Neuroactive steroids and rat CNS

Sammanfattning: Several studies suggest profound effects on mood and cognition by neuroactive steroids. Estrogen alone or in combination with antidepressant drugs affecting the serotonin system has been used to treat mood disorders. On the other hand, progesterone is related to negative effects on mood and memory. A major part of the progesterone effects on the brain can be mediated by its metabolite allopregnanolone, which is also de novo synthesized in the brain, and affects the GABAA receptors. It would be of great importance to find a substance that antagonize allopregnanolone adverse effects. To investigate how long term supplementation of estradiol and progesterone, resembling postmenopausal hormone replacement therapy, affects serotonin receptors in different brain areas important for mood and memory functions, we used ovariectomized female rats. After 2 weeks of supplementation with 17β-estradiol alone or in combination with progesterone, or placebo pellets, estradiol alone decreases but estradiol supplemented together with progesterone increases 5HT1A mRNA expression in the hippocampus. Estradiol decreases the 5HT2C receptor gene expression, while estradiol in combination with progesterone increases the 5HT2A mRNA expression in the ventral hippocampus. Thus, estradiol alone has opposite effects compared to the estradiol/progesterone combination. To detect if acute tolerance develops to allopregnanolone, an EEG method was used where male rats by continuous allopregnanolone infusion were kept on anesthesia level of the silent second (SS). After different time intervals (first SS, 30 min or 90 min of anesthesia) several GABAA receptor subunit mRNAs were measured for detecting if changed expression of any GABAA receptor subunits is involved in development of acute tolerance. There is development of acute tolerance to allopregnanolone and brain regions of importance are hippocampus, thalamus and hypothalamus. The GABAA receptor alpha4 subunit in thalamus and alpha2 subunit in the dorsal hippocampus are related to development of acute tolerance. For assessing allopregnanolone behavioral effects, we studied how this neurosteroid affects spatial learning in the Morris water maze task Allopregnanolone inhibits spatial learning short after the injection and shows a specific behavioral pattern with swimming close to the pool wall. The steroid UC1011 can inhibit the increase in chloride ion uptake induced by allopregnanolone. UC1011 decreases allopregnanoloneinduced impairment of spatial learning in the water maze, as well as the specific behavioral swim pattern. In conclusion, the present work demonstrates that neuroactive steroids affect the 5HT and GABA systems in a brain region specific way. GABAA receptor subunit changes in hippocampus and thalamus are related to acute allopregnanolone tolerance. Allopregnanolone induces cognitive deficits, like spatial learning impairment and UC1011 can inhibit allopregnanolone-induced effects in vitro and in vivo. Key words: Estradiol, progesterone, HRT, allopregnanolone, UC1011, serotonin receptor, GABAA receptor, mRNA, Morris water maze, silent second, tolerance.