Corticosteroid-binding Globulin Capacity Research Articles

Corticosterone exposure during development has sustained but not lifelong effects on body size and total and free corticosterone responses in the zebra finch

Animals exposed to stress during development experience sustained morphological, physiological, neurological, and behavioral consequences. For example, elevated glucocorticoids (GCs) during development can increase GC secretion in adults. Studies have examined the sustained effects of elevated developmental GCs on total GC responses, but no study to date has examined the effect of developmental stress on corticosteroid binding globulin (CBG). CBG is a protein which binds to GCs and facilitates their transportation in blood. When bound to CBG, GCs are unavailable to interact with target tissues. Exposure to stress can decrease CBG capacity and, thus, increase free GCs (the portion of unbound GCs). We examined the long-term effects of elevated corticosterone (CORT) during development (12–28days post-hatch) on acute stress responses, negative feedback, and CBG capacity at 30, 60, and 90days post-hatch in zebra finches. Additionally, we evaluated the effect of CORT treatment on body size and condition at 28, 60, and 90days post-hatch. CORT exposed birds had higher acute stress responses at 30days post-hatch compared to control birds. However, there was no treatment effect at 60 or 90days post-hatch. CBG levels were not affected by treatment, and so free CORT estimations reflected patterns in total CORT. CORT treatment decreased growth and condition in zebra finches at 28days post-hatch, but these differences were not present at later life history stages. However, brood size had a sustained effect on body size such that birds reared in medium sized broods were larger at 28, 60, and 90days post-hatch. These results demonstrate the complexity of early environmental effects on adult phenotype and suggest that some conditions may have stronger programmatic effects than others.

How acute is the acute stress response? Baseline corticosterone and corticosteroid-binding globulin levels change 24 h after an acute stressor in Japanese quail

Changes in plasma corticosteroid-binding globulin (CBG) capacity can alter free plasma concentration and tissue availability of glucocorticoids (GC) and hence alter the organismal response to stress. However, CBG change in response to stress has not been extensively studied. While it is clear that chronic stress can causes CBG decline and in some species acute stressors can reduce CBG during the 30–60 min of the stressor, more long-term changes in CBG following an acute stressor has received less attention. Here we investigated corticosterone (CORT: the primary GC in birds) and CBG levels 24 h after an acute stressor in a unique study system: Japanese quail divergently selected for CORT reactivity to acute stress. Using this model, we examined the interaction of selected CORT reactivity with CBG response to determine if CBG shows a delayed decline in response to an acute stressor and if that decline varies by selected genetic background. We found lowered CBG capacity, elevated total CORT and free CORT 24 h after acute stress in all three quail groups. These results demonstrate for the first time in an avian species that exposure to an acute stressor can affect CBG and CORT 24 h later.

Ontogeny and Individual Variation in the Adrenocortical Response of Zebra Finch (Taeniopygia guttata) Nestlings

Numerous studies indicate interspecies variation in the ontogeny of the adrenocortical response in birds; however, little is known about the extent of interindividual variation in avian young. Toward this end, we examined the ontogeny and interindividual variation in the adrenocortical response in zebra finch (Taeniopygia guttata) nestlings. We measured baseline and stress-induced total (bound and free) corticosterone, corticosteroid binding globulin capacity, and resulting estimated free corticosterone levels in nestlings of four different ages (days 5, 10, 16, and 21). In addition, we investigated the potential correlates of interindividual variation (brood size and mass). Nestlings at days 5 and 10 post-hatching showed no significant increase in total or free corticosterone levels in response to a standardized handling stress, whereas an adult-like stress response was seen by day 16 post-hatching. There was large interindividual (fivefold) variation in both baseline and stress-induced corticosterone among individual nestlings at any age. We estimate that half of this individual variation in the adrenocortical response could be explained by between-clutch variation (e.g., genetics), while the other half could be explained by other factors such as rearing environment (based on estimated intraclass correlation coefficients). Total baseline corticosterone, but not stress-induced corticosterone, was negatively correlated with fledging mass in this species.

Effects of corticosterone pellets on baseline and stress-induced corticosterone and corticosteroid-binding-globulin

Exogenous administration of glucocorticoids is a widely used and efficient tool to investigate the effects of elevated concentrations of these hormones in field studies. Because the effects of corticosterone are dose and duration-dependent, the exact course of plasma corticosterone levels after exogenous administration needs to be known. We tested the performance of self-degradable corticosterone pellets (implanted under the skin) in elevating plasma corticosterone levels. We monitored baseline (sampled within 3 min after capture) total corticosterone levels and investigated potential interactions with corticosteroid-binding-globulin (CBG) capacity and the endogenous corticosterone response to handling in Eurasian kestrel Falco tinnunculus and barn owl Tyto alba nestlings. Corticosterone pellets designed for a 7-day-release in rodents elevated circulating baseline total corticosterone during only 2–3 days compared to placebo-nestlings. Highest levels occurred 1–2 days after implantation and levels decreased strongly thereafter. CBG capacity was also increased, resulting in a smaller, but still significant, increase in baseline free corticosterone levels. The release of endogenous corticosterone as a response to handling was strong in placebo-nestlings, but absent 2 and 8 days after corticosterone pellet implantation. This indicates a potential shut-down of the hypothalamo–pituitary–adrenal axis after the 2–3 days of elevated baseline corticosterone levels. 20 days after pellet implantation, the endogenous corticosterone response to handling of nestlings implanted with corticosterone pellets attained similar levels as in placebo-nestlings. Self-degradable pellets proved to be an efficient tool to artificially elevate circulating baseline corticosterone especially in field studies, requiring only one intervention. The resulting peak-like elevation of circulating corticosterone, the concomitant elevation of CBG capacity, and the absence of an endogenous corticosterone response to an acute stressor have to be taken into account.

Interaction of testosterone, corticosterone and corticosterone binding globulin in the white-throated sparrow ( Zonotrichia albicollis)

Plasma binding globulins bind steroid hormones and are thought to regulate hormone access to tissues. Mammals have both sex steroid binding globulin (SSBG) and corticosteroid binding globulin (CBG). Birds, however, have no detectable SSBG, leading to the early conclusion that birds have no plasma regulation of sex steroids. CBG, however, can bind androgens with relatively high affinity. In birds, therefore, the control of androgenic effects may be tightly regulated by glucocorticoid physiology because glucocorticoids compete with androgens for CBG binding sites. We report levels of total testosterone (T), total corticosterone, CBG, and estimated free T in the males, the more aggressive morph had higher levels of total T; female morphs did not differ. Approximately 96% of T was bound to CBG, but a lack of morph or sex-specific differences in corticosterone titers or CBG capacity caused patterns of free T to mirror those of total T. While CBG has the potential to greatly influence T availability to tissues, in this species interactions between T, CBG and corticosterone do not appear to alter general patterns of T availability to tissues.

Corticosteroid binding globulin: a new target for cortisol-driven obesity.

We present data suggesting that corticosteroid-binding globulin (CBG) may be the causal gene of a previously identified quantitative trait locus (QTL) associated with cortisol levels, fat, and muscle content in a pig intercross. Because Cbg in human and mouse maps in the region orthologous to the pig region containing this QTL, we considered Cbg as an interesting positional candidate gene because CBG plays a major role in cortisol bioavailability. Firstly, we cloned pig Cbg from a bacterial artificial chromosome library and showed by fluorescent in situ hybridization and radiation hybrid mapping that it maps on 7q26 at the peak of the QTL interval. Secondly, we detected in a subset of the pig intercross progeny a highly significant genetic linkage between CBG plasma binding capacity values and the chromosome 7 markers flanking the cortisol-associated QTL. In this population, CBG capacity is correlated positively to fat and negatively to muscle content. Thirdly, CBG capacity was three times higher in Meishan compared with Large White parental breeds and a 7-fold difference was found in Cbg mRNA expression between the two breeds. Overall, the data accumulated in this study point to Cbg gene as a key regulator of cortisol levels and obesity susceptibility.

Neuroendocrine responses to stress in mice: hyporesponsiveness in pregnancy and parturition.

Hypothalamo-pituitary-adrenal axis secretory responses to stress were compared in female virgin, late pregnant, parturient, and lactating mice. The basal plasma ACTH concentration was not different in pregnancy or lactation compared with virgins, but corticosterone concentration and corticosteroid-binding globulin capacity were greatly elevated in late pregnancy. Secretory responses to novel environment were attenuated in pregnant, but not lactating, mice compared with virgin females, whereas ACTH responses to forced swimming were attenuated in both groups. The expression of immediate early gene (nur77) mRNA increased in paraventricular nucleus neurons after stress exposure in virgin and lactating, but not pregnant, mice. During parturition, the basal ACTH concentration was similar to virgin and pregnant controls and did not increase with stress. Oxytocin secretion in response to either novel environment or forced swimming was unchanged in any reproductive group, whereas vasopressin secretion was decreased by both stressors, but only in virgins. Pretreatment with oxytocin receptor antagonist centrally had no effect on ACTH responses to stress in either virgin or pregnant mice. Pretreatment with an opioid receptor antagonist increased ACTH responses to stress in virgin mice, indicating opioid inhibition, but had no effect in pregnancy. Thus, in mice hypothalamo-pituitary-adrenal hyporesponsiveness in late pregnancy is a consequence of reduced responsiveness of paraventricular neurons, but inhibition by opioids or intracerebral oxytocin does not appear to be involved.

The effect of social stress on adrenal axis activity in horses: the importance of monitoring corticosteroid-binding globulin capacity.

Plasma cortisol is largely bound to corticosteroid-binding globulin (CBG), which regulates its bioavailability by restricting exit from capillaries. Levels of CBG may be altered by several factors including stress and this can influence the amount of cortisol reaching cells. This study investigated the effect of social instability on plasma concentrations of CBG, total and free (not protein bound) cortisol in horses. Horses new to our research herd ('newcomers') were confined in a small yard with four dominant resident horses for 3-4 h daily for 3-4 (n = 5) or 9-14 (n = 3) days. Jugular blood was collected in the mornings from newcomers before the period of stress began ('pre-stress'), and then before each day's stress. Residents were bled before stress on the first and thirteenth day. Residents always behaved aggressively towards newcomers. By the end of the stress period, all newcomers were subordinate to residents. In newcomers (n = 8) after 3-4 days of social stress, CBG binding capacity had fallen (P = 0.0025), while free cortisol concentrations had risen (P = 0.0016) from pre-stress values. In contrast, total cortisol did not change. In residents, CBG had decreased slightly but significantly (P = 0.0162) after 12 days of stress. Residents and newcomers did not differ in pre-stress CBG binding capacity, total or free cortisol concentrations. However, by the second week of stress, CBG binding capacity was lower (P = 0.015) and free cortisol higher (P = 0.030) in newcomers (n = 3) than in residents. Total cortisol did not differ between the groups. In conclusion social stress clearly affected the adrenal axis of subordinate newcomer horses, lowering the binding capacity of CBG and raising free cortisol concentrations. However, no effect of stress could be detected when only total cortisol was measured. Therefore, to assess adrenal axis status accurately in horses, it is essential to monitor the binding capacity of CBG and free cortisol concentrations in addition to total cortisol levels.

Seasonal Changes in Adrenal Sensitivity Alter Corticosterone Levels in Gambel's White-Crowned Sparrows ( Zonotrichia leucophrys gambelii)

Recent work indicates that Gambel's white-crowned sparrows modulate corticosterone release seasonally in response to capture and restraint. Free-living white-crowned sparrows dramatically elevate both baseline and stress-induced corticosterone levels during the breeding season compared with either wintering or migrating birds. Although corticosteroid binding globulin capacity also rises during breeding, it does not fully compensate for elevated glucocorticoid levels. Consequently, we examined what hypothalamic-pituitary-adrenal axis changes could account for seasonal changes in glucocorticoid levels. During winter and fall migration, exogenous ACTH failed to elevate corticosterone levels beyond the response to capture and handling, suggesting that the adrenal's ability to release corticosterone limits circulating levels during these seasons. In contrast, not only were corticosterone levels higher during breeding, but adrenals further responded to an exogenous ACTH signal, indicating a dramatic enhancement of the adrenal's ability to secrete corticosterone. Furthermore, we inferred the pituitary's ACTH secretory ability by injecting exogenous corticotrophin-releasing factor (CRF), arginine vasotocin (AVT) and mesotocin and measured corticosterone release. Pituitaries in breeding birds appear to respond to exogenous CRF and AVT, suggesting hypothalamic control of corticosterone release during this season. Taken together, these results suggest that seasonal modulation of corticosterone release in this species is controlled primarily at the adrenal.

Changes in Free and Total Levels of Plasma Cortisol and Thyroxine Following Thermal Injury in Man

Changes in endocrine status are known to occur following thermal injury. Therefore, the effect of burn injury on plasma corticosteroid and and thyroid hormones and their respective binding proteins was examined. Thirty-one subjects with 18%-99% TBSA burn were compared to 50 normal controls with respect to plasma levels of total cortisol, free cortisol, total thyroxine (T4), free thyroxine index (FTI), corticosteroidbinding globulin capacity ICBGC), tri-iodothyronine uptake (T3U), and albumin. Compared to controls, plasma concentrations of total and free cortisol in thermally injured patients were elevated at all time points tested. However, there was no significant difference in total or free cortisol values for survivors and nonsurvivors. Both total T4 and the FTI were significantly decreased compared to normal, and nonsurvivors in turn had significantly lower T4 and FTI values than did survivors. In nonsurviving patients, values of T4 and the FTI were about 50% of the mean value for normal controls, and these levels showed only a slight increase by postburn day (PBD) 21. For both survivors and nonsurvivors, CBGC was decreased relative to normals in the 48-hour period following burn, but by PBD 7 survivors and nonsuivivors had mean levels of CBGC within the normal range. Plasma albumin was uniformly decreased throughout, and there was no difference in levels for survivors and nonsurvivors. These data confirm that thermally injured human beings have increased levels of free and total corticosteroids and decreased levels of T4, and that these changes are more extreme in patients with greater illness who eventually die.

Thyroid hormones and plasma corticosteroid binding globulin capacity in fetal and newborn lambs.

Although the increase in plasma corticosteroid binding globulin (CBG) capacity during late gestation in the fetal sheep is dependent on an intact fetal pituitary, the possible role of thyroid hormones is unknown. In rats, thyroid hormones control the postnatal increase in CBG. In the present study we investigated the effect of thyroidectomy and T3 treatment on CBG capacity in both the fetal and newborn lamb. Fifteen fetal lambs were thyroidectomized in utero at 119-121 days gestation and then infused with either vehicle or T3 (8-50 micrograms/h) for 8 days. Thyroidectomy alone did not prevent the expected increase in CBG capacity. T3 treatment of thyroidectomized fetuses caused a dose-dependent reduction in CBG capacity (maximum decrease = 31% of control). By contrast, plasma concentrations of total protein, albumin, and beta-globulins were unaltered. T3 treatment reduced plasma total corticoid concentrations, but the unbound level was unchanged. Administration of T3 (maximum dose 8 micrograms/h) to one intact fetus did not affect CBG capacity. Eleven newborn lambs were thyroidectomized between 2 and 8 days of age and infused with either vehicle or T3 (25 micrograms/h) for 8 days. Thyroidectomy alone did not prevent the normal decrease in CBG capacity after birth. T3 administration to the thyroidectomized lambs resulted in even lower plasma CBG capacity. We conclude that the prenatal increase in CBG capacity of the fetal sheep is not regulated by thyroid hormones, and that pharmacological concentrations of T3 decrease plasma CBG capacity and secondarily plasma corticoids. These findings indicate that CBG in the fetal sheep is apparently controlled by a different hormone(s) than in other species which have been studied.

Ontogeny and regulation of corticosteroid binding globulin capacity in plasma of fetal and newborn lambs.

The developmental pattern for plasma corticosteroid binding globulin (CBG) and unbound corticoids was studied in fetal and newborn sheep. CBG capacity in the fetus increased progressively from a value of 1.6 micrograms cortisol bound/dl plasma at 75 days gestation to 7.1 micrograms/dl at 141 days (n = 249), with a greater rate of increase after 120 days. By contrast total plasma proteins increased by about 50% during the third trimester. There was an accelerated increase in the concentration of both CBG and total proteins with labor, apparently reflecting hemoconcentration. The rate of increase in total plasma corticoids was similar to that for CBG capacity; however, corticoids began their major rise later in gestation (approximately 130 days). The concentration of unbound corticoids was 0.09 micrograms/dl at 115 days and did not increase until after 135 days, reaching a maximal level of 1.87 micrograms/dl on the day before spontaneous term delivery. After birth, CBG capacity decreased from a prepartum value of 11.4 micrograms/dl to 0.6 micrograms/dl at 14 days of age, whereas total plasma proteins were constant. Total and free corticoid levels also decreased postnatally, but at a slower rate than for CBG. The expected rise in fetal CBG capacity during the third trimester did not occur in hydranencephalic, stalk-sectioned, and hypophysectomized fetuses with loss of pituitary function. Under the conditions used, there was no precocious increase in CBG concentrations with infusion of hydrocortisone, PRL, or 17 beta-estradiol to intact fetuses, and infusion of ACTH to a hypophysectomized fetus did not affect the level of CBG. Thus, there is a marked increase in CBG capacity prenatally and a rapid decrease after birth. These changes in CBG may be responsible, at least in part, for the similar changes in total corticoid concentrations. We conclude that pituitary hormone(s) regulate CBG in the sheep fetus and speculate that this effect involves the placenta.

EINE EINFACHE METHODE ZUR BESTIMMUNG DER KAPAZITÄT DES CORTICOSTEROIDBINDENDEN GLOBULINS IM PLASMA

ABSTRACT The capacity of the corticosteroid-binding globulin (CBG) in serum was determined under equilibrium conditions based on the partition of cortisol between CBG, albumin and an adsorbent. A simplified mathematical model was used to calculate the optimal range of serum dilution, cortisol concentration and Florisil. To one ml of a 1:10 diluted serum 1 μg cortisol, a trace amount of 3H-cortisol and 40 mg Florisil are added. After reaching the equilibrium at 4°C the radioactivity of the supernatant is measured and the cortisol concentration is calculated. This procedure is repeated, but instead of the whole serum an albumin solution correspondent to the concentration in the individual sample is used. The CBG capacity is calculated by substracting the albumin bound from the whole serum bound cortisol fraction. The reliability of the proposed method was established by analysing the samples also by equilibrium dialyses. The correlation coefficient was 0.98, the regression line y = 0.97x. The average CBG capacity of normal males and females was 22.1 μg cortisol/100 ml (n = 13), of oestrogen treated females 49.6 (n = 7).

Serum corticosteroid binding following thermal injury.

SummaryAn adsorption technique has been adapted to measure both the total corticosteroid-binding capacity (CBC) and corticosteroid-binding globulin (CBG) capacity of rat serum following thermal injury. A maximum increase in the rat serum CBC was seen 4 days following a third-degree burn. This increase in CBC activity was accounted for by a twofold increase in the activity of the CBG fraction. Changes in corticosterone levels paralleled alterations in the CBG capacity. Increased levels of CBG were still indicated when CBG capacity was corrected for changes in serum globulin levels following a burn. The binding by non-CBG serum elements remained relatively constant postburn.In contrast to the rat, no significant alteration in CBC or CBG capacity occurred in human burn patient serum. As a result, the level of cortisol in excess of CBC was as much as 5 times normal in the early postburn period and remained elevated up to 40 days after injury. It is concluded that the human, unlike the rat, does not respond wi...