The brain has both estrogen and progesterone receptors. In women who have epilepsy, seizures are known to occur more frequently during times of high estrogen (late follicular phase and ovulation) and they are decreased when progesterone is high. In this sense, progesterone acts a a brain anesthetic to some degree. High doses of progesterone can be very sedating.
Women who have depression, have lower brain levels of serotonin, thus the success of medications that block the body's degradation of serotonin and allow brain levels to remain higher. Estrogens are known to block one of the enzymes (monoamine oxidase - MAO) which degrades serotonin with the result of elevating mood. Progestogens, probably more so than natural progesterone, increase MAO concentration thus producing depression and irritability. Pure progestogen treatment without estrogen, such as DepoProvera® is know to worsen depression in women who already have a tendency toward or clinical signs of depression. The combination of estrogen plus progestogens such as used in birth control pills and menopausal hormonal replacement therapy does not tend to worsen mood because the compounds are neutralizing each other. There are some women who are more sensitive to certain hormones so their doses may need to be adjusted.
SOURCE OF QUOTE
P4 = progesterone
Here: metabolism of L-tryptophan to serotonin, melatonin, and niacin
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