Estrogen-induced region specific decrease in the density of 5-bromo-2-deoxyuridine-labeled cells in the olfactory bulb of adult female rats

Hormonal Regulation of Mammalian Adult Neurogenesis: A Multifaceted Mechanism

Adult neurogenesis—resulting in adult-generated functioning, integrated neurons—is still one of the most captivating research areas of neuroplasticity. The addition of new neurons in adulthood follows a seemingly consistent multi-step process. These neurogenic stages include proliferation, differentiation, migration, maturation/survival, and integration of new neurons into the existing neuronal network. Most studies assessing the impact of exogenous (e.g., restraint stress) or endogenous (e.g., neurotrophins) factors on adult neurogenesis have focused on proliferation, survival, and neuronal differentiation. This review will discuss the multifaceted impact of hormones on these various stages of adult neurogenesis. Specifically, we will review the evidence for hormonal facilitation (via gonadal hormones), inhibition (via glucocorticoids), and neuroprotection (via recruitment of other neurochemicals such as neurotrophin and neuromodulators) on newly adult-generated neurons in the mammalian brain.

GPER1 Signaling Initiates Migration of Female V-SVZ-Derived Cells

In the rodent ventricular-subventricular zone (V-SVZ) neurons are generated throughout life. They migrate along the rostral migratory stream (RMS) into the olfactory bulb before their final differentiation into interneurons and integration into local circuits. Estrogen receptors (ERs) are steroid hormone receptors with important functions in neurogenesis and synaptic plasticity. In this study, we show that the ER GPER1 is expressed in subsets of cells within the V-SVZ of female animals and provide evidence for a potential local estrogen source from aromatase-positive astrocytes surrounding the RMS. Blocking of GPER1 in Matrigel cultures of female animals significantly impairs migration of V-SVZ-derived cells. This outgrowth is accompanied by regulation of phosphorylation of the actin-binding protein cofilin by GPER1 signaling including an involvement of the p21-Ras pathway. Our results point to a prominent role of GPER1 in the initiation of neuronal migration from the V-SVZ to the olfactory bulb.

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GPER1 is expressed within all cell types of the stem cell lineage in the V-SVZ

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Blocking of GPER1 leads to a decrease in migration of V-SVZ-derived neuroblasts

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GPER1 signaling in V-SVZ Matrigel cultures involves Ras-induced p21

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Blocking of GPER1 signaling leads to an increase in the ratio of p-cofilin/cofilin

Testosterone and Adult Neurogenesis

It is now well established that neurogenesis occurs throughout adulthood in select brain regions, but the functional significance of adult neurogenesis remains unclear. There is considerable evidence that steroid hormones modulate various stages of adult neurogenesis, and this review provides a focused summary of the effects of testosterone on adult neurogenesis. Initial evidence came from field studies with birds and wild rodent populations. Subsequent experiments with laboratory rodents have tested the effects of testosterone and its steroid metabolites upon adult neurogenesis, as well as the functional consequences of induced changes in neurogenesis. These experiments have provided clear evidence that testosterone increases adult neurogenesis within the dentate gyrus region of the hippocampus through an androgen-dependent pathway. Most evidence indicates that androgens selectively enhance the survival of newly generated neurons, while having little effect on cell proliferation. Whether this is a result of androgens acting directly on receptors of new neurons remains unclear, and indirect routes involving brain-derived neurotrophic factor (BDNF) and glucocorticoids may be involved. In vitro experiments suggest that testosterone has broad-ranging neuroprotective effects, which will be briefly reviewed. A better understanding of the effects of testosterone upon adult neurogenesis could shed light on neurological diseases that show sex differences.

Sex Steroids and Adult Neurogenesis in the Ventricular-Subventricular Zone

The forebrain ventricular-subventricular zone (V-SVZ) continuously generates new neurons throughout life. Neural stem cells (type B1 cells) along the lateral ventricle become activated, self-renew, and give rise to proliferating precursors which progress along the neurogenic lineage from intermediate progenitors (type C cells) to neuroblasts (type A cells). Neuroblasts proliferate and migrate into the olfactory bulb and differentiate into different interneuronal types. Multiple factors regulate each step of this process. Newly generated olfactory bulb interneurons are an important relay station in the olfactory circuits, controlling social recognition, reproductive behavior, and parental care. Those behaviors are strongly sexually dimorphic and changes throughout life from puberty through aging and in the reproductive age during estrous cycle and gestation. Despite the key role of sex hormones in regulating those behaviors, their contribution in modulating adult neurogenesis in V-SVZ is underestimated. Here, we compare the literature highlighting the sexual dimorphism and the differences across the physiological phases of the animal for the different cell types and steps through the neurogenic lineage.

Cellular and molecular attributes of neural stem cell niches in adult zebrafish brain

Adult neurogenesis is a complex, presumably conserved phenomenon in vertebrates with a broad range of variations regarding neural progenitor/stem cell niches, cellular composition of these niches, migratory patterns of progenitors and so forth among different species. Current understanding of the reasons underlying the inter-species differences in adult neurogenic potential, the identification and characterization of various neural progenitors, characterization of the permissive environment of neural stem cell niches and other important aspects of adult neurogenesis is insufficient. In the last decade, zebrafish has emerged as a very useful model for addressing these questions. In this review, we have discussed the present knowledge regarding the neural stem cell niches in adult zebrafish brain as well as their cellular and molecular attributes. We have also highlighted their similarities and differences with other vertebrate species. In the end, we shed light on some of the known intrinsic and extrinsic factors that are assumed to regulate the neurogenic process in adult zebrafish brain. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1188-1205, 2017.

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.

The interplay between reproductive social stimuli and adult olfactory bulb neurogenesis

Adult neurogenesis is a striking form of structural plasticity that adapts the brain to the changing world. Accordingly, new neuron production is involved in cognitive functions, such as memory, learning, and pattern separation. Recent data in rodents indicate a close link between adult neurogenesis and reproductive social behavior. This provides a key to unravel the functional meaning of adult neurogenesis in biological relevant contexts and, in parallel, opens new perspectives to explore the way the brain is processing social stimuli. In this paper we will summarize some of the major achievements on cues and mechanisms modulating adult neurogenesis during social behaviors related to reproduction and possible role/s played by olfactory newborn neurons in this context. We will point out that newborn interneurons in the accessory olfactory bulb (AOB) represent a privileged cellular target for social stimuli that elicit reproductive behaviors and that such cues modulate adult neurogenesis at two different levels increasing both proliferation of neuronal progenitors in the germinative regions and integration of newborn neurons into functional circuits. This dual mechanism provides fresh neurons that can be involved in critical activities for the individual fitness, that is, the processing of social stimuli driving the parental behavior and partner recognition.

Estradiol treatment decreases cell proliferation in the neurogenic zones of adult female zebrafish (Danio rerio) brain

While estrogens are known to play a crucial role in the neurogenesis of the mammalian and avian brain, their role in teleost adult proliferation pattern is not yet fully understood. The present study aimed to determine the estrogen effects in adult brain proliferation zones, using zebrafish, as a model organism. Indeed, teleost fish brain provides a unique adult neurogenesis model, based on its extensive proliferation, contrasting the restricted adult telencephalic neurogenesis observed in birds and mammals. To determine the effect of estrogens, 17-β estradiol was administrated for 7 days in adult female zebrafish, followed by bromodeoxyuridine (BrdU)-immunohistochemistry and double immunofluorescence. Stereological analyses of the BrdU-positive cells within the neurogenic zones, showed region-specific decreases of actively proliferating cells in the estrogen-treated animals, compared to matched controls. Interestingly, the most prominent estradiol effects were found in the number of cycling cells of the ventral nucleus of ventral telencephalic area (Vv) and cerebellar areas. Significant decreases were also determined in the dorso-lateral telencephalic, preoptic and dorsal hypothalamic areas. In contrast, medial dorsal telencephalic, caudal (Hc) and ventral (Hv) hypothalamic areas were unaffected by estrogen treatment. The majority of the BrdU-labeled cells were found to co-express PCNA proliferating marker in Hc, Hv and Vv. Additionally, a population of proliferating cells co-expressed the early neuronal marker TOAD in all areas studied. These results provide significant evidence on the 17-β estradiol impact on adult neurogenesis, down-regulating the fast-cycling and post-mitotic cells within the female zebrafish brain neurogenetic zones.

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.

Postnatal and adult exposure to estradiol differentially influences adult neurogenesis in the main and accessory olfactory bulb of female mice

Neurons incorporated into the adult main olfactory bulb (MOB) and accessory olfactory bulb (AOB) derive from the subventricular zone (SVZ). Despite some recent studies on the role of olfactory neurogenesis in sociosexual behaviors mediated by hormones, data on the implication of estrogens are still lacking. Taking advantage of female aromatase-knockout (ArKO) mice, which are unable to produce estradiol across their life span, we investigated the role of estradiol exposure during early postnatal and adult periods on adult neurogenesis in the MOB and AOB. We found that proliferation of progenitor cells in the adult female SVZ was not influenced by estradiol. However, whereas adult exposure to estradiol influences the turnover of MOB newborn neurons, the survival of those in the AOB depends on exposure to estradiol during the early postnatal period. Finally, based on their expression of Zif268, we showed that newborn neurons in the MOB responded to sociosexual odors, albeit to a lesser extent in ArKO females, suggesting a contribution of estradiol during the early postnatal period to this response. Together, these results suggest that the survival and functional integration of newborn neurons in the adult female MOB and AOB are differentially influenced by estrogens from the early postnatal period to adulthood.

Agonistic behavior in males and females: effects of an estrogen receptor beta agonist in gonadectomized and gonadally intact mice

Affiliative and agonistic social interactions are mediated by gonadal hormones. Research with estrogen receptor alpha (ERalpha) or beta (ERbeta) knockout (KO) mice show that long-term inactivation of ERalpha decreases, while inactivation of ERbeta increases, male aggression. Opposite effects were found in female alphaERKO and betaERKO mice. The role of acute activation of ERalpha or ERbeta in the agonistic responses of adult non-KO mice is unknown. We report here the effects of the ERbeta selective agonist WAY-200070 on agonistic and social behavior in gonadally intact and gonadectomized (gonadex) male and female CD-1 mice towards a gonadex, same-sex intruder. All 15min resident-intruder tests were videotaped for comprehensive behavioral analysis. Separate analyses assessed: (1) effects of WAY-200070 on each sex and gonadal condition; (2) differences between sexes, and between gonadally intact and gonadex mice, in untreated animals. Results show that in gonadally intact male and female mice, WAY-200070 increased agonistic behaviors such as pushing down the intruder and aggressive grooming, while leaving attacks unaffected. In untreated mice, males attacked more than females, and gonadex animals showed less agonistic behavior than same-sex, gonadally intact mice. Overall, our detailed behavioral analysis suggested that in gonadally intact male and female mice, ERbeta mediates patterns of agonistic behavior that are not directly involved in attacks. This suggests that specific aspects of aggressive behavior are acutely mediated by ERbeta in adult mice. Our results also showed that, in resident-intruder tests, female mice spend as much time in intrasexual agonistic interactions as males, but use agonistic behaviors that involve extremely low levels of direct attacks. This non-attack aggression in females is increased by acute activation of ERbeta. Thus, acute activation of ERbeta similarly mediates agonistic behavior in adult male and female CD-1 mice.

Sexual dimorphic stages affect both proliferation and serotonergic innervation in the adult rostral migratory stream

One of the sexual dimorphic differences in adult rodents is neural proliferation. Here we demonstrate that physiological hormone stages can modulate this proliferation in the adult forebrain. Female mice, both pregnant and synchronized in oestrus, exhibited higher proliferating cell percentages than males in both the rostral migratory stream (RMS) and the olfactory bulb (OB). Moreover, although the hormonal component also influenced the subventricular zone (SVZ), no differences in proliferation were observed in this region. In addition, both groups of females had higher numbers of serotonergic fibres in these regions. Serotonin may therefore be related to the mechanism of action by which hormones can affect cell proliferation of this brain region. We also evaluated cell death in the SVZ in males and females, finding that this was higher in the former. Taken together, our results support the idea that in female rodents more neuroblasts are able to reach the RMS and then proliferate, apoptosis being an additional mechanism affecting the low proliferation of cells in the RMS and OB in males. Thus, proliferation in the RMS is influenced by sexual dimorphism.

Sexually dimorphic enhancement by estradiol of male urinary odor detection thresholds in mice

We asked whether sex and adult estrogen exposure influence the detection thresholds for urinary odors used by mice to guide their social behaviors. Gonadectomized (GDX) male and female mice were trained on a two-choice food-motivated task to determine detection thresholds for male urinary odors. There was no significant sex difference in the detection of these odors by GDX subjects without hormone replacement. However, during treatment with estradiol benzoate (EB), GDX females, but not GDX males, showed an enhanced ability to detect these odors. To investigate a possible mechanism for this effect, the authors measured GDX females' odor-sampling behavior (sniffing) by monitoring intranasal pressure transients during performance of the urinary odor detection task with and without EB treatment. Under both hormone conditions, females decreased their sniffing frequency as the urinary odor concentration decreased, with this decrease being significantly greater while GDX females received EB. Thus, estradiol enhanced detection thresholds for male urine in a sex-specific manner, and this enhanced sensitivity in females was correlated with altered odor-sampling behavior.

Restoration of female genital vasocongestive arousal responses in young and aged rats

INTRODUCTION

Treatments of aged, male hypertensive rats that induce vascular remodeling or that normalize endothelial function are known to produce sustained improvements in erectile function. Whether the treatments targeting these processes benefit female genital vasocongestive arousal (GVA) responses is currently not known.

AIM

To determine whether the actions of nitric oxide (NO) are critical to the apomorphine (APO)-generated GVA responses in both intact and ovariectomized OVX young adult female rats (before any aging-associated decreases in the responses). In addition, we also investigated whether the diminished GVA responses in aged rats could be restored, at least in part, using an antihypertensive treatment, which is known to enhance erectile responses and improve general vascular function in male rats.

METHODS

In female Wistar rats, APO-induced GVA responses (80 microg/kg, subcutaneously [sc], 30 minutes) were assessed by videomonitoring following various treatments. Young adult females were ovariectomized or were treated with the nitric oxide synthase (NOS) inhibitor N-nitro-L-arginine methyl ester (30 mg/kg, iv), followed by an NO mimetic, sodium nitroprusside (10 microg/kg/minute, intravenous). Aged females (18 months) were treated for 2 weeks with the angiotensin converting enzyme (ACE) inhibitor, enalapril (30 mg/kg/day, orally) plus low sodium (0.04%).

MAIN OUTCOME MEASURES

APO-induced GVA responses in female rats.

RESULTS

There was an age-associated reduction in sexual responses in normotensive rats that was greatly enhanced (fourfold) by brief, aggressive antihypertensive treatment. The enhanced vasocongestive responses persisted for a 5-week off-treatment. Both OVX and NOS inhibition significantly decreased sexual responses by approximately 80% in young female rats. Systemic administration of an NO mimetic recovered vasocongestive responses in the NOS-blocked rats, but not in OVX animals.

CONCLUSIONS

Although mechanisms were not established, the major findings were that brief aggressive ACE inhibitor treatment markedly improved sexual responses in aged female rats, and systemic delivery of an NO mimetic recovered sexual responses in globally NOS-blocked animals.