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

Gonadal hormones modulate neurogenesis in the dentate gyrus differentially in male and female adult rodents. Neurogenesis is comprised of at least two components: cell proliferation (the production of new cells) and cell survival (the number of new neurons that survive to maturity). Previous studies have found sex differences in the level of cell proliferation in the dentate gyrus only when comparing females in a high estrogen state to males. This review focuses on the effects of acute and chronic levels of estrogens or androgens on hippocampal neurogenesis in the adult male and female rodent. Evidence is also reviewed for the co-localization of androgen receptors and estrogen receptors (ER) with markers for cell proliferation or immature new cell survival. Briefly, evidence suggests that acute estradiol initially enhances and subsequently suppresses cell proliferation in the dentate gyrus of adult female rodents but may have limited effects in male rodents. Both the two known ER subtypes, ER and β upregulate hippocampal neurogenesis via cell proliferation. Intriguingly, repeated exposure to estradiol modulates hippocampal neurogenesis and cell death in adult female, but not male, rodents. However short-term estradiol treatment (5 days) in male meadow voles enhances new cell survival in the dentate gyrus but only when administered during the ‘axon extension’ phase. Furthermore, evidence is also reviewed showing a difference in response to acute and chronic estradiol treatment in older female rats compared to younger female rats. Recent findings from our laboratory indicate that testosterone and dihydrotestosterone upregulate hippocampal neurogenesis (via cell survival), but not cell proliferation, in adult male rodents. Effects of endogenous fluctuations in gonadal hormones on adult neurogenesis are observed across the seasons in meadow voles and during pregnancy and lactation in the rat dam. Pregnancy and motherhood differentially regulate adult hippocampal neurogenesis in the adult female rodent, with primiparous rats displaying lower levels of hippocampal cell proliferation and survival after parturition. Few studies have compared males and females but existing research suggests a sex difference in the hormonal regulation of hippocampal neurogenesis in the adult. Clearly more work is needed to elucidate the effects of gonadal hormones on neurogenesis in the dentate gyrus of both male and female rodents across the lifespan, especially if we are to use our knowledge of how adult neurogenesis is regulated to develop strategies to repair neuron loss in neurodegenerative diseases.     

Gonadal hormone modulation of hippocampal neurogenesis in the adult

Gonadal hormones modulate neurogenesis in the dentate gyrus (DG) of adult rodents in complex ways. Estradiol, the most potent estrogen, initially enhances and subsequently suppresses cell proliferation in the dentate gryus of adult female rodents. Much less is known about how estradiol modulates neurogenesis in the adult male rodent; however, recent evidence suggests that estradiol may have a moderate effect on cell proliferation but enhances cell survival in the DG of newly synthesized cells but only when estradiol is administered during a specific stage in the cell maturation cycle in the adult male rodent. Testosterone likely plays a role in adult neurogenesis, although there have been no direct studies to address this. However, pilot studies from our laboratory suggest that testosterone up-regulates cell survival but not cell proliferation in the DG of adult male rats. Progesterone appears to attenuate the estradiol-induced enhancement of cell proliferation. Neurosteroids such as allopregnalone decrease neurogenesis in adult rodents, while pregnancy and motherhood differentially regulate adult neurogenesis in the adult female rodent. Very few studies have investigated the effects of gonadal hormones on male rodents; however, studies have indicated that there is a gender difference in the response to hormone-regulated hippocampal neurogenesis in the adult. Clearly, more work needs to be done to elucidate the effects of gonadal hormones on neurogenesis in the DG of both male and female rodents.     

Gonadal steroids regulate dendritic spine density in hippocampal pyramidal cells in adulthood

Gonadal steroids are known to influence hippocampal physiology in adulthood. It is presently unknown whether gonadal steroids influence the morphology of hippocampal neurons in the adult intact rat brain. In order to determine whether female sex hormones influence hippocampal morphology in the intact adult, we performed Golgi impregnation on brains from ovariectomized rats and ovariectomized rats which received estradiol or estradiol and progesterone replacement. Removal of circulating gonadal steroids by ovariectomy of adult female rats resulted in a profound decrease in dendritic spine density in CA1 pyramidal cells of the hippocampus. Estradiol replacement prevented the observed decrease in dendritic spine density; progesterone augmented the effect of estradiol within a short time period (5 hr). Ovariectomy or gonadal steroid replacement did not affect spine density of CA3 pyramidal cells or granule cells of the dentate gyrus. These results demonstrate that gonadal steroids are necessary for the maintenance of normal adult CA1 hippocampal pyramidal cell structure. The short time course required to observe these effects (3 d for the estradiol effect and 5 hr for the progesterone effect) implies that CA1 pyramidal cell dendritic spine density may fluctuate during the normal (4-5 d) rat estrous cycle.     

Estradiol tends to improve inhibitory avoidance performance in adrenalectomized male rats and reduces pyknotic cells in the dentate gyrus of adrenalectomized male and female rats

Estradiol or vehicle was administered daily to gonadectomized, adrenalectomized (ADX) male (experiment 1) and female (experiment 2) Long-Evans rats. Four and 5 days after ADX, respectively, animals were trained and tested in an inhibitory avoidance task. Male and female ADX rats had shorter cross over latencies on the test day than did nonADX rats. In males, estradiol administration to ADX rats improved inhibitory avoidance performance to levels that were comparable to nonADX, estradiol-administered male rats. No effects of estradiol were seen on inhibitory avoidance performance of female rats. In both males and females, ADX increased the number of pyknotic cells in the dentate gyrus compared to nonADX rats; estradiol also reduced pyknotic cell number compared to vehicle administration. These findings suggest that estradiol's effects on inhibitory avoidance and pyknosis in the dentate gyrus may be independent of each other.     

Neuroprotective effects of estradiol in newborn female rat hippocampus

Perinatal brain injury, consequent to hypoxic/ischemic events, is associated with the release of excess excitatory neurotransmitters, including glutamate. We have previously shown that administration of a glutamate receptor agonist, kainic acid (KA), to postnatal day 0 (PN0) and PN1 rats results in damage selective to the dentate gyrus of females. Pretreatment with the gonadal steroid estradiol prevents KA-induced damage to the female dentate gyrus. To begin to elucidate the cellular mechanism of the neuroprotective effects of estradiol in neonatal females, we have employed the estrogen receptor antagonists Tamoxifen and ICI 182,780 in vivo and in vitro, respectively. Peripheral administration of Tamoxifen, which crosses the blood-brain barrier, prevented estradiol-mediated neuroprotection against KA-induced damage in the dentate gyrus. The highly selective estrogen receptor antagonist ICI 182,780, which does not penetrate into the brain from the periphery, also prevented estradiol's protective effects on KA-induced cell death in cultured hippocampal neurons but only late in the time course of injury. The data suggest that the neuroprotection afforded by estradiol against KA-induced injury in the female is estrogen receptor mediated but may include an additional mechanism that is not antagonized at the receptor.     

Sex-related asymmetries in the morphology of the left and right hippocampi?

Cell densities were determined in left and right surgically removed hippocampal tissue of epileptic patients. Pyramidal cells were studied in CA1, CA4, and the dentate gyrus. Lower densities of nucleolated cells were found for males in the right CA1 and CA4 than on the left while there was no significant left-right difference in females. Moreover, we found a probable sex difference in intercorrelations of nucleolated cells among the three subfields. In males, they were positive and significant on the left while they were low on the right. In females, positive significant intercorrelations were obtained between some subfields and not between other subfields, on either side. The present findings suggest greater hippocampal lateralization in males than in females with higher hippocampal neuronal connectivity on the left in males than on the right.     

Autoradiographic localization of estradiol-binding neurons in the rat hippocampal formation and entorhinal cortex

This study has examined the distribution of [3H]estradiol and [1 alpha,2 alpha-3H]testosterone uptake in the hippocampal formation and entorhinal cortex of male and female rats. In both males and females, [3H]estradiol-binding neurons in Ammon's horn are located deep in stratum pyramidale and may correspond either to polymorphic interneurons or to early maturing pyramidal cells. Interneurons of strata oriens, lucidum and radiatum of Ammon's horn and of stratum moleculare of the subiculum also bind [3H]estradiol, as do basket cell interneurons in the polymorphic, infragranular layer of the dentate gyrus. While no granule cells appear to accumulate [3H]estradiol, these cells may be affected transsynaptically by gonadal steroids via their afferent contacts with the entorhinal cortex, which, of the areas examined, contains the greatest number of [3H]estradiol-binding neurons. While relatively few neurons concentrate [3H]estradiol in the hippocampal formation, these are localized to specific subpopulations, which may enhance their functional significance. Because there is no significant nuclear accumulation of [3H]-alpha-testosterone in either the entorhinal cortex or hippocampal formation, it appears that aromatase enzyme activity is not a major contributor to estrogen receptor occupancy in adult rats.     

Influence of estradiol, stress, and 5-HT2A agonist treatment on brain-derived neurotrophic factor expression in female rats.

BACKGROUND: Estradiol affects neuronal plasticity, mood, and cognition. We examined the effects of the estrous cycle, acute and chronic estradiol treatments on BDNF mRNA expression in the hippocampus and cortex of female rats. The roles of 5-HT2A receptors and of stress on the BDNF mRNA regulation were also explored. METHODS: BDNF mRNA levels were measured using in situ hybridization at proestrus and estrus, and following acute and chronic estradiol treatment of acutely and chronically ovariectomized (OVX) female rats. Some rats were pretreated with 5-HT2A agonist and antagonist, and another group was subjected to two-hour immobilization stress. RESULTS: BDNF mRNA levels in the dentate gyrus and the medial prefrontal cortex were decreased during estrus, when estradiol levels are highest. Acute estradiol treatment decreased hippocampal BDNF mRNA in acutely OVX rats, but neither acute nor chronic estradiol had effect in chronically OVX rats. Estradiol pretreatment reduced the 5-HT2A receptor-mediated cortical upregulation in BDNF mRNA and did not effect the stress-induced down-regulation of BDNF mRNA in the dentate gyrus. CONCLUSIONS: The duration of the estradiol treatment and the duration of the ovarian hormone deprivation are important factors in the regulation of BDNF synthesis and possibly in the functional outcome of estrogen treatment.     

Granule cells in aging rats are sexually dimorphic in their response to estradiol.

Normal aging comprises cognitive decline, including deterioration of memory. It has been suggested that this decline in memory is sexually dimorphic because of the cessation in gonadal steroid secretion that occurs during reproductive aging in female, but not male, mammals. We wondered whether neurons in brain regions associated with learning and memory underwent morphological changes that were dimorphic as well and whether cessation of the secretion of gonadal steroids influenced these morphological changes. To explore these questions, we deprived and restored estrogens to young and old gonadectomized females and males and studied the morphology of dentate granule cells by intracellular dye filling in a lightly fixed slice preparation. We found the following: (1) Aged female dentate granule cells deprived of gonadal steroids long-term have a paucity of dendritic spines compared with young females deprived short-term; however, aged male dentate granule cells deprived of gonadal steroids long-term have no decrease in dendritic spines compared with young males deprived short-term. (2) Aged female dentate granule cells with long-term estrogen replacement at either high or low levels still had a decline in spine density. (3) Aged female dentate granule cells with short-term estradiol replacement had spine density increased to levels normally observed in young adults, whereas aged males with short-term estradiol replacement had decreased spine density. These data suggest that the response of rat dentate granule cells to aging and estradiol is sexually dimorphic and that, in females, the responsiveness of granule cells depends on the temporal pattern of estradiol replacement.     

Influence of a natural stressor (predator odor) on locomotor activity in the meadow vole (Microtus pennsylvanicus): modulation by sex, reproductive condition and gonadal hormones

Sex differences in a variety of non-reproductive behaviors have been indicated to occur in seasonally breeding polygynous promiscuous rodents such as the meadow vole, Microtus pennsylvanicus. The present study was designed to assess the effects of reproductive and hormonal status on the locomotor responses of meadow voles following brief exposure to the odors of a natural predator, the Red fox (Vulpes vulpes). Adult male and female meadow voles, which are seasonal photoperiodically-induced breeders, were housed in either mixed sex pairs under a long, reproductively stimulatory photoperiod (simulating breeding: long light cycle, paired: LLC + P) or in same-sex pairs under a short, reproductively inhibitory photoperiod (simulated non-breeding: short light cycle, non-paired: SLC-NP). On 2 consecutive days following 1 day of baseline activity monitoring, voles were exposed individually for 3 min to fox odor and a novel pungent control odor (extract of almond). The levels of various measures of activity that were displayed by the voles were assessed by an automated Digiscan activity monitoring system. LLC + P (simulated breeding) voles displayed higher basal levels of activity relative to SLC + NP (simulated non-breeding) voles, with males displaying greater activity than females. LLC + P (simulated breeding) males displayed a significant reduction in activity levels following exposure to fox odor relative to control odor. The reductions in activity following fox odor exposure were related to plasma testosterone levels such that a larger behavioral response (i.e. greater reduction) was associated with higher levels of testosterone. Furthermore, dividing males into high and low testosterone groups based on the median levels of testosterone revealed that high but not low testosterone males displayed reductions in activity following exposure to fox odor relative to control odor. No changes in activity levels following exposure to fox odor were noted in SLC-NP males, and either SLC-NP or LLC + P females. These results show that this sexually dimorphic non-reproductive behavior is significantly influenced by reproductive condition and gonadal hormone levels.     

Density of mu-opioid receptors in the hippocampus of adult male and female rats is altered by prenatal morphine exposure and gonadal hormone treatment

The present in vitro autoradiography study demonstrates that prenatal exposure to morphine alters the density of mu-opioid receptors in the hippocampus of adult female but not adult male rats. Prenatal morphine exposure increased the mu-opioid receptor density in the CA1 of ovariectomized (OVX) females and in the CA3 of OVX, estradiol benzoate-plus progesterone (EB+P)-treated females, but decreased it in CA3 of OVX females. There were also hormonal effects on mu-opioid receptor density in adult female rats. In the CA1, only morphine-exposed but not saline-exposed, hormone-treated females (EB, P, or EB+P) had a decrease in mu-opioid receptor density relative to OVX females. Both saline-exposed and morphine-exposed, OVX females after gonadal hormone replacement had a lower density of mu-opioid receptors in the CA3 and in the dentate gyrus (DG) than OVX females. In male rats, there was a decrease in mu-opioid receptor density in the CA1 and CA3 of gonadectomized (GNX), testosterone 17beta-proprionate (TP)-treated males relative to GNX males regardless of prenatal morphine exposure. In the DG, the mu-opioid receptor density was reduced only in morphine-exposed but not in saline-exposed, TP-treated males compared with GNX males. Thus, our data demonstrate that mu-opioid receptor density in the hippocampus is affected by prenatal morphine exposure and by male and female gonadal hormones.     

Sex differences in the effects of adolescent stress on adult brain inflammatory markers in rats

Both basic and clinical research indicates that females are more susceptible to stress-related affective disorders than males. One of the mechanisms by which stress induces depression is via inflammatory signaling in the brain. Stress during adolescence, in particular, can also disrupt the activation and continued development of both the hypothalamic–pituitary–adrenal (HPA) and –gonadal (HPG) axes, both of which modulate inflammatory pathways and brain regions involved in affective behavior. Therefore, we tested the hypothesis that adolescent stress differentially alters brain inflammatory mechanisms associated with affective-like behavior into adulthood based on sex. Male and female Wistar rats underwent mixed-modality stress during adolescence (PND 37–48) and were challenged with lipopolysaccharide (LPS; 250 μg/kg, i.p.) or saline 4.5 weeks later (in adulthood). Hippocampal inflammatory marker gene expression and circulating HPA and HPG axes hormone concentrations were then determined. Despite previous studies indicating that adolescent stress induces affective-like behaviors in female rats only, this study demonstrated that adolescent stress increased hippocampal inflammatory responses to LPS in males only, suggesting that differences in neuroinflammatory signaling do not drive the divergent affective-like behaviors. The sex differences in inflammatory markers were not associated with differences in corticosterone. In females that experienced adolescent stress, LPS increased circulating estradiol. Estradiol positively correlated with hippocampal microglial gene expression in control female rats, whereas adolescent stress negated this relationship. Thus, estradiol in females may potentially protect against stress-induced increases in neuroinflammation.     

Repetitive noxious neonatal stimuli increases dentate gyrus cell proliferation and hippocampal brain‐derived neurotrophic factor levels

Neonatal noxious stimulation has been proposed to model pain triggered by diagnostic/therapeutic invasive procedures in premature infants. Previous studies have shown that hippocampal neurogenesis rate and the behavioral repertoire of adult rats may be altered by neonatal noxious stimuli. The purpose of this study was to evaluate whether noxious stimulation during neonatal period alters the nociceptive response and dentate gyrus neurogenesis when compared to rats subjected to a single noxious stimulus in late infancy. Plasma corticosterone and hippocampal brain‐derived neurotrophic factor (BDNF) levels were measured. Neurogenesis in the dentate gyrus was evaluated in adolescent rats (postnatal day 40; P40) exposed twice to intra‐plantar injections of Complete Freund's adjuvant (CFA) on P1 and P21 (group P1P21) or P8 and P21 (P8P21) or exposed once on P21 (pubertal). On P21, one subset of animals received 5‐bromo‐2′‐deoxyuridine (BrdU) and was euthanized on P40 for identification of proliferating cells in the dentate gyrus. Another subset was sampled for thermal response or plasma corticosterone measurement and hippocampal BDNF levels. Proliferative cell rate in dentate gyrus was the highest in all re‐exposed groups (P < 0.001), except for P8 females (P8P21F), revealing also a sex difference, where P8P21 males showed higher rate than females (P < 0.001). Stimulated groups took longer than CTL animals to lick the paws (P < 0.001), regardless of the age when the noxious stimulus was applied. Re‐exposed groups had lower corticosterone plasma level (P1P21 M and F, P8P21M) than controls. On the contrary, hippocampal BDNF was increased in males from both re‐exposed groups. These results show that infant noxious stimulation in neonatally previously stimulated rats is related to high proliferation in the DG and this association seems to be modified by the animal's sex. The new generated dentate granule cells in the hippocampus may have a role in the long‐term behavioral responses to neonatal nociceptive stimulation. Noxious stimulation in the neonatal period results in sex‐dependent neurogenic response. © 2013 Wiley Periodicals, Inc.     

"Tectonic" hippocampal malformations in patients with temporal lobe epilepsy

Histological analysis of hippocampi removed en bloc during surgical treatment of temporal lobe epilepsy revealed a subgroup of patients with bulbous expansions of the CA1 pyramidal cell/subicular layers that were consistently accompanied by "tectonic" invaginations of the adjacent dentate gyrus. Most hippocampi containing the CA1/subicular anomaly and the tectonically deformed dentate gyrus exhibited minor cell loss compared to hippocampi with typical hippocampal sclerosis, and retrospective analysis revealed that conventional imaging methods usually failed to detect subtle hippocampal atrophy or abnormal signal characteristics in patients with this anomaly. Cells within the anomaly exhibited the spherical appearance of undifferentiated pyramidal layer neurons, and were immunopositive for the neuronal marker NeuN. Immunostaining for the synaptic marker beta-synuclein suggested abnormal dentate gyrus lamination in segments containing the pyramidal cell layer anomaly, but not in unaffected areas of the same specimens. Despite differences in the extent of neuronal loss between patients with hippocampal sclerosis and those with the CA1/subicular anomaly, the incidence of antecedent febrile seizures was similar in both groups. In a comparison group of hippocampi obtained at autopsy, structural irregularities were evident, but were consistently less disruptive to hippocampal architecture than the anomalies observed in epilepsy patients. We hypothesize that developmental malformation of the CA1 pyramidal cell/subicular layers may adversely influence the subsequent development of the adjacent dentate gyrus, and may render temporal lobe structures hyperexcitable and more vulnerable to relatively innocuous seizures and injuries. Thus, these presumably developmental hippocampal anomalies may serve as substrates for early febrile seizures and subsequent epilepsy.     

Sex and species differences in tyrosine hydroxylase-synthesizing cells of the rodent olfactory extended amygdala

The bed nucleus of the stria terminalis (BST) and the medial amygdala (MeA) are anatomically connected sites necessary for chemosensory regulation of social behaviors in rodents. Prairie voles (Microtus ochrogaster) are a valuable model for studying the neural regulation of social behaviors because, unlike many other rodents, they are gregarious, pair bond after copulating, and are biparental. We herein describe sex and species differences in immunoreactivity for tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine synthesis, in the BST and MeA. Virgin male prairie voles had a large number of TH-immunoreactive cells in areas analogous to the rat principal nucleus of the BST (pBST) and the posterodorsal medial amygdala (MeAPd). Virgin female prairie voles had far fewer TH-immunoreactive cells in these sites ( approximately 17% of the number of cells as males in the pBST, approximately 35% of the number of cells in the MeAPd). A few TH-immunoreactive cells were found in the BST of male and female hamsters and meadow voles, but not in rats. The MeApd also contained a few TH-immunoreactive cells in male and female hamsters and male meadow voles, but not rats. Castration greatly reduced the number of TH-immunoreactive cells in the male prairie vole pBST and MeAPd, an effect that could be reversed with testosterone. Furthermore, treating ovariectomized females with testosterone substantially increased TH-immunoreactive cells in both sites. Therefore, a species-specific sex difference in TH expression is found in a chemosensory pathway in prairie voles. Expression of TH in these sites is influenced by circulating gonadal hormones in adults, which may be related to changes in their display of social behaviors across the reproductive cycle.     

A retrospective analysis of hippocampal pathology in human temporal lobe epilepsy: evidence for distinctive patient subcategories

PURPOSE: This study is a retrospective analysis of the pathology of the hippocampus from patients with medically intractable temporal lobe epilepsy. We attempted to relate neuronal density, immunohistochemistry, electrophysiologic data, and surgical outcome. METHODS: Immunostaining patterns for neuropeptide Y, somatostatin, substance P, and dynorphin defined the immunohistochemical characteristics of the hippocampi. Neuronal densities were determined by microscopic cell counts. Sharp electrode recordings from dentate granule cells determined measures of inhibition and excitation. RESULTS: Patient hippocampi without evidence of sclerosis generally resembled autopsy controls on the basis of neuronal densities of hippocampal subfields and patterns of immunostaining. The nonsclerotic hippocampi were divisible into two subgroups on the basis of neuronal density correlations between hippocampal subfields, the excitability of dentate granule cells, etiology, and surgical outcome. Hippocampi with sclerosis were divisible into those with significant neuronal loss confined to area CA1 and those with neuronal loss throughout the hippocampus and dentate gyrus. In the former, the dentate gyrus resembled in morphology the nonsclerotic hippocampi but with slightly increased excitability of the dentate granule cells. The hippocampi with more extensive neuronal loss had changes in immunostaining patterns associated with the dentate gyrus, correlated with significant hyperexcitability of dentate granule cells. The surgical outcome, with the exception of one group, was good in approximately 70-90%. CONCLUSIONS: Hippocampi from patients with intractable temporal lobe epilepsy can be assigned to several groups on the basis of pathophysiology. Different pathologies may represent differing causative mechanisms of intractable temporal lobe epilepsy and be predictive of surgical outcome.     

Influence of gonadal hormones on the development of parental behavior in adult virgin prairie voles (Microtus ochrogaster)

Prairie voles (Microtus ochrogaster) are a socially monogamous species and both sexes are parental after the birth of pups. In contrast, sexually inexperienced adult prairie voles differ in their behavior towards pups such that virgin males are paternal whereas virgin females are often infanticidal. To test whether there exists a discrete perinatal 'sensitive period' during which gonadal hormones influence this behavior, and to distinguish between the relative contributions of estrogenic and androgenic mechanisms to this influence, prairie voles were exposed to testosterone propionate (TP), the anti-androgen flutamide, or the aromatase inhibitor 1,4,6-androstatriene-3,17-doine (ATD) either prenatally via their pregnant dam for the last 15-19 days of the 22-day gestational period or postnatally on days 1-7. None of the treatments altered the high paternal responsiveness of males or the high infanticide rate in females when compared with controls. Females exposed prenatally to ATD, however, had levels of parental behavior that were significantly higher than the lowest levels observed in prenatally TP-treated females. These results suggest that sex differences in the parental behavior of adult virgin prairie voles are not generated exclusively by androgenic or estrogenic mechanisms during a restricted prenatal or early postnatal 'sensitive period' and that the parental behavior of virgin females may be more susceptible to any influence of gonadal hormones during development than males.     

Long-Term Adrenalectomy can Decrease or Increase Hippocampal Dentate Gyrus Volumes

Male and female Long-Evans adult rats were adrenalectomized and sacrificed 6 weeks later to determine whether dentate gyrus damage would differ in females and males. A subset of adrenalectomized rats of both sexes had significantly reduced dentate gyrus volumes compared to the same sex SHAM operated rats. The remainder of the male and female adrenalectomized rats which did not have clear dentate gyrus damage had significantly larger dentate gyrus volumes compared to the same sex SHAM rats. The dentate gyrus volumes of all adrenalectomized rats were significantly correlated with two indices of residual hormonal levels (Na⁺/K⁺ ratios and body weight gain 6 weeks after surgery), indicating that endogenous corticosterone levels may be a determining factor in the response of the dentate gyrus to adrenalectomy. These dentate gyrus volumetric changes could not be attributed to tissue shrinkage as there were no changes in CA3 volumes in any of the groups. These results suggest that long-term adrenalectomy can result in either increased or decreased dentate gyrus volumes and that the adrenal steroid levels of each individual adrenalectomized rat may be the factor determining the direction of the dentate gyrus volumetric response.     

Gonadal hormones down-regulate reactive gliosis and astrocyte proliferation after a penetrating brain injury

Astrocytes are a target for gonadal steroids in the normal brain. The putative modulation by gonadal hormones of the astrocytic reaction to brain injury was assessed in this study. Male and female adult Wistar albino rats were gonadectomized and, one month later, their brains were lesioned by a longitudinal incision crossing the parietal cerebral cortex, the CA1 field of the dorsal hippocampus and the dentate gyrus. Males were injected either with testosterone (20 μg/rat) or vehicle immediately after surgery. Females were injected either with 17β estradiol (250 μg/rat), progesterone (500 μg/rat) or vehicle. Hormonal injections were repeated 24 and 48 h after brain injury. All animals received injections of 5′-bromodeoxyuridine (BrdU) to label proliferating cells. Histological sections from the brain of animals killed 72 h after surgery were used for the double immunohistochemical localization of BrdU and glial fibrillary acidic protein (GFAP). The number of GFAP-immunoreactive astrocytes and the number of double labelled astrocytes (GFAP+BrdU) were recorded as a function of the distance to the lesion site in the parietal cerebral cortex, the CA1 field of the hippocampus and the dentate gyrus. Testosterone, estradiol and progesterone treatments resulted in a significant decrease in the number of GFAP-immunolabelled reactive astrocytes in the vicinity of the wound. The number of double labelled cells and the labelling index (proportion of GFAP-immunoreactive astrocytes labelled with BrdU) varied according to the cerebral area, the distance to the wound and the sex of the animals, and were significantly decreased by gonadal steroids in all the areas examined. These results ndicate that gonadal hormones may decrease gliosis and astrocyte proliferation after a penetrating brain injury.     

Sex and ovarian steroids modulate brain-derived neurotrophic factor (BDNF) protein levels in rat hippocampus under stressful and non-stressful conditions

Abnormal levels of brain-derived neurotrophic factor (BDNF) are associated with major depression, a disorder with a higher incidence in women than men. Stress affects BDNF levels in various brain regions and thus, a heightened stress response in females could contribute to the development of depression. As well, ovarian hormones directly affect brain levels of BDNF mRNA and protein. Two experiments were performed to investigate the effects of stress and sex and gonadal hormones on BDNF protein levels in CA1, CA3, and dentate gyrus (DG) subregions of the hippocampus. In the first experiment, male and female Sprague-Dawley rats were subjected to one hour of restraint stress or control handling prior to sacrifice. In the second experiment, fifty-one female rats were ovariectomized and separated into stress and control conditions, as described for the first experiment. Stressed and handled groups received a single injection of estrogen (E; 53h prior to stress), estrogen and progesterone (EP; E given at 53h and P given 5h prior to stress), or vehicle (OVX). In both experiments BDNF protein was quantified using an enzyme-linked immunosorbent enzyme assay (ELISA) in micropunches of hippocampus. Gonadally intact females had significantly higher levels of BDNF in CA3, but significantly lower levels in DG, relative to males. In CA3, stress significantly decreased BDNF in both males and females. In DG of ovariectomized female rats, the effects of stress were significantly different following EP vs. vehicle treatment. Thus, stress increased BDNF levels in EP-treated rats but decreased BDNF levels in vehicle-treated rats. Reduced trophic support in DG in the presence of estrogen and progesterone could jeopardize neurogenesis and under certain conditions could be a contributing factor to the hippocampal atrophy associated with stress-induced affective disorders. These results emphasize the need to consider sex, gonadal steroids, and hippocampal subregion when examining the effects of stress on the brain.