Regional melatonin and somatostatin dependent mechanisms in pancreatic incretory activity and in intestinal bacterial homeostasis.

Abstract

For many years, it was generally accepted that melatonin and somatostatin have intracerebral sources of synthesis. Melatonin is produced in the pineal gland and somatostatin in the hypothalamus. However, there are very informative data regarding the extracerebral sources of melatonin and somatostatin synthesis in numerous literary sources published over the past 60 years. At the same time, the biological purpose of extracerebral somatostatin and melatonin has been insufficiently studied. This article presents the readers with an analysis of modern literature and the results of own researches on the biological role of melatonin and somatostatin produced in the pancreas and intestinal tract. Under experimental conditions on arginine-induced acute pancreatitis model in rats, structural and hormonal changes were studied in the pancreas, mucosa of the small and large intestines in regional pathological process using morphological, morphometric, immunomorphological and enzyme immunoassay methods. According to the morphological and morphometric analysis results, gross dystrophic changes in the pancreas and intestinal mucosa were observed at relatively early stages of the pathological process – in 2 and 24 hours after the arginine administration. Immunomorphological studies established that the content of melatonin-positive secretory cells noticeably decreased, especially in the areas of mucosal damage in this very period of the pathological process in pancreas, in the mucous membrane of the small and large intestines. At the same time, the content of somatotropin in blood serum and pancreas markedly increased during the indicated period of the experiment. Severe dystrophic processes on small intestinal mucosa, which are particularly expressed by inhibition of local melatonin-secreting cell function, apparently exclude the inhibitory effect of melatonin on somatostatin synthesis process in pancreas. It is not excluded that there is a directed activation of pancreatic delta cells responsible for the somatostatin synthesis at relatively early stages of “arginine pancreatitis”. Based on our own researches in context with available very informative literature data, we can conclude that extrapineal melatonin and extrahypothalamic somatostatin should be given an important role in the integrative activity of organ-digestive system – the pancreas and small intestine.