Differences in control of descending inhibition in the proximal and distal regions of rat colon

Functional and Transcriptomic Characterization of Postnatal Maturation of ENS and SIP Syncytium in Mice Colon

The interplay of the enteric nervous system (ENS) and SIP syncytium (smooth muscle cells-interstitial cells of Cajal-PDGFRα+ cells) plays an important role in the regulation of gastrointestinal (GI) motility. This study aimed to investigate the dynamic regulatory mechanisms of the ENS-SIP system on colon motility during postnatal development. Colonic samples of postnatal 1-week-old (PW1), 3-week-old (PW3), and 5-week-old (PW5) mice were characterized by RNA sequencing, qPCR, Western blotting, isometric force recordings (IFR), and colonic motor complex (CMC) force measurements. Our study showed that the transcriptional expression of Pdgfrα, c-Kit, P2ry1, Nos1, and Slc18a3, and the protein expression of nNOS, c-Kit, and ANO1 significantly increased with age from PW1 to PW5. In PW1 and PW3 mice, colonic migrating movement was not fully developed. In PW5 mice, rhythmic CMCs were recorded, similar to the CMC pattern described previously in adult mice. The inhibition of nNOS revealed excitatory and non-propulsive responses which are normally suppressed due to ongoing nitrergic inhibition. During postnatal development, molecular data demonstrated the establishment and expansion of ICC and PDGFRα+ cells, along with nitrergic and cholinergic nerves and purinergic receptors. Our findings are important for understanding the role of the SIP syncytium in generating and establishing CMCs in postnatal, developing murine colons.

Analysis of spatiotemporal pattern and quantification of gastrointestinal slow waves caused by anticholinergic drugs

Anticholinergic drugs are well-known to cause adverse effects, such as constipation, but their effects on baseline contractile activity in the gut driven by slow waves is not well established. In a video-based gastrointestinal motility monitoring (GIMM) system, a mouse's small intestine was placed in Krebs solution and recorded using a high definition camera. Untreated controls were recorded for each specimen, then treated with a therapeutic concentration of the drug, and finally, treated with a supratherapeutic dose of the drug. Next, the video clips showing gastrointestinal motility were processed, giving us the segmentation motions of the intestine, which were then converted via Fast Fourier Transform (FFT) into their respective frequency spectrums. These contraction quantifications were analyzed from the video recordings under standardised conditions to evaluate the effect of drugs. Six experimental trials were included with benztropine and promethazine treatments. Only the supratherapeutic dose of benztropine was shown to significantly decrease the amplitude of contractions; at therapeutic doses of both drugs, neither frequency nor amplitude was significantly affected. We have demonstrated that intestinal slow waves can be analyzed based on the colonic frequency or amplitude at a supratherapeutic dose of the anticholinergic medications. More research is required on the effects of anticholinergic drugs on these slow waves to ascertain the true role of ICC in neurologic control of gastrointestinal motility.

Effects of 5-HT2B, 5-HT3 and 5-HT4 receptor antagonists on gastrointestinal motor activity in dogs

AIM: To study the effects of 5-hydroxytryptamine (5-HT) receptor antagonists on normal colonic motor activity in conscious dogs.

METHODS: Colonic motor activity was recorded using a strain gauge force transducer in 5 dogs before and after 5-HT2B, 5-HT3 and 5-HT4 receptor antagonist administration. The force transducers were implanted on the serosal surfaces of the gastric antrum, terminal ileum, ileocecal sphincter and colon. Test materials or vehicle alone was administered as an intravenous bolus injection during a quiescent period of the whole colon in the interdigestive state. The effects of these receptor antagonists on normal gastrointestinal motor activity were analyzed.

RESULTS: 5-HT2B, 5-HT3 and 5-HT4 receptor antagonists had no contractile effect on the fasting canine terminal ileum. The 5-HT3 and 5-HT4 receptor antagonists inhibited phase III of the interdigestive motor complex of the antrum and significantly inhibited colonic motor activity. In the proximal colon, the inhibitory effect was dose dependent. Dose dependency, however, was not observed in the distal colon. The 5-HT2B receptor antagonist had no contractile effect on normal colonic motor activity.

CONCLUSION: The 5-HT3 and 5-HT4 receptor antagonists inhibited normal colonic motor activity. The 5-HT2B receptor antagonist had no contractile effect on normal colonic motor activity.

Muscular vs. Neural Activation in Propulsion Induced by Electrical Stimulation in the Descending Colon of Rats

The present experiments were performed on rat colon to study neurogenic and myogenic elicited propulsion induced by 0.3 and 30 msec long current pulses. The colon segments were stimulated sequentially and randomly. The obtained contractions displaced the intraluminal content in individual propulsion steps. The propulsion steps differed in displacement onset latency, distance, and velocity; the latency decreased while the distance and velocity increased from the proximal to the distal colon segments when performing sequential stimulation; the propulsion steps differed in latency when stimulation was performed randomly; the latency in the first propulsion step was three times longer when using 0.3 vs. 30 msec long pulses. When inhibiting cholinergic transmission by atropine, the propulsion induced by 0.3 msec pulses was blocked, while partially inhibited when using 30 msec pulses. Inhibiting nitric oxide synthesis by N(G) -nitro-L-arginine methyl ester (L-NAME) blocked propulsion induced by both of the pulse durations. In conclusion, electrical stimulation induces propulsion when using both 0.3 and 30 msec long pulses; stimulation using 0.3 msec pulses activates neurons, whereas 30 msec pulses depolarize muscles; in the absence of nitrergic transmission, propulsion cannot be induced by electrical stimulation.

Colon emptying induced by sequential electrical stimulation in rats

Electrical stimulation could be used to induce colon emptying. The present experiments were performed to establish a stimulation pattern to optimize the stimulation parameters and to test neural involvement in propulsion induced by electrical stimulation. Colon segments were sequentially stimulated using rectangular pulses. The resulting propulsive activity displaced intraluminal content in consecutive propulsion steps. The propulsion steps differed in displacement latency, distance, and velocity along the stimulated colon. Increasing the pulse duration or amplitude resulted in a decrease of the latency. Increasing the stimulation amplitude doubled the displacement distance. The frequencies tested in the present study did not affect propulsion. Inhibition of cholinergic and nitrergic pathways inhibited propulsion. Electrical stimulation can induce colonic propulsion. Motor differences are present along the descending colon. The most suitable combination of pulse parameters regarding colon stimulation is 0.3 ms, 5 mA, 10 Hz. Neural circuits are involved in propulsion when using these values.

Neonatal maternal separation causes colonic dysfunction in rat pups including impaired host resistance

Previous studies have shown that early life stress in the form of intermittent maternal separation (MS) predisposes adult rats to develop stress-induced intestinal mucosal dysfunction and visceral hypersensitivity. However, the mechanism involved in the functional abnormalities is unclear. Our aim was to study immature animals during or shortly after exposure to MS to determine whether there are early pathophysiological changes in the gut. Sprague-Dawley rat pups were individually separated from the dam for 3 h/d from 4 to 21 d of age; nonseparated (NS) control pups remained in the home cage with the dam. On d 19-20, d 24-25, and d 29-30, blood was collected for corticosterone measurement, and colonic tissues were removed for functional and morphologic assessment. Corticosteroid levels were elevated in MS pups compared with NS, indicating that MS was indeed stressful. The distal colon demonstrated significantly enhanced ion secretion and macromolecular permeability at d 19-20 and d 24-25, returning to normal by d 29-30. Electron microscopy and bacterial culture studies indicated bacteria adhering to and penetrating into the colonic epithelium of the MS pups at all time points, while such events were rare in NS pups. The pathophysiological changes were inhibited by injecting pups sc with a corticotropin-releasing hormone (CRH) receptor antagonist daily during MS. Our studies indicate that early psychological trauma predisposes neonatal rats to develop persistent mucosal barrier dysfunction, including impaired host defense to luminal bacteria, by a mechanism involving peripheral CRH receptors.

Examination of the role of cholinergic myenteric neurons with the impairment of neural reflexes in the ileum of c-kit mutant mice

Our previous study showed that impairment of ascending and descending neural reflexes in the ileum of the c-kit mutant, W/W(V), mice is due to a loss of interstitial cells of Cajal present at the myenteric plexus region (ICC-MY) in the mutant. In the present study, cholinergic interneurons were thought to be involved in these pathways, since hexamethonium, an antagonist of the nicotinic ACh receptor, significantly inhibited both neural reflexes in wild type mice. Therefore, we examined whether the loss of ICC-MY affects cholinergic interneurons involved in these pathways. Immunohistochemistry with anti-choline acetyltransferase revealed that there was no difference in the numbers of immunopositive cells in the myenteric plexus region between the wild type and mutant mice. In addition, there was no difference in the extent of spontaneous and EFS-evoked ACh release from longitudinal muscle with myenteric plexus preparations between the wild type and mutant mice. Exogenously added nicotine induced contraction or relaxation of ileal circular muscle in the absence or presence of atropine, respectively, to a similar extent in both the wild type and mutant mice. These results suggest that loss of ICC-MY resulted in an impairment of the ascending and descending reflex pathways at the step before activation of cholinergic interneurons.

Ascending contraction and descending relaxation in the distal colon of mice lacking interstitial cells of Cajal

Recently an essential role of interstitial cells of Cajal (ICC) within myenteric plexus (ICC-MY) was suggested in ascending contraction and descending relaxation in the mouse ileum. The role of ICC in these neural reflexes was examined in the distal colonic segments prepared from the wild type and c-kit mutant, W/W(V) mice, in the present study. Localized distension of the segments from the wild type mice by using a small balloon resulted in ascending contraction and descending relaxation. In the segments from the mutant mice, localized distension also induced these neural reflexes similar to those observed in the wild type mice. Immunohistochemical examination demonstrated that ICC-MY and ICC present in muscle layers (ICC-IM) were severely disrupted in the mutant mouse, but only ICC, present within submucosal plexus (ICC-SMP), remained unchanged. In the small strips with ICC-SMP absent prepared from the mutant mouse, electrical field stimulation induced contraction or relaxation in the absence or presence of atropine, respectively. It was suggested that ICC have no important role in the ascending and descending neural reflexes in the mouse distal colon, this is in direct contrast to the role of ICC-MY in the ileum.

Role of the interstitial cells distributed in the myenteric plexus in neural reflexes in the mouse ileum

We examined the role of interstitial cells of Cajal (ICC) in the ascending and descending neural reflexes in the ileal segments prepared from wild type mice and c-kit mutant W/WV mice. Localized distension of the ileal segments from wild type mice with a small balloon caused contraction or relaxation of the circular muscle on the oral or anal side of the distended region, respectively. However, these intestinal reflexes were not induced in the ileal segments from the mutant mice. In the small strips that include the step of the pathways from efferent motor neurons to smooth muscle cells, nerve stimulation induced contraction of circular muscle in the absence of atropine and relaxation in the presence of atropine. The extent of nerve stimulation-induced contractions and relaxations of the ileal circular muscle were similar in wild type and W/WV mice. The responsiveness of ileal circular muscle to exogenously added acetylcholine and Nor-1, a nitric oxide donor, was also unaffected in the mutant ileum. Since previous immunohistochemical study had revealed selective loss of ICC within the myenteric plexus (ICC-MY) in the mutant ileum, it was concluded that ICC-MY have an essential role in ascending and descending neural pathways in the mouse ileum.

Mechanisms involved in carbachol-induced Ca(2+) sensitization of contractile elements in rat proximal and distal colon

1. Mechanisms involved in Ca(2+) sensitization of contractile elements induced by the activation of muscarinic receptors in membrane-permeabilized preparations of the rat proximal and distal colon were studied. 2. In alpha-toxin-permeabilized preparations from the rat proximal and distal colon, Ca(2+) induced a rapid phasic and subsequent tonic component. After Ca(2+)-induced contraction reached a plateau, guanosine 5'-triphosphate (GTP) and carbachol (CCh) in the presence of GTP further contracted preparations of both the proximal and distal colon (Ca(2+) sensitization). Y-27632, a rho-kinase inhibitor, inhibited GTP plus CCh-induced Ca(2+) sensitization more significantly in the proximal colon than in the distal colon. 3. Y-27632 at 10 microm had no effect on Ca(2+)-induced contraction or slightly inhibited phorbol-12,13-dibutyrate-induced Ca(2+) sensitization in either proximal or distal colon. Chelerythrine, a protein kinase C inhibitor, inhibited GTP plus CCh-induced Ca(2+) sensitization in the distal colon, but not in the proximal colon. The component of Ca(2+) sensitization that persisted after the chelerythrine treatment was completely inhibited by Y-27632. 4. In beta-escin-permeabilized preparations of the proximal colon, C3 exoenzyme completely inhibited GTP plus CCh-induced Ca(2+) sensitization, but PKC(19-31) did not. In the distal colon, C3 exoenzyme abolished GTP-induced Ca(2+) sensitization. It inhibited CCh-induced sensitization by 50 % and the remaining component was inhibited by PKC(19-31). 5. These results suggest that both protein kinase C and rho pathways in parallel mediate the Ca(2+) sensitization coupled to activation of muscarinic receptors in the rat distal colon, whereas the rho pathway alone mediates this action in the proximal colon.

[Roles of interstitial cells of Cajal in regulation of motility of the mouse intestine]

The role of interstitial cells of Cajal (ICC) in contractile activity of the gastrointestinal tract has been intensely studied. Among ICC present within various regions of the gastrointestinal tissue, ICC within the myenteric plexus (ICC-MY) and within the submuscular plexus (ICC-SMP) are regarded as the pacemaker. Action potentials initiated in ICC were suggested to propagate to adjacent smooth muscle cells and to induce the spontaneous activity. It was suggested that ACh-mediated contraction and nitric oxide-mediated relaxation were induced after these neuronal signals were transmitted to ICC and then to smooth muscle via the gap junction. This suggestion was based on the findings mainly in the esophagus, stomach, and small intestine by using ICC-deficient mice (W/W(V) and Sl/Sl(d)). In our studies, ICC-MY were shown to be closely associated with neuron-mediated contractile and relaxant responses and to be associated with neural reflexes for peristalsis, since ascending and descending reflexes were not seen in W/W(V) mice. In the distal colon of W/W(V) mice, in which ICC-MY and -IM were lost, these neural responses remained unchanged. It seems likely that ICC-MY and ICC-IM do not have any role in inducing these responses in the distal colon. It is concluded that the extent of association of ICC with the motility and the manner of the association vary from region to region in the gastrointestinal tract.

Morphological properties of zero-stress state in rat large intestine during systemic EGF treatment

Systemic treatment with epidermal growth factor (EGF) induce growth of the large intestine. The aim of this study was to investigate the morphological properties early in the course of EGF-induced large intestinal growth. The effects of systemic EGF treatment on the morphological properties at the zero-stress state along the large intestine were investigated. EGF-treated rats and control rats were allocated into group with EGF treatment for 2, 4, 7, and 14 days (N = 6 for each EGF treatment group except N = 4 for the 14-day group). The controls had saline injected (N = 3 for each group). The excised large intestine was subdivided into four segments: the ascending colon, transverse colon, descending colon, and rectum. The length and weight of each segment was measured. The zero-stress state was obtained by cutting rings of the large intestine radially, and the opening angle was measued on video images. The thickness and cross-sectional area of the mucosa and muscle layers, and the inner and outer circumference were measured. The total colon length did not increase, whereas the weight of the large intestine, muscosal thickness and mass, and inner and outer circumference increased significantly (P < 0.05). The increase was most prominent in the proximal colon after 7 days of EGF treatment. Later no further morphological changes were observed, except for a decrease in mucosal thickness in most segments and in mucosal cross-sectional area in the descending colon. In the controls and during the first week of EGF treatment, the opening angle was approximately 100 degrees. After 14 days the opening angle increased significantly in the ascending and transverse colon to approximately 172 and 135 degrees (P < 0.05). In conclusion, this study demonstrates that systemic EGF treatment caused remodeling of the morphology of the zero-stress state in the large intestine in a time-dependent manner. The growth was most pronounced in the ascending and transverse colon and involved mainly the mucosal layer.

Contribution of NK3 tachykinin receptors to propulsion in the rabbit isolated distal colon

The role of NK3 receptors in rabbit colonic propulsion has been investigated in vitro with the selective agonist, senktide, and two selective antagonists, SR142801 and SB222200. Peristalsis was elicited by distending a rubber balloon with 0.3 and 1.0 mL of water leading to a velocity of 2.2 and 2.8 mm s-1, respectively. At concentrations of 1 nM, senktide inhibited propulsion evoked by both distensions (range 25-40%), whereas at 6 and 60 nmol L-1 facilitated 'submaximal' propulsion by 30%. In the presence of Nomega-nitro-L-arginine (L-NNA, 200 micromol L-1), which per se caused a slight prokinetic effect, 1 nmol L-1 senktide markedly accelerated propulsion (range 35-50%). Hexamethonium (200 micromol L-1) had minor effects on propulsion. In its presence, 60 nmol L-1 senktide significantly inhibited propulsion induced by both stimuli (range 20-50%). SR142801 (0.3, 3 nmol L-1) and SB222200 (30, 300 nmol L-1) facilitated 'submaximal' propulsion (range 20-40%). Conversely, higher antagonist concentrations (SR142801: 30, 300 nM; SB222200: 1, 10 micromol L-1) inhibited propulsion to both distensions by 20%. A combination of SR142801 (300 nmol L-1) plus hexamethonium (200 micromol L-1) induced an approximately four-fold greater inhibition of propulsion than that induced by SR142801 alone. In conclusion, in the rabbit-isolated distal colon, a subset of NK3 receptors located on descending pathways mediates an inhibitory effect on propulsion by activating a NO-dependent mechanism. Another subset of NK3 receptors, located on ascending pathways mediates a facilitative effect involving a synergistic interaction with cholinergic nicotinic receptors.

Orphanin FQ causes contractions via inhibiting purinergic pathway in the rat colon

BACKGROUND & AIMS

We have previously shown that orphanin FQ (OFQ) preferentially stimulates muscle contraction in the rat colon. However, the mechanism of action of OFQ remains unclear.

METHODS

We studied the effects of OFQ on muscle contractions and inhibitory junction potentials (IJPs) in rat colon. The site of action of OFQ was also investigated by in situ hybridization of OFQ receptors.

RESULTS

OFQ (10(-10) to 10(-6) mol/L) caused circular muscle contractions that were blocked by tetrodotoxin (10(-7) mol/L), suggesting the contractions were nerve mediated. Suramin (a nonselective P(2)-purinoceptor antagonist; 10(-4) mol/L) and reactive blue 2 (a P(2Y)-purinoceptor antagonist; 3 x 10(-5) mol/L), but not pyridoxalphosphate-6-azophenyl-2',4' disulfonic acid (PPADS; a P(2X)-purinoceptor antagonist; 3 x 10(-5) mol/L), abolished OFQ-induced colonic contractions. Focal stimulation of interganglionic fiber tracts evoked biphasic IJPs in colonic circular muscle cells. Suramin and reactive blue 2 inhibited the peak amplitude of the IJP, whereas PPADS had no effect. Cumulative addition of OFQ (10(-10) to 10(-6 )mol/L) significantly inhibited the IJPs. In situ hybridization revealed that OFQ receptor messenger RNA was expressed in the colonic myenteric plexus but not in the smooth muscle cells, suggesting that the site of action of OFQ is neuronal.

CONCLUSIONS

These results suggest that OFQ causes muscle contractions by inhibiting purinergic inhibitory motorneurons in the rat colon.

Biomechanical and morphological properties in rat large intestine

Intestinal stress-strain distributions are important determinants of intestinal function and are determined by the mechanical properties of the intestinal wall, the physiological loading conditions and the zero-stress state of the intestine. In this study the distribution of morphometric measures, residual circumferential strains and stress-strain relationships along the rat large intestine were determined in vitro. Segments from four parts of the large intestine were excised, closed at both ends, and inflated with pressures up to 2kPa. The outer diameter and length were measured. The zero-stress state was obtained by cutting rings of large intestine radially. The geometric configuration at the zero-stress state is of fundamental importance because it is the basic state with respect to which the physical stresses and strains are defined. The outer and inner circumferences, wall thickness and opening angle were measured from digitised images. Subsequently, residual strain and stress-strain distributions were calculated. The wall thickness and wall thickness-to-circumference ratio increased in the distal direction. The opening angle varied between approximately 40 and approximately 125 degrees with the highest values in the beginning of proximal colon (F=1.739, P<0.05). The residual strain at the inner surface was negative indicating that the mucosa-submucosal layers of the large intestine in no-load state are in compression. The four segments showed stress-strain distributions that were exponential. All segments were stiffer in longitudinal direction than in the circumferential direction (P<0.05). The transverse colon seemed stiffest both in the circumferential and longitudinal directions. In conclusion, significant variations were found in morphometric and biomechanical properties along the large intestine. The circumferential residual strains and passive elastic properties must be taken into account in studies of physiological problems in which the stress and strain are important, e.g. large intestinal bolus transport function.

Differences in mediator of nonadrenergic, noncholinergic relaxation of the distal colon between Wistar-ST and Sprague-Dawley strains of rats

Participation of nitric oxide and vasoactive intestinal peptide (VIP) in electrical field stimulation-induced nonadrenergic, noncholinergic (NANC) relaxation of longitudinal muscle and in balloon distension-induced descending NANC relaxation of circular muscle were studied in the distal colon of Wistar-ST and Sprague-Dawley rats. The extent of the nitric oxide-mediated component was approximately 50% in longitudinal and circular muscle of Sprague-Dawley rats, whereas this component was absent in both muscles of Wistar-ST rats. The extent of the VIP-mediated component was approximately 40% in longitudinal muscle of Wistar-ST rats and circular muscle of Sprague-Dawley rats, whereas this component was absent in circular muscle of Wistar-ST rats and longitudinal muscle of Sprague-Dawley rats. In circular muscle of Sprague-Dawley rats, in which participation of both nitric oxide and VIP in the relaxation was suggested, inhibition of descending relaxation by N(G)-nitro-L-arginine (L-NOARG) together with VIP-(10-28) was similar to that by either of the antagonists, and exogenous VIP-induced relaxation was not affected by L-NOARG, but exogenous nitric oxide-induced relaxation was partly inhibited by VIP-(10-28). These results suggest a linkage of the pathways mediated by nitric oxide and VIP. In the immunohistochemical studies, nitric oxide synthase or VIP immunoreactive neurons were seen in the ganglia, primary internodal strands of the myenteric plexus and in the circular muscle layer. However, the overall appearance of immunoreactive cell bodies in the myenteric plexus and the numbers of immunoreactive fibers in the circular muscle layer appeared to be similar in Wistar-ST and Sprague-Dawley rats. These results suggest that mediators of NANC relaxation in the distal colon are different in different strains of rats, i.e., Wistar-ST and Sprague-Dawley, although no such difference was seen in immunohistochemical studies.

Characterization of 5-hydroxytryptamine (5-HT) receptor subtypes influencing colonic motility in conscious dogs

We examined the effects of exogenous 5-hydroxytryptamine (5-HT) and selective 5-HT receptor agonists and antagonists on proximal, middle and distal colonic motility in conscious fasted dogs with extraluminal force transducers implanted chronically. 5-HT (0.003-0.1 mg/kg i.v.) dose-dependently enhanced motility along the entire length of the colon. The 5-HT (0.03 mg/kg i.v.)-induced response was inhibited by 0.1-1.0 mg/kg i.v. methysergide, a 5-HT1/2 antagonist, at all recording sites and by 0.1-1.0 mg/kg i.v. ketanserin, a 5-HT2A antagonist, at the middle and distal sites only. At 1 mg/kg i.v., YM060, a 5-HT3 antagonist, reduced the amplitude of the initial transient high-amplitude contractions induced by 5-HT, but did not affect the tonic contraction induced by 5-HT. At doses up to 3 mg/kg i.v., 2-methoxy-4-amino-5-chlorobenzoic acid 2-(diethylamino) ethyl ester (SDZ205-557), a 5-HT4 antagonist, and hexamethonium (up to 10 mg/kg i.v.) did not affect 5-HT-induced responses at any recording site. Renzapride, a 5-HT4 agonist, also stimulated motility along the entire length of the colon at 0.3 mg/kg i.v.. The renzapride-induced response was inhibited by 1 mg/kg i.v. SDZ205-557 or 3 mg/kg i.v. hexamethonium. m-Chlorophenylbiguanide (m-CPBG), a 5-HT3 agonist, (1 mg/kg i.v.) produced a transient high-amplitude contraction at all recording sites and this contraction was eliminated by pretreatment with 0.03 mg/kg i.v. YM060. The contraction produced by m-CPBG declined rapidly, so the increase in the motility index by m-CPBG was not significant at any recording site. Of the antagonists tested, 0.1-1 mg/kg i.v. methysergide produced a delayed and prolonged contractile response at the middle and distal sites. The onset of the response was delayed about 20 min after application and the response was maintained over the subsequent 60-min observation period. The methysergide (1 mg/kg i.v.)-induced response was inhibited by 3 mg/kg i.v. hexamethonium. The other antagonists, ketanserin, YM060 and SDZ205-557, had no contractile effect at any recording site. These results indicate that exogenous 5-HT stimulates motility along the entire length of the fasted canine colon and that 5-HT-induced responses in the proximal colon are mediated mainly by 5-HT1, whereas those in the middle and distal colon are mediated by both 5-HT1 and 5-HT2 receptors. Renzapride and methysergide also stimulate colonic motility via additional mechanisms. The activation of 5-HT4 receptors and the blockade of endogenous 5-HT inhibitory regulation via 5-HT1 receptors may be involved in the action of renzapride and methysergide respectively.

Nitric oxide-mediated relaxation without concomitant changes in cyclic GMP content of rat proximal colon

1. We studied the relation of nitric oxide-mediated relaxation of longitudinal muscle to changes in cyclic GMP content of the tissue in the proximal colon of rats. 2. Dimethylphenylpiperazinium (DMPP) and electrical field stimulation (EFS) induced nitric oxide-mediated relaxation of the segments with a concomitant increase in cyclic GMP content. 3. LY 83583 and methylene blue, soluble guanylyl cyclase inhibitors, significantly inhibited the stimulatory effects of DMPP and EFS on the cyclic GMP content, but did not affect the relaxant responses of the segments to DMPP and EFS. 4. Rp-8 bromo cyclic GMPS, an inhibitor of cyclic GMP-dependent protein kinase had no effect on DMPP- and EFS-induced relaxation. 5. These data strongly suggested that nitric oxide-mediated relaxation of the rat proximal colon is not associated with change in cyclic GMP content of the tissue.

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.

Manometric patterns of rat colonic motor activity and defecation. Effect of selective 5HT1A agonist 8-OH-DPAT

We investigated in conscious and unrestrained rats, the major patterns of colonic pressure waves, as related to defecation. A manometric low compliance perfusion system, which was set at a very low flow rate (0.03 ml/min), permitted simultaneous recordings of intraluminal pressure in the proximal, transverse, and distal colon. Pressure waves in control rats reflected two types of motor activity: short-duration waves (< 15 sec), that were frequent throughout the colon (about 40-90/hr with aborally decreasing frequency), and propulsive, long-duration, high-amplitude waves (> 15 sec, > 15 mm Hg) that occurred only occasionally (1/hr or less) in the transverse and distal, but not in the proximal colon; these waves appeared to migrate aborally and were associated with defecation. The serotonin 5HT1A agonist 8-OH-DPAT dramatically and dose-dependently increased the frequency of long-duration, high-amplitude waves in the transverse and distal colon, and concurrently promoted defecation; these effects were prevented by the putative 5HT1A antagonist pindolol. We conclude that 5HT1A agonists such as 8-OH-DPAT may promote defecation and occurrence of propulsive waves through the same serotoninergic mechanism.

Nitric oxide-mediated inhibitory response of rat proximal colon: independence from changes in membrane potential

1. We studied the relation of nitric oxide-mediated relaxation of smooth muscle to changes in membrane potential of cells in the proximal colon of rats. 2. The resting membrane potential and electrical field stimulation (EFS)-induced junction potentials were recorded from the circular and longitudinal muscle cells. 3. Localized distension with a small balloon caused relaxation of the circular muscle on the anal side of the distended region (descending relaxation). Relaxation of the longitudinal muscle was also induced by EFS. 4. Inhibitory junction potentials (i.j.ps) were recorded from all circular muscle cells tested, but rarely from the longitudinal muscle cells. 5. The i.j.ps were recorded only in the presence of atropine but relaxations of both muscles were induced even in the absence of atropine. 6. Apamin (100 nM) completely abolished the i.j.ps recorded in both circular and longitudinal muscle cells, but had no significant effect on the relaxations of either. 7. In contrast to apamin, Ng nitro-L-arginine (10 microM) inhibited the relaxations of both muscles, but did not affect the i.j.ps. 8. Exogenously added nitric oxide (0.1-10 microM) induced relaxations of both muscles concentration-dependently, but did not affect the membrane potentials at these concentrations. 9. These data strongly suggest that nitric oxide-mediated relaxation of rat proximal colon is not associated with the i.j.ps of the cell membrane.

Neuronal pathway involved in nitric oxide-mediated descending relaxation in rat ileum

The neuronal pathway that initiates nitric oxide-mediated descending relaxation in rat ileum was studied. The descending relaxation, which was suggested to be mediated by nitric oxide from our previous study, was selectively inhibited by 5-HT3 receptor antagonists. It was also inhibited by a nicotinic acetylcholine receptor antagonist. Exogenous 5-hydroxytryptamine (5-HT) and a selective 5-HT3 receptor agonist induced dose-dependent relaxation of ileal circular muscle. 5-HT-induced relaxation was selectively inhibited by 5-HT3 receptor antagonists. Nicotine also induced relaxation of the circular muscle, and its effect was inhibited by 5-HT3 receptor antagonists. 5-HT and nicotine increased the cyclic GMP content of the ileal tissue. Nitro-L-arginine inhibited the increases induced by both compounds in the cyclic GMP content, and a 5-HT3 receptor antagonist also inhibited that induced by nicotine. These results indicate that activation of a cholinergic neuron-5-HT neuron pathway initiates nitric oxide-mediated descending relaxation in rat ileum.