The progesterone metabolite isoallopregnanolone is a subunit-selective antagonist of the GABA-A receptor
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Allopregnanolone is a progesterone metabolite that can negatively affect learning and induce anaesthesia in rats. It also impairs episodic memory in women. Allopregnanolone levels are elevated during the luteal phase of the menstrual cycle, during pregnancy, and during stress. Allopregnanolone is a strong positive modulator of the GABAA receptor. The subunit composition of the GABAA receptor is of importance for effects of modulators, and GABAA receptors including the α5-subunit are of significance for learning, while receptors with other subunits are involved in e.g. anesthesia. Isoallopregnanolone, a natural 3β-epimer of allopregnanolone, has been shown to antagonize allopregnanolone-induced anesthesia in rats.
We tried to block the allopregnanolone-induced impairment of learning in rats in the Morris water maze test, using isoallopregnanolone (4–32 mg/kg). We also determined steroid concentrations in blood and brain tissue, and with whole-cell patch clamp we studied the effects of isoallopregnanolone and tetrahydrodeoxycorticosterone (a neurosteroid similar to allopregnanolone) on HEK-293 cells expressing the human α5β2γ2L GABAA receptor.
Isoallopregnanolone did not block the negative effects of allopregnanolone (2 mg/kg) in the Morris water maze test. Our presumed antagonist actually had an agonistic effect on the tetrahydrodeoxycorticosterone-mediated potentiation of the GABA effect on the α5β2γ2L GABAA receptor. The baseline shift induced by tetrahydrodeoxycorticosterone alone was not reversed by isoallopregnanolone. A bidirectional epimerisation between allopregnanolone and isoallopregnanolone was also identified in the rat.
The lack of antagonistic effect at the α5β2γ2L GABAA receptor together with the epimerisation of isoallopregnanolone to allopregnanolone would probably explain the lack of effect of our proposed antagonist on the allopregnanolone-induced impairment of learning
IdentifiersURN: urn:nbn:se:umu:diva-27069OAI: oai:DiVA.org:umu-27069DiVA: diva2:276032