Male Wistar rats were exposed to whole body irradiation with 14.35 Gy gamma rays after the adaptation to light/dark cycle (LD 12:12). Three groups of rats were examined: A) rats irradiated in the night and placed in the 12 h LD cycle again, B) rats irradiated in the day-time and placed in the 12 h LD cycle, and C) rats irradiated in the night and kept in constant darkness. All analyses were carried out in the dark. Radiation enhanced the activity of pineal N-acetyltransferase 3-4 days after exposure in all groups, in the C group significantly on the 4th day. Different light regimens during and after irradiation did not to affect the activity of this key enzyme of melatonin synthesis substantially.
Circadian and circaannual oscillations of tissue lipid peroxides (LPO) were studied in young male Wistar rats. The concentration of malondialdehyde, one of LPO degradation products, was measured at 3-h intervals during 24 hours in rats, adapted to lightrdark 12:12 h regimen in the course of the year. LPO in the liver, thymus and bone marrow oscillated rhythmically in the course of the day and year. Circadian oscillations in all tissues were two-peaked, with zeniths at various times of the light and dark parts of the day. In the liver and thymus, the highest mesors were found during the winter, in the bone marrow during the spring. The same holds for amplitude values, with the exception of the bone marrow which exhibited the highest values during the summer. The reason for the LPO oscillations is probably resulting from the changing ratio of pro- and anti-oxidative capacities in various tissues during the day and the year.
The seasonal influence on circadian oscillations of serum thyroid hormones has been confirmed in the laboratory rat, an animal exhibiting low photoperiodic activity. The aim of this paper was to study the influence of various photoperiods, applied in a single season, on circadian variations in the levels of thyroid hormones in male Wistar rats. After 6-weeks of adaptation to artificial light-dark regimens (LD) 08:16 h, 12:12 h, 16:08 h, and to the standard housing conditions, the rats were examined in 3 h intervals in the course of 24 h in December. The concentrations of thyroxine (T4), triiodothyronine (T3) and reverse T3 (rT3) were examined in the serum. The curves of T4 circadian oscillations showed two peaks in all the photoperiods followed. Computative acrophases were localized between 07.00 and 08.00 h, the amplitude in the LD 12:12 regimen was twice that observed in LD 08:16 and 16:08, the rhythm was present and the mesors were approximately the same. Circadian oscillations of T3 exhibited rhythmicity in all the photoperiods with computative acrophases localized between 07.30 and 09.00 h, and the values of mesors in LD 08:16 and 16:08 regimens were significantly lower in comparison with those in the LD 12:12 regimen. The rT3 circadian variations in the LD 12:12 regimen showed rhythmicity with acrophase at 06.00 h. The rhythm in the LD 16:08 regimen was of borderline significance, the computative acrophase occurred at 8.16 h, and the mesor value was significantly higher than those in the LD 12:12 regimen. The decrease in the amplitude of T4 oscillations and the lower T3 mesors in LD 08:16 and 16:08 regimens in comparison with the LD 12:12 values indicated only minor modification in circadian oscillations of T4 and T3 resulting from artificial photoperiods. In comparison with our previous studies these data suggest that changes in circadian oscillations of serum thyroid hormones might reflect the effect of the season of the year rather than the effect of day duration, i.e. the photoperiod.
Male SPF bred Wistar rats were adapted to natural light (N) and to a 12 : 12 h (light-dark) artificial light (A) regimen in the course of the year. The rats were analyzed at 3 h intervals during 24 h approximately at the time of the vernal and autumnal equinox and at the winter and summer solistice. Serum insulin circadian oscillations depended on the season, being different in various light regimens. The mesors were the highest during summer, the lowest during winter in both regimens. The external acrophases of insulin in the N differed from those in the A group, contrary to the computative ones. The annual mean of serum insulin concentration was lower in the N than in the A group. The circadian oscillations of corticosterone were influenced primarily by the time of year. The mesors were the highest during summer, lower in winter and spring in N and A group. The computative acrophases were similar in both groups in all seasons except spring. The external acrophase was similar in both regimens during the year. The response of insulin, a major anabolic hormone, to various light regimens during the day and year was different from that of corticosterone, a major hormone of the stress reaction.
The effects of ionizing radiation on pineal melatonin and on key enzymes of its metabolism have been studied in our laboratory. After adaptation to an artificial light/dark cycle of 12:12 h, male Wistar rats were fractionally whole-body irradiated with a dose of 2.4 Gy of gamma-rays 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 were 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 oxidase (MAO) were determined. The NAT activities in the rats irradiated with 4.8 and 9.6 Gy decreased at some intervals without changes of melatonin concentration. Irradiation with a total dose of 14.4 Gy decreased NAT activity and the concentration of pineal and serum melatonin 6 h and 3 days after the last exposure. The activity of MAO, estimated only in the rats irradiated with the dose of 14.4 Gy, increased significantly 3 days after irradiation. The fractionated irradiation up to the dose of 14.4 Gy caused a transient decrease in pineal melatonin synthesis. This could be the consequence of preferential oxidative deamination of serotonin in comparison with its N-acetylation, leading to melatonin biosynthesis.
Suspension hypokinesia is a new model which can simulate some effects of microgravity on the organism of laboratory animals. Two groups of male SPF-bred Wistar rats were suspended for 24 h. In the first group hypokinesia began in the morning (M) (1 h after light onset, 0800 h), whereas the other group was subjected to this treatment from the evening (E) (1 h after dark onset, 2000 h). In the serum, there was a statistically significant increase in non-esterified fatty acids, triacylglycerols (TG) and glucose and a decrease in triiodothyronine concentration in the M group, while only a significant increase in phospholipids (PL) was found in the E group. The serum corticosterone level was increased in both groups, more markedly in the M group. There was an increase in TG and PL in the liver in M rats. In the bone marrow (femur), an increase of triglycerols in E rats and an increase of phospholipids in M rats were found. The concentration of glycogen in the heart muscle, m. quadriceps femoris and m. soleus rose in the M group only. The changes in the analyzed parameters predominate in the rats subjected to hypokinesia in the morning period. This fact confirmed the hypothesis about a higher sensitivity of rats to the stressor acting in the period of inactivity.
Male Wistar rats adapted to a light/dark cycle (LD) 12:12 h were exposed in the darkness to a single dose of 14.35 Gy gamma rays on the head with the body shielded. Irradiated and sham-irradiated rats were kept again in the 12 h LD cycle with a free access to food and water till the analysis performed in the darkness. Pineal N-acetyltransferase activity and melatonin content, the serum concentration of the melatonin, corticosterone, thyrotropin and thyroid hormones were determined. N-acetyltransferase activity was lower 2-24 h after irradiation non-significantly whereas between 3-10 days it did not differ from the controls. Radiation decreased the pineal melatonin content and its serum concentration 2 h after exposure and increased them significantly 1-3 days after irradiation. No changes in melatonin levels were found on postirradiation days 5-10. The corticosterone concentration was increased 2 h after exposure only. Local head irradiation changed neither thyrotropin nor thyroid hormone levels.
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 (Kassayova et al. 1993a). The underlying mechanisms require further research.
The effect of various photoperiods on circadian rhythms of chosen parameters was investigated in laboratory rats. SPF male Wistar rats were adapted for six weeks to artificial light-dark cycles (LD 8 : 16, 12 : 12, 16 : 8). The light was switched on at 07.00 h in all regimens. The rats were killed at 3-hour intervals within 24 h, the serum concentration of corticosterone, insulin,glucose, food and water intake was determined. The external and computative acrophases of corticosterone varied in every photoperiod being dependent on the duration of light, the mesor values decreased in LD 16 : 8 in comparison with other photoperiods. The external acrophase of insulin was located 4 h after light onset in LD 8 : 16 and 12 : 12, in LD 16 : 8 one hour before light onset. The mesor values were approximately equal in all photoperiods. The circadian rhythms of glucose were similar in all regimens. Circadian variation of food and water consumption culminated at the same time in all regimens, the amount of food consumed in light increased with the light duration. Various photoperiods remarkably influenced circadian oscillations of corticosterone and in part food and water intake which could be considered as photoperiodic traits.
Male Wistar rats were irradiated continuously with a daily dose of 0.19 Gy (120 days), 037 Gy (90 days) and 0.96 Gy (35 days) of gamma rays. An other group of rats was irradiated continuously with graded doses of gamma rays, up to total exposures ranging from 3.83-19.15 Gy. Depending on both the daily dose and total exposure, there was a decrease in phospholipid content in the thymus which correlated well with thymus weight changes. The decrease in triacylglycerol content was a less reliable sign of radiation damage. The phospholipid content reflecting the patterns of organ cellularity is a valuable indicator of the extent as well as recovery from radiation - induced injury to the thymus.