title: Long-term enteral administration of melatonin reduces plasma insulin and increases expression of pineal insulin receptors in both Wistar and type 2-diabetic Goto-Kakizaki rats.
abstract: This paper represents an essential aspect of recent investigations into the functional and clinical implications of insulin-melatonin interrelationships. The aim of the study was to analyze whether melatonin reduces insulin secretion in an animal in a manner comparable to the pattern observed in previous in vitro experiments; to this end, we used two models: Wistar and type 2-diabetic Goto-Kakizaki (GK) rats. Thirty-two Wistar and 32 GK rats were divided into two subgroups of 16 rats each; each subgroup was treated either with or without melatonin. The daily administration of melatonin, starting in 8-wk-old rats, was adjusted to 2.5 mg/kg body weight. Melatonin was given daily during the dark period for 12 hr. After 9 wk of treatment, the rats were sacrificed in the middle of the dark period. Melatonin administration strongly enhanced the plasma melatonin level and diminished the expression of pancreatic melatonin receptor-mRNA, whereas the expression of pineal AA-NAT and HIOMT was unchanged. Furthermore, the experiments showed in agreement with recent in vitro results of pancreatic islets that plasma insulin levels were diminished after melatonin treatment. However, the pineal insulin receptor expression was increased after melatonin administration. The pancreatic expression of glucagon, GLUT2, and glucokinase was decreased in GK rats, whereas the glucose levels, as well as the parameters of glucose sensing, GLUT2-mRNA, and glucokinase-mRNA, were unchanged after melatonin administration in both Wistar and GK rats. In summary, the results show that melatonin administration decreases plasma insulin levels in vivo and, furthermore, that an insulin-melatonin antagonism exists.
authors.affiliation: Institute of Anatomy and Cell Biology, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany. elmar.peschke@medizin.uni-halle.de
authors.forename: Elmar
authors.initials: E
authors.lastname: Peschke
authors.forename: Helena
authors.initials: H
authors.lastname: Schucht
authors.forename: Eckhard
authors.initials: E
authors.lastname: Mühlbauer
mesh_terms: D000284:Administration, Oral; D000704:Analysis of Variance; D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D003924:Diabetes Mellitus, Type 2 / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D005934:Glucagon / Q000096:biosynthesis / Q000235:genetics; D051273:Glucose Transporter Type 2 / Q000096:biosynthesis / Q000235:genetics; D007328:Insulin / Q000097:blood* / Q000235:genetics; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D010870:Pineal Gland / Q000187:drug effects* / Q000378:metabolism; D051381:Rats; D055647:Rats, Transgenic; D017208:Rats, Wistar; D011972:Receptor, Insulin / Q000096:biosynthesis* / Q000235:genetics; D044122:Receptor, Melatonin, MT1 / Q000096:biosynthesis / Q000235:genetics; D044123:Receptor, Melatonin, MT2 / Q000235:genetics / Q000378:metabolism; D013004:Somatostatin / Q000096:biosynthesis / Q000235:genetics
chemical_list: D001786:Blood Glucose; D051273:Glucose Transporter Type 2; D007328:Insulin; D044122:Receptor, Melatonin, MT1; D044123:Receptor, Melatonin, MT2; C495683:Slc2a2 protein, rat; D013004:Somatostatin; D005934:Glucagon; D011972:Receptor, Insulin; D008550:Melatonin