FEATURES OF THE FORMATION OF THE ESTRAL CYCLE OF RATS UNDER CONDITIONS OF EXPERIMENTAL HYPOTHYROIDSIS

Abstract

In a chronic experiment, the influence of hypothyroidism developing in the prepubertal period on the formation of sexual cycles and the onset of puberty was studied. It has been established that hypothyroidism affects the hormonal function of the ovaries and has an antiestrogenic effect. Hypothyroidism does not affect the formation of the hypothalamic-pituitary-gonadal axis.

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Relevance of the topic.
The study of the reproductive cycle of female rats and the influence of hypothyroidism on its process, despite the existing impressive list of scientific and educational sources, remains relevant at the present time. There are several reasons for studying the stages of the estrous cycle in experimental hypothyroidism: conducting pharmacological and toxicological studies on female laboratory rodents; choosing the optimal time for crossing when creating transgenic animals or artificial insemination, maintaining colonies of laboratory animals, especially lines with low fertility [1]. Morphological studies require determining the stage of the estrous cycle and the effect of diseases on it, in this case hypothyroidism. Taking this factor into account can optimize the performance of “profile” experiments and increase the efficiency of reproduction of laboratory animals [2].

The purpose of this study was to study the peculiarities of the formation of the estrous cycle in rats under conditions of experimental hypothyroidism using the drug “merkazolil”.

Materials and research methods
The experiments were carried out on 10 outbred female rats weighing 73–146.5 grams. The animals were kept under standard vivarium conditions. The rats were kept in a cage for small rodents.
Preparation and analysis of smears to determine the phases of the estrous cycle were performed according to standard methods [6]. Smears were taken with a thin cotton swab moistened with saline, placed on a glass slide, stained with azur-eosin solution according to Romanovsky and examined using a BIOLAM light microscope at 120x magnification.
The structure of the smears was studied for four days in the daytime before taking Mercazolil (the active ingredient is thiamazole) and after two weeks of taking Mercazolil at a dose of 5 mg per 10 rats every 2 days dissolved in water for six days with a break of 5 days. Mercazolil blocks the binding of iodine to tyrosine in thyroglobulin. Mercazolil does not penetrate through GEP (intercellular contacts that provide direct transfer of ions and small molecules between neighboring cells) [2]. Statistical analysis of the results was carried out using the SigmaPlot 12.5 program. The average values of indicators and standard deviation were calculated. Comparison of samples was carried out using the Studet method.

Main results
On vaginal smears, we clearly observed the following phases of the cycle: diestrus (leukocytes and mucus), proestrus (appearance of epithelial cells with nuclei), estrus - karyopyknosis occurs (appearance of scales) and metaestrus (leukocytes, scales, cells with nuclei) [3].

Rice. 1 Diestrus
Rice. 2 Proestrus
Rice. 3 Estrus
Rice. 4 Metaestrus

At the beginning of the experiment, most of the animals were in the prepubertal period, since we did not observe cyclic changes in smears corresponding to the phases of the estrous cycle. As the body weight of the animals increased, the appearance of cyclic changes in smears was also noted, which indicates the onset of puberty.

Rice. 5 Dependence of the onset of puberty on the weight of the rat

During the experiment, it was noted that the median weight at the onset of puberty in half of the female rats was 70.5 g, and the average weight at which puberty occurred was 88.7 ± 23.9 grams. No differences were found in the onset of puberty between rats not exposed to antithyroid drugs and intact animals. The development of puberty was facilitated by tactile stimulation of the vaginal opening with a cotton swab moistened with saline; this was proven by the fact that at the beginning of the experiment the vagina was closed, and after a long period of taking vaginal smears, the genital opening increased in size.
The duration of the sexual cycle before exposure to Mercazolil averaged 5.2±1.0 days. After a two-week administration of the antithyroid (inhibits the synthesis of thyroxine) drug Mercazolil, the median duration of the sexual cycle shifted and amounted to 6 days, and the estrus stage passed in a few hours and was clearly noticed only in the second week. The differences between the duration of the sexual cycle before and after the administration of Mercazolil were significant (p <0.05).

Rice. 6 Duration of the estrous cycle in rats before and during administration of Mercazolil

Thus, hypofunction of the thyroid gland in the prepubertal period has virtually no effect on the timing of puberty, but does affect the duration of the sexual cycle in pubertal animals [5]. The cellularity of the smears did not change; this fact may indicate that hypothyroidism primarily affects the hormonal function of the ovaries and has an antiestrogenic effect, but not on the formation of the hypothalamic-pituitary-gonadal axis [7].

Conclusions
Thyroid hormones during puberty are involved in the formation of the sexual cycle. Hypothyroidism leads to prolongation of the sexual cycle. The estrus stage occurred within a few hours and was noticed only in the second week of the experiment.

Hypothyroidism had no effect on the timing of puberty, but did affect the formation of the sexual cycle.