Characterization of muscarinic receptor-mediated cationic currents in longitudinal smooth muscle cells of mouse small intestine

Functions of Muscarinic Receptor Subtypes in Gastrointestinal Smooth Muscle: A Review of Studies with Receptor-Knockout Mice

Parasympathetic signalling via muscarinic acetylcholine receptors (mAChRs) regulates gastrointestinal smooth muscle function. In most instances, the mAChR population in smooth muscle consists mainly of M2 and M3 subtypes in a roughly 80% to 20% mixture. Stimulation of these mAChRs triggers a complex array of biochemical and electrical events in the cell via associated G proteins, leading to smooth muscle contraction and facilitating gastrointestinal motility. Major signalling events induced by mAChRs include adenylyl cyclase inhibition, phosphoinositide hydrolysis, intracellular Ca2+ mobilisation, myofilament Ca2+ sensitisation, generation of non-selective cationic and chloride currents, K+ current modulation, inhibition or potentiation of voltage-dependent Ca2+ currents and membrane depolarisation. A lack of ligands with a high degree of receptor subtype selectivity and the frequent contribution of multiple receptor subtypes to responses in the same cell type have hampered studies on the signal transduction mechanisms and functions of individual mAChR subtypes. Therefore, novel strategies such as genetic manipulation are required to elucidate both the contributions of specific AChR subtypes to smooth muscle function and the underlying molecular mechanisms. In this article, we review recent studies on muscarinic function in gastrointestinal smooth muscle using mAChR subtype-knockout mice.

Further characterization of the synergistic activation mechanism of cationic channels by M2 and M3 muscarinic receptors in mouse intestinal smooth muscle cells

In mouse ileal myocytes, muscarinic receptor-mediated cationic current (mI_cat) occurs mainly through synergism of M₂ and M₃ subtypes involving G_i/o-type GTP-binding proteins and phospholipase C (PLC). We have further studied the M₂/M₃ synergistic pathway. Carbachol-induced mI_cat was markedly depressed by YM-254890, a G_q/11 protein inhibitor. However, the mI_cat was unaffected by heparin, calphostin C, or chelerythrine, suggesting that mI_cat activation does not involve signaling molecules downstream of phosphatidylinositol 4,5-bisphosphate (PIP₂) breakdown. M₂-knockout (KO) mice displayed a reduced mI_cat (~10% of wild-type mI_cat) because of the lack of M₂-G_i/o signaling. The impaired mI_cat was insensitive to neuropeptide Y possessing a G_i/o-stimulating activity. M₃-KO mice also displayed a reduced mI_cat (~6% of wild-type mI_cat) because of the lack of M₃-G_q/11 signaling, and the mI_cat was insensitive to prostaglandin F_2α possessing a G_q/11-stimulating activity. These results suggest the importance of G_q/11/PLC-hydrolyzed PIP₂ breakdown itself in mI_cat activation and also support the idea that the M₂/M₃ synergistic pathway represents a signaling complex consisting of M₂-G_i/o and M₃-G_q/11-PLC systems in which both G proteins are special for this pathway but not general in receptor coupling.

Regulation of the pacemaker activities in cultured interstitial cells of Cajal by Citrus unshiu peel extracts

The Citrus unshiu peel has been widely used for the treatment of gastrointestinal (GI) disorders in Eastern traditional medicine. The present study aimed to investigate the effects of Citrus unshiu peel extract (CPE) on the pacemaker activity of the GI tract in cultured interstitial cells of Cajal (ICCs) derived from the mouse small intestine. The whole‑cell patch‑clamp configuration was used to record pacemaker potentials. In current clamp mode, exposure to CPE caused membrane pacemaker depolarization in a concentration‑dependent manner. In the presence of the muscarinic M2 receptor antagonist, methoctramine, CPE induced membrane pacemaker depolarization, whereas treatment with the muscarinic M3 receptor antagonist, 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide, inhibited CPE‑induced responses. When the pipette solution contained guanosine 5'-(β-thio) diphosphate trilithium salt (1 mM), CPE marginally induced membrane pacemaker depolarization. In addition, CPE‑induced membrane pacemaker depolarization was inhibited following exposure to the active phospholipase C (PLC) inhibitor U‑73122, but not the inactive PLC inhibitor U‑73343. In the presence of a p42/p44 mitogen‑activated protein kinase (MAPK) inhibitor (PD98059), a p38 MAPK inhibitor (SB203580) or a c‑jun NH2‑terminal kinase (JNK) II inhibitor, CPE failed to induce membrane pacemaker depolarization. These results suggest that CPE may affect GI motility through modulating ICC pacemaker activity by activating the muscarinic M3 receptor and inducing the G‑protein dependent PLC and MAPK signaling pathways.

Effects of the roots of Liriope Platyphylla Wang Et tang on gastrointestinal motility function

ETHNOPHARMACOLOGICAL RELEVANCE

Liriope platyphylla Wang et Tang continues to be used in Korea as a traditional medicine for the treatment of gastrointestinal (GI) disorders related to constipation and abnormal GI motility.

AIM OF THE STUDY

Because GI disorders, especially GI motility dysfunctions, are major lifelong problems, the authors investigated the effects of a water extract of the roots of L. platyphylla Wang et Tang (LPE) on the pacemaker potentials (PPTs) of interstitial cells of Cajal (ICCs) and on GI motility in male ICR mice.

MATERIALS AND METHODS

Enzymatic digestions were used to dissociate ICCs from small intestines, and the whole-cell patch-clamp configuration was used to record PPTs generated by cultured ICCs in vitro. In vivo effects of LPE on GI motility were investigated by measuring intestinal transit rates (ITRs) of Evans blue in normal mice and in acetic acid (AA) and streptozotocin (STZ)-induced diabetic mouse models of GI motility dysfunction.

RESULTS

LPE dose-dependently depolarized PPTs in ICCs. Pretreatment with methoctramine (a muscarinic M2 receptor antagonist) did not block LPE-induced PPT depolarization. However, pretreatment with 4-DAMP (a muscarinic M3 receptor antagonist) blocked LPE-induced PPT depolarization. In addition, treatment with LY294002 (a phosphoinositide 3-kinase (PI3K) inhibitor) also blocked LPE-induced PPT depolarization. Intracellular GDPβS inhibited LPE-induced PPT depolarization, and LPE-induced PPT depolarization was found to occur in a phospholipase C (PLC)- and a protein kinase C (PKC)-dependent manner. Pretreatment with Ca(2+)free solution or thapsigargin (a Ca(2+)-ATPase inhibitor in endoplasmic reticulum) abolished PPTs, and under these conditions, LPE did not depolarize ICC PPTs. In normal mice, ITRs were significantly and dose-dependently increased by LPE (0.01-1g/kg administered intragastrically (i.g.)). In addition, LPE (i.g.) significantly recovered GI motility dysfunctions in both animal models.

CONCLUSION

LPE dose-dependently depolarizes ICC PPTs through M3 receptors via external and internal Ca(2+)regulation and via G protein-, PI3K-, PLC- and PKC- dependent pathways in vitro. Also, in vivo, LPE increased ITRs in treatment naïve mice and our two mouse models of GI dysfunction. Therefore, this study shows that LPE offers a basis for the development of a prokinetic agent that prevents or alleviates GI motility dysfunctions.

Inhibitory effects of SKF96365 on the activities of K(+) channels in mouse small intestinal smooth muscle cells

In order to investigate the effects of SKF96365 (SKF), which is a non-selective cationic channel blocker, on K⁺ channel currents, we recorded currents through ATP sensitive K⁺ (I_KATP), voltage-gated K⁺ (I_Kv) and Ca²⁺ activated K⁺ channels (I_BK) in the absence and presence of SKF in single small intestinal myocytes of mice with patch-clamp techniques. SKF (10 µM) reversibly abolished I_KATP that was induced by cromakalim (10 µM), which is a selective ATP sensitive K⁺ channel opener. These inhibitory effects were induced in a concentration-dependent and voltage-independent manner. The 50% inhibitory concentration (IC₅₀) was 0.85 µM, which was obviously lower than that reported for the muscarinic cationic current. In addition, SKF (1 µM ≈ the IC₅₀ value in I_KATP suppression) reversibly inhibited the I_Kv that was induced by repetitive depolarizing pulses from −80 to 20 mV. However, the extent of the inhibitory effects was only ~30%. In contrast, SKF (1 µM) had no significant effects on spontaneous transient I_BK and caffeine-induced I_BK. These results indicated that SKF inhibited ATP sensitive K⁺ channels and voltage-gated K⁺ channels, with the ATP sensitive K⁺ channels being more sensitive than the voltage-gated K⁺ channels. These inhibitory effects on K⁺ channels should be considered when SKF is used as a cationic channel blocker.

Modulation of pacemaker potentials by pyungwi-san in interstitial cells of cajal from murine small intestine: pyungwi-san and interstitial cells of cajal

Objective:

Pyungwi-san (PWS) plays a role in a number of physiologic and pharmacologic functions in many organs. Interstitial cells of Cajal (ICCs) are pacemaker cells that generate slow waves in the gastrointestinal (GI) tract. We aimed to investigate the beneficial effects of PWS in mouse small-intestinal ICCs.

Methods:

Enzymatic digestion was used to dissociate ICCs from the small intestine of a mouse. The wholecell patch-clamp configuration was used to record membrane potentials from the cultured ICCs.

Results:

ICCs generated pacemaker potentials in the GI tract. PWS produced membrane depolarization in the current clamp mode. Pretreatment with a Ca²⁺ -free solution and a thapsigargin, a Ca²⁺ -ATPase, inhibitor in the endoplasmic reticulum, eliminated the generation of pacemaker potentials. However, only when the thapsigargin was applied in a bath solution, the membrane depolarization was not produced by PWS. Furthermore, the membrane depolarizations due to PWS were inhibited not by U-73122, an active phospholipase C inhibitor, but by chelerythrine and calphostin C, protein kinase C inhibitors.

Conclusions:

These results suggest that PWS might affect GI motility by modulating the pacemaker activity in the ICCs.

Shengmaisan regulates pacemaker potentials in interstitial cells of cajal in mice

OBJECTIVES

Shengmaisan (SMS) is a traditional Chinese medicine prescription widely used for the treatment of diverse organs in Korea. The interstitial cells of Cajal (ICCs) are pacemaker cells that play an important role in the generation of coordinated gastrointestinal (GI) motility. We have aimed to investigate the effects of SMS in the ICCs in the mouse small intestine.

METHODS

To dissociate the ICCs, we used enzymatic digestions from the small intestine in a mouse. After that, the ICCs were identified immunologically by using the anti-c-kit antibody. In the ICCs, the electrophysiological whole-cell patch-clamp configuration was used to record pacemaker potentials in the cultured ICCs.

RESULTS

The ICCs generated pacemaker potentials in the mouse small intestine. SMS produced membrane depolarization with concentration-dependent manners in the current clamp mode. Pretreatment with a Ca(2+) free solution and thapsigargin, a Ca(2+)-ATPase inhibitor in the endoplasmic reticulum, stopped the generation of the pacemaker potentials. In the case of Ca(2+)-free solutions, SMS induced membrane depolarizations. However, when thapsigargin in a bath solution was applied, the membrane depolarization was not produced by SMS. The membrane depolarizations produced by SMS were inhibited by U-73122, an active phospholipase C (PLC) inhibitors. Furthermore, chelerythrine and calphostin C, a protein kinase C (PKC) inhibitors had no effects on SMS-induced membrane depolarizations.

CONCLUSIONS

These results suggest that SMS might affect GI motility by modulating the pacemaker activity through an internal Ca(2+)- and PLC-dependent and PKC-independent pathway in the ICCs.

Involvement of MAPKs and PLC pathways in modulation of pacemaking activity by So-Cheong-Ryong-Tang in interstitial cells of Cajal from murine small intestine

Purpose. Interstitial cells of Cajal (ICCs) are the pacemaker cells that generate slow waves in the gastrointestinal (GI) tract. We have aimed to investigate the effects of Socheongryong-Tang (SCRT) in ICCs from mouse's small intestine. Methods. The whole-cell patch-clamp configuration was used to record membrane potentials from cultured ICCs. Intracellular Ca²⁺ ([Ca²⁺]_i) increase was studied in cultured ICCs using fura-2 AM. Results. ICCs generated pacemaker potentials in mouse's small intestine. SCRT produced membrane depolarization in current clamp mode. Y25130 (5-HT₃ receptor antagonist) and RS39604 (5-HT₄ receptor antagonist) blocked SCRT-induced membrane depolarizations, whereas SB269970 (5-HT₇ receptor antagonist) did not. When GDP-β-S (1 mM) was in the pipette solution, SCRT did not induce the membrane depolarizations. [Ca²⁺]_i analysis showed that SCRT increased [Ca²⁺]_i. In the presence of PD98059 (p42/44 MAPK inhibitor), SCRT did not produce membrane depolarizations. In addition, SB203580 (p38 MAPK inhibitor) and JNK inhibitors blocked the depolarizations by SCRT in pacemaker potentials. Furthermore, the membrane depolarizations by SCRT were not inhibited by U-73122, an active phospholipase C (PLC) inhibitor, but by U-73343, an inactive PLC inhibitor. Conclusion. These results suggest that SCRT might affect GI motility by the modulation of pacemaker activity through MAPKs and PLC pathways in the ICCs.

Effects of Lizhong Tang on cultured mouse small intestine interstitial cells of Cajal

AIM: To investigate the effects of Lizhong Tang, an herbal product used in traditional Chinese medicine, on mouse small intestine interstitial cells of Cajal (ICCs).

METHODS: Enzymatic digestions were used to dissociate ICCs from mouse small intestine tissues. The ICCs were morphologically distinct from other cell types in culture and were identified using phase contrast microscopy after verification with anti c-kit antibody. A whole-cell patch-clamp configuration was used to record potentials (current clamp) from cultured ICCs. All of the experiments were performed at 30-32  °C.

RESULTS: ICCs generated pacemaker potentials, and Lizhong Tang produced membrane depolarization in current-clamp mode. The application of flufenamic acid (a nonselective cation channel blocker) abolished the generation of pacemaker potentials by Lizhong Tang. Pretreatment with thapsigargin (a Ca²⁺-ATPase inhibitor in the endoplasmic reticulum) also abolished the generation of pacemaker potentials by Lizhong Tang. However, pacemaker potentials were completely abolished in the presence of an external Ca²⁺-free solution, and under this condition, Lizhong Tang induced membrane depolarizations. Furthermore, When GDP-β-S (1 mmol/L) was in the pipette solution, Lizhong Tang still induced membrane depolarizations. In addition, membrane depolarizations were not inhibited by chelerythrine or calphostin C, which are protein kinase C inhibitors, but were inhibited by U-73122, an active phospholipase C inhibitors.

CONCLUSION: These results suggest that Lizhong Tang might affect gastrointestinal motility by modulating pacemaker activity in interstitial cells of Cajal.

Evidence for M2 and M3 muscarinic receptor involvement in cholinergic excitatory junction potentials through synergistic activation of cation channels in the longitudinal muscle of mouse ileum

Cholinergic nerve-mediated excitatory junction potentials (EJPs) in the longitudinal muscle of mouse ileum were characterized by using M2 or M3 muscarinic receptor-knockout (KO) mice and 1-[β-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SK&F 96365) and pertussis toxin (PTX). EJPs evoked by electrical field stimulation (EFS) in wild-type preparations, initially determined to be cholinergic in origin using tetrodotoxin, atropine, and eserine, were profoundly depressed after SK&F 96365 treatment known to block muscarinic receptor-operated cation channels. A similar depression of the EJPs was also observed by PTX treatment, which is predicted to disrupt M2-mediated pathways linked to cation channel activation. In M2-KO mouse preparations, cholinergic EJPs were evoked by EFS with their relative amplitude of 20%-30% to the wild-type EJP and strongly inhibited by SK&F 96365. No cholinergic EJP was seen in M3-KO as well as M2/M3 double-KO preparations. The results suggest that the wild-type cholinergic EJP is not a simple mixture of M2 and M3 responses, but due to synergistic activation of cation channels by both M2 and M3 receptors in the murine ileal longitudinal muscle.

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

Arecoline excites the contraction of distal colonic smooth muscle strips in rats via the M3 receptor – extracellular Ca2+ influx – Ca2+ store release pathway

Areca is a Chinese herbal medicine that is widely used for constipation. However the mechanisms of its action are not clear. We investigated the effects of arecoline, the most active component of areca, on the motility of rat distal colonic smooth muscle strips. In longitudinal muscle of distal colon (LMDC) and circular muscle of distal colon (CMDC), arecoline increased the contraction in a dose-dependent manner. Tetrodotoxin (TTX) did not inhibit the effects of arecoline. The contractile response to arecoline was completely antagonized by atropine. 4-Diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) strongly depressed the response to arecoline, but gallamine and methoctramine did not. Nifedipine, 2-aminoethoxydiphenyl borate (2-APB), and Ca2+-free Krebs solution with EGTA partly inhibited the effects of arecoline. The sum of Ca2+-free Krebs solution, EGTA, and 2-APB completely inhibited the effects of arecoline. The results show that arecoline stimulates distal colonic contraction in rats via the muscarinic (M3) receptor – extracellular Ca2+ influx – Ca2+ store release pathway. It is likely that the action of areca in relieving constipation is due to its stimulation of muscle contraction.

Multiple muscarinic pathways mediate the suppression of voltage-gated Ca2+ channels in mouse intestinal smooth muscle cells

BACKGROUND AND PURPOSE

Stimulation of muscarinic receptors in intestinal smooth muscle cells results in suppression of voltage-gated Ca2+ channel currents (I(Ca)). However, little is known about which receptor subtype(s) mediate this effect.

EXPERIMENTAL APPROACH

The effect of carbachol on I(Ca) was studied in single intestinal myocytes from M2 or M3 muscarinic receptor knockout (KO) and wild-type (WT) mice.

KEY RESULTS

In M2KO cells, carbachol (100 microM) induced a sustained I(Ca) suppression as seen in WT cells. However, this suppression was significantly smaller than that seen in WT cells. Carbachol also suppressed I(Ca) in M3KO cells, but with a phasic time course. In M2/M3-double KO cells, carbachol had no effect on I(Ca). The extent of the suppression in WT cells was greater than the sum of the I(Ca) suppressions in M2KO and M3KO cells, indicating that it is not a simple mixture of M2 and M3 receptor responses. The G(i/o) inhibitor, Pertussis toxin, abolished the I(Ca) suppression in M3KO cells, but not in M2KO cells. In contrast, the G(q/11) inhibitor YM-254890 strongly inhibited only the I(Ca) suppression in M2KO cells. Suppression of I(Ca) in WT cells was markedly reduced by either Pertussis toxin or YM-254890.

CONCLUSION AND IMPLICATIONS

In intestinal myocytes, M2 receptors mediate a phasic I(Ca) suppression via G(i/o) proteins, while M3 receptors mediate a sustained I(Ca) suppression via G(q/11) proteins. In addition, another pathway that requires both M2/G(i/o) and M3/G(q/11) systems may be operative in inducing a sustained I(Ca) suppression.

Deletion of TRPC4 and TRPC6 in mice impairs smooth muscle contraction and intestinal motility in vivo

BACKGROUND & AIMS

Downstream effects of muscarinic receptor stimulation in intestinal smooth muscle include contraction and intestinal transit. We thought to determine whether classic transient receptor potential (TRPC) channels integrate the intracellular signaling cascades evoked by the stimulated receptors and thereby contribute to the control of the membrane potential, Ca-influx, and cell responses.

METHODS

We created trpc4-, trpc6-, and trpc4/trpc6-gene-deficient mice and analyzed them for intestinal smooth muscle function in vitro and in vivo.

RESULTS

In intestinal smooth muscle cells TRPC4 forms a 55 pS cation channel and underlies more than 80% of the muscarinic receptor-induced cation current (mI(CAT)). The residual mI(CAT) depends on the expression of TRPC6, indicating that TRPC6 and TRPC4 determine mI(CAT) channel activity independent of other channel subunits. In TRPC4-deficient ileal myocytes the carbachol-induced membrane depolarizations are diminished greatly and the atropine-sensitive contraction elicited by acetylcholine release from excitatory motor neurons is reduced greatly. Additional deletion of TRPC6 aggravates these effects. Intestinal transit is slowed down in mice lacking TRPC4 and TRPC6.

CONCLUSIONS

In intestinal smooth muscle cells TRPC4 and TRPC6 channels are gated by muscarinic receptors and are responsible for mI(CAT). They couple muscarinic receptors to depolarization of intestinal smooth muscle cells and voltage-activated Ca(2+)-influx and contraction, and thereby accelerate small intestinal motility in vivo.

Carbachol regulates pacemaker activities in cultured interstitial cells of Cajal from the mouse small intestine

We studied the effect of carbachol on pacemaker currents in cultured interstitial cells of Cajal (ICC) from the mouse small intestine by muscarinic stimulation using a whole cell patch clamp technique and Ca2+-imaging. ICC generated periodic pacemaker potentials in the current-clamp mode and generated spontaneous inward pacemaker currents at a holding potential of-70 mV. Exposure to carbachol depolarized the membrane and produced tonic inward pacemaker currents with a decrease in the frequency and amplitude of the pacemaker currents. The effects of carbachol were blocked by 1-dimethyl-4-diphenylacetoxypiperidinium, a muscarinic M(3) receptor antagonist, but not by methotramine, a muscarinic M(2) receptor antagonist. Intracellular GDP-beta-S suppressed the carbachol-induced effects. Carbachol-induced effects were blocked by external Na+-free solution and by flufenamic acid, a non-selective cation channel blocker, and in the presence of thapsigargin, a Ca2+-ATPase inhibitor in the endoplasmic reticulum. However, carbachol still produced tonic inward pacemaker currents with the removal of external Ca2+. In recording of intracellular Ca2+ concentrations using fluo 3-AM dye, carbachol increased intracellular Ca2+ concentrations with increasing of Ca2+ oscillations. These results suggest that carbachol modulates the pacemaker activity of ICC through the activation of non-selective cation channels via muscarinic M(3) receptors by a G-protein dependent intracellular Ca2+ release mechanism.

Induction of pacemaker currents by DA-9701, a prokinetic agent, in interstitial cells of Cajal from murine small intestine

The interstitial cells of Cajal (ICC) are pacemaking cells required for gastrointestinal motility. The possibility of whether DA-9701, a novel prokinetic agent formulated with Pharbitis Semen and Corydalis Tuber, modulates pacemaker activities in the ICC was tested using the whole cell patch clamp technique. DA-9701 produced membrane depolarization and increased tonic inward pacemaker currents in the voltage-clamp mode. The application of flufenamic acid, a non-selective cation channel blocker, but not niflumic acid, abolished the generation of pacemaker currents induced by DA-9701. Pretreatment with a Ca2+-free solution and thapsigargin, a Ca2+-ATPase inhibitor in the endoplasmic reticulum, abolished the generation of pacemaker currents. In addition, the tonic inward currents were inhibited by U-73122, an active phospholipase C inhibitor, but not by GDP-beta-S, which permanently binds G-binding proteins. Furthermore, the protein kinase C inhibitors, chelerythrine and calphostin C, did not block the DA-9701-induced pacemaker currents. These results suggest that DA-9701 might affect gastrointestinal motility by the modulation of pacemaker activity in the ICC, and the activation is associated with the non-selective cationic channels via external Ca2+ influx, phospholipase C activation, and Ca2+ release from internal storage in a G protein-independent and protein kinase C-independent manner.

A non-selective cationic channel activated by diacylglycerol in mouse intestinal myocytes

Application of 1-oleoyl-2-acetyl-sn-glycerol (OAG), an analogue of diacylglycerol (DAG) formed via M(3) muscarinic receptors, induced inward cationic currents via a protein kinase C-independent mechanism and produced membrane depolarization with increased action potential discharges in mouse intestinal myocytes. Outside-out patches from the myocytes responded to OAG with openings of 115-pS channels characterized by a mean open time (O(tau)) of 0.15 ms. M(3) receptor stimulation is reportedly capable of causing brief openings (O(tau)=0.23 ms) of 120-pS cationic channels in intestinal myocytes, thus the present results strongly support the idea that the M(3)-mediated 120-pS channel opening is brought about via DAG-dependent mechanisms.

Evolutionary changes of astroglia in Elasmobranchii comparing to amniotes: a study based on three immunohistochemical markers (GFAP, S-100, and glutamine synthetase)

This paper supplements former studies on elasmobranch species with an immunohistochemical investigation into glutamine synthetase and S-100 protein, in addition to GFAP, and extends its scope to the representatives of almost every group of Elasmobranchii: squalomorph sharks, galeomorph sharks, skates (Rajiformes) and rays (Torpediniformes and Myliobatifomes). More glial elements were labeled by S-100 protein, and even more so by using glutamine synthetase immunostaining than by GFAP: more astrocytes (mainly non-perivascular ones) were detected in the telencephalon of sharks, skates and rays. Only the markers S-100 and glutamine synthetase, but not GFAP, characterized the Bergmann-glia of skates and rays and astrocyte-like non-ependymal cells in Squalus acanthias. Another squalomorph shark species, Pristiophorus cirratus, however, had GFAP immunopositive astrocytes. Of all the species studied, the greatest number of GFAP positive astrocytes could be observed in Mobula japanica (order Myliobatiformes), in each major brain part. According to anatomical location, perivascular glia comprised varied types, including even a location in Mobula, which can also be found in mammals. Remnants of radial glia were found in confined areas of skates, less so in rays. In the rhombencephalon and in the spinal cord modified ependymoglia predominated in every group. In conclusion, there was no meaningful difference between the astroglial architectures of squalomorph and galeomorph sharks. The difference in the astroglial structure between sharks and batoids, however, was confined to the telencephalon and mesencephalon, and did not take place in the rhombencephalon, the latter structure being quite similar in all the species studied. The appearance of astrocytes in the relatively thin-walled shark telencephalon, however, indicates that the brain thickening promoted the preponderance of astrocytes rather than their appearance itself. Although the evolutionary changes of astroglia had some similarities in Elasmobranchii and Amniota, there was one meaningful difference: in Elasmobranchii astrocytes did not prevail in conservative brain regions as they did in the progressive brain regions.

Three distinct muscarinic signalling pathways for cationic channel activation in mouse gut smooth muscle cells

Using mutant mice genetically lacking certain subtypes of muscarinic receptor, we have studied muscarinic signal pathways mediating cationic channel activation in intestinal smooth muscle cells. In cells from M2 subtype-knockout (M2-KO) or M3-KO mice, carbachol (100 microM) evoked a muscarinic cationic current (mI(Cat)) as small as approximately 10% of mI(Cat) in wild-type (WT) cells. No appreciable current was evoked in M2/M3 double-KO cells. All mutant type cells preserved normal G-protein-cationic channel coupling. The M3-KO and WT mI(Cat) each showed a U-shaped current-voltage (I-V) relationship, whereas the M2-KO mI(Cat) displayed a linear I-V relationship. Channel analysis in outside-out patches recognized 70-pS and 120-pS channels as the major muscarinic cationic channels. Active patches of M2-KO cells exhibited both 70-pS and 120-pS channel activity usually together, either of which consisted of brief openings (the respective mean open times O(tau) = 0.55 and 0.23 ms). In contrast, active M3-KO patches showed only 70-pS channel activity, which had three open states (O(tau) = 0.55, 3.1 and 17.4 ms). In WT patches, besides the M2-KO and M3-KO types, another type of channel activity was also observed that consisted of 70-pS channel openings with four open states (O(tau) = 0.62, 2.7, 16.9 and 121.1 ms), and patch current of this channel activity showed a U-shaped I-V curve similar to the WT mI(Cat). The present results demonstrate that intestinal myocytes are endowed with three distinct muscarinic pathways mediating cationic channel activation and that the M2/M3 pathway targeting 70-pS channels, serves as the major contributor to mI(Cat) generation. The delineation of this pathway is consistent with the formation of a functional unit by the M2-Go protein and the M3-PLC systems predicted to control cationic channels.

Roles of M2 and M3 muscarinic receptors in cholinergic nerve-induced contractions in mouse ileum studied with receptor knockout mice

BACKGROUND AND PURPOSE

The functional roles of M(2) and M(3) muscarinic receptors in neurogenic cholinergic contractions in gastrointestinal tracts remain to be elucidated. To address this issue, we studied cholinergic nerve-induced contractions in the ileum using mutant mice lacking M(2) or M(3) receptor subtypes.

EXPERIMENTAL APPROACH

Contractile responses to transmural electrical (TE) stimulation were isometrically recorded in ileal segments from M(2)-knockout (KO), M(3)-KO, M(2)/M(3)-double KO, and wild-type mice.

KEY RESULTS

TE stimulation at 2-50 Hz frequency-dependently evoked a fast, brief contraction followed by a slower, longer one in wild-type, M(2)-KO or M(3)-KO mouse preparations. Tetrodotoxin blocked both the initial and later contractions, while atropine only inhibited the initial contractions. The initial cholinergic contractions were significantly greater in wild-type than M(2)-KO or M(3)-KO mice; the respective mean amplitudes at 50 Hz were 91, 74 and 68 % of 70mM K(+)-induced contraction. Pretreatment with pertussis toxin blocked the cholinergic contractions in M(3)-KO but not in M(2)-KO mice. Cholinergic contractions also remained in wild-type preparations, but their sizes were reduced by 20-30 % at 10-50 Hz. In M(2)/M(3)-double KO mice, TE stimulation evoked only slow, noncholinergic contractions, which were significantly greater in sizes than in any of the other three mouse strains.

CONCLUSION AND IMPLICATIONS

These results demonstrate that M(2) and M(3) receptors participate in mediating cholinergic contractions in mouse ileum with the latter receptors assuming a greater role. Our data also suggest that the lack of both M(2) and M(3) receptors causes upregulation of noncholinergic excitatory innervation of the gut smooth muscle.

Regulation of TRP-like muscarinic cation current in gastrointestinal smooth muscle with special reference to PLC/InsP3/Ca2+ system

Acetylcholine, the main enteric excitatory neuromuscular transmitter, evokes membrane depolarization and contraction of gastrointestinal smooth muscle cells by activating G protein-coupled muscarinic receptors. Although the cholinergic excitation is generally underlined by the multiplicity of ion channel effects, the primary event appears to be the opening of cation-selective channels; among them the 60 pS channel has been recently identified as the main target for the acetylcholine action in gastrointestinal myocytes. The evoked cation current, termed mI(CAT), causes either an oscillatory or a more sustained membrane depolarization response, which in turn leads to increases of the open probability of voltage-gated Ca2+ channels, thus providing Ca2+ entry in parallel with Ca2+ release for intracellular Ca2+ concentration rise and contraction. In recent years there have been several significant developments in our understanding of the signaling processes underlying mICAT generation. They have revealed important synergistic interactions between M2 and M3 receptor subtypes, single channel mechanisms, and the involvement of TRPC-encoded proteins as essential components of native muscarinic cation channels. This review summarizes these recent findings and in particular discusses the roles of the phospholipase C/InsP3/intracellular Ca2+ release system in the mI(CAT) physiological regulation.