The effect of the photoperiod on the serotonergic system of the thymus and its role in the ­implementation of the effects of exogenous melatonin

Background. It was believed that the functioning of the immune organs is photoindependent, but there are a number of works that testify to the effect of the photoperiod shift on the functioning of the immune organs. We hypothesized that the absence of a photoperiod under constant light or constant dark would affect the serotonergic system of the thymus gland, as well as the direction and intensity of melatonin exposure.

Aim. Study of serotonin-containing thymus cells under conditions of experimental desynchronosis and the role of the photoperiod in the realization of the effects of exogenous melatonin.

Material and methods. A comparative study of serotonin-containing thymus cells of 8-week-old Wistar rats, which were divided into 6 groups, was carried out. The first and second groups were kept under conditions of natural photoperiod, the third and fourth — in conditions of constant darkening, the fifth and sixth groups — in conditions of constant illumination within 4 weeks. Melatonin ad libitum at a concentration of 4 mg/l with water was received by animals of the second, fourth and sixth groups for 4 weeks. To determine the level of serotonin in cells, monoclonal antibodies to 5-HT were used. Conclusions about the serotonin content in the cells were made by measuring the optical density of the substance in 100 cells for each animal using the SigmaScan Pro5 program. Descriptive statistical processing was performed using the Statistica 17 program. The data obtained for each group of animals were averaged, the standard error and standard deviation were calculated.

Results. Artificial darkening reduced the serotonin content in 5-HT-immunoreactive cells of the cortical substance by 2.4 times (p=0.001), and constant illumination increased this indicator by 1.9 times (p=0.001). When melatonin was administered to animals kept in dark conditions for 4 weeks, the optical density of serotonin increased by 3.6 times (p <0.0001). Administration of melatonin to animals under constant illumination led to an increase in the optical density of serotonin in the cells of the diffuse endocrine system at the border of the cortex and medulla of the lobules by 1.5 times (p=0.002).

Conclusion. The cells of the diffuse endocrine system of the thymus are sensitive to changes in the photoperiod, and the introduction of melatonin has a multidirectional effect on the amount and optical density of serotonin under different light conditions.