Serum melatonin, corticosterone and thyroid hormones in irradiated rats: effect of exposure during various times of day and of light regimen.

Time of day and lighting regimen could modify the results of experiments, analyzing the effect of various stimuli. Male Wistar rats adapted to an artificial light/dark regimen (LD) 12:12 h (light 07-19 h) were whole-body irradiated with 14.4 Gy of gamma-rays. The experiment was divided into three parts: A) rats irradiated in the dark and placed in the LD regimen, B) rats irradiated in the light and housed in the LD regimen, C) rats irradiated in the dark and kept in the constant dark. Examinations were performed in the dark, 6 h to 4 day after exposure except the 24 h interval in B group. Serum concentrations of melatonin (Mel), thyrotropin (TSH), thyroxine (T4), 3,5,3'-triiodothyronine (T3) and corticosterone (CS) were determined. Irradiation enhanced the concentrations of Mel within days 3-4 in the animals of groups A, B and of TSH on day 4 postexposure in group C only. Radiation decreased the levels of T4 and T3 6 h and 72 h in group C, in group A at 72 h, in group B at 24 h postexposure. CS level was increased 6 h to 60 h after irradiation in all groups. Lethal whole-body gamma irradiation of rats changed the hormone levels with unsubstantionally influence of the time of day when the exposure were done and lighting regimen used.