The effect of forced molt on pituitary function and other endocrine parameters was investigated in three year old hens subjected to a dietary forced molting procedure. In addition to molting, fasting caused cessation of egg production, body and organ weight loss, alterations in hormone secretion and morphological changes in some endocrine glands. Body and ovary weights decreased but returned to normal with ad libitum feeding. Pituitary, thyroid and adrenal weights were not affected but serum hormone levels measured by RIA revealed a decrease in LH, FSH and PRL and increases in TSH, T₃ and GH all of which returned to higher levels with ad libitum feeding. Serum P₄ levels remained low (and egg-laying stopped) until ad libitum feeding was resumed and then increased and egg-laying returned to a typically productive level. Serum ACTH and T₄ increased with fasting and remained elevated. Gonadotrophs and corticotrophs increased in numbers with fasting and/or food restriction but thyrotrophs, somatotrophs and lactotrophs decreased. Correlations between cell populations and serum hormone levels was quite common. Colloid-filled follicles resembling a hypertrophic thyroid gland occurred throughout the pituitary pars distalis. Granules appear to be discharged into the follicular lumen through exocytotic pores in the apical plasmalemma of follicular cells. Lactotrophs, corticotrophs and somatotrophs are commonly arranged in follicles or clusters. PRL-containing granules are in the center of some follicles and are concentrated near pituitary cysts. Pituitary cysts, lined with ciliated epithelium and sparse mucous cells, are more prevalent in fasted hens and decline with the resumption of feeding. Reduced lactotroph populations and presumptively degenerated lactotrophs in cyst lumens are correlated with reduced serum PRL levels. Necrotic cells occurred in the pituitary parenchyma of fasted birds but dilated RER in the thyrotrophs of fasted hens indicate enhanced activity of these cells. Ultrastructural evidence presented here indicates that pituitary secretion by lactotrophs occurs both intraluminally and perivascularly.

Investigations have shown that the hypothalamus and pituitary respond to decreases in body temperature by stimulating thyroid release of T_3 and T_4 . This study was designed to bypass the control of the hypothalamus and pituitary gland and investigate the direct effect of temperature on the thyroid gland. Hypothermia was by an in vivo isolated perfusion of the thyroid gland. Radio-immunoassay was used to measure T_3 and T_4 concentrations. Significant increases were observed in animals perfused between 36º and 25ºc. These results indicate that the thyroid gland is directly effected by decreased temperature and that it is capable of exerting control over body temperature independent of the hypothalamus and pituitary gland. Lower perfusion temperatures produced no significant increases.

A parte ventral da área pré-óptica medial (MPOA) apresenta o RNAm do prépró-hormônio concentrador de melanina (ppMCH) e o MCH apenas durante a lactação, especialmente entre os dias 19º e 21º. Fibras imunorreativas ao MCH (MCH-ir) e ocitocinérgicas (OT-ir) de origem desconhecida transitam na lâmina interna da eminência mediana (MEi) e na hipófise posterior (PPit). Utilizamos ratas lactantes da linhagem Long-Evans para identificar a origem dessas fibras, caracterizar sua relação na MEi e PPit e quantificar o ppMCH e OT na PPit. A imuno-histoquímica mostrou que não há colocalização entre as fibras MCH-ir e OT-i e há uma proximidade entre suas fibras na MEi e PPit. A dupla hibridização in situ e imuno-histoquímica mostrou que os neurônios retrogradamente marcados com FG após injeção intravascular expressam o RNAm do ppMCH na MPOA e no núcleo paraventricular do hipotálamo. O Western Blotting mostrou uma maior presença do ppMCH na PPit no 19º dia de lactação. Os dados sugerem que o MCH atua no controle neuroendócrino para o término do período de lactação. / The ventral part of the medial preoptic area (MPOA) has been described as the novel harbor of the expression of prepro-melanin-concentrating hormone (ppMCH) mRNA and MCH peptide, only during the end of lactation, especially between the 19th and 21st days. MCH-ir and OT-ir fibers are described as passing in the internal layer of the median eminence (MEi). However, the origin of MCH-ir fibers and its possible relationship with the OT-ir fibers are not known. Long-Evans rats were used to identify the origin of these fibers and characterize their relationship into MEi and PPit and quantify ppMCH and OT in PPit. Immunohistochemistry showed no colocalization between MCH-ir and OT-ir fibers, but a closeness between these fibers into MEi and PPit. Using a combination of in situ hybridization and immunohistochemistry we have showed that neurons retrogradely labeled with FG after intravascular injection express the ppMCH mRNA into MPOA and paraventricular nucleus of the hypothalamus. The Western blotting showed an increase of ppMCH in the PPit parallel with a decrease of OT-ir on the 19th day of lactation. The data suggest that MCH acts on the neuroendocrine control towards the end of the lactation period.

A partir da hipófise de Astyanax altiparanae foram clonados e caracterizados os cDNAs completos para fshb, lhb e gpha. Das gônadas, foram clonados os cDNAs para fshr e lhr. As análises filogenéticas destes genes demonstram uma proximidade com as sequências homólogas das ordens Siluriformes e Cypriniformes. Por meio de qPCR, detectamos que a expressão dos mRNAs para as gonadotropinas foi restrita à hipófise, ao passo que dos receptores foi restrita às gônadas. Foi detectado que os mRNAs/proteínas para subunidades beta são expressas por diferentes populações de células gonadotrópicas. Nas gônadas, ambos receptores foram localizados nas células de Sertoli e Leydig, bem como nas células foliculares e da teca de oócitos a partir de crescimento secundário. Então, os níveis de expressão gênica das subunidades das gonadotropinas e seus receptores, juntamente como os níveis plasmáticos de esteroides sexuais foram avaliados e relacionados com a cinética da gametogênese durante o ciclo reprodutivo anual de machos e fêmeas em cativeiro. / From Astyanax altiparanae pituitaries, we cloned the full-leght cDNAs coding for fshb, lhb e gpha. From gonads, we cloned the full-length cDNAs for fshr and lhr. Phylogenetic analysis revealed that all five sequences showed the highest identity degree with homologous sequences from Siluriformes and Cypriniformes, respectively. Using qPCR, we observed that gonadotropin subunits expression were restricted to the pituitary gland, just like their receptors were restricted to the gonads. We also observed that both gonadotropin beta subunits mRNAs/proteins are expressed by distinct populations of gonadotropes at pituitary. In the gonads, both receptors were detected on Sertoli and Leydig cell, as well as on ovaries were located on follicular and theca cells, in oocytes starting from secondary growth. Then, gene expression levels of gonadotropins subunits mRNAs and their receptors were assessed, in parallel to plasma level of sex steroids, and evaluated according to gemetogenesis during the annual reproductive cycle of males and females kept in captivity.

A partir da hipófise de Astyanax altiparanae foram clonados e caracterizados os cDNAs completos para fshb, lhb e gpha. Das gônadas, foram clonados os cDNAs para fshr e lhr. As análises filogenéticas destes genes demonstram uma proximidade com as sequências homólogas das ordens Siluriformes e Cypriniformes. Por meio de qPCR, detectamos que a expressão dos mRNAs para as gonadotropinas foi restrita à hipófise, ao passo que dos receptores foi restrita às gônadas. Foi detectado que os mRNAs/proteínas para subunidades beta são expressas por diferentes populações de células gonadotrópicas. Nas gônadas, ambos receptores foram localizados nas células de Sertoli e Leydig, bem como nas células foliculares e da teca de oócitos a partir de crescimento secundário. Então, os níveis de expressão gênica das subunidades das gonadotropinas e seus receptores, juntamente como os níveis plasmáticos de esteroides sexuais foram avaliados e relacionados com a cinética da gametogênese durante o ciclo reprodutivo anual de machos e fêmeas em cativeiro. / From Astyanax altiparanae pituitaries, we cloned the full-leght cDNAs coding for fshb, lhb e gpha. From gonads, we cloned the full-length cDNAs for fshr and lhr. Phylogenetic analysis revealed that all five sequences showed the highest identity degree with homologous sequences from Siluriformes and Cypriniformes, respectively. Using qPCR, we observed that gonadotropin subunits expression were restricted to the pituitary gland, just like their receptors were restricted to the gonads. We also observed that both gonadotropin beta subunits mRNAs/proteins are expressed by distinct populations of gonadotropes at pituitary. In the gonads, both receptors were detected on Sertoli and Leydig cell, as well as on ovaries were located on follicular and theca cells, in oocytes starting from secondary growth. Then, gene expression levels of gonadotropins subunits mRNAs and their receptors were assessed, in parallel to plasma level of sex steroids, and evaluated according to gemetogenesis during the annual reproductive cycle of males and females kept in captivity.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

published_or_final_version / abstract / toc / Zoology / Master / Master of Philosophy

The hypothalamic-pituitary-thyroid axis operates to maintain the circulating concentration of thyroid hormones. Thyrotropin releasing hormone (TRH) is the major hypothalamic messenger controlling the pituitary-thyroid unit. However, the pituitary-thyroid unit responses to various modalities of TRH exposure are not well characterized. Also, interactions between the thyroid axis and other mammalian organ systems, specifically other hormone axes, to maintain the organism's homeostatic state are not well characterized. This work was designed to clarify the response of the pituitary-thyroid unit to TRH and to assess the effects of physiological levels of the rat's primary adrenal cortical hormone, corticosterone, on the thyroid axis. Adult rats were given equal amounts of TRH by intravenous (I.V.) bolus injection or constant intraperitoneal (I.P.) infusion. Both methods resulted in significant increases in plasma thyroid stimulating hormone (TSH), although the time course and peak plasma value varied with the TRH dosage and administration method. Despite the differences in plasma TSH elicited, the thyroid gland responses were similar. Thus, the pituitary is sensitive to the rate and dose of TRH administration. Also, the thyroid is sensitive to plasma levels of TSH but reaches maximum stimulation at submaximal circulating TSH levels. Adrenalectomized female rats, with I.P. and I.V. catheters, were infused with corticosterone (B) to achieve plasma levels within the rat's physiological range. Plasma samples were drawn before and after submaximal TRH (250 ng/100 g Body Weight) administration for assay of TSH and B concentrations. B in the lower half of its physiological range significantly inhibited the increase in plasma TSH observed 10 and 30 minutes after TRH administration. Also, direct stereotaxic infusion of B (50 ng) followed by TRH (1 ng) into the anterior pituitary inhibited the observed increase in plasma TSH. These studies indicate that homeostatic thyroid axis hormone concentrations are maintained by a feedback loop mechanism which is modulated by adrenal hormones. Specifically, physiological levels of corticosterone decrease pituitary sensitivity to TRH in the rat. In addition, the pituitary and thyroid gland exhibit different response patterns to hormonal stimulation.

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