Neurochemical, endocrine and immunological responses to stress in young and old Fischer 344 male rats

Heterogeneity and Dynamics of Vasculature in the Endocrine System During Aging and Disease

The endocrine system consists of several highly vascularized glands that produce and secrete hormones to maintain body homeostasis and regulate a range of bodily functions and processes, including growth, metabolism and development. The dense and highly vascularized capillary network functions as the main transport system for hormones and regulatory factors to enable efficient endocrine function. The specialized capillary types provide the microenvironments to support stem and progenitor cells, by regulating their survival, maintenance and differentiation. Moreover, the vasculature interacts with endocrine cells supporting their endocrine function. However, the structure and niche function of vasculature in endocrine tissues remain poorly understood. Aging and endocrine disorders are associated with vascular perturbations. Understanding the cellular and molecular cues driving the disease, and age-related vascular perturbations hold potential to manage or even treat endocrine disorders and comorbidities associated with aging. This review aims to describe the structure and niche functions of the vasculature in various endocrine glands and define the vascular changes in aging and endocrine disorders.

Isolation-induced vocalization in the infant rat depends on the nucleus accumbens

Mammalian infants vocalize when socially isolated. Vocalization guides the return of the caregiver and thereby maintains an environment critical to the infant's survival. Although the role of the periaqueductal gray area (PAG) in these vocalizations is established, other aspects of the relevant neural circuitry remain under-studied. Here we report that output from the nucleus accumbens (Acb) is necessary for isolation-induced vocalizations of infant rats aged postnatal days (PND) 11-13. Local inhibition via infusion of the GABA_A agonist muscimol (.8 μg/side) of the Acb, but not the dorsolateral striatum, blocked isolation-induced vocalizations, independent of whether the isolation was at room temperature, followed a brief reunion with the dam, or occurred in a cool (10 °C) environment. These findings highlight a possible anatomical area mediating the mammalian infant response to social separation and, more generally, to the development of social attachment.

Effects of aging on stress-related responses of serotonergic neurons in the dorsal raphe nucleus of male rats

Responses to various stressors in the brain change with age. However, little is known about the neural mechanisms underlying age-dependent changes in stress responses. It is known that serotonin, a stress-related transmitter, is closely related with the regulation of stress responses in the brain and that serotonergic function is modulated by various factors, including estrogen, in both sexes. In the present study, to elucidate the effects of aging on stress responses in serotonergic neurons, we examined the expression levels of tryptophan hydroxylase (TPH; a marker of serotonergic neurons) in the dorsal, ventral and lateral parts of the dorsal raphe nucleus (DRN) in young and old intact male rats. In young males, repeated restraint stress significantly increased the number of TPH-positive cells in all subdivisions of the DRN. In contrast, the stress-induced increase in TPH expression was only observed in the ventral part of the DRN in old males. Pretreatment with an estrogen receptor β antagonist had no effect on the number of TPH-positive cells in the dorsal and lateral DRN in young stressed males, whereas the antagonist decreased the number of TPH-positive cells in all DRN subdivisions in old stressed males. Our results suggest that the effects of repeated stress exposure on the expression of TPH in serotonergic neurons in the DRN change with age and that estrogenic effects via estrogen receptor β on TPH expression in stressed old males differ from those in young males.

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We examined the effect of aging on stress-induced TPH expression in male rats.

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The effect of stress exposure on TPH expression in the DRN changed with age.

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The effect of ER-β blockade on stress-induced TPH expression changed with age.

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Stress experience in young adulthood changed serotonergic response in old age.

Age-related declines in thirst and salt appetite responses in male Fischer 344×Brown Norway rats

The F344×BN strain is the first generational cross between Fischer 344 (F344) and Brown Norway (BN) rats. The F344×BN strain is widely used in aging studies as it is regarded as a model of "healthy" aging (Sprott, 1991). In the present work, male F344×BN rats aged 4mo (young, n=6) and 20mo (old, n=9) received a series of experimental challenges to body fluid homeostasis to determine their thirst and salt appetite responses. Corresponding urinary responses were measured in some of the studies. Following sodium depletion, old rats ingested less saline solution (0.3M NaCl) than young rats on a body weight basis, but both ages drank enough saline solution to completely repair the accrued sodium deficits. Following intracellular dehydration, old rats drank less water than young rats, again on a body weight basis, and were less able than young rats to drink amounts of water proportionate to the osmotic challenge. Compared with young rats, old rats drank less of both water and saline solution after combined food and fluid restriction, and also were refractory to the stimulatory effects of low doses of captopril on water drinking and sodium ingestion. Age differences in urinary water and sodium excretion could not account for the age differences in accumulated water and sodium balances. These results extend observations of diminished behavioral responses of aging animals to the F344×BN rat strain and support the idea that impairments in behavior contribute more to the waning ability of aging animals to respond to body fluid challenges than do declines in kidney function. In addition, the results suggest that behavioral defense of sodium homeostasis is less diminished with age in the F344×BN strain compared to other strains so far studied.

Vasopressin indirectly excites dorsal raphe serotonin neurons through activation of the vasopressin1A receptor

The neuropeptide vasopressin (AVP; arginine-vasopressin) is produced in a handful of brain nuclei located in the hypothalamus and extended amygdala and is released both peripherally as a hormone and within the central nervous system as a neurotransmitter. Central projections have been associated with a number of functions including regulation of physiological homeostasis, control of circadian rhythms, and modulation of social behavior. The AVP neurons located in the bed nucleus of the stria terminalis and medial amygdala (i.e., extended amygdala) in particular have been associated with affiliative social behavior in multiple species. It was recently demonstrated that in the mouse AVP projections emanating from extended amygdala neurons innervate a number of forebrain and midbrain brain regions including the dorsal raphe nucleus (DR), the site of origin of most forebrain-projecting serotonin neurons. Based on the presence of AVP fibers in the DR, we hypothesized that AVP would alter the physiology of serotonin neurons via AVP 1A receptor (V1AR) activation. Using whole-cell electrophysiology techniques, we found that AVP increased the frequency and amplitude of excitatory post-synaptic currents (EPSCs) in serotonin neurons of male mice. The indirect stimulation of serotonin neurons was AMPA/kainate receptor dependent and blocked by the sodium channel blocker tetrodotoxin, suggesting an effect of AVP on glutamate neurons. Further, the increase in EPSC frequency induced by AVP was blocked by selective V1AR antagonists. Our data suggest that AVP had an excitatory influence on serotonin neurons. This work highlights a new target (i.e., V1AR) for manipulating serotonin neuron excitability. In light of our data, we propose that some of the diverse effects of AVP on physiology and behavior, including social behavior, may be due to activation of the DR serotonin system.

3alpha-androstanediol, but not testosterone, attenuates age-related decrements in cognitive, anxiety, and depressive behavior of male rats

Some hippocampally-influenced affective and/or cognitive processes decline with aging. The role of androgens in this process is of interest. Testosterone (T) is aromatized to estrogen, and reduced to dihydrotestosterone (DHT), which is converted to 5α-androstane, 3α, 17α-diol (3α-diol). To determine the extent to which some age-related decline in hippocampally-influenced behaviors may be due to androgens, we examined the effects of variation in androgen levels due to age, gonadectomy, and androgen replacement on cognitive (inhibitory avoidance, Morris water maze) and affective (defensive freezing, forced swim) behavior among young (4 months), middle-aged (13 months), and aged (24 months) male rats. Plasma and hippocampal levels of androgens were determined. In experiment 1, comparisons were made between 4-, 13-, and 24-month-old rats that were intact or gonadectomized (GDX) and administered a T-filled or empty silastic capsule. There was age-related decline in performance of the inhibitory avoidance, water maze, defensive freezing, and forced swim tasks, and hippocampal 3α-diol levels. Chronic, long-term (1–4 weeks) T-replacement reversed the effects of GDX in 4- and 13-month-old, but not 24-month-old, rats in the inhibitory avoidance task. Experiments 2 and 3 assessed whether acute subcutaneous T or 3α-diol, respectively, could reverse age-associated decline in performance. 3α-diol, but not T, compared to vehicle, improved performance in the inhibitory avoidance, water maze, forced swim, and defensive freezing tasks, irrespective of age. Thus, age is associated with a decrease in 3α-diol production and 3α-diol administration reinstates cognitive and affective performance of aged male rats.

Aging increases the susceptibility to develop anhedonia in male rats

The objective of this study was to establish the effect of aging on the development of anhedonia, a core feature of depression. Young and old male Wistar rats (of around 3-5 and 12-15 months, respectively) were exposed to a chronic variable stress (CVS) schedule for 3 weeks. CVS produced anhedonia, indicated by a reduction in the intake of a sucrose solution (1%), in 8 out of 23 (35%) young rats and in 19 out of 26 (73%) old rats, implying that old animals are more susceptible to stress and develop anhedonia more readily than young animals. Young and old anhedonic rats showed a similar temporal course in the reduction of sucrose consumption, reaching the anhedonic state after 2 weeks of CVS exposure. Compared with young animals, old rats had lower basal serum testosterone and estradiol levels. The systemic levels of corticosterone did not vary between both age groups. No significant pathological condition was detected in old animals. It is suggested that the higher susceptibility to develop anhedonia in male rats could be associated to neuroendocrine changes consequent to aging.

Social buffering in rats: prolactin attenuation of active interaction

Stress may result when the present environment is interpreted as threatening, and stress is known to increase the prolactin-secretory response. In the present study, rats (N=83) were exposed to a conditioned-fear paradigm (environment paired with footshock), and on testing day, rats were exposed to the experimental chamber without shock while alone (Alone n=16), with an object (Object n=17), with a euthanized conspecific (Euthanized n=16), or with a social partner (Social n=19). The control group (Control n=15) was exposed to the experimental chamber but was never shocked. The Control group had significantly lower levels of prolactin than the Alone, Object, and Euthanized groups; however, the Control group's levels of prolactin were not significantly different than that of the Social group, which was significantly lower than that for the Alone group. Social interaction decreased fear independent of the distraction provided by a stimulus in the chamber. Active touch appeared to be crucial for social buffering to occur.

Noradrenaline transmission within the ventral bed nucleus of the stria terminalis is critical for fear behavior induced by trimethylthiazoline, a component of fox odor

The bed nucleus of the stria terminalis (BNST) is involved in the mediation of fear behavior in rats. A previous study of our laboratory demonstrated that temporary inactivation of the BNST blocks fear behavior induced by exposure to trimethylthiazoline (TMT), a component of fox odor. The present study investigates whether noradrenaline release within the BNST is critical for TMT-induced fear behavior. First, we confirmed previous studies showing that the ventral BNST is the part of the BNST that receives the densest noradrenaline innervation. Second, using in vivo microdialysis, we showed that noradrenaline release within the BNST is strongly increased during TMT exposure, and that this increase can be blocked by local infusions of the alpha2-receptor blocker clonidine. Third, using intracerebral injections, we showed that clonidine injections into the ventral BNST, but not into neighboring brain sites, completely blocked TMT-induced potentiation of freezing behavior. The present data clearly show that the noradrenergic innervation of the ventral BNST is important for the full expression of behavioral signs of fear to the predator odor TMT.

Mid-life stress and cognitive deficits during early aging in rats: individual differences and hippocampal correlates

We explored here the possibility that mid-life stress in rats could have deleterious effects on cognitive abilities during early aging, as well as the potential role of inter-individual differences on the development of such effects. Male Wistar rats were classified according to their reactivity to novelty (4 months old) as highly (HR) or low (LR) reactive and, at mid-life (12 months old), either submitted to chronic stress (28 days) or left undisturbed. At early aging (18 months old), their learning abilities were tested in the water maze, and a number of neuroendocrine (plasma corticosterone; hippocampal corticosteroid receptors) and neurobiological (hippocampal expression of neuronal cell adhesion molecules) parameters were evaluated. Impaired performance was observed in stressed HR rats, as compared to unstressed HR and stressed LR rats. Increased hippocampal mineralocorticoid receptors were found in stressed LR rats when compared with stressed HR and control LR groups. In addition, mid-life stress-induced an increased corticosterone response and a reduction in NCAM-180 isoform and L1 regardless of the behavioral trait of novelty reactivity. These findings highlight a role of stress experienced throughout life on cognitive impairment occurring during the early aging period, as well as the importance of taking into account individual differences to understand variability in such cognitive decline.

Stress, depression, and anhedonia: caveats concerning animal models

Numerous animal models of depression have been advanced, each having multiple attributes and some limitations. This review provides caveats concerning etiologically valid animal models of depression, focusing on characteristics of the depressive subtype being examined (e.g. typical vs atypical major depression, dysthymia, melancholia), and factors that contribute to the interindividual behavioral variability frequently evident in stressor-related behavioral paradigms. These include the stressor type (processive vs systemic stressors), and characteristics of the stressor (controllability, predictability, ambiguity, chronicity, intermittence), as well as organismic variables (genetic, age, sex), experiential variables (stressor history, early life events) and psychosocial and personality factors that moderate stressor reactivity. Finally, a model of depression is reviewed that evaluates the effects of stressors on hedonic processes, reflected by responding for rewarding brain stimulation. Anhedonia is a fundamental feature of depression, and assessment of stressor-related reductions in the rewarding value of brain stimulation, especially when coupled with other potential symptoms of depression, provides considerable face, construct and predictive validity. Stressful events markedly impact rewarding brain stimulation, and this effect varies across strains of mice differentially reactive to stressors, is modifiable by antidepressant treatments, and allows for analyses of the contribution of different brain regions to anhedonic processes. The paradigm is sensitive to several factors known to acts as moderators of stress responses, but analyses remain to be conducted with regard to several such variables.

Dihydrotestosterone may inhibit hypothalamo–pituitary–adrenal activity by acting through estrogen receptor in the male mouse

The corticosterone (CORT) response to environmental perturbation has been shown to be enhanced by estrogen but inhibited by the androgen dihydrotestosterone (DHT). However, the mechanism of androgen's action has not been identified. This study examined the effects of estradiol benzoate (EB), the non-aromatizable androgen DHT, and the DHT metabolite 5alpha-androstan-3beta, 17beta-diol (3beta-diol) on the corticosterone response to stress. Adult male CBB6/F1 mice were gonadectomized and injected subcutaneously (once a day for 4 days) with the above compounds (controls received oil vehicle injections). Animals (within treatments) were randomly assigned to stress or non-stress conditions. The non-stress animals were taken directly from their home cages and killed. Animals were stressed by a 30 min restraint prior to being killed. Hormone levels were determined in plasma via radioimmunoassay. In agreement with previous studies, the CORT response to immobilization was enhanced by EB and inhibited by DHT. Surprisingly, 3beta-diol inhibited the CORT response similar to the effect of DHT. In a second study, concomitant injections of the androgen receptor antagonist flutamide only partially blocked DHT's, but had no effect on 3beta-diol's, inhibitory action. In contrast, injections with the estrogen receptor antagonist tamoxifen completely blocked the effects of 3beta-diol and partially blocked DHT's effect. Taken together these studies suggest that DHT's inhibitory effects may be, at least in part, via the estrogen receptor, through its conversion to 3beta-diol. These studies also suggest that the DHT metabolites may be functionally relevant when considering hormonal responses to stress.

A heparin derived oligosaccharide normalizes the fear response of old Brown Norway rats

Old animals exhibit impaired spatial learning and an exaggerated response to stress. It was predicted that the heparin derived oligosaccharide (HDO), C3, would reverse these age-related deficits. Young (4-5 months) and old (19-20 months) Brown Norway male rats ingested C3 (25 mg/kg per day, p.o.) or vehicle (drinking water) daily for 42-44 days. Two weeks after the initiation of drug treatment, the rats were examined using a series of behavioral tests. The old control rats evidenced: (1) increased neophobia and reduced exploratory behavior in a novel open field (OF); and (2) exaggerated freezing during the acquisition and retention of a conditioned response (CR) to a Pavlovian light-shock pairing ("foreground" conditioning). C3 treatment attenuated or reversed this age-related impairment of emotional behavior. Analysis of spatial learning using the Morris water maze (MWM), and of CR formation to the place in which the foreground conditioning was conducted ("background" conditioning) during context dependent fear conditioning (CDFC) did not reveal major age or drug effects on memory processes. It is hypothesized that C3 repairs damage to the extracellular matrix (ECM) that occurs during the aging process and thereby normalizes age-related exacerbated fear behaviors without affecting mnestic processes.

Age-dependent changes in 24-hour rhythms of catecholamine content and turnover in hypothalamus, corpus striatum and pituitary gland of rats injected with Freund's adjuvant

BACKGROUND

Little information is available on the circadian sequela of an immune challenge in the brain of aged rats. To assess them, we studied 24-hour rhythms in hypothalamic and striatal norepinephrine (NE) content, hypothalamic and striatal dopamine (DA) turnover and hypophysial NE and DA content, in young (2 months) and aged (18-20 months) rats killed at 6 different time intervals, on day 18th after Freund's adjuvant or adjuvant's vehicle administration.

RESULTS

Aging decreased anterior and medial hypothalamic NE content, medial and posterior hypothalamic DA turnover, and striatal NE concentration and DA turnover. Aging also decreased NE and DA content in pituitary neurointermediate lobe and augmented DA content in the anterior pituitary lobe. Immunization by Freund's adjuvant injection caused: (i) reduction of DA turnover in anterior hypothalamus and corpus striatum; (ii) acrophase delay of medial hypothalamic DA turnover in old rats, and of striatal NE content in young rats; (iii) abolition of 24-h rhythm in NE and DA content of neurointermediate pituitary lobe, and in DA content of anterior lobe, of old rats.

CONCLUSIONS

The decline in catecholamine neurotransmission with aging could contribute to the decrease of gonadotropin and increase of prolactin release reported in similar groups of rats. Some circadian responses to immunization, e.g. suppression of 24-h rhythms of neurointermediate lobe NE and DA and of anterior lobe DA were seen only in aged rats.

Do glucocorticoids contribute to brain aging?

The hippocampus, an area with abundant glucocorticoid receptors, continues to be the focus of research on effects of glucocorticoids on the aging brain. Based on recent studies, the primary structural change found during aging is synaptic loss, rather than neuronal loss. High levels of glucocorticoids are associated with synaptic loss in the hippocampus, hippocampal atrophy, and cognitive decline during aging in some individuals. However, increasing levels of glucocorticoid are not always found since early experiences can alter sensitivity to negative feedback and the level of activation of the hypothalamic-pituitary-adrenal axis in aged individuals. New ways in which glucocorticoids may contribute to brain aging are discussed, including decreased responses to glucocorticoids possibly as a result of decreased glucocorticoid receptors and also altered regulation of neuronal turnover in the dentate gyrus. Decreased responsiveness of glial fibrillary acidic protein to glucocorticoids during aging could facilitate reactive gliosis and loss of synapses by altering neuron-astrocyte interactions. Neuronal turnover is regulated by glucocorticoids in the dentate gyrus where ongoing neurogenesis may be important for hippocampal-based memory formation in adulthood. Although the age-related decline in neurogenesis can be reversed by removal of adrenal steroids, the death of dentate granule neurons is also greatly increased by this treatment. Recent studies show age-related resistance to induced apoptosis and neurogenesis in the dentate gyrus following adrenalectomy, which is associated with increased expression of transforming growth factor-beta1. Therefore, the contribution of glucocorticoids to brain aging depends on the physiological and cellular context and some of these effects are reversible.

Behavioral and neurochemical changes following predatory stress in mice

This article had several objectives. First it aimed at investigating the anxiogenic-like behaviors elicited by unavoidable cat exposure and/or cat odor across nine strains of mice (BALB/c, C57BL/6, C3H, CBA, DBA/2, NMRI, NZB, SJL, Swiss) in a modified version of the free-exploration test. The second objective was to investigate possible neurochemical changes following cat exposure in Swiss mice by measuring the turnover of dopamine (DA), noradrenaline (NA) and serotonin (5-HT) in several brain regions known to be involved in the modulation of emotional processes (hippocampus, hypothalamus and striatum). Finally, the third objective was to examine the effects of anxiolytic drug treatments on the anxiogenic responses elicited by a cat odor (i.e. a feces) in Swiss mice previously exposed to a cat using the free-exploration test. Results from the strain comparison showed that mice could be divided into three distinct groups: two non-reactive strains (NZB and SJL) which were relatively insensitive to predatory exposure and/or odor; five intermediate-reactive strains (Swiss, NMRI, CBA, C3H and BALB/c) which displayed clear anxiogenic-like responses only when exposed to both cat and, subsequently, to feces; and two high reactive strains (C57BL/6 and DBA/2) which showed anxiogenic-like reactions following cat exposure, regardless of the stimulus (clay or feces) present in the free-exploration cage. Neurochemical data revealed that, while brain levels of NA, DA, 5-HT in cat exposed Swiss mice were not significantly different from those of control animals, turnover rates of these monoamines were increased in the hippocampus (NA and 5-HT), hypothalamus and striatum (DA) after cat exposure. Results from pharmacological experiments indicated that repeated administration of the 5-HT reuptake inhibitor fluoxetine (5-20 mg/kg, twice a day, for 5 days) completely abolished avoidance of the cat feces in Swiss mice previously exposed to the predator. Neither acute nor repeated administration of the classical anxiolytic diazepam was able to reduce avoidance behavior of the anxiogenic stimulus in the free-exploration test. Taken together, these findings indicate that the exposure of mice to unavoidable predatory stimuli is associated with behavioral and neurochemical changes consistent with increased anxiety.

Differential and age-dependent effects of maternal deprivation on the hypothalamic-pituitary-adrenal axis of brown norway rats from youth to senescence

In this study, the hypothesis was tested that infants deprived from maternal care show persistent changes in hypothalamic-pituitary-adrenal activity. For this purpose, we studied the effect of maternal deprivation in one cohort of the healthy ageing Brown Norway rat strain showing still more than 80% survival rate at 32 months of age. Three-day-old male Brown Norway rats were either maternally deprived for 24 h or remained with the dam. In 3, 12 and 30-32 months (young, adult, senescent) deprived rats and their nondeprived littermates (controls), we determined basal resting and stress-induced plasma adrenocorticotropic hormone (ACTH) and corticosterone as well as corticotropin releasing hormone (CRH) mRNA expression in the paraventricular nucleus (PVN) of the hypothalamus. Mineralocorticoid (MR) and glucocorticoid receptors (GR) in hippocampus and PVN were also assessed using in vitro cytosol binding and in situ hybridization. The effect of ageing per se showed that in the control nondeprived Brown Norway rats, basal corticosterone and ACTH concentrations did not change during life. However, with age, the corticosterone response to novelty stress became progressively attenuated, but prolonged, while there was an age-related increase in the ACTH response. CRH mRNA expression in PVN decreased with age. Hippocampal MR binding and MR mRNA expression in the dentate gyrus were reduced at senescence, as were the GR binding capacities in hippocampus and hypothalamus. Maternal deprivation did not affect survival rate, body weight, nor adrenal weight of the ageing Brown Norway rats. Basal corticosterone and ACTH levels were not affected by deprivation, except for a rise in basal corticosterone concentrations at 3 months. At this age, the corticosterone output in response to novelty was attenuated in the deprived rats. In contrast, a striking surge in novelty stress-induced corticosterone output occurred at midlife while, at senescence, the corticosterone and ACTH responses were attenuated again in the deprived animals, particularly after the more severe restraint stressor. CRH mRNA expression was reduced only during adulthood in the deprived animals. After maternal deprivation, the MR mRNA in dentate gyrus showed a transient midlife rise. GR binding in hypothalamus and hippocampus GR binding was reduced in young rats while, in the senescent deprived animals, a reduced GRmRNA expression was observed in PVN and hippocampal CA1. In conclusion, in the Brown Norway rat, ageing causes a progressive decline in corticosterone output after stress, which is paralleled at senescence by decreased MR and GR mRNA expression in hippocampus and hypothalamus. The long-term effects of maternal deprivation become manifest differently at different ages and depend on test conditions. The deprivation effect culminates in a midlife corticosterone surge and results at senescence in a strongly reduced corticosterone output.

Immunohistochemical and neurochemical correlates of learning deficits in aged rats

This study examined whether cholinergic and monoaminergic dysfunctions in the brain could be related to spatial learning capabilities in 26-month-old, as compared to three-month-old, Long-Evans female rats. Performances were evaluated in the water maze task and used to constitute subgroups with a cluster analysis statistical procedure. In the first experiment (histological approach), the first cluster contained young rats and aged unimpaired rats, the second one aged rats with moderate impairment and the third one aged rats with severe impairment. Aged rats showed a reduced number of choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis, and choline acetyltransferase-positive neurons in the striatum. In the second experiment (neurochemical approach), the three clusters comprised young rats, aged rats with moderate impairment and aged rats with severe impairment. Alterations related to aging consisted of reduced concentration of acetylcholine, norepinephrine and serotonin in the striatum, serotonin in the occipital cortex, dopamine and norepinephrine in the dorsal hippocampus, and norepinephrine in the ventral hippocampus. In the first experiment, there were significant correlations between water maze performance and the number of; (i) choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis; (ii) choline acetyltransferase-positive neurons in the striatum and; (iii) p75(NTR)-positive neurons in the medial septum. In the second experiment, water maze performance was correlated with the concentration of; (i) acetylcholine and serotonin in the striatum; (ii) serotonin and norepinephrine in the dorsal hippocampus; (iii) norepinephrine in the frontoparietal cortex and; (iv) with other functional markers such as the 5-hydroxyindoleacetic acid/serotonin ratio in the striatum, 3,4-dihydroxyphenylacetic acid/dopamine ratio in the dorsal hippocampus, 5-hydroxyindoleacetic acid/serotonin and homovanillic acid/dopamine ratios in the frontoparietal cortex, and 3,4-dihydroxyphenylacetic acid/dopamine ratio in the occipital cortex. The results indicate that cognitive deficits related to aging might involve concomitant alterations of various neurochemical systems in several brain regions such as the striatum, the hippocampus or the cortex. It also seems that these alterations occur in a complex way which, in addition to the loss of cholinergic neurons in the basal forebrain, affects dopaminergic, noradrenergic and serotonergic processes.

1999 Curt P. Richter award. Glucocorticoids and the regulation of memory consolidation

This paper summarizes recent findings on the amygdala's role in mediating acute effects of glucocorticoids on memory consolidation in rats. Posttraining activation of glucocorticoid-sensitive pathways involving glucocorticoid receptors (GRs or type II) enhances memory consolidation in a dose-dependent inverted-U fashion. Selective lesions of the basolateral nucleus of the amygdala (BLA) or infusions of beta-adrenoceptor antagonists into the BLA block the memory-modulatory effects of systemic injections of glucocorticoids. Additionally, posttraining infusions of a specific GR agonist administered directly into the BLA enhance memory consolidation, whereas those of a GR antagonist impair. These findings indicate that glucocorticoid effects on memory consolidation are mediated, in part, by an activation of GRs in the BLA and that the effects require beta-adrenergic activity in the BLA. Other findings indicate that the BLA interacts with the hippocampus in mediating glucocorticoid-induced modulatory influences on memory consolidation. Lesions of the BLA or inactivation of beta-adrenoceptors within the BLA also block the memory-modulatory effects of intrahippocampal administration of a GR agonist or antagonist. These findings are in agreement with the general hypothesis that the BLA integrates hormonal and neuromodulatory influences on memory consolidation. However, the BLA is not a permanent locus of storage for this information, but modulates consolidation processes for explicit/associative memories in other brain regions, including the hippocampus.

Prolactin levels in juvenile and adult rats following acute restraint and the open field

Acute restraint and exposure to a novel environment alter behavior and increase prolactin levels in rats quickly and reliably. However, little research is available that examines behavior and levels of prolactin as a result of acute exposure to one stressor immediately followed by a second stressor. Similarly, a relationship between prolactin and behavior has not been established. In the present study, juvenile (35-day-old) and adult (5-month-old) rats were either placed in a novel open field for 10 min or restrained for 10 min prior to exposure to the open field. Restrained juveniles groomed more than control juveniles and restrained adults. Conversely, restraint + open field reduced ambulation and rearing among juvenile males and females, and adult females ambulated less than control females and restrained males across both behaviors. In addition, results from the present study demonstrated the first reported relationship between prolactin and open-field behaviors. Prolactin was positively correlated with rearing and number of fecal boli, and levels were negatively correlated with freezing. Among adult females, prolactin levels were lower following restraint + open field than after exposure only to the open field. This may be explained by the psychological response to the open field when it directly followed the physical stressor of acute restraint.

5-HT-moduline controls serotonergic activity: implication in neuroimmune reciprocal regulation mechanisms

The serotonergic neurotransmission is known as a neuromodulatory system exerting its activity in the central nervous system (CNS) as well as at the periphery. The anatomical and morphological organization of the system based on a marked centralization of the cellular bodies and the large, almost ubiquitary, presence of axonal projections of the neurons is in good agreement with this modulatory role. Furthermore, a very high number of varicosities located along the axonal branches are capable of releasing serotonin (5-HT). The amine stimulates a number of different specific receptor types which allows 5-HT to exert different activities on its various cellular targets. Among these receptors, the 5-HT1B subtypes play a particular role as they are autoreceptors located on 5-HT neurons terminals and heteroreceptors located on non-serotonergic terminals where they control the release of the neurotransmitter. 5-HT-moduline, an endogenous tetrapeptide, regulates the efficacy of these 5-HT1B receptors, hence, is able to control the serotonergic activity in a synchronous manner for the various varicosities from a single neuron and thus may favour the differential effect of that neuron on distinct cerebral functions. Accordingly, the peptide allows the 'fine tuning' of the cerebral activity by the serotonergic system to elaborate the response given by the brain to a particular stimulus, that is, stress situations. At the periphery, the serotonergic system also appears to possess a regulatory activity via 5-HT1B receptors. In particular, the receptors located on immunocompetent cells control their activity and are themselves regulated by 5-HT-moduline likely originating from adrenal medulla and released after acute stress. The serotonergic system appears to play a major role in the reciprocal signalling existing between the neuronal and the immune system. The participation of 5-HT-moduline is likely in physiological functions as well as in pathological disorders affecting central and peripheral activities.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

Estrogens normalize the hypothalamic-pituitary-adrenal axis response to stress and increase glucocorticoid receptor immuno-reactivity in hippocampus of aging male rats

Aging is associated with a disturbance in the regulation of the hypothalamic-pituitary-adrenal axis (HPA) and reduced levels of glucocorticoid receptors (GR) in the hippocampus. To compensate for these effects, we have investigated whether estrogen therapy normalized the HPA response to stress and GR in hippocampus and paraventricular (PVN) nucleus. Young (3-4 months) and old (20 months) male Sprague-Dawley rats were bled by tail cut in the basal state and following ether stress. While basal and ether-stimulated levels of plasma corticosterone (CORT) were similar in the two groups, old animals presented a delayed termination of the response to ether stress. A dexamethasone inhibition test carried out in old animals, showed a failure to completely block plasma CORT after ether stimulation. Furthermore, in old rats GR-immunoreactive levels were reduced in CA1-CA2 hippocampal subfields and subiculum, while normal levels were obtained in CA3-CA4 and PVN. We observed that prolonged estrogen treatment (6 weeks) of old rats normalized the termination of the stress response, restored dexamethasone inhibition of plasma CORT, and increased GR immunoreactivity in CA1 and CA2 hippocampal subfields and subiculum. The results suggest that estrogen treatment enhanced the glucocorticoid feedback signal by increasing GR in hippocampus, and corrected the disturbances in HPA axis regulation. These animal experiments may be important to elucidate the effects of estrogenic on the hippocampal and HPA dysfunction associated with aging and Alzheimer's disease in humans.

Maintenance of hippocampal cell numbers in young and aged rats submitted to chronic unpredictable stress. Comparison with the effects of corticosterone treatment

Exposure of rats to sustained stress has been associated with behavioural impairments, the degree of impairment being greater with increasing age of the subject. Although the behavioural deficits have been frequently attributed to stress-induced neuronal loss in the hippocampus, the validity of that view may be disputed since it is based on data collected using conventional morphometric methods which are subject to bias. The question of whether stress per se does indeed induce hippocampal cell losses was therefore re-examined using unbiased stereological tools in the present work. Specifically, we used the optical fractionator and the Cavalieri principle, to respectively estimate the total number of neurons and volumes of the main divisions of the hippocampal formation of young and old rats which had been exposed for 1 month to an unpredictable stress paradigm. The efficacy of the treatment was confirmed by elevated serum corticosterone levels measured at various intervals during the experimental period. In order to evaluate whether any deleterious effects might have occurred merely due to the stress-induced elevations in corticosterone secretion, we conducted a parallel study on animals that were injected with corticosterone over a similar duration. Neither stress nor treatment with corticosterone was found to result in significant cell losses in any division of the hippocampal formation; likewise, neither treatment produced significant volumetric differences. Further, these results were not influenced by age of the experimental subjects. The present findings therefore call for a reappraisal of the hypothesis that hippocampal cell loss accounts for the behavioural impairments observed by others following prolonged stress and/or chronic elevation of serum corticosterone levels.

Changes with age in the modulation of natural killer activity of murine leukocytes by gastrin-releasing peptide, neuropeptide Y and sulfated cholecystokinin octapeptide

Several investigations have suggested that the interactions between the nervous and immune systems are modified with age. The aim of the present work was to study the effect of three neuropeptides: gastrin-releasing peptide (GRP), neuropeptide Y (NPY) and sulfated cholecystokinin octapeptide (CCK-8 s) on natural killer (NK) activity of spleen, thymus and axillary node leukocytes from BALB/c male, young (8+/-1 weeks), adult (24+/-2 weeks) and old (72+/-2 weeks) mice. We used cells from murine lymphoma YAC-1 as targets for the cytotoxic assay and three physiological concentrations of the neuropeptides (10(-8), 10(-10) and 10(-12) M). In control samples, in the absence of neuropeptide, we observed a decreased NK activity in young and old mice with respect to the adults in the three organs studied. Regarding the effect of the neuropeptides, GRP stimulates the cytotoxic activity of leukocytes from all locations, in adult animals. At the same age, NPY also stimulates the NK activity of leukocytes from axillary nodes and thymus, whereas it decreases the NK activity of spleen leukocytes from young mice. CCK-8 s has an inhibitory effect on the axillary node leukocytes from young mice and spleen leukocytes from old animals. However, CCK-8 s increased the NK activity of thymus leukocytes from young and adult mice. The results indicate that the highest values of NK activity are found in adult mice, and that the stimulating effect of the three neuropeptides studied on NK activity of leukocytes from adult mice are reduced or disappeared, in general, in old as well as in young animals. Furthermore, the changes observed with ageing in the modulation of NK activity by the neuropeptides studied suggest an altered integration of the nervous and immune systems.

Continuous blockade of brain glucocorticoid receptors facilitates spatial learning and memory in rats

Previously, a corticosterone surge associated with a learning task was shown to facilitate cognitive processes through brain glucocorticoid receptors (GR) while chronic overexposure to this stress hormone impaired cognition. In the present study we tested the hypothesis that opposing effects on learning and memory might also occur after either phasic or continuous blockade of brain GR by intracerebroventricular (i.c.v.) administration of the GR antagonist RU38486 (aGR). We used a Morris water maze procedure to assess spatial learning and memory abilities in male Wistar rats. The effect of phasic brain GR blockade was studied following daily pretraining administration of 10 and 100 ng/microL aGR i.c.v. on 3 consecutive days. This repetitive aGR treatment impaired spatial learning and memory dose-dependently in comparison with vehicle controls. For continuous brain GR blockade, animals received an i.c.v., infusion of aGR (10 and 100 ng/0.5 microL per h or vehicle) over 10 days. Infusion of 100 ng aGR per hour resulted in a long-lasting facilitation of spatial performance. The 10 ng aGR infusion also caused initially a facilitating effect, which was, however, transient and performance became impaired during retest. Possible anxiolytic properties of the drugs were excluded in view of the animals' behaviour in the elevated plus maze. Both doses of aGR infusion reduced the number of mineralocorticoid receptors in the hippocampus, but only the high dose of aGR resulted in a significant reduction of available GR sites. In conclusion, continuous administration of GR antagonist improves cognitive function, while phasic blockade of brain GR function causes a cognitive deficit.

Mineralocorticoid receptor also modulates basal activity of hypothalamus-pituitary-adrenocortical system in humans

Hippocampal mineralocorticoid (MRs) and glucocorticoid receptors (GRs) have been demonstrated to regulate the activity of the hypothalamus-pituitary-adrenocortical (HPA) system. To elucidate the role of the hippocampal MR in the circadian activity of the human HPA system, we studied diurnal secretory profiles of corticotropin (ACTH) and cortisol in 10 healthy male humans before and after an 8-day treatment with the MR antagonist spironolactone. 24-hour blood sampling at 30-min intervals was performed for estimation of cortisol (q30) and ACTH (q120). Saliva cortisol was measured for estimation of unbound cortisol. At the end of the 24-hour sampling period a corticotropin-releasing hormone (CRH) challenge was performed. High plasma concentrations of the active metabolite canrenone were achieved (begin of sampling: 2,653 +/- 693 nmol/l; end of sampling: 747 +/- 177 nmol/l). There was a significant increase in the diurnal minima (37.1 +/- 13.3 vs. 23.7 +/- 8.9 nmol/l, p < 0.02) and mean cortisol (193.5 +/- 25.8 vs. 173.0 +/- 23. 0 nmol/l, p < 0.03) plasma concentrations. However, the diurnal peak concentrations and pulsatile secretory features were unchanged after spironolactone treatment. For saliva cortisol, the only significant treatment difference was a decrease in the diurnal amplitude of cortisol relative to the diurnal mean concentration (2.56 +/- 0.47 vs. 3.11 +/- 0.87, p < 0.03). After spironolactone treatment there was a decrease in diurnal mean ACTH concentrations (46.2 +/- 14.4 vs. 41.8 +/- 10.3 pmol/l). There was no difference in the ACTH and cortisol response after infusion of CRH before and after spironolactone treatment. CBG plasma concentrations were significantly increased (22.4 +/- 2.3 vs. 19.2 +/- 2.7 mg/l, p < 0. 01) after spironolactone treatment, which possibly contributed to the observed increase in plasma cortisol. In summary, as predicted from animal studies we found significant effects of MR antagonization to be restricted to time windows of low HPA system activity. These findings are similar to the effects of aging upon the HPA system. However, the effect of spironolactone treatment was small, suggesting that the HPA system activity in humans is modulated but not regulated by the hippocampal MR.

Peripheral markers in testing pathophysiological hypotheses and diagnosing Alzheimer's disease

Alterations in amyloid precursor protein (APP) metabolism, calcium regulation, oxidative metabolism, and transduction systems have been implicated in Alzheimer's disease (AD). Limitations to the use of postmortem brain for examining molecular mechanisms underscore the need to develop a human tissue model representative of the pathophysiological processes that characterize AD. The use of peripheral tissues, particularly of cultured skin fibroblasts derived from AD patients, could complement studies of autopsy samples and provide a useful tool with which to investigate such dynamic processes as signal transduction systems, ionic homeostasis, oxidative metabolism, and APP processing. Peripheral cells as well as body fluids (i.e., plasma and CSF) could also provide peripheral biological markers for the diagnosis of AD. The criteria required for a definite diagnosis of AD presently include clinical criteria in association with histopathologic evidence obtained from biopsy or autopsy. Thus, the use of peripheral markers as a diagnostic tool, either to predict or at least to confirm a diagnosis, may be of great importance.

Forced swim test-induced neurochemical endocrine, and immune changes in the rat

The forced swim test (FST) is a behavioral paradigm that is widely used as a screening test for antidepressant activity in rodents. The objectives of the present study were to characterize the corticosterone and immune responses and in addition to examine neurotransmitter levels, in five brain regions at intervals (15, 30, 60, 90, and 120 min) following the second exposure to the FST. There was a significant but transient reduction in noradrenaline and 5-HT concentrations, in the hypothalamus 15 min post-FST exposure. 5-HT turnover in the frontal cortex and amygdala was significantly increased between 20-120 min post-FST exposure. The FST elicited a robust corticosterone response that peaked significantly at 30 min and had almost returned to baseline 120 min after exposure. There was a significant reduction in total white blood cell count 120 min after the FST, which was accompanied by a significantly reduced percentage of lymphocytes 90 and 120 min post-FST exposure. In addition, there was a significant but transient suppression of both PHA and Con A-induced lymphocyte proliferation 15 min following FST exposure. This study demonstrates that there are neurochemical changes that are coincident with the endocrine and immune changes associated with FST exposure in rats. Furthermore, this model could be used to examine the effects of manipulation of this stress response by antidepressant drugs. Such an investigation could add to our understanding of the interactions between antidepressants, stress and the neuroendocrine and immune systems.

Time-course of the murine lymphoid tissue involution during and following stressor exposure

Groups of 35-day-old male C57BL/6 mice were stressed 1 hour per day by immobilization for 1, 2, 3, 5, 8, 11 or 14 consecutive days. Control groups were left undisturbed. The animals were then killed and body weight and the weights of the thymus, spleen and axillary lymph nodes determined. Chronic immobilization stress caused involution of the thymus, spleen and lymph nodes to an extent depending on the number of days of stress. The thymus showed the fastest response: thymus weight was significantly lower in stressed animals than in controls by the third day of stress while significant effects on spleen and lymph node weight were not observed until day 5. Fast recovery of lymphoid organ weight was observed after the stress period. The thymus recovered most quickly: control values were re-attained approximately 8 days after cessation of stress, and indeed by day 20 thymus weight was about 12% higher than in normal animals. The spleen and lymph nodes recuperated weight more slowly, re-attaining control values after about 20 days.

Age-related decrease in the hypothalamic CRH mRNA expression is reduced by dehydroepiandrosterone (DHEA) treatment in male and female rats

Aging is frequently associated with changes in physiological and cognitive processes. Among these changes, it has been shown that aging has a profound impact on the hypothalamo-pituitary-adrenocortical (HPA) axis. Since there is controversy about the influence of aging on the HPA axis, the first objective of the present study was to clarify the effects of aging on the HPA axis using the genetic expression of hypothalamic corticotropin-releasing hormone (CRH). Changes in the CRH mRNA expression were quantified in the hypothalamic paraventricular nucleus (PVN) using in situ hybridization, in young (50 days) and old (18 months) rats of both sexes. The second objective was to determine the effects of dehydroepiandrosterone (DHEA) treatment on the age-related changes in CRH mRNA in both sexes. In aged animals of both sexes, CRH mRNA levels in PVN were significantly reduced by 20%. DHEA treatment increased the CRH mRNA expression in young rats and reversed the decrease in the CRH mRNA expression in old rats. Young males were more sensitive to DHEA than young females (+15% vs. +9%) whereas in old animals females showed more striking changes than males (+38% vs. +24%). These findings clearly demonstrate that in old apparently healthy rats of both sexes aging process induced a significant decrease in basal CRH mRNA levels and that DHEA treatment which exerts a positive influence on CRH gene expression completely reversed the decrease of the CRH mRNA levels associated with aging.

Neuroendocrine correlates of separation stress in the Siberian dwarf hamster (Phodopus sungorus)

Siberian dwarf hamsters form monogamous male-female pair bonds. Disruption of the pair bond results in increases in body mass and behavioral alterations similar to profiles seen in human atypical depression. We examined behavioral and neuroendocrine correlates associated with separation of the male from his mate. Animals were paired (n = 28 pairs) for 3 weeks, then 15 pairs were separated and 13 pairs remained as controls. Behaviors of the males were observed in a novel environment following 3 weeks of pairing and 4 weeks of separation. The 5-min behavioral test monitored exploratory, territorial behavior, and locomotor activity. Separated males showed a significant increase in body mass (p < 0.01). paralleled by an increase in food consumption (p < 0.01). Separated males had decreased seminal vesicle mass (p < 0.05) and testicular mass (p < 0.05). Behavioral analysis revealed that separated males showed no significant differences in grooming, scent-marking, alert on 2 feet, or escape behavior when compared to paired males. Separated males did show significant increases in inactivity (p < 0.05). Plasma cortisol levels were significantly increased in separated animals (p < 0.05), but there were no significant effects on testosterone. Resting levels of plasma norepinephrine and epinephrine were less in separated males, but this was significant only for norepinephrine (p < 0.05). In conclusion, separation stress was accompanied by increased hypothalamic-pituitary-adrenal axis function and decreased peripheral sympathetic nervous system activity and decreased reproductive profiles.

Androgen inhibits neurotransmitter turnover in the medial prefrontal cortex of the rat following exposure to a novel environment

Previous studies have demonstrated that gonadal steroid hormones affect the neuroendocrine response to a novel environment and other stressors. Introduction to a novel environment also increases neurotransmitter turnover in the medial prefrontal cortex (MPFC). In this study, we examined the possibility that gonadal steroid hormones could similarly modulate the neurotransmitter response to a novel environment in the MPFC of the male rat. Male Fischer 344 rats at 3 months of age were gonadectomized (GDX'd) and implanted with Silastic capsules containing dihydrotestosterone propionate (DHTP, a non-aromatizable form of androgen), 17 beta-estradiol (E), or placebo. Control animals were left intact. Each of these groups was further divided into a group introduced to a novel environment or a home cage control group. Animals exposed to a novel environment were killed after spending 20 min in a novel open field, whereas control animals were killed immediately upon removal from their home cage. Using high performance liquid chromatography, the MPFC was assayed for tissue levels of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylalanine (DOPAC) and homovanillic acid (HVA); norepinephrine (NE) and its metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG); or serotonin (5-HT) and its metabolite 5-hydroxyindole acetic acid (5-HIAA). The introduction to a novel environment caused significant increases in turnover of all three neurochemicals examined as estimated by metabolite/precursor ratios. These increases were characterized by increases in DOPAC, HVA, MHPG and 5-HIAA coupled with decreases in DA, NE and 5-HT. There was no effect of GDX on neurotransmitter turnover, however, treatment of GDX animals with DHTP prevented the open field induced increase in DOPAC/DA, MHPG/NE, and 5-HIAA/5-HT ratio. Treatment of GDX animals with estrogen had the opposite effect of DHTP, DOPAC/DA and MHPG/NE ratios increased to a greater level following the introduction to a novel environment than in GDX or intact animals. Examination of behavior in the open field showed significant decreases in activity in the DHTP-treated group but not in any other behavioral parameter (rears, nose pokes). Since the non-aromatizable androgen, DHTP, is presumably acting via androgen receptors, and E is presumably acting via estrogen receptors, these data suggest that, in the MPFC of male rats, androgen and estrogen receptors act in an opposing fashion to modify neurotransmitter turnover. This suggests that local changes in the relative levels of androgen and estrogen can have profound effects on the neurobiological response of the medial prefrontal cortex to stimuli.

Serotonin transporter expression in rat brain regions and blood platelets: aging and glucocorticoid effects

Hyperactivity of the hypothalamus-pituitary-adrenal axis is more common in elderly depression than in younger cohorts and glucocorticoids are known to influence serotonergic systems. The current study explores the interaction of glucocorticoids with aging on serotonin transporter expression and function. Continuous infusions of dexamethasone (26 days) reduced transporter expression in the aged brain but the ability of imipramine to inhibit synaptosomal [3H]serotonin uptake was unimpaired. These effects were unique to aged animals, as prior work with young adults found no effects of dexamethasone on transporter expression. In contrast to the effects in the brain, there were no differences in platelet transporter expression between young and old rats nor did dexamethasone treatment affect the values in the aged group: thus, the platelet may not reliably model these aspects of CNS function. The results suggest that there are basic biologic differences in the effects of glucocorticoids in aged vs. young brain that could contribute to lowered effectiveness to antidepressants in elderly depression; if transport capacity is already reduced by the effects of increased glucocorticoids, further inhibition of transport by antidepressants would have proportionally less impact on synaptic serotonin concentrations.

Stage-specific effects of prenatal d-methamphetamine exposure on behavioral and eye development in rats

Pregnant Sprague-Dawley rats were treated with 0, 5, 10, 15, or 20 mg/kg d-methamphetamine (MA), expressed as the free base, by SC injection (b.i.d., 8 h apart) on days 7-12 or 13-18 of gestation. Plasma concentration of MA and amphetamine were determined after the last dose. MA reduced gestation weight gain. The late exposure resulted in an increase in maternal and offspring mortality and reduced offspring growth. Offspring treated early in gestation with MA showed delayed development of early locomotion. In addition, memory impairment, evidenced by decreased target quadrant times and platform crossings on test trials and increased latency on reversal trials in the Morris spatial navigation maze, reduced spontaneous alternation, and lengthened passive avoidance retention latency was seen in the early treated high-dose groups. A reduction of serotonin was found in the nucleus accumbens following late exposure to MA at 20 mg/kg. Animals in both exposure groups had eye defects; however, the type of defect was dependent on the developmental stage at the time of dosing. Anophthalmia occurred only after early MA exposure, whereas folded retina was drug related only after late MA exposure. The behavioral effects did not show graded dose dependency; however, the effects were sensitive to exposure period. The early exposed animals had more alterations in behavior whereas the late exposed group showed higher mortality, reduced body weights, and neurochemical alterations.

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.

Effects of chronic alcohol consumption and aging on dopamine D2 receptors in Fischer 344 rats

Aging and chronic alcohol consumption are each accompanied by significant changes in dopamine and dopamine receptors. This study extended previous work by investigating the combined effects of chronic alcoholism and aging on total dopamine D2 receptors in brain areas associated with the nigrostriatal and mesocorticolimbic systems. In addition, the effects of chronic alcohol consumption and aging on the high-affinity state of D2 receptors and their conversion to the low-affinity form is included. Quantitative autoradiography was used to assess [3H]spiperone-labeled D2 receptors in tissue sections from 5- to 14- and 24-month Fischer 344 rats that were pair-fed a control or 6.6% (v/v) ethanol-containing liquid diet for 6 weeks. In addition, D2 receptors were determined in rats given the control liquid diet ad libitum. The results of these experiments demonstrated age-related changes in the nigrostriatal system. There was an age-related loss of total dopamine D2 receptors in the rostral and caudal striatum (approximately 25% decrease in Bmax). This decline in D2 receptors may be associated with changes in motor function. Despite the age-related decline in D2 receptors, there were no significant differences in the proportion of striatal receptors in the high-affinity form or in their conversion to the low-affinity state. Both aging and chronic alcohol consumption produced significant changes in the concentration of D2 receptors in brain areas associated with the mesocorticolimbic system. That is, the specific binding of [3H]spiperone was decreased in the frontal cortex of aged rats. In addition, chronic alcoholism was associated with a significant increase (approximately 20%) in the Bmax for D2 receptors in the nucleus accumbens. Nonetheless, neither age nor chronic alcohol consumption altered the proportion of high-affinity D2 receptors in the nucleus accumbens or their conversion to the lower affinity state. The observed changes in D2 receptors in the frontal cortex and nucleus accumbens are of interest because of the involvement of the mesocorticolimbic dopamine areas in the rewarding properties of alcohol and other drugs of abuse. Although aging and chronic alcoholism both produced significant changes in dopamine D2 receptor concentrations, alcohol did not accentuate the age-related loss of D2 receptors. We cannot eliminate the possibility that a more prolonged exposure of higher ethanol dose may potentiate age-related changes in the dopaminergic system.

Subchronic and chronic exposure to d-fenfluramine dose-dependently enhances splenic immune functions in young and old male Fischer-344 rats

Serotonin (5-HT) has been shown to modulate various arms of the rodent immune system in an age- and sex-dependent fashion. Thus, only young (5-6 months) male and old (21-23 months) female Fischer-344 (F344) rats demonstrated an elevation in ex vivo assessed basal and IL-2 stimulated splenic NK activity as well as CON-A induced T cell proliferation after subchronic (30-44 days) administration of low doses (0.6-1.8 mg/kg per day, p.o.) of the 5-HT releaser and reuptake inhibitor, d-fenfluramine (d-FEN). In the present study when young male F344 rats were administered higher doses of d-FEN (3-9 mg/kg per day, p.o.) for 30-39 days, there was a dose-dependent decrease in basal NK activity which returned to control levels after overnight incubation with IL-2. Further, only the rats receiving 6 mg/kg per day of d-FEN exhibited an elevation (40%) in CON-A mitogenesis compared to controls. When 15-month-old male rats were treated with d-FEN (0.6 mg/kg per day, p.o.) for 8 months, their NK and T cell activities at 23 months were not statistically different from young (7 months old) control animals. Importantly, neither the old rats treated with d-FEN nor the young control animals evidenced splenic or hepatic hypertrophy and lesions. In contrast, the old control animals showed increased NK activity (250%) and decreased T cell mitogenesis (300%) which correlated with a high incidence of splenic pathology. Thus, long-term exposure to d-FEN appears to maintain the NK and T cell arms of the immune system at youthful levels and prophylactically reduce the splenic pathology associated with advancing age. These results suggest that long-term exposure to increased levels of 5-HT may be beneficial to the immune system of the aging male F344 rat.

Effects of chronic alcohol consumption and aging on dopamine D1 receptors in Fischer 344 rats

The present study examined the hypothesis that chronic alcoholism augments the age-related loss of dopamine D1 receptors. This hypothesis was investigated because previous studies reported that both aging and chronic alcoholism produce significant changes in dopaminergic systems, and because chronic alcoholism potentiates some age-related CNS losses. In addition, this study investigated the effects of aging on D1 receptors in animals 1 and 7 days after withdrawal from chronic ethanol. Quantitative autoradiography was used to measure [3H]SCH 23390 binding to D1 receptors in brain areas associated with both the nigrostriatal and mesocorticolimbic dopamine systems. Receptors were assessed in 5-, 14-, and 24-month-old male Fischer 344 rats that were pair-fed a control or 6.6% (v/v) ethanol-containing liquid diet for 6 weeks. The results of these studies demonstrated that aging is associated with a significant decline in D1 receptors in the rostral and caudal striatum, and substantia nigra of both control and ethanol-fed rats. These receptor changes in the nigrostriatal system may be associated with motor abnormalities. In addition, there was an age-related decline in D1 receptors in two brain areas of the mesocorticolimbic system: the nucleus accumbens and frontal cortex. The latter findings may be important because of the involvement of this system with the rewarding properties of ethanol and other drugs of abuse. There were no age-related differences in the response of D1 receptors to ethanol withdrawal in the caudal and rostral striatum, substantia nigra, and nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoids, hippocampal corticosteroid receptor gene expression and antidepressant treatment: relationship with spatial learning in young and aged rats

The emergence of cognitive deficits in a subgroup of aged rats is associated with increased hypothalamic-pituitary-adrenal axis activity, decreased hippocampal mineralocorticoid and/or glucocorticoid receptor gene expression and neuronal loss. Short-term treatment with antidepressant drugs in young rats increases hippocampal corticosteroid receptor gene expression. In this study, the effects of chronic antidepressant administration on hippocampal mineralocorticoid and glucocorticoid receptor gene expression and spatial memory in young and aged rats were investigated. Young (eight months) and old (22 +/- 1 months) Lister-hooded rats were ranked according to watermaze performance. Matched pairs of rats were treated with amitriptyline (10 mg/kg) or saline daily for nine weeks, then reassessed in the watermaze. Amitriptyline significantly improved spatial memory in the young rats (33% increase in transfer test time) and increased hippocampal mineralocorticoid, but not glucocorticoid receptor messenger RNA expression. By contrast, in aged rats, amitriptyline had no effect on spatial memory or hippocampal corticosteroid receptor gene expression, either in cognitively unimpaired or cognitively-impaired animals. In aged rats, basal plasma corticosterone levels, which were significantly higher than in young animals, correlated negatively with spatial memory, while hippocampal glucocorticoid receptor mRNA expression correlated negatively with plasma corticosterone levels and positively with spatial memory. Amitriptyline had no significant effect on basal morning plasma corticosterone levels in either young or aged rats, but significantly decreased evening corticosterone levels in aged rats. Our data support the notion that corticosterone exerts a concentration-dependent biphasic influence, via selective activation of hippocampal mineralocorticoid and glucocorticoid receptor, on spatial memory. Amitriptyline improves spatial memory in young rats and increases hippocampal mineralocorticoid receptor gene expression. The lack of amitriptyline effect on spatial memory in aged rats may reflect decreased plasticity of both the synaptic processes underlying spatial memory and the regulation of hippocampal mineralocorticoid/glucocorticoid receptor expression, with mineralocorticoid receptors fully occupied due to elevated basal plasma corticosterone levels (in part a consequence of inadequate glucocorticoid receptor function).

Hypothalamic-pituitary-adrenal system function in patients with Alzheimer's disease

The neuropathologic hallmarks of Alzheimer's disease (AD) are very prominent in the hippocampus, a brain site which is pivotal for the regulation of the hypothalamic-pituitary-adrenal (HPA) system. Thus, the combined dexamethasone-suppression/CRH-stimulation-test outcome in patients with AD was compared to that of healthy elderly controls to assess--with a more refined neuroendocrine challenge procedure--HPA function in AD. Cortisol secretion after dexamethasone (DEX) pretreatment and before CRH was increased in Alzheimer's patients and 21% of this group were DST-nonsuppressors. None of the healthy control subjects escaped DEX-induced suppression of cortisol. However, after additional CRH administration, AD patients released significantly less cortisol and ACTH than the control subjects. No correlations were found between any of the endocrine parameters and degree of severity of dementia. It is concluded that the DST part of the DEX/CRH test better reflects glucocorticoid feedback disturbances, probably at a suprapituitary level. The CRH part of the DEX/CRH-test outcome might indicate the loss of endogenous CRH-Arginine-Vasopressin (AVP) synergism of the HPA system of these patients.

The effect of age on the monoamines of the hypothalamus

Measurement of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) homovanillic acid (HVA), 3-methoxytyramine (3-MT), noradrenaline (NA), 3-methoxy-4-hydroxyphenyl glycol (MHPG) and serotonin (5-HT) and its main metabolite, 5-hydroxyindol-3-acetic acid (5-HIAA) was assessed in hypothalamus and median eminence of aged rats. Age-related changes were not observed in the concentration of NA and its metabolites in median eminence. In contrast, there was a significant NA decrease in aged hypothalamus compared with 12 months (no differences were found compared with 3 months). No significant differences were found in DA concentration and its metabolites in hypothalamus but DA decreased significantly in aged median eminence compared with 12 months. The ratio 5-HIAA/5-HT, indicative of 5-HT turnover, appeared to increase in the hypothalamus and median eminence of the aged rat. Morphological dissimilarities between hypothalamus of young and aged rats were demonstrated using serotonin-immunocytochemistry. A degeneration of the serotoninergic system, denoted by the appearance of enlarged or swollen varicosities, was observed in the hypothalamus of the aged rat. These aberrant serotoninergic fibers may reflect the local degeneration of serotoninergic hypothalamic afferents during ageing. Such differential age-dependent alterations of the serotoninergic system might be responsible for at least some of the functional deficits in aged animals.

Alterations in central catecholamines associated with immune responding in adult and aged mice

Central catecholamine alterations associated with immune activity are similar to those seen following stressor exposure. Inasmuch as aged animals exhibit more pronounced stressor-provoked alterations of central amines relative to younger animals, it was of interest to determine whether immune challenge would similarly induce more pronounced central amine variations in older animals. Fifteen-month old CD-1 mice challenged with 10(7) sheep red blood cells (SRBC) revealed an equivalent peak splenic plaque-forming cell response (4 days after antigen challenge) to that of 3-month-old mice challenged with 10(6) cells. Neither plasma adrenocorticotropic hormone (ACTH) nor corticosterone levels varied over days following immunization, although ACTH levels were generally higher in the older mice. In both age groups reductions of hypothalamic and locus coeruleus norepinephrine (NE) and increased accumulation of the metabolite MHPG coincided with (or preceded by 24 h) the peak immune response. However, increased accumulation of MHPG in the hypothalamus was greater and occurred earlier in the locus coeruleus of the aged mice. Likewise, at or about the time of peak immune responses nucleus accumbens dopamine (DA) levels were reduced and metabolites elevated in both age groups, while in the prefrontal cortex only DA metabolite levels were elevated. These data are commensurate with previous findings showing that SRBC inoculation may influence central neurotransmitters and that such effects correspond with the time of the peak immune responses. Moreover, in so far as hypothalamic NE utilization is concerned, it seems that the effects of SRBC inoculation are more pronounced in aged animals.

Hippocampal corticosteroid receptor mRNA expression and spatial learning in the aged Wistar rat

The expression of mineralocorticoid (MR) and glucocorticoid receptor (GR) mRNA in the hippocampus of aged (18-24 months) and young (7 months) Wistar rats in relation to spatial learning in the water maze was investigated. All aged rats showed impaired performance of the task to locate a hidden platform compared to young controls, but the Wistar strain as a whole showed inferior performance to a group of young Lister hooded controls (added for the purposes of comparison) with even young Wistar rats apparently failing to learn platform location. The aged Wistar rats spent nearly 70% of their time near the side walls of the pool throughout the 5 days of training, while the young Wistar controls showed a gradual reduction in the percentage time spent there. MR and GR mRNA expression were unaltered in any hippocampal subregion in aged rats compared to young controls. Plasma corticosterone levels were also unchanged in aged rats. These results show that alterations in hippocampal corticosteroid receptor gene expression are not an inevitable consequence of aging in the rat, but that aging may be associated with strategy changes in the performance of a spatial learning task that are independent of glucocorticoid hypersecretion or alterations in hippocampal corticosteroid receptor mRNA expression.