Concentrations Of Glucocorticoid Hormones Research Articles

Great tits differ in glucocorticoid plasticity in response to spring temperature.

Fluctuations in environmental temperature affect energy metabolism and stimulate the expression of reversible phenotypic plasticity in vertebrate behavioural and physiological traits. Changes in circulating concentrations of glucocorticoid hormones often underpin environmentally induced phenotypic plasticity. Ongoing climate change is predicted to increase fluctuations in environmental temperature globally, making it imperative to determine the standing phenotypic variation in glucocorticoid responses of free-living populations to evaluate their potential for coping via plastic or evolutionary changes. Using a reaction norm approach, we repeatedly sampled wild great tit (<i>Parus major</i>) individuals for circulating glucocorticoid concentrations during reproduction across five years to quantify individual variation in glucocorticoid plasticity along an environmental temperature gradient. As expected, baseline and stress-induced glucocorticoid concentrations increased with lower environmental temperatures at the population and within-individual level. Moreover, we provide unique evidence that individuals differ significantly in their plastic responses to the temperature gradient for both glucocorticoid traits, with some displaying greater plasticity than others. Average concentrations and degree of plasticity covaried for baseline glucocorticoids, indicating that these two reaction norm components are linked. Hence, individual variation in glucocorticoid plasticity in response to a key environmental factor exists in a wild vertebrate population, representing a crucial step to assess their potential to endure temperature fluctuations.

Corticosterone Metabolite Concentration Is Not Related to Problem Solving in the Fawn-Footed Mosaic-Tailed Rat Melomys Cervinipes.

Animals can respond physiologically, such as by adjusting glucocorticoid hormone concentrations, to sudden environmental challenges. These physiological changes can then affect behavioural and cognitive responses. While the relationships between adrenocortical activity and behaviour and cognition are well documented, results are equivocal, suggesting species-specific responses. We investigated whether adrenocortical activity, measured using corticosterone metabolite concentration, was related to problem solving in an Australian rodent, the fawn-footed mosaic-tailed rat (Melomys cervinipes). Mosaic-tailed rats live in complex environments that are prone to disturbance, suggesting a potential need to solve novel problems, and have been found to show relationships between physiology and other behaviours. We measured problem solving using five food-baited puzzles (matchbox and cylinder in the home cage, and activity board with pillars to push, tiles to slide and levers to lift in an open field), and an escape-motivated obstruction task in a light/dark box. Faecal samples were collected from individuals during routine cage cleaning. Adrenocortical activity was evaluated non-invasively by measuring faecal corticosterone metabolites using an enzyme immunoassay, which was biochemically and biologically validated. Despite varying over time, adrenocortical activity was not significantly related to problem solving success or time spent interacting for any task. However, as adrenocortical activity is reflective of multiple physiological processes, including stress and metabolism, future studies should consider how other measures of physiology are also linked to problem solving.

Causal mediation analysis for sparse and irregular longitudinal data

Causal mediation analysis seeks to investigate how the treatment effect of an exposure on outcomes is mediated through intermediate variables. Although many applications involve longitudinal data, the existing methods are not directly applicable to settings where the mediator and outcome are measured on sparse and irregular time grids. We extend the existing causal mediation framework from a functional data analysis perspective, viewing the sparse and irregular longitudinal data as realizations of underlying smooth stochastic processes. We define causal estimands of direct and indirect effects accordingly and provide corresponding identification assumptions. For estimation and inference, we employ a functional principal component analysis approach for dimension reduction and use the first few functional principal components instead of the whole trajectories in the structural equation models. We adopt the Bayesian paradigm to accurately quantify the uncertainties. The operating characteristics of the proposed methods are examined via simulations. We apply the proposed methods to a longitudinal data set from a wild baboon population in Kenya to investigate the causal relationships between early adversity, strength of social bonds between animals and adult glucocorticoid hormone concentrations. We find that early adversity has a significant direct effect (a 9–14% increase) on females’ glucocorticoid concentrations across adulthood but find little evidence that these effects were mediated by weak social bonds.

Influence of a Maximal Incremental Test Until Exhaustion on the Urinary Excretion of Steroid Hormones in Trained Cyclists.

This study aimed to assess the effect of a maximum incremental stress test through urinary concentrations of steroid hormones in trained cyclists. Twenty male cyclists participated in the study (23.83 ± 2.3 years; 1.76 ± 0.03 m; 66.94 ± 3.59 kg; training volume: 20.50 ± 2.35 h/week). Athletes performed a maximum incremental test until exhaustion on a cycle ergometer and urine samples were collected at three different time points: before, immediately after, and 48 h after the test. Lactate, creatinine and urinary concentrations of testicular and adrenal androgens were obtained as well as urinary concentrations of glucocorticoid hormones. An increase in lactate was observed after the test (p < 0.01). There were decreases in the urinary excretion of androgenic hormones after the test, which were significant in testosterone, androsterone, androstenedione, total adrenal androgen and total testicular androgen (p < 0.05). The values were restored after 48 h (p < 0.05). Urinary cortisol concentrations decreased after the test (p < 0.05). A decrease was also observed in the ratio of anabolic/catabolic hormones (p < 0.05) increasing 48 h after the test (p < 0.05). Increased acute physical exercise until exhaustion causes variations in the urinary excretions of steroid hormones which were restored 48 h after exercise. Urinary excretion of steroid hormones could be a valid method of monitoring training loads.

Increased glucocorticoid concentrations in early life cause mitochondrial inefficiency and short telomeres.

Telomeres are DNA structures that protect chromosome ends. However, telomeres shorten during cell replication and at critically low lengths can reduce cell replicative potential, induce cell senescence and decrease fitness. Stress exposure, which elevates glucocorticoid hormone concentrations, can exacerbate telomere attrition. This phenomenon has been attributed to increased oxidative stress generated by glucocorticoids ('oxidative stress hypothesis'). We recently suggested that glucocorticoids could increase telomere attrition during stressful periods by reducing the resources available for telomere maintenance through changes in the metabolic machinery ('metabolic telomere attrition hypothesis'). Here, we tested whether experimental increases in glucocorticoid levels affected telomere length and mitochondrial function in wild great tit (Parus major) nestlings during the energy-demanding early growth period. We monitored resulting corticosterone (Cort) concentrations in plasma and red blood cells, telomere lengths and mitochondrial metabolism (metabolic rate, proton leak, oxidative phosphorylation, maximal mitochondrial capacity and mitochondrial inefficiency). We assessed oxidative damage caused by reactive oxygen species (ROS) metabolites as well as the total non-enzymatic antioxidant protection in plasma. Compared with control nestlings, Cort-nestlings had higher baseline corticosterone, shorter telomeres and higher mitochondrial metabolic rate. Importantly, Cort-nestlings showed increased mitochondrial proton leak, leading to a decreased ATP production efficiency. Treatment groups did not differ in oxidative damage or antioxidants. Hence, glucocorticoid-induced telomere attrition is associated with changes in mitochondrial metabolism, but not with ROS production. These findings support the hypothesis that shortening of telomere length during stressful periods is mediated by glucocorticoids through metabolic rearrangements.

Exogenous glucocorticoids amplify the costs of infection by reducing resistance and tolerance, but effects are mitigated by co-infection.

Individual variation in parasite defences, such as resistance and tolerance, can underlie heterogeneity in fitness and could influence disease transmission dynamics. Glucocorticoid hormone concentrations often change in response to fluctuating environmental conditions and mediate changes in immune function, resource allocation and tissue repair. Thus, changes in glucocorticoid hormone concentrations might mediate individual variation in investment in resistance versus tolerance. In this study, we experimentally increased glucocorticoid concentrations in red-winged blackbirds ( Agelaius phoeniceus) that were naturally infected with haemosporidian parasites, and assessed changes in resistance and tolerance of infection. Glucocorticoid treatment increased burdens of Plasmodium, the parasite causing avian malaria, but only in the absence of co-infection with another Haemosporidian, Haemoproteus. Thus, glucocorticoids might reduce resistance to infection, but co-infection can mitigate the negative consequences of increased hormone concentrations. Glucocorticoid treatment also decreased tolerance of infection. We found no evidence that the inflammatory immune response or rate of red blood cell production underlie the effects of glucocorticoids on resistance and tolerance. Our findings suggest that exogenous glucocorticoids can increase the costs of haemosporidian infections by both increasing parasite numbers and reducing an individual's ability to cope with infection. These effects could scale up to impact populations of both host and parasite.

The Evolutionary Endocrinology of Circulating Glucocorticoids in Free-Living Vertebrates: Recent Advances and Future Directions across Scales of Study.

Circulating glucocorticoid hormone concentrations are dynamic, flexible, and promote adaptive responses following perturbations in an animal's environment. As a result, circulating glucocorticoid levels are thought to shape fitness and have been suggested to be a key trait for predicting how species will cope with novel environmental change. Nevertheless, the factors that shape variation in glucocorticoid-mediated coping mechanisms remain unclear because the evolutionary underpinnings of the function and regulation of these hormones are poorly understood. Here, I summarize recent advances in our understanding of the evolution of circulating glucocorticoids, which have included (i) longitudinal studies exploring microevolutionary processes that shape within- and between-individual variation in glucocorticoids, (ii) interspecific comparative studies highlighting macro-evolutionary patterns of among-species variation in glucocorticoids, and (iii) intraspecific comparative studies which help to disentangle the relative roles of environment, life-history, and behavior in shaping among-population variation in glucocorticoids. Important avenues for future research will include exploring how natural selection may act on different components of the hypothalamus-pituitary-adrenal axis, characterizing patterns of phenotypic plasticity in circulating glucocorticoids across populations and species, as well as exploring how microevolutionary processes differ across taxa or gradients of environmental conditions.

A 12.175-Day Periodicity in Heliogeophysical, Atmospheric, and Biological Processes

The dynamics of several heliophysical and atmospheric processes were compared with 12.175- and 4.06-day biorhythms observed earlier in the mitotic activity of epithelial tissues and glucocorticoid hormone concentrations in mammals and birds. A 12.175-day period was detected in sign reversals of the mean solar magnetic field and sharp drops in atmospheric pressure. A 4.06-day period was discerned in changes from plus to minus of interplanetary magnetic-field (IMF) sectors, but only during a solar maximum, which is the period of the greatest Solar activity during an 11-year solar cycle. Similar 4.06-day periods were observed in the dynamics of atmospheric pressure and averaged daily air temperature regardless of the phase of solar activity, but only from January through April and from July through October. The studied heliophysical and atmospheric processes are synchronous in terms of the 12.175- and 4.06-day biorhythms. However, the processes lack a direct relationship because the periodicities are detectable only at certain time intervals, while the biorhythms are equally detectable in all seasons of the year and all phases of the solar cycle.

The effects of body region, season and external arsenic application on hair cortisol concentration.

Hair cortisol analysis has been used to quantify hormone levels in circulation in several mammal species. Hair remains stable for decades or centuries, allowing researchers to use archived hair samples to investigate hormone levels that span long time periods. However, several studies have found that intra-individual variability, driven by the body region from which a sample is derived, confounds measurements of systemic glucocorticoid hormone concentrations. In addition, the external application of chemical agents to hair can remove or concentrate molecules of interest. These may preclude the use of samples that have been collected opportunistically and/or those that have been housed in museum collections. Using a captive population of Vancouver Island marmots (Marmota vancouverensis), we found a strong effect of body region on the concentration of cortisol within hair, as well as an effect of season. Using a collection of American mink (Neovison vison) pelts, we found that application of the preservative arsenic in the form of a soap does not cause a significant decrease in cortisol. The marmot results suggest that intra-individual variability is not stable through time. The reason for these seasonal effects is not clear and further study is necessary. Researchers using samples from an unknown body region should exercise caution in interpreting their results. The mink results suggest that samples held in museum collections can be used to quantify cortisol, even when arsenic preservation is suspected.

Chronic exposure to a low dose of ingested petroleum disrupts corticosterone receptor signalling in a tissue-specific manner in the house sparrow (Passer domesticus).

Stress-induced concentrations of glucocorticoid hormones (including corticosterone, CORT) can be suppressed by chronic exposure to a low dose of ingested petroleum. However, endocrine-disrupting chemicals could interfere with CORT signalling beyond the disruption of hormone titres, including effects on receptors in different target tissues. In this study, we examined the effects of 6 weeks of exposure to a petroleum-laced diet (1% oil weight:food weight) on tissue mass and intracellular CORT receptors in liver, fat, muscle and kidney (metabolic tissues), spleen (an immune tissue) and testes (a reproductive tissue). In the laboratory, male house sparrows were fed either a 1% weathered crude oil (n = 12) or a control diet (n = 12); glucocorticoid receptors and mineralocorticoid receptors were quantified using radioligand binding assays. In oil-exposed birds, glucocorticoid receptors were lower in one metabolic tissue (liver), higher in another metabolic tissue (fat) and unchanged in four other tissues (kidney, muscle, spleen and testes) compared with control birds. We saw no differences in mineralocorticoid receptors between groups. We also saw a trend towards reduced mass of the testes in oil-exposed birds compared with controls, but no differences in fat, kidney, liver, muscle or spleen mass between the two groups. This is the first study to examine the effects of petroleum on CORT receptor density in more than one or two target tissues. Given that a chronic low dose of ingested petroleum can affect stress-induced CORT titres as well as receptor density, this demonstrates that oil can act at multiple levels to disrupt an animal's response to environmental stressors. This also highlights the potential usefulness of the stress response as a bioindicator of chronic crude oil exposure.

Hydrocortisone infusion exerts dose- and sex-dependent effects on attention to emotional stimuli

Glucocorticoid administration has been shown to exert complex effects on cognitive and emotional processing. In the current study we investigated the effects of glucocorticoid administration on attention towards emotional words, using an Affective Go/No-go task on which healthy humans have shown an attentional bias towards positive as compared to negative words. Healthy volunteers received placebo and either low-dose (0.15mg/kg) or high-dose (0.45mg/kg) hydrocortisone intravenously during two separate visits in a double-blind, randomized design. Seventy-five minutes post-infusion, the subjects performed tests of attention (Rapid Visual Information Processing [RVIP]), spatial working memory (Spatial Span) and emotional processing (Affective Go/No-go task [AGNG]). On the attention task, performance was impaired under both hydrocortisone doses relative to placebo, though the effect on error rate was not significant after controlling for age; Spatial Span performance was unaffected by hydrocortisone administration. On the AGNG task, relative to the placebo condition the low-dose hydrocortisone infusion decreased response time to emotional words while high-dose hydrocortisone increased response time. In the females specifically, both high and low dose hydrocortisone administration attenuated the normal attentional bias toward positively valenced words. These data suggest that, in healthy women, the modulation of attention by the emotional salience of stimuli is influenced by glucocorticoid hormone concentrations.

The functional roles of 11β-HSD1: vascular tissue, testis and brain

Glucocorticoid hormones bind both glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) exerting a broad spectrum of actions in various tissues. The concentrations of glucocorticoid hormones in the target cells are regulated by 11β-hydroxysteroid dehydrogenases, type 1 (11β-HSD1) and type 2 (11β-HSD2). 11β-HSD2 is a unidirectional dehydrogenase, which inactivates biologically active glucocorticoid into inert metabolite, while 11β-HSD1 is a bi-directional oxidoreductase, which either inactivates biologically active glucocorticoid or activates inert metabolite into active forms. GRs and MRs are present in various tissues and mediate a broad spectrum of physiological actions. The co-existence of 11β-HSD1 with these two types of receptors plays an important role in regulation of glucocorticoid actions. This review examines the roles of 11β-HSD1 in vascular tissues, testis, brain and other tissues such as placental, retinal and adipose tissues.

Positive Relationship of Cortisol Concentrations and Oral Nutrition to Epidermal Growth Factor Concentrations in Preterm Infants

We have suggested from previous studies that increases in early neonatal epidermal growth factor (EGF) concentrations were dependent on adequate glucocorticoid hormone concentrations. In order to examine this relationship, matched values for cortisol and EGF in 193 preterm infants on days 2 and 6 were compared. Gestational age had a significant positive effect on EGF concentrations for those infants receiving oral nutrition and there was also a positive relationship between nutrition and cortisol concentration. Cortisol was then used as an independent factor and was significantly (p = 0.01) related to EGF values such that as cortisol concentrations increased, EGF values also increased. In summary, we suggest that these results are consistent with a role for cortisol in the control of the EGF pattern in the newborn period.

Adrenalectomy Lowers the Body Weight Set-Point in Rats

Gosselin, C. and M. Cabanac. Adrenalectomy lowers the body weight set-point in rats. Physiol Behav 62(3) 519–523, 1997.—In order to evaluate the impact of a complete depletion of glucocorticoids on rats’ body weight set-point, rats were adrenalectomized, and their set-points were estimated before, and after surgery. Body weight set-points were obtained from a quantitative behavioral method based on the rats’ food-hoarding response to weight deficit. In addition, body fat contents were measured in vivo using a total body electrical conductivity analyzer (TOBEC). The hoarding of food showed that adrenalectomized rats had significantly lower body weight set-points than the sham-operated controls (337 ± 11 g vs. 385 ± 8 g) and were also significantly lighter. TOBEC measurements showed that the sham-operated control rats had a higher body fat content than the adrenalectomized rats (16.7 ± 1.1% vs. 10.2 ± 1.2%). The present study demonstrates that adrenalectomy lowers the body weight set-point in rats, suggesting that the concentrations of glucocorticoid hormones, and in turn, the hypothalamic corticotrophin releasing hormone (CRH) participates in the adjustment of the body weight set-point.

Regulation of type I 5'-deiodinase by thyroid hormone and dexamethasone in rat liver and kidney cells.

Type I 5'-deiodinase (5'D-I) is crucial for the generation of circulating 3,5,3'-triiodothyronine (T3) and so is a major determinant of thyroid hormone action. Liver and kidney 5'D-I activity is reduced in nonthyroidal illness (NTI), but the exact cause of reduced 5'D-I activity remains unknown. Elevated circulating glucocorticoid hormone concentrations are one factor postulated to play a role. We have studied regulation of 5'D-I expression in cultured rat liver (phi 1) and kidney (NRK-52E) cells in response to treatment with T3 and the glucocorticoid dexamethasone. 5'D-I mRNA was measured by Northern hybridization and 5'D-I activity was measured in a sub-strate conversion assay. Expression of the sodium pump alpha 1-subunit (Na+/K(+)-ATPase alpha 1-subunit) mRNA was measured as an index of T3 and dexamethasone effects. In ø1 liver cells, T3 increased 5'D-I mRNA by 76 +/- 17% and 5'D-I activity by 101 +/- 30% (all results mean +/- SEM; n = 9). Dexamethasone increased 5'D-I mRNA by 55 +/- 16% and 5'D-I activity by 128 +/- 6%. In NRK-52E rat kidney cells, 5'D-I mRNA increased by 87 +/- 15% with T3 treatment and by 76 +/- 14% with dexamethasone. 5'D-I activity in NRK-52E cells was measurable only after stimulation by combined T3 and dexamethasone treatment. Na+/K(+)-ATPase alpha 1-subunit mRNA expression was stimulated following T3 and dexamethasone treatment in both cell lines. These results provide evidence for direct pretranslational regulation of 5'D-I by T3 and dexamethasone in both rat liver and kidney cells. Our findings do not support the hypothesis that glucocorticoids are directly responsible for inhibition of 5'D-I enzyme activity in NTI.

Effects of the adrenal steroidogenesis inhibitor trilostane on serum cortisol concentrations and growth in pigs

ABSTRACTThere is evidence that the growth of female rats and sheep may be constrained by physiological concentrations of glucocorticoid hormones. This study was designed to examine the effects of trilostane, an inhibitor of adrenal steroidogenesis, on serum cortisol concentrations and growth in pigs.First, trilostane was tested at three doses (250 mg, 500 mg and 1000 mg per pig) to examine its effect on serum cortisol concentrations. A reduction in serum cortisol concentration was observed 2 to 4 h after oral administration of trilostane (P &lt; 0·05) and the adrenal response to a challenge dose of ACTH was reduced 4 to 6 h after administration (P &lt; 0·05), at all doses tested. In a second experiment, trilostane (9·24 mg/kg body weight) was administered orally to 10 pigs twice daily for 35 days to examine its effects on growth rate. A further 10 pigs received a placebo. Trilostane did not affect weight gain or carcass composition, but caused a reduction in food intake (P &lt; 0·05). Adrenal weight was increased by trilostane treatment and liver weight was reduced.These results show that trilostane is effective at lowering serum cortisol concentrations acutely, but suggest that physiological concentrations of glucocorticoid hormones may not retard growth rate in barrows.

The roles of glucagon, insulin and glucocorticoid hormones in the effects of sub-lethal doses of endotoxin on glucose homeostasis in rats

(1) The effects of sub-lethal doses of endotoxin on plasma glucose, glucagon, insulin, glucocorticoids and non-esterified fatty acids (NEFA) were determined in rats. (2) Endotoxin did not change the plasma concentration of glucocorticoids, but blocked the effects of elevated glucocorticoid hormone concentrations on both plasma glucose and hepatic tryptophan dioxygenase activity. (3) Endotoxin increased the plasma concentrations of glucose, glucagon and insulin in rats with basal glucocorticoid concentrations, and changed the observed relationships between glucose, glucagon and insulin in a manner consistent with an increased sensitivity of glucagon secretion to lowered glucose concentrations. (4) At the highest dose of endotoxin used, 20 mg/kg over 6hr, a substantial decrease (>7-fold) in the insulin/glucagon ratio provides evidence for changes in basal (as opposed to hormone-stimulated) glucose production and/or utilisation in vivo.

Glucocorticoid hormones inhibit DNA synthesis in glial cells cultured in chemically defined medium

Primary cultures of glial cells from 2-day-old rat cerebellum were used to examine the growth control properties of steroid hormones. Immunocytochemical staining with anti-glial fibrillary acidic protein (anti-GFAP) demonstrated that the cultures were highly enriched for astrocytes (90%). In an effort to avoid the potential influence of serum-borne steroids, cultures were switched from serum-supplemented to serum-free, chemically defined medium prior to experimentation. Assays for DNA synthesis used [ 3H]thymidine incorporation with either liquid scintillation counting of TCA-insoluble material or light microscopic autoradiography. Glial cells grown in serum-free, chemically defined medium (F12 basal medium supplemented with putrescine, selenium, insulin, transferrin, and BSA) replicated their DNA to a limited extent even in the absence of serum mitogens. When the glial cells were shifted to defined medium supplemented with various steroid hormones (corticosterone, dexamethasone, hydrocortisone, 17β-estradiol, progesterone, or testosterone) at a concentration of 10 −7 M, it was found that the glucocorticoids, corticosterone and dexamethasone inhibited synthesis of DNA by 49.6 and 56.9%, respectively. Hydrocortisone, another glucocorticoid, caused only a small reduction in DNA synthesis that was not statistically significant ( p<0.05). Moreover, the gonadal steroids, 17β-estradiol, progesterone, and testosterone, failed to show any reduction on DNA synthesis. The growth-controlling activity of the glucocorticoids was dose-dependent with concentrations of 10 −7- 10 −6 M showing maximal effect on DNA synthesis. These results suggest that physiological concentrations of glucocorticoid hormones may exert negative control over DNA synthesis of glial cells in the developing or injured central nervous system.

Growth inhibition of clonal osteosarcoma cell-lines by low concentrations of glucocorticoid hormones

Clonal osteosarcoma cell line, ROS 2 3 , showed marked inhibition of [ 3H]thymidine incorporation in response to low concentrations (10 −10 M) of triamcinolone acetonide and dexamethasone. Hydrocortisone and corticosterone induced inhibition at somewhat higher concentrations. The osteosarcoma cell line ROS 17 2 responded similarly to triamcinolone acetonide and dexamethasone but at higher concentrations of the hormones. In ROS 2 3 the inhibitory effects of triamcinolone acetonide were accompanied by only slight elevation in the amount of intracellular exchangeable Ca 2+. In contrast, in primary cultures of normal rat-calvarian bone cells, [ 3H]thymidine incorporation was inhibited to a much lesser extent only at higher concentrations of triamcinolone acetonide (10 −7 M). The difference in the susceptibility of normal and malignant bone cells to the inhibitory effects of glucocorticoids may have potential therapeutic importance.

Glucocorticoid-Induced Inhibition of T Cell Growth Factor Production

Abstract The previous communication presented data detailing that glucocorticoid hormones exhibited slight but significant inhibitory effects on the proliferation of cytolytic T cells maintained in long-term T cell growth factor (TCGF)-dependent cultures. However, most importantly, glucocorticoids were shown to inhibit T-lymphocyte proliferation via a profound effect on TCGF production. Addition of exogenous TCGF to glucocorticoid-treated human, mouse, or rat T cell mitogen-stimulated lymphocytes restored normal levels of proliferation. These observations led us to propose that the immunosuppressive effects of glucocorticoid hormones were mediated by controlling the production of the T cell proliferation-inducing agent, TCGF. Results of experimentation detailed in this communication provide further evidence in support of this hypothesis. We found that although glucocorticoid hormones had little effect on the cytolytic reactivity of T cells harvested from TCGF-dependent culture, treatment of mixed lymphocyte cultures (MLC) with identical glucocorticoid hormone concentrations completely abrogated in situ TCGF production and alloantigen-directed cytolysis. The inhibitory effects of glucocorticoids in MLC could be eliminated once again simply by the delayed addition of exogenous TCGF. These observations coupled with those detailed in the previous report provide evidence that, rather than effecting lysis of potentially reactive mature lymphocytes, glucocorticoids influence the development of T cell-mediated immune reactivity by inhibiting TCGF production, which in turn serves to inhibit the clonal expansion of activated T cells.