Modulation of the neonatal pituitary and adrenocortical responses to stress by thyroid hormones in the rat: effects of hypothyroidism and hyperthyroidism

Thyroid Hormone Deficiency Suppresses Fetal Pituitary-Adrenal Function Near Term: Implications for the Control of Fetal Maturation and Parturition

Background: The fetal hypothalamic-pituitary-adrenal (HPA) axis plays a key role in the control of parturition and maturation of organ systems in preparation for birth. In hypothyroid fetuses, gestational length may be prolonged and maturational processes delayed. The extent to which the effects of thyroid hormone deficiency in utero on the timing of fetal maturation and parturition are mediated by changes to the structure and function of the fetal HPA axis is unknown. Methods: In twin sheep pregnancies where one fetus was thyroidectomized and the other sham-operated, this study investigated the effect of hypothyroidism on circulating concentrations of adrenocorticotrophic hormone (ACTH) and cortisol, and the structure and secretory capacity of the anterior pituitary and adrenal glands. The relative population of pituitary corticotrophs and the masses of the adrenal zones were assessed by immunohistochemical and stereological techniques. Adrenal mRNA abundances of key steroidogenic enzymes and growth factors were examined by quantitative polymerase chain reaction. Results: Hypothyroidism in utero reduced plasma concentrations of ACTH and cortisol. In thyroid-deficient fetuses, the mass of corticotrophs in the anterior pituitary gland was unexpectedly increased, while the mass of the zona fasciculata and its proportion of the adrenal gland were decreased. These structural changes were associated with lower adrenocortical mRNA abundances of insulin-like growth factor (IGF)-I and its receptor, and key steroidogenic enzymes responsible for glucocorticoid synthesis. The relative mass of the adrenal medulla and its proportion of the adrenal gland were increased by thyroid hormone deficiency in utero, without any change in expression of phenylethanolamine N-methyltransferase or the IGF system. Conclusions: Thyroid hormones are important regulators of the structure and secretory capacity of the pituitary-adrenal axis before birth. In hypothyroid fetuses, low plasma cortisol may be due to impaired adrenocortical growth and steroidogenic enzyme expression, secondary to low circulating ACTH concentration. Greater corticotroph population in the anterior pituitary gland of the hypothyroid fetus indicates compensatory cell proliferation and that there may be abnormal corticotroph capacity for ACTH synthesis and/or impaired hypothalamic input. Suppression of the development of the fetal HPA axis by thyroid hormone deficiency may contribute to the delay in fetal maturation and delivery observed in hypothyroid offspring.

Influence of maternal thyroid hormones during gestation on fetal brain development

Thyroid hormones (THs) play an obligatory role in many fundamental processes underlying brain development and maturation. The developing embryo/fetus is dependent on maternal supply of TH. The fetal thyroid gland does not commence TH synthesis until mid gestation, and the adverse consequences of severe maternal TH deficiency on offspring neurodevelopment are well established. Recent evidence suggests that even more moderate forms of maternal thyroid dysfunction, particularly during early gestation, may have a long-lasting influence on child cognitive development and risk of neurodevelopmental disorders. Moreover, these observed alterations appear to be largely irreversible after birth. It is, therefore, important to gain a better understanding of the role of maternal thyroid dysfunction on offspring neurodevelopment in terms of the nature, magnitude, time-specificity, and context-specificity of its effects. With respect to the issue of context specificity, it is possible that maternal stress and stress-related biological processes during pregnancy may modulate maternal thyroid function. The possibility of an interaction between the thyroid and stress systems in the context of fetal brain development has, however, not been addressed to date. We begin this review with a brief overview of TH biology during pregnancy and a summary of the literature on its effect on the developing brain. Next, we consider and discuss whether and how processes related to maternal stress and stress biology may interact with and modify the effects of maternal thyroid function on offspring brain development. We synthesize several research areas and identify important knowledge gaps that may warrant further study. The scientific and public health relevance of this review relates to achieving a better understanding of the timing, mechanisms and contexts of thyroid programing of brain development, with implications for early identification of risk, primary prevention and intervention.

Development of the ACTH and corticosterone response to acute hypoxia in the neonatal rat

Acute episodes of severe hypoxia are among the most common stressors in neonates. An understanding of the development of the physiological response to acute hypoxia will help improve clinical interventions. The present study measured ACTH and corticosterone responses to acute, severe hypoxia (8% inspired O(2) for 4 h) in neonatal rats at postnatal days (PD) 2, 5, and 8. Expression of specific hypothalamic, anterior pituitary, and adrenocortical mRNAs was assessed by real-time PCR, and expression of specific proteins in isolated adrenal mitochondria from adrenal zona fascisulata/reticularis was assessed by immunoblot analyses. Oxygen saturation, heart rate, and body temperature were also measured. Exposure to 8% O(2) for as little as 1 h elicited an increase in plasma corticosterone in all age groups studied, with PD2 pups showing the greatest response ( approximately 3 times greater than PD8 pups). Interestingly, the ACTH response to hypoxia was absent in PD2 pups, while plasma ACTH nearly tripled in PD8 pups. Analysis of adrenal mRNA expression revealed a hypoxia-induced increase in Ldlr mRNA at PD2, while both Ldlr and Star mRNA were increased at PD8. Acute hypoxia decreased arterial O(2) saturation (SPo(2)) to approximately 80% and also decreased body temperature by 5-6 degrees C. The hypoxic thermal response may contribute to the ACTH and corticosterone response to decreases in oxygen. The present data describe a developmentally regulated, differential corticosterone response to acute hypoxia, shifting from ACTH independence in early life (PD2) to ACTH dependence less than 1 wk later (PD8).

Developmental exposure to pentachlorophenol affects the expression of thyroid hormone receptor beta1 and synapsin I in brain, resulting in thyroid function vulnerability in rats

Pentachlorophenol (PCP), a component of biocides and a contaminant in diverse tissue samples from humans from various geographic areas, disrupts regulatory effects of thyroid hormones. Here we examined the effects of developmental exposure of rats to PCP on various aspects of brain development, male reproductive function, and adrenal function, all of which are under thyroid hormones regulation. PCP was administered to dams and their offspring via drinking water (6.6 mg l(-1)) during gestation and lactation. Tissue samples were obtained from dams, 3-week-old weanling pups, and 12-week-old pups. Gene expressions of thyroid hormone receptor beta1 and synapsin I, factors that promote brain growth, was increased in the cerebral cortex of PCP-treated weanling females, whereas plasma concentrations of total thyroxine were decreased in dams and weanling pups, and plasma thyroid-stimulating hormone concentrations were higher in PCP-treated weanling males. PCP caused a decrease in plasma corticosterone concentrations in 12-week-old female rats, but not in male rats or weanling females. PCP-treated male pups had significantly increased testis weight at 12 week of age. No overt signs of toxicity were noted throughout this study. Our results show that PCP exposure during development causes thyroid function vulnerability, testicular hypertrophy in adults, and aberrations of brain gene expression.

Assessment of the efficacy of different statins in murine collagen-induced arthritis

OBJECTIVE

Hydroxymethylglutaryl-coenzyme A reductase inhibitors (statins) are widely used lipid-lowering agents. In addition to their well-known effect on cholesterol levels, statins have been reported to display antiinflammatory activities both in vitro and in vivo. In this context, in vivo prophylactic and therapeutic effects of simvastatin were recently demonstrated in mouse collagen-induced arthritis, a well-described experimental model for human rheumatoid arthritis (RA). The aim of this study was to further investigate in vivo effects of 3 different statins, atorvastatin, rosuvastatin, and simvastatin, using the same experimental model.

METHODS

Different doses and routes of administration were used for the various statins in an attempt to elicit antiarthritic activity in preventive and curative treatment protocols.

RESULTS

Atorvastatin and rosuvastatin had no in vivo efficacy, as indicated by clinical, histologic (synovial hyperplasia, exudate, and cartilage damage), immunologic (anti-type II collagen IgG production), and biochemical (interleukin-6, serum amyloid A, and glucocorticoid production) parameters of inflammation and autoimmunity. The previously described beneficial effects of administration of intraperitoneal simvastatin were reproduced in our experiments, but could be accounted for by very severe side effects of the treatment, leading to increased glucocorticoid levels.

CONCLUSION

This work shows that different statins have no effect in a murine model of arthritis, an unexpected observation given the previously described therapeutic effect of statins in immune-mediated inflammatory diseases. It is still unclear whether statins will have benefit in the treatment of RA.

Delayed resolution of acute inflammation during zymosan-induced arthritis in leptin-deficient mice

The severity of antigen-induced arthritis (AIA) is decreased in leptin-deficient ob/ob mice. However, joint inflammation in AIA depends on the immune response, which is impaired in ob/ob mice. In the present study we investigated the effects of leptin deficiency on zymosan-induced arthritis (ZIA), which is independent of adaptive immunity. Arthritis was induced by injection of zymosan into the knee joint. Joint swelling was similar after 6 and 24 hours in ob/ob and control mice. However, it remained elevated in ob/ob animals on day 3 whereas values normalized in controls. Histology revealed similar articular lesions in all animals on day 3, but on days 14 and 21 arthritis tended to be more severe in ob/ob mice. The acute phase response, reflected by circulating levels of IL-6 and serum amyloid A, was also more pronounced in ob/ob mice, although corticosterone was significantly elevated in these animals. Similar results were obtained in leptin receptor-deficient db/db mice. Thus, in contrast to AIA, ZIA is not impaired in leptin-deficient animals. On the contrary, resolution of acute inflammation appears to be delayed in the absence of leptin or leptin signalling, suggesting that chronic leptin deficiency interferes with adequate control of the inflammatory response in ZIA.

Neonatal thyroxine treatment: changes in the number of corticotropin-releasing-factor (CRF) and neuropeptide Y (NPY) containing neurons and density of tyrosine hydroxylase positive fibers (TH) in the amygdala correlate with anxiety-related behavior of wistar rats

Neonatal hyperthyroidism induces persisting alterations in the adult brain, e.g. in spatial learning and hippocampal morphology. In the present study, the relationship between anxiety-related behavior and amygdala morphology was investigated in the adult rat after transient neonatal hyperthyroidism (daily s.c. injections of 7.5 microg L-thyroxine in 0.5 ml 0.9% NaCl solution from postnatal day p1 to p12). The behavioral tests used to study anxiety-related behavior were the motility test, elevated plus-maze and fear-sensitized acoustic startle response. In the amygdala, the number of neurons containing the anxiogenic peptide corticotropin releasing factor (CRF-ir and CRF mRNA) and anxiolytic neuropeptide Y (NPY-ir), the total number of neurons and the density of tyrosine hydroxylase immunoreactive (TH-ir) fibers were quantified. Thyroxine-treated pups presented an accelerated development including opening of eyes and snout elongation as typical signs of hyperthyroidism. Thyroxine-treated adult animals displayed a reduced anxiety in the motility box and elevated plus maze, a reduction in the number of CRF-ir neurons in the central nucleus of the amygdala, as well as an increase in the number of NPY-ir neurons and density of TH-ir fibers in nuclei of the basolateral complex of the amygdala. Moreover, there was a reduction in the total number of neurons in all nuclei of the basolateral complex (despite the higher number of NPY-ir neurons), but not central nucleus of the amygdala. The number of CRF-ir neurons in the central nucleus correlated positively with anxiety-related behavior, and the number of NPY-ir neurons and the density of TH-ir fibers in the basolateral complex correlated inversely with anxiety-related behavior. The findings suggested a shift toward an anxiolytic rather than anxiogenic distribution of peptidergic neurons and fibers in the amygdala at adult age following transient neonatal hyperthyroidism.

Glucocorticoids aggravate hyperoxia-induced lung injury through decreased nuclear factor-kappa B activity

We previously reported that exposure of mice to hyperoxia is characterized by extensive lung cell necrosis and apoptosis, mild inflammatory response, and elevated circulating levels of corticosterone. Administration of hydroxycortisone acetate during hyperoxia aggravated lung injury. Using adrenalectomized (ADX) and sham-operated (sham) mice, we studied the role of the glucocorticoids in hyperoxia-induced lung injury. Lung damage was attenuated in ADX mice as measured by lung weight and protein and cell content in bronchoalveolar lavage and as seen by light microscopy. Mortality was delayed by 10 h. Nuclear factor-kappaB (NF-kappaB) activity was significantly decreased in lungs of sham mice exposed to hyperoxia but was preserved in ADX mice. There was a correlation between NF-kappaB activity in ADX mice and decreased levels of IkappaBalpha. In contrast, activator protein-1 activity increased similarly in both groups of mice. Levels of interleukin-6 (IL-6), a transcriptional target of NF-kappaB, were higher in bronchoalveolar lavage and serum of ADX than sham mice. However, the protective effect of ADX was not mediated by IL-6, because administration of recombinant human IL-6 to sham mice did not prevent lung damage. These results demonstrate that the adrenal response aggravates alveolar injury and is likely to be mediated by the decrease of NF-kappaB function involved in cell survival.

Hyperoxia increases leptin production: a mechanism mediated through endogenous elevation of corticosterone

Leptin, a cytokine involved in the regulation of food intake, has been reported to be decreased in lung diseases such as chronic obstructive pulmonary disease and cystic fibrosis and increased in critically ill patients with sepsis. We investigated the role of leptin during hyperoxia in mice, which results in alveolar edema, severe weight loss, and death within 3-4 days. In oxygen-breathing mice, serum leptin was increased six- to sevenfold and its mRNA was upregulated in white adipose tissue. Leptin elevation could not be attributed to changes in circulating tumor necrosis factor-alpha but was completely dependent on endogenous corticosterone elevation because adrenalectomized mice did not exhibit any increase in leptin levels. Using leptin-deficient mice and wild-type mice treated with anti-leptin antibody, we demonstrate that weight loss was leptin independent. Lung damage was moderately attenuated in leptin-deficient mice but was not modified by anti-leptin antibody or leptin administration, suggesting that leptin does not play an essential role in the direct and short-term effects of oxygen-induced injury.

Thyroxine modulates corticotropin-releasing factor but not arginine vasopressin gene expression in the hypothalamic paraventricular nucleus of the developing Rat

Neonatal rats were daily injected with 100 microg/kg T4 and killed at 4, 8 or 15 days. Circulating corticosterone and corticosteroid binding globulin concentrations increased in 8- and 15-day-old rats after T4 treatment. Plasma adrenocorticotropic hormone (ACTH) concentrations, pituitary ACTH content and pro-opiomelanocortin mRNA expression were unaffected in T4-treated rats. T4 treatment induced an increase in corticotropin-releasing factor (CRF) mRNA expression in the whole population of CRF synthesizing cells of the paraventricular nucleus (PVN) that became significant at day 8 and disappeared at day 15. Double labelling in situ hybridization revealed that CRF gene expression in the CRF+/arginine vasopressin (AVP)+ subpopulation was increased at days 4 and 8 and decreased at day 15. CRF immunoreactivity in the zona externa of the median eminence increased with age but was not affected by the experimental hyperthyroidism. The degree of CRF and AVP colocalization, the concentration of AVP mRNA in the parvo and magnocellular cell bodies of the PVN and the density of immunoreactive AVP in the zona interna or zona externa of the median eminence did not change after T4 treatment. Our data demonstrate that experimental hyperthyroidism accelerates the maturation of hypothalamic CRF gene expression, including in particular in the CRF+/AVP+ subpopulation, during the stress hyporesponsive period. These observations suggest that the physiological peak of plasma thyroxine that occurs between days 8-12 may participate in the maturation of hypothalamic CRF cells.

Effect of chronic thyroxine treatment on pregnancy in rats: effects on oestrogen, progesterone, prolactin and GH receptors in uterus, liver and mammary gland

We have previously shown that experimental hyperthyroidism produces premature and difficult delivery and absence of lactation in spite of apparently adequate luteolysis and lactogenesis. To study the possible causes of these alterations we measured the effect of treatment with T4 (0.25 or 1 mg kg(-1), s.c., daily, started 10-15 days before mating, HT0.25 and HT1) on serum hormones and their receptor (R) concentrations in reproductive tissues on day 20 of pregnancy (1800 hours), comparing them with controls on the same day (C20), or on day 21 of pregnancy (1800 hours) (C21). Serum prolactin (PRL) and corticosterone (B) concentrations increased in the HT groups, progesterone (Pg) and GH decreased and estradiol (E2) did not change, compared with C20 group. C21 rats had increased serum PRL and decreased Pg and GH. In HT rats mammary DNA and protein tissue content was doubled. Receptor concentrations were expressed per mg DNA. Mammary PRL-R were increased in HT1 rats, while E-R and Pg-R were significantly lower in both HT groups. HT0.25 and HT1 rats had increased uterine E-R and Pg-R and decreased liver PRL-R and GH-R as well as their mRNAs. Liver E-R, PRL-R and GH-R were decreased in C21 rats, while uterine Pg-R were increased. Thus, some of the observed changes (serum Pg and GH, mammary and uterine Pg-R, and liver GH-R and PRL-R decreases and serum PRL increase) may be due at least partially to the advancement in luteolysis and delivery, being similar to the changes observed between days 20 and 21. The changes in serum B, mammary PRL-R, and mammary and uterine E-R may be caused solely by the T4 treatments and may play a role in the alterations previously observed.

Postnatal leptin surge and regulation of circadian rhythm of leptin by feeding. Implications for energy homeostasis and neuroendocrine function

Leptin is thought to regulate energy balance through effects on food intake and thermogenesis. In addition, leptin may serve as a mediator of the neuroendocrine response to starvation, and may modulate the stress response and the timing of puberty. A role for leptin in development is suggested by the presence of neuroendocrine and structural neuronal abnormalities in ob/ob mice with genetic leptin deficiency. Here, we sought to determine the ontogeny of leptin expression and its relationship to the developing neuroendocrine axis. Leptin increased 5-10-fold in female mice during the second postnatal week independent of fat mass, and declined after weaning. The rise in leptin preceded the establishment of adult levels of corticosterone, thyroxine, and estradiol. In contrast to adult mice, leptin was not acutely regulated by food deprivation during the early postnatal period. Circadian rhythms of leptin, corticosterone, and thyroxine were regulated by food intake in adult mice. When ad libitum feeding was restricted to the light cycle, peak corticosterone levels were shifted to the beginning of the light cycle and coincided with the nadir of leptin. The inverse relationship between leptin and corticosterone was maintained such that a rise in leptin after feeding was associated with a decline in corticosterone. To determine whether changes in corticosterone during food restriction are mediated by leptin, we compared the patterns of corticosterone levels among ob/ob, db/db, and lean mice. Despite their higher basal levels of corticosterone, leptin deficiency in ob/ ob mice did not prevent the nocturnal rise in corticosterone. In contrast, the nocturnal surge of corticosterone was blunted in db/db mice. Therefore, it is likely that factors in addition to leptin are involved in the regulation of the circadian rhythm of corticosterone. The temporal relationship between leptin and other hormones in neonatal and adult mice suggests that leptin is involved in the maturation and function of the neuroendocrine axis.

Changes in plasma adrenocorticotropin, corticosterone, corticosteroid-binding globulin, and hippocampal glucocorticoid receptor occupancy/translocation in rat pups in response to stress

Pituitary-adrenal responses to stress in the neonatal rat have been reported to be substantially reduced compared to older animals (i.e. a stress hyporesponsive period). This supposed period of endocrine quiescence is characterized by reduced stress-induced increases in both plasma ACTH and corticosterone. At the same time a number of authors have noted the decreased plasma corticosteroid-binding globulin (CBG) levels of the neonate, and there is evidence for an increased percentage of free corticosterone as well as age-related changes in the volume of distribution for corticosterone. These findings suggest that the reduced CBG levels might enhance the biological significance of existing glucocorticoid levels, beyond that assumed on the basis of plasma total corticosterone levels. We examined this question by estimating hippocampal glucocorticoid receptor occupancy and 'translocation' in Day 6, Day 15, and adult animals under basal and stressful conditions. The results showed that: 1) plasma ACTH levels were elevated in Day 6 animals in response to acute exposure to ether, maternal separation, and maternal separation + ether, however, ACTH responses were substantially lower than in Day 15 or adult animals; 2) Plasma total corticosterone levels followed a similar pattern; most noteworthy was the potent glucocorticoid response in Day 15 animals to the combination of maternal separation + ether; 3) Plasma CBG levels in Day 6 animals were extremely low (< 3% adult values); by Day 15 CBG levels were about 25% of adult levels. Interestingly, maternal separation was associated with a substantial decrease in plasma CBG levels; 4) Hippocampal glucocorticoid receptor occupancy/translocation was similar at all ages under both basal and stress conditions. The only notable exception occurred during maternal separation in Day 15 animals, where the percentage of hippocampal glucocorticoid receptor occupancy/translocation was higher than that observed at any time in either Day 6 or adult animals. This finding is likely related to the decrease in plasma CBG that occurs following separation of Day 15 pups from the dam. Thus, despite the higher corticosterone level in the adult, the increase in glucocorticoid receptor occupancy/translocation was generally comparable across all ages either under basal conditions, or following stress. These receptor data underscore the importance of developmental changes in plasma CBG levels.