Spontaneous migrating motor complexes occur in both the terminal ileum and colon of the C57BL/6 mouse in vitro

Abstract

We have studied migrating motor complexes (MMCs) in the isolated terminal ileum or colon (IMMCs and CMMCs respectively) of the C57BL/6 mouse. Periodic contractions occurred spontaneously in both preparations in the absence of intraluminal stimulation. After an initial period, complexes became synchronized between the oral and anal ends of the tissue, and could be observed for in excess of 7 h. The propagation velocity was 3.1±1.0 and 3.9±0.6 mm s⁻¹ in the ileum and colon respectively. IMMCs occurred every 6.01±0.39 min and had a duration of 86.3±10.4 s. The interval between CMMCs was smaller (3.52±0.31 min) and contractions were shorter in duration (30.7±3.6 s). In both preparations, these motor events were dependent on cholinergic transmission: blocked by hexamethonium (500 μM) and attenuated or blocked by atropine (1 μM). This study is the first demonstration of spontaneous migrating contractions in the isolated ileum or colon of the C57BL/6 mouse, the strain of choice for neurological transgenic and targeted mice.

Introduction

Organized groups of contractions have been observed to migrate periodically down both the small and large intestine in a variety of animals, including humans, in vivo (Sarna, 1985, Wingate, 1981). The initiation and propagation of these migrating motor complexes (MMC) is dependent on the enteric nervous system. Complexes continue to occur after extrinsic denervation, and are blocked by drugs that interfere with nerve conduction (tetrodotoxin) and cholinergic neurotransmission (hexamethonium and atropine) (Galligan et al., 1986, Galligan et al., 1989). Extrinsic nerves and hormones do however appear to modulate the duration and frequency of MMCs. Each segment of intestine appears also to have its own intrinsic neural rhythm generator or oscillator since, after transection, each segment of bowel can cycle independently (Bueno et al., 1979, Christensen et al., 1974, Grivel and Ruckebusch, 1972). Periodic migrating contractions and electrical activity, with somewhat similar properties to that recorded in vivo, have also been recorded from isolated segments of both rabbit small (Marzio et al., 1994) and large intestine of mouse and cat (Christensen et al., 1974, Fida et al., 1997, Wood, 1973). These in vitro studies give further support to the hypothesis that this activity is generated within the enteric nervous system.

Since the turn of the century, the dog and guinea-pig have been the experimental animals of choice for investigating gastrointestinal motility. However, with the explosive growth in both transgenic and genomic technology, the mouse is becoming an increasingly important model. Therefore a full description of basal activity in the mouse is a prerequisite to exploit and apply these new technologies.

Migrating activity in the isolated mouse colon was first described by Wood using the piebald lethal strain (Wood, 1973). Mice homozygous for the mutant allele have a reduction in the number of ganglion cells in the large intestine that causes megacolon (Lane, 1966). The contractile activity of the tissue was not associated with electrical slow waves but rather with periodic action potentials. The latter events can therefore be considered the myoelectric complex (Christensen, 1994). Later studies of the colon have investigated these myoelectric and motor complexes in the non-affected littermates of this strain (Fida et al., 1997, Lyster et al., 1995, Lyster et al., 1993) and dissected them pharmacologically (Spencer et al., 1998a, Spencer et al., 1998b). Likewise, motility studies using mutant mice for the c-kit gene (W/W^v) have been conducted in vitro (Huizinga et al., 1998, Ward et al., 1994) and in vivo (Der-Silaphet et al., 1998). W/W^v mutants lack the class of interstitial cells of Cajal that serve as pacemakers in the intestine and have disrupted rhythmicity of the gastrointestinal tract. However, an important caveat must be considered when interpreting data from mutant animal studies. Over consecutive generations, deleterious consequences of the mutation may exert selective pressures for compensatory changes (Banbury Conference on genetic background in mice, 1997). This can be avoided by using inbred mice that are genetically identical and therefore phenotypically more uniform than outbred stocks. To date, only one study has investigated spontaneous contractions in preparations of an inbred strain, C57BL/10 (Mule et al., 1999). However, this study was limited to small segments of colon in which propagation of MMCs could not be assessed.

The present study demonstrated that periodic migrating motor complexes occurred spontaneously not only in the isolated colon but also in the isolated small intestine of the C57BL/6 mouse. This inbred strain was selected because it is the background of choice for neurological transgenic and ‘knockout’ mice (Banbury Conference on genetic background in mice, 1997).