THE INFLUENCE OF PHOTOPERIOD ON SEROTONIN-CONTAINING CELLS IN THE MOUSE THYMUS DURING INVOLUTION

INTRODUCTION. In the modern world, ancient biological mechanisms that synchronize circadian rhythms are increasingly being disrupted. The disruption of these biorhythms leads to depression, decreased immunity, and consequently, to neurological, autoimmune, endocrine, and oncological diseases. However, the mechanisms of interaction between serotonin-producing cells and lymphocytes–both under a natural light/dark cycle and in its absence–remain poorly understood.

OBJECTIVE. This study aimed to investigate the effect of different photoperiods on serotonin-containing cells of the diffuse endocrine system and on thymus involution.

MATERIALS AND METHODS. Three-week-old mice weighing 18–20 g were divided into three groups of 20 animals each. The first group was maintained under natural photoperiod conditions (9/15 light/dark), the second under constant darkness (0/24), and the third under constant illumination (24/0) for 4 weeks. An immunocytochemical reaction for serotonin was used to detect serotonin-containing thymus cells. Nuclei were counterstained with DAPI (Dianova, Germany) to count lymphocytes. The optical density of the reaction product was measured in 500 cells per experimental group using SigmaScan Pro 5 software.

RESULTS. Constant illumination led to a 1.7-fold increase in the number of serotonin-producing cells at the corticomedullary border and a 1.6-fold increase in serotonin-absorbing cells in the cortex of the thymic lobules. Furthermore, constant illumination resulted in an increased optical density of serotonin in cortical lymphocytes, accompanied by a decrease in the total lymphocyte count. Multivariate factor analysis revealed a strong factor loading between the number of serotonin-positive cells and their serotonin content.

CONCLUSION. The findings demonstrate a clear relationship between the number of serotonin-containing cells, the area of thymus involution, and the duration of photoperiod exposure. In the modern world, ancient biological mechanisms of synchronization of circadian biological rhythms are being disrupted. Violation of the body's biorhythms leads to depression, decreased immunity and, as a result, to neurological, autoimmune, endocrine and oncological diseases. The mechanisms of interaction between serotonin-producing cells and lymphocytes under conditions of a natural change of dark and light phases in the light/dark rhythm and in the absence of this rhythm are poorly studied.  

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