Differential regulation by pregnenolone sulfate of intracellular Ca2+ increase by amino acids in primary cultured rat cortical neurons

Gut microbiota dysbiosis: The potential mechanisms by which alcohol disrupts gut and brain functions

Alcohol use disorder (AUD) is a high-risk psychiatric disorder and a key cause of death and disability in individuals. In the development of AUD, there is a connection known as the microbiota-gut-brain axis, where alcohol use disrupts the gut barrier, resulting in changes in intestinal permeability as well as the gut microbiota composition, which in turn impairs brain function and worsens the patient’s mental status and gut activity. Potential mechanisms are explored by which alcohol alters gut and brain function through the effects of the gut microbiota and their metabolites on immune and inflammatory pathways. Alcohol and microbiota dysregulation regulating neurotransmitter release, including DA, 5-HT, and GABA, are also discussed. Thus, based on the above discussion, it is possible to speculate on the gut microbiota as an underlying target for the treatment of diseases associated with alcohol addiction. This review will focus more on how alcohol and gut microbiota affect the structure and function of the gut and brain, specific changes in the composition of the gut microbiota, and some measures to mitigate the changes caused by alcohol exposure. This leads to a potential intervention for alcohol addiction through fecal microbiota transplantation, which could normalize the disruption of gut microbiota after AUD.

GABAA receptor polymorphisms in alcohol use disorder in the GWAS era

Alcohol use disorder (AUD) is a chronic, relapsing, neuro-psychiatric illness of high prevalence and with a serious public health impact worldwide. It is complex and polygenic, with a heritability of about 50%, and influenced by environmental causal heterogeneity. Risk factors associated with its etiology have a genetic component. GABA (γ-aminobutyric acid) is a major inhibitory neurotransmitter in mammalian brain. GABA_A receptors are believed to mediate some of the physiological and behavioral actions of alcohol. In this critical review, relevant genetic terms and type and methodology of the genetic studies are briefly explained. Postulated candidate genes that encode subunits of GABA_A receptors, with all the reported SNPs, are presented. Genetic studies and meta-analyses examining polymorphisms of the GABA_A receptor and their association with AUD predisposition are presented. The data are critically examined with reference to recent GWAS studies that failed to show relations between GABA_A receptors and AUD. Restrictions and perspectives of the different findings are discussed.

Calcium-engaged Mechanisms of Nongenomic Action of Neurosteroids

BACKGROUND

Neurosteroids form the unique group because of their dual mechanism of action. Classically, they bind to specific intracellular and/or nuclear receptors, and next modify genes transcription. Another mode of action is linked with the rapid effects induced at the plasma membrane level within seconds or milliseconds. The key molecules in neurotransmission are calcium ions, thereby we focus on the recent advances in understanding of complex signaling crosstalk between action of neurosteroids and calcium-engaged events.

METHODS

Short-time effects of neurosteroids action have been reviewed for GABAA receptor complex, glycine receptor, NMDA receptor, AMPA receptor, G protein-coupled receptors and sigma-1 receptor, as well as for several membrane ion channels and plasma membrane enzymes, based on available published research.

RESULTS

The physiological relevance of neurosteroids results from the fact that they can be synthesized and accumulated in the central nervous system, independently from peripheral sources. Fast action of neurosteroids is a prerequisite for genomic effects and these early events can significantly modify intracellular downstream signaling pathways. Since they may exert either positive or negative effects on calcium homeostasis, their role in monitoring of spatio-temporal Ca2+ dynamics, and subsequently, Ca2+-dependent physiological processes or initiation of pathological events, is evident.

CONCLUSION

Neurosteroids and calcium appear to be the integrated elements of signaling systems in neuronal cells under physiological and pathological conditions. A better understanding of cellular and molecular mechanisms of nongenomic, calcium-engaged neurosteroids action could open new ways for therapeutic interventions aimed to restore neuronal function in many neurological and psychiatric diseases.

Acyl-CoA:cholesterol acyltransferases (ACATs/SOATs): Enzymes with multiple sterols as substrates and as activators

Cholesterol is essential to the growth and viability of cells. The metabolites of cholesterol include: steroids, oxysterols, and bile acids, all of which play important physiological functions. Cholesterol and its metabolites have been implicated in the pathogenesis of multiple human diseases, including: atherosclerosis, cancer, neurodegenerative diseases, and diabetes. Thus, understanding how cells maintain the homeostasis of cholesterol and its metabolites is an important area of study. Acyl-coenzyme A:cholesterol acyltransferases (ACATs, also abbreviated as SOATs) converts cholesterol to cholesteryl esters and play key roles in the regulation of cellular cholesterol homeostasis. ACATs are most unusual enzymes because (i) they metabolize diverse substrates including both sterols and certain steroids; (ii) they contain two different binding sites for steroidal molecules. In mammals, there are two ACAT genes that encode two different enzymes, ACAT1 and ACAT2. Both are allosteric enzymes that can be activated by a variety of sterols. In addition to cholesterol, other sterols that possess the 3-beta OH at C-3, including PREG, oxysterols (such as 24(S)-hydroxycholesterol and 27-hydroxycholesterol, etc.), and various plant sterols, could all be ACAT substrates. All sterols that possess the iso-octyl side chain including cholesterol, oxysterols, various plant sterols could all be activators of ACAT. PREG can only be an ACAT substrate because it lacks the iso-octyl side chain required to be an ACAT activator. The unnatural cholesterol analogs epi-cholesterol (with 3-alpha OH in steroid ring B) and ent-cholesterol (the mirror image of cholesterol) contain the iso-octyl side chain but do not have the 3-beta OH at C-3. Thus, they can only serve as activators and cannot serve as substrates. Thus, within the ACAT holoenzyme, there are site(s) that bind sterol as substrate and site(s) that bind sterol as activator; these sites are distinct from each other. These features form the basis to further pursue ACAT structure-function analysis, and can be explored to develop novel allosteric ACAT inhibitors for therapeutic purposes. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

PREGS induces LTP in the hippocampal dentate gyrus of adult rats via the tyrosine phosphorylation of NR2B coupled to ERK/CREB [corrected] signaling

An acute application of neurosteroid pregnenolone sulfate (PREGS) to hippocampal slices from adult rats induced a long-lasting potentiation (LLP PREGS) at the perforant path-granule cell synapse. As a partial mechanism of the LLP PREGS, we previously revealed that PREGS transiently increases the probability of presynaptic glutamate release via a sensitization of alpha7-nicotinic acetylcholine receptor (alpha7nAChR). We herein demonstrate that the LLP PREGS could be separated into two independent processes: the above-mentioned early presynaptic-origin short-term potentiation (STP PREGS) and a delayed postsynaptic N-methyl-d-aspartate receptor (NMDAr)-dependent long-term potentiation termed LTP(PREGS). This study focused on the analysis of the signaling mechanism underlying the LTP PREGS. PREGS increased the tyrosine phosphorylation of NR2B, a subunit of NMDAr, and the NMDAr-mediated Ca2+ influx in the granule cells. The enhanced Ca2+ influx was largely attenuated by the NR2B subunit inhibitor ifenprodil and the Src kinase family inhibitor PP2. PREGS also triggered a persistent phosphorylation of extracellular signal-regulated kinase 2 (ERK2) followed by an ERK-dependent phosphorylation of cAMP-response element-binding protein (CREB), which was crucial for the LTP PREGS induction and was sensitive to ifenprodil. These results suggest that PREGS induces an acute increase in the NR2B tyrosine phosphorylation which enhances the Ca2+ influx through NMDAr, followed by an activation of the ERK/CREB signaling cascade that leads to LTP PREGS.

Ethanol regulation of γ-aminobutyric acidA receptors: genomic and nongenomic mechanisms

gamma-Aminobutyric acid(A) (GABA(A)) receptors are ligand-gated ion channels that, predominantly, mediate inhibitory synaptic transmission in the CNS. These receptors are pentameric complexes that are comprised of subunits from several classes (alpha, beta, gamma, delta, ), with each class consisting of several isoforms. Chronic ethanol consumption alters GABA(A) receptor function producing cellular tolerance to GABA and ethanol, cross-tolerance to benzodiazepines and barbiturates, and sensitization to inverse agonists. Recent studies have clearly demonstrated that GABA(A) receptors play an important role in ethanol dependence and functional properties of GABA(A) receptor are altered following chronic ethanol administration. However, the exact mechanisms that account for alterations in GABA(A) receptor function following chronic ethanol administration have not been resolved. The mechanisms responsible for adaptation of GABA(A) receptors to chronic ethanol exposure may involve ethanol-induced changes in cell surface expression, subcellular localization, synaptic localization, receptor phosphorylation, neurosteroids, and/or changes in GABA(A) receptor subunit composition. In this review, we provide an overview of recent data pertaining to mechanisms that could be responsible for altered properties and expression of GABA(A) receptors following chronic ethanol administration.

Neuroactive steroids: new biomarkers of cognitive aging

Intensive studies in animals established that neuroactive steroids display neuronal actions and influence behavioral functions. We describe here investigations on the role of neuroactive steroids in learning and memory processes during aging and suggest their role as biomarkers of cognitive aging. Our work demonstrated the role of the steroid pregnenolone (PREG) sulfate as a factor underlying an individual's age-related cognitive decline in animals. As new perspectives of research we argue that knowing whether neuroactive steroids exist as endogenous neuromodulators and modulate physiologically behavioral functions is essential. To this end, a new approach using the sensitive, specific, and accurate quantitative determination of neuroactive steroids by mass spectrometry seems to have potential for examining the role of each steroid in discrete brain areas in learning and memory alterations, as observed during aging.

Functional cooperation between neurosteroids and D2 dopamine antagonists on KCl-evoked [3H]noradrenaline release: modulation by calcium channel blockers

It has recently been proposed that neurosteroids, such as dehydroepiandrosterone sulphate and pregnenolone sulphate, interfere with the dopamine system in the central nervous system. According to our previous report showing that the butyrophenone, spiperone, slightly enhances the evoked release of [3H]-noradrenaline ([3H]NA) in the presence of these sulphated steroids, the present study was carried out to document the putative interplay between steroids and spiperone, which is known to be a prototypic D2 dopamine antagonist and also a 5-HT2 serotonin antagonist. For this purpose, the paradigm of KCl-evoked [3H]NA release from preloaded rat hippocampal slices was used to investigate the interactions between neurosteroids, spiperone and the voltage-sensitive calcium channels (VSCCs). The selective 5-HT2 serotonin antagonist ritanserine was ineffective, whereas sulpiride, a selective D2 dopamine antagonist mimicked the action of spiperone, thus suggesting that the blockade of D2 dopamine receptors accounted for the modulatory effect of spiperone on neurosteroid-induced modulation of evoked [3H]NA release. In addition, this facilitation of KCl-evoked [3H]NA release by the combination of a steroid and a D2 dopamine antagonist was partially inhibited by the L- and N-type VSCC blockers nifedipine and omega-conotoxin GVIA, respectively. The present results provide in-vitro functional evidence for the putative role of VSCCs in the interplay between steroids and D2 dopamine receptors.

Steroid structure and pharmacological properties determine the anti-amnesic effects of pregnenolone sulphate in the passive avoidance task in rats

Pregnenolone sulphate (PREGS) has generated interest as one of the most potent memory-enhancing neurosteroids to be examined in rodent learning studies, with particular importance in the ageing process. The mechanism by which this endogenous steroid enhances memory formation is hypothesized to involve actions on glutamatergic and GABAergic systems. This hypothesis stems from findings that PREGS is a potent positive modulator of N-methyl-d-aspartate receptors (NMDARs) and a negative modulator of gamma-aminobutyric acid(A) receptors (GABA(A)Rs). Moreover, PREGS is able to reverse the amnesic-like effects of NMDAR and GABA(A)R ligands. To investigate this hypothesis, the present study in rats examined the memory-altering abilities of structural analogs of PREGS, which differ in their modulation of NMDAR and/or GABA(A)R function. The analogs tested were: 11-ketopregnenolone sulphate (an agent that is inactive at GABA(A)Rs and NMDARs), epipregnanolone ([3beta-hydroxy-5beta-pregnan-20-one] sulphate, an inhibitor of both GABA(A)Rs and NMDARs), and a newly synthesized (-) PREGS enantiomer (which is identical to PREGS in effects on GABA(A)Rs and NMDARs). The memory-enhancing effects of PREGS and its analogs were tested in the passive avoidance task using the model of scopolamine-induced amnesia. Both PREGS and its (-) enantiomer blocked the effects of scopolamine. The results show that, unlike PREGS, 11-ketopregnenolone sulphate and epipregnanolone sulphate failed to block the effect of scopolamine, suggesting that altering the modulation of NMDA receptors diminishes the memory-enhancing effects of PREGS. Moreover, enantioselectivity was demonstrated by the ability of natural PREGS to be an order of magnitude more effective than its synthetic enantiomer in reversing scopolamine-induced amnesia. These results identify a novel neuropharmacological site for the modulation of memory processes by neuroactive steroids.

Effects of neurosteroids on Ca(2+) signaling mediated by recombinant N-methyl-D-aspartate receptor expressed in Chinese hamster ovary cells

Pregnenolone sulfate (PREGS) potentiates the N-methyl-D-aspartate (NMDA) receptor-mediated Ca(2+)-signals in cultured hippocampal neurons. The NMDA receptor family has several different subunits whose expression in neurons has distinct spatial and temporal patterns. To examine subunit specificity of the PREGS action, we have investigated the effect of PREGS on recombinant GluR epsilon2/zeta1 (NR2B/NR1) type NMDA receptors stabley expressed in Chinese hamster ovary cells using heat shock promoters. PREGS enhanced the Ca(2+) influx through the GluR epsilon2/zeta1 receptors in a dose-dependent manner. The EC(50) of PREGS for the GluR epsilon2/zeta1 receptors was 8.6 microM. Other sulfated neurosteroids, dehydroepiandrosterone sulfate (DHEAS), 17beta-estradiol sulfate and 3alpha-ol-5beta-pregnan-20-one sulfate (3alpha5betaS), inhibited the positive modulatory effect of PREGS on the GluR epsilon2/zeta1 NMDA receptors. Ifenprodil, a specific inhibitor of GluR epsilon2 subunit, abolished the NMDA-induced Ca(2+) influx even in the presence of PREGS. These results imply that PREGS positively modulates the Ca(2+) influx through the GluR epsilon2/zeta1 receptors which are expressed from the embryonic period.

Neuroactive neurosteroids as endogenous effectors for the sigma1 (sigma1) receptor: pharmacological evidence and therapeutic opportunities

Neuroactive neurosteroids, including progesterone, allopregnanolone, pregnenolone and dehydroepiandrosterone, represent steroid hormones synthesized de novo in the brain and acting locally on nervous cells. Neurosteroids modulate several neurotransmitter systems such as gamma-aminobutyric acid type A (GABA(A)), N-methyl-D-aspartate (NMDA) and acetylcholine receptors. As physiologic consequences, they are involved in neuronal plasticity, learning and memory processes, aggression and epilepsy, and they modulate the responses to stress, anxiety and depression. The sigma1-receptor protein was recently purified and its cDNA was cloned in several species. The amino-acid sequences are structurally unrelated to known mammalian proteins, but shared homology with a fungal sterol C8-C7 isomerase. The sigma1-receptor ligands exert a potent neuromodulation on excitatory neurotransmitter systems, including the glutamate and cholinergic systems. Consequently, selective sigma1 agonists show neuroprotective properties and beneficial effects in memory processes, stress and depression. The evidence of a direct interaction between neurosteroids and sigma1 receptors was first suggested by the ability of several steroids to inhibit the binding of sigma1-receptor radioligands in vitro and in vivo. A crossed pharmacology between neurosteroids and sigma1-receptor ligands was described in several physiological tests and behavioral responses. This review will detail the recent evidence for a common mechanism of action between neurosteroids and sigma1-receptor ligands and focus on the potential therapeutic interests of such interaction in the physiopathology of learning and memory impairments, stress, depression and neuroprotection.