Abstract
Melatonin(MEL) is a neuroendocrine hormone secreted by the pineal gland that has a wide range of effects, including sleep regulation, anti-aging, anti-inflammatory,reproduction and anti-oxidative activities. Melatonin helps bone regulation metabolism through promotion of alkaline phosphatase (ALP) activity in blood and bone tissues. The present study was designed to examine the effect of using different light regimes on melatonin secretion and bone remodeling .Forty male albino rats (6 months old) were divided into four equal groups as follow, the 1st control (exposed to 12 hours light : 12 hours dark), The 2nd exposed to 24 hours dark, the 3rd exposed to 24 hours light and the 4th MEL treated(subcutaneously with 50 μg/100 g b.w) and exposed to 24 hours light. After 12 weeks of treatment, blood and bone samples were collected. Bones grossly examined and fixed in neutral formalin for histopathological examination and measurement of RANKL mRNA levels by real time PCR . Serum total and bone specific alkaline phosphatase (TALP and BALP) respectively , corticosterone, malondialdehyde (MDA), reduced glutathione (GSH), and superoxide dismutase (SOD) activity were measured .Moreover, blood and bone minerals contentment as calcium ( Ca), phosphorus (P),zinc (Zn), copper (Cu), iron ( Fe) and manganese (Mn) were determined. There were significant (P<0.01) increase in bone mass,GSH, BALP and SOD activities in the 2nd and the 4th groups . Moreover, a significant (P<0.05) increase in serum Ca level and serum and bone Zn and Fe. Moreover, there were significant (P<0.05) decrease in serum MDA and corticosterone levels in the 2nd and the 4th groups were observed and there was down regulation to RANKL mRNA in the 2nd group.
In conclusion : the findings of the present study may be support a positive effect of MEL on bone formation and oxidative stress in dark exposed group.

Key words: melatonin , hormone ,bone, minerals, RANKL, rat