Possible role of transient receptor potential melastatin 4 channels in adrenergic contractions in mouse prostate smooth muscles.

Transient receptor potential melastatin 4 (TRPM4) cation channels are expressed in prostate glands. However, the precise role of these channels in prostate contractility remains unclear. In this study, we examined whether TRPM4 channels were involved in adrenergic contractions in the mouse prostate gland. Adrenergic contractile responses elicited by noradrenaline or electrical field stimulation of the sympathetic nerve were isometrically recorded, and the effects of 9-phenanthrol, a specific TRPM4 channel inhibitor, on those contractile responses were investigated in mouse ventral prostate preparations. 9-phenanthrol (10 or 30 µM) inhibited noradrenaline- and sympathetic nerve-evoked contractions in a concentration-dependent manner. A similar inhibitory effect was observed with another TRPM4 channel inhibitor, 4-chloro-2-(2-(naphthalene-1-yloxy) acetamido) benzoic acid (NBA; 10 µM). Inhibition by 9-phenanthrol and NBA were much greater at lower noradrenaline concentrations and lower stimulus frequencies than those of higher concentrations or frequencies. However, 9-phenanthrol did not inhibit the noradrenaline-induced contractile response when the membrane potential was decreased to approximately 0 mV in the 140 mM K+ medium. Moreover, 9-phenanthrol does not affect noradrenaline-induced increases in spontaneous contractions of cardiac atrial preparation. This agent inhibited noradrenaline-induced contractions in the posterior aorta preparation. However, the inhibitory effect was significantly weaker than that observed in the prostate gland. These results suggest that TRPM4 channels are involved in adrenergic contractions in the mouse prostate gland, possibly through membrane depolarization by their opening; therefore, they might be potential candidates for treating benign prostatic hyperplasia.

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 (mIcat) occurs mainly through synergism of M2 and M3 subtypes involving Gi/o-type GTP-binding proteins and phospholipase C (PLC). We have further studied the M2/M3 synergistic pathway. Carbachol-induced mIcat was markedly depressed by YM-254890, a Gq/11 protein inhibitor. However, the mIcat was unaffected by heparin, calphostin C, or chelerythrine, suggesting that mIcat activation does not involve signaling molecules downstream of phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown. M2-knockout (KO) mice displayed a reduced mIcat (~10% of wild-type mIcat) because of the lack of M2-Gi/o signaling. The impaired mIcat was insensitive to neuropeptide Y possessing a Gi/o-stimulating activity. M3-KO mice also displayed a reduced mIcat (~6% of wild-type mIcat) because of the lack of M3-Gq/11 signaling, and the mIcat was insensitive to prostaglandin F2α possessing a Gq/11-stimulating activity. These results suggest the importance of Gq/11/PLC-hydrolyzed PIP2 breakdown itself in mIcat activation and also support the idea that the M2/M3 synergistic pathway represents a signaling complex consisting of M2-Gi/o and M3-Gq/11-PLC systems in which both G proteins are special for this pathway but not general in receptor coupling.

Involvement of transient receptor potential melastatin 4 channels in the resting membrane potential setting and cholinergic contractile responses in mouse detrusor and ileal smooth muscles.

Here, we investigated the effects of 9-hydroxyphenanthrene (9-phenanthrol), a potent and selective transient receptor potential melastatin 4 (TRPM4) channel blocker, on the resting membrane potential and cholinergic contractile responses to elucidate the functional role of TRPM4 channels in the contractile activities of mouse detrusor and ileal longitudinal smooth muscles. We observed that, 9-phenanthrol (3-30 µM) did not significantly inhibit high K+-induced contractions in both preparations; however, 9-phenanthrol (10 µM) strongly inhibited cholinergic contractions evoked by electrical field stimulation in detrusor preparations compared to inhibitions in ileal preparations. 9-Phenanthrol (10 µM) significantly inhibited the muscarinic agonist, carbachol-induced contractile responses and slowed the maximum upstroke velocities of the contraction in detrusor preparations. However, the agent (10 µM) did not inhibit the contractions due to intracellular Ca2+ release evoked by carbachol, suggesting that the inhibitory effect of 9-phenanthrol may primarily be due to the inhibition of the membrane depolarization process incurred by TRPM4 channels. On the other hand, 9-phenanthrol (10 µM) did not affect carbachol-induced contractile responses in ileal preparations. Further, 9-phenanthrol (10 µM) significantly hyperpolarized the resting membrane potential and decreased the basal tone in both detrusor and ileal muscle preparations. Taken together, our results suggest that TRPM4 channels are constitutively active and are involved in setting of the resting membrane potential, thereby regulating the basal tone in detrusor and ileal smooth muscles. Thus, TRPM4 channels play a significant role in cholinergic signaling in detrusor, but not ileal, smooth muscles.

Muscarinic suppression of ATP-sensitive K+ channels mediated by the M3/Gq/11/phospholipase C pathway contributes to mouse ileal smooth muscle contractions.

ATP-sensitive K+ (KATP) channels are expressed in gastrointestinal smooth muscles, and their activity is regulated by muscarinic receptor stimulation. However, the physiological significance and mechanisms of muscarinic regulation of KATP channels are not fully understood. We examined the effects of the KATP channel opener cromakalim and the KATP channel blocker glibenclamide on electrical activity of single mouse ileal myocytes and on mechanical activity in ileal segment preparations. To explore muscarinic regulation of KATP channel activity and its underlying mechanisms, the effect of carbachol (CCh) on cromakalim-induced KATP channel currents ( IKATP) was studied in myocytes of M2 or M3 muscarinic receptor-knockout (KO) and wild-type (WT) mice. Cromakalim (10 µM) induced membrane hyperpolarization in single myocytes and relaxation in segment preparations from WT mice, whereas glibenclamide (10 µM) caused membrane depolarization and contraction. CCh (100 µM) induced sustained suppression of IKATP in cells from both WT and M2KO mice. However, CCh had a minimal effect on IKATP in M3KO and M2/M3 double-KO cells. The Gq/11 inhibitor YM-254890 (10 µM) and PLC inhibitor U73122 (1 µM), but not the PKC inhibitor calphostin C (1 µM), markedly decreased CCh-induced suppression of IKATP in WT cells. These results indicated that KATP channels are constitutively active and contribute to the setting of resting membrane potential in mouse ileal smooth muscles. M3 receptors inhibit the activity of these channels via a Gq/11/PLC-dependent but PKC-independent pathways, thereby contributing to membrane depolarization and contraction of smooth muscles. NEW & NOTEWORTHY We systematically investigated the regulation of ATP-sensitive K+ channels by muscarinic receptors expressed on mouse ileal smooth muscles. We found that M3 receptors inhibit the activity of ATP-sensitive K+ channels via a Gq/11/PLC-dependent, but PKC-independent, pathway. This muscarinic suppression of ATP-sensitive K+ channels contributes to membrane depolarization and contraction of smooth muscles.

Possible antagonistic effects of the TRPC4 channel blocker ML204 on M2 and M3 muscarinic receptors in mouse ileal and detrusor smooth muscles and atrial myocardium.

ML204, a potent transient receptor potential canonical 4 (TRPC4) channel blocker, is often used to elucidate the involvement of TRPC4 channels in receptor-operated signaling processes in visceral smooth muscles. In the present study, we investigated the possible antagonistic actions of ML204 on M2 and M3 muscarinic receptors, which mediate contractions in mouse ileal and detrusor smooth muscles. In ileal and detrusor smooth muscle preparations, ML204 (3 or 10 µM) significantly inhibited electrical field stimulation (EFS)-evoked cholinergic contractions. However, it did not significantly inhibit high K+-induced and EFS-evoked non-cholinergic contractions in the ileal preparations. When the muscarinic agonist, carbachol was cumulatively applied, ML204 (1, 3 and 10 µM) caused a rightward parallel shift of the concentration-response curves of carbachol. Additionally, ML204 (1, 3 and 10 µM) inhibited carbachol-induced negative chronotropic response in atrial preparations, which is mediated by M2 muscarinic receptors. Furthermore, ML204 significantly inhibited the contractions evoked by carbachol-induced intracellular Ca2+ release, which is mediated by M3 muscarinic receptors. These results suggested that ML204 might exhibit antagonistic actions on M2 and M3 muscarinic receptors; in addition, the inhibitory effects of ML204 against EFS-induced cholinergic contractions might be attributed to this receptor antagonism rather than inhibition of TRPC4 channel activity. Therefore, these effects should be considered when ML204 is used as a TRPC4 channel blocker.

Muscarinic M2 receptor promotes vasopressin synthesis in mice supraoptic nuclei.

Muscarinic acetylcholine receptors have been suggested to be implicated in arginine-vasopressin secretion because intracerebroventricular muscarinic agonist administration induces arginine-vasopressin release into the circulation. Although which subtype is involved in the regulation of arginine-vasopressin secretion is unclear, M2 receptors have been reported to be highly expressed in the hypothalamus. In the present study, M2 receptor-knockout mice were used to elucidate whether M2 receptor regulates arginine-vasopressin synthesis in the paraventricular nuclei and supraoptic nuclei of the hypothalamus. The number of arginine-vasopressin-immunoreactive neurons in M2 receptor-knockout mice was significantly decreased in the supraoptic nuclei, but not in the paraventricular nuclei compared with wild-type mice. Plasma arginine-vasopressin level in M2 receptor-knockout mice was also significantly lower than in the wild-type mice. Urinary volume and frequency as well as water intake in M2 receptor-knockout mice were significantly higher than those in wild-type mice. The V2 vasopressin receptor expression in kidneys of M2 receptor-knockout mice was comparable with that of wild-type mice, and increased urination in M2 receptor-knockout mice was significantly decreased by administration of desmopressin, a specific V2 receptor agonist, suggesting that V2 receptors in the kidneys of M2 receptor-knockout mice are intact. These results suggest that M2 receptors promote arginine-vasopressin synthesis in the supraoptic nuclei and play a role in the regulation and maintenance of body fluid.

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(+) (IKATP), voltage-gated K(+) (IKv) and Ca(2+) activated K(+) channels (IBK) in the absence and presence of SKF in single small intestinal myocytes of mice with patch-clamp techniques. SKF (10 µM) reversibly abolished IKATP 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 IKATP suppression) reversibly inhibited the IKv 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 IBK and caffeine-induced IBK. 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.

Cholinergic neuromuscular transmission mediated by interstitial cells of Cajal in the myenteric layer in mouse ileal longitudinal smooth muscles.

To elucidate the roles played by the interstitial cells of Cajal in the myenteric layer (ICC-MY) in cholinergic neuromuscular transmission, we recorded mechanical and electrical activities in response to electrical field stimulation (EFS) of the ileal longitudinal muscle strips from WBB6F1-W/W(V) (W/W(V)) mutant mice, that lacked ICC-MY and compared with those in WBB6F1-+/+ (+/+) control mice. In +/+ muscle strips, EFS induced phasic contractions, which were abolished or strongly attenuated by atropine or tetrodotoxin. In W/W(V) preparations, EFS induced similar phasic contractions, but the cholinergic component was smaller than that in +/+ strips. This was despite of the fact that the contractions because of exogenous applications of carbachol and high K(+) solution in W/W(V) strips were comparable to or rather greater than those in the +/+ preparations. EFS induced atropine-sensitive excitatory junction potentials (EJPs) in the +/+ longitudinal smooth muscle cells but not in W/W(V) cells. In the presence of eserine, EFS induced atropine-sensitive EJPs in W/W(V) cells. These results suggest that ICC-MY mediate the cholinergic neuromuscular transmission in mouse ileal longitudinal smooth muscles. In addition, the other pathway in which ICC-MY are not involved can operate concomitantly.

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-[ß-[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.

Nitrergic inhibition of tachykininergic neuro-muscular transmission via cyclic GMP in the hamster ileum.

The present study was designed to explore the inhibitory mechanism by nitric oxide (NO) of the tachykininergic neuro-muscular transmissions in the hamster ileum. In the presence of guanethidine (1 µM), atropine (0.5 µM), nifedipine (0.1 µM) and apamin (100 nM), electrical field stimuli (EFS; 0.5 ms duration, 15 V) evoked non-adrenergic, non-cholinergic excitatory junction potentials (EJPs) in circular smooth muscle cells. The EJPs were markedly inhibited by the tachykinin NK1 receptor antagonists [D-Pro(4), D-Trp(7,9)]-SP(4-11) (3 µM). Both the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 200 µM) and the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ, 10 µM), did not affect on the resting membrane potentials, but enhanced the tachykininergic EJPs. In the presence of L-NAME (200 µM), exogenously applied NO (10 µM) and the membrane permeable analogue of guanosine 3',5'-cyclic monophosphate (cGMP), 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP, 3 mM), significantly inhibited the tachykininergic EJPs. Application of EFS (0.5 msec duration, 15 V) with trains of 20 pulses at 20 Hz increased amount of released substance P (SP). The release of SP was further increased by the treatment of L-NAME or ODQ, but markedly reduced by exogenously applied NO and 8-Br-cGMP. These results suggest that the endogenous NO may inhibit the tachykininergic neuro-muscular transmissions by the decrease of SP release from the tachykininergic neurons, possibly through a guanylate cyclase-cGMP-dependent mechanism in the hamster ileum.

Muscarinic receptor subtypes mediating Ca2+ sensitization of intestinal smooth muscle contraction: studies with receptor knockout mice.

In the present study, we have characterized muscarinic receptor subtypes that mediate carbachol-induced Ca2+ sensitization of contraction in intestinal smooth muscle, using mutant mice lacking M(2) or M(3) muscarinic receptors or both receptor subtypes. In alpha-toxin-permeabilized muscle strips from wild-type (WT) mice, isometric tension responses to Ca2+ applied cumulatively (pCa 7.0-5.0) were increased when the muscarinic agonist carbachol (100 microM) was added to the medium, as judged from shifts of pCa-tension curves in both 50% effective concentration (EC(50)) and maximum response (E(max)) of pCa-tension curve. In preparations from M(2)-knockout (KO) mice, pCa-tension curves were also shifted by carbachol (100 microM), and the extents of the EC(50) and E(max) changes resembled those observed in preparations from WT mice. In preparations from M(3)-KO or M(2)/M(3)-double KO mice, however, no significant changes in pCa-tension curves were obtained after carbachol application. The G(q/11)-type G-protein inhibitor YM-254890 (1 microM) completely blocked the Ca2+ sensitization of contraction induced by carbachol in M(2)-KO or WT preparations. The results strongly support the idea that the muscarinic activation of Ca2+ sensitization in intestinal smooth muscles is mediated by the M(3) muscarinic receptor coupled to G(q/11)-type G-proteins, without any significant involvement of the other muscarinic receptor subtypes including M(2).

Tributyltin-induced Ca(2+) mobilization via L-type voltage-dependent Ca(2+) channels in PC12 cells.

The effects of tributyltin (TBT) on cytosolic Ca(2+) concentration ([Ca(2+)](c)) and cell viability were investigated in nerve growth factor-differentiated PC12 cells. TBT concentration dependently increased [Ca(2+)](c) with an EC(50) value of 0.07µM. This effect was markedly reduced by removal of the extracellular Ca(2+) or membrane depolarization with a high K(+) medium, but unaffected by thapsigargin causing depletion of intracellular Ca(2+) stores. The L-type voltage-dependent Ca(2+) channel (VDCC) blocker nicardipine blocked the effect of TBT, but the N-type VDCC blocker ω-conotoxin did not. TBT decreased the number of viable cells with an EC(50) value of 0.09µM. The TBT-induced cell death was prevented by nicardipine or by chelating the cytosolic Ca(2+) with BAPTA-AM, but not by ω-conotoxin. The results show that TBT causes an increase in [Ca(2+)](c) via activating L-type VDCCs, and support the idea that the organotin-induced cell death arises through Ca(2+) mobilization via L-type VDCCs.

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.

Two types of cation channel activated by stimulation of muscarinic receptors in guinea-pig urinary bladder smooth muscle.

The present study, aiming to elucidate ion channel mechanisms underlying muscarinic receptor-induced depolarization, has characterized membrane currents induced by carbachol in single guinea-pig urinary bladder myocytes. Application of carbachol to cells that were voltage-clamped at -50 mV produced an atropine-sensitive, biphasic inward current consisting of an initial peak followed by a smaller sustained phase. Replacing the extracellular Na+ and intracellular Cl(-) with impermeable tris+ and glutamate(-), respectively, demonstrated that the biphasic current is entirely composed of cation currents. Its initial peak phase was abolished by buffering intracellular Ca2+ to a constant level of 100 nM or depleting intracellular Ca2+ stores, and it was mimicked by the Ca2+ releaser caffeine. Ca2+ entry evoked by voltage steps in the sustained phase induced no noticeable change, indicating that this phase of cation current is insensitive to a rise of [Ca2+]i. These results demonstrate that muscarinic receptor stimulation invokes the openings of two types of cation channel, a Ca2+-activated and a receptor-operated type; the former channels are gated by a rise in [Ca2+]i upon intracellular Ca2+ release, and the latter are gated through other muscarinic receptor-coupled signal transduction mechanisms.

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.

Inflammation and inflammatory agents activate protein kinase C epsilon translocation and excite guinea-pig submucosal neurons.

BACKGROUND & AIMS: Properties of enteric neurons are transformed by inflammation and protein kinase C (PKC) isoforms are involved both in long-term changes in enteric neurons, and in transducing the effects of substances released during inflammation. We investigated roles of PKCepsilon in submucosal neurons by studying translocation in response to inflammatory mediators, effects on neuron excitability, and the changes in PKCepsilon distribution in a trinitrobenzene sulphonate model of ileitis. METHODS: Immunohistochemical detection and analysis of association with membrane and cytosolic fractions, and Western blot analysis of cytosolic and particulate fractions were used to quantify translocation. Electrophysiology methods were used to measure effects on neuron excitability. RESULTS: All submucosal neurons were immunoreactive for the novel PKC, PKCepsilon, and direct PKC activators, phorbol 12,13-dibutyrate, ingenol 3,20-dibenzoate, and the PKCepsilon-specific activator, transactivator of transduction-Psiepsilon receptor for activated C kinase, all caused PKCepsilon translocation from cytoplasm to surfaces of the neurons. Electrophysiologic studies showed that the stimulant of novel PKCs, ingenol (1 micromol/L), increased excitability of all neurons. Stimulation of protease-activated receptors caused PKCepsilon translocation selectively in vasoactive intestinal peptide secretomotor neurons, whereas a neurokinin 3 tachykinin receptor agonist caused translocation in neuropeptide Y and calretinin neurons. In all cases translocation was reduced significantly by a PKCepsilon-specific translocation inhibitor peptide. Increased PKCepsilon at the plasma membrane occurred in all neurons 6-7 days after an inflammatory stimulus. CONCLUSIONS: Major targets for PKCepsilon include ion channels near the plasma membrane. PKCepsilon is likely to have a significant role in controlling the excitability of submucosal neurons and is probably an intermediate in causing hyperexcitability after inflammation.

Phenotypic changes of morphologically identified guinea-pig myenteric neurons following intestinal inflammation.

We investigated the responses of morphologically identified myenteric neurons of the guinea-pig ileum to inflammation that was induced by the intraluminal injection of trinitrobenzene sulphonate, 6 or 7 days previously. Electrophysiological properties were examined with intracellular microelectrodes using in vitro preparations from the inflamed or control ileum. The neurons were injected with marker dyes during recording and later they were recovered for morphological examination. A proportion of neurons with Dogiel type I morphology, 45% (32/71), from the inflamed ileum had a changed phenotype. These neurons exhibited an action potential with a tetrodotoxin-resistant component, and a prolonged after-hyperpolarizing potential followed the action potential. Of the other 39 Dogiel type I neurons, no changes were observed in 36 and 3 had increased excitability. The afterhyperpolarizing potential (AHP) in Dogiel type I neurons was blocked by the intermediate conductance, Ca(2+)-dependent K(+) channel blocker TRAM-34. Neurons which showed these phenotypic changes had anally directed axonal projections. Neither a tetrodotoxin-resistant action potential nor an AHP was seen in Dogiel type I neurons from control preparations. Dogiel type II neurons retained their distinguishing AH phenotype, including an inflection on the falling phase of the action potential, an AHP and, in over 90% of neurons, an absence of fast excitatory transmission. However, they became hyperexcitable and exhibited anodal break action potentials, which, unlike control Dogiel type II neurons, were not all blocked by the h current (I(h)) antagonist Cs(+). It is concluded that inflammation selectively affects different classes of myenteric neurons and causes specific changes in their electrophysiological properties.

Effects of compounds that influence IK (KCNN4) channels on afterhyperpolarizing potentials, and determination of IK channel sequence, in guinea pig enteric neurons.

The late afterhyperpolarizing potential (AHP) that follows the action potential in intrinsic primary afferent neurons of the gastrointestinal tract has a profound influence on their firing patterns. There has been uncertainty about the identity of the channels that carry the late AHP current, especially in guinea pigs, where the majority of the physiological studies have been made. In the present work, the late AHP was recorded with intracellular microelectrodes from myenteric neurons in the guinea pig small intestine. mRNA was extracted from the ganglia to determine the identity of the guinea pig intermediate conductance potassium (I(K)) channel gene transcript. The late AHP was inhibited by two blockers of I(K) channels, TRAM34 (0.1-1 microM) and clotrimazole (10 microM), and was enhanced by the potentiator of the opening of these channels, DC-EBIO (100 nM). Action potential characteristics were unchanged by TRAM34 or DC-EBIO. The full sequence of the gene transcript and the deduced amino acid sequence were determined from extracts including myenteric ganglia and from bladder urothelium, which is a rich source of I(K) channel mRNA. This showed that the guinea pig sequence has a high degree of homology with other mammalian sequences but that the guinea pig channel lacks a phosphorylation site that was thought to be critical for channel regulation. It is concluded that the channels that carry the current of the late afterhyperpolarizing potential in guinea pig enteric neurons are I(K) channels.