Synchronicity, cycles and synaptic signalling in the colon

Interstitial cells of Cajal (ICCs) in the gastrointestinal tract are pacemaker cells that synchronize electrical activity and contractions of the muscle layers. ICCs may also be intermediaries of neurotransmission to the muscle layers (Ward & Sanders, 2006), although this proposal is controversial (Sarna, 2008; Huizinga et al. 2008). In the stomach and small intestine, there are two types of ICCs, ICC-myenteric (ICC-MY) and ICC-intramuscular (ICC-IM). ICC-MY are located at the border of the myenteric plexus and circular muscle while ICC-IM are within the muscle layers. In the colon, there are ICC-MY and ICC-deep muscular plexus (DMP) which reside at the border between the circular muscle and submucosal layer. In addition to anatomical heterogeneity there is also functional heterogeneity. ICC-DMP in the colon generate slow waves which propagate into the bulk circular muscle and they act as intermediaries between nerves and muscle. The ICC-MY generate myenteric potential oscillations (MPOs) that propagate into the muscle to reinforce slow waves and muscle contractions. ICC-MY also synchronize activity of the longitudinal and circular muscle layers.

The colon exhibits periodic propagating contractions that propel colonic content in an oral to anal direction. These colonic migrating motor complexes (CMMCs) occur in vitro in the mouse colon and they can occur spontaneously or they can be evoked by mechanical stimulation of the mucosa. These properties have enabled detailed studies of the underlying neural circuitry and contributions of ICC activity to the CMMCs. This has been accomplished through mechanical and electrical recordings of muscle activity and high resolution imaging using activity-dependent fluorescent dyes to monitor activity in muscle, neurons and ICC. This latter approach was used in the study done by Bayguinov et al. (2010) in a recent issue of The Journal of Physiology.

Bayguinov et al. (2010) used high speed imaging to measure intracellular calcium changes using the indicator dye fluo-4. They also used post-fixed tissues and immunohistochemical techniques to identify the structures providing the calcium signals. The authors focused on the relationship between activity in myenteric neurons and nerve fibres, ICC-MY and smooth muscle cells. The main findings from this study were that activity in the muscle and ICC-MY is suppressed in the intervals between CMMCs. This is likely to be due to ongoing activity in inhibitory neurons that supply ICC-MY and the muscle layers. Secondly, during the CMMC there are coordinated increases in activity of cholinergic varicosities and calcium transients in ICC-MY and smooth muscle. The increases in calcium transients in ICC-MY and muscle were blocked by atropine and a neurokinin-1 (NK-1) receptor antagonist. The authors showed that there was a sequential relationship between activity in the nerve varicosities and nearby ICC-MY and muscle cells. Calcium transients occurred first in varicosities of excitatory motorneurons, which presumably link to acetylcholine, and neurokinin peptide release followed by ICC-MY activation, which preceded calcium transients in the muscle. Calcium transients in the muscle, but not ICC-MY or neurons, were caused by activation of L-type calcium channels. These data do not provide decisive support for an obligatory role of ICC in neurotransmission to the muscle. However, the data demonstrate a spatial and temporal relationship between activity in nerve terminals, ICC and smooth muscle during propulsive contractions in the mouse colon in vitro.

Perhaps the most interesting finding was that ICC-MY do not appear to be coupled together in a synchronized network as occurs in other classes of ICC. It was found that small groups of ICC-MY are activated by a few varicosities of excitatory motor neurons and this activity was then conducted into surrounding smooth muscle cells. This suggests that there may be local units or clusters of cells that control local contractions. These units include excitatory and inhibitory varicosities, ICC-MY and nearby muscle cells. Coordinated activation or inhibition of these units would be driven by the nerve circuits that control the CMMC. It would be interesting to determine whether the composition of the units is fixed or dynamic. For example, do the number of nerve fibres, ICC-MY and muscle cells composing a functional unit change under physiological or pathophysiological conditions, such as inflammation? It would also be interesting to determine if there are minimum and maximum sizes of the functional units in terms of both the number of cells and the area of muscle controlled by individual units. Enteric glia may also contribute to modulation of synaptic activity in the myenteric plexus during propulsive motility patterns. It would be informative to apply high resolution imaging of glial activity during the CMMC (Gulbransen & Sharkey, 2009). Finally, it will be important to determine if the platelet derived growth factor positive (PDGF⁺) cells (Iino et al. 2009) that are found in gastrointestinal muscle bundles are also activated during the CMMC.

The CMMC in the mouse colon provides a valuable model to study the contribution of multiple cell types to an integrative motor behaviour. The spatial and temporal resolution of imaging, mechanical and electrical techniques allows precise measurements of the events responsible for propulsive motility. This approach should also be feasible for studies of motor reflexes in specimens from the human colon.