Background: Biogenic amines (BAs) secreted by the sympathetic neural apparatus of rat uterus is reported to be conducive to the uterine functional activity during postpartum involution; the imbalance in BAs ratio could confer postpartum reproductive disorders including improper postpartum involution. Objective: The changes in density of uterine sympathetic nerves implicated in the pathology of endometriosis, adenomyosis, and delayed uterine involution. The present study is aimed to ascertain ‘serotonin’ and ‘catecholamine’ concentrations in mesenteric mast cells (MCs), and structural elements of nerve fibers across the perivascular plexuses (PPs) and single sympathetic nerve terminals (SST). Methods: Furthermore, the density of their spatial distribution (SDP and SDT) in the uterine body, cervix, and mesometrium was determined during postpartum involution. Tissue specimens of postpartum uterus were obtained from 55 nulliparous female Wistar outbred strain rats, which were grouped according to the days after parturition at the time of sacrifice. The nerve fibers of PP and SST exhibited emerald green fluorescence, which was detected by glyoxylic acid fluorescence technique; the fluorescence invoked by BAs was identified by microspectrofluorimetry. Results: Concentrations of BAs were extensive in the varicosities of PP and SST on the 10th day. However, the highest BA concentrations were found in structural elements of PP in the uterine mesometrium in the initial days of postpartum. In mesenteric MC, serotonin and catecholamines were at the highest concentration on 10th day postpartum. Histamines peaked on the 6th day. Conclusion: SDP and SDT were increased significantly in all structural elements of uterine nerve fibers in the uterine body and cervix compared to SDP in mesentery. Considering that catecholamines and serotonin are antagonists in many aspects of their biological action, the ratio of BAs should be well-balanced to maintain anabolic-catabolic equilibrium in the rat uterus.

The uterus is a unique organ of its kind due to the constant remodeling during the reproductive period of the tissues that make it up. Violations of the involution processes occurring in this organ after part can cause the development of pathological conditions of the uterus. The relevance of the study is determined by the active participation of biogenic amines in the regulation of tissue homeostasis and uterine histogenesis. The aim of the work was to study the patterns of spatial cooperation of mast cells in the uterus of rats and the content of catecholamine, serotonin and histamine in them during the postpartum period. Using fluorescent-histochemical methods, mast cells of the uterus of rats were studied in 100 outbred female rats of reproductive age, divided into 11 groups according to the postpartum period. The study was performed on cryostat sections of preparations of the uterine body. The density of the spatial distribution of endometrial and myometrium mast cells was assessed. The content of histamine, serotonin and catecholamine was determined by cytospectrofluorimetry in conventional units of the recording device in the cells. As a result of the study, it was found that the density of the spatial distribution of fluorescent mast cells in the mucosa and muscular layers of the uterine body is minimal on the first day after childbirth. The largest number of mast cells is observed on the 10th day after birth. The content of histamine, serotonin and catecholamine in the mast cells of the myometrium of the uterine body is maximum on the first day after birth and gradually decreasing, reaching the lowest values ​​on the 10–15th day of postpartum involution. Mast cells of the endometrium are characterized by a higher saturation of serotonin and catecholamine compared to the myometrium. In tissue basophils of the myometrium, a higher level of histamine is noted. Correlations of changes in the content of bio amines have been established. The density of the spatial distribution of mast cells of the endometrium and myometrium and the content of neurotransmitter bioamines in them during the postpartum involution of the uterus changes unidirectional, despite regional differences. It is possible that mast cells contribute to the restoration of sympathetic innervation of the uterus in the postpartum period.