tonic inhibition: nootropics and compounds | sci-wiki

Pregnanolone

Pregnanolone (3α-hydroxy-5β-pregnan-20-one, also written 3α,5β-tetrahydroprogesterone) is an endogenous neurosteroid, meaning a steroid that acts rapidly on neuronal ion channels rather than through classical intracellular hormone receptors, formed as a reduced metabolite of progesterone. It is the 5β epimer of allopregnanolone and a potent positive allosteric modulator of the GABA-A receptor (the pentameric chloride channel that carries most fast inhibitory signalling in the brain), giving it sedative, anxiolytic, anticonvulsant and anaesthetic properties. Under the International Nonproprietary Name eltanolone it was developed in the 1990s as an intravenous general anaesthetic, formulated in a soybean-oil emulsion after earlier castor-oil-solubilised steroid anaesthetics proved allergenic; smooth induction and cardiovascular stability were offset by slow recovery and skin reactions, and it was never marketed. Its sulfate and synthetic glutamate esters are studied separately as use-dependent inhibitors of the NMDA receptor, an excitatory glutamate channel implicated in excitotoxicity and neuroprotection.

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Hormone

Progesterone

Progesterone (P4; pregn-4-ene-3,20-dione) is an endogenous pregnane steroid hormone best known for its reproductive roles and is the prototypical neuroactive precursor within the neurosteroid system. Although the parent hormone signals principally through classical nuclear progesterone receptors and through membrane-associated receptors such as PGRMC1 and the mPR/PAQR family, most of its rapid effects on neuronal excitability arise only after sequential metabolism to 5-alpha-dihydroprogesterone and then to allopregnanolone, one of the most potent endogenous positive allosteric modulators of the GABA-A receptor (the brain's principal inhibitory chloride channel). Progesterone and its metabolites are synthesized within the nervous system itself, where they regulate myelination, neuronal survival, neuroinflammation, and inhibitory tone. It has been studied extensively as a neuroprotective agent, with strong preclinical support but negative large-scale human trials in acute traumatic brain injury.

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Nootropic

3α-Androstanediol

3α-Androstanediol (5α-androstane-3α,17β-diol) is an endogenous neurosteroid formed as the terminal 3α-reduced metabolite of dihydrotestosterone (DHT, the most potent natural androgen). Despite its androgenic origin it binds the androgen receptor only weakly; its defining pharmacology is potent positive allosteric modulation of the GABA-A receptor (the brain's principal inhibitory chloride ion channel), which produces anxiolytic (anxiety-reducing) and anticonvulsant (seizure-suppressing) effects in animal models. It is widely regarded as the androgenic counterpart of allopregnanolone (the analogous progesterone-derived neurosteroid), and is proposed to mediate much of the influence that testosterone exerts on seizure threshold, anxiety, and mood in males. It additionally acts as a ligand of estrogen receptor beta (ERβ), which may underlie some of its cognitive and neuroprotective actions.

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Anesthetic

Alfaxalone

Alfaxalone (also spelled alphaxalone) is a synthetic neuroactive steroid of the pregnane class that produces general anesthesia by acting as a positive allosteric modulator (a molecule that amplifies a receptor's response to its own neurotransmitter) at the GABA-A receptor, the principal inhibitory ion channel of the central nervous system. First introduced in 1971 as the main active component of the intravenous anesthetics Althesin (human) and Saffan (veterinary), it was withdrawn from human use in the 1980s because the solubilizing vehicle Cremophor EL provoked anaphylactoid reactions, then reintroduced in a cyclodextrin formulation marketed for veterinary anesthesia as Alfaxan. Because it lacks classical hormonal activity yet retains rapid, non-cumulative central depressant effects, alfaxalone occupies a distinct pharmacological niche among intravenous anesthetics and now serves as a template for a new generation of water-soluble neurosteroid sedatives such as Phaxan.

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Neurosteroid

Allopregnanolone

Allopregnanolone (chemically 3-alpha-hydroxy-5-alpha-pregnan-20-one, also known as 3-alpha,5-alpha-tetrahydroprogesterone or 3-alpha,5-alpha-THP) is an endogenous neurosteroid synthesized in the brain, adrenal glands and gonads as a downstream metabolite of the hormone progesterone. It is the prototypical inhibitory neuroactive steroid and one of the most potent known positive allosteric modulators (molecules that amplify a receptor's response to its natural transmitter) of the GABA-A receptor (the brain's principal inhibitory chloride ion channel), acting at both synaptic receptors and extrasynaptic delta-subunit-containing receptors. The pharmaceutical formulation of this exact molecule, brexanolone (trade name Zulresso), became in 2019 the first drug ever approved by the United States Food and Drug Administration specifically for postpartum depression, and its orally active analog zuranolone followed in 2023. Beyond its rapid actions on inhibitory neurotransmission, allopregnanolone participates in the stress response, ovarian-cycle and pregnancy physiology, seizure regulation and, as more recently characterized, the suppression of innate-immune inflammatory signaling.

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Neurosteroid

Deoxycorticosterone

Deoxycorticosterone (11-deoxycorticosterone, 21-hydroxyprogesterone; also called cortexone or desoxycortone) is an endogenous steroid hormone made by the adrenal cortex that functions both as a mineralocorticoid and as the metabolic precursor to a potent neurosteroid. Acting through the mineralocorticoid receptor (a ligand-activated transcription factor that governs sodium and water balance), it promotes renal salt retention with an affinity close to that of aldosterone. Its principal relevance to neuropharmacology is as the parent compound of 3-alpha,5-alpha-tetrahydrodeoxycorticosterone (THDOC), a positive allosteric modulator of the GABA-A receptor (the brain's main inhibitory chloride channel) that is released during stress and shapes seizure threshold, anxiety, and hypothalamic-pituitary-adrenal (HPA) axis activity. Deoxycorticosterone should not be confused with the psychedelic amphetamine also abbreviated DOC (2,5-dimethoxy-4-chloroamphetamine), which is an entirely unrelated compound.

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Anticonvulsant

Ganaxolone

Ganaxolone (research code CCD-1042; brand name Ztalmy) is a synthetic neuroactive steroid, specifically the 3-beta-methyl analog of the endogenous neurosteroid allopregnanolone (a metabolite of progesterone). It is a positive allosteric modulator (a molecule that amplifies a receptor's response to its natural activator) of the GABA-A receptor, the brain's principal inhibitory chloride channel, and it enhances both synaptic (phasic) and extrasynaptic (tonic) inhibition, with particularly strong activity at delta-subunit-containing receptors. In March 2022 it became the first medicine approved by the United States Food and Drug Administration for seizures associated with CDKL5 deficiency disorder (a rare genetic developmental and epileptic encephalopathy caused by mutations in the cyclin-dependent kinase-like 5 gene), and the first synthetic neurosteroid approved as an anticonvulsant. The defining 3-beta-methyl group makes ganaxolone orally bioavailable, metabolically stable, and non-hormonal, distinguishing it from its parent neurosteroid, which requires intravenous administration.

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Neurosteroid

SAGE-324

SAGE-324 (also designated BIIB124) is an investigational, orally bioavailable synthetic neuroactive steroid that acts as a positive allosteric modulator (a compound that amplifies a receptor's response to its natural signal) of GABA-A receptors (the brain's principal inhibitory chloride ion channels). Developed on the Sage Therapeutics neuroactive-steroid platform and co-developed with Biogen, it is a next-generation analog engineered for a long duration of action suitable for once-daily oral dosing, distinguishing it from the intravenous and short-acting neurosteroids that preceded it. Its lead clinical indication is essential tremor (a common movement disorder marked by rhythmic shaking of the hands and arms), where the randomized Phase 2 KINETIC trial met its primary endpoint; it has also been explored in the context of epilepsy. As a synthetic congener within the same chemical lineage as brexanolone and zuranolone, SAGE-324 represents an effort to translate the potent inhibitory pharmacology of endogenous neurosteroids into a chronically dosable oral therapeutic.

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Neurosteroid

THDOC

THDOC (3-alpha,5-alpha-tetrahydrodeoxycorticosterone) is an endogenous neurosteroid (a steroid that acts rapidly on neuronal membrane receptors rather than on classical nuclear hormone receptors) and the fully reduced 3-alpha,5-alpha metabolite of the adrenal steroid deoxycorticosterone. It is one of the most potent naturally occurring positive allosteric modulators (compounds that amplify a receptor's response to its own transmitter) of the GABA-A receptor, the brain's principal inhibitory chloride ion channel, where it enhances both fast synaptic and sustained extrasynaptic inhibition. Because its synthesis is driven by adrenocorticotropic hormone and by acute stress, THDOC is regarded as a stress-responsive inhibitory neurosteroid that provides negative feedback on the hypothalamic-pituitary-adrenal axis and transiently raises the seizure threshold. Alongside allopregnanolone, it is a prototypical member of the stress-derived GABAergic neurosteroid family.

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Antidepressant

Zuranolone

Zuranolone (development code SAGE-217; marketed as Zurzuvae) is an orally bioavailable synthetic analog of allopregnanolone (a naturally occurring neurosteroid derived from progesterone) that acts as a positive allosteric modulator (a compound that amplifies a receptor's response to its own transmitter) of the GABA-A receptor (the brain's principal inhibitory chloride channel). It was engineered by Sage Therapeutics from the same neurosteroid scaffold as the intravenous agent brexanolone, but with a modified 3-hydroxy pyrazole substitution that confers metabolic stability suitable for once-daily oral tablets rather than a continuous infusion. In August 2023 the United States Food and Drug Administration approved zuranolone for postpartum depression (a major depressive episode arising in the weeks after childbirth), making it the first oral drug indicated specifically for that condition. It is notable for producing an antidepressant response within days when given as a short, fixed two-week course, in contrast with the weeks-long onset of conventional monoamine antidepressants.

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