Estrogen regulation of the nucleus accumbens as a gateway to understanding menopause associated metabolic dysfunction

Abstract

The nucleus accumbens (NAc) may be a link between metabolic and behavioral changes due to menopause. Loss of estrogen (E2) activation in the NAc might cause reduced physical activity and adipose tissue dysfunction. We review potential mechanisms by which NAc might communicate with adipose tissue and how menopause may impact this pathway. A better understanding of the NAc-adipose tissue axis will pave the way for improved treatments in menopausal women.

The utterance of “menopause” conjures up a variety of negative connotations. Besides the societal stigmatism of being “old” and unable to reproduce, menopausal women face a myriad of health concerns that are precipitated by the dramatic reduction in circulating estrogen (E2) levels, as well as other hormonal fluctuations. Physiological, neurological, metabolic changes take place during this time, culminating in, not only increased risk of disease, but an overall reduction in quality of life. Mood changes likely include a diminished motivation to engage in physical activity, which exacerbates the already increased risk for obesity and keeps women from achieving sufficient levels of physical activity and its host of benefits that are particularly critical in this population (e.g., cognitive/mood enhancement, improved sleep, improved glucose metabolism, and other physical and mental health attributes)¹. Particularly concerning is that the increased weight gain coincides with dysfunctional (i.e., that which is insulin resistant and inflammatory with suppressed mitochondrial activity) adipose tissue (AT), which is an important predictor of metabolic dysfunction^1,2. Comprehensive approaches to support the neurological and metabolic health of aging women are critically needed. Despite this need and the fact that women spend approximately 1/3 of their lives in a post-menopausal state, this topic has received insufficient attention. For decades, the “change” women underwent at this time was considered a taboo topic, yet shifting public awareness has finally spurred media attention, as evidenced by a recent article in Salon titled, “Gen X feels left in the dark with menopause due to a lack of research and support”³. Other media resources, including the New York Times and Forbes, have also highlighted critical gaps in our knowledge and need for improved therapies to help menopausal women^4,5. These cries for improving the lives of aging women has resulted in the Biden White House health initiative to devote much needed research dollars to this topic (https://www.whitehouse.gov/womenshealthresearch/) The fate of this important research funding likely is in jeopardy with the new Administration, and yet, it is more important than ever to understand how to improve the lives of menopausal women.

Circulating E2 is metabolically protective in many tissues, including and especially AT⁶. While E2 loss causes the constellation of symptoms described above via complex and multifaceted mechanisms, such actions involve signaling via its two major nuclear receptors, estrogen receptor 1 (ESR1) or ERα and estrogen receptor 2 (ESR2) or ERβ, which are widely distributed throughout the body, including the cardiovascular system, brain and AT⁷, the latter two being the primary focus of this review.

Role of suppressed brain estrogen signaling on menopause

Besides weight gain, neurological symptoms are among the most pressing health concerns in aging women. Vasomotor symptoms, memory impairments, sleep disturbances, new onset of depression, and cognitive decline are all symptoms of neurological origin⁸. Research over the past several decades supports the notion of the brain being the key tissue driving menopausal symptoms including, in addition to classical reproductive effects, regulation of blood flow, inflammation, thermoregulation, mood, sleep, and cognition⁹. Not surprisingly, ESRs are densely populated among diverse brain regions. Their temporal increase in the early years following menopause is likely indication of the importance of E2 brain signaling. In fact, the brain adapts quickly to increase expression of these receptors to remain responsive to even minimal circulating E2 concentrations⁹. However, this compensation is short-lived and the adverse effects of E2 loss ensue thereafter.

The nucleus accumbens (NAc) brain region, considered the brain’s “reward circuit”, is known to regulate behaviors associated with reward and positive reinforcement, and is associated with addictive, impulsive, and pleasurable behaviors. In fact, this is a primary region, in addition to the gut, that is affected by the increasingly popular class of obesity/anti-diabetic drugs, the Glucagon-Like Peptide-1 (GLP1) agonists¹⁰. This brain region has previously been unrecognized as being critical to menopause associated changes, but our recent studies provide evidence that it may serve as an important link between AT metabolism and neurobehavioral manifestations of menopause, and loss of E2 signaling in the NAc might be a mechanism by which menopause affects metabolic programming.

In relation to estrogen receptor mediated mechanisms, ESRs, including G-protein coupled estrogen receptor 1 (GPER1), are known to influence various brain functions, including metabolism and behavior. Studies have indicated that the NAc shell exhibits low levels of nuclear ERα and ERβ, which may not fully explain E2’s rapid effects in this region. However, levels of GPER1 are also low, suggesting that other mechanisms or receptor distributions might contribute to E2 signaling within the NAc¹¹.

The estrogenic effects on the brain may involve its influence on the mitochondria of neurons and other cells in the brain. Mechanistically, estrogens have been shown to affect mitochondrial function through various pathways, involving both membrane-bound and nuclear ESRs. These effects include alterations in mitochondrial protein content, oxidative phosphorylation, and calcium retention capacities, and may be mediated by E2’s signaling through mitochondrial ESRs. Indeed, ESRs have been identified within mitochondria on many cell types¹², indicating a direct role in modulating mitochondrial activity. Furthermore, research demonstrates that mitochondrial ESRs are indeed present in the NAc brain region, with ERβ being the major receptor subtype expressed in mitochondria of this brain region¹³. While specific studies detailing the distribution of mitochondrial ESRs within the NAc are limited, the presence of ESRs in brain mitochondria suggests potential regulatory roles in mitochondrial metabolism, which likely impacts metabolism and neuronal function of the NAc. Further research is necessary to elucidate the precise mechanisms and effects of mitochondrial ESRs in the NAc, and how E2 signaling through brain mitochondrial ESRs may affect both metabolism and behavior.

Menopause-related changes in physical activity behavior: Role of NAc?

Another troublesome, yet underappreciated side effect of menopause is reduced motivation to engage in physical activity. Lack of exercise in particular may be contributing to and further exacerbate many of the metabolic symptoms associated with menopause (Fig. 1)^1,14. Thus, discovering a central intervention point to improve both mood/physical activity behavior and metabolism has major clinical relevance.

Fig. 1 |. Model of relationship between how low E2 due to menopause affects physical activity and obesity.

This figure shows the key inter-relationships between how declining estrogen levels brought upon by menopause can lead to decreased physical activity and ensuing cardiometabolic and neurobehavioral changes. Decreased physical activity during menopause can result in increased body weight gain and adiposity, detrimental cardiovascular changes, including decreased blood vessel elasticity, decreased insulin sensitivity, decreased bone density, decreased sleep, reduced balance, and decreased emotional happiness or increased depression or mood disorders. This figure was published in ref. 14 and reproduced in the current work with permission from Georg Thieme Verlag KG and Copyright Clearance Center.

In rodent models, endogenous and exogenous estrogenic compounds promote voluntary physical activity as assessed via wheel running^15,16. Even prenatal exposure to estrogens has been shown to affect wheel running behavior; mice exposed during gestation to an endocrine disrupting chemical known to affect ESRs have longstanding reductions in physical activity¹⁷. While several brain regions regulate locomotor activity, the NAc may be particularly important in regulating motivated physical activity behavior¹⁸. Aromatase knockout (ArKO) mice who lack the ability to convert testosterone to E2, exhibit reduced cage physical activity, a finding demonstrated across laboratories^19,20. We demonstrated that such changes in the ArKO mouse are accompanied by significant changes in NAc transcriptome profiles²⁰. Estrogen deficiency due to ovariectomy also causes gene expression changes in the NAc that correlate strongly with suppressed physical activity behavior^21–23, supporting the notion that reduced estrogen signaling in the NAc is a determinant of E2 loss mediated reduction in physical activity²⁰.

The NAc is part of the basal forebrain that is located rostral to the preoptic area of the hypothalamus, as shown in Fig. 2. Along with the olfactory tubercle, the left and right NAc forms part of the ventral striatum. It is comprised of a core and shell with aggregate neurons within this region, and it receives substantial input from a number of brain regions that are subdivided into two primary categories: (1) those providing glutamatergic inputs, such as the prefrontal cortex, basolateral amygdala, hippocampus, thalamus, and ventral tegmental area (VTA) and (2) dopaminergic input that arises also from the VTA and regulates GABAergic neurons within the NAc. Axons originating from the NAc in turn synapse within several brain region that range from the basal ganglia, ventral pallidum (VP), thalamus, prefrontal cortex, dorsal striatum, VTA, and pons. The GABAergic medium spiny neurons within the NAc are further subdivided based on the type of dopamine (D) receptors they express. Those that express D1 receptors tend to mediate reward-associated traits, and those that express D2 receptors are involved in regulating fear responses²⁴. Figure 3 shows the inter-relationships between the NAc and other brain regions and these signaling pathways within this brain region. Most of the neurons fall into one of these two categories. Isolated cholinergic interneurons have also been identified within NAc²⁵. The NAc responds to a variety of hormones and neurotransmitters, including estrogens, glucocorticoids, GABA, dopamine, serotonin (5-HT), and glutamate. Estrogens act on the diverse types of neurons within the NAc by binding and activating ESRs^{11,20,21,25–27}.

Fig. 2 |. Diagrammatic location of nucleus accumbens (NAc), which is the brain region critical for regulating voluntary physical activity or motivation to engage in exercise.

The figure has been modified from pikovit – stock.adobe.com and is freely available.

Fig. 3 |. Interactions between the nucleus accumbens (NAc) and other mesocorticolimbic brain regions regulated by glutaminergic and dopamine signaling.

The medium spiny neurons shown in red extended to the ventral tegmental area (VTA) and indirectly to the ventral pallidum (VP) region. These neurons have dopamine1 receptor D1 and D2 coexpression. Both dopaminergic and glutamatergic neurons have dopamine receptors. The diagram shows the potential of the hypothalamus, thalamus, basolateral amygdala, orbifrontal, prelimbic, and infralimbic cortex to impact the NAc via glutamatergic inputs. This diagram has been modified from ref. 24 in accordance with Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License (CC-BY-NC-SA), which is the license under which material was published in this journal from 2010 to 2014. This license allows individuals to adapt- remix, transform, and build upon the material. NMDA N-methyl-D-aspartate receptor. AMPA α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor.

Our recent work suggests that, even in ovary-intact animals, genetic manipulation of E2 receptor signaling in the NAc region leads to changes in WAT metabolism²⁷, highlighting for the first time a potential direct connection between the NAc and adipocytes. It is important to determine the effects of E2 on the “NAc-AT axis” vs. its direct effects, as this will deepen our understanding of the underpinning mechanisms by which E2 loss affects both the brain and systemic metabolism and may point to an integration point which can be therapeutically targeted. Interestingly, in addition to modulating reward-initiated behavior, dopamine also directly affects AT metabolism via its conversion into the neurotransmitter, norepinephrine, which induces mitochondrial activity in AT^28,29. This may lend insight into an important yet underappreciated link between E2 loss-mediated behavior changes and disruptions in AT metabolism, which precede systemic metabolic dysfunction.

Estrogen loss affects adipose tissue through direct and indirect mechanisms

A body of evidence supports the AT as being a major target tissue of E2, and menopause undoubtedly affects AT via direct and indirect mechanisms. The following is a summary of the known menopause-related changes in metabolic activity of AT cells. In general, E2 is protective against adipocyte metabolic dysfunction. Following menopause, adipocytes become insulin resistant, secrete less of the insulin-sensitizing adipokine, adiponectin, and become more susceptible to inflammation, which exacerbates insulin resistance². In turn, adipocyte insulin resistance leads to higher levels of blood glucose as well as elevated levels of circulating non-esterified fatty acids (NEFA), due to loss of insulin-mediated NEFA suppression. This adipocyte dysfunction leads to systemic metabolic dysfunction by increasing hepatic triglyceride content and impairing systemic insulin sensitivity³⁰. E2 receptor-mediated pathways improve insulin signaling, and facilitate expandability of adipocytes³¹, as well as increase adipocyte mitochondrial activity^32–34, facilitating lipolytic flux, and ultimately increasing energy expenditure of adipocytes³⁵. E2, via binding to its nuclear receptors, also affects expression of classes of genes associated with natural antioxidant mechanisms, mitochondrial activity, and fatty acid metabolism³⁶. A rather newly discovered effect of E2 is its ability to induce and/or facilitate browning of white AT, as well as activation of brown adipose tissue (BAT). While the mechanisms are likely complex and incompletely understood, the sympathetic nervous system is likely involved, as environmental conditions that activate the sympathetic nervous system to release catecholamines (e.g., norepinephrine) also induce browning and activate BAT. Cold exposure is a prime example of one such factor and is the most well studied stimulus for this process. In fact, this relatively recent discovery has led to an onslaught of individuals opting to receive cold exposure (i.e., “cryotherapy”) to improve metabolism and facilitate fat loss and there are now cryotherapy clinics popping up all over the nation. One study showed efficacy in post-menopausal women in particular³⁷. Interestingly, recent studies in adults have shown that young women are more susceptible than age-matched males to cold-induced BAT activation, whereas postmenopausal women are not. That study went on to show that E2 repression in young women reduced their cold-induced BAT activity³⁸. It is likely also the case that E2 plays a role in the established sex difference in relative BAT, with young females having more than males. Collectively, these data point to a critical role played by E2 in susceptibility to browning and BAT activation, although mechanisms remain unknown. Thus, the reduction in circulating E2 that accompanies menopause absolutely affects AT. Moreover, it is likely that E2 affect AT both directly and in-directly. In menopause, the AT increases its production of estrogens due to increased activity of the enzyme aromatase³⁹. This suggests that centrally mediated E2-involving mechanisms are particularly important following menopause.

E2 signaling in the central nervous system affects AT and this occurs via multiple brain regions. Indeed, ESRs are heavily expressed across brain regions and are present on both neurons and glia⁴⁰. The region most heavily studied in this regard is the hypothalamus, which has been shown to play an important role in regulating AT distribution, “beiging” of WAT, and BAT thermogenesis, a topic that has been summarized in a recent review⁴¹.

In addition to affecting peripheral metabolism via central nervous system signaling, estrogen has also been shown to directly affect brain lipid metabolism⁴². Our data support this in that, in comparing aromatase knockout and wildtype female mice, loss of estrogen via aromatase ablation significantly impacted genes associated with lipid metabolism in the brain, in particular, the NAc brain region²⁰. The implications of shifts in brain lipid metabolism are unclear, and no studies to our knowledge have addressed how menopause per se affects brain lipid metabolism, an area demanding further research.

The NAc-AT axis

Our recent publication suggests that loss of ESR1 (ERα) selectively in the NAc affects WAT and BAT²⁷, suggesting a direct linkage between the NAc and AT that is governed by E2 signaling in this brain region. In a recent study not yet published, we demonstrated that NAc-direct E2 injection over the course of 5 days significantly increased voluntary wheel running in ovariectomized rats (Shay et al., unpublished, In Prep), indicating that E2 signaling in this region directly affects physical activity behavior; ongoing studies will assess AT phenotypic changes.

Other emerging findings support a connection between the NAc and AT. Prior work has shown positive associations between total AT, subcutaneous AT, and white matter connectivity in the NAc and other brain regions involved in reward processing and appetite regulation⁴³. It is possible that catecholamines from the NAc might transit to the WAT (e.g., via extracellular vesicles (EVs), as discussed below) to directly influence this tissue to regulate adiposity. Catecholamines directly affect AT by increasing adipocyte lipolysis and thus may may fat accumulation in AT⁴⁴. Other yet unknown factors (e.g., mRNAs and miRNAs) originating in the brain, might also affect adipose as well as other peripheral tissues. At least one published study has shown that AT-derived EVs mediate connectivity between the brain and AT as AT-derived vesicles produced during obesity lead to cognitive impairments⁴⁵. Obesity has been shown to change the NAc, as evidenced by one study where mice provided a high fat diet (HFD) showed reductions in miR-155 and miR-146a in the NAc⁴⁶. Moreover, deletion of miR-155 resulted in increased weight gain and food ingestion. Thus, miR-155, and other miRs, expressed within the NAc may regulate AT and behavioral responses. Further work in this area is needed to establish how these two structures might communicate with each other via E2 signaling within the NAc.

Potential for the NAc to communicate with metabolic organs via extracellular vesicles (EVs) and clinical significance of EVs

Extracellular vesicles (EVs) have been shown to be released from the brain⁴⁷ and might serve as primary mediators of how the NAc communicates with white AT, BAT, and other metabolic organs. EVs are categorized into microvesicles, exosomes, and apoptotic bodies. Exosomes (30–160 nm diameter) are released from cells upon fusion of an endocytic compartment, multivesicular body with the plasma membrane⁴⁸. They are known to contain proteins, lipids, mRNA, miRNA, other small RNA, and as we have shown recently, catecholamines⁴⁹. With their protective membrane shell, EVs provide a mechanism for shuttling a variety of biomolecules to target organs without risk of metabolism, and upon being internalized by target cells, they can induce a wide range of cellular changes. EVs are considered a research ‘gold mine’, as elucidation of how their composition changes under varying physiological and pathological conditions might unlock novel approaches for early diagnostic and therapeutic strategies, e.g., liquid biopsies and engineered EVs. Characterizing the contents of EVs from the NAc per se has gained increasing interest⁵⁰. Substances released from EVs have indeed been shown to cross the blood brain barrier and affect metabolism. MicroRNAs, including reductions in miR-122, miR-192, and miR-22 within EVs circulating in the serum following aerobic training exercise in mice altered AT metabolism⁵¹. The researchers in that study did not determine the source of the EVs but speculate they might originate from the liver; however, the NAc and other brain regions regulating PA are as likely possibilities.

Serum might be considered as a superhighway for EVs traveling from one cell type to another. The isolation and characterization of contents within EVs, including proteins, lipid, mRNA, and small RNA, is considered one of the leading liquid biopsy approaches for early diagnosis of a variety of cancers and most recently, other diseases including cardiovascular, neurological, obesity, and other metabolic disorders^51–66. Further, EVs might be leveraged in targeted drug delivery methods for prevention and treatment. Precision medicine is dependent upon the development of non-invasive biomarkers for early disease detection. Leveraging of EV composition in different pathophysiological conditions, including menopause, might pave the way for NextGen diagnostic and therapeutic strategies. In this respect, exosomes have been postulated to be a potential treatment in perimenopausal women. In one study, treatment of ovarian cortex samples from perimenopausal women with exosomes from human umbilical cord mesenchymal stem cells revealed a significant increase in estrogen production, ESR1 expression, increased cellular proliferation, and reduced expression of apoptotic cell markers⁶⁷.

Conclusions and future directions

Women are no longer willing to accept the decline physical and mental health brought upon by menopause, a period currently comprising one-third of a woman’s life, and this number may increase as individuals increasingly live into their 80’s and 90’s. Thus, it is imperative to understand the multifaceted nature of the physiological changes women encounter during the menopausal transition and thereafter, and to develop improved and precision-targeted strategies to promote health. Declining ovarian hormones, especially E2, triggers detrimental physical and mental health impairments that can lead to disease and compromise the quality of life for many aging women⁷. It is critical to establish the underpinning mechanisms to stem this tide such that menopausal/post-menopausal women can lead their best lives. Menopausal women should no longer have to acquiesce or be misled into accepting declining health following menopause and the notion that that there is nothing that can be done about it.

The decline in physical activity associated with menopause, which may be caused by E2 loss-mediated changes in the NAc brain region, may precipitate the increased risk for depression and cardiometabolic diseases in menopausal/post-menopausal women (Fig. 2)¹⁴. Based on our recent discoveries with a mouse model our group created that lacks ESR1 selectively in the NAc, we are proposing that this brain region has a direct influence on AT, and it is primarily regulated by E2 signaling²⁷. Our data support the existence of a NAc-AT axis, as reinforced by other studies^43,46. EVs might act to safely shepherd such molecules from the NAc to the WAT and protect them from metabolic degradation. Recent attention has turned to isolating and characterizing contents of EVs from the NAc, although no studies to date have analyzed the molecular contents from EVs derived from this brain region and how the biomolecules might be affecting by declining E2 levels.

The isolation and characterization of contents within EVs, including proteins, lipid, mRNA, and small RNA, is considered one of the leading liquid biopsy approaches for early diagnosis of a variety of cancers and most recently, other diseases including cardiovascular, neurological, obesity, and other metabolic disorders^51–66. There are several potential applications for tissue-derived EVs in the setting of menopause, including allowing for early diagnosis of menopause, and leveraging targeted drug delivery methods for prevention and treatment of its metabolic and neurological/behavioral symptoms. Leveraging of EV composition in different pathophysiological conditions, including menopause might pave the way for NextGen diagnostic and therapeutic strategies.

Data Availability

No new datasets were generated or analysed during the current study.