Network of Gut Pacemaker Cells: Spatio-Temporal Evaluation of Electrical Activity by Use of Microelectrode Array

The word ‘pacemaker’ prompts us to imagine ‘heart beats’ and also an ‘electric cardiogram’. However, spontaneous electrical activity occurs in numerous tissues and organs in the autonomic nervous system, such as in the gastrointestinal and urinary tracts, trachea, uterus, lymph ducts, etc. (Takaki et al, 2010). Recent studies have revealed that special interstitial cells in these tissues and organs act as pacemaker cells. In particular, in the gastrointestinal tract, pacemaker cells surround the myenteric plexus between the circular and longitudinal muscle layers. Since these cells express abundant tyrosine kinase, Kit receptors, on the surface, Kit-immunoreactivity and kit cDNA are used as markers for special pacemaker cells. Moreover, these cells are now referred to as interstitial cells of Cajal (ICC) due to their histological features of the network (Maeda et al. 1992; FaussonePellegrini and Thuneberg, 1999; Sanders et al. 1999; Rumessen and Vandervinden, 2003; Takaki, 2003). It is widely accepted that peristaltic movement of the gastrointestinal tract is organized by a network of enteric neurons in the myenteric plexus: Ascending contraction and descending relaxation simultaneously occur to forward luminal contents. It is hypothesized that ICC may also make an essential contribution to the co-ordinated actions of gastrointestinal motility through their network (Nakayama et al. 2006; 2009, see also Fig. 7). There are an increasing number of papers that report the impairment of ICC in GI motility disorders, for example, in inflammatory bowel diseases (IBD) (Suzuki et al. 2004; Wang et al. 2002; 2007; Kinoshita et al. 2007), diabetic gastroparesis (Camilleri, 2002; Ordög et al. 2000; Vittal et al. 2007), slow transit constipation (He et al. 2000; Lyford et al. 2002; Wedel et al. 2002), Hirschsprung's disease (Vanderwinden et al. 1996), etc. In these studies, the impairment of ICC has been evaluated in histological terms, such as apoptotic changes and reduction of ICC number. However, in some stage of these diseases, GI motility disorders may be caused by functional impairment of ICC preceded by or not linked with histological changes. Therefore, tools for evaluating ICC spatio-temporal activity are now anticipated. We have thus employed a microelectrode array (MEA) in order to study ICC pacemaker activity. In this chapter, we describe our methods of MEA measurements and provide evidence showing that ICC play crucial roles in propagating electrical activity as well as in pacemaking. In some experiments, we compare ileal muscle preparations of wild-type (WT)