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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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Neurosteroid

Allotetrahydrocorticosterone

Allotetrahydrocorticosterone (3α,5α-tetrahydrocorticosterone, or allo-THB) is an endogenous neurosteroid generated by sequential 5α-reduction and 3α-hydroxylation of the adrenal glucocorticoid corticosterone. It is the corticosterone-derived structural counterpart of THDOC (which is formed the same way from deoxycorticosterone) and behaves as a positive allosteric modulator (an enhancer that boosts a receptor's response without activating it directly) of the GABA-A receptor (the brain's principal inhibitory chloride ion channel); the additional 11β-hydroxyl group it carries, however, makes it a considerably weaker potentiator than THDOC or allopregnanolone. Beyond GABA-A it shows an unusually broad ion-channel profile, inhibiting glycine receptors (a second inhibitory chloride channel, prominent in the brainstem and spinal cord) and N-type calcium channels (presynaptic channels that gate neurotransmitter release), opening large-conductance calcium-activated potassium channels (BK or Maxi-K channels) in sensory nerves, and retaining residual glucocorticoid-receptor agonism. Clinically it appears mainly as a minor urinary corticosteroid metabolite that indexes 5α-reductase activity in steroid profiling.

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Nootropic

DHEA Sulfate

DHEA sulfate (DHEAS) is the 3-beta sulfate ester of dehydroepiandrosterone and the most abundant circulating steroid in the human body, present in plasma at concentrations roughly a thousandfold higher than unconjugated DHEA. Synthesized principally in the zona reticularis of the adrenal cortex, it functions as a stable, long-lived reservoir that is interconverted with DHEA and supplies a precursor pool for downstream androgens and estrogens. Within the nervous system it is classified as a neurosteroid (a steroid synthesized in or acting directly upon nervous tissue); it acts as an agonist at the sigma-1 receptor (an intracellular chaperone protein), a positive modulator of the NMDA receptor (a glutamate-gated excitatory ion channel), and a negative allosteric modulator of the GABA-A receptor (the brain's principal inhibitory ion channel), giving it a net excitatory, pro-cognitive neuromodulatory profile. Circulating concentrations fall markedly with age, a decline termed adrenopause that has made DHEAS a widely studied biomarker of adrenal function, cognitive aging, and longevity.

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Nootropic

Epiandrosterone

Epiandrosterone (3β-hydroxy-5α-androstan-17-one; also called isoandrosterone) is an endogenous 5-alpha-reduced steroid formed from dehydroepiandrosterone (DHEA) and androstenedione, and it is the 3-beta-hydroxyl epimer of androsterone. It circulates in humans predominantly as its sulfate ester and functions as a weak neurosteroid (a steroid that acts on neuronal receptors rather than classical nuclear hormone receptors), producing only mild modulation of the GABA-A receptor (the brain's principal inhibitory chloride ion channel) while inhibiting glycine receptors (another inhibitory ligand-gated channel concentrated in the spinal cord and brainstem). Beyond the nervous system it is a well-characterized uncompetitive inhibitor of glucose-6-phosphate dehydrogenase (G6PD, the rate-limiting enzyme of the pentose phosphate pathway) and a recognized urinary marker of 5-alpha-reductase activity. Because it can be metabolized toward dihydrotestosterone (DHT, the most potent natural androgen), it is also sold over the counter as a non-methylated androgen prohormone.

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Anesthetic

Minaxolone

Minaxolone is a synthetic water-soluble aminosteroid (a steroid carrying a basic amino group) developed by Glaxo in the late 1970s as an intravenous general anaesthetic and a successor to Althesin. Structurally it is a 2-beta-ethoxy, 11-alpha-(dimethylamino) derivative of the neuroactive pregnane skeleton, and it acts as a potent positive allosteric modulator at the neurosteroid recognition site of the GABA-A receptor (the brain's principal inhibitory chloride ion channel), enhancing inhibitory conductance to produce hypnosis and surgical anaesthesia. Its basic amino group conferred solubility in water, avoiding the Cremophor EL vehicle (a polyethoxylated castor oil solubiliser) that made earlier lipophilic steroid anaesthetics prone to anaphylactoid reactions. After clinical trials conducted between roughly 1979 and 1982, its development was halted over toxicity and genotoxicity concerns arising in long-term rodent studies, and it was never marketed.

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Anesthetic

Org 21465

Org 21465 (also written ORG-21465) is a water-soluble aminosteroid intravenous anaesthetic developed by Organon in the 1990s as a companion candidate to the earlier compound Org 20599. It belongs to a series of 2-substituted (C2-morpholinyl) pregnane steroids engineered to combine the favourable receptor pharmacology of neuroactive steroids with the aqueous solubility needed for a practical injectable formulation, acting principally as a positive allosteric modulator (an agent that amplifies a receptor's response without being the primary trigger) of the GABA-A receptor (the brain's main inhibitory chloride ion channel). In computer-controlled infusion studies in healthy male volunteers the compound produced dose-related sedation and reversible loss of consciousness, but it was accompanied by prominent involuntary excitatory movements, injection-site venous pain, and slow equilibration with the effect site, and its clinical development was subsequently abandoned. It survives in the literature as a historical, niche illustration of the difficulty of translating neurosteroid GABA-A pharmacology into a usable anaesthetic.

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Anesthetic

Renanolone

Renanolone (3alpha-hydroxy-5beta-pregnane-11,20-dione) is a synthetic pregnane steroid that was investigated as an intravenous general anaesthetic during the Glaxo steroid-anaesthetic program of the 1960s and 1970s. It is the 5beta-configured epimer of alphaxalone (alfaxalone), differing only in the stereochemistry of the A/B ring fusion, and it belongs to the same 3alpha-hydroxy-pregnane pharmacophore that defines neurosteroid modulators of the GABA-A receptor (the brain's principal inhibitory chloride ion channel). Like other steroid anaesthetics it produces sedation, hypnosis and anaesthesia by acting as a positive allosteric modulator of GABA-A receptors, although modern photoaffinity work shows it occupies the intersubunit neurosteroid pocket roughly twenty times more weakly than its 5alpha counterpart. Never marketed, it survives as a historical and mechanistic reference compound within the neurosteroid anaesthetic class.

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