DHEA attenuates catecholamine secretion from bovine adrenal chromaffin cells

Neurosteroids as positive and negative allosteric modulators of ligand-gated ion channels: P2X receptor perspective

Neurosteroids are steroids synthesized de novo in the brain from cholesterol in an independent manner from peripheral steroid sources. The term "neuroactive steroid" includes all steroids independent of their origin, and newly synthesized analogs of neurosteroids that modify neuronal activities. In vivo application of neuroactive steroids induces potent anxiolytic, antidepressant, anticonvulsant, sedative, analgesic and amnesic effects, mainly through interaction with the γ-aminobutyric acid type-A receptor (GABA_AR). However, neuroactive steroids also act as positive or negative allosteric regulators on several ligand-gated channels including N-methyl-d-aspartate receptors (NMDARs), nicotinic acetylcholine receptors (nAChRs) and ATP-gated purinergic P2X receptors. Seven different P2X subunits (P2X1-7) can assemble to form homotrimeric or heterotrimeric ion channels permeable for monovalent cations and calcium. Among them, P2X2, P2X4, and P2X7 are the most abundant within the brain and can be regulated by neurosteroids. Transmembrane domains are necessary for neurosteroid binding, however, no generic motif of amino acids can accurately predict the neurosteroid binding site for any of the ligand-gated ion channels including P2X. Here, we will review what is currently known about the modulation of rat and human P2X by neuroactive steroids and the possible structural determinants underlying neurosteroid-induced potentiation and inhibition of the P2X2 and P2X4 receptors.

The role of dehydroepiandrosterone on functional innate immune responses to acute stress

The androgen dehydroepiandrosterone (DHEA) responds to stress activation, exhibits anti-glucocorticoid properties, and modulates immunity in diverse ways, yet little is known of its role in acute stress responses. In this study, the effects of DHEA and its sulfate ester DHEA-S on human male immune function during exposure to an acute stressor is explored. Variation in DHEA, DHEA-S, testosterone, and cortisol, along with bacterial killing assays, was measured in response to a modified Trier Social Stress test in 27 young adult males. Cortisol was positively related to salivary innate immunity but only for participants who also exhibited high DHEA responses. Additionally, DHEA positively and DHEA-S negatively predicted salivary immunity, but the opposite was observed for serum-based innate immunity. The DHEA response to acute stress appears to be an important factor in stress-mediated immunological responses, with differential effects on immunity dependent upon the presence of other hormones, primarily cortisol and DHEA-S. These results suggest that DHEA plays an important role, alongside other hormones, in modulating immunological shifts during acute stress.

Dehydroepiandrosterone, its metabolites and ion channels

This review is focused on the physiological and pathophysiological relevance of steroids influencing the activities of the central and peripheral nervous systems with regard to their concentrations in body fluids and tissues in various stages of human life like the fetal development or pregnancy. The data summarized in this review shows that DHEA and its unconjugated and sulfated metabolites are physiologically and pathophysiologically relevant in modulating numerous ion channels and participate in vital functions of the human organism. DHEA and its unconjugated and sulfated metabolites including 5α/β-reduced androstane steroids participate in various physiological and pathophysiological processes like the management of GnRH cyclic release, regulation of glandular and neurotransmitter secretions, maintenance of glucose homeostasis on one hand and insulin insensitivity on the other hand, control of skeletal muscle and smooth muscle activities including vasoregulation, promotion of tolerance to ischemia and other neuroprotective effects. In respect of prevalence of steroid sulfates over unconjugated steroids in the periphery and the opposite situation in the CNS, the sulfated androgens and androgen metabolites reach relevance in peripheral organs. The unconjugated androgens and estrogens are relevant in periphery and so much the more in the CNS due to higher concentrations of most unconjugated steroids in the CNS tissues than in circulation and peripheral organs. This article is part of a Special Issue entitled "Essential role of DHEA".

Modulation of nicotinic acetylcholine receptor conformational state by free fatty acids and steroids

Steroids and free fatty acids (FFA) are noncompetitive antagonists of the nicotinic acetylcholine receptor (AChR). Their site of action is purportedly located at the lipid-AChR interface, but their exact mechanism of action is still unknown. Here we studied the effect of structurally different FFA and steroids on the conformational equilibrium of the AChR in Torpedo californica receptor-rich membranes. We took advantage of the higher affinity of the fluorescent AChR open channel blocker, crystal violet, for the desensitized state than for the resting state. Increasing concentrations of steroids and FFA decreased the K(D) of crystal violet in the absence of agonist; however, only cis-unsaturated FFA caused an increase in K(D) in the presence of agonist. This latter effect was also observed with treatments that caused the opposite effects on membrane polarity, such as phospholipase A(2) treatment or temperature increase (decreasing or increasing membrane polarity, respectively). Quenching by spin-labeled fatty acids of pyrene-labeled AChR reconstituted into model membranes, with the label located at the gammaM4 transmembrane segment, disclosed the occurrence of conformational changes induced by steroids and cis-unsaturated FFA. The present work is a step forward in understanding the mechanism of action of this type of molecules, suggesting that the direct contact between exogenous lipids and the AChR transmembrane segments removes the AChR from its resting state and that membrane polarity modulates the AChR activation equilibrium by an independent mechanism.

Effects of dehydroepiandrosterone on aldosterone release in rat zona glomerulosa cells

The present study was to investigate the effects and action mechanisms of dehydroepiandrosterone (DHEA) on steroidogenesis in rat adrenal zona glomerulosa cells (ZG). ZG cells were incubated with DHEA in the presence or absence of angiotensin II (AngII), a high concentration of potassium, 8-Br-cAMP, forskolin, 25-OH-cholesterol, pregnenolone, progesterone, deoxycorticosterone, corticosterone, A23187, or cyclopiazonic acid (CPA) at 37 degrees C for 1 h. The concentration of aldosterone or pregnenolone in the culture medium was then measured by radioimmunoassay (RIA). The cells were used to determine the cellular cAMP content. The data demonstrated that: (1) DHEA inhibited AngII-, high concentration of KCl-, forskolin-, 8-Br-cAMP-, 25-OH-cholesterol-, pregnenolone-, progesterone-, deoxycorticosterone-, corticosterone-, A23187-, or CPA-stimulated aldosterone release; (2) DHEA increased 25-OH-cholesterol-stimulated pregnenolone release but not when 25-OH-cholesterol was combined with trilostane; (3) DHEA noncompetitively inhibited aldosterone synthase but showed uncompetitive inhibition of P450scc. These results suggest that DHEA acts directly on rat ZG cells to diminish aldosterone secretion by inhibition of a post-cAMP pathway or by acting on intracellular Ca2+ mobilization. In addition it affects the function of post-P450scc steroidogenic enzymes.

Orphan seven transmembrane receptor screening

Drug discovery has successfully exploited the superfamily of seven transmembrane receptors (7TMR), with over 35% of clinically marketed drugs targeting them. However, it is clear that there remains an undefined potential within this protein family for successful drugs of the future. The human genome sequencing project identified approximately 720 genes that belong to the 7TMR superfamily. Around half of these genes encode sensory receptors, while the other half are potential drug targets. Natural ligands have been identified for approximately 215 of these, leaving 155 receptors classified as orphan 7TMRs having no known ligand. Deorphanisation of these receptors by identification of natural ligands has been the traditional method enabling target validation by use of these ligands as tools to define biological relevance and disease association. Such ligands have been paired with their cognate receptor experimentally by screening of small molecule and peptide ligands, reverse pharmacology and the use of bioinformatics to predict candidate ligands. In this manuscript, we review the methodologies developed for the identification of ligands at orphan 7TMRs and exemplify these with case studies.

Butyl benzyl phthalate blocks Ca2+ signaling coupled with purinoceptor in rat PC12 cells

Butyl benzyl phthalate (BBP) is a plasticizer and causes public concern because of its genomic estrogenic effects via estrogen receptors. We previously found that BBP has non-genomic effects, exerting inhibitory effects on the functional activities of nicotinic acetylcholine receptors (nAChR) in bovine adrenal chromaffin cells. nAChR belongs to the superfamily of neurotransmitter-gated channels, so does P2X purinoceptor that is widely distributed in the nervous system and play a role in pain reactions. In this study, we investigated the effects of BBP on the change of [Ca2+]c (cytosolic calcium ion concentration) under the stimulation of purinoceptors in PC12 cells and found that BBP inhibited ATP-induced [Ca2+]c rise (IC50=8.3 microM). The inhibitory rate of BBP remained under the increase of ATP concentration; therefore, the possibility of competitive inhibition was excluded. The inhibition of BBP on P2Y was excluded because its inhibition on ATP-induced [Ca2+]c rise was not found in the absence of extracellular Ca2+. BBP might have some actions on voltage-operated Ca2+ channels (VOCCs) since BBP inhibited the Ca2+ signaling responding to high K+ stimulation (IC50=1.2 microM). We suggest that BBP inhibits the ATP-induced [Ca2+]c rise via its non-competitive inhibition on P2X purinoceptors and VOCCs in the plasma membrane.

Inhibition by 2,4-toluene diisocyanate of the calcium signaling of neuronal nicotinic acetylcholine receptors in human neuroblastoma SH-SY5Y cells

Toluene diisocyanate (TDI) is widely used as a chemical intermediate in the production of polyurethane products such as foams, coatings, and elastomers. In exposed workers, chronic inhalation of TDI has resulted in significant decreases in lung function. TDI-induced asthma is related to its disturbance of acetylcholine in most affected workers but the actions of TDI on nicotinic acetylcholine receptors (nAChR) are unclear. In order to understand the role of TDI acting on nAChR, we used human neuroblastoma SH-SY5Y cells to investigate the effects of TDI on cytosolic free calcium concentration ([Ca2+]c) changes under the stimulation of nAChR. The results showed that TDI was capable of inhibiting the [Ca2+]c rise induced by nicotinic ligands, epibatidine, DMPP and nicotine. The inhibition was remained, even increased after chronic treatment of TDI. Our study of TDI acting on human nAChR suggests a possibility that the human nerve system plays some role in the toxicity of TDI in the pulmonary system.

Progesterone regulation of catecholamine secretion from chromaffin cells

Stress stimulates the adrenal medulla to rapidly secrete catecholamines (CAs), and the adrenal cortex to release progesterone (PROG), which may locally regulate stress-induced CA release. We used bovine chromaffin cells to investigate the effects of PROG on CA secretion. PROG dose-dependently inhibited CA secretion induced by nicotinic acetylcholine receptor (nAChR) agonist 1,1-dimethyl-4-phenlypiperazinium iodide (DMPP) up to 77%. Pre-incubation with PROG up to 1 h increased this inhibition. 3alpha,5alpha-Tetrahydroprogesterone (3alpha,5alpha-THP) and dexamethasone were less potent inhibitors. Patch-clamp techniques revealed that PROG co-applied with DMPP inhibited peak DMPP-induced current up to 68% and with 3 min pre-incubation inhibited both peak and integrated current up to approximately 95%. Monitoring of FURA-2 showed that PROG similarly inhibited parallel changes in intracellular-free Ca(++) concentration. PROG also inhibited CA secretion elicited by elevated K(+) (38%), and, in single cells, suppressed Ca(++) current evoked by step depolarization, inhibiting amplitude by 15%, and reducing the time constant of current decay during depolarization by 57%. In contrast to the immediate inhibition of nicotinic current, inhibition of Ca(++) current became statistically significant only after 1 min exposure to PROG. PROG did not inhibit secretion stimulated by high Ca(++) perfusion of permeabilized cells. These data suggest that PROG inhibits CA secretion from chromaffin cells predominantly by rapidly inhibiting nAChRs, and by gradually enhancing the inactivation of voltage-dependent Ca(++) channels (VDCCs), but not by affecting secretory processes downstream of Ca(++) influx. This study supports a role for adrenocortical PROG in the regulation of CA secretion during stress.