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

The effects of amfonelic acid, a dopamine uptake inhibitor, on methamphetamine-induced dopaminergic terminal degeneration and astrocytic response in rat striatum

Administration of methamphetamine (MA) induces degeneration of dopaminergic nerve terminals and astrogliosis, such as hypertrophy and an increase in apparent number, in the neostriatum. In this experiment adult rats were treated with MA (10 mg/kg, i.p.) 4 times in one day at 2 h intervals. Amfonelic acid (AFA), a dopamine reuptake inhibitor, was administered (20 mg/kg, i.p.) at the same time the last MA dose was given. Three days later, dopaminergic terminals and astrocytes were examined immunohistochemically and the contents of striatal dopamine and its metabolites were analyzed by HPLC. The results showed that MA-induced the typical depletion of dopaminergic terminals, reduction of dopamine content and astrogliosis in the neostriatum. AFA treatment completely prevented the effects of MA on the dopaminergic system, both morphologically and biochemically. However, the reaction of astrocytes remained in the region where the most severe depletion of dopaminergic terminals was seen in MA treated animals (ventral-lateral portion of neostriatum). The results support the concept that the dopamine transporter is involved in MA-induced dopaminergic nerve terminal degeneration. The results also indicate that blocking the dopamine transporter cannot completely prevent the reaction of astrocytes in the neostriatum, which indicates that the astrocytic reaction can be induced by factors other than degeneration of dopaminergic terminals in this region. Based on these and other data, it is hypothesized that MA may cause degeneration of corticostriatal glutamate pathways and this effect may be responsible for the astrogliosis in MA-AFA treated animals.

Age-associated changes of pituitary-adrenocortical hormone regulation in humans: importance of gender

In the present study the hypothesis was tested that in normal human aging an insensitivity of the glucocorticoid feedback signals is acquired. Thus, 40 healthy elderly (mean age: 69 +/- 5 years) and 20 younger (mean age: 34 +/- 8 years) individuals underwent a combined dexamethasone suppression/CRH-stimulation test. Cortisol secretion after dexamethasone (DEX) pretreatment and before CRH was increased in the older age group, but none of the subjects escaped DEX-induced suppression of cortisol. However, after additional CRH administration to the DEX-pretreated volunteers, the older group released significantly more cortisol than their young counterparts. Within the group of the elderly only, a positive correlation between BASAL, DEX-pretreated cortisol concentration and post-CRH steroid responses was found. Gender profoundly affected DEX/CRH-test outcome: females, regardless of age, had an increased hormonal secretion in comparison to males. It is concluded that, during human aging, adaptive changes in glucocorticoid receptors take place, allowing for the system to maintain "peripheral" glucocorticoid homeostasis, but that more sophisticated challenge procedures such as the DEX/CRH test reveal an age-related increase in HPA system activity.

Spatial learning deficits in the aged rat: neuroanatomical and neurochemical correlates

To assess neurochemical and neuroanatomical correlates of age and spatial learning, aged Sprague-Dawley male rats (20-22 mo) were divided into two groups based on their ability to locate a hidden platform in a Morris water maze. An "old good" group of rats acquired the task as rapidly as young (3-6 mo) animals, whereas an "old poor" group of rats failed to show improvement on subsequent testing days. Age-related changes included (a) a significant decrease in the number of choline acetyltransferase (CHAT) immunoreactive cells in the ventral cell group of the septal complex (28%); (b) a decrease in caudate dopamine levels (-11%); and (c) an increase in 5-HIAA levels in the n. accumbens (+25%) and hippocampus (+18%). Spatial learning related changes in aged rats included: (a) an increase in medial frontal cortex 5-HIAA levels (52%) in the old good learners but not old poor learners with (b) a decrease in medial frontal cortex dopamine levels (-24%) only in the old poor learners group and (c) a decrease in n. accumbens DOPAC (-22%) and HVA (-23%) in the old good learners group only. The present study demonstrates age-related but not spatial learning related decrease in CHAT immunoreactive cells in the ventral cell group of the septal complex. Therefore, either the cholinergic cell loss in the septum is unrelated to the acquisition of spatial learning measured by the Morris water maze, or it is a permissive effect along with specific alterations in forebrain dopaminergic and serotonergic systems, particularly in the medial frontal cortex and n. accumbens. The above findings are consistent with findings seen in Alzheimer's disease where both basal forebrain cholinergic nuclei and cortical projecting brainstem monoamine systems are affected.

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.

Age-related alteration in signal transduction: involvement of the cAMP cascade

In the present study, we have investigated the involvement of the cAMP signal transduction pathways in young and aged rats. A significantly higher endogenous adenosine 3':5'-cyclic monophosphate (cAMP) level and a significant decline of the adenylate cyclase [AC, ATP pyrophosphate-lyase (cyclizing), EC.4.6.1.1.] activity were observed in striatal tissue from young rats (3 months) in comparison to aged rats (approximately 40 months). In the nucleus accumbens (NA), no age-dependent changes in the cAMP concentration and in the AC basal activity were found. To address the question, whether the interactions of guanine nucleotide-binding protein (G-protein) subunits (G alpha s and Gi) with AC have changed in the aging process, various pharmacological agents that modulate the AC activity (e.g., beta, tau-imidoguanine 5'-triphosphate (GppNHp), sodium fluoride (NaF), forskolin (FSK), and the combinations of GppNHp plus FSK, NaF plus FSK, and NaF plus ethanol (ETOH)) were applied. In addition, a [3H]FSK binding test was carried out. In striatal and NA tissue, the stimulation of the AC activity by FSK was inhibited by GppNHp (via Gi-protein) and was superadditive by the combination of FSK and NaF (via Gs-protein). The absolute AC activity upon stimulation by all agents used was significantly lower in the aged striatum compared to young striatum. In the NA, however, the AC activity showed an age-dependent reduction only upon FSK and upon FSK plus GppNHp stimulation. There was no difference in the specific [3H]FSK binding to the G alpha s protein-coupled catalytic subunit of the AC between young and aged animals both in the striatum and NA.(ABSTRACT TRUNCATED AT 250 WORDS)

Generation of cellular immune responses against a glioma-associated antigen(s)

The study demonstrated that RT2, a highly malignant anaplastic glioma, expresses antigens that make it susceptible to in vivo adoptive immunotherapy with cytotoxic T lymphocyte-containing immune cell populations. Rats were immunized with irradiated RT2 tumor cells and the adjuvant C. parvum. Lymphocytes from immunized rats were restimulated in vitro with irradiated RT2 tumor cells plus interleukin-2 (IL-2). The cells that proliferated and differentiated in vitro effectively killed RT2, but only low levels of cytotoxicity were observed against other histopathologically related and unrelated, syngeneic and allogeneic target cells. Adoptive transfer of immune cells combined with a 5-day course of systemic IL-2 produced specific regression of brain tumors growing as lung microfoci.

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.

Effects of hypothalamic peptides on the aging brain

The hypothalamic peptide hormones, TRH, LHRH (GnRH), CRH, GHRH, and GHIRH (somatostatin), influence the release of the anterior pituitary hormones, which in turn promote the release of target endocrine gland hormones and other metabolites. These latter compounds feed back to the brain to help control the secretion of the hypothalamic hormones. This is a dynamic interaction that is influenced by the aging process: Most of these hormones systems become less responsive with advancing age, due to decreased function of peptide-containing secretory neurons, a loss of hormone receptor sensitivity, and/or a reduction in the output of the target endocrine glands. That the hypothalamic peptides themselves can influence brain function is supported by the fact that most are found in areas of the brain other than the hypothalamus and that receptors for them exist in these other areas. For example, CRH is contained in a number of central neural systems that can influence behavior, including limbic areas, the hypothalamus, locus coeruleus, median raphé nuclei, and cortical interneurons. CRH has been shown to be anxiogenic in animal models, and its effect can be blocked by CRH receptor antagonists. CRH content in the locus coeruleus is particularly increased by stress and may influence norepinephrine neurotransmitter function in this structure. In aging there is a gradual reduction of the sensitivity of the brain to the negative feedback of corticosteroids, such that CRH secretion becomes somewhat increased under basal conditions. The behavioral effects of this change are unclear, however, as is the influence of stress-related activation of CRH, ACTH, and glucocorticoid secretion on behavior in the elderly. Other hypothalamic peptides have different patterns of change with aging, and some are markedly altered in pathological conditions; for example, in Alzheimer's disease the content of CRH and somatostatin in certain brain areas is decreased. However, whether the changes in hypothalamic peptides precede or follow the pathological behavioral changes, and how they participate in the changes, is still unclear.

Conditioned neuroendocrine and cardiovascular stress responsiveness accompanying behavioral passivity and activity in aged and in young rats

Mean arterial pressure (MAP), heart rate (HR), plasma epinephrine (E), plasma norepinephrine (NE), and plasma corticosterone (CORT) were measured in 3-month- and 24-month-old male Wistar rats exposed to a conditioned emotional stress response (CER) paradigm and a conditioned defensive burying (CDB) paradigm. In the CER situation blood samples were taken during reexposure to the training environment one day after a single inescapable footshock (0.6 mA, AC for 3 s) had been administered. In the CER paradigm the young rats displayed passive behavior (immobility) accompanied by an increase in plasma levels of CORT and E, whereas both the control and conditioned animals showed increased NE responses. Previously shocked aged rats exhibited an attenuated plasma NE response, whereas levels of E remained elevated to a greater extent. Aged animals showed elevated basal levels of CORT one day after footshock administration. Stress-induced immobility was preserved in the aged rats. These animals had an increase in basal MAP values and a decrease in basal HR values compared to young ones. In the CDB paradigm, rats were exposed to a nonelectrified probe 1 day after the repeated shock (2 mA/contact) procedure. Young rats displayed defensive burying accompanied by increments in MAP, HR, CORT, and NE. The aged animals showed similar hormonal, autonomic, and behavioral stress responses. Thus, the age-related alterations in neuroendocrine and autonomic response patterns are apparent in stressed animals during behavioral passivity in absence of control (CER) rather than during active control (defensive burying).

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.

Preliminary evidence for methamphetamine-induced behavioral and ocular effects in rat offspring following exposure during early organogenesis

Gravid Sprague-Dawley CD (VAF) rats received 50 mg/kg (d,l)-methamphetamine (MA) HCl (expressed as free base, N = 15) or distilled water (N = 6) by SC injection x 2/day in a 3 ml/kg volume on embryonic (E) days 7-12. Control rats were pair-fed to MA-exposed dams on days E7-18. No control dams failed to deliver; however, of 15 MA-exposed dams 4 did not deliver (2 died and 2 had completely resorbed litters). One additional MA litter had all the offspring die shortly after birth. There was no difference between groups on offspring postnatal (P) body weight. The offspring exposed prenatally to MA had significantly lower olfactory orientation scores (P9, 11, 13) to their home cage scent. In a test of early activity (P10, 12, 14) the MA-exposed progeny were marginally less active than controls. MA-exposed offspring exhibited hyperreactivity and marginally shortened response latency on a test of acoustic startle (P27). Motor activity showed no differential response in MA treated or control offspring to MA (P63) or fluoxetine challenge (P70). However, the MA offspring were more active than controls with respect to central and side activity during the second week of testing. No group differences were found for performance in a straight swimming channel or on the number of errors committed or latency to escape in a complex (Cincinnati) water maze (P84). Prenatal exposure to MA also induced eye defects (i.e., anophthalmia, microphthalmia and folded retina) in 16.7% of the progeny. However, MA did not effect hippocampal or neostriatal monoamine levels when measured on P28.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of subchronic d-fenfluramine on splenic immune functions in young and old male and female Fischer 344 rats

The present study was designed to demonstrate age- and sex-related differences in immune functions, and to determine whether subchronic elevations in serotonin (5-HT) availability in vivo would alter immune functions assessed subsequently in vitro. Male and female F344 rats (5 and 21 months of age) were administered the 5-HT releaser and reuptake inhibitor, d-fenfluramine (d-Fen), in their drinking water for 30-38 days then killed. The young animals received a higher dose (1.8 mg/kg/day) of d-Fen than the old rats (0.6 mg/kg/day) in order to compensate for age-related decreases in drug biotransformation and clearance. Brain and spleen d-Fen and metabolite concentrations, however, were considerably higher in the young than in the old rats. d-Fen treatment did not affect body weight or fluid intake. Although substantial sex differences in immune function were not discerned, age-related decreases were observed in absolute splenic cellularity, recombinant interleukin-2 (rIL-2) stimulated natural killer (NK) cytotoxicity, LPS stimulated B-cell mitogenesis, and in the level of Ox19 (CD5) positive cells. d-Fen caused an increase in absolute spleen weight and a decrease in absolute splenic cellularity only in the old rats of both sexes. Spleen cells from young male and old female rats receiving d-Fen had relatively more large granular lymphocytes and enhanced baseline and rIL-2 activated killing of YAC-1 cells than their vehicle matched or opposite sex counterparts. The drug also increased Con A-induced T-cell proliferation in young males and LPS induced B-cell proliferation in old females. d-Fen decreased Ox39 (CD25) levels by 19%, but did not affect any of the other phenotypes examined. The results suggest that 5-HT has a selective stimulatory effect on young male and old female NK activity, and that old female rats are more sensitive to the immunological effects of d-Fen than old male rats.

Brain corticosteroid receptor gene expression and neuroendocrine dynamics during aging

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 in old (30 months) and young (3 months) male Brown Norway rats. The data demonstrate that circulating ACTH rather than the corticosteroid plasma level was elevated under basal conditions and following stress. Furthermore, a reduction of mineralocorticoid receptor (MR) number in the hippocampus and of glucocorticoid receptor (GR) number in the hypothalamus and the pituitary correspond to increased neuroendocrine responsiveness and negative feedback following stress. The changes in receptor binding do not parallel the changes in the amount of MR and GR mRNA measured with in situ hybridization. This suggests that the processing rather than the receptor gene expression is affected in senescence.