Orexin A affects ascending contraction depending on downstream cholinergic neurons and descending relaxation through independent pathways in mouse jejunum

Abstract

The involvement of orexin in neural pathways for peristalsis was examined in mouse jejunal segments. Localized distension of the segments using a small balloon resulted in ascending contraction and descending relaxation. Ascending contraction was abolished by atropine and tetrodotoxin. Desensitization to orexin A (OXA) and SB-334867-A, an orexin-1 receptor antagonist, significantly inhibited ascending contraction. Hexamethonium also produced a significant inhibition. Exogenous administration of either OXA or nicotine induced a transient contraction that was completely inhibited by atropine and tetrodotoxin. The OXA-induced contraction was significantly inhibited by hexamethonium and SB-334867-A, whereas the nicotine-induced contraction was not inhibited by SB-334867-A. Descending relaxation was either partially or completely inhibited by l-nitroarginine and tetrodotoxin, respectively. Both SB-334867-A and hexamethonium partially inhibited descending relaxation. A combination of SB-334867-A and hexamethonium had an additive inhibitory effect on descending relaxation. Exogenous OXA, in the presence of atropine, induced a relaxation that was significantly inhibited by both l-nitroarginine and SB-334867-A, but not by hexamethonium. Nicotine in the presence of atropine relaxed the jejunal segment. SB-334867-A, unlike hexamethonium, did not affect nicotine-induced relaxation. These results suggest that OXA plays an important role in the ascending and descending neural reflexes in the mouse jejunum.

Introduction

The regulation of gut motility is complex involving interactions between the enteric nervous system, the central nervous system and various local circulating peptides (Furness, 2006). Orexins are neuropeptides that are localized in the central nervous system and stimulate food consumption (Sakurai et al., 1998, Sweet et al., 1999). Recently, It was reported that orexin A and B (OXA and OXB), and two orexin receptors (OX1R and OX2R) are present in the enteric nervous system of various species of animals (Kirchgessner and Liu, 1999). OXA caused contractions that were significantly inhibited by atropine and tetrodotoxin and increased the motility in the isolated guinea pig ileum and colon (Kirchgessner and Liu, 1999, Matsuo et al., 2002), respectively. The excitatory effect of OXA was shown to be due to stimulating the release of acetylcholine (ACh) from the enteric cholinergic neurons (Matsuo et al., 2002). In addition, OXA caused membrane depolarization and augmented nicotinic fast excitatory postsynaptic potentials in isolated myenteric neurons of the guinea pig ileum (Katayama et al., 2003). In contrast, OXA significantly increased the myoelecric motor complex (MMC) cycle length (Naslund et al., 2002), and this effect was suggested to depend on, at least partly, nitrergic neurons in the rat small intestine (Ehrstrom et al., 2003). We also reported a possible role of OXA in non-adrenergic, non-cholinergic (NANC) inhibitory response in the mouse small intestine (Satoh et al., 2001): OXA partially mediated NANC relaxation of the longitudinal muscle of the intestinal regions tested, the duodenum, jejunum and ileum of ICR mouse via activation of nitrergic neurons. Thus, orexins have been suggested to have important roles to play peripherally in regulating motility of the intestine, but their actions are complex and remain to be elucidated.

Myenteric neurons are known to contain neural pathways responsible for peristalsis of the intestine. The neural pathways of the ascending and descending reflex in peristalsis contain afferent sensory neurons, interneurons, and excitatory and inhibitory motor neurons, respectively (Furness, 2000). Peristalsis movements were recorded in a series of in vitro studies by using the intestinal segments of the guinea pig and rat (Grider, 1994, Kanada et al., 1993, Okishio et al., 2000, Spencer et al., 1999). In these studies, local stimuli applied to the intestinal wall produced contraction on the oral side (ascending contraction) and relaxation on the anal side (descending relaxation) of the stimulated region. As a result, it has been clarified that a variety of different transmitters and modulators contributed to the neural pathway connecting from stimulated region to responding site (Bornstein et al., 2004, Furness, 2000). Recently, the ascending contraction and descending relaxation in the mouse intestine were studied for the first time using specially devised small balloon (Fujita et al., 2004, Okishio et al., 2005b). The study demonstrated the presence of cholinergic interneurons in the neural pathways of the mouse ileum (Okishio et al., 2005a). However, there are still a lot of uncertain points with regard to neuronal pathway in the mouse intestinal tracts.

In OXA immunoreactivity of the rat duodenum, OXA-immunoreactive neurons were suggested to be interneurons and/or motor neurons (Naslund et al., 2002). In the present study, we expanded our studies of the neural pathways responsible for ascending contraction and descending relaxation in the mouse jejunum, and examined the involvement of orexin in these pathways. The involvement of orexin and cholinergic neurons in these neural pathways is clarified in the present report.