Pregnenolone (P5), or pregn-5-en-3β-ol-20-one, is an endogenous steroid and precursor/metabolic intermediate in the biosynthesis of most of the steroid hormones, including the progestogens, androgens, estrogens, glucocorticoids, and mineralocorticoids. In addition, pregnenolone is biologically active in its own right, acting as a neurosteroid.
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Pregnenolone and its 3β-sulfate, pregnenolone sulfate, like DHEA, DHEA sulfate, and progesterone, belong to the group of neurosteroids that are found in high concentrations in certain areas of the brain, and are synthesized there. Neurosteroids affect synaptic functioning, are neuroprotective, and enhance myelinization. Pregnenolone and its sulfate ester are under investigation for their potential to improve cognitive and memory functioning. Pregnenolone is also being considered as a potential treatment for schizophrenia.
Although pregnenolone itself does not possess these activities, its metabolite pregnenolone sulfate is a negative allosteric modulator of the GABAA receptor as well as a positive allosteric modulator of the NMDA receptor. In addition, pregnenolone sulfate has been shown to activate the transient receptor potential M3 (TRPM3) ion channel in hepatocytes and pancreatic islets causing calcium entry and subsequent insulin release.
Pregnenolone is an allosteric endocannabinoid, as it is a negative allosteric modulator of the CB1 receptor. Pregnenolone is involved in a natural negative feedback loop against CB1 receptor activation in animals. It prevents CB1 receptor agonists like tetrahydrocannabinol, the main active constituent in cannabis, from fully activating the CB1 
Microtubule-associated protein 2Edit
Pregnenolone has been found to bind with high, nanomolar affinity to microtubule-associated protein 2 (MAP2) in the brain. In contrast to pregnenolone, pregnenolone sulfate did not bind to microtubules. However, progesterone did and with similar affinity to pregnenolone, although unlike pregnenolone, it did not increase binding of MAP2 to tubulin. Pregnenolone was found to induce tubule polymerization in neuronal cultures and to increase neurite growth in PC12 cells treated with nerve growth factor. As such, pregnenolone may control formation and stabilization of microtubules in neurons and may affect both neural development during prenatal development and neural plasticity during aging. The 3β-methyl ether of pregnenolone, 3β-methoxypregnenolone (MAP-4343), retains similar activity to pregnenolone in regards to interaction with MAP2, and is under development for potential clinical use for indications such as the treatment of brain and spinal cord injury and depressive disorders.
Pregnenolone is synthesized from cholesterol. This conversion involves hydroxylation of the side chain at the C20 and C22 positions, with cleavage of the side chain. The enzyme performing this task is cytochrome P450scc, located in the mitochondria, and controlled by anterior pituitary trophic hormones, such as adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormone, in the adrenal glands and gonads. There are two intermediates in the transformation of cholesterol into pregnenolone, 22R-hydroxycholesterol and 20α,22R-dihydroxycholesterol, and all three steps in the transformation are catalyzed by P450scc.
Pregnenolone undergoes further steroid metabolism in one of several ways:
- Pregnenolone can be converted into progesterone. The critical enzyme step is two-fold using a 3β-hydroxysteroid dehydrogenase and a Δ5-4 isomerase. The latter transfers the double bond from C5 to C4 on the A ring. Progesterone is the entry into the Δ4 pathway, resulting in production of 17α-hydroxyprogesterone and androstenedione, precursor to testosterone and estrone. Aldosterone and corticosteroids are also derived from progesterone or its derivatives.
- Pregnenolone can be converted to 17α-hydroxypregnenolone by the enzyme 17α-hydroxylase (CYP17A1). Using this pathway, termed Δ5 pathway, the next step is conversion to dehydroepiandrosterone (DHEA) via 17,20-lyase (CYP17A1). DHEA is the precursor of androstenedione.
- Pregnenolone can be converted to androstadienol by 16-ene synthase (CYP17A1).
- Pregnenolone can be converted to pregnenolone sulfate by steroid sulfotransferase, and this conversion can be reversed by steroid sulfatase.
Oral administration of 50 or 100 mg pregnenolone has been found to have minimal or negligible effect on urinary levels of testosterone and testosterone metabolites, including of androsterone, etiocholanolone, 5β-androstanediol, androstadienol, and androstenol (and/or their conjugates), and this suggests that only a small amount of pregnenolone is converted into testosterone. This is in accordance with findings on the conversion of DHEA into testosterone, in which only 1.5% of an oral dose of DHEA was found to be converted into testosterone. In contrast to the androstanes, 50 or 100 mg oral pregnenolone has been found to significantly and in fact "strongly" increase urinary levels of the progesterone metabolites pregnanediol and pregnanolone (and/or their conjugates), whereas pregnanetriol was unaffected. Unlike the case of oral administration, transdermal administration of 30 mg/day pregnenolone cream has not been found to affect urinary levels of metabolites of any other steroids, including of progesterone.
Sripada et al. reported that oral pregnenolone is preferentially metabolized into the neurosteroid allopregnanolone rather than into other steroids such as DHEA or cortisol. In further research by their group, a single 400 mg dose of oral pregnenolone at 3 hours post-administration was found to result in a 3-fold elevation in serum levels of pregnenolone and a 7-fold increase in allopregnanolone levels. Pregnanolone levels increased by approximately 60% while DHEA levels decreased non-significantly by approximately 5% and cortisol levels were not affected. Another study found that allopregnanolone levels were increased by 3-fold at 2 hours post-administration following a single 400 mg oral dose of pregnenolone.
Pregnenolone is also known chemically as pregn-5-en-3β-ol-20-one. Like other steroids, it consists of four interconnected cyclic hydrocarbons. The compound contains ketone and hydroxyl functional groups, two methyl branches, and a double bond at C5, in the B cyclic hydrocarbon ring. Like many steroid hormones, it is hydrophobic. The sulfated derivative, pregnenolone sulfate, is water-soluble.
A few synthetic ester derivatives of pregnenolone exist. These include pregnenolone acetate (Antofin, Previsone, Pregno-Pan) and pregnenolone succinate (Panzalone, Formula 405). Prebediolone acetate (Acetoxanon, Acetoxy-Prenolon, Artisone, Artivis, Pregnartrone, Sterosone), the 21-acetate ester of 21-hydroxypregnenolone, also exists. These esters are all described as glucocorticoids similarly to pregnenolone.
Society and cultureEdit
Pregnenolone is or has been marketed as a pharmaceutical drug in some countries. Brand names of pregnenolone have included Arthenolone, Bina-Skin, Enelone, Natolone, Pregnetan, Pregneton, Pregnolon, Prenolon, Regnosone, Sharmone, and Skinostelon. It is described in the context of medical use as an "adrenal cortex hormone" or a glucocorticoid for the treatment of rheumatoid arthritis.
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