Gender and endogenous levels of estradiol do not influence adult hippocampal neurogenesis in mice

Multiparity-induced enhancement of hippocampal neurogenesis and spatial memory depends on ovarian hormone status in middle age

Menopause is associated with cognitive decline, and previous parity can increase or delay the trajectory of cognitive aging. Furthermore, parity enables the hippocampus to respond to estrogens in middle age. The present study investigated how previous parity and estrogens influence cognition, neurogenesis, and neuronal activation in response to memory retrieval in the hippocampus of middle-aged females. Multiparous and nulliparous rats were ovariectomized (OVX) or received sham surgery and were treated with vehicle, 17β-estradiol, 17α-estradiol, or estrone. Rats were trained on the spatial working and reference memory versions of the Morris water maze. Multiparous rats had a significantly greater density of immature neurons in the hippocampus, enhanced acquisition of working memory, but poorer reference memory compared with nulliparous rats. Furthermore, OVX increased, while treatment with estrogens reduced, the density of immature neurons, regardless of parity. OVX improved reference memory only in nulliparous rats. Thus, motherhood has long-lasting effects on the neuroplasticity and function of the hippocampus. These findings have wide-ranging implications for the treatment of age-associated decline in women.

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.

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.

Sex differences and the modulating effects of gonadal hormones on basal and the stressor-decreased newly proliferative cells and neuroblasts in dentate gyrus

This study was undertaken to assess sex differences and the modulating effects of gonad intactness and the estrous phase on basal and the stressor-decreased cell proliferation and early differentiation in Balb/C mouse dentate gyrus (DG). Besides, we compared the stress-reversing effects exerted by the presence of male and female Balb/C mouse odors in stressed male and female mouse DG in this regard. Female mice had lower baselines in the number of newly proliferated cells and neuroblasts than male mice. Although the stressor induced decreases in the number of newly proliferative cells and neuroblasts in both male and female DG, an obvious decrease in neuronal lineage commitment was observed in female DG. Moreover, ovariectomy induced decreases in baselines in the number of proliferative cells and neuroblasts but did not affect the stressor-induced decrease in neuronal lineage commitment in female DG. Interestingly, pro-estrous mice exhibited the stressor-decreased neuronal lineage commitment, while estrous and diestrous mice did not display such a decrease. Furthermore, orchidectomy did not affect basal or the stressor-decreased newly proliferative cells or neuroblasts in male DG. Finally, male odors were less effective than female odors in abolishing the stressor-decreased neuronal lineage commitment in female mice, while male and female odors were comparable in reversing the stressor-decreased newly proliferated cells and neuroblasts in male mice. The protective effects of mouse odors' company in the stressed male mouse DG were associated with local BDNF and NGF replenishment. Taken together, sexual differences in baselines in the number of newly proliferative cells, neuroblasts, and the sensitivity to stress-altered neuronal lineage commitment in the DG could be, in part, due to gonadal hormone differences between the two sexes. Mouse odors may reverse stressor-decreased newly proliferative cells and neuroblasts in male, but not in female, mouse DG by restoring BDNF and NGF levels.

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.

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.

Sex, hormones and neurogenesis in the hippocampus: hormonal modulation of neurogenesis and potential functional implications

The hippocampus is an area of the brain that undergoes dramatic plasticity in response to experience and hormone exposure. The hippocampus retains the ability to produce new neurones in most mammalian species and is a structure that is targeted in a number of neurodegenerative and neuropsychiatric diseases, many of which are influenced by both sex and sex hormone exposure. Intriguingly, gonadal and adrenal hormones affect the structure and function of the hippocampus differently in males and females. Adult neurogenesis in the hippocampus is regulated by both gonadal and adrenal hormones in a sex- and experience-dependent way. Sex differences in the effects of steroid hormones to modulate hippocampal plasticity should not be completely unexpected because the physiology of males and females is different, with the most notable difference being that females gestate and nurse the offspring. Furthermore, reproductive experience (i.e. pregnancy and mothering) results in permanent changes to the maternal brain, including the hippocampus. This review outlines the ability of gonadal and stress hormones to modulate multiple aspects of neurogenesis (cell proliferation and cell survival) in both male and female rodents. The function of adult neurogenesis in the hippocampus is linked to spatial memory and depression, and the present review provides early evidence of the functional links between the hormonal modulation of neurogenesis that may contribute to the regulation of cognition and stress.

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.

Effects of altered levels of extracellular superoxide dismutase and irradiation on hippocampal neurogenesis in female mice

PURPOSE

Altered levels of extracellular superoxide dismutase (EC-SOD) and cranial irradiation have been shown to affect hippocampal neurogenesis. However, previous studies were only conducted in male mice, and it was not clear if there was a difference between males and females. Therefore, female mice were studied and the results compared with those generated in male mice from an earlier study.

METHODS AND MATERIALS

Female wild-type, EC-SOD-null (KO), and EC-SOD bigenic mice with neuronal-specific expression of EC-SOD (OE) were subjected to a single dose of 5-Gy gamma rays to the head at 8 weeks of age. Progenitor cell proliferation, differentiation, and long-term survival of newborn neurons were determined.

RESULTS

Similar to results from male mice, EC-SOD deficiency and irradiation both resulted in significant reductions in mature newborn neurons in female mice. EC-SOD deficiency reduced long-term survival of newborn neurons whereas irradiation reduced progenitor cell proliferation. Overexpression of EC-SOD corrected the negative impacts from EC-SOD deficiency and irradiation and normalized the production of newborn neurons in OE mice. Expression of neurotrophic factors brain-derived neurotrophic factor and neurotrophin-3 were significantly reduced by irradiation in wild-type mice, but the levels were not changed in KO and OE mice even though both cohorts started out with a lower baseline level.

CONCLUSION

In terms of hippocampal neurogenesis, EC-SOD deficiency and irradiation have the same overall effects in males and females at the age the studies were conducted.

Bi-parental care contributes to sexually dimorphic neural cell genesis in the adult mammalian brain

Early life events can modulate brain development to produce persistent physiological and behavioural phenotypes that are transmissible across generations. However, whether neural precursor cells are altered by early life events, to produce persistent and transmissible behavioural changes, is unknown. Here, we show that bi-parental care, in early life, increases neural cell genesis in the adult rodent brain in a sexually dimorphic manner. Bi-parentally raised male mice display enhanced adult dentate gyrus neurogenesis, which improves hippocampal neurogenesis-dependent learning and memory. Female mice display enhanced adult white matter oligodendrocyte production, which increases proficiency in bilateral motor coordination and preference for social investigation. Surprisingly, single parent-raised male and female offspring, whose fathers and mothers received bi-parental care, respectively, display a similar enhancement in adult neural cell genesis and phenotypic behaviour. Therefore, neural plasticity and behavioural effects due to bi-parental care persist throughout life and are transmitted to the next generation.

Hippocampus-dependent learning influences hippocampal neurogenesis

The structure of the mammalian hippocampus continues to be modified throughout life by continuous addition of neurons in the dentate gyrus. Although the existence of adult neurogenesis is now widely accepted the function that adult generated granule cells play is a topic of intense debate. Many studies have argued that adult generated neurons, due to unique physiological characteristics, play a unique role in hippocampus-dependent learning and memory. However, it is not currently clear whether this is the case or what specific capability adult generated neurons may confer that developmentally generated neurons do not. These questions have been addressed in numerous ways, from examining the effects of increasing or decreasing neurogenesis to computational modeling. One particular area of research has examined the effects of hippocampus dependent learning on proliferation, survival, integration and activation of immature neurons in response to memory retrieval. Within this subfield there remains a range of data showing that hippocampus dependent learning may increase, decrease or alternatively may not alter these components of neurogenesis in the hippocampus. Determining how and when hippocampus-dependent learning alters adult neurogenesis will help to further clarify the role of adult generated neurons. There are many variables (such as age of immature neurons, species, strain, sex, stress, task difficulty, and type of learning) as well as numerous methodological differences (such as marker type, quantification techniques, apparatus size etc.) that could all be crucial for a clear understanding of the interaction between learning and neurogenesis. Here, we review these findings and discuss the different conditions under which hippocampus-dependent learning impacts adult neurogenesis in the dentate gyrus.

Female mice lacking cholecystokinin 1 receptors have compromised neurogenesis, and fewer dopaminergic cells in the olfactory bulb

Neurogenesis in the adult rodent brain is largely restricted to the subependymal zone (SVZ) of the lateral ventricle and subgranular zone (SGZ) of the dentate gyrus (DG). We examined whether cholecystokinin (CCK) through actions mediated by CCK1 receptors (CCK1R) is involved in regulating neurogenesis. Proliferating cells in the SVZ, measured by 5-bromo-2-deoxyuridine (BrdU) injected 2 h prior to death or by immunoreactivity against Ki67, were reduced by 37 and 42%, respectively, in female (but not male) mice lacking CCK1Rs (CCK1R(-/-)) compared to wild-type (WT). Generation of neuroblasts in the SVZ and rostral migratory stream (RMS) was also affected, since the number of doublecortin (DCX)-immunoreactive (ir) neuroblasts in these regions decreased by 29%. In the SGZ of female CCK1R(-/-) mice, BrdU-positive (+), and Ki67-ir cells were reduced by 38 and 56%, respectively, while DCX-ir neuroblasts were down 80%. Subsequently, the effect of reduced SVZ/SGZ proliferation on the generation and survival of mature adult-born cells in female CCK1R(-/-) mice was examined. In the OB granule cell layer (GCL), the number of neuronal nuclei (NeuN)-ir and calretinin-ir cells was stable compared to WT, and 42 days after BrdU injections, the number of BrdU+ cells co-expressing GABA- or NeuN-like immunoreactivity (LI) was similar. Compared to WT, the granule cell layer of the DG in female CCK1R(-/-) mice had a similar number of calbindin-ir cells and BrdU+ cells co-expressing calbindin-LI 42 days after BrdU injections. However, the OB glomerular layer (GL) of CCK1R(-/-) female mice had 11% fewer NeuN-ir cells, 23% less TH-ir cells, and a 38% and 29% reduction in BrdU+ cells that co-expressed TH-LI or GABA-LI, respectively. We conclude that CCK, via CCK1Rs, is involved in regulating the generation of proliferating cells and neuroblasts in the adult female mouse brain, and mechanisms are in place to maintain steady neuronal populations in the OB and DG when the rate of proliferation is altered.

Differential effects of stress and glucocorticoids on adult neurogenesis

Stress is known to inhibit neuronal growth in the hippocampus. In addition to reducing the size and complexity of the dendritic tree, stress and elevated glucocorticoid levels are known to inhibit adult neurogenesis. Despite the negative effects of stress hormones on progenitor cell proliferation in the hippocampus, some experiences which produce robust increases in glucocorticoid levels actually promote neuronal growth. These experiences, including running, mating, enriched environment living, and intracranial self-stimulation, all share in common a strong hedonic component. Taken together, the findings suggest that rewarding experiences buffer progenitor cells in the dentate gyrus from the negative effects of elevated stress hormones. This chapter considers the evidence that stress and glucocorticoids inhibit neuronal growth along with the paradoxical findings of enhanced neuronal growth under rewarding conditions with a view toward understanding the underlying biological mechanisms.

Determinants of central nervous system adult neurogenesis are sex, hormones, mouse strain, age, and brain region

Multiple sclerosis is a sexually dimorphic (SD) disease that causes oligodendrocyte death, but SD of glial cells is poorly studied. Here, we analyze SD of neural progenitors in 6-8 weeks and 6-8 months normal C57BL/6, SJL/J, and BALB/c mice in the subventricular zone (SVZ), dorsolateral horn (DLC), corpus callosum (CC), and parenchyma. With a short 2-h bromodeoxyuridine (BrdU) pulse, no gender and strain differences are present at 6-8 weeks. At 6-8 months, the number of BrdU(+) cells decreases twofold in each sex, strain, and region, indicating that a common aging mechanism regulates BrdU incorporation. Strikingly, 2× more BrdU(+) cells are found in all brain regions in 6-8 months C57BL/6 females versus males, no gender differences in 6-8 months SJL/J, and fewer BrdU(+) cells in females versus males in BALB/cs. The number of BrdU(+) cells modestly fluctuates throughout the estrous cycle in C57BL/6 and SJLs. Castration causes a dramatic increase in BrdU(+) cells in SVZ and DLC. These findings indicate that testosterone is a major regulator of adult neural proliferation. At 6-8 months, the ratio of PDGFRα(+) cells in the CC to BrdU(+) cells in the DLC of both strains, sexes, estrous cycle, and castrated mice was essentially the same, suggesting that BrdU(+) cells in the DLC differentiate into CC oligodendrocytes. The ratio of TUNEL(+) to BrdU(+) cells does not match proliferation, indicating that these events are differentially regulated. Differential regulation of these two processes leads to the variation in glial numbers between gender and strain. Explanations of neural proliferation based upon data from one sex or strain may be very misleading.

Impact of treadmill running and sex on hippocampal neurogenesis in the mouse model of amyotrophic lateral sclerosis

Hippocampal neurogenesis in the subgranular zone (SGZ) of dentate gyrus (DG) occurs throughout life and is regulated by pathological and physiological processes. The role of oxidative stress in hippocampal neurogenesis and its response to exercise or neurodegenerative diseases remains controversial. The present study was designed to investigate the impact of oxidative stress, treadmill exercise and sex on hippocampal neurogenesis in a murine model of heightened oxidative stress (G93A mice). G93A and wild type (WT) mice were randomized to a treadmill running (EX) or a sedentary (SED) group for 1 or 4 wk. Immunohistochemistry was used to detect bromodeoxyuridine (BrdU) labeled proliferating cells, surviving cells, and their phenotype, as well as for determination of oxidative stress (3-NT; 8-OHdG). BDNF and IGF1 mRNA expression was assessed by in situ hybridization. Results showed that: (1) G93A-SED mice had greater hippocampal neurogenesis, BDNF mRNA, and 3-NT, as compared to WT-SED mice. (2) Treadmill running promoted hippocampal neurogenesis and BDNF mRNA content and lowered DNA oxidative damage (8-OHdG) in WT mice. (3) Male G93A mice showed significantly higher cell proliferation but a lower level of survival vs. female G93A mice. We conclude that G93A mice show higher hippocampal neurogenesis, in association with higher BDNF expression, yet running did not further enhance these phenomena in G93A mice, probably due to a 'ceiling effect' of an already heightened basal levels of hippocampal neurogenesis and BDNF expression.

Melatonin's stimulatory effect on adult hippocampal neurogenesis in mice persists after ovariectomy

In this study, we examined whether melatonin treatment would increase new cell formation in the hippocampus in ovariectomized (OVX) mice. Chronic exogenous melatonin administration increased bromodeoxyuridine (BrdU) (OVX-sham 72 ± 3.2 versus OVX-mel 122 ± 12.0; P < 0.05) and doublecortin (DCX) (OVX-sham 88 ± 3.1 versus OVX-mel 176 ± 9.9; P < 0.05) immunoreactive cells in the hippocampus of ovariectomized mice. This neuronal development was correlated with synaptic plasticity, identified using the Golgi impregnation method to quantify dendritic spines in mouse dentate gyrus (DG). Finally, the antidepressant-like state of the animals was evaluated by the tail suspension test. The results indicate that melatonin acts on birth, survival, and differentiation of new neurons in the hippocampus, stimulates maturation of spines, and exerts an antidepressant-like action under estrogen-deprived conditions, in both a strain- and gender-independent manner, suggesting that this indoleamine may be useful in improving brain functions.

A novel flow cytometry-based technique to measure adult neurogenesis in the brain

The stimulation of neurogenesis is an exciting novel therapeutic option for diseases of the central nervous system, ranging from depression to neurodegeneration. One major bottleneck in screening approaches for neurogenesis-inducing compounds is the very demanding in vivo quantification of newborn neurons based on stereological techniques. To effectively develop compounds in this area, novel fast and reliable techniques for quantification of in vivo neurogenesis are needed. In this study, we introduce a flow cytometry-based method for quantifying newly generated neurons in the brain based on the counting of cell nuclei from dissected brain regions. Important steps involve density sedimentation of the cell nuclei, and staining for the proliferation marker bromodeoxy uridine and nuclear cell type markers such as NeuN. We demonstrate the ability of the technique to detect increased neurogenesis in the hippocampus of animals which underwent physical exercise and received fluoxetine treatment.

Different regulation of adult hippocampal neurogenesis in Western house mice (Mus musculus domesticus) and C57BL/6 mice

Adult hippocampal neurogenesis (AHN) of laboratory rodents is enhanced by physical exercise in a running wheel. However, little is known about modulation of AHN in wild-living rodent species. The finding that AHN cannot be modulated by voluntary exercise in wild wood mice suggests that AHN may be regulated differently under natural conditions than in laboratory adapted animals. In order to minimize genetic influences, we aimed to investigate the genetically closest wild-living relatives of laboratory mice. Here, C57BL/6 mice and F1 offspring of wild house mice (Mus musculus domesticus) were tested in two different running paradigms: voluntary running and running-for-food--a condition in which mice had to run for their daily allowance of food. In house mice, we found a non-significant trend towards increased numbers of proliferating cells and doublecortin-positive immature neurons in both voluntary runners and runners-for-food. Voluntary running in C57BL/6 mice resulted in a 30% increase in cell proliferation and a pronounced 70% increase in doublecortin-positive cells. C57BL/6 runners-for-food ran as much as voluntary runners, but they showed no enhancement of cell proliferation, a small increase in the number of doublecortin-positive cells and more pyknotic cells compared to controls. Taken together, these findings suggest that motivational aspects of running are critical determinants of the increased cell proliferation in C57BL/6 mice. In contrast, running has smaller and context-independent effects in house mice. The findings imply a difference in the regulation of AHN in C57BL/6 mice and their wild-derived conspecifics.

Nanoparticle-rich diesel exhaust affects hippocampal-dependent spatial learning and NMDA receptor subunit expression in female mice

We investigated the effect of exposure to nanoparticle-rich diesel exhaust (NRDE) on hippocampal-dependent spatial learning and memory function-related gene expressions in female mice. Female BALB/c mice were exposed to clean air, middle-dose NRDE (M-NRDE), high-dose NRDE (H-NRDE) or filtered diesel exhaust (F-DE) for three months. A Morris water maze apparatus was used to examine spatial learning. The expression levels of the N-methyl-D-aspartate (NMDA) receptor subunit, proinflammatory cytokines and neurotrophin mRNAs in the hippocampus were then investigated using real-time RT-PCR. Mice exposed to H-NRDE required a longer time to reach the hidden platform and showed higher mRNA expression levels of the NMDA receptor subunit NR2A, the proinflammatory cytokine CCL3, and brain-derived neurotrophic factor (BDNF) in the hippocampus, compared with the findings in the control group. These results indicate that three months of exposure to NRDE affected spatial learning and memory function-related gene expressions in the female mouse hippocampus.

Stress, stress hormones, and adult neurogenesis

The dentate gyrus of the hippocampus continues to produce new neurons throughout adulthood. Adult neurogenesis has been linked to hippocampal function, including learning and memory, anxiety regulation and feedback of the stress response. It is thus not surprising that stress, which affects hippocampal function, also alters the production and survival of new neurons. Glucocorticoids, along with other neurochemicals, have been implicated in stress-induced impairment of adult neurogenesis. Paradoxically, increases in corticosterone levels are sometimes associated with enhanced adult neurogenesis in the dentate gyrus. In these circumstances, the factors that buffer against the suppressive influence of elevated glucocorticoids remain unknown; their discovery may provide clues to reversing pathological processes arising from chronic exposure to aversive stress.

Long-term ovariectomy enhances anxiety and depressive-like behaviors in mice submitted to chronic unpredictable stress

Ovarian hormones exert anti-depressive and anxiolytic actions. In this study we have analyzed the effects of ovariectomy on the development of anxiety and depression-like behaviors and on cell proliferation in the hippocampus of mice submitted to chronic unpredictable stress. Animals submitted to stress 4 months after ovariectomy showed a significant increase in immobility behavior in the forced swimming test compared to animals submitted to stress 2 weeks after ovariectomy. In addition, long-term ovariectomy resulted in a significant decrease on the time spent in the open arms in the elevated plus-maze test compared to control animals. Stress did not significantly affect cell proliferation in the hilus of the dentate gyrus. However, ovariectomy resulted in a significant decrease in cell proliferation. These results indicate that long-term deprivation of ovarian hormones enhances the effect of chronic unpredictable stress on depressive- and anxiety-like behaviors in mice. Therefore, a prolonged deprivation of ovarian hormones may represent a risk factor for the development of depressive and anxiety symptoms after the exposure to stressful experiences.

Fluoxetine treatment induces dose dependent alterations in depression associated behavior and neural plasticity in female mice

Antidepressant-induced increases in neurogenesis and neurotrophin mobilization in rodents and primates are proposed to be necessary for behavioral efficacy. The current study examines the relationship between the effects of fluoxetine treatment on behavior, cell proliferation and the neurotrophin BDNF in females. Female MRL/MpJ mice were treated acutely (5 and 10mg/kg) or chronically (2.5, 5 and 10mg/kg b.i.d.) with fluoxetine and tested in the tail suspension test (TST) and or novelty-induced hypophagia test (NIH), respectively. Mice treated chronically with fluoxetine received 4 (100mg/kg) injections of 5-bromo-2'-deoxyuridine (BrdU) on the last 4 days of treatment to measure DNA synthesis. The other half of the hippocampus and the frontal cortex was removed and examined for BDNF levels. Fluoxetine treatment decreased immobility in the TST and latency to eat in the NIH test, but only the highest dose of fluoxetine significantly altered behavior in both tests. Chronic treatment with 5 and 10mg/kg of fluoxetine significantly increased cell proliferation and BDNF levels in the hippocampus. Only chronic treatment with the highest of fluoxetine increased BDNF levels in the frontal cortex. Behavioral measures in the NIH test correlated with BDNF levels in the frontal cortex but not in the hippocampus or with cell proliferation in the hippocampus. These data suggest that females require high doses of fluoxetine for behavioral efficacy regardless of elevations of neurogenesis and BDNF mobilization in the hippocampus. Elevations in BDNF levels in the frontal cortex are related to the behavioral efficacy of fluoxetine.

Long-term gonadal hormone treatment and endogenous neurogenesis in the dentate gyrus of the adult female monkey

Neurogenesis occurs continually throughout life in all mammals and the extent of neurogenesis is influenced by many factors including gonadal hormones. Most research regarding hormones and neurogenesis has been performed on non-primate species. To determine whether gonadal hormones can modulate endogenous neurogenesis in the dentate gyrus (DG) of the hippocampus in non-human primates, ovariectomized (OVX) female rhesus monkeys received continuous, unopposed beta-estradiol (OVX-E-Con), cyclic unopposed beta-estradiol (OVX-E-Cyc), continuous beta-estradiol+cyclic progesterone (OVX-E-Con+P-Cyc), or control (OVX-Veh) treatments. At week 29, all monkeys received BrdU injections for 4 consecutive days, in addition to the ongoing treatment. Twenty days after the last BrdU injection, all animals were sacrificed for tissue collection. In DG of hippocampus, scattered BrdU-ir cells were observed mainly in the subgranular zone (SGZ) and in the granule cell layer and occasionally these BrdU-ir cells in the SGZ formed clusters containing between 2 and 5 cells. In the granule cell layer and SGZ, virtually none of the BrdU-ir cells were either Dcx, a marker of immature neurons, or GFAP positive. However, an occasional BrdU-ir cell was positive for both neuronal marker NeuN or beta III-tubulin. Unbiased stereological analysis of BrdU-ir cells within the SGZ and the granule cell layer of DG revealed that among the experimental groups, there was no significant difference in number of BrdU-ir cells within the SGZ and the granule cell layer of the DG: OVX-E-Con (1801+/-218.7), OVX-E-Cyc (1783+/-415.6), OVX-E-Con+/-P-Cyc (1721+/-229.6), and OVX-Veh (1263+/-106.3), but a trend towards increased BrdU-ir cells was observed in all the experimental groups.

Castration and training in a spatial task alter the number of immature neurons in the hippocampus of male mice

New neurons are generated in the granule cell layer of the dentate gyrus (GCL) throughout adulthood. This process is modulated by many environmental and neurochemical factors. We previously observed that castrated mice, compared to sham-operated mice, perform poorly in the delayed matching to place water-maze task (DMTP). In this study, we quantified the number of doublecortin expressing (DCX+) immature neurons and Ki-67 expressing (Ki-67+) proliferating progenitors in mice previously tested in a spatial DMTP task, a nonspatial DMTP, or that received equivalent amounts of handling only. Regardless of DMTP training experience, castration reduced immature neuron number in the GCL but had no effect on proliferating progenitors. Compared to handling only, visible DMTP training reduced the immature neuron number, but hidden DMTP training had no effect. Castration did not alter these environmental effects. Finally, performance on the spatial DMTP task did not correlate with immature neuron number. In addition, while the number of immature neurons was strongly reduced following cranial irradiation with (137)Cs, this treatment did not affect spatial DMTP performance. Thus, in mice, castration disrupts spatial memory and reduces immature neuron number, but there is no strong link between these effects.

Enhanced sensitivity of the MRL/MpJ mouse to the neuroplastic and behavioral effects of acute and chronic antidepressant treatments

Adult hippocampal neurogenesis has been implicated in the pathophysiology of depression and in the therapeutic effects of antidepressant drugs. Current immunohistochemical methods that study neurogenesis are time consuming and labor intensive. Therefore, a significantly more rapid flow cytometric method was characterized to measure neurogenesis in the adult mouse brain. The sensitivity of mice to the effects of antidepressant treatments is dependent on genetic background. Thus, studies were conducted comparing the responsiveness of 2 inbred mouse strains, MRL/MpJ and C57BL/6J, to the acute and chronic effects of antidepressants on neurochemistry and behavior. Acutely, MRL/MpJ mice displayed more robust behavioral and neurochemical responses to pharmacologically distinct antidepressants than C57BL/6J mice. Chronic administration of the antidepressant drugs fluoxetine and desipramine produced robust elevations in hippocampal cell proliferation and brain-derived neurotrophic factor (BDNF) protein levels in MRL/MpJ mice. C57BL/6J mice treated similarly with antidepressant drugs were mainly unresponsive on these measures. Mice were tested in the novelty-induced hypophagia (NIH) paradigm to examine a behavioral response associated with chronic, but not acute, antidepressant treatment. Only MRL/MpJ mice were behaviorally responsive to chronic antidepressant administration in the NIH paradigm. The positive effects of chronic antidepressants on hippocampal cell proliferation and BDNF paralleled the ability of these drugs to produce changes in NIH behavior. These studies highlight the advantages of using flow cytometry to study hippocampal neurogenesis and identify the MRL/MpJ mouse as a strain with superior response to antidepressant drug treatments that may lead to a better understanding of the genetics behind antidepressant efficacy and sensitivity.

Hippocampal neurogenesis as a target for the treatment of mental illness: a critical evaluation

Over one-quarter of adult Americans are diagnosed with a mental illness like Major Depressive Disorder (MDD), Post-Traumatic Stress Disorder (PTSD), schizophrenia, and Alzheimer's Disease. In addition to the exceptional personal burden these disorders exert on patients and their families, they also have enormous cost to society. Although existing pharmacological and psychosocial treatments alleviate symptoms in many patients, the comorbidity, severity, and intractable nature of mental disorders strongly underscore the need for novel strategies. As the hippocampus is a site of structural and functional pathology in most mental illnesses, a hippocampal-based treatment approach has been proposed to counteract the cognitive deficits and mood dysregulation that are hallmarks of psychiatric disorders. In particular, preclinical and clinical research suggests that hippocampal neurogenesis, the generation of new neurons in the adult dentate gyrus, may be harnessed to treat mental illness. There are obvious applications and allures of this approach; for example, perhaps stimulating hippocampal neurogenesis would reverse the overt and noncontroversial hippocampal atrophy and functional deficits observed in Alzheimer's Disease and schizophrenia, or the more controversial hippocampal deficits seen in MDD and PTSD. However, critical examination suggests that neurogenesis may only correlate with mental illness and treatment, suggesting targeting neurogenesis alone is not a sufficient treatment strategy. Here we review the classic and causative links between adult hippocampal neurogenesis and mental disorders, and provide a critical evaluation of how (and if) our basic knowledge of new neurons in the adult hippocampus might eventually help combat or even prevent mental illness.

Stress differentially regulates the effects of voluntary exercise on cell proliferation in the dentate gyrus of mice

It has been well-established that cell proliferation and neurogenesis in the adult mouse dentate gyrus (DG) can be regulated by voluntary exercise. Recent evidence has suggested that the effects of voluntary exercise can in turn be influenced by environmental factors that regulate the amount of stress an animal is exposed to. In this study, we use bromodeoxyuridine and proliferating cell nuclear antigen immunohistochemistry to show that voluntary exercise produces a significant increase in cell proliferation in the adult mouse DG in both isolated and socially housed mice. This effect on proliferation translates into an increase in neurogenesis and neuronal branching of new neurons in the mice that exercised. Although social condition did not regulate proliferation in young adult mice, an effect of social housing could be observed in mice exposed to acute restraint stress. Surprisingly, only exercising mice housed in isolated conditions showed an increase in cellular proliferation following restraint stress, whereas socially housed, exercising mice, failed to show a significant increase in proliferation. These findings indicate that social housing may increase the effects of any stressful episodes on hippocampal neurogenesis in the mouse DG.

Sex-specific effects of chronic fluoxetine treatment on neuroplasticity and pharmacokinetics in mice

Neurogenesis is a mechanism through which antidepressants may produce therapeutic effects. There is a dearth of information regarding the effects of antidepressants on neurogenesis and neurotrophin mobilization in females. This study examined sex differences in the alteration of cell proliferation and survival in multiple regions of the brain. Additional experiments examined brain-derived neurotrophic factor (BDNF) levels and pharmacokinetics of fluoxetine to determine whether they mediate sex differences. MRL/MpJ mice were treated with fluoxetine (5 and 10 mg/kg b.i.d.) for 21 days and received injections of 5-bromo-2'-deoxyuridine (200 mg/kg) to measure DNA synthesis. In the hippocampus, fluoxetine increased cell proliferation at both doses; females treated with 10 mg/kg produced more new cells than males. Fluoxetine did not alter survival in males, but 10 mg/kg reduced survival in females. In the frontal cortex, fluoxetine increased cell proliferation and survival in males treated with 10 mg/kg. In the cerebellum and amygdala, 10 mg/kg fluoxetine increased cell proliferation in both sexes but did not alter the incorporation of the new cells. Fluoxetine increased BDNF levels in the hippocampus of both sexes. BDNF levels correlated with cell proliferation in males but not females. Brain and plasma levels indicated that females metabolized fluoxetine faster than males and produced more of the metabolite norfluoxetine. These data suggest that fluoxetine acts on multiple areas of the brain to increase cell proliferation, and the pattern of activation differs between males and females. Sex-specific effects of fluoxetine on neurotrophin mobilization and pharmacokinetics may contribute to these differences in neural plasticity.

Sex differences in a hypoxia model of preterm brain damage

Male sex is a well-established risk factor for poor neurodevelopmental outcome after premature birth. The mechanisms behind this sex-related difference are unknown. The damage associated with prematurity can be mimicked in rodents by prolonged exposure to sublethal postnatal hypoxia. This chronic hypoxia leads to anatomical changes in mice that strongly resemble the loss of volume, decreased myelination, and ventriculomegaly seen in preterm newborns. However, no sex differences have been previously noted in this rodent model. We hypothesized that sex comparisons in hypoxic mice would show sex-related differences in brain volume and white matter loss in response to the same degree of hypoxic insult. Mice were placed in chronic sublethal hypoxia from postnatal day 3-11. Cortical, hippocampal, and cerebellar volumes and myelination indices were measured. We found that the male hippocampus, normally larger than the female, undergoes a greater volume loss compared with females (p < 0.05). Myelination, generally greater in males, was significantly disrupted by hypoxia in neonatal male forebrain. These results support the use of this rodent model to investigate the basis of sex-related susceptibility to brain damage and develop new sex-based neuroprotective strategies.

Hippocampus-dependent learning is associated with adult neurogenesis in MRL/MpJ mice

The hippocampus is involved in declarative memory and produces new neurons throughout adulthood. Numerous experiments have been aimed at testing the possibility that adult neurogenesis is required for learning and memory. However, progress has been encumbered by the fact that abating adult neurogenesis usually affects other biological processes, confounding the interpretation of such experiments. In an effort to circumvent this problem, we used a reverse approach to test the role of neurogenesis in hippocampus-dependent learning, exploiting the low levels of adult neurogenesis in the MRL/MpJ strain of mice compared with other mouse strains. We observed that adult MRL/MpJ mice produce 75% fewer new neurons in the dentate gyrus than age-matched C57BL/6 mice. Learning-induced synaptic remodeling, spatial learning, and visual recognition learning were reduced in MRL/MpJ mice compared with C57BL/6 mice. When MRL/MpJ mice were allowed unlimited access to running wheels, neurogenesis along with spatial learning and visual recognition learning were increased to levels comparable to those in running C57BL/6 mice. Together, these results suggest that adult neurogenesis is correlated with spatial learning and visual recognition learning, possibly by modulating morphological plasticity in the dentate gyrus.

Effects of steroid hormones on neurogenesis in the hippocampus of the adult female rodent during the estrous cycle, pregnancy, lactation and aging

Adult neurogenesis exists in most mammalian species, including humans, in two main areas: the subventricular zone (new cells migrate to the olfactory bulbs) and the dentate gyrus of the hippocampus. Many factors affect neurogenesis in the hippocampus and the subventricular zone, however the focus of this review will be on factors that affect hippocampal neurogenesis, particularly in females. Sex differences are often seen in levels of hippocampal neurogenesis, and these effects are due in part to differences in circulating levels of steroid hormones such as estradiol, progesterone, and corticosterone during the estrous cycle, in response to stress, with reproduction (including pregnancy and lactation), and aging. Depletion and administration of these same steroid hormones also has marked effects on hippocampal neurogenesis in the adult female, and these effects are dependent upon reproductive status and age. The present review will focus on current research investigating how hippocampal neurogenesis is altered in the adult female rodent across the lifespan.

Prozac during puberty: distinctive effects on neurogenesis as a function of age and sex

Neurogenesis is a possible substrate through which antidepressants alleviate symptoms of depression. In adult male rodents and primates, chronic treatment with fluoxetine increases neurogenesis in the hippocampal formation. Little is known about the effects of the antidepressant on neurogenesis during puberty or in female animals at any age. Therefore we examined the effects of chronic fluoxetine treatment on cell proliferation and survival in male and female rats during puberty and adulthood. Adult and peri-pubescent male and female rats were treated chronically with fluoxetine (Prozac, 5 mg/kg) or saline. Subsequently rats received a single injection of 5-bromo-2'-deoxyuridine (BrdU; 200 mg/kg) to label DNA synthesis. Rats were sacrificed 2 h, 24 h, or 28 days after BrdU injection to examine cell proliferation, survival and cell fate. Fluoxetine increased cell proliferation in adult male rats but not in peri-pubescent males or female rats of any age or stage of the estrous cycle. Treatment did not alter the number of surviving cells in the male hippocampus but decreased survival in the female hippocampus. Thus, fluoxetine has distinctive effects on neurogenesis as a function of age and sex. Circulating levels of the stress hormone corticosterone were also examined. Treatment of female rats with fluoxetine during puberty decreased circulating levels of corticosterone in adults, even in the absence of the drug suggesting disruption of maturation of the hypothalamic-pituitary-adrenal axis.

Enhanced sensitivity of the MRL/MpJ mouse to the neuroplastic and behavioral effects of chronic antidepressant treatments

Chronic administration of antidepressant drugs produce changes in neuroplasticity and behavior in rodents, effects that may be associated with the slow emergence of clinical therapeutic effects. Owing to the uncertainty over the effects of chronic antidepressant treatments in mice, these experiments compared the regulation of neurogenesis, neurotrophin levels, and behavior produced by chronic antidepressant treatments between two inbred mouse strains, MRL/MpJ and C57BL/6J. The MRL/MpJ strain is associated with enhanced wound healing and tissue regeneration, whereas C57BL/6J mice are used commonly for behavioral studies. Proliferation and survival of hippocampal progenitor cells were measured using flow cytometry, a new platform that rapidly quantifies the incorporation of 5-bromo-2-deoxyuridine (BrdU). Hippocampal cell proliferation was increased significantly after chronic administration of fluoxetine (FLX: 5, 10 mg/kg, intraperitoneal (i.p.), b.i.d.) or desipramine (DMI: 5, 10 mg/kg, i.p., b.i.d.) for 21 days in MRL/MpJ mice, but not in C57BL/6J mice. Hippocampal progenitor cells born prior to chronic antidepressant treatments were not affected in either mouse strain. Protein levels of brain-derived neurotrophic factor (BDNF) in MRL/MpJ mice were elevated significantly in the frontal cortex, hippocampus, and amygdala after chronic FLX treatment, but increased only in the frontal cortex by chronic DMI. In contrast, BDNF levels in C57BL/6J mice were decreased in the hippocampus and increased in the amygdala after chronic FLX, and were decreased in the brain stem after chronic DMI. Novelty-induced hypophagia (NIH) was used to examine a behavioral effect produced by chronic antidepressant treatment. MRL/MpJ mice, chronically administered FLX or DMI, had significantly shorter latencies to consume food when exposed to a novel environment than untreated mice, whereas there were no effects on the behavior of C57BL/6J mice. In conclusion, robust effects of chronic antidepressant treatments on hippocampal cell proliferation and BDNF levels paralleled the ability of these drugs to produce changes in NIH behavior in MRL/MpJ, while none of these effects were produced in C57BL/6J mice. The greater responsiveness of MRL/MpJ mice may be important for drug discovery, for genetic studies, and for understanding the neural mechanisms underlying the physiological and behavioral effects of chronic antidepressant treatments.

Role of nuclear factor kappaB in ovarian hormone-mediated stress hypersensitivity in female mice

BACKGROUND

The molecular mechanisms of stress-induced depressive behaviors have been characterized extensively in male rodents; however, much less is known about female subjects, despite the fact that human depression is far more prevalent in women.

METHODS

To gain insight into these mechanisms, we performed microarray analysis in nucleus accumbens (NAc), a key brain reward region implicated in depression, in ovariectomized (OVX) and gonadally intact female mice after chronic unpredictable stress and measured stress-induced depression-like behavior in the forced swim test (FST). Male mice were studied in the FST for comparison.

RESULTS

We find that stress regulation of genes in NAc of gonadally intact female mice is blunted in OVX mice. This pattern of gene regulation is consistent with behavioral findings on the FST: the pro-depression-like effect of stress in intact female mice is absent in OVX female and gonadally intact male mice. We identified, among many genes regulated by stress, several nuclear factor kappaB (NFkappaB) subunits-a pro-survival transcription factor involved in cellular responses to stress-as being highly upregulated in NAc of OVX mice. Given the role of NFkappaB during stress, we hypothesized that upregulation of NFkappaB by OVX decreases susceptibility to stress. Indeed, we show that inhibition of NFkappaB in NAc of OVX animals increases susceptibility to stress-induced depressive behaviors, whereas activation of NFkappaB in NAc of intact female subjects blocks susceptibility.

CONCLUSIONS

These results suggest a hormonal mechanism of NFkappaB regulation that contributes to stress-induced depressive behaviors in female subjects and might represent a mechanism for gender differences in prevalence rates of these disorders in humans.

Rapid integration of young newborn dentate gyrus granule cells in the adult hippocampal circuitry

Newborn dentate gyrus granule cells (DGCs) are integrated into the hippocampal circuitry and contribute to the cognitive functions of learning and memory. The dendritic maturation of newborn DGCs in adult mice occurs by the first 3-4 weeks, but DGCs seem to receive a variety of neural inputs at both their dendrites and soma even shortly after their birth. However, few studies on the axonal maturation of newborn DGCs have focused on synaptic structure. Here, we investigated the potentiality of output and input in newborn DGCs, especially in the early period after terminal mitosis. We labeled nestin-positive progenitor cells by injecting GFP Cre-reporter adenovirus into Nestin-Cre mice, enabling us to trace the development of progenitor cells by their GFP expression. In addition to GABAergic input from interneurons, we observed that the young DGCs received axosomatic input from the medial septum as early as postinfection day 7 (PID 7). To evaluate the axonal maturation of the newborn DGCs compared with mature DCGs, we performed confocal and electron microscopic analyses. We observed that newborn DGCs projected their mossy fibers to the CA3 region, forming small terminals on hilar or CA3 interneurons and large boutons on CA3 pyramidal cells. These terminals expressed vesicular glutamate transporter 1, indicating they were glutamatergic terminals. Intriguingly, the terminals at PID 7 had already formed asymmetric synapses, similar to those of mature DGCs. Together, our findings suggest that newborn DGCs may form excitatory synapses on both interneurons and CA3 pyramidal cells within 7 days of their terminal mitosis.

Flow cytometric analysis of BrdU incorporation as a high-throughput method for measuring adult neurogenesis in the mouse

INTRODUCTION

The generation of new neurons occurs throughout adulthood in discrete brain regions, and may be regulated by neuropsychiatric diseases and therapeutic drug treatments. Most current methods that study this process measure the labeling of newborn cells by 5-bromo-2-deoxyuridine (BrdU) using immunohistochemical methods followed by the microscopic counting of BrdU positive cells. This method is time consuming and labor intensive, typically taking several weeks to analyze.

METHODS

Therefore, we characterized a method to measure BrdU incorporation in the adult mouse hippocampus in vivo by using flow cytometry, which normally allows analysis of data within a single day.

RESULTS

The present study compared multiple BrdU dosing and loading protocols to determine a dosing strategy that produced the best signal to noise ratio. BrdU incorporation was also compared across different brain regions. The method was sensitive to a number of experimental disease manipulations. Induction of type-1 diabetes and depletion of norepinephrine reduced hippocampal cell proliferation. In contrast, chronic administration of electroconvulsive shock, a somatic treatment for depression, as well as chronic treatment with the antidepressant fluoxetine elevated hippocampal cell proliferation. This increase in cell proliferation with fluoxetine was detected as early as 14 days into treatment. Moreover, comparing measures of cell proliferation obtained by immunohistochemical and flow cytometric methods within the same animals were convergent and significantly correlated to each other. Flow cytometry was also sufficiently sensitive to quantify the survival of newly born cells.

DISCUSSION

These experiments validate the utility of flow cytometry in analyzing hippocampal cell proliferation and survival in a reliable and high-throughput fashion. The speedy analysis afforded by flow cytometry lends itself to be utilized in novel drug discovery and physiology.

Time course of morphine's effects on adult hippocampal subgranular zone reveals preferential inhibition of cells in S phase of the cell cycle and a subpopulation of immature neurons

Opiates, such as morphine, decrease neurogenesis in the adult hippocampal subgranular zone (SGZ), raising the possibility that decreased neurogenesis contributes to opiate-induced cognitive deficits. However, there is an incomplete understanding of how alterations in cell cycle progression and progenitor maturation contribute to this decrease. The present study examined how morphine regulates progenitor cell cycle, cell death and immature SGZ neurons (experiment 1) as well as the progression of SGZ progenitors through key stages of maturation (experiment 2). In experiment 1, mice received sham or morphine pellets (s.c., 0 and 48 h) and bromodeoxyuridine (BrdU) 2 h prior to sacrifice (24, 72 or 96 h). Morphine decreased both the number of S phase and total cycling cells, as there were fewer cells immunoreactive (IR) for the S phase marker BrdU and the cell cycle marker Ki67. The percentage of Ki67-IR cells that were BrdU-IR was decreased after 24 but not 96 h of morphine, suggesting a disproportionate effect on S phase cells relative to all cycling cells at this time point. Cell death (activated caspase-3 counts) was increased after 24 but not 96 h. In experiment 2, nestin-green fluorescent protein (GFP) mice given BrdU 1 day prior to morphine or sham surgery (0 and 48 h, sacrifice 96 h) had fewer Ki67-IR cells, but no change in BrdU-IR cell number, suggesting that this population of BrdU-IR cells was less sensitive to morphine. Interestingly, examination of key stages of progenitor cell maturation revealed that morphine increased the percent of BrdU-IR cells that were type 2b and decreased the percent that were immature neurons. These data suggest that chronic morphine decreases SGZ neurogenesis by inhibiting dividing cells, particularly those in S phase, and progenitor cell progression to a more mature neuronal stage.

Estrous cycle regulates activation of hippocampal Akt, LIM kinase, and neurotrophin receptors in C57BL/6 mice

Estradiol modulates dendritic spine morphology and synaptic protein expression in the rodent hippocampus, as well as hippocampal-dependent learning and memory. In the rat, these effects may be mediated through nongenomic steroid signaling such as estradiol activation of the Akt and LIM kinase (LIMK) pathways, in addition to genomic signaling involving estradiol upregulation of brain-derived neurotrophic factor expression (BDNF). Due to the many species differences between mice and rats, including differences in the hippocampal response to estradiol, it is unclear whether estradiol modulates these pathways in the mouse hippocampus. Therefore, we investigated whether endogenous fluctuations of gonadal steroids modulate hippocampal activation of the Akt, LIMK, and the BDNF receptor TrkB in conjunction with spatial memory in female C57BL/6 mice. We found that Akt, LIMK, and TrkB were activated throughout the dorsal hippocampal formation during the high-estradiol phase, proestrus. Cycle phase also modulated expression of the pre- and post-synaptic markers synaptophysin and post-synaptic density 95. However, cycle phase did not influence performance on an object placement test of spatial memory, although this task is known to be sensitive to the complete absence of ovarian hormones. The findings suggest that endogenous estradiol and progesterone produced by the ovaries modulate specific signaling pathways governing actin remodeling, cell excitability, and synapse formation.

Calcium-sensitive adenylyl cyclases in depression and anxiety: behavioral and biochemical consequences of isoform targeting

BACKGROUND

Adenylyl cyclases (ACs) represent a diverse family of enzymes responsible for the generation of cyclic adenosine monophosphate (cAMP), a key intracellular second messenger. The Ca(2+)/calmodulin-stimulated AC1 and AC8 isoforms as well as the calcium-inhibited AC5 isoform are abundantly expressed within limbic regions of the central nervous system. This study examines the contribution of these AC isoforms to emotional behavior.

METHODS

Male and female AC1/8 double knockout mice (DKO) and AC5 knockout mice (AC5KO) were examined on a series of standard laboratory assays of emotionality. Mice were also assayed for hippocampal cell proliferation and for changes in brain-derived neurotrophic factor signaling in the nucleus accumbens, amygdala, and hippocampus, three forebrain structures involved in the regulation of mood and affect.

RESULTS

The AC5KO mice showed striking anxiolytic and antidepressant phenotypes on standard behavioral assays. In contrast, AC1/8 DKO mice were hypoactive, exhibited diminished sucrose preference, and displayed alterations in neurotrophic signaling, generally consistent with a prodepressant phenotype. Neither line of mice displayed alterations in hippocampal cell proliferation.

CONCLUSIONS

These data illustrate the complex manner in which Ca(2+)/calmodulin-stimulated ACs contribute to emotional behavior. In addition, they support the possibility that a selective AC5 antagonist would be of therapeutic value against depression and anxiety disorders.

Impaired adult neurogenesis in the dentate gyrus of a triple transgenic mouse model of Alzheimer's disease

It has become generally accepted that new neurones are added and integrated mainly in two areas of the mammalian CNS, the subventricular zone and the subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus, which is of central importance in learning and memory. The newly generated cells display neuronal morphology, are able to generate action potentials and receive functional synaptic inputs, i.e. their properties are similar to those found in mature neurones. Alzheimer's disease (AD) is the primary and widespread cause of dementia and is an age-related, progressive and irreversible neurodegenerative disease that deteriorates cognitive functions. Here, we have used male and female triple transgenic mice (3xTg-AD) harbouring three mutant genes (beta-amyloid precursor protein, presenilin-1 and tau) and their respective non-transgenic (non-Tg) controls at 2, 3, 4, 6, 9 and 12 months of age to establish the link between AD and neurogenesis. Using immunohistochemistry we determined the area density of proliferating cells within the SGZ of the DG, measured by the presence of phosphorylated Histone H3 (HH3), and their possible co-localisation with GFAP to exclude a glial phenotype. Less than 1% of the HH3 labeled cells co-localised with GFAP. Both non-Tg and 3xTg-AD showed an age-dependent decrease in neurogenesis. However, male 3xTg-AD mice demonstrated a further reduction in the production of new neurones from 9 months of age (73% decrease) and a complete depletion at 12 months, when compared to controls. In addition, female 3xTg-AD mice showed an earlier but equivalent decrease in neurogenesis at 4 months (reduction of 63%) with an almost inexistent rate at 12 months (88% decrease) compared to controls. This reduction in neurogenesis was directly associated with the presence of beta-amyloid plaques and an increase in the number of beta-amyloid containing neurones in the hippocampus; which in the case of 3xgTg females was directly correlated. These results suggest that 3xTg-AD mice have an impaired ability to generate new neurones in the DG of the hippocampus, the severity of which increases with age and might be directly associated with the known cognitive impairment observed from 6 months of age onwards . The earlier reduction of neurogenesis in females, from 4 months, is in agreement with the higher prevalence of AD in women than in men. Thus it is conceivable to speculate that a recovery in neurogenesis rates in AD could help to rescue cognitive impairment.

Adult hippocampal cell proliferation is suppressed with estrogen withdrawal after a hormone-simulated pregnancy

Estradiol withdrawal after pregnancy is hypothesized to precipitate depressive symptoms in vulnerable women. A hormone-simulated pregnancy was induced in female rats and the effects of a 'postpartum' drop in estradiol on hippocampal cell proliferation were examined. All groups were ovariectomized or given sham surgery prior to treatment. Rats were randomly assigned to 'postpartum', 'postpartum'+EB (estradiol benzoate), 'postpartum'+DPN (diarylpropionitrile; an ERbeta agonist), 'postpartum'+IMI (imipramine; a tricyclic antidepressant), sham, ovariectomized (OVX), sham+IMI or OVX+IMI groups. All 'postpartum' groups received hormone injections (estradiol and progesterone) over 23 days to simulate pregnancy, while IMI groups also received daily imipramine injections. After day 23, 'postpartum' rats were withdrawn from the hormone-simulated pregnancy (mimicking the postpartum drop in gonadal hormones), while other 'postpartum' treatment groups received daily injections of DPN, EB or IMI. On day 3 'postpartum' all rats were injected with bromodeoxyuridine (BrdU; a DNA synthesis marker) and perfused 24 h later to assess cell proliferation and cell death in the dentate gyrus. 'Postpartum' hormone withdrawal decreased hippocampal cell proliferation in the 'postpartum' and 'postpartum'+EB groups only. Chronic imipramine significantly increased hippocampal cell proliferation in sham+IMI, but not OVX+IMI rats suggesting that imipramine's effects to increase hippocampal cell proliferation in female rats is related to reproductive status. Cell death (pyknotic cells) was decreased only in the 'postpartum' group. Together, these results suggest an important, though complex, role for gonadal hormones in the cellular changes accompanying this model of postpartum depression.