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Membrane-Initiated Estradiol Signaling in the Central Nervous System  

Paul E. Micevych and Melinda A. Mittelman-Smith

In the last two decades of the 20th century, key findings in the field of estrogen signaling completely changed our understanding of hormones: first, steroidogenesis was demonstrated in the CNS; second, a vast majority of cells in the nervous system were shown to have estrogen receptors; third, a second nuclear estrogen receptor (ERß) was cloned; and finally, “nuclear” receptors were shown to be present and functional in the cell membrane. Shortly thereafter, even more membrane estrogen receptors were discovered. Steroids (estrogens, in particular) began to be considered as neurotransmitters and their receptors were tethered to G protein-coupled receptor signaling cascades. In some parts of the brain, levels of steroids appeared to be independent of those found in the circulation and yet, circulating steroids had profound actions on the brain physiology. In this review, we discuss the interaction of peripheral and central estrogen action in the context of female reproduction—one of the best-studied aspects of steroid action. In addition to reviewing the evidence for steroidogenesis in the hypothalamus, we review membrane-localized nuclear receptors coupling to G protein-signaling cascades and the downstream physiological consequences for reproduction. We will also introduce newer work that demonstrates cell signaling for a common splice variant of estrogen receptor-α (ERα), and membrane action of neuroprogesterone in regulating estrogen positive feedback.


Molecular Regulation of Energy Balance  

D. Grahame Hardie and A. Mark Evans

AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that monitors the levels of AMP and ADP relative to ATP. If increases in AMP:ATP and/or ADP:ATP ratios are detected (indicating a reduction in cellular energy status), AMPK is activated by the canonical mechanism involving both allosteric activation and enhanced net phosphorylation at Thr172 on the catalytic subunit. Once activated, AMPK phosphorylates dozens of downstream targets, thus switching on catabolic pathways that generate ATP and switching off anabolic pathways and other energy-consuming processes. AMPK can also be activated by non-canonical mechanisms, triggered either by glucose starvation by a mechanism independent of changes in adenine nucleotides, or by increases in intracellular Ca 2 + in response to hormones, mediated by the alternate upstream kinase CaMKK2. AMPK is expressed in almost all eukaryotic cells, including neurons, as heterotrimeric complexes comprising a catalytic α subunit and regulatory β and γ subunits. The α subunits contain the kinase domain and regulatory regions that interact with the other two subunits. The β subunits contain a domain that, with the small lobe of the kinase domain on the α subunit, forms the “ADaM” site that binds synthetic drugs that are potent allosteric activators of AMPK, while the γ subunits contain the binding sites for the classical regulatory nucleotides, AMP, ADP, and ATP. Although much undoubtedly remains to be discovered about the roles of AMPK in the nervous system, emerging evidence has confirmed the proposal that, in addition to its universal functions in regulating energy balance at the cellular level, AMPK also has cell- and circuit-specific roles at the whole-body level, particularly in energy homeostasis. These roles are mediated by phosphorylation of neural-specific targets such as ion channels, distinct from the targets by which AMPK regulates general, cell-autonomous energy balance. Examples of these cell- and circuit-specific functions discussed in this review include roles in the hypothalamus in balancing energy intake (feeding) and energy expenditure (thermogenesis), and its role in the brainstem, where it supports the hypoxic ventilatory response (breathing), increasing the supply of oxygen to the tissues during systemic hypoxia.


Neural Control of Lower Urinary Tract Function  

Jonathan M. Beckel and William C. de Groat

Functions of the lower urinary tract to store and periodically eliminate urine are regulated by a complex neural control system in the brain and lumbosacral spinal cord that coordinates the activity of smooth and striated muscles of the bladder and urethral outlet via a combination of voluntary and reflex mechanisms. Many neural circuits controlling the lower urinary tract exhibit switch-like patterns of activity that turn on and off in an all-or-none manner. During urine storage, spinal sympathetic and somatic reflexes are active to maintain a quiescent bladder and a closed outlet. During micturition, these spinal storage reflexes are suppressed by input from the brain, while parasympathetic pathways in the brain are activated to produce a bladder contraction and relaxation of the urethra. The major component of the micturition switching circuit is a spinobulbospinal parasympathetic pathway that consists of essential relay circuitry in the periaqueductal gray and pontine micturition center. These circuits in the rostral brain stem are, in turn, regulated by inputs from the forebrain that mediate voluntary control of micturition. Thus neural control of micturition is organized as a hierarchical system in which spinal storage reflexes and supraspinal voiding reflexes are regulated voluntarily by higher centers in the brain. In young children the voluntary mechanisms are undeveloped and voiding is purely reflex. Voluntary control emerges during maturation of the nervous system and depends on learned behavior. Diseases or injuries of the nervous system in adults cause re-emergence of involuntary micturition, leading to urinary incontinence.


Regulation of Gonadotropins  

Daniel J. Bernard, Yining Li, Chirine Toufaily, and Gauthier Schang

The gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are glycoproteins produced by gonadotrope cells of the anterior pituitary gland. The two hormones act on somatic cells of the gonads in both males and females to regulate fundamental aspects of reproductive physiology, including gametogenesis and steroidogenesis. In males, LH stimulates testosterone production and sperm maturation. FSH also regulates spermatogenesis, though the importance of the hormone in this process differs across species. In females, FSH stimulates ovarian follicle maturation. Follicles are structures composed of oocytes surrounded by two types of somatic cells, granulosa and theca cells. FSH stimulates granulosa cells to proliferate and to increase their production of the aromatase enzyme. LH stimulates theca cells to make androgens, which are converted into estrogens by aromatase in granulosa cells. A surge of LH also stimulates ovulation of mature follicles. Gonadotropin-releasing hormone (GnRH) from the brain is the principal stimulator of gonadotropin synthesis and secretion from the pituitary. The sex steroids (androgens and estrogens) that are produced by the gonads in response to the gonadotropins feedback to the brain and pituitary gland. In the brain, these hormones usually slow the release of GnRH through a process called negative feedback, which in turn leads to decreases in FSH and LH. The steroids also modulate the sensitivity of the pituitary to GnRH in addition to directly regulating expression of the genes that encode the gonadotropin subunits. These effects are gene- and species-specific. In females, estrogens also have positive feedback actions in the brain and pituitary in a reproductive cycle stage-dependent manner. This positive feedback promotes GnRH and LH release, leading to the surge of LH that triggers ovulation. The gonadotropins are dimeric proteins. FSH and LH share a common α-subunit but have hormone-specific subunits, FSHβ and LHβ. The β subunits provide a means for differential regulation and action of the two hormones. In the case of FSH, there is a second gonadal feedback system that specifically regulates the FSHβ subunit. The gonads produce proteins in the transforming growth factor β (TGFβ) family called inhibins, which come in two forms (inhibin A and inhibin B). The ovary produces both inhibins whereas the testes make inhibin B alone. Inhibins selectively suppress FSH synthesis and secretion, without affecting LH. The pituitary produces additional TGFβ proteins called activins, which are structurally related to inhibins. Activins, however, stimulate FSH synthesis by promoting transcription of the FSHβ subunit gene. Inhibins act as competitive receptor antagonists, binding to activin receptors and blocking activin action, and thereby leading to decreases in FSH. Together, GnRH, sex steroids, activins, and inhibins modulate and coordinate gonadotropin production and action to promote proper gonadal function and fertility.


Autonomic Control of Immune Function  

Eric S. Wohleb

Proper immune function is critical to maintain homeostasis, recognize and eliminate pathogens, and promote tissue repair. Primary and secondary immune organs receive input from the autonomic nervous system and immune cells express receptors for epinephrine, norepinephrine, and/or acetylcholine. Through direct signaling the autonomic nervous system controls immune function by altering immune cell development, initiating redistribution of immune cells throughout the body, and promoting molecular pathways that shift immune cell reactivity. This neuroimmune communication allows the autonomic nervous system to shape immune function based on physiological and psychological demands.


Neurobiology of Anabolic-Androgenic Steroid Abuse  

Ruth I. Wood and Kathryn G. Wallin-Miller

Anabolic-androgenic steroids (AAS) are both performance-enhancing substances and drugs of abuse. Although AAS are banned in competitive sports, they are widely used by both elite and rank-and-file athletes. All AAS are derived from testosterone, the principle endogenous androgen produced by the testes of adult men. While AAS increase muscular strength and athletic performance, they also have serious consequences for health and behavior. AAS are implicated in maladaptive behavioral and cognitive changes such as increased risk-taking and altered decision-making. However, effects of AAS on cognition are not well understood. Studies of human AAS users are limited by an inability to control for pre-existing psychopathology and behavioral differences. Furthermore, in order to understand AAS effects on behavior, it is important to discover how AAS impact the brain. Animal models of AAS abuse parallel human studies to uncover effects on cognition, decision-making, and underlying neurobiological mechanisms. In operant discounting tests, rats treated with chronic high-dose testosterone are less sensitive to effort, punishment, and delay but are more sensitive to uncertainty. Likewise, they demonstrate impaired cognitive flexibility when tested for set-shifting and reversal learning. It appears that AAS induce many of these cognitive changes via effects on the mesocorticolimbic dopamine system, particularly through the dopamine D1- and D2-like receptors in subnuclei of the nucleus accumbens. AAS also have rewarding effects mediated by similar neural circuits. In preclinical studies, animals will voluntarily self-administer AAS. Human users may develop dependence. These findings highlight the vulnerability of brain circuits controlling cognition and reward to androgens at high doses.


Neuroendocrine Mechanisms of Psychological Stress: Age and Sex Differences in Adults  

Allison E. Gaffey and Brandy S. Martinez

There are two main branches of the human stress response. The autonomic nervous system acts rapidly and is often referred to as our fight or flight response. The slow-acting arm of the stress response refers to the hypothalamic-pituitary-adrenal (HPA) axis, which triggers a hormone cascade resulting in the release of various hormones including cortisol. Healthy functioning of the HPA axis is tightly regulated by negative feedback, the endogenous self-regulatory mechanism of the system that terminates cortisol production. Alterations in HPA axis functioning are characterized by both hypo- and hypersecretion of cortisol in response to psychological stress and are typically associated with negative physical health outcomes as well as clinical pathology. What remains poorly understood is how HPA activity changes with age and the pathways through which these changes occur. In addition to changes associated with the normative aging process, age-related changes in cortisol may also be driven by the cumulative effects of stress experienced across the life span (e.g., traumatic stress); stressors unique to later life (e.g., caring for an ailing loved one); or health problems. Although research examining how the HPA axis might change with age is inconsistent, there appears to be reasonable evidence to suggest that: (1) both stress-induced and diurnal cortisol output may increase with age, potentially beginning with changes in the cortisol awakening response, (2) variability in cortisol production increases with age, (3) diurnal (i.e., daily) cortisol rhythms are preserved in later life, and (4) age-related differences in cortisol may be more distinct in men than in women. However, it remains unknown whether these changes in older adults’ physiology reflect maladaptive functioning of the HPA axis or interact with other health concerns to negatively affect overall psychophysiological health. Further research is needed to disentangle the interplay between aging and HPA axis functioning to better understand what alterations are associated with the normative aging process, when they occur, and how they influence longevity.


Inflammatory Signals and Sexual Differentiation of the Brain  

Margaret M. McCarthy

Sex differences in the brain are established by the differential gonadal steroid hormonal milieu experienced by developing male and female fetuses and newborns. Androgen production by the testis starts in males prior to birth resulting in a brief developmental window during which the brain is exposed to high levels of steroid. Androgens and aromatized estrogens program the developing brain toward masculinized physiology and behavior that is then manifest in adulthood. In rodents, the perinatal programming of sex-specific adult mating behavior provides a model system for exploring the mechanistic origins of brain sex differences. Microglia are resident in the brain and provide innate immunity. Previously considered restricted to response to injury, these cells are now thought to be major contributors to the sculpting of developing neural circuits. This role extends to being an important component of the sexual differentiation process and has opened the door for exploration into myriad other aspects of neuroimmunity and inflammation as possible determinants of sex differences. In humans, males are at greater risk for more frequent and/or more severe neuropsychiatric and neurological disorders of development, many of which include prenatal inflammation as an additional risk factor. Emerging clinical and preclinical evidence suggests male brains experience a higher inflammatory tone early in development, and this may have its origins in the maternal immune system. Collectively, these disparate observations coalesce into a coherent picture in which brain development diverges in males versus females due to a combination of gonadal steroid action and neuroinflammation, and the latter increases the risk to males of developmental disorders.


The Hypocretin Arousal Network  

Jeremy C. Borniger and Luis de Lecea

The hypocretins (also known as orexins) are selectively expressed in a subset of lateral hypothalamic neurons. Since the reports of their discovery in 1998, they have been intensely investigated in relation to their role in sleep/wake transitions, feeding, reward, drug abuse, and motivated behavior. This research has cemented their role as a subcortical relay optimized to tune arousal in response to various salient stimuli. This article reviews their discovery, physiological modulation, circuitry, and integrative functionality contributing to vigilance state transitions and stability. Specific emphasis is placed on humoral and neural inputs regulating hcrt neural function and new evidence for an autoimmune basis of the sleep disorder narcolepsy. Future directions for this field involve dissection of the heterogeneity of this neural population using single-cell transcriptomics, optogenetic, and chemogenetics, as well as monitoring population and single cell activity. Computational models of the hypocretin network, using the “flip-flop” or “integrator neuron” frameworks, provide a fundamental understanding of how this neural population influences brain-wide activity and behavior.


Suprachiasmatic Nucleus Anatomy, Physiology, and Neurochemistry  

Rae Silver

We live in an approximately 24-hour world and circadian rhythms have evolved to adapt organisms to the opportunities presented by Earth’s 24-hour cycle of light and dark. A “master clock” located in the suprachiasmatic nucleus (SCN) of the brain orchestrates daily rhythms in all manner of behavioral, endocrine, metabolic, autonomic, and homeostatic systems in our bodies. The SCN is comprised of about 20,000 neurons and about one third as many astroglia. How can so few neurons and astroglia guide so many rhythms? How do neurons time out an interval as long as a day? The answers are a case study in understanding how genes within cells, and cells within circuits, function together to perform complex activities and optimize bodily functions. While individual clock cells are found in virtually all bodily tissues, the unique connectome of the SCN, its specialized afferent inputs from the retinohypothalamic tract, and its neural and humoral outputs enable its “babel” of neuronal types to synchronize their activity and signal time to the rest of the body. At the molecular-cellular level, circadian rhythms are regulated by a 24-hour transcriptional–translational feedback loop. At the SCN tissue level, individual SCN neurons coordinate their gene expression and electrical activity, working together in circuits that sustain coherent rhythms. The SCN has many distinct cell types based on their neurotransmitters, neuropeptides, and afferent and efferent connections. There has been much progress in unraveling the dynamic network organization that underlies the SCN network’s communications. Though the precise anatomical connections underlying interneuronal communication in the SCN are not completely understood, key signaling mechanisms that sustain the SCN’s intrinsic rhythmicity have been tackled using intersectional genomic tools. Transgenic animals that permit the visualization of clock gene–protein expression have enabled analysis of SCN network activity over time. Availability of animals bearing mutations in clock genes or proteins enable the determination of changes within neurons, among neurons in networks, and their impact on behavior. The use of continuous readouts of circadian activity that track behavior, or clock gene expression, or electrical activity changes over time, within an SCN or a single neuron, leads the way to unraveling mechanisms sustaining the circadian timing system. Because the results of circadian studies generate huge amounts of data, the entry of mathematical modelers and statisticians into the field has begun to yield useful and testable predictions on how these multiplexed systems work to adapt to our 24-hour world.