Distribution of nitric oxide synthase in the esophagus of the cat and monkey

Abstract

The distribution of nitrergic neurons and processes in the esophagus of the cat and monkey was studied by light microscopic immunocytochemistry using a specific antibody against purified rat brain nitric oxide synthase and immunoperoxidase procedures. Immunoreactive nerve fibers were found pervading the myenteric plexus, submucous plexus and plexus of the muscularis mucosae, and particularly in the lower esophagus a few immunoreactive fibers entered the epithelium as free nerve endings, some of which derived from perivascular fibers. In the upper esophagus immunoreactive motor end-plates were found in the striated muscle. Thirty–forty-five percent of neuronal cell bodies found in the intramural ganglia and along the course of nerve fiber bundles were immunoreactive and were of the three morphological types earlier described. In the intramural ganglia immunoreactive nerve fibers formed a plexus in which varicose nerve terminals were in close relation to immunoreactive and non-immunoreactive neurons. The intramural blood vessels that crossed the different layers of the esophageal wall were surrounded by paravascular and perivascular plexuses containing immunoreactive nerve fibers. The anatomical findings suggest that nitric oxide is involved in neural communication and in the control of peristalsis and vascular tone in the esophagus. In the lower esophagus a few nitrergic nerve fibers are anatomically disposed to subserve a sensory-motor function.

Introduction

Nitric oxide (NO) acts as a neurotransmitter in both the central (Bredt et al., 1992; Rodrigo et al., 1994b) and peripheral nervous system (Ceccatelli et al., 1994; De Man et al., 1991; Li and Furness, 1993). As a highly diffusible gas it cannot be stored in vesicles but is produced at the time of transmission by calcium-dependent activation of the constitutive enzyme, neuronal nitric oxide synthase (nNOS), which catalyses its formation from l-arginine (Knowles et al., 1989). Previous studies have demonstrated the presence of nitrergic structures in the enteric nervous systems of the dog (Ward et al., 1992), guinea pig (Furness et al., 1994) and rat (Belai et al., 1992), and NOS immunoreactivity or NADPH-diaphorase activity has also been found in the enteric nervous systems of elasmobranch (Olsson and Karila, 1995), teleost (Li and Furness, 1993), amphibian (Murphy et al., 1994) and avian (Li et al., 1994) species.

The role of NO in the gastrointestinal tract has been the subject of several studies focusing on individual organs such as the stomach (Barrachina et al., 1994; Yano et al., 1995), duodenum (Postorino et al., 1995), ileum (Costa et al., 1992) and colon (McConalogue and Furness, 1993). In the esophagus, the presence of nitrergic nerve fibers has mainly been studied in the lower esophageal sphincter in man (McKirdy et al., 1992; Mearin et al., 1993; Singaram et al., 1995), where NO is a putative nonadrenergic noncholinergic (NANC) inhibitory neurotransmitter (McKirdy et al., 1992). In the opossum NO mediates inhibitory potentials in the esophageal circular smooth muscle (Cayabyab and Daniel, 1995; Murray et al., 1995). NO is also involved, at least in part, in the relaxation of rat (Postorino et al., 1995), mouse (Yano et al., 1995), porcine (Stebbing et al., 1995) and human (McKirdy et al., 1992) GIT smooth muscle induced by electrical stimulation or demonstrated by organ superfusion.

The present study of the cat and monkey esophagus was carried out to examine the general distribution of nitrergic neural structures along the length of the esophagus and to analyze their reactions with non-neural efferent structures in the esophageal wall, in an attempt to obtain anatomical information relevant to the possible role of NO in esophageal motility and vasomotor control.