Oestradiol as a neuromodulator of learning and memory.

Although hormones such as glucocorticoids have been broadly accepted in recent decades as general neuromodulators of memory processes, sex steroid hormones such as the potent oestrogen 17ß-oestradiol have been less well recognized by the scientific community in this capacity. The predominance of females in studies of oestradiol and memory and the general (but erroneous) perception that oestrogens are 'female' hormones have probably prevented oestradiol from being more widely considered as a key memory modulator in both sexes. Indeed, although considerable evidence supports a crucial role for oestradiol in regulating learning and memory in females, a growing body of literature indicates a similar role in males. This Review discusses the mechanisms of oestradiol signalling and provides an overview of the effects of oestradiol on spatial, object recognition, social and fear memories. Although the primary focus is on data collected in females, effects of oestradiol on memory in males will be discussed, as will sex differences in the molecular mechanisms that regulate oestrogenic modulation of memory, which may have important implications for the development of future cognitive therapeutics.

Astrocytic glutamate transport is essential for the memory-enhancing effects of 17ß-estradiol in ovariectomized mice.

Infusion of 17ß-estradiol (E2) into the dorsal hippocampus (DH) of ovariectomized (OVX) mice enhances memory consolidation, an effect that depends on rapid phosphorylation of extracellular signal-regulated kinase (ERK) and Akt. Astrocytic glutamate transporter 1 (GLT-1) modulates neurotransmission via glutamate uptake from the synaptic cleft. However, little is known about the contribution of DH astrocytes, and astrocytic glutamate transport, to the memory-enhancing effects of E2. This study was designed to test whether DH astrocytes contribute to estrogenic modulation of memory consolidation by determining the extent to which DH GLT-1 is necessary for E2 to enhance memory in object recognition and object placement tasks and trigger rapid phosphorylation events in DH astrocytes. OVX female mice were bilaterally cannulated into the DH or the DH and dorsal third ventricle (ICV). Post-training DH infusion of the GLT-1 inhibitor dihydrokainic acid (DHK) dose-dependently impaired memory consolidation in both tasks. Moreover, the memory-enhancing effects of ICV-infused E2 in each task were blocked by DH DHK infusion. E2 increased p42 ERK and Akt phosphorylation in DH astrocytes, and these effects were blocked by DHK. Results suggest the necessity of DH GLT-1 activity for object and spatial memory consolidation, and for E2 to enhance consolidation of these memories and to rapidly activate cell signaling in DH astrocytes. Findings indicate that astrocytic function in the DH of OVX females is necessary for memory formation and is regulated by E2, and suggest an essential role for DH astrocytic GLT-1 activity in the memory-enhancing effects of E2.

G protein-coupled estrogen receptor (GPER) in the dorsal hippocampus regulates memory consolidation in gonadectomized male mice, likely via different signaling mechanisms than in female mice.

Studies in ovariectomized (OVX) female rodents suggest that G protein-coupled estrogen receptor (GPER) is a key regulator of memory, yet little is known about its importance to memory in males or the cellular mechanisms underlying its mnemonic effects in either sex. In OVX mice, bilateral infusion of the GPER agonist G-1 into the dorsal hippocampus (DH) enhances object recognition and spatial memory consolidation in a manner dependent on rapid activation of c-Jun N-terminal kinase (JNK) signaling, cofilin phosphorylation, and actin polymerization in the DH. However, the effects of GPER on memory consolidation and DH cell signaling in males are unknown. Thus, the present study first assessed effects of DH infusion of G-1 or the GPER antagonist G-15 on object recognition and spatial memory consolidation in gonadectomized (GDX) male mice. As in OVX mice, immediate post-training bilateral DH infusion of G-1 enhanced, whereas G-15 impaired, memory consolidation in the object recognition and object placement tasks. However, G-1 did not increase levels of phosphorylated JNK (p46, p54) or cofilin in the DH 5, 15, or 30 min after infusion, nor did it affect phosphorylation of ERK (p42, p44), PI3K, or Akt. Levels of phospho-cAMP-responsive element binding protein (CREB) were elevated in the DH 30 min following G-1 infusion, indicating that GPER in males activates a yet unknown signaling mechanism that triggers CREB-mediated gene transcription. Our findings show for the first time that GPER in the DH regulates memory consolidation in males and suggests sex differences in underlying signaling mechanisms.

Sex differences in training-induced activity of the ubiquitin proteasome system in the dorsal hippocampus and medial prefrontal cortex of male and female mice.

The ubiquitin proteasome system (UPS) is a primary mechanism through which proteins are degraded in cells. UPS activity in the dorsal hippocampus (DH) is necessary for multiple types of memory, including object memory, in male rodents. However, sex differences in DH UPS activation after fear conditioning suggest that other forms of learning may also differentially regulate DH UPS activity in males and females. Here, we examined markers of UPS activity in the synaptic and cytoplasmic fractions of DH and medial prefrontal cortex (mPFC) tissue collected 1 h following object training. In males, training increased phosphorylation of proteasomal subunit Rpt6, 20S proteasome activity, and the amount of PSD-95 in the DH synaptic fraction, as well as proteasome activity in the mPFC synaptic fraction. In females, training did not affect measures of UPS or synaptic activity in the DH synaptic fraction or in either mPFC fraction but increased Rpt6 phosphorylation in the DH cytoplasmic fraction. Overall, training-induced UPS activity was greater in males than in females, greater in the DH than in the mPFC, and greater in synaptic fractions than in cytosol. These data suggest that object training drives sex-specific alterations in UPS activity across brain regions and subcellular compartments important for memory.

On the role of sex steroids in biological functions by classical and non-classical pathways. An update.

The sex steroid hormones (SSHs) play several roles in regulation of various processes in the cardiovascular, immune, muscular and neural systems. SSHs affect prenatal and postnatal development of various brain structures, including regions associated with important physiological, behavioral, cognitive, and emotional functions. This action can be mediated by either intracellular or transmembrane receptors. While the classical mechanisms of SSHs action are relatively well examined, the physiological importance of non-classical mechanism of SSHs action through membrane-associated and transmembrane receptors in the brain remains unclear. The most recent summary describing the role of SSHs in different body systems is lacking. Therefore, the aim of this review is to discuss classical and non-classical signaling pathways of testosterone and estradiol action via their receptors at functional, cellular, tissue level and to describe the effects on various body systems and behavior. Particular emphasis will be on brain regions including the hippocampus, hypothalamus, frontal cortex and cerebellum.

APOE4 genotype or ovarian hormone loss influence open field exploration in an EFAD mouse model of Alzheimer's disease.

Anxiety is a prominent and debilitating symptom in Alzheimer's disease (AD) patients. Carriers of APOE4, the greatest genetic risk factor for late-onset AD, may experience increased anxiety relative to carriers of other APOE genotypes. However, whether APOE4 genotype interacts with other AD risk factors to promote anxiety-like behaviors is less clear. Here, we used open field exploration to assess anxiety-like behavior in an EFAD mouse model of AD that expresses five familial AD mutations (5xFAD) and human APOE3 or APOE4. We first examined whether APOE4 genotype exacerbates anxiety-like exploratory behavior in the open field relative to APOE3 genotype in a sex-specific manner among six-month-old male and female E3FAD (APOE3+/+/5xFAD+/-) and E4FAD mice (APOE4+/+/5xFAD+/-). Next, we determined whether circulating ovarian hormone loss influences exploratory behavior in the open field among female E3FAD and E4FADs. APOE4 genotype was associated with decreased time in the center of the open field, particularly among female EFADs. Furthermore, ovariectomy (OVX) decreased time in the center of the open field among female E3FADs to levels similar to intact and OVXed E4FAD females. Our results suggest that APOE4 genotype increased anxiety-like behavior in the open field, and that ovarian hormones may protect against an anxiety-like phenotype in female E3FAD, but not E4FAD mice.

It takes a neural village: Circuit-based approaches for estrogenic regulation of episodic memory.

Cognitive behaviors, such as episodic memory formation, are complex processes involving coordinated activity in multiple brain regions. However, much of the research on hormonal regulation of cognition focuses on manipulation of one region at a time or provides a single snapshot of how a systemic treatment affects multiple brain regions without investigating how these regions might interact to mediate hormone effects. Here, we use estrogenic regulation of episodic memory as an example of how circuit-based approaches may be incorporated into future studies of hormones and cognition. We first review basic episodic memory circuitry, rapid mechanisms by which 17ß-estradiol can alter circuit activity, and current knowledge about 17ß-estradiol's effects on episodic memory. Next, we outline approaches that researchers can employ to consider circuit effects in their estrogen research and provide examples of how these methods have been used to examine hormonal regulation of memory and other behaviors.

Chemogenetic inactivation of the nucleus reuniens impairs object placement memory in female mice.

Episodic memory is a complex process requiring input from several regions of the brain. Emerging evidence suggests that coordinated activity between the dorsal hippocampus (DH) and medial prefrontal cortex (mPFC) is required for episodic memory consolidation. However, the mechanisms through which the DH and mPFC interact to promote memory consolidation remain poorly understood. A growing body of research suggests that the nucleus reuniens of the thalamus (RE) is one of several structures that facilitate communication between the DH and mPFC during memory and may do so through bidirectional excitatory projections to both regions. Furthermore, recent work from other labs indicates that the RE is necessary for spatial working memory. However, it is not clear to what extent the RE is necessary for memory of object locations. The goal of this study was to determine whether activity in the RE is necessary for spatial memory as measured by the object placement (OP) task in female mice. A kappa-opioid receptor DREADD (KORD) virus was used to inactivate excitatory neurons in the RE pre- or post-training to establish a role for the RE in spatial memory acquisition and consolidation, respectively. RE inactivation prior to, or immediately after, object training blocked OP memory formation relative to chance and to control mice. Moreover, expression of the immediate early gene EGR-1 was reduced in the RE 1 hour after an object training trial, supporting the conclusion that reduced neuronal activity in the RE impairs the formation of object location memories. In summary, the findings of this study support a key role for the RE in spatial memory acquisition and consolidation.

Long-term oral administration of a novel estrogen receptor beta agonist enhances memory and alleviates drug-induced vasodilation in young ovariectomized mice.

Development of estrogen therapies targeting the ß (ERß) but not α (ERα) estrogen receptor is critically needed for the treatment of negative menopausal symptoms, as ERα activation increases health risks like cancer. Here, we determined the effects of long-term oral treatment with EGX358, a novel highly selective ERß agonist, on memory, vasodilation, and affect in young ovariectomized mice. Mice were orally gavaged daily for 9 weeks with vehicle, 17ß-estradiol (E2), the ERß agonist diarylpropionitrile (DPN), or EGX358 at doses that enhance memory when delivered acutely. Tail skin temperature was recorded as a proxy for vasodilation following injection of vehicle or senktide, a tachykinin receptor 3 agonist used to model hot flashes. Anxiety-like behavior was assessed in the open field (OF) and elevated plus maze (EPM), and depression-like behavior was measured in the tail suspension (TST) and forced swim tests (FST). Finally, memory was assessed in object recognition (OR) and object placement (OP) tasks. E2, DPN, and EGX358 reduced senktide-mediated increases in tail skin temperature compared to vehicle. All three treatments also enhanced memory in the OR and OP tasks, whereas vehicle did not. Although E2 increased time spent in the center of the OF, no other treatment effects were observed in the OF, EPM, TST, or FST. These data suggest that long-term ERß activation can reduce hot flash-like symptoms and enhance spatial and object recognition memories in ovariectomized mice. Thus, the highly selective ERß agonist EGX358 may be a promising avenue for reducing menopause-related hot flashes and memory dysfunction.

Dorsal Hippocampal Actin Polymerization Is Necessary for Activation of G-Protein-Coupled Estrogen Receptor (GPER) to Increase CA1 Dendritic Spine Density and Enhance Memory Consolidation.

Activation of the membrane estrogen receptor G-protein-coupled estrogen receptor (GPER) in ovariectomized mice via the GPER agonist G-1 mimics the beneficial effects of 17ß-estradiol (E2) on hippocampal CA1 spine density and memory consolidation, yet the cell-signaling mechanisms mediating these effects remain unclear. The present study examined the role of actin polymerization and c-Jun N-terminal kinase (JNK) phosphorylation in mediating effects of dorsal hippocampally infused G-1 on CA1 dendritic spine density and consolidation of object recognition and spatial memories in ovariectomized mice. We first showed that object learning increased apical CA1 spine density in the dorsal hippocampus (DH) within 40 min. We then found that DH infusion of G-1 increased both CA1 spine density and phosphorylation of the actin polymerization regulator cofilin, suggesting that activation of GPER may increase spine morphogenesis through actin polymerization. As with memory consolidation in our previous work (Kim et al., 2016), effects of G-1 on CA1 spine density and cofilin phosphorylation depended on JNK phosphorylation in the DH. Also consistent with our previous findings, E2-induced cofilin phosphorylation was not dependent on GPER activation. Finally, we found that infusion of the actin polymerization inhibitor, latrunculin A, into the DH prevented G-1 from increasing apical CA1 spine density and enhancing both object recognition and spatial memory consolidation. Collectively, these data demonstrate that GPER-mediated hippocampal spinogenesis and memory consolidation depend on JNK and cofilin signaling, supporting a critical role for actin polymerization in the GPER-induced regulation of hippocampal function in female mice.SIGNIFICANCE STATEMENT Emerging evidence suggests that G-protein-coupled estrogen receptor (GPER) activation mimics effects of 17ß-estradiol on hippocampal memory consolidation. Unlike canonical estrogen receptors, GPER activation is associated with reduced cancer cell proliferation; thus, understanding the molecular mechanisms through which GPER regulates hippocampal function may provide new avenues for the development of drugs that provide the cognitive benefits of estrogens without harmful side effects. Here, we demonstrate that GPER increases CA1 dendritic spine density and hippocampal memory consolidation in a manner dependent on actin polymerization and c-Jun N-terminal kinase phosphorylation. These findings provide novel insights into the role of GPER in mediating hippocampal morphology and memory consolidation, and may suggest first steps toward new therapeutics that more safely and effectively reduce memory decline in menopausal women.

Activation of androgen receptors protects intact male mice from memory impairments caused by aromatase inhibition.

Although 17ß-estradiol (E2) is known to regulate hippocampal function, the specific contributions of hippocampally-synthesized E2 remain unclear. Infusion of the aromatase inhibitor letrozole into the dorsal hippocampus (DH) of ovariectomized mice disrupts object recognition and object placement memory consolidation, suggesting that DH-synthesized E2 is essential for memory. However, the role of DH-synthesized E2 in memory among male rodents is unknown. Here, we examined effects of aromatase inhibition on memory consolidation in male mice. Intact and gonadectomized mice were infused with vehicle or letrozole into the DH immediately post-training in object placement and object recognition tasks. Letrozole blocked memory in both tasks among gonadectomized males only, suggesting that circulating androgens, or a rise in hippocampal androgens due to aromatase inhibition, may support memory consolidation in intact males. To test this hypothesis, intact males were infused with the androgen receptor antagonist flutamide into the DH after object training. A dose-dependent impairment was observed in both tasks, indicating that blocking androgen signaling can impair memory consolidation. To test if hippocampal androgen receptor activation protected intact males from the impairing effects of letrozole, a non-impairing dose of flutamide was co-infused with letrozole. Co-administration of both drugs blocked object placement and object recognition memory consolidation, demonstrating that letrozole impairs memory in intact males only if androgen receptors are blocked. Together, these data suggest that DH-synthesized E2 and androgen receptor activation may work in concert to mediate memory consolidation in intact males, such that androgen receptor activation protects against memory impairments caused by aromatase inhibition.

Dickkopf-1 blocks 17ß-estradiol-enhanced object memory consolidation in ovariectomized female mice.

The memory-enhancing effects of 17ß-estradiol (E2) depend upon rapid activation of several cell-signaling cascades within the dorsal hippocampus (DH). Among the many cell-signaling pathways that mediate memory processes, Wnt/ß-catenin signaling has emerged as a potential key player because of its importance to hippocampal development and synaptic plasticity. However, whether E2 interacts with Wnt/ß-catenin signaling to promote memory consolidation is unknown. Therefore, the present study examined whether Wnt/ß-catenin signaling within the DH is necessary for E2-induced memory consolidation in ovariectomized mice tested in the object recognition and object placement tasks. Ovariectomized C57BL/6 mice received immediate post-training infusions of E2 or vehicle into the dorsal third ventricle plus the endogenous Wnt/ß-catenin antagonist Dickkopf-1 (Dkk-1) or vehicle into the DH to assess whether the memory-enhancing effects of E2 depend on activation of Wnt/ß-catenin signaling. Our results suggest that Dkk-1 blocks E2-induced memory enhancement as hypothesized, but may do so by only moderately blunting Wnt/ß-catenin signaling while concurrently activating Wnt/JNK signaling. The current study provides novel insights into the mechanisms through which E2 enhances memory consolidation in the DH, as well as critical information about the mechanistic actions of Dkk-1.

Chemogenetic inactivation of the dorsal hippocampus and medial prefrontal cortex, individually and concurrently, impairs object recognition and spatial memory consolidation in female mice.

The dorsal hippocampus (DH) and medial prefrontal cortex (mPFC) are brain regions essential for processing and storing episodic memory. In rodents, the DH has a well-established role in supporting the consolidation of episodic-like memory in tasks such as object recognition and object placement. However, the role of the mPFC in the consolidation of episodic-like memory tasks remains controversial. Therefore, the present study examined involvement of the DH and mPFC, alone and in combination, in object and spatial recognition memory consolidation in ovariectomized female mice. To this end, we utilized two types of inhibitory Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to inactivate the DH alone, the mPFC alone, or both brain regions concurrently immediately after object training to assess the role of each region in the consolidation of object recognition and spatial memories. Our results using single and multiplexed DREADDS suggest that excitatory activity in the DH and mPFC, alone or in combination, is required for the successful consolidation of object recognition and spatial memories. Together, these studies provide critical insight into how the DH and mPFC work in concert to facilitate memory consolidation in female mice.

Mechanisms underlying the rapid effects of estradiol and progesterone on hippocampal memory consolidation in female rodents.

Contribution to Special Issue on Fast effects of steroids. Although rapid effects of 17ß­estradiol (E2) and progesterone on cellular functions have been observed for several decades, a proliferation of data in recent years has demonstrated the importance of these actions to cognition. In particular, an emerging literature has demonstrated that these hormones promote the consolidation of spatial and object recognition memories in rodents via rapid activation of numerous cellular events including cell signaling, histone modifications, and local protein translation in the hippocampus. This article provides an overview of the evidence demonstrating that E2 and progesterone enhance hippocampal memory consolidation in female rodents, and then discusses numerous molecular mechanisms thus far shown to mediate the beneficial effects of these hormones on memory formation.

Estradiol-Mediated Spine Changes in the Dorsal Hippocampus and Medial Prefrontal Cortex of Ovariectomized Female Mice Depend on ERK and mTOR Activation in the Dorsal Hippocampus.

Dendritic spine plasticity underlies the formation and maintenance of memories. Both natural fluctuations and systemic administration of 17ß-estradiol (E2) alter spine density in the dorsal hippocampus (DH) of rodents. DH E2 infusion enhances hippocampal-dependent memory by rapidly activating extracellular signal-regulated kinase (ERK)-dependent signaling of mammalian target of rapamycin (mTOR), a key protein synthesis pathway involved in spine remodeling. Here, we investigated whether infusion of E2 directly into the DH drives spine changes in the DH and other brain regions, and identified cell-signaling pathways that mediate these effects. E2 significantly increased basal and apical spine density on CA1 pyramidal neurons 30 min and 2 h after infusion. DH E2 infusion also significantly increased basal spine density on pyramidal neurons in the medial prefrontal cortex (mPFC) 2 h later, suggesting that E2-mediated activity in the DH drives mPFC spinogenesis. The increase in CA1 and mPFC spine density observed 2 h after intracerebroventricular infusion of E2 was blocked by DH infusion of an ERK or mTOR inhibitor. DH E2 infusion did not affect spine density in the dentate gyrus or ventromedial hypothalamus, suggesting specific effects of E2 on the DH and mPFC. Collectively, these data demonstrate that DH E2 treatment elicits ERK- and mTOR-dependent spinogenesis on CA1 and mPFC pyramidal neurons, effects that may support the memory-enhancing effects of E2. SIGNIFICANCE STATEMENT: Although systemically injected 17ß-estradiol (E2) increases CA1 dendritic spine density, the molecular mechanisms regulating E2-induced spinogenesis in vivo are largely unknown. We found that E2 infused directly into the dorsal hippocampus (DH) increased CA1 spine density 30 min and 2 h later. Surprisingly, DH E2 infusion also increased spine density in the medial prefrontal cortex (mPFC), suggesting that estrogenic regulation of the DH influences mPFC spinogenesis. Moreover, inhibition of ERK and mTOR activation in the DH prevented E2 from increasing DH and mPFC spines, demonstrating that DH ERK and mTOR activation is necessary for E2-induced spinogenesis in the DH and mPFC. These findings provide novel insights into the molecular mechanisms through which E2 mediates dendritic spine density in CA1 and mPFC.

17ß-Estradiol and Agonism of G-protein-Coupled Estrogen Receptor Enhance Hippocampal Memory via Different Cell-Signaling Mechanisms.

The ability of 17ß-estradiol (E2) to enhance hippocampal object recognition and spatial memory depends on rapid activation of extracellular signal-regulated kinase (ERK) in the dorsal hippocampus (DH). Although this activation can be mediated by the intracellular estrogen receptors ERα and ERß, little is known about the role that the membrane estrogen receptor GPER plays in regulating ERK or E2-mediated memory formation. In this study, post-training DH infusion of the GPER agonist G-1 enhanced object recognition and spatial memory in ovariectomized female mice, whereas the GPER antagonist G-15 impaired memory, suggesting that GPER activation, like E2, promotes hippocampal memory formation. However, unlike E2, G-1 did not increase ERK phosphorylation, but instead significantly increased phosphorylation of c-Jun N-terminal kinase (JNK) in the DH. Moreover, DH infusion of the JNK inhibitor SP600125 prevented G-1 from enhancing object recognition and spatial memory, but the ERK inhibitor U0126 did not. These data suggest that GPER enhances memory via different cell-signaling mechanisms than E2. This conclusion was supported by data showing that the ability of E2 to facilitate memory and activate ERK signaling was not blocked by G-15 or SP600125, which demonstrates that the memory-enhancing effects of E2 are not dependent on JNK or GPER activation in the DH. Together, these data indicate that GPER regulates memory independently from ERα and ERß by activating JNK signaling, rather than ERK signaling. Thus, the findings suggest that GPER in the DH may not function as an estrogen receptor to regulate object recognition and spatial memory. SIGNIFICANCE STATEMENT: Although 17ß-estradiol has long been known to regulate memory function, the molecular mechanisms underlying estrogenic memory modulation remain largely unknown. Here, we examined whether the putative membrane estrogen receptor GPER acts like the classical estrogen receptors, ERα and ERß, to facilitate hippocampal memory in female mice. Although GPER activation did enhance object recognition and spatial memory, it did so by activating different cell-signaling mechanisms from ERα, ERß, or 17ß-estradiol. These data indicate that 17ß-estradiol and GPER independently regulate hippocampal memory, and suggest that hippocampal GPER may not function as an estrogen receptor in the dorsal hippocampus. These findings are significant because they provide novel insights about the molecular mechanisms through which 17ß-estradiol modulates hippocampal memory.

Sex differences in hippocampal function.

Sex differences in the function of the hippocampus have been observed in numerous mammalian species. However, the magnitude, extent, and specificity of these differences are unclear because they can depend on factors including age, methodology, and environment. This Review will discuss seminal studies examining sex differences in hippocampal memory, neuronal morphology, synaptic plasticity, and cell signaling in humans and rodents. We also describe possible organizational and activational effects of sex steroid hormones during early development, puberty, and adulthood that may lead to sex differences observed in the hippocampus. We conclude by discussing the implications of sex differences in hippocampal function for mental health. © 2016 Wiley Periodicals, Inc.

Inhibition of local estrogen synthesis in the hippocampus impairs hippocampal memory consolidation in ovariectomized female mice.

The potent estrogen 17ß-Estradiol (E2) plays a critical role in mediating hippocampal function, yet the precise mechanisms through which E2 enhances hippocampal memory remain unclear. In young adult female rodents, the beneficial effects of E2 on memory are generally attributed to ovarian-synthesized E2. However, E2 is also synthesized in the adult brain in numerous species, where it regulates synaptic plasticity and is synthesized in response to experiences such as exposure to females or conspecific song. Although de novo E2 synthesis has been demonstrated in rodent hippocampal cultures, little is known about the functional role of local E2 synthesis in mediating hippocampal memory function. Therefore, the present study examined the role of hippocampal E2 synthesis in hippocampal memory consolidation. Using bilateral dorsal hippocampal infusions of the aromatase inhibitor letrozole, we first found that blockade of dorsal hippocampal E2 synthesis impaired hippocampal memory consolidation. We next found that elevated levels of E2 in the dorsal hippocampus observed 30min after object training were blocked by dorsal hippocampal infusion of letrozole, suggesting that behavioral experience increases acute and local E2 synthesis. Finally, aromatase inhibition did not prevent exogenous E2 from enhancing hippocampal memory consolidation, indicating that hippocampal E2 synthesis is not necessary for exogenous E2 to enhance hippocampal memory. Combined, these data are consistent with the hypothesis that hippocampally-synthesized E2 is necessary for hippocampus-dependent memory consolidation in rodents.

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

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

The mTOR and canonical Wnt signaling pathways mediate the mnemonic effects of progesterone in the dorsal hippocampus.

Although much is known about the neural mechanisms responsible for the mnemonic effects of 17ß-estradiol (E2 ), very little is understood about the mechanisms through which progesterone (P4 ) regulates memory. We previously showed that intrahippocampal infusion of P4 in ovariectomized female mice enhances object recognition (OR) memory consolidation in a manner dependent on activation of dorsal hippocampal ERK and mTOR signaling. However, the role of specific progesterone receptors (PRs) in mediating the effects of progesterone on memory consolidation and hippocampal cell signaling are unknown. Therefore, the goals of this study were to investigate the roles of membrane-associated and intracellular PRs in mediating hippocampal memory consolidation, and identify downstream cell signaling pathways activated by PRs. Membrane-associated PRs were targeted using bovine serum albumin-conjugated progesterone (BSA-P), and intracellular PRs (PR-A, PR-B) were targeted using the intracellular PR agonist R5020. Immediately after OR training, ovariectomized mice received bilateral dorsal hippocampal infusion of vehicle, P4 , BSA-P, or R5020. OR memory consolidation was enhanced by P4 , BSA-P, and R5020. However, only P4 and BSA-P activated ERK and mTOR signaling. Furthermore, dorsal hippocampal infusion of the ERK inhibitor U0126 blocked the memory-enhancing effects of BSA-P, but not R5020. The intracellular PR antagonist RU486 blocked the memory-enhancing effects of R5020, but not BSA-P. Interestingly, P4 robustly activated canonical Wnt signaling in the dorsal hippocampus, which is consistent with our recent findings that canonical Wnt signaling is necessary for OR memory consolidation. R5020, but not BSA-P, also elicited a modest increase in canonical Wnt signaling. Collectively, these data suggest that activation of ERK signaling is necessary for membrane-associated PRs to enhance OR, and indicate a role for canonical Wnt signaling in the memory-enhancing effects of intracellular PRs. This study provides the first evidence that membrane and intracellular PRs may employ different molecular mechanisms to enhance hippocampal memory.