RESEARCH ARTICLEEffect of thyroxin on cell morphology and hormone secretion of pituitary grafts in rats
Introduction
The pituitary gland is composed of several cell types that secrete a variety of hormones which regulate the function of various endocrine glands including the thyroid.
Thyrotroph cells are the least abundant cell type in the anterior pituitary, comprising approximately 5–8% of all cells (Villalobos et al., 2004) producing thyroid-stimulating hormone (TSH) under control of the hypothalamic thyrotrophin-releasing hormone (TRH). The effect of TRH on TSH gene regulation is mediated by several nuclear proteins, such as Pit-1 which is essential for the development of several pituitary cell lines, and it is expressed in somatotrophs, lactotrophs and thyrotrophs. TSH is the main regulator of thyroid development and growth, and stimulates all of the steps involved in the synthesis and secretion of thyroid hormones (Ortiga-Carvalho et al., 2016). Thyroxin (T4) is the main secretory product of thyroid follicular epithelial cells, but has low biological activity. Through the process of deiodination, T4 is converted to the active form 3,5,3′-triiodothyronine (T3) (Brook and Marshal, 2001). The final cellular physiological response to thyroid hormones is a result of a complex mechanism involving both genomic and non-genomic actions (Bargi-Souza et al., 2017).
In rats and humans, growth hormone (GH) is synthesized in somatotroph cells of the adenohypophysis. TRH, as well as thyroid hormones, can stimulate GH synthesis and release from the somatotroph cells (Harvey, 1990; Giustina and Wehrenberg, 1995) upregulating Gh transcription. T3-mediated activation of rat Gh gene expression presented a synergism between transcriptional factor Pit-1 and T3, increasing the Gh mRNA content (Bargi-Souza et al., 2017). In rodents, hypothyroidism conspicuously reduced the expression of GH, while increasing the expression of both TSH subunits (Bargi-Souza et al., 2017). Additionally, thyroid hormones play an important role in the maintenance of prolactin (PRL) cell function and its reactivity to TRH. In the presence of thyroid hormones, TRH stimulates PRL release from lactotrophs in a dose-dependent manner, both in vitro and in vivo (Tashjian et al., 1971; Blake, 1974). TRH is secreted into the bloodstream of the hypophysial stalk and it binds to two types of receptors TRH-R1/R2. TRH-R1 mediates neuroendocrine hormonal functions, while TRH-R2, which is highly expressed in the brain region, likely mediates neurotransmitter effects (Deflorian et al., 2008). TRH-R1 is most important for activity in thyrotropes, although it is also expressed in lactotropes and a fraction of somatotropes (Ortiga-Carvalho et al., 2016). Hypothyroidism results in elevation of TRH that, contrary to the expected increase in PRL secretion, induces a deficiency in lactotrophs (Stahl et al., 1999).
Low levels of thyroid hormones after thyroidectomy lead not only to the proliferation of thyrothroph cells and their hypertrophy, but also to a decrease in GH secretion and a reduction in somatotroph quantity (Quintanar-Stephano and Valverde, 1997; Stahl et al., 1999; Nolan et al., 2004). It can also lead to decreased PRL secretion and morphological changes in lactotroph cells, therefore, when in hypothyroid state, there is a complex hormonal dysfunction rather than a single hormonal defect. These effects are reversible by T4 administration (Gómez Dumm et al., 1985; Nolan et al., 2004).
Since the hypothalamus plays a major role in regulating pituitary hormone secretion, pituitary grafts have been used as experimental models to study the effects of different substances on pituitary cells not connected with the hypothalamus. In pituitary grafts, PRL hypersecretion is induced by the lack of hypothalamic inhibition, while the failure of the hypophysiotropic releasing hormones to reach the pituitary graft causes low secretion of the remaining adenohypophysial hormones. Hence, pituitary grafted rats are not only valid models of excessive and abnormal PRL secretion, but may also be useful for distinguishing between stimuli requiring an intact hypothalamic-pituitary axis and agents which act directly on the pituitary (Adler, 1986). Autotransplantation of the rat anterior pituitary under the kidney capsule results in chronic hyperprolactinemia and hypopituitarism where GH secretion and other anterior pituitary functions, as well as growth, are diminished. In contrast, in non-hypophysectomised rats implanted with anterior pituitary glands from littermate donors, GH secretion, growth and thyroid function remain normal even though they have high levels of circulating PRL (Adler, 1986).
Herein, we studied the effect of T4 on pituitary hormone secretion and the functional morphology of pituitary secretory cells autografted (AUTO) or allografted (ALLO) under the kidney capsule, with no hypothalamic influence under an integrative point of view, including body and endocrine organ weights, hormonal levels, functional morphology and cell proliferation.
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Animals
Male Wistar rats of three months of age weighing 250 g were divided into eight groups of five to nine animals each (Table 1): 1) intact untreated controls (CO); 2) thyroidectomised (TX); 3) pituitary autografted under the kidney capsule (AUTO); 4) rats with intact pituitaries and an isogenic female pituitary allografted under the kidney capsule (ALLO); the remaining four groups were similar but injected intramuscularly (i.m.) with T4 (6 μg/100 g of body weight) daily for two weeks. Injections
Body weights
Body weights were assessed weekly for six weeks Fig. 2A. The percentage of body weight variation is summarized in Fig. 2B.
The body weights of CO animals increased until the end of the experiment (average of 48.5 g), representing 19.5% of increment in their body weight. In contrast, CO + T4 animals increased in weight an average of only 4.9%, which was four times less than the CO group. The TX group body weight decreased 3.3% and their final body weights were 18% lower than those in CO group.
Occurrence of thyroid deficient cells in pituitary grafts
In normal animals, thyroidectomy results in the development of thyroidectomy cells in the pituitaries (Fig. 5). It is well-known that the feedback mechanism between the thyroid gland and the pituitary is affected from reduced thyroid function. Thyroidectomy cells are supposed to be very active TSH cells activated by the increased hypothalamic TRH level, attempting to compensate the missing T3 and T4. In our experiment, there is no TRH nearby the explanted pituitaries. Consequently, the most
Conclusion
In summary, thyroid hormones (T4) induced changes in pituitary cell morphology, as seen by light microscopy and TEM. The development of thyroidectomy cells in pituitaries grafted under the kidney capsule (disconnected from hypothalamus) is a remarkable phenomenon that was not previously reported. A long term hypothyroid state (six weeks) induces the development of thyroidectomy cells, even in the pituitary grafts, denoting that hypothalamic control is not necessary for its occurrence. In
Ethical approval
All procedures performed in this animal study were in accordance with the ethical standards of the institution or practice at which the study was conducted.
Acknowledgments
This work was supported by CONACYT221262 grant in México (A.Q-S). Authors are also grateful to the Jarislowsky and Lloyd Carr-Harris Foundations for their support.
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