Male Wistar rats adapted to an artificial light-dark regimen (12 h light: 12 h darkness) were whole-body irradiated with a dose of 14.35 Gy of gamma rays. Irradiation,
sham-irradiation and
decapitation 30, 60 and 120 min after the exposure were performed between 2000 h and 0100 h in the darkness. The
serotonin N-acetyltransferase activity (
NAT), the concentration of
melatonin,
dopamine,
norepinephrine and
epinephrine were measured in the pineal gland. The serum levels of
melatonin and
corticosterone were also determined. Ionizing radiation did not change the activity of the key
enzyme of
melatonin synthesis,
NAT, but decreased the concentration of pineal
melatonin. The concentration of pineal
dopamine and
norepinephrine decreased 30 and 120 min after exposure, while the concentration of
epinephrine was elevated 30 min after irradiation, though later it was markedly decreased. The serum
melatonin level was not changed, but an increase in
corticosterone level was observed. In the early period after the exposure, a decrease in pineal
melatonin occurred, accompanied by a decrease in pineal
catecholamines. On the contrary, in the phase of developed
radiation injury the signs of increased
melatonin synthesis were observed on days 3 and 4 after the exposure (Kassayová et al. 1993a). The underlying mechanisms require further research.