Two-phase response of rat pineal melatonin to lethal whole-body irradiation with gamma rays

Male Wistar rats adapted to artificial light:dark (LD) regimen 12:12 h were whole-body irradiated with a single dose of 9.6 Gy of gamma rays and sham/ irradiated in the night in darkness. The rats were examined 60 min, 1, 3 an d 5 days after exposure between 22:00 and 01:30 h in the darkness. The resu lts obtained indicate a two-phase reaction of pineal melatonin after the le thal irradiation of rats: the decline of melatonin concentration early afte r the exposure (at 60 min) with unchanged serotonin N-acetyltransferase (NA T) activity followed by an increase of melatonin synthesis, accompanied by an increase of pineal and serum melatonin on day 5 after the exposure. NAT activity was increased on day 3 after the exposure. Serum corticosterone co ncentrations in irradiated rats were increased 60 min and 3 days after expo sure. With respect to the antioxidant, immunomodulating and stress-diminish ing properties of melatonin, we consider the increase in melatonin synthesi s during later periods after irradiation as part of adaptation of the organ ism to overcome radiation stress.