Ovarian hormones and cortical excitability. An rTMS study in humans

Objective: Ovarian steroids influence neural excitability. Using repetitive transcranial magnetic stimulation (rTMS) we investigated changes in cortical excitability during the menstrual cycle. Methods: Eight women underwent rTMS on Days 1 and 14 of the menstrual cycle. As a control group, 8 age-matched men were also tested twice, with a 14-day interval between the two experimental sessions. Repetitive magnetic pulses were delivered in trains of 10 stimuli (5 Hz frequency and 120% of the motor threshold calculated at rest) to the left motor area of the first dorsal interosseous muscle. Results: In women, the motor evoked potential (MEP) size did not increase on Day 1, but it increased progressively during the train on Day 14. The duration of the silent period progressively lengthened during the train on both days. In men the MEP increased in size, and the silent period lengthened to a similar extent on both days. Conclusions: In women, hormone changes related to the menstrual cycle alter cortical excitability. Significance: Low estrogen levels probably reduce cortical excitability because their diminished action on sodium channels reduces recruitment of excitatory interneurons during rTMS thus abolishing the MEP facilitation.