The use of intramuscularly administered methyl-TRH to evaluate the pituitary-thyroid axis.

The present study was carried out to evaluate the effectiveness of intramuscular administration of methyl-TRH, a potent analogue of thyrotropin-releasing hormone, for assessing pituitary reserve of TSH and prolactin and for distinguishing euthyroid, hypothyroid and hyperthyroid individuals. Serum samples were taken for 24 hours after intramuscular injection of methyl-TRH, 200 microgram, in 19 euthyroid subjects, 9 hypothyroid men and 9 hyperthyroid men. The mean serum prolactin and TSH concentrations were significantly elevated over baseline levels at 30 min in the euthyroid individuals and remained elevated for 3 to 4 hours. The serum TSH, T3 and T4 responses after intramuscular methyl-TRH in euthyroid subjects were clearly distinguishable from those of hyperthyroid and hypothyroid patients. Significant elevation of the serum T3 and T4 concentrations at 24 hours after intramuscular injection of methyl-TRH shows the sustained effect of this TRH analogue in euthyroid subjects.     

The age at onset of diabetes influences functional and structural changes in the pituitary-thyroid axis of streptozocin-diabetic male rats

Severe structural changes leading to marked alterations in secretory activity are known to occur in the pituitary-thyroid axis 1 month after induction of postpuberal streptozocin (SZ)-diabetes. However, SZ-diabetic rats of different age groups have not been compared, nor has the maturity of the pituitary and thyroid glands at the onset of diabetes been correlated with the type and evolution of functional and structural changes. We thus induced diabetes in 1-month (prepuberal of 3-month (postpuberal) old male rats and compared diabetic with control groups 4 and 8 months after SZ or saline injection. We determined: 1) pituitary and thyroid weights, 2) the basal plasma TSH, T3, and T4 concentrations, and 3) several morphometrical measurements in the pituitary and thyroid glands. After 4 months, 1) the pituitary and thyroid weights were decreased, 2) plasma TSH and T3 were unchanged, plasma T4 was reduced. and 3) the number of thyrotropes, degenerative changes of follicle cells, and colloid area were increased, the follicle cell height as well as the number of fused cold follicles decreased, and the follicle area was unchanged in diabetic compared with control rats. The lesions were more conspicuous in pre- than in postpuberal diabetic animals. After 8 months, plasma TSH, T3, and T4 were decreased in diabetic compared with control rats. Except for the increased colloid area, all other lesions were similar, though more severe in prepuberal diabetic rats after 8 than 4 months. Few changes were found in postpuberal diabetic rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute psychological stress increases plasma levels of cortisol, prolactin and TSH.

The effects of acute stress during a parachute jump on hormonal responses were studied in 12 experienced and 11 inexperienced military parachutists. Each subject performed two jumps. Prior to and immediately after each jump blood samples were drawn and analysed for plasma levels of cortisol, prolactin, thyrotropin (TSH), somatotropin (STH), and luteinizing hormone (LH). While there was a significant increase in cortisol, prolactin and TSH levels after both jumps, no alterations could be observed in STH and LH levels. Stress-induced hormonal responses were not affected by jump experience. There was also no association between the endocrine variables and anxiety scores.     

The effect of dexamethasone on TSH and prolactin secretion after TRH stimulation

Serum thyrotropin (TSH) and prolactin levels were measured after intravenous administration of 400 μg of synthetic thyrotropin-releasing hormone (TRH) in 13 normal subjects and six hypothyroid patients before and after three days of administration of dexamethasone 2 mg per day. In the normal subjects dexamethasone suppressed baseline serum levels and secretion of TSH after TRH stimulation. On the other hand, it had no effect on the hypothyroid patients. In the control group dexamethasone also suppressed baseline serum levels but not secretion of prolactin after TRH stimulation. Dexamethasone had no effect on prolactin levels in the hypothyroid group. It is concluded that in normal patients short-term administration of dexamethasone has an inhibitory effect on TSH secretion at the pituitary level. As for prolactin, our results could indicate that TRH is a more potent stimulator of prolactin secretion than of TSH secretion, or that TSH and prolactin pituitary thresholds for TRH are different.     

Hormonal changes during the early development of ovarian cysts in the rat

The daily administration of human chorionic gonadotropin (hCG) to rats with thiouracil-induced hypothyroidism results in the development of cystic ovaries. This study was undertaken to delineate hormonal changes during the first 48 h of hCG treatment. Groups of euthyroid and hypothyroid rats were injected daily with hCG or saline for up to two days and killed at 0, 12, 24, or 48 h after the initial hCG injection. Sera were analyzed for progesterone (P), testosterone (T), 17 beta-estradiol (E2), and prolactin (Prl) by specific radioimmunoassay (RIA). Serum levels of these hormones were not significantly different in the euthyroid and hypothyroid rats. However, P was significantly elevated at 12, 24, and 48 h in the hypothyroid/hCG rats. T and Prl were significantly elevated at 12 and 48 h in the hypothyroid/hCG rats. T levels were also elevated at 12 and 48 h in the euthyroid rats receiving hCG. In contrast, hCG had no effect on P and Prl levels in the euthyroid rats. E2 levels were undetectable in the euthyroid and hypothyroid rats. The administration of hCG increased E2 in both the euthyroid and hypothyroid rats at 48 h with significantly more E2 detected in the hypothyroid rats. These results show that ovarian steroids and Prl levels increase during the early stages of cyst induction and suggest they may be important in triggering ovarian cyst formation.     

Changes in TSH-immunoreactivity in the pars tuberalis and pars distalis of the fetal rat hypophysis following maternal administration of propylthiouracil and thyroxine

Summary The pars tuberalis (pt) of the adenohypophysis is unique in its close spatial relationship to the neurohemal contact area of the median eminence. The morphology of pt-specific secretory cells does not resemble cell types of the pars distalis (pd); the functional role of these cells within the endocrine system is still unknown. One group of young mature female Wistar rats received propylthiouracil (PTU), a second group thyroxine (T4) (10 mg/l each in drinking water) from about 3 weeks prior to the expected pregnancy and throughout the experiment. On gestation day 20, the fetuses were obtained by laparatomy. Serial sections from the rostral portion of the pt and from the pd were immunostained using the peroxidase-antiperoxidase method. TSH concentrations were determined by RIA in serum and pituitaries; T4 was measured in serum. An antiserum against rat (r) TSH revealed a moderate positive reaction of nearly all cells of the pt in the control group. In both experimental groups the pt-specific cells showed weak or no immunoreactivity. Sections of all groups were negative with anti(r)-LH,-GH,-PRL. In contrast to controls, only a few immature TSH-cells could be found in sections of the pd in the T4-group, while concentrations of TSH in blood and hypophysis were very low. TSH-cells in the PTU-group were enlarged and less intensely stained. TSH-concentrations were decreased in the hypophysis, blood levels were elevated. All sections of the pd-specific cell populations showed positive immunoreactions with anti(r)-LH,-GH,-PRL. The present results suggest that pt-specific secretory cells of the fetal rat possess TSH immunoreactivity but do not resemble the thyrotropes of the pd. Marked differences in immunoreactivity displayed by the experimental groups indicate that pt-specific cells respond to changes in the fetal thyroid status and are a component of the thyroid-regulating system in addition to the thyrotropes of the pd. This novel aspect of pt function is discussed in connection with recent results concerning melatonin receptors found in the pt and the inhibitory influence of the pineal gland exerted on the thyroid gland.The study was supported by the Deutsche Forschungsgemeinschaft (Wi 558/4-1)     

Changes in thyrotroph and somatotroph cell populations induced by stimulation and inhibition of their secretory activity

Summary The populations of cells which produce immunoreactive growth hormone (GH) and thyroid stimulating hormone (TSH) in the rat pituitary gland do not occur in fixed percentages but vary greatly under different physiological and experimental conditions. These variations can be directly correlated to the levels of stimulation and/or inhibition of the specific secretory activity. In both types of cell, sustained stimulation with trophic hormones or blockage of the feedback mechanisms induces remarkable growth in the specific cell population. Conversely, the interruption or inhibition of the stimulus thwarted the hormonal secretion and caused a massive degeneration of redundant cells. The stimulation of both GH and TSH cells is accompanied by an enhanced secretory activity as judged by their higher concentrations in serum and hypertrophy of the cytoplasmic organelles involved in synthesis and intracellular processing of the hormones. By contrast, interruption of the stimulus is followed by a variable degree of disruption of the cytoplasmic organization, including a sizable degeneration of cells. In stimulated rats, the concentrations of both GH and TSH decreased significantly in pituitary tissue due to mobilization of the hormonal stores contained in secretory granules. On the other hand, the withdrawal of stimuli blocked the hormonal release; this is reflected by the accumulation of both hormones and secretory granules in pituitary tissue. The strict correlation between the size of the GH and TSH populations with stimulation and inhibition of hormonal secretory activity reported in this investigation further supports the critical role played by the cell renewal process in endocrine secretion.     

Morphofunctional features of the secretory cells of the adenohypophysis and adrenal glands of the rat during aging

Karyometric and electron microscopic investigation has been performed in the adenohypophyseal secretory cells, in the cortex and medulla of the adrenal glands in mature (6 months) and ageing (25-26 months) white male rats. The diameter of the corticotropic cell nuclei of the adenohypophysis significantly increases with age. In the secretory cells of the adrenal cortex and medulla no important changes in their parameters are revealed. With age destructive changes of the ultrastructures are manifested variously in different types of the secretory cells in the endocrinic glands studied. In the corticotropic, somatotropic, thyreotropic cells of the adenohypophysis, in spongiocytes of the adrenal cortex, compensatory ultrastructural rearrangements develop; they are aimed to preserve functional activity of the cells mentioned. No age changes are revealed in ultrastructure of chromaffin cells of the adrenal medulla.     

Quantitative histochemical investigation of rat salivary gland at different age stages of involution of the endocrine system

A complex quantitative histochemical investigation of the submandibular salivary glands in female albino rats at different age periods (4-5 month, 12 to 14 month- and 20 to 25 month-old) revealed some structural and functional changes during the oestrus cycle. The animals were grouped according to the age changes of the endocrine system. The salivary glands were sensitive to hormonal balance changes at all age periods but their metabolic interrelations varied. The functional changes in the salivary glands of young rats were accompanied by synchronous changes in the indices of energy, synthesis and transport metabolism. The gradual increase of disintegration of the endocrine system resulted in the uncoupling between the indices of the parenchyma and the microcirculation, as well as between nucleo-cytoplasmic relationships and intracellular transport processes in the salivary glands. That was a condition under which the impairment of cellularly excretory processes occurred (secretory stasis). The intercalated ducts and the striated tubules were especially sensitive to hormonal balance fluctuations which is consistent with the hypothesis of the endocrine nature of their function.     

Effect of the removal of different endocrine glands upon insulin secretion and B-cell ultrastructure

Glucose-induced insulin secretion and B-cell ultrastructure were studied in islets obtained from normal, adrenalectomized, radiothyroidectomized, ovariectomized and orchidectomized rats. Both parameters were also studied in the same experimental groups submitted to specific substitutive therapy. Insulin secretion in response to high glucose was significantly diminished in adrenalectomized, hypothyroid and male castrated rats. Conversely, this secretion was enhanced in ovariectomized rats. These abnormal insulin responses were restored to normal range by specific substitutive therapy. B-cell ultrastructure was markedly altered in hypothyroid and in female and male castrated rats. No significant changes were observed in the adrenalectomized rats. No conspicuous alterations were depicted in the other islet cell populations. The features of the morphological alterations were mainly related to changes in the B-granules and the rough endoplasmic reticulum. Modifications of the other B-cell organelles were less frequent. In the castrated rats, a distinctive feature was the appearance of a finely granulated colloid material. These B-cell alterations, consecutive to changes in the circulating levels of a given hormone, seemed to depend on the chemical structure of the hormone itself rather than on the changes induced in the B-cell secretory function. The ultrastructural changes described were reversed, as in the case of insulin release, by specific substitutive therapy. It is concluded that changes in the circulating levels of the hormones studied are followed by specific alterations in both B-cell secretion and ultrastructure.     

Prenatal programming of adult thyroid function by alcohol and thyroid hormones

Increasing evidence associates environmental challenges early in life with permanent alterations of physiological functions in adulthood. These changes in fetal environment can trigger physiological adaptations by the fetus, called fetal programming, which may be beneficial before birth but permanently influence the physiology of the organism. In this study, we investigated the potential connection between alcohol-induced decreased maternal thyroid function and the hypothalamic-pituitary-thyroid (HPT) function of adult rat offspring. Plasma 3,5,3'-triiodothyronine (T(3)), thyroxine (T(4)), and thyroid-stimulating hormone (TSH) levels were decreased in alcohol-consuming (E) dams on gestational day 21 compared with ad libitum- (C) and pair-fed (PF) controls. No significant differences were found in HPT function in young offspring (3 wk of age) between diet groups. However, adult fetal alcohol-exposed (FAE) offspring had significantly decreased levels of T(3) along with elevated TSH compared with control offspring. T(4) administration to the mother did not normalize the hypothyroid state of the adult FAE offspring. Interestingly, administration of T(4) to control pregnant dams decreased plasma T(3) of the adult female offspring only, whereas T(4) together with maternal alcohol consumption or pair-feeding led to decreased TSH and T(4) in the adult female offspring. Our results suggest that ethanol consumption and T(4) administration alter maternal HPT function, leading to prenatally programmed permanent alterations in the thyroid function of the adult offspring.     

Effect of single neonatal melatonin treatment on in vitro thyroxin secretion and TSH or melatonin-modified cAMP level of one-month-old rats

At the age of one month, incubation with melatonin of the thyroid glands of rats having received a single melatonin treatment at the age of three days resulted in increased thyroxine production. TSH was unable to enhance the thyroxine production of animals treated with melatonin neonatally, while its considerable increase could be observed in the case of control animals. Simultaneous TSH and melatonin treatment applied in vitro at the age of one month resulted in an approximately twofold increase of thyroid T4 production in rats having received neonatal melatonin treatment. In vitro alteration of the cyclic AMP level of the thyroid glands of intact and neonatally melatonin treated rats ran practically parallel, except that in the melatonin treated animals the cAMP level was higher after TSH administration. At the same time the cAMP level decreased in the thyroid gland of animals treated with TSH + melatonin. There was no exact correlation between the alterations of cAMP and T4 levels in the given experimental system.     

Changes in somatotropic and lactotropic functions of the adenohypophysis of rats with acute alloxan diabetes

Increased growth hormone and prolactin contents of the rat adenohypophysis during the development of experimental diabetes were found by colorimetric studies of stained electrophoregrams. 4 to 5 days after alloxan administration the levels of somatotropic hormone (STH) and prolactin were higher in comparison to those in intact animals by 58% and 43%, respectively. Experiments on the primary cell culture using the precursor 14C-L-leucine revealed an enhanced secretion of somatotropic hormone and prolactin by cells of the rats with alloxan diabetes. A possible role of the adenohypophyseal changes in the development of experimental diabetes is discussed.     

Prolactin receptor expression by splenocytes from rats in various hormonal states

Prolactin (PRL) is mitogenic for lymphocytes in vitro , but the responsiveness of lymphocytes depends on the in vivo hormonal status of the rats from which the cells were obtained. Lymphocytes from ovariectomized (OVX) rats, but not from rats in oestrus or from male rats, respond to prolactin; administration of oestradiol to OVX rats diminishes the response. In order to determine if a correlation exists between lymphocyte responsiveness to prolactin and levels of cell surface prolactin receptors (PRL-R) expression, the percentage of splenocytes and each splenocyte subpopulation expressing surface PRL-R from rats of various hormonal states (OVX, oestradiol-injected OVX, oestrus and male) was analysed by single-colour and dual-colour flow cytometric analysis. We found that approximately 20% of splenocytes expressed surface PRL-R regardless of hormonal states ( n =16). The majority (85%) of PRL-R positive splenocytes were B lymphocytes whereas 11.1% and 4.8% of splenocytes expressing the PRL-R were CD4 positive T-helper (T_H) and CD8 positive T-cytotoxic (T_C) lymphocytes, respectively. B lymphocytes also stained more brightly than T lymphocytes. This distribution of PRL-R expression did not show significant alterations on total splenocytes or T_H and T_C lymphocytes during various hormonal stages. However, the percentage of PRL-R-positive B lymphocytes increased markedly in OVX rats (twofold), compared to rats at oestrus. In summary, no correlation was found between the responsiveness to prolactin as a mitogen and levels of PRL-R expression by lymphocytes from rats at different hormonal states. This result suggests that sex steroid hormones may control prolactin responsiveness of lymphocytes by affecting the signal transduction pathway through PRL-R rather than by altering the level of the cell surface receptor expression.     

Secretory granules of prolactin cells of neonatally thyroidectomized female rats

In the anterior pituitary glands of neonatally thyroidectomized female rats sacrificed at 30 days of age, the prolactin granules were small and spherical in shape. The administration of thyroxine to neonatally thyroidectomized rats produced an obvious increase in the number and size of secretory granules in prolactin cells; comparatively large, pleomorphic secretory granules were frequently observed in these cells. These enlarged and pleomorphic granules closely resembled those observed in the prolactin cells of sham-operated control rats. These results may indicate that thyroxine stimulates the basic metabolism or cellular function of prolactin cells of neonatally thyroidectomized rats and leads to the formation of prolactin granules that are similar to those of sham-operated control rats.     

Evidence that thyroxine inhibits either basal or TRH-induced TSH secretion only after conversion to triiodothyronine

When TRH was administered every 15 min for 2 hr in euthyroid rats, equivalent modestly supraphysiologic doses of either T4 or T3 suppressed TRH-induced TSH secretion after 45 min. Pretreatment with iopanoic acid blocked the ability of T4 but not of T3 to suppress TRH-induced TSH secretion 2 hr after administration of the respective thyroid hormone. Pretreatment with iopanoic acid also blocked the ability of T4, but not of T3, to depress the elevated basal plasma TSH concentration of hypothyroid rats within 2 hr. Propylthiouracil did not significantly inhibit the ability of T4 to depress TRH-induced TSH secretion and only slightly depressed the ability of T4 to reduce the elevated plasma TSH of hypothyroid rats. Our data support the concept that although equivalent physiologic doses of T4 or T3 inhibit basal or TRH-induced TSH secretion equally rapidly, TSH inhibition produced by T4 is probably dependent on its rapid conversion to T3, either within the pituitary or peripherally. T3 thus seems to be exerting almost all the negative feedback effects on TSH secretion under the conditions of our experiments.     

Aminoglutethimide-stimulated corticotrophs

Thirty-six rats of both sexes in two groups were treated with aminoglutethimide (AG), a steroid synthesis inhibitor, for 1 and 8 weeks respectively. The anterior pituitaries were investigated by light and electron microscopy, the techniques used including immunocytochemical and morphometric methods. Corticotrophs were identified by the avidinbiotin-peroxidase complex technique at the light and electron microscopic levels. AG administration resulted in hyperplasia of ACTH-containing cells. The increase in the volume density of corticotrophs was prevented by simultaneous medication with 3 mg corticosterone in male rats, whereas in female rats a larger dose of corticosterone was required to suppress pituitary corticotroph hyperplasia. Electron microscopy revealed that in AG-treated rats, corticotrophs were larger and contained more secretory granules than those in controls, the mean secretory granule diameter increasing from 100 nm to 165 nm. Under AG stimulation, corticotrophs showed considerable variations in structural features probably reflecting differences in their functional state. It was apparent that only one ACTH-producing cell type existed in the pars distalis of the rat adenohypophysis, although this cell type may undergo substantial morphologic alterations due to changes in endocrine activity. Gonadotrophs and thyrotrophs showed morphologic signs of stimulation and exhibited hyperplasia following AG treatment indicating that the effect of the drug was not restricted to corticotrophs.     

Pituitary-thyroid function of fetuses of hypothyroid and growth hormone treated hypothyroid rats.

Maternal hypothyroidism induced by surgical thyroidectomy (Tx) of the rat resulted in significantly higher fetal serum levels of thyroid stimulating hormone (TSH) and thyroxine (T4) on day 22 of gestation. Surprisingly, administration of growth hormone (GH) to hypothyroid mothers increased further the fetal serum T4 and TSH. The in vitro uptake of 131I-T4 by erythrocytes was elevated significantly when incubated with serum from fetuses of both hypothyroid and hypothyroid GH-treated mothers. Although the plasma protein levels of hypothyroid mothers and their fetuses are decreased significantly as compared to controls this is not true of hypothyroid GH-treated mothers and their fetuses. The T4 levels of both groups of Tx mothers were significantly below that of controls. However, as in the case of their fetuses, the serum T4 of GH-treated hypothyroid mothers was elevated from that of Tx only animals. It is concluded that the pituitary-thyroid system of fetuses of hypothyroid mothers is activated excessively during late gestation, that considerable T4 can be transported from the fetus to the mother during this period and that these fetuses are in fact born in a hyperthyroid state which is aggravated by maternal treatment with GH.     

Age-dependent effect of Freund's adjuvant on 24-hour rhythms in plasma prolactin, growth hormone, thyrotropin, insulin, follicle-stimulating hormone, luteinizing hormone and testosterone in rats

The effect of Freund's adjuvant administration on 24-hour changes of plasma prolactin, growth hormone (GH), thyrotropin (TSH), insulin, follicle-stimulating hormone (FSH), luteinizing hormone (LH) and testosterone were studied in young (2 months) and aged (18 months) male Wistar rats. Rats were injected s.c. with Freund's adjuvant or adjuvant's vehicle and, 18 days later, they were killed at 6 different time intervals throughout a 24-hour cycle to measure circulating hormone levels by specific RIAs. Young rats receiving adjuvant's vehicle exhibited significant time-of-day-dependent variations in plasma TSH, LH and testosterone, with maximal levels at 1300 h, 0100 h and 1700 h, respectively. Prolactin and insulin levels, analyzed globally in a factorial ANOVA, showed significant time-of-day changes with maximal levels at 1300 - 1700 h and 2100 h, respectively. The daily rhythms in plasma LH and testosterone found in young rats were not longer observed in Freund's adjuvant-injected rats, while as far as TSH, a second peak was observed at 0100 h after Freund's adjuvant administration. Twenty-four hour rhythms in circulating TSH, LH and testosterone were blunted in old rats receiving either Freund's adjuvant or its vehicle. Aged rats exhibited significantly higher circulating levels of prolactin, and lower levels of GH, TSH, FSH and testosterone. The results indicate that secretion of prolactin, GH, TSH, FSH and testosterone are age-dependent, as are the responses of TSH, LH and testosterone to Freund's adjuvant administration.     

Alteration of stimulated TSH and prolactin response in children treated with growth hormone.

The plasma TSH and prolactin responses to thyrotropin releasing hormone (TRH) were measured in 5 children with isolated growth hormone deficiency prior to, during and after the administration of human growth hormone (hGH). TSH and prolactin secretory patterns were not uniformly concordant. TSH responses to TRH infusion were suppressed in 4 subjects after 5 days or 1 month of hGH administration despite normal serum thyroxin concentrations. Prolactin responses were suppressed in all 5 subjects after 5 days of hGH administration. After 8 months of hGH therapy both TSH and prolactin responses returned toward pre-hGH values. Our finding that suppression of the TRH-induced TSH and prolactin secretory responses are reversible during hGH administration supports the concept of altered neuroregulation in this form of hypothalamic disorder.