Hippocampal Aromatization Modulates Spatial Memory and Characteristics of the Synaptic Membrane in the Male Zebra Finch

Letrozole delays acquisition of water maze task in female BALB/c mice: Possible involvement of anxiety

Letrozole, an aromatase inhibitor preventing estrogen synthesis from testosterone, is used as an adjuvant therapy in estrogen receptor-positive breast cancer patients. However, like other aromatase inhibitors, it induces many side effects, including impaired cognition. Despite its negative effect in humans, results from animal models are inconsistent and suggest that letrozole can either impair or improve cognition. Here, we studied the effects of chronic letrozole treatment on cognitive behavior of adult female BALB/c mice, a relevant animal model for breast cancer studies, to develop an appropriate animal model aimed at testing therapies to mitigate side effects of letrozole. In Morris water maze, letrozole 0.1 mg/kg impaired reference learning and memory. Interestingly, most of the letrozole 0.1 mg/kg-treated mice were able to learn the new platform position in reversal training and performed similar to control mice in a reversal probe test. Results of the reversal test suggest that letrozole did not completely disrupt spatial navigation, but rather delayed acquisition of spatial information. The delay might be related to increased anxiety as suggested by increased thigmotactic behavior during the reference memory training. The learning impairment was water maze-specific since we did not observe impairment in other spatial tasks such as in Y-maze or object location test. In contrast, the dose of 0.3 mg/kg did not have effect on water maze learning and facilitated locomotor habituation and recognition in novel object recognition test. The current study shows that letrozole dose-dependently modulates behavioral response and that its effects are task-dependent.

Differences in vocal brain areas and astrocytes between the house wren and the rufous-tailed hummingbird

The house wren shows complex song, and the rufous-tailed hummingbird has a simple song. The location of vocal brain areas supports the song's complexity; however, these still need to be studied. The astrocytic population in songbirds appears to be associated with change in vocal control nuclei; however, astrocytic distribution and morphology have not been described in these species. Consequently, we compared the distribution and volume of the vocal brain areas: HVC, RA, Area X, and LMAN, cell density, and the morphology of astrocytes in the house wren and the rufous-tailed hummingbird. Individuals of the two species were collected, and their brains were analyzed using serial Nissl- NeuN- and MAP2-stained tissue scanner imaging, followed by 3D reconstructions of the vocal areas; and GFAP and S100β astrocytes were analyzed in both species. We found that vocal areas were located close to the cerebral midline in the house wren and a more lateralized position in the rufous-tailed hummingbird. The LMAN occupied a larger volume in the rufous-tailed hummingbird, while the RA and HVC were larger in the house wren. While Area X showed higher cell density in the house wren than the rufous-tailed hummingbird, the LMAN showed a higher density in the rufous-tailed hummingbird. In the house wren, GFAP astrocytes in the same bregma where the vocal areas were located were observed at the laminar edge of the pallium (LEP) and in the vascular region, as well as in vocal motor relay regions in the pallidum and mesencephalon. In contrast, GFAP astrocytes were found in LEP, but not in the pallidum and mesencephalon in hummingbirds. Finally, when comparing GFAP astrocytes in the LEP region of both species, house wren astrocytes exhibited significantly more complex morphology than those of the rufous-tailed hummingbird. These findings suggest a difference in the location and cellular density of vocal circuits, as well as morphology of GFAP astrocytes between the house wren and the rufous-tailed hummingbird.

What do zebra finches learn besides singing? Systematic mapping of the literature and presentation of an efficient associative learning test

The process of learning in birds has been extensively studied, with a focus on species such as pigeons, parrots, chickens, and crows. In recent years, the zebra finch has emerged as a model species in avian cognition, particularly in song learning. However, other cognitive domains such as spatial memory and associative learning could also be critical to fitness and survival, particularly during the intensive juvenile period. In this systematic review, we provide an overview of cognitive studies on zebra finches, with a focus on domains other than song learning. Our findings indicate that spatial, associative, and social learning are the most frequently studied domains, while motoric learning and inhibitory control have been examined less frequently over 30 years of research. All of the 60 studies included in this review were conducted on captive birds, limiting the generalizability of the findings to wild populations. Moreover, only two of the studies were conducted on juveniles, highlighting the need for more research on this critical period of learning. To address this research gap, we propose a high-throughput method for testing associative learning performance in a large number of both juvenile and adult zebra finches. Our results demonstrate that learning can occur in both age groups, thus encouraging researchers to also perform cognitive tests on juveniles. We also note the heterogeneity of methodologies, protocols, and subject exclusion criteria applied by different researchers, which makes it difficult to compare results across studies. Therefore, we call for better communication among researchers to develop standardised methodologies for studying each cognitive domain at different life stages and also in their natural conditions.

Estradiol decreases the excitability of RA projection neurons in adult male zebra finches

Zebra finches are essential animal models for studying learned vocal signals. The robust nucleus of the arcopallium (RA) plays an important role in regulating singing behavior. Our previous study showed that castration inhibited the electrophysiological activity of RA projection neurons (PNs) in male zebra finches, demonstrating that testosterone modulates the excitability of RA PNs. Testosterone can be converted into estradiol (E2) in the brain through aromatase; however, the physiological functions of E2 in RA are still unknown. This study aimed to investigate the electrophysiological activities of E2 on the RA PNs of male zebra finches through patch-clamp recording. E2 rapidly decreased the rate of evoked and spontaneous action potentials (APs) of RA PNs, hyperpolarized the resting membrane potential, and decreased the membrane input resistance. Moreover, the G-protein-coupled membrane-bound estrogen receptor (GPER) agonist G1 decreased both the evoked and spontaneous APs of RA PNs. Furthermore, the GPER antagonist G15 had no effect on the evoked and spontaneous APs of RA PNs; E2 and G15 together also had no effect on the evoked and spontaneous APs of RA PNs. These findings suggested that E2 rapidly decreased the excitability of RA PNs and its binding to GPER suppressed the excitability of RA PNs. These pieces of evidence helped us fully understand the principle of E2 signal mediation via its receptors to modulate the excitability of RA PNs in songbirds.

Spatial and temporal specificity of neuroestradiol provision in the songbird

Steroid hormones are often synthesized in multiple tissues, affect several different targets, and modulate numerous physiological endpoints. The mechanisms by which this modulation is achieved with temporal and spatial specificity remain unclear. 17β-estradiol for example, is made in several peripheral tissues and in the brain, where it affects a diverse set of behaviors. How is estradiol delivered to the right target, at the right time, and at the right concentration? In the last two decades, we have learned that aromatase (estrogen-synthase) can be induced in astrocytes following damage to the brain and is expressed at central synapses. Both mechanisms of estrogen provision confer spatial and temporal specificity on a lipophilic neurohormone with potential access to all cells and tissues. In this review, I trace the progress in our understanding of astrocytic and synaptic aromatization. I discuss the incidence, regulation, and functions of neuroestradiol provision by aromatization, first in astrocytes and then at synapses. Finally, I focus on a relatively novel hypothesis about the role of neuroestradiol in the orchestration of species-specific behaviors.

The form, function, and evolutionary significance of neural aromatization

Songbirds have emerged as exceptional research subjects for helping us appreciate and understand estrogen synthesis and function in brain. In the context of recognizing the vertebrate-wide importance of brain aromatase expression, in this review we highlight where we believe studies of songbirds have provided clarification and conceptual insight. We follow by focusing on more recent studies of aromatase and neuroestrogen function in the hippocampus and the pallial auditory processing region NCM of songbirds. With perspectives drawn from this body of work, we speculate that the evolution of enhanced neural estrogen signaling, including in the mediation of social behaviors, may have given songbirds the resilience to radiate into one of the most successful vertebrate groups on the planet.

Neuron-Derived Estrogen-A Key Neuromodulator in Synaptic Function and Memory

In addition to being a steroid hormone, 17β-estradiol (E₂) is also a neurosteroid produced in neurons in various regions of the brain of many species, including humans. Neuron-derived E₂ (NDE₂) is synthesized from androgen precursors via the action of the biosynthetic enzyme aromatase, which is located at synapses and in presynaptic terminals in neurons in both the male and female brain. In this review, we discuss evidence supporting a key role for NDE₂ as a neuromodulator that regulates synaptic plasticity and memory. Evidence supporting an important neuromodulatory role of NDE₂ in the brain has come from studies using aromatase inhibitors, aromatase overexpression in neurons, global aromatase knockout mice, and the recent development of conditional forebrain neuron-specific knockout mice. Collectively, these studies demonstrate a key role of NDE₂ in the regulation of synapse and spine density, efficacy of excitatory synaptic transmission and long-term potentiation, and regulation of hippocampal-dependent recognition memory, spatial reference memory, and contextual fear memory. NDE₂ is suggested to achieve these effects through estrogen receptor-mediated regulation of rapid kinase signaling and CREB-BDNF signaling pathways, which regulate actin remodeling, as well as transcription, translation, and transport of synaptic proteins critical for synaptic plasticity and function.

Brain-derived estrogen and neural function

Although classically known as an endocrine signal produced by the ovary, 17β-estradiol (E₂) is also a neurosteroid produced in neurons and astrocytes in the brain of many different species. In this review, we provide a comprehensive overview of the localization, regulation, sex differences, and physiological/pathological roles of brain-derived E₂ (BDE₂). Much of what we know regarding the functional roles of BDE₂ has come from studies using specific inhibitors of the E₂ synthesis enzyme, aromatase, as well as the recent development of conditional forebrain neuron-specific and astrocyte-specific aromatase knockout mouse models. The evidence from these studies support a critical role for neuron-derived E₂ (NDE₂) in the regulation of synaptic plasticity, memory, socio-sexual behavior, sexual differentiation, reproduction, injury-induced reactive gliosis, and neuroprotection. Furthermore, we review evidence that astrocyte-derived E₂ (ADE₂) is induced following brain injury/ischemia, and plays a key role in reactive gliosis, neuroprotection, and cognitive preservation. Finally, we conclude by discussing the key controversies and challenges in this area, as well as potential future directions for the field.

Brain-Generated 17β-Estradiol Modulates Long-Term Synaptic Plasticity in the Primary Auditory Cortex of Adult Male Rats

Neuron-derived 17β-estradiol (E2) alters synaptic transmission and plasticity in brain regions with endocrine and non-endocrine functions. Investigations into a modulatory role of E2 in synaptic activity and plasticity have mainly focused on the rodent hippocampal formation. In songbirds, E2 is synthesized by auditory forebrain neurons and promotes auditory signal processing and memory for salient acoustic stimuli; however, the modulatory effects of E2 on memory-related synaptic plasticity mechanisms have not been directly examined in the auditory forebrain. We investigated the effects of bidirectional E2 manipulations on synaptic transmission and long-term potentiation (LTP) in the rat primary auditory cortex (A1). Immunohistochemistry revealed widespread neuronal expression of the E2 biosynthetic enzyme aromatase in multiple regions of the rat sensory and association neocortex, including A1. In A1, E2 application reduced the threshold for in vivo LTP induction at layer IV synapses, whereas pharmacological suppression of E2 production by aromatase inhibition abolished LTP induction at layer II/III synapses. In acute A1 slices, glutamate and γ-aminobutyric acid (GABA) receptor-mediated currents were sensitive to E2 manipulations in a layer-specific manner. These findings demonstrate that locally synthesized E2 modulates synaptic transmission and plasticity in A1 and suggest potential mechanisms by which E2 contributes to auditory signal processing and memory.

The G protein-coupled estrogen receptor (GPER) regulates recognition and aversively-motivated memory in male rats

Estrogens, particularly 17β-estradiol (estradiol, E₂), regulate memory formation. E2 acts through its intracellular receptors, estrogen receptors (ER) ERα and ERβ, as well as a recently identified G protein-coupled estrogen receptor (GPER). Although the effects of E2 on memory have been investigated, studies examining the effects of GPER stimulation are scarce. Selective GPER agonism improves memory in ovariectomized female rats, but little information is available regarding the effects of GPER stimulation in male rodents. The aim of the present study was to investigate the effects of the GPER agonist, G1, on consolidation and reconsolidation of inhibitory avoidance (IA) and object recognition (OR) memory in male rats. Animals received vehicle, G1 (15, 75, 150 µg/kg; i.p.), or the GPER antagonist G15 (100 µg/kg; i.p.) immediately after training, or G1 (150 µg/kg; i.p.) 3 or 6 h after training. To investigate reconsolidation, G1 was administered immediately after IA retention Test 1. Results indicated that G1 administered immediately after training at the highest dose enhanced both OR and IA memory consolidation, while GPER blockade immediately after training impaired OR. No effects of GPER stimulation were observed when G1 was given 3 or 6 h after training or after Test 1. The present findings provide evidence that GPER is involved in the early stages of memory consolidation in both neutral and emotional memory tasks in male adult rats.

Rapid Estrogenic and Androgenic Neurosteroids Effects in the Induction of Long-Term Synaptic Changes: Implication for Early Memory Formation

Mounting experimental evidence demonstrate that sex neuroactive steroids (neurosteroids) are essential for memory formation. Neurosteroids have a profound impact on the function and structure of neural circuits and their local synthesis is necessary for the induction of both long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission and for neural spine formation in different areas of the central nervous system (CNS). Several studies demonstrated that in the hippocampus, 17β-estradiol (E2) is necessary for inducing LTP, while 5α-dihydrotestosterone (DHT) is necessary for inducing LTD. This contribution has been proven by administering sex neurosteroids in rodent models and by using blocking agents of their synthesis or of their specific receptors. The general opposite role of sex neurosteroids in synaptic plasticity appears to be dependent on their different local availability in response to low or high frequency of synaptic stimulation, allowing the induction of bidirectional synaptic plasticity. The relevant contribution of these neurosteroids to synaptic plasticity has also been described in other brain regions involved in memory processes such as motor learning, as in the case of the vestibular nuclei, the cerebellum, and the basal ganglia, or as the emotional circuit of the amygdala. The rapid effects of sex neurosteroids on neural synaptic plasticity need the maintenance of a tonic or phasic local steroid synthesis determined by neural activity but might also be influenced by circulating hormones, age, and gender. To disclose the exact mechanisms how sex neurosteroids participate in finely tuning long-term synaptic changes and spine remodeling, further investigation is required.

Estrogen as a Neuroprotectant in Both Sexes: Stories From the Bird Brain

Estrogens such as estradiol (E2) are potent effectors of neural structure and function via peripheral and central synthesis. In the zebra finch (Taeniopygia guttata), neural E2 synthesis is among the highest reported in homeotherms due to the abundant constitutive expression of aromatase (E-synthase) in discrete neuronal pools across the forebrain. Following penetrating or concussive trauma, E2 synthesis increases even further via the induced expression of aromatase in reactive astrocytes around the site of damage. Injury-associated astrocytic aromatization occurs in the brains of both sexes regardless of the site of injury and can remain elevated for weeks following trauma. Interestingly, penetrating injury induces astrocytic aromatase more rapidly in females compared to males, but this sex difference is not detectable 24 h posttrauma. Indeed, unilateral penetrating injury can increase E2 content 4-fold relative to the contralateral uninjured hemisphere, suggesting that glial aromatization may be a powerful source of neural E2 available to circuits. Glial aromatization is neuroprotective as inhibition of injury-induced aromatase increases neuroinflammation, gliosis, necrosis, apoptosis, and infarct size. These effects are ameliorated upon replacement with E2, suggesting that the songbird may have evolved a rapidly responsive neurosteroidogenic system to protect vulnerable brain circuits. The precise signals that induce aromatase expression in astrocytes include elements of the inflammatory cascade and underscore the sentinel role of the innate immune system as a crucial effector of trauma-associated E2 provision in the vertebrate brain. This review will describe the inductive signals of astroglial aromatase and the neuroprotective role for glial E2 synthesis in the adult songbird brains of both sexes.

De Novo Synthesized Estradiol: A Role in Modulating the Cerebellar Function

The estrogen estradiol is a potent neuroactive steroid that may regulate brain structure and function. Although the effects of estradiol have been historically associated with gonadal secretion, the discovery that this steroid may be synthesized within the brain has expanded this traditional concept. Indeed, it is accepted that de novo synthesized estradiol in the nervous system (nE2) may modulate several aspects of neuronal physiology, including synaptic transmission and plasticity, thereby influencing a variety of behaviors. These modulations may be on a time scale of minutes via non-classical and often membrane-initiated mechanisms or hours and days by classical actions on gene transcription. Besides the high level, recent investigations in the cerebellum indicate that even a low aromatase expression can be related to the fast nE2 effect on brain functioning. These pieces of evidence point to the importance of an on-demand and localized nE2 synthesis to rapidly contribute to regulating the synaptic transmission. This review is geared at exploring a new scenario for the impact of estradiol on brain processes as it emerges from the nE2 action on cerebellar neurotransmission and cerebellum-dependent learning.

Neuroestrogen synthesis modifies neural representations of learned song without altering vocal imitation in developing songbirds

Birdsong learning, like human speech, depends on the early memorization of auditory models, yet how initial auditory experiences are formed and consolidated is unclear. In songbirds, a putative cortical locus is the caudomedial nidopallium (NCM), and one mechanism to facilitate auditory consolidation is 17β-estradiol (E2), which is associated with human speech-language development, and is abundant in both NCM and human temporal cortex. Circulating and NCM E2 levels are dynamic during learning, suggesting E2’s involvement in encoding recent auditory experiences. Therefore, we tested this hypothesis in juvenile male songbirds using a comprehensive assessment of neuroanatomy, behavior, and neurophysiology. First, we found that brain aromatase expression, and thus the capacity to synthesize neuroestrogens, remains high in the auditory cortex throughout development. Further, while systemic estrogen synthesis blockade suppressed juvenile song production, neither systemic nor unilateral E2 synthesis inhibition in NCM disrupted eventual song imitation. Surprisingly, early life neuroestrogen synthesis blockade in NCM enhanced the neural representations of both the birds’ own song and the tutor song in NCM and a downstream sensorimotor region, HVC, respectively. Taken together, these findings indicate that E2 plays a multifaceted role during development, and that, contrary to prediction, tutor song memorization is unimpaired by unilateral estrogen synthesis blockade in the auditory cortex.

The role of hippocampal estradiol in synaptic plasticity and memory: A systematic review

The consolidation of long-term memory is influenced by various neuromodulators. One of these is estradiol, a steroid hormone that is synthesized both in peripheral endocrine tissue and in the brain, including the hippocampus. Here, we examine the evidence regarding the role of estradiol in the hippocampus, specifically, in memory formation and its effects on the molecular mechanisms underlying synaptic plasticity. We conclude that estradiol improves memory consolidation and, thereby, long-term memory. Previous studies have shown that it does this in three, interconnected ways: (1) via functional changes in excitatory activity, (2) signaling changes in calcium dynamics, protein phosphorylation and protein expression, and (3) structural changes to synaptic morphology. Through a functional network analysis of proteins affected by estradiol, we identify potential protein-protein interactions that further support a role for estradiol in modulating synaptic plasticity as well as highlight signaling pathways that may be involved in these changes within the hippocampus.

G Protein-Coupled Estrogen Receptor Immunoreactivity Fluctuates During the Estrous Cycle and Show Sex Differences in the Amygdala and Dorsal Hippocampus

G protein-coupled estrogen receptor (GPER) in the amygdala and the dorsal hippocampus mediates actions of estradiol on anxiety, social recognition and spatial memory. In addition, GPER participates in the estrogenic regulation of synaptic function in the amygdala and in the process of adult neurogenesis in the dentate gyrus. While the distribution of the canonical estrogen receptors α and β in the amygdala and dorsal hippocampus are well characterized, little is known about the regional distribution of GPER in these brain regions and whether this distribution is affected by sex or the stages of the estrous cycle. In this study we performed a morphometric analysis of GPER immunoreactivity in the posterodorsal medial, anteroventral medial, basolateral, basomedial and central subdivisions of the amygdala and in all the histological layers of CA1 and the dentate gyrus of the dorsal hippocampal formation. The number of GPER immunoreactive cells was estimated in these different structures. GPER immunoreactivity was detected in all the assessed subdivisions of the amygdaloid nucleus and dorsal hippocampal formation. The number of GPER immunoreactive cells was higher in males than in estrus females in the central (P = 0.001) and the posterodorsal medial amygdala (P < 0.05); higher in males than in diestrus females in the strata orients (P < 0.01) and radiatum-lacunosum-moleculare (P < 0.05) of CA1-CA3 and in the molecular layer of the dentate gyrus (P < 0.01); higher in diestrus females than in males in the basolateral amygdala (P < 0.05); higher in diestrus females than in estrus females in the central (P < 0.01), posterodorsal medial (P < 0.01) and basolateral amygdala (P < 0.01) and higher in estrus females than in diestrus females in the strata oriens (P < 0.05) and radiatum-lacunosum-moleculare (P < 0.05) of CA1-CA3 and in the molecular layer (P < 0.05) and the hilus of the dentate gyrus (P < 0.05). The findings suggest that estrogenic regulation of the amygdala and hippocampus through GPER may be different in males and in females and may fluctuate during the estrous cycle.

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

Although 17β-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.

Detection of estradiol in rat brain tissues: Contribution of local versus systemic production

Estrogens play important roles in regulating brain development, brain function, and behavior. Many studies have evaluated these effects using ovariectomized (OVX) rats or mice with different doses of estrogen replacement, assuming that estradiol levels in all regions of the brain are the same as levels achieved in the serum. It is well known, however, that the brain contains all the enzymes necessary to produce estrogens, and that estrogen levels in the brain are determined by both systemic and local production and are region-specific. The present study conducted a detailed analysis of the relationship between systemic levels of 17-β-estradiol (E2) achieved by estrogen replacement and levels achieved in specific regions of the brain. Levels of E2 were measured in both brain and serum in OVX rats treated with different doses of estradiol benzoate (EB) using a novel and recently validated UPLC-MS/MS method. Results confirmed significantly higher levels of E2 in the brain than in serum in brain regions known to contain aromatase (ARO) activity, both in OVX controls and in rats treated with physiological doses of EB. Additional studies compared the level of E2 and testosterone (T) in the brain and serum between testosterone propionate (TP) treated OVX and male. This demonstrated higher levels of E2 in certain brain regions of males than in TP treated OVX females even though T levels in the brain and serum were similar between the two groups. Studies also demonstrated that the differences between serum and brain levels of E2 can be eliminated by letrozole (ARO inhibitor) treatment, which indicates that the differences are due to local ARO activity. Collectively the results provide a detailed analysis of brain region-specific E2 concentrations in OVX, E2-, and T-treated rats and demonstrate the degree to which these concentrations are ARO-dependent.

Neuron-Derived Estrogen Regulates Synaptic Plasticity and Memory

17β-estradiol (E2) is produced from androgens via the action of the enzyme aromatase. E2 is known to be made in neurons in the brain, but its precise functions in the brain are unclear. Here, we used a forebrain-neuron-specific aromatase knock-out (FBN-ARO-KO) mouse model to deplete neuron-derived E2 in the forebrain of mice and thereby elucidate its functions. FBN-ARO-KO mice showed a 70–80% decrease in aromatase and forebrain E2 levels compared with FLOX controls. Male and female FBN-ARO-KO mice exhibited significant deficits in forebrain spine and synaptic density, as well as hippocampal-dependent spatial reference memory, recognition memory, and contextual fear memory, but had normal locomotor function and anxiety levels. Reinstating forebrain E2 levels via exogenous in vivo E2 administration was able to rescue both the molecular and behavioral defects in FBN-ARO-KO mice. Furthermore, in vitro studies using FBN-ARO-KO hippocampal slices revealed that, whereas induction of long-term potentiation (LTP) was normal, the amplitude was significantly decreased. Intriguingly, the LTP defect could be fully rescued by acute E2 treatment in vitro. Mechanistic studies revealed that FBN-ARO-KO mice had compromised rapid kinase (AKT, ERK) and CREB-BDNF signaling in the hippocampus and cerebral cortex. In addition, acute E2 rescue of LTP in hippocampal FBN-ARO-KO slices could be blocked by administration of a MEK/ERK inhibitor, further suggesting a key role for rapid ERK signaling in neuronal E2 effects. In conclusion, the findings provide evidence of a critical role for neuron-derived E2 in regulating synaptic plasticity and cognitive function in the male and female brain.

SIGNIFICANCE STATEMENT The steroid hormone 17β-estradiol (E2) is well known to be produced in the ovaries in females. Intriguingly, forebrain neurons also express aromatase, the E2 biosynthetic enzyme, but the precise functions of neuron-derived E2 is unclear. Using a novel forebrain-neuron-specific aromatase knock-out mouse model to deplete neuron-derived E2, the current study provides direct genetic evidence of a critical role for neuron-derived E2 in the regulation of rapid AKT-ERK and CREB-BDNF signaling in the mouse forebrain and demonstrates that neuron-derived E2 is essential for normal expression of LTP, synaptic plasticity, and cognitive function in both the male and female brain. These findings suggest that neuron-derived E2 functions as a novel neuromodulator in the forebrain to control synaptic plasticity and cognitive function.

Adverse Effects of Aromatase Inhibition on the Brain and Behavior in a Nonhuman Primate

Breast cancer patients using aromatase inhibitors (AIs) as an adjuvant therapy often report side effects, including hot flashes, mood changes, and cognitive impairment. Despite long-term use in humans, little is known about the effects of continuous AI administration on the brain and cognition. We used a primate model of human cognitive aging, the common marmoset, to examine the effects of a 4-week daily administration of the AI letrozole (20 μg, p.o.) on cognition, anxiety, thermoregulation, brain estrogen content, and hippocampal pyramidal cell physiology. Letrozole treatment was administered to both male and female marmosets and reduced peripheral levels of estradiol (E2), but unexpectedly increased E2 levels in the hippocampus. Spatial working memory and intrinsic excitability of hippocampal neurons were negatively affected by the treatment possibly due to increased hippocampal E2. While no changes in hypothalamic E2 were observed, thermoregulation was disrupted by letrozole in females only, indicating some impact on hypothalamic activity. These findings suggest adverse effects of AIs on the primate brain and call for new therapies that effectively prevent breast cancer recurrence while minimizing side effects that further compromise quality of life.SIGNIFICANCE STATEMENT Aromatase inhibitors (AIs) are used as an adjuvant therapy for estrogen-receptor-positive breast cancer and are associated with side effects, including hot flashes, depression/anxiety, and memory deficits severe enough for many women to discontinue this life-saving treatment. AIs are also used by men, yet sex differences in the reported side effects have not been systematically studied. We show that AI-treated male and female marmosets exhibit behavioral changes consistent with these CNS symptoms, as well as elevated hippocampal estradiol and compromised hippocampal physiology. These findings illustrate the need for (1) a greater understanding of the precise mechanisms by which AIs impact brain function and (2) the development of new treatment approaches for breast cancer patients that minimize adverse effects on the brain.

Non-reproductive Functions of Aromatase in the Central Nervous System Under Physiological and Pathological Conditions

The modulation of brain function and behavior by steroid hormones was classically associated with their secretion by peripheral endocrine glands. The discovery that the brain expresses the enzyme aromatase, which produces estradiol from testosterone, expanded this traditional concept. One of the best-studied roles of brain estradiol synthesis is the control of reproductive behavior. In addition, there is increasing evidence that estradiol from neural origin is also involved in a variety of non-reproductive functions. These include the regulation of neurogenesis, neuronal development, synaptic transmission, and plasticity in brain regions not directly related with the control of reproduction. Central aromatase is also involved in the modulation of cognition, mood, and non-reproductive behaviors. Furthermore, under pathological conditions aromatase is upregulated in the central nervous system. This upregulation represents a neuroprotective and likely also a reparative response by increasing local estradiol levels in order to maintain the homeostasis of the neural tissue. In this paper, we review the non-reproductive functions of neural aromatase and neural-derived estradiol under physiological and pathological conditions. We also consider the existence of sex differences in the role of the enzyme in both contexts.

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 17β‑estradiol (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.

Neuroestrogens rapidly shape auditory circuits to support communication learning and perception: Evidence from songbirds

Contribution to Special Issue on Fast effects of steroids. Steroid hormones, such as estrogens, were once thought to be exclusively synthesized in the ovaries and enact transcriptional changes over the course of hours to days. However, estrogens are also locally synthesized within neural circuits, wherein they rapidly (within minutes) modulate a range of behaviors, including spatial cognition and communication. Here, we review the role of brain-derived estrogens (neuroestrogens) as modulators within sensory circuits in songbirds. We first present songbirds as an attractive model to explore how neuroestrogens in auditory cortex modulate vocal communication processing and learning. Further, we examine how estrogens may enhance vocal learning and auditory memory consolidation in sensory cortex via mechanisms similar to those found in the hippocampus of rodents and birds. Finally, we propose future directions for investigation, including: 1) the extent of developmental and hemispheric shifts in aromatase and membrane estrogen receptor expression in auditory circuits; 2) how neuroestrogens may impact inhibitory interneurons to regulate audition and critical period plasticity; and, 3) dendritic spine plasticity as a candidate mechanism mediating estrogen-dependent effects on vocal learning. Together, this perspective of estrogens as neuromodulators in the vertebrate brain has opened new avenues in understanding sensory plasticity, including how hormones can act on communication circuits to influence behaviors in other vocal learning species, such as in language acquisition and speech processing in humans.

Estrogenic regulation of memory consolidation: A look beyond the hippocampus, ovaries, and females

The potent estrogen 17β-estradiol (E2) has long been known to regulate the hippocampus and hippocampal-dependent memories in females, and research from the past decade has begun to shed light on the molecular mechanisms through which E2 mediates memory formation in females. Although E2 can also regulate hippocampal function in males, relatively little is known about how E2 influences memory formation in males, or whether sex differences in underlying mechanisms exist. This review, based on a talk given in April 2017 at the American University symposium entitled, "Sex Differences: From Neuroscience to the Clinic and Beyond", first provides an overview of the molecular mechanisms in the dorsal hippocampus through which E2 enhances memory consolidation in ovariectomized female mice. Next, newer research is described demonstrating key roles for the prefrontal cortex and de novo hippocampal E2 synthesis to the memory-enhancing effects of E2 in females. The review then discusses the effects of de novo and exogenous E2 on hippocampal memory consolidation in both sexes, and putative sex differences in the underlying molecular mechanisms through which E2 enhances memory formation. The review concludes by discussing the importance and implications of sex differences in the molecular mechanisms underlying E2-induced memory consolidation for human health.

A Membrane G-Protein-Coupled Estrogen Receptor Is Necessary but Not Sufficient for Sex Differences in Zebra Finch Auditory Coding

Estradiol acts as a neuromodulator in brain regions important for cognition and sensory processing. Estradiol also shapes brain sex differences but rarely have these concepts been considered simultaneously. In male and female songbirds, estradiol rapidly increases within the auditory forebrain during song exposure and enhances local auditory processing. We tested whether G-protein-coupled estrogen receptor 1 (GPER1), a membrane-bound estrogen receptor, is necessary and sufficient for neuroestrogen regulation of forebrain auditory processing in male and female zebra finches (Taeniopygia guttata). At baseline, we observed that females had elevated single-neuron responses to songs vs males. In males, narrow-spiking (NS) neurons were more responsive to conspecific songs than broad-spiking (BS) neurons, yet cell types were similarly auditory responsive in females. Following acute inactivation of GPER1, auditory responsiveness and coding were suppressed in male NS yet unchanged in female NS and in BS of both sexes. By contrast, GPER1 activation did not mimic previously established estradiol actions in either sex. Lastly, the expression of GPER1 and its coexpression with an inhibitory neuron marker were similarly abundant in males and females, confirming anatomical similarity in the auditory forebrain. In this study, we found: (1) a role for GPER1 in regulating sensory processing and (2) a sex difference in auditory processing of complex vocalizations in a cell type-specific manner. These results reveal sex specificity of a rapid estrogen signaling mechanism in which neuromodulation accounts and/or compensates for brain sex differences, dependent on cell type, in brain regions that are anatomically similar in both sexes.

Cognitive Effects of Aromatase and Possible Role in Memory Disorders

Diverse cognitive functions in many vertebrate species are influenced by local conversion of androgens to 17β-estradiol (E2) by aromatase. This enzyme is highly expressed in various brain regions across species, with some inter-species variation in terms of regional brain expression. Since women with breast cancer and men and women with other disorders are often treated with aromatase inhibitors (AI), these populations might be especially vulnerable to cognitive deficits due to low neuroE2 synthesis, i.e., synthesis of E2 directly within the brain. Animal models have been useful in deciphering aromatase effects on cognitive functions. Consequences of AI administration at various life cycle stages have been assessed on auditory, song processing, and spatial memory in birds and various aspects of cognition in rodent models. Additionally, cognitive deficits have been described in aromatase knockout (ArKO) mice that systemically lack this gene throughout their lifespan. This review will consider evidence to date that AI treatment in male and female rodent models, birds, and humans results in cognitive impairments. How brain aromatase regulates cognitive function throughout the lifespan, and gaps in current knowledge will be considered, along with future directions to better define how aromatase might guide learning and memory from early development through the geriatric period. Better understanding the importance of E2 synthesis on neurobehavioral responses at various ages will likely aid in the discovery of therapeutic strategies to prevent potential cognitive deficits, including Alzheimer's Disease, in individuals treated with AI or those possessing CYP19 gene polymorphisms, as well as cognitive effects of normal aging that may be related to changes in brain aromatase activity.

Sensory Coding and Sensitivity to Local Estrogens Shift during Critical Period Milestones in the Auditory Cortex of Male Songbirds

Vocal learning occurs during an experience-dependent, age-limited critical period early in development. In songbirds, vocal learning begins when presinging birds acquire an auditory memory of their tutor's song (sensory phase) followed by the onset of vocal production and refinement (sensorimotor phase). Hearing is necessary throughout the vocal learning critical period. One key brain area for songbird auditory processing is the caudomedial nidopallium (NCM), a telencephalic region analogous to mammalian auditory cortex. Despite NCM's established role in auditory processing, it is unclear how the response properties of NCM neurons may shift across development. Moreover, communication processing in NCM is rapidly enhanced by local 17β-estradiol (E2) administration in adult songbirds; however, the function of dynamically fluctuating E₂ in NCM during development is unknown. We collected bilateral extracellular recordings in NCM coupled with reverse microdialysis delivery in juvenile male zebra finches (Taeniopygia guttata) across the vocal learning critical period. We found that auditory-evoked activity and coding accuracy were substantially higher in the NCM of sensory-aged animals compared to sensorimotor-aged animals. Further, we observed both age-dependent and lateralized effects of local E₂ administration on sensory processing. In sensory-aged subjects, E₂ decreased auditory responsiveness across both hemispheres; however, a similar trend was observed in age-matched control subjects. In sensorimotor-aged subjects, E₂ dampened auditory responsiveness in left NCM but enhanced auditory responsiveness in right NCM. Our results reveal an age-dependent physiological shift in auditory processing and lateralized E₂ sensitivity that each precisely track a key neural "switch point" from purely sensory (pre-singing) to sensorimotor (singing) in developing songbirds.