Expression and estrogen regulation of brain-derived neurotrophic factor gene and protein in the forebrain of female prairie voles

Dynamic regulation of BDNF gene expression by estradiol and lncRNA HOTAIR

Brain derived neurotrophic factor (BDNF) is a major neurotransmitter that controls growth and maintenance of neurons and its misregulation is linked to neurodegeneration and human diseases. Estradiol (E2) is well-known to regulate the process of differentiation and plasticity of hippocampal neurons. Here we examined the mechanisms of BDNF gene regulation under basal conditions and under stimuli such as E2. Our results demonstrated that BDNF expression is induced by E2 in vitro in HT22 cells (hippocampal neuronal cells) and in vivo (in ovariectomized mouse brain under E2-treatment). Using chromatin immunoprecipitation assay, we demonstrated that estrogen receptors (ERα, ERβ) were enriched at the BDNF promoter in presence of E2. Additionally, ER-coregulators (e.g., CBP/p300, MLL3), histone acetylation, H3K4-trimethylation, and RNA polymerase II levels were also elevated at the BDNF promoter in an E2-dependent manner. Additionally, under the basal conditions (in the absence of E2), the long noncoding RNA HOTAIR and its interacting partners PRC2 and LSD1 complexes binds to the promoter of BDNF and represses its expression. HOTAIR knockdown -relieves the repression resulting in elevation of BDNF expression. Further, levels of HOTAIR-interacting partners, EZH2 and LSD1 were reduced at the BDNF promoter upon HOTAIR-knockdown revealing that HOTAIR plays a regulatory role in BDNF gene expression by modulating promoter histone modifications. Additionally, we showed that E2 induced-BDNF expression is mediated by the displacement of silencing factors, EZH2 and LSD1 at BDNF promoter and subsequent recruitment of active transcription machinery. These results reveal the mechanisms of BDNF gene regulation under the basal condition and in presence of a positive regulator such as E2 in neuronal cells.

Parvalbumin as a sex-specific target in Alzheimer's disease research - A mini-review

Alzheimer's disease (AD) is the most common form of dementia, and both the incidence of this disease and its associated cognitive decline disproportionally effect women. While the etiology of AD is unknown, recent work has demonstrated that the balance of excitatory and inhibitory activity across the brain may serve as a strong predictor of cognitive impairments in AD. Across the cortex, the most prominent source of inhibitory signalling is from a class of parvalbumin-expressing interneurons (PV+). In this mini-review, the impacts of sex- and age-related factors on the function of PV+ neurons are examined within the context of vulnerability to AD pathology. These primary factors of influence include changes in brain metabolism, circulating sex hormone levels, and inflammatory response. In addition to positing the increased vulnerability of PV+ neurons to dysfunction in AD, this mini-review highlights the critical importance of presenting sex stratified data in the study of AD.

Short-Term Irisin Treatment Enhanced Neurotrophin Expression Differently in the Hippocampus and the Prefrontal Cortex of Young Mice

As a result of physical exercise, muscle releases multiple exerkines, such as "irisin", which is thought to induce pro-cognitive and antidepressant effects. We recently demonstrated in young healthy mice the mitigation of depressive behaviors induced by consecutive 5 day irisin administration. To understand which molecular mechanisms might be involved in such effect, we here studied, in a group of mice previously submitted to a behavioral test of depression, the gene expression of neurotrophins and cytokines in the hippocampus and prefrontal cortex (PFC), two brain areas frequently investigated in the depression pathogenesis. We found significantly increased mRNA levels of nerve growth factor (NGF) and fibroblast growth factor 2 (FGF-2) in the hippocampus and brain-derived growth factor (BDNF) in the PFC. We did not detect a difference in the mRNA levels of interleukin 6 (IL-6) and IL-1β in both brain regions. Except for BDNF in the PFC, two-way ANOVA analysis did not reveal sex differences in the expression of the tested genes. Overall, our data evidenced a site-specific cerebral modulation of neurotrophins induced by irisin treatment in the hippocampus and the PFC, contributing to the search for new antidepressant treatments targeted at single depressive events with short-term protocols.

A High Throughput Cell-Based Screen Assay for LINE-1 ORF1p Expression Inhibitors Using the In-Cell Western Technique

Long interspersed nuclear element 1 (LINE-1) is a dominant autonomous retrotransposon in human genomes which plays a role in affecting the structure and function of somatic genomes, resulting in human disorders including genetic disease and cancer. LINE-1 encoded ORF1p protein which possesses RNA-binding and nucleic acid chaperone activity, and interacts with LINE-1 RNA to form a ribonucleoprotein particle (RNP). ORF1p can be detected in many kinds of tumors and its overexpression has been regarded as a hallmark of histologically aggressive cancers. In this study, we developed an In-Cell Western (ICW) assay in T47D cells to screen the compounds which can decrease the expression of ORF1p. Using this assay, we screened 1,947 compounds from the natural products library of Target Mol and Selleckchem, among which three compounds, Hydroxyprogesterone, 2,2':5′,2″-Terthiophene and Ethynyl estradiol displayed potency in diminishing LINE-1 ORF1p expression level. Further mechanistic studies indicated the compounds act by affecting LINE-1 RNA transcription. Notably, we demonstrated that the compounds have an inhibitory effect on the proliferation of several lung and breast cancer cell lines. Taken together, we established a high throughput screening system for ORF1p expression inhibitors and the identified compounds provide some clues to the development of a novel anti-tumor therapeutic strategy by targeting ORF1p.

Xenoestrogens impact brain estrogen receptor signaling during the female lifespan: A precursor to neurological disease?

Xenoestrogens, foreign synthetic chemicals mimicking estrogens, are lurking in our surroundings. Climate change may alter their toxicity and bioavailability. Since xenoestrogens have extremely high lipid solubility and are structurally similar to natural endogenous estrogens, they can bind to estrogen receptors (ERs) -alpha (ER-α) and -beta (ER-β). Scientific evidence accumulated over the past decades have suggested that natural 17β-estradiol (E2; a potent estrogen), via activation of its receptors, plays a pivotal role in regulation of brain development, differentiation, metabolism, synaptic plasticity, neuroprotection, cognition, anxiety, body temperature, feeding and sexual behavior. In the brain, ER-β is predominantly expressed in the various regions, including cerebral cortex and hippocampus, that have been shown to play a key role in cognition. Therefore, disturbances in function of ER-β mediated E2 signaling by xenoestrogens can lead to deleterious effects that potentiate a variety of neurological diseases starting from prenatal to post-menopause in women. The goal of this review is to identify the possible neurological effects of xenoestrogens that can alter estrogen receptor-mediated signaling in the brain during different stages of the female lifespan.

The different role of G-protein-coupled receptor 30 (GPR30) in the interaction effects of marijuana and estradiol on spatial learning and memory at different ages

Some studies suggest that the effect of cannabis on behavior performance depends on the presence of ovarian hormones and the age of use initiation. Estradiol is the main ovarian hormone that can interact with cannabinoids. It has been suggested that cannabinoids exert some of their effects directly through estrogen receptors (ERs). A novel G-protein-coupled receptor (GPR30) was described as mediating estrogen signaling in various cell lines. Since there are few studies on the interaction of cannabis and ovarian hormones on cognitive behaviors, so, this study evaluated the role of GPR30 in the effects of marijuana (M) and estrogen, alone and in combination, on spatial learning and memory of young (non-ovarian(OVX)) and old female rats. Young (5-7 months) and old (22-24 months) female rats received an intraperitoneal injection (i.p) of 17β-estradiol (E2), G1 (GPR30 agonist), and G15 (GPR30 antagonist) every four days, and M (every day), either alone or in combination, for 28 days. One hour after the last injection, the Morris water maze (MWM) test was conducted to evaluate of spatial learning and memory. Moreover, hippocampal BDNF level was assessed by the ELISA method. The results showed a positive effect of M on spatial learning in both young and old rats, however, E2 showed beneficial effects on the memory of young, but not old rats. Our results showed that GPR30 does not have any role in the interaction effects of M and E2 in young rats. Although both E2 and M alone showed positive effects on spatial learning and memory in old rats, however, our results showed a negative interaction between marijuana and E2 combined effects on spatial learning and memory in old female rats which is mediated by GPR30. Our results showed that the effects of GPR30 on spatial learning and memory is age dependent. Furthermore, this study showed that hippocampal BDNF does not have any role in the interaction effects of M and E2 on spatial learning and memory in young and old rats.

Previous estradiol treatment during midlife maintains transcriptional regulation of memory-related proteins by ERα in the hippocampus in a rat model of menopause

Previous midlife estradiol treatment, like continuous treatment, improves memory and results in lasting increases in hippocampal levels of estrogen receptor (ER) α and ER-dependent transcription in ovariectomized rodents. We hypothesized that previous and continuous midlife estradiol act to specifically increase levels of nuclear ERα, resulting in transcriptional regulation of proteins that mediate estrogen effects on memory. Ovariectomized middle-aged rats received estradiol or vehicle capsule implants. After 40 days, rats initially receiving vehicle received another vehicle capsule (ovariectomized controls). Rats initially receiving estradiol received either another estradiol (continuous estradiol) or a vehicle (previous estradiol) capsule. One month later, hippocampi were dissected and processed. Continuous and previous estradiol increased levels of nuclear, but not membrane or cytosolic ERα and had no effect on Esr1. Continuous and previous estradiol impacted gene expression and/or protein levels of mediators of estrogenic action on memory including ChAT, BDNF, and PSD-95. Findings demonstrate a long-lasting role for hippocampal ERα as a transcriptional regulator of memory following termination of previous estradiol treatment in a rat model of menopause.

Marijuana and β-estradiol interactions on spatial learning and memory in young female rats: Lack of role of the G protein-coupled estrogen receptor (GPR30)

It has been shown that 17β-estradiol (E2) hormone is an essential biological factor for increasing the sensitivity of women to drug abuse. Recent studies have shown a potential overlap between the molecular pathways of cannabinoids and ovarian hormones. The current study evaluated the interference between the marijuana and E2 effect on spatial learning and memory and the role of the G protein-coupled estrogen receptor (GPR30) in young female rats. The animals were separated into two main groups: intact-ovary and ovariectomized (OVX) rats. The latter group received intraperitoneal injections of E2, G-1 (GPR30 agonist), G-15 (GPR30 antagonist), marijuana, and different combinations of these substances for 28 days. Spatial learning and memory were evaluated by the Morris water maze (MWM) test. We also assessed the BDNF (brain-derived neurotrophic factor) concentration and the hippocampal level of GPR30. The results showed a significant reduction of spatial learning and memory in OVX rats compared to intact-ovary rats, which were restored by E2 replacement. Moreover, treatment with G-1 mimicked E2 effects on spatial learning and memory. Marijuana impaired spatial learning and memory in intact-ovary rats, while improved in OVX rats. We also found that treatment with M + E2 induced significant impairment in spatial learning and memory; however, treatment with M + G1 and M + G15 + E2 showed no significant difference. No significant differences in BDNF expression were observed in experimental groups. These results suggest that marijuana and E2 interact in their effect on spatial learning and memory in young female rats, but GPR30 seems to play no role in this interaction.

17β-estradiol activation of dorsal hippocampal TrkB is independent of increased mature BDNF expression and is required for enhanced memory consolidation in female mice

The potent estrogen 17β-estradiol (E₂) is known to enhance hippocampal memory and plasticity, however the molecular mechanisms underlying these effects remain unclear. Brain derived neurotrophic factor (BDNF) and its receptor tropomyosin receptor kinase B (TrkB) are regulated by E₂, but the potential mechanistic roles of neurotrophic signaling in E₂-induced enhancement of memory are not well understood. Here, we examined the effects of hippocampal TrkB signaling on E₂-induced enhancement of memory consolidation in the object placement and recognition tasks. Bilateral infusion of the TrkB antagonist ANA-12 into the dorsal hippocampus of ovariectomized female mice blocked E₂-induced enhancement of memory consolidation, supporting a role for TrkB-mediated signaling in estrogenic regulation of memory. Although dorsal hippocampal E₂ infusion increased levels of phospho-TrkB and mature BDNF (mBDNF) in the dorsal hippocampus within 4-6 h, E₂-induced increases in hippocampal mBDNF expression were not required for hippocampal TrkB activation and were not inhibited by TrkB antagonism. Thus, E₂ regulates TrkB signaling to facilitate memory consolidation in a manner independent of mBDNF expression. Together these results provide the first direct evidence that E₂ modulation of hippocampal TrkB signaling is required for its beneficial effects on memory consolidation and provide additional characterization of the ways in which TrkB/BDNF signaling is regulated by E₂ in the hippocampus.

Brain-Derived Neurotrophic Factor/Tropomyosin Receptor Kinase B Signaling Controls Excitability and Long-Term Depression in Oval Nucleus of the BNST

Dysregulation of proteins involved in synaptic plasticity is associated with pathologies in the CNS, including psychiatric disorders. The bed nucleus of the stria terminalis (BNST), a brain region of the extended amygdala circuit, has been identified as the critical hub responsible for fear responses related to stress coping and pathologic systems states. Here, we report that one particular nucleus, the oval nucleus of the BNST (ovBNST), is rich in brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) receptor. Whole-cell patch-clamp recordings of neurons from male mouse ovBNST in vitro showed that the BDNF/TrkB interaction causes a hyperpolarizing shift of the membrane potential from resting value, mediated by an inwardly rectifying potassium current, resulting in reduced neuronal excitability in all major types of ovBNST neurons. Furthermore, BDNF/TrkB signaling mediated long-term depression (LTD) at postsynaptic sites in ovBNST neurons. LTD of ovBNST neurons was prevented by a BDNF scavenger or in the presence of TrkB inhibitors, indicating the contribution to LTD induction. Our data identify BDNF/TrkB signaling as a critical regulator of synaptic activity in ovBNST, which acts at postsynaptic sites to dampen excitability at short and long time scales. Given the central role of ovBNST in mediating maladaptive behaviors associated with stress exposure, our findings suggest a synaptic entry point of the BDNF/TrkB system for adaptation to stressful environmental encounters.

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.

Effects of estrogen deprivation on memory and expression of related proteins in ovariectomized mice

Background

The ovariectomized (OVX) rodent model is most widely used for studying the influence of estrogen deprivation on memory. However, the results of these studies are inconsistent, in that the memory of OVX rodents shows either impairment or no change. These inconsistent outcomes increase the difficulty of researching neurochemical mechanisms and evaluating drug efficacy. One possible explanation for these discrepancies might be that the time point for memory examination after OVX varies considerably among studies. The aim of our study was to investigate the effects of estrogen deprivation on memory and the expression of memory-related proteins at different times after OVX.

Methods

Novel object recognition (NOR), step-through passive avoidance (STPA) and the Morris water maze (MWM) were performed to evaluate the memory performance of mice at different times after OVX. The expressions of BDNF, TrkB, ULK1 and LC3II/LC3I in the hippocampus were also assessed to explore the relevant mechanisms.

Results

After OVX, a significant memory impairment was found in the STPA test at 4 weeks. In the NOR and MWM tests, however, memory deficits were not observed until 8 weeks post-OVX. Interestingly, at 8 weeks, a memory rebound was found in the STPA test. In the hippocampus, the levels of BDNF and TrkB in OVX mice were markedly decreased at 4 and 8 weeks. Subsequently, a significant decrease in the ULK1 and LC3II/LC3I level in OVX mice was observed at 8 weeks.

Conclusions

Memory impairment in mice was observed as early as 4 weeks after OVX, although there was a possibility of memory rebound with the prolongation of estrogen deprivation. Eight weeks of estrogen deprivation would be more likely to induce hippocampus-dependent memory impairment. This progressive impairment of memory might be due to the downregulation of the BDNF/TrkB signaling pathway at the early post-OVX stage, while the decrease of autophagy level in the later stage might also contribute to these progressive alterations. The underlying relationship between the BDNF/TrkB signaling pathway and autophagy in this progressive impairment of memory requires further study.

Acute ovarian hormone treatment in freely cycling female rats regulates distinct aspects of heroin seeking

Females are at higher risk for certain opioid addictive behaviors, but the influence of ovarian hormones is unknown. In our rat model of heroin self-administration, females exhibited higher relapse rates that correlated with rates of heroin seeking on the first extinction session. We administered estradiol alone, or in combination with progesterone, 30 min prior to the first extinction session in freely cycling, heroin-seeking female rats. Although neither treatment produced long-term effects on relapse, each treatment regulated distinct aspects of heroin seeking. Estradiol treatment enhanced extinction memory retention, whereas the combination treatment acutely reduced expression of heroin seeking.

Infralimbic Estradiol Enhances Neuronal Excitability and Facilitates Extinction of Cocaine Seeking in Female Rats via a BDNF/TrkB Mechanism

Women are more susceptible to developing cocaine dependence than men, but paradoxically, are more responsive to treatment. The potent estrogen, 17β-estradiol (E₂), mediates these effects by augmenting cocaine seeking but also promoting extinction of cocaine seeking through E₂'s memory-enhancing functions. Although we have previously shown that E₂ facilitates extinction, the neuroanatomical locus of action and underlying mechanisms are unknown. Here we demonstrate that E₂ infused directly into the infralimbic-medial prefrontal cortex (IL-mPFC), a region critical for extinction consolidation, enhances extinction of cocaine seeking in ovariectomized (OVX) female rats. Using patch-clamp electrophysiology, we show that E₂ may facilitate extinction by potentiating intrinsic excitability of IL-mPFC neurons. Because the mnemonic effects of E₂ are known to be regulated by brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin-related kinase B (TrkB), we examined whether BDNF/TrkB signaling was necessary for E₂-induced enhancement of excitability and extinction. We found that E₂-mediated increases in excitability of IL-mPFC neurons were abolished by Trk receptor blockade. Moreover, blockade of TrkB signaling impaired E₂-facilitated extinction of cocaine seeking in OVX female rats. Thus, E₂ enhances IL-mPFC neuronal excitability in a TrkB-dependent manner to support extinction of cocaine seeking. Our findings suggest that pharmacological enhancement of E₂ or BDNF/TrkB signaling during extinction-based therapies would improve therapeutic outcome in cocaine-addicted women.

Increased plasma Brain-Derived Neurotrophic Factor (BDNF) levels in females with schizophrenia

Brain-Derived Neurotrophic Factor (BDNF) acts as a critical regulator of synaptogenesis and synaptic plasticity. Sex differences have been demonstrated in many aspects of schizophrenia. This study tested for sex-specific differences in peripheral BDNF levels in people with schizophrenia and healthy controls. We measured circulating plasma BDNF levels in 95 people with schizophrenia and 80 healthy controls. Plasma BDNF levels were significantly elevated in females with schizophrenia compared to males with schizophrenia and to female healthy controls. These results suggest that sex differences in peripheral BDNF levels may contribute to other sex related differences in schizophrenia.

Sex differences in neural mechanisms mediating reward and addiction

There is increasing evidence in humans and laboratory animals for biologically based sex differences in every phase of drug addiction: acute reinforcing effects, transition from occasional to compulsive use, withdrawal-associated negative affective states, craving, and relapse. There is also evidence that many qualitative aspects of the addiction phases do not differ significantly between males and females, but one sex may be more likely to exhibit a trait than the other, resulting in population differences. The conceptual framework of this review is to focus on hormonal, chromosomal, and epigenetic organizational and contingent, sex-dependent mechanisms of four neural systems that are known-primarily in males-to be key players in addiction: dopamine, mu-opioid receptors (MOR), kappa opioid receptors (KOR), and brain-derived neurotrophic factor (BDNF). We highlight data demonstrating sex differences in development, expression, and function of these neural systems as they relate-directly or indirectly-to processes of reward and addictive behavior, with a focus on psychostimulants and opioids. We identify gaps in knowledge about how these neural systems interact with sex to influence addictive behavior, emphasizing throughout that the impact of sex can be highly nuanced and male/female data should be reported regardless of the outcome.

Brain-derived neurotrophic factor and addiction: Pathological versus therapeutic effects on drug seeking

Many abused drugs lead to changes in endogenous brain-derived neurotrophic factor (BDNF) expression in neural circuits responsible for addictive behaviors. BDNF is a known molecular mediator of memory consolidation processes, evident at both behavioral and neurophysiological levels. Specific neural circuits are responsible for storing and executing drug-procuring motor programs, whereas other neural circuits are responsible for the active suppression of these "seeking" systems. These seeking-circuits are established as associations are formed between drug-associated cues and the conditioned responses they elicit. Such conditioned responses (e.g. drug seeking) can be diminished either through a passive weakening of seeking- circuits or an active suppression of those circuits through extinction. Extinction learning occurs when the association between cues and drug are violated, for example, by cue exposure without the drug present. Cue exposure therapy has been proposed as a therapeutic avenue for the treatment of addictions. Here we explore the role of BDNF in extinction circuits, compared to seeking-circuits that "incubate" over prolonged withdrawal periods. We begin by discussing the role of BDNF in extinction memory for fear and cocaine-seeking behaviors, where extinction circuits overlap in infralimbic prefrontal cortex (PFC). We highlight the ability of estrogen to promote BDNF-like effects in hippocampal-prefrontal circuits and consider the role of sex differences in extinction and incubation of drug-seeking behaviors. Finally, we examine how opiates and alcohol "break the mold" in terms of BDNF function in extinction circuits.

Estrogenic mediation of serotonergic and neurotrophic systems: implications for female mood disorders

Clinical research has demonstrated a significant sex difference in the occurrence of depressive disorders. Beginning at pubertal onset, women report a higher incidence of depression than men. Women are also vulnerable to the development of depressive disorders such as premenstrual dysphoric disorder, postpartum depression, and perimenopausal depression. These disorders are associated with reproductive stages involving changes in gonadal hormone levels. Specifically, female depression and female affective behaviors are influenced by estradiol levels. This review argues two major mechanisms by which estrogens influence depression and depressive-like behavior: through interactions with neurotrophic factors and through an influence on the serotonergic system. In particular, estradiol increases brain derived neurotrophic factor (BDNF) levels within the brain, and alters serotonergic expression in a receptor subtype-specific manner. We will take a regional approach, examining these effects of estrogens in the major brain areas implicated in depression. Finally, we will discuss the gaps in our current knowledge of the effects of estrogens on female depression, and the potential utility for estrogen receptor modulators in treatment for this disorder.

Dibutyl phthalate-induced neurotoxicity in the brain of immature and mature rat offspring

PURPOSE

We aim to investigate the neurotoxicity induced by perinatal exposure of dibutyl phthalate (DBP) on the immature and mature offspring animals using a rodent model.

METHODS

Pregnant rats were given intragastric administration of 500mg/kg body weight DBP daily from gestational day 6 to postnatal day 21 while control animals received the same volume of edible corn oil. Serum estradiol and testosterone levels of the offspring were evaluated. Protein levels of AROM, ER-β, BDNF and p-CREB in the hippocampus were also measured.

RESULTS

Perinatal exposure of DBP significantly up-regulated the serum estradiol levels in both immature and mature offspring rats. DBP exposure also significantly down-regulated the testosterone levels in immature male and female rats and mature male rats but had no influence on the testosterone levels in mature female rats. DBP exposure up-regulated AROM, but down-regulated ER-β, BDNF and p-CREB expression in the hippocampus of immature rat offspring, while it had no influence on the levels of these proteins in the mature animals.

CONCLUSIONS

Perinatal exposure of DBP could induce neurotoxicity in immature offspring rats through regulation of AROM, ER-β, BDNF and p-CREB expression, while it has no influence on mature offspring animals.

Synaptic and cellular profile of neurons in the lateral habenula

The lateral habenula (LHb) is emerging as a crucial structure capable of conveying rewarding and aversive information. Recent evidence indicates that a rapid increase in the activity of LHb neurons drives negative states and avoidance. Furthermore, the hyperexcitability of neurons in the LHb, especially those projecting to the midbrain, may represent an important cellular correlate for neuropsychiatric disorders like depression and drug addiction. Despite the recent insights regarding the implications of the LHb in the context of reward and aversion, the exact nature of the synaptic and cellular players regulating LHb neuronal functions remains largely unknown. Here we focus on the synaptic and cellular physiology of LHb neurons. First, we discuss the properties of excitatory transmission and the implications of glutamate receptors for long-term synaptic plasticity; second, we review the features of GABAergic transmission onto LHb neurons; and finally, we describe the contribution that neuromodulators such as dopamine (DA) and serotonin may have for LHb neuronal physiology. We relate these findings to the role that the LHb can play in processing aversive and rewarding stimuli, both in health and disease states.

Neurotoxicity of dibutyl phthalate in brain development following perinatal exposure: a study in rats

Dibutyl-phthalate (DBP) is a ubiquitous environmental contaminant. However, its neurotoxic effects on neonatal, immature or mature brains remain unclear. Here, we aimed to investigate the neurotoxicity of perinatal exposure of DBP on rodent offspring animals. Pregnant rats received intragastric DBP (500mg/kg body weight) daily from gestational day (GD) 6 to postnatal day (PND) 21. Animals in the control group received the same volume of edible corn oil. Brain sections or tissues from offspring rats on PND5, PND21 and PND60 were collected for analysis. Histological examination demonstrated that perinatal exposure of DBP resulted in hippocampal neuron loss and structural alternation in neonatal and immature offspring rats (PND5 and PND21), while no significant change was found in mature rats (PND60). DBP exposure induced cell apoptosis in hippocampal neurons of these neonatal and immature animals, as evidenced by the increased number of TUNEL-positive and Annexin V-propidium iodide (PI) positive cells and up-regulated caspase-3 activity. Moreover, DBP exposure decreased the expression of synaptophysin in the hippocampus and reduced both the slope and amplitude of field excitatory postsynaptic potentials (fEPSPs). DBP also impaired the spatial learning and memory of offspring rats. However, no significant difference in the susceptibility to DBP-induced neurotoxicity was found between male and female offspring rats. Our findings indicated that perinatal exposure of DBP could induce neurotoxicity in neonatal and immature offspring animals, but had no influence on mature animals after DBP withdrawal. These results may provide basic experimental evidence for better understanding the neurotoxic effects of DBP on neonatal, immature and mature brains.

Differential regulation of BDNF, synaptic plasticity and sprouting in the hippocampal mossy fiber pathway of male and female rats

Many studies have described potent effects of BDNF, 17β-estradiol or androgen on hippocampal synapses and their plasticity. Far less information is available about the interactions between 17β-estradiol and BDNF in hippocampus, or interactions between androgen and BDNF in hippocampus. Here we review the regulation of BDNF in the mossy fiber pathway, a critical part of hippocampal circuitry. We discuss the emerging view that 17β-estradiol upregulates mossy fiber BDNF synthesis in the adult female rat, while testosterone exerts a tonic suppression of mossy fiber BDNF levels in the adult male rat. The consequences are interesting to consider: in females, increased excitability associated with high levels of BDNF in mossy fibers could improve normal functions of area CA3, such as the ability to perform pattern completion. However, memory retrieval may lead to anxiety if stressful events are recalled. Therefore, the actions of 17β-estradiol on the mossy fiber pathway in females may provide a potential explanation for the greater incidence of anxiety-related disorders and post-traumatic stress syndrome (PTSD) in women relative to men. In males, suppression of BDNF-dependent plasticity in the mossy fibers may be protective, but at the 'price' of reduced synaptic plasticity in CA3. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Mechanisms underlying the interactions between rapid estrogenic and BDNF control of synaptic connectivity

The effects of the steroid hormone 17β-estradiol and the neurotrophin brain-derived neurotrophic factor (BDNF) on neuronal physiology have been well investigated. Numerous studies have demonstrated that each signal can exert powerful influences on the structure and function of synapses, and specifically on dendritic spines, both within short and long time frames. Moreover, it has been suggested that BDNF is required for the long-term, or genomic, actions of 17β-estradiol on dendritic spines, via its ability to regulate the expression of neurotrophins. Here we focus on the acute, or rapid effects, of 17β-estradiol and BDNF, and their ability to activate specific signalling cascades, resulting in alterations in dendritic spine morphology. We first review recent literature describing the mechanisms by which 17β-estradiol activates these pathways, and the resulting alterations in dendritic spine number. We then describe the molecular mechanisms underlying acute modulation of dendritic spine morphology by BDNF. Finally, we consider how this new evidence may suggest that the temporal interactions of 17β-estradiol and BDNF can occur more rapidly than previously reported. Building on these new data, we propose a novel model for the interactions of this steroid and neurotrophin, whereby rapid, non-genomic 17β-estradiol and acute BDNF signal in a co-operative manner, resulting in dendritic spine formation and subsequent stabilization in support of synapse and circuit plasticity. This extended hypothesis suggests an additional mechanism by which these two signals may modulate dendritic spines in a time-specific manner.

Brain-derived neurotrophic factor and androgen interactions in spinal neuromuscular systems

Neurotrophic factors and steroid hormones interact to regulate a variety of neuronal processes such as neurite outgrowth, differentiation, and neuroprotection. The coexpression of steroid hormone and neurotrophin receptor mRNAs and proteins, as well as their reciprocal regulation provides the necessary substrates for such interactions to occur. This review will focus on androgen brain-derived neurotrophic factor (BDNF) interactions in the spinal cord, describing androgen regulation of BDNF in neuromuscular systems following castration, androgen manipulation, and injury. Androgens interact with BDNF during development to regulate normally-occurring motoneuron death, and in adulthood, androgen-BDNF interactions are involved in the maintenance of several features of neuromuscular systems. Androgens regulate BDNF and trkB expression in spinal motoneurons. Androgens also regulate BDNF levels in the target musculature, and androgenic action at the muscle regulates BDNF levels in motoneurons. These interactions have important implications for the maintenance of motoneuron morphology. Finally, androgens interact with BDNF after injury, influencing soma size, dendritic morphology, and axon regeneration. Together, these findings provide further insight into the development and maintenance of neuromuscular systems and have implications for the neurotherapeutic/neuroprotective roles of androgens and trophic factors in the treatment of motoneuron disease and recovery from injury.

Interactions between estradiol, BDNF and dendritic spines in promoting memory

Several lines of evidence have converged to indicate that memory formation involves plasticity of dendritic spines in the medial prefrontal cortex (PFC) and the hippocampus. Memory varies with estrogen levels throughout the lifespan of the female. Generally, increased levels of estrogen are related to greater dendritic spine density on pyramidal cells in the PFC and the hippocampus and to improved memory function. Brain-derived neurotrophic factor (BDNF) is a growth factor which increases dendritic spines and enhances memory function. Estrogens increase BDNF levels in the PFC and the hippocampus. In the present review we provide evidence that estradiol and BDNF may work in concert to enhance cognition. In adult females, fluctuations in recognition memory following ovariectomy and estradiol replacement, during the estrous cycle, in pregnancy and with aging are accompanied by similar changes in circulating estradiol, BDNF levels and spine density alterations in the PFC and the hippocampus. In addition, both estradiol and BDNF induce spine plasticity via rapid membrane effects and slower transcriptional regulation via the CREB pathway. Moreover, estradiol increases BDNF levels through action on nuclear receptors. While the exact mechanism(s) for the influence of estrogens and BDNF on memory remain unclear, this combination may provide the basis for new and more effective strategies for treating age-related and neurodegenerative memory loss.

Brain-derived neurotrophic factor Val⁶⁶Met polymorphism affects resting regional cerebral blood flow and functional connectivity differentially in women versus men

The human Val⁶⁶Met single nucleotide polymorphism in the brain-derived neurotrophic factor (BDNF) gene impacts BDNF signaling at the cellular level. At the neural-systems level, it is associated with differences in prefrontal cortex (PFC) and hippocampal function during performance of cognitive and affective tasks. Because the impact of this variant on basal prefrontal and hippocampal activity is not known but may be relevant to understanding the function of this gene in health and disease, we studied 94 healthy individuals with H₂ ¹⁵O PET to assess regional cerebral blood flow (rCBF) during rest and tested for between-genotype differences. Because BDNF and gonadal steroid hormones conjointly influence neuronal growth, survival, and plasticity in hippocampus and PFC, we also tested for sex × genotype interactions. Finally, in light of the known impact of BDNF on plasticity and dendritic arborization, we complimented direct rCBF comparisons with connectivity analyses to determine how activity in hippocampal and prefrontal regions showing between-genotype group differences covaries with rCBF in other nodes throughout the brain in a genotype- or sex-dependent manner. Compared with Val homozygotes, Met carriers had higher rCBF in prefrontal (BA25 extending into BA10) and hippocampal/parahippocampal regions. Moreover, there were significant sex × genotype interactions in regions (including frontal, parahippocampal, and lateral temporal cortex) in which Val homozygotes showed higher rCBF in females than males, but Met carriers showed the opposite relationship. Functional connectivity analysis demonstrated that correlations of BA25, hippocampus, and parahippocampus with frontal and temporal networks were positive for Val homozygotes and negative for Met carriers. In addition, sex × genotype analysis of functional connectivity revealed that genotype affected directionality of the inter-regional correlations differentially in men versus women. Our data indicate that BDNF allelic variation and sex interactively affect basal prefrontal and hippocampal function.

Interaction of PFOS and BDE-47 co-exposure on thyroid hormone levels and TH-related gene and protein expression in developing rat brains

Perfluorooctane sulfonate (PFOS) and 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) are two persistent environmental contaminants that are toxic to developing nervous systems, particularly via their disruption of thyroid hormone (TH) function. To investigate whether an interaction existed between PFOS and BDE-47 on TH-mediated pathways, adult female Wistar rats were exposed to 3.2 and 32 mg/kg of PFOS or BDE-47 in their diet and co-exposed to a combination of each chemical (3.2 mg/kg) from gestational day 1 to postnatal day (PND) 14. Serum and brain tissues from both male and female neonates were collected on PNDs 1, 7, and 14 to examine TH-regulated gene and protein expression. The results revealed that (1) a significant accumulation difference occurred between the two chemicals; (2) On a equimolar basis, BDE-47 and PFOS affected serum total triiodothyronine and total thyroxine differently in adults and offspring; (3) there were region-specific and exposure- and time-dependent alterations in TH concentrations and tested gene and protein expression levels; and (4) interaction for the combined chemicals was only observed for brain-derived neurotrophic factor (BDNF), which exhibited a synergistic effect on PND 1 in the cortex and an antagonistic effect on PND 14 in the hippocampus. Our results suggest a complex TH-mediated gene and protein response to BDE-47 and/or PFOS exposure that seems little related to TH homeostasis and that little combined interaction of co-exposures was observed except on BDNF. The underlying mechanisms remain uncertain but seem to involve more actions than just TH-regulated pathway.

Androgen regulates brain-derived neurotrophic factor in spinal motoneurons and their target musculature

Trophic factors maintain motoneuron morphology and function in adulthood. Brain-derived neurotrophic factor (BDNF) interacts with testosterone to maintain dendritic morphology of spinal motoneurons. In addition, testosterone regulates BDNF's receptor (trkB) in motoneurons innervating the quadriceps muscles as well as in motoneurons of the highly androgen-sensitive spinal nucleus of the bulbocavernosus (SNB). Given these interactive effects, we examined whether androgen might also regulate BDNF in quadriceps and SNB motoneurons and their corresponding target musculature. In both motoneuron populations, castration of males reduced BDNF immunolabeling, and this effect was prevented with testosterone replacement. ELISA for BDNF in the target musculature of quadriceps (vastus lateralis, VL) and SNB (bulbocavernosus, BC) motoneurons revealed that BDNF in the VL and BC muscles was also regulated by androgen. However, although castration significantly decreased BDNF concentration in the VL muscle, BDNF concentration in the BC muscle was significantly increased in castrates. Treatment of castrated males with testosterone maintained BDNF levels at those of intact males in both sets of muscles. Together, these results demonstrate that androgens regulate BDNF in both a sexually dimorphic, highly androgen-sensitive neuromuscular system as well as a more typical somatic neuromuscular system. Furthermore, in addition to the regulation of trkB, these studies provide another possible mechanism for the interactive effects of testosterone and BDNF on motoneuron morphology. More importantly, by examining both the motoneurons and the muscles they innervate, these results demonstrate that within a neural system, BDNF levels in different components are differentially affected by androgen manipulation.

Oestrogen modulates hypothalamic control of energy homeostasis through multiple mechanisms

The control of energy homeostasis in women is correlated with the anorectic effects of oestrogen, which can attenuate body weight gain and reduce food intake in rodent models. This review investigates the multiple signalling pathways and cellular targets that oestrogen utilises to control energy homeostasis in the hypothalamus. Oestrogen affects all of the hypothalamic nuclei that control energy homeostasis. Oestrogen controls the activity of hypothalamic neurones through gene regulation and neuronal excitability. Oestrogen's primary cellular pathway is the control of gene transcription through the classical oestrogen receptors (ERs) (ERalpha and ERbeta) with ERalpha having the primary role in energy homeostasis. Oestrogen also controls energy homeostasis through membrane-mediated events via membrane-associated ERs or a novel, putative membrane ER that is coupled to G-proteins. Therefore, oestrogen is coupled to at least two receptors with multiple signalling and transcriptional pathways to mediate immediate and long-term anorectic effects. Ultimately, it is the interactions of all the receptor-mediated processes in hypothalamus and other areas of the central nervous system that will determine the anorectic effects of oestrogen and its control of energy homeostasis.

Estrogen and adult neurogenesis in the amygdala and hypothalamus

In mammals, adult neurogenesis has been extensively studied in the dentate gyrus of the hippocampus and subventricular zone. However, newly proliferated neurons have also been documented in other brain regions, including the amygdala and hypothalamus. In this review, we will examine the evidence for new neurons in the adult amygdala and hypothalamus and then discuss how environmental influences can alter cell proliferation. As some of these environmental effects may be attributed to changes in the levels of circulating hormones, we will provide evidence for estrogen-mediated cell proliferation among different species and between sexes. Finally, we will review recent data suggesting that new neurons may become functionally significant in adulthood.

Androgen regulates trkB immunolabeling in spinal motoneurons

Neurotrophic factors and steroid hormones have been shown to have neuroprotective/neurotherapeutic effects, and it has been shown previously that brain-derived neurotrophic factor (BDNF) and testosterone have a combinatorial effect in the maintenance of motoneurons. Given that gonadal hormones regulate the BDNF receptor, tyrosine receptor kinase B (trkB), we hypothesized that such a regulatory effect could mediate the interactive effects of BDNF and testosterone. Using immunohistochemical methods, we examined the frequency of cells immunolabeled for trkB receptors in two populations of spinal motoneurons, the hormone-sensitive, sexually dimorphic motoneurons of the spinal nucleus of the bulbocavernosus (SNB) and the nondimorphic motoneurons innervating the muscles of the quadriceps. In both the highly androgen-sensitive SNB motoneurons and the more typical somatic motoneurons innervating the quadriceps, the frequency of motoneurons intensely immunolabeled for trkB receptors was regulated by the presence of testosterone. Castrated animals deprived of testosterone showed a reduced frequency of intensely labeled motoneurons compared with intact animals or castrated animals given testosterone replacement. This finding suggests that the combinatorial effect of BDNF and testosterone in the maintenance of motoneurons could occur at least in part through an androgen-mediated expression of the BDNF receptor.

Estrogen-BDNF interactions: implications for neurodegenerative diseases

Since its' discovery over 20 years ago, BDNF has been shown to play a key role in neuronal survival, in promoting neuronal regeneration following injury, regulating transmitter systems and attenuating neural-immune responses. Estrogen's actions in the young and mature brain, and its role in neurodegenerative diseases in many cases overlaps with those observed for BDNF. Reduced estrogen and BDNF are observed in patients with Parkinson's disease and Alzheimer's disease, while high estrogen levels are a risk factor for development of multiple sclerosis. Estrogen receptors, which transduce the actions of estrogen, colocalize to cells that express BDNF and its receptor trkB, and estrogen further regulates the expression of this neurotrophin system. This review describes the distribution of BDNF and trkB expressing cells in the forebrain, and the roles of estrogen and the BDNF-trkB neurotrophin system in Parkinson's disease, Alzheimer's disease and multiple sclerosis.

Estrogen and brain-derived neurotrophic factor (BDNF) in hippocampus: complexity of steroid hormone-growth factor interactions in the adult CNS

In the CNS, there are widespread and diverse interactions between growth factors and estrogen. Here we examine the interactions of estrogen and brain-derived neurotrophic factor (BDNF), two molecules that have historically been studied separately, despite the fact that they seem to share common targets, effects, and mechanisms of action. The demonstration of an estrogen-sensitive response element on the BDNF gene provided an impetus to explore a direct relationship between estrogen and BDNF, and predicted that the effects of estrogen, at least in part, might be due to the induction of BDNF. This hypothesis is discussed with respect to the hippocampus, where substantial evidence has accumulated in favor of it, but alternate hypotheses are also raised. It is suggested that some of the interactions between estrogen and BDNF, as well as the controversies and implications associated with their respective actions, may be best appreciated in light of the ability of BDNF to induce neuropeptide Y (NPY) synthesis in hippocampal neurons. Taken together, this tri-molecular cascade, estrogen-BDNF-NPY, may be important in understanding the hormonal regulation of hippocampal function. It may also be relevant to other regions of the CNS where estrogen is known to exert profound effects, such as amygdala and hypothalamus; and may provide greater insight into neurological disorders and psychiatric illness, including Alzheimer's disease, depression and epilepsy.

Estrogenic support of motoneuron dendritic growth via the neuromuscular periphery in a sexually dimorphic motor system

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). In males, the growth of SNB dendrites is steroid-dependent: dendrites fail to grow after castration, but grow in castrates treated with androgens or estrogens. Blocking estradiol synthesis or estrogen receptors in gonadally intact males attenuates SNB dendritic growth, suggesting that estrogens are required and must be able to act at their receptors to support normal masculine dendritic growth. However, SNB motoneurons do not accumulate estrogens, suggesting that estrogens act indirectly to support SNB dendritic growth. In this experiment, we examined whether local estrogen action in the neuromuscular periphery was involved in the postnatal development of SNB motoneurons. Motoneuron morphology was assessed in gonadally intact and castrated males. Gonadally intact males were left untreated or given either blank or tamoxifen implants sutured to the target musculature, or tamoxifen interscapular implants. Castrated males were left untreated or were given estradiol by muscle or interscapular implants or systemic injection during the period of SNB dendritic growth. At postnatal day 28, when SNB dendritic length is normally maximal, SNB motoneurons were retrogradely labeled with cholera toxin-HRP and reconstructed in three dimensions. While interscapular tamoxifen implants were ineffective, blocking estrogen receptors at the target musculature resulted in attenuation of SNB dendritic growth. In contrast, while interscapular implants of estradiol were ineffective, local treatment with estradiol at the target musculature in castrated males resulted in masculinization of dendritic growth. Thus, estrogens may act by an indirect action in the neuromuscular periphery to support SNB dendritic growth.

Gender-dependent modulation of brain monoamines and anxiety-like behaviors in mice with genetic serotonin transporter and BDNF deficiencies

1. Brain-derived neurotrophic factor (BDNF) supports serotonergic neuronal development and our recent study found that heterozygous mice lacking one BDNF gene allele interbred with male serotonin transporter (SERT) knockout mice had greater reductions in brain tissue serotonin concentrations, greater increases in anxiety-like behaviors and greater ACTH responses to stress than found in the SERT knockout mice alone. 2. We investigated here whether there might be gender differences in these consequences of combined SERT and BDNF deficiencies by extending the original studies to female mice, and also to an examination of the effects of ovariectomy and tamoxifen in these female mice, and of 21-day 17-beta estradiol implantation to male mice. 3. We found that unlike the male SERTxBDNF-deficient mice, female SERTxBDNF mice appeared protected by their gender in having significantly lesser reductions in serotonin concentrations in hypothalamus and other brain regions than males, relative to controls. Likewise, in the elevated plus maze, female SERTxBDNF-deficient mice demonstrated no increases in the anxiety-like behaviors previously found in males. 4. Furthermore, female SERTxBDNF mice did not manifest the approximately 40% reduction in the expression of TrkB receptors or the approximately 30% reductions in dopamine and its metabolites that male SERTxBDNF did. After estradiol implantation in male SERTxBDNF mice, hypothalamic serotonin was significantly increased compared to vehicle-implanted mice. These findings support the hypothesis that estrogen may enhance BDNF function via its TrkB receptor, leading to alterations in the serotonin circuits, which modulate anxiety-like behaviors. 5. This double-mutant mouse model contributes to the knowledge base that will help in understanding genexgenexgender interactions in studies of SERT and BDNF gene polymorphisms in human genetic diseases such as anxiety disorders and depression.

Differential distributions of the Ca2+ -dependent activator protein for secretion family proteins (CAPS2 and CAPS1) in the mouse brain

The Ca(2+)-dependent activator protein for secretion (CAPS/Cadps) family consists of two members, CAPS1 and CAPS2, and plays an important role in secretory granule exocytosis. It has been shown that CAPS1 regulates catecholamine release from neuroendocrine cells, whereas CAPS2 is involved in the release of two neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), from parallel fibers of cerebellar granule cells. Although both CAPS proteins are expressed predominantly in the brain, their cellular and regional distributions in the brain are largely unknown. In this study we analyzed the immunohistochemical distributions of the CAPS family proteins in the mouse brain. In most areas of the embryonic nervous system CAPS1 and CAPS2 proteins were complementarily expressed. In the postnatal brain, CAPS1 was widespread at different levels. On the other hand, CAPS2 was localized to distinct cell types and fibers of various brain regions, including the olfactory bulb, cerebrum, hippocampal formation, thalamus, mesencephalic tegmentum, cerebellum, medulla, and spinal cord, except for some regions that overlapped with CAPS1. These CAPS2 cellular distribution patterns had the marked feature of coinciding with those of BDNF in various brain regions. Immunolabels for CAPS2 were also colocalized with those for some proteins related to exocytosis (VAMP and SNAP-25) and endocytosis (Dynamin I) in the cell soma and processes of the mesencephalic tegmentum and cerebellum, suggesting that these proteins might be involved in the dynamics of CAPS2-associated vesicles, although their colocalization on vesicles remains elusive. These results demonstrate that the CAPS family proteins are involved in the secretion of different secretory substances in developing and postnatal brains, and that CAPS2 is probably involved in BDNF secretion in many brain areas.

Effects of castration on the expression of brain-derived neurotrophic factor (BDNF) in the vas deferens and male accessory genital glands of the rat

Brain-derived neurotrophic factor (BDNF) is a growth factor belonging to the family of neurotrophins. Although neurotrophins in the male genital organs have been well documented, their role in the biology of these organs is far from clear. In particular, little is known about the influence of sex hormones on neurotrophin expression. In the present study, using immunohistochemistry and enzyme-linked immunosorbent assay (ELISA), we investigated the distribution and tissue concentration of BDNF in the vas deferens and accessory male genital glands in normal and castrated rats. The expression of BDNF mRNA was also investigated. In normal rats, BDNF immunoreactivity was localized in the musculature of the vas deferens and vesicular gland and in the fibromuscular stromal cells of the prostate. In the ventral prostatic lobes, BDNF immunoreactivity was localized in basal, secretory and neuroendocrine epithelial cells. Innervating ganglia and nerves were immunoreactive in all the examined tracts. After castration, BDNF immunoreactivity increased in the musculature of the vesicular gland and in the fibromuscular stromal cells of both dorsal and ventral prostatic lobes. BDNF immunoreactivity also increased in the nerves. ELISA and reverse transcription/real-time polymerase chain reaction confirmed the findings of the immunohistochemical study. In the accessory glands, castration induced an increase of both BDNF tissue concentration and mRNA expression. These results suggest that BDNF is expressed in the internal male genital organs of the rat and that its expression is downregulated by androgen hormones. We hypothesize that the observed BDNF increases are related to the castration-induced regression of the sympathetic nerves.

Estradiol to aged female or male mice improves learning in inhibitory avoidance and water maze tasks

Although 17beta-Estradiol (E2) improves cognitive performance of aged female mice, its mnemonic effects when administered post-training to aged male mice have not been examined. E2 (10 microg, SC) or oil vehicle was administered to intact, 24-month-old female or male congenic (primarily C57BL/6 background) mice immediately after training in the inhibitory avoidance or water maze tasks. Following behavioral testing, effects of 1 or 24 h of E2 exposure on hippocampal levels of E2 and brain-derived neurotrophic factor (BDNF) were examined. Female and male mice administered E2 showed significantly better performance in the inhibitory avoidance task than did vehicle-administered mice. When tested 24 h after training, mice that received E2 had significantly longer latencies to cross-over to the shock-associated side of the chamber than did vehicle-administered mice. Female or male mice administered E2 showed significantly better performance in the reference memory aspect of the spatial water maze task. When tested 30 min after training, mice administered E2 had shorter latencies to, and spent longer swimming in, the quadrant that the hidden platform had previously been located in. E2 administration produced physiological levels of E2 in the hippocampus 1 and 24 h after E2. BDNF levels in the hippocampus were decreased following 1 h of E2 exposure compared to vehicle. These findings suggest that E2 to female and male mice may overcome age-related deficits in reference memory in an emotional or spatial learning task.

Offer and demand: proliferation and survival of neurons in the dentate gyrus

The proliferation and survival of new cells in the dentate gyrus of mammals is a complex process that is subject to numerous influences, presenting a confusing picture. We suggest regarding these processes on the level of small networks, which can be simulated in silico and which illustrate in a nutshell the influences that proliferating cells exert on plasticity and the conditions they require for survival. Beyond the insights gained by this consideration, we review the available literature on factors that regulate cell proliferation and neurogenesis in the dentate gyrus in vivo. It turns out that the rate of cell proliferation and excitatory afferents via the perforant path interactively determine cell survival, such that the best network stability is achieved when either of the two is increased whereas concurrent activation of the two factors lowers cell survival rates. Consequently, the mitotic activity is regulated by systemic parameters in compliance with the hippocampal network's requirements. The resulting neurogenesis, in contrast, depends on local factors, i.e. the activity flow within the network. In the process of cell differentiation and survival, each cell's spectrum of afferent and efferent connections decides whether it will integrate into the network or undergo apoptosis, and it is the current neuronal activity which determines the synaptic spectrum. We believe that this framework will help explain the biology of dentate cell proliferation and provide a basis for future research hypotheses.

Environmental enrichment reduces the mnemonic and neural benefits of estrogen

The degree to which memory is enhanced by estrogen replacement in postmenopausal women may depend on environmental factors such as education. The present study utilized an animal model of environmental enrichment to determine whether environmental factors influence the mnemonic and neural response to estrogen. Female mice were raised in standard (SC) or enriched (EC) conditions from weaning until adulthood (7 months). All mice were ovariectomized at 10 weeks, and tested in object recognition and water-escape motivated radial arm maze (WRAM) tasks at 6 months. Each day at the completion of training, mice received injections of 0.1 mg/kg cyclodextrin-encapsulated 17-beta-estradiol (E2), 0.2 mg/kg E2, or cyclodextrin vehicle (VEH). At the completion of behavioral testing, hippocampal levels of the presynaptic protein synaptophysin and of brain-derived neurotrophic factor (BDNF) were measured. Enrichment effects were evident in VEH-treated mice; relative to SC-VEH females, EC-VEH females committed fewer working memory errors in the WRAM and exhibited increased hippocampal synaptophysin levels. Estrogen effects depended on environmental conditions. E2 (0.2 mg/kg) improved object memory only in SC females. The same dose improved working memory in SC females, but somewhat impaired working memory in EC females. Furthermore, both doses reduced hippocampal synaptophysin levels in EC, but not SC, females. In contrast, E2 reduced hippocampal BDNF levels in SC, but not EC, females. This study is the first to compare the effects of estrogen on memory and hippocampal function in enriched and non-enriched female mice. The results suggest that: (1) estrogen benefits object and working memory more in mice raised in non-enriched environments than in those raised in enriched environments, and (2) the changes induced by estrogen and/or enrichment may be associated with alterations in hippocampal synaptic plasticity.

Effects of estrogen treatment on expression of brain-derived neurotrophic factor and cAMP response element-binding protein expression and phosphorylation in rat amygdaloid and hippocampal structures

Clinical studies indicate an effect of estrogen (E2) on affect and cognition, which may be mediated by the cAMP response element-binding protein (CREB) pathway and CREB-related gene target brain-derived neurotrophic factor (BDNF). We investigated the effect of E2 on CREB expression and phosphorylation and BDNF expression in the amygdala and hippocampus, areas involved in emotional processing. Ovariectomized rats were given 10 microg 17beta-estradiol or vehicle for 14 days and expression of components of the CREB signaling pathway, i.e., CREB, phosphorylated CREB (pCREB), and BDNF in amygdala and hippocampus were investigated using immunogold labeling. Levels of BDNF mRNA were determined by in situ reverse-transcriptase polymerase chain reaction. We also examined the effect of E2 on calcium/calmodulin kinase (CaMK IV) immunolabeling in the hippocampus. E2 increased immunolabeling and mRNA levels of BDNF in the medial and basomedial amygdala and CA1 and CA3 regions of the hippocampus, but not in any other amygdaloid or hippocampal regions examined. E2 increased immunolabeling of CREB and pCREB in the medial and basomedial, but not central or basolateral amygdala. E2 also increased CaMK IV and pCREB immunolabeling in the CA1 and CA3 regions, but not CA2 region or dentate gyrus, of the hippocampus. There was no change in immunolabeling of CREB in any hippocampal region. These data identify a signaling pathway through which E2 increases BDNF expression that may underlie some actions of E2 on affective behavior and indicate neuroanatomical heterogeneity in the E2 effect within the amygdala and hippocampus.

Circadian firing-rate rhythms and light responses of rat habenular nucleus neurons in vivo and in vitro

The suprachiasmatic nuclei of the anterior hypothalamus serve as the principal pacemaker of the mammalian circadian system. Among its efferent targets are the habenular nucleus (Hb), especially the lateral Hb (LHb), which plays an important role in conveying input from the limbic forebrain to midbrain structures. We recorded extracellularly from single neurons in the LHb and medial Hb (MHb), both in vivo and using an in vitro slice preparation, to assess their responses to retinal illumination and the rhythmicity of their firing rates. Of cells recorded in the LHb, 42% were tonically activated or suppressed by retinal illumination, while significantly fewer cells recorded in the MHb responded to retinal illumination (19%). Of photically responsive cells, 68% in the LHb were activated and the remainder suppressed, while only 25% of those recorded in the MHb were activated. Cells in both the LHb and MHb showed higher baseline firing rates during the day than during the night in vivo, while photic responses were of significantly larger amplitude among LHb cells during the projected night than during the projected day. LHb cells recorded in vitro maintained their rhythmicity for two circadian cycles, but MHb cells did not show a rhythm in vitro. The habenula may play a role in linking circadian and motivational systems and may contribute to photic regulation of these systems, as well as to the rhythmicity of their function.

Chronic oral estrogen affects memory and neurochemistry in middle-aged female mice

This study tested whether chronic oral estrogen could improve memory and alter neural plasticity in the hippocampus and neocortex of middle-aged female mice. Ovariectomized C57BL/6 mice were administered 1,000, 1,500, or 2,500 nM 17beta-estradiol in drinking water for 5 weeks prior to and during spatial and object memory testing. Synaptophysin, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) levels were then measured in hippocampus and neocortex. The medium dose impaired spatial reference memory in the radial-arm maze, whereas all doses improved object recognition. The high dose increased hippocampal synaptophysin and NGF levels, whereas the medium dose decreased these neocortical levels. The high dose decreased neocortical BDNF levels. These data suggest that chronic oral estrogen selectively affects memory and neural function in middle-aged female mice.

Developmental and Genetic Influences upon Gender Differences in Methamphetamine-Induced Nigrostriatal Dopaminergic Neurotoxicity

The gonadal steroid hormone estrogen (E) may play an important role in sex differences in methamphetamine (MA)-induced neurotoxicity of the nigrostriatal dopaminergic (NSDA) system because E can serve as a neuroprotectant in female, but not male, mice. Gonadal steroid hormones also exert important organizational/developmental effects upon the brain at critical developmental periods. In Part 1 we assessed whether organizational (neonatal) or developmental (prepubertal) effects of gonadal steroids would alter gender/E-dependent neuroprotection of MA-induced NSDA neurotoxicity. Attempts to feminize male mice by gonadectomy at either the neonatal or prepubertal period failed to enable E to function as a neuroprotectant within the adult male mouse. Attempts to masculinize the female by testosterone administration at the neonatal period did not abolish the capacity for E to function as a neuroprotectant. However, prepubertal gonadectomy of female mice did disrupt E's capacity to serve as a neuroprotectant. These results suggest that genetic sex may prove the primary determinant for the sex differences observed in response to MA-induced NSDA neurotoxicity. In Part 2 we examined whether gender differences in response to MA-induced NSDA neurotoxicity would interact with a specific genetic alteration in a neurotrophic factor, brain-derived neurotrophic factor (BDNF). Female and male mice that were either deficient (+/- BDNF) or overexpressing (DBH:BDNF+) BDNF were treated with MA. Sex differences in MA-induced NSDA neurotoxicity remained present in +/- BDNF mice and were less severe as compared with their wild-type controls. A similar result was obtained in mice that overexpress BDNF, with female and mutant mice showing less NSDA neurotoxicity. In both BDNF-deficient mice and mice that overexpress BDNF, the relative degree of MA-induced NSDA neurotoxicity was lower in males. Taken together, these results suggest that a selective alteration in BDNF expression offers some neuroprotective potential against MA-induced NSDA neurotoxicity, and the relative degree of this neuroprotection may interact with the gender of the subject.

Hippocampal excitability increases during the estrous cycle in the rat: a potential role for brain-derived neurotrophic factor

To test the hypothesis that induction of BDNF may contribute to changes in hippocampal excitability occurring during the female reproductive cycle, we examined the distribution of BDNF immunoreactivity and changes in CA1 and CA3 electrophysiology across the estrous cycle in rats. Hippocampal BDNF immunoreactivity increased on the day of proestrus as well as on the following morning (estrus), relative to metestrus or ovariectomized animals. Changes in immunoreactivity were clearest in mossy fiber axons of dentate gyrus granule cells, which contain the highest concentration of BDNF. Increased immunoreactivity was also apparent in the neuropil-containing dendrites of CA1 and CA3 neurons. Electrophysiological recordings in hippocampal slices showed robust cycle-dependent differences. Evoked responses of CA1 neurons to Schaffer collateral stimulation changed over the cycle, with larger maximum responses at both proestrus and estrus relative to metestrus. In area CA3, repetitive hilar stimuli frequently evoked multiple population spikes at proestrus and estrus but only rarely at other cycle stages, and never in slices of ovariectomized rats. Hyperexcitability in area CA3 at proestrus was blocked by exposure to the high-affinity neurotrophin receptor antagonist K252a, or an antagonist of the alpha7 nicotinic cholinergic receptor, whereas it was induced at metestrus by the addition of BDNF to hippocampal slices. These studies suggest that hippocampal BDNF levels change across the estrous cycle, accompanied by neurophysiological responses that resemble the effects of BDNF treatment. An estrogen-induced interaction of BDNF and alpha7 nicotinic receptors on mossy fibers seems responsible for estrous cycle changes in area CA3. Periovulatory changes in hippocampal function may, thus, involve estrogen-induced increases in BDNF expression.

Newly proliferated cells in the adult male amygdala are affected by gonadal steroid hormones

Gonadal steroid hormones play an important role in the proliferation, survival, and activation of neurons. The present study was performed to examine the effects of testosterone and its metabolites on newly proliferated cells in the amygdala of adult male meadow voles (Microtus pennsylvanicus). Treatment with testosterone propionate (TP) in castrated males resulted in plasma testosterone levels similar to males following mating. TP-treated males displayed a significant increase in the density of cells labeled with a cell proliferation marker (BrdU) in the amygdala. Treatment with estradiol benzoate (EB) exerted a similar effect as TP on the density of BrdU-labeled cells, whereas 5alpha-dihydrotestosterone (DHT) was ineffective. A larger proportion (approximately 44%) of the BrdU-labeled cells in the amygdala displayed a neuronal phenotype, and a lesser percentage (approximately 35%) displayed a glial progenitor phenotype; however, treatment effects were not found in either population of cells. Hormonal effects appeared to be site-specific as no group differences were found in the dentate gyrus of the hippocampus or ventromedial hypothalamus. Finally, a time course study indicated that BrdU-labeled cells in the amygdala are present as early as 30 min following an acute injection of BrdU. Together, these data suggest that gonadal steroid hormones influence the number of newly proliferated cells in the amygdala, most likely by acting through an estrogenic mechanism, and these effects may be exerted on locally proliferating progenitors within the amygdala.

Estradiol enhances neurogenesis in the dentate gyri of adult male meadow voles by increasing the survival of young granule neurons

This study investigated whether estradiol influenced the survival of new granule neurons, independent of altering cell proliferation, in the adult rodent dentate gyrus and whether estradiol-induced changes in new neuron number relate to any observed changes in hippocampus-dependent behavior. To test whether estradiol specifically promotes the survival of new neurons we injected castrated adult male meadow voles with the cell synthesis marker bromodeoxyuridine (BrdU; 50 mg/kg) twice on day 0 and then injected either estradiol (10 microg) or vehicle for 5 consecutive days either over days 1-5, days 6-10 or days 11-15 and perfused them on day 16. Estradiol doubled the number of hippocampal BrdU-labeled neurons but only when administered during a discrete period (days 6-10; P< or =0.01) when most new neurons extend their axons [J Comp Neurol 413 (1999) 146]. To test whether the estradiol-induced increase in new neuron number was related to hippocampus-dependent behavior, males were injected with BrdU twice on day 0 and with estradiol or vehicle over days 6-10 before standard Morris water maze training commenced on day 16, 5 days after the final hormone injection. As in the first study, estradiol-treated males had more BrdU-labeled cells than vehicle-treated males. On a probe trial, estradiol-treated males spent significantly more time in the training quadrant than vehicle-treated males despite similar performance between groups during acquisition and reversal training trials. Thus estradiol enhanced the survival of young neurons but only when administered during their 'axon extension' phase and this effect was related to better spatial memory in male voles.