Cloning of glucocorticoid receptor and mineralocorticoid receptor cDNA and gene expression in the central nervous system of the tree shrew (Tupaia belangeri)

Increased oxidative stress, hyperphosphorylation of tau, and dystrophic microglia in the hippocampus of aged Tupaia belangeri

Aging is a major risk factor for the development of neurodegenerative diseases. Alzheimer's disease and other neurodegenerative diseases are characterized by abnormal and prominent protein aggregation in the brain, partially due to deficiency in protein clearance. It has been proposed that alterations in microglia phagocytosis and debris clearance hasten the onset of neurodegeneration. Dystrophic microglia are abundant in aged humans, and it has been associated with the onset of disease. Furthermore, alterations in microglia containing ferritin are associated with neurodegenerative conditions. To further understand the process of microglia dysfunction during the aging process, we used hippocampal sections from Tupaia belangeri (tree shrews). Adult (mean age 3.8 years), old (mean age 6 years), and aged (mean age 7.5 years) tree shrews were used for histochemical and immunostaining techniques to determine ferritin and Iba1 positive microglia, iron tissue content, tau hyperphosphorylation and oxidized-RNA in dentate gyrus, subiculum, and CA1-CA3 hippocampal regions. Our results indicated that aged tree shrews presented an increased number of activated microglia containing ferritin, but microglia labeled with Iba1 with a dystrophic phenotype was more abundant in aged individuals. With aging, oxidative damage to RNA (8OHG) increased significantly in all hippocampal regions, while tau hyperphosphorylation (AT100) was enhanced in DG, CA3, and SUB in aged animals. Phagocytic inclusions of 8OHG- and AT100-damaged cells were observed in activated M2 microglia in old and aged animals. These data indicate that aged tree shrew may be a suitable model for translational research to study brain and microglia alterations during the aging process.

11β-HSD Types 1 and 2 in the Songbird Brain

Glucocorticoid (GC) hormones act on the brain to regulate diverse functions, from behavior and homeostasis to the activity of the hypothalamic-pituitary-adrenal axis. Local regeneration and metabolism of GCs can occur in target tissues through the actions of the 11β-hydroxysteroid dehydrogenases [11 beta-hydroxysteroid dehydrogenase type 1 (11β-HSD1) and 11 beta-hydroxysteroid dehydrogenase type 2 (11β-HSD2), respectively] to regulate access to GC receptors. Songbirds have become especially important model organisms for studies of stress hormone action; however, there has been little focus on neural GC metabolism. Therefore, we tested the hypothesis that 11β-HSD1 and 11β-HSD2 are expressed in GC-sensitive regions of the songbird brain. Localization of 11β-HSD expression in these regions could provide precise temporal and spatial control over GC actions. We quantified GC sensitivity in zebra finch (Taeniopygia guttata) brain by measuring glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) expression across six regions, followed by quantification of 11β-HSD1 and 11β-HSD2 expression. We detected GR, MR, and 11β-HSD2 mRNA expression throughout the adult brain. Whereas 11β-HSD1 expression was undetectable in the adult brain, we detected low levels of expression in the brain of developing finches. Across several adult brain regions, expression of 11β-HSD2 covaried with GR and MR, with the exception of the cerebellum and hippocampus. It is possible that receptors in these latter two regions require direct access to systemic GC levels. Overall, these results suggest that 11β-HSD2 expression protects the adult songbird brain by rapid metabolism of GCs in a context and region-specific manner.

Animal models of major depression and their clinical implications

Major depressive disorder is a common, complex, and potentially life-threatening mental disorder that imposes a severe social and economic burden worldwide. Over the years, numerous animal models have been established to elucidate pathophysiology that underlies depression and to test novel antidepressant treatment strategies. Despite these substantial efforts, the animal models available currently are of limited utility for these purposes, probably because none of the models mimics this complex disorder fully. It is presumable that psychiatric illnesses, such as affective disorders, are related to the complexity of the human brain. Here, we summarize the animal models that are used most commonly for depression, and discuss their advantages and limitations. We discuss genetic models, including the recently developed optogenetic tools and the stress models, such as the social stress, chronic mild stress, learned helplessness, and early-life stress paradigms. Moreover, we summarize briefly the olfactory bulbectomy model, as well as models that are based on pharmacological manipulations and disruption of the circadian rhythm. Finally, we highlight common misinterpretations and often-neglected important issues in this field.

Distribution of the glucocorticoid receptor in the human amygdala; changes in mood disorder patients

Exposure to stress activates the hypothalamic-pituitary-adrenal (HPA) axis that stimulates glucocorticoid (GC) release from the adrenal. These hormones exert numerous effects in the body and brain and bind to a.o. glucocorticoid receptors (GR) expressed in the limbic system, including the hippocampus and amygdala. Hyperactivity of the HPA axis and disturbed stress feedback are common features in major depression. GR protein is present in the human hypothalamus and hippocampus, but little is known-neither in healthy subjects nor in depressed patients-about GR expression in the amygdala, a brain structure involved in fear and anxiety. Since chronic stress in rodents affects GR expression in the amygdala, altered GR protein level in depressed versus healthy controls can be expected. To test this, we investigated GR-α protein expression in the post-mortem human amygdala and assessed changes in ten major or bipolar depressed patients and eight non-depressed controls. Abundant GR immunoreactivity was observed in the human amygdala, both in neurons and astrocytes, with a similar pattern in its different anatomical subnuclei. In major depression, GR protein level as well as the percentage of GR-containing astrocytes was significantly higher than in bipolar depressed patients or in control subjects. Taken together, the prominent expression of GR protein in the human amygdala indicates that this region can form an important target for corticosteroids and stress, while the increased GR expression in major, but not bipolar, depression suggests possible involvement in the etiology of major depression.

Exploration of the Hypothalamic-Pituitary-Adrenal Axis to Improve Animal Welfare by Means of Genetic Selection: Lessons from the South African Merino

It is a difficult task to improve animal production by means of genetic selection, if the environment does not allow full expression of the animal's genetic potential. This concept may well be the future for animal welfare, because it highlights the need to incorporate traits related to production and robustness, simultaneously, to reach sustainable breeding goals. This review explores the identification of potential genetic markers for robustness within the hypothalamic-pituitary-adrenal axis (HPAA), since this axis plays a vital role in the stress response. If genetic selection for superior HPAA responses to stress is possible, then it ought to be possible to breed robust and easily managed genotypes that might be able to adapt to a wide range of environmental conditions whilst expressing a high production potential. This approach is explored in this review by means of lessons learnt from research on Merino sheep, which were divergently selected for their multiple rearing ability. These two selection lines have shown marked differences in reproduction, production and welfare, which makes this breeding programme ideal to investigate potential genetic markers of robustness. The HPAA function is explored in detail to elucidate where such genetic markers are likely to be found.

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.

Plasticity of the parental brain: a case for neurogenesis

Profound behavioural changes occur in the mother at parturition, together with extensive remodelling of neural circuits. These changes include neurochemical, morphological and functional plasticity. The continuous generation of new neurones in the hippocampus and the olfactory system is an additional form of neuroplasticity that contributes to motherhood. This review describes the reciprocal relationships between hippocampal and olfactory neurogenesis and parental behaviour. Studies in rodents demonstrate that parturition and interactions with the young affect both cell proliferation and survival in a different manner across neurogenic zones. Species in which an individual recognition of the offspring is formed, such as sheep, show a down-regulation of neurogenesis during the perinatal period. This would function to decrease cell competition, favouring the selection of newborn neurones involved in olfactory recognition of the young. Also, in biparental species, increases in olfactory neurogenesis occur in the father in response to pup exposure during the early postpartum period. Oestradiol, corticosterone and prolactin changes associated with parturition are the main physiological factors involved in the regulation of neurogenesis that have been determined so far. In the father, prolactin mediates an enhancement of olfactory neurogenesis. Contradictory evidence indicates a functional link between neurogenesis and parenting behaviour. Mice receiving focal irradiation of the olfactory neurogenic subventricular zone show few disturbances in the expression of maternal behaviour, whereas a reduction of both hippocampal and olfactory neurogenesis as a result of the infusion of an anti-mitotic agent induces behavioural deficits. Disrupting prolactin signalling abolished increased paternal neurogenesis and offspring recognition by the father, and rescuing this neurogenesis restored recognition behaviour. More studies that selectively suppress the changes of neurogenesis are needed to confirm the role of new neurones in regulating parenting behaviour.

Decreased expression of mineralocorticoid receptor mRNA and its splice variants in postmortem brain regions of patients with major depressive disorder

Appropriate signaling in the brain by the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) is critical in regulation of the hypothalamic-pituitary-adrenal (HPA) axis, emotional arousal and cognitive performance. To date, few data exist on MR (and GR) expression in the brain of patients suffering from major depressive disorder (MDD). With the help of quantitative PCR we assessed MR and GR mRNA expression, including the splice variants MRα and MRβ, in tissue samples from the hippocampus, amygdala, inferior frontal gyrus, cingulate gyrus and nucleus accumbens. Expression levels were compared between tissue samples from six MDD patients and six non-depressed subjects. Relative to total GR, total MR mRNA expression was higher in hippocampus and lower in the amygdala, inferior frontal gyrus and nucleus accumbens. Both MRα and MRβ could be detected in all brain regions that were analyzed, although MRβ expression was low. Significantly lower expression levels (30-50%) were detected for MR or GR in hippocampal, inferior frontal gyrus and cingulate gyrus tissue from MDD patients (p < .05), while no differences were found in the amygdala or nucleus accumbens. The data show that both MRα and MRβ mRNA are expressed throughout the human limbic brain with highest expressions in the hippocampus. A decreased expression of corticosteroid receptors in specific brain regions of MDD patients could underlie HPA hyperactivity, mood and cognitive disturbances often observed in patients suffering from stress-related psychopathologies.

Evidence for cell proliferation in the sheep brain and its down-regulation by parturition and interactions with the young

Production of new neurons continues throughout life in the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus and is influenced by both endocrine and social factors. In sheep parturition is associated with the establishment of a selective bond with the young based on an olfactory learning. The possibility exists that endocrine changes at parturition together with interactions with the young modulate cell proliferation in the neurogenic zones. In the present study, we first investigated the existence of cell proliferation in sheep. Newly born cells labeled by the cell proliferation marker 5-bromo-2'-deoxyuridine (BrdU) were found in the SVZ, the main olfactory bulb (MOB) and the DG and completely co-localized with Ki-67, another mitotic marker. Forty to 50% of the BrdU-labeled cells contained GFAP suggestive of the presence of neural stem cells. Secondly, parturition with or without interactions with the lamb for 2 days, down-regulated the number of BrdU-labeled cells in the 3 proliferation sites in comparison to no pregnancy. An additional control provided evidence that this effect is specific to early postpartum period: estrus with interactions with males did not affect cell proliferation. Our results provide the first characterization of neural cell proliferation in the SVZ, the DG and unexpectedly in the MOB of adult sheep. We hypothesize that the down-regulation of cell proliferation observed in the early postpartum period could facilitate the olfactory perceptual and memory demands associated with maternal behavior by favouring the survival and integration of neurons born earlier.

Selective cancer targeting via aberrant behavior of cancer cell-associated glucocorticoid receptor

Glucocorticoid receptors (GRs) are ubiquitous, nuclear hormone receptors residing in cell types of both cancer and noncancerous origin. It is not known whether cancer cell-associated GR alone can be selectively manipulated for delivery of exogenous genes to its nucleus for eliciting anticancer effect. We find that GR ligand, dexamethasone (Dex) in association with cationic lipoplex (termed as targeted lipoplex) could selectively manipulate GR in cancer cells alone for the delivery of transgenes in the nucleus, a phenomenon that remained unobserved in normal cells. The targeted lipoplex (i) showed GR-targeted transfections in all cancer cells experimented (P < 0.01), (ii) significantly diminished transfection in cancer cells when GR is downregulated (P < 0.01), and (iii) elicited specific nuclear translocation of targeted lipoplex in cancer cells, followed by upregulated transactivation of glucocorticoid response element (GRE)- promoted gene. Using anticancer gene, targeted lipoplex induced significant tumor growth retardation in mice in comparison to different control groups (P < 0.05). Interestingly, cell surface-associated Hsp90 in cancer cells assisted the intracellular uptake of GR-targeted lipoplex. Moreover, selective inhibition of Hsp90 in noncancer cells resulted in cancer cell-like, aberrant, GR activation. The current study discovers a therapeutically important, unique property of cancer cell associated-GR that may be linked to a compromised role of Hsp90.Molecular Therapy (2009) 17 4, 623-631 doi:10.1038/mt.2009.4.

Melatonin, consciousness, and traumatic stress

Descartes intuitively anticipated the so-called 'binding problem' of consciousness and thought that the pineal gland enables spatio-temporal integration in cognitive processing. Recent findings indicate that a major role in the process of temporal integration and binding involve neurons in suprachiasmatic nuclei, specifically targeting the pineal gland and other structures, and control the neuroendocrine rhythms. Melatonin is an endocrine output signal of the clock and provides circadian information as an endogenous synchronizer which stabilizes and reinforces circadian rhythms. This integrative process occurs at the different levels of the circadian network via gene expression in some brain regions and peripheral structures that enables integration of circadian, hormonal, and metabolic information and creating temporal order of bodily and mental experience. This specific temporal order is reflected in associative sequentiality that is necessary for cognition, behavior and all processes of memory consolidation that must preserve all information in the temporal causal order and synchrony. In this context, recent findings suggest that melatonin could be a potential regulator in the processes that contribute to memory formation, long-term potentiation, and synaptic plasticity in the hippocampus and other brain regions. There is evidence that stress disrupts normal activity and memory consolidation in the hippocampus and prefrontal cortex, and this process leads to memories that are stored without a contextual or spatiotemporal frame. These findings emphasize a specific role of melatonin in mechanisms of consciousness, memory and stress and are also consistent with reported studies that indicate melatonin alterations under stressful conditions and in mental disorders.

Postnatal ontogeny of expression of the corticosteroid receptor genes in mammalian brains: inter-species and intra-species differences

Corticosteroids are important mediators of homeostasis and stress, and exert their effects via two transcription-factor receptors, mineralocorticoid receptor (MR) and glucocorticoid receptor (GR). Both receptors are expressed in the brain in a region-specific manner, and regulate neuroendocrine and behavioral functions. Stress during early development has been demonstrated to lead to long-term alterations in MR and GR levels and in the phenotypes that they mediate. To date, however, nearly all of this evidence has been obtained in rats, and there is actually no clear basis for extrapolation to other species. The current comparative review presents data, as available, on the following aspects of GR and MR gene expression in mouse and rat (Rodentia), tree shrew (Scandentia), common marmoset, squirrel monkey, rhesus macaque and human (Primates): (1) species-typical adult expression of MR mRNA and GR mRNA in hypothalamus, amygdala, hippocampus and neocortex; (2) species-typical neonate, infant, juvenile/adolescent and adult expression of MR mRNA and GR mRNA in hippocampus. (1) and (2) allow for identification of inter-species consistencies and differences in the relative levels of MR and GR expression across brain regions and ontogenetic stages. In addition, data are presented on (3) within-species inter-individual variation in MR and GR expression and causes thereof, including polymorphism and early life stress. Integrating the evidence in (1)-(3), it is noted that, should the expression levels of MR and GR at the time of early-life stress determine the latter's effects on the formers' long-term expression levels and functioning, then the long-term effects of early life stress on corticosteroid receptor expression and function will be species-, brain-region- and receptor-type-specific.

The role of protein turnover in regulating MKP-1 levels in rat pinealocytes

We have previously shown that mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) is induced at night under the control of a photoneural system in the rat pineal gland. Because of the established roles of MAPKs, glucocorticoids and proteasome activity in regulating MKP-1 expression in other cell types, their relative contributions to MKP-1 regulation were investigated in rat pinealocytes. We found that neither inhibition of MAPKs nor treatment with dexamethasone affected norepinephrine-stimulated MKP-1 expression. In contrast, treatment with proteasome inhibitors increased norepinephrine-stimulated MKP-1 protein levels and abolished the decline in norepinephrine-stimulated MKP-1 protein levels caused by inhibition of transcription or translation, or blockade of alpha-adrenergic receptors. Taken together, our results indicate that in rat pinealocytes, the continuous and rapid turnover of MKP-1 protein allows for its rapid induction but is not sufficient to generate the sustained increase in MKP-1 expression post-adrenergic stimulation.

Chronic stress decreases the expression of sympathetic markers in the pineal gland and increases plasma melatonin concentration in rats

Chronic stress affects brain areas involved in learning and emotional responses. Although most studies have concentrated on the effect of stress on limbic-related brain structures, in this study we investigated whether chronic stress might induce impairments in diencephalic structures associated with limbic components of the stress response. Specifically, we analyzed the effect of chronic immobilization stress on the expression of sympathetic markers in the rat epithalamic pineal gland by immunohistochemistry and western blot, whereas the plasma melatonin concentration was determined by radioimmunoassay. We found that chronic stress decreased the expression of three sympathetic markers in the pineal gland, tyrosine hydroxylase, the p75 neurotrophin receptor and alpha-tubulin, while the same treatment did not affect the expression of the non-specific sympathetic markers Erk1 and Erk2, and glyceraldehyde-3-phosphate dehydrogenase. Furthermore, these results were correlated with a significant increase in plasma melatonin concentration in stressed rats when compared with control animals. Our findings indicate that stress may impair pineal sympathetic inputs, leading to an abnormal melatonin release that may contribute to environmental maladaptation. In addition, we propose that the pineal gland is a target of glucocorticoid damage during stress.

Chronic stress induces upregulation of brain-derived neurotrophic factor (BDNF) mRNA and integrin alpha5 expression in the rat pineal gland

Chronic stress affects brain areas involved in learning and emotional responses. These alterations have been related with the development of cognitive deficits in major depression. Moreover, stress induces deleterious actions on the epithalamic pineal organ, a gland involved in a wide range of physiological functions. The aim of this study was to investigate whether the stress effects on the pineal gland are related with changes in the expression of neurotrophic factors and cell adhesion molecules. Using reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, we analyzed the effect of chronic immobilization stress on the BDNF mRNA and integrin alpha5 expression in the rat pineal gland. We found that BDNF is produced in situ in the pineal gland. Chronic immobilization stress induced upregulation of BDNF mRNA and integrin alpha5 expression in the rat pineal gland but did not produce changes in beta-actin mRNA or in GAPDH expression. Stressed animals also evidenced an increase in anxiety-like behavior and acute gastric lesions. These results suggest that BDNF and integrin alpha5 may have a counteracting effect to the deleterious actions of immobilization stress on functionally stimulated pinealocytes. Furthermore, this study proposes that the pineal gland may be a target of glucocorticoid damage during stress.

Mossy cells and different subpopulations of pyramidal neurons are immunoreactive for cocaine- and amphetamine-regulated transcript peptide in the hippocampal formation of non-human primates and tree shrew (Tupaia belangeri)

Cocaine- and amphetamine-regulated transcript peptide mRNA was discovered in the rat striatum following cocaine and amphetamine administration. Since both psychostimulants elicit memory-related effects, localization of cocaine- and amphetamine-regulated transcript peptide in the hippocampal formation may have functional importance. Previous studies demonstrated different cellular localizations of cocaine- and amphetamine-regulated transcript peptide in humans and in rodents. Mossy cells were cocaine- and amphetamine-regulated transcript-positive in the human dentate gyrus, whereas granule cells contained this peptide in the rat. In the present study, the localization of cocaine- and amphetamine-regulated transcript peptide was examined using immunohistochemistry in the hippocampal formation of the rhesus monkey (Macaca mulatta), the common marmoset monkey (Callithrix jacchus) and in the tree shrew (Tupaia belangeri). In these species principal neurons of the hippocampal formation were cocaine- and amphetamine-regulated transcript-immunoreactive. In both monkeys and tree shrews, mossy cells of the hilus were cocaine- and amphetamine-regulated transcript-positive whereas granule cells of the dentate gyrus were cocaine- and amphetamine-regulated transcript-negative. The dense cocaine- and amphetamine-regulated transcript-immunoreactive axonal plexus of the associational pathway outlined the inner one-third of the dentate molecular layer. In the hippocampus of the tree shrew and marmoset monkey, a subset of CA3 pyramidal cells were cocaine- and amphetamine-regulated transcript-immunoreactive. In the marmoset monkey, cocaine- and amphetamine-regulated transcript labeling was found only in layer V pyramidal cells of the entorhinal cortex, while in the rhesus monkey, pyramidal cells of layers II and III were cocaine- and amphetamine-regulated transcript-immunopositive. Our results show that cocaine- and amphetamine-regulated transcript positive neurons in the dentate gyrus of non-human primates are similar to that of the human. Furthermore, in the hippocampal formation of the tree shrew similar cocaine- and amphetamine-regulated transcript-immunoreactive cell-types were observed as in monkeys, supporting their evolutionary relationship with primates. Mossy cells and granule cells are members of a mutual excitatory intrahippocampal circuitry, therefore cocaine- and amphetamine-regulated transcript-immunoreactivity of these neurons in primates and rodents suggests that psychostimulants cocaine and amphetamine may induce memory-related effects at different points of the same excitatory circuitry in the hippocampal formation.

Social stress in tree shrews as an animal model of depression: an example of a behavioral model of a CNS disorder

Animal models are invaluable in preclinical research on human psychopathology. Valid animal models to study the pathophysiology of depression and specific biological and behavioral responses to antidepressant drug treatments are of prime interest. In order to improve our knowledge of the causal mechanisms of stress-related disorders such as depression, we need animal models that mirror the situation seen in patients. One promising model is the chronic psychosocial stress paradigm in male tree shrews. Coexistence of two males in visual and olfactory contact leads to a stable dominant/subordinate relationship, with the subordinates showing obvious changes in behavioral, neuroendocrine, and central nervous activity that are similar to the signs and symptoms observed during episodes of depression in patients. To discover whether this model, besides its "face validity" for depression, also has "predictive validity," we treated subordinate animals with the tricyclic antidepressant clomipramine and found a time-dependent recovery of both endocrine function and normal behavior. In contrast, the anxiolytic diazepam was ineffective. Chronic psychosocial stress in male tree shrews significantly decreased hippocampal volume and the proliferation rate of the granule precursor cells in the dentate gyrus. These stress-induced changes can be prevented by treating the animals with clomipramine, tianeptine, or the selective neurokinin receptor antagonist L-760,735. In addition to its apparent face and predictive validity, the tree shrew model also has a "molecular validity" due to the degradation routes of psychotropic compounds and gene sequences of receptors are very similar to those in humans. Although further research is required to validate this model fully, it provides an adequate and interesting non-rodent experimental paradigm for preclinical research on depression.

Examining novel concepts of the pathophysiology of depression in the chronic psychosocial stress paradigm in tree shrews

Despite decades of research on psychiatric disorders, the aetiology and precise biological mechanisms that underlie depressive diseases are still poorly understood. There is increasing evidence that psychiatric disorders not only have a neurochemical basis but are also associated with morphological alterations in central nervous neurons and/or glial cells. Antidepressants may act by restoring structure as well as function of neural networks, meaning that they may, as a fundamental principle, affect neural plasticity underlying normal brain functioning. To examine these novel concepts of the pathophysiology of depression and antidepressant medication we have carried out a series of experiments using the chronic psychosocial stress paradigm in male tree shrews, an animal model with a high validity for the pathophysiology of depressive disorders, in which the animals were treated with the tricyclic antidepressant compound clomipramine. We found that one month of stress reduced cell proliferation in the dentate gyrus, and decreased the total hippocampal volume. Gene transcription analysis revealed that, under these experimental conditions, expression of genes known to be involved in processes of cell differentiation is suppressed. These effects of social conflict on hippocampal cells, including gene transcription, and on the entire hippocampal volume could be counteracted by chronic treatment with the antidepressant clomipramine. Stress also induced a constant hyperactivity of the hypothalamic-pituitary-adrenal axis, and suppressed both motor and marking behaviour. These neuroendocrine and behavioural stress-induced changes were also re-normalized by clomipramine.

Preservation of hippocampal neuron numbers and hippocampal subfield volumes in behaviorally characterized aged tree shrews

Aging is associated with a decreased ability to store and retrieve information. The hippocampal formation plays a critical role in such memory processes, and its integrity is affected during normal aging. We used tree shrews (Tupaia belangeri) as an animal model of aging, because in many characteristics, tree shrews are closer to primates than they are to rodents. Young and aged male tree shrews performed a holeboard spatial memory task, which permits assessment of reference and working memory. Upon completion of the behavioral measurements, we carried out modified stereological analyses of neuronal numbers in various subdivisions of the hippocampus and used the Cavalieri method to calculate the volumes of these subfields. Results showed that the working memory of aged tree shrews was significantly impaired compared with that of young animals, whereas the hippocampus-dependent reference memory remained unchanged by aging. Estimation of the number of neurons revealed preserved neuron numbers in the subiculum, in the subregions CA1, CA2, CA3, and in the hilus of the dentate gyrus. Volume measurements showed no aging-related changes in the volume of any of these hippocampal subregions, or in the molecular and granule cell layers of the dentate gyrus of tree shrews. We conclude that the observed changes in memory performance in aging tree shrews are not accompanied by observable reductions of hippocampal neuron numbers or hippocampal volume, rather, the changes in memory performance are more likely the result of modified subcellular mechanisms that are affected by the aging process.

Homeostatic maintenance in excitability of tree shrew hippocampal CA3 pyramidal neurons after chronic stress

The experience of chronic stress induces a reversible regression of hippocampal CA3 apical neuron dendrites. Although such postsynaptic membrane reduction will obviously diminish the possibility of synaptic input, the consequences for the functional membrane properties of these cells are not well understood. We tested the hypothesis that chronic stress affects the input-output characteristics and excitability of CA3 pyramidal cells. Somatic whole-cell current-clamp recording with parallel intracellular biocytin labeling was performed on CA3 neurons from in vitro hippocampal slices from male tree shrews, which were collected after 28 days of psychosocial stress exposure and compared to recordings obtained from control animals. Post hoc morphometric analysis of biocytin-labeled CA3 cells revealed branch regression, by fewer dendritic crossings and length, limited to a distance of approximately 280-340 microm from the soma only. The results from whole-cell recording indicate that chronic stress surprisingly reduced the apparent membrane time constant and input resistance 20-25%, accompanied by increased amplitude of the hyperpolarization-induced voltage "sag." All active membrane properties, including depolarization-induced action potential kinetics, complex spiking patterns, and afterhyperpolarization voltages, were indistinguishable from control recordings. Although linear association analysis confirmed that differences in geometry, such as apical length or branch number, were correlated to functional variability in properties of the AP current and voltage threshold, these changes were too marginal to be reflected in the group differences. However, the individual adrenal hormone status was associated significantly with the selective changes in subthreshold excitability. Taken together, the data provide evidence that despite long-term stress induces morphological changes, upregulates cortisol release and shifts the intrinsic membrane properties, the efficacy of somatic excitability of CA3 pyramidal neurons is largely preserved.

Identification of genes regulated by chronic psychosocial stress and antidepressant treatment in the hippocampus

Analysis of differentially expressed genes in the brain is a promising tool for elucidating pathological mechanisms that lead to central nervous disorders. Stress is known to be involved in the development of psychopathologies such as depression. In the present study, we searched for differentially expressed genes in the hippocampal formation after chronic psychosocial stress and after treatment with the antidepressant clomipramine. Experiments were conducted in male tree shrews, a valid psychosocial stress model in which antidepressant drugs prevent diverse effects of stress. Because many effects of stress have been attributed to the stress-induced elevation in glucocorticoids, we screened two subtractive hippocampal cDNA libraries generated from RNA of chronic cortisol-treated animals. Using real-time PCR to measure mRNA amounts, we identified five sequences whose expression levels differed between stressed animals and controls. Transcript levels of four of them, nerve growth factor (NGF), membrane glycoprotein 6a (M6a), CDC-like kinase 1 (CLK-1) and G-protein alpha q (GNAQ) were reduced by chronic psychosocial stress. Reduced amounts of these genes, which are all related to processes of cell differentiation, is in agreement with previous findings showing a retraction of dendrites and an impairment of neurogenesis in the hippocampal formation after chronic stress. An additional expressed sequence that was also regulated by stress could not be assigned to any known gene. Treatment with the antidepressant clomipramine prevented stress effects on expression of M6a, CLK-1, GNAQ and the novel sequence, but showed no effect on NGF stress-induced down-regulation. These findings support the concept that depressive disorders are accompanied by processes of neuronal dedifferentiation, at least in the hippocampal formation, and that antidepressants prevent these processes.

Bidirectional shift in the cornu ammonis 3 pyramidal dendritic organization following brief stress

The negative impact of chronic stress at the structure of apical dendrite branches of cornu ammonis 3 (CA3) pyramidal neurons is well established. However, there is no information available on the CA3 dendritic organization related to short-lasting stress, which suffices to produce long-term habituation or sensitization of anxiety behaviors and neuroendocrine responses. Here, we tested the effects evoked by brief stress on the arrangements of CA3 pyramidal neuron dendrites, and the activity-dependent properties of the commissural-associational (C/A) excitatory postsynaptic potentials (EPSPs). Adult male rats were socially defeated followed by 3 weeks without further treatment or as comparison exposed to a regimen of a social defeat every second day for the same time period. We assessed CA3 pyramidal neurons with somatic whole-cell recording and neurobiotin application in acute hippocampal slices. The results from morphometric analysis of post hoc reconstructions demonstrated that CA3 dendrites from repeatedly stressed rats were reduced in surface area and length selectively at the apical cone (70% of control, approximately 280 microm from the soma). Brief stress, however, produced a similar decrease in apical dendritic length (77% of control, approximately 400 microm from the soma), accompanied by an increased length (167% of control) and branch complexity at the basal cone. The structural changes of the dendrites significantly influenced signal propagation by shortening the onset latency of EPSPs and increasing input resistance (r=0.45, P<0.01), of which the first was significantly changed in repeatedly stressed animals. Both brief and repeated stress long-lastingly impaired long-term potentiation of C/A synapses to a similar degree (P<0.05). These data indicate that the geometric plasticity of CA3 dendrites is dissociated from repetition of aversive experiences. A double social conflict suffices to drive a dynamic reorganization, by site-selective elimination and de novo growth of dendrite branches over the course of weeks after the actual experience.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

Chronic social stress: effects on limbic brain structures

Different types of stressors are known to activate distinct neuronal circuits in the brain. Acute physiological stimuli that are life threatening and require immediate reactions lead to a rapid stimulation of brainstem and hypothalamus to activate efferent visceral pathways. In contrast, psychological stressors activate higher-order brain structures for further interpretations of the perceived endangerment. Common to the later multimodal stressors is that they need cortical processing and, depending on previous experience or ongoing activation, the information is assembled within limbic circuits connecting, e.g., the hippocampus, amygdala and prefrontal cortex to induce neuroendocrine and behavioral responses. In view of the fact that stressful life events often contribute to the etiology of psychopathologies such as depressive episodes, several animal models have been developed to study central nervous mechanisms that are induced by stress. The present review summarizes observations made in the tree shrew chronic psychosocial stress paradigm with particular focus on neurotransmitter systems and structural changes in limbic brain regions.

A cytoarchitectonic study of the hippocampal formation of the tree shrew (Tupaia belangeri)

Tree shrews constitute an interesting animal model to study the impact of stress or aging on the hippocampal formation, a brain structure known to be affected under such environmental or internal influences. To perform detailed investigations of the hippocampal formation, adequate knowledge of its anatomy should be present. Until now, the hippocampal formation of the tree shrew has not yet been studied extensively. The main objective of this study, therefore, was to describe the subfield boundaries in various levels of the dorsoventral hippocampal axis of the tree shrew (Tupaia belangeri) in detail. The secondary aim was to clarify whether a separate CA2 field can actually be distinguished in the tree shrew hippocampus, a fact that was denied in former reports. In addition, we aimed at investigating whether or not a CA4 subfield can be identified in the tree shrew's hippocampus. The immunocytochemical distribution of microtubule-associated protein 2 and the calcium-binding proteins, parvalbumin and calbindin, and the characteristics of Nissl staining in adjacent sections were compared. Because of the rather dorsoventral orientation of the long hippocampal axis in tree shrews, staining patterns were analyzed mainly in horizontal sections. The subiculum and the hippocampal CA1 and CA3 areas were easily identified. Moreover, we were able to demonstrate the existence of a distinct CA2 subfield in the tree shrew's Ammon's horn, contrary to previous reports. However, our results indicate that a CA4 field in the tree shrew hippocampal formation cannot be identified with the methods that we used. Therefore, supposed CA4 pyramidal neurons should be included into the CA3 field.

Blockade of glucocorticoid receptors with ORG 34116 does not normalize stress-induced symptoms in male tree shrews

Glucocorticoid receptors play an important role in the regulation of the activity of the hypothalamo-pituitary-adrenal axis, and are thought to be involved in the pathophysiology of depressive disorders. The present study investigated the effect of the specific glucocorticoid receptor antagonist ORG 34116 (a substituted 11,21 bisarylsteroid compound) in the tree shrew (Tupaia belangeri) chronic psychosocial stress model, an established animal model for depressive disorders. Animals were stressed for 10 days before treatment with ORG 34116 started (25 mg/kg p.o. for 28 days). Stress induced a decrease in body weight, which just failed significance, whereas ORG 34116 did not affect body weight in stress and control animals. ORG 34116 enhanced the stress-induced increase in the concentration of urinary-free cortisol, although no differences between the different experimental groups existed during the last week of treatment. In stressed animals, ORG 34116 did not affect marking behavior, but decreased locomotor activity. Post mortem analysis of 5-HT(1A) receptors revealed a decreased affinity of 3[H]-8-OH-DPAT (3[H]-8-hydroxy-2-[di-n-propylamino]tetralin) binding sites in the hippocampus of animals treated with the glucocorticoid receptor antagonist. In conclusion, under our experimental conditions, the glucocorticoid receptor antagonist ORG 34116 did not normalize the depressive-like symptoms in the psychosocial stress model of male tree shrews. This finding, however, does not exclude that specific central, neuroendocrine and behavioral features are affected by the compound.

Social stress in tree shrews: effects on physiology, brain function, and behavior of subordinate individuals

Social stress is known to be involved in the etiology of central nervous disorders such as depression. In recent years, animal models have been developed that use chronic stress to induce neuroendocrine and central nervous changes that might be similar to those occurring in the course of the development of depressive disorders. The present review gives a summary of observations made in the tree shrew chronic social stress model. During periods of daily social stress, male tree shrews develop symptoms that are known from many depressed patients such as persistent hyperactivities of both the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic nervous system, disturbances in sleeping patterns, and reduced motor activity. Moreover, various physiological parameters indicate an acceleration of the over all metabolic rate in socially stressed tree shrews. Some of these parameters can be renormalized by antidepressants thus supporting the view of the tree shrew social stress paradigm as model for major depression. In the brains of socially stressed animals, monoamine receptors show dynamic changes that reflect adaptation to the persistent monoaminergic hyperactivity during periods of chronic stress. In addition to the changes in neurotransmitter systems, there are structural changes in neurons, e.g., retraction of the dendrites of hippocampal pyramidal neurons. Together, these processes are suggested as a cause of behavioral alterations that can be counteracted by antidepressants in this naturalistic social stress model.

Chronic psychosocial stress regulates the expression of both GR and MR mRNA in the hippocampal formation of tree shrews

A persistent hyperactivity of the hypothalamic-pituitary-adrenal axis and thus elevated glucocorticoid levels are main neuroendocrine features of depressive symptomatology in humans. The broad range of effects that are set off by glucocorticoids is mediated by glucocorticoid receptors (GRs) and mineralocorticoid receptors (MRs), which themselves are subject to autoregulation. In order to investigate the impact of long-lasting psychological stress on corticosteroid receptor mRNA expression in the hippocampal formation, we employed the psychosocial stress paradigm in male tree shrews (Tupaia belangeri). By in situ hybridization studies and semiquantitative evaluation of stress-induced changes of GR and MR mRNA expression at the single-cell level, brain tissue from subordinate animals which were exposed to 27 days (1 h/day) of social confrontation was compared to that of nonstressed animals. Four weeks of stress exposure resulted in a downregulation of GR mRNA in the dentate gyrus and hippocampal subfields CA1 and CA3 of subordinate male tree shrews compared to controls. The MR mRNA content in these subfields of the anterior hippocampus was also clearly reduced. On the contrary, in a more posterior location on the longitudinal axis of the tree shrew hippocampus, the MR message was increased in subfields CA1 and CA3 and in the dentate gyrus. These results suggest a relevance of the stress-induced regulation of both corticosteroid receptor subtype mRNAs in a naturalistic challenging situation. Moreover, the differential regulation of MR mRNA along the rostrocaudal axis of the hippocampus adds another feature to the heterogenous composition of this structure.

Gender-specific alterations of cerebral metabolites with aging and cortisol treatment

Excess availability of the adrenocortical glucocorticoid hormone cortisol has been correlated with structural brain changes and a decline of cognitive functions during aging. Pertinent studies need to consider gender as a potential confound because of sexual dimorphism in the regulation of hypothalamus-pituitary-adrenal axis activity. In vivo localized proton magnetic resonance spectroscopy of male and female tree shrews revealed similar concentrations of cerebral metabolites in young adult animals but gender-specific alterations with aging as well as in response to cortisol treatment. In comparison with adult tree shrews, aged males had reduced concentrations of N-acetylaspartate (-33%; P<0.01) and total creatine (-34%; P< 0.01). These findings are in line with the occurrence of neuronal loss. In contrast, aged females exhibited increased concentrations of choline-containing compounds (+27%; P<0.05) which--together with a tendency for increased creatine (+24%) and myo-inositol (+14%)--is indicative of glial proliferation. After chronic administration of cortisol (4 mg/day for 28 days), male but not female tree shrews showed a specific reduction of the choline-containing compounds (-29%; P< 0.05). The observed sex differences with age are likely to result from differences in the regulation of stress-related hormones which is further supported by the gender-specific responses to cortisol.

Glucocorticoid receptors and serotonin N-acetyltransferase activity in the fish pineal organ

This study aimed to determine whether glucocorticoid receptors are expressed in the photosensitive trout pineal organ, and whether glucocorticoids modulate melatonin secretion. On Western blots from pineal extracts, an antibody directed against trout glucocorticoid receptor labeled a single band at the expected size (approximately 100 kDa). Dexamethasone inhibited pineal arylalkylamine N-acetyltransferase activity (AANAT2; serotonin --> N-acetylserotonin) in a dose-dependent manner after 6 h of culture in the dark (IC50 2.10(-8) M). RU486 (10(-7) M) alone had a partial agonistic activity, whereas it antagonized the effects of 10(-8) M dexamethasone. Hydroxyindole-O-methyltransferase activity (N-acetylserotonin --> melatonin) remained unaffected. This is the first demonstration that glucocorticoid receptors are present in the pineal organ and that glucocorticoids modulate melatonin production.

Glucocorticoids and the ageing hippocampus

Approximately 30 % of human and mammalian populations develop cognitive impairments with ageing. Many of these impairments have been linked to dysfunction of the hippocampus, a well studied area of the medial-temporal lobe, which is involved in episodic memory and control of the hypothalamo-pituitary-adrenal stress axis and, thus, of glucocorticoid secretion. This paper reviews the growing body of studies which explore a possible relationship between lifetime exposure to glucocorticoids and hippocampal impairment. There is now strong evidence which associates hypercortisolemia in aged men with later cognitive dysfunction and this complements a wealth of rodent and other human data. We conclude with a discussion of possible pharmacological and behavioural interventions.

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.

Regulation of alpha(2A)-adrenoceptor expression by chronic stress in neurons of the brain stem

Alpha(2)-Adrenoceptors are supposed to be important regulatory elements in responses to stress. Previous receptor binding studies in male tree shrews have shown that chronic psychosocial stress down-regulates binding sites for alpha(2)-adrenergic ligands in several brain stem nuclei. The aim of the present study was to quantify effects of chronic subordination stress on expression of the alpha(2)-adrenoceptor subtype A gene in identified neurons of the brain stem. We partially cloned the alpha(2A)-adrenoceptor cDNA of the tree shrew (1.22 kb) and localized receptor RNA expression in brain stem neurons by in situ hybridization using a 35S-labeled cRNA probe (1.06 kb). To identify neurons expressing receptor mRNA, brain sections were first immunocytochemically stained with antibodies against tyrosine hydroxylase, phenylethanolamine-N-methyltransferase, or glutamate, and then processed for in situ hybridization. Furthermore, expression of receptor-specific RNA was quantified in single neurons of animals which had been psychosocially stressed during 4 weeks and in unstressed controls. We found strong in situ hybridization in the noradrenergic neurons of the locus coeruleus, but only weak labeling of A2 neurons in the solitary tract nucleus and no labeling of A1 neurons in the caudal ventrolateral medulla. Adrenergic neurons in the solitary tract nucleus (group C2) did not express the alpha(2A)-adrenoceptor, and C1 neurons in the rostral ventrolateral medulla showed only a minor labeling by the in situ probe. In contrast, large glutamatergic neurons in the lateral reticular nucleus were strongly labeled by the probe. Chronic psychosocial stress reduced alpha(2A)-adrenoceptor RNA expression in locus coeruleus neurons (-24.0%), in solitary tract neurons (-31.0%), and in neurons of the lateral reticular nucleus (-18.8%). These findings show that stress not only decreases the expression of the alpha(2A)-adrenergic autoreceptor in the locus coeruleus but also of alpha(2A)-heteroreceptors in glutamatergic neurons.

A mineralocorticoid-like receptor in the rainbow trout, Oncorhynchus mykiss: cloning and characterization of its steroid binding domain

Using reverse transcriptase polymerase chain reaction (PCR) (RT-PCR) with degenerate primers followed by 3' rapid amplification of cDNA ends PCR (3'Race-PCR) we have isolated a new fish steroid receptor cDNA sequence of 1806 bp from rainbow trout (Oncorhynchus mykiss) testis. This sequence has clear homology with various mineralocorticoid receptor cDNA sequences (rat, human, African toad: 68-70% amino acid identity), and encompasses the second part of DNA binding domain (C domain), the whole hinge region (D domain) and the steroid binding domain (E domain) plus 726 bp of 3'untranslated sequence. COS-1 cells transfected with a pCMV5 expression vector containing the whole E domain (pCMV5-rtMR) showed high affinity binding for cortisol (K(a) = 0.53+/-0.03 nM, K(d) = 1.9 nM) in the cytosol, which could not be detected in untransfected cells. Aldosterone displaced (3)H-cortisol binding, though was less effective by than unlabeled cortisol (P<0.05). Competition experiments with other steroids gave the following hierarchy for the displacement of the (3) dexamethasone, whereas 17, 20beta-dihydroxy-4-pregnen-3-one and 17,20beta,21beta-trihydroxy-4 pregnen-3-one (two fish specific progestins) did not show any specific binding. These results strongly suggest that this cDNA sequence encodes a rainbow trout mineralocorticoid-like receptor, and represent the first description of such a receptor in teleost fish where aldosterone, the classic mineralocorticoid, is believed to be absent.

Isolation and pharmacological characterization of two functional splice variants of corticotropin-releasing factor type 2 receptor from Tupaia belangeri

From brain, heart and muscle tissue of the tree shrew (Tupaia belangeri), a higher order mammal, cDNA clones were isolated that encoded two functional splice variants of the corticotropin-releasing factor (CRF) type 2 receptor (CRF-R2). The first, full-length splice variant, amplified from brain and heart tissue, encoded a CRF receptor protein that is 410 amino acids in length and approximately 96% homologous to human CRF-R2alpha. The second, full-length splice variant, derived from skeletal muscle tissue, encoded a 437-amino acid CRF receptor protein that is approximately 92% homologous to human CRF-R2beta. Semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) amplifications and RNase protection analyses, showed that tree shrew CRF-R2alpha (tCRF-R2alpha) and tree shrew CRF-R2beta (tCRF-R2beta) were coexpressed in brain tissue but not in heart and skeletal muscle tissue. Finally, human embryonic kidney 293 (HEK293) cells stably transfected with tCRF-R2alpha and tCRF-R2beta were used to demonstrate that the CRF analogs urocortin and sauvagine bind with significantly greater affinity (21- to 140-fold) to these two CRF-R2 splice variants than do human/rat and ovine CRF analogs. In keeping with these results of our CRF binding studies, EC50 values were substantially lower for urocortin-and sauvagine-stimulated than for h/rCRF-and oCRF-stimulated cyclic AMP accumulation in HEK293 cells stably transfected with tCRF-R2alpha or tCRF-R2beta cDNAs. The tree shrew therefore constitutes an important animal model in which to investigate the role of CRF receptor subtypes in the stress response.

Primate-like amyloid-beta sequence but no cerebral amyloidosis in aged tree shrews

A central pathological feature of Alzheimer's disease is the profuse deposition of amyloid-beta protein (Abeta) in the brain parenchyma and vessel walls. Abeta also forms deposits in the brains of a variety of mammals, including all aged non-human primates studied to date. The sequence of Abeta in these animals is identical to that in humans. No Abeta deposits have been found in the brains of wild-type rats and mice, suggesting that the three amino acid differences between their Abeta and that of amyloid-bearing mammals impedes the fibrillogenicity of Abeta. Analysis of the primary sequence of the beta-amyloid precursor protein in tree shrews revealed a 98% similarity and 97% identity with the human protein. Furthermore, the predicted amino acid sequence of Abeta in tree shrews is identical to that in humans. However, immunohistochemical analysis failed to reveal beta-amyloid deposits in the neural parenchyma or vasculature of eight aged (7-8 years) tree shrews (Tupaia belangeri). The lack of correlation between the Abeta sequence and amyloid formation suggests that other factors contribute to cerebral amyloid deposition in aged animals.