Tonic activity of submucosal neurons influences basal ion transport

VPAC1 receptors regulate intestinal secretion and muscle contractility by activating cholinergic neurons in guinea pig jejunum

In the gastrointestinal tract, vasoactive intestinal peptide (VIP) is found exclusively within neurons. VIP regulates intestinal motility via neurally mediated and direct actions on smooth muscle and secretion by a direct mucosal action, and via actions on submucosal neurons. VIP acts via VPAC1 and VPAC2 receptors; however, the subtype involved in its neural actions is unclear. The neural roles of VIP and VPAC1 receptors (VPAC1R) were investigated in intestinal motility and secretion in guinea pig jejunum. Expression of VIP receptors across the jejunal layers was examined using RT-PCR. Submucosal and myenteric neurons expressing VIP receptor subtype VPAC1 and/or various neurochemical markers were identified immunohistochemically. Isotonic muscle contraction was measured in longitudinal muscle-myenteric plexus preparations. Electrogenic secretion across mucosa-submucosa preparations was measured in Ussing chambers by monitoring short-circuit current. Calretinin(+) excitatory longitudinal muscle motor neurons expressed VPAC1R. Most cholinergic submucosal neurons, notably NPY(+) secretomotor neurons, expressed VPAC1R. VIP (100 nM) induced longitudinal muscle contraction that was inhibited by TTX (1 μM), PG97-269 (VPAC1 antagonist; 1 μM), and hyoscine (10 μM), but not by hexamethonium (200 μM). VIP (50 nM)-evoked secretion was depressed by hyoscine or PG97-269 and involved a small TTX-sensitive component. PG97-269 and TTX combined did not further depress the VIP response observed in the presence of PG97-269 alone. We conclude that VIP stimulates ACh-mediated longitudinal muscle contraction via VPAC1R on cholinergic motor neurons. VIP induces Cl(-) secretion directly via epithelial VPAC1R and indirectly via VPAC1R on cholinergic secretomotor neurons. No evidence was obtained for involvement of other neural VIP receptors.

Lubiprostone reverses the inhibitory action of morphine on mucosal secretion in human small intestine

BACKGROUND AND AIMS

Treatments with morphine or opioid agonists cause constipation. Lubiprostone is approved for treatment of adult idiopathic constipation and constipation-predominant IBS in adult women. We tested whether lubiprostone can reverse morphine-suppression of mucosal secretion in human intestine and explored the mechanism of action.

METHODS

Fresh segments of jejunum discarded during Roux-En-Y gastric bypass surgeries were used. Changes in short-circuit current (ΔIsc) were recorded in Ussing flux chambers as a marker for electrogenic chloride secretion during pharmacological interactions between morphine, prostaglandin receptor antagonists, chloride channel blockers and lubiprostone.

RESULTS

Morphine suppressed basal Isc. Lubiprostone reversed morphine suppression of basal Isc. Lubiprostone, applied to the mucosa in concentrations ranging from 3 nM to 30 μM, evoked increases in Isc in concentration-dependent manner when applied to the mucosal side of muscle-stripped preparations. Blockade of enteric nerves did not change stimulation of Isc by lubiprostone. Removal of chloride or application of bumetanide or NPPB suppressed or abolished responses to lubiprostone. Antagonists acting at CFTR channels and prostaglandin EP(4) receptors, but not at E(1), EP(1-3) receptors, partially suppressed stimulation of Isc by lubiprostone.

CONCLUSIONS

Antisecretory action of morphine results from suppression of excitability of secretomotor neurons in the enteric nervous system. Lubiprostone, which does not affect enteric neurons directly, bypasses the action of morphine by directly opening mucosal chloride channels.

Lubiprostone reverses the inhibitory action of morphine on intestinal secretion in guinea pig and mouse

Lubiprostone activates ClC-2 chloride channels in epithelia. It is approved for treatment of chronic idiopathic constipation in adults and constipation-predominate irritable bowel syndrome in women. We tested a hypothesis that lubiprostone can reverse the constipating action of morphine and investigated the mechanism of action. Short-circuit current (Isc) was recorded in Ussing chambers as a marker for chloride secretion during pharmacological interactions between morphine and lubiprostone. Measurements of fecal wet weight were used to obtain information on morphine-lubiprostone interactions in conscious mice. Morphine decreased basal Isc, with an IC(50) of 96.1 nM. The action of dimethylphenylpiperazinium (DMPP), a nicotinic receptor agonist that stimulates neurogenic Isc, was suppressed by morphine. Lubiprostone applied after pretreatment with morphine reversed morphine suppression of both basal Isc and DMPP-evoked chloride secretion. Electrical field stimulation (EFS) of submucosal neurons evoked biphasic increases in Isc. Morphine abolished the first phase and marginally suppressed the second phase. Lubiprostone reversed, in concentration-dependent manner, the action of morphine on the first and second phases of the EFS-evoked responses. Subcutaneous lubiprostone increased fecal wet weight and numbers of pellets expelled. Morphine significantly reduced fecal wet weight and number of pellets. Injection of lubiprostone, 30-min after morphine, reversed morphine-induced suppression of fecal wet weight. We conclude that inhibitory action of morphine on chloride secretion reflects suppression of excitability of cholinergic secretomotor neurons in the enteric nervous system. Lubiprostone, which does not directly affect enteric neurons, bypasses the neurogenic constipating effects of morphine by directly opening chloride channels in the mucosal epithelium.

Stimulation of mucosal secretion by lubiprostone (SPI-0211) in guinea pig small intestine and colon

Actions of lubiprostone, a selective type-2 chloride channel activator, on mucosal secretion were investigated in guinea pig small intestine and colon. Flat-sheet preparations were mounted in Ussing flux chambers for recording short-circuit current (Isc) as a marker for electrogenic chloride secretion. Lubiprostone, applied to the small intestinal mucosa in eight concentrations ranging from 1-3000 nM, evoked increases in Isc in a concentration-dependent manner with an EC50 of 42.5 nM. Lubiprostone applied to the mucosa of the colon in eight concentrations ranging from 1-3000 nM evoked increases in Isc in a concentration-dependent manner with an EC50 of 31.7 nM. Blockade of enteric nerves by tetrodotoxin did not influence stimulation of Isc by lubiprostone. Antagonists acting at prostaglandin (PG)E2, EP1-3, or EP4 receptors did not suppress stimulation of Isc by lubiprostone but suppressed or abolished PGE2-evoked responses. Substitution of gluconate for chloride abolished all responses to lubiprostone. The selective CFTR channel blocker, CFTR(inh)-172, did not suppress lubiprostone-evoked Isc. The broadly acting blocker, glibenclamide, suppressed (P<0.001) lubiprostone-evoked Isc. Lubiprostone, in the presence of tetrodotoxin, enhanced carbachol-evoked Isc. The cholinergic component, but not the putative vasoactive intestinal peptide component, of neural responses to electrical field stimulation was enhanced by lubiprostone. Application of any of the prostaglandins, E2, F2, or I2, evoked depolarization of the resting membrane potential in enteric neurons. Unlike the prostaglandins, lubiprostone did not alter the electrical behavior of enteric neurons. Exposure to the histamine H2 receptor agonists increased basal Isc followed by persistent cyclical increases in Isc. Lubiprostone increased the peak amplitude of the dimaprit-evoked cycles.

Neurogastroenterology of tegaserod (HTF 919) in the submucosal division of the guinea-pig and human enteric nervous system

Actions of the 5-HT(4) serotonergic receptor partial agonist, tegaserod, were investigated on mucosal secretion in the guinea-pig and human small intestine and on electrophysiological behaviour of secretomotor neurons in the guinea-pig small intestinal submucosal plexus. Expression of 5-HT(4) receptor protein and immunohistochemical localization of the 5-HT(4) receptor in the submucosal plexus in relation to expression and localization of choline acetyltransferase and the vesicular acetylcholine (ACh) transporter were determined for the enteric nervous system of human and guinea-pig small intestine. Immunoreactivity for the 5-HT(4) receptor was expressed as ring-like fluorescence surrounding the perimeter of the neuronal cell bodies and co-localized with the vesicular ACh transporter. Exposure of mucosal/submucosal preparations to tegaserod in Ussing chambers evoked increases in mucosal secretion reflected by stimulation of short-circuit current. Stimulation of secretion had a relative high EC(50) of 28.1 +/- 1.3 mumol L(-1), was resistant to neural blockade and appeared to be a direct action on the secretory epithelium. Tegaserod acted at presynaptic 5-HT(4) receptors to facilitate the release of ACh at nicotinic synapses on secretomotor neurons in the submucosal plexus. The 5-HT(2B) receptor subtype was not involved in actions at nicotinic synapses or stimulation of secretion.

Recurrent networks of submucous neurons controlling intestinal secretion: a modeling study

Secretomotor neurons, immunoreactive for vasoactive intestinal peptide (VIP), are important in controlling chloride secretion in the small intestine. These neurons form functional synapses with other submucosal VIP neurons and transmit via slow excitatory postsynaptic potentials (EPSPs). Thus they form a recurrent network with positive feedback. Intrinsic sensory neurons within the submucosa are also likely to form recurrent networks with positive feedback, provide substantial output to VIP neurons, and receive input from VIP neurons. If positive feedback within recurrent networks is sufficiently large, then neurons in the network respond to even small stimuli by firing at their maximum possible rate, even after the stimulus is removed. However, it is not clear whether such a mechanism operates within the recurrent networks of submucous neurons. We investigated this question by performing computer simulations of realistic models of VIP and intrinsic sensory neuron networks. In the expected range of electrophysiological properties, we found that activity in the VIP neuron network decayed slowly after cessation of a stimulus, indicating that positive feedback is not strong enough to support the uncontrolled firing state. The addition of intrinsic sensory neurons produced a low stable firing rate consistent with the common finding that basal secretory activity is, in part, neurogenic. Changing electrophysiological properties enables these recurrent networks to support the uncontrolled firing state, which may have implications with hypersecretion in the presence of enterotoxins such as cholera-toxin.

Interrelationship between colonic muscularis mucosae activity and changes in transmucosal potential difference

This in vitro study investigated the relationship between rabbit colonic muscularis mucosae motor activity and changes in transmucosal potential difference. Spontaneous muscle contractions and potential difference oscillations occurred independently and were not neurally driven. ACh and histamine directly stimulated the muscularis mucosae, but their mucosal effects were largely indirect, suggesting that muscularis mucosae contractions promote epithelial secretion. 1,1-Dimethyl-4-phenyl-piperazinium iodide and vasoactive intestinal polypeptide induced large potential difference changes but small muscularis mucosae contractions, demonstrating mucosal secretion without significant muscle activity. Lowered intraluminal pH directly stimulated the muscle, whereas a bile salt-lipid mixture evoked TTX- and atropine-sensitive increases in its contractile activity. Increased intraluminal pressure and hypertonic luminal perfusion did not elicit muscularis mucosae excitation. Thus under basal conditions muscle and mucosal activities are independent, but evoked muscularis mucosae contractions can stimulate epithelial secretion. In response to specific luminal stimuli, muscularis mucosae motor activity is increased via the activation of cholinergic nerves. These data suggest that muscularis mucosae and mucosal functions are physiologically linked and that their activities can be coordinated by multiple mechanisms.

Intestinal transport during fasting and malnutrition

Fasting or malnutrition (FM) has dramatic effects on small intestinal mucosal structure and transport function. Intestinal secretion of ions and fluid is increased by FM both under basal conditions and in response to secretory agonists. Intestinal permeability to ions and macromolecules may also be elevated by FM, which increases the potential for fluid and electrolyte losses and for anaphylactic responses to luminal antigens. Mucosal atrophy induced by FM reduces total intestinal absorption of nutrients, but nutrient absorption normalized to mucosal mass may actually be enhanced by a variety of mechanisms, including increased transporter gene expression, electrochemical gradients, and ratio of mature to immature cells. These observations underscore the value of enteral feeding during health and disease.

Neural control of intestinal ion transport and paracellular permeability is altered by nutritional status

This study examined the effect of fasting on the neural control of ion transport and paracellular permeability in piglet jejunum. Muscle-stripped tissues from fed or 48-h fasted piglets were mounted in Ussing chambers. Neural blockade with tetrodotoxin (TTX) or antagonists of muscarinic or nicotinic receptors caused reductions in basal short-circuit current that were approximately threefold greater in fasted piglets. The TTX-induced reduction in short-circuit current in fasted piglets was due to a decrease in residual ion flux and was abolished in the absence of HCO(-)(3). Intestinal paracellular permeability, as indicated by tissue conductance (G(t)) and fluxes of inulin and mannitol, was significantly increased by fasting. TTX increased inulin flux and G(t) in fed but not fasted piglets. In fasted piglets, carbachol reduced G(t) by 29% and mannitol flux by 27% but had no effect on these parameters in the fed state. We conclude that fasting enhances enteric neural control of basal ion transport and increases paracellular permeability in piglet jejunum. Tonic release of enteric neurotransmitters regulates paracellular permeability in the fed state, and cholinergic stimulation restores fasting-induced elevations in paracellular permeability to fed levels.

Gastrointestinal complications of cocaine abuse

The mechanisms thought to be responsible for cocaine-associated gastrointestinal injury are presented. In addition, the literature available concerning the enteric consequences of cocaine use is reviewed.

Brain Na+,K(+)-ATPase regulation in vivo: reduction in activity and response to sodium by intracerebroventricular tetrodotoxin

We investigated the effects of intracerebroventricular infusion of tetrodotoxin on activity and function of brain Na+,K(+)-ATPase. Infusion of 1 or 3 micrograms/h for 2, 4 or 7 days by osmotic minipump reduced the number of Na+,K(+)-ATPase sites as measured by ouabain binding in cerebral cortex. Tetrodotoxin infusions substantially reduced the functional transport capacity of Na+,K(+)-ATPase, measured by the maximal increase in synaptoneurosomal 86Rb+ uptake in the presence of monensin. The effects were maximal at 4 days, with a possible partial recovery of activity at 7 days. Results of ouabain inhibition curves suggested that the effect of tetrodotoxin was not specific for enzyme with high or low affinity for ouabain.