Increased melatonin synthesis in pineal glands of rats in streptozotocin induced type 1 diabetes.

Abstract:  It is well-documented that melatonin influences insulin secretion. The effects are mediated by specific, high-affinity, pertussis-toxin-sensitive, G protein-coupled membrane receptors (MT1 as well MT2), which are present in both the pancreatic tissue and islets of rats and humans, as well as in rat insulinoma cells (INS1). Via the Gi-protein-adenylatecyclase-3′,5′-cyclic adenosine monophosphate (cAMP) and, possibly, the guanylatecyclase-cGMP pathways, melatonin decreases insulin secretion, whereas, by activating the Gq-protein-phospholipase C-IP3 pathway, it has the opposite effect. For further analysis of the interactions between melatonin and insulin, diabetic rats were investigated with respect to melatonin synthesis in the pineal gland and plasma insulin levels. In this context, recent investigations have proven that type 2 diabetic rats and humans display decreased melatonin levels, whereas type 1 diabetic IDDM rats or those with diabetes induced by streptozotocin (STZ) of the present study show increased plasma melatonin levels and elevated AA-NAT-mRNA. Furthermore, the mRNA of pineal insulin receptors and β1-adrenoceptors, including the clock genes Per1 and Bmal1 and the clock-controlled output gene Dbp, increases in both young and middle-aged STZ rats. The results therefore indicate that the decreased insulin levels in STZ-induced type 1 diabetes are associated with higher melatonin plasma levels. In good agreement with earlier investigations, it was shown that the elevated insulin levels observed in type 2 diabetes, are associated with decreased melatonin levels. The results thus prove that a melatonin–insulin antagonism exists. Astonishingly, notwithstanding the drastic metabolic disturbances in STZ-diabetic rats, the diurnal rhythms of the parameters investigated are maintained.