Women with epilepsy have known for some time that female hormones affect seizures. Female sex hormones change the excitability of brain neurons by increasing excitation or inhibition. These hormones act on the cell membrane, changing the threshold for firing, change the rate at which neurons manufacture excitatory and inhibitory brain chemicals, and even change the shape of neurons, altering the way brain cells connect to one another. Steroid molecules: estrogen and progesterone easily pass through the cell membrane and are able to find receptor molecules within the cell. Progesterone can depress brain excitability, and therefore may reduce seizure activity. The other major female sex steroid hormone, estrogen, has an almost opposite effect from progesterone. The most direct, fastest, and obvious effect of estrogen is to increase the excitatory neurotransmitters in brain regions such as the hippocampus which are thought to be responsible for the generation of temporal lobe seizures.