Neurogenic control of myoelectric complexes in the mouse isolated colon

Motility in the isolated mouse colon: migrating motor complexes, myoelectric complexes and pressure waves

This study has used mechanical, together with pressure/volume recordings or electrophysiological recordings, to investigate the spontaneous activity in isolated preparations of mouse colon. In the former preparations, when not distended with fluid, spontaneous colonic migrating motor complexes (CMMCs) were observed using isotonic transducers. When the colons were distended with fluid, CMMCs continued at an increased frequency and in addition were associated temporally, with rises in intraluminal pressure and pulses of distally ejected fluid. 5-Hydroxytryptamine (1 micro mol L-1) or NG-nitro-l-arginine (100 micro mol L-1) increased the frequency of propulsive activity and this activity was abolished by hexamethonium (500 micro mol L-1). In a second preparation, myoelectric complexes recorded from circular muscle cells in colons using intracellular microelectrodes, were found to correlate in frequency and phase with CMMCs. The experiments indicate that CMMCs are intimately related to pressure waves in the fluid-filled viscus and the muscle membrane potential changes that have been recorded during myoelectric complexes are likely to be analogous to those occurring during fluid-filled propulsive activity.

Murine intestinal migrating motor complexes: longitudinal components

Spontaneous migrating contractions have been described in the circular muscle of the isolated mouse colon and terminal ileum, however, spontaneous events equivalent to these have not been reported in the longitudinal muscle. The longitudinal muscle shortenings in the colon and ileum, which are of similar form, frequency and pharmacology to the circular muscle colonic and ileal migrating motor complexes (CMMCs and IMMCs), are recorded in the present study. The spontaneous ileal and colonic longitudinal muscle shortenings appear to be neurally organized as they are abolished by tetrodotoxin (1 micro mol L-1), hexamethonium (500 micro mol L-1) and morphine (1 micro mol L-1). Endogenously released nitric oxide slowed the frequency of spontaneous ileal and colonic longitudinal muscle shortenings and 5-hydroxytryptamine increased their frequency. Hyoscine (1 micro mol L-1) abolished longitudinal shortenings in the ileum and reduced the amplitude of longitudinal shortening by approximately 44% in the colon. Shortenings were effectively abolished by nifedipine (1 micro mol L-1). Surgical sectioning of the colon identified that each region of the colon contracted longitudinally in an independent fashion; the distal colon contracted to the greatest amplitude and lowest frequency. The longitudinal preparation is suitable to initially assess the actions of novel pharmacological agents on spontaneous, neurally coordinated, CMMCs and IMMCs in emptied isolated murine intestines.

Correlation of electrophysiological and morphological characteristics of enteric neurons in the mouse colon

We report on the first correlative study of the electrophysiological properties, shapes, and projections of enteric neurons in the mouse. Neurons in the myenteric plexus of the mouse colon were impaled with microelectrodes containing biocytin, their passive and active electrophysiological properties determined, and their responses to activation of synaptic inputs investigated. Biocytin, injected into the neurons from which recordings were made, was converted to an optically dense product and used to determine the shapes of neurons. By electrophysiological properties, almost all neurons belonged to one of two classes, AH neurons or S neurons. AH neurons had a biphasic repolarization of the action potential, and slow afterhyperpolarizing potentials usually followed the action potentials. S neurons had monophasic repolarizations, no slow afterhyperpolarization, and fast excitatory postsynaptic potentials in response to fibre tract stimulation. By shape, neurons were divided into Dogiel type II (28/136 neurons) and uniaxonal neurons. Dogiel type II neurons had large, smooth-surfaced cell bodies and several long processes that supplied branches within myenteric ganglia. All Dogiel type II neurons had AH electrophysiology; conversely, most AH neurons had Dogiel type II morphology. The majority of uniaxonal neurons had lamellar dendrites, i.e., Dogiel type I morphology. They projected to the circular muscle (circular muscle motor neurons), to the longitudinal muscle (longitudinal muscle motor neurons), and to other myenteric ganglia (interneurons) and in some cases could not be traced to target cells. All S neurons were uniaxonal. A small proportion of uniaxonal neurons (3/70) had AH electrophysiology. Fast excitatory synaptic potentials were only recorded from uniaxonal neurons and were in most cases blocked by nicotinic receptor antagonists. A small component of fast excitatory transmission in some neurons was antagonized by the purine receptor antagonist PPADS. Slow excitatory postsynaptic potentials were observed in both AH and S neurons. Slow inhibitory postsynaptic potentials were recorded from S neurons. We conclude that the major classes of neurons are Dogiel type II neurons with AH electrophysiological properties and Dogiel type I neurons with S electrophysiological properties. The S/Dogiel type I neurons include circular muscle motor neurons, longitudinal muscle motor neurons, and interneurons.

Neural integrity is essential for the propagation of colonic migrating motor complexes in the mouse

The mechanisms that underlie the propagation of contractions along the colon are uncertain. We have examined whether spontaneous colonic migrating motor complexes (CMMCs) migrate through a region of muscle paralysis, or through a region where neural transmission was disrupted in the isolated mouse colon. Mouse colon was mounted in a separately perfused three-compartment organ bath and recordings of circular muscle tension were made. Drug application was restricted to the middle compartment. Application of nifedipine (1 micromol L(-1)), an l-type calcium channel antagonist, reduced the contraction amplitude by approximately 94%, without affecting the form of contractions in the proximal and distal compartments. Moreover, CMMCs appeared to remain temporally related in all compartments. In contrast, interruption of neural transmission in the middle compartment by either tetrodotoxin (1.6 micromol L(-1)), hexamethonium (500 micromol L(-1)) or a low-calcium, high-magnesium solution abolished CMMCs in this compartment; contractions recorded in the proximal and distal compartments became slower in frequency and were no longer synchronized. The experiments suggest that there may be more than one 'pacemaker' generating spontaneous CMMCs and that CMMCs can migrate through a region where there is minimal tension generation, but not through a region where neural integrity has been compromised.

Analysis of motor patterns in the isolated guinea-pig large intestine by spatio-temporal maps

We investigated and quantified the spontaneous patterns of motility in the isolated guinea-pig proximal and distal colon taken from adult animals. During spontaneous emptying, profiles of proximal and distal colon were recorded with a video camera, and image analysis was used to construct spatio-temporal maps of the motions of the intestinal wall. Four patterns of motility were recorded. In the proximal colon there were neurally mediated contractions that propagated in the aboral direction at 4.1 mm s(-1), gently pushing the soft contents aborally; these are likely to represent spontaneous peristaltic behaviour. A second pattern, insensitive to tetrodotoxin (TTX; 0.6 microM), consisted, in both oral and aboral propagation, of shallow contractions of the circular muscle (ripples). These contractions propagated aborally at 2.8 +/- 0.45 mm s(-1) and orally at 2.03 +/- 0.31 mm s(-1) (n=10). Of these TTX-resistant contractions, 22.5% propagated both orally and aborally from a common origin. The orally propagated component of these myogenic contractions is likely to correspond to the antiperistalsis widely described in the proximal colon. In the distal colon, two patterns of motor activity were observed. One, induced by natural or artificial pellets, consisted of peristaltic contractions that pushed the pellets aborally at 0.8 mm s(-1) and expelled a pellet every 108 s. In the interval between pellet propulsion and after the distal colon had emptied all of its pellets a second, nerve-mediated pattern of motor activity, consisting of clusters of annular circular muscle contractions separated by short dilated regions, slowly propagated aborally at 0.3 mm s(-1). Both of these motor patterns were abolished by TTX (0.6 microM). A latex balloon, inserted at the oral end of the empty isolated distal colon and inflated to a size similar to faecal pellets, was propelled at 1.4 mm s(-1). Epoxy resin-covered natural pellets were propelled at a similar speed of 1.6 mm s(-1). Our data revealed that myogenic and neurogenic patterns of propagated contractions in the colon occur in isolated preparations and are involved in emptying the colon.

Ongoing nicotinic and non-nicotinic inputs to inhibitory neurons in the mouse colon

1. Intracellular microelectrodes were used to record spontaneous and evoked inhibitory junction potentials (IJP) from the circular muscle layer of the mid-distal region of the mouse isolated colon in the presence of nifedipine (1 micromol/L) and hyoscine (1 micromol/L). 2. The length of the tissue preparation (> 1 cm) or the presence of the mucosa had no effect on the frequency of spontaneous IJP. 3. Hexamethonium (500 micromol/L) reduced the frequency of spontaneous IJP to approximately 70% of the control frequency, whereas D-tubocurarine (280 micromol/L) reduced the frequency to approximately 17% of control. Apamin (250 nmol/L) abolished all spontaneous IJP activity. 4. The greater inhibition of spontaneous IJP in the presence of D-tubocurarine compared with hexamethonium is discussed as a possible 'apamin-like' effect. 5. Although electrically evoked IJP (single pulse at 15 V, 0.6 msec) were not significantly affected by hexamethonium, D-tubocurarine and apamin reduced the amplitude of evoked IJP to approximately 65 and 50% of control, respectively. 6. These results suggest that the properties of spontaneous IJP cannot be inferred by a study of evoked IJP alone.

Evidence supporting presence of two pacemakers in rat colon

Intracellular microelectrodes and organ bath techniques were used to study spontaneous cyclic electrical and mechanical activity in the rat colon. Electron microscopy and immunohistochemical studies showed two major populations of interstitial cells of Cajal (ICC): one associated with Auerbach's plexus (ICC-AP) and one with the submuscular plexus (ICC-SMP). The ICC-SMP network partly adhered to the submucosa when removed and was generally strongly damaged after separation of musculature and submucosa. Similarly, longitudinal muscle removal severely damaged AP. Two electrical and mechanical activity patterns were recorded: pattern A, low-frequency (0.5--1.5 cycles/min), high-amplitude oscillations; and pattern B, high-frequency (13--15 cycles/min), low-amplitude oscillations. Pattern A was recorded in preparations with intact AP but absent in those without intact AP. Pattern B was recorded in preparations with intact SMP but was absent in those lacking SMP. With full-thickness strips, the superimposed patterns A and B were recorded in circular muscle. When longitudinal muscle mechanical activity was recorded, only pattern A was present. We conclude that two pacemakers regulate rat colonic cyclic activity: the ICC-SMP network (responsible for cyclic slow waves and small-amplitude contractions) and the ICC-AP network (which may drive the cyclic depolarizations responsible for high-amplitude contractions). This is the first report showing consistent slow wave activity in the rodent colon.

Endogenous nitric oxide release modulates the direction and frequency of colonic migrating motor complexes in the isolated mouse colon

Spontaneous colonic migrating motor complexes (CMMCs) were recorded from circular muscle at three sites along the isolated mouse colon. The interval between CMMCs was decreased from approximately 3 min in control solution, by approximately 55% in a nitric oxide synthase (NOS) inhibitor, N-nitro-L-arginine (L-NNA; 100 micromol L-1). This was associated with a shift in migration direction of CMMCs, such that CMMCs migrated in an oral direction. Application of the endogenous substrate for NOS, L-arginine, at a low concentration used to mimic plasma concentration (134 micromol L-1), or a high concentration (5 mmol L-1) suppressed CMMCs (for at least 15 min) which were replaced by high frequency (10-15 min-1), short duration (half width approximately 1.5 s) contractions of variable amplitudes (largest in the proximal region) until CMMCs resumed. CMMCs remained in the presence of D-arginine (134 micromol L-1 and 5 mmol L-1). Apamin (250 nmol L-1) did not alter the interval between CMMCs, however, additional nonmigrating contractions were observed between the CMMCs in the distal region. In addition to its effects on smooth muscle tone, NO, but not apamin-sensitive channels, plays an important role in suppressing the frequency of migrating contractions in the isolated mouse colon. Consideration should be given to the inclusion of L-arginine, in in vitro experiments where there may be spontaneous activity in NOS containing neurones.

Altered electrical activity in colonic smooth muscle cells from dystrophic (mdx) mice

Because the colon from dystrophic (mdx) mice shows an altered motor pattern, probably due to neural disorders, our aim was to examine the electrophysiological properties of muscle cells and the functionality of nitrergic transmission in circular muscle from normal and mdx colon. Normal colonic cells (resting membrane potential [RMP] about -50 mV) showed spontaneous hyperpolarizations (inhibitory junction potentials; IJPs) and cyclic slow depolarizations were sometimes recorded. Mdx colon had a depolarized RMP (about -36 mV) and spontaneous IJPs, but the cyclic activity was never observed. In the normal colon, Nomega-nitro-L-arginine methyl ester (L-NAME) induced depolarization and abolished the cyclic activity. In the mdx colon, L-NAME caused a slight depolarization. Both preparations displayed the same value of RMP in the presence of L-NAME. In normals, neural stimulation induced nonadrenergic, noncholinergic IJPs composed of fast hyperpolarizations followed by a nitrergic slow hyperpolarization, selectively abolished by L-NAME. In the mdx colon the evoked IJPs were composed only of the initial fast hyperpolarization, the nitrergic component being absent. The hyperpolarization to sodium nitroprusside was not significantly different in both preparations. We conclude that the colon from animals lacking in dystrophin displays different electrophysiological features because of an impairment of nitric oxide function.

The myogenic component in distention-induced peristalsis in the guinea pig small intestine

In an in vitro model for distention-induced peristalsis in the guinea pig small intestine, the electrical activity, intraluminal pressure, and outflow of contents were studied simultaneously to search for evidence of myogenic control activity. Intraluminal distention induced periods of nifedipine-sensitive slow wave activity with superimposed action potentials, alternating with periods of quiescence. Slow waves and associated high intraluminal pressure transients propagated aborally, causing outflow of content. In the proximal small intestine, a frequency gradient of distention-induced slow waves was observed, with a frequency of 19 cycles/min in the first 1 cm and 11 cycles/min 10 cm distally. Intracellular recording revealed that the guinea pig small intestinal musculature, in response to carbachol, generated slow waves with superimposed action potentials, both sensitive to nifedipine. These slow waves also exhibited a frequency gradient. In addition, distention and cholinergic stimulation induced high-frequency membrane potential oscillations (~55 cycles/min) that were not associated with distention-induced peristalsis. Continuous distention produced excitation of the musculature, in part neurally mediated, that resulted in periodic occurrence of bursts of distally propagating nifedipine-sensitive slow waves with superimposed action potentials associated with propagating intraluminal pressure waves that caused pulsatile outflow of content at the slow wave frequency.

Neural mechanisms underlying migrating motor complex formation in mouse isolated colon

1. Little is known about the intrinsic enteric reflex pathways associated with migrating motor complex (MMC) formation. Acetylcholine (ACh) mediates the rapid component of the MMC, however a non-cholinergic component also exists. The present study investigated the possible role of endogenous tachykinins (TKs) in the formation of colonic MMCs and the relative roles of excitatory and inhibitory pathways. 2. MMCs were recorded from the circular muscle at four sites (proximal, proximal-mid, mid-distal and distal) along the mouse colon using force transducers. 3. The tachykinin (NK(1) and NK(2)) receptor antagonists SR-140 333 (250 nM) and SR-48 968 (250 nM) reduced the amplitude of MMCs at all recording sites, preferentially abolishing the long duration contraction. Residual MMCs were abolished by the subsequent addition of atropine (1 microM). 4. The neuronal nitric oxide synthase inhibitor, N(omega)nitro-L-arginine (L-NOARG, 100 microM), increased MMC amplitude in the distal region, whilst reducing the amplitude in the proximal region. In preparations where MMCs did not migrate to the distal colon, addition of L-NOARG resulted in the formation of MMCs. Subsequent addition of apamin (250 nM) or suramin (100 microM) further increased MMC amplitude in the distal region, whilst suramin increased MMC amplitude in the mid-distal region. Apamin but not suramin reduced MMC amplitude in the proximal region. Subsequent addition of SR-140 333 and SR-48 968 reduced MMC amplitude at all sites. Residual MMCs were abolished by atropine (1 microM). 5. In conclusion, TKs, ACh, nitric oxide (NO) and ATP are involved in the neural mechanisms underlying the formation of MMCs in the mouse colon. Tachykinins mediate the long duration component of the MMC via NK(1) and NK(2) receptors. Inhibitory pathways may be involved in determining whether MMCs are formed.

Effects of VIP and NO on the motor activity of vascularly perfused rat proximal colon

The effects of vasoactive intestinal polypeptide (VIP) and nitric oxide (NO) on the motor activity of the rat proximal colon were examined in an ex vivo model of vascularly perfused rat proximal colon. VIP reduced motor activity and this inhibitory effect was not altered by either atropine, hexamethonium, tetrodotoxin (TTX) nor TTX plus acetylcholine (ACh), but was completely antagonized by NO synthase inhibitor N(G)-nitro-L-arginine (L-NA) and by VIP receptor antagonist, VIP(10-28). These results suggest that VIP may exert a direct inhibitory effect on the motor activity of the rat proximal colon via a VIP receptor located on the smooth muscle and this effect is mediated by NO but not by cholinergic pathways. Atropine and hexamethonium reduced but ACh stimulated motor activity and the effect of ACh was not changed by TTX, suggesting that the cholinergic pathway may exert a direct stimulatory effect on motor activity. Single injection of TTX, VIP(10-28) or L-NA induced a marked increase in motor activity, suggesting that the motor activity of rat proximal colon is tonically suppressed by VIP and NO generating pathways, and elimination of inhibitory neurotransmission by TTX may induce an abnormal increase of the motor activity. The interaction between VIP and NO in regulation of motor activity was further examined by a measurement of NO release from vascularly perfused rat proximal colon. Results showed that NO release was significantly increased during infusion of VIP and this response was reversed by L-NA. These results suggest that VIP generating neurons may inhibit colonic motility by stimulating endogenous NO production in either smooth muscle cells or nerve terminals.

Spontaneous migrating motor complexes occur in both the terminal ileum and colon of the C57BL/6 mouse in vitro

We have studied migrating motor complexes (MMCs) in the isolated terminal ileum or colon (IMMCs and CMMCs respectively) of the C57BL/6 mouse. Periodic contractions occurred spontaneously in both preparations in the absence of intraluminal stimulation. After an initial period, complexes became synchronized between the oral and anal ends of the tissue, and could be observed for in excess of 7 h. The propagation velocity was 3.1+/-1.0 and 3.9+/-0.6 mm s(-1) in the ileum and colon respectively. IMMCs occurred every 6.01+/-0.39 min and had a duration of 86.3+/-10.4 s. The interval between CMMCs was smaller (3.52+/-0.31 min) and contractions were shorter in duration (30.7+/-3.6 s). In both preparations, these motor events were dependent on cholinergic transmission: blocked by hexamethonium (500 microM) and attenuated or blocked by atropine (1 microM). This study is the first demonstration of spontaneous migrating contractions in the isolated ileum or colon of the C57BL/6 mouse, the strain of choice for neurological transgenic and targeted mice.

Electrical and mechanical effects of vasoactive intestinal peptide and pituitary adenylate cyclase-activating peptide in the rat colon involve different mechanisms

This work aimed to study the effects of pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP) on the mechanical and electrical activity of the circular muscle of the rat colon and the mechanisms involved in such effects. Spontaneous mechanical activity was studied in vitro in an organ bath and the membrane potential was recorded using the microelectrode technique. Both VIP and PACAP (0.1 microM) caused an immediate, sustained and tetrodotoxin (1 microM)-resistant inhibition of the cyclic spontaneous mechanical activity and hyperpolarization. The small-conductance Ca(2+)-activated K(+) channel blocker, apamin (1 microM), did not change the VIP- and PACAP-induced relaxation but reduced the hyperpolarization induced by PACAP whereas it did not change that induced by VIP. In contrast, the purinoceptor antagonist, suramin (100 microM), blocked the hyperpolarization caused by PACAP and VIP but failed to change their mechanical inhibitory effects. Moreover, the putative PACAP and VIP receptor antagonists, PACAP-(6-38) and VIP-(10-28), respectively, both 3 microM, failed to change the effects of either peptide and modified neither the inhibitory junction potential nor the relaxation induced by electrical-field stimulation. Thus, these results suggest that the mechanisms mediating relaxation are not strictly coupled to the mechanisms mediating hyperpolarization. This could be due to activation of two distinct mechanisms of action after agonist receptor interaction.

Gastrointestinal peristalsis: joint action of enteric nerves, smooth muscle, and interstitial cells of Cajal

Peristalsis is a propulsive motor pattern orchestrated by neuronal excitation and inhibition in cooperation with intrinsic muscular control mechanisms, including those residing in interstitial cells of Cajal (ICC). Interstitial cells of Cajal form a network of cells in which electrical slow waves originate and then propagate into the musculature initiating rhythmic contractile activity upon excitaton by enteric nerves. Interstitial cells of Cajal have now been isolated and their intrinsic properties reveal the presence of rhythmic inward currents not found in smooth muscle cells. In tissues where classical slow waves are not present, enteric cholinergic excitation will evoke slow wave-like activity that forces action potentials to occur in a rhythmic manner. Intrinsic and induced slow wave activity directs many of the peristaltic motor patterns in the gut.

Rebound excitation and alternating slow wave patterns depend upon eicosanoid production in canine proximal colon

1. We tested the hypothesis that eicosanoid production could be related to the long-duration slow waves that occur after brief periods of inhibitory neurotransmission (rebound excitation) and the alternating patterns of long- and short-duration slow waves observed in the canine proximal colon. 2. Electrical field stimulation of colonic muscles inhibited slow waves during the stimulus and a long-duration slow wave occurred after the stimulus. Indomethacin reduced the post-stimulus response without affecting the inhibitory response. 3. ATP or 2-methylthio-ATP produced post-stimulus rebound responses similar to the response to field stimulation. Indomethacin inhibited the rebound response caused by ATP or 2-methylthio-ATP. 4. Alternating patterns consisting of long- and short-duration slow waves occurred spontaneously in some colonic muscles. These patterns could also be induced with acetylcholine. 5. Indomethacin, acetylsalicylic acid and ibuprofen abolished the alternating pattern and shifted the bimodal distribution of slow wave durations toward an intermediate duration. 6. Patch clamp experiments on isolated colonic myocytes showed that indomethacin blocked L-type Ca2+ currents. The effects of indomethacin on rebound excitation and alternating slow waves were accomplished at concentrations that blocked cyclooxygenase activity without significantly inhibiting L-type Ca2+ currents. 7. The results demonstrate that rebound excitation and alternating slow wave patterns in the canine colon have similar dependence on endogenous eicosanoid production. Rebound excitation may result from reduced production of an inhibitory eicosanoid during inhibitory nerve stimulation, and the alternating pattern may result from oscillations in eicosanoid production as a function of changes in cytoplasmic Ca2+ during long and short slow waves.

Cyclic GMP-associated apamin-sensitive nitrergic slow inhibitory junction potential in the hamster ileum

1. The mediators of non-adrenergic, non-cholinergic (NANC) inhibitory junction potentials (i.j.ps) in the circular smooth muscle cells of the hamster ileum were studied. 2. Electrical field stimulation (EFS; 0.5 ms duration, 15 V) of the intramural nerves with a train of five pulses at 20 Hz evoked a rapidly developing hyperpolarization (fast i.j.p.) followed by a sustained hyperpolarization (slow i.j.p.). 3. NG-nitro-L-arginine methyl ester (L-NAME; 50 - 200 microM) and NG-nitro-L-arginine (L-NNA; 50 - 200 microM), NO synthase inhibitors, inhibited or abolished the EFS-induced fast and slow NANC i.j.ps. The effects of these NO synthase inhibitors were reversed by L-arginine (5 mM) but not by D-arginine (5 mM). 4. Exogenously applied nitric oxide (NO; 1 - 100 microM) induced concentration-dependent hyperpolarizations. 5. Oxyhaemoglobin (5 - 50 microM), NO scavenger, inhibited only the slow i.j.p., and the NO-induced hyperpolarization. 6. 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxaline-1-one (ODQ; 10 microM) and cystamine (10 mM), guanylate cyclase inhibitors, inhibited only the slow i.j.p. Zaprinast (100 microM), a phosphodiesterase type V inhibitor, enhanced the amplitude and duration of the slow i.j.p. 7. Apamin (100 nM), a small conductance Ca2+-activated K+ channel blocker, inhibited only the slow i.j.p., and NO-induced hyperpolarization. A high concentration of 8-bromoguanosine 3':5'-cyclic monophosphate (8-bromo-cGMP; 1 mM)-induced membrane hyperpolarization which was blocked by apamin. 8. These results suggest that NO, or a related compound, may be the inhibitory transmitter underlying the apamin-sensitive NANC slow i.j.p. and cyclic GMP mediates the slow i. j.p. in the hamster ileum. It is also likely that NO, without involvement of guanylate cyclase is associated with the fast i.j.p.

Mechanical activity of small and large intestine in normal and mdx mice: a comparative analysis

The aim of this study was to compare the motor pattern (recorded as changes in intraluminal pressure) of isolated duodenum and proximal colon between dystrophic mdx and normal mice. When duodenal recordings from control preparations were compared with mdx mice there was no significant difference in the spontaneous motor pattern, responses to electrical nerve stimulation or sensitivity to pharmacological agents. Colonic segments from mdx mice showed a more complex motor pattern, consisting of contractions with amplitude and frequency similar to those of controls and by additional contractions with lower amplitude and higher frequency. Moreover, 70% of the colonic preparations from mdx mice developed active tone. TTX (1 microM), both in control and in mdx mice, changed the motor pattern, revealing regular rhythmic contractions similar in both preparations. L-NAME (100 microM) in both preparations increased contractile activity, revealing additional low contractions in control and potentiating them in mdx colon. In both control and mdx mice, inhibitory responses elicited by electrical field stimulation (EFS) were significantly attenuated by L-NAME. Our results provide evidence for the presence of a different motor pattern in mdx proximal colon and suggest that mdx mice can be considered a suitable animal model for investigating the dystrophic process.

Nitric oxide regenerates the normal colonic peristaltic activity in mdx dystrophic mouse

We demonstrated in vitro that the colonic peristaltic activity is modified in dystrophin-deficient mdx mouse indicating a defect in the enteric nervous system (ENS). Since nitric oxide (NO) has been proposed as a putative inhibitory mediator of ENS, here we have examined the effects of both L-Arginine (L-Arg) and Nomega-nitro-L-arginine methyl ester (L-NAME) on the peristaltic activity of mdx mouse distal colon. The motor pattern of colonic segment showed irregular peristaltic waves. L-Arg (10(-7) - 10(-5) M) induced the peristaltic activity to slow down. At a concentration of 10(-5) M, L-Arg produced hypomotility, characterised by a decrease in amplitude, frequency and ejected fluid volume. Conversely, L-NAME elicited hypermotility, this effect being reversed once again by the subsequent addition of L-Arg. Interestingly the addition of 10(-5) M L-Arg to the organ bath led to the normal progression, in an oral to aboral direction, of 90% of the peristaltic waves. This last result strongly suggests that exogenous application of L-Arg restores the integrative circuits of the ENS responsible for programming and co-ordinating peristaltic activity in the distal colon of mdx mouse.

Neural modulation of the cyclic electrical and mechanical activity in the rat colonic circular muscle: putative role of ATP and NO

1. The rat colonic circular muscle displays cyclic episodes of myenteric potential oscillations (MPOs), each of them associated with a spontaneous contraction. Nifedipine 1 microM abolished both MPOs and their associated contractions. TTX (1 microM) increased the amplitude and frequency of spontaneous contractions. 2. Electrical field stimulation (EFS) induced a non-adrenergic non-cholinergic (NANC) inhibitory junction potential (IJP), with two phases: an initial fast hyperpolarization (characterized by IJP amplitude) and a sustained hyperpolarization (characterized by IJP duration). 3. Sodium nitroprusside (10 microM) hyperpolarized and abolished spontaneous contractions even in presence of TTX or 1 microM apamin. ATP (100 microM) also hyperpolarized and abolished spontaneous contractions but its effects were decreased by TTX and abolished by apamin. 4. Suramin (100 microM) or apamin reduced the amplitude of the IJPs, but did not affect their duration. Incubation with L-NOARG (1 mM) reduced the duration but not the amplitude of the IJPs. In presence of L-NOARG plus suramin or L-NOARG plus apamin, both duration and amplitude of the IJPs were reduced but a residual IJP could still be recorded. 5. We conclude that the mechanical and electrical cyclic activity of the rat colonic circular muscle is modulated but not originated by the enteric nervous system and involves L-type calcium channel activity. EFS induces release of NANC inhibitory neurotransmitters which hyperpolarize and relax smooth muscle cells. Both ATP and NO are involved in IJP generation: ATP is responsible for the first phase of the IJPs involving activation of apamin-sensitive potassium channels, whereas NO initiates the second phase which is independent of the activation of such channels.

Effects of sympathetic nerve stimulation on membrane potential in the circular muscle layer of mouse distal colon

Little is known about the effects of sympathetic nerve stimulation on the membrane potential of colonic smooth muscle. In the distal colon of the mouse, intracellular microelectrodes were used to record the effects of lumbar colonic (LCN) and intermesenteric nerve (IMN) stimulation on circular muscle membrane potential in vitro. A two-compartment organ bath was used to selectively perfuse the colon and inferior mesenteric ganglion (IMG). In the presence of nifedipine (1-2 microM) (to the colonic compartment only), spontaneous depolarizations (myoelectric complexes, MCs) were recorded about every 4 min. MCs consisted of a prolonged burst of rapid oscillations in membrane potential (approximately 2 Hz) that were superimposed on a slow depolarization (mean amplitude 12 mV). Single electrical stimuli (600 microseconds duration) delivered to the LCN or IMN did not elicit a detectable change in the membrane potential. However, trains of stimuli (e.g., 60 pulses at 10-20 Hz) to the LCN or IMN during the intervals between MCs evoked a depolarization (with superimposed action potentials in the absence of nifedipine). Trains of stimuli delivered during the plateau phase of the MC reduced or abolished the rapid oscillations, without a further membrane depolarization. The MC period was unaffected by stimulation of the IMN or LCN. Responses were abolished by the selective perfusion of guanethidine (1 microM) to the colon, or by severing the LCN. Hexamethonium (500 microM) (to the colon) abolished MCs, induced sustained depolarization and attenuated the amplitude of the sympathetic depolarizations by 74%. In hexamethonium, sympathetic responses remained attenuated when the membrane of the circular muscle was repolarised by sodium nitroprusside (1 microM). Immunohistochemical studies of the colon revealed intense immunoreactivity for tyrosine hydroxylase in the myenteric plexus but not in the circular muscle layer. It is suggested that responses to sympathetic nerve stimulation in the circular muscle layer of the mouse colon are secondary to actions on the enteric nervous system.

Second-, minute- and hour-metronomes of intestinal pacemakers

1. Movements of the gastrointestinal tract are required for the digestion of food and the expulsion of waste products. 2. The present paper will discuss the nature of electrical rhythms underlying some intestinal motility patterns. 3. The rhythms are generated by pacemakers with cycle rates appropriate to controlling individual contractions, motor patternings or switching between different motor programmes. 4. Electrical rhythms are discussed with periods of the order of seconds, minutes and hours. 5. Particular discussion is centred on rhythms recorded from the small and large intestine of the mouse.

Evidence that myoelectric complexes in the isolated mouse colon may not be of myogenic origin

The hypothesis that spontaneous depolarisations (myoelectric complexes, MCs) can occur in the absence of neuronal activity, depending on the level of the membrane potential, was systematically studied. In control Krebs' solution, MCs were recorded approximately every 5 min and were abolished by tetrodotoxin (TTX, 1.6 microM). However, TTX also induced sustained membrane depolarisation (19 mV) in the circular muscle. To test whether MCs were blocked by the depolarisation induced by TTX, graded membrane repolarisations were generated, in the continuing presence of TTX, using sodium nitroprusside (SNP, 10 nM-1 microM). Under these conditions, MC activity was not restored. The addition of SNP (1 microM) to control preparations, in normal Krebs' solution, hyperpolarised the membrane of the circular muscle cells, but did not inhibit ongoing MC activity. It is suggested that the underlying mechanisms involved in MC generation are unlikely to be dependent upon the level of membrane potential in circular smooth muscle.

Control of motility patterns in the human colonic circular muscle layer by pacemaker activity

1. This study characterized the electrical and mechanical activities of human colonic muscle strips obtained from either the ascending, descending or sigmoid colon of patient volunteers during elective colon resections. 2. Rhythmic contractile activity was observed in colonic circular muscle strips in the absence of external stimuli. This activity persisted in the presence of atropine, phentolamine, propranolol, tetrodotoxin and Nomega-nitro-L-arginine but was abolished by nifedipine. 3. The activity of whole circular muscle (WCM) was compared with that of the myenteric half (MCM), the submucosal half (SCM) and the interior (ICM) of the circular muscle layer. WCM exhibited a prominent 2-4 contractions min-1 contractile pattern which was also present in strips of SCM. In contrast, MCM and ICM exhibited slow (0.3-0.6 contractions min-1), long duration contractions with superimposed higher frequency contractions (17-18 contractions min-1). 4. Resting membrane potential (Vm), recorded at various positions through the thickness of WCM strips did not differ and averaged -50 mV. 5. Slow waves were observed in 83 % of muscles. They averaged 12 mV in amplitude, 9.4 s in duration and had a frequency of 2-4 contractions min-1. Slow waves were greatest in amplitude near the submucosal edge and decreased with distance away from this edge. Each slow wave was associated with a transient contraction. 6. Near the myenteric edge, rapid fluctuations of Vm with a mean frequency of 18 contractions min-1 were recorded in 67 % of muscles. Spiking activity was common and was superimposed upon slow waves and rapid Vm fluctuations. 7. In summary, slow waves were identified in the human colonic circular muscle layer which arise at or near the submucosal edge. These electrical events give rise to a 2-4 contractions min-1 contractile rhythm which is characteristic of the intact muscle layer. Thus, the nature and spatial organization of pacemaker activity in the human colon bears significant resemblance to other animal models, such as the dog and pig.

Disinhibition during myoelectric complexes in the mouse colon

Intracellular microelectrodes were used to record electrically evoked inhibitory junction potentials (IJPs) and electrotonic potentials during spontaneous cyclical depolarisations (myoelectric complexes, MCs) in the circular muscle layer of mouse colon in vitro. In the presence of nifedipine (1-2 microM) and atropine (1 microM), MCs were recorded every 264 +/- 18 s. Between MCs, single electrical stimuli (15 V, 0.6 ms, every 8 s) elicited IJPs whose amplitudes remained constant. In comparison, during the depolarising phase of MCs, the mean IJP amplitude was reduced by 61 +/- 7%, while during the late plateau and early repolarising phase of MCs, IJP amplitude was increased (up to 20%). NG-nitro-L-arginine (NOLA, 100 microM) abolished the repolarisation phase between MCs, so that the circular muscle remained depolarised and the amplitude of MCs was reduced by 73 +/- 6%. However, the amplitude of evoked IJPs was unaffected, as was the decrease in their amplitude during the depolarising phase of the residual MCs. In the presence of NOLA (100 microM), the further addition of apamin (250 nM) reduced the amplitude of evoked IJPs by approximately half. However, the amplitudes of NOLA- and apamin-resistant IJPs were also attenuated by 82 +/- 5% during the depolarising phase of residual MCs (amplitude: 1.9 +/- 1 mV). However, during this phase, the amplitude of an electrotonic potential (evoked by extracellular current application) was not attenuated. Addition of hexamethonium (500 microM), or tetrodotoxin (TTX) (1.6 microM) to solutions containing NOLA and apamin were without effect on membrane potential, but the residual MCs and the cyclical attenuation in IJP amplitude were abolished. During the intervals between MCs, membrane potential is maintained under tonic inhibition, via spontaneous release of inhibitory neurotransmitter(s), predominantly through nitrergic mechanisms. The cyclical attenuation in the amplitude of the non-nitrergic IJP does not arise from cyclical postjunctional changes in membrane resistance or potential. Moreover, the generation of the depolarising phase of MCs involves the simultaneous suppression of both nitrergic and non-nitrergic inhibitory neurotransmission. It is suggested that MCs arise from presynaptic suppression of ongoing inhibitory neurotransmitter release.

Inhibitory neurotransmission in the circular muscle layer of mouse colon

Intracellular electrophysiological techniques were used to record the spontaneous myoelectric activity in the circular muscle layer of an in vitro preparation of whole mouse colon. In 34 out of 58 preparations, spontaneous depolarisations (myoelectric complexes, MCs) were recorded cyclically, about every 4 min. In these preparations, apamin (250 nM) and NG-nitro-L-arginine (NOLA, 100 microM) depolarised the membrane potential between MCs by 8 mV or 13 mV, respectively. Tetrodotoxin (1.6 microM) abolished MCs and also induced depolarisation (17 mV). In the remaining 24 preparations, MCs were not recorded and the membrane potential was significantly depolarised compared to the membrane potential between MCs. NOLA (100 microM), apamin (250 nM) and tetrodotoxin (1.6 microM) were without significant effect on membrane potential. It is suggested that in preparations that exhibit MC cycling, membrane potential between MCs is maintained in a state of tonic inhibition, predominantly mediated by nitrergic mechanisms generated via spontaneously active inhibitory neurons. Apamin-sensitive channels may also be involved in the inhibition.

Basal release of nitric oxide induces an oscillatory motor pattern in canine colon

1. The consequences of intrinsic, basal nitric oxide release on electrical and contractile activity of canine proximal colon were examined. Membrane potential and contraction were simultaneously recorded from the circular muscle in the presence of drugs to block adrenergic and cholinergic responses. 2. Electrical slow waves were recorded from muscle cells near the submucosal surface of the circular layer. Spontaneous contractions were initiated by each slow wave. Contractile amplitude increased 1.9-fold when nerves were blocked with tetrodotoxin (TTX, 1 microM). 3. Muscle cells near the myenteric surface displayed myenteric potential oscillations (MPOs) averaging 16 cycles per minute (c.p.m.) in frequency and 10 mV in amplitude. Twenty-five per cent of muscles displayed an additional slow, neurogenic oscillation (mean frequency, 1 c.p.m.; amplitude, 14 mV) superimposed upon the MPO rhythm. 4. The nitric oxide (NO) synthase inhibitor N omega -nitro-L-arginine (L-NA, 100 microM; n = 16) abolished neurogenic oscillations, depolarized cells, and increased MPO upstroke velocity, amplitude and frequency. The actions of L-NA were mimicked by N omega-nitro-L-arginine methylester (L-NAME, 100 microM) and oxyhaemoglobin (3%). 5. Spontaneous contractions were increased 2.3-fold by L-NA, and TTX had no effect on contractions after addition of L-NA. 6. The NO-donor sodium nitroprusside (SNP, 1 microM) reversed the electrical and mechanical effects of L-NA and initiated slow oscillations similar to the neurogenic oscillations. Slow oscillations were also evoked with S-nitroso-N-acetylpenicillamine (SNAP, 1 microM). The effects of NO donors were blocked by oxyhaemoglobin. 7. Slow electrical oscillations could not be elicited by SNP after removal of a thin strip of circular muscle along the myenteric edge. 8. These data suggest that the spontaneous electrical and contractile activity of the proximal colon is tonically suppressed by basal release of NO. Basal NO causes an oscillatory pattern of electrical and mechanical activity. This activity does not require patterned firing of nerves; rather a continuous, low level release of NO would be capable of producing the neurogenic oscillatory behaviour. The slow oscillatory activity depends upon the presence of the myenteric region of the circular muscle layer, which contains cell bodies of enteric neurons and interstitial cells of Cajal.

Regulation of citrulline recycling in nitric oxide-dependent neurotransmission in the murine proximal colon

1. We investigated the contribution of nitric oxide (NO) to inhibitory neuromuscular transmission in murine proximal colon and the possibility that citrulline is recycled to arginine to maintain the supply of substrate for NO synthesis. 2. Intracellular microelectrode recordings were made from circular smooth muscle cells in the presence of nifedipine and atropine (both 1 microM). Electrical field stimulation (EFS, 0.3-20 Hz) produced inhibitory junction potentials (i.j.ps) composed of an initial transient hyperpolarization (fast component) followed by a slow recovery to resting potential (slow component). 3. L-Nitro-arginine-methyl ester (L-NAME, 100 microM) selectively abolished the slow component of i.j.ps. The effects of L-NAME were reversed by L-arginine (0.2-2 mM) but not by D-arginine (2 mM). Sodium nitroprusside (an NO donor, 1 microM) reversibly hyperpolarized muscle cells. This suggests that NO mediates the slow component of i.j.ps. 4. L-Citrulline (0.2 mM) also reversed the effects of L-NAME, and this action was maintained during sustained exposures to L-citrulline (0.2 mM). This may reflect intraneuronal recycling of L-citrulline to L-arginine. 5. Higher concentrations of L-citrulline (e.g. 2 mM) had time-dependent effects. Brief exposure (15 min) reversed the effects of L-NAME, but during longer exposures (30 min) the effects of L-NAME gradually returned. In the continued presence of L-citrulline, L-arginine (2 mM) readily restored nitrergic transmission, suggesting that during long exposures to high concentrations of L-citrulline, the ability to generate arginine from citrulline was reduced. 6. Aspartate (2 mM) had no effect on i.j.ps, the effects of L-NAME, or the actions of L-citrulline in the presence of L-NAME, L-Citrulline (0.2-2 mM) alone had no effect on i.j.ps under control conditions. 7. S-methyl-L-thiocitrulline (10 microM), a novel NOS inhibitor, blocked the slow component of i.j.ps. The effects of this inhibitor were reversed by L-arginine (2 mM), but not by L-citrulline (2 mM). 8. These results suggest that i.j.ps in the murine colon result from release of multiple inhibitory neurotransmitters. NO mediates a slow component of enteric inhibitory neurotransmission. Recycling of L-citrulline to L-arginine may sustain substrate concentrations in support of NO synthesis and this pathway may be inhibited when concentrations of L-citrulline are elevated.