Persistent stress-induced elevations of urinary corticosterone in rats

Exposure of rats to inescapable stressors (IS) results in persistent elevations in plasma corticosterone (CORT), which are selective to the trough of the circadian rhythm. Although affective disorders (depression, anxiety) in humans are also characterized by persistent hypothalamic-pituitary-adrenal axis (HPAA) activation, the predominant measure of HPAA activation in clinical studies is 24-h urinary cortisol. To facilitate interspecies comparisons regarding the persistent effects of stress on HPAA activity, we compared the effects of IS on plasma and urinary CORT in rats. Male Sprague-Dawley rats were exposed to three 2-h sessions of IS (40, 2.0 mA tailshocks) or remained in their home cages. The 24-h urine samples were collected daily from 2 days prior to stress to 5 days after stressor cessation, then weekly for 3 weeks. In addition, plasma samples were obtained at 08:00 (trough) and 20:00 hours (peak) for the first 3 days after stressor cessation and weekly for 3 weeks thereafter. Consistent with our earlier work, plasma CORT elevations were apparent in the trough, but not the peak samples for 3 days after stressor cessation. The 24-h urinary CORT levels were elevated during stressor exposure, and remained elevated for 3 days after stressor cessation. Persistent stress-induced urinary CORT elevations in rats are reminiscent of the clinical HPAA abnormalities described for major depression and affective disorders.     

Plasma corticosterone fluctuations in an infant-learning paradigm

Plasma corticosterone fluctuations of infant rats were examined in a learning task. A blood sample for analysis of plasma corticosterone was collected from groups of 10- and 15-day-old pups following either no disturbance, standard maternal deprivation only, or deprivation and training in an established approach-mother, avoid-shock conflict task. In the latter condition, pups remained in the goalbox either alone or with an anesthetized dam for either 15 or 60 min before blood sample collection. Plasma corticosterone levels were elevated following deprivation plus training in pups of both ages and following deprivation only in the 15-day-old pups. Further, the presence of the dam in the goalbox reduced plasma corticosterone elevations, particularly among 15-day-old pups and at 60 min. These findings suggest that the mother's capacity to moderate the pup's plasma corticosterone response may contribute to her reinforcement value in infant-learning paradigms.     

Differential plasma corticosterone responses to hippocampal stimulation

Summary The effect of limbic forebrain stimulation on pituitary-adrenal function was assessed by evaluating plasma corticosterone obtained prior to and following sham or electrical stimulation of urethane (1.20 g/kg) anesthetized female rats. Cortical electroencephalogram (EEG), electrocardiogram (ECG), heart rate (HR), mean arterial blood pressure (MAP), and respiration were routinely monitored. Timed blood samples (0.25 ml) were obtained from a catherized femoral artery. The HR (Bts/min), MAP (mm of Hg), and corticosterone levels (g/dl) for 7 non-stimulated rats averaged over 6 sampling periods were 385±19, 95±6, and 70.3±5.8 respectively. In electrically or sham stimulated rats, blood samples were taken just prior to stimulation (biphasic square waves, 100 A, 50 or 60 Hz, 1 ms, 1 s on/1 s off for 15 or 30 min) and 5, 10, 15, and 30 min after initiation of stimulation.Significant changes in plasma corticosterone levels were obtained following stimulation of hippocampal and amygdaloid areas. In contrast, no change in corticosterone concentration was observed following stimulation of cortex, corpus callosum, fornix and a variety of other CNS areas. Detailed analysis of hippocampal influence on urethane stimulated plasma corticosterone levels showed increased plasma corticosterone levels following stimulation of CA1. In contrast, stimulation of CA3, dentate (includes CA4) and the subiculum produced significant decreases in plasma corticosterone levels. No change in corticosterone levels was observed following sham stimulation. Collectively, these data indicate that consideration must be given to the possibility that differential neuroendocrine regulatory mechanisms reside within various limbic forebrain complexes and that electrical stimulation of limbic forebrain sites of urethane anesthetized rats may provide information regarding sites inhibitory to pituitary-adrenal activity.     

Prevention of adrenalectomy-induced brain growth stimulation by corticosterone treatment

Previous research in our laboratory has shown that adrenalectomizing rats on day 11 postnatal leads to subsequent increases in brain weight, cellularity (measured in terms of DNA content), and myelination. The present study was designed to determine whether these effects are due to removal of circulating glucocorticoids. Male and female albino rats were adrenalectomized (ADX) on day 11 and then injected daily with either 7.0 mg/kg corticosterone in a steroid suspension vehicle or vehicle alone. A third group of animals was sham-operated (SHAM) and given daily vehicle injections. Subjects were sacrificed at 60 days of age for measurement of brain and body weights, cerebral DNA content, and the activity of 2',3'-cyclic nucleotide phosphodiesterase (CNP), a myelin marker enzyme. As expected from our earlier findings, brain weights and DNA and CNP levels were all significantly elevated in ADX-untreated rats compared to SHAMs. More importantly, all of these changes were completely prevented by chronic corticosterone administration. These results are consistent with previous reports that corticosterone treatment blocks increased brain growth in ADX weanling rats and support our hypothesis that the brain growth stimulating effects of day-11 adrenalectomy are likewise mediated by the removal of glucocorticoids. It appears that glucocorticoid hormones exert a tonic inhibition of at least some growth related processes in the brains of intact developing organisms.     

Sensitization of the plasma corticosterone response to novel environments

In Experiment 1, ICR Swiss mice were exposed to one of two novel cages for 1, 6, or 10 30-min trials. Plasma corticosterone elevations were greater in the more novel than in the less novel cage and following the sixth as compared to the first and tenth exposures. In Experiment 2, BALB/c and C57BL/6J mice and Sprague-Dawley rats also exhibited significant, though modest, increases in corticosterone levels from their first to their sixth 30-min exposures to the more novel cage. These findings demonstrate generalization of an apparent sensitization of the plasma corticosterone response across the two levels of novelty and four strains of laboratory rodents tested here. The transience of the effect may explain some earlier inconsistencies in the literature. In Experiment 3, five daily 30-min exposures to the more novel environment minimally raised resting corticosterone levels of ICR Swiss mice. Further, corticosterone levels during a first exposure were no greater at 15 or 45 min than at 30 min. These results suggest that the increase in corticosterone with repeated exposures reflects a true sensitization of the corticosterone response rather than an increase in resting levels with repeated exposures or a shift in the time course of the corticosterone response.     

Changes in corticosterone levels in iron deficient rats

The hypothesis that behavioural changes in iron deficiency are a product of stress was tested by monitoring corticosterone levels. Adult hooded rats were placed on an iron deficient diet for 2 (Stage 1), 8 (Stage 2) or 12 (Stage 3) weeks. When trained and tested on a single-trial taste aversion task, iron deficient animals showed the same performance as seen in previous experiments. Poor performance in early iron deficiency were followed by a temporary return to normal levels as the deficiency progressed, and falling below normal with continued iron depletion. Iron deficient animals failed to demonstrate the predicted stress-related changes in cortocosterone levels. Instead of an increasing in corticosterone levels after conditioning, levels fell to well below normal at all stages of deficiency following conditioning. Iron deficiency probably limits the action of certain enzymes responsible for corticosterone production. Due to the similarity of changes in catecholamines in stress and iron deficiency, it is possible that these changes will explain the pattern of behavioural response in iron depletion.     

Stress-induced pressor and corticosterone responses in oxytocin-deficient mice

We used oxytocin knockout (OTKO) mice to investigate the role of oxytocin in regulation of blood pressure, heart rate and stress reactivity (pressure reactivity and plasma corticosterone). Male OTKO and control wild-type mice with carotid arterial catheters were exposed to intermittent shaker stress for 7 days (2 min stressors, 45 times per day). Mean arterial pressure (MAP) and heart rate (HR) were recorded continuously (24 h) before stress (basal), on stress days 1, 3 and 7 (S1, S3 and S7) and 1 day poststress (recovery). Plasma corticosterone (Cort) was measured before stress and 30 min after the last stress on day 7. Twenty-four hour averages of MAP and HR were lower in OTKO mice than in controls (P < 0.0001 and P < 0.005, respectively) with a significant diurnal rhythm. Chronic stress (S1 and S3) produced an increase in 24 h average MAP in OTKO mice, but not in controls. There were no stress-related changes in 24 h average HR values between control and OTKO mice. The immediate pressor responses were analysed during the dark and light periods (19.00 and 08.00 h). During the dark period, stress-induced pressor responses were observed only in OTKO mice (S1 and S3). In the light period, stress-induced MAP increases were seen on all days in OTKO mice and on days S1 and S3 in controls. There were no differences in baseline Cort between the groups; however, OTKO mice showed a reduced response to chronic stress (+298 versus+411%, OTKO mice versus controls, P < 0.005). In conclusion, oxytocin deficiency alters the endocrine and pressor responses to chronic stress, suggesting that the endogenous oxytocin system is important in regulating the stress-induced pressor response.     

Thymocyte apoptosis induced by elevated endogenous corticosterone levels

A well-known model of apoptosis is induction in thymocytes by injection of pharmacological doses of exogenous steroids. The aim of this study was to investigate whether this process also occurs under physiological conditions, i.e. by stimulation of endogenous glucocorticoid release, using the chicken as an experimental model. Endogenous glucocorticoid levels can be elevated by immunization with exogenous antigens or by injection of conditioned medium, e.g. supernatant of mitogen-stimulated spleen cells. This effect is mediated by so-called glucocorticoid-increasing factors, and is considered to act as an immunoregulatory principle. Thymocyte DNA of so treated birds showed a typical “ladder” pattern after electrophoresis in a 1.8% agarose gel, and degradation could be prevented by RU 38486. This provides evidence that apoptosis can be induced by elevating endogenous corticosterone levels in vivo. By means of in situ nick translation (ISNT) and simultaneous immunofluorescence tests, it was possible to analyze various thymic subpopulations during apoptosis after treatment with exogenous glucocorticoids. Additionally, using confocal microscopical techniques, apoptosis of the same cells as analyzed by ISNT is shown. The possible role of elevated concentrations of endogenous glucocorticoids in regulating thymocyte cell death and autoimmune diseases is discussed.     

ACTH-dependent corticosterone secretion inhibited by the antioxidant indol

Research Institute of General Pathology and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Central Research Laboratory, Irkutsk Medical Institute. Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 107, No. 1, pp. 42–43, January, 1989.     

Absence of effects of corticosterone given at 22 days.

Mice given a single large dose of corticosterone at 2 days of age, and tested when adult, have shown lasting alterations in operant behavior, associated with reductions in cerebral weight and DNA content. When mice were given corticosterone at 22 days, no changes appeared in cerebral weight, DNA, or in operant behavior, but an apparent decrease did occur in open-field activity. The results suggest that the effects of corticosterone treatment at 2 days on the developing brain are mediated by one or more types of sensitivity to the steroid that have been largely lost by 22 days.