Motor behavior of mouse large intestine: A Minireview

Mice with a recessive gene which reduces the number of ganglion cells of the distal colon and rectum and produces megacolon, imitating Hirschsprung disease, are discussed as a model for integrative control of the large intestinal smooth musculature by the enteric division of the autonomic nervous system (ie, the brain-in-the-gut). Investigative approaches, such as propulsion of artificial pellets in preparations of whole colon in organ baths in vitro and innovative approaches capitalizing on neurogenetic technologies (eg, optogenetics), are considered in view of potential application in the development of novel therapeutic mechanisms to selectively evoke and control gastrointestinal motility patterns, such as the small intestinal digestive motility pattern, interdigestive pattern, and reversed direction of powerful propulsive motility during emesis. This minireview relates to the paper titled: "Motor patterns in the proximal and distal mouse colon which underlie formation and propulsion of feces," appearing in this issue of Neurogastroenterology and Motility.

Motility changes induced by intraluminal FeSO4 in guinea pig jejunum

BACKGROUND

Dietary iron supplementation is associated with gastrointestinal (GI) side effects including vomiting, nausea, and diarrhea. Although inorganic iron in high concentrations may be damaging to the intestinal mucosa, we hypothesize that there are physiological effects on the GI tract that occur at concentrations achieved by supplementation. Thus, our aim was to investigate the effect of intraluminal ferrous sulfate (FeSO4 ) on jejunal motility.

METHODS

Segments of guinea pig jejunum were cannulated and the intraluminal pressure recorded with a transducer, while movements were recorded with a video camera. Peristaltic threshold was the oral pressure that evoked four consecutive propulsive contractions. The nutrients decanoic acid (1 mM), l-phenylalanine (50 mM), or the micronutrient FeSO4 (1 mM) were infused intraluminally. We also tested the effect of FeSO4 on electrochemically detected serotonin (5-HT, 5-hydroxytryptamine) released from in vitro tissues, both at rest and following mechanical stimulation.

KEY RESULTS

The jejuna peristaltic threshold was significantly decreased by all three nutrients: FeSO4 : 31 ± 2-23 ± 3 mmH2 O; decanoic acid: 27 ± 2-14 ± 2 mmH2 O; and l-phenylalanine: 30 ± 3-14 ± 3mmH2 O. Of the three, only decanoic acid induced segmentation, while FeSO4 inhibited decanoic acid-induced segmentation. Resting 5-HT release was increased by FeSO4 (128% of control), but mechanically evoked 5-HT release was reduced (70% of control).

CONCLUSIONS & INFERENCES

These data suggest that some luminal effects of inorganic iron on jejunal motility could be mediated through a pathway involving altered release of 5-HT. A better understanding of the interaction between luminal iron and 5-HT containing enterochromaffin cells could improve iron supplementation strategies, thus reducing side effects.

Synaptic transmission at functionally identified synapses in the enteric nervous system: roles for both ionotropic and metabotropic receptors

Digestion and absorption of nutrients and the secretion and reabsorption of fluid in the gastrointestinal tract are regulated by neurons of the enteric nervous system (ENS), the extensive peripheral nerve network contained within the intestinal wall. The ENS is an important physiological model for the study of neural networks since it is both complex and accessible. At least 20 different neurochemically and functionally distinct classes of enteric neurons have been identified in the guinea pig ileum. These neurons express a wide range of ionotropic and metabotropic receptors. Synaptic potentials mediated by ionotropic receptors such as the nicotinic acetylcholine receptor, P2X purinoceptors and 5-HT(3) receptors are seen in many enteric neurons. However, prominent synaptic potentials mediated by metabotropic receptors, like the P2Y(1) receptor and the NK(1) receptor, are also seen in these neurons. Studies of synaptic transmission between the different neuron classes within the enteric neural pathways have shown that both ionotropic and metabotropic synaptic potentials play major roles at distinct synapses within simple reflex pathways. However, there are still functional synapses at which no known transmitter or receptor has been identified. This review describes the identified roles for both ionotropic and metabotropic neurotransmission at functionally defined synapses within the guinea pig ileum ENS. It is concluded that metabotropic synaptic potentials act as primary transmitters at some synapses. It is suggested identification of the interactions between different synaptic potentials in the production of complex behaviours will require the use of well validated computer models of the enteric neural circuitry.

Influence of body posture on intestinal transit of gas

BACKGROUND

Patients describe that body posture may affect their abdominal bloating, distension, and flatulence, but whether changes in position have objectively demonstrable effects, either beneficial or deleterious, has not been investigated.

AIM

To determine the effect of body posture, upright versus supine, on intestinal transit of gas loads.

SUBJECTS

Eight healthy subjects without gastrointestinal symptoms.

METHODS

In each subject a gas mixture was continuously infused into the jejunum (12 ml/min) for three hours, and gas evacuation, clearance of a non- absorbable gaseous marker, perception, and abdominal girth were measured. Paired studies were randomly performed in each subject on separate days in the upright and supine positions.

RESULTS

In the upright position, intestinal gas retention was much smaller than when supine (13 (52) ml v 146 (75) ml retention at 60 minutes, respectively; p<0.05), and clearance of the gas marker was expedited (72 (10)% clearance v 49 (16)% at 60 minutes, respectively; p<0.05). The gas challenge test was well tolerated both in the upright and supine positions without abdominal distension.

CONCLUSION

Body posture has a significant influence on intestinal gas propulsion: transit is faster in the upright position than when supine.