Receptor-induced Ca(2+) signaling in cultured myenteric neurons

Butyrate Protects Myenteric Neurons Loss in Mice Following Experimental Ulcerative Colitis

The enteric nervous system is affected by inflammatory bowel diseases (IBD). Gut microbiota ferments dietary fibers and produces short-chain fatty acids, such as Butyrate, which bind to G protein-coupled receptors, such as GPR41, and contribute to maintaining intestinal health. This work aimed to study the GPR41 in myenteric neurons and analyze the effect of Butyrate in mice submitted to experimental ulcerative colitis. The 2, 4, 6 trinitrobenzene sulfonic acid (TNBS) was injected intrarectally in C57BL/6 mice (Colitis). Sham group received ethanol (vehicle). One group was treated with 100 mg/kg of Sodium Butyrate (Butyrate), and the other groups received saline. Animals were euthanized 7 days after colitis induction. Analyzes demonstrated colocalization of GPR41 with neurons immunoreactive (-ir) to nNOS and ChAT-ir and absence of colocalization of the GPR41 with GFAP-ir glia. Quantitative results demonstrated losses of nNOS-ir, ChAT-ir, and GPR41-ir neurons in the Colitis group and Butyrate treatment attenuated neuronal loss. The number of GFAP-ir glia increased in the Colitis group, whereas Butyrate reduced the number of these cells. In addition, morphological alterations observed in the Colitis group were attenuated in the Butyrate group. The presence of GPR41 in myenteric neurons was identified, and the treatment with Butyrate attenuated the damage caused by experimental ulcerative colitis.

The gut brain in a dish: Murine primary enteric nervous system cell cultures

BACKGROUND

The enteric nervous system (ENS) is an extensive neural network embedded in the wall of the gastrointestinal tract that regulates digestive function and gastrointestinal homeostasis. The ENS consists of two main cell types; enteric neurons and enteric glial cells. In vitro techniques allow simplified investigation of ENS function, and different culture methods have been developed over the years helping to understand the role of ENS cells in health and disease.

PURPOSE

This review focuses on summarizing and comparing available culture protocols for the generation of primary ENS cells from adult mice, including dissection of intestinal segments, enzymatic digestions, surface coatings, and culture media. In addition, the potential of human ENS cultures is also discussed.

Fast calcium and voltage-sensitive dye imaging in enteric neurones reveal calcium peaks associated with single action potential discharge

Slow changes in [Ca(2+)](i) reflect increased neuronal activity. Our study demonstrates that single-trial fast [Ca(2+)](i) imaging (≥200 Hz sampling rate) revealed peaks each of which are associated with single spike discharge recorded by consecutive voltage-sensitive dye (VSD) imaging in enteric neurones and nerve fibres. Fast [Ca(2+)](i) imaging also revealed subthreshold fast excitatory postsynaptic potentials. Nicotine-evoked [Ca(2+)](i) peaks were reduced by -conotoxin and blocked by ruthenium red or tetrodotoxin. Fast [Ca(2+)](i) imaging can be used to directly record single action potentials in enteric neurones. [Ca(2+)](i) peaks required opening of voltage-gated sodium and calcium channels as well as Ca(2+) release from intracellular stores.

Prolonged IL-1beta exposure alters neurotransmitter and electrically induced Ca(2+) responses in the myenteric plexus

BACKGROUND Infection and inflammatory diseases of the gut results in profound changes of intestinal motor function. Acute administration of the pro-inflammatory cytokine interleukin-1beta (IL-1beta) was shown to have excitatory and neuromodulatory roles in the myenteric plexus. Here we aimed to study the effect of prolonged IL-1beta incubation on the response of myenteric neurones to different stimuli. METHODS Longitudinal muscle myenteric plexus preparations (LMMP's) of the guinea pig jejunum were incubated for 24 h in medium with or without IL-1beta. After loading with Fluo-4, calcium imaging was used to visualize activation of neurones. The response to application of serotonin (5-HT), substance P (SP) and ATP or to electrical fibre tract stimulation (eFTS) was tested. Expression of nNOS, HuD, calbindin and calretinin was compared by immunohistochemistry. KEY RESULTS IL-1beta concentration-dependently influenced the neuronal responsiveness and duration of the [Ca(2+)](i) rises to 5-HT and ATP, while it also affected the Ca(2+)-transient amplitudes induced by 5-HT, ATP and SP. Ca(2+)-transients in response to eFTS were observed in significantly more neurones per ganglion after IL-1beta (10(-10) and 10(-11) mol L(-1)). Peak [Ca(2+)](i) rise after eFTS was concentration-dependently decreased by IL-1beta. The duration of the [Ca(2+)](i) rise after eFTS was prolonged after IL-1beta 10(-12) mol L(-1). IL-1beta (10(-9) mol L(-1)) incubation did not affect the number of nNOS, calretinin and calbindin expressing neurones, nor did it induce neuronal loss (HuD). CONCLUSIONS & INFERENCES In this study, IL-1beta differentially modulates the neuronal response to eFTS and neurotransmitter application in the myenteric plexus of guinea pigs. This cytokine could be implicated in the motility disturbances observed during gastrointestinal inflammation.

Electrochemical detection of neurotransmitters in the gut wall

Cells interact with each other by releasing signalling molecules, which can activate or inactivate target cells. In order to understand how coordination results from this communication, accurate measurements of these signalling molecules are prerequisite. Several different techniques exist to monitor and quantify these compounds, including enzymatic and histochemical assays, electrophysiological and optical recordings. However, there has been little use of electrochemical recordings in gastroenterological research, although these are very fast and sensitive. Electrochemical techniques rely on the simple fact that electroactive molecules can be oxidized at a given potential. The currents, elicited by the oxidation, are directly proportional to the concentration of the compound. In the current issue of Neurogastroenterology and Motility, electrochemical detection was successfully applied to measure nitric oxide (NO) from intestinal preparations. Although there are some important specificity, timing and spatial aspects to consider, this direct NO-probing technique is definitely a great asset to the field of gastrointestinal research and advances our understanding of NO signalling in the intestinal wall.

CRF-induced calcium signaling in guinea pig small intestine myenteric neurons involves CRF-1 receptors and activation of voltage-sensitive calcium channels

Corticotropin-releasing factor (CRF) is a 41-amino acid peptide with distinct effects on gastrointestinal motility involving both CRF-1 and CRF-2 receptor-mediated mechanisms that are generally claimed to be centrally mediated. Evidence for a direct peripheral effect is rather limited. Electrophysiological studies showed a cAMP-dependent prolonged depolarization of guinea pig myenteric neurons on application of CRF. The current study aimed to test the direct effect of CRF on myenteric neurons and to identify the receptor subtype and the possible mechanisms involved. Longitudinal muscle myenteric plexus preparations and myenteric neuron cultures of guinea pig small intestine were incubated with the calcium indicator Fluo-4. Confocal Ca(2+) imaging was used to visualize activation of neurons on application of CRF. All in situ experiments were performed in the presence of nicardipine 10(-6) M to reduce tissue movement. Images were analyzed using Scion image and a specifically developed macro to correct for residual minimal movements. A 75 mM K(+)-Krebs solution identified 1,076 neurons in 46 myenteric ganglia (16 animals). Administration of CRF 10(-6) M and CRF 10(-7) M during 30 s induced a Ca(2+) response in 22.4% of the myenteric neurons (n = 303). Responses were completely abolished in the presence of the nonselective CRF antagonist astressin (n = 55). The selective CRF-1 receptor antagonist CP 154,526 (n = 187) reduced the response significantly to 2.1%. Stresscopin, a CRF-2 receptor agonist, could not activate neurons at 10(-7) M, and its effect at 10(-6) M (15.3%, n = 59) was completely blocked by CP 154,526. TTX 10(-6) M (n = 70) could not block the CRF-induced Ca(2+) transients but reduced the amplitude of the signals significantly. Removal of extracellular Ca(2+) blocked all responses to CRF (n = 47). L-type channels did not contribute to the CRF-induced Ca(2+) transients. Blocking N- or P/Q-type Ca(2+) channels did not reduce the responses significantly. Combined L- and R-type Ca(2+) channel blocking (SNX-482 10(-8) M, n = 64) abolished nearly all responses in situ. Combined L-, N-, and P/Q-type channel blocking also significantly reduced the response to 8.6%. Immunohistochemical staining for CRF-1 receptors clearly labeled individual cell bodies in the ganglia, whereas the CRF-2 receptor staining was barely above background. CRF induces Ca(2+) transients in myenteric neurons via a CRF-1 receptor-dependent mechanism. These Ca(2+) transients highly depend on somatic calcium influx through voltage-operated Ca(2+) channels, in particular R-type channels. Action potential firing through voltage-sensitive sodium channels increases the amplitude of the Ca(2+) signals. Besides centrally mediated effects, CRF is likely to modulate gastrointestinal motility on the myenteric neuronal level.

P2X2 receptors are essential for [Ca2+]i increases in response to ATP in cultured rat myenteric neurons

We characterized ATP-induced changes in intracellular Ca2+ concentration ([Ca2+]i) and membrane current in cultured rat myenteric neurons using ratiometric Ca2+ imaging with fura-2 and the whole cell patch-clamp technique, respectively. Neuronal cells were functionally identified by [Ca2+]i responses to high K+ and nicotine, which occurred only in cells positive for neuron-specific protein gene product 9.5 immunoreactivity. ATP evoked a dose-dependent increase of [Ca2+]i that was greatly decreased by the removal of extracellular Ca2+ concentration ([Ca2+]o). The amplitude of the [Ca2+]i response to ATP was reduced by half in the presence of voltage-dependent Ca2+ channel blockers. In [Ca2+]o-free solution, ATP produced a small transient rise in [Ca2+]i similar to that induced by P2Y agonists. At -60 mV, ATP evoked a slowly inactivating inward current that was suppressed by the removal of extracellular Na+ concentration. The current-voltage relation for ATP showed an inward rectification with the reversal potential of about 0 mV. The apparent rank order of potency for the purinoceptor agonist-induced increases of [Ca2+]i was ATP > or = adenosine 5'-O-3-triphosphate > or = CTP > or = 2-methylthio-ATP > benzoylbenzoyl-ATP. A similar potency order was obtained with current responses to these agonists. P2 antagonists inhibited inward currents induced by ATP. Ca2+ and Mg2+ suppressed the ATP-induced current, and Zn2+, Cu2+, and protons potentiated it. RT-PCR and immunocytochemical studies showed the expression of P2X2 receptors in cultured rat myenteric neurons. These results suggest that ATP mainly activates ionotropic P2X2 receptors, resulting in a [Ca2+]i increase dependent on [Ca2+]o in rat myenteric neurons. A small part of the ATP-induced [Ca2+]i increase may be also mediated via a P2Y receptor-related mechanism.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Inhibitory effects of galanin on evoked [Ca2+]i responses in cultured myenteric neurons

Galanin modulates gastrointestinal motility by inhibiting the release of ACh from enteric neurons. It is, however, not known whether galanin also inhibits neuronal cholinergic transmission postsynaptically and whether galanin also reduces the action of other excitatory neurotransmitters. The aim of the present study was thus to investigate the effect of galanin on the evoked intracellular Ca(2+) concentration ([Ca(2+)](i)) responses in myenteric neurons. Cultured myenteric neurons from small intestine of adult guinea pigs were loaded with the Ca(2+) indicator fluo-3 AM, and the [Ca(2+)](i) responses following the application of different stimuli were quantified by confocal microscopy and expressed as a percentage of the response to high-K(+) solution (75 mM). Trains of electrical pulses (2 s, 10 Hz) were applied to stimulate the neuronal fibers before and after a 30-s superfusion with galanin (10(-6) M). Substance P (SP), 5-HT, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP), and carbachol were used as direct postsynaptic stimuli (10(-5) M, 30 s) and were applied alone or after galanin perfusion. Galanin significantly reduced the responses induced by electrical fiber stimulation (43 +/- 2 to 35 +/- 3%, P = 0.01), SP (15.4 +/- 1 to 8.0 +/- 0.3%, P < 0.01), and 5-HT (26 +/- 2 to 21.4 +/- 1.5%, P < 0.05). On the contrary, galanin did not affect the responses induced by local application of DMPP and carbachol. We conclude that in cultured myenteric neurons, galanin inhibits the excitatory responses induced by electrical stimulation, SP, and 5-HT. Finally, the inhibitory effect of galanin on electrical stimulation, but not on DMPP- and carbachol-induced responses, suggests that, at least for the cholinergic component, galanin acts at the presynaptic level.

Characteristics of intermittent mitochondrial transport in guinea pig enteric nerve fibers

Enteric neurons controlling various gut functions are prone to oxidative insults that might damage mitochondria (e.g., intestinal inflammation). To resume local energy supply, mitochondria need to be transported. We used MitoTracker dyes and confocal microscopy to investigate basic characteristics of mitochondrial transport in guinea pig myenteric neurites. During a 10-s observation of 1 mm nerve fiber, on average, three mitochondria were transported at an average speed of 0.41 +/- 0.02 microm/s. Movement patterns were clearly erratic, and velocities were independent of mitochondrial size. The velocity oscillated periodically ( approximately 6 s) but was not consistently affected by structures such as en route boutons, bifurcations, or stationary mitochondria. Also, mitochondria transported in opposite directions did not necessarily affect each others' mobility. Transport was blocked by microtubule disruption (100 microM colchicine), and destabilization (1 microM cytochalasin-D) or stabilization (10 microM phalloidin) of actin filaments, respectively, decreased (0.22 +/- 0.02 microm/s, P < 0.05) or increased (0.53 +/- 0.02 microm/s, P < 0.05) transport speed. Transport was inhibited by TTX (1 microM), and removal of extracellular Ca(2+) (plus 2 mM EGTA) had no effect. However, depletion of intracellular stores (thapsigargin) reduced (to 33%) and slowed the transport significantly (0.18 +/- 0.02 microm/s, P < 0.05), suggesting an important role for stored Ca(2+) in mitochondrial transport. Transport was also reduced (to 21%) by the mitochondrial uncoupler FCCP (1 microM) in a time-dependent fashion and slowed by oligomycin (10 microM). We conclude that mitochondrial transport is remarkably independent of structural nerve fiber properties. We also show that mitochondrial transport is TTX sensitive and speeds up by stabilizing actin and that functional Ca(2+) stores are required for efficient transport.

Calcium imaging of gut activity

The major cell types regulating gut motility include enteric neurones, interstitial cells of Cajal (ICC) and their effector smooth muscle cells. These cells are arranged conveniently in nested layers through the gut wall. Our knowledge of how many of these cells in each layer are integrated to produce the various patterns of motility is largely unknown. So far, much of our knowledge of gut motility has usually been obtained by examining point sources of activity (e.g. intracellular recordings from enteric neurones, ICC and smooth muscle cells), rather than the spread of activity through these spatially distributed nerve and ICC networks, or smooth muscle syncitia. Our understanding of how these cells are integrated to produce gut movements would be greatly enhanced if we could image the activity in many of these cells in each layer, or many cells in several layers, simultaneously. Calcium (Ca2+) is a major signalling and regulatory molecule in most cells. In fact, electrical excitability in enteric neurones, ICC and smooth muscle is associated with robust rises in intracellular Ca2+ that long outlast the electrical events (e.g. action potentials in neurones and smooth muscle) that gave rise to them. These prolonged Ca2+ responses, together with the development of several high quality Ca2+ indicators, has provided a unique opportunity to image many cells in intact tissues simultaneously using ICCD video-rate cameras along with conventional microscopy. However, confocal microscopy has also been used, and has several advantages over the above systems. These include reduced photo-toxicity and bleaching and the elimination of out of focus light from different layers within the tissue. So far, despite some limitations with the calcium imaging techniques, the spread of activity through the two layers of smooth muscle, ICC networks and myenteric neurones in intact preparations, or cultured myenteric neuronal networks, is beginning to yield exciting new data about how these different cells interact and process information.

Electrical stimulation reveals complex neuronal input and activation patterns in single myenteric guinea pig ganglia

The myenteric plexus plays a key role in the control of gastrointestinal motility. We used confocal calcium imaging to study responses to electrical train stimulation (ETS) of interganglionic fiber tracts in entire myenteric ganglia of the guinea pig small intestine. ETS induced calcium transients in a subset of neurons: 52.2% responded to oral ETS, 65.4% to aboral ETS, and 71.7% to simultaneous oral and aboral ETS. A total of 41.3% of the neurons displayed convergence of oral and aboral ETS-induced responses. Responses could be reversibly blocked with TTX (10(-)6 M), demonstrating involvement of neuronal conduction, and by removal of extracellular calcium. omega-Conotoxin (5 x 10(-7) M) blocked the majority of responses and reduced the amplitude of residual responses by 45%, indicating the involvement of N-type calcium channels. Staining for calbindin and calretinin did not reveal different response patterns in these immunohistochemically identified neurons. We conclude that, at least for ETS close to a ganglion, confocal calcium imaging reveals complex oral and aboral input to individual myenteric neurons rather than a polarization in spread of activity.

GABA(A) receptor antagonists enhance cortical acetylcholine release induced by 5-HT(3) receptor blockade in freely moving rats

ACh release from the rat frontal cortex was increased by both local, 0.1-1 microM, and systemic, 0.1-10 microg/kg, administration of the 5-HT(3) receptor antagonist ondansetron, reaching a maximum peak of 143% over basal values. Bicuculline, 1-10 microM, and flumazenil, 5-10 mg/kg, antagonists at different sites of the GABA(A) receptor, also enhanced ACh release, with maximum effects of 85 and 124% above baseline, respectively. GABA(A) receptor antagonists potentiated the effect induced by ondansetron on ACh release, reaching a peak increase of 238% (with bicuculline) and 259% (with flumazenil) over basal levels. These results suggest an interaction of ondansetron with GABAergic neurons modulating ACh release in the rat frontal cortex in vivo.

Differential Ca(2+) signaling characteristics of inhibitory and excitatory myenteric motor neurons in culture

Physiological studies on functionally identified myenteric neurons are scarce because of technical limitations. We combined retrograde labeling, cell culturing, and fluorescent intracellular Ca(2+) concentration ([Ca(2+)](i)) signaling to study excitatory neurotransmitter responsiveness of myenteric motor neurons. 1, 1-Didodecyl-3,3,3',3'-tetramethyl indocarbocyanine (DiI) was used to label circular muscle motor neurons of the guinea pig ileum. DiI-labeled neurons were easily detectable in cultures prepared from these segments. The excitatory neurotransmitters (10(-5) M) acetylcholine, substance P, and serotonin induced a transient rise in [Ca(2+)](i) in subsets of DiI-labeled neurons (66.7, 56.5, and 84. 3%, respectively). DiI-labeled motor neurons were either inhibitory (23.8%) or excitatory (76.2%) as assessed by staining for nitric oxide synthase or choline acetyltransferase. Compared with excitatory motor neurons, significantly fewer inhibitory neurons in culture responded to acetylcholine (0 vs. 69%) and substance P (12.5 vs. 69.2%). We conclude that combining retrograde labeling and Ca(2+) imaging allows identification of differential receptor expression in functionally identified neurons in culture.