Effects of estrogens on cholinergic neurons in the rat basal nucleus

Estradiol treatment in young postmenopausal women with self-reported cognitive complaints: Effects on cholinergic-mediated cognitive performance

OBJECTIVE

Older women are at increased risk of developing Alzheimer's disease compared to men. One proposed reason is that following menopause there is a decline in estrogens. Estrogens are important for cholinergic functioning and attenuate the impact of cholinergic antagonists on cognitive performance in postmenopausal women. Self-reported or subjective cognitive complaints in middle or older age may represent a harbinger of cognitive decline and those who endorse cognitive complaints appear more likely to develop future cognitive impairment. However, the response of individuals with cognitive complaints after menopause to estrogen and the relationship to cholinergic functioning has not been investigated. This study investigated the effect of estrogen treatment using 17β-estradiol on cognitive performance following anticholinergic blockade in postmenopausal women and the relationship of this interaction with the level of self-reported (subjective) postmenopausal cognitive complaints.

METHODS

Forty postmenopausal women (aged 50-60 years) completed a 3-month treatment regimen of either 1 mg oral estradiol or placebo. Participants then completed four challenge days in which they completed cognitive and behavioral tasks after one of four cholinergic antagonist drug conditions (oral mecamylamine (MECA), intravenous scopolamine, combined MECA and scopolamine, or PLC).

RESULTS

Compared to PLC, the estradiol treated group performed worse on attention tasks under cholinergic challenge including the choice reaction time task and the critical flicker fusion task. In addition, participants who endorsed greater cognitive complaints showed reduced performance on the N-back working memory task, regardless of whether they received estradiol treatment.

CONCLUSIONS

The findings of this study indicate that estradiol treatment was unable to mitigate anticholinergic blockade in postmenopausal women with subjective cognitive complaints, and worsened performance on attention tasks. Moreover, the present study suggests that greater levels of cognitive complaints following menopause may be associated with an underlying decline in cholinergic function that may manifest as an inability to compensate during working memory tasks.

A computational lens on menopause-associated psychosis

Psychotic episodes are debilitating disease states that can cause extreme distress and impair functioning. There are sex differences that drive the onset of these episodes. One difference is that, in addition to a risk period in adolescence and early adulthood, women approaching the menopause transition experience a second period of risk for new-onset psychosis. One leading hypothesis explaining this menopause-associated psychosis (MAP) is that estrogen decline in menopause removes a protective factor against processes that contribute to psychotic symptoms. However, the neural mechanisms connecting estrogen decline to these symptoms are still not well understood. Using the tools of computational psychiatry, links have been proposed between symptom presentation and potential algorithmic and biological correlates. These models connect changes in signaling with symptom formation by evaluating changes in information processing that are not easily observable (latent states). In this manuscript, we contextualize the observed effects of estrogen (decline) on neural pathways implicated in psychosis. We then propose how estrogen could drive changes in latent states giving rise to cognitive and psychotic symptoms associated with psychosis. Using computational frameworks to inform research in MAP may provide a systematic method for identifying patient-specific pathways driving symptoms and simultaneously refine models describing the pathogenesis of psychosis across all age groups.

Osteocalcin, ovarian senescence, and brain health

Menopause, an inevitable event in a woman's life, significantly increases risk of bone resorption and diseases such as Alzheimer's, vascular dementia, cardiac arrest, and stroke. The sole role of bones, as traditionally regarded, is to provide structural support for skeletal muscles and allow for ambulation, however this concept is becoming quickly outdated. New literature has emerged that suggests the bone cell-derived hormone osteocalcin (OCN) plays a pivotal role in cognition. OCN levels are correlated with bone mass density and bone turnover, and thus are strongly influenced by the changes associated with menopause. The goal of the current review is to discuss potential gaps in our knowledge of OCN and cognition, discrepancies in methods of OCN quantification, and therapies to enhance circulating OCN. A discussion on implementing exercise or low frequency vibration interventions at the menopausal transition to reduce risk and severity of neurological diseases and associated cognitive decline is included.

Brain-Derived Acetylcholine Maintains Peak Bone Mass in Adult Female Mice

Preclinical and clinical data support a role of the sympathetic nervous system in the regulation of bone remodeling, but the contribution of parasympathetic arm of the autonomic nervous system to bone homeostasis remains less studied. In this study, we sought to determine whether acetylcholine (ACh) contributes to the regulation of bone remodeling after peak bone mass acquisition. We show that reduced central ACh synthesis in mice heterozygous for the choline transporter (ChT) leads to a decrease in bone mass in young female mice, thus independently confirming the previously reported beneficial effect of ACh signaling on bone mass accrual. Increasing brain ACh levels through the use of the blood brain barrier (BBB)-permeable acetylcholinesterase inhibitor (AChEI) galantamine increased trabecular bone mass in adult female mice, whereas a peripheral increase in ACh levels induced by the BBB-impermeable AChEI pyridostigmine caused trabecular bone loss. AChEIs did not alter skeletal norepinephrine level, and induced an overall increase in osteoblast and osteoclast densities, two findings that do not support a reduction in sympathetic outflow as the mechanism involved in the pro-anabolic effect of galantamine on the skeleton. In addition, we did not detect changes in the commitment of skeletal progenitor cells to the osteoblast lineage in vivo in AChEI-treated mice, nor a direct impact of these drugs in vitro on the survival and differentiation of osteoblast and osteoclast progenitors. Last, ChT heterozygosity and galantamine treatment triggered bone changes in female mice only, thus revealing the existence of a gender-specific skeletal response to brain ACh level. In conclusion, this study supports the stimulatory effect of central ACh on bone mass accrual, shows that it also promotes peak bone mass maintenance in adult mice, and suggests that central ACh regulates bone mass via different mechanisms in growing versus sexually mature mice. © 2020 American Society for Bone and Mineral Research.

Sex-dependent effects of nicotine on the developing brain

The use of tobacco products represents a major public health concern, especially among women. Epidemiological data have consistently demonstrated that women have less success quitting tobacco use and a higher risk for developing tobacco-related diseases. The deleterious effects of nicotine are not restricted to adulthood, as nicotinic acetylcholine receptors regulate critical aspects of neural development. However, the exact mechanisms underlying the particular sensitivity of women to develop tobacco dependence have not been well elucidated. In this mini-review, we show that gonadal hormone-mediated sexual differentiation of the brain may be an important determinant of sex differences in the effects of nicotine. We highlight direct interactions between sex steroid hormones and ligand-gated ion channels critical for brain maturation, and discuss the extended and profound sexual differentiation of the brain. We emphasize that nicotine exposure during the perinatal and adolescent periods interferes with normal sexual differentiation and can induce long-lasting, sex-dependent alterations in neuronal structure, cognitive and executive function, learning and memory, and reward processing. We stress important age and sex differences in nicotine's effects and emphasize the importance of including these factors in preclinical research that models tobacco dependence. © 2016 Wiley Periodicals, Inc.

Estrogen-cholinergic interactions: Implications for cognitive aging

This article is part of a Special Issue "Estradiol and Cognition". While many studies in humans have investigated the effects of estrogen and hormone therapy on cognition, potential neurobiological correlates of these effects have been less well studied. An important site of action for estrogen in the brain is the cholinergic system. Several decades of research support the critical role of CNS cholinergic systems in cognition in humans, particularly in learning and memory formation and attention. In humans, the cholinergic system has been implicated in many aspects of cognition including the partitioning of attentional resources, working memory, inhibition of irrelevant information, and improved performance on effort-demanding tasks. Studies support the hypothesis that estradiol helps to maintain aspects of attention and verbal and visual memory. Such cognitive domains are exactly those modulated by cholinergic systems and extensive basic and preclinical work over the past several decades has clearly shown that basal forebrain cholinergic systems are dependent on estradiol support for adequate functioning. This paper will review recent human studies from our laboratories and others that have extended preclinical research examining estrogen-cholinergic interactions to humans. Studies examined include estradiol and cholinergic antagonist reversal studies in normal older women, examinations of the neural representations of estrogen-cholinergic interactions using functional brain imaging, and studies of the ability of selective estrogen receptor modulators such as tamoxifen to interact with cholinergic-mediated cognitive performance. We also discuss the implications of these studies for the underlying hypotheses of cholinergic-estrogen interactions and cognitive aging, and indications for prophylactic and therapeutic potential that may exploit these effects.

Genistein partly eases aging and estropause-induced primary cortical neuronal changes in rats

Gonadal hormones can modulate brain morphology and behavior. Recent studies have shown that hypogonadism could result in cortical function deficits. To this end, hormone therapy has been used to ease associated symptoms but the risk may outweigh the benefits. Here we explored whether genistein, a phytoestrogen, is effective in restoring the cognitive and central neuronal changes in late middle age and surgically estropause female rats. Both animal groups showed poorer spatial learning than young adults. The dendritic arbors and spines of the somatosensory cortical and CA1 hippocampal pyramidal neurons were revealed with intracellular dye injection and analyzed. The results showed that dendritic spines on these neurons were significantly decreased. Remarkably, genistein treatment rescued spatial learning deficits and restored the spine density on all neurons in the surgically estropause young females. In late middle age females, genistein was as effective as estradiol in restoring spines; however, the recovery was less thorough than on young OHE rats. Neither genistein nor estradiol rectified the shortened dendritic arbors of the aging cortical pyramidal neurons suggesting that dendritic arbors and spines are differently modulated. Thus, genistein could work at central level to restore excitatory connectivity and appears to be potent alternative to estradiol for easing aging and menopausal syndromes.

Tamoxifen improves cholinergically modulated cognitive performance in postmenopausal women

Tamoxifen (TMX) is a selective estrogen receptor modulator that is used as an estrogen receptor antagonist for the treatment and prevention of breast cancer. Whether TMX has antagonist activities in the human brain is less clear and its effects on cognitive function have not been experimentally explored. This study examined how TMX affected cognitive performance in older women using a model of anticholinergic drug-induced cognitive dysfunction. Twenty-one postmenopausal women were administered 20 mg of oral TMX or placebo for 3 months. Participants then took part in five drug challenges using the anticholinergic antinicotinic agent mecamylamine (MECA) and antimuscarinic agent scopolamine (SCOP) and were tested on a comprehensive battery including tasks of attention and psychomotor function, verbal episodic memory, and spatial navigation. After a 3-month placebo washout, participants were then crossed over to the alternate treatment and repeated the drug challenges after 3 months. Compared with placebo treatment, TMX significantly attenuated the impairment from cholinergic blockade on tasks of verbal episodic memory and spatial navigation, but effects on attentional/psychomotor tasks were more variable. Analysis by APOE genotype showed that APO ɛ4+ women showed a greater beneficial effect of TMX on reversing the cholinergic impairment than APO ɛ4- women on most tasks. This study provides evidence that TMX may act as an estrogen-like agonist to enhance cholinergic system activity and hippocampally mediated learning.

Polymorphisms in HSD17B1: Early Onset and Increased Risk of Alzheimer's Disease in Women with Down Syndrome

Background/Aims. Genetic variants that affect estrogen activity may influence the risk of Alzheimer's disease (AD). In women with Down syndrome, we examined the relation of polymorphisms in hydroxysteroid-17beta-dehydrogenase (HSD17B1) to age at onset and risk of AD. HSD17B1 encodes the enzyme 17β-hydroxysteroid dehydrogenase (HSD1), which catalyzes the conversion of estrone to estradiol. Methods. Two hundred and thirty-eight women with DS, nondemented at baseline, 31-78 years of age, were followed at 14-18-month intervals for 4.5 years. Women were genotyped for 5 haplotype-tagging single-nucleotide polymorphisms (SNPs) in the HSD17B1 gene region, and their association with incident AD was examined. Results. Age at onset was earlier, and risk of AD was elevated from two- to threefold among women homozygous for the minor allele at 3 SNPs in intron 4 (rs676387), exon 6 (rs605059), and exon 4 in COASY (rs598126). Carriers of the haplotype TCC, based on the risk alleles for these three SNPs, had an almost twofold increased risk of developing AD (hazard ratio = 1.8, 95% CI, 1.1-3.1). Conclusion. These findings support experimental and clinical studies of the neuroprotective role of estrogen.

Methamphetamine exposure during brain development alters the brain acetylcholine system in adolescent mice

Children exposed to methamphetamine during brain development as a result of maternal drug use have long-term hippocampus-dependent cognitive impairments, but the mechanisms underlying these impairments are not understood. The acetylcholine system plays an important role in cognitive function and potential methamphetamine-induced acetylcholine alterations may be related to methamphetamine-induced cognitive impairments. In this study, we investigated the potential long-term effects of methamphetamine exposure during hippocampal development on the acetylcholine system in adolescence mice on postnatal day 30 and in adult mice on postnatal day 90. Methamphetamine exposure increased the density of acetylcholine neurons in regions of the basal forebrain and the area occupied by acetylcholine axons in the hippocampus in adolescent female mice. In contrast, methamphetamine exposure did not affect the density of GABA cells or total neurons in the basal forebrain. Methamphetamine exposure also increased the number of muscarinic acetylcholine receptors in the hippocampus of adolescent male and female mice. Our results demonstrate for the first time that methamphetamine exposure during hippocampal development affects the acetylcholine system in adolescent mice and that these changes are more profound in females than males.

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

17β-Estradiol (E2) treatment exerts rapid, nonclassical actions via intracellular signal transduction system in basal forebrain cholinergic (BFC) neurons in vivo. Here we examined the effect of E2 treatment on lesioned BFC neurons in ovariectomized mice and the role of E2-induced nonclassical action in this treatment. Mice given an N-methyl-d-aspartic acid (NMDA) injection into the substantia innominata-nucleus basalis magnocellularis complex (SI-NBM) exhibited cholinergic cell loss in the SI-NBM and ipsilateral cholinergic fiber loss in the cortex. A single injection of E2 after NMDA lesion did not have an effect on cholinergic cell loss in the SI-NBM, but it restored the ipsilateral cholinergic fiber density in the cortex in a time- and dose-dependent manner. The most effective cholinergic fiber restoration was observed with 33 ng/g E2 treatment at 1 h after NMDA lesion. The E2-induced cholinergic fiber restoration was absent in neuron-specific estrogen receptor-α knockout mice in vivo. Selective activation of nonclassical estrogen signaling in vivo by estren induced E2-like restorative actions. Selective blockade of the MAPK or protein kinase A pathway in vivo prevented E2's ability to restore cholinergic fiber loss. Finally, studies in intact female mice revealed an E2-induced restorative effect that was similar to that of E2-treated ovariectomized mice. These observations demonstrate that a single E2 treatment restores the BFC fiber loss in the cortex, regardless of endogenous E2 levels. They also reveal the critical role of nonclassical estrogen signaling via estrogen receptor-α and protein kinase A-MAPK pathways in E2-induced restorative action in the cholinergic system in vivo.

Asymmetric regulation by estrogen at the cholinergic gene locus in differentiated NG108-15 neuronal cells

AIMS

Estrogen acts as a neurogenerative and neuroprotective factor in the cholinergic system. Choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) are regarded as markers of cholinergic neurons. The genes coding these proteins are located at a common locus, the cholinergic gene locus. However, few details concerning activation of the locus have been obtained. We examined the effect of estrogen on the activation pattern of the locus using a cholinergic cell line.

MAIN METHODS

NG108-15 neuronal cells, as a model of cholinergic neurons, were used. Dose-dependent effects of estradiol (E2) on the gene expression of ChAT and VAChT were quantitatively determined by a real-time RT-PCR. The expression of ChAT mRNA variants was qualitatively evaluated by RT-PCR using specific primers.

KEY FINDINGS

The expression of ChAT and VAChT mRNA was strongly enhanced with the induction of differentiation. The enhanced expression of ChAT mRNA was further increased dose-dependently by E2 (10(-10) to 10(-7)M), while that of VAChT mRNA did not respond to E2. The up-regulation of ChAT mRNA expression by E2 was abolished by co-treatment with a pure-antagonist of estrogen receptors. A qualitative analysis of ChAT mRNA variants revealed the R types, which share a common sequence with the VAChT gene, and type M ChAT mRNA to mainly be expressed, and that the appearance of these variants was not altered by E2.

SIGNIFICANCE

The cholinergic gene locus in differentiated NG108-15 neuronal cells is further activated by E2, but the effect is restricted to the transcription of ChAT gene.

Menopause and mitochondria: windows into estrogen effects on Alzheimer's disease risk and therapy

Metabolic derangements and oxidative stress are early events in Alzheimer's disease pathogenesis. Multi-faceted effects of estrogens include improved cerebral metabolic profile and reduced oxidative stress through actions on mitochondria, suggesting that a woman's endogenous and exogenous estrogen exposures during midlife and in the late post-menopause might favourably influence Alzheimer risk and symptoms. This prediction finds partial support in the clinical literature. As expected, early menopause induced by oophorectomy may increase cognitive vulnerability; however, there is no clear link between age at menopause and Alzheimer risk in other settings, or between natural menopause and memory loss. Further, among older post-menopausal women, initiating estrogen-containing hormone therapy increases dementia risk and probably does not improve Alzheimer's disease symptoms. As suggested by the 'critical window' or 'healthy cell' hypothesis, better outcomes might be expected from earlier estrogen exposures. Some observational results imply that effects of hormone therapy on Alzheimer risk are indeed modified by age at initiation, temporal proximity to menopause, or a woman's health. However, potential methodological biases warrant caution in interpreting observational findings. Anticipated results from large, ongoing clinical trials [Early Versus Late Intervention Trial with Estradiol (ELITE), Kronos Early Estrogen Prevention Study (KEEPS)] will help settle whether midlife estrogen therapy improves midlife cognitive skills but not whether midlife estrogen exposures modify late-life Alzheimer risk. Estrogen effects on mitochondria adumbrate the potential relevance of estrogens to Alzheimer's disease. However, laboratory models are inexact embodiments of Alzheimer pathogenesis and progression, making it difficult to surmise net effects of estrogen exposures. Research needs include better predictors of adverse cognitive outcomes, biomarkers for risks associated with hormone therapy, and tools for monitoring brain function and disease progression.

Premarin improves memory, prevents scopolamine-induced amnesia and increases number of basal forebrain choline acetyltransferase positive cells in middle-aged surgically menopausal rats

Conjugated equine estrogen (CEE) is the most commonly prescribed estrogen therapy, and is the estrogen used in the Women's Health Initiative study. While in-vitro studies suggest that CEE is neuroprotective, no study has evaluated CEE's effects on a cognitive battery and brain immunohistochemistry in an animal model. The current experiment tested whether CEE impacted: I) spatial learning, reference memory, working memory and long-term retention, as well as ability to handle mnemonic delay and interference challenges; and, II) the cholinergic system, via pharmacological challenge during memory testing and ChAT-immunoreactive cell counts in the basal forebrain. Middle-aged ovariectomized (Ovx) rats received chronic cyclic injections of either Oil (vehicle), CEE-Low (10 microg), CEE-Medium (20 microg) or CEE-High (30 microg) treatment. Relative to the Oil group, all three CEE groups showed less overnight forgetting on the spatial reference memory task, and the CEE-High group had enhanced platform localization during the probe trial. All CEE groups exhibited enhanced learning on the spatial working memory task, and CEE dose-dependently protected against scopolamine-induced amnesia with every rat receiving the highest CEE dose maintaining zero errors after scopolamine challenge. CEE also increased number of ChAT-immunoreactive neurons in the vertical diagonal band of the basal forebrain. Neither the ability to remember after a delay nor interference, nor long-term retention, was influenced by the CEE regimen used in this study. These findings are similar to those reported previously for 17 beta-estradiol, and suggest that CEE can provide cognitive benefits on spatial learning, reference and working memory, possibly through cholinergic mechanisms.