The effects of ionizing radiation on pineal melatonin and on key enzym
es of its metabolism have been studied in our laboratory. After adapta
tion to an artificial light/dark cycle of 12:12 h, male Wistar rats we
re fractionally whole-body irradiated with a dose of 2.4 Gy of gamma-r
ays twice a week up to total doses of 4.8, 9.6 or 14.4 Gy. Irradiation
and sham-irradiation were performed in the late afternoon. The rats w
ere sacrificed at 24:00 to 01:00 h in darkness, 6 h, 3 or 5 days after
the last exposure. Pineal and serum melatonin concentrations, pineal
activities of serotonin N-acetyltransferase (NAT) and of monoamine oxi
dase (MAO) were determined. The NAT activities in the rats irradiated
with 4.8 and 9.6 Gy decreased at some intervals without changes of mel
atonin concentration. Irradiation with a total dose of 14.4 Gy decreas
ed NAT activity and the concentration of pineal and serum melatonin 6
h and 3 days after the last exposure. The activity of MAO, estimated o
nly in the rats irradiated with the dose of 14.4 Gy, increased signifi
cantly 3 days after irradiation. The fractionated irradiation up to th
e dose of 14.4 Gy caused a transient decrease in pineal melatonin synt
hesis. This could be the consequence of preferential oxidative deamina
tion of serotonin in comparison with its N-acetylation, leading to mel
atonin biosynthesis.