Increased hypothalamic somatostatin mRNA following dexamethasone administration in rats

Cushing's Syndrome Effects on the Thyroid

The most known effects of endogenous Cushing’s syndrome are the phenotypic changes and metabolic consequences. However, hypercortisolism can exert important effects on other endocrine axes. The hypothalamus–pituitary–thyroid axis activity can be impaired by the inappropriate cortisol secretion, which determinates the clinical and biochemical features of the “central hypothyroidism”. These findings have been confirmed by several clinical studies, which also showed that the cure of hypercortisolism can determine the recovery of normal hypothalamus–pituitary–thyroid axis activity. During active Cushing’s syndrome, the “immunological tolerance” guaranteed by the hypercortisolism can mask, in predisposed patients, the development of autoimmune thyroid diseases, which increases in prevalence after the resolution of hypercortisolism. However, the immunological mechanism is not the only factor that contributes to this phenomenon, which probably includes also deiodinase-impaired activity. Cushing’s syndrome can also have an indirect impact on thyroid function, considering that some drugs used for the medical control of hypercortisolism are associated with alterations in the thyroid function test. These considerations suggest the utility to check the thyroid function in Cushing’s syndrome patients, both during the active disease and after its remission.

A study of thyroid functions in patients with Cushing's syndrome: a single-center experience

OBJECTIVE

The aim of this study was to evaluate thyroid functions in Cushing's syndrome (CS), the dynamic changes of thyroid hormones and antithyroid antibodies in Cushing's disease (CD) pre- and postoperatively.

DESIGN AND METHODS

This is a retrospective study enrolling 118 patients with CS (102 CD, 10 adrenal CS and 6 ectopic adrenocorticotropic syndrome (EAS)). Thyroid functions (thyroid-stimulation hormone (TSH), T3, free T3 (FT3), T4 and free T4 (FT4)) were measured in all CS at the time of diagnosis and in all CD 3 months after transsphenoidal pituitary tumor resection. Postoperative hormone monitoring within 3 months was conducted in 9 CD patients completing remission. Twenty-eight remitted CD patients experienced hormone and antithyroid antibody evaluation preoperatively and on the 3rd, 6th and 12th month after surgery.

RESULTS

TSH, T3 and FT3 were below the reference range in 31%, 69% and 44% of the 118 CS patients. Remitted CD patients (81/102) had significantly higher TSH (P = 0.000), T3 (P = 0.000) and FT3 (P = 0.000) than those in the non-remission group (21/102). After remission of CD, TSH, T3 and FT3 showed a significant increase, with a few cases above the reference range. By 12 months, most CD patients' thyroid functions returned to normal. Thyroid hormones (including TSH, T3 and FT3) were negatively associated with serum cortisol levels both before and after surgery. No significant changes of antithyroid autoantibodies were observed.

CONCLUSIONS

TSH, T3 and FT3 are suppressed in endogenous hypercortisolemia. After remission of CD, TSH, T3 and FT3 increased significantly, even above the reference range, but returned to normal 1 year after surgery in most cases. Antithyroid antibodies did not change significantly after remission of CD.

Thyroid Hormone and Thyrotropin Concentrations and Responses to Thyrotropin-Stimulating Hormone in Horses with PPID Compared with Age-Matched Normal Horses

Glucocorticoids are known to exert inhibitory action on the hypothalamic-pituitary-thyroid axis. With recent evidence that free plasma cortisol and urinary excretion of cortisol metabolites may be increased in horses with pituitary pars intermedia dysfunction (PPID), it is important to further examine thyroid function in horses with PPID. To test the hypothesis that serum thyrotropin (TSH) concentrations are decreased in horses with PPID, baseline serum thyroid hormone and TSH concentrations, and responses to TSH-releasing hormone (TRH), were compared between 12 horses diagnosed as having PPID and 14 age-matched normal horses. Horses with PPID had resting serum concentrations of free thyroxine by equilibrium dialysis (fT4D) and TSH that were significantly lower than serum concentrations of fT4D and TSH in age-matched normal horses. Serum concentrations of total T4 and total and free triiodothyronine (T3) were also lower in horses with PPID compared with normal horses, but the differences did not reach statistical significance. Thyroid hormone and TSH responses to TRH administration were not different between horses with PPID and normal horses. In conclusion, serum fT4D concentrations are decreased in horses with PPID without an appropriate increase in serum TSH concentrations. Normal serum thyroid hormone and TSH concentration responses to exogenous TRH administration support the theory that increased glucocorticoid activity in horses with PPID may exert prolonged tonic suppression, but not complete inhibition, of TRH and subsequent TSH release, similar to what has been observed in other species.

Regulatory aspects of the human hypothalamus-pituitary-thyroid axis

Thyroid hormones are essential for growth, differentiation and metabolism during prenatal and postnatal life. The hypothalamus-pituitary-thyroid (HPT)-axis is optimized for these actions. Knowledge of this hormonal axis is derived from decades of experiments in animals and man, and more recently from spontaneous mutations in man and constructed mutations in mice. This review examines the HPT-axis in relation to 24 h TSH profiles in men in various physiological and pathophysiological conditions, including obesity, age, longevity, and primary as well as central hypothyroidism. Hormone rhythms can be analyzed by quantitative methods, e.g. operator-independent deconvolution, approximate entropy and fitting the 24-h component by Cosinor analysis or related procedures. These approaches have identified some of the regulatory components in (patho)physiological conditions.

A Mechanistic Basis for the Beneficial Effects of Caloric Restriction On Longevity and Disease: Consequences for the Interpretation of Rodent Toxicity Studies

Caloric restriction in rodents has been repeatedly shown to increase life span while reducing the severity and retarding the onset of both spontaneous and chemically induced neoplasms. These effects of caloric restriction are associated with a spectrum of biochemical and physiological changes that characterize the organism's adaptation to reduced caloric intake and provide the mechanistic basis for caloric restriction's effect on longevity. Here, we review evidence suggesting that the primary adaptation appears to be a rhythmic hypercorticism in the absence of elevated adrenocorticotropin (ACTH) levels. This characteristic hypercorticism evokes a spectrum of responses, including reduced body temperature and increased metabolic efficiency, decreased mitogenic response coupled with increased rates of apoptosis, reduced inflammatory response, reduced oxidative damage to proteins and DNA, reduced reproductive capacity, and altered drug-metabolizing enzyme expression. The net effect of these changes is to (1) decrease growth and metabolism in peripheral tissues to spare energy for central functions, and (2) increase the organism's capacity to withstand stress and chemical toxicity. Thus, caloric restriction research has uncovered an evolutionary mechanism that provides rodents with an adaptive advantage in conditions of fluctuating food supply. During periods of abundance, body growth and fecundity are favored over endurance and longevity. Conversely, during periods of famine, reproductive performance and growth are sacrificed to ensure survival of individuals to breed in better times. This phenomena can be observed in rodent populations that are used in toxicity testing. Improvements over the last 30 years in animal husbandry and nutrition, coupled with selective breeding for growth and fecundity, have resulted in several strains now exhibiting larger animals with reduced survival and increased incidence of background lesions. The mechanistic data from caloric restriction studies suggest that these large animals will also be more susceptible to chemically induced toxicity. This creates a problem in comparing tests performed on animals of different weights and comparing data generated today with the historical database. The rational use of caloric restriction to control body weight to within preset guidelines is a possible way of alleviating this problem.

Thyrotropin secretion patterns in health and disease

Thyroid hormones are extremely important for metabolism, development, and growth during the lifetime. The hypothalamo-pituitary-thyroid axis is precisely regulated for these purposes. Much of our knowledge of this hormonal axis is derived from experiments in animals and mutations in man. This review examines the hypothalamo-pituitary-thyroid axis particularly in relation to the regulated 24-hour serum TSH concentration profiles in physiological and pathophysiological conditions, including obesity, primary hypothyroidism, pituitary diseases, psychiatric disorders, and selected neurological diseases. Diurnal TSH rhythms can be analyzed with novel and precise techniques, eg, operator-independent deconvolution and approximate entropy. These approaches provide indirect insight in the regulatory components in pathophysiological conditions.

Endotoxin administration increases hypothalamic somatostatin mRNA through nitric oxide release

Acute inflammation induced by endotoxin (LPS) administration inhibits insulin-like growth factor (IGF-I) and growth hormone (GH) secretion. The aim of this study was to elucidate the role of glucocorticoids and nitric oxide (NO) in the effect of LPS on hypothalamic somatostatin gene expression. Adult male Wistar rats were injected with different doses of LPS (5, 10 and 100 microg/kg). Rats received two i.p. injections of LPS (at 17:30 and 8:30 h the following day) and were killed 4 h after the second injection. LPS administration at the dose of 100 microg/kg increased the hypothalamic somatostatin mRNA content, as well as the serum concentrations of corticosterone. Glucocorticoids do not seem to be involved in LPS-induced increase in hypothalamic somatostatin mRNA since adrenalectomy did not prevent this effect. In order to analyze the possible effect of NO, aminoguanidine, an inducible nitric oxide synthase inhibitor, was injected (100 mg/kg s.c.) simultaneously with LPS injection. Aminoguanidine administration did not modify somatostatin mRNA in saline injected rats, but it prevented LPS-induced increase in hypothalamic somatostatin mRNA. These data suggest that the stimulatory effect of endotoxin on hypothalamic somatostatin gene expression is not mediated by glucocorticoids, but instead by the increase in NO release.

Pulsatile secretion pattern of growth hormone in dogs with pituitary-dependent hyperadrenocorticism

The amplitude and frequency of growth hormone (GH) secretory pulses are influenced by a variety of hormonal signals, among which glucocorticoids play an important role. The aim of this study was to investigate the pulsatile secretion pattern of GH in dogs in which the endogenous secretion of glucocorticoids is persistently elevated, i.e. in dogs with pituitary-dependent hyperadrenocorticism (PDH). Blood samples for the determination of the pulsatile secretion pattern of GH were collected at 10-min interval between 08:00 and 14:00 h in 16 dogs with PDH and in 6 healthy control dogs of comparable age. The pulsatile secretion patterns of GH were analyzed using the Pulsar program. GH was secreted in a pulsatile fashion in both dogs with PDH and control dogs. There was no statistical difference between the mean (+/-S.E.M.) basal GH level in dogs with PDH (0.7+/-0.1 microg/l) and the control dogs (0.6+/-0.1 microg/l). The mean area under the curve (AUC) for GH above the zero-level in dogs with PDH (4.6+/-0.6 microg/l per 6 h) was significantly lower than that in the control dogs (7.3+/-1.0 microg/l per 6 h). Likewise, the mean AUC for GH above the base-level in dogs with PDH (0.6+/-0.1 microg/l per 6 h) was significantly lower than that in the control dogs (3.7+/-1.0 microg/l per 6 h). The median GH pulse frequency in the dogs with PDH (2 pulses/6 h, range 0-7 pulses/6 h) was significantly lower (P = 0.04) than that (5 pulses/6 h, range 3-9 pulses/6 h) in the control group. The results of this study demonstrate that PDH in dogs is associated with less GH secreted in pulses than in control dogs, whereas the basal plasma GH concentrations were similarly low in both groups. It is discussed that the impaired pulsatile GH secretion in dogs with PDH is the result of alterations in function of pituitary somatotrophs and changes in supra-pituitary regulation.

Growth hormone (GH) secretion in primary adrenal insufficiency: effects of cortisol withdrawal and patterned replacement on GH pulsatility and circadian rhythmicity

We studied the effects of cortisol withdrawal and patterned replacement upon spontaneous GH secretion and circadian rhythmicity in 7 patients with Addison's disease. Hydrocortisone was administered in physiological daily total dosages, and all resulting plasma cortisol values were 2-15 micrograms/dl. It was given in 3 pulsatile modes: simulating "physiological" rhythm, "reverse" diurnal rhythmicity and "continuous" pulsatility. All modes of cortisol administration increased mean 24 h, GH pulse amplitude and interpulse GH levels. During saline infusions circadian GH rhythmicity was preserved, with GH being at its highest between 2400-0400 h. Administration of hydrocortisone in any mode did not modify circadian GH rhythmicity. We conclude: Cortisol replacement in physiological daily doses increases GH output in patients with Addison's disease by augmenting GH pulse amplitude and interpulse levels. This is likely due to the attenuation of hypothalamic somatostatin (SRIF) secretion by physiologic levels of cortisol. By inference, it implies that cortisol deficiency leads to diminution of GH output with low GH pulse amplitude, likely as a result of an augmented hypothalamic somatostatin secretion. However, circadian rhythmicity of GH secretion is glucocorticoid-independent.

Glucocorticoids regulate pituitary growth hormone secretagogue receptor gene expression

Glucocorticoids regulate growth hormone (GH) secretion by modulating both hypothalamic and pituitary function. At the level of the pituitary, glucocorticoids increase GH and GH-releasing hormone receptor (GHRH-R) gene expression. To test if glucocorticoids might also regulate the pituitary expression of the recently identified GH secretagogue (GHS) receptor, GHS-R; adult male rats were adrenalectomized or sham operated, and treated with the synthetic glucocorticoid (dexamethasone, 200 microg/day) or vehicle for 8 days. Pituitary GHS-R mRNA levels were assessed by reverse transcriptase polymerase chain reaction (RT-PCR). Adrenalectomy decreased pituitary GHS-R mRNA to 45% of vehicle-treated, sham-operated rats (P < 0.05). Administration of dexamethasone increased GHS-R mRNA levels in sham-operated as well as in adrenalectomized rats (199 +/- 24% (P < 0.05) and 369 +/- 48% (P < 0.01) of vehicle-treated controls). Addition of dexamethasone to primary rat pituitary cell cultures increased GHS-R mRNA levels in a dose- and time-dependent manner while the transcriptional inhibitor, actinomycin D, completely blocked the stimulatory action of dexamethasone. Taken together, these results suggest glucocorticoids directly increase pituitary GHS-R mRNA levels by stimulating GHS-R gene transcription.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Psychoneuroendocrinology of depression. Growth hormone

The release of growth hormone (GH) from the anterior pituitary is regulated by hypothalamic peptides especially GH-releasing hormone (GHRH) and somatostatin, which in turn are controlled by classic neurotransmitters such as noradrenaline, dopamine, and acetylcholine, as well as negative feedback from GH and insulin-like growth factor-1. There has been extensive investigation of this axis in patients with depression. The most consistently reported abnormality is in noradrenergic-mediated GH release, which probably occurs via GHRH containing neurones. ACh-induced GH release through the somatostatin system, GABA, and also GHRH-stimulated release are reported as abnormal by some researchers.

Cortisol enhances non-REM sleep and growth hormone secretion in elderly subjects

Aging is accompanied by a continuous decline in slow wave sleep (SWS) and in growth hormone (GH) secretion, particularly during the sleeping period. Because short-term pulsatile administration of cortisol increases GH release and SWS in young adults, we wondered whether similar effects can be induced also in elderly men. Hourly injections of cortisol between 1700 and 600 h increased stage 2 and SWS and decreased rapid eye movement sleep. Spectral analysis revealed significant increases in delta and theta power. Cortisol infusions increased the GH secretion prior to sleep onset, but remained largely unchanged during sleep. Thus, sleep EEG and GH release are modulated by cortisol administration in a manner similar to that in young subjects, but to a lesser extent. The stimulatory effect of cortisol on both GH release and SWS points to a mechanism involving glucocorticoid-enhanced production and release of GH-releasing hormone that activates pituitary GH release and simultaneously antagonizes the effects of corticotropin-releasing hormone and somatostatin.

Effect of glucocorticoids on the paradoxical growth hormone response to thyrotropin-releasing hormone in patients with acromegaly

It has been hypothesized that in acromegalic patients, as well as in normal subjects, acute increases in serum cortisol levels may cause an enhancement of hypothalamic somatostatin secretion, which in turn may be responsible for the glucocorticoid-mediated growth hormone (GH) inhibition. The aim of this study was to investigate short-term effects of an intravenous (i.v.) infusion of hydrocortisone on the GH response to thyrotropin-releasing hormone (TRH) in acromegaly. We studied six adult patients with active acromegaly. The group was composed of four women and two men with a mean age of 55.8 +/- 6.4 years (range, 27 to 68) and a mean body mass index of 26.7 +/- 1 kg/m2 (range, 23.3 to 30). All patients underwent the following treatments: (1) hydrocortisone alone: a bolus i.v. injection of hydrocortisone succinate 100 mg in 2 mL saline at time -60 minutes, followed by a 120-minute i.v. infusion of hydrocortisone succinate 250 mg in 250 mL saline from -60 to 60 minutes; (2) TRH+hydrocortisone: a bolus i.v. injection of TRH 200 micrograms 60 minutes after initiation of a 2-hour hydrocortisone infusion; (3) TRH alone: a bolus i.v. injection of TRH at time 0, 60 minutes after initiation of a 2-hour saline infusion. In all six patients, TRH induced large GH increases (absolute peak GH level, 58.1 +/- 23.2 micrograms/L; maximum % GH change with respect to baseline, 1,397.8% +/- 807.8%; range, 205% +/- 5,219%).(ABSTRACT TRUNCATED AT 250 WORDS)

Galanin counteracts the inhibitory effects of glucocorticoids on growth hormone secretion in the rat

The aim of our study was to investigate the effect of galanin on baseline and growth hormone (GH)-releasing hormone (GHRH)-stimulated GH concentrations in conscious, freely moving rats receiving long-term glucocorticoid treatment. Animals were treated for 7 days with an intraperitoneal injection of either vehicle or dexamethasone ([dex] 40 micrograms/d). Rats underwent the following experimental trials: at -15 minutes animals received an intravenous injection of saline or galanin (12.5 micrograms/kg), and at 0 minutes rats received a second intravenous injection of saline or rat GHRH (500 ng/kg). Blood samples were drawn every 5 minutes from -15 to +15 minutes and then at 30 minutes. The GH response to saline + GHRH alone was significantly higher (P < .05) in chronically vehicle-treated rats as compared with chronically dex-treated ones. In contrast, galanin + saline increased serum GH levels in a similar fashion in both chronically vehicle- and dex-treated rats. The response to galanin + GHRH was similar to galanin + saline in chronically vehicle-treated rats, but was significantly enhanced in chronically dex-treated rats. These results suggest that galanin-mediated GH release in rats may involve somatostatinergic pathways.

Lowering cortisol enhances growth hormone response to growth hormone releasing hormone in healthy subjects

Cortisol is known to influence growth hormone release probably by modulating somatostatin tone. We examined the effect of metyrapone (the 11 beta-hydroxylase inhibitor) treatment on growth hormone response to growth hormone releasing hormone (1 microgram kg-1 body wt). Six healthy male subjects were tested on two occasions 1 wk apart. On one occasion they received metyrapone followed by growth hormone releasing hormone and on the other placebo followed by growth hormone releasing hormone. In all subjects metyrapone produced a significant drop in cortisol levels. Together with this drop there was a significant enhancement of growth hormone response to growth hormone releasing hormone. The GH response was negatively correlated with the cortisol level. Growth hormone release in response to growth hormone releasing hormone challenge is thus seen to be heavily influenced by cortisol levels.

Pulsatile thyrotropin secretion in patients with Cushing's syndrome

Pulsatile and circadian thyrotropin (TSH) secretion were studied in 16 healthy controls and in three patients with Cushing's syndrome who were studied twice (before and after treatment). Blood was sampled every 10 minutes over 24 hours for TSH (immunoradiometric assay [IRMA]). Mean 24-hour TSH in Cushing's syndrome was lower than in controls (0.4 +/- 0.2 v 1.7 +/- 0.7 mU/L, P < .001), related to a lower mean 24-hour TSH pulse amplitude (Desade: 0.16 +/- 0.15 v 0.44 +/- 0.20 mU/L, P < .001; Cluster: 0.17 +/- 0.14 v 0.39 +/- 0.20 mU/L, P < .001; mean +/- SD). TSH pulse frequency was unchanged with approximately 10 pulses/24 h. The nocturnal TSH surge was diminished relative to controls (median-0, range- -0.03 to 0.2 mU/L v 0.9 and 0.3 to 2.5 mU/L, respectively), related to a loss of the usual nocturnal increase in TSH pulse amplitude, but not of TSH pulse frequency. In the eucorticoid state, there was an increase of mean 24-hour TSH to 1.3 +/- 0.8 mU/L (P < .001) under normalization of the mean 24-hour TSH pulse amplitude. The nocturnal TSH surge also increased (median, 0.21; range, 0.15 to 0.4 mU/L) under restoration of the usual nocturnal increase in TSH pulse amplitude. In conclusion, Cushing's syndrome is associated with a decrease of mean 24-hour plasma TSH, related to a decrease of TSH pulse amplitude (not of TSH pulse frequency). The nocturnal TSH surge is decreased in Cushing's syndrome associated with a loss of the usual nocturnal increase of TSH pulse amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of adrenocorticosteroid receptor mRNA expression in the central nervous system

1. The adrenocorticosteroid receptors are hormone-activated transcription factors that have the potential to influence gene expression in a wide variety of CNS neurons. This review summarizes the present state of knowledge regarding the localization and regulation of glucocorticoid (or type II corticosteroid) receptor and mineralocorticoid (or type I corticosteroid) receptor mRNAs in brain, from the perspective of their potential influence on a wide variety of hormone-responsive genes. 2. Corticosteroid receptors are widely but not uniformly localized in the CNS and exhibit very complex regulation by glucocorticoids, gonadal steroids, neurotransmitter systems, and endogenous circadian drive. Both receptor species are present during development, implying an ability for these transcription factors to interact with neuronal differentiation, growth, and viability, and both receptors appear to regulate with age, suggesting relationships between adrenocorticosteroid receptor populations and brain aging. Regulation of adrenocorticosteroid receptor mRNA expression at the level of polyadenylation and splicing indicates that GR and MR biosynthesis is a dynamic process susceptible to numerous classes of information. 3. Further study of GR and MR biosynthesis at the gene, mRNA, and protein level is required to determine the true meaning of the regulatory complexities seen in defined neuronal circuits.