The influence of exogenous steroids on macromolecule uptake by the small intestine of the new-born rat

1. Plasma concentrations of cortisol and corticosterone measured by competitive protein binding in rats between 5 and 28 days after birth have been related to the intestinal uptake of [(125)I]polyvinyl pyrrolidone (PVP).2. Plasma cortisol concentration was consistently low throughout the period studied, but there was an increase in plasma corticosterone concentration at the time (18-21 days) when PVP uptake declined to zero (closure).3. Injection of a large dose of cortisone acetate 5 days after birth resulted in precocious closure; PVP uptake declined progressively to zero during the 6 days following the injection. Injection of this steroid at 12 days of age caused closure within 4 days.4. Precocious closure induced by cortisone acetate was closely comparable histologically with natural closure; the decline in PVP uptake was associated with the progressive displacement of vacuolated cells from the villi of the terminal intestine.5. Injection of corticosterone at either 5 or 12 days after birth also reduced PVP uptake. However, the reduction was transient and uptake returned to control levels some days after the injection.6. The temporary reduction in PVP uptake following corticosterone injection was not associated with any change in the histological appearance of the small intestine at the light microscope level.7. The injection of either cortisone acetate or corticosterone was followed by a period of impaired body growth and also a reduction of adrenal weight in animals injected at 12 days but not in animals injected at 5 days.     

Suppression of the hypothalamic-pituitary-adrenal axis after subcutaneous cortisone acetate administration in rats.

Groups of female rats were injected daily for 14 days with 10 mg of cortisone acetate subcutaneously, to study the mechanisms of glucocorticoid suppression on the hypothalamic-pituitary-adrenal axis. Pituitary adrenocorticotropic hormone (ACTH) content, plasma ACTH, adrenal venous corticosterone, adrenal weights, and the catabolic effects on body weight were studied simultaneously (under stressful and non-stressful conditions) before, during, and up to six weeks after cortisone. This study confirmed the results of other investigators that cortisone acetate caused catabolic weight loss and adrenal atrophy, but it was noted to persist up to six weeks after the injections. Glucocorticoid acetate was more effective in causing ACTH-axis suppression than succinate or phosphate preparations, and the effects were dose and time related. Significant depletion of pituitary ACTH content, suppression of plasma ACTH, and corticosterone secretion occurred five to seven days after beginning cortisone acetate (p=<0.001); it was continuous throughout the injection schedule (p=<0.001); it remained for two to four weeks after the cortisone was discontinued (p=<0.001). The animals showed minimum plasma ACTH responsiveness to severe acute stress during this two to four-week suppression phase, but rapid recovery occurred thereafter. Plasma ACTH was undetectable up to six weeks post-cortisone when the animals were not under stress. This may be related to residual cortisone acetate found at the injection sites, or to an altered or different ACTH-axis control mechanism. The sequence of events during recovery from cortisone suppression appeared to be (1) repletion of corticotrophin-releasing hormone (by inference), (2) repletion of pituitary ACTH content, (3) secretion of plasma ACTH, (4) reversal of adrenal atrophy, and (5) subsequent secretion of corticosterone.     

Factors influencing the uptake of [125I]polyvinyl pyrrolidone by the intestine of the young rat

1. The uptake of orally administered [(125)I]polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000 (K. 60) by the small intestine of young rats was determined 4 hr after feeding by measurement of the radioactivity remaining in the wall after flushing out the intestinal contents.2. PVP uptake was proportional to the administered dose of PVP for doses of 1 mg and below. At higher doses, uptake was no longer in linear proportion, suggesting a degree of saturation of uptake.3. Litters were reared in one of three environmental temperatures: 13, 20 and 30 degrees C, and were tested to determine the age at which PVP uptake ceased (;closure'). The results were evaluated statistically (see Appendix).4. There was no significant difference between the time of closure in animals reared at 20 and 30 degrees C, but, relative to these groups, closure was significantly delayed, by approximately 48 hr, in animals reared at 13 degrees C.5. Once closure had begun, there were no significant differences in the rate of decline in PVP uptake with age between animals reared at different environmental temperatures.6. Separation of the young rats from their mother both before and after the test-feed of PVP caused a considerable reduction in PVP uptake. Provided that the body temperature of the young rats was maintained, the main factor contributing to the reduction in PVP uptake appeared to be the absence of milk, rather than separation from the mother.     

Corticosterone secretion in the rat after DSP-4 treatment

Functioning of the hypothalamo-pituitary-adrenocortical axis was assessed in rats treated with DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine), a selective noradrenergic neurotoxin. Although adrenal weights were slightly increased 10 days after DSP-4 injection, basal plasma corticosterone levels were unaltered compared to those of control animals. Moreover, the elevation of corticosterone levels induced by the stress of ether inhalation did not appreciably differ from that in control rats. These various results suggest that DSP-4 treatment has no marked effect on the normal functioning of the hypothalamo-pituitary-adrenocortical axis.     

Influence of infantile stimulation on the response to stress during preweaning development

New born rats were stimulated in infancy and the maturation of the adrenal response to stress or to exogenous adrenocorticotropic hormone (ACTH) was examined. Control animals (nonhandled) showed little or no response to ether and surgical trauma until about 18 days of age. In contrast, handled newborn rats had a significant elevation in plasma corticosterone concentration following stress introduced as early as 3 days of age, whereas at 21 days of age, nonhandled animals showed a significantly higher plasma corticosterone concentration than their handled counterparts. The response to ACTH was equivalent in both groups on each test day. Plasma steroid levels were elevated after ACTH injection at 3 days of age. This was followed by a period of marked suppression of the response to ACTH lasting until about 15 days of age in both groups.     

Structural changes in the small intestine associated with the uptake of polyvinyl pyrrolidone by the young ferret, rabbit, guinea-pig, cat and chicken

1. The entry of [(125)I]polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000 (K. 60) into the epithelial cells of the small intestine has been measured in new-born animals of five species.2. The distribution along the intestine of cells capable of taking up [(125)I]PVP and the decrease and eventual cessation of uptake (closure) with increasing age have been investigated, and have been related to changes in the histological appearance of the small intestine.3. The small intestine of the ferret took up PVP readily until 33-34 days after birth. From 34 to 37 days of age PVP uptake declined sharply and disappeared completely by 40-45 days.4. In the ferret, unlike other species studied, some PVP was taken up by the duodenum. This continued for the first 4 weeks after birth. Thereafter PVP uptake gradually became confined to the terminal ileum.5. In the guinea-pig, PVP uptake was limited to the first 48 hr after birth. During this period the site of uptake was progressively restricted to the terminal ileum.6. In the rabbit, PVP could be taken up in the distal two-thirds of the small intestine for at least 20 days after birth. A decline in uptake occurred between 20 and 22 days after birth in most animals.7. Wide individual variations were seen in the kitten, but PVP uptake was seen in some animals up to 14 days after birth.8. Newly hatched chicks and chicks tested 48 hr after hatching did not take up PVP.9. Histological examination of the small intestine with the light microscope demonstrated that in all species PVP uptake was associated with the presence of vacuoles in the epithelial cells of the villus.10. In the young guinea-pig, large PAS-positive granules were seen in the macrophages of the lamina propria. These appeared to migrate through the epithelium into the intestinal lumen. The significance of this finding and its relation to macromolecular uptake remain unclear.     

The functional activity of the adrenocortical system and the transcortin-binding capacity of the blood plasma in rats during the development of tolerance to the narcotic action of ethanol

Corticosterone levels were studied using HPLC in the blood plasma and tissues of rats after 1, 4, 7 and 10 injections of ethanol (3.5 g/kg, daily). Main parameters of binding of corticosteroid-binding globulin (CBG) were calculated using Scatchard's plots. It is shown that development of tolerance to the hypnotic action of ethanol in rats is accompanied with: 1) increased levels of corticosterone in the adrenal tissue, 2) gain of the adrenals' weight, 3) inhibition of the stressogenic elevation of corticosterone plasma levels, 4) decline in the binding capacity of plasma CBG, 5) reduced concentration of dexamethasone-sensitive steroid receptors in the liver. Negative correlations between the duration of ethanol-induced sleep and concentration of corticosterone were found: for adrenal tissue after 7 and for plasma after 10 injections of ethanol (24 hours after the injection).     

Differential potencies of corticosterone and hydrocortisone in immune and immune-related processes in the mouse

The effects of corticosterone and hydrocortisone on the thymus, the pituitary--adrenal axis, delayed hypersensitivity, the corticosterone plasma level and the numbers of circulating nucleated and monocytic cells were investigated in the mouse. Short-term effects within 48 h after one or two corticoid injections and late effects 7 days after a regimen of 4 corticoid injections were discerned. In short-term experiments hydrocortisone was more active than corticosterone upon the induction of leukopenia and monocytopenia and the inhibition of delayed hypersensitivity. However, regarding late effects and the short-term effect on adrenal weight, corticosterone far exceeded hydrocortisone in activity. Our results could be explained by assuming two feedback-inhibition systems for glycocorticoids. The first, likely to be responsible for the changes observed for the adrenal weight and the numbers of ciruclating white cells after a single glucocorticoid injection, was shown to be expressed in a soluble factor released in the blood stream tentatively designated "glucocorticoid inhibiting factor. The factor was more readily induced by hydrocortisone but displayed a greater specificity in inhibiting effects of corticosterone. The second feedback-inhibition system, responsible for increased numbers of circulating monocytes paralleled by an enhanced delayed hypersensitivity response, was expressed in a decreased corticosterone plasma level, most probably secondary to a diminished release of ACTH from the pituitary gland. With the glucocorticoid doses we used the second feedback-inhibition system was only triggered by the more physiological hormone, corticosterone.     

Effect of Neonatal Handling in Rats with Hereditary Stress-Induced Arterial Hypertension (NISAG Rats)

NISAG rats were subjected to handling on days 1-21 after birth. Blood pressure and plasma corticosterone concentration were measured in 6-month-old handled and control NISAG rats at rest and under stress conditions. Animal behavior was studied in the open-field test. Handling had no effect on body weight and relative weights of the heart and adrenal glands. In rats subjected to handling, changes in blood pressure and adrenal cortex produced by acute emotional stress (30-min restriction) were less pronounced than in control animals. Handled rats demonstrated less fear in a new environment and exhibited high exploratory activity in the open-field test. Our findings suggest that neonatal handling reduces stress reactivity and decreases the severity of hypertension in adult NISAG rats.     

Increased glucocorticoid response to a novel stress in rats that have been restrained

Rats exposed to repeated restraint stress (3 h of restraint on each of 3 days) lose weight during stress and do not return to the weight of nonstressed controls once stress ends. Others have reported that chronic stress raises the daily nadir of corticosterone release and increases the adrenal response to subsequent stress; therefore, we examined glucocorticoid release in rats that had been exposed to repeated restraint. Repeated restraint had no effect on the diurnal pattern of corticosterone or insulin release, measured 12 days after restraint had ended, indicating that the reduced weight of the rats is not associated with an elevated corticosterone-insulin ratio. In contrast, rats that had been exposed to repeated restraint, 12 days previously, showed a blunted corticosterone release during a second restraint stress, a normal response to the novel physiological stress of 2-deoxy glucose (2-DG) injection, but an exaggerated corticosterone response to the novel mild stress (MS) of either placement in a unfamiliar environment or an intraperitoneal injection of saline. Mice exposed to repeated restraint showed a similar hyperresponsiveness to novel MS, suggesting that repeated restraint lowers the threshold for stress-induced activation of the adrenal gland. MS caused a small, but significant, degree of hypophagia in rats that had been exposed to repeated restraint stress. Therefore, multiple aspects of the stress response may be exaggerated in these animals and contribute to the chronic reduction in body weight.     

Role of the adrenal cortex in maturation of the lymphoid system and immunological competence; the effects of aminoglutethimide in suckling mice

Newborn rats and mice have rudimentary peripheral lymphoid tissues and are immunologically incompetent. The lymphoid system matures late in the third week after birth, shortly before weaning. The adrenal cortex also is relatively inactive neonatally and begins to secrete glucocorticoids in a mature fashion two weeks after birth. These experiments were designed to test the hypothesis that adrenal glucocorticoids induce maturation of the lymphoid system and immunological competence during the third postnatal week. Mice 10–12 days old were injected daily for 4–11 days with aminoglutethimide, an inhibitor of steroid biosynthesis. The adrenal glands and lymphoid tissues were prepared for histological examination 24 hours after the last injection and evaluated without knowledge of the treatment received. Aminoglutethimide caused adrenal dysplasia with a frequency proportional to dose. Effective doses also inhibited growth, and slightly larger doses were fatal. Therefore it was concluded that large doses of aminoglutethimide caused adrenal insufficiency, but the completeness of this insufficiency in surviving animals was not ascertained. Three phases in postnatal maturation of the lymphoid system were identified by examination of untreated littermate controls and reference to previous work. During the first week after birth, the thymus-dependent lymphoid tissues grow by immigration of thymus-derived cells that soon have the capacity for cell-mediated immunity. During the second postnatal week, a new population of wandering lymphoid cells, presumptively derived from bone marrow, settles in lymphoid organs in response to antigenic stimulation, to form primary lymphoid nodules and a few plasma cells. Late in the third week after birth the machinery for humoral antibody synthesis matures with the appearance of germinal centers and numerous plasma cells, coincident with a great increase in production of immunoglobulin. This third phase of maturation was retarded in mice injected with near-fatal doses of aminoglutethimide. Because these mice suffered neither involution of lymphoid tissue nor suppression of proliferation in thymus or thymus-dependent lymphoid-tissue, it was concluded that the effects of aminoglutethimide upon the development of germinal centers and plasma cells were selective and specific. Therefore these experiments support the hypothesis that glucocorticoid secretion plays a decisive role in maturation of immunological competence during the third week after birth in mice. This role appears to be the potentiation of cellular proliferation and differentiation of B cells in response to antigens, culminating in antibody synthesis. Maturation of adrenal glucocorticoid synthesis has also been implicated in the initiation of physiological involution of the thymus and cessation of intestinal absorption of antibodies during the third postnatal week in rats and mice.     

Paraventricular hypothalamic obesity in rats: role of corticosterone.

The role of adrenal glucocorticoid hormones in the weight gain produced by lesions of the paraventricular nuclei was explored in two experiments. In the first experiment, female rats with PVN lesions were found to have normal a.m. plasma corticosterone concentrations and blunted, albeit still elevated, p.m. concentrations. Nighttime corticosterone levels were moderately correlated with plasma insulin levels. In the second experiment, adrenalectomy markedly suppressed weight gain in animals with very large PVN lesions (mean weight gain of 33.0 g/20 days compared to 137.6 g/20 days for PVN rats with sham adrenalectomies). In the ADX-PVN group, there was a +0.90 correlation between plasma corticosterone levels and weight gain. Administration of corticosterone restored the abnormal weight gain in ADX-PVN animals. It is concluded that the steroid receptors mediating this effect of corticosterone lie outside the hypothalamus.     

Opposite effects of dexamethasone and ACTHon the adrenal cortex response to ethane dimethanesulphonate (EDS)

Recently we have reported that ethane dimethane-sulphonate (EDS), the Leydig cell cytotoxin, caused marked atrophy of the adrenal cortex of adult male rats. The aim of this work was to examine whether a 9-day treatment with dexamethasone (0.25 mg/kg/d) or ACTH (40 IU/kg/d), which started 4 days prior to administration of a single dose of EDS (75 mg/kg), influenced the response of the inner adrenocortical zones to the toxin. On day 15 after administration of EDS, adrenal weight was significantly decreased in saline treated rats, but glandular and serum corticosterone levels were not altered. In dexamethasone-suppressed rats, the effect of EDS was augmented; an additional decrease in adrenal weight was accompanied by reduced adrenal and serum corticosterone levels. In ACTH-treated animals EDS was ineffective. These results demonstrate that the deleterious effects of EDS on rat adrenal cortex can be prevented by ACTH and potentiated by dexamethasone.     

Effect of Intraperitoneal Administration of Bacterial Lipopolysaccharide on Synthesis of Pro-Opiomelanocortin mRNA in Rat Tanycytes

Intensity of pro-opiomelanocortin mRNA synthesis in tanycytes of adrenalectomied and sham-operated rats was studied by in situ hybridization 4 h after intraperitoneal injection of bacterial LPS. The results were compared with pro-opiomelanocortin mRNA expression in rats subjected to immobilization stress. Immobilization and LPS injection to adrenalectomied and sham-operated animals sharply reduced pro-opiomelanocortin mRNA expression in tanycytes. Injection of LPS to sham-operated animals and immobilization stress caused a significant increase in blood corticosterone level. On the other hand, LPS injection did not change low blood level of corticosterone in adrenalectomied rats. The data indicate a synergic inhibitory effect of glucocorticoids and LPS on synthetic activity of tanycytes.     

The acute effects of single and repeated injections of acrolein and other aldehydes

The irritating aldehyde acrolein was injected intraperitoneally into mice. A single injection at 4 mg/kg gave rise to a 5-fold increase in plasma total lactate dehydrogenase (LDH) activity, with the peak after approximately 10 h. The pattern of LDH isoenzymes was not altered. Repeated injections (daily or weekly) caused a progressively less pronounced effect on the LDH activity. Experiments with formaldehyde and crotonaldehyde gave essentially the same results. The LD50 for acrolein i.p. in mice was increased from a level of 7 mg/kg to a level of 12 mg/kg by pretreatment with sublethal doses of 4 mg/kg/day for 5 days. Thus, the response to repeated acrolein injections, in terms of LDH and LD50, indicates an acquired tolerance against the irritant. Likewise, pretreatment with formaldehyde or crotonaldehyde could induce tolerance, in terms of LDH activity, towards a subsequent injection of acrolein. Histopathological examination revealed that spleen, adrenals and thymus were affected. The thymus markedly decreased in size after repeated injections of acrolein, crotonaldehyde or formaldehyde. Adrenalectomized mice given acrolein showed no thymus atrophy. A single injection of aldehyde caused an increased level of the adrenal hormone corticosterone in blood plasma. Adrenalectomized mice still showed a certain tolerance, in terms of LDH activity, after repeated injections of acrolein, but the increase in plasma LDH activity was smaller than for normal animals. Treatment with acrolein for six days did not change the level of reduced glutathione or the glutathione S-transferase activity in liver cytosol, but the rate of glutathione synthesis was increased. It is concluded that adrenalectomy does not completely prevent the development of tolerance in mice. It is possible that an increased metabolism can partially explain the acquired tolerance.     

The effects of corticosterone and cortisone on the uptake of polyvinyl pyrrolidone and the transmission of immunoglobulin G by the small intestine in young rats.

1. The distribution of polyvinyl pyrrolidone along the intestinal lumen and in the intestinal wall, following oral administration to normal and corticosterone treated rats, was found to be extremely variable. Valid comparisons between the two groups of animals could not be made using this technique. 2. Three, 4 and 5 days after corticosterone treatment there was no significant change in the uptake of 125I-labelled polyvinyl pyrrolidone from standard doses injected into ligated segments of the distal small intestine; nor did the treatment induce precocious replacement of the absorptive cells in this region. Cortisone induced precocious cell replacement, a process which took up to 4 days to complete, and also led to a marked reduction in the uptake of 125I-labelled polyvinyl pyrrolidone from ligated segments of the distal intestine. 3. Three days after treatment with corticosterone (5 mg I.P. at 12 days) there was a marked reduction of labelled immunoglobulin G transport into the blood. Four and 5 days after treatment there was some recovery of the immunoglobulin G transport function. Three days after treatment with cortisone (5 mg I.P. at 12 days) there was closure of the gut to labelled immunoglobulin G. 4. The relevance of these results to antibody transmission and the termination of immunoglobulin transport is discussed.     

Suppression of the hypothalamic-pituitary-adrenal axis after oral hydrocortisone succinate ingestion in rats.

Groups of Holtzman female rats were fed 10 mg/day of hydrocortisone succinate orally to study the responsiveness of the hypothalamic-pituitary-adrenal axis to acute stress. Pituitary ACTH content, plasma ACTH, adrenal venous corticosterone, and adrenal weights were studied simultaneously in experimental and control rats before, during, and up to two weeks after oral hydrocortisone administration. There was a significant decrease in pituitary ACTH content (p=<0.001), suppression of plasma ACTH and corticosterone in response to acute stress (p=<0.001), and adrenal atrophy during and following oral hydrocortisone administration. After discontinuing the hydrocortisone it required three to five days for the rats to respond adequately to acute stress. However, it was seven to ten days post-hydrocortisone before plasma ACTH and corticosterone responses to acute stress had returned to basal values, but decreased pituitary ACTH content and partial adrenal atrophy continued throughout the ten-day post-hydrocortisone study interval. Recovering from the suppressive effects of oral hydrocortisone was more rapid than following parenteral hydrocortisone. However, oral hydrocortisone causes identical but less sustained suppression of the hypothalamic-pituitary-adrenal axis as observed in animals treated with parenteral glucocorticoid preparations.     

The effects of infusions of synthetic adrenocorticotrophin in the conscious calf

1. A technique is described by which the whole of the effluent blood from the right adrenal gland can be collected as required from conscious, unrestrained calves. The technique may be used to measure adrenal blood flow gravimetrically and to compute the output of adrenal hormones under various conditions in the normal calf.2. In a group of seven calves mean cortisol output from the right adrenal gland was found to vary between 20 and 40 ng.kg(-1) min(-1) and corticosterone between 6 and 18 ng.kg(-1) min(-1) during a 2 hr period, 24 hr after surgery.3. Intravenous infusions of synthetic adrenocorticotrophin (5 ng.kg(-1) min(-1)) produced a significant increase in the output of both cortisol and corticosterone within 5 min. The output of both hormones rose to maximal values within 10-20 min and mean values of approximately 300 ng.kg(-1) min(-1) (cortisol) and 120 ng.kg(-1) min(-1) (corticosterone) were maintained thereafter for the duration of the infusion (120 min). The output of both steroids fell to values comparable with those observed initially within 45-60 min after the infusion was discontinued.4. These changes in glucocorticoid output in response to adrenocorticotrophin produced a significant rise in the concentration of both cortisol and corticosterone in peripheral plasma. It is noteworthy that the rise in the mean corticosterone concentration in the peripheral plasma was substantially less than that which might be expected from relating the rise in mean plasma cortisol concentration to cortisol output.5. The results of control experiments have eliminated the possibility that the sampling procedure might itself increase steroid output or peripheral plasma concentration. Comparison of results from calves of widely disparate ages (8-38 days) provided no evidence that either the resting output of cortisol or corticosterone or the response to adrenocorticotrophin changes with age within the range examined.6. Infusion of adrenocorticotrophin (5 ng.kg(-1) min(-1)) also stimulated an abrupt rise in adrenal blood flow; mean resting flow (210 +/- 23 mul.kg(-1)) increased by approximately 30% within 5 min and attained peak values (355-365 mul.kg(-1) min(-1)) between 10 and 30 min. Thereafter, adrenal blood flow steadily decreased and then fell rapidly to within the resting range when the infusion was terminated. No significant changes in heart rate or aortic blood pressure occurred during these infusions.7. The results are discussed in relation to those obtained in other species and under differing conditions by other workers.     

Corticosteroids and ACTH are not required for compensatory adrenal growth.

We have tested the hypothesis that unilateral adrenalectomy results in decreased glucocorticoid secretion, reflexly elevated ACTH secretion, and consequently, compensatory adrenal growth. Plasma ACTH and corticosterone and right adrenal weight were measured during the first 10 days after left adrenalectomy or sham adrenalectomy in young male rats. There is a decrease in plasma corticosterone after unilateral adrenalectomy compared to sham adrenalectomy that persists for 1 h. ACTH is elevated only at 2 h after unilateral adrenalectomy compared to shamoperated rats. Treatment with dexamethasone, shown to abolish the ACTH and corticosterone responses to laparotomy with intestinal traction, resulted in significantly increased adrenal weight after unilateral adrenalectomy by 6 h (wet or dry weight), and at 24 h. Compensatory adrenal growth also occurs after unilateral adrenalectomy in hypophysectomized rats (wet or dry weight). We conclude the compensatory adrenal growth after unilateral adrenalectomy requires neither a virtual decrease in circulating corticosterone levels nor elevated ACTH levels, and speculate that the phenomenon is neurally mediated.     

Effects of prolonged leptin infusion on rat pituitary-adrenocortical function.

Compelling evidence suggests that a regulatory loop between leptin and the hypothalamo-pituitary-adrenal (HPA) axis is operative, where ACTH inhibits leptin secretion by adipose tissue and in turn leptin increases expression and secretion of ACTH. However, conflicting findings have been obtained in vivo on the acute and chronic effects of leptin on the HPA axis. Adult female Wistar rats, kept in metabolic cages, were intraperitoneally infused for 2, 4, 8 or 16 days with leptin (10 nmol/kg. 24 h); control animals were infused with the vehicle only. The rate of body-weight gain was similar in control and leptin-infused rats. At day 16 of treatment relative pituitary weight was higher and relative adrenal weight smaller in leptin-infused than control rats. Pituitary ACTH concentration gradually decreased with the duration of treatment, and the drop was significantly higher in leptin-infused than control rats. During the entire experimental period the blood level of aldosterone was similar in both groups of rats. Conversely, at days 2 and 4 of treatment the blood concentration of corticosterone was lower in leptin-infused than in control rats, which at these times displayed elevated levels of circulating corticosterone. Taken together, these findings allow us to conclude that in the rat the prolonged infusion of low doses of leptin i) primarily depresses pituitary ACTH production, the effect being probably mediated by the hypothalamus; and ii) inhibits corticosterone response to the stress evoked by placing of animals in the metabolic cages.