Inhibitory neurotransmission in lower oesophagus

Abstract: The inhibitory neuromuscular control of the human and cat lower oesophagus was investigated by morphological, functional, and biochemical methods. A supply of nerve fibres and myenteric cell bodies, containing nitric oxide synthase (NOS), haem oxygenase (HO)-2, vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase activating peptide (PACAP) was observed. The highest density of nerves was seen at the oesophagogastric junction area with an abundace in the circular muscle layer. As judged by confocal microscopy, NOS, HO-2, VIP and PACAP were frequently colocalized within the same nerve structures. In circular smooth muscle strips from the lower oesophageal sphincter (LOS), electrical field stimulation (EFS) induced nerve-mediated relaxations that were abolished by the nitric oxide (NO) synthesis inhibitor NG-nitro-L-arginine (L-NOARG). The response was prevented and partially restored by L-arginine, but not by D-arginine. The relaxation was associated with an increase in cyclic GMP levels. A L-NOARG resistant relaxation could be induced, electrically or by the spider venom alpha-latrotoxin, which was not associated with changes in cyclic GMP or cyclic AMP levels, but inhibited by VIP-antiserum. The HO inhibitors, zinc protoporphyrin-IX or tin protoporphyrin-IX did not affect EFS induced relaxations. NO and carbon monoxide (CO) induced concentration-dependent relaxations associated with an increase of cyclic GMP levels, whereas VIP and PACAP induced concentration-dependent relaxations associated with an increase in cyclic AMP levels. In circular smooth muscle stips from the oesophageal body (OB), L-NOARG reduced the latency period and increased the amplitude in EFS-evoked contractions, and in carbachol-contracted strips, EFS induced relaxations that was abolished by L-NOARG. NO, VIP and alpha-latrotoxin all produced a hyperpolarization of oesophageal smooth muscle. In cat tissue, activities of NOS and HO could be demonstrated. In LOS tissue of the cat, VIP concentrations were 50-fold higher than PACAP concentrations; in human tissue they were 10-fold higher. These results suggest that the L-arginine/NO system plays an important role in the inhibitory neuronal control of oesophageal motility in cat and man. Also NO independent inhibitory pathways are present, involving other mediators of which VIP, PACAP, and CO are putative candidates. The L-arginine/NO system seems to act in concert with the NO-independent mechanisms to produce relaxant effects.