Distribution of hippocampal mineralocorticoid and glucocorticoid receptor mRNA in a glucocorticoid resistant nonhuman primate

Corticosteroid-Binding Globulin Deficiency Specifically Impairs Contextual and Recognition Memory Consolidation in Male Mice

BACKGROUND/AIMS

Glucocorticoids are essential in modulating memory processes of emotionally arousing experiences and we have shown that corticosteroid-binding globulin (CBG) influences glucocorticoid delivery to the brain. Here, we investigated the role of CBG in contextual and recognition long-term memory according to stress intensity.

METHOD

We used adult male mice totally deficient in CBG (Cbg KO) or brain-specific Cbg KO (CbgCamk KO) to examine their performance in contextual fear conditioning (CFC) and au-ditory fear conditioning, both at short (1 h) and long-term (24 h). Long-term memory in Cbg KO was further analyzed in conditioned odor aversion and in novel object recognition task (NORT) with different paradigms, that is, with and without prior habituation to the context, with a mild or strong stressor applied during consolidation. In the NORT experiments, total and free glucocorticoid levels were measured during consolidation.

RESULTS

Impaired memory was observed in the Cbg KO but not in the CbgCamk KO in the CFC and the NORT without habituation when tested 24 h later. However, Cbg KO displayed normal behavior in the NORT with previous habituation and in the NORT with a mild stressor. In condition of the NORT with a strong stressor, Cbg KO retained good 24 h memory performance while controls were impaired. Total and free glucocorticoids levels were always higher in controls than in Cbg KO except in NORT with mild stressor where free glucocorticoids were equivalent to controls.

CONCLUSIONS

These data indicate that circulating but not brain CBG influences contextual and recognition long-term memory in relation with glucocorticoid levels.

Modulation of central glucocorticoid receptors in short- and long-term experimental hyperthyroidism

Hyperthyroidism is associated with a significant increase in circulating glucocorticoid levels and hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis. The aim of this study was to examine whether the HPA axis hyperactivity observed in hyperthyroidism may be explained by a disturbed feedback inhibition of endogenous glucocorticoids through two specific intracellular receptors in the brain: the high affinity mineralocorticoid receptor (MR) and the lower affinity glucocorticoid receptor (GR). Cytosolic receptor binding and gene expression was assessed in rats with short (7 days) and long standing (60 days) eu- and hyperthyroidism. Glucocorticoid receptor number and binding affinity (Kd) in the hippocampus were measured using [(3)H2]-dexamethasone radioreceptor assay. In situ hybridization was employed to examine the effects of hyperthyroidism on the GR and MR mRNA levels in the hippocampus and the pituitary. Both short- and long-term hyperthyroid rats showed pronounced reduction in the concentration of cytosolic GR in the hippocampus, without changes in binding affinity or changes in GR expression. In contrast, GR mRNA in the pituitary increased after 7 days and decreased after 60 days of thyroxin treatment. MR mRNA was moderately affected. Hyperthyroidism is associated with significant decreases in hippocampal GR levels supporting the hypothesis that hyperactivity of the HPA axis observed in experimentally induced hyperthyroidism may be attributed, at least in part, to decreased negative feedback at the level of the hippocampus. These findings further support the notion that a central locus is principally responsible for the hyperactivity of the HPA axis observed in hyperthyroidism.

Glucocorticoid receptor gene expression and promoter CpG modifications throughout the human brain

Glucocorticoids and the glucocorticoid (GR) and mineralocorticoid (MR) receptors have been implicated in many processes, particularly in negative feedback regulation of the hypothalamic-pituitary-adrenal axis. Epigenetically programmed GR alternative promoter usage underlies transcriptional control of GR levels, generation of GR 3' splice variants, and the overall GC response in the brain. No detailed analysis of GR first exons or GR transcript variants throughout the human brain has been reported. Therefore we investigated post mortem tissues from 28 brain regions of 5 individuals. GR first exons were expressed throughout the healthy human brain with no region-specific usage patterns. First exon levels were highly inter-correlated suggesting that they are co-regulated. GR 3' splice variants (GRα and GR-P) were equally distributed in all regions, and GRβ expression was always low. GR/MR ratios showed significant differences between the 28 tissues with the highest ratio in the pituitary gland. Modification levels of individual CpG dinucleotides, including 5-mC and 5-hmC, in promoters 1D, 1E, 1F, and 1H were low, and diffusely clustered; despite significant heterogeneity between the donors. In agreement with this clustering, sum modification levels rather than individual CpG modifications correlated with GR expression. Two-way ANOVA showed that this sum modification was both promoter and brain region specific, but that there was however no promoter*tissue interaction. The heterogeneity between donors may however hide such an interaction. In both promoters 1F and 1H modification levels correlated with GRα expression suggesting that 5-mC and 5-hmC play an important role in fine tuning GR expression levels throughout the brain.

Glucocorticoid receptor protein expression in human hippocampus; stability with age

The glucocorticoid receptor (GR) exerts numerous functions in the body and brain. In the brain, it has been implicated, amongst others, in feedback regulation of the hypothalamic-pituitary-adrenal axis, with potential deficits during aging and in depression. GRs are abundantly expressed in the hippocampus of rodent, except for the Ammon's horn (CA) 3 subregion. In rhesus monkey however, GR protein was largely absent from all hippocampal subregions, which prompted us to investigate its distribution in human hippocampus. After validation of antibody specificity, we investigated GRα protein distribution in the postmortem hippocampus of 26 human control subjects (1-98 years of age) and quantified changes with age and sex. In contrast to monkey, abundant GR-immunoreactivity was present in nuclei of almost all neurons of the hippocampal CA subfields and dentate gyrus (DG), although neurons of the CA3 subregion displayed lower levels of immunoreactivity. Colocalization with glial fibrillary acidic protein confirmed that GR was additionally expressed in approximately 50% of the astrocytes in the CA regions, with lower levels of colocalization (approximately 20%) in the DG. With increased age, GR expression remained stable in the CA regions in both sexes, whereas a significant negative correlation was found with age only in the DG of females. Thus, in contrast to the very low levels previously reported in monkey, GR protein is prominently expressed in human hippocampus, indicating that this region can form an important target for corticosteroid effects in human.

Chronic stress-induced hippocampal vulnerability: the glucocorticoid vulnerability hypothesis

The hippocampus, a limbic structure important in learning and memory, is particularly sensitive to chronic stress and to glucocorticoids. While glucocorticoids are essential for an effective stress response, their oversecretion was originally hypothesized to contribute to age-related hippocampal degeneration. However, conflicting findings were reported on whether prolonged exposure to elevated glucocorticoids endangered the hippocampus and whether the primate hippocampus even responded to glucocorticoids as the rodent hippocampus did. This review discusses the seemingly inconsistent findings about the effects of elevated and prolonged glucocorticoids on hippocampal health and proposes that a chronic stress history, which includes repeated elevation of glucocorticoids, may make the hippocampus vulnerable to potential injury. Studies are described to show that chronic stress or prolonged exposure to glucocorticoids can compromise the hippocampus by producing dendritic retraction, a reversible form of plasticity that includes dendritic restructuring without irreversible cell death. Conditions that produce dendritic retraction are hypothesized to make the hippocampus vulnerable to neurotoxic or metabolic challenges. Of particular interest is the finding that the hippocampus can recover from dendritic retraction without any noticeable cell loss. When conditions surrounding dendritic retraction are present, the potential for harm is increased because dendritic retraction may persist for weeks, months or even years, thereby broadening the window of time during which the hippocampus is vulnerable to harm, called the 'glucocorticoid vulnerability hypothesis'. The relevance of these findings is discussed with regard to conditions exhibiting parallels in hippocampal plasticity, including Cushing's disease, major depressive disorder (MDD), and post-traumatic stress disorder (PTSD).

Postnatal ontogeny of hippocampal expression of the mineralocorticoid and glucocorticoid receptors in the common marmoset monkey

The mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) are nuclear transcription factors that mediate many of the basal and stress functions and effects of the corticosteroid hormones, including those related to brain development. Despite this, relatively little is known about the postnatal ontogeny of MR and GR gene and protein expression in the central nervous system, and this is particularly true of the primates, including humans. Here we describe the postnatal ontogeny of central MR and GR gene and protein expression in the common marmoset monkey. By developing marmoset-specific riboprobes and using in situ hybridization, it was demonstrated that MR mRNA expression in the dentate gyrus and Ammon's horn was significantly greater in marmoset infants (aged 4-6 weeks) than in neonates (1-2 days), juveniles (4-5 months) and adults (3-6 years), with expression in the latter three ontogenetic stages being broadly similar. In the same subjects and ontogenetic stages, GR mRNA expression was developmentally consistent in the marmoset dentate gyrus and Ammon's horn, as well as in the paraventricular nucleus of the hypothalamus. Qualitative immunohistochemical comparison of infants and adults demonstrated that MR protein expression in the hippocampus was, as for mRNA, also greater in infants than adults, and that hippocampal GR protein was, as for mRNA, also similar in infants and adults. The increase in MR mRNA expression between the stages of neonate and infant co-occurred with a reduction in basal plasma ACTH and cortisol titres. The ontogenetic profiles of MR and GR gene expression in the marmoset monkey are therefore fundamentally different from those described for the rat and the mouse. This evidence for the postnatal ontogeny of central corticosteroid nuclear receptor expression in a primate is important for understanding both the developmental stage-specific significance of stress exposure and its potential long-term effects on health and disease.

CoGrafting of Hamster (Phodopus sungorus) and Marmoset (Callithrix jacchus) Testicular Tissues into Nude Mice Does Not Overcome Blockade of Early Spermatogenic Differentiation in Primate Grafts1

The ectopic xenotransplantation of testicular tissues into nude mice is a tool to generate sperm from immature testes. Immunodeficient mice as recipients of xenografts offered an appropriate microenvironment for differentiation of testicular tissue from hamsters, goats, pigs, and macaques. One exception is the neotropical primate Callithrix jacchus. Spermatogenesis in testicular grafts from marmosets does not proceed beyond the spermatogonial stage. The most likely cause for the poor graft development of marmosets is a deletion of exon 10 in the luteinizing hormone-receptor (LHR) gene, which renders this species insensitive to LH but responsive to chorionic gonadotropin (CG). We investigated whether cografting of testicular tissue from Djungarian hamsters would overcome the blockade in marmoset graft development. We also tested if exogenous administration of human CG (hCG) to the recipient would stimulate development of the marmoset tissue. No difference in graft survival was noted between hamster and monkey tissue. Seminiferous lumina were present in marmoset and hamster grafts but were significantly larger in hamster grafts. In the hamster grafts, a high proportion of the tubules contained meiotic and postmeiotic germ cells. In contrast, the marmoset tubules were populated with gonocytes and premeiotic spermatogonia. These results indicate that neither normal serum androgen levels nor the high local testosterone levels were sufficient to initiate marmoset spermatogenesis, nor was administration of hCG successful in overcoming the developmental blockade in marmoset tissue. Our results indicate that the conditions needed for initiation of spermatogenesis in the marmoset are remarkably different from those present in most other mammals.

Stress, memory, and the hippocampus: can't live with it, can't live without it

Since the 1968s discovery of receptors for stress hormones (corticosteroids) in the rodent hippocampus, a tremendous amount of data has been gathered on the specific and somewhat isolated role of the hippocampus in stress reactivity. The hippocampal sensitivity to stress has also been extended in order to explain the negative impact of stress and related stress hormones on animal and human cognitive function. As a consequence, a majority of studies now uses the stress-hippocampus link as a working hypothesis in setting up experimental protocols. However, in the last decade, new data were gathered showing that stress impacts on many cortical and subcortical brain structures other than the hippocampus. The goal of this paper is to summarize the four major arguments previously used in order to confirm the stress-hippocampus link, and to describe new data showing the implication of other brain regions for each of these previously used arguments. The conclusion of this analysis will be that scientists should gain from extending the impact of stress hormones to other brain regions, since hormonal functions on the brain are best explained by their modulatory role on various brain structures, rather than by their unique impact on one particular brain region.

Glucocorticoid and mineralocorticoid receptor mRNA expression in squirrel monkey brain

Corticosteroids have been implicated in hippocampal atrophy in patients with severe psychiatric disorders, but little is known about receptor expression for corticosteroids in human or nonhuman primate brain. Both the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) were surveyed in this study of squirrel monkey brain using in situ hybridization histochemistry. Regions of high GR mRNA levels included CA1 and CA2 of hippocampus, dentate gyrus, paraventricular hypothalamus, lateral geniculate, lateral>medial amygdala, and cerebellum. Western analysis confirmed that GR immunoreactivity in squirrel monkey brain tissue most likely reflects the alpha isoform. Regions of high MR mRNA levels included all hippocampal pyramidal cell fields, dentate gyrus granule cell layer, lateral septum, medial>lateral amygdala, and to a lesser extent, cerebellum. Low levels of MR were also expressed in caudate and putamen. Receptor expression for corticosteroids in deep brain structures and the hippocampal formation was similar to that previously reported in rodents, but GR and MR mRNA were expressed at higher levels in squirrel monkey cerebral cortex. GR expression was evident in all cortical layers, particularly the pyramidal cell-rich layers II/III and V. MR expression was restricted to the more superficial cortical layers, and was only moderately represented in layer V. Laminar patterns were apparent in all regions of cortex for GR expression in squirrel monkeys, but low MR mRNA levels were found in dorsomedial prefrontal cortex (PFC). Different subregional distributions and distinctive laminar patterns suggest specialized functions or coordinated interactions between GR and MR mediated functions in primate PFC.

Distribution of corticosteroid receptors in the rhesus brain: relative absence of glucocorticoid receptors in the hippocampal formation

Chronic stress has been associated with degenerative changes in the rodent and primate hippocampus, presumably mediated in part via neuronal glucocorticoid receptors (GRs). In the rat brain, GRs are widely distributed and are particularly dense in the hippocampus. The distribution of GRs in the primate brain, however, has not been fully characterized. In this study, we used in situ hybridization histochemistry and immunohistochemistry to map the distribution of GR mRNA and GR protein, respectively, in adult rhesus monkeys (Macaca mulatta). In contrast to its well established distribution in the rat brain, GR mRNA was only weakly detected in the dentate gyrus (DG) and Cornu Ammonis (CA) of the macaque hippocampus, whereas it was abundant in the pituitary (PIT), cerebellum (CBL), hypothalamic paraventricular nucleus (PVN), and, to a lesser extent, the neocortex. Immunohistochemical staining indicated a very low density of GR-like immunoreactive cells within the macaque hippocampal formation in contrast to the high density observed within the PVN, prefrontal and entorhinal cortices, and cerebellar cortex. Relative to the low level of GR, mineralocorticoid receptor (MR) mRNA and protein expression were abundant within the DG and CA of the rhesus monkey hippocampal formation. These results indicate that, in the primate, neocortical and hypothalamic areas may be more important targets for GR-mediated effects of glucocorticoids than the hippocampus. Alternatively, it is also possible that glucocorticoid effects are mediated through the MRs present in the hippocampal formation.

Characterization of mineralocorticoid and glucocorticoid receptors in pigs: comparison of Meishan and Large White breeds

Corticosteroids receptors were characterized and compared in central and peripheral tissues of two pig breeds, the Meishan (MS) and the Large White (LW) pigs, that display differences in the basal activity and stress-induced reactivity of the hypothalamic-pituitary-adrenal (HPA) axis. In vitro kinetic experiments on kidney and liver cytosols from adrenalectomized pigs allowed to identify two distinct corticosteroid receptors referred to as mineralocorticoid (MR) and glucocorticoid (GR) receptors. The binding specificities were determined for kidney and hippocampal MR and for liver and hippocampal GR. In hippocampus and peripheral tissues, cortisol showed a greater affinity for MR than for GR. As already described in the dog, mouse and human, dexamethasone and progesterone display a moderate affinity for MR. Putative differences in corticosteroid receptors binding capacities and affinities were investigated by saturation binding studies in specific regions implicated in the regulation of HPA axis (hippocampus and pituitary). The MS pigs evidenced higher densities of hippocampal MR, while LW pigs had higher densities of pituitary GR. Thus, this study suggests that a difference in the MR/GR balance in hippocampus and pituitary could be implicated in the different HPA activity between MS and LW pigs.

Effects of weaning and restraint stress on glucocorticoid receptor binding capacity in limbic areas of domestic pigs

Changes in glucocorticoid receptor (GR) binding in different brain areas were investigated in neonatal and adult pigs exposed to psychological stress (weaning) and a physical stressor (repeated snaring). The GR binding was significantly decreased 4 days after weaning in both the hippocampus and the amygdala, but there were no changes in the hypothalamus. Repeated snaring of adult pigs resulted in a significant diminished GR binding only in the hippocampus.

The hypothalamic-pituitary-adrenal axis of prairie voles (Microtus ochrogaster): evidence for target tissue glucocorticoid resistance

Basal plasma corticosterone levels in prairie voles (Microtus ochrogaster) are extremely high, in the absence of any apparent negative consequences of glucocorticoid excess. We tested the hypothesis that prairie voles are a novel rodent model of target tissue resistance to glucocorticoids. Prairie voles had a significantly higher adrenal-to-body weight ratio, 5- to 10-fold greater basal plasma corticosterone, and 2- to 3-fold greater basal plasma ACTH concentrations than montane voles (Microtus montanus) and rats. While plasma corticosterone binding globulin (CBG) was 2-fold higher in prairie voles than in rats, both estimated and directly measured plasma free corticosterone were significantly higher in prairie voles than in rats. Plasma corticosterone levels in prairie voles were responsive to both circadian cues and a stressor, but were resistant to suppression by the synthetic glucocorticoid, dexamethasone (DEX). Western blots of brain and liver protein extracts, using a glucocorticoid receptor (GR) antibody, revealed the presence of a approximately 97 kDa immunoreactive band, the expected size for GR. Binding assays revealed significantly lower DEX affinity of corticosteroid receptors (CR) in cytosol of prairie vole brain and liver than that in the same tissues in rats. We conclude that prairie voles are a novel rodent model of glucocorticoid resistance, and that decreased affinity of CR for ligand might be partially responsible for this phenomenon.