High doses of systemic DHEA-sulfate do not affect sleep structure and elicit moderate changes in non-REM sleep EEG in rats

Neurobehavioural complications of sleep deprivation: Shedding light on the emerging role of neuroactive steroids

Sleep deprivation (SD) is associated with a broad spectrum of cognitive and behavioural complications, including emotional lability and enhanced stress reactivity, as well as deficits in executive functions, decision making and impulse control. These impairments, which have profound negative consequences on the health and productivity of many individuals, reflect alterations of the prefrontal cortex (PFC) and its connectivity with subcortical regions. However, the molecular underpinnings of these alterations remain elusive. Our group and others have begun examining how the neurobehavioural outcomes of SD may be influenced by neuroactive steroids, a family of molecules deeply implicated in sleep regulation and the stress response. These studies have revealed that, similar to other stressors, acute SD leads to increased synthesis of the neurosteroid allopregnanolone in the PFC. Whereas this up-regulation is likely aimed at counterbalancing the detrimental impact of oxidative stress induced by SD, the increase in prefrontal allopregnanolone levels contributes to deficits in sensorimotor gating and impulse control, signalling a functional impairment of PFC. This scenario suggests that the synthesis of neuroactive steroids during acute SD may be enacted as a neuroprotective response in the PFC; however, such compensation may in turn set off neurobehavioural complications by interfering with the corticolimbic connections responsible for executive functions and emotional regulation.

Neurosteroid influences on sensitivity to ethanol

This review will highlight a variety of mechanisms by which neurosteroids affect sensitivity to ethanol, including physiological states associated with activity of the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes, and the effects of chronic exposure to ethanol, in addition to behavioral implications. To date, γ-aminobutyric acid (GABA(A)) receptor mechanisms are a major focus of the modulation of ethanol effects by neuroactive steroids. While NMDA receptor mechanisms are gaining prominence in the literature, these complex data would be best discussed separately. Accordingly, GABA(A) receptor mechanisms are emphasized in this review with brief mention of some NMDA receptor mechanisms to point out contrasting neuroactive steroid pharmacology. Overall, the data suggest that neurosteroids are virtually ubiquitous modulators of inhibitory neurotransmission. Neurosteroids appear to affect sensitivity to ethanol in specific brain regions and, consequently, specific behavioral tests, possibly related to the efficacy and potency of ethanol to potentiate the release of GABA and increase neurosteroid concentrations. Although direct interaction of ethanol and neuroactive steroids at common receptor binding sites has been suggested in some studies, this proposition is still controversial. It is currently difficult to assign a specific mechanism by which neuroactive steroids could modulate the effects of ethanol in particular behavioral tasks.

Neurochemical regulation of sleep

This review summarizes recent developments in the field of sleep regulation, particularly in the role of hormones, and of synthetic GABA(A) receptor agonists. Certain hormones play a specific role in sleep regulation. A reciprocal interaction of the neuropeptides growth hormone (GH)-releasing hormone (GHRH) and corticotropin-releasing hormone (CRH) plays a key role in sleep regulation. At least in males GHRH is a common stimulus of non-rapid-eye-movement sleep (NREMS) and GH and inhibits the hypothalamo-pituitary adrenocortical (HPA) hormones, whereas CRH exerts opposite effects. Furthermore CRH may enhance rapid-eye-movement sleep (REMS). Changes in the GHRH:CRH ratio in favor of CRH appear to contribute to sleep EEG and endocrine changes during depression and normal ageing. In women, however, CRH-like effects of GHRH were found. Besides CRH somatostatin impairs sleep, whereas ghrelin, galanin and neuropeptide Y promote sleep. Vasoactive intestinal polypeptide appears to be involved in the temporal organization of human sleep. Beside of peptides, steroids participate in sleep regulation. Cortisol appears to promote REMS. Various neuroactive steroids exert specific effects on sleep. The beneficial effect of estrogen replacement therapy in menopausal women suggests a role of estrogen in sleep regulation. The GABA(A) receptor or GABAergic neurons are involved in the action of many of these hormones. Recently synthetic GABA(A) agonists, particularly gaboxadol and the GABA reuptake inhibitor tiagabine were shown to differ distinctly in their action from allosteric modulators of the GABA(A) receptor like benzodiazepines as they promote slow-wave sleep, decrease wakefulness and do not affect REMS.

Neuroactive steroids modulate HPA axis activity and cerebral brain-derived neurotrophic factor (BDNF) protein levels in adult male rats

Depression is characterized by hypothalamo-pituitary-adrenocortical (HPA) axis hyperactivity. In this major mood disorder, neurosteroids and neurotrophins, particularly brain-derived neurotrophic factor (BDNF), seem to be implicated and have some antidepressant effects. BDNF is highly involved in regulation of the HPA axis, whereas neurosteroids effects have never been clearly established. In this systematic in vivo study, we showed that the principal neuroactive steroids, namely dehydroepiandrosterone (DHEA), pregnenolone (PREG) and their sulfate esters (DHEA-S and PREG-S), along with allopregnanolone (ALLO), stimulated HPA axis activity, while also modulating central BDNF contents. In detail, DHEA, DHEA-S, PREG, PREG-S and ALLO induced corticotropin-releasing hormone (CRH) and/or arginine vasopressin (AVP) synthesis and release at the hypothalamic level, thus enhancing plasma adrenocorticotropin hormone (ACTH) and corticosterone (CORT) concentrations. This stimulation of the HPA axis occurred concomitantly with BDNF modifications at the hippocampus, amygdala and hypothalamus levels. We showed that these neurosteroids induced rapid effects, probably via neurotransmitter receptors and delayed effects perhaps after metabolization in other neuroactive steroids. We highlighted that they had peripheral effects directly at the adrenal level by inducing CORT release, certainly after estrogenic metabolization. In addition, we showed that, at the dose used, only DHEA, DHEA-S and PREG-S had antidepressant effects. In conclusion, these results highly suggest that part of the HPA axis and antidepressant effects of neuroactive steroids could be mediated by BDNF, particularly at the amygdala level. They also suggest that neurosteroids effects on central BDNF could partially explain the trophic properties of these molecules.

Roles of peptides and steroids in sleep disorders

A bidirectional interaction exists between the electrophysiological and neuroendocrine components of sleep. The first is represented by the nonrapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS) cycles, the latter by distinct patterns of the secretion of various hormones. Certain hormones (neuropeptides and steroids) play a specific role in sleep regulation. Changes in their activity contribute to the pathophysiology of sleep disorders. A reciprocal interaction of the peptides growth hormone-releasing hormone (GHRH) and corticotropin-releasing hormone (CRH) plays a key role in sleep regulation. GHRH promotes growth hormone secretion and, at least in males, NREMS, whereas CRH impairs NREMS, promotes REMS and stimulates the secretion of adrenocorticotropic hormone and cortisol. Changes in the CRH:GHRH ratio in favor of CRH contribute to impaired sleep, elevated cortisol secretion and blunted GH levels during depression and normal aging. However, in women, GHRH exerts CRH-like effects. Galanin, ghrelin and neuropeptide Y are other sleep-promoting peptides, whereas somatostatin impairs sleep. A decline of orexin activity causes narcolepsy. In addition to CRH overactivity, hypercortisolism appears to be involved in the pathophysiology of sleep- electroencephalogram (EEG) changes in depression. Various neuroactive steroids exert specific effects on sleep. The changes of sleep EEG in women after the menopause are related to the decline of estrogen and progesterone. Furthermore, sleep-EEG changes in dwarfism, acromegaly, Addison's disease, Cushing's disease, brain injury, sleep apnea syndrome, primary insomnia, prolactinoma and dementia appear to be related to changes in the activity of peptides and steroids.

Neurosteroids and cholinergic systems: implications for sleep and cognitive processes and potential role of age-related changes

RATIONALE

The neurosteroids pregnenolone sulfate (PREGS), dehydroepiandrosterone sulfate (DHEAS) and allopregnanolone (3alpha,5alpha THPROG) have been implicated as powerful modulators of memory processes and sleep states in young and aged subjects with memory impairment. As these processes depend on the integrity of cholinergic systems, a specific effect of neurosteroids on these systems may account for their effects on sleep and memory.

OBJECTIVE

To review the evidence for a specific and differential effect of neurosteroids on cholinergic systems.

METHODS

We carried out keyword searches in "Medline" to identify articles concerning (1) the effects of neurosteroids on cholinergic systems, sleep and memory processes, and (2) changes in neurosteroid concentrations during aging. Few results are available for humans. Most data concerned rodents.

RESULTS

Peripheral and central administrations of PREGS, DHEAS, and 3alpha,5alpha THPROG modulate the basal forebrain and brainstem projection cholinergic neurons but not striatal cholinergic interneurons. Local administration of neurosteroids to the basal forebrain and brainstem cholinergic neurons alters sleep and memory in rodents. There are a few conflicting reports concerning the effects of aging on neurosteroid concentrations in normal and pathological conditions.

CONCLUSIONS

The specific modulation of basal forebrain and brainstem cholinergic systems by neurosteroids may account for the effects of these compounds on sleep and memory processes. To improve our understanding of the role of neurosteroids in cholinergic systems during normal and pathological aging, we need to determine whether there is specific regionalization of neurosteroids, and we need to investigate the relationship between neurosteroid concentrations in cholinergic nuclei and age-related sleep and memory impairments.

Ex vivo studies on the acute and chronic effects of DHEA and DHEA-sulfate on melatonin synthesis in young- and old-rat pineal glands

This study investigates the effects of acute and chronic injections of the neurosteroid dehydroepiandrosterone (DHEA) and its sulfate DHEA-S on pineal gland melatonin synthesis. Pineal melatonin production and plasma melatonin levels were investigated in young (9-week-old) and old (27-month-old) male Wistar rats. DHEA or DHEA-S have been administered acutely in a single intraperitoneal injection at a dosage of 50, 250, or 500 microg per animal, or on a long-term basis, i.e., for 8 days at a dosage of 100 microg per animal, 1 h before the onset of darkness. DHEA, at a dose of 50, 250, or 500 microg per animal, administered acutely to rats had no significant effects on pineal melatonin production whatever the age of the animals. In contrast, 500 microg DHEA-S induced a significant increase in the pineal melatonin content (15% in young animals and 35% in old animals) and the activity of N-acetyltransferase, the rate-limiting enzyme for melatonin synthesis in the pineal gland, (40% in young animals and 20% in old animals), without altering the activity of hydroxyindole-O-methyltransferase whatever the age of the animals. At lower concentrations (50 or 250 microg) DHEA-S had no effect on pineal melatonin production regardless of the age of the rats. Chronic injection of DHEA or DHEA-S at a dose of 100 microg had no effect on pineal melatonin or NAT and HIOMT activities in the two age groups. This work shows that DHEA-S (and not DHEA) is able, at pharmacological concentrations, to stimulate melatonin production by rat pineal glands regardless of the age of the animals.

Sleep and endocrinology

A bidirectional interaction between sleep electroencephalogram and endocrine activity is well established in various species including humans. Various hormones (peptides and steroids) participate in sleep regulation. A key role was shown for the reciprocal interaction between sleep-promoting growth hormone-releasing hormone (GHRH) and sleep-impairing corticotropin-releasing hormone (CRH). Changes in the GHRH : CRH ratio result in changes of sleep-endocrine activity. It is thought that the change of this ratio in favour of CRH contributes to aberrations of sleep during ageing and depression (shallow sleep, blunted GH and elevated cortisol). Besides GHRH, ghrelin and galanin enhance slow wave sleep. Somatostatin is another sleep-impairing factor. Neuropeptide Y acts as a CRH antagonist and induces sleep onset. There are hints that CRH promotes rapid eye movement sleep (REMS). In animals prolactin enhances REMS. In humans vasoactive intestinal polypeptide (VIP) appears to play a role in the temporal organization of sleep as, after VIP, the non-REMS-REMS cycle decelerated. Cortisol appears to enhance REMS. Finally, gonadal hormones participate in sleep regulation. Oestrogen replacement therapy and CRH-1 receptor antagonism in depression are beneficial clinical applications of sleep-endocrine research.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Dehydroepiandrosterone--a neurosteroid

Dehydroepiandrosteone (DHEA) and its sulfate ester (DHEAS) are the major secretory products of the human adrenal glands and serve as precursors for both androgenic and estrogenic steroids. DHEA/S concentrations are particularly high in the brain, and DHEA/S and related steroids can be synthesized de novo in brain glial cells. Therefore, the term 'neurosteroids' has been coined for these compounds. This review summarizes findings in neurosteroid physiology on a cellular and molecular level, and outlines current concepts of how these compounds modulate physiological functions of the brain. Today, despite promising preclinical and human data the present clinical studies provide only weak evidence, if any, in favour of a DHEA replacement therapy.