Acute stress increases the synthesis of 7α-hydroxypregnenolone, a new key neurosteroid stimulating locomotor activity, through corticosterone action in newts

Toward understanding the endocrine regulation of diverse facultative migration strategies

Migration is an important event in the annual cycle of many animals that facilitates the use of resources that vary across space and time. It can occur with regular and predictable timing, as in obligate migration, or with much greater flexibility, as in facultative migration. Most research aimed at understanding the endocrine mechanisms regulating the transition to a migratory stage has focused on obligate migration, whereas less is known about facultative forms of migration. One challenge for research into the endocrine regulation of facultative migration is that facultative migrations encompass a diverse array of migratory movements. Here, we present a framework to describe and conceptualize variation in facultative migrations that focuses on conditions at departure. Within the context of this framework, we review potential endocrine mechanisms involved in the initiation of facultative migrations in vertebrates. We first focus on glucocorticoids, which have been the subject of most research on the topic. We then examine other potential hormones and neurohormones that have received less attention, but are exciting candidates to consider. We conclude by highlighting areas where future research is particularly needed.

The serotonin reuptake blocker citalopram destabilizes fictive locomotor activity in salamander axial circuits through 5-HT1A receptors

Serotoninergic (5-HT) neurons are powerful modulators of spinal locomotor circuits. Most studies on 5-HT modulation focused on the effect of exogenous 5-HT and these studies provided key information about the cellular mechanisms involved. Less is known about the effects of increased release of endogenous 5-HT with selective serotonin reuptake inhibitors. In mammals, such molecules were shown to destabilize the fictive locomotor output of spinal limb networks through 5-HT_1A receptors. However, in tetrapods little is known about the effects of increased 5-HT release on the locomotor output of axial networks, which are coordinated with limb circuits during locomotion from basal vertebrates to mammals. Here, we examined the effect of citalopram on fictive locomotion generated in axial segments of isolated spinal cords in salamanders, a tetrapod where raphe 5-HT reticulospinal neurons and intraspinal 5-HT neurons are present as in other vertebrates. Using electrophysiological recordings of ventral roots, we show that fictive locomotion generated by bath-applied glutamatergic agonists is destabilized by citalopram. Citalopram-induced destabilization was prevented by a 5-HT_1A receptor antagonist, whereas a 5-HT_1A receptor agonist destabilized fictive locomotion. Using immunofluorescence experiments, we found 5-HT-positive fibers and varicosities in proximity with motoneurons and glutamatergic interneurons that are likely involved in rhythmogenesis. Our results show that increasing 5-HT release has a deleterious effect on axial locomotor activity through 5-HT_1A receptors. This is consistent with studies in limb networks of turtle and mouse, suggesting that this part of the complex 5-HT modulation of spinal locomotor circuits is common to limb and axial networks in limbed vertebrates.NEW & NOTEWORTHY Little is known about the modulation exerted by endogenous serotonin on axial locomotor circuits in tetrapods. Using axial ventral root recordings in salamanders, we found that a serotonin reuptake blocker destabilized fictive locomotor activity through 5-HT_1A receptors. Our anatomical results suggest that serotonin is released on motoneurons and glutamatergic interneurons possibly involved in rhythmogenesis. Our study suggests that common serotoninergic mechanisms modulate axial motor circuits in amphibians and limb motor circuits in reptiles and mammals.

Pineal Neurosteroids: Biosynthesis and Physiological Functions

Similar to the adrenal glands, gonads, and placenta, vertebrate brains also produce various steroids, which are known as "neurosteroids." Neurosteroids are mainly synthesized in the hippocampus, hypothalamus, and cerebellum; however, it has recently been discovered that in birds, the pineal gland, a photosensitive region in the brain, produces more neurosteroids than other brain regions. A series of experiments using molecular and biochemical techniques have found that the pineal gland produces various neurosteroids, including sex steroids, de novo from cholesterol. For instance, allopregnanolone and 7α-hydroxypregnenolone are actively produced in the pineal gland, unlike in other brain regions. Pineal 7α-hydroxypregnenolone, an up-regulator of locomotion, enhances locomotor activity in response to light stimuli in birds. Additionally, pineal allopregnanolone acts on Purkinje cells in the cerebellum and prevents neuronal apoptosis within the developing cerebellum in juvenile birds. Furthermore, exposure to light during nighttime hours can cause loss of diurnal variations of pineal allopregnanolone synthesis during early posthatch life, eventually leading to cerebellar Purkinje cell death in juvenile birds. In light of these new findings, this review summarizes the biosynthesis and physiological functions of pineal neurosteroids. Given that the circadian rhythms of individuals in modern societies are constantly interrupted by artificial light exposure, these findings in birds, which are excellent model diurnal animals, may have direct implications for addressing problems regarding the mental health and brain development of humans.

Light-at-night exposure affects brain development through pineal allopregnanolone-dependent mechanisms

The molecular mechanisms by which environmental light conditions affect cerebellar development are incompletely understood. We showed that circadian disruption by light-at-night induced Purkinje cell death through pineal allopregnanolone (ALLO) activity during early life in chicks. Light-at-night caused the loss of diurnal variation of pineal ALLO synthesis during early life and led to cerebellar Purkinje cell death, which was suppressed by a daily injection of ALLO. The loss of diurnal variation of pineal ALLO synthesis induced not only reduction in pituitary adenylate cyclase-activating polypeptide (PACAP), a neuroprotective hormone, but also transcriptional repression of the cerebellar Adcyap1 gene that produces PACAP, with subsequent Purkinje cell death. Taken together, pineal ALLO mediated the effect of light on early cerebellar development in chicks.

7α-Hydroxypregnenolone regulating locomotor behavior identified in the brain and pineal gland across vertebrates

The brain synthesizes steroids de novo from cholesterol, which are called neurosteroids. Based on extensive studies on neurosteroids over the past thirty years, it is now accepted that neurosteroidogenesis in the brain is a conserved property across vertebrates. However, the formation of bioactive neurosteroids in the brain is still incompletely elucidated in vertebrates. In fact, we recently identified 7α-hydroxypregnenolone (7α-OH PREG) as a novel bioactive neurosteroid stimulating locomotor behavior in the brain of several vertebrates. The follow-up studies have demonstrated that the stimulatory action of brain 7α-OH PREG on locomotor behavior is mediated by the dopaminergic system across vertebrates. More recently, we have further demonstrated that the pineal gland, an endocrine organ located close to the brain, is a major site of the formation of bioactive neurosteroids. In addition to the brain, the pineal gland actively produces 7α-OH PREG de novo from cholesterol as a major pineal neurosteroid that acts on the brain to control locomotor rhythms. This review summarizes the identification, biosynthesis and mode of action of brain and pineal 7α-OH PREG, a new bioactive neurosteroid regulating locomotor behavior, across vertebrates.

Quercetin protects against stress-induced anxiety- and depression-like behavior and improves memory in male mice

The present study evaluates the protective role of Quercetin (Quer), against immobilization stress- induced anxiety, depression and cognition alteration in mice using behavioral and biochemical parameters. 24 adult Albino mice were distributed into 2 groups vehicle (n=12; 1 ml/kg) and Quer injected (n=12; 20 mg/kg/ml). The animals received their respective treatment for 14 days. On day 15, after the drug administration, animals were sub-divided into 4 groups (n=6); (i) unstressed + vehicle; (ii) stressed + vehicle; (iii) unstressed + Quer; (iv) stressed + Quer. On day 16, 24 h after the immobilization stress behavioral activities (light-dark activity, elevated plus maze, Morris water maze, and forced swim test) monitored and then animals were decapitated 1 h after the drug administration. Brain samples were collected for biochemical (antioxidant enzymes, AChE, ACh, 5-HT and its metabolite) analysis. The present study indicates the Quer reversed the stress-induced anxiety and depression, in addition, memory performance was more enhanced in stressed group. Following the treatment of Quer, stress-induced elevation of lipid peroxidation and suppression of antioxidant enzymes were also reversed. Administration of Quer decreased AChE in unstressed, while levels of acetylcholine were increased in vehicle and Quer treated stressed animals. The metabolism of 5-HT was increased in Quer treated stressed than unstressed animals. In conclusion, the present finding showed that Quer could prevent the impairment of antioxidant enzymes and also regulate the serotonergic and cholinergic neurotransmission and produce antianxiety, antidepressant effect and enhance memory following 2 h immobilization stress in mice.

Expression of steroidogenic enzymes and metabolism of steroids in COS-7 cells known as non-steroidogenic cells

The COS-7 (CV-1 in Origin with SV40 genes) cells are known as non-steroidogenic cells because they are derived from kidney cells and the kidney is defined as a non-steroidogenic organ. Therefore, COS-7 cells are used for transfection experiments to analyze the actions of functional molecules including steroids. However, a preliminary study suggested that COS-7 cells metabolize [³H]testosterone to [³H]androstenedione. These results suggest that COS-7 cells are able to metabolize steroids. Therefore, the present study investigated the expression of steroidogenic enzymes and the metabolism of steroids in COS-7 cells. RT-PCR analyses demonstrated the expressions of several kinds of steroidogenic enzymes, such as cytochrome P450 side-chain cleavage enzyme, 3β-hydroxysteroid dehydrogenase/Δ⁵-Δ⁴ isomerase, cytochrome P450 7α-hydroxylase, cytochrome P450 17α-hydroxylase/17,20-lyase, 17β-hydroxysteroid dehydrogenase, 5α-reductase, cytochrome P450 21-hydroxylase, cytochrome P450 11β-hydroxylase, and cytochrome P450 aromatase in COS-7 cells. In addition, steroidogenic enzymes 3β-HSD, P4507α, 5α-reductase, P450c17, P450c21, P450c11β, and 17β-HSD actively metabolized various steroids in cultured COS-7 cells. Finally, we demonstrated that 17β-HSD activity toward androstenedione formation was greater than other steroidogenic enzyme activities. Our results provide new evidence that COS-7 cells express a series of steroidogenic enzyme mRNAs and actively metabolize a variety of steroids.

Brain-Derived Steroids, Behavior and Endocrine Conflicts Across Life History Stages in Birds: A Perspective

Biological steroids were traditionally thought to be synthesized exclusively by the adrenal glands and gonads. Recent decades have seen the discovery of neurosteroid production that acts locally within the central nervous system to affect physiology and behavior. These actions include, for example, regulation of aggressive behavior, such as territoriality, and locomotor movement associated with migration. Important questions then arose as to how and why neurosteroid production evolved and why similar steroids of peripheral origin do not always fulfill these central roles? Investigations of free-living vertebrates suggest that synthesis and action of bioactive steroids within the brain may have evolved to regulate expression of specific behavior in different life history stages. Synthesis and secretion of these hormones from peripheral glands is broadcast throughout the organism via the blood stream. While widespread, general actions of steroids released into the blood might be relevant for regulation of morphological, physiological, and behavioral traits in one life history stage, such hormonal release may not be appropriate in other stages. Specific and localized production of bioactive steroids in the brain, but not released into the periphery, could be a way to avoid such conflicts. Two examples are highlighted. First, we compare the control of territorial aggression of songbirds in the breeding season under the influence of gonadal steroids with autumnal (non-breeding) territoriality regulated by sex steroid production in the brain either from circulating precursors such as dehydroepiandrosterone or local central production of sex steroids de novo from cholesterol. Second, we outline the production of 7α-hydroxypregnenolone within the brain that appears to affect locomotor behavior in several contexts. Local production of these steroids in the brain may provide specific regulation of behavioral traits throughout the year and independently of life history stage.

How to contribute to the progress of neuroendocrinology: New insights from discovering novel neuropeptides and neurosteroids regulating pituitary and brain functions

Obtaining new insights by discovering novel neuropeptides and neurosteroids regulating pituitary and brain functions is essential for the progress of neuroendocrinology. At the beginning of 1970s, gonadotropin-releasing hormone (GnRH) was discovered in mammals. Since then, it was generally accepted that GnRH is the only hypothalamic neuropeptide regulating gonadotropin release in vertebrates. In 2000, however, gonadotropin-inhibitory hormone (GnIH), a novel hypothalamic neuropeptide that actively inhibits gonadotropin release, was discovered in quail. The follow-up studies demonstrated that GnIH acts as a new key player for regulation of reproduction across vertebrates. It now appears that GnIH acts on the pituitary and the brain to serve a number of behavioral and physiological functions. On the other hand, a new concept has been established that the brain synthesizes steroids, called neurosteroids. The formation of neurosteroids in the brain was originally demonstrated in mammals and subsequently in other vertebrates. Recently, 7α-hydroxypregnenolone was discovered as a novel bioactive neurosteroid inducing locomotor behavior of vertebrates, indicating that neurosteroidogenesis in the brain is still incompletely elucidated in vertebrates. At the beginning of 2010s, it was further found that the pineal gland actively produces neurosteroids. Pineal neurosteroids act on the brain to regulate locomotor rhythms and neuronal survival. Furthermore, the interaction of neuropeptides and neurosteroids is becoming clear. GnIH decreases aggressive behavior by regulating neuroestrogen synthesis in the brain. This review summarizes these new insights by discovering novel neuropeptides and neurosteroids in the field of neuroendocrinology.

Neurosteroids; potential underpinning roles in maintaining homeostasis

The neuroactive steroids which are synthesized in the brain and nervous system are known as "Neurosteroids". These steroids have crucial functions such as contributing to the myelination and organization of the brain connectivity. Under the stressful circumstances, the concentrations of neurosteroid products such as allopregnanolone (ALLO) and allotetrahydrodeoxycorticosterone (THDOC) alter. It has been suggested that these stress-derived neurosteroids modulate the physiological response to stress. Moreover, it has been demonstrated that the hypothalamic-pituitary-adrenal (HPA) axis mediates the physiological adaptation following stress in order to maintain homeostasis. Although several regulatory pathways have been introduced, the exact role of neurosteroids in controlling HPA axis is not clear to date. In this review, we intend to discern specific pathways associated with regulation of HPA axis in which neuroactive steroids have the main role. In this respect, we propose pathways that may be initiated after neurosteroidogenesis in different brain subregions following acute stress which are potentially capable of activating or inhibiting the HPA axis.

7α-Hydroxypregnenolone, a key neuronal modulator of locomotion, stimulates upstream migration by means of the dopaminergic system in salmon

Salmon migrate upstream against an opposing current in their natal river. However, the molecular mechanisms that stimulate upstream migratory behavior are poorly understood. Here, we show that 7α-hydroxypregnenolone (7α-OH PREG), a newly identified neuronal modulator of locomotion, acts as a key factor for upstream migration in salmon. We first identified 7α-OH PREG and cytochrome P450 7α-hydroxylase (P450_7α), a steroidogenic enzyme producing 7α-OH PREG, in the salmon brain and then found that 7α-OH PREG synthesis in the brain increases during upstream migration. Subsequently, we demonstrated that 7α-OH PREG increases upstream migratory behavior of salmon. We further found that 7α-OH PREG acts on dopamine neurons in the magnocellular preoptic nucleus during upstream migration. Thus, 7α-OH PREG stimulates upstream migratory behavior through the dopaminergic system in salmon. These findings provide new insights into the molecular mechanisms of fish upstream migration.

Breakthrough in neuroendocrinology by discovering novel neuropeptides and neurosteroids: 2. Discovery of neurosteroids and pineal neurosteroids

Bargmann-Scharrer's discovery of "neurosecretion" in the first half of the 20th century has since matured into the scientific discipline of neuroendocrinology. Identification of novel neurohormones, such as neuropeptides and neurosteroids, is essential for the progress of neuroendocrinology. Our studies over the past two decades have significantly broadened the horizons of this field of research by identifying novel neuropeptides and neurosteroids in vertebrates that have opened new lines of scientific investigation in neuroendocrinology. We have established de novo synthesis and functions of neurosteroids in the brain of various vertebrates. Recently, we discovered 7α-hydroxypregnenolone (7α-OH PREG), a novel bioactive neurosteroid that acts as a key regulator for inducing locomotor behavior by means of the dopaminergic system. We further discovered that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol (CHOL). The pineal gland secretes 7α-OH PREG and 3α,5α-tetrahydroprogesterone (3α,5α-THP; allopregnanolone) that are involved in locomotor rhythms and neuronal survival, respectively. Subsequently, we have demonstrated their mode of action and functional significance. This review summarizes the discovery of these novel neurosteroids and its contribution to the progress of neuroendocrinology.

Biosynthesis and biological action of pineal allopregnanolone

The pineal gland transduces photoperiodic changes to the neuroendocrine system by rhythmic secretion of melatonin. We recently provided new evidence that the pineal gland is a major neurosteroidogenic organ and actively produces a variety of neurosteroids de novo from cholesterol in birds. Notably, allopregnanolone is a major pineal neurosteroid that is far more actively produced in the pineal gland than the brain and secreted by the pineal gland in juvenile birds. Subsequently, we have demonstrated the biological action of pineal allopregnanolone on Purkinje cells in the cerebellum during development in juvenile birds. Pinealectomy (Px) induces apoptosis of Purkinje cells, whereas allopregnanolone administration to Px chicks prevents cell death. Furthermore, Px increases the number of Purkinje cells that express active caspase-3, a crucial mediator of apoptosis, and allopregnanolone administration to Px chicks decreases the number of Purkinje cells expressing active caspase-3. It thus appears that pineal allopregnanolone prevents cell death of Purkinje cells by suppressing the activity of caspase-3 during development. This paper highlights new aspects of the biosynthesis and biological action of pineal allopregnanolone.

Brain and pineal 7α-hydroxypregnenolone stimulating locomotor activity: identification, mode of action and regulation of biosynthesis

Biologically active steroids synthesized in the central and peripheral nervous systems are termed neurosteroids. However, the biosynthetic pathways leading to the formation of neurosteroids are still incompletely elucidated. 7α-Hydroxypregnenolone, a novel bioactive neurosteroid stimulating locomotor activity, has been recently identified in the brain of newts and quail. Subsequently, the mode of action and regulation of biosynthesis of 7α-hydroxypregnenolone have been determined. Moreover, recent studies on birds have demonstrated that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol. 7α-Hydroxypregnenolone is a major pineal neurosteroid that stimulates locomotor activity in juvenile chickens, connecting light-induced gene expression with locomotion. This review summarizes the advances in our understanding of the identification, mode of action and regulation of biosynthesis of brain and pineal 7α-hydroxypregnenolone, a potent stimulator of locomotor activity.

A genetically female brain is required for a regular reproductive cycle in chicken brain chimeras

Sexual differentiation leads to structural and behavioural differences between males and females. Here we investigate the intrinsic sex identity of the brain by constructing chicken chimeras in which the brain primordium is switched between male and female identities before gonadal development. We find that the female chimeras with male brains display delayed sexual maturation and irregular oviposition cycles, although their behaviour, plasma concentrations of sex steroids and luteinizing hormone levels are normal. The male chimeras with female brains show phenotypes similar to typical cocks. In the perinatal period, oestrogen concentrations in the genetically male brain are higher than those in the genetically female brain. Our study demonstrates that male brain cells retain male sex identity and do not differentiate into female cells to drive the normal oestrous cycle, even when situated in the female hormonal milieu. This is clear evidence for a sex-specific feature that develops independent of gonadal steroids.

Possible role of pineal allopregnanolone in Purkinje cell survival

It is believed that neurosteroids are produced in the brain and other nervous systems. Here, we show that allopregnanolone (ALLO), a neurosteroid, is exceedingly produced in the pineal gland compared with the brain and that pineal ALLO acts on the Purkinje cell, a principal cerebellar neuron, to prevent apoptosis in the juvenile quail. We first demonstrated that the pineal gland is a major organ of neurosteroidogenesis. A series of experiments using molecular and biochemical techniques has further demonstrated that the pineal gland produces a variety of neurosteroids de novo from cholesterol in the juvenile quail. Importantly, ALLO was far more actively produced in the pineal gland than in the brain. Pinealectomy (Px) decreased ALLO concentration in the cerebellum and induced apoptosis of Purkinje cells, whereas administration of ALLO to Px quail chicks prevented apoptosis of Purkinje cells. We further found that Px significantly increased the number of Purkinje cells that expressed active caspase-3, a key protease in apoptotic pathway, and daily injection of ALLO to Px quail chicks decreased the number of Purkinje cells expressing active caspase-3. These results indicate that the neuroprotective effect of pineal ALLO is associated with the decrease in caspase-3 activity during the early stage of neuronal development. We thus provide evidence that the pineal gland is an important neurosteroidogenic organ and that pineal ALLO may be involved in Purkinje cell survival during development. This is an important function of the pineal gland in the formation of neuronal circuits in the developing cerebellum.

N-Methyl-D-aspartate (NMDA) receptor involvement in central nervous system prostaglandin production during the relapse phase of chronic relapsing experimental autoimmune encephalomyelitis (CR EAE)

Our previous studies have established that major changes in central nervous system (CNS) prostaglandin (PG) levels occur during the relapse phase of chronic relapsing experimental autoimmune encephalomyelitis (CR EAE), an animal model of the human demyelinating disease multiple sclerosis. PG production is controlled through a series of enzymic pathways that, in EAE, are influenced by neuroantigen-driven autoimmune events. In non-immune-based models of CNS disease, endogenous glucocorticoids have been proposed as instigators of PG synthesis via activation of the N-methyl-D-aspartate (NMDA) receptor. Glucocorticoids have an important regulatory role in the pathogenesis EAE and the NMDA receptor is intimately involved in many of the characteristic neuroinflammatory processes that govern the disease. Therefore, the alterations in prostanoid concentrations during the relapse stage of CR EAE may ultimately be governed by glucocorticoid-induced NMDA receptor activation. The current investigation has examined the proposed glucocorticoid-NMDA receptor link by determining the effects of the receptor antagonist, (+) MK-801, on CNS PGE 2 and PGD 2 levels in Biozzi mice with relapse symptoms of CR EAE. Prostanoid concentrations in the cerebral cortex were not altered by drug administration, and in cerebellar tissues, a vehicle effect negated any drug-induced changes. However, the level of PGD 2 in spinal cords from (+) MK-801-dosed mice was significantly lower, compared to controls, but PGE 2 concentrations remained unchanged. The results suggest that glucocorticoid-NMDA receptor-linked events are not primarily responsible for PG generation in the brain but may influence prostanoid production in discrete areas of the CNS.

7α-Hydroxypregnenolone, a new key regulator of amphibian locomotion: discovery, progress and prospect

Seasonally-breeding amphibians have served as excellent animal models to investigate the biosynthesis and biological actions of neurosteroids. Previous studies have demonstrated that the brain of amphibians possesses key steroidogenic enzymes and produces pregnenolone, a precursor of steroid hormones, and other various neurosteroids. We recently found that the brain of seasonally-breeding newts actively produces 7α-hydroxypregnenolone, a previously undescribed amphibian neurosteroid. This novel amphibian neurosteroid acts as a neuronal modulator to stimulate locomotor activity in newts. Subsequently, the mode of action of 7α-hydroxypregnenolone has been demonstrated in the newt brain. 7α-Hydroxypregnenolone stimulates locomotor activity through activation of the dopaminergic system. To understand the functional significance of 7α-hydroxypregnenolone in the regulation of locomotor activity, diurnal and seasonal changes in synthesis of 7α-hydroxypregnenolone have also been demonstrated in the newt brain. Melatonin derived from the pineal gland and eyes regulates 7α-hydroxypregnenolone synthesis in the brain, thus inducing diurnal locomotor changes. Prolactin, an adenohypophyseal hormone, regulates 7α-hydroxypregnenolone synthesis in the brain, and also induces seasonal locomotor changes. In addition, 7α-hydroxypregnenolone mediates corticosterone action to increase locomotor activity under stress. This review summarizes the discovery, progress and prospect of 7α-hydroxypregnenolone, a new key regulator of amphibian locomotion.