Sodium conductance in cultured myenteric AH-type neurons from guinea-pig small intestine

Luminal administration ex vivo of a live Lactobacillus species moderates mouse jejunal motility within minutes

Gut commensals modulate host immune, endocrine, and metabolic functions. They also affect peripheral and central neural reflexes and function. We have previously shown that daily ingestion of Lactobacillus reuteri (LR) for 9 d inhibits the pseudoaffective cardiac response and spinal single-fiber discharge evoked by visceral distension, and decreases intestinal motility and myenteric AH cell slow afterhyperpolarization (sAHP) by inhibiting a Ca-activated K (IK(Ca)) channel. We tested whether luminal LR could acutely decrease motility in an ex vivo perfusion model of naive Balb/c jejunum. Live LR dose dependently decreased motor complex pressure wave amplitudes with 9- to 16-min onset latency and an IC(50) of 5 × 10(7) cells/ml Krebs. Heat-killed LR or another live commensal, Lactobacillus salivarius, were without effect. The IK(Ca) channel blocker TRAM-34, but neither the opener (DCEBIO) nor the hyperpolarization-activated cationic channel inhibitor ZD7288 (5 μM) (or TTX 1 μM), mimicked the LR effect on motility acutely ex vivo. We provide evidence for a rapid, strain-specific, dose-dependent action of a live Lactobacillus on small intestinal motility reflexes that recapitulates the long-term effects of LR ingestion. These observations may be useful as a first step to unraveling the pathways involved in bacteria to the nervous system communication.

Nav1.7 and Nav1.3 are the only tetrodotoxin-sensitive sodium channels expressed by the adult guinea pig enteric nervous system

The types of sodium channels that are expressed by neurons shape the rising phase of action potentials and influence patterns of action potential discharge. With regard to the enteric nervous system (ENS), there is uncertainty about which channels are expressed, and in particular it is unknown whether Na(v)1.7 is present. We designed specific probes for the guinea pig Na(v)1.7 alpha subunit as well as for the other tetrodotoxin (TTX)-sensitive alpha subunits (Na(v)1.1, Na(v)1.2, Na(v)1.3, and Na(v)1.6) in order to perform in situ hybridization (ISH) histochemistry on guinea pig myenteric ganglia. We established that only Na(v)1.7 mRNA and Na(v)1.3 mRNA are expressed in these ganglia. The ISH signal for Na(v)1.7 transcripts was found in seemingly all the myenteric neurons. The expression of the Na(v)1.3 alpha subunit was confirmed by immunohistochemistry in a large proportion (62%) of the myenteric neuron population. This population included enteric sensory neurons. Na(v)1.6 immunoreactivity, absent from myenteric neurons, was detected in glial cells only when a high anti-Na(v)1.6 antibody concentration was used. This suggests that the Na(v)1.6 alpha subunit and mRNA are present only at low levels, which is consistent with the fact that no Na(v)1.6 mRNA could be detected in the ENS by ISH. The fact that adult myenteric neurons are endowed with only two TTX-sensitive alpha subunits, namely, Na(v)1.3 and Na(v)1.7, emphasizes the singularity of the ENS. Both these subunits, known to have slow-inactivation kinetics, are well adapted for generating action potentials from slow excitatory postsynaptic potentials, a mode of synaptic transmission that applies to all ENS neuron types.

Enteric nervous system

PURPOSE OF THE REVIEW

The purpose of this review is to provide a synopsis of how the field of enteric neurobiology has advanced during the past 2 years.

RECENT FINDINGS

With more than 500 studies from which to choose, the authors have focused on several themes that illustrate recent progress. There has been an explosion of interest in the development of the enteric nervous system driven by the need to understand development abnormalities, particularly in Hirschsprung disease, and fueled by technical advances for investigating how neural crest-derived cells migrate, proliferate, and differentiate into enteric neurons and glia. The use of neural stem cells as a therapeutic strategy aimed at repopulating regions of bowel, where enteric neurones are reduced or absent, is on the horizon. Enteric reflexes involve interactions between sensory neurons, interneurons, and motor neurons. Recent findings suggest this distinction may be blurred, with neurons having multifunctional properties, perhaps because enteric neurons, unlike their central nervous system counterparts, are directly exposed to mechanical forces that they regulate. Another topic the authors have highlighted is pharmacology, with new tools for investigating ion channels, receptors, and transporters, leading to an expanding list of molecular mechanisms that regulate neuronal excitability. Long-term alterations in the expression of these molecules during disease or injury may underlie many gastrointestinal disorders that currently have unknown etiology. The authors finish with a look to the future and what may be the subject of this review next time.

SUMMARY

Basic science information gathered during the past 2 years provides insight into pathophysiologic processes and will pave the wave for improved understanding of both organic and 'functional' gastrointestinal disorders.

Expression and distribution of TTX-sensitive sodium channel alpha subunits in the enteric nervous system

The expression and distribution of TTX-sensitive voltage-gated sodium channel (VGSC) alpha subunits in the enteric nervous system (ENS) has not been described. Using RT-PCR, expression of Na(v)1.2, Na(v)1.3, Na(v)1.6, and Na(v)1.7 mRNA was detected in small and large intestinal preparations from guinea pigs. Expression of Na(v)1.1 mRNA as well as Na(v)1.1-like immunoreactivity (-li) were not observed in any intestinal region investigated. Na(v)1.2-li was primarily observed within the soma of the majority of myenteric and submucosal neurons, although faint immunoreactivity was occasionally observed in ganglionic and internodal fibers. Na(v)1.3-li was observed in dendrites, soma, and axons in a small group of myenteric neurons, as well as in numerous myenteric internodal fibers; immunoreactivity was rarely observed in the submucosal plexus. Na(v)1.6-li was primarily observed in the initial axonal segment of colonic myenteric neurons. Na(v)1.7-li was observed in dorsal root ganglia neurons but not in the myenteric plexus of the small and large intestine. In the ileum, 37% of Na(v)1.2-li cell bodies colocalized with calbindin-li while colocalization with calretinin-li was rare. In contrast, 22% of Na(v)1.3-li cell bodies colocalized with calretinin-li but colocalization with calbindin-li was not observed. In the colon, both Na(v)1.2-li and Na(v)1.3-li cell bodies frequently colocalized with either calretinin-li or calbindin-li. Na(v)1.2-li cell bodies also colocalized with the majority of NeuN-li cells in the small and large intestine. These data suggest that Na(v)1.1 may not be highly expressed in the ENS, but that Na(v)1.2, Na(v)1.3, and Na(v)1.6, and possibly Na(v)1.7, have broadly important and distinct functions in the ENS.

Intrinsic primary afferent neurons and nerve circuits within the intestine

Intrinsic primary afferent neurons (IPANs) of the enteric nervous system are quite different from all other peripheral neurons. The IPANs are transducers of physiological stimuli, including movement of the villi or distortion of the mucosa, contraction of intestinal muscle and changes in the chemistry of the contents of the gut lumen. They are the first neurons in intrinsic reflexes that influence the patterns of motility, secretion of fluid across the mucosal epithelium and local blood flow in the small and large intestines. In the guinea pig small intestine, where they have been characterized in detail, IPANs have Dogiel type II morphology, that is they are large round or oval neurons with multiple processes, some of which end close to the luminal surface of the intestine, and some of which form synapses with enteric interneurons, motor neurons and with other IPANs. The IPANs have well-defined ionic currents through which their excitability, and their functions in enteric nerve circuits, is determined. These include voltage-gated Na(+) and Ca(2+) currents, a long lasting calcium-activated K(+) current, and a hyperpolarization-activated cationic current. The IPANs exhibit long-term changes in their states of excitation that can be induced by extended periods of low frequency activity in synaptic inputs and by inflammatory mediators, either applied directly or released during an inflammatory challenge. The IPANs may be involved in pathological changes in enteric function following inflammation.

Altered excitability of intestinal neurons in primary culture caused by acute oxidative stress

Neurons were isolated from the intestine of guinea pigs and grown in primary culture for < or =15 days. Using conventional whole cell recording techniques, we demonstrated that the majority of neurons express a prolonged poststimulus afterhyperpolarization (slow AHP). These neurons also had large-amplitude (approximately 100 mV), broad-duration (approximately 2 ms) action potentials and generated a hyperpolarization activated inward current (Ih). Application of H2O2 (0.22-8.8 mM) hyperpolarized these neurons but not those lacking slow AHPs. The H2O2-induced hyperpolarization was followed by irreversible depolarization at higher concentrations (more than approximately 1 mM) of H2O2 while it was maintained after washout of submillimolar H2O2. The ionic mechanisms underlying the hyperpolarization included the suppression of Ih and the activation of an inwardly rectifying outward current, which was blocked by glybenclamide (25-50 microM) and TEA (30 mM). In addition, H2O2 suppressed the slow AHP and its underlying current. Internal perfusion of catalase and glutathione opposed the H2O2-mediated decrease in IsAHP. Our results indicate that acute oxidative stress has neuron- and conductance-specific actions in intestinal neurons that may underlie pathophysiological conditions.

Selective expression of a persistent tetrodotoxin-resistant Na+ current and NaV1.9 subunit in myenteric sensory neurons

Voltage-gated Na(+) currents play critical roles in shaping electrogenesis in neurons. Here, we have identified a TTX-resistant Na(+) current (TTX-R I(Na)) in duodenum myenteric neurons of guinea pig and rat and have sought evidence regarding the molecular identity of the channel producing this current from the expression of Na(+) channel alpha subunits and the biophysical and pharmacological properties of TTX-R I(Na). Whole-cell patch-clamp recording from in situ neurons revealed the presence of a voltage-gated Na(+) current that was highly resistant to TTX (IC(50), approximately 200 microm) and selectively distributed in myenteric sensory neurons but not in interneurons and motor neurons. TTX-R I(Na) activated slowly in response to depolarization and exhibited a threshold for activation at -50 mV. V(1/2) values of activation and steady-state inactivation were -32 and -31 mV in the absence of fluoride, respectively, which, as predicted from the window current, generated persistent currents. TTX-R I(Na) also had prominent ultraslow inactivation, which turns off 50% of the conductance at rest (-60 mV). Substituting CsF for CsCl in the intracellular solution shifted the voltage-dependent parameters of TTX-R I(Na) leftward by approximately 20 mV. Under these conditions, TTX-R I(Na) had voltage-dependent properties similar to those reported previously for NaN/Na(V)1.9 in dorsal root ganglion neurons. Consistent with this, reverse transcription-PCR, single-cell profiling, and immunostaining experiments indicated that Na(V)1.9 transcripts and subunits, but not Na(V)1.8, were expressed in the enteric nervous system and restricted to myenteric sensory neurons. TTX-R I(Na) may play an important role in regulating subthreshold electrogenesis and boosting synaptic stimuli, thereby conferring distinct integrative properties to myenteric sensory neurons.

Ca2+-activated Cl- current in cultured myenteric neurons from murine proximal colon

Whole cell patch-clamp recordings were made from cultured myenteric neurons taken from murine proximal colon. The micropipette contained Cs(+) to remove K(+) currents. Depolarization elicited a slowly activating time-dependent outward current (I(tdo)), whereas repolarization was followed by a slowly deactivating tail current (I(tail)). I(tdo) and I(tail) were present in approximately 70% of neurons. We identified these currents as Cl(-) currents (I(Cl)), because changing the transmembrane Cl(-) gradient altered the measured reversal potential (E(rev)) of both I(tdo) and I(tail) with that for I(tail) shifted close to the calculated Cl(-) equilibrium potential (E(Cl)). I(Cl) are Ca(2+)-activated Cl(-) current [I(Cl(Ca))] because they were Ca(2+) dependent. E(Cl), which was measured from the E(rev) of I(Cl(Ca)) using a gramicidin perforated patch, was -33 mV. This value is more positive than the resting membrane potential (-56.3 +/- 2.7 mV), suggesting myenteric neurons accumulate intracellular Cl(-). omega-Conotoxin GIVA [0.3 microM; N-type Ca(2+) channel blocker] and niflumic acid [10 microM; known I(Cl(Ca)) blocker], decreased the I(Cl(Ca)). In conclusion, these neurons have I(Cl(Ca)) that are activated by Ca(2+) entry through N-type Ca(2+) channels. These currents likely regulate postspike frequency adaptation.