Progesterone metabolites : learning, tolerance, antagonism & metabolism

Abstract: Progesterone metabolites as allopregnanolone, isoallopregnanolone and tetrahydrodeoxy-corticosterone (THDOC) are increased in the luteal phase of the menstrual cycle, throughout pregnancy and during stress. Allopregnanolone and THDOC are neurosteroids with 3α-hydroxy, 5α-configurations and positive modulating effect on the GABAA receptor. They have similar properties and effect, and share the same binding sites on the GABAA receptor. Isoallopregnanolone has a 3β-hydroxy, 5α-configuration and a diverse effect as a proposed antagonist to both allopregnanolone and THDOC. Neurosteroids are thought to exert their effect predominantly at extrasynaptic GABAA receptors, containing for example α4- or α5-subunits. Such receptors are involved in the tonic response. Different subunits have diverse distribution pattern in the brain and are involved in different functions. The α5-subunit, mainly expressed in the hippocampus, is involved in learning, while α4 is more widespread and involved in e.g. anxiety and anaesthesia.The aim of the present thesis was to contribute to the knowledge about selected progesterone metabolites and their effects on learning and tolerance development, as well as their metabolism. Also basic characteristics between different α-subunits of the GABAA receptor were evaluated.The thesis shows that the effect of bicuculline and pentobarbital is not dependent on the α-subunit isoform of the GABAA receptor expressed in oocytes. Acute tolerance developed after allopregnanolone-induced anaesthesia with a decrease at both mRNA and protein levels of the GABAA receptor α4-subunit in the thalamus VPM nucleus. A negative correlation between the α4 mRNA and the increased dose of allopregnanolone needed to maintain the anaesthesia level was also shown. In addition, allopregnanolone induces a learning impairment in the Morris water maze test, when high concentrations of allopregnanolone are present in the brain. This impairment is not possible to reverse by isoallopregnanolone. In α5β3γ2L-transfected HEK-293 cells THDOC induces a baseline shift of its own and also potentiate the GABA-current. Neither of those THDOC effects can be inhibited by isoallopregnanolone. Instead isoallopregnanolone shows an agonistic effect on the THDOC-potentiation of the GABA-response. The main allopregnanolone metabolites identified, 5α-DHP and isoallopregnanolone, as well as allopregnanolone itself are mainly localized to the brain after an i.v. injection. After an isoallopregnanolone injection there is a more even distribution of the given steroid and the metabolites between plasma and brain. There is an epimerisation between isoallopregnanolone and allopregnanolone and vice versa.In conclusion, the present thesis shows that the α4-subunit in the thalamus VPM nucleus is likely to be involved in the acute tolerance development against allopregnanolone and that allopregnanolone-induced learning impairment is likely to be hippocampus dependent. The lack of antagonistic effect of isoallopregnanolone on the THDOC-induced α5β3γ2L-GABAA response, together with epimerisation of isoallopregnanolone to allopregnanolone, could explain why isoallopregnanolone does not work as an antagonist to the allopregnanolone-induced learning impairment in a hippocampus dependent learning task.