Pineal melatonin production in Streptozotocin-diabetic rats : mechanisms and microdialysis daily profile

AMARAL, F. G. Pineal melatonin production in Streptozotocin-diabetic rats: mechanisms and microdialysis daily profile. 2009. 181 p. Doctorade Thesis (Physiology) Instituto de Ciências Biomédicas, Universidade de São Paulo, São Paulo, 2009. Melatonin is mainly synthesized by the pineal gland in a strictly nocturnal pattern. The resulting increase in circulating melatonin provides a signal of night time and is used to optimally synchronize physiological functions, including the energy metabolism, to daily changes in the environment. Diabetes mellitus is a syndrome characterized by disordered metabolism and hyperglycemia resulting from either a progressive loss in insulin secretion and/or from abnormal resistance to insulin's effects. Evidences in the literature show that melatonin is involved in the regulation of insulin synthesis and secretion, as well as in the peripheral sensitivity to this hormone. On the other hand, insulin was shown to increase melatonin synthesis induced by norepinephrine. Considering the possibility of a crosstalk involving both hormones, the controversial available data on melatonin synthesis and diabetes, and the lack of information on the status of the molecular mechanisms involved in melatonin synthesis in diabetic rats, this study was designed to analyze the daily pineal melatonin production in streptozotocin-diabetic male Wistar rats (60mg/kg, i.p.), as well as the activity pattern and gene expression profiles of the enzymes involved in this process. The animals (240–280 g) were kept under a 12-hr light/12-hr dark cycle in a temperaturecontrolled room (21 ± 2°C), with water and food ad libitum. Diabetes was determined by tail blood glucose measurement and the animals were sacrificed on day 3 and day 15 after induction. The techniques used here include flow cytometry, microdialysis, HPLC, enzyme activity assay, qPCR and Western blot. The results show that pineal melatonin production is decreased (50%) in streptozotocin-induced diabetic animals, both 3 and 15 days after induction, with an additional loss in the mono/biphasic circadian profile. These alterations were not due to necrosis or apoptosis in the pinealocytes, but to impairment in the pineal metabolism that involved the important decrease in the AANAT activity (55%), resulting in the observed indolamine synthesis reduction. In addition, other key factors involved in melatonin production presented an imbalance in the rhytmic profile, such as the ß1-adrenergic receptor gene expression and the TPH1 and HIOMT genes expressions and activities. Besides that, the already known decrease in circulating insulin and leptin levels in diabetic animals may also have a role in the observed melatonin synthesis reduction. This decrease may contribute to the development of diabetes and its cronic effects by undermining the organism capability of overcoming the oxidative stress derived from the disease. Future studies should include melatonin rhythmically reposition to check if it would be able to diminish some of the symptoms, as well as insulin treatment to try to restore melatonin levels.