A synthetic form of the hormone progesterone increases aggression and anxiety in female monkeys, according to research conducted at the Yerkes National Primate Research Center.
Medroxyprogesterone acetate (MPA), used in contraceptives and hormone replacement therapy, has been reported anecdotally to cause depression, mood changes, and loss of libido in some women. To determine if this finding could be substantiated in a research setting, Karen Pazol and colleagues tested six female pigtail macaques under three conditions, each lasting one week: estrogen only, estrogen plus natural progesterone, and estrogen plus MPA.
The researchers report that the monkeys exhibited significantly more aggressive and anxious behavior when taking estrogen plus MPA than they did in either of the other test conditions. They also were much less interested in sexual activity.
Pazol says, "In comparison to natural progesterone, MPA binds to glucocorticoid receptors with a much higher affinity and may have a greater impact on the brain's stress system." Also, unlike natural progesterone, MPA cannot be converted to allopregnanolone. Abnormal levels of allopregnanolone are tentatively linked to depression, anxiety disorders, and premenstrual mood disorders.
The researchers say their findings suggest that "production of negative affect may be a particularly serious side.
"Medroxyprogesterone acetate antagonizes the effects of estrogen treatment on social and sexual behavior in female macaques," Karen Pazol, Mark E. Wilson and Kim Wallen, Journal of Clinical Endocrinology and Metabolism, Vol. 89, No. 6, June 2004, 2998-3006. Address: Karen Pazol, Yerkes National Primate Research Center, Emory University, 954 Gatewood Dr., Atlanta, GA 30322.
"Synthetic hormone used in contraceptives and HRT produces negative effects in monkey studies," news release, Emory University Health Sciences Center, June 7, 2004.