| Paper authors: | Maurizio Inghilleri, Antonella Conte, Antonio Currá, Vittorio Frasca, Cinzia Lorenzano, Alfredo N. Berardelli |
| Year of paper publication: | 2004 |
| Post authors: | Caleb Pozdnikoff, Lisa Ridgway, Fidel Vila-Rodriguez |
| Check out the research article: | Inghilleri et al. (2004) Ovarian hormones and cortical excitability. An rTMS study in humans |
Introduction
Researchers have previously found that sex hormones produced by the ovaries can influence how easily individual brain cells can be excited. Repetitive transcranial magnetic stimulation (rTMS) functions by exciting brain cells. Does this mean researchers need to account for fluctuating sex hormone levels when people are undergoing rTMS treatments?
This study aimed to investigate the changes in how excitable the brain is at different points in the menstrual cycle, when hormones are fluctuating. The researchers examined 2 main hormones:
- Progesterone: thought to make the brain less excitable (harder to activate).
- Estrogens (i.e. estradiol): thought to make the brain more excitable (easier to activate).
The researchers used rTMS to measure how excitable the brain was. They measured brain excitability by stimulating a part of the brain called the motor cortex. The part of the motor cortex they stimulated controls muscle movements in the hand. So, researchers measured the amount of electrical activity in the hand after they stimulated the motor cortex using rTMS. Higher electrical activity meant that the brain was more excitable, and lower activity meant the brain was less excitable.
The researchers thought if the brain was stimulated when estrogen was high and progesterone was low, the brain should be more excitable, leading to a progressive increase in electrical activity in the hand. The two measurements here are called:
- Motor evoked potentials (MEPs): the amount of electrical activity in the hand.
- Silent period (SP): the amount of time that electrical activity is suppressed after a hand contraction occurs.
The goal of this study was to investigate the changes in excitability of the brain during the menstrual cycle in females using rTMS.
Methods
This study was done in 16 healthy volunteers. Among these volunteers were 8 females with consistent menstrual cycles and 8 males included for comparison. The female participants had rTMS performed on days 1 and 14 of their menstrual cycle. The male participants were also tested twice, 14 days apart.
The two phases in the menstrual cycle that researchers examined are called the early follicular phase and the late follicular (ovulatory) phase. The early follicular phase (day 1) is associated with low estrogens and low progesterone levels. The late follicular phase (day 14) is associated with high estrogens and low progesterone levels.
Researchers measured motor evoked potentials, which is the electrical activity detected in the hand muscle. They also measured the silent period duration, which measures the suppression of electrical activity that happens after stimulation. Levels of estrogens and progesterone were also tested in most of the females before each session.
Results
- High estrogen levels (day 14) promote the progressive increase of brain excitability during repetitive stimulation.
- This effect is linked to brain cells branching out and making new connections with each other, and this is absent when estrogen levels are low (Day 1).
- The lack of excitability on Day 1 suggests that low estrogen hinders the brain’s ability to strengthen electrical signals.
- Researchers believe this happens because less estrogen is acting on the brain cells’ electrical gateways and prevents the required firing of excitatory brain cells.
- The brain’s inhibitory circuits, measured by the silent period, were unaffected by hormonal changes.
Conclusion
This study shows that hormone changes seem to alter cortical excitability. More specifically, when estrogen levels are high, a progressive increase in brain excitability can occur. This suggests that females enrolled in repeated session rTMS treatments or research should be tested during the same phase of their menstrual cycle to ensure consistent and reliable results.