Postlesion estradiol treatment increases cortical cholinergic innervations via estrogen receptor-α dependent nonclassical estrogen signaling in vivo

Estradiol and Estrogen-like Alternative Therapies in Use: The Importance of the Selective and Non-Classical Actions

Estrogen is one of the most important female sex hormones, and is indispensable for reproduction. However, its role is much wider. Among others, due to its neuroprotective effects, estrogen protects the brain against dementia and complications of traumatic injury. Previously, it was used mainly as a therapeutic option for influencing the menstrual cycle and treating menopausal symptoms. Unfortunately, hormone replacement therapy might be associated with detrimental side effects, such as increased risk of stroke and breast cancer, raising concerns about its safety. Thus, tissue-selective and non-classical estrogen analogues have become the focus of interest. Here, we review the current knowledge about estrogen effects in a broader sense, and the possibility of using selective estrogen-receptor modulators (SERMs), selective estrogen-receptor downregulators (SERDs), phytoestrogens, and activators of non-genomic estrogen-like signaling (ANGELS) molecules as treatment.

Ovariectomy-induced hormone deprivation aggravates Aβ1-42 deposition in the basolateral amygdala and cholinergic fiber loss in the cortex but not cognitive behavioral symptoms in a triple transgenic mouse model of Alzheimer's disease

Alzheimer's disease is the most common type of dementia, being highly prevalent in elderly women. The advanced progression may be due to decreased hormone synthesis during post-menopause as estradiol and progesterone both have neuroprotective potentials. We aimed to confirm that female hormone depletion aggravates the progression of dementia in a triple transgenic mouse model of Alzheimer's disease (3xTg-AD). As pathological hallmarks are known to appear in 6-month-old animals, we expected to see disease-like changes in the 4-month-old 3xTg-AD mice only after hormone depletion. Three-month-old female 3xTg-AD mice were compared with their age-matched controls. As a menopause model, ovaries were removed (OVX or Sham surgery). After 1-month recovery, the body composition of the animals was measured by an MRI scan. The cognitive and anxiety parameters were evaluated by different behavioral tests, modeling different aspects (Y-maze, Morris water maze, open-field, social discrimination, elevated plus maze, light-dark box, fox odor, operant conditioning, and conditioned fear test). At the end of the experiment, uterus was collected, amyloid-β accumulation, and the cholinergic system in the brain was examined by immunohistochemistry. The uterus weight decreased, and the body weight increased significantly in the OVX animals. The MRI data showed that the body weight change can be due to fat accumulation. Moreover, OVX increased anxiety in control, but decreased in 3xTg-AD animals, the later genotype being more anxious by default based on the anxiety z-score. In general, 3xTg-AD mice moved less. In relation to cognition, neither the 3xTg-AD genotype nor OVX surgery impaired learning and memory in general. Despite no progression of dementia-like behavior after OVX, at the histological level, OVX aggravated the amyloid-β plaque deposition in the basolateral amygdala and induced early cholinergic neuronal fiber loss in the somatosensory cortex of the transgenic animals. We confirmed that OVX induced menopausal symptoms. Removal of the sexual steroids aggravated the appearance of AD-related alterations in the brain without significantly affecting the behavior. Thus, the OVX in young, 3-month-old 3xTg-AD mice might be a suitable model for testing the effect of new treatment options on structural changes; however, to reveal any beneficial effect on behavior, a later time point might be needed.

Targeting the non-classical estrogen pathway in neurodegenerative diseases and brain injury disorders

Estrogens can alter the biology of various tissues and organs, including the brain, and thus play an essential role in modulating homeostasis. Despite its traditional role in reproduction, it is now accepted that estrogen and its analogues can exert neuroprotective effects. Several studies have shown the beneficial effects of estrogen in ameliorating and delaying the progression of neurodegenerative diseases, including Alzheimer's and Parkinson's disease and various forms of brain injury disorders. While the classical effects of estrogen through intracellular receptors are more established, the impact of the non-classical pathway through receptors located at the plasma membrane as well as the rapid stimulation of intracellular signaling cascades are still under active research. Moreover, it has been suggested that the non-classical estrogen pathway plays a crucial role in neuroprotection in various brain areas. In this mini-review, we will discuss the use of compounds targeting the non-classical estrogen pathway in their potential use as treatment in neurodegenerative diseases and brain injury disorders.

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.

Cholinergic modulation of sensory processing in awake mouse cortex

Cholinergic modulation of brain activity is fundamental for awareness and conscious sensorimotor behaviours, but deciphering the timing and significance of acetylcholine actions for these behaviours is challenging. The widespread nature of cholinergic projections to the cortex means that new insights require access to specific neuronal populations, and on a time-scale that matches behaviourally relevant cholinergic actions. Here, we use fast, voltage imaging of L2/3 cortical pyramidal neurons exclusively expressing the genetically-encoded voltage indicator Butterfly 1.2, in awake, head-fixed mice, receiving sensory stimulation, whilst manipulating the cholinergic system. Altering muscarinic acetylcholine function re-shaped sensory-evoked fast depolarisation and subsequent slow hyperpolarisation of L2/3 pyramidal neurons. A consequence of this re-shaping was disrupted adaptation of the sensory-evoked responses, suggesting a critical role for acetylcholine during sensory discrimination behaviour. Our findings provide new insights into how the cortex processes sensory information and how loss of acetylcholine, for example in Alzheimer's Disease, disrupts sensory behaviours.

The Role of Estradiol in Traumatic Brain Injury: Mechanism and Treatment Potential

Patients surviving traumatic brain injury (TBI) face numerous neurological and neuropsychological problems significantly affecting their quality of life. Extensive studies over the past decades have investigated pharmacological treatment options in different animal models, targeting various pathological consequences of TBI. Sex and gender are known to influence the outcome of TBI in animal models and in patients, respectively. Apart from its well-known effects on reproduction, 17β-estradiol (E2) has a neuroprotective role in brain injury. Hence, in this review, we focus on the effect of E2 in TBI in humans and animals. First, we discuss the clinical classification and pathomechanism of TBI, the research in animal models, and the neuroprotective role of E2. Based on the results of animal studies and clinical trials, we discuss possible E2 targets from early to late events in the pathomechanism of TBI, including neuroinflammation and possible disturbances of the endocrine system. Finally, the potential relevance of selective estrogenic compounds in the treatment of TBI will be discussed.

Genetic deletion of TRPA1 receptor attenuates amyloid beta- 1-42 (Aβ1-42)-induced neurotoxicity in the mouse basal forebrain in vivo

Amyloid β 1-42 peptide (Aβ_1-42) accumulates in Alzheimer's disease (AD) that is toxic to the basal forebrain cholinergic (BFC) neurons in substantia innominata-nucleus basalis magnocellularis complex (SI-NBM). Transient Receptor Potential Ankyrin1 (TRPA1) receptor is present in murine brain, however its role in neurotoxic processes is unclear. We investigated the Aβ_1-42-induced neurotoxicity in TRPA1 wild-type (TRPA1^+/+) and knockout (TRPA1^-/-) mice. Expression and neuroanatomical localization of TRPA1 receptor were examined using RT qPCR. Cholinergic fibre loss was determined on acetylcholinesterase (AChE) stained brain slices, and choline acetyltransferase (ChAT) immunohistochemistry was used to assess the cholinergic cell loss. Novel object recognition (NOR), radial arm maze (RAM) and Y-maze tests were used to investigate memory loss. Aβ_1-42-injected WT mice showed marked loss of cholinergic fibres and cell bodies, which was significantly attenuated in TRPA1^-/- animals. According to the NOR and RAM tests, pronounced memory loss was detected in Aβ_1-42-injected TRPA1^+/+ mice, but not in TRPA1^-/- group. Our findings demonstrate that TRPA1 KO animals show substantially reduced morphological damage and memory loss after Aβ_1-42 injection in the SI-NBM. We conclude that TRPA1 receptors may play an important deteriorating role in the Aβ_1-42-induced cholinergic neurotoxicity and the consequent memory loss in the murine brain.

Sex differences in rapid nonclassical action of 17β-oestradiol on intracellular signalling and oestrogen receptor α expression in basal forebrain cholinergic neurones in mouse

Rapid nonclassical effects of 17β-oestradiol (E₂ ) on intracellular signalling have been identified in the basal forebrain, although the extent to which these actions may be different in males and females is unknown. Previous work has shown that E₂ rapidly phosphorylates cAMP responsive element binding protein (CREB) via ΕRα in female cholinergic neurones. Using this indicator, the present study examined whether nonclassical actions of E₂ occur in a sexually dimorphic manner within basal forebrain cholinergic neurones in mice. In addition, we investigated the expression and intracellular distribution of oestrogen receptor (ΕR)α in cholinergic neurones in female and male mice. Animals were gonadectomised and treated 2 weeks later with E₂ . The number of CREB-expressing cholinergic neurones was not altered in any of the brain regions after E₂ treatment in both males and females. However, E₂ treatment rapidly (< 15 minutes) increased (P < 0.05) the number of cholinergic neurones expressing phosphorylated CREB (pCREB) in the substantia innominata and medial septum but not in the striatum in female mice. By contrast, E₂ did not change pCREB expression in cholinergic neurones in male mice at any time point (15 minutes, 1 hour, 4 hours), irrespective of the neuroanatomical location. We also observed that, in females, more cholinergic neurones expressed nuclear ΕRα in all regions, whereas males showed more cholinergic neurones with cytoplasmic or both nuclear and cytoplasmic expression of ΕRα. Taken together, these results demonstrate a marked sex difference in the E₂ -induced nonclassical effect and intracellular distribution of ΕRα in basal forebrain cholinergic neurones in vivo.

Effect of Estradiol on Neurotrophin Receptors in Basal Forebrain Cholinergic Neurons: Relevance for Alzheimer's Disease

The basal forebrain is home to the largest population of cholinergic neurons in the brain. These neurons are involved in a number of cognitive functions including attention, learning and memory. Basal forebrain cholinergic neurons (BFCNs) are particularly vulnerable in a number of neurological diseases with the most notable being Alzheimer's disease, with evidence for a link between decreasing cholinergic markers and the degree of cognitive impairment. The neurotrophin growth factor system is present on these BFCNs and has been shown to promote survival and differentiation on these neurons. Clinical and animal model studies have demonstrated the neuroprotective effects of 17β-estradiol (E2) on neurodegeneration in BFCNs. It is believed that E2 interacts with neurotrophin signaling on cholinergic neurons to mediate these beneficial effects. Evidence presented in our recent study confirms that altering the levels of circulating E2 levels via ovariectomy and E2 replacement significantly affects the expression of the neurotrophin receptors on BFCN. However, we also showed that E2 differentially regulates neurotrophin receptor expression on BFCNs with effects depending on neurotrophin receptor type and neuroanatomical location. In this review, we aim to survey the current literature to understand the influence of E2 on the neurotrophin system, and the receptors and signaling pathways it mediates on BFCN. In addition, we summarize the physiological and pathophysiological significance of E2 actions on the neurotrophin system in BFCN, especially focusing on changes related to Alzheimer's disease.

Treatment of beta amyloid 1-42 (Aβ(1-42))-induced basal forebrain cholinergic damage by a non-classical estrogen signaling activator in vivo

In Alzheimer's disease (AD), there is a loss in cholinergic innervation targets of basal forebrain which has been implicated in substantial cognitive decline. Amyloid beta peptide (Aβ(1-42)) accumulates in AD that is highly toxic for basal forebrain cholinergic (BFC) neurons. Although the gonadal steroid estradiol is neuroprotective, the administration is associated with risk of off-target effects. Previous findings suggested that non-classical estradiol action on intracellular signaling pathways has ameliorative potential without estrogenic side effects. After Aβ(1-42) injection into mouse basal forebrain, a single dose of 4-estren-3α, 17β-diol (estren), the non-classical estradiol pathway activator, restored loss of cholinergic cortical projections and also attenuated the Aβ(1-42)-induced learning deficits. Estren rapidly and directly phosphorylates c-AMP-response-element-binding-protein and extracellular-signal-regulated-kinase-1/2 in BFC neurons and restores the cholinergic fibers via estrogen receptor-α. These findings indicated that selective activation of non-classical intracellular estrogen signaling has a potential to treat the damage of cholinergic neurons in AD.

Neurite outgrowth promoting effect of 17-β estradiol is mediated through estrogen receptor alpha in an olfactory epithelium culture

Olfactory deficits are observed early in the course of chronic neurological disorders including Alzheimer's disease (AD). Estrogen treatment in post-menopausal women reduced the incidence of olfactory dysfunction, raising the possibility that estrogen treatment can cure olfactory deficits in preclinical stages of AD. In this study, we examined the estradiol׳s effects on neurite outgrowth in explant cultures of mouse olfactory epithelium (OE). We found that neurons in OE cultures treated with 100 pM 17-β estradiol (estradiol) had significantly longer neurite outgrowth than cultures treated with ethanol alone (vehicle). The OE neurons expressed estrogen receptors alpha (ERα) and ER beta (ERβ). Estrogen treatment upregulated both ERα and ERβ expression in OE culture. Treatment of OE cultures with propyl pyrazole triol (PPT), a selective agonist for ERα increased neurite outgrowth to comparable extent as estradiol treatment. In contrast, 2,3-bis-4-hydroxyphenyl (DPN), a specific agonist for ERβ, had no effect on neurite outgrowth. Furthermore, estradiol treatment increased neurite outgrowth in OE cultures derived from ERβ-deficient/knockout mice and wild-type littermates, but not in ERα-deficient/knockout mice. These data suggest that ERα mediates the neurite outgrowth promoting effects of estradiol in OE cultures. We propose that olfactory dysfunction in chronic neurological disorders, where estrogen deficiency is a risk factor, is an indicator of compromised axonal regeneration of olfactory sensory neurons.

Estradiol modulation of neurotrophin receptor expression in female mouse basal forebrain cholinergic neurons in vivo

The neuroprotective effect of estradiol (E2) on basal forebrain cholinergic neurons (BFCNs) has been suggested to occur as a result of E2 modulation of the neurotrophin system on these neurons. The present study provides a comprehensive examination of the relationship between E2 and neurotrophin signaling on BFCNs by investigating the effect of E2 deficiency on the expression levels of neurotrophin receptors (NRs), TrkA, TrkB, and p75 on BFCNs. The number of TrkA receptor-expressing choline acetyltransferase-positive neurons was significantly reduced in the medial septum (MS) in the absence of E2. A significant reduction in TrkB-expressing choline acetyltransferase-positive cells was also observed in ovariectomized mice in the MS and nucleus basalis magnocellularis (NBM). p75 receptor expression was reduced in the NBM and striatum but not in the MS. We also showed that estrogen receptor (ER)-α was expressed by a small percentage of TrkA- and TrkB-positive neurons in the MS (12%) and NBM (19%) and by a high percentage of TrkB-positive neurons in the striatum (69%). Similarly, ERα was expressed at low levels by p75 neurons in the MS (6%) and NBM (9%) but was not expressed on striatal neurons. Finally, ERα knockout using neuron-specific estrogen receptor-α knockout transgenic mice abolished all E2-mediated changes in the NR expression on BFCNs. These results indicate that E2 differentially regulates NR expression on BFCNs, with effects depending on the NR type and neuroanatomical location, and also provide some evidence that alterations in the NR expression are, at least in part, mediated via ERα.

Non-classical effects of estradiol on cAMP responsive element binding protein phosphorylation in gonadotropin-releasing hormone neurons: mechanisms and role

Gonadotropin-releasing hormone (GnRH) is produced by a heterogenous neuronal population in the hypothalamus to control pituitary gonadotropin production and reproductive function in all mammalian species. Estradiol is a critical component for the communication between the gonads and the central nervous system. Resolving the mechanisms by which estradiol modulates GnRH neurons is critical for the understanding of how fertility is regulated. Extensive studies during the past decades have provided compelling evidence that estradiol has the potential to alter the intracellular signal transduction mechanisms. The common target of many signaling pathways is the phosphorylation of a key transcription factor, the cAMP response element binding protein (CREB). This review first addresses the aspects of estradiol action on CREB phosphorylation (pCREB) in GnRH neurons. Secondly, this review considers the receptors and signaling network that regulates estradiol's action on pCREB within GnRH neurons and finally it summarizes the physiological significance of CREB to estrogen feedback.

Rat uterine oxytocin receptor and estrogen receptor α and β mRNA levels are regulated by estrogen through multiple estrogen receptors

Estrogen action is mediated through several types of receptors (ERs), such as ERα, ERβ and putative membrane ERs. Oxytocin receptor (OTR) and ER expression levels in the rat uterus are regulated by estrogen; however, which types of ERs are involved has not been elucidated. This study examined OTR, ERα and ERβ levels in ovariectomized rats treated with 17β-estradiol (E2), an ERα agonist (PPT), an ERβ agonist (DPN) or estren (Es). E2 and PPT increased OTR mRNA levels and decreased ERα and ERβ mRNA levels 3 and 6 h posttreatment. DPN decreased ERα and ERβ mRNA levels at 3 and 6 h, while OTR mRNA levels increased at 3 h and decreased at 6 h. OTR mRNA levels increased 3 h after the Es treatment and then declined until 6 h. ERα and ERβ mRNA levels decreased by 3 h and remained low until 6 h posttreatment with Es. The ER antagonist ICI182,780 (ICI) suppressed the increases in OTR mRNA levels induced 3 h after the Es treatment. However, ICI and tamoxifen (Tam) had no significant effect on ERα and ERβ mRNA levels in the Es-treated or vehicle-treated group. In intact rats, proestrus-associated increases in OTR mRNA levels were antagonized by both ICI and Tam. However, decreases in ERα and ERβ mRNA levels were not antagonized by Tam and ICI, respectively. Therefore, uterine OTR gene expression is upregulated by estrogen through the classical nuclear (or non-nuclear) ERs, ERα and ERβ, while the levels of these ERs are downregulated by estrogen through multiple pathways including Es-sensitive nonclassical ERs.

Effect of ageing on post-lesion oestradiol treatment on mouse cholinergic neurones in vivo

A single 17β-oestradiol (E(2)) treatment reduces the loss in cholinergic fibre density in the cortex after NMDA lesion into the nucleus basalis magnocellularis (NBM) of the basal forebrain (BF) in young female mice. In the present study, we examined whether age influences this protective effect of E(2) on cholinergic neurones in male and female mice. Gonad-intact young and aged animals of both sexes were treated with E(2) after unilateral NMDA lesion into the NBM. NMDA lesion elicited ipsilateral cholinergic cell loss in the NBM and ipsilateral fibre loss in the somatosensory cortex to the same extent, irrespective of age or sex. A single E(2) injection performed 1 h post-lesion did not affect the cholinergic cell loss but reduced the loss of fibres in the ipsilateral cortex in young male and female mice. By contrast, E(2) did not have an effect on the NMDA-induced cholinergic cell and fibre loss in aged male or female mice. The oestrous stage of young female mice did not alter the number of cholinergic cells/fibres or the protective effect of E(2) on cholinergic fibres after NMDA injection. Our results show that E(2) has a protective action on BF cholinergic fibres in young males and females, although the treatment potential of E(2) declines with age.