The effect of single whole-body irradiation on pineal functions

The influence of the single lethal dose of gamma rays on the functional parameters of the pineal gland was studied in experiments on Wistar rats. Male rats, adapted to an artificial light:dark regimen (LD) 12:12 h were exposed to whole-body irradiation with gamma rays at a dose of 9.6 Gy. Irradiation, sham-irradiation and decapitation were carried out in darkness. The animals were analyzed 60 min, 1, 3 and 5 days after irradiation. The ionizing radiation increased the activity of the rate-limiting enzyme of melatonin synthesis - serotonin N-acetyltransferase (NAT) - on day 3 post-irradiation, without any change in the remaining intervals investigated. The changes in pineal melatonin concentration occured in two phases: a decrease at 60 min was followed by an increase, significant on day 5 after exposure. The serum concentration of melatonin started to increase on day 3 post-irradiation and on day 5 was significantly higher in comparison with the control. The results obtained in the present experiment and in our studies published previously suggest that there are two phases in the response of the mt pineal parameters to the single whole-body lethal irradiation: during the early phase up to 2 hours postirradiation the concentration of pineal melatonin decreases, without any change of NAT. The reason may be the inbalance, induced by ionizing radiation, between N-acetylation and oxidation of serotonin. During the late phase after exposure the NAT activity increases, as well as the pineal and serum melatonin concentration, which is considered an adaptive response of the pineal gland to the radiation stress.