Fast nongenomic effects of steroids on synaptic transmission and role of endogenous neurosteroids in spinal pain pathways

17-β-estradiol potentiates the neurotrophic and neuroprotective effects mediated by the dopamine D3/acetylcholine nicotinic receptor heteromer in dopaminergic neurons

Dopaminergic neurons express a heteromer composed of the dopamine D3 receptor and the α4β2 nicotinic acetylcholine receptor, the D3R-nAChR heteromer, activated by both nicotine and dopamine D2 and D3 receptors agonists, such as quinpirole, and crucial for dopaminergic neuron homeostasis. We now report that D3R-nAChR heteromer activity is potentiated by 17-β-estradiol which acts as a positive allosteric modulator by binding a specific domain on the α4 subunit of the nicotinic receptor protomer. In mouse dopaminergic neurons, in fact, 17-β-estradiol significantly increased the ability of nicotine and quinpirole in promoting neuron dendritic remodeling and in protecting neurons against the accumulation of α-synuclein induced by deprivation of glucose, with a mechanism that does not involve the classical estrogen receptors. The potentiation induced by 17-β-estradiol required the D3R-nAChR heteromer since either nicotinic receptor or dopamine D3 receptor antagonists and interfering TAT-peptides, but not the estrogen receptor antagonist fulvestrant, specifically prevented 17-β-estradiol effects. Evidence of estrogens neuroprotection, mainly mediated by genomic mechanisms, have been provided, which is in line with epidemiological data reporting that females are less likely to develop Parkinson's Disease than males. Therefore, potentiation of D3R-nAChR heteromer activity may represent a further mechanism by which 17-β-estradiol reduces dopaminergic neuron vulnerability.

Short-term plasticity in the spinal nociceptive system

ABSTRACT

Somatosensory information is delivered to neuronal networks of the dorsal horn (DH) of the spinal cord by the axons of primary afferent neurons that encode the intensity of peripheral sensory stimuli under the form of a code based on the frequency of action potential firing. The efficient processing of these messages within the DH involves frequency-tuned synapses, a phenomenon linked to their ability to display activity-dependent forms of short-term plasticity (STP). By affecting differently excitatory and inhibitory synaptic transmissions, these STP properties allow a powerful gain control in DH neuronal networks that may be critical for the integration of nociceptive messages before they are forwarded to the brain, where they may be ultimately interpreted as pain. Moreover, these STPs can be finely modulated by endogenous signaling molecules, such as neurosteroids, adenosine, or GABA. The STP properties of DH inhibitory synapses might also, at least in part, participate in the pain-relieving effect of nonpharmacological analgesic procedures, such as transcutaneous electrical nerve stimulation, electroacupuncture, or spinal cord stimulation. The properties of target-specific STP at inhibitory DH synapses and their possible contribution to electrical stimulation-induced reduction of hyperalgesic and allodynic states in chronic pain will be reviewed and discussed.

Neurosteroids are reduced in diabetic neuropathy and may be associated with the development of neuropathic pain

Introduction: Peripheral and central sensitisation are implicated in the development of neuropathic pain. Hypersensitivity of pain pathway neurons has been described in animal models of diabetic neuropathy, which is postulated to be related to an imbalance between inhibitory and excitatory signals within the spinal cord. GABAergic neurons within the pain pathway are vital for the transmission of painful stimuli to higher centres. A developmental change in the rate of exponential decay of GABAergic synaptic events has been observed in other types of neurons and this may be associated with fluctuations in endogenous neurosteroid tone. 

Methods: The whole-cell patch-clamp technique was used on slices of neural tissue. Electrophysiological recordings were obtained from wild type mice between the ages of 6 and 80 days in the spinal cord, the nucleus reticularis of the thalamus and the cerebral cortex. Recordings were also obtained from mice with diabetic neuropathy (ob/ob and db/db) between the ages of 60 and 80 days. Behavioural experiments were performed to examine mechanical and thermal nociception.

Results: Electrophysiological recordings from cortical pain pathway neurons from mature type-2 diabetic mice revealed that the endogenous neurosteroid tone is reduced compared to control. However, selected neurosteroid compounds had a more pronounced effect on the GABA _A receptors of these diabetic mice. ob/ob mice exhibit mechanical hyperalgesia and allodynia, which was reduced by neurosteroids applied exogenously.

Conclusions: The reduced endogenous neurosteroid tone in ob/ob mice may be linked to their hypersensitivity. Neurosteroids may exert analgesic effects in pathological pain states by attempting to restore the physiological GABAergic inhibitory tone.

TSPO PIGA Ligands Promote Neurosteroidogenesis and Human Astrocyte Well-Being

The steroidogenic 18 kDa translocator protein (TSPO) is an emerging, attractive therapeutic tool for several pathological conditions of the nervous system. Here, 13 high affinity TSPO ligands belonging to our previously described N,N-dialkyl-2-phenylindol-3-ylglyoxylamide (PIGA) class were evaluated for their potential ability to affect the cellular Oxidative Metabolism Activity/Proliferation index, which is used as a measure of astrocyte well-being. The most active PIGA ligands were also assessed for steroidogenic activity in terms of pregnenolone production, and the values were related to the metabolic index in rat and human models. The results showed a positive correlation between the increase in the Oxidative Metabolism Activity/Proliferation index and the pharmacologically induced stimulation of steroidogenesis. The specific involvement of steroid molecules in mediating the metabolic effects of the PIGA ligands was demonstrated using aminoglutethimide, a specific inhibitor of the first step of steroid biosynthesis. The most promising steroidogenic PIGA ligands were the 2-naphthyl derivatives that showed a long residence time to the target, in agreement with our previous data. In conclusion, TSPO ligand-induced neurosteroidogenesis was involved in astrocyte well-being.

Elevated Neurosteroids in the Lateral Thalamus Relieve Neuropathic Pain in Rats with Spared Nerve Injury

Neurosteroids are synthesized in the nervous system from cholesterol or steroidal precursors imported from peripheral sources. These compounds are important allosteric modulators of γ-aminobutyric acid A receptors (GABAARs), which play a vital role in pain modulation in the lateral thalamus, a main gate where somatosensory information enters the cerebral cortex. Using high-performance liquid chromatography/tandem mass spectrometry, we found increased levels of neurosteroids (pregnenolone, progesterone, deoxycorticosterone, allopregnanolone, and tetrahydrodeoxycorticosterone) in the chronic stage of neuropathic pain (28 days after spared nerve injury) in rats. The expression of the translocator protein TSPO, the upstream steroidogenesis rate-limiting enzyme, increased at the same time. In vivo stereotaxic microinjection of neurosteroids or the TSPO activator AC-5216 into the lateral thalamus (AP -3.0 mm, ML ±3.0 mm, DV 6.0 mm) alleviated the mechanical allodynia in neuropathic pain, while the TSPO inhibitor PK 11195 exacerbated it. The analgesic effects of AC-5216 and neurosteroids were significantly attenuated by the GABAAR antagonist bicuculline. These results suggested that elevated neurosteroids in the lateral thalamus play a protective role in the chronic stage of neuropathic pain.

Neurosteroids Allopregnanolone Sulfate and Pregnanolone Sulfate Have Diverse Effect on the α Subunit of the Neuronal Voltage-gated Sodium Channels Nav1.2, Nav1.6, Nav1.7, and Nav1.8 Expressed in Xenopus Oocytes

Background:

The neurosteroids allopregnanolone and pregnanolone are potent positive modulators of γ-aminobutyric acid type A receptors. Antinociceptive effects of allopregnanolone have attracted much attention because recent reports have indicated the potential of allopregnanolone as a therapeutic agent for refractory pain. However, the analgesic mechanisms of allopregnanolone are still unclear. Voltage-gated sodium channels (Nav) are thought to play important roles in inflammatory and neuropathic pain, but there have been few investigations on the effects of allopregnanolone on sodium channels.

Methods:

Using voltage-clamp techniques, the effects of allopregnanolone sulfate (APAS) and pregnanolone sulfate (PAS) on sodium current were examined in Xenopus oocytes expressing Nav1.2, Nav1.6, Nav1.7, and Nav1.8 α subunits.

Results:

APAS suppressed sodium currents of Nav1.2, Nav1.6, and Nav1.7 at a holding potential causing half-maximal current in a concentration-dependent manner, whereas it markedly enhanced sodium current of Nav1.8 at a holding potential causing maximal current. Half-maximal inhibitory concentration values for Nav1.2, Nav1.6, and Nav1.7 were 12 ± 4 (n = 6), 41 ± 2 (n = 7), and 131 ± 15 (n = 5) μmol/l (mean ± SEM), respectively. The effects of PAS were lower than those of APAS. From gating analysis, two compounds increased inactivation of all α subunits, while they showed different actions on activation of each α subunit. Moreover, two compounds showed a use-dependent block on Nav1.2, Nav1.6, and Nav1.7.

Conclusion:

APAS and PAS have diverse effects on sodium currents in oocytes expressing four α subunits. APAS inhibited the sodium currents of Nav1.2 most strongly.

Analgesic strategies aimed at stimulating the endogenous production of allopregnanolone

A growing number of studies indicate that 3-alpha reduced neurosteroids are remarkable analgesics in various pain states. This is the case for allopregnanolone (AP), one of the most potent endogenous positive allosteric modulators of GABA_A receptor function. From the pioneering work of Hans Selye, who described the sedative properties of steroids, synthetic compounds resembling the progesterone metabolite AP have been developed. If some of them have been used as anesthetics, it seems difficult to propose them as a therapeutic option for pain since they display several adverse side effects such as sedation, amnesia and functional tolerance. An alternative strategy, chosen by few laboratories around the world, is aimed at stimulating the local production of 3-alpha reduced neurosteroids in order to limit these well-known side effects. This pharmacological approach has the advantage of targeting specific structures, fully equipped with the necessary biosynthetic enzymatic machinery, where neurosteroids already act as endogenous pain modulators. The various pharmacological trials which attempted to treat pain symptoms by stimulating the production of 3-alpha reduced neurosteroids are reviewed here, as well as novel neurotransmitter systems possibly regulating their endogenous production.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Etifoxine stimulates allopregnanolone synthesis in the spinal cord to produce analgesia in experimental mononeuropathy

BACKGROUND

Pathological pain states are often associated with neuronal hyperexcitability in the spinal cord. Reducing this excitability could theoretically be achieved by amplifying the existing spinal inhibitory control mediated by GABAA receptors (GABAARs). In this study, we used the non-benzodiazepine anxiolytic etifoxine (EFX) to characterize its interest as pain killer and spinal mechanisms of action. EFX potentiates GABAAR function but can also increase its function by stimulating the local synthesis of 3α-reduced neurosteroids (3αNS), the most potent endogenous modulators of this receptor.

METHODS

The efficacy of EFX analgesia and the contribution of 3αNS were evaluated in a rat model of mononeuropathy. Spinal contribution of EFX was characterized through changes in pain symptoms after intrathecal injections, spinal content of EFX and 3αNS, and expression of FosB-related genes, a marker of long-term plasticity.

RESULTS

We found that a 2-week treatment with EFX (>5 mg/kg, i.p.) fully suppressed neuropathic pain symptoms. This effect was fully mediated by 3αNS and probably by allopregnanolone, which was found at a high concentration in the spinal cord. In good agreement, the level of EFX analgesia after intrathecal injections confirmed that the spinal cord is a privileged target as well as the limited expression of FosB/ΔFosB gene products that are highly expressed in persistent pain states.

CONCLUSIONS

This preclinical study shows that stimulating the production of endogenous analgesics such as 3αNS represents an interesting strategy to reduce neuropathic pain symptoms. Since EFX is already prescribed as an anxiolytic in several countries, a translation to the human clinic needs to be rapidly evaluated.

GABA(A) receptor modulation: potential to deliver novel pain medicines?

GAB(A) (γ-aminobutyric acid) is abundantly expressed within the brain, and spinal cord pain circuits where it acts as the principal mediator of fast inhibitory neurotransmission. However, drugs that target GABA(A) receptor function such as the classical benzodiazepines have not been optimised to promote analgesia, are limited by side effects and are not routinely used for this purpose in humans. Compounds such as NS11394, L-838,417, HZ166 and TPA023 all bind to the same benzodiazepine site on the GABA(A) receptor to allosterically modulate receptor function and enhance the actions of GABA. By virtue of their ability to activate selected subtypes of GABA(A) receptors (principally those containing α2, α3 and α5 subunits) these compounds have been shown to possess excellent tolerability profiles in animals. Importantly, a number of these molecules also mediate profound analgesia in animal models of inflammatory and neuropathic pain. Other modulators such as neurosteroids bind to distinct sites on GABA(A) receptor α subunits, possess a unique pharmacology and are capable of targeting alternative GABA(A) receptor expressing populations. Moreover, neurosteroids also have pronounced analgesic actions in animal pain models. The continuing call for novel mechanism of action analgesics to target specific pathologies, especially in clinical neuropathic conditions, emphasizes the need to test modulators of GABA(A) receptor function in both human experimental pain models and pain patients.

Mechanisms underlying the interactions between rapid estrogenic and BDNF control of synaptic connectivity

The effects of the steroid hormone 17β-estradiol and the neurotrophin brain-derived neurotrophic factor (BDNF) on neuronal physiology have been well investigated. Numerous studies have demonstrated that each signal can exert powerful influences on the structure and function of synapses, and specifically on dendritic spines, both within short and long time frames. Moreover, it has been suggested that BDNF is required for the long-term, or genomic, actions of 17β-estradiol on dendritic spines, via its ability to regulate the expression of neurotrophins. Here we focus on the acute, or rapid effects, of 17β-estradiol and BDNF, and their ability to activate specific signalling cascades, resulting in alterations in dendritic spine morphology. We first review recent literature describing the mechanisms by which 17β-estradiol activates these pathways, and the resulting alterations in dendritic spine number. We then describe the molecular mechanisms underlying acute modulation of dendritic spine morphology by BDNF. Finally, we consider how this new evidence may suggest that the temporal interactions of 17β-estradiol and BDNF can occur more rapidly than previously reported. Building on these new data, we propose a novel model for the interactions of this steroid and neurotrophin, whereby rapid, non-genomic 17β-estradiol and acute BDNF signal in a co-operative manner, resulting in dendritic spine formation and subsequent stabilization in support of synapse and circuit plasticity. This extended hypothesis suggests an additional mechanism by which these two signals may modulate dendritic spines in a time-specific manner.

Neurosteroids, stress and depression: potential therapeutic opportunities

Neurosteroids are potent and effective neuromodulators that are synthesized from cholesterol in the brain. These agents and their synthetic derivatives influence the function of multiple signaling pathways including receptors for γ-aminobutyric acid (GABA) and glutamate, the major inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Increasing evidence indicates that dysregulation of neurosteroid production plays a role in the pathophysiology of stress and stress-related psychiatric disorders, including mood and anxiety disorders. In this paper, we review the mechanisms of neurosteroid action in brain with an emphasis on those neurosteroids that potently modulate the function of GABA(A) receptors. We then discuss evidence indicating a role for GABA and neurosteroids in stress and depression, and focus on potential strategies that can be used to manipulate CNS neurosteroid synthesis and function for therapeutic purposes.

Evaluation of Epidural Analgesic Paste Components in Lumbar Decompressive Surgery

BACKGROUND:

Adjuncts for pain management in lumbar decompressive surgery are needed to reduce narcotic consumption and promote early mobility.

OBJECTIVE:

To evaluate the efficacy and active components of a previously described epidural analgesic paste in controlling postoperative pain and facilitating early discharge from hospital after lumbar decompressive surgery.

METHODS:

A randomized double-blind controlled trial was conducted. Two-hundred and one patients were randomized to 1 of 4 analgesic epidural pastes at the time of lumbar spinal surgery: combination paste (morphine + methylprednisolone), steroid paste (methylprednisolone alone), morphine paste (morphine alone), or placebo. The primary outcome measures used were analgesic consumption and the McGill Pain Questionnaire (MPQ). Secondary outcome measures were: modified American Spinal Cord Injury Association (ASIA) score, Short Form 36 General Health Survey (SF-36), Aberdeen Pain Index (ABPI), time to ambulation and time to discharge from hospital.

RESULTS:

Administration of combination and steroid paste, but not morphine paste, resulted in a statistically significant reduction in mean pain rating index (PRI) and present pain intensity (PPI) components of the MPQ in the first 3 days after surgery. Likewise, postoperative in-patient narcotic analgesic consumption was reduced in the combination paste and steroid paste group, but not in the morphine paste group. No difference in time to ambulation or discharge, SF-36 scores, ABPI scores, or neurologic recovery was observed.

CONCLUSION:

An analgesic paste containing methylprednisolone acetate is effective at reducing postoperative pain after lumbar decompressive surgery. Mixing effective doses of morphine sulfate in the paste abrogates the expected analgesic effects of epidural morphine.

Kinetic and structural determinants for GABA-A receptor potentiation by neuroactive steroids

Endogenous neurosteroids and synthetic neuroactive steroid analogs are among the most potent and efficacious potentiators of the mammalian GABA-A receptor. The compounds interact with one or more sites on the receptor leading to an increase in the channel open probability through a set of changes in the open and closed time distributions. The endogenous neurosteroid allopregnanolone potentiates the α1β2γ2L GABA-A receptor by enhancing the mean duration and prevalence of the longest-lived open time component and by reducing the prevalence of the longest-lived intracluster closed time component. Thus the channel mean open time is increased and the mean closed time duration is decreased, resulting in potentiation of channel function. Some of the other previously characterized neurosteroids and steroid analogs act through similar mechanisms while others affect a subset of these parameters. The steroids modulate the GABA-A receptor through interactions with the membrane-spanning region of the receptor. However, the number of binding sites that mediate the actions of steroids is unclear. We discuss data supporting the notions of a single site vs. multiple sites mediating the potentiating actions of steroids.

A portable site: a binding element for 17β-estradiol can be placed on any subunit of a nicotinic α4β2 receptor

Endogenous steroids can modulate the activity of transmitter-gated channels by directly interacting with the receptor. 17β-Estradiol potentiates activation of neuronal nicotinic α4β2 receptors by interacting with a 4 aa sequence at the extreme C terminus of the α4 subunit, but it is not known whether potentiation requires that the sequence be placed on a specific subunit (e.g., an α4 subunit that is involved in forming an acetylcholine-binding site). By using concatemers of subunits and chimeric subunits, we have found that the C-terminal domain can be moved from the α4 to the β2 subunit and still result in potentiation. In addition, the sequence can be placed on a subunit that contributes to an acetylcholine-binding site or on the structural subunit. The data indicate that this estradiol-binding element is a discrete sequence and suggest that the effect of 17β-estradiol is mediated by actions on single subunits and that the overall consequences for gating occur because of the summation of independent energetic contributions to overall gating of this receptor.

A mutant residue in the third transmembrane region of the GABA(A) alpha1 subunit causes increased agonistic neurosteroid responses

Pregnane derived steroids have agonistic and antagonistic actions at GABA(A) receptors. Putative binding sites for agonistic neurosteroids are located within the transmembrane (TM) regions. A mutation within the rat α(1) TM3 region, S299C, caused the expressed receptors to have unusual and extreme sensitivity to agonistic neurosteroids. For mutant α1S299C receptors, with wild type β and γ subunits, expressed in Xenopus oocytes, steroids activated the GABA(A) receptors in the absence of GABA. Maximal steroid induced currents were about half of maximal GABA currents. The steroid activation was biphasic with EC(50)'s much lower than wild type, in subnanomolar and nanomolar concentrations, while the wild type had only one activation peak with near micromolar EC(50). These currents could be blocked by both picrotoxin and an antagonist neurosteroid. The steroids did not seem to potentiate significantly submaximal GABA currents. The α1S299C mutation did not affect responses to the extracellularly acting partial agonist piperidine-4-sulfate. Substituted cysteine experiments indicate that this mutant can be modified by pCMBS(-) when the sulfhydryl reagent is added with the higher steroid concentration for activation but not the lower steroid concentration. The pCMBS(-) will also immediately block the high concentration steroid current. Taken together the data suggest that α1S299 is important in at least the in transduction of the steroid binding to the rest of the receptor.

Increase in formalin-induced tonic pain by 5alpha-reductase and aromatase inhibition in female rats

Little is known about the role of steroidogenic enzymes in pain modulation. This study examined the effects of 5α-reductase and aromatase inhibition on formalin-induced tonic pain (FITP) in adult female rats. The animals received subcutaneous injection (5 mg/kg) of finasteride (an inhibitor of 5α-reductase) and letrozole (an inhibitor of aromatase), either separately or in combination, 15 min before formalin injection at a low (0.25%) and high (2.5%) concentration. Pretreatment with inhibitors increased FITP evoked by injection of 0.25% formalin, but they were not effective on 2.5% formalin pain. The enhancing effects of finasteride and letrozole on FITP induced by 2.5% formalin was demonstrated by inhibitory actions of these drugs on morphine (7 and 10 mg/kg, intraperitoneal) induced antinociception. The nervous system could be considered as the main target of the enzymes inhibition, since the pronociceptive effect was also observed after administration of inhibitors to ovariectomized rats. Altogether, these findings suggest that the biological activity of the enzymes 5α-reductase and aromatase modulates FITP and may help to develop effective therapeutic strategies to counteract pain.

An increase in spinal dehydroepiandrosterone sulfate (DHEAS) enhances NMDA-induced pain via phosphorylation of the NR1 subunit in mice: involvement of the sigma-1 receptor

Our laboratory has recently demonstrated that an increase in the spinal neurosteroid, dehydroepiandrosterone sulfate (DHEAS) facilitates nociception via the activation of sigma-1 receptors and/or the allosteric inhibition GABA(A) receptors. Several lines of evidence have suggested that DHEAS positively modulates N-methyl-d-aspartate (NMDA) receptor activity within the central nervous system. Moreover, we have demonstrated that the activation of sigma-1 receptors increases NMDA receptor activity. Since NMDA receptors play a key role in the enhancement of pain perception, the present study was designed to determine whether spinally administered DHEAS modulates NMDA receptor-mediated nociceptive activity and whether this effect is mediated by sigma-1 or GABA(A) receptors. Intrathecal (i.t.) DHEAS was found to significantly potentiate i.t. NMDA-induced spontaneous pain behaviors. Subsequent immunohistochemical analysis demonstrated that i.t. DHEAS also increased protein kinase C (PKC)- and protein kinase A (PKA)-dependent phosphorylation of the NMDA receptor subunit NR1 (pNR1), which was used as a marker of NMDA receptor sensitization. The sigma-1 receptor antagonist, BD-1047, but not the GABA(A) receptor agonist, muscimol, dose-dependently suppressed DHEAS's facilitatory effect on NMDA-induced nociception and pNR1 expression. In addition, pretreatment with either a PKC or PKA blocker significantly reduced the facilitatory effect of DHEAS on NMDA-induced nociception. Conversely the GABA(A) receptor antagonist, bicuculline did not affect NMDA-induced pain behavior or pNR1 expression. The results of this study suggest that the DHEAS-induced enhancement of NMDA-mediated nociception is dependent on an increase in PKC- and PKA-dependent pNR1. Moreover, this effect of DHEAS on NMDA receptor activity is mediated by the activation of spinal sigma-1 receptors and not through the inhibition of GABA(A) receptors.

GABA and neuroactive steroid interactions in glia: new roles for old players?

In recent years it has becoming clear that glial cells of the central and peripheral nervous system play a crucial role from the earliest stages of development throughout adult life. Glial cells are important for neuronal plasticity, axonal conduction and synaptic transmission. In this respect, glial cells are able to produce, uptake and metabolize many factors that are essential for neuronal physiology, including classic neurotransmitters and neuroactive steroids. In particular, neuroactive steroids, which are mainly synthesized by glial cells, are able to modulate some neurotransmitter receptors affecting both glia and neurons. Among the signaling systems that are specialized for neuron-glial communication, we can include neurotransmitter GABA.The main focus of this review is to illustrate the cross-talk between neurons and glial cells in terms of GABA neurotransmission and actions of neuroactive steroids. To this purpose, we will review the presence of the different GABA receptors in the glial cells of the central and peripheral nervous system. Then, we will discuss their modulation by some neuroactive steroids.

Dissociated non-steroidal glucocorticoids: tuning out untoward effects

The endogenous glucocorticoid (GC), cortisol, is involved in maintaining homeostatic balance in glucose regulation and immune response while allowing stress adaptation. The glucocorticoid receptor (GR) is required to maintain life and is the target of numerous FDA-approved drugs. Synthetic steroidal GCs are useful in a plethora of conditions characterized by excessive inflammatory or immune responses. Unfortunately, the GCs used at present have potentially dose-limiting and debilitating side effects, some of which derive from the glucose regulatory role of GCs. Consequently, there is a great need to find agents which preserve the potent immune effects without the side effects. This manuscript reviews the existing patent literature on these intensely sought non-steroidal agents that dissociate the therapeutic from metabolic effects, or specifically retain certain GR target effects with attenuated untoward effects. The chemical classes and underlying mechanisms (when known) for these non-steroidal GCs are discussed.

Selective potentiation of homomeric P2X2 ionotropic ATP receptors by a fast non-genomic action of progesterone

P2X receptors are ligand-gated ion channels activated by ATP that are widely expressed in the organism and regulate many physiological functions. We have studied the effect of progesterone (PROG) on native P2X receptors present in rat dorsal root ganglion (DRG) neurons and on recombinant P2X receptors expressed in HEK293 cells or Xenopus laevis oocytes. The effects of PROG were observed and already maximal during the first coapplication with ATP and did not need any preincubation of the cells with PROG, indicating a fast mechanism of action. In DRG neurons, PROG rapidly and reversibly potentiated submaximal but not saturating plateau-like currents evoked by ATP, but had no effect on the currents activated by alpha,beta-methylene ATP, an agonist of homomeric or heteromeric receptors containing P2X1 or P2X3 subunits. In cells expressing homomeric P2X2 receptors, responses to submaximal ATP, were systematically potentiated by PROG in a dose-dependent manner with a threshold between 1 and 10 nM. PROG had no effect on ATP currents carried by homomeric P2X1, P2X3, and P2X4 receptors or by heteromeric P2X1/5 and P2X2/3 receptors. We conclude that PROG selectively potentiates homomeric P2X2 receptors and, in contrast with dehydroepiandrosterone (DHEA), discriminates between homomeric and heteromeric P2X2-containing receptors. This might have important physiological implications since the P2X2 subunit is the most widely distributed P2X subunit in the organism. Moreover, DHEA and PROG might be useful tools to clarify the distribution and the role of native homo- and heteromeric P2X2 receptors.

Neurosteroids' effects and mechanisms for social, cognitive, emotional, and physical functions

Hormones are trophic factors that integrate central and peripheral nervous system functions, and can influence social, cognitive, emotional and physical (SCEP) processes. Greater understanding of behavioral and neurobiological underpinnings of mental, cognitive, and/or physical changes with maturation is becoming increasingly important as the world's population ages. There are individual differences in how people age, but the factors that influence these differences are not well understood. Social supports are one factor that may influence the trajectory of age-related processes. The loss of close relationships, especially among older persons, is one of the greatest risk factors for mental and physical decline. Progesterone, secreted by the ovaries, or produced de novo in the brain, is readily converted centrally to 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), and can influence SCEP, through rapid, non-classical steroid-mediated actions. Our hypothesis is that 3alpha,5alpha-THP is a key trophic factor in SCEP and development. Our research has demonstrated that 3alpha,5alpha-THP facilitates social and sexual behavior of rodents, which evokes further increases in 3alpha,5alpha-THP in midbrain and hippocampus, brain areas involved in SCEP. The role of 3alpha,5alpha-THP to influence social and/or sexual experience, and thereby SCEP, is discussed in this review. Further understanding of these neurobiological and/or behavioral factors may lead to findings that ultimately can promote health and prevent disease.

Spinal translocator protein (TSPO) modulates pain behavior in rats with CFA-induced monoarthritis

Translocator protein 18 kDa (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), is predominantly located in the mitochondrial outer membrane and plays an important role in steroidogenesis, immunomodulation, cell survival and proliferation. Previous studies have shown an increased expression of TSPO centrally in neuropathology, as well as in injured nerves. TSPO has also been implicated in modulation of nociception. In the present study, we examined the hypothesis that TSPO is involved in the initiation and maintenance of inflammatory pain using a rat model of Complete Freund's Adjuvant (CFA)-induced monoarthritis of the tibio-tarsal joint. Immunohistochemistry was performed using Iba-1 (microglia), NeuN (neurons), anti-Glial Fibrillary Acidic Protein, GFAP (astrocytes) and anti-PBR (TSPO) on Days 1, 7 and 14 after CFA-induced arthritis. Rats with CFA-induced monoarthritis showed mechanical allodynia and thermal hyperalgesia on the ipsilateral hindpaw, which correlated with the increased TSPO expression in ipsilateral laminae I-II on all experimental days. Iba-1 expression in the ipsilateral dorsal horn was also increased on Days 7 and 14. Moreover, TSPO was colocalized with Iba-1, GFAP and NeuN within the spinal cord dorsal horn. The TSPO agonist Ro5-4864, given intrathecally, dose-dependently retarded or prevented the development of mechanical allodynia and thermal hyperalgesia in rats with CFA-induced monoarthritis. These findings provide evidence that spinal TSPO is involved in the development and maintenance of inflammatory pain behaviors in rats. Thus, spinal TSPO may present a central target as a complementary therapy to reduce inflammatory pain.

Fast non-genomic effects of progesterone-derived neurosteroids on nociceptive thresholds and pain symptoms

Fast Inhibitory controls mediated by glycine (GlyRs) and GABAA receptors (GABAARs) play an important role to prevent the apparition of pathological pain symptoms of allodynia and hyperalgesia. The use of positive allosteric modulators of these receptors, specifically expressed in the spinal cord, may represent an interesting strategy to limit or block pain expression. In this study, we have used stereoisomers of progesterone metabolites, acting only via non-genomic effects, in order to evaluate the contribution of GlyRs and GABAARs for the reduction of mechanical and thermal heat hypernociception. We show that 3alpha neurosteroids were particularly efficient to elevate nociceptive thresholds in naive animal. It also reduced mechanical allodynia and thermal heat hyperalgesia in the carrageenan model of inflammatory pain. This effect is likely to be mediated by GABAA receptors since 3beta isomer was inefficient. More interestingly, 3alpha5beta neurosteroid was only efficient on mechanical allodynia while having no effect on thermal heat hyperalgesia. We characterized these paradoxical effects of 3alpha5beta neurosteroid using the strychnine and bicuculline models of allodynia. We clearly show that 3alpha5beta neurosteroid exerts an antinociceptive effect via a positive allosteric modulation of GABAARs but, at the same time, is pronociceptive by reducing GlyR function. This illustrates the importance of the inhibitory amino acid receptor channels and their allosteric modulators in spinal pain processing. Moreover, our results indicate that neurosteroids, which are synthesized in the dorsal horn of the spinal cord and have limited side effects, may be of significant interest in order to treat pathological pain symptoms.

Reflexive testosterone release: a model system for studying the nongenomic effects of testosterone upon male behavior

Male mammals of many species exhibit reflexive testosterone release in mating situations. In house mice (Mus musculus), the dramatic robustness of such release, occurring primarily in response to a novel female, suggests some function. The resulting testosterone elevations typically peak during copulatory behavior and may serve to activate transitory motivational and physiological responses that facilitate reproduction. However, such a function requires that testosterone be working through either nongenomic, or very quick genomic, mechanisms. The first part of the review describes reflexive sex hormone release in house mice. The second part summarizes research implicating testosterone's fast actions in affecting anxiety, reward, learning, analgesia, and penile reflexes in rodents, all of which could optimize male mating success. The review concludes with a speculative model of how spontaneous and reflexive hormone release might interact to regulate reproductive behavior and why mice appear to be an ideal species for examining testosterone's quick effects.

Regional differences in the decay kinetics of GABA(A) receptor-mediated miniature IPSCs in the dorsal horn of the rat spinal cord are determined by mitochondrial transport of cholesterol

We examined the possibility of a differential spatial control in the endogenous production of 3alpha5alpha-reduced steroids and its consequences on GABA(A) receptor-mediated miniature IPSCs (mIPSCs) in laminas II and III-IV of the rat spinal cord dorsal horn (DH). Early in postnatal development [younger than postnatal day 8 (P8)], mIPSCs displayed slow decay kinetics in laminas II and III-IV resulting from a continuous local production of 3alpha5alpha-reduced steroids. This was mediated by the tonic activity of the translocator protein of 18 kDa (TSPO), which controls neurosteroid synthesis by regulating the transport of cholesterol across the mitochondrial membrane system. TSPO activity disappeared in laminas III-IV after P8 and was functionally downregulated in lamina II after P15, resulting in a marked reduction of mIPSC duration in these laminas. TSPO-mediated synthesis of 3alpha5alpha-reduced steroids was spatially restricted, because, at P9-P15, when their production was maximal in lamina II, no sign of spillover to laminas III-IV was apparent. Interestingly, after P8, the enzymes necessary for the synthesis of 3alpha5alpha-reduced steroids remained functional in laminas III-IV and could produce such steroids from various precursors or after a single subcutaneous injection of progesterone. Moreover, induction of an acute peripheral inflammation by intraplantar injection of carrageenan, restored a maximal TSPO-mediated neurosteroidogenesis in laminas III-IV. Our results indicate that the decay kinetics of GABA(A) receptor-mediated mIPSCs in the DH of the spinal cord are primarily controlled by 3alpha5alpha-reduced steroids, which can be produced from circulating steroid precursors and/or in a spatially restricted manner by the modulation of the activity of TSPO.

Differential contribution of GABAergic and glycinergic components to inhibitory synaptic transmission in lamina II and laminae III-IV of the young rat spinal cord

Using whole-cell patch-clamp recordings from spinal cord slices of young (10-15 days old) rats, we have characterized and compared the properties of inhibitory synaptic transmission in lamina II and laminae III-IV of the dorsal horn, which are involved in the processing of nociceptive and non-nociceptive sensory information, respectively. All (100%) of laminae III-IV neurons, but only 55% of lamina II neurons, received both gamma-aminobutyric acid (GABA)ergic and glycinergic inputs. The remaining 45% of lamina II neurons received only GABAergic synapses. Neurons receiving only glycinergic synapses were never observed. Among the 55% of lamina II neurons receiving both GABAergic and glycinergic inputs, all displayed a small proportion (approximately 10%) of mixed miniature inhibitory postsynaptic currents (mIPSCs), indicating the presence of a functional GABA/glycine co-transmission at a subset of synapses. Such a co-transmission was never observed in laminae III-IV neurons. The presence of mixed mIPSCs and the differences in decay kinetics of GABAA-type receptor mIPSCs between lamina II and laminae III-IV were due to the endogenous tonic production of 3alpha5alpha-reduced steroids (3alpha5alpha-RS) in lamina II. Stimulation of the local production of 3alpha5alpha-RS was possible in laminae III-IV after incubation of slices with progesterone, subcutaneous injection of progesterone or induction of a peripheral inflammation. This led to the prolongation of GABAergic mIPSCs, but failed to induce the appearance of mixed mIPSCs in laminae III-IV. Our results indicate that, compared with lamina II, inhibitory synaptic transmission in laminae III-IV is characterized by a dominant role of glycinergic inhibition and the absence of a functional GABA/glycine co-transmission.

Progesterone: Therapeutic opportunities for neuroprotection and myelin repair

Progesterone and its metabolites promote the viability of neurons in the brain and spinal cord. Their neuroprotective effects have been documented in different lesion models, including traumatic brain injury (TBI), experimentally induced ischemia, spinal cord lesions and a genetic model of motoneuron disease. Progesterone plays an important role in developmental myelination and in myelin repair, and the aging nervous system appears to remain sensitive to some of progesterone's beneficial effects. Thus, the hormone may promote neuroregeneration by several different actions by reducing inflammation, swelling and apoptosis, thereby increasing the survival of neurons, and by promoting the formation of new myelin sheaths. Recognition of the important pleiotropic effects of progesterone opens novel perspectives for the treatment of brain lesions and diseases of the nervous system. Over the last decade, there have been a growing number of studies showing that exogenous administration of progesterone or some of its metabolites can be successfully used to treat traumatic brain and spinal cord injury, as well as ischemic stroke. Progesterone can also be synthesized by neurons and by glial cells within the nervous system. This finding opens the way for a promising therapeutic strategy, the use of pharmacological agents, such as ligands of the translocator protein (18 kDa) (TSPO; the former peripheral benzodiazepine receptor or PBR), to locally increase the synthesis of steroids with neuroprotective and neuroregenerative properties. A concept is emerging that progesterone may exert different actions and use different signaling mechanisms in normal and injured neural tissue.

Novel perspectives for progesterone in hormone replacement therapy, with special reference to the nervous system

The utility and safety of postmenopausal hormone replacement therapy has recently been put into question by large clinical trials. Their outcome has been extensively commented upon, but discussions have mainly been limited to the effects of estrogens. In fact, progestagens are generally only considered with respect to their usefulness in preventing estrogen stimulation of uterine hyperplasia and malignancy. In addition, various risks have been attributed to progestagens and their omission from hormone replacement therapy has been considered, but this may underestimate their potential benefits and therapeutic promises. A major reason for the controversial reputation of progestagens is that they are generally considered as a single class. Moreover, the term progesterone is often used as a generic one for the different types of both natural and synthetic progestagens. This is not appropriate because natural progesterone has properties very distinct from the synthetic progestins. Within the nervous system, the neuroprotective and promyelinating effects of progesterone are promising, not only for preventing but also for reversing age-dependent changes and dysfunctions. There is indeed strong evidence that the aging nervous system remains at least to some extent sensitive to these beneficial effects of progesterone. The actions of progesterone in peripheral target tissues including breast, blood vessels, and bones are less well understood, but there is evidence for the beneficial effects of progesterone. The variety of signaling mechanisms of progesterone offers exciting possibilities for the development of more selective, efficient, and safe progestagens. The recognition that progesterone is synthesized by neurons and glial cells requires a reevaluation of hormonal aging.

Neuregulin 1 is a pronociceptive cytokine that is regulated by progesterone in the spinal cord: implications for sex specific pain modulation

Sex differences in the magnitude of response to thermal and tactile stimuli have been demonstrated in both clinical and animal studies. Females typically display lower threshold responses to painful stimuli as compared to males. We have previously observed sexually dimorphic expression of the growth factor, neuregulin 1 (NRG1) following L5 nerve root ligation (LR) in male and female rats. In the present study, we sought to determine which gonadal hormones were involved in regulating NRG1 expression following L5 nerve root ligation. We observed that expression of NRG1 mRNA and the neuregulin receptors, ErbB2 and ErbB4 in the lumbar spinal cord was facilitated by the presence of progesterone in female rats following L5 nerve root ligation. An increase in NRG1 protein and NRG1 immunoreactivity was also observed in the ipsilateral spinal cord of progesterone treated female rats as compared to ovariectomized female rats and male rats at day 14 following LR. NRG1 immunoreactivity was equally colocalized with either the astrocytic marker, GFAP, and with NeuN labeled neurons 14days following L5 nerve root ligation. Intrathecal administration of recombinant NRG1-beta1 protein significantly decreased the hindpaw tactile withdrawal threshold in male rats, ovariectomized female rats, and progesterone treated female rats. These results demonstrate a role for progesterone-dependent regulation of glial and/or neuronal neuregulin 1 in female rats in mediating sex differences in nociception. Furthermore, our results suggest that NRG1 may be involved in central sensitization during the maintenance phase, but not in the initiation of persistent pain in female rats.

The many faces of progesterone: a role in adult and developing male brain

In addition to its well documented action in female-typical behaviors, progesterone exerts an influence on the brain and behavior of males. This review will discuss the role of progesterone and its receptor in male-typical reproductive behaviors in adulthood and the role of progesterone and its receptor in neural development, in both sexual differentiation of the brain as well as in the development of "non-reproductive" functions. The seemingly inconsistent and contradictory results on progesterone in males that exist in the literature illustrate the complexity of progesterone's actions and illuminate the need for further research in this area. As progestin-containing contraceptives in men are currently being tested and progesterone administration to pregnant women and premature newborns increases, a better understanding of the role of this hormone in behavior and brain development becomes essential.