The pituitary-adrenal response to novel stimulation and ether stress in young adult and aged rats

The role of melatonin deficiency induced by pinealectomy on motor activity and anxiety responses in young adult, middle-aged and old rats

BACKGROUND

Aging affects anxiety levels in rats while the pineal gland, via its hormone melatonin, could modulate their inherited life "clock." The present study aimed to explore the impact of plasma melatonin deficiency on anxiety responses and the possible involvement of the hypothalamic-pituitary-adrenocortical (HPA) axis and heat shock proteins (Hsp) 70 and 90 in the frontal cortex (FC) and the hippocampus in young adult, middle-aged and elderly rats with pinealectomy.

RESULTS

Melatonin deficiency induced at different life stages did not affect the lifespan of rats. Pinealectomy abolished the circadian rhythm of motor activity, measured for 48 h in the actimeter, in young adult but not in middle-aged rats. Pinealectomy reduced the motor activity of the young adult rats during the dark phase and impaired the diurnal activity variations of old rats. The same generations (3- and 18 month-old rats with pinealectomy) had lower anxiety levels than the matched sham groups, measured in three tests: elevated-plus maze, light-dark test, and novelty-suppressed feeding test. While the activity of the HPA axis remained intact in young adult and middle-aged rats with melatonin deficiency, a high baseline corticosterone level and blunted stress-induced mechanism of its release were detected in the oldest rats. Age-associated reduced Hsp 70 and 90 levels in the FC but not in the hippocampus were detected. Pinealectomy diminished the expression of Hsp 70 in the FC of middle-aged rats compared to the matched sham rats.

CONCLUSIONS

Our results suggest that while melatonin hormonal dysfunction impaired the motor activity in the actimeter and emotional behavior in young adult and elderly rats, the underlying pathogenic mechanism in these generations might be different and needs further verification.

Maternal deprivation during early infancy in rats increases oxytocin immunoreactivity in females and corticosterone reactivity to a social test in both sexes without changing emotional behaviour

Impairment of social behaviour is a hallmark of emotional disorders, with increased avoidance of social contact. In rats, the 24 h maternal deprivation (DEP) paradigm is used to understand the impact of extreme neglect on neurodevelopment. Due to the distinct immediate effects of DEP on postnatal days (PND) 3 (DEP3) or 11 (DEP11), in the present study we investigated the long-term effects of DEP at these ages on anxiety-like behaviour, by recording the visits and time spent in the centre part of the open-field, social investigation of a confined, same-sex, unfamiliar animal, basal and post-social test corticosterone plasma levels and the immunoreactivity to oxytocin in the paraventricular (PVN) and supraoptic nuclei of the hypothalamus (SON). Whole litters were distributed into control (CTL), DEP3 or DEP11 groups and behavioural tests and biological samples were collected between PNDs 40 and 45 in males and females. There were no differences in the exploration of the central part of the open field or on the time investigating the unfamiliar rat. However, the percent increase in post-test corticosterone secretion from baseline was greater for both DEP3 male and female subgroups than their CTL and DEP11 counterparts. DEP3 females showed more oxytocin staining than DEP11 counterparts in magnocellular neurons of the SON and PVN. These results suggest that DEP at the ages chosen does not alter social investigation, although it results in distinct neurobiological outcomes, depending on the developmental phase when it is imposed.

Age-related changes in endogenous glucocorticoids, gonadal steroids, endocannabinoids and their ratios in plasma and hair from the male C57BL/6 mice

Age-related level changes of hormones, endocannabinoids and their ratios are of pathophysiological significance for understanding functions, activities and interactions of the endocrine systems, including the hypothalamic-pituitaryadrenal (HPA), hypothalamic-pituitary-gonadal (HPG) axes and endogenous cannabinoid system (ECS). The present study aimed to investigate the age-dependent fluctuations of glucocorticoids, gonadal steroids, endocannabinoids and their ratios from 21 days to 10 months in both plasma and hair from the male C57BL/6 mice. A novel framework based on the liquid chromatography-tandem mass spectrometry was developed to simultaneously determine ten hormones and two endocannabinoids in plasma and hair. Results showed that glucocorticoids, corticosterone (CORT), aldosterone (ALD), 11-dehydrocorticosterone (11-DHC), gonadal steroids, progesterone (P), dehydroepiandrosterone (DHEA), testosterone (T) and dihydrotestosterone (DHT) in plasma were unimodally fluctuated (ps < 0.001) along age with the maximum value at 2.7-month-old. In contrast, the other two gonadal steroids, estrone (E1) and estradiol (E2) were declined with age (ps < 0.001). Differently, endocannabinoids, N-arachidonoyl-ethanolamine (AEA) and 1-arachydonoyl glycerol (1-AG) showed nadir and zenith values at 2.7-month-old and 3.4-month-old, respectively (ps < 0.001). Additionally, the ratios of CORT to 11-DHC and ALD in plasma were dropped similarly with age (ps < 0.001). The ratios of 1-AG to AEA, and of T to A4 and DHT, and of DHEA to A4 were unimodally changed (ps < 0.001) along age with maximum value at 2.7- or 3.4-month-old. In contrast, the ratios of E2 to T and E1 to A4 were decreased with age (ps < 0.05). The rest six ratios that reflected the interactions among the three endocrine systems, were similar age-dependent and showed nadir and zenith values at 2.7-month-old and 3.4-month-old, respectively (ps < 0.05). Most importantly, these findings in light of age-related changing patterns in plasma were repeated in hair, suggesting that the fi41-ndings in the two matrices were mutually validated. However, it was worth noting that their magnitude of levels in the two bio-matrices were markedly different. The current findings could provide reliable hormone and endocannabinoid signatures with age on neuroendocrine profiles as well as their ratios for the male C57BL/6 mice.

Neurobiological and Hormonal Mechanisms Regulating Women's Sleep

Sleep is crucial for optimal well-being, and sex differences in sleep quality have significant implications for women's health. We review the current literature on sex differences in sleep, such as differences in objective and subjective sleep measures and their relationship with aging. We then discuss the convincing evidence for the role of ovarian hormones in regulating female sleep, and survey how these hormones act on a multitude of brain regions and neurochemicals to impact sleep. Lastly, we identify several important areas in need of future research to narrow the knowledge gap and improve the health of women and other understudied populations.

Sex-dependent programming effects of prenatal glucocorticoid treatment on the developing serotonin system and stress-related behaviors in adulthood

Prenatal stress and overexposure to glucocorticoids (GC) during development may be associated with an increased susceptibility to a number of diseases in adulthood including neuropsychiatric disorders, such as depression and anxiety. In animal models, prenatal overexposure to GC results in hyper-responsiveness to stress in adulthood, and females appear to be more susceptible than males. Here, we tested the hypothesis that overexposure to GC during fetal development has sex-specific programming effects on the brain, resulting in altered behaviors in adulthood. We examined the effects of dexamethasone (DEX; a synthetic GC) during prenatal life on stress-related behaviors in adulthood and on the tryptophan hydroxylase-2 (TpH2) gene expression in the adult dorsal raphe nucleus (DRN). TpH2 is the rate-limiting enzyme for serotonin (5-HT) synthesis and has been implicated in the etiology of human affective disorders. Timed-pregnant rats were treated with DEX from gestational days 18-22. Male and female offspring were sacrificed on the day of birth (postnatal day 0; P0), P7, and in adulthood (P80-84) and brains were examined for changes in TpH2 mRNA expression. Adult animals were also tested for anxiety- and depressive- like behaviors. In adulthood, prenatal DEX increased anxiety- and depressive- like behaviors selectively in females, as measured by decreased time spent in the center of the open field and increased time spent immobile in the forced swim test, respectively. Prenatal DEX increased TpH2 mRNA selectively in the female caudal DRN at P7, whereas it decreased TpH2 mRNA selectively in the female caudal DRN in adulthood. In animals challenged with restraint stress in adulthood, TpH2 mRNA was significantly lower in rostral DRN of prenatal DEX-treated females compared to vehicle-treated females. These data demonstrated that prenatal overexposure to GC alters the development of TpH2 gene expression and these alterations correlated with lasting behavioral changes found in adult female offspring.

Novel environment influences the effect of paradoxical sleep deprivation upon brain and peripheral cytokine gene expression

Sleep loss increases inflammatory mediators in brain and peripheral tissues, but the mechanisms underlying this association are not fully understood. Male C57BL/6j mice were exposed to paradoxical sleep deprivation (PSD) for 24h using the modified multiple platform (MMP) technique (platforms over water) or two different controls: home cage or a dry platform cage, which constituted a novel environment. PSD mice exhibited increased IL-1β and TNF-α pro-inflammatory gene expression in brain (hypothalamus, hippocampus, pre-frontal cortex), as well as in peripheral tissues (liver, spleen), when compared with home-cage controls. In addition, among PSD mice, TGFβ1, an anti-inflammatory cytokine, was increased in pre-frontal cortex, liver, and spleen in conjunction with elevated serum corticosterone concentration relative to home-cage controls. However, these differences were nearly abolished when PSD mice were compared with control mice subjected to a dry MMP cage, suggesting that simply exposing mice to a novel environment can induce an acute inflammatory response.

The prudent parent meets old age: a high stress response in very old seabirds supports the terminal restraint hypothesis

The reproductive success of wild animals usually increases with age before declining at the end of life, but the proximate mechanisms underlying those patterns remain elusive. Young animals are expected to invest less in current reproduction due to high prospects for future reproduction (the "restraint" hypothesis). The oldest animals may also show restraint when conditions are sub-optimal where even a small increase in reproductive investment may lead to death ("terminal restraint"). Alternatively, reproduction may be constrained by lack of experience and senescence (the "constraint" hypothesis). In two species of breeding seabirds, behavioural (time to return the offspring, calmness during restraint) and physiological (metabolism, glucose and corticosterone) parameters responded similarly to stress with advancing age, implying a generalized stress response. Across those parameters, birds were "shy" (high stress response) when young or old, and "bold" (low stress response) when middle-aged. Specifically, free corticosterone, the principal avian glucocorticoid responsible for directing energy away from reproduction and towards immediate survival following stress, was highest in both young and very old stressed birds. All age groups had a similar adrenal capacity to produce corticosterone, implying that middle-aged birds were showing restraint. Because the stress response, was highest at ages when the probability of current reproduction was lowest rather than at ages when the probability of future reproduction was highest we concluded that birds restrained reproductive investment based on current conditions rather than potential future opportunities. In particular, old birds showed terminal restraint when stressed. Hormonal cues promoted investment in adult survival over reproductive output at both the start and end of life consistent with the restraint hypothesis.

Effects of aging on hypothalamic-pituitary-adrenal (HPA) axis activity and reactivity in virgin male and female California mice (Peromyscus californicus)

Life history theory posits that organisms face a trade-off between current and future reproductive attempts. The physiological mechanisms mediating such trade-offs are still largely unknown, but glucocorticoid hormones are likely candidates as elevated, post-stress glucocorticoid levels have been shown to suppress both reproductive physiology and reproductive behavior. Aged individuals have a decreasing window in which to reproduce, and are thus predicted to invest more heavily in current as opposed to future reproduction. Therefore, if glucocorticoids are important in mediating the trade-off between current and future reproduction, aged animals are expected to show decreased hypothalamic-pituitary-adrenal (HPA) axis responses to stressors and to stimulation by corticotropin-releasing hormone (CRH), and enhanced responses to glucocorticoid negative feedback, as compared to younger animals. We tested this hypothesis in the monogamous, biparental California mouse by comparing baseline and post-stress corticosterone levels, as well as corticosterone responses to dexamethasone (DEX) and CRH injections, between old (∼18-20months) and young (∼4months) virgin adults of both sexes. We also measured gonadal and uterine masses as a proxy for investment in potential current reproductive effort. Adrenal glands were weighed to determine if older animal had decreased adrenal mass. Old male mice had lower plasma corticosterone levels 8h after DEX injection than did young male mice, suggesting that the anterior pituitary of older males is more sensitive to DEX-induced negative feedback. Old female mice had higher body-mass-corrected uterine mass than did young females. No other differences in corticosterone levels or organ masses were found between age groups within either sex. In conclusion, we did not find strong evidence for age-related change in HPA activity or reactivity in virgin adult male or female California mice; however, future studies investigating HPA activity and reproductive outcomes in young and old breeding adults would be illuminating.

Age-related differences in baseline and stress-induced corticosterone in Florida scrub-jays

In physiological studies of free-living species, it is essential to consider the context of the life history stage at which an individual was observed in order to link measures of physiology with ecological parameters. One such measure that is important to consider is the age of an individual. We tested whether baseline or stress-induced corticosterone levels vary with age in free-living Florida scrub-jays (Aphelocoma coerulescens) during the pre-breeding period. Corticosterone (CORT), the primary avian stress hormone, is released in response to stressful stimuli, and stimulates gluconeogenesis; however, it also serves as a chemical messenger that can influence other physiological processes, reproduction, and behavior. We monitored both baseline CORT levels longitudinally throughout a five-year period and stress-induced CORT responses over a shorter two-year period. We predicted that older jays would have lower baseline CORT levels and a dampened stress response compared to younger birds, as has been shown in other avian species. We found no significant differences in baseline CORT levels with age. We found a decrease in total corticosterone responses to a stressor with age, however, the oldest birds in the population showed greater total corticosterone responses to a stressor. These results may be a product of age-related changes in physiological processes related to the stress response or a result of selection acting on the population, resulting in only the most responsive individuals surviving to old age.

3xTgAD mice exhibit altered behavior and elevated Aβ after chronic mild social stress

Chronic stress may be a risk factor for developing Alzheimer's disease (AD), but most studies of the effects of stress in models of AD utilize acute adverse stressors of questionable clinical relevance. The goal of this work was to determine how chronic psychosocial stress affects behavioral and pathological outcomes in an animal model of AD, and to elucidate underlying mechanisms. A triple-transgenic mouse model of AD (3xTgAD mice) and nontransgenic control mice were used to test for an affect of chronic mild social stress on blood glucose, plasma glucocorticoids, plasma insulin, anxiety, and hippocampal amyloid β-particle (Aβ), phosphorylated tau (ptau), and brain-derived neurotrophic factor (BDNF) levels. Despite the fact that both control and 3xTgAD mice experienced rises in corticosterone during episodes of mild social stress, at the end of the 6-week stress period 3xTgAD mice displayed increased anxiety, elevated levels of Aβ oligomers and intraneuronal Aβ, and decreased brain-derived neurotrophic factor levels, whereas control mice did not. Findings suggest 3xTgAD mice are more vulnerable than control mice to chronic psychosocial stress, and that such chronic stress exacerbates Aβ accumulation and impairs neurotrophic signaling.

REM sleep loss increases brain excitability: role of noradrenaline and its mechanism of action

Ever since the discovery of rapid eye movement sleep (REMS), studies have been undertaken to understand its necessity, function and mechanism of action on normal physiological processes as well as in pathological conditions. In this review, first, we briefly surveyed the literature which led us to hypothesise REMS maintains brain excitability. Thereafter, we present evidence from in vivo and in vitro studies tracing behavioural to cellular to molecular pathways showing REMS deprivation (REMSD) increases noradrenaline level in the brain, which stimulates neuronal Na-K ATPase, the key factor for maintaining neuronal excitability, the fundamental property of a neuron for executing brain functions; we also show for the first time the role of glia in maintaining ionic homeostasis in the brain. As REMSD exerts a global effect on most of the physiological processes regulated by the brain, we propose that REMS possibly serves a housekeeping function in the brain. Finally, subject to confirmation from clinical studies, based on the results reviewed here, it is being proposed that the subjects suffering from REMS loss may be effectively treated by reducing either noradrenaline level or Na-K ATPase activity in the brain.

Physiological and hormonal responses to novelty exposure in rats are mainly related to ongoing behavioral activity

Stress research has been dominated by a circular type of reasoning that occurrence of a stress response is bad. Consequently, the stimulus is often interpreted as stressful in terms of aversiveness involving uncontrollability and unpredictability, which may have maladaptive and pathological consequences. However, the hypothalamic-pituitary-adrenal (HPA) axis and sympathico-adrenomedullary (SAM) system are not only activated in response of the organism to challenges, but also prepare and support the body for behavior. Therefore, a considerable part of the physiological and hormonal responses to a certain situation can be a direct reflection of the metabolic requirements for the normal ongoing behavioral activity, rather than of the stressful nature. In order to clarify this, behavioral, physiological, hormonal and electroencephalographic (EEG) responses to novel cage exposure were studied in male Sprague-Dawley rats. Forced confrontation with a novel cage has been interpreted as a psychological and aversive stressor. However, this interpretation is simply based on the occurrence of a stress response. This study aimed at detailed analysis of the time course of the novelty-induced responses. Different parameters were measured simultaneously in freely moving rats, which allowed correlational comparisons. Hereto, radio telemetry using a small implantable transmitter combined with permanent catheters and an automated blood sampling system was used. A camera placed above the cage allowed behavioral observations. The results show that novelty exposure induced significant increases in locomotor activity, heart rate, blood pressure and plasma corticosterone together with a complete lack of sleep as compared to the undisturbed control situation. The latency to reach significance and the duration of responses varied across parameters but all had recovered within 30min after termination of novelty. The behavioral activity (locomotor activity and EEG wakefulness duration) response pattern was significantly correlated with that of heart rate, blood pressure and plasma corticosterone. Behavioral observations showed mainly explorative behavior in response to novelty. Therefore, the present results indicate that the novelty-induced physiological and hormonal responses are closely related to the ongoing, mainly explorative behavioral activity induced by novelty. An interpretation in terms of metabolic support of ongoing behavior seems to be more appropriate than the frequently used stress interpretation. The present study also emphasizes the added value of simultaneous assessment of behavioral, physiological and hormonal parameters under controlled, non-confounding conditions.

Physiological response of wild guanacos to capture for live shearing

Context The use of wild guanacos (Lama guanicoe) through live capture and shearing may contribute to their conservation by providing an economic alternative to rural inhabitants. However, none of the biological impacts of this activity, including the physiological ones, have been addressed. Aims The aim of this work was to characterise the acute response of guanacos to stress after capture and shearing in terms of serum cortisol levels, heart rate, and body temperature. Methods The study was performed during 2006 and 2007 in La Payunia Reserve in western Argentina. In order to determine serum cortisol concentration, 128 blood samples were obtained and the unextracted sera were analysed by radioimmunoanalyses (RIA). Sex, age category, heart rate, body temperature and total restraint time (TRT) were also registered for each animal captured. Key results Serum cortisol levels were higher in guanacos captured and sheared during 2007 than in 2006, and male cortisol levels were consistently lower than female levels. No significant differences were observed in cortisol levels of the different age categories. A positive correlation was observed between TRT and serum cortisol concentration. The analyses of cortisol levels in relation to TRT showed differences between males and females. With handling periods longer than 80 minutes, females showed a delayed stabilisation in cortisol response when compared with males. Heart rate and body temperature showed no differences between year, sex or age categories. Conclusions The present results show that the stress response to capture and shearing in wild guanacos increased significantly with handling time. We recommend avoiding capture of large numbers of animals and keeping roundup duration short to reduce TRT. Implications This work provides new information that can improve guanaco welfare during handling and shearing and may have implications for the conservation of the species.

Noradrenergic neurons expressing Fos during waking and paradoxical sleep deprivation in the rat

Noradrenaline is known to induce waking (W) and to inhibit paradoxical sleep (PS or REM). Both roles have been exclusively attributed to the noradrenergic neurons of the locus coeruleus (LC, A6), shown to be active during W and inactive during PS. However, the A1, A2, A5 and A7 noradrenergic neurons could also be responsible. Therefore, to determine the contribution of each of the noradrenergic groups in W and in PS inhibition, rats were maintained in continuous W for 3h in a novel environment or specifically deprived of PS for 3 days, with some of them allowed to recover from this deprivation. A double immunohistochemical labeling with Fos and tyrosine hydroxylase was then performed. Thirty percent of the LC noradrenergic cells were found to be Fos-positive after exposure to the novel environment and less than 2% after PS deprivation. In contrast, a significant number of double-labeled neurons (up to 40% of the noradrenergic neurons) were observed in the A1/C1, A2 and A5 groups, after both novel environment and PS deprivation. After PS recovery and in control condition, less than 1% of the noradrenergic neurons were Fos-immunoreactive, regardless of the noradrenergic group. These results indicate that the brainstem noradrenergic cell groups are activated during W and silent during PS. They further suggest that the inhibitory effect of noradrenaline on PS may be due to the A1/C1, A2 and to a lesser degree to A5 neurons but not from those of the LC as previously hypothesized.

Early brain development disruption from NMDA receptor hypofunction: relevance to schizophrenia

Disruption to brain development at an early stage can potentially alter chemically coded neural networks and can affect behavior in later life. During early brain development antagonism of glutamate NMDA receptors, which play an important role in neuronal outgrowth and survival, leads to neuronal damage in several brain regions and causes behavioral alterations in rodents that mimic schizophrenia symptoms and endophenotypes. There are several lines of evidence implicating involvement of a dysfunctional glutamate system in schizophrenia. In normal subjects, NMDA receptor antagonists produce behavioral and neurochemical changes that mimic schizophrenia symptoms better than any other psychotomimetic drug. Moreover, these drugs worsen symptoms in schizophrenia patients and can trigger a recrudescence of the acute psychotic state in patients with stable chronic schizophrenia. In addition, genes consistently reported as being altered in schizophrenia play roles in development, neuroplasticity and glutamate/GABAergic neurotransmission. Perinatal NMDA receptor antagonist treatment is a useful model for studying the neurodevelopmental and NMDA receptor hypofunction hypotheses of schizophrenia because neurochemical and behavioral changes, reminiscent of those seen in schizophrenia, are present long after cessation of drug administration, which suggests that a permanent change in brain structure and organization has occurred during brain development.

Testosterone suppression of CRH-stimulated cortisol in men

Despite observations of age-dependent sexual dimorphisms in hypothalamic-pituitary-adrenal (HPA) axis activity, the role of androgens in the regulation of HPA axis activity in men has not been examined. We assessed this role by performing CRH stimulation tests in 10 men (ages 18-45 years) during gonadal suppression with leuprolide acetate and during testosterone addition to leuprolide. CRH-stimulated cortisol levels as well as peak cortisol and greatest cortisol excursion were significantly lower (p<0.05, 0.005, and 0.01, respectively) during testosterone replacement compared with the induced hypogonadal condition (leuprolide plus placebo); cortisol area under the curve was lower at a trend level (p<0.1). Paradoxically, CRH-stimulated corticotropin (ACTH) was increased significantly during testosterone replacement (p<0.05). The cortisol : ACTH ratio, a measure of adrenal sensitivity, was lower during testosterone replacement (p<0.1). A mixed effects regression model showed that testosterone but not estradiol or CBG significantly contributed to the variance of cortisol. These data demonstrate that testosterone regulates CRH-stimulated HPA axis activity in men, with the divergent effects on ACTH and cortisol suggesting a peripheral (adrenal) locus for the suppressive effects on cortisol. Our results further demonstrate that the enhanced stimulated HPA axis activity previously described in young men compared with young women cannot be ascribed to an activational upregulation of the axis by testosterone.

Selective hippocampal lesions do not increase adrenocortical activity

It has been proposed that the hippocampus exerts a tonic inhibitory influence on the hypothalamic-pituitary-adrenal (HPA) stress axis. This claim rests, in particular, on the upregulation of corticosterone secretion and other measures of HPA activity after nonselective lesions of the hippocampal formation. We measured plasma corticosterone concentrations after selective neurotoxic damage to the hippocampus and the subiculum in rats. Concentrations were estimated during rest in the rat's home cage and at several time points after varying degrees of stress. Lesions of the hippocampus did not increase the concentration of corticosterone relative to control rats in any condition. Temporary inactivation of the hippocampus or the ventral subiculum by infusion of the GABAA receptor agonist muscimol also failed to induce hypersecretion, although hippocampal infusions did impair spatial memory. These results suggest that the hippocampus is not necessary for tonic inhibition of adrenocortical activity and imply that the HPA axis receives efficient negative feedback inhibition from other brain systems too.

Maternal regulation of the hypothalamic-pituitary-adrenal axis in the 20-day-old rat: consequences of laboratory weaning

There is a large body of evidence that the development of the hypothalamic-pituitary-adrenal (HPA) system in the rat is under maternal regulation. One method used to study the influence of the dam-pup interaction in neonates and weanlings is the separation of mother and litter for 24 h. Previous studies showed that, even at the time of weaning, maternal deprivation results in a dysregulation of the HPA axis at multiple levels. However, the maternal deprivation paradigm usually includes deprivation of food and water, and it was not clear to which extent the observed effects are due to either maternal cues or dehydration and fasting. The primary purpose of the present study was to determine the role of fasting and/or maternal separation on the HPA axis at the time of weaning. Pups at 20 days after parturition are capable of self-feeding and no longer require tactile stimulation to induce eliminative functions. The results indicated that 24 h of fasting led to increased basal levels and further increases in stress induced corticosterone secretion. Fasting also appeared to contribute to the down regulation of basal glucocorticoid receptor mRNA in the hippocampus. In contrast, abrupt weaning irrespective of fasting or dehydration resulted in a suppressed adrenocorticotropin hormone response to an injection of isotonic saline. Although there was an effect of maternal separation on corticotropin-releasing factor mRNA in the paraventricular nucleus, this effect was further exacerbated by the absence of food. Finally, all rats that were separated from their dams showed more efficient negative-feedback. Thus, different aspects of the HPA system appear to respond differentially to either the absence of food or the absence of the mother or both.

Glucocorticoids and 11 beta-hydroxysteroid dehydrogenase type 2 gene expression in the aging kidney

BACKGROUND

Aging is associated with increased concentrations of circulating glucocorticoids, a situation expected to induce a glucocorticoid-mediated mineralocorticoid effect, resulting in sodium retention and hypertension unless counteracting mechanisms are operative. Conversion of glucocorticoids to inert 11 beta-keto compounds by the enzyme 11 beta-hydroxysteroid dehydrogenase type 2 (11 beta-HSD2) is one of these mechanisms. We hypothesized therefore that 11 beta-HSD2 gene expression and/or activity increase with age in male WAG/Rij rats, a strain without increased blood pressure with age or senescence-related obesity or kidney disease.

MATERIALS AND METHODS

Corticosterone (B) concentrations in plasma and urinary excretion of corticosterone and dehydrocorticosterone (A) tetrahydro metabolites, THB + 5 alpha-THB + THA, were assessed by gas chromatography-mass spectrometry (GC-MS) in 10-month-old-rats (n = 6) and in 30-month-old rats (n = 6). Renal 11 beta-HSD2 messenger ribonucleic acid (mRNA) abundance was measured by real-time quantitative TaqMan polymerase chain reaction and microarray assays.

RESULTS

Thirty-month-old rats had significantly higher corticosterone concentrations in plasma and increased urinary excretion of corticosterone and dehydrocorticosterone tetrahydro metabolites. Conversion of B to A in kidney microsomes from 30-month-old rats was moderately but not significantly increased compared with 10-month-old rats. The urinary ratios of (THB + 5 alpha-THB)/THA and free B/A and renal 11 beta-HSD2 mRNA abundance were equal in 10- and 30-month-old rats.

CONCLUSIONS

There is no evidence for an enhanced gene expression or activity of renal 11 beta-HSD2 in these aging rats, suggesting either that endogenous 11 beta-HSD2 is able to cope with the increased corticosterone concentrations characteristic of the aging process or that alternative mechanisms contribute to the maintenance of a normal sodium excretion in these animals.

Functional sex differences ('sexual diergism') of central nervous system cholinergic systems, vasopressin, and hypothalamic-pituitary-adrenal axis activity in mammals: a selective review

Sexual dimorphism of the mammalian central nervous system (CNS) has been widely documented. Morphological sex differences in brain areas underlie sex differences in function. To distinguish sex differences in physiological function from underlying sexual dimorphisms, we use the term, sexual diergism, to encompass differences in function between males and females. Whereas the influence of sex hormones on CNS morphological characteristics and function of the hypothalamic-pituitary-gonadal axis has been well-documented, little is known about sexual diergism of CNS control of the hypothalamic-pituitary-adrenal (HPA) axis. Many studies have been conducted on both men and women but have not reported comparisons between them, and many animal studies have used males or females, but not both. From a diergic standpoint, the CNS cholinergic system appears to be more responsive to stress and other stimuli in female than in male mammals; but from a dimorphic standpoint, it is anatomically larger, higher in cell density, and more stable with age in males than in females. Dimorphism often produces diergism, but age, hormones, environment and genetics contribute differentially. This review focuses on the sexual diergism of CNS cholinergic and vasopressinergic systems and their relationship to the HPA axis, with resulting implications for the study of behavior, disease, and therapeutics.

Age-related differences in corticosterone secretion in female rats

The effects of age on steroidogenesis in rat zona fasciculata-reticularis (ZFR) cells were studied. Young, adult, and middle-aged rats were ovariectomized (Ovx) and received replacement therapy with oil or estradiol benzoate ([EB] 25 microg/mL/kg). Rat ZFR cells were incubated with corticotropin (ACTH), prolactin (PRL), or forskolin at 37 degrees C for 1 hour. The effects of age on the activity of steroidogenic enzymes of ZFR cells were measured by the amount of intermediate steroidal products separated by thin-layer chromatography. Plasma levels were higher for PRL (54% to 254%) and corticosterone (179% to 257%) in middle-aged versus young rats. In oil-treated Ovx rats, basal and ACTH-stimulated corticosterone release by ZFR cells were also greater in middle-aged compared with young rats. Replacement with EB in Ovx rats increased the ACTH-stimulated release of corticosterone. Administration of ovine PRL in vitro resulted in a dose-dependent increase of corticosterone production. In oil-treated middle-aged rats, ovine PRL-stimulated corticosterone release was higher than in young rats. Forskolin-induced production of cyclic adenosine 3',5'-monophosphate (cAMP) was greater in middle-aged versus young rats and correlated with the increase of corticosterone production. The activity of steroidogenic enzymes in rat ZFR cells was unchanged by age. These results suggest that the age-related increase of corticosterone production in female rats is associated with the stimulatory effect of PRL on ZFR cells and is due in part to an increase of cAMP generation.

Locomotion elicited by MK801 in developing and adult rats: temporal, environmental, and gender effects

The effects of environmental novelty on locomotion elicited by an N-methyl-D-aspartate (NMDA) receptor antagonist, (+)MK-801 hydrogen maleate [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine], were investigated. Male and female rats aged 10, 20, 30 or 54-68 days were injected s.c. with MK801 and placed in activity monitors either immediately (no-delay) or after a 60 min delay (delay). In the no-delay condition, MK801 induced an inverse U-shaped dose-response effect on locomotion; peak activation occurred with 0.1 mg/kg and ataxia occurred with higher doses. The introduction of a novel environment 60 min after drug injection shifted the dose-effect function of MK801 to the left; i.e., in rats 20 days of age and older, the activity induced by 0.1 mg/kg MK801 was potentiated in the delay condition. For the 0.5 mg/kg dose, 20-day-olds showed activation in the no-delay condition but ataxia in the delay condition. This dose induced ataxia followed by activation in 30-day-olds and adult males or ataxia in adult females, regardless of delay condition. Age-, gender-, and novelty-dependent variations in MK801-induced locomotion may reflect differences in limbic-motor circuitry.

Response to surgical stress in elderly patients and Alzheimer's disease

PURPOSE

To determine the effect of surgical stress on plasma epinephrine, norepinephrine, ACTH and cortisol concentrations in patients aged 80-99 yr and in patients complicated with Alzheimer's disease.

METHODS

A prospective controlled study was undertaken in 55 undergoing surgical reduction of femur neck fracture in Hirosaki University hospital and Hakodate Watanabe hospital; 18 patients in 80-99 yr (Group 1) and 18 patients in 40-59 yr (Group 2) and 7 patients (Group A) with and 12 patients (Group B) without Alzheimer's dementia (AD) aged 60-79 yr.

RESULTS

The increase in plasma norepinephrine level (274%) of group 1 patients, 15 min after skin incision was higher than that in group 2 (191%) (P < 0.01). Mean plasma cortisol levels (40.4+/-4.7 and 44.1+/-5.2 microg x dl(-1)) of group 1, 15 min after skin incision and 60 min after the end of surgery were significantly higher than the 29.8+/-3.5 and 22.3+/-3.0 microg x dl(-1) of group 2 (P < 0.05). Plasma norepinephrine (1092.9+/-112.0 pg x ml(-1)) and cortisol concentrations (53.4+/-5.8 microg x dl(-1)) in group A were higher than in group B (772.6+/-82.4 pg x ml(-1) and 41.7+/-4.3 microg x dl(-1)) 15 min after skin incision (P < 0.05).

CONCLUSIONS

Plasma norepinephrine and cortisol responses to surgical stress are activated in elderly patients and in patients with Alzheimer's disease.

Glucocorticoid response to forced exercise in laboratory house mice (Mus domesticus)

We examined the time course and sex differences of the glucocorticoid response to forced, moderate-intensity treadmill exercise in outbred laboratory house mice. Mice (n = 64 total) were divided into eight groups, each of four males and four females, which were run on a motorized treadmill at 1.0 km/h for either 0, 2, 5, 10, 15, 25, 40, or 60 min. Serum samples were taken immediately after exercise and corticosterone (CORT) concentration was determined by radioimmunoassay. Resting CORT levels ranged between 11.6 and 29.5 ng/mL for both sexes. CORT levels increased with length of exercise and then exhibited a plateau by 25 min in females and by 40 min in males. Females displayed a significantly more rapid increase in serum CORT levels and attained higher maximal CORT levels than males. Females also had significantly larger adrenal glands, both in absolute terms and relative to body mass.

Differential induction of nerve growth factor and basic fibroblast growth factor mRNA in neonatal and aged rat brain

Stimulation of glucocorticoid or beta-adrenergic receptors (BAR) has been shown to increase nerve growth factor (NGF) biosynthesis in adult rat brain. Little is known about the role of these receptors in the regulation of NGF expression in neonatal and aged brain. We have examined the effect of the synthetic glucocorticoid dexamethasone (DEX) and the BAR agonist clenbuterol (CLE) on the levels of NGF mRNA in neonatal (8 day old), adult (3 month old) and aged (24 month old) rats. By 3 h, DEX (0.5 mg/kg, s.c.) evoked a comparable increase in NGF mRNA in the cerebral cortex and hippocampus in both 8-day and 3-month-old rats. In contrast, CLE (10 mg/kg, i.p.) failed to change NGF mRNA levels in neonatal rats, while increasing (2-3-fold) NGF mRNA levels in the cerebral cortex of adult rats. In 24-month-old rats, both DEX and CLE elicited only a modest increase in NGF mRNA. This increase was, however, anatomically and temporally similar to that observed in adult animals. The weak effect of DEX or CLE was not related to a down-regulation of receptor function because both DEX and CLE were able to elicit a comparable increase in the mRNA levels for basic fibroblast growth factor (FGF2) in neonatal, adult and aged rat brain. Our data demonstrate that induction of NGF expression by neurotransmitter/hormone receptor activation varies throughout life and suggest that pharmacological agents might be useful tools to enhance trophic support in aging.

Selective decline of 5-HT1A receptor binding sites in rat cortex, hippocampus and cholinergic basal forebrain nuclei during aging

The effect of aging on 5-HT1A receptor binding in several forebrain areas associated with the basal forebrain cholinergic system was investigated in rats of 3-, 24- and 30-months-old by receptor autoradiography and biochemical binding assay using [3H]8-OH-DPAT as a ligand. Autoradiographic measurements demonstrated a marked region-specific decline of ligand binding in: (i) regions of the basal forebrain cholinergic cell groups, i.e. the medial septum, diagonal band nuclei and magnocellular nucleus basalis, (ii) the frontal and parietal neocortex and (iii) the dentate gyrus of the hippocampus. No change or only a slight decrease of the 5-HT1A receptor density was found in other areas investigated: the CA1 and CA3 sectors of hippocampus, the cingular and perirhinal cerebral cortex and the lateral septum. The autoradiographic findings were substantiated by the biochemical binding assay, which revealed a comparable loss of 5-HT1A receptor in the hippocampus and neocortex at the age of 30 months. The results clearly show that with increasing age the decrement of 5-HT1A receptor binding in the rat forebrain is remarkably region-selective and particularly affects the cholinergic cell groups that innervate cortex and hippocampus. This phenomenon appears to be especially significant in relation to the neuronal substrates underlying the age-related alterations of mood and cognition.

Responses to novelty stress in female F344 rats: effects of age and d-fenfluramine treatment

To elucidate some of the mechanisms underlying the neuroendocrine and neurochemical changes associated with age in female rats, we administered the serotonin (5-HT) releaser and reuptake inhibitor, d-fenfluramine (d-FEN; 0.0 or 0.6 mg/kg/day, PO) for 30-38 days to young (4 month) and old (21 month) F-344 female rats. Animals were placed into a novel open field (OF) for 20 min before sacrifice. Control animals were sacrificed immediately upon removal from their home cage (HC). Old rats exhibited significantly (p < 0.05) less exploratory behavior and a smaller CORT response to OF than young animals. d-FEN treatment had no effect on plasma ACTH and CORT levels or exploratory behavior. The old HC rats had significantly (p < 0.05) higher plasma levels of prolactin (PRL) than the young HC rats. A stress induced increase in PRL secretion was observed in the old rats only, which was attenuated by d-FEN treatment. In the OF groups, both the young and old rats showed elevated medial frontal cortex (MFC) dopamine turnover (DOPAC/DA ratio), but only the young rats exhibited an elevation in norepinephrine (NE) turnover (MHPG/NE ratio). d-FEN treatment blocked the stress-induced increase in NE turnover in the young rats and the increase in DA turnover in the old rats. These data suggest that 5-HT activity could be involved in the age-related changes in the MFC catecholamine and PRL responses to stress in female rats.

Sex differences in corticosteroid binding in the rat brain: an in vitro autoradiographic study

Several previous studies have raised the possibility of sex differences in the distribution of corticosteroid receptors in the brain. The direction and magnitude of these differences have, however, remained controversial. In the present study, we have re-examined the concentrations of mineralocorticoid (MR) and glucocorticoid (GR) receptors in the brains of male and female rats at varying times (1 to 6 days) after combined gonadectomy (GDX) and adrenalectomy (ADX). Cytosol binding assays confirmed the presence of higher MR levels in short-term (3-day) GDX-ADX males. This difference disappeared by 6 days after surgery, as receptor levels in females rose to be equivalent to those in males. Using an improved in vitro autoradiographic method, the distribution of MR and GR was studied in males and females 3 days after GDX-ADX. The distribution of MR and GR in the brains of these rats was similar in the two sexes. MR binding in the male, however, was significantly greater than that in the female throughout the principal cell fields of the hippocampus. Measurements of circulating corticosterone levels at the time of GDX-ADX suggest that this sex difference may reflect a more rapid recovery of the MR system in males than in females following the stress-induced rise in corticosterone secretion occurring at the time of surgery.

Gender differences in patterns of HPA axis response to challenge: Macarthur studies of successful aging

Previous research has suggested that women may have greater and more prolonged hypothalamic-pituitary-adrenal response to challenge at older ages. Data on patterns of ACTH and cortisol responses to a "naturalistic" driving simulation challenge were examined to test the hypothesis that older women (aged 70-79 years) would show greater response than a comparable group of older men. Analyses of mean ACTH or cortisol responses in terms of maximal increase, area under the curve and repeated measures ANOVA did not reveal significant gender differences. By contrast, analyses of the joint occurrence of ACTH and cortisol responses above the respective sample medians indicated that women were significantly more likely to respond to the challenge with elevated ACTH and cortisol responses. The lack of significant gender difference in the analyses of mean scores resulted from the sensitivity of this measure of typically (or average response) to outliers in the data, thereby substantially distorting the report of typical (or average) responses for the male population. Analyses of the temporal patterns of ACTH and cortisol response in terms of the stability of the subject's relative rank at each sampling time indicated that: (1) there was considerable stability of rankings based on the initial cortisol response (though not ACTH) through the recovery period; and (2) women exhibited greater instability than men, being more likely to exhibit shifts in rank from high to low over time. These findings suggest that: (1) as a group, older women are more likely to show larger responses than comparably healthy older men to a "naturalistic" driving simulation challenge; (2) there is considerable stability in patterns of cortisol response from initial response at 20 min through the recovery period, indicating that those showing greater initial response continue to experience higher levels of cortisol during the post-challenge recovery period; and (3) instability is more common among women than men.

Life-spanning behavioural and adrenal dysfunction induced by prenatal hypoxia in the rat is prevented by the calcium antagonist nimodipine

The long-term behavioural effects of prenatal chronic anaemic hypoxia were investigated in young (5 months old), late adult (19 months) and aged Wistar rats (23-26 months). Sodium nitrite (2 g/l) offered in the drinking water during the second half of pregnancy served to evoke prenatal hypoxia. In parallel to nitrite treatment the Ca2+ channel blocker nimodipine (10 mg/kg) or vehicle alone was administered intragastrically once daily. Open-field activity, intermale social behaviour, learning ability in a black-white discrimination paradigm and fear-induced emotionality were assessed at different ages. Plasma corticosterone response to novelty stress was measured by blood sampling through chronic venous canulas at the age of 28 months. The nitrite-exposed 5-month-old offspring started exploration in a novel open-field with considerable delay. This delayed start-latency was augmented in 19- and 23-month-old rats, pointing to exaggerated suppression of behavioural arousal. Nitrite-induced hypoxia decreased the duration of social interactions during ageing. Aged rats exposed to nitrite were unable to learn a black-white discrimination but showed a normal generalized conditioned fear response (immobility) to the test situation as a whole. The conditioned fear-induced vocalization was more frequent among hypoxic aged animals. The aged hypoxic rats displayed a prolonged plasma corticosterone stress response and had higher adrenal weight than their controls. The abnormal open-field, social, learning and emotional behaviours, as well as the altered plasma corticosterone response, were prevented by prenatal nimodipine treatment.

Androgen regulation of adrenocorticotropin and corticosterone secretion in the male rat following novelty and foot shock stressors

To examine mechanisms responsible for sex differences in hypothalamo-pituitary-adrenal (HPA) axis responsiveness to stress, we studied the role of androgens in the regulation of the adrenocorticotropin (ACTH) and corticosterone (CORT) responses to foot shock and novelty stressors in gonadectomized (GDX) or intact male F344 rats. Foot shock or exposure to a novel open field increased plasma ACTH and CORT, which was significantly greater in GDX vs. intacts. Testosterone (T) or dihydrotestosterone propionate (DHT) treatment of GDX animals returned poststress levels of ACTH and CORT to intact levels. Estrogen treatment of GDX males further increased poststress CORT secretion above GDX levels. There was no difference in the ACTH response of anterior pituitaries from intact, GDX, and GDX+DHT animals to CRF using an in vitro perifusion system. There were no differences in beta max or binding affinity of type I or II CORT receptors in the hypothalamus or hippocampus of intact, GDX, or GDX+DHT groups. These data demonstrate an effect of GDX on hormonal indices of stress. The increased response in GDX rats appears to be due to the release from androgen receptor mediated inhibition of the HPA axis. This inhibition by androgen is not due to changes in anterior pituitary sensitivity to CRH, nor to changes in type I or type II corticosteroid receptor concentrations.

Neuroendocrine and neurochemical responses to novelty stress in young and old male F344 rats: effects of d-fenfluramine treatment

To understand some of the mechanisms underlying the neuroendocrine and neurochemical changes associated with aging, we administered the serotonin [5-hydroxytryptamine (5-HT)] releaser and reuptake inhibitor d-fenfluramine (d-FEN; 0.0, 0.2, or 0.6 mg/kg/day, p.o) for 30-38 days to young (4 months) and old (22 months) F344 male rats. Rats were stressed by placement into a novel open field (OF) for 20 min before sacrifice. Control animals were sacrificed immediately upon removal from their home cage (HC). Old rats exhibited less (p < 0.05) exploratory behavior than young rats, which was not altered by treatment with d-FEN. Old HC rats also had higher (p < 0.05) basal plasma levels of adrenocorticotropic hormone (ACTH) and prolactin (PRL) than young HC rats. Old OF rats showed higher (p < 0.05) levels of ACTH and corticosterone (CORT) than young OF animals. A stress-induced increase in PRL secretion was not observed in old rats. Subchronic low-dose d-FEN normalized the enhanced ACTH and CORT responses of old animals to novelty. In addition to these endocrine changes, stress-induced increases in medial frontal cortex (MFC) dopamine (DA) and norepinephrine (NE) turnover also were observed. The increase in NE turnover was greater (p < 0.01) in old than in young rats. d-FEN treatment blocked the stress-induced increase in MFC NE but not MFC DA turnover in both young and old rats. These data support a role for 5-HT and/or NE in some age-related neuroendocrine perturbations and suggest that increased 5-HT neurotransmission can normalize the hyperactivation of the hypothalamo-pituitary-adrenal axis of old male rats.

Stress, glucocorticoids, and aging

Attention has long been focused on the relationship between stress and aging, both under the guise of stress as an accelerator of normal aging and of aging as a time of impaired ability to cope with stress. This review examines the considerable amount of evidence in support of these views. We address these ideas with respect to glucocorticoids, the adrenal steroid hormones secreted during stress. In particular, we concentrate on three model systems: 1) programmed senescence in marsupial mice and semelparous fish as mediated by glucocorticoid excess; 2) glucocorticoid hypersecretion in rats and its role in damaging the aging brain; and 3) potential human and primate adrenocortical dysfunction during aging. We discuss physical and cognitive consequences of adrenocortical dysfunction in these systems, and how they may relate to human aging.

Age-related alterations in gonadotropin, adrenocorticotropin and growth hormone secretion

This review addresses some relevant aspects of the aging of the neuroendocrine system, particularly the reproductive and the adrenocortical axis. Deterioration of the reproductive function, one of the most striking endocrine alterations occurring in aging, is related to a complex interplay of factors. They comprise alterations occurring at the level of all the three components of the reproductive axis, the gonads, the pituitary and the brain, acting synergistically to disrupt the normal pulsatile release of gonadotropins. Particular relevance is given to the neurotoxic action of estrogens during the constant estrous phase occurring in aged female rodents, at the level of hypothalamic nuclei regulating gonadotropin secretion. This effect, to be found also in women during the anovulatory period of the perimenopause, would worsen the dysregulation of the central mechanisms controlling the reproductive function. The activity of the adrenocortical (HPA) axis increases with advancing age in rodents but also, although less strikingly, in humans. The main alteration which can be evidenced in both species is a delayed post-stimulus decline in plasma corticosteroid levels, indicating a diminished sensitivity to glucocorticoids of HPA axis feedback regulation in the elderly. Increased exposure to the highly catabolic adrenal glucocorticoids appears to be associated to a loss of cerebral neurons, particularly in the hippocampus, and the emergence of cognitive deficits in the aged rats. The relevance of experimental data performed in rodents to healthy and pathological human aging is extensively discussed. Finally, this chapter considers the age-related impairment in growth hormone secretion, a common finding of all the animal species investigated so far. The etiology of the hyposomatotropism of aging is namely linked to a progressive defect in growth hormone releasing hormone-producing hypothalamic neurons, although alterations of somatostatin-producing neurons have also been described. This background knowledge makes the use of neuroactive compounds aimed at restoring the physiologic function of hypothalamic hypophysiotropic hormones a rational approach to rectify the alterations of the neuroendocrine system occurring in elderly individuals.

The effect of aging on stress responsiveness and central corticosteroid receptors in the brown Norway rat

The present study examined the stress responsiveness of the hypothalamic-pituitary-adrenal axis in relation to the properties of corticosteroid receptors in the brain and pituitary of old (30 months) and young (3 months) male Brown Norway rats. Adrenocorticotropin hormone (ACTH) and corticosterone (B) were measured following exposure to novelty and to a conditioned emotional stimulus in blood samples sequentially obtained from chronically cannulated animals. Mineralocorticoid (MR) and glucocorticoid (GR) receptors were quantified by radioligand binding assay and in situ hybridization. The receptor binding constants were determined in tissue of rats that were adrenalectomized 24 hours previously, whereas gene expression was measured in the brain of intact animals. Aged Brown Norway rats showed a small but significant elevation in basal circulating ACTH level. The conditioned emotional stimulus, rather than the exposure to novelty, triggered a more than two-times higher ACTH response in the aged compared to the young rat. The termination of the stress-induced ACTH response seemed to proceed more efficiently in the aged rat. Basal and stress-induced total plasma B level did not differ in the young and old rats. The latter showed a 65% lower binding capacity of corticosteroid-binding globulin (CBG). Interestingly, in the aged rat the stress-induced rise in free circulating plasma B level was not elevated, but only prolonged. The hippocampus of aged rats displayed a decrease of maximally 44% in the apparent Bmax of MR, but no change in GR number. The Bmax of GR showed a 40% reduction in the hypothalamus and a 50% reduction in the anterior pituitary. GR affinity was considerably increased in the anterior pituitary, but was unchanged in the hippocampus and hypothalamus. Old age affected MR and GR gene expression differentially. GR mRNA was significantly reduced in cell field CA3 (-42%), CA4 (-41%) and the dentate gyrus (-26%) of the dorsal hippocampus, but did not change either in hippocampal cell field CA1 or in the hypothalamic paraventricular nucleus (PVN) of the old rat. There was no significant difference in MR mRNA between young and aged rats in the different cell fields of the hippocampus. The aged rat, therefore, is characterized by site- and receptor-specific changes in binding constants as well as by changes in receptor transcription and translation. The data demonstrate that in the old Brown Norway rats, a conditioned emotional stimulus results in enhanced pituitary ACTH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Do glucocorticoid concentrations rise with age in the rat?

While the secretion of glucocorticoids by the adrenal gland is essential for survival of various stressors, glucocorticoid excess can be pathogenic. This two-edged quality to glucocorticoid action makes it of interest whether glucocorticoid concentrations change with age. Numerous studies have examined this in the rat but have failed to reach consensus. The present report analyzes this literature and concludes that the lack of consensus cannot be attributed to strain or sex differences or differences in the point in the circadian cycle at which rats were studied. Instead, it appears that a critical variable is how truly "basal" (i.e., unstressed) basal samples were; in studies in which basal glucocorticoid concentrations in young control subjects were in a range reflecting unstressed basal conditions, there is a robust increase in hormone concentrations with age. In contrast, the bulk of studies reporting no increase with age were those in which young subjects had elevated basal glucocorticoid concentrations (perhaps reflecting the method and speed of obtaining the blood sample, the social conditions of the rat housing, and/or the recency with which there was a disturbance in the animal room). Thus, it appears that once this source of variability is recognized and factored out, there is a considerable increase in basal glucocorticoid concentrations in aged rats.

Age-related changes in glucocorticoid receptor binding and mRNA levels in the rat brain and pituitary

Hypothalamic-pituitary-adrenal (HPA) activity under both basal and poststress conditions is often increased in the aged rat. This change has been associated with the loss of corticosteroid receptors in specific regions that mediate glucocorticoid negative feedback. In order to study the cellular basis for the loss of receptors, we measured glucocorticoid (type II corticosteroid) receptor binding and mRNA levels in pituitary and selected brain regions in rats at 6, 12, and 24 months of age. Receptor binding, measured using [3H]RU 28362, was stable in all regions (pituitary, hypothalamus, amygdala, and frontal cortex) except the hippocampus, where there was about a 40% decrease in binding capacity, with no change in the affinity of the receptor for RU 28362. The loss of receptors in the hippocampus was apparent in animals at 12 months of age, and binding was further decreased at 24 months. Glucocorticoid receptor mRNA levels were significantly higher in all regions at 12 months of age than at 6 months. By 24 months, however, receptor mRNA levels in most regions had returned to levels that were similar to those at 6 months of age. In contrast, glucocorticoid receptor mRNA levels in the hippocampus were significantly decreased at 24 months of age compared to levels at both 6 and 12 months of age. Thus, in general, variations in receptor mRNA levels parallel those in receptor binding in animals 6 and 24 months of age, with the hippocampus as the only region showing a significant loss of receptors and a decrease in mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of neonatal handling on the development of the adrenocortical response to stress: implications for neuropathology and cognitive deficits in later life

Several years ago Levine, Denenberg, Weininger, Ader, and others described the effects of postnatal "handling" on the development of behavioral and endocrine responses to stress. The handling procedure usually involved removing rat pups from their cages, placing the animals together in small containers, and 15-20 min later, returning the animals to their cages and their mothers. The manipulation was performed daily for the first 21 days of life. As adults, handled (H) rats exhibited attenuated fearfulness (e.g., decreased freezing, increased exploration) in novel environments and a less pronounced increase in the secretion of adrenal glucocorticoids in response to a variety of stressors. These findings clearly demonstrated that the development of rudimentary, adaptive responses to stress could be modified by environmental events. We have followed on these earlier handling studies, convinced that this paradigm provides a marvelous opportunity to examine how subtle variations in the early environment alter the development of specific biochemical systems in the brain, leading to stable individual differences in biological responses to stimuli that threaten homeostasis. In this work we have shown how early handling influences the neurochemical development of certain brain regions that regulate the adrenocortical response to stress. Neonatal handling increases the efficiency of this endocrine response to stress, preventing excessive exposure to the highly catabolic adrenal steroids. In later life, this effect appears to protect the animal from potentially damaging effects of these steroids, ensuring the anatomical integrity of brain structures involved in cognitive functioning.

Age-related changes in the dog hypothalamic-pituitary-adrenocortical system: neuroendocrine activity and corticosteroid receptors

Aging is associated with a progressive dysfunctioning of the hypothalamic-pituitary-adrenocortical (HPA) axis. We have studied the response of the HPA axis to stress and a hormonal (ovine corticotropin releasing factor (o-CRF) challenge in young (1.5-2 years) and aged (greater than 11 years) dogs. Compared to the young dogs, the aged animals displayed an increased basal concentration of both ACTH and cortisol. In addition, in response to an o-CRF challenge (1 microgram/kg i.v.) or an electric footshock (1 mA, alternatively on/off for 2 s) or immobilization (45 min) stress, the aged dogs showed significantly larger increments in ACTH and cortisol. Following the challenge test, the young and aged dogs reached their respective basal hormone levels at the same time, except for the o-CRF test. In the latter case, in contrast to the young controls, the aged dogs still showed an increased plasma cortisol level compared to the pre-challenge basal hormone concentration. Concerning the effect of aging on the brain and hypophyseal corticosteroid receptors, a selective decline (minus 50-75%) in mineralocorticoid receptor (MR) was observed in all measured brain regions (dorsal and ventral hippocampus, septum, hypothalamus) and anterior pituitary, whereas no change was found in brain glucocorticoid receptor (GR) number. The GR level in the anterior pituitary was even increased by 70%. In light of the role that MR and GR seem to play in the regulation of the HPA axis, it is concluded that the diminished MR number in the aged dog brain may underly the increased basal hormone levels and the elevated responsiveness of the HPA axis in these animals. The observation that the stress-induced elevations of cortisol and ACTH were not prolonged at senescence suggests that the GR-mediated negative feedback action of glucocorticoids is not altered, which is in line with the unchanged brain GR numbers in the aged dogs.

Postnatal handling attenuates certain neuroendocrine, anatomical, and cognitive dysfunctions associated with aging in female rats

Hippocampal degeneration with aging is associated with increased hypothalamic-pituitary-adrenal (HPA) activity and, in male rats, both are attenuated by postnatal handling. Considering the important sex differences in the effects of handling and in HPA responses to stress in older rats, we have examined the effects of postnatal handling on aging in females. Female, Long-Evans rats were handled (H) during the first 3 weeks of life and later compared with nonhandled (NH) controls at various ages. Handling resulted in permanently increased hippocampal type II, glucocorticoid receptor binding. Relative to H females, NH females showed increased basal corticosterone levels in later life and hypersecreted corticosterone following stress at all ages examined. Both effects are similar to those reported in males. However, unlike males, H and NH females did not differ in corticosterone levels achieved during stress, a finding that may be related to sex-dependent effects of handling on pituitary transcortin receptors. There were no differences in hippocampal neuron density in 6-month-old animals. However, the older NH animals showed considerable neuron loss in the CA1 and CA3 hippocampal cellfields. There was little or no neuron loss in the H animals. Finally, the NH animals exhibited age-related spatial memory impairments, such that by 24 months of age the performance of the NH females was profoundly worse than that of the younger NHs and same-aged H animals. These data suggest that early handling permanently alters CNS systems that regulate hypothalamic-pituitary-adrenal (HPA) function, although the effect may depend on the gender of the animal. In both males and females, however, handling appears to prevent (or minimize) increased adrenal secretion in later life and to attenuate hippocampal cell loss and spatial memory impairments.

Behavioral assessment of forgetting in aged rodents and its relationship to peripheral sympathetic function

Observation of age-related memory deficits in rodents is often dependent on the behavioral task used to assess these changes, rather than being universal to all memories. A review of studies using aversively-motivated multiple trial training paradigms suggests that the apparent acquisition deficits common to older animals may instead be due to a confounding tendency of these animals to behavioral rigidity or perseveration. Data obtained using single trial training paradigms, such as the one-trial passive avoidance task, indicate that young and old rodents can learn tasks with equal facility, that retention in young and old animals is similar at short training-testing intervals, but that retention is impaired in aged animals at longer training-testing intervals. We suggest that the adrenal medulla, a peripheral source of catecholamines, secretes catecholamines that act outside the blood-brain barrier to modulate memory processes. Further, we review evidence suggesting that the ability of the adrenal medulla to respond to the stresses of footshock during aversively-motivated training are impaired in aged rodents, and that this impairment may contribute to the rapid forgetting observed in senescent animals.

Neurotrophic ACTH analogue promotes plasticity of type I corticosteroid receptor in brain of senescent male rats

Age-related changes were studied in the concentration of type-I and type-II corticosteroid receptors in the hippocampus of young adult (3 months) and aged (28.5 to 30.5 months) male rats. Using 3H-labelled ligands, in vitro binding of type-I and type-II corticosteroid receptors in the soluble cell fraction (cytosol) revealed an age-related decrease in concentration of both receptor types of 52% and 28%, respectively. Infusion of young and aged male rats for 2 weeks with the ACTH4-9 [adrenocorticotropin4-9] peptide analogue ORG 2766 (0.5 micrograms/0.5 microliter/hr) resulted in only a minor increase (+8%) in the number of type-I receptors in young rats. In the aged animals, however, the type-I receptor concentration was 68% higher than in the vehicle-treated aged animals. In contrast, no effect of the peptide treatment was noted on the concentration of type-II receptors in either young or aged rats. Furthermore, no effect was found for either age or treatment with peptide on the affinity of type-I and type-II receptors for their respective ligands. Binding of 3H-labelled ligands to brain sections of young and aged rats was performed using in vitro autoradiography. Quantitative image analysis of the film showed that in senescence there is a marked reduction in both type-I (62-75%) and type-II (29-56%) receptor concentrations in the hippocampal subregions (CA1, CA2, CA3 and dentate gyrus) as well as in the lateral septum. Treatment of aged rats with ORG 2766 selectively reversed the age-associated reduction in type-I receptors, while the peptide did not affect the type-II receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Bidirectional responsiveness of the pituitary-adrenal system in old and young male and female rats

Bidirectional responsiveness of the pituitary-adrenal system was examined in young adult (3-5 months) and old (24-26 months) male and female rats. In Experiment 1, an approach-avoidance conflict was created by exposing food- and water-deprived subjects to a flavored solution which had previously been paired with lithium chloride-induced illness in a conditioned taste aversion paradigm. Young and old males and young females elevated plasma corticosterone in response to deprivation; males showed a further elevation when reexposed to the solution, whereas young females did not. Old females did not exhibit a corticosterone elevation to deprivation or upon reexposure. In Experiment 2, water was restricted to a fixed period in the morning when corticosterone levels are typically low. Plasma corticosterone was measured before and after watering. Young and old males and young females reentrained their circadian corticosterone rhythm so that levels were elevated just prior to watering; consumption was followed by a drop in corticosterone. Old females failed to reentrain or to suppress corticosterone secretion upon drinking. These findings indicate that the lability of the pituitary-adrenal system is more markedly affected by senescence in females than in males.

Adrenocortical responsiveness to novelty in the hamster

The adrenocortical response to increasing periods of exposure to a novel cage, as well as the effects of returning hamsters to their familiar home cages, were examined. We found that a five min exposure to a novel cage was insufficient to activate the pituitary-adrenal system, but that cortisol levels increased significantly as duration of exposure increased from 15 to 30 min. In contrast, returning hamsters to their home cages for at least 15 min following the 15 min novel cage exposure produced a significant drop in plasma levels of cortisol. Finally we found that males exhibit a greater cortisol response to novelty than females, indicating a sexual dimorphism which is the reverse of that reported in other rodents.