Estrogens have long been implicated in influencing cognitive processes, yet the molecular mechanisms underlying these effects and the roles of the estrogen receptors alpha (ERalpha) and beta (ERbeta) remain unclear. Using pharmacological, biochemical and behavioral techniques, we demonstrate that the effects of estrogen on hippocampal synaptic plasticity and memory are mediated through ERbeta. Selective ERbeta agonists increased key synaptic proteins in vivo, including PSD-95, synaptophysin and the AMPA-receptor subunit GluR1. These effects were absent in ERbeta knockout mice. In hippocampal slices, ERbeta activation enhanced long-term potentiation, an effect that was absent in slices from ERbeta knockout mice. ERbeta activation induced morphological changes in hippocampal neurons in vivo, including increased dendritic branching and increased density of mushroom-type spines. An ERbeta agonist, but not an ERalpha agonist, also improved performance in hippocampus-dependent memory tasks. Our data suggest that activation of ERbeta can regulate hippocampal synaptic plasticity and improve hippocampus-dependent cognition.

In the present study, we have characterized properties of steroid withdrawal using a pseudopregnant rat model. This paradigm results in increased production of endogenous progesterone from ovarian sources and as such is a useful physiological model. "Withdrawal" from progesterone induced by ovariectomy on day 12 of pseudopregnancy resulted in increased anxiety, as determined by a decrease in open arm entries on the elevated plus maze compared to control rats and pseudopregnant animals not undergoing withdrawal. Similar findings were obtained 24 hr after administration of a 5␣-reductase blocker to a pseudopregnant animal, suggesting that it is the GABA Amodulatory 3␣-OH-5␣-pregnan-20-one (3␣,5␣-THP) that produces anxiogenic withdrawal symptoms. Twenty-four hours after steroid withdrawal, the time constant for decay of GABA Agated current was also reduced sixfold, assessed using wholecell patch-clamp procedures on pyramidal neurons acutely dissociated from CA1 hippocampus. Thus, 3␣,5␣-THP withdrawal results in a marked decrease in total GABA A current, a possible mechanism for its anxiogenic, proconvulsant sequelae. In addition, 3␣,5␣-THP withdrawal resulted in insensitivity to the normally potentiating effect of the benzodiazepine lorazepam (LZM) on GABA A -gated Cl Ϫ current. This withdrawal profile is similar to that reported for other GABA A -modulatory drugs such as the benzodiazepines (BDZs), barbiturates, and ethanol. These changes were also associated with significant two and threefold increases in both the mRNA and protein for the ␣4 subunit of the GABA A receptor, respectively, in hippocampus. The pseudopregnancy paradigm may be a useful model for periods of endogenous 3␣,5␣-THP withdrawal such as premenstrual syndrome and postpartum or postmenopausal dysphoria, when increased emotional lability and BDZ insensitivity have been reported.

No abstract

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)

No abstract