anesthesia: 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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Anesthetic

Alphadolone

Alphadolone (alfadolone; clinically alphadolone acetate) is a synthetic neuroactive pregnane steroid (a laboratory-made steroid that acts on the nervous system) which potentiates the GABA-A receptor, the brain's principal fast inhibitory chloride channel, to produce sedation and anaesthesia. It is best known as the minor component of the intravenous anaesthetic Althesin (for humans) and Saffan (for animals), where it was combined with the more potent alfaxalone in a 3:1 ratio, added chiefly to improve the poor water solubility of alfaxalone while still contributing roughly half of that agent's anaesthetic potency in its own right. Both preparations were dissolved in the surfactant Cremophor EL, whose tendency to trigger histamine release and anaphylactoid reactions led to the withdrawal of Althesin from human use in 1984. Alphadolone later attracted independent research interest because, unlike alfaxalone, it produces spinally mediated analgesia without sedation when given by mouth or intraperitoneally, an effect thought to depend on an analgesic metabolite formed in the liver.

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Anesthetic

Hydroxydione

Hydroxydione is a synthetic pregnane neuroactive steroid that, introduced by Pfizer in 1955 under the trade name Viadril (also Presuren), became the first steroid used as an intravenous general anesthetic. The parent steroid, 21-hydroxy-5-beta-pregnane-3,20-dione, is almost insoluble in water; esterifying its 21-hydroxyl group with succinic acid and forming the sodium salt yields hydroxydione sodium succinate, a water-soluble prodrug that plasma esterases (enzymes that cleave ester bonds) hydrolyze in vivo to release the active steroid. Like later neurosteroid anesthetics such as alfaxalone, it produces hypnosis by acting as a positive allosteric modulator of the GABA-A receptor (the brain's principal inhibitory chloride ion channel), without meaningful analgesic or classical hormonal activity. Although valued for a wide margin of cardiovascular and respiratory safety, its very slow onset and a high incidence of injection-site thrombophlebitis (painful inflammation and clotting of the vein) led to its abandonment, yet it remains historically pivotal as the origin of the entire field of steroid anesthesia.

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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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