Gonadal hormone modulation of neurogenesis in the dentate gyrus of adult male and female rodents

Brain-specific ablation of Efr3a promotes adult hippocampal neurogenesis via the brain-derived neurotrophic factor pathway

Efr3 is a newly identified plasma membrane protein and plays an important role in the phosphoinositide metabolism on the plasma membrane. However, although it is highly expressed in the brain, the functional significance of Efr3 in the brain is not clear. In the present study, we generated Efr3a^f/f mice and then crossed them with Nestin-Cre mice to delete Efr3a, one of the Efr3 isoforms, specifically in the brain. We found that brain-specific ablation of Efr3a promoted adult hippocampal neurogenesis by increasing survival and maturation of newborn neurons without affecting their dendritic tree morphology. Moreover, the brain-derived neurotrophic factor (BDNF)-tropomyosin-related kinase B (TrkB) signaling pathway was significantly enhanced in the hippocampus of Efr3a-deficient mice, as reflected by increased expression of BDNF, TrkB, and the downstream molecules, including phospho-MAPK and phospho-Akt. Furthermore, the number of TUNEL⁺ cells was decreased in the subgranular zone of dentate gyrus in Efr3a-deficient mice compared with that of control mice. Our data suggest that brain-specific deletion of Efr3a could promote adult hippocampal neurogenesis, presumably by upregulating the expression of BDNF and its receptor, TrkB, and therefore provide new insight into the roles of Efr3 in the brain.-Qian, Q., Liu, Q., Zhou, D., Pan, H., Liu, Z., He, F., Ji, S., Wang, D., Bao, W., Liu, X., Liu, Z., Zhang, H., Zhang, X., Zhang, L., Wang, M., Xu, Y., Huang, F., Luo, B., Sun B. Brain-specific ablation of Efr3a promotes adult hippocampal neurogenesis via the brain-derived neurotrophic factor pathway.

Ameliorative Effects of Endurance Exercise with Two Different Intensities on Nandrolone Decanoate-Induced Neurodegeneration in Rats: Involving Redox and Apoptotic Systems

Despite the importance of this issue, less has been paid to the influence of exercise on the neural side effects of anabolic androgenic steroids and mechanisms. We investigated the effects of two levels of endurance exercise on neurodegeneration side effects of nandrolone. The study period was 8 weeks. Wistar rats were divided into nine groups including the control (CTL) group, mild exercise (mEx) group, and vehicle (Arach) group which received arachis oil intramuscularly, nandrolone (Nan) group which received nandrolone decanoate 5 mg/kg two times weekly, mEx+Arach group which treated with arachis oil along with mild exercise, mEx+Nan group which treated with nandrolone along with mild exercise, severe exercise (sEx) group, sEx+Arach, and sEx+Nan groups. Finally, brain samples were taken for histopathological, biochemical, and western blot analysis. Nandrolone significantly decreased the intact cells of the hippocampus, total antioxidant capacity (TAC) (P < 0.05 versus CTL and Arach groups), TAC to malondialdehyde ratio (TAC/MDA), and Bcl-2. Nandrolone increased the Bax/Bcl-2 ratio of the brain tissue (P < 0.01 versus CTL and Arach groups). Combination of mild exercise and nandrolone rescued the intact cells to some extent, and this effect was associated with the improvement of Bcl-2 level and Bax/Bcl-2 ratio of brain tissue. Combination of severe exercise and nandrolone rescued the intact cells and improved the TAC, TAC/MDA, and Bax/Bcl-2 ratios. The findings suggest that low- and high-intensity endurance exercise decreased the risk of neurodegeneration effect of nandrolone in the hippocampus of rats. This effect can be explained by the regulation of the redox system and cell homeostasis.

Hippocampal serotonin-2A receptor-immunoreactive neurons density increases after testosterone therapy in the gonadectomized male mice

The change of steroid levels may also exert different modulatory effects on the number and class of serotonin receptors present in the plasma membrane. The effects of chronic treatment of testosterone for anxiety were examined and expression of 5-HT_2A serotonergic receptor, neuron, astrocyte, and dark neuron density in the hippocampus of gonadectomized male mice was determined. Thirty-six adult male NMRI mice were randomly divided into six groups: intact-no testosterone treatment (No T), gonadectomy (GDX)-No T, GDX-Vehicle, GDX-6.25 mg/kg testosterone (T), GDX-12.5 mg/kg T, and GDX-25 mg/kg T. Anxiety-related behavior was evaluated using elevated plus maze apparatus. The animals were anesthetized after 48 hours after behavioral testing, and decapitated and micron slices were prepared for immunohistochemical as well as histopathological assessment. Subcutaneous injection of testosterone (25 mg/kg) may induce anxiogenic-like behavior in male mice. In addition, immunohistochemical data reveal reduced expression of 5-HT_2A serotonergic receptor after gonadectomy in all areas of the hippocampus. However, treatment with testosterone could increase the mean number of dark neurons as well as immunoreactive neurons in CA1 and CA3 area, dose dependently. The density of 5-HT_2A receptor-immunoreactive neurons may play a crucial role in the induction of anxiety like behavior. As reduction in such receptor expression have shown to significantly enhance anxiety behaviors. However, replacement of testosterone dose dependently enhances the number of 5-HT_2A receptor-immunoreactive neurons and interestingly also reduced anxiety like behaviors.

Early exposure to Aroclor 1254 in vivo disrupts the functional synaptic development of newborn hippocampal granule cells

Neurogenesis in the dentate gyrus is sensitive to endogenous and exogenous factors that influence hippocampal function. Ongoing neurogenesis and the integration of these new neurons throughout life thus may provide a sensitive indicator of environmental stress. We examined the effects of Aroclor 1254 (A1254), a mixture of polychlorinated biphenyls (PCBs), on the development and function of newly generated dentate granule cells. Early exposure to A1254 has been associated with learning impairment in children, suggesting potential impact on the development of hippocampus and/or cortical circuits. Oral A1254 (from the 6th day of gestation to postnatal day 21) produced the expected increase in PCB levels in brain at postnatal day 21, which persisted at lower levels into adulthood. A1254 did not affect the proliferation or survival of newborn neurons in immature animals nor did it cause overt changes in neuronal morphology. However, A1254 occluded the normal developmental increase in sEPSC frequency in the third post-mitotic week without altering the average sEPSC amplitude. Our results suggest that early exposure to PCBs can disrupt excitatory synaptic function during a period of active synaptogenesis, and thus could contribute to the cognitive effects noted in children exposed to PCBs.

Estradiol-induced neurogenesis in the female accessory olfactory bulb is required for the learning of the male odor

Odors processed by the main and accessory olfactory bulbs (MOB, AOB) are important for sexual behavior. Interestingly, both structures continue to receive new neurons during adulthood. A role for olfactory neurogenesis in sexual behavior in female mice has recently been shown and gonadal hormones such as estradiol can modulate adult neurogenesis. Therefore, we wanted to determine the role of estradiol in learning the odors of sexual partners and in the adult neurogenesis of female aromatase knockout mice (ArKO), unable to produce estradiol. Female wild-type (WT) and ArKO mice were exposed to male odors during 7 days, and olfactory preferences, cell proliferation, cell survival and functional involvement of newborn neurons were analyzed, using BrdU injections, in combination with a marker of cell activation (Zif268) and neuronal fate (doublecortin, NeuN). Behavioral tasks indicated that both WT and ArKO females were able to discriminate between the odors of two different males, but ArKO mice failed to learn the familiar male odor. Proliferation of newborn cells was reduced in ArKO mice only in the dentate gyrus of the hippocampus. Olfactory exposure decreased cell survival in the AOB in WT females, suggesting a role for estradiol in a structure involved in sexual behavior. Finally, newborn neurons do not seem to be functionally involved in the AOB of ArKO mice compared with WT, when females were exposed to the odor of a familiar male, suggesting that estradiol-induced neurogenesis in the AOB is required for the learning of the male odor in female mice. Aromatase knockout mice (ArKO) presented deficits in olfactory preferences without affecting their olfactory discrimination abilities, and showed no functional involvement of newborn neurons in the accessory olfactory bulb (AOB) in response to the odor of a familiar male. These results suggest that estradiol-induced neurogenesis in the female AOB is required for the learning of the male odor.

Decreased left hippocampal volumes in parents with or without posttraumatic stress disorder who lost their only child in China

BACKGROUND

Limbic structural changes have been found in people with post-traumatic stress disorder (PTSD). However, the results were controversial, and no study has examined the hippocampal and amygdala volume changes in parents with or without PTSD who had lost their only child and could no longer conceive in China.

METHODS

Hippocampal and amygdala volumes of 57 parents with PTSD (PTSD+), 11 trauma-exposed parents without PTSD (PTSD-) and 39 non-traumatized controls were examined using magnetic resonance imaging. Correlations of the volumes with the time since trauma, Clinician-Administered PTSD Scale (CAPS) scores, age, gender and intracranial volume (ICV) were investigated in the PTSD+ group.

RESULTS

left hippocampal volumes were significantly smaller in the PTSD+ and PTSD- groups than in the controls, but there were no significant differences between the PTSD+ and PTSD- groups. Furthermore, there was no significant difference in the right hippocampus or bilateral amygdala volumes. Additionally, the hippocampal and amygdala volumes showed no correlation with the time since trauma, CAPS score and gender, whereas the left hippocampal volumes were correlated with ICV, and the bilateral amygdala volumes were correlated with ICV and age in the PTSD+ group.

LIMITATIONS

The PTSD- group included only 11 participants.

CONCLUSIONS

left hippocampal volumes decreased in parents who lost their only child, with or without PTSD. Our results suggest a potentially unique role of the trauma of losing an only child, which is extremely painful and may induce a decrease in the left hippocampal volume independent of PTSD effects.

Contrasting effects of opposite- versus same-sex housing on hormones, behavior and neurogenesis in a eusocial mammal

Competitive interactions can have striking and enduring effects on behavior, but the mechanisms underlying this experience-induced plasticity are unclear, particularly in females. Naked mole-rat (NMR) colonies are characterized by the strictest social and reproductive hierarchy among mammals, and represent an ideal system for studies of social competition. In large matriarchal colonies, breeding is monopolized by one female and 1-3 males, with other colony members being socially subordinate and reproductively suppressed. To date, competition for breeding status has been examined in-colony, with female, but not male, aggression observed following the death/removal of established queens. To determine whether this sex difference extends to colony-founding contexts, and clarify neural and endocrine mechanisms underlying behavioral change in females competing for status, we examined neurogenesis and steroid hormone concentrations in colony-housed subordinates, and NMRs given the opportunity to transition status via pair-housing. To this end, Ki-67 and doublecortin immunoreactivity were compared in the hippocampal dentate gyrus (DG) and basolateral amygdala (BLA) of colony-housed subordinates, and subordinates housed with a same-sex (SS) or opposite-sex (OS) conspecific. Results suggest that OS pairing in eusocial mammals promotes cooperation and enhances hippocampal plasticity, while SS pairing is stressful, resulting in enhanced HPA activation and muted hippocampal neurogenesis relative to OS pairs. Data further indicate that competition for status is confined to females, with female-female housing exerting contrasting effects on hippocampal and amygdalar neurogenesis. These findings advance understanding of social stress effects on neuroplasticity and behavior, and highlight the importance of including female-dominated species in research on aggression and intrasexual competition.

The maternal brain under stress: Consequences for adaptive peripartum plasticity and its potential functional implications

The peripartum period represents a time during which all mammalian species undergo substantial physiological and behavioural changes, which prepare the female for the demands of motherhood. In addition to behavioural and physiological alterations, numerous brain regions, such as the medial prefrontal cortex, olfactory bulb, medial amygdala and hippocampus are subject to substantial peripartum-associated neuronal, dendritic and synaptic plasticity. These changes, which are temporally- and spatially-distinct, are strongly influenced by gonadal and adrenal hormones, such as estrogen and cortisol/corticosterone, which undergo dramatic fluctuations across this period. In this review, we describe our current knowledge regarding these plasticity changes and describe how stress affects such normal adaptations. Finally, we discuss the mechanisms potentially underlying these neuronal, dendritic and synaptic changes and their functional relevance for the mother and her offspring.

Effect of estrogen on vagal afferent projections to the brainstem in the female

The effects of 17β-estradiol (E) on the distribution and density of brainstem projections of small or large diameter primary vagal afferents were investigated in Wistar rats using transganglionic transport of wheat germ agglutinin- (WGA; preferentially transported by non-myelinated afferent C-fibers; 2%), or cholera toxin B-subunit- (CTB, 5%; preferentially transported by large myelinated afferent A-fibers) conjugated horseradish peroxidase (HRP) in combination with the tetramethylbenzidine method in age matched ovariectomized (OVX) only or OVX and treated with E (OVX+E; 30 pg/ml plasma) females for 12 weeks. Additionally, these projections were compared to aged matched males. Unilateral microinjection of WGA-HRP into the nodose ganglion resulted in dense anterograde labeling bilaterally, with an ipsilateral predominance in several subnuclei of the nucleus of the solitary tract (NTS) and in area postrema that was greatest in OVX+E animals compared to OVX only and males. Moderately dense anterograde labeling was also observed in paratrigeminal nucleus (PAT) of the OVX+E animals. CTB-HRP produced less dense anterograde labeling in the NTS complex, but had a wider distribution within the brainstem including the area postrema, dorsal motor nucleus of the vagus, PAT, the nucleus ambiguus complex and ventrolateral medulla in all groups. The distribution of CTB-HRP anterograde labeling was densest in OVX+E, less dense in OVX only females and least dense in male rats. Little, if any, labeling was found within PAT in males using either WGA-or CTB-HRP. Taken together, these data suggest that small, non-myelinated (WGA-labeled) and large myelinated (CTB-labeled) diameter vagal afferents projecting to brainstem autonomic areas are differentially affected by circulating levels of estrogen. These effects of estrogen on connectivity may contribute to the sex differences observed in central autonomic mechanisms between gender, and in females with and without estrogen.

Gender Differences in the Neurobiology of Anxiety: Focus on Adult Hippocampal Neurogenesis

Although the literature reports a higher incidence of anxiety disorders in women, the majority of basic research has focused on male rodents, thus resulting in a lack of knowledge on the neurobiology of anxiety in females. Bridging this gap is crucial for the design of effective translational interventions in women. One of the key brain mechanisms likely to regulate anxious behavior is adult hippocampal neurogenesis (AHN). This review paper aims to discuss the evidence on the differences between male and female rodents with regard to anxiety-related behavior and physiology, with a special focus on AHN. The differences between male and female physiologies are greatly influenced by hormonal differences. Gonadal hormones and their fluctuations during the estrous cycle have often been identified as agents responsible for sexual dimorphism in behavior and AHN. During sexual maturity, hormone levels fluctuate cyclically in females more than in males, increasing the stress response and the susceptibility to anxiety. It is therefore of great importance that future research investigates anxiety and other neurophysiological aspects in the female model, so that results can be more accurately applicable to the female population.

Sex differences in drug addiction and response to exercise intervention: From human to animal studies

Accumulated research supports the idea that exercise could be an option of potential prevention and treatment for drug addiction. During the past few years, there has been increased interest in investigating of sex differences in exercise and drug addiction. This demonstrates that sex-specific exercise intervention strategies may be important for preventing and treating drug addiction in men and women. However, little is known about how and why sex differences are found when doing exercise-induced interventions for drug addiction. In this review, we included both animal and human that pulled subjects from a varied age demographic, as well as neurobiological mechanisms that may highlight the sex-related differences in these potential to assess the impact of sex-specific roles in drug addiction and exercise therapies.

Testosterone Depletion Induces Demethylation of Murine Reelin Promoter CpG Dinucleotides: A Preliminary Study

Schizophrenia (SZ) is a debilitating mental disorder characterized by psychotic events, abnormal social behavior, false beliefs, and auditory hallucinations. Hypermethylation of the promoter region of reelin (RELN), a gene involved in regulation of neuronal positioning during telencephalic development, is strongly associated with low protein expression in several cortical structures and promoter hypermethylation in brain from postmortem SZ subjects. Recent experimental data suggests that testosterone is able to promote RELN demethylation, although no direct evidence of hormonal influence on reelin promoter methylation was obtained. We investigated if reduced levels of plasma testosterone in adult male mice lead to Reln promoter demethylation. Animals were administered with flutamide, an antiandrogenic compound, and reelin promoter methylation was assessed using methylationspecific PCR using bisulfite DNA from cerebellum. We found that flutamide was able to significantly lower plasma testosterone when compared to control mice, and treatment did not influence animal survival and body weight. We also show that low plasma testosterone was associated with demethylation of a cytosine residue located at -860 in reelin promoter region. These preliminary data suggest that androgenic hormones can influence cerebral reelin demethylation. To our knowledge, this is the first experimental approach directly linking testosterone depletion and RELN promoter methylation.

Seipin knockout in mice impairs stem cell proliferation and progenitor cell differentiation in the adult hippocampal dentate gyrus via reduced levels of PPARγ

The seipin gene (BSCL2) was originally identified in humans as a loss-of-function gene associated with congenital generalized lipodystrophy type 2 (CGL2). Neuronal seipin-knockout (seipin-nKO) mice display a depression-like phenotype with a reduced level of hippocampal peroxisome proliferator-activated receptor gamma (PPARγ). The present study investigated the influence of seipin deficiency on adult neurogenesis in the hippocampal dentate gyrus (DG) and the underlying mechanisms of the effects. We show that the proliferative capability of stem cells in seipin-nKO mice was substantially reduced compared to in wild-type (WT) mice, and that this could be rescued by the PPARγ agonist rosiglitazone (rosi). In seipin-nKO mice, neuronal differentiation of progenitor cells was inhibited, with the enhancement of astrogliogenesis; both of these effects were recovered by rosi treatment during early stages of progenitor cell differentiation. In addition, rosi treatment could correct the decline in hippocampal ERK2 phosphorylation and cyclin A mRNA level in seipin-nKO mice. The MEK inhibitor U0126 abolished the rosi-rescued cell proliferation and cyclin A expression in seipin-nKO mice. In seipin-nKO mice, the hippocampal Wnt3 protein level was less than that in WT mice, and there was a reduction of neurogenin 1 (Neurog1) and neurogenic differentiation 1 (NeuroD1) mRNA, levels of which were corrected by rosi treatment. STAT3 phosphorylation (Tyr705) was enhanced in seipin-nKO mice, and was further elevated by rosi treatment. Finally, rosi treatment for 10 days could alleviate the depression-like phenotype in seipin-nKO mice, and this alleviation was blocked by the MEK inhibitor U0126. The results indicate that, by reducing PPARγ, seipin deficiency impairs proliferation and differentiation of neural stem and progenitor cells, respectively, in the adult DG, which might be responsible for the production of the depression-like phenotype in seipin-nKO mice.

Regulation of Adult Neurogenesis and Plasticity by (Early) Stress, Glucocorticoids, and Inflammation

Exposure to stress is one of the best-known negative regulators of adult neurogenesis (AN). We discuss changes in neurogenesis in relation to exposure to stress, glucocorticoid hormones, and inflammation, with a particular focus on early development and on lasting effects of stress. Although the effects of acute and mild stress on AN are generally brief and can be quickly overcome, chronic exposure or more severe forms of stress can induce longer lasting reductions in neurogenesis that can, however, in part, be overcome by subsequent exposure to exercise, drugs targeting the stress system, and some antidepressants. Exposure to stress, particularly during the sensitive period of early life, may (re)program brain plasticity, in particular, in the hippocampus. This may increase the risk to develop cognitive or anxiety symptoms, common to brain diseases like dementia and depression in which plasticity changes occur, and a normalization of neurogenesis may be required for a successful treatment response and recovery.

Sex steroid hormones matter for learning and memory: estrogenic regulation of hippocampal function in male and female rodents

Ample evidence has demonstrated that sex steroid hormones, such as the potent estrogen 17β-estradiol (E2), affect hippocampal morphology, plasticity, and memory in male and female rodents. Yet relatively few investigators who work with male subjects consider the effects of these hormones on learning and memory. This review describes the effects of E2 on hippocampal spinogenesis, neurogenesis, physiology, and memory, with particular attention paid to the effects of E2 in male rodents. The estrogen receptors, cell-signaling pathways, and epigenetic processes necessary for E2 to enhance memory in female rodents are also discussed in detail. Finally, practical considerations for working with female rodents are described for those investigators thinking of adding females to their experimental designs.

Influenza A(H1N1) vaccination during early pregnancy transiently promotes hippocampal neurogenesis and working memory. Involvement of Th1/Th2 balance

The 2009 influenza A(H1N1) pandemic led to a particularly high risk of morbidity and mortality among pregnant women. Therefore, inactivated influenza vaccines have been widely recommended for women in any period of gestation. Recent studies have shown that the peripheral adaptive immune system plays an important role in the function of the central nervous system (CNS). The present study was conducted to explore if influenza vaccination, aiming to induce protective immune activation, affects maternal neurogenesis and cognitive ability. The results showed that A(H1N1) pregnant mice (AIV+Pre) had superior spatial working memory performance compared with pregnant controls (Pre). At the cellular level, a transient increase in both cell proliferation and neuronal differentiation in the dentate gyrus (DG) was found in the AIV+Pre group compared with the Pre group when BrdU was injected on gestational day 14 (G14). However, there were no obvious differences between A(H1N1) virgin mice (AIV+Vir) and virgin controls (Vir) in both hippocampal neurogenesis and working memory. Our findings further indicated that prolactin (PRL) concentrations were not overtly different between the AIV+Pre group and the Pre group at any time. Interestingly, IL-4 and IFN-γ levels were obviously increased both in the serum and hippocampus of the AIV+Pre group (with a T helper-1 like response; Th1) compared with the Pre group (with a T helper-2 like response; Th2) at G14, whereas the expression of IL-6 and TNF-α, the proinflammatory factors, was significantly reduced. Altogether, the results suggest that A(H1N1) vaccination during early pregnancy may contribute to adult hippocampal neurogenesis and spatial working memory and that the improvements were, at least in part, associated with Th1/Th2 balance.

Testosterone-induced adult neurosphere growth is mediated by sexually-dimorphic aromatase expression

We derived adult neural stem/progenitor cells (NSPCs) from the sub-ventricular zone of male and female mice to examine direct responses to principal sex hormones. In the presence of epidermal growth factor (EGF) and fibroblast growth factor-2 (FGF2) NSPCs of both sexes expressed nestin and sox2, and could be maintained as neurospheres without addition of any sex hormones. The reverse was not observed; neither testosterone (T), 17β-estradiol (E₂) nor progesterone (P₄) was able to support neurosphere growth in the absence of EGF and FGF2. Ten nanomolar T, E₂ or P₄ induced nestin(+) cell proliferation within 20 min and enhanced neurosphere growth over 7 days irrespective of sex, which was abolished by Erk inhibition with 20 μM U0126. Maintaining neurospheres with each sex hormone did not affect subsequent neuronal differentiation. However, 10 nM T, E₂ or P₄ added during differentiation increased βIII tubulin(+) neuron production with E₂ being more potent compared to T and P₄ in both sexes. Androgen receptor (AR) inhibition with 20 μM flutamide but not aromatase inhibition with 10 μM letrozole reduced basal and T-induced neurosphere growth in females, while only concurrent inhibition of AR and aromatase produced the same effect in males. This sex-specific effect was supported by higher aromatase expression in male neurospheres compared to females measured by Western blot and green fluorescent protein (GFP) reporter. Ten micromolar menadione induced oxidative stress, impaired neurosphere growth and up-regulated aromatase expression in both sexes. However, under oxidative stress letrozole significantly exacerbated impaired neurosphere growth in males only. While both E₂ and T could prevent oxidative stress-induced growth reduction in both sexes, the effects of T were dependent on innate aromatase activity. We show for the first time that intrinsic androgen and estrogen signaling may impact the capacity of NSPCs to produce neural progenitors under pathological conditions of oxidative stress.

Depression, depressive symptoms, and rate of hippocampal atrophy in a longitudinal cohort of older men and women

BACKGROUND

Several studies have reported smaller hippocampal volume (HcV) in depression patients; however, the temporality of the association remains unknown. One proposed hypothesis is that depression may cause HcV loss. This study evaluates whether previous depression and recent depressive symptoms are associated with HcV and HcV loss.

METHOD

We used a prospective cohort of older adults (n = 1328; age = 65-80 years) with two cerebral magnetic resonance imaging examinations at baseline and 4-year follow-up. Using multivariable linear regression models, we estimated, in stratified analyses by gender, the association between indicators of history of depression and its severity (age at onset, recurrence, hospitalization for depression), proximal depressive symptoms [Center for Epidemiologic Studies-Depression (CES-D) scale], baseline antidepressant use, and the outcomes: baseline HcV and annual percentage change in HcV.

RESULTS

At baseline, women with more depressive symptoms had smaller HcV [-0.05 cm3, 95% confidence interval (CI) -0.1 to -0.01 cm3 per 10-unit increase in CES-D scores]. History of depression was associated with a 0.2% faster annual HcV loss in women (95% CI 0.01-0.36%). More baseline depressive symptoms and worsening of these symptoms were also associated with accelerated HcV loss in women. No associations were observed in men. Treatment for depression was associated with slower HcV loss in women and men.

CONCLUSIONS

While only concomitant depressive symptoms were associated with HcV, both previous depression and more proximal depressive symptoms were associated with faster HcV loss in women.

Developmental exposure to SSRIs, in addition to maternal stress, has long-term sex-dependent effects on hippocampal plasticity

RATIONALE

During pregnancy and postpartum period, 20 % of women are affected by depression, which is a growing health concern. Selective serotonin reuptake inhibitor (SSRI) medications are popular treatments for maternal depression; however, the effect of maternal depression and perinatal SSRI exposure on offspring's neural development needs further investigation.

OBJECTIVES

This study aims to determine the role of developmental fluoxetine exposure on hippocampal plasticity in the adult offspring.

METHODS

Sprague-Dawley rat offspring were exposed to fluoxetine beginning on postnatal day 1. Offspring were also exposed to prenatal maternal stress. Four groups of male and female offspring were used: (1) prenatal stress + fluoxetine, (2) prenatal stress + vehicle, (3) fluoxetine alone, and (4) vehicle alone. Hippocampi were analyzed for levels of cell proliferation, immature neurons, and new cell survival (3 weeks after 5-bromo-2-deoxyuridine injection) in the granule cell layer, as well as synaptophysin density in the CA3 region and granule cell layer. TPH staining was assessed in the dorsal raphe nucleus.

RESULTS

Developmental fluoxetine exposure to prenatally stressed offspring reversed the effect of prenatal stress or fluoxetine exposure alone on the number of immature neurons. Prenatal stress alone, regardless of developmental exposure to fluoxetine, markedly decreased hippocampal cell proliferation and tended to decrease new cell survival. Furthermore, in adult female offspring, developmental fluoxetine exposure greatly increased new cell survival and significantly decreased synaptophysin density in the granule cell layer.

CONCLUSIONS

There are long-term effects of developmental SSRI exposure on hippocampal plasticity that is differentially affected by expose to maternal adversity and offspring sex.

Relationship of monoamine oxidase-A distribution volume to postpartum depression and postpartum crying

Postpartum depression (PPD) has a prevalence rate of 13% and a similarly high proportion of women report a subclinical state of one or more major depressive episode symptoms. The aim was to investigate whether monoamine oxidase-A (MAO-A) VT, an index of MAO-A density, is increased in the prefrontal and anterior cingulate cortex (PFC and ACC), during PPD or when a PPD spectrum symptom, greater predisposition to crying, is present. MAO-A is an enzyme that increases in density after estrogen decline, and has several functions including creating oxidative stress, influencing apoptosis and monoamine metabolism. Fifty-seven women were recruited including 15 first-onset, antidepressant naive, PPD subjects, 12 postpartum healthy who cry due to sad mood, 15 asymptomatic postpartum healthy women, and 15 healthy women not recently pregnant. Each underwent [(11)C]-harmine positron emission tomography scanning to measure MAO-A VT. Both PPD and greater predisposition to crying were associated with greater MAO-A VT in the PFC and ACC (multivariate analysis of variance (MANOVA), group effect, F21,135=1.856; p=0.019; mean combined region elevation 21% and 14% in PPD and crying groups, respectively, relative to postpartum asymptomatic). Greater MAO-A VT in the PFC and ACC represents a new biomarker in PPD, and the PPD symptom of predisposition to crying. Novel strategies for preventing PPD (and some PPD symptoms) may be possible by avoiding environmental conditions that elevate MAO-A level and enhancing conditions that normalize MAO-A level. These findings also argue for clinical trials in PPD with the newer, well-tolerated MAO-A inhibitor antidepressants.

The Antidepressant-like Effects of Estrogen-mediated Ghrelin

Ghrelin, one of the brain-gut peptides, stimulates food-intake. Recently, ghrelin has also shown to play an important role in depression treatment. However, the mechanism of ghrelin's antidepressant-like actions is unknown. On the other hand, sex differences in depression, and the fluctuation of estrogens secretion have been proved to play a key role in depression. It has been reported that women have higher level of ghrelin expression, and ghrelin can stimulate estrogen secretion while estrogen acts as a positive feedback mechanism to up-regulate ghrelin level. Ghrelin may be a potential regulator of reproductive function, and estrogen may have additional effect in ghrelin's antidepressantlike actions. In this review, we summarize antidepressant-like effects of ghrelin and estrogen in basic and clinical studies, and provide new insight on ghrelin's effect in depression.

Influences of prenatal and postnatal stress on adult hippocampal neurogenesis: the double neurogenic niche hypothesis

Adult hippocampal neurogenesis (AHN) is involved in learning, memory, and stress, and plays a significant role in neurodegenerative and psychiatric disorders. As an age-dependent process, AHN is largely influenced by changes that occur during the pre- and postnatal stages of brain development, and constitutes an important field of research. This review examines the current knowledge regarding the regulators of AHN and the influence of prenatal and postnatal stress on later AHN. In addition, a hypothesis is presented suggesting that each kind of stress influences a specific neurogenic pool, developmental or postnatal, that later becomes a precursor with important repercussions for AHN. This hypothesis is referred to as "the double neurogenic niche hypothesis." Discovering what receptors, transcription factors, or genes are specifically activated by different stressors is proposed as an essential line of future research in the field. Such knowledge shall constitute an important starting point toward the goal of modifying AHN in neurodegenerative or psychiatric diseases.

Could androgens maintain specific domains of mental health in aging men by preserving hippocampal neurogenesis?

Interest surrounds the role of sex-hormones in regulating brain function outside of reproductive behaviour. Declining androgen production in aging males has been associated with cognitive impairment, depression and increased risk of developing Alzheimer's disease. Indication for testosterone replacement therapy is based on biochemically determined low circulating testosterone combined with manifest symptoms. However, which aspects of age-related cognitive decline are attributable to low circulating testosterone remain ambiguous. Studies examining cognition in aging men receiving testosterone replacement therapy have yielded equivocal results. The exact role of testosterone in maintaining cognitive function and the underlying neural mechanisms are largely unknown, though it would appear to be domain specific. Clarity in this area will provide clinical direction toward addressing an increasing healthcare burden of mental health decline coincident with increasing longevity. The premise that androgens contribute to maintaining aspects of mental health in aging men by preserving hippocampal neurogenesis will be used as a forum in this review to discuss current knowledge and the need for further studies to better define testosterone replacement strategies for aging male health.

The motirod: a novel physical skill task that enhances motivation to learn and thereby increases neurogenesis especially in the female hippocampus

Males and females perform differently on a variety of training tasks. In the present study we examined performance of male and female rats while they were trained with a gross motor skill in which they learn to maintain their balance on an accelerating rotating rod (the accelerating rotarod). During training, many animals simply step off the rod, thus terminating the training. This problem was addressed by placing cold water below the rod. We termed the new training procedure "motirod" training because the trained animals were apparently motivated to remain on the rod for longer periods of time. Groups of male and female adult Sprague-Dawley rats were trained on either the standard accelerating rotarod or the motirod for four trials per day on four consecutive days. Latency to fall from the rod (in seconds) was recorded. The motivating feature increased performance especially in females (p=.001). As a consequence of enhanced performance, females retained significantly more new cells in the dentate gyrus of the hippocampus than those trained on the accelerating rotarod or those that received no training. In addition, individuals that learned well retained more new cells, irrespective of sex or task conditions. Previous studies have established that new cells rescued from death by learning remain in the hippocampus for months and mature into neurons (Leuner et al., 2004a; Shors, 2014). These data suggest that sex differences in physical skill learning can arise from sex differences in motivation, which thereby influence how many new neurons survive in the adult brain. This article is part of a Special Issue entitled SI: Brain and Memory.

Testosterone and estradiol differentially affect cell proliferation in the subventricular zone of young adult gonadectomized male and female rats

Steroid hormones are important players to regulate adult neurogenesis in the dentate gyrus of the hippocampus, but their involvement in the regulation of the same phenomenon in the subventricular zone (SVZ) of the lateral ventricles is not completely understood. Here, in male rats, we tested the existence of activational effects of testosterone (T) on cell proliferation in the adult SVZ. To this aim, three groups of male rats: castrated, castrated and treated with T, and controls were treated with 5-bromo-2'-deoxyuridine (BrdU) and killed after 24h. The density of BrdU-labeled cells was significantly lower in castrated animals in comparison to the other two groups, thus supporting a direct correlation between SVZ proliferation and levels of circulating T. To clarify whether this effect is purely androgen-dependent, or mediated by the T metabolites, estradiol (E2) and dihydrotestosterone (DHT), we evaluated SVZ proliferation in castrated males treated with E2, DHT and E2+DHT, in comparison to T- and vehicle-treated animals, and sham-operated controls. The stereological analysis demonstrated that E2 and T, but not DHT, increase proliferation in the SVZ of adult male rats. Quantitative evaluation of cells expressing the endogenous marker of cell proliferation phosphorylated form of Histone H3 (PHH3), or the marker of highly dividing SVZ progenitors Mash1, indicated the effect of T/E2 is mostly restricted to SVZ proliferating progenitors. The same experimental protocol was repeated on ovariectomized female rats treated with E2 or T. In this case, no statistically significant difference was found among groups. Overall, our results clearly show that the gonadal hormones T and E2 represent important mediators of cell proliferation in the adult SVZ. Moreover, we show that such an effect is restricted to males, supporting adult neurogenesis in rats is a process differentially modulated in the two sexes.

Androgen Modulation of Hippocampal Structure and Function

Androgens have profound effects on hippocampal structure and function, including induction of spines and spine synapses on the dendrites of CA1 pyramidal neurons, as well as alterations in long-term synaptic plasticity (LTP) and hippocampally dependent cognitive behaviors. How these effects occur remains largely unknown. Emerging evidence, however, suggests that one of the key elements in the response mechanism may be modulation of brain-derived neurotrophic factor (BDNF) in the mossy fiber (MF) system. In male rats, orchidectomy increases synaptic transmission and excitability in the MF pathway. Testosterone reverses these effects, suggesting that testosterone exerts tonic suppression on MF BDNF levels. These findings suggest that changes in hippocampal function resulting from declining androgen levels may reflect the outcome of responses mediated through normally balanced, but opposing, mechanisms: loss of androgen effects on the hippocampal circuitry may be compensated, at least in part, by an increase in BDNF-dependent MF plasticity.

Oestradiol and diet modulate energy homeostasis and hypothalamic neurogenesis in the adult female mouse

Leptin and oestradiol have overlapping functions in energy homeostasis and fertility, and receptors for these hormones are localised in the same hypothalamic regions. Although, historically, it was assumed that mammalian adult neurogenesis was confined to the olfactory bulbs and the hippocampus, recent research has found new neurones in the male rodent hypothalamus. Furthermore, some of these new neurones are leptin-sensitive and affected by diet. In the present study, we tested the hypothesis that diet and hormonal status modulate hypothalamic neurogenesis in the adult female mouse. Adult mice were ovariectomised and implanted with capsules containing oestradiol (E2 ) or oil. Within each group, mice were fed a high-fat diet (HFD) or maintained on standard chow (STND). All animals were administered i.c.v. 5-bromo-2'-deoxyuridine (BrdU) for 9 days and sacrificed 34 days later after an injection of leptin to induce phosphorylation of signal transducer of activation and transcription 3 (pSTAT3). Brain tissue was immunohistochemically labelled for BrdU (newly born cells), Hu (neuronal marker) and pSTAT3 (leptin sensitive). Although mice on a HFD became obese, oestradiol protected against obesity. There was a strong interaction between diet and hormone on new cells (BrdU+) in the arcuate, ventromedial hypothalamus and dorsomedial hypothalamus. HFD increased the number of new cells, whereas E2 inhibited this effect. Conversely, E2 increased the number of new cells in mice on a STND diet in all hypothalamic regions studied. Although the total number of new leptin-sensitive neurones (BrdU-Hu-pSTAT3) found in the hypothalamus was low, HFD increased these new cells in the arcuate, whereas E2 attenuated this induction. These results suggest that adult neurogenesis in the hypothalamic neurogenic niche is modulated by diet and hormonal status and is related to energy homeostasis in female mice.

Investigation of the neuroprotective impact of nimodipine on Neuro2a cells by means of a surgery-like stress model

Nimodipine is well characterized for the management of SAH (subarachnoid hemorrhage) and has been shown to promote a better outcome and less DIND (delayed ischemic neurological deficits). In rat experiments, enhanced axonal sprouting and higher survival of motoneurons was demonstrated after cutting or crushing the facial nerve by nimodipine. These results were confirmed in clinical trials following vestibular Schwannoma surgery. The mechanism of the protective competence of nimodipine is unknown. Therefore, in this study, we established an in vitro model to examine the survival of Neuro2a cells after different stress stimuli occurring during surgery with or without nimodipine. Nimodipine significantly decreased ethanol-induced cell death of cells up to approximately 9% in all tested concentrations. Heat-induced cell death was diminished by approximately 2.5% by nimodipine. Cell death induced by mechanical treatment was reduced up to 15% by nimodipine. Our findings indicate that nimodipine rescues Neuro2a cells faintly, but significantly, from ethanol-, heat- and mechanically-induced cell death to different extents in a dosage-dependent manner. This model seems suitable for further investigation of the molecular mechanisms involved in the neuroprotective signal pathways influenced by nimodipine.

Hippocampal atrophy and subsequent depressive symptoms in older men and women: results from a 10-year prospective cohort

Several studies have reported smaller hippocampal volume in patients with depression. However, the temporality of the association is undetermined. One hypothesis is that hippocampal atrophy might be a susceptibility factor for depression. In the present study, we assessed whether hippocampal atrophy was associated with subsequent depressive symptoms in a cohort of older French adults (n = 1,309) who were 65-80 years of age and enrolled into the study in 1999-2001 in Dijon, France. Subjects were followed for more than 10 years. Participants underwent 2 cerebral magnetic resonance imaging scans, one at baseline and one at the 4-year follow-up. We used linear mixed models to estimate the associations of hippocampal atrophy with 1) the average depressive symptom scores over follow-up (using the Center for Epidemiologic Studies-Depression scale) measured biennially over the subsequent 6 years and 2) changes in symptom scores over follow-up. In women, a 2-standard-deviation increase in annual hippocampal atrophy was associated with a 1.67-point (95% confidence interval: 0.59, 2.77) increase in the average depressive symptom score over follow-up and with a 1.97-point (95% confidence interval: 0.68, 3.24) increase in scores over the 2 subsequent years but not with later changes in symptoms. No association was detected in men. Accounting for potential selective attrition (using inverse probability weights) did not alter results. Hippocampal atrophy was associated with more subsequent depressive symptoms and with shorter-term worsening of symptoms in women.

Adult hippocampal neurogenesis and its role in cognition

Adult hippocampal neurogenesis (AHN) has intrigued neuroscientists for decades. Several lines of evidence show that adult-born neurons in the hippocampus are functionally integrated and contribute to cognitive function, in particular learning and memory processes. Biological properties of immature hippocampal neurons indicate that these cells are more easily excitable compared with mature neurons, and demonstrate enhanced structural plasticity. The structure in which adult-born hippocampal neurons are situated-the dentate gyrus-is thought to contribute to hippocampus function by disambiguating similar input patterns, a process referred to as pattern separation. Several ideas about AHN function have been put forward; currently there is good evidence in favor of a role for AHN in pattern separation. This function of AHN may be understood within a 'representational-hierarchical' view of brain organization. WIREs Cogn Sci 2014, 5:573-587. doi: 10.1002/wcs.1304 For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Effects of chronic oestradiol, progesterone and medroxyprogesterone acetate on hippocampal neurogenesis and adrenal mass in adult female rats

Both natural oestrogens and progesterone influence synaptic plasticity and neurogenesis within the female hippocampus. However, less is known of the impact of synthetic hormones on hippocampal structure and function. There is some evidence that the administration of the synthetic progestin, medroxyprogesterone acetate (MPA) is not as beneficial as natural progesterone and can attenuate oestrogen-induced neuroprotection. Although the effects of oestradiol have been well studied, little is known about the effects of natural and synthetic progestins alone and in combination with oestradiol on adult neurogenesis in females. In the present study, we investigated the effects of chronic oestradiol, progesterone, MPA and the co-administration of each progestin with oestradiol on neurogenesis within the dentate gyrus of adult ovariectomised female rats. Twenty-four hours after a bromodeoxyuridine (BrdU; 200 mg/kg) injection, female rats were repeatedly administered either progesterone (1 or 4 mg), MPA (1 or 4 mg), oestradiol benzoate (EB), progesterone or MPA in combination with EB (10 μg), or vehicle for 21 days. Rats were perfused on day 22 and brain tissue was analysed for the number of BrdU-labelled and Ki67 (an endogenous marker of cell proliferation)-expressing cells. EB alone and MPA + EB significantly decreased neurogenesis and the number of surviving BrdU-labelled cells in the dorsal region of the dentate gyrus, independent of any effects on cell proliferation. Furthermore, MPA (1 and 4 mg) and MPA + EB treated animals had significantly lower adrenal/body mass ratios and reduced serum corticosterone (CORT) levels. By contrast, progesterone + EB treated animals had significantly higher adrenal/body mass ratios and 1 mg of progesterone, progesterone + EB, and EB significantly increased CORT levels. The results of the present study demonstrate that different progestins alone and in combination with oestradiol can differentially affect neurogenesis (via cell survival) and regulation of the hypothalamic-pituitary-adrenal axis. These findings have implications for women using hormone replacement therapies with MPA for both neuroprotection and stress-related disorders.

Effects of continuous vs. cycling estrogen replacement on the acquisition, retention and expression of place- and response-learning in the open-field tower maze

Estrogen has been shown to either enhance or impair learning and memory in female rats. The use of different experimental paradigms or estrogen treatment regimens may contribute to these disparate findings. In order to assess the effect of different estradiol (E2) treatments on several aspects of cognition, we trained ovariectomized female rats with either continuous, cycling, or vehicle E2 replacement, in an open-field tower maze task (OFTM) designed to test reference memory in a low-stress environment. In addition, in order to compare two distinct learning and memory systems, rats were trained to use either a dorsolateral striatum-based response type learning or a hippocampal-based place type learning to solve the maze. Results showed that cyclic, but not continuous, E2 replacement facilitated the acquisition of spatial memory in place-learners. Neither E2 regimen affected acquisition in response-learners. Additionally, when all experimental groups were performing at asymptote, rats were evaluated for performance stability by changing the location of their start position in the OFTM. Both regimens of E2 disrupted the expression of spatial memory in place-learners following the novel start position. However, E2 replacement protected ovariectomized female rats from the disruption of memory expression following a start position change in response-learners. Additionally all experimental groups performed equally well when tested following a 21-day period during which rats were absent from the maze. These results suggest that E2 fluctuation is particularly important in the acquisition of hippocampal-mediated spatial learning, and that hippocampal-based memory may be subject to disruption following environmental change, while striatum-based memory is subject to protection.

The effects of season and sex on dentate gyrus size and neurogenesis in a wild rodent, Richardson's ground squirrel (Urocitellus richardsonii)

Sex and reproductive status affect hippocampal neurogenesis and dentate gyrus (DG) size in rodents. Relatively few studies, however, address these two effects simultaneously and even fewer studies address this issue in wild populations. Here, we examined seasonal and sex differences in neurogenesis and DG size in a wild, polygynous and social rodent, Richardson's ground squirrel (Uriocitellus richardsonii). Based on the behavioral ecology of this species, we predicted that both neurogenesis and DG size would be sexually dimorphic and the degree of dimorphism would be greatest in the breeding season. Using unbiased stereology and doublecortin (DCX) immunohistochemistry, we found that brain volume, DG size and number of DCX cells varied significantly between breeding and non-breeding seasons, but only brain volume and the number of DCX labeled cells differed between the sexes. Both sex and seasonal differences likely reflect circulating hormone levels, but the extent to which these differences relate to space use in this species is unclear. Based on the degree of seasonal differences in neurogenesis and the DG, we suggest that ground squirrels could be considered model species in which to examine hippocampal plasticity in an ecologically valid context.

Gender differences and lateralization in the distribution pattern of insulin-like growth factor-1 receptor in developing rat hippocampus: an immunohistochemical study

Numerous investigators have provided data supporting essential roles for insulin-like growth factor-I (IGF-I) in development of the brain. The aim of this study was to immunohistochemically determine the distinct regional distribution pattern of IGF-1 receptor (IGF-IR) expression in various portions of newborn rat hippocampus on postnatal days 0 (P0), 7 (P7), and 14 (P14), with comparison between male/female and right/left hippocampi. We found an overall significant increase in distribution of IGF-IR-positive (IGF-IR+) cells in CA1 from P0 until P14. Although, no marked changes in distribution of IGF-IR+ cells in areas CA2 and CA3 were observed; IGF-IR+ cells in DG decreased until P14. The smallest number of immunoreactive cells was present in CA2 and the highest number in DG at P0. Moreover, in CA1, CA3, and DG, the number of IGF-IR+ cells was markedly higher in both sides of the hippocampus in females. Our data also showed a higher mean number of IGF-IR+ cells in the left hippocampus of female at P7. By contrast, male pups showed a significantly higher number of IGF-IR+ cells in the DG of the right hippocampus. At P14, the mean number of immunoreactive cells in CA1, CA3, and DG areas found to be significantly increased in left side of hippocampus of males, compared to females. These results indicate the existence of a differential distribution pattern of IGF-IR between left-right and male-female hippocampi. Together with other mechanisms, these differences may underlie sexual dimorphism and left-right asymmetry in the hippocampus.

Sex differences and laterality of insulin receptor distribution in developing rat hippocampus: an immunohistochemical study

This study aimed to compare the regional distribution of insulin receptor in various portions of newborn rat hippocampus on postnatal days 0 (P0), 7 (P7), and 14 (P14) between male/female and right/left hippocampi. We found that the number of insulin receptor (InsR)-immunoreactive-positive (InsR+) cells in CA1 continued to increase until P7 and remained unchanged thereafter. A marked increase in distribution of InsR+ cells in CA3 from P0 to P14 was observed, although there was a significant decline in the number of InsR+ cells in dentate gyrus (DG) at the same time. No differences between the right/left and male/female hippocampi were detected at P0 (P > 0.05). Seven-day-old female rats showed a higher number of labeled cells in the left than in the right hippocampus. Moreover, the differences between the number of InsR+ cells in area CA1 and CA3 were statistically significant between males and females (P < 0.05). At P14, the number of InsR+ cells was significantly higher in CA1 and DG of males, especially in the right one (P < 0.05). These results indicate the existence of a differential distribution pattern of InsR between the left/right and male/female hippocampi. Together with other mechanisms, these differences may underlie sexual dimorphism and left/right asymmetry in the hippocampus.

The trouble with circadian clock dysfunction: multiple deleterious effects on the brain and body

This review consolidates research employing human correlational and experimental work across brain and body with experimental animal models to provide a more complete representation of how circadian rhythms influence almost all aspects of life. In doing so, we will cover the morphological and biochemical pathways responsible for rhythm generation as well as interactions between these systems and others (e.g., stress, feeding, reproduction). The effects of circadian disruption on the health of humans, including time of day effects, cognitive sequelae, dementia, Alzheimer's disease, diet, obesity, food preferences, mood disorders, and cancer will also be discussed. Subsequently, experimental support for these largely correlational human studies conducted in non-human animal models will be described.

Neuropathology of stress

Environmental challenges are part of daily life for any individual. In fact, stress appears to be increasingly present in our modern, and demanding, industrialized society. Virtually every aspect of our body and brain can be influenced by stress and although its effects are partly mediated by powerful corticosteroid hormones that target the nervous system, relatively little is known about when, and how, the effects of stress shift from being beneficial and protective to becoming deleterious. Decades of stress research have provided valuable insights into whether stress can directly induce dysfunction and/or pathological alterations, which elements of stress exposure are responsible, and which structural substrates are involved. Using a broad definition of pathology, we here review the "neuropathology of stress" and focus on structural consequences of stress exposure for different regions of the rodent, primate and human brain. We discuss cytoarchitectural, neuropathological and structural plasticity measures as well as more recent neuroimaging techniques that allow direct monitoring of the spatiotemporal effects of stress and the role of different CNS structures in the regulation of the hypothalamic-pituitary-adrenal axis in human brain. We focus on the hypothalamus, hippocampus, amygdala, nucleus accumbens, prefrontal and orbitofrontal cortex, key brain regions that not only modulate emotions and cognition but also the response to stress itself, and discuss disorders like depression, post-traumatic stress disorder, Cushing syndrome and dementia.

Calorie restriction alleviates the age-related decrease in neural progenitor cell division in the aging brain

Production of new neurons from stem cells is important for cognitive function, and the reduction of neurogenesis in the aging brain may contribute to the accumulation of age-related cognitive deficits. Restriction of calorie intake and prolonged treatment with rapamycin have been shown to extend the lifespan of animals and delay the onset of the age-related decline in tissue and organ function. Using a reporter line in which neural stem and progenitor cells are marked by the expression of green fluorescent protein (GFP), we examined the effect of prolonged exposure to calorie restriction (CR) or rapamycin on hippocampal neural stem and progenitor cell proliferation in aging mice. We showed that CR increased the number of dividing cells in the dentate gyrus of female mice. The majority of these cells corresponded to nestin-GFP-expressing neural stem or progenitor cells; however, this increased proliferative activity of stem and progenitor cells did not result in a significant increase in the number of doublecortin-positive newborn neurons. Our results suggest that restricted calorie intake may increase the number of divisions that neural stem and progenitor cells undergo in the aging brain of females.

Sex, drugs, and adult neurogenesis: sex-dependent effects of escalating adolescent cannabinoid exposure on adult hippocampal neurogenesis, stress reactivity, and amphetamine sensitization

Cannabinoid exposure during adolescence has adverse effects on neuroplasticity, emotional behavior, cognition, and reward sensitivity in adult rats. We investigated whether escalating doses of the cannabinoid receptor 1 (CB1 R) agonist, HU-210, in adolescence would affect adult hippocampal neurogenesis and behavioral processes putatively modulated by hippocampal neurogenesis, in adult male and female Sprague-Dawley rats. Escalating doses of HU-210 (25, 50, and 100 µg/kg), or vehicle were administered from postnatal day (PND) 35 to 46. Animals were left undisturbed until PND 70, when they were treated with 5-bromo-2-deoxyuridine (BrdU; 200 mg/kg) and perfused 21 days later to examine density of BrdU-ir and BrdU/NeuN cells in the dentate gyrus. In another cohort, hypothalamic-pituitary-adrenal (HPA) axis reactivity to an acute restraint stress (30 min; PND 75) and behavioral sensitization to d-amphetamine sulfate (1-2 mg/kg; PND 105-134) were assessed in adulthood. Adolescent HU-210 administration suppressed the density of BrdU-ir cells in the dentate gyrus in adult male, but not adult female rats. Adolescent HU-210 administration also induced significantly higher peak corticosterone levels and reminiscent of the changes in neurogenesis, this effect was more pronounced in adult males than females. However, adolescent cannabinoid treatment resulted in significantly higher stereotypy scores in adult female, but not male, rats. Thus, adolescent CB1 R activation suppressed hippocampal neurogenesis and increased stress responsivity in adult males, but not females, and enhanced amphetamine sensitization in adult female, but not male, rats. Taken together, increased CB1 R activation during adolescence results in sex-dependent, long-term, changes to hippocampal structure and function, an effect that may shed light on differing vulnerabilities to developing disorders following adolescent cannabinoid exposure, based on sex.

Effects of androgens on early post-ischemic neurogenesis in mice

Although androgens are reported to affect stroke outcomes by altering ischemic tissue damage, their effect on post-injury repair is unknown. Since neurogenesis has recently been recognized as contributing to stroke outcomes, we investigated the role of androgens on stroke-induced neurogenesis. Adult male mice were subjected to transient middle cerebral artery occlusion (MCAO) and neurogenesis was examined 1 week later by quantifying BrdU/doublecortin-positive and BrdU/NeuN-positive neurons in brain germinal regions as well as the injured striatum. To elucidate the role of endogenous androgens, post-MCAO neurogenesis was examined in gonadally intact males, intact males implanted with the androgen receptor antagonist flutamide, and surgically castrated males. Surgical castration or pharmacologic androgen receptor blockade had no effects on post-ischemic neurogenesis, except that continuous androgen receptor blockade unexpectedly suppressed maturation of newborn neurons (BrdU/NeuN-positive cells) in the dentate gyrus. Post-MCAO neurogenesis was also examined in surgically castrated mice treated with continuous release implants containing testosterone or dihydrotestosterone (DHT). Testosterone and DHT robustly inhibited post-ischemic neurogenesis in the dentate gyrus, and the more potent androgen DHT virtually abolished the presence of immature newborn neurons (BrdU/doublecortin-positive cells) in the injured striatum. Our data suggest that endogenous androgens do not alter post-stroke neurogenesis quantitatively, but the presence of supra-physiological androgen stimulation profoundly suppresses early neurogenesis in germinal brain areas and reduces cellular repair in injured tissue after cerebral ischemia. These results advance the understanding of the role that androgens play in stroke outcomes.

The touchscreen operant platform for testing working memory and pattern separation in rats and mice

The automated touchscreen operant chamber for rats and mice allows for the assessment of multiple cognitive domains within the same testing environment. This protocol presents the location discrimination (LD) task and the trial-unique delayed nonmatching-to-location (TUNL) task, which both assess memory for location. During these tasks, animals are trained to a predefined criterion during ∼20-40 daily sessions. In LD sessions, touching the same location on the screen is rewarded on consecutive trials, followed by a reversal of location-reward contingencies. TUNL, a working memory task, requires animals to 'nonmatch' to a sample location after a delay. In both the LD and TUNL tasks, spatial similarity can be varied, allowing assessment of pattern separation ability, a function that is thought to be performed by the dentate gyrus (DG). These tasks are therefore particularly useful in animal models of hippocampal, and specifically DG, function, but they additionally permit discernment of changes in pattern separation from those in working memory.

Increased hippocampal neurogenesis and p21 expression in depression: dependent on antidepressants, sex, age, and antipsychotic exposure

The mammalian hippocampus continues to generate new neurons throughout life. The function of adult-generated neurons remains controversial, but adult neurogenesis in the hippocampus is related to depression. Studies show that neurogenesis in the hippocampus is regulated by antidepressants in both humans and rodents, but no studies have examined the effects of age, sex, or antipsychotic exposure on the relationship between depression, antidepressant exposure, and hippocampal neurogenesis in humans. Hippocampal sections were obtained from the Stanley Medical Research Institute and were immunohistochemically labeled for the immature neuron marker doublecortin and the cell cycle arrest marker p21. We compared the number of cells in the granule cell layer and subgranular zone that expressed these proteins in brains from control subjects (n=12), patients with major depressive disorder (MDD) without psychotic symptoms (n=12), and patients with MDD and psychotic symptoms (n=12). We show here that the density of doublecortin/NeuN expression was increased in MDD patients compared with controls and MDD patients with psychosis, with the effect greater in women. Further, we show that older depressed patients without psychosis had higher levels of p21/NeuN expression and that depressed individuals prescribed antidepressants had higher levels of p21/NeuN expression, but only in older women. We show for the first time that changes in neurogenesis due to prescribed antidepressants or depression are dependent on age, sex, and the presence of antipsychotics or psychotic symptoms.