Doodipala Samba Reddy

Neurosteroids: Endogenous role in the human brain and therapeutic potentials

This chapter provides an overview of neurosteroids, especially their impact on the brain, sex differences and their therapeutic potentials. Neurosteroids are synthesized within the brain and rapidly modulate neuronal excitability. They are classified as pregnane neurosteroids, such as allopregnanolone and allotetrahydrodeoxycorticosterone, androstane neurosteroids, such as androstanediol and etiocholanolone, and sulfated neurosteroids such as pregnenolone sulfate. Neurosteroids such as allopregnanolone are positive allosteric modulators of GABA-A receptors with powerful anti-seizure activity in diverse animal models. Neurosteroids increase both synaptic and tonic inhibition. They are endogenous regulators of seizure susceptibility, anxiety, and stress. Sulfated neurosteroids such as pregnenolone sulfate, which are negative GABA-A receptor modulators, are memory-enhancing agents. Sex differences in susceptibility to brain disorders could be due to neurosteroids and sexual dimorphism in specific structures of the human brain. Synthetic neurosteroids that exhibit better bioavailability and efficacy and drugs that enhance neurosteroid synthesis have therapeutic potential in anxiety, epilepsy, and other brain disorders. Clinical trials with the synthetic neurosteroid analog ganaxolone in the treatment of epilepsy have been encouraging. Neurosteroidogenic agents that lack benzodiazepine-like side effects show promise in the treatment of anxiety and depression.

Possible role of nitric oxide in the nootropic and antiamnesic effects of neurosteroids on aging- and dizocilpine-induced learning impairment

The ability of the nitric oxide (NO) synthase inhibitor, NG-nitro-l-arginine methyl ester (L-NAME), to modulate the attenuating effects of neurosteroids on the aging- and NMDA receptor antagonist dizocilpine-induced learning impairment, was tested in mice using two different behavioral models of long-term memory. The performance of aged mice (16 months old) in step-down type of passive-avoidance and elevated plus-maze paradigms was significantly impaired compared to that of young mice (3 months old). Neurosteroids pregnenolone sulfate (PS) and dehydroepiandrosterone sulfate (DHEAS), at 1-20 mg/kg, s.c., significantly improved the passive-avoidance and plus-maze performances in aged mice. Neurosteroids PS and DHEAS, at doses 1-20 mg/kg, s.c., significantly attenuated dizocilpine (0.1 mg/kg, i.p.)-induced amnesia, without producing any promnestic effects alone in adult mice. In both cognitive tasks, the effects exhibited by the neurosteroids tested had a bell-shaped curve. Preadministration of L-NAME (10 and 20 mg/kg, i.p.), at doses that did not disrupt cognition alone in either young or aged mice, significantly blocked the beneficial and antiamnesic effects of neurosteroids PS (5 mg/kg) and DHEAS (10 mg/kg). A selective action of L-NAME on the effects of neurosteroids was indicated, since the effects of L-NAME were completely reversed by L-arginine (300 mg/kg, i.p.), a competitive substrate for NO synthase. Neither L-NAME nor L-arginine alone affected the antinociception, locomotor activity or rota-rod performance of young or aged mice. These observations suggest that a NO-dependent mechanism may be involved in the beneficial and antiamnesic effects of neurosteroids PS and DHEAS on the aging- and dizocilpine-induced impairment of learning and memory processes.

Sigma (σ1) receptor mediated antidepressant-like effects of neurosteroids in the Porsolt forced swim test

THIS study examined the effects of neurosteroids dehydroepiandrosterone sulfate (DHEAS) and pregnenolone sulfate (PS) and progesterone on the Porsolt forced swim test of depression in mice, and investigated the possible involvement o σ receptors. The immobility time in the mouse forced swimming test was significantly reduced by DHEAS (5 and 2 0 mg/kg, s.c.) and PS (5 mg/kg) without accompanying changes in the ambulatory or open-field activity. Pretreatment with DHEAS (&PHgr; mg/kg) or PS (10 and &PHgr; mg/kg), however, failed to modify the immobility. The relief of behavioral despair in the immobility test by DHEAS (5 and 20 mg/kg) was dose-dependently blocked by preadministration of NE-100 (N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl-ethylamine monohydrochloride; 0.5 and 1 mg/kg), a putativeσ1 receptor antagonist, or progesterone (1 0 mg/kg), aσ receptor antagonistic neurosteroid. On the other hand, PS (5 mg/kg)-induced decrease in the immobility was significantly blocked by NE-100 (0. 5 mg/kg), but not by progesterone (1 0 mg/kg). Neither NE-100 nor progesterone influenced the immobility alone. These data suggest a role for centralσ receptor in the antidepressant-like effects of neurosteroids, and reinforced their potential therapeutic use in depression.

Anxiolytic activity of progesterone in progesterone receptor knockout mice.

Progesterone is an anxiolytic steroid that could play a role in the regulation of anxiety in women. However, the mechanism by which progesterone decreases anxiety is incompletely understood. Progesterone affects the function of the brain by two distinct mechanisms. Progesterone regulates reproductive behavior by activating intracellular progesterone receptors (PRs). In addition, progesterone is believed to influence neuronal activity through its conversion to allopregnanolone, a neurosteroid that acts as a positive allosteric modulator of GABAA receptors. The extent to which the anxiolytic action of progesterone requires PRs is uncertain. In this study, we utilized PR knockout (PRKO) mice bearing a targeted null mutation of the PR gene that abrogates the function of both PR-A and PR-B subtypes to determine the requirement for PRs in the anxiolytic actions of progesterone. The absence of PR receptor protein expression in PRKO brain was confirmed by immunocytochemistry. In PRKO mice and their isogenic wild-type (WT) littermates, progesterone administration was associated with a dose-dependent rise in plasma allopregnanolone concentrations and corresponding anxiolytic effects in the elevated plus maze test. PRKO mice exhibited a greater anxiolytic response than WT animals although the allopregnanolone levels were similar in the two genotypes. Allopregnanolone also exhibited anxiolytic effects, but the magnitude of the response was similar in both genotypes. Pretreatment of PRKO mice with finasteride, a 5alpha-reductase inhibitor that blocks the conversion of progesterone to allopregnanolone, completely prevented the anxiolytic activity of progesterone, but had no effect on the response to allopregnanolone, demonstrating that allopregnanolone (or other 5alpha-reduced metabolites of progesterone) accounts for the response to the parent steroid hormone. These results provide direct evidence that the anxiolytic action of progesterone does not require PRs. However, PR activation by progesterone may influence the anxiolytic response since PRKO mice were more sensitive to progesterone.

Neurosteroid Withdrawal Model of Perimenstrual Catamenial Epilepsy

Summary:  Purpose: Perimenstrual catamenial epilepsy, the increase in seizure frequency that some women with epilepsy experience near the time of menstruation, may in part be related to withdrawal of the progesterone metabolite allopregnanolone, an endogenous anticonvulsant neurosteroid that is a potent positive allosteric γ‐aminobutyric acidA (GABAA) receptor modulator. The objective of this study was to develop an animal model of perimenstrual catamenial epilepsy for use in evaluating drug‐treatment strategies.

Enhanced Anticonvulsant Activity of Neuroactive Steroids in a Rat Model of Catamenial Epilepsy

Summary:  Purpose: Perimenstrual catamenial epilepsy may in part be due to withdrawal of the endogenous progesterone‐derived neurosteroid allopregnanolone that potentiates γ‐aminobutyric acidA (GABAA) receptor–mediated inhibition. Here we sought to determine whether the anticonvulsant potencies of neuroactive steroids, benzodiazepines, phenobarbital (PB), and valproate (VPA) are altered during the heightened seizure susceptibility accompanying neurosteroid withdrawal in a rat model of perimenstrual catamenial epilepsy.

Testosterone modulation of seizure susceptibility is mediated by neurosteroids 3α-androstanediol and 17β-estradiol

Abstract Testosterone modulates seizure susceptibility in animals and humans, but the underlying mechanisms are obscure. Here, testosterone modulation of seizure susceptibility is hypothesized to occur through its conversion to neurosteroids with “anticonvulsant” and “proconvulsant” actions, and hence the net effect of testosterone on neural excitability and seizure activity depends on the levels of distinct testosterone metabolites. Testosterone undergoes metabolism to neurosteroids via two distinct pathways. Aromatization of the A-ring converts testosterone into 17β-estradiol. Reduction of testosterone by 5α-reductase generates 5α-dihydrotestosterone (DHT), which is then converted to 3α-androstanediol (3α-Diol), a powerful GABA A receptor-modulating neurosteroid with anticonvulsant properties. Systemic doses of testosterone decreased seizure threshold in rats and increased the incidence and severity of pentylenetetrazol (PTZ)-induced seizures in mice. These proconvulsant effects of testosterone were associated with increases in plasma 17β-estradiol and 3α-Diol concentrations. Pretreatment with letrozole, an aromatase inhibitor that blocks the conversion of testosterone to 17β-estradiol, significantly inhibited testosterone-induced exacerbation of seizures. The 5α-reductase inhibitor finasteride significantly reduced 3α-Diol levels and also blocked letrozoles ability to inhibit the proconvulsant effects of testosterone. The 5α-reduced metabolites of testosterone, DHT and 3α-Diol, had powerful anticonvulsant activity in the PTZ test. Letrozole or finasteride had no effect on seizure protection by DHT and 3α-Diol, but indomethacin partially reversed DHT actions. 3α-Diol but not 3β-androstanediol, a GABA A receptor-inactive stereoisomer, suppressed 4-aminopyridine-induced spontaneous epileptiform bursting in rat hippocampal slices. Thus, testosterone-derived neurosteroids 3α-Diol and 17β-estradiol could contribute to the net cellular actions of testosterone on neural excitability and seizure susceptibility.

Neurosteroid Replacement Therapy for Catamenial Epilepsy

SummaryPerimenstrual catamenial epilepsy, the cyclical occurrence of seizure exacerbations near the time of menstruation, affects a high proportion of women of reproductive age with drug-refractory epilepsy. Enhanced seizure susceptibility in perimenstrual catamenial epilepsy is believed to be due to the withdrawal of the progesterone-derived GABAA receptor modulating neurosteroid allopregnanolone as a result of the fall in progesterone at the time of menstruation. Studies in a rat pseudopregnancy model of catamenial epilepsy indicate that after neurosteroid withdrawal there is enhanced susceptibility to chemoconvulsant seizures. There is also a transitory increase in the frequency of spontaneous seizures in epileptic rats that had experienced pilocarpine-induced status epilepticus. In the catamenial epilepsy model, there is a marked reduction in the antiseizure potency of anticonvulsant drugs, including benzodiazepines and valproate, but an increase in the anticonvulsant potency and protective index of neurosteroids such as allopregnanolone and the neurosteroid analog ganaxolone. The enhanced seizure susceptibility and benzodiazepine-resistance subsequent to neurosteroid withdrawal may be related to reduced expression and altered kinetics of synaptic GABAA receptors and increased expression of GABAA receptor subunits (such as α4) that confer benzodiazepine insensitivity. The enhanced potency of neurosteroids may be due to a relative increase after neurosteroid withdrawal in the expression of neurosteroid-sensitive δ-subunit-containing perisynaptic or extrasynaptic GABAA receptors. Positive allosteric modulatory neurosteroids and synthetic analogs such as ganaxolone may be administered to prevent catamenial seizure exacerbations, in what we call neurosteroid replacement therapy.

Role of neurosteroids in catamenial epilepsy.

Catamenial epilepsy is a menstrual cycle-related seizure disorder that affects up to 70% of women with epilepsy. Catamenial epilepsy is characterized by an increase in seizures during particular phases of the menstrual cycle. Three distinct patterns of catamenial epilepsy - perimenstrual, periovulatory, and inadequate luteal phase - have been described. Currently, there is no specific treatment for catamenial epilepsy. The molecular mechanisms involved in the pathophysiology of catamenial epilepsy are not well understood. Recent studies suggest that cyclical changes of ovarian hormones estrogens (proconvulsant) and progesterone (anticonvulsant) appear to play a key role in the genesis of catamenial seizures. Progesterone reduces seizure susceptibility partly through conversion to neurosteroids such as allopregnanolone, which enhances GABA(A) receptor function and thereby inhibits neuronal excitability. In animal models, withdrawal from chronic progesterone and, consequently, of allopregnanolone levels in brain, has been shown to increase seizure susceptibility. Natural progesterone therapy has proven effective in women with epilepsy. Moreover, neurosteroids have been shown to be very effective inhibitors of catamenial seizures in animal models. Thus, synthetic neuroactive steroids, such as ganaxolone, which are orally active and devoid of hormonal side effects, represent a novel treatment strategy for catamenial epilepsy. However, their clinical efficacy in catamenial epilepsy has yet to be explored. A greater understanding of the molecular mechanisms is clearly needed for designing effective treatment and prevention strategies of catamenial epilepsy in women at risk.

Is there a physiological role for the neurosteroid THDOC in stress-sensitive conditions?

Endogenous neurosteroids affect brain excitability during physiological states such as pregnancy and the menstrual cycle, and during conditions of acute and chronic stress. The neurosteroid allotetrahydrodeoxycorticosterone (THDOC) is an allosteric modulator of the GABA(A) receptor. Although the role of THDOC within the brain is undefined, recent studies indicate that stress induces THDOC to levels that can activate GABA(A) receptors. These results might have significant implications for human stress-sensitive conditions such as epilepsy, post-traumatic stress disorder and depression.