Presynaptic inhibition decreases when estrogen level rises

The objective was to determine estrogen's influence on control of a skeletal muscle through measurements of motorneuron excitability (H:M ratio) and presynaptic inhibition (PI). Estrogen serum concentrations were measured at menses and ovulation of female subjects and compared to male controls. Data were analyzed from 12 women and 13 men reporting no history of knee ligament injury. Women reported regular menstrual cycles and no hormone-based contraceptive use for the previous year. Women were tested at menses (Time1) and ovulation (Time2). Men were tested twice, approximately 14 days apart. Analysis indicated no difference in the H:M ratio between the sexes at either time point. A significant difference for the sexes was detected in the magnitude of estrogen change (∆EST) between observations. At Time1, the male and female estrogen concentrations were not different; however, they were different at Time2, primarily due to the large rise observed in the women. A significant difference between the sexes was also seen in the magnitude of change for PI (∆PI) between observations. As with EST, the levels of PI between the sexes at Time1 were not different; however, a difference existed at Time 2. Estrogen interacts with GABA at several nervous system locations affecting inhibition of synaptic transmission. This is the first study to investigate changes in PI of a skeletal muscle between times of low and high estrogen. Improving the understanding of estrogen's influence on skeletal muscles may provide answers to why noncontact anterior cruciate ligament injuries of the knee occur more frequently in women.