Stress induces atrophy of apical dendrites of hippocampal CA3 pyramidal neurons

Neonatal stress alters LTP in freely moving male and female adult rats

We previously reported that neonatal isolation stress significantly changes measures of hippocampal long-term potentiation (LTP) in male and female juvenile rats, i.e., at 30 days of age. The changes in dentate granule population measures, i.e., excitatory postsynaptic potential (EPSP) and population spike amplitude (PSA), evoked by tetanization of the medial perforant pathway, indicated that juvenile rats exposed to neonatal isolation exhibit different enhancement profiles with respect to both the magnitude and duration of LTP in a sex-specific manner. Isolated males showed a significantly greater enhancement of LTP, while female "isolates" showed significantly longer LTP duration when compared to all other groups. The present study was designed to determine whether the effects of the neonatal isolation stress paradigm endures into adulthood. Rats isolated from their mothers for 1 h per day during postnatal days 2-9 were surgically prepared at 70-90 days of age, with stimulating and recording electrodes placed in the medial perforant pathway and the hippocampal dentate gyrus, respectively. Prior to tetanization, no significant effect of sex or treatment was obtained for baseline measures of EPSP slope or PSA. In order to rule out baseline differences in hippocampal cell excitability in female adult rats, we measured the response of dentate granule cells for one estrus cycle and found no pretetanization enhancement in the evoked response in either controls or previously stressed rats. Following tetanization, there was a significant treatment and sex effect. During the induction of LTP, PSA values were significantly enhanced in both isolated males and females and had significantly longer LTP duration when compared to the unhandled control group. Additionally, we observed that females took longer to reach baseline levels than males. Taken together, these results indicate that repeated infant isolation stress enhances LTP induction and duration in both males and females. These results indicate that infant stress alters hippocampal neuroplasticity in such a way that its effect endures into adulthood.

Decrease in cortisol reverses human hippocampal atrophy following treatment of Cushing's disease

BACKGROUND

Decreased hippocampal volume is observed in patients with Cushing's syndrome and other conditions associated with elevated cortisol levels, stress, or both. Reversibility of hippocampal neuronal atrophy resulting from stress occurs in animals. Our study investigated the potential for reversibility of human hippocampal atrophy.

METHODS

The study included 22 patients with Cushing's disease. Magnetic resonance brain imaging was performed prior to transsphenoidal microadenomectomy and again after treatment.

RESULTS

Following treatment, hippocampal formation volume (HFV) increased by up to 10%. The mean percent change (3.2 +/- 2.5) was significantly greater (p < .04) than that of the comparison structure, caudate head volume (1.5 +/- 3.4). Increase in HFV was significantly associated with magnitude of decrease in urinary free cortisol (r = -.61, p < .01). This relationship strengthened after adjustments for age, duration of disease, and months elapsed since surgery (r = -.70, p < .001). There was no significant correlation between caudate head volume change and magnitude of cortisol decrease.

CONCLUSIONS

Changes in human HFV associated with sustained hypercortisolemia are reversible, at least in part, once cortisol levels decrease. While many brain regions are likely affected by hypercortisolemia, the human hippocampus exhibits increased sensitivity to cortisol, affecting both volume loss and recovery.

Stress and hippocampal neurogenesis

The dentate gyrus of the hippocampal formation develops during an extended period that begins during gestation and continues well into the postnatal period. Furthermore, the dentate gyrus undergoes continual structural remodeling in adulthood. The production of new granule neurons in adulthood has been documented in a number of mammalian species, ranging from rodents to primates. The late development of this brain region makes the dentate gyrus particularly sensitive to environmental and experience-dependent structural changes. Studies have demonstrated that the proliferation of granule cell precursors, and ultimately the production of new granule cells, are dependent on the levels of circulating adrenal steroids. Adrenal steroids inhibit cell proliferation in the dentate gyrus during the early postnatal period and in adulthood. The suppressive action of glucocorticoids on cell proliferation is not direct but occurs through an NMDA receptor-dependent excitatory pathway. Stressful experiences, which are known to elevate circulating levels of glucocorticoids and stimulate hippocampal glutamate release, inhibit the proliferation of granule cell precursors. Chronic stress results in persistent inhibition of granule cell production and changes in the structure of the dentate gyrus, raising the possibility that stress alters hippocampal function through this mechanism. This review considers the unusual developmental profile of the dentate gyrus and its vulnerability to environmental perturbations. The long-term impact of developmental events on hippocampal function is considered.

Behavioural trait of reactivity to novelty is related to hippocampal neurogenesis

The hippocampal formation is one of the brain areas where neurogenesis persists during adulthood, with new neurons being continuously added to the population of dentate granule cells. However, the functional implications of this neurogenesis are unknown. On the other hand, the hippocampal formation is particularly concerned with the detection of novelty, and there are indications that dentate granule cells play a significant role in this function. Recently, the existence of inter-individual differences in behavioural reactivity to novelty has been evidenced, related to differences in the reactivity of the hypothalamic-pituitary-adrenal axis (HPA). Rats that are highly reactive to novelty (HR) exhibit a prolonged corticosterone secretion in response to novelty and to stress when compared with low reactive rats (LR). Taking advantage of the existence of these inter-individual differences, we investigated whether neurogenesis in the dentate gyrus is correlated with the behavioural trait of reactivity to novelty. Rats were first selected according to their locomotor reactivity to a novel environment. Two weeks later, cell proliferation, evaluated by the incorporation of 5-bromo-2'-deoxyuridine (BrdU) in progenitors, was studied by immunohistochemistry. We found that cell proliferation in the dentate gyrus was negatively correlated with locomotor reactivity to novelty. Indeed, cell proliferation in LR rats was twice that observed in HR rats. In contrast, survival of nascent neurons was not influenced by the behavioural trait of reactivity to novelty. Using an unbiased stereology, we show that LR rats had more cells within the granule cell layer of the dentate gyrus than did HR rats. These results demonstrate the existence of inter-individual differences in neurogenesis and total granule cell number within the dentate gyrus. These differences in hippocampal plasticity can be predicted by the behavioural trait of reactivity to novelty.

Brain opioid receptor density relates to stereotypies in chronically stressed pigs

Opioid receptor densities were measured in the hippocampus of chronically stressed (tethered) pigs to study the involvement of endogenous opioid systems in stereotypy performance. Three groups of animals were housed tethered for 2 (n=12), 5.5 (n=12) and 8-9 months (n=8), respectively, and the intensity of stereotypy performance was determined. Opioid receptor densities were measured post mortem using membrane binding assays with [(3)H]naloxone as a ligand. A negative correlation was found between the density of opioid receptors and the intensity of stereotypy performance in the animals that had been housed tethered for 2 months. This correlation seemed to disappear with increasing duration of tethered housing. The data further suggest that, associated with the duration of tediered housing, there was a gradual decrease in the density of opioid receptors in the left hippocampal lobe of the low-stereotyping animals, but not in the right lobe, nor in the left and right lobes of the high-stereotypers. This suggests that chronic stress leads to a (asymmetrically expressed) progressive loss of opioid receptors in the hippocampus, and that stereotypies exert a mitigating effect on stress-induced changes in opioid receptor densities, supporting the hypothesis that stereotypies help the animals cope wife the adverse effects of chronic stress.

The effects of prenatal stress on the development of hypothalamic paraventricular neurons in fetal rats

The present experiments focused on the influence of prenatal stress on the development of neurons of the hypothalamic paraventricular nucleus in the fetal rat, including corticotropin-releasing factor-containing neurons. Prenatal stress was administered by restraining pregnant rats in a small cage for either 30 (30-min stress group) or 240 min (240-min stress group) daily for three days from embryonic day 15 to 17, and the fetal brains were taken on embryonic day 18 for later analysis. Golgi-impregnated neurons of the paraventricular nucleus in the 240-min stress group revealed that the total length of the processes was significantly shorter than in the control (unstressed) and 30-min stress groups. In addition, the 240-min stress group showed an increase in the number of apoptotic cells in the fetal paraventricular nucleus. On the other hand, Golgi-impregnated neurons of the paraventricular nucleus in the 30-min stress group had a greater degree of cell differentiation as manifested by an increase in both the number of branch points and the total length of the processes from the cell body. Furthermore, the fetal paraventricular nucleus in the 30-min stress group showed enhanced corticotropin-releasing factor messenger RNA expression, while the varicosities of corticotropin-releasing factor-containing axons at the median eminence revealed more matured morphology such as shorter intervals between the varicosities. These findings suggest the duration-dependent effects of prenatal stress on the development of fetal hypothalamic paraventricular nucleus neurons, including corticotropin-releasing factor-containing neurons: long-lasting stress causes neurotoxic changes of fetal paraventricular nucleus neurons, whereas short-lasting stress facilitates the development of these fetal brain neurons. These morphological changes induced by prenatal stress may contribute to behavioral changes of the offspring after birth.

Depression duration but not age predicts hippocampal volume loss in medically healthy women with recurrent major depression

This study takes advantage of continuing advances in the precision of magnetic resonance imaging (MRI) to quantify hippocampal volumes in a series of human subjects with a history of depression compared with controls. We sought to test the hypothesis that both age and duration of past depression would be inversely and independently correlated with hippocampal volume. A sample of 24 women ranging in age from 23 to 86 years with a history of recurrent major depression, but no medical comorbidity, and 24 case-matched controls underwent MRI scanning. Subjects with a history of depression (post-depressed) had smaller hippocampal volumes bilaterally than controls. Post-depressives also had smaller amygdala core nuclei volumes, and these volumes correlated with hippocampal volumes. In addition, post-depressives scored lower in verbal memory, a neuropsychological measure of hippocampal function, suggesting that the volume loss was related to an aspect of cognitive functioning. In contrast, there was no difference in overall brain size or general intellectual performance. Contrary to our initial hypothesis, there was no significant correlation between hippocampal volume and age in either post-depressive or control subjects, whereas there was a significant correlation with total lifetime duration of depression. This suggests that repeated stress during recurrent depressive episodes may result in cumulative hippocampal injury as reflected in volume loss.

Ligand and subfield specificity of corticoid-induced neuronal loss in the rat hippocampal formation

Adult male rats were treated chronically with the selective type II corticosteroid receptor agonist dexamethasone, with dexamethasone plus aldosterone, a selective type I receptor agonist, and with a supraphysiological dose of corticosterone sufficient to occupy both type I and type II receptors; injection-free and oil (vehicle)-treated rats served as controls. Following one month of treatment, the animals were killed and their brains were processed for stereological assessment of volumes and total numbers of neurons in the hippocampal formation. Dexamethasone treatment resulted in significant reductions in the total number of dentate granule and the CA3 pyramidal cells and in the volumes of some layers of these subfields; however, this steroid did not influence any morphometric parameter in the CA1 subfield, and the number of hilar cells was also unaltered. In contrast to the results obtained with dexamethasone, the other two groups of corticoid injected animals did not reveal changes in total cell numbers in any of the subfields of the hippocampal formation, although in the corticosterone-treated group a reduction in the volumes of the hilus and of the stratum radiatum of the CA3 subfield was observed. The present data show that the exclusive activation of type II corticosteroid receptors results in subfield-specific neuronal loss in the hippocampal formation of rats. This type II receptor-mediated neuronal loss can, however, be abrogated by the simultaneous stimulation of type I corticosteroid receptors. Together, these findings extend and support previous studies which suggested that activation of type I corticosteroid receptors may promote neuronal survival and that neurodegeneration may be triggered by type II corticosteroid receptor stimulation. An important implication of this result is that elevated levels of the endogenous corticosteroid receptor ligands (e.g., during stress) is unlikely to cause severe structural damage to the hippocampal formation due to the contemporaneous occupation of type I receptors.

Self-stimulation rewarding experience induced alterations in dendritic spine density in CA3 hippocampal and layer V motor cortical pyramidal neurons

Self-stimulation rewarding experience induced alterations in the numerical density of spines in CA3 hippocampal and layer V motor cortical pyramidal neurons in adult male Wistar rats was evaluated. Self-stimulation experience was provided 1 h daily over a period of 10 days through stereotaxically implanted bipolar stainless steel electrodes bilaterally in lateral hypothalamus and substantia nigra-ventral tegmental area. After 10 days, rats were killed and the hippocampus and motor cortex were processed for rapid Golgi staining procedure. The dendritic spine densities were studied in CA3 hippocampal and layer V motor cortical pyramidal neurons. The spine densities were quantified in five successive segments of 15.2 microm up to a distance of 76 microm. Apical dendrites were classified as mainshaft, sub branch, oblique shaft-I, oblique shaft-II, primary branch; and basal dendrites as main shaft, primary branch and secondary branch. A grand total of 864 CA3 hippocampal and 1008 layer V motor cortical dendrites were analysed for spine counting in different groups of rats. The results revealed a significant (P<0.001; ANOVA, F-test) increase in the number of spines in all the categories of dendrites in apical and basal regions in both hippocampal and motor cortical neurons in self-stimulation group of rats. Such changes were not observed either in sham control, experimenter-administered or normal control groups of rats. The self-stimulation induced increase in the spine density suggests an increase in the postsynaptic receptive field in CA3 hippocampal and layer V motor cortical neurons. This might enhance the efficacy of synaptic transmission in these neurons. Our study clearly demonstrated the self-stimulation rewarding experience induced postsynaptic plasticity in hippocampal and motor cortical pyramidal neurons.

Opposite effects on NCAM expression in the rat frontal cortex induced by acute vs. chronic corticosterone treatments

The temporal pattern of exposure to glucocorticoids has been reported to be a critical factor in determining the outcome of glucocorticoid actions at the brain. In this work, the effects of different regimes of subcutaneous corticosterone administration (acute-single injection-vs. chronic-daily injection for 21 days) on the expression of the neural cell adhesion molecule (NCAM) were evaluated in different rat brain regions (CA1-CA4, dentate gyrus, frontal cortex, striatum, and hypothalamus). The treatments were selected according to previous studies in which we showed biphasic effects of corticosterone on memory formation, with acute corticosterone effects being facilitating and chronic effects being deleterious. In addition, the chronic treatment was shown by others to result in structural alterations at the hippocampus. NCAM was evaluated given its cell-cell recognition and adhesion properties, and the involvement on synaptic stabilisation subserving long-term memory formation. The results showed a biphasic modulation of NCAM levels at the frontal cortex, with acute corticosterone resulting in enhanced NCAM levels at 8 h and 24 h posttraining, and the chronic treatment decreasing its expression. None of the other brain areas examined showed significant changes in NCAM expression with corticosterone treatments, except for the hypothalamus that showed reduced NCAM levels after the chronic corticosterone regime. These results support the view that NCAM regulation at the frontal cortex might be a mechanism by which corticosterone treatments influence memory formation. They also highlight the hypothalamus as a brain area particularly sensitive to NCAM regulation by prolonged exposure to elevated glucocorticoids.

Is learning blocked by saturation of synaptic weights in the hippocampus?

Long-term potentiation (LTP) has become a leading candidate mechanism for memory formation. The proposed link between LTP and memory rests primarily on a single type of behavioural evidence: disruption of learning by interventions that block critical steps in the induction of LTP. As such blockade may disrupt non-mnemonic functions also, the LTP-learning question should be approached with multiple strategies. One alternative approach is to determine whether hippocampus-dependent learning is blocked by saturation of hippocampal LTP before training. Early investigations found that spatial learning was impaired after cumulative LTP in dentate perforant-path synapses. Several groups failed to replicate these findings, but it is now clear that hippocampus-dependent spatial learning is disrupted only if LTP is saturated throughout the terminal field of the tetanized pathway. Moreover, to prevent compensatory modifications in the hippocampal network, a massed tetanization and training protocol may be required. The blockade of learning by repetition of the very same stimulus that induces LTP suggests that LTP-like modifications are necessary for memory encoding in the hippocampus.

Regulation of GLUT-3 glucose transporter in the hippocampus of diabetic rats subjected to stress

Previous studies from our laboratory have demonstrated that chronic stress produces molecular, morphological, and ultrastructural changes in the rat hippocampus that are accompanied by cognitive deficits. Glucocorticoid attenuation of glucose utilization is proposed to be one of the causative factors involved in stress-induced changes in the hippocampus, producing an energy-compromised environment that may make hippocampal neuronal populations more vulnerable to neurotoxic insults. Similarly, diabetes potentiates neuronal damage in acute neurotoxic events, such as ischemia and stroke. Accordingly, the current study examined the regulation of the neuron-specific glucose transporter, GLUT-3, in the hippocampus of streptozotocin-induced diabetic rats subjected to restraint stress. Diabetes leads to significant increases in GLUT-3 mRNA and protein expression in the hippocampus, increases that are not affected by stress. Collectively, these results suggest that streptozotocin-induced increases in GLUT-3 mRNA and protein expression in the hippocampus may represent a compensatory mechanism to increase glucose utilization during diabetes and also suggest that modulation of GLUT-3 expression is not responsible for glucocorticoid impairment of glucose utilization.

Long-lasting effects of stress on glucocorticoid receptor gene expression in the rat brain

Stressful stimuli are known to affect glucocorticoid receptor (GR) mRNA levels in the rat brain. The aim of this study was to examine the duration of chronic stress-induced changes in GR gene expression in the male rat hippocampus and cerebellum. By using in situ hybridization histochemistry, we detected a statistically significant down-regulation of GR mRNA both in the hippocampus and in the cerebellum of rats stressed for 8, 10 and 14 days. The same degree of down-regulation could also be detected in the above brain areas of rats stressed for 14 days and left undisturbed for 48 h or 8 days after stress. To examine the effects of subsequent stressors on the expression of down-regulated GR mRNA in the hippocampus of chronically stressed rats, we determined, by Northern blotting, GR mRNA levels in the hippocampi of rats stressed for 14 days and subsequently exposed to either short- or long-duration stressors. The down-regulated levels of GR mRNA remained practically unaffected when a subsequent new stressor was applied. Our results show that chronic stress-induced down-regulation of GR mRNA in the rat brain can be extended for periods longer than the initial/causative stimulus, irrespective of the presence of a novel stimulus.

Effects of antidepressants and benzodiazepine treatments on the dendritic structure of CA3 pyramidal neurons after chronic stress

Both repeated stress and corticosterone administration induce remodeling of apical dendrites of hippocampal CA3 pyramidal neurons. Circulating glucocorticoids are involved in the mechanism that produces atrophy, along with excitatory amino acids and serotonin (5-hydroxytryptamine, 5-HT). We used 5-HT-related antidepressants and a benzodiazepine in order to explore indirectly the role of serotonin and GABA(A)-benzodiazepine receptors in the stress-induced structural changes visualized by the Golgi impregnation of the rat hippocampus. The 5-HT reuptake enhancer (+/-)-tianeptine prevented the dendritic atrophy caused by repeated restraint stress in a non-stereoselective fashion and two 5-HT reuptake antagonists, fluoxetine and fluvoxamine, failed to block dendritic atrophy. Tianeptine also functions as a therapeutic tool since it reversed the already established hippocampal atrophy caused by treatment with corticosterone for 3 weeks. Finally, the benzodiazepine agonist adinazolam was effective in preventing the stress-induced dendritic atrophy. These findings suggest that the synaptic availability of 5-HT is involved in the mechanism leading to stress-induced dendritic remodeling and supports the idea that the hippocampal inhibitory GABAergic tone may play a regulatory role.

Effect of chronic high-dose exogenous cortisol on hippocampal neuronal number in aged nonhuman primates

Chronic exposure to increased glucocorticoid concentrations appears to lower the threshold for hippocampal neuronal degeneration in the old rat. It has been proposed that increased brain exposure to glucocorticoids may lower the threshold for hippocampal neuronal degeneration in human aging and Alzheimer's disease. Here, we asked whether chronic administration of high-dose cortisol to older nonhuman primates decreases hippocampal neuronal number as assessed by unbiased stereological counting methodology. Sixteen Macaca nemestrina (pigtailed macaques) from 18 to 29 years of age were age-, sex-, and weight-matched into pairs and randomized to receive either high-dose oral hydrocortisone (cortisol) acetate (4-6 mg/kg/d) or placebo in twice daily palatable treats for 12 months. Hypothalamic-pituitary-adrenal activity was monitored by measuring plasma adrenocorticotropin and cortisol, 24 hr urinary cortisol, and CSF cortisol. Urinary, plasma, and CSF cortisol were elevated, and plasma adrenocorticotropin was reduced in the active treatment group. Total hippocampal volume, subfield volumes, subfield neuronal density, and subfield total neuronal number did not differ between the experimental groups. These findings suggest that chronically elevated cortisol concentrations, in the absence of stress, do not produce hippocampal neuronal loss in nonhuman primates.

Does stress damage the brain?

Studies in animals showed that stress results in damage to the hippocampus, a brain area involved in learning and memory, with associated memory deficits. The mechanism involves glucocorticoids and possibly serotonin acting through excitatory amino acids to mediate hippocampal atrophy. Patients with posttraumatic stress disorder (PTSD) from Vietnam combat and childhood abuse had deficits on neuropsychological measures that have been validated as probes of hippocampal function. In addition, magnetic resonance imaging (MRI) showed reduction in volume of the hippocampus in both combat veterans and victims of childhood abuse. In combat veterans, hippocampal volume reduction was correlated with deficits in verbal memory on neuropsychological testing. These studies introduce the possibility that experiences in the form of traumatic stressors can have long-term effects on the structure and function of the brain.

Reduced corticotropin-releasing factor and enhanced vasopressin gene expression in brains of mice with autoimmunity-induced behavioral dysfunction

The spontaneous development of autoimmune disease in MRL-lpr mice induces behavioral and endocrine changes that resemble effects of chronic stressors. To further examine the correspondence between autoimmune disease and chronic stress, we asked whether the brains of autoimmune mice show a shift in the corticotropin-releasing factor (CRF) to vasopressin (AVP) ratio. Using in situ hybridization histochemistry with 35S-labelled mouse riboprobes, the levels of mRNA transcripts encoding CRF and AVP were compared between autoimmune MRL-lpr and control MRL +/+ brains. CRF transcript levels were lower in the hypothalamic paraventricular nucleus and in the central nucleus of the amygdala in MRL-lpr mice. AVP transcript levels were higher in the paraventricular and the supraoptic nuclei in MRL-lpr mice compared to controls. CRF mRNA levels were inversely related to performance in stress-sensitive tasks and to measures of autoimmunity. As found previously for behavioral performance, immunosuppressive treatment with cyclophosphamide abolished the group difference in neuropeptide gene expression. These results indicate that an autoimmune disease process is necessary for the shift in the brain CRF:AVP ratio. Furthermore, they support the parallel between chronic stress and chronic autoimmunity/inflammation, and suggest common central mechanisms relevant to endocrine function and behavior.

Hippocampal mossy fiber sprouting induced by chronic electroconvulsive seizures

Stress, which can precipitate and exacerbate depression, causes atrophy and in severe cases death of hippocampal neurons. Atrophy of the hippocampus has also been observed in patients suffering from recurrent major depression. The present study examines the influence of electroconvulsive seizures, one of the most effective treatments for depression, on the morphology and survival of hippocampal neurons. The results demonstrate that chronic administration of electroconvulsive seizures induces sprouting of the granule cell mossy fiber pathway in the hippocampus. This sprouting is dependent on repeated administration of electroconvulsive seizures, reaches a maximum 12 days after the last treatment and is long lasting (i.e. up to six months). Electroconvulsive seizure-induced sprouting occurs in the absence of neuronal loss, indicating that sprouting is not a compensatory response to cell death. This is different from the sprouting induced by kindling or excitotoxin treatment, which induce cell death along with recurrent seizures. Electroconvulsive seizure-induced sprouting is significantly diminished in brain-derived neurotrophic factor heterozygote knockout mice, indicating that this neurotrophic factor contributes to mossy fiber sprouting. However, infusion of brain-derived neurotrophic factor into the hippocampus does not induce sprouting of the mossy fiber pathway. The results demonstrate that chronic administration of electroconvulsive seizures induces mossy fiber sprouting and suggest that increased expression of brain-derived neurotrophic factor is necessary, but not sufficient for the induction of this sprouting. Although the functional consequences remain unclear, sprouting of the mossy fiber pathway would appear to oppose the actions of stress and could thereby contribute to the therapeutic actions of electroconvulsive seizure therapy.

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.

Stress, glucocorticoids and structural plasticity of the hippocampus

Sustained stress can have numerous pathophysiological effects. Adrenal glucocorticoid hormones are principal effectors in the stress response. They have profound effects on mood and behavior and affect neurochemical transmission and neuroendocrine control. We have used the experimental paradigm of chronic psychosocial stress in tree shrews to investigate the impact of aversive social encounters on brain structures. Chronic stress in male tree shrews which is accompanied by constantly elevated levels of glucocorticoids leads to structural changes in hippocampal neurons. Whereas dendritic atrophy of hippocampal pyramidal neurons and impairment of neurogenesis in the dentate gyrus could be demonstrated in chronically stressed tree shrews, a loss of hippocampal neurons was not observed in this animal model. The present review summarizes recent results on the question which structural changes occur during chronic stress in neurons of the brain and whether glucocorticoids might be responsible for such stress effects. The role of transmitter systems in stress-related neuronal plasticity is also discussed.

Adverse psychological impact, glutamatergic dysfunction, and risk factors for Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cell loss and pathological changes in neuronal transmission. In particular, malfunction in glutamatergic activity may be associated with the impairment of memory seen in Alzheimer patients. Both hypoactivation and hyperactivation of glutamatergic systems seem to cause impeded cognitive processing in animals. Rats subjected to rearing in isolation display reduced levels of glutamate in temporal regions accompanied by impaired learning and memory. Similar cognitive deficits are also seen in animals exposed to behavioral stress. Stress appears to have deleterious effects on cognition caused by glutamate neurotoxicity leading to attenuated synaptic activity. It is suggested that stress may represent a potential risk factor for AD. The known risk factors for AD (age, heredity, head trauma, low education, depression) may all be related to glutamatergic dysfunction. Some difficulties with pharmacological approaches based on glutamatergic agonists are discussed. It is suggested that optimal glutamate-mediated neurotransmission throughout life may prevent the occurrence of mental decline associated with AD.

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.

Stress: metaplastic effects in the hippocampus

Memory impairments, which occur regularly across species as a result of aging, disease and psychological insults (for example, stress), constitute a useful area for investigation into the neurobiological basis of learning and memory. Memory researchers have identified the hippocampus as a crucial brain structure involved in key aspects of memory formation. The most widely accepted putative mechanisms of memory storage in this structure are LTP and LTD. The hippocampus is enriched with receptors for corticosterone (a glucocorticoid hormone released in response to stress) and it plays a role in glucocorticoid negative feedback and, therefore, some hippocampal functioning might be particularly susceptible to stress. In support of this view, stress-induced modifications in learning, synaptic plasticity and endangerment of neurons have been seen in the hippocampus. Stress and glucocorticoids appear to exert a metaplastic effect through the modulation of Ca2+ levels. We propose a synaptic model that provides a conceptual scaffold to structure our understanding of the manifold actions of stress on the hippocampus. Accordingly, we suggest that stress-induced metaplasticity could disrupt Ca2+ homeostasis and thus endanger hippocampal neurons.

Morphological changes in the hippocampal CA3 region induced by non-invasive glucocorticoid administration: a paradox

Repeated stress induces atrophy, or remodeling, of apical dendrites in hippocampal CA3 pyramidal neurons. In rats, the stress effect is blocked by adrenal steroid synthesis inhibitors, and mimicked by daily injection of corticosterone. We report that non-invasive administration of corticosterone in the drinking water (400 micrograms/ml) also produced atrophy of apical dendrites in CA3. Unexpectedly, the combination of daily stress and oral corticosterone negated the effects of either treatment alone, and no changes in the apical dendritic length or branching pattern of CA3 pyramidal neurons were observed compared to control unstressed rats.

Explicit memory in pregnant women

OBJECTIVE

The study was conducted to systematically investigate previous anecdotal reports of memory decline during pregnancy.

STUDY DESIGN

We used a longitudinal design to investigate memory in women throughout pregnancy and in the postpartum period. Closely matched, nonpregnant women were similarly studied at equivalent intervals. We also assessed degree of depression and anxiety.

RESULTS

There was a significant time-by-group interaction (P < .01) for both immediate and delayed recall of paragraph length material. Contrasts showed a significant decline in memory for the pregnant group from the second to the third trimester (P < .01). No significant changes in memory were noted for the control group. The pregnant women scored higher on both depression and anxiety scales; however, somatic rather than cognitive items accounted for the elevated scores. Fluctuations in mood and memory did not coincide.

CONCLUSION

There is a pregnancy-related decline in memory, which is limited to the third trimester. The decline is not attributable to depression, anxiety, sleep deprivation, or other physical changes associated with pregnancy.

Effect of chronic restraint stress and tianeptine on growth factors, growth-associated protein-43 and microtubule-associated protein 2 mRNA expression in the rat hippocampus

Chronic restraint stress of rats for three weeks produces an atrophy of apical dendrites in the CA3 region of the hippocampus. This alteration is blocked by the novel antidepressant, tianeptine. In order to investigate the underlying mechanism of these phenomena, we evaluated the effect of chronic restraint and tianeptine on mRNA expression of neurotrophic factors such as brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and basic fibroblast growth factor (bFGF). Chronic restraint and tianeptine treatment did not change the expression of these neurotrophins in the rat hippocampus. We also evaluated the effects of stress and tianeptine on GAP-43 and MAP2, both of which are known to be related to the development of neurons. Chronic restraint resulted in a small decrease in GAP-43 mRNA expression in the CA3 region of the hippocampus, which was not prevented by the concomitant administration of tianeptine. MAP2 mRNA expression was not changed by either chronic stress or tianeptine treatment. We conclude that these neurotrophins, GAP-43 and MAP2 are not likely to be directly related to the chronic stress-induced dendritic atrophy or the prevention of the atrophy by tianeptine.

Effects of microgravity and bone morphogenetic protein II on GFAP in rat brain

This study evaluated effects of bone morphogenetic protein II (BMP) on glial fibrillary acidic protein (GFAP) in the brain of female Fischer 344 rats during 14 days of spaceflight. GFAP mRNA decreased in vehicle-implanted rats flown on the space shuttle by 53 and 48% in the stratum moleculare and stratum lacunosum moleculare hippocampal subregions, respectively. GFAP mRNA was not significantly affected by BMP implantation during spaceflight. Rats returning from space exhibited a 56% increase in serum corticosterone. BMP treatment did not additively increase corticosterone elevations in microgravity but appeared to increase serum corticosterone and reduce GFAP mRNA in the stratum moleculare in control rats. These data suggest that exposure to microgravity reduces GFAP expression in hippocampal astrocytes.

Morphological features of the entorhinal-hippocampal connection

The goal of this review in an overview of the structural elements of the entorhinal-hippocampal connection. The development of the dendrites of hippocampal neurons will be outlined in relation to afferent pathway specificity and the mature dendritic structure compared. Interneurons will be contrasted to pyramidal cells in terms of processing of physiological signals and convergence and divergence in control of hippocampal circuits. Mechanisms of axonal guidance and target recognition, target structures, the involvement of receptor distribution on hippocampal dendrites and the involvement of non-neuronal cellular elements in the establishment of specific connections will be presented. Mechanisms relevant for the maintenance of shape and morphological specializations of hippocampal dendrites will be reviewed. One of the significant contexts in which to view these structural elements is the degree of plasticity in which they participate, during development and origination of dendrites, mature synaptic plasticity and after lesions, when the cells must continue to maintain and reconstitute function, to remain part of the circuitry in the hippocampus. This review will be presented in four main sections: (1) interneurons-development, role in synchronizing influence and hippocampal network functioning; (2) principal cells in CA1, CA3 and dentate gyrus regions-their development, function in terms of synaptic integration, differentiating structure and alterations with lesions; (3) glia and glia/neuronal interactions-response to lesions and developmental guidance mechanisms; and (4) network and circuit aspects of hippocampal morphology and functioning. Finally, the interwoven role of these various elements participating in hippocampal network function will be discussed.

Activity-stress induces atrophy of apical dendrites of hippocampal pyramidal neurons in male rats

Recently, researchers have demonstrated the damaging effect of restraint-stress on hippocampal neurons. The purpose of the present study was to determine if a more chronic stressor, i.e., activity-stress (A-S), would also result in hippocampal dendritic atrophy. When activity-stress (n = 6) rats showed evidence of the criteria "stress symptoms" (after an average of 6 days), they were sacrificed and their brains were quickly removed, blocked, and placed in Golgi-Cox solution. Food-yoked control animals (n = 6) were sacrificed on the following day. Serial coronal sections (150 um) of the rostral hippocampus were cut so that the CA3 and CAI areas could be analyzed. Stressed short-shaft neurons were significantly shorter and had fewer branch points in CA1 and CA3 neurons than the control neurons. A similar nonsignificant trend was observed in long-shaft neurons. These data suggest that a short period of chronic stress (6 days as opposed to 21 days in prior studies) induces neuronal atrophy in the hippocampus.

Corticosterone peak is responsible for stress-induced elevation of glutamate in the hippocampus

Effect of ether stress on dialysate concentration of extracellular amino acids in the hippocampus was studied by microdialysis in freely moving rats that have been either sham operated (SHAM) or adrenalectomized and supplemented with subcutaneous steroid pellets (ADX+CORT) providing constant corticosterone (CORT) plasma levels. In SHAM rats, ether stress resulted in a peak of glutamate and taurine 30 min after stress, while extracellular aspartate concentration was increased 120 min after challenge. These changes in amino acid levels as well as in glutamate/glutamine ratio were paralleled by stress-induced rise of plasma CORT. No significant alterations were detected in the concentration of hippocampal arginine, alanine, glycine, glutamine, threonine or serine. In contrast to SHAM animals, ether stress failed to have an effect on dialysate concentration of amino acid transmitters in the hippocampus of adrenalectomized rats supplemented with 50 mg CORT-pellets. Our results demonstrate that ether stress alters aspartate, glutamate, glutamate/glutamine ratio and taurine concentration in the hippocampus and indicate that stress-induced CORT release in plasma may be responsible for these amino acid alterations. These changes may also contribute to negative feedback effect of CORT on hypothalamo-pituitary-adrenocortical (HPA) axis via the hippocampus during stress.

Effects of corticosterone treatment and rehabilitation on the hippocampal formation of neonatal and adult rats. An unbiased stereological study

Elevations in the plasma levels of glucocorticoids are associated with cognitive impairments that have been ascribed to loss of neurons in the hippocampal formation. However, recent studies have strongly challenged this view. In order to clarify this issue, we have employed for the first time the optical fractionator and the Cavalieri principle, two unbiased stereological tools, to estimate respectively the total number of neurons and the volumes of the main subdivisions of the hippocampal formation of rats submitted to corticosterone treatment for different periods, either neonatally or in adulthood. A significant reduction in the number of neurons and in the volumes of the layers of the dentate gyrus and CA3 hippocampal field was found in rats exposed to glucocorticoids in the neonatal period; furthermore, animals treated with corticosterone from birth until 180 days of age had also a reduction in the volume of the stratum radiatum of the CA1 hippocampal field. Conversely, when the exposure occurred only during adulthood, no significant neuronal loss was observed, but there were significant reductions in the volume of layers in the dentate gyrus and CA3 hippocampal field. To search for signs of structural recovery, we incorporated a group of rats submitted to corticosterone treatment during the neonatal period in which the hormonal conditions were restored thenceforth. In this group we found a significant increase in the volume of the molecular layer of the dentate gyrus when compared with rats that were kept under corticosteroid treatment. In conclusion, these data provide a sound structural basis for the cognitive deficits observed during, and following, exposure to increased levels of glucocorticoids.

Brain corticosteroid receptor balance in health and disease

In this review, we have described the function of MR and GR in hippocampal neurons. The balance in actions mediated by the two corticosteroid receptor types in these neurons appears critical for neuronal excitability, stress responsiveness, and behavioral adaptation. Dysregulation of this MR/GR balance brings neurons in a vulnerable state with consequences for regulation of the stress response and enhanced vulnerability to disease in genetically predisposed individuals. The following specific inferences can be made on the basis of the currently available facts. 1. Corticosterone binds with high affinity to MRs predominantly localized in limbic brain (hippocampus) and with a 10-fold lower affinity to GRs that are widely distributed in brain. MRs are close to saturated with low basal concentrations of corticosterone, while high corticosterone concentrations during stress occupy both MRs and GRs. 2. The neuronal effects of corticosterone, mediated by MRs and GRs, are long-lasting, site-specific, and conditional. The action depends on cellular context, which is in part determined by other signals that can activate their own transcription factors interacting with MR and GR. These interactions provide an impressive diversity and complexity to corticosteroid modulation of gene expression. 3. Conditions of predominant MR activation, i.e., at the circadian trough at rest, are associated with the maintenance of excitability so that steady excitatory inputs to the hippocampal CA1 area result in considerable excitatory hippocampal output. By contrast, additional GR activation, e.g., after acute stress, generally depresses the CA1 hippocampal output. A similar effect is seen after adrenalectomy, indicating a U-shaped dose-response dependency of these cellular responses after the exposure to corticosterone. 4. Corticosterone through GR blocks the stress-induced HPA activation in hypothalamic CRH neurons and modulates the activity of the excitatory and inhibitory neural inputs to these neurons. Limbic (e.g., hippocampal) MRs mediate the effect of corticosterone on the maintenance of basal HPA activity and are of relevance for the sensitivity or threshold of the central stress response system. How this control occurs is not known, but it probably involves a steady excitatory hippocampal output, which regulates a GABA-ergic inhibitory tone on PVN neurons. Colocalized hippocampal GRs mediate a counteracting (i.e., disinhibitory) influence. Through GRs in ascending aminergic pathways, corticosterone potentiates the effect of stressors and arousal on HPA activation. The functional interaction between these corticosteroid-responsive inputs at the level of the PVN is probably the key to understanding HPA dysregulation associated with stress-related brain disorders. 5. Fine-tuning of HPA regulation occurs through MR- and GR-mediated effects on the processing of information in higher brain structures. Under healthy conditions, hippocampal MRs are involved in processes underlying integration of sensory information, interpretation of environmental information, and execution of appropriate behavioral reactions. Activation of hippocampal GRs facilitates storage of information and promotes elimination of inadequate behavioral responses. These behavioral effects mediated by MR and GR are linked, but how they influence endocrine regulation is not well understood. 6. Dexamethasone preferentially targets the pituitary in the blockade of stress-induced HPA activation. The brain penetration of this synthetic glucocorticoid is hampered by the mdr1a P-glycoprotein in the blood-brain barrier. Administration of moderate amounts of dexamethasone partially depletes the brain of corticosterone, and this has destabilizing consequences for excitability and information processing. 7. The set points of HPA regulation and MR/GR balance are genetically programmed, but can be reset by early life experiences involving mother-infant interaction. 8. (ABSTRACT TRUNCATED)

Chronic stress alters synaptic terminal structure in hippocampus

Repeated psychosocial or restraint stress causes atrophy of apical dendrites in CA3 pyramidal neurons of the hippocampus, accompanied by specific cognitive deficits in spatial learning and memory. Excitatory amino acids mediate this atrophy together with adrenal steroids and the neurotransmitter serotonin. Because the mossy fibers from dentate granule neurons provide a major excitatory input to the CA3 proximal apical dendrites, we measured ultrastructural parameters associated with the mossy fiber-CA3 synapses in control and 21-day restraint-stressed rats in an effort to find additional morphological consequences of stress that could help elucidate the underlying anatomical as well as cellular and molecular mechanisms. Although mossy fiber terminals of control rats were packed with small, clear synaptic vesicles, terminals from stressed animals showed a marked rearrangement of vesicles, with more densely packed clusters localized in the vicinity of active zones. Moreover, compared with controls, restraint stress increased the area of the mossy fiber terminal occupied by mitochondrial profiles and consequently, a larger, localized energy-generating capacity. A single stress session did not produce these changes either immediately after or the next day following the restraint session. These findings provide a morphological marker of the effects of chronic stress on the hippocampus that points to possible underlying neuroanatomical as well as cellular and molecular mechanisms for the ability of repeated stress to cause structural changes within the hippocampus.

Sex differences in dendritic atrophy of CA3 pyramidal neurons in response to chronic restraint stress

The present study investigated the effects of 21 days of chronic restraint stress on neural and endocrine parameters in male and female rats. Consistent with previous results, repeated restraint stress induced apical dendritic atrophy (a decrease in the number of apical branch points and dendritic length) of the CA3c pyramidal neurons in male rats. In contrast, female rats did not show significant dendritic atrophy in the apical field in response to repeated restraint stress. Female rats did show a decrease in the number of branch points in the basal dendritic tree compared to male rats in response to repeated restraint stress. Baseline and stress levels of plasma corticosterone were higher in female rats compared to male rats. Females exhibited slightly longer increases in corticosterone levels throughout the 21 days of restraint stress than males, indicating that the male corticosterone response to stress exhibited greater habituation. Plasma corticosteroid-binding globulin levels of female rats were also higher than those of male rats throughout the experiment. There was no change in plasma corticosteroid-binding globulin levels in male rats during the restraint stress, while there was a decrease in plasma corticosteroid-binding globulin levels in female rats during the restraint stress. Plasma estradiol levels in female rats also decreased in response to the chronic stress. In view of the qualitatively different dendritic atrophy found in males and females in appears unlikely that sex differences in the corticosteroid-binding globulin and corticosterone response can account for these morphological differences.

Entorhinal cortex lesioning protects hippocampal CA3 neurons from stress-induced damage

The role of the entorhinal cortex (EC) in stress-induced damage in terms of dendritic branching points and intersections of hippocampal CA3 neurons has been investigated. Following bilateral electrolytic lesions of the EC, the rats were subjected to restraint stress, 6 h per day for 21 days. Chronic restraint stress resulted in the atrophy of hippocampal CA3 neurons and the lesioning of the EC prior to stress significantly (P < 0.001) reduced this dendritic atrophy. These results show that the neuronal vulnerability to chronic stress can be attenuated by entorhinal glutamatergic denervation.

Prevention of stress-induced morphological and cognitive consequences

Atrophy and dysfunction of the human hippocampus is a feature of aging in some individuals, and this dysfunction predicts later dementia. There is reason to believe that adrenal glucocorticoids may contribute to these changes, since the elevations of glucocorticoids in Cushing's syndrome and during normal aging are associated with atrophy of the entire hippocampal formation in humans and are linked to deficits in short-term verbal memory. We have developed a model of stress-induced atrophy of the hippocampus of rats at the cellular level, and we have been investigating underlying mechanisms in search of agents that will block the atrophy. Repeated restraint stress in rats for 3 weeks causes changes in the hippocampal formation that include suppression of 5-HT1A receptor binding and atrophy of dendrites of CA3 pyramidal neurons, as well as impairment of initial learning of a radial arm maze task. Because serotonin is released by stressors and may play a role in the actions of stress on nerve cells, we investigated the actions of agents that facilitate or inhibit serotonin reuptake. Tianeptine is known to enhance serotonin uptake, and we compared it with fluoxetine, an inhibitor of 5-HT reuptake, as well as with desipramine. Tianeptine treatment (10 mg/kg/day) prevented the stress-induced atrophy of dendrites of CA3 pycamidal neurons, whereas neither fluoxetine (10 mg/kg/day) nor desipramine (10 mg/kg/day) had any effect. Tianeptine treatment also prevented the stress-induced impairment of radial maze learning. Because corticosterone- and stress-induced atrophy of CA3 dendrites is also blocked by phenytoin, an inhibitor of excitatory amino acid release and actions, these results suggest that serotonin released by stress or corticosterone may interact pre- or post-synaptically with glutamate released by stress or corticosterone, and that the final common path may involve interactive effects between serotonin and glutamate receptors on the dendrites of CA3 neurons innervated by mossy fibers from the dentate gyrus. We discuss the implications of these findings for treating cognitive impairments and the risk for dementia in the elderly.

Chronic glucocorticoid administration as well as repeated stress affects the subsequent acute immobilization stress-induced expression of immediate early genes but not that of NGFI-A

We reported that repeated immobilization for six days attenuates the subsequent acute immobilization stress-induced expression of the immediate early genes c-fos, fos B, jun B and nerve growth factor-induced gene-B (NGFI-B), but not of NGFI-A, in the rat paraventricular hypothalamic nucleus. In this study, we confirmed these findings by means of a time-course study, and further investigated whether the elevated plasma basal glucocorticoid level induced by repeated stress underlies the attenuated response of immediate early genes and the preserved reactivity of NGFI-A. Rats implanted with 100, 200 or 400 mg corticosterone or placebo pellets (control), were immobilized for 1 h and decapitated seven days later. In control rats acute immobilization induced c-fos, fos B, jun B, NGFI-A and NGFI-B messenger RNA in the paraventricular hypothalamic nucleus, whereas all of them except NGFI-A, were significantly reduced in rats given 200 and 400 mg corticosterone implants. The similarity of the results from the two procedures suggests that glucocorticoid is involved in regulating immediate early genes in the paraventricular hypothalamic nucleus under repeated stress and that the NGFI-A gene is not regulated by this mechanism. However, the plasma basal corticosterone level in repeatedly stressed rats was lower than that of rats implanted with 100 mg corticosterone, suggesting that a repetitive stress-induced corticosterone surge also contributes to this mechanism.

Steroid Hormone Modulation of Hippocampal Dependent Spatial Memory

Recent studies show that the adult CNS is capable of considerable re-structuring and re-growth, a property previously thought limited to the developmental period. Hormones play an important role in many of these plastic processes, and the hippocampus, as a target for all the major classes of steroid hormones, undergoes considerable remodeling. Since it is critical for mediating tasks requiring spatial memory, the hippocampus can serve as an important model for understanding not only the mechanisms underlying plasticity of brain circuits but also how these changes impact on a higher order function, spatial memory. In this review, the effects of steroid hormones on hippocampal remodeling are discussed, and the ability of estradiol to enhance spatial memory as well as the ability of both excessive or diminished corticosteroid levels to impair spatial memory are described. The neural mechanisms for these effects, as well as previous and current contributions of McEwen and colleagues to this expanding area of research, are discussed.

Immobilization stress reduced the expression of neurotrophins and their receptors in the rat brain

Exposure to stressful events and elevated level of stress hormones are associated with impaired spatial memory and neuronal damage in the hippocampus. These neurons are considered to be maintained by neurotrophins such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) and trk family of neurotrophin receptors. Male Wistar rats (6 weeks old) were exposed to immobilization stress for 8 h and their brains were processed for in situ hybridization histochemistry. Exposure to long-lasting immobilization stress reduced mRNA levels for neurotrophins and their high affinity receptors in the brain, especially in the hippocampus. Our results provide, some new information that may be relevant to the pathogenesis of stress-induced disturbances of memory and learning.

Chronic corticosterone treatment induces parallel changes in N-methyl-D-aspartate receptor subunit messenger RNA levels and antagonist binding sites in the hippocampus

Some of the effects of glucocorticoids on the function and neuronal plasticity of the hippocampus are mediated by N-methyl-D-aspartate receptor activation. We tested the hypothesis that chronic corticosterone administration increases N-methyl-D-aspartate receptor expression in the hippocampus of the rat. We used in situ hybridization histochemistry to measure the messenger RNA levels for the NR1, NR2A and NR2B subunits of the N-methyl-D-aspartate receptor and [3H]dizocilpine maleate (a non-competitive antagonist) binding to measure N-methyl-D-aspartate receptor density. Since corticosterone depresses circulating testosterone levels, we also examined whether the effects of corticosterone are mediated by or interact with the effects of testosterone. In the intact animal, corticosterone increased messenger RNA levels for NR2A and NR2B but not NR1 subunits of the N-methyl-D-aspartate receptor in all regions of the hippocampus. Testosterone had no significant effect on messenger RNA levels of any of the subunits. The subunit composition determines the functional and pharmacological properties of the N-methyl-D-aspartate receptor. We used ifenprodil inhibition of [3H]dizocilpine maleate binding, which has been used to distinguish NR2A- from NR2B-containing receptors, to determine whether corticosterone altered the proportion of high- and low-affinity sites for ifenprodil in parallel with the changes in subunit messenger RNA levels. Corticosterone increased the density of [3H]dizocilpine maleate binding sites without changing the dissociation constant for [3H]dizocilpine maleate or the proportion of high- and low-affinity sites for ifenprodil. These data suggest that the effects of corticosterone on hippocampal function are mediated, in part, by parallel increases in NR2A and NR2B subunit levels and the number of receptor channel binding sites.

Effects of neonatal stimulation on later cognitive function and hippocampal nerve growth factor

This study examined the behavioural and physiological effects of chronic mild stress on neonatally handled and non-handled rats. Neonatally handled and non-handled rats were exposed to chronic mild stress from weaning time to 6 months of age. They were behaviourally tested at 6 months of age, and sacrificed for analysis of nerve growth factor (NGF) in the hippocampus and hypothalamus. In contrast to the reported deleterious effect of acute strong stress, mild stress appeared to stimulate production of NGF in the hippocampus and improve spatial learning in both handled and nonhandled rats. Because neonatal handling produces neuroanatomical changes in the rat hippocampus and enhances cognitive function throughout the rats life span, these results implicate hippocampal NGF in the neuroprotective effects of handling.

Anterograde transport of endogenous brain-derived neurotrophic factor in hippocampal mossy fibers

Neurotrophic factors such as brain-derived neurotrophic factor (BDNF) are assumed to provide trophic support via a target-derived, retrograde mechanism of action. However, recent studies suggest that neurotrophic factors can act in an autocrine fashion and perhaps even in an anterograde direction similar to neurotransmitters. To further explore this hypothesis, we compared the neuroanatomical pattern of BDNF mRNA and protein in response to electroconvulsive seizures (ECS) or kainic acid-induced seizure activity. Using in situ hybridization, we found that chronic ECS induced BDNF mRNA predominantly in the granule neurons of the dentate gyrus. However, immunohistochemistry with an anti-BDNF antibody revealed that ECS increased endogenous BDNF protein in the mossy fibers, which are composed of axons projecting from the granule neurons of the dentate gyrus to the CA3 pyramidal layer of the hippocampus. Kainic acid administration (10 mg/kg, i.p., once) was used to lesion CA3 neurons selectively, as these are a possible retrograde source of BDNF protein in mossy fibers. Three weeks later, a prolonged elevation of BDNF mRNA in granule neurons, but not elsewhere in hippocampus, was accompanied by an increase in BDNF protein in the mossy fibers. These results suggest that BDNF was transcribed and translated in granule neuron cell bodies but transported in an anterograde direction to provide trophic support of CA3 pyramidal neurons.

Temporal dynamics of glutamate efflux in the prefrontal cortex and in the hippocampus following repeated stress: effects of pretreatment with saline or diazepam

Acute stress has been associated with activation of glutamate efflux in forebrain structures. The present study sought to characterize the extracellular dynamics of glutamate in response to acute and repeated stress in the prefrontal cortex and hippocampus in rats. One-minute sampling of extracellular glutamate levels was performed during repeated tail-pinch stress. Animals were stressed three times, beginning at approximately 10.30 a.m. and continuing at 2.5-h intervals. In the prefrontal cortex, the initial 10-min tail pinch produced a robust increase in extracellular levels of glutamate. This increase was apparent immediately (i.e. 1 min) after the start of the stress procedure. The second tail pinch produced a smaller increase in glutamate levels while the third tail pinch did not significantly increase these levels. In the hippocampus, the initial stress response was smaller in magnitude than that observed in the prefrontal cortex. Furthermore, responses to subsequent tail pinches were similar to that seen following the first tail pinch. Treatment with diazepam (3 mg/kg/i.p.) 30 min before the first stress session abolished the stress response in the prefrontal cortex and hippocampus. However, in the prefrontal cortex, the second tail pinch (performed approximately 3 h after diazepam administration) produced a robust increase in glutamate efflux. In contrast, in the hippocampus of diazepam-treated rats, the second tail pinch produced a small delayed response. Pretreatment with saline resulted in non-significant responses to all three tail pinches in the prefrontal cortex. The present study suggests that: (i) stress produces a rapid increase in glutamate efflux in the prefrontal cortex and hippocampus, (ii) repeated stress reveals tolerance in the glutamatergic response in the prefrontal cortex, (iii) saline and diazepam pretreatment reduce the stress-induced efflux of glutamate in the prefrontal cortex, and (iv) exposure to diazepam may prevent the prefrontal cortex from adapting its response to the subsequent stressor. These finding are consistent with the role of the prefrontal cortex as a region which may regulate reactions to aversive stimuli.

Exposure to chronic psychosocial stress and corticosterone in the rat: effects on spatial discrimination learning and hippocampal protein kinase Cgamma immunoreactivity

Previous reports have demonstrated a striking increase of the immunoreactivity of the gamma-isoform of protein kinase C (PKCgamma-ir) in Ammon's horn and dentate gyrus (DG) of rodent hippocampus after training in a spatial orientation task. In the present study, we investigated how 8 days of psychosocial stress affects spatial discrimination learning in a hole board and influences PKCgamma-ir in the hippocampal formation. The acquisition of both reference memory and working memory was significantly delayed in the stressed animals during the entire training period. With respect to cellular plasticity, the training experience in both nonstressed and stressed groups yielded enhanced PKCgamma-ir in the CA1 and CA3 regions of the posterior hippocampus but not in subfields of the anterior hippocampus. Stress enhanced PKCgamma-ir in the DG and CA3 pyramidal cells of the anterior hippocampus. In stressed animals that were subsequently trained, the PKCgamma-ir was increased in the posterior CA1 region to the same level as that found in nonstressed trained animals. Stress apparently abrogated the PKCgamma-ir training response in the CA3 region. In a second experiment, the elevation of plasma corticosterone levels to values that are found during stress did not significantly influence reference memory scores but slightly and temporarily affected working memory. The training-induced enhancement of PKCgamma-ir in the CA1 region was similar in trained and corticosterone-treated trained animals, but the learning-induced PKCgamma-ir response in the posterior CA3 area was absent after corticosterone pretreatment. These results reveal that prolonged psychosocial stress causes spatial learning deficits, whereas artificial elevation of corticosterone levels to the levels that occur during stress only mildly affects spatial memory performance. The spatial learning deficits following stress are reflected only in part in the redistribution of hippocampal PKCgamma-ir following training.

Magnetic resonance imaging study of hippocampal volume in chronic, combat-related posttraumatic stress disorder

This study used quantitative volumetric magnetic resonance imaging techniques to explore the neuroanatomic correlates of chronic, combat-related posttraumatic stress disorder (PTSD) in seven Vietnam veterans with PTSD compared with seven nonPTSD combat veterans and eight normal nonveterans. Both left and right hippocampi were significantly smaller in the PTSD subjects compared to the Combat Control and Normal subjects, even after adjusting for age, whole brain volume, and lifetime alcohol consumption. There were no statistically significant group differences in intracranial cavity, whole brain, ventricles, ventricle:brain ratio, or amygdala. Subarachnoidal cerebrospinal fluid was increased in both veteran groups. Our finding of decreased hippocampal volume in PTSD subjects is consistent with results of other investigations which utilized only trauma-unexposed control groups. Hippocampal volume was directly correlated with combat exposure, which suggests that traumatic stress may damage the hippocampus. Alternatively, smaller hippocampi volume may be a pre-existing risk factor for combat exposure and/or the development of PTSD upon combat exposure.