Anatomical and cellular localization of neuroactive 5?/3?-reduced steroid-synthesizing enzymes in the spinal cord

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.

Cellular distribution of the GABAA receptor-modulating 3alpha-hydroxy, 5alpha-reduced pregnane steroids in the adult rat brain

The 3alpha-hydroxy,5alpha-reduced pregnane steroids, allopregnanolone and allotetrahydrodeoxycorticosterone, are the most potent endogenous positive modulators of GABA(A) receptor-mediated inhibition. This study presents the first immunohistochemical examination of the cellular distribution of 3alpha-hydroxy,5alpha-reduced pregnane steroids across the brain. We found a widespread distribution in the adult rat, with dense immunolabelling in the olfactory bulb, striatum and cerebral cortex, and lower density labelling in the brainstem reticular formation. In general terms, this distribution accords with the regional concentrations of 3alpha-hydroxy,5alpha-reduced steroids determined, in other laboratories, by brain region sampling and either gas chromatography-mass fragmentography or radioimmunoassay. However, immunohistochemistry allowed for a more detailed examination of regional distribution and cellular specificity. All immunoreactivity was confined to the cell bodies and thick dendrites of neurones; no identifiable glia were labelled. In most brain areas, the location and morphology of labelled cells identified them as excitatory neurones. In addition, cell populations known to be projecting GABAergic neurones (e.g. cerebellar Purkinje cells) were immunoreactive, whereas local inhibitory neurones generally were not. The cellular distribution of 3alpha-hydroxy,5alpha-reduced steroids suggests that sensory, motor, limbic and homeostatic systems can be influenced by neurosteroids at multiple stages of processing.

Neurosteroids Involved in Regulating Inhibition in the Inferior Colliculus

Fast inhibitory neurotransmission in the brain is largely mediated by the γ-aminobutyric acid-type A (GABAA) receptor. The 3α,5α-reduced neurosteroids (e.g., allopregnanolone) are the most potent endogenous modulators of the GABAA receptor. Although it is known that 3α,5α-reduced neurosteroid levels change during stress or depression and over the estrus cycle, a basic physiological role consistent with their pharmacological action remains elusive. We used the unique architecture of the auditory midbrain to reveal a role for 3α,5α-reduced neurosteroids in regulating inhibitory efficacy. After blocking the massive GABAergic projection from the dorsal nucleus of the lateral lemniscus (DNLL) to the contralateral central nucleus of the inferior colliculus (ICC) in anesthetized rats, a reactive increase in the efficacy of other inhibitory circuits in the ICC (separable because of the dominant ear that drives each circuit) was demonstrated with physiological measures—single-neuron activity and a neural-population-evoked response. This effect was prevented by blocking 3α,5α-reduced neurosteroid synthesis with a 5α-reductase inhibitor: finasteride. Immunohistochemistry confirmed that the DNLL blockade induced an increase in 3α,5α-reduced neurosteroids in the contralateral ICC. This study shows that when GABAergic inhibition is reduced, the brain compensates within minutes by locally increasing synthesis of neurosteroids, thereby balancing excitatory and inhibitory inputs in complex neural circuits.

Decreased cerebrospinal fluid allopregnanolone levels in women with posttraumatic stress disorder

BACKGROUND

Alterations in the gamma-amino-butyric acid (GABA) neurotransmitter system have been identified in some populations with posttraumatic stress disorder (PTSD).

METHODS

To further investigate factors of relevance to GABAergic neurotransmission in PTSD, we measured cerebrospinal fluid (CSF) levels of allopregnanolone and pregnanolone combined (ALLO: congeners that potently and positively modulate effects of GABA at the GABA(A) receptor), 5alpha-dihydroprogesterone (5alpha-DHP: the immediate precursor for allopregnanolone), dehydroepiandrosterone (DHEA: a negative modulator of GABA(A) receptor function), and progesterone with gas chromatography, mass spectrometry in premenopausal women with (n = 9) and without (n = 10) PTSD. Subjects were free of psychotropic medications, alcohol, and illicit drugs; all were in the follicular phase of the menstrual cycle except three healthy and four PTSD subjects receiving oral contraceptives.

RESULTS

There were no group differences in progesterone, 5alpha-DHP, or DHEA levels. The PTSD group ALLO levels were < 39% of healthy group levels. The ALLO/DHEA ratio correlated negatively with PTSD re-experiencing symptoms (n = -.82, p < 008; trend) and with Profile of Mood State depression/dejection scores (n = -0.70, p < 0008).

CONCLUSION

Low CSF ALLO levels in premenopausal women with PTSD might contribute to an imbalance in inhibitory versus excitatory neurotransmission, resulting in increased PTSD re-experiencing and depressive symptoms.

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

Steroids exert long-term modulatory effects on numerous physiological functions by acting at intracellular/nuclear receptors influencing gene transcription. Steroids and neurosteroids can also rapidly modulate membrane excitability and synaptic transmission by interacting with ion channels, that is, ionotropic neurotransmitter receptors or voltage-dependent Ca2+ or K+ channels. More recently, the cloning of a plasma membrane-located G protein-coupled receptor for progestins in various species has suggested that steroids/neurosteroids could also influence second-messenger pathways by directly interacting with specific membrane receptors. Here we review the experimental evidence implicating steroids/neurosteroids in the modulation of synaptic transmission and the evidence for a role of endogenously produced neurosteroids in such modulatory effects. We present some of our recent results concerning inhibitory synaptic transmission in lamina II of the spinal cord and show that endogenous 5alpha-reduced neurosteroids are produced locally in lamina II and modulate synaptic gamma-aminobutyric acid A(GABAA) receptor function during development, as well as during inflammatory pain. The production of 5alpha-reduced neurosteroids is controlled by the endogenous activation of the peripheral benzodiazepine receptor (PBR), which initiates the first step of neurosteroidogenesis by stimulating the translocation of cholesterol across the inner mitochondrial membrane. Tonic neurosteroidogenesis observed in immature animals was decreased during postnatal development, resulting in an acceleration of GABAA receptor-mediated miniature inhibitory postsynaptic current (mIPSC) kinetics observed in the adult. Stimulation of the PBR resulted in a prolongation of GABAergic mIPSCs at all ages and was observed during inflammatory pain. Neurosteroidogenesis might play an important role in the control of nociception at least at the spinal cord level.

A neuroactive steroid inhibits guinea pig airway sensory nerves via Maxi-K channel activation

BACKGROUND

Although neurogenic inflammation via the activation of C-fibers in the airway may have an important role in the pathogenesis of asthma, their regulatory mechanism remains uncertain.

OBJECTIVE

The pharmacological profiles of a neuroactive steroid, allotetrahydrocorticosterone, on the activation of C-fibers in airway tissues were investigated, and the mechanism how a neuroactive steroid regulates airway inflammatory reactions was clarified.

METHODS

The effects of allotetrahydrocorticosterone on electrical field stimulation-induced bronchial smooth muscle contraction in guinea pig airway tissues were investigated. The influences of K+ channel blockers and intracellular protein inhibitors on the effects of allotetrahydrocorticosterone were examined.

RESULTS

Allotetrahydrocorticosterone dose-dependently inhibited electrical field stimulation-induced guinea pig bronchial smooth muscle contraction. The inhibitory effects of allotetrahydrocorticosterone on electrical field stimulation-induced bronchial contraction were reduced by the pretreatment of Maxi-K+ channel blockers, iberiotoxin and charybdotoxin, but not other K+ channel blockers, dendrotoxin or glibenclamide. Pretreatment with pertussis toxin diminished the inhibitory effect of allotetrahydrocorticosterone, but not an adenylate cyclase inhibitor, SQ 22536, nor a specific inhibitor of mitogen-activated protein kinase kinase, PD 98059.

CONCLUSIONS

These findings suggest that allotetrahydrocorticosterone negatively modulates the activation of C-fibers in guinea pig airway tissues via the opening of Maxi-K+ channels and a pertussis toxin-sensitive G-protein-coupled mechanism.

Inflammatory pain upregulates spinal inhibition via endogenous neurosteroid production

Inhibitory synaptic transmission in the dorsal horn (DH) of the spinal cord plays an important role in the modulation of nociceptive messages because pharmacological blockade of spinal GABAA receptors leads to thermal and mechanical pain symptoms. Here, we show that during the development of thermal hyperalgesia and mechanical allodynia associated with inflammatory pain, synaptic inhibition mediated by GABAA receptors in lamina II of the DH was in fact markedly increased. This phenomenon was accompanied by an upregulation of the endogenous production of 5alpha-reduced neurosteroids, which, at the spinal level, led to a prolongation of GABAA receptor-mediated synaptic currents and to the appearance of a mixed GABA/glycine cotransmission. This increased inhibition was correlated with a selective limitation of the inflammation-induced thermal hyperalgesia, whereas mechanical allodynia remained unaffected. Our results show that peripheral inflammation activates an endogenous neurosteroid-based antinociceptive control, which discriminates between thermal and mechanical hyperalgesia.

Injury elicited increase in spinal cord neurosteroid content analyzed by gas chromatography mass spectrometry

The effects of spinal cord injury (SCI), combined with castration and adrenalectomy, and of progesterone (PROG) treatment on neurosteroid levels and steroidogenic enzyme expression were investigated in the adult male rat spinal cord (SC). Steroid levels were quantified by gas chromatography/mass spectrometry in SC and plasma, and mRNAs of enzymes by quantitative real-time RT-PCR. The levels of pregnenolone (PREG), PROG, 5alpha-dihydroprogesterone, 3alpha,5alpha-tetrahydroprogesterone increased in SC 75 h after transection without significant increase in the plasma. After combined adrenalectomy and gonadectomy, significant levels of PREG and PROG remained in the SC, suggesting their local biosynthesis. In the SC of adrenalectomized and gonadectomized rats, there was an increase of PREG 24 h after SCI, followed at 75 h by a concomitant increase in its direct metabolite, PROG. These observations are consistent with a sequential increase of PREG biosynthesis and its conversion to PROG within the SC in response to injury. However, no significant change in P450-side chain cleavage and 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase mRNA levels was observed after SCI. Systemic PROG treatment after SCI, resulted in a very large increase in PROG, 5alpha-dihydroprogesterone, and 3alpha,5alpha-tetrahydroprogesterone in both plasma and SC. Furthermore, high levels of 3beta,5alpha-tetrahydroprogesterone were detected in SC, whereas their plasma levels remained barely detectable. Because the ratio of reduced metabolites to PROG was 65-times higher in SC than in the plasma, it appears likely that reduced metabolites mainly originated from local biosynthesis. Our results strongly suggest an important role for locally biosynthesized neurosteroids in the response of the SC to injury.

Mechanisms of modulation of pregnanolone on glycinergic response in cultured spinal dorsal horn neurons of rat

The glycine receptors and neurosteroids in spinal cord are both implicated in nociceptive signal processing. However, the modulatory effects of neurosteroid pregnanolone (5beta-pregnan-3alpha-ol-20-one) on native glycine receptors remain unclear. In the present study, we examined the effects of pregnanolone and its three isomers on glycine receptors by using whole-cell patch-clamp technique. Our results showed that pregnanolone reversibly inhibited the amplitude of glycine-induced current mediated by native glycine receptors and recombinant alpha1-, alpha2-, alpha3- and alpha1beta-glycine receptors. In cultured spinal dorsal horn neurons of rats, pregnanolone inhibited the glycine-induced current in dose-dependent manner, with an antagonist concentration inducing half-maximal response of 1.0+/-0.3 microM. The inhibitory effect of pregnanolone on glycine-induced current was voltage-independent and pregnanolone shifted the concentration-response curve for glycine-induced current rightward in a parallel manner without altering the maximal value and Hill coefficient. The isomer of pregnanolone, allopregnanolone (5alpha-pregnan-3alpha-ol-20-one) slightly enhanced glycine-induced current, whereas iso-pregnanolone (5beta-pregnan-3beta-ol-20-one) and iso-allopregnanolone (5alpha-pregnan-3beta-ol-20-one) did not affect the glycine-induced current significantly in cultured spinal dorsal horn neurons. Thus, our results suggest that the inhibitory effect of pregnanolone on glycine-induced current is of a competitive type and depends on the stereo structure of pregnanolone. Furthermore, pregnanolone decreased the amplitude and frequency of the glycinergic miniature inhibitory postsynaptic currents. Through modulating the glycinergic inhibitory neurotransmission, pregnanolone may affect the nociceptive sensory processing under physiological and pathological conditions.

The progesterone derivative dydrogesterone down-regulates neurokinin 1 receptor expression on lymphocytes, induces a Th2 skew and exerts hypoalgesic effects in mice

Accumulating evidence indicates that the neuropeptide substance P (SP) is predominantly involved in neurogenic inflammation and pain perception via its high-affinity neurokinin 1 receptor (NK-1R). Intriguingly, decreased pain sensitivity is found to be associated with high plasma progesterone levels. We hypothesize that progesterone may attenuate nociception and associated inflammatory response via NK-1R-dependent pathways. To address our hypothesis, we incubated splenic lymphocytes from CBA/J female mice with different concentrations of the progesterone derivative dydrogesterone. Subsequently, the expressions of NK-1R and T helper (Th1)-type cytokines were analyzed by flow cytometry. Next, we subcutaneously injected CBA/J mice with 1.25 mg of dydrogesterone in 200-microl sesame oil; control mice were sham-injected. Tail flick test to detect the nociceptive threshold was performed in 30-min intervals upon injection. Lymphocytes were isolated from blood and uterus and analyzed for NK-1R surface expression. Immunohistochemical analyses were performed to investigate the uterine tissue distribution of NK-1R. Dydrogesterone induced a decrease in the percentage of NK-1R+ lymphocytes in vitro and in vivo. Additionally, an increase in Th2-type and a decrease in Th1-type cytokines could be detected in vitro after incubation with dydrogesterone. An increased tail flick latency following dydrogesterone injection supported the concept that decreased expression of the NK-1R on lymphocytes is associated with an increased pain threshold. Taken together, these results clearly reveal a pathway by which dydrogesterone or progesterone respectively modulates the cross talk of the nervous, endocrine and immune systems in inflammation and pain.

A neuroactive steroid, allotetrahydrocorticosterone inhibits sensory nerves activation in guinea-pig airways

We examined the effects of a neuroactive steroid, allotetrahydrocorticosterone on the activation of capsaicin-sensitive afferent sensory nerves (C-fibers). Allotetrahydrocorticosterone (0.0001-1.0 microg/ml) dose-dependently inhibited electrical field stimulation-induced guinea-pig bronchial smooth muscle contraction, but not the substance P-induced contraction at 1.0 microg/ml. Allotetrahydrocorticosterone (0.01-1.0 microg/ml) also reduced the capsaicin-induced release of substance P-like immunoreactivity from guinea-pig airway tissues in a dose-dependent manner. The inhibitory effect of allotetrahydrocorticosterone on electrical field stimulation-induced bronchial contraction were reduced by the pretreatment of voltage-dependent K+ channel blockers, tetraethylammonium (1 mM). This evidence suggests that allotetrahydrocorticosterone negatively modulate the activation of C-fibers and substance P release from their endings in airway tissues via the opening of voltage-dependent K+ channels.

Molecular and neurochemical evidence for the biosynthesis of dehydroepiandrosterone in the adult rat spinal cord

Various studies have indicated that exogenous dehydroepiandrosterone (DHEA) modulates several mechanisms in the CNS of rodents. As adult rodent glands do not secrete significant amounts of DHEA, its role as endogenous modulator of the CNS remains possible only if DHEA is produced by nerve cells. Therefore, the last decade has been marked by diverse unsuccessful investigations aiming to demonstrate the activity of cytochrome P450c17 (P450c17), the key DHEA-synthesizing enzyme, in adult rodent CNS. Here, we combined molecular, anatomical, cellular and neurochemical approaches to provide the first demonstration of the existence of P450c17 and bioactivity in adult rat spinal cord (SC). Real-time RT-PCR revealed P450c17 gene expression in all SC segments. Western blot analyses allowed identification of a specific P450c17 protein in the SC and immunohistochemical studies localized P450c17 in neurones and glial cells. Pulse-chase experiments combined with HPLC and radioactive steroid detection showed that SC slices converted [3H]pregnenolone into [3H]DHEA, a conversion markedly reduced by ketoconazole, a P450c17 inhibitor. Kinetics studies revealed accumulation of [3H]DHEA newly synthesized by SC slices in the incubation medium as its amount declined slowly. This first cellular mapping of an active P450c17 in adult rodent SC suggests that endogenous DHEA synthesized in spinal neural networks may control various spinally-mediated activities.

Substance P inhibits progesterone conversion to neuroactive metabolites in spinal sensory circuit: a potential component of nociception

A crucial biochemical reaction in vertebrates is progesterone conversion into neuroactive metabolites such as dihydroprogesterone (5alpha-DHP) and tetrahydroprogesterone (3alpha,5alpha-THP), which regulate several neurobiological processes, including stress, depression, neuroprotection, and analgesia. 3alpha,5alpha-THP is a potent stimulator of type A receptors of GABA, the main inhibitory neurotransmitter. Here, we show that in the spinal sensory circuit progesterone conversion into 5alpha-DHP and 3alpha,5alpha-THP is inhibited dose-dependently by substance P (SP), a major mediator of painful signals. We developed a triple-labeling approach coupled with multichannel confocal microscope analysis, which revealed that, in the spinal cord (SC), SP-releasing afferents project on sensory neurons expressing simultaneously neurokinin 1 receptors (rNK1) and key enzymes catalyzing progesterone metabolism. Evidence for a potent inhibitory effect of SP on 5alpha-DHP and 3alpha,5alpha-THP formation in the SC was provided by combining pulse-chase experiments using [3H]progesterone as precursor, HPLC, recrystallization of [3H]metabolites to constant specific activity, and continuous flow detection of radioactive steroids. The action of SP on progesterone metabolism was mimicked by the rNK1-specific agonist [Sar-9,Met(O2)11]-SP. The selective rNK1 antagonist SR140333 totally reversed the effect of SP on progesterone conversion into 5alpha-DHP and 3alpha,5alpha-THP. These results provide direct evidence for the occurrence of anatomical and functional interactions between the SP-rNK1 system and neuroactive steroid-producing cells in the SC. The data suggest that, through the local control of 3alpha,5alpha-THP concentration in spinal sensory circuit, the SP-rNK1 system may indirectly interfere with GABA(A) receptor activity in the modulation of nociceptive transmission.

Effect of streptozotocin-induced diabetes on the gene expression and biological activity of 3β-hydroxysteroid dehydrogenase in the rat spinal cord

Abnormal secretion of steroids by the adrenals and gonads is one of the disturbances occurring in diabetics but the impact of diabetes on steroid formation in the nervous system has never been studied. However, it is well known that numerous actions of peripheral steroids on the nervous system require their conversion into neuroactive metabolites within the neural tissue. As this in situ steroid synthesis/metabolism is crucial for the control of several neurobiological functions, we investigated the effects of streptozotocin-induced diabetes on the gene expression and activity of 3beta-hydroxysteroid dehydrogenase in the spinal cord, a pivotal structure involved in sensorimotor and neurovegetative mechanisms. 3beta-Hydroxysteroid dehydrogenase is a key enzyme which participates to the biosynthesis of all classes of steroids by converting delta5-3beta-hydroxysteroids such as pregnenolone and dehydroepiandrosterone into delta4-3-ketosteroids as progesterone and androstenedione, respectively. Reverse transcription coupled with quantitative real-time polymerase chain reaction revealed that 3beta-hydroxysteroid dehydrogenase gene was over-expressed in the spinal cord of streptozotocin-treated rats compared with controls. Pulse-chase experiments combined with high performance liquid chromatography and continuous flow detection of newly-synthesized steroids showed an increase of 3beta-hydroxysteroid dehydrogenase activity responsible for a hyper-production of progesterone in the spinal cord of diabetic rats. This up-regulation of progesterone biosynthesis was concomitant with a decrease of its transformation into tetrahydroprogesterone, a process which facilitated progesterone accumulation in the spinal cord of streptozotocin-treated rats. Since progesterone is a potent neuroprotective steroid, increase of its production appeared as an endogenous molecular and biochemical mechanism triggered by spinal nerve cells to cope with degenerative effects of streptozotocin-induced diabetes. Our results constitute the first direct evidence showing an impact of diabetes on steroid biosynthetic and metabolic pathways in the nervous system. The data open new perspectives for the modulation of deleterious effects of diabetes by neuroprotective steroids.