title: Antinociceptive profile of ring A-reduced progesterone metabolites in the formalin test.
abstract: The present study investigated the effect of acute systemic administration of six progesterone metabolites on formalin-induced pain in the rat. The 3alpha-hydroxylated metabolites allopregnanolone and pregnanolone are highly potent positive modulators at the GABA(A) receptor and produced a biphasic effect on pain in the formalin test. Dose-dependent antinociception was observed at lower doses (maximal antinociception at 0.16mg/kg) and was reversed at higher doses. Bicuculline abolished the antinociceptive effect. The 3beta-hydroxylated epipregnanolone and isopregnanolone are inactive or only weekly active at the GABA(A) receptor, and did not affect formalin-induced pain. 5alpha- and 5beta-dihydroprogesterone have also been shown to have low affinity for the GABA(A) receptor, but can be rapidly metabolized to their 3alpha-hydroxylated counterparts. In the formalin test, they produced a biphasic effect on pain similar to that of pregnanolone and allopregnanolone, but with lower potency. The effect was reversible by bicuculline, showing involvement of the GABA(A) receptor, and was blocked by indomethacin, implying that the antinociceptive effect is dependent on their conversion to allopregnanolone or pregnanolone. The results indicate that GABA-ergic progesterone metabolites modulate nociception. A change in levels of GABA-ergic progesterone metabolites, such as is observed in depression, chronic fatigue and premenstrual dysphoric disorder could, therefore, contribute to the pain complaints associated with these disorders.
authors.affiliation: Department of Experimental Medicine, McGill University, Lady Meredith House, 1110 Pine Avenue West, Montreal, Que., Canada H3A 1A3 Department of Medicine, Montreal General Hospital, 1650 Cedar Avenue, R. C6260, Montreal, Que., Canada H3G 1A4 Department of Psychiatry, McGill University, 1033 Pine Avenue West, Montreal, Que., Canada H3A 1A1 Department of Psychology, McGill University, 1205 Dr. Penfield Avenue, Montreal, Que., Canada H3A 1B1.
authors.forename: Rok
authors.initials: R
authors.lastname: Ocvirk
authors.forename: Beverley E
authors.initials: BE
authors.lastname: Pearson Murphy
authors.forename: Keith B J
authors.initials: KBJ
authors.lastname: Franklin
authors.forename: Frances V
authors.initials: FV
authors.lastname: Abbott
mesh_terms: D000700:Analgesics / Q000378:metabolism* / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004394:Dydrogesterone / Q000378:metabolism / Q000494:pharmacology; D008297:Male; D010147:Pain Measurement / Q000187:drug effects* / Q000379:methods; D011280:Pregnanolone / Q000378:metabolism / Q000494:pharmacology; D011374:Progesterone / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology*; D051381:Rats; D020318:Rats, Long-Evans
chemical_list: D000700:Analgesics; D011374:Progesterone; D004394:Dydrogesterone; D011280:Pregnanolone