Gastrointestinal Motility Monitor (GIMM)

Nerve-mediated propulsive motility in the gastrointestinal tract was first described over one century ago by Bayliss and Starling (Bayliss and Starling, 1899). This observation led to the designation of the nerves of the gut as the enteric nervous system (ENS), a distinct division of the autonomic nervous system (Langley, 1921). Neurogenic intestinal peristalsis involves stretch and/or mucosal stimulation at a given point and reflex mediated contractions above, or oral to, the level of stimulus and relaxation in the aboral direction. The result is the generation of a pressure gradient that propels the luminal contents in an oral to aboral direction. In the small intestines of a variety of species, and in the rat large intestine, peristaltic waves of contractions can be activated by infusion of fluid into the lumen of segments of bowel. In the guinea pig distal colon, natural or artificial fecal pellets can be inserted into the oral end of the colon and their progress along the segment of colon can be easily monitored. Thus, the guinea pig distal colon provides a simple and useful assay for investigating propulsive motility in the bowel.

Peristalsis is a complex neural reflex that involves a number of neuroactive compounds and receptors. As a result, compounds that affect neurotransmission in the ENS affect the rate of propulsive motility. Serotonin, released primarily from enterochromaffin cells in the intestinal mucosa, is involved in the initiation of the peristaltic reflex. Using the guinea pig distal colon, Grider and colleagues demonstrated that propulsive motility is enhanced by intralumenal administration of 5-HT₄ receptor agonists (Foxx-Orenstein et al., 1998; Grider et al., 1998), while bath of application of 5-HT₃ and 5-HT₄ receptor antagonists decreases the rate of propulsion (Kadowaki et al., 1996; Linden et al., 2003b). Furthermore, inhibition of the serotonin transporter with serotonin selective reuptake inhibitors (SSRIs) increases peristalsis at low concentrations, yet decreases peristalsis at higher concentrations, likely due to desensitization of 5-HT receptors (Kadowaki et al., 1996; Wade et al., 1996). Other signaling molecules that contribute to guinea pig propulsive motility include acetylcholine acting at nicotinic receptors, which is the primary mediator of interneuronal signaling in the intestines. Blocking synaptic transmission with the nicotinic receptor antagonist hexamethonium decreases pellet propulsion at high concentrations, while lesser concentrations do not affect the rate of propulsion (Kadowaki et al., 1996). Opiates, and opioid receptor agonists, which have long been known to have inhibitory effects on gastrointestinal motility, decrease pellet propulsion in the guinea pig propulsive motility model (Foxx-Orenstein et al., 1998; Wood et al., 2009). Interestingly, daikenchuto, a traditional Japanese herbal medicine used clinically to treat vomiting, stomachache, and disordered motility, decreases the rate of pellet propulsion when administered with the opioid antagonist naloxone, and when administered alone, results in reverse peristalsis (Wood et al., 2009). These findings illustrate the usefulness of the guinea pig propulsive motility model to assess the contributions of various compounds and their receptors on colonic motility patterns.

A number of studies have demonstrated that inflammation leads to changes in the electrical and synaptic properties of colonic neurons (Linden et al., 2003a; Lomax et al., 2005), and that these changes can persist for weeks following recovery from inflammation (Krauter et al., 2007; Lomax et al., 2007). The impact of colitis-induced neuroplastic changes on propulsive motility can be investigated in the guinea pig distal colon using the GIMM system. This system has several advantages over the traditional method of monitoring colonic motility, which assesses propulsive motility by simply measuring the time a fecal pellet travels a given distance. Previous studies of inflammation-induced neural plasticity in the colon have indicated there is a decrease in the velocity of pellet propulsion (Linden et al., 2003a; Linden et al., 2005), yet more recent investigations using the GIMM system have revealed more complex changes (Strong et al., 2010). For example, the rate of pellet propulsion is decreased in ulcerated regions of the inflamed distal colon, yet motility is accelerated in adjacent regions, a phenomenon that was not recognized by the earlier “stopwatch” method. Other studies have shown that dysmotility during the active phase of inflammation can be restored with inhibition of COX-2 (Linden et al., 2004), yet altered motility that persists beyond the resolution of inflammation is COX-2 insensitive (Krauter et al., 2007). Furthermore, in these later studies, spatiotemporal maps generated by the GIMM system revealed a stepwise pattern of colonic motility that was observed in post-inflammatory animals that contributes to the decreased rates of propulsion.

In conclusion, the guinea pig distal colon propulsive motility assay represents a high throughput means of measuring the effects of test compounds and pathological conditions on colonic motility, Furthermore, the GIMM system provides a simple and straightforward approach to assess colonic propulsive motility in vitro. It allows for continuous measurements of pellet propulsion, as well as the generation of spatiotemporal maps to study spontaneous activity patterns. As compared to traditional methods, it can also yield more complex phenomenon that can be quantitated and reanalyzed multiple ways as digital files are saved to the system and are easily accessible.