treated SHR was 139 +/- 2 mm Hg as compared with 195 +/- 8 mm Hg in untreated SHR. The extent of sympathectomy was verified by histofluorescence. Fluorescence histochemistry for catecholamine-containing nerves showed a complete absence of adrenergic nerves in the mesenteric arteries of treated rats. A supersensitivity to norepinephrine was exhibited by mesenteric arteries, anococcygeus muscle, and tail arteries from the treated SHR and WKY. In the mesenteric vascular bed, maximal response to norepinephrine was significantly reduced by sympathectomy. Sympathectomy also abolished the responses (e.g., generation of excitatory junctional potentials) of tail arteries to electrical stimulation of perivascular nerves. Morphometric measurements of three categories of mesenteric arteries showed that sympathectomy had no effect on the hypertrophic change of smooth muscle cells in the conducting vessels, but it prevented the hyperplastic changes of the muscle cells from reactive, muscular arteries and small resistance vessels. These results suggest that one of the primary roles of the overactive sympathetic nervous system in the development of hypertension in SHR is manifested through its trophic effect on the arteries of SHR. This trophic effect appears to cause a hyperplastic change in the smooth muscle cells in the reactive and resistance vessels, thereby contributing to the development of hypertension in older SHR.

Distribution of peptide-containing neurons and endocrine cells in the rabbit gastrointestinal tract, with particular reference to the mucosa

The distribution patterns of peptide-containing neurons and endocrine cells were mapped in sections of oesophagus, stomach, small intestine and large intestine of the rabbit, by use of standard immunohistochemical techniques. Whole mounts of separated layers of ileum were similarly examined. Antibodies raised against vasoactive intestinal peptide (VIP), substance P (SP), somatostatin (SOM), neuropeptide Y (NPY), enkephalins (ENK) and gastrin-releasing peptide (GRP) were used, and for each of these antisera distinct populations of immunoreactive (IR) nerve fibres were observed. Endocrine cells were labelled by the SP, SOM or NPY antisera in some regions. VIP-IR nerve fibres were common in each layer throughout the gastrointestinal tract. With the exception of the oesophagus, GRP-IR nerve fibres also occurred in each layer of the gastrointestinal tract; they formed a particularly rich network in the mucosa of the stomach and small intestine. Fewer nerve fibres containing NPY-IR or SOM-IR were seen in all areas. SOM-IR nerve fibres were very scarce in the circular and longitudinal muscle layers of each area and were absent from the gastric mucosa. The SP-IR innervation of the external musculature and ganglionated plexuses in most regions was rather extensive, whereas the mucosa was only very sparsely innervated. ENK-IR nerve fibres were extremely rare or absent from the mucosa of all areas, although immunoreactive nerve fibres were found in other layers. These studies illustrate the differences in distribution patterns of peptide-containing nerve fibres and endocrine cells along the gastrointestinal tract of the rabbit and also show that there are some marked differences in these patterns, in comparison with other mammalian species.

Neurotensin receptors in canine intestinal smooth muscle: preparation of plasma membranes and characterization of (Tyr3-125I)-labelled neurotensin binding

To study the binding of (Tyr3-125I)-labelled neurotensin to intestinal muscle, plasma membranes have been purified from dog intestinal circular smooth muscle. Purification was done by differential centrifugation followed by separation on a sucrose gradient. Electron microscopic study revealed that the dissected circular muscles used as the source of membranes were free of myenteric plexus and that the plasma membrane fraction obtained was free of any mitochondria or synaptosomes. The fraction used was obtained at the interface of 14%-33% sucrose density on the gradient and was 25-times enriched in the plasma membrane marker enzyme 5'-nucleotidase activity as compared to post-nuclear supernatant. This fraction contained negligible activity of mitochondrial membrane marker enzyme cytochrome c oxidase and low activity of a putative endoplasmic reticulum marker enzyme NADPH-cytochrome-c reductase. This membrane fraction contained a high density of neurotensin binding sites. This binding was studied by kinetic and by saturation approaches. Analysis of data from saturation binding studies by the computer programs (EBDA and LIGAND) suggested the presence of a two-site model (Kd1 = 0.118 nM, Kd2 = 3.18 nM, Bmax1 = 9.73 fmol/mg and Bmax2 = 129.8 fmol/mg). A part of specifically bound neurotensin was rapidly dissociated. No cooperativity between the two receptor types could be detected. A kinetic analysis of binding gave the Kd value equal to 0.107 nM. Carboxy terminal amino acid residues 8-13 were found to be essential for the binding activity and replacement of Tyr11 by tryptophan reduced the affinity of the peptide by 10 times in displacement studies. Binding was modulated by sodium ions and a guanine nucleotide Gpp[NH]p. MgCl2, CaCl2 and KCl were also found to reduce the specific binding. Evidence was found of a high specific binding to another membrane fraction poor in plasma membranes and rich in synaptosomes. We concluded that plasma membrane of canine intestinal circular muscle contains neurotensin receptors with recognition properties distinct from those obtained in previous studies of neurotensin binding sites in murine tissues. Another neurotensin binding site may be present on neuronal membranes.

Tachykinin activation of muscarinic inhibition in canine small intestine is SPP in nature

Substance P when injected intraarterially into the small intestine of the anaesthetized dog during phasic activity produces three concentration dependent responses of the circular muscle. At lowest doses (approximately 10(-12) moles) inhibition occurs via release of acetylcholine to a muscarinic auto-receptor. At slightly higher doses (10(-10) moles) inhibition is preceeded by excitation via release of acetylcholine to muscarinic receptors on the smooth muscle. At still higher doses (10(-9) moles) substance P excites the smooth muscle directly. The present study demonstrates that other members of the tachykinin family also produce inhibition in vivo. The potency sequence was found to be physalaemin greater than or equal to substance P = neuromedin K greater than kassinin greater than alpha neurokinin = eledoisin. Such a sequence suggests that substance P is a natural stimulant of this pathway and that the receptor is SPP-like. The C-terminal fragment, substance P8-11, was a weak agonist at this receptor, while substance P1-9 was ineffective.