Objectives: We investigated the day-night differences in oxidative-injury and the

inflammatory response in rat lung following total body irradiation by measuring

thiobarbituric acid reactive substances levels and myeloperoxidase activity.

Materials and Methods: 63 male rats were randomly divided into 6 groups and

were exposed to a dose of 8 Gy to the total body region either in the morning

or evening together with vehicle or melatonin administration, immediately before,

immediately after and 24 h preceeding irradiation (10, 20 and 10 mg/kg, ip, respectively).

Rats in the control group didn’t receive any treatments. 48 h after irradiation,

all animals were sacrificed.

Results: Irradiation increased thiobarbituric acid reactive substances levels. Melatonin

administration decreased both thiobarbituric acid reactive substances levels

and myeloperoxidase activity when the time of irradiation is not taken into consideration.

Depending on the time of irradiation, thiobarbituric acid reactive substances

levels increased after total body irradiation only in morning group; treatment with

melatonin significantly decreased thiobarbituric acid reactive substances levels.

Total body irradiation did not increase myeloperoxidase levels but melatonin treatment

in both time-points caused a significant decrease.

Conclusion: The effect of melatonin can be attributed to its antioxidant and free radical

scavenger properties. Although further studies are required to define underlying

mechanism(s) of time dependent variations in thiobarbituric acid reactive substances

levels in rats to which total body irradiation and total body irradiation plus melatonin

were applied, this study might help to optimize the treatment time with irradiation.