The Role of Estrogen and Testosterone in Determining Brain Blood Flow and Metabolic Regulation in Humans

University of British Columbia

Status: Begins enrollment in 1 month

Conditions: Cerebral Blood Flow; Sex Differences; Vascular; Cerebral Blood Flow Regulation; Exercise

Treatments: Drug: Estradiol (E2); Drug: Anastrozole (Arimidex); Drug: Testosterone; Drug: GnRH antagonist

Study type: Interventional

Funder types: Other

Identifiers

NCT07212712

H25-00385

Details and patient eligibility

About

Due to historical exclusion of females from research, there are gaps in the understanding of female physiology, how it differs from males, and how sex-specific hormones contribute. As a result, many diagnoses and treatments are based on male physiology and may not be appropriate or effective for females. Females consistently experience greater risk and report worse neurological outcomes in many diseases, including stroke, cardiac arrest, and dementia. As research in females progresses, differences between sexes and changes throughout the lifespan (e.g., puberty, menopause) highlight the importance of understanding the effects of sex and sex-specific hormones on the body. The brain is arguably the most important organ in the body, consuming 20% of the body's total energy. Previous research supports higher blood flow to the brain in females, and research in animals suggests hormones such as estrogen, progesterone, and testosterone are responsible. However, it is extremely difficult to isolate these hormones in humans, due to natural fluctuations (i.e., menstrual cycle). Therefore, the investigators plan to explore the direct role of these sex-specific hormones in regulating blood flow to the brain by blocking hormone production in healthy males and females and giving back testosterone and estrogen, respectively. The investigators will then conduct a range of tests to look at blood flow to the brain at rest and during various stressors. This research will provide crucial insight into how males and females differ in regulation of brain blood flow and inform new treatments and therapies to a wide range of brain injuries and diseases, improving outcomes and reducing the sex disparity in clinical pathways.

Full description

Sex hormones have been demonstrated to influence cardiovascular function in humans and cerebrovascular function in preclinical models. Declines in gonadal function have been attributed to the development of chronic disease risk in humans, including stroke, cognitive decline and dementia but the pathophysiology of these disorders remains unclear, in part, because there remains a gap-in-knowledge of the regulatory actions of sex hormones on cerebrovascular function in males and females. Therefore, the major goal of this application is to determine the independent regulatory actions of testosterone and estradiol on cerebrovascular function in males and females, respectively. Determination of this fundamental physiology is needed to inform future studies evaluating the sex-specific pathology physiology of chronic diseases and/or the development of efficacious sex-specific interventions for improving chronic disease risk.

To our knowledge, there is minimal evidence in humans isolating the influence of these hormones on basic mechanisms of brain blood flow regulation in vivo. Cross-sectional studies demonstrate younger females show significantly higher resting cerebral blood flow (CBF) compared to young males however, this sex difference in CBF is lost after midlife. This loss of female 'protection' on CBF is often attributed to hormonal differences 1 following menopause. 2 Further evidence in animal models support a key role of estrogen in both resting CBF and reactivity. 3-5 Crucially, these findings have not been replicated in humans, due to natural fluctuations in sex hormones. Research investigating natural hormonal fluctuations throughout the menstrual cycle and menopause provides conflicting findings, falling short on isolating the specific hormonal influences on cerebrovascular physiology likely due large interindividual variations in hormone levels and cycle lengths. 6 Similarly, the presence of normal physiological levels of testosterone has been shown to influence chronic disease risk including cardiovascular disease risk and peripheral vascular function. However, whether testosterone concentrations influence CBF in males remains incompletely understood with one study showing testosterone may enhance CBF in older hypogonadal men.7 Furthermore, testosterone can be converted to estradiol via a specific enzyme, aromatase; 8 thus, it is unclear whether the findings of prior study were related to changes in testosterone or estradiol concentrations in hypogonadal males. A definitive role of these hormones in brain blood flow regulation remains to be elucidated.

Current published studies suffer from several key limitations. First, many studies are unable to isolate the impact of hormones and their causality on cerebrovascular function in humans. 2,9-11 The inability to replicate findings from direct hormone manipulation in animals with observational data during natural hormone cycling in humans suggests measuring females during their menstrual cycles is not adequate for understanding the direct role of these hormones. There is a very large degree of variation both between and within individuals throughout their menstrual cycles, 6,12,13 proving difficult to relate changes in function with hormone levels. In males, very limited research exists investigating how testosterone influences cerebrovascular function. Further, this research will have direct implications for peri- and postmenopausal women, who are regularly prescribed exogenous estradiol for symptom management. Understanding the role of hormones in healthy physiology will allow direct translation to a myriad of hormonal disorders, including endometriosis, polycystic ovary syndrome, uterine fibroids, and hypogonadism. Individuals with endometriosis are commonly prescribed oral GnRH antagonist, which can be combined with estradiol for prolonged use (> 6 months). 14 Similarly, males with hypogonadism often supplement testosterone for prolonged periods with minimal negative effects.

This study will allow for isolation of estradiol in females and testosterone in males to investigate their role on cerebrovascular regulation. In contrast with previous observational studies throughout the menstrual cycle and menopause in females and aging in males, this study will provide a controlled environment to determine causal relationships between hormones and physiology without the presence of confounding variables. We will be able to address the gap in literature between interventional models in animals and observational studies in healthy humans. This study will provide fundamental understandings of healthy physiology that will shape future studies to investigate the role of estradiol and testosterone in clinical populations.

The primary objective of this study is to determine how estrogen and testosterone influence cerebral blood flow and metabolism.

Primary Hypotheses:

• Cerebral blood flow and cerebral metabolic rate of oxygen consumption will be reduced in females with hormone blockade but augmented with estrogen add-back.
• Cerebral blood flow and cerebral metabolic rate of oxygen consumption will be increased in males with hormone blockade but attenuated with testosterone add-back.

Enrollment: 50 estimated patients

Sex: All

Ages: 18 to 40 weeks old

Volunteers: Accepts Healthy Volunteers

Inclusion and exclusion criteria

Inclusion Criteria:

The investigators will recruit healthy young (18-40 years) males (n = 25) and females (n = 25). Females must be naturally cycling (i.e., no oral or hormonal contraceptives), not pregnant, and premenopausal. Participants must be normotensive (<140/90 mmHg & >90/60 mmHg) and have no medical history of cerebrovascular, cardiopulmonary, cardiovascular, neuromuscular, and renal disease assessed by a study questionnaire. A recent physical examination (within 1 year of participation) from a family physician or a completed physical examination by the study doctor is required prior to participation. All subjects will sign an informed consent form prior to participation.

Participants will be excluded if they meet any of the following criteria:

• Hypertensive (≥140/90 mmHg) or hypotensive (≤90/60 mmHg)
• BMI ≥ 30 kg/m2
• Fasting blood glucose ≥ 100 mg/dL
• Abnormal results on standard blood hematology tests (e.g., complete blood count, comprehensive metabolic panel)
• Males with hypogonadism (total testosterone ≤ 262 ng/dL, per Endocrine Society recommendations)
• Any history of cerebrovascular, cardiopulmonary, cardiovascular, neuromuscular, or renal disease
• Any liver dysfunction or disease
• Taking any cardiovascular medications or medications that would interfere with study medications (e.g., strong organic anion transporting polypeptide 1B1 inhibitors)
• Contraindications or sensitivities to study medications
• Current eating disorder (defined as >2 on the 'SCOFF' questionnaire 67)
• Current smokers
• Females with an irregular menstrual cycle or any medical conditions affecting the menstrual cycle (normal cycle defined as 21-35 days from menstrual cycle diary)
• Females who are pregnant (assessed with pregnancy test at screening and prior to starting medications), breastfeeding, or planning to conceive within 3 months
• Females using hormonal birth control (Copper intrauterine devices [IUDs] will be allowed as they do not change systemic hormone levels)
• Females who are peri- or postmenopausal (as determined by STRAW criteria 68)
• History of hormone-sensitive cancers (i.e., breast, ovarian, uterine or prostate)
• Z-score < -2 as assessed with DXA scan (to determine risk of decrease in bone mineral density)
• Assessed using the Center for Epidemiologic Studies Depression (CES-D) scale, participants with a score > 16 at screening unless otherwise approved by the study MD

Trial design

Primary purpose: Basic Science

Allocation: N/A

Interventional model: Single Group Assignment

Masking: None (Open label)

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50 participants in 1 patient group

Hormone Manipulation

Experimental group

Description:

Participants will act as their own controls, completing the experiment 3 times. First, with no hormone manipulation; second, with blockade of hormone production; third, with hormone add-back.

Treatment:

Drug: Testosterone

Drug: GnRH antagonist

Drug: Anastrozole (Arimidex)

Drug: Estradiol (E2)