Calcitonin gene-related peptide: a sensory and motor neurotransmitter in the feline lower esophageal sphincter

Abstract

The effect of calcitonin gene-related peptide (CGRP) on the feline lower esophageal sphincter (LES) was determined and correlated with its anatomic distribution as determined by immunohistochemistry. Intraluminal pressures of the esophagus and LES were recorded in anesthetized cats. In separate cats, gastroesophageal junctions were removed after locating the LES manometrically and stained for CGRP-like immunoreactivity (LI) and substance P-LI (SP-LI) by indirect immunohistochemistry. CGRP-LI in the LES was most prominent in large nerve fascicles between the circular and longitudinal muscle layers and only rarely seen in nerve fibers within the circular muscle. The myenteric plexus contained numerous CGRP-LI nerve fibers but cell bodies were not seen. Many CGRP-LI nerve fibers in the myenteric plexus and occasional varicose nerves in the circular muscle demonstrated colocalization with SP-LI. Colocalization of CGRP-LI with SP-LI was also seen in the perivascular nerves of the submucosal and intramural blood vessels and in varicose fibers in the lamina propria of the gastric fundic mucosa. In the esophagus, CGRP-LI nerves extended through the muscularis mucosa and penetrated the squamous epithelium to the lumen. CGRP, given intra-arterially, caused a dose-dependent fall in basal LES pressure, with a threshold dose of 10 −8 g/kg (2.63 pmol/kg). At the maximal effective dose, 5 × 10 −6 g/kg (1.31 × 10 3 pmol/kg), CGRP produced 61.0 ± 6.0% decrease in basal LES pressure. At this dose, mean systemic blood pressure fell by 40.9 ± 7.8%. The LES relaxation induced by a submaximal dose of CGRP (10 −6 g/kg, 262.7 pmol/kg), 50.3 ± 3.2% relaxation was partially inhibited by tetrodotoxin (26.9 ± 10.8% relaxation, P < 0.025). The inhibitory effect of CGRP was not affected by cervical vagotomy, hexamethonium, atropine, propranolol, or naloxone. The LES contractile response to the D 50 of SP (5 × 10 −8 g/kg, 37.1 pmol/kg) was not altered by CGRP 10 −8 or 10 −6 g/kg and the CGRP relaxation effect was not altered by the threshold dose of substance P (5 × 10 −9 g/kg, 3.71 pmol/kg). Conclusions: (1) CGRP-LI is present at the feline LES and is primarily seen in large nerve fascicles which pass from the intermuscular plane and through the circular muscle layer to the submucosa and in mucosal nerves. (2) CGRP colocalizes with SP-LI in some varicose nerve fibers of the circular muscle of the esophagus, LES and fundus, in perivascular nerves of the submucosal and intramucosal blood vessels, and in nerves of the lamina propria of the gastric fundus. (3) The luminal penetration of CGRP-LI nerves in the squamous mucosa of the esophagus suggests a sensory function for some CGRP-LI nerves. (4) CGRP exerts a dose-dependent inhibitory effect at the LES by both a neural and a direct smooth muscle effect. (5) Despite colocalization in nerve fibers in the muscularis propria at the LES, CGRP exerts no influence on SP-induced LES contractions.