Abstract We propose to test the hypothesis that progesterone receptor (PR) activation enhances the AMPA receptor (AMPAR)-mediated synaptic transmission of hippocampal principal neurons and increases seizure susceptibility. Cyclic fluctuations in progesterone levels in women of reproductive age with epilepsy are linked to catamenial seizure exacerbation. Furthermore, perimenstrual progesterone withdrawal is linked to migraines and dysphoria. Whether PR activation regulates these disorders is not known. Progesterone is historically thought to exert anticonvulsant effects via the potentiation of GABA-A receptor- mediated inhibition. In contrast, the role of PRs, which are also expressed in the brain, in regulating neuronal activity is poorly understood outside of the hypothalamic-pituitary axis. Our preliminary studies show that PR activation increases AMPAR expression in the hippocampus and enhances the AMPAR-mediated synaptic transmission of CA1 pyramidal neurons. These changes were associated with the increased activity of CA1 neurons. Furthermore, the blockade of PR by the anti-progestin RU-486 or by the genetic deletion of PRs prevented the observed upregulation of AMPARs. Finally, the specific activation of PRs with a synthetic agonist, nestorone, increased seizure susceptibility, while RU-486 treatment suppressed neurosteroid withdrawal-induced seizures in an experimental animal model. We propose to directly test the role of PRs in regulating AMPAR expression and the excitability of hippocampal principal neurons and to assess whether PRs regulate seizure susceptibility. In the first aim, we will activate PRs using nestorone in wild-type and knockout animals and determine its effect on the neuronal excitability and AMPAR-mediated synaptic transmission of CA1 pyramidal neurons and dentate granule cells, and on the expression of GluA1 and GluA2 subunit proteins and mRNA. We will use western blotting, qRT-PCR, and electrophysiological techniques in these studies. In the second aim, we will determine whether PR expression in CA1 neurons and DGCs changes during the estrous cycle, using fluorescent in situ hybridization assay, qRT-PCR and a mouse expressing myc-tagged PR. In the third aim, we will test whether PR activation reduces the seizure threshold and alters the susceptibility to status epilepticus and epileptogenesis by using continuous video-EEG recording.