Correlation between hippocampal BDNF mRNA expression and memory performance in senescent rats.

Brain-derived neurotrophic factor (BDNF) has been suggested to be involved in memory processes. In the present study, the association between memory impairment at senescence and BDNF expression in the hippocampus was studied in 30-32-month-old Brown Norway rats, which had been maternally deprived early in life. These animals display a bimodal distribution in their spatial learning ability: rats are either non-impaired or impaired. BDNF mRNA expression in the hippocampus was compared between non-impaired and impaired rats. We measured BDNF mRNA expression in the hippocampus 3 h after training in the Morris water maze ('post-training') and at 1 month after training ('basal'). Non-impaired performers displayed a higher post-training BDNF mRNA level in the CA1 region than impaired rats. In addition, only in the non-impaired performers post-training BDNF mRNA levels in CA1 and dentate gyrus were increased as compared to basal levels. Thus, we have demonstrated that in senescent rats, hippocampal BDNF expression in response to water maze training is associated with memory performance.

Maternal deprivation increases 5-HT(1A) receptor expression in the CA1 and CA3 areas of senescent Brown Norway rats.

Maternally-deprived male Brown Norway rats were classified as non-impaired or impaired according to their performance in the water maze when 3 and 30-32 months old. Age and spatial learning ability did not affect the pattern and density of hippocampal 5-HT(1A)-receptor mRNA in mother-reared control rats. However, senescent maternally-deprived rats with impaired spatial learning ability showed increased expression of 5-HT(1A)-receptor mRNA in the hippocampal CA1 (14%) and CA3 (13%) areas but not in the dentate gyrus.

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

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

Maternal deprivation affects behaviour from youth to senescence: amplification of individual differences in spatial learning and memory in senescent Brown Norway rats.

Previous studies have shown that deprivation of the infant rat from maternal care has pronounced effects on the stress system during ontogeny. Here we test the hypothesis that 24 h of maternal deprivation at postnatal day 3 will cause persistent changes in behaviour. Spatial learning and memory of male Brown Norway rats deprived as infants were observed in the Morris water maze at 3, 12, 24 and 30-32 months of age (young, adult, aged, senescent). Their nondeprived mother-reared littermates served as controls. (i) With increasing age, water maze performance declined in deprived and nondeprived groups. However, once the task was learned the animals maintained their good performance during retest at later ages. (ii) Maternal deprivation delayed acquisition until adulthood and caused at every age a higher degree of persistent behaviour as judged from the performance of deprived rats' free swim trials and reversal trials. (iii) At senescence the mean performance in the water maze did not differ between the groups. Instead, the individual performance was strikingly different within each group. Senescent deprived rats were either nonimpaired or impaired with only a few animals showing an intermediate performance. Thus, a large group of animals ( approximately 40%) ages successfully as they are resistant to the effect of maternal deprivation. In contrast, the majority of the control animals displayed intermediate performance. Taken together, maternal deprivation has life-long consequences for behaviour and culminates at senescence in amplification of individual differences in learning ability rather than in a generalized deterioration of cognitive functions.

Penetration of dexamethasone into brain glucocorticoid targets is enhanced in mdr1A P-glycoprotein knockout mice.

Mice with a genetic disruption of the multiple drug resistance (mdr1a) gene were used to examine the effect of the absence of its drug-transporting P-glycoprotein product from the blood-brain barrier on the distribution and cell nuclear uptake of [3H]-dexamethasone in the brain. [3H]-dexamethasone (4 microg/kg mouse) was administered s.c. to adrenalectomized mdr1a (-/-) and mdr1a (+/+) mice. One hour later, the mice were decapitated, and the radioactivity was measured in homogenates of cerebellum, blood, and liver following extraction of the radioactive steroid. The frontal brain was cut in sections for autoradiography. In the cerebellum of the mdr1a mutants, the amount of [3H]-dexamethasone relative to blood was about 5-fold higher than observed in the controls, whereas the ratio in blood vs. liver was not different. Using autoradiography, it was found that brain areas expressing the glucocorticoid receptor (GR) in high abundance, such as the hippocampal cell fields and the paraventricular nucleus (PVN), showed a 10-fold increase in cell nuclear uptake of radiolabeled steroid. The amount of retained steroid increased toward levels observed in the pituitary, which contains a similar density of GRs. The [3H]-dexamethasone concentration in pituitary was not affected by mdr1a gene disruption. The GR messenger RNA expression pattern in hippocampus was not different between the wild types and mdr1a mutants, which rules out altered receptor expression as a cause of the enhanced dexamethasone uptake. In conclusion, the present study demonstrates that the brain is resistant to penetration by dexamethasone because of mdr1a activity at the level of the blood-brain barrier. The data support the concept of a pituitary site of action of dexamethasone in blockade of stress-induced ACTH release. Dexamethasone poorly substitutes for depletion of the endogenous glucocorticoid from the brain and therefore, in this tissue, may cause a condition resembling that of adrenalectomy.

The Brown Norway rat displays enhanced stress-induced ACTH reactivity at day 18 after 24-h maternal deprivation at day 3.

The effect of a 24-h maternal deprivation at day 3 was studied on the hypothalamus-pituitary-adrenal axis of 18-day-old Brown Norway pups, whose stress-hyporesponsive period is similar to other rat strains. Deprivation resulted at day 18 in reduced basal ACTH levels. The rate of onset and the duration of stress-induced ACTH release were enhanced. CRH mRNA, brain corticosteroid mRNA levels and corticosterone receptor levels were not affected by deprivation, but adrenal weight was increased. It is concluded that maternal deprivation has persistently diminished adrenocortical function in containment of the ACTH response to stress.

Chronic brain glucocorticoid receptor blockade enhances the rise in circadian and stress-induced pituitary-adrenal activity.

This study examined the hypothesis that experimentally induced corticosteroid resistance in the brain would lead to adaptations in the activity of the hypothalamic-pituitary-adrenal (HPA) axis similar to the endocrine features of the endogenous resistance accompanying the pathogenesis of depression. For this purpose, the glucocorticoid antagonist RU 38486 (aGC) was infused intracerebroventricularly (i.c.v.) (100 ng/h) via Alzet minipumps for several days. During this chronic receptor blockade, parameters for basal and stress-induced HPA activity were measured in a longitudinal study design. Chronic i.c.v. infusion of the aGC did not affect basal morning levels of ACTH and corticosterone. During the afternoon phase of the circadian cycle, the aGC caused gradual and sequential changes in the HPA axis. After aGC infusion, the circadian rise of ACTH levels was enhanced in the afternoon of day 1, but was normal on subsequent days. For corticosterone, basal afternoon levels towards the diurnal peak were increased at days 1, 3, and 4 in aGC-treated rats. On day 2, in contrast, corticosterone levels did not differ from vehicle-infused controls. Paraventricular CRH messenger RNA, as measured at day 4, was not altered by aGC treatment. After 10 days of aGC treatment, the adrenal weight was increased, and the sensitivity of adrenocortical cells in vitro to ACTH was enhanced. Corticosteroid receptor binding in vitro in hippocampus, hypothalamus, and pituitary was not different between the aGC and vehicle-treated rats. In a second series of experiments, the HPA responsiveness to the stress of a novel environment at day 2 in the morning was increased after chronic aGC infusion, at a time basal hormone levels were not affected. The data show that 1) chronic i.c.v. infusion of aGC readily enhances the amplitude of circadian corticosterone changes, presumably by increasing the adrenocortical sensitivity to ACTH; 2) chronic aGC-treated animals show an enhanced ACTH and corticosterone response to stress, which is delayed in termination; 3) corticosteroid receptor expression, basal CRH messenger RNA, and ACTH levels are not altered after prolonged chronic aGC treatment. It is concluded that, over a period of a few days, aGC-induced corticosteroid resistance triggers a sequelae of pituitary-adrenal adaptations ultimately resulting in hypercorticism. Paradoxically, however, this hypercorticism develops because of increased peak levels of corticosteroid hormone rather than through elevated trough levels as is commonly observed during depressive illness.

Neonatal maternally deprived rats have as adults elevated basal pituitary-adrenal activity and enhanced susceptibility to apomorphine.

Maternal deprivation of neonatal rats for 24 h enhances the adrenocortical response to stress and/or adrenocorticotropin hormone (ACTH) stimulation during the stress hyporesponsive period (SHRP). The present study tests the hypothesis that such maternally deprived neonatal male rats show altered hypothalamic-pituitary-adrenal (HPA) regulation not only immediately after deprivation but also in later life. In addition, we found previously that neonatal changes in HPA activity preceded modulation of nigrostriatal dopamine function. Therefore, we also measured dopamine responsiveness in adult rats which were deprived of their mother during infancy. Neonatal male rats were maternally deprived for 24 h at the age of 3 days, whereas rats of the control group were left undisturbed. At 60 days of age deprived and non-deprived rats were decapitated and brain, adrenal glands and thymus were removed. Trunk blood was collected for determination of plasma ACTH, corticosterone and prolactin concentrations using radioimmunoassay procedures. mRNA levels of mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs), corticotropin-releasing hormone (CRH) mRNA and tyrosine hydroxylase (TH) mRNA were measured in brain sections with in situ hybridization. In a second group of male deprived and non-deprived rats apomorphine-induced stereotypic gnawing behaviour was examined at 60 days of age as a measure for functional activity of the dopamine system. Deprived neonatal rats showed the following characteristics as compared with non-deprived rats: (i) lower basal CRH mRNA concentration in parvocellular neurons of the paraventricular nucleus of the hypothalamus (PVN), while basal plasma ACTH and corticosterone concentrations were significantly elevated. Basal prolactin levels were not different. (ii) Similar hippocampal MR and GR mRNA levels. (iii) Significantly reduced GR mRNA levels in PVN and anterior pituitary. (iv) Significantly enhanced apomorphine-induced stereotypic gnawing behaviour and (v) higher TH mRNA levels in substantia nigra, while no changes were found in the ventral tegmental area (VTA). It is concluded that maternally deprived neonatal male rats display as young adults elevated basal pituitary-adrenal activity and enhanced apomorphine susceptibility.