Presynaptic inhibition decreases when estrogen level rises

Abstract

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.

Sex differences in soleus muscle H‐reflex and V‐wave excitability

“…Alterations in the levels of neuroactive sex hormones, such as oestradiol and progesterone, might also provide a partial explanation to such discrepant findings. However, with the exception of one experimental design in which the female H max /M max ratio was shown to be higher in conditions compatible with similar levels of oestrogen and progesterone between sexes (Hoffman et al., 2017), no other studies were able to replicate these findings (Hoffman et al., 2008, 2018). In fact, in one of these reports it was even shown that the average correlation between hormones and the H max /M max ratio was close to zero, and that this occurred similarly in both men and women (Hoffman et al., 2008).…”

“…Third, we did not control for the effects of the menstrual cycle on H‐reflex excitability. Even though there is some evidence that acute fluctuations of endogenous oestradiol and progesterone do not modulate H‐reflex excitability (Casey et al., 2016), it has also been shown that changes in oestrogen during the menstrual cycle interact with presynaptic inhibition (Hoffman et al., 2018). Presynaptic inhibition exerts a strong influence on H‐reflex excitability (Knikou, 2008).…”

“…findings (e.g. levels of physical activity of the participants were not controlled, nerve stimulation was frequently elicited at rest and no data were reported on the magnitude of background EMG, an index of a subliminal fringe of the MN pool) (Hoffman et al, 2008(Hoffman et al, , 2017(Hoffman et al, , 2018Johnson et al, 2012).…”

“…However, based on the current literature, it is difficult draw definite conclusions because all previous reports have important constraints that limit the validity of their observations and may even explain the inconsistent findings (e.g. levels of physical activity of the participants were not controlled, nerve stimulation was frequently elicited at rest and no data were reported on the magnitude of background EMG, an index of a subliminal fringe of the MN pool) (Hoffman et al., 2008, 2017, 2018; Johnson et al., 2012).…”

“…Other experimental designs focused primarily on H‐reflex responses, and their findings were contradictory. For instance, while two studies reported no sex differences for the H max /M max ratio (Hoffman, Doeringer, Norcross, Johnson, & Chappell, 2018; Hoffman, Harter, Hayes, Wojtys, & Murtaugh, 2008), one observed greater H‐reflex excitability in women (Hoffman, Norcross, & Johnson, 2017). Although the exact mechanisms remain unknown, sex‐related differences in brain maturation (De Bellis et al., 2001; Lenroot et al., 2007), as well as the differential activation of cortical areas during motor tasks (Lenroot et al., 2007; Lissek et al., 2007) might contribute to discrepancies in neural activation, thus affecting V‐wave amplitude and motor performance.…”