Anxiolytic effect of progesterone is mediated by the neurosteroid allopregnanolone at brain GABAA receptors.

Previous studies from this laboratory have shown that progesterone (PROG) treatment in ovariectomized rats produces an anti-anxiety response similar to that observed after the administration of prototypical anxiolytic benzodiazepine (BDZ) compounds. The PROG-induced anxiolytic response was highly correlated with an increased level of 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone) in the blood and brain, and was also associated with a facilitation of GABA-stimulated chloride ion (Cl-) influx in cortical synaptoneurosomes. This correlative evidence suggested that the anxiolytic effect of PROG was a result of its in vivo reduction to the neuroactive steroid, allopregnanolone. In this report, a series of studies was conducted to determine the mechanism(s) by which PROG alters behavior in animal models of anxiety. In the first experiment, ovariectomized rats were injected with PROG (1 mg/0.2 ml, SC) 4 h prior to a test in the elevated plus-maze. Some animals also received an injection of picrotoxin (0.75 mg/kg, IP), a GABAA receptor-gated Cl- channel antagonist, whereas other animals were pretreated with RU 38486 (5 mg/0.2 ml, SC), a progestin receptor antagonist. PROG elicited anxiolytic behavior in the plus-maze, an effect that was blocked by picrotoxin administration. Pretreatment with RU 38486 was not effective in altering PROG-induced anxiolytic behavior in the plus-maze. In a second experiment, the effect of PROG on behavior in the plus-maze was determined in the presence of N,N-diethyl-4-methyl-3-oxo-4-aza-5 alpha-androstane-17 beta-carboxamide (4-MA; 10 mg/0.2 ml, SC), a 5 alpha-reductase inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)