In mammals, day-length-sensitive (photoperiodic) seasonal breeding cycles depend on the pineal hormone melatonin, which modulates secretion of reproductive hormones by the anterior pituitary gland.  It is thought that melatonin acts in the hypothalamus to control reproduction through the release of neurosecretory signals into the pituitary portal blood supply, where they act on pituitary endocrine cells. Contrastingly, the work presented here shows that during the reproductive response of Soay sheep exposed to summer day lengths, the reverse applies. Melatonin acts directly on anterior-pituitary cells, and these then relay the photoperiodic message back into the hypothalamus to control neuroendocrine output. The switch to long days causes melatonin-responsive cells in the pars tuberalis (PT) of the anterior pituitary to increase production of thyrotrophin (TSH).  This acts locally on TSH receptor (TSH-R)-expressing cells in the adjacent mediobasal hypothalamus, leading to increased expression of type II thyroid hormone deiodinase (DIO2).  DIO2 initiates the summer response by increasing hypothalamic triiodothyronine (T3) levels. The present work also reports similar expression of TSH-R, TSH and DIO2 in European hamster, while DIO3 was oppositely regulated. These data and recent findings in quail indicate that the TSH-expressing cells of the PT play an ancestral role in seasonal reproductive control in vertebrates.  In mammals this provides the missing link between the pineal melatonin signal and thyroid-dependent seasonal biology.

Thesis (M.Sc. (Biochemistry))--Mahidol University, 1979.

Thesis (M.S.)--Florida State University, 2005. / Advisor: J. Michael Overton, Florida State University, College of Human Sciences, Dept. of Nutrition, Food, and Exercise Sciences. Title and description from dissertation home page (viewed Jan. 26, 2006). Document formatted into pages; contains vii, 34 pages. Includes bibliographical references.

The objective of these studies was to contribute to the understanding and assessment of thyroid function in horses. The first study evaluated methods of assessing thyroid function in horses, including validation of an enzyme immunoassay (EIA) for measuring equine thyroid hormones and development and assessment of a thyrotropin releasing hormone (TRH) response test. Our data indicated that EIA is an acceptable method for the measurement of total (T) and free (F) thyroxine (T4) and triiodothyronine (T3) in equine plasma. Its measurements are not equivalent to values obtained by radioimmunoassay (RIA), but they can be calibrated to predict corresponding RIA values. A protocol was developed for TRH response tests involving administration of 1 mg TRH intravenously, with blood sample collection immediately before, 2.5, 5.0, and 24 h after administration. Analysis of plasma TT4, FT4, TT3, and FT3 revealed that the magnitude of hormone response was best approximated by the area under the curve of hormone plotted against time and by the absolute change in thyroid hormone concentration. Baseline concentrations, peak concentrations, and percent of baseline values were not as well able to predict the magnitude of hormone response. The second study assessed the effects of exercise and feed composition on thyroid status. Thirteen mature Arabian geldings, adapted to either a high sugar and starch (SS) or high fat and fiber (FF) feed, underwent 15 wk of exercise training followed by a treadmill exercise test. The TRH response tests performed before training, after training, and the morning after the exercise test revealed that the exercise test decreased the TT4 and FT4 response, whereas feeding of high levels of sugars and starches increased the response of TT3 and FT3. During the first four weeks of training, increased TT4 and FT4 concentrations occurred simultaneously with increased nonesterified fatty acid concentrations, decreased triglyceride concentrations, and increased insulin sensitivity. The increase in TT4 and FT4 may have provided the cellular signaling necessary for increased lipolysis and insulin sensitivity. These metabolic changes facilitate the increases in lipid and carbohydrate metabolism that are needed to fulfill the additional energy requirements of regular exercise. The third study assessed thyroid status in ponies with different laminitic histories. Total T4, FT4, TT3, and FT3 were measured during March and May 2004 in 126 ponies that were categorized as either previously laminitic (PL; n = 54) or never laminitic (NL; n = 72) and evaluated for current laminitis in May (CL; n = 13). Decreased concentrations of TT4 and FT4 were found in PL ponies when compared to NL ponies in March (P = 0.018, 0.020) and May (P = 0.018, 0.001). However, TT4 and FT4 concentrations in CL ponies were not different than concentrations in NL ponies in May (P = 0.82, 0.72), and when retrospectively separated out in March, were not different than NL ponies (P = 0.90, 0.84). Therefore, basal thyroid hormone concentrations are not useful as a predictor or hormonal characteristic of pasture-associated laminitis. The decreased TT4 and FT4 in PL ponies may be an indication of a response or compensation to laminitis and may facilitate the metabolic changes necessary to cope with the disease. / Master of Science

Thyroid hormones are well known important to be in development and growth in birds and that signaling of thyrotropin (TSH) regulates the photo induced seasonal reproduction. A mutation at the thyroid stimulating hormone receptor (TSHR) gene in domestic breeds of chicken could be involved in the release of the photoperiodic regulation. Furthermore, TSH can affect a wide range of domestication related phenotypes, such as behaviour, growth rate and pigmentation. The aim of this study was to investigate the behaviours expressed in the different genotypes on the TSHR gene in chickens. Four standard tests were conducted, aerial predator, fear of human, social dominance and tonic immobility. An advanced intercross line of chickens between red junglefowl and White leghorn was used. Male domestic type chickens explored more, showed more less fear behaviours and showed least fear behaviours in the fear of human test. Increased activity and flight response has been interpreted as a lower fear response, which is in line with this study. The wild type chickens showed more social dominance than domestic type chickens which are in line with previous results. In tonic immobility there was a difference between the wild type male and heterozygous male chickens in latency until first head movement. The conclusion of this study is that there is a difference between the wild type and domestic type chickens. This indicates that the TSHR gene is involved in behavioural changes during domestication, but whether it is due to passive or active selection is the question.

Thyrotropin (TSH) is a glycoprotein hormone released from the pituitary gland to promote the synthesis and secretion of thyroid hormone. The existence of well-established peripheral mechanisms for regulation of thyroid hormone delivery to targets has called into question the significance of TSH as a primary regulator of circulating thyroid hormone concentrations in fish. However, relatively little is known about the regulation or action of endogenously secreted teleost TSH, largely due to lack of purified TSH suitable for biological testing and immunoassay development. I developed a red drum in vivo bioassay to aid in the production and purification of recombinant TSH from the red drum, a perciform fish demonstrating dynamic daily thyroxine (T4) cycles hypothesized to be driven by TSH. Exogenous bovine TSH injection resulted in a time and dose-dependent increase in circulating TSH and T4 in red drum. However, the sensitivity of the red drum thyroid gland to stimulation by bovine TSH was lost during growth under controlled laboratory conditions, even when circulating levels of exogenously-administered mammalian TSH remained elevated. The insensitivity of the thyroid was not due to prior TSH injection or feed source. Because insensitivity of the Thyrotropin (TSH) is a glycoprotein hormone released from the pituitary gland to promote the synthesis and secretion of thyroid hormone. The existence of well-established peripheral mechanisms for regulation of thyroid hormone delivery to targets has called into question the significance of TSH as a primary regulator of circulating thyroid hormone concentrations in fish. However, relatively little is known about the regulation or action of endogenously secreted teleost TSH, largely due to lack of purified TSH suitable for biological testing and immunoassay development. I developed a red drum in vivo bioassay to aid in the production and purification of recombinant TSH from the red drum, a perciform fish demonstrating dynamic daily thyroxine (T4) cycles hypothesized to be driven by TSH. Exogenous bovine TSH injection resulted in a time and dose-dependent increase in circulating TSH and T4 in red drum. However, the sensitivity of the red drum thyroid gland to stimulation by bovine TSH was lost during growth under controlled laboratory conditions, even when circulating levels of exogenously-administered mammalian TSH remained elevated. The insensitivity of the thyroid was not due to prior TSH injection or feed source. Because insensitivity of the red drum thyroid precluded their use as a bioassay species, the plasma TSH and T4 response to exogenous TSH was next characterized in goldfish. The T4 response in goldfish was stable and repeatable, with T4 levels peaking at 5 hours and remaining elevated for more than 11 hours after bovine TSH injection. Plasma TSH peaked from 2-5 hours following TSH injection with more than 90 percent cleared by 11 hours. The goldfish bioassay was further utilized to evaluate the effects of structural modifications on TSH biological activity. Substitution of four positively charged amino acids at the n-recombinant human TSH, had the same effect in goldfish. The heterothyrotropic potency of mammalian follicle stimulating hormone in goldfish was also enhanced by the same amino acid substitutions. Finally, the importance of oligosaccharides to TSH bioactivity was also examined in goldfish. Deglycosylation abolished TSH bioactivity, even when immunoreactivity persisted in circulation. Furthermore, recombinant canine TSH was less potent when produced in cell lines generating insect-type glycosylation than when produced in a cell line capable of mammalian-type glycosylation. These studies utilizing recombinant mammalian demonstrated conservation of mammalian TSH hormone-receptor interactions in goldfish, suggesting TSH function might likewise be conserved. Thus, I have established goldfish as a sensitive and stable bioassay which can now be utilized to monitor the biological activity of teleost TSH expressed in vitro as well as to evaluate how structural modifications of the TSH molecule influence its vivo biological activity.

Cyclic AMP-dependent protein kinase (cAMP-PK) is an important mediator of hormone action. Its activity ratio is an accurate indicator of cellular activity under various experimental conditions including: (1) age and sex, (2) hormone administration and (3) temperature and photoperiod. Pituitary activity in unstimulated birds is not altered by age, but thyroid activity is much higher in old birds than in young animals. Thyrotropin releasing hormone (TRH) increases pituitary, thyroid and liver activity of prepubescent chickens, but has no effect on aged males and increases only thyroid and liver activities in aged females, suggesting a reduction in pituitary-thyroid function with advancing age. In prepubertal females, TRH increases pituitary and thyroid cAMP-PK activity, plasma T₃ and T₄ levels and liver T₄ monodeiodination. Thyroid activity reaches maximum activity before the pituitary, while plasma T₄ and liver T₄ monodeodinating activity reach their highest levels 20 minutes before plasma T₃. These findings suggest that fluctuations in liver T₄ 5' monodeiodinating activity might be responsible for the cyclic response of plasma T₃ and T₄. Castrated cockerels have larger pituitaries than untreated birds, but contain the same amount of DNA. Methimazole-fed cockerels have pituitaries significantly smaller than controls, while castrated cockerels fed methimazole have pituitaries the same size as untreated birds. Pituitary DNA is less than controls in both groups of methimazole-fed birds. These results are considered to be due to a change in the thyrotroph population, without an increase in total cell numbers, and may indicate a transformation of basophils. Pituitary cAMP-PK activity during cold stress substantiates this conclusion. Thyroid glands of castrated and untreated cockerels are smaller in size, histological appearance and DNA content; however, cAMP-PK activity is much greater in the castrated birds. Methimazole-fed cockerels have enlarged thyroid glands, elevated cAMP-PK activity, increased DNA and cellular hypertrophy; however, these effects may be mitigated by castration. Seven days after removal of testosterone supplements, photostimulated castrates have a higher thyroid cAMP-PK activity ratio than short day castrates; however, both groups are elevated above control, suggesting that long photoperiods enhance the stimulatory effects of castration on thyroid activity. Pituitary activity is elevated in long and short day birds seven days after removal of testosterone, but remains high only in short day castrates. Therefore, a reduction in the sensitivity of the hypothalamic-pituitary axis to testosterone may occur only in long day cockerels.

No description available.

The role of thyroid-stimulating hormone (TSH) in the regulation of peripheral thyroid function in non-mammalian species is still poorly understood. Thyroxine (T₄), the principal hormone released from the thyroid gland in response to TSH stimulation, circulates with a robust daily rhythm in the sciaenid fish, red drum. Previous research has suggested that the red drum T₄ cycle is circadian in nature, driven by TSH secretion in the early photophase and inhibited by T₄ feedback in the early scotophase. To determine whether TSH is produced in a pattern consistent with driving this T₄ cycle, I developed quantitative real time RT-PCR (qPCR) techniques to quantify the daily cycle of expression of the pituitary TSH subunits GSU[alpha], and TSH[beta]. I found that pituitary TSH expression cycled inversely to, and 6-12 hours out of phase with, the T₄ cycle, consistent with the hypothesis that TSH secretion drives the T₄ cycle. To examine the potential role of deiodinases in negative feedback regulation of this TSH cycle, I also utilized qPCR to assess the pituitary expression patterns of the TH activating enzyme outer-ring deiodinase (Dio2) and the TH deactivating enzyme inner ring deiodinase (Dio3). Whereas Dio2 was not expressed with an obvious daily cycle, Dio3 was expressed in the pituitary mirroring the TSH cycle. These results are consistent with T₄ negative feedback on TSH and suggest that TH inactivation by pituitary cells is an important component of the negative feedback system. To further examine the TH regulation of this Dio3 cycle, I developed an immersion technique to administer physiological doses of T₃ and T₄ in vivo. Both hormones persist in static tank water for at least 40 hours. Immersion in 200ng/ml T₄ significantly increased both plasma T₄ and T₃ within physiological ranges above control at 4.5 hours. Immersion in 100ng/ml T₃ increased plasma T₃ within physiological ranges over control by 22 hours while significantly decreasing plasma T₄ below control, presumably through inhibition of TSH secretion. T₄ also significantly inhibited the expression of the TSH [alpha] and [beta] subunits at 4.5 and 22 hours of immersion whereas T₃ immersion significantly inhibited the expression of the [alpha] and [beta] subunits of TSH by 22 hours. Both Dio2 and Dio3 expression were significantly diminished by T3 and T₄ at 22 hours. Inhibition of circulating THs with the goitrogen methimazole significantly increased the expression of TSH. These results indicate that both T₄ and T₃ are capable of negative feedback regulation of TSH expression in red drum on a time scale consistent with the T₄ daily cycle, and further support Dio3 destruction of THs in the pituitary, potentially regulated by circulating T₄, as a critical component of negative feedback on TSH. This study supports the importance of central mechanisms acting through pituitary TSH secretion in regulating thyroid function in red drum.

Thesis (M.S.)--Ohio University, March, 2010. / Title from PDF t.p. Release of full electronic text on OhioLINK has been delayed until October 1, 2010. Includes bibliographical references.