Effect of Prenatal Stress on the Pituitary–Adrenal Axis in Blue Foxes

Handling is a source of stress for farm bred blue foxes. The influence of handling during the late gestation period on the pituitary–adrenal axis was studied in 10-day old male and female blue foxes. Cortisol and progesterone were measured by radioimmunoassay in the plasma, adrenal homogenates, and in vitro incubates from animals of both sexes. Adrenals were incubated in vitro in the absence or presence of ACTH. In addition, the adrenal weight and plasma concentration of ACTH were assessed. In cubs of both sexes, the adrenal weight was decreased after prenatal stress treatment. The plasma concentration of progesterone and the adrenal cortisol in vitro production were elevated in the prenatally stressed female cubs, as compared to the control, along with the adrenal progesterone in vitro production in prenatally stressed male cubs. The adrenal cortisol and progesterone content and plasma ACTH and cortisol concentrations were not affected by prenatal stress. In conclusion, the results of this study suggest that the prenatal stress induced by handling pregnant vixens can affect the pituitary–adrenal axis in neonatal offspring, this effect being more pronounced in female cubs.     

The effect of prenatal stress on steroidogenesis in the gonads of Arctic foxes (Alopex lagopus)

Handling is a source of stress for farm-bred blue foxes. The influence of handling during the late gestational period was investigated in 10-day old male and female blue foxes. Testosterone and estradiol were measured by RIA in the plasma, gonadal homogenates and in vitro incubates from blue foxes of both sexes. The gonads were incubated in vitro without or with human chorionic gonadotropin. In cubs of both sexes, the gonad weights and ovarian estradiol production were decreased by stress. The testicular testosterone and ovarian estradiol contents were increased in prenatally stressed cubs as compared to the controls. The testicular content and baseline in vitro production of testosterone were not affected by prenatal stress, but the testicular response to human chorionic gonadotropin was higher in the stressed group. This study suggests that prenatal stress induced by handling pregnant vixens may influence gonadal steroidogenesis and this effect was more pronounced in female cubs.     

Influence of prenatal stress on steroidogenesis in gonads of blue foxes

Handling is a source of stress for farm bred blue foxes. The influence of handling during the late gestational period was investigated in 10-day-old male and female blue foxes. Testosterone and estradiol were measured by RIA in the plasma, gonadal homogenates andin vitro incubates, from blue foxes of both sexes. The gonads were incubatedin vitro without or with human chorionic gonadotropin. In cubs of both sexes, the gonad weights and ovarian estradiol production were decreased by stress. The testicular testosterone and ovarian estradiol contents were increased in prenatally stressed cubs as compared to the controls. The testicular content and baselinein vitro production of testosterone were not affected by prenatal stress, but the testicular response to human chorionic gonadotropin was higher in the stressed group. This study suggests that prenatal stress induced by handling pregnant vixens may influence gonadal steroidogenesis and that this effect was more pronounced in female cubs.     

Effects of prenatal maternal stress on the pituitary adrenocortical reactivity in guinea-pig pups

Seven days before term, pregnant guinea-pigs were subjected to a psychosomatic stress. Adrenocorticotrophin (ACTH), cortisol and aldosterone concentrations increased in maternal and fetal plasma immediately after stress. In the offspring born from prenatally-stressed mothers that were resubjected to stress, the increases in plasma ACTH and cortisol level were lower than in the control offspring. Plasma aldosterone levels increased after stress in 12 days-old pups but did not change neither in 50 days-old animals nor in 12 days-old guinea-pigs born of prenatally stressed mothers. Thus prenatal stress of mother resulted in lowered response of pituitary-adrenocortical axis of offspring subjected to stress.     

Gender-specific effects of prenatal stress on emotional reactivity and stress physiology of goat kids

The aims of this study were to investigate the effects of maternal stress during pregnancy on the emotional reactivity, the hypothalamo-pituitary-adrenocortical (HPA) axis, and the sympatho-adrenomedullary (SAM) system of goat offspring according to their gender, and to investigate the role of maternal cortisol in prenatal stress effects. Goats were exposed to ten transports in isolation or ten ACTH injections (0.125 IU/kg body weight) during the last third of pregnancy. Control goats remained undisturbed. No effect of repeated transport during the last third of pregnancy was found on basal cortisol concentrations of the offspring. However, an increase in phenylethanolamine N-methyl transferase activity in the adrenals was observed in prenatally stressed kids compared to control kids (P = 0.031). In the presence of novelty, prenatally stressed female kids were more active (P = 0.049) than control females; they also showed more signs of arousal (P = 0.039) and tended to explore more of their environment (P = 0.053) in reaction to a startling stimulus. On the contrary, prenatally stressed male kids tended to be less active (P = 0.051) than control male kids but showed more signs of distress (P = 0.047) in the presence of novelty. Intermediate effects were found on the emotional reactivity to novelty of kids born from dams given injections of ACTH. In conclusion, transport stress in pregnant goats affects the sympatho-adrenomedullary system and the emotional reactivity of their offspring in a gender-specific manner. Moreover, the effects of prenatal transport and ACTH injections showed some similarities but differed in some critical details.     

Activity of the hypothalamic–pituitary–adrenal axis of prenatally stressed male rats in experimental model of depression

Changes in activity of the hypothalamic–pituitary–adrenal (HPA) axis were examined in adult, prenatally stressed male rats in the experimental depression model of ‘learned helplessness’. It was shown that in males descending from intact mothers a depressive-like state was accompanied by an increase in activity of the entire HPA axis. Namely, expression of corticotropin-releasing hormone (CRH) in the hypothalamic paraventricular nucleus (PVN) increased coupled to a rise in plasma levels of ACTH and corticosterone as well as in adrenal weight. At the same time, in males born to mothers who suffered stress during the last week of pregnancy a decrease was detected in activity both of the central (hypothalamus) and peripheral (adrenal cortex) parts of this regulatory hormonal axis, analogous to that we revealed previously in the ‘stress–restress’ experimental model. It is concluded that prenatal stress modifies the sensitivity of animals to inescapable intense stress impacts, as manifested in the specific pattern of HPA axis activity after stressing.     

Stress- and (1-24)ACTH-induced alterations in adrenal and testicular steroidogenesis in Mongolian gerbils (Meriones unguiculatus): comparison of plasma levels, tissue content and in vitro secretion

Plasma glucocorticosteroid levels were significantly elevated 1 hr after confinement stress or (1-24)ACTH administration. Both adrenal content and in vitro secretion of glucocorticosteroids and progesterone from adrenals of stressed or (1-24)ACTH-injected animals were higher than values measured in controls. Neither adrenal testosterone content nor output of testosterone or progesterone from superfused testes were changed. Significant correlations were obtained between glucocorticosteroid plasma levels and corresponding adrenal content/in vitro secretion, adrenal progesterone content and output, and between adrenal glucocorticosteroid and progesterone content.     

“Green odor” inhalation by stressed rat dams reduces behavioral and neuroendocrine signs of prenatal stress in the offspring

Chronic maternal stress during pregnancy results in the “prenatally stressed” offspring displaying behavioral and neuroendocrine alterations that persist into adulthood. We investigated how inhalation of green odor (a mixture of equal amounts of trans-2-hexenal and cis-3-hexenol) by stressed dams might alter certain indices of prenatal stress in their offspring. These indices were depression-like behavior (increased immobility time in the forced-swim test) and acute restraint stress-induced changes in hypothalamo-pituitary-adrenocortical (HPA) axis activity [plasma corticosterone (CORT) and ACTH levels and the number of Fos-immunoreactive cells in the hypothalamic paraventricular nucleus (an index of neuronal activity)]. Pregnant rats were exposed to restraint stress for 60 min/day for 10 days (gestational days 10-19). The prenatally stressed offspring exhibited significant increases in depression-like behavior and in restraint stress-induced ACTH, CORT, and Fos responses, unless their dam had been exposed to green odor. The behavioral effect of the odor was also seen in offspring that were fostered by unstressed dams. The results obtained in the dams themselves were as follows. In vehicle-exposed stressed dams, but not in green odor-exposed ones, total body and adrenal weights were significantly decreased or increased, respectively. Depression-like behavior was not observed in the vehicle-exposed stressed dams themselves. Green odor inhalation prevented the impairment of maternal behavior induced by restraint stress. Thus, exposure of dams to stress may affect both the fetal brain and fetal HPA axis, and also maternal behavior, leading to altered behavioral and neuroendocrine responses in the offspring. Such effects may be prevented by the stressed dams inhaling green odor.     

Cortisol production in fetal adrenals of the silver fox

The present study was designed to examine cortisol production by the silver fox fetal adrenals and their response to ACTH at different periods of prenatal life. Serum levels of cortisol were determined on days 35, 40, 45 and 50 of gestation (term on day 52) in embryos of both sexes. Cortisol content in adrenal tissue homogenates and its in vitro adrenal production were also investigated at the same time points. Hormones were measured by RIA. The levels of cortisol changed slightly during embryonic life. The adrenal content and the in vitro production of cortisol increased sharply and progressively (by 4-10 times from days 35 to 50 of gestation). The rises in cortisol content and in vitro production were associated with a rapid growth of the fetal adrenals. There were no sex differences in cortisol level and its adrenal content. ACTH (50 mIU per sample) increased the in vitro cortisol production by the adrenals in the two sexes on all the studied days (by 3-4 times on day 35 and by 1.4-1.7 times on day 50). These results indicate that 1) silver fox fetal adrenals are capable of synthesising cortisol; 2) ACTH is a potent activator of the in vitro cortisol production during embryonic life in this species.     

Responses of the hypothalamic-pituitary-adrenal axis to noradrenergic and hormonal stimulation in prenatally stressed rats

We studied the effects of maternal stress (the so-called prenatal stress, PS, provided by immobilization of pregnant female Wistar rats for 1 h daily during the 15–21st gestational days) on the corticosterone response in the blood plasma evoked by infusion of 10 μg noradrenaline bitartrate into the III cerebral ventricle or by injection of β-1-24-corticotropin in 3-month-old male and female offspring. The animals were bearing an intracerebroventricular stainless steel guide cannula implanted eight to nine days before the experiment, and a Silastic catheter inserted into the external jugular vein 24 h prior to the experiment. Blood samples were periodically taken from conscious unrestrained rats (before and then 30, 60 and 90 or 120 min after noradrenaline or corticotropin challenge). In the male offspring PS augmented and prolonged an increase in the plasma corticosterone level resulting from adrenergic stimulation of the hypothalamus, as compared with that in non-stressed animals. In prenatally stressed female offspring tested in diestrus, there was no response of the hypothalamic-pituitary-adrenal (HPA) axis to intracerebroventricular noradrenaline stimulation, in contrast to what was observed in the control. Prenatal stress did not modify the adrenal cortex responsiveness to corticotropin either in male or in female offspring. The results demonstrate differential effects of PS on the adrenergic activation of the HPA axis in males and females. A decrease in the acute HPA stress-responsiveness in prenatally stressed male rats, which was demonstrated in an earlier study, and the maintenance or even enhancement of this effect in prenatally stressed females are not likely to be connected with the state of hypothalamic adrenergic reactivity.     

Effect of prenatal stress on the hormonal response to acute and chronic stress and on immune parameters in the offspring

The effect of prenatal stress on the time course of the corticosterone response to acute and chronic stress and on hematological and immunological parameters in the offspring were analized in the present study. Pregnant Sprague-Dawley rats were stressed daily for 2 hours during the last week of gestation, and female and male off-spring were studied during adulthood. Corticosterone response to acute immobilization stress was not significantly different in either control or prenatally stressed rats. However, after 10 days of immobilization stress the corticosterone response completely disappeared in the control animals but not in the prenatally stressed group: high levels of corticosterone were found during the first hour of stress, although they were lower than those found in acutely stressed rats. Adrenal hypertrophy in response to prenatal stress was observed in females but not in male offspring, and chronic stress only increased adrenal weights in the male control group. Prenatal stress decreased the total peripheral leukocyte count, altered its diferential count decreasing lymphocytes and increasing neutrophil and eosinhophil counts, and significantly reduced the percentage of peripheral lymphocyte T CD8+ subset in male offspring. Chronic stress also reduced the percentage of the peripheral T CD8+ lymphocyte subset in the control group but not in the prenatally stressed group. These results suggest that the exposure to stress during pregnancy alters the adaptative response of the hypothalamus-pituitary-adrenocortical axis to chronic stress and presumably the immune competence in the offspring.     

Antenatal glucocorticoids reset the level of baseline and hypoxemia-induced pituitary-adrenal activity in the sheep fetus during late gestation

This study examined the effects of dexamethasone treatment on basal hypothalamo-pituitary-adrenal (HPA) axis function and HPA responses to subsequent acute hypoxemia in the ovine fetus during late gestation. Between 117 and 120 days (term: approximately 145 days), 12 fetal sheep and their mothers were catheterized under halothane anesthesia. From 124 days, 6 fetuses were continuously infused intravenously with dexamethasone (1.80 +/- 0.15 microg.kg(-1).h(-1) in 0.9% saline at 0.5 ml/h) for 48 h, while the remaining 6 fetuses received saline at the same rate. Two days after infusion, when dexamethasone had cleared from the fetal circulation, acute hypoxemia was induced in both groups for 1 h by reducing the maternal fraction of inspired O2. Fetal dexamethasone treatment transiently lowered fetal basal plasma cortisol, but not ACTH, concentrations. However, 2 days after treatment, fetal basal plasma cortisol concentration was elevated without changes in basal ACTH concentration. Despite elevated basal plasma cortisol concentration, the ACTH response to acute hypoxemia was enhanced, and the increment in plasma cortisol levels was maintained, in dexamethasone-treated fetuses. Correlation of fetal plasma ACTH and cortisol concentrations indicated enhanced cortisol output without a change in adrenocortical sensitivity. The enhancements in basal cortisol concentration and the HPA axis responses to acute hypoxemia after dexamethasone treatment were associated with reductions in pituitary and adrenal glucocorticoid receptor mRNA contents, which persisted at 3-4 days after the end of treatment. These data show that prenatal glucocorticoids alter the basal set point of the HPA axis and enhance HPA axis responses to acute stress in the ovine fetus during late gestation.     

Modification of expression of neurohormones in hypothalamus of prenatally stressed male rats in model of posttraumatic stress disorder

By the method of quantitative immunohistochemistry there has been studied expression of corticotrophin-releasing hormone (CRH) and vasopressin in hypothalamic paraventricular nucleus (PVN) of prenatally stressed rats in the experimental model of the posttraumatic stress disorder-the paradigm “stress-restress”. The prenatal stress was modeled by immobilization of pregnant female rats for 1 h from the 15th to the 19th day of pregnancy. It has been shown that in sexually mature males-descendants of stressed mothers-a decrease in immunoreactivity to CRH and vasopressin is observed in the parvocellular and magnocellular PVN areas 10 days after the restress. In the control group males born by intact mothers the level of immunoreactivity to CRH was increased in both PVN areas, whereas with respect to vasopressin-in the magnocellular area. Only in the prenatally stressed males there is detected a decrease in the corticosterone level in the blood plasma 10 days after the restress. It is concluded that in the control group males the manifestation of the pathological state in the paradigm “stress-restress” consists in hyperactivation of the hypothalamic chain of regulation of the hypothalamus-pituitary-adrenocortical system, whereas in the prenatally stressed animals, on the contrary, there is observed a decrease in activity both of the central (PVN) and of the peripheral (adrenal cortex) chain of this hormonal axis.     

Evaluation of adrenocortical function in Florida manatees (Trichechus manatus latirostris)

The study objectives were to determine the predominant manatee glucocorticoid; validate assays to measure this glucocorticoid and adrenocorticotropic hormone (ACTH); determine diagnostic thresholds to distinguish physiological vs. pathological concentrations; identify differences associated with sex, age class, female reproductive status, capture time, and lactate; and determine the best methods for manatee biologists and clinicians to diagnose stress. Cortisol is the predominant manatee glucocorticoid. IMMULITE 1000 assays for cortisol and ACTH were validated. Precision yielded intra‐ and inter‐assay coefficients of variation for serum cortisol: ≤23.5 and ≤16.7%; and ACTH: ≤6.9 and ≤8.5%. Accuracy resulted in a mean adjusted R²≥0.87 for serum cortisol and ≥0.96 for ACTH. Assay analytical sensitivities for cortisol (0.1 µg/dl) and ACTH (10.0 pg/ml) were verified. Methods were highly correlated with another IMMULITE 1000 for serum cortisol (r=0.97) and ACTH (r=0.98). There was no significant variation in cortisol or ACTH with sex or age class and no correlation with female progesterone concentrations. Cortisol concentrations were highest in unhealthy manatees, chronically stressed by disease or injury. ACTH was greatest in healthy free‐ranging or short‐term rehabilitating individuals, peracutely stressed by capture and handling. Cortisol concentrations ≥1.0 µg/dl were diagnostic of chronic stress; ACTH concentrations ≥87.5 pg/ml were diagnostic of peracute stress. In healthy long‐term captive manatees, cortisol (0.4±0.2 µg/dl) and ACTH (47.7±15.9 pg/ml) concentrations were lower than healthy free‐ranging, short‐term rehabilitated or unhealthy manatees. Capture time was not significantly correlated with cortisol; ACTH correlation was borderline significant. Cortisol and ACTH were positively correlated with lactate. Zoo Biol 30:17–31, 2011. © 2010 Wiley‐Liss, Inc.     

Ovine fetal adrenal maturation at term and during fetal ACTH administration: evidence that the modulating effect of cortisol may involve cAMP

Exogenous ACTH1-24 promotes adrenal maturation in fetal sheep, and this effect appears to be modulated in part by cortisol (Challis et al. 1985). We have examined whether similar changes in adrenal metabolism of progesterone occur with ACTH-induced labour as at spontaneous term and whether the site of cortisol modulation is on adrenal steroidogenesis or at the level of cAMP generation. Chronically catheterized fetal sheep were infused in utero for 100 h between days 127 and 131 of pregnancy with P-ACTH, P-ACTH + metopirone, P-ACTH + metopirone + cortisol, or saline. After 100 h the metabolism of [3H]progesterone was measured in adrenal homogenates. Similar incubations were performed with adrenal tissue from fetal sheep at day 130 of pregnancy and at spontaneous labour. In the treatment groups of sheep, cAMP output by dispersed adrenal cells in response to ACTH added in vitro was also determined. Similar qualitative patterns of [3H]progesterone metabolism were found in adrenal homogenates after in vivo ACTH or at term. At both times there was an increase in cortisol and in total 17 alpha-hydroxycorticosteroid accumulation and also evidence for increased activity of 11 beta-hydroxylase enzyme. The formation of total 17 alpha-hydroxycorticosteroids was not affected significantly by concurrent treatment in vivo with metopirone +/- cortisol. The accumulation of cAMP in vitro was increased after in vivo ACTH, attenuated after ACTH + metopirone, but statistical significance over controls was restored after ACTH + metopirone + cortisol treatment. We conclude that ACTH-induced labour and spontaneous parturition in sheep is associated with qualitatively similar changes in progesterone metabolism by the fetal adrenal gland.(ABSTRACT TRUNCATED AT 250 WORDS)     

Transient sex-related changes in the mice hypothalamo-pituitary-adrenal (HPA) axis during the acute phase of the inflammatory process

The potential role of endogenous sex hormones in regulating hypothalamo-pituitary-adrenal (HPA) axis function was investigated after a single injection of endotoxin in adult (8 week old) BALB/c mice of both sexes. The effect of LPS on plasma ACTH, corticosterone (B), testosterone and oestradiol (E) levels and on anterior pituitary (AP) ACTH and adrenal B contents at different times after treatment was studied. The results indicate that: (a) basal B but not ACTH plasma levels were significantly higher in female than in male mice; (b) LPS significantly increased both ACTH and B plasma levels over the baseline 2 h after injection, both hormone levels being higher in female than in male mice; (c) although plasma ACTH concentrations recovered the basal value at 72 h after LPS in animals of both sexes, plasma B levels returned to the baseline only at 120 h after treatment; (d) E plasma levels significantly increased 2 h after LPS and returned to the baseline at 72 h post-treatment, in both sexes; (e) at 2 h after LPS, testosterone plasma levels significantly decreased in male mice and increased in female mice, recovering the baseline level at 120 and 72 h after LPS, respectively; (f) AP ACTH content was similar in both sexes in basal condition and it was significantly diminished 72 h post-treatment without sex difference; whereas AP ACTH returned to basal content 120 h after LPS in males, it remained significantly decreased in females; (g) basal adrenal B content was higher in female than in male mice, and it significantly increased in both sexes 2 h post-LPS, maintaining this sex difference. Whereas adrenal B returned to basal content 72 h after treatment in male mice, it remained significantly enhanced up to 120 h post-LPS in female animals. The data demonstrate the existence of a clear sexual dimorphism in basal condition and during the acute phase response as well as in the recovery of the HPA axis function shortly after infection.     

Effects of repeated stress during pregnancy in ewes on the behavioural and physiological responses to stressful events and birth weight of their offspring

Stressors are frequently encountered during the pregnancies of farm animals. However, very few studies have investigated the effects of repeated stress in pregnant females on their offspring. Forty-eight ewes at 2.5 months of pregnancy were selected for their high or low cortisol response to isolation. Half of the ewes were subjected, twice a week during the last 5 weeks of gestation, to isolation. The first isolation bout of each week consisted only of isolation from conspecifics, while the second bout of each week consisted of isolation in the presence of a dog. The cortisol responses of ewes were measured during the treatment. The other half of the ewes was drafted and released without further disturbance. The emotional reactivity of the lambs to a light contrast test, a startling stimulus test and a social isolation test were studied at 25 days of age. The lambs were retested at 8 months of age in the same tests and also in a novel arena and a novel object tests. Cardiac responses to the startling stimulus were studied at 8 months of age. Basal cortisol concentrations and cortisol response after the isolation test were studied at both ages and cortisol response to ACTH was studied at 25 days of age. Ewes repeatedly exposed to the treatment of isolation plus presence of a dog habituated to the treatment. Moreover, the initial differences between the high and low reactivity ewes were not apparent when the stress treatment was subsequently imposed. Lambs prenatally stressed were heavier at birth than control lambs. The prenatally stressed lambs showed more exploration and locomotion in the behavioural tests at 8 months of age but not at 25 days of age. At 25 days of age, the basal cortisol concentrations were higher in the prenatally stressed lambs compared to the control lambs. Whereas these results suggest that mild prenatal stress of lambs has no adverse effect on the behavioural and cortisol responses to a range of stressors, there is evidence that this treatment increases birth weight and exploratory behaviour of prenatally stressed lambs. Clearly these results require confirmation.     

Endocrine patterns before and after examination stress in males and females

Plasma levels of cortisol, adrenocorticotropin (ACTH), and growth hormone (GH), were determined by radioimmunoassay in medical students just before (Exp.1) plasma values of cortisol were higher than their control values in both sexes, statistically significantly, however, only in males. The difference of cortisol behavior between the sexes was not significant. The males' ACTH values were lower than their control levels, but there was no such difference in the females. Plasma levels and differences of growth hormone varied greatly in both sexes. After the examination (Exp.2) plasma values of cortisol were unaffected compared with their control values in both sexes. The ACTH values were lower than the control values in both sexes, significantly though only in the males. The GH values were higher than control values in the males but not in the females. The results are in line with previous observations suggesting that psychological stress is associated with slightly different hormonal effects in males and females.     

Sex differences in adrenal function in the lizard Cnemidophorus sexlineatus: II. Responses to acute stress in the laboratory.

Circulating concentrations of plasma corticosterone and gonadal steroids were measured in intact and gonadectomized male and female lizards (Cnemidophorus sexlineatus) following acute stress (handling) in the laboratory. There was a significant increase in plasma corticosterone after stress. Whereas intact females exhibited greater concentrations of corticosterone relative to intact males, ovariectomized females exhibited lower concentrations of corticosterone relative to castrated males. In addition to sex differences in corticosterone responses to gonadectomy, progesterone was elevated by stress in both intact and ovariectomized females but not in males. Corticosterone adjusted for castration and handling in males was negatively correlated with the plasma androgen level. The adrenal responsiveness of males to acute stress may be attenuated by androgens presumably secreted by the testis. Not only does adrenal function influence reproduction, but adrenal responses differ between males and females, and appear to be influenced by the gonadal axis. The sex differences in adrenal responses to stress likely reflect different reproductive strategies and nutritional requirements of males and females during the breeding season.     

Response of the interrenal to adrenocorticotropic hormone after short-term thyroxine treatment of coho salmon (Oncorhynchus kisutch)

The effects of experimentally elevated plasma thyroxine levels on the subsequent response of interrenals of coho salmon (Oncorhynchus kisutch) to adrenocorticotropic hormone (ACTH) in vitro were examined. Animals were treated with thyroxine by immersion (200 micrograms/L) for 3 days, which resulted in physiological elevations in circulating thyroxine. In animals treated before the parr-smolt transformation was completed (early smolts), thyroxine had no effect on plasma cortisol levels but significantly enhanced the sensitivity of the interrenal to ACTH in vitro. In animals treated after the period of smoltification (postsmolts), plasma cortisol levels were significantly higher than those of controls; both experimental and control animals had plasma cortisol levels higher than normally observed at this stage of development. The response of the interrenals of thyroxine-treated postsmolts to ACTH in vitro was significantly lower than that of controls. Results from the experiments using early smolts are in agreement with studies in other vertebrates showing that thyroid hormones are involved in maintaining the normal functioning of corticosteroidogenic tissue and suggest that thyroid hormones may be involved in the activation of the interrenal that occurs during smoltification. The results obtained using postsmolts are more difficult to interpret because of the possibility that these animals were physiologically stressed by the treatment. Increased ACTH release in vivo resulting from stress may have led to a depression of interrenal sensitivity to ACTH in vitro and may have masked a refractoriness of the pituitary-interrenal axis to thyroxine.