It is known that women suffer mood changes along the menstrual cycle, driven by hormonal changes.
But what's the mechanism that makes hormones act like that?
A team at the National Institute of Mental Health (NIMH), a component of the National Institutes of Health (NIH), showed that changes in sex hormone levels during women's menstrual cycle affect the sensitivity of their brain's reward circuitry. This circuitry proved more active if they were in a menstrual phase before ovulation and dominated by estrogen, compared to a phase when both estrogen and progesterone are present.
"These first pictures of sex hormones influencing reward-evoked brain activity in humans may provide insights into menstrual-related mood disorders, women's higher rates of mood and anxiety disorders, and their later onset and less severe course in schizophrenia," said Karen Berman, chief of the NIMH Section on Integrative Neuroimaging. "The study may also shed light on why women are more vulnerable to addictive drugs during the pre-ovulation phase of the cycle."
The reward system circuitry is made of the prefrontal cortex (the center of thinking and planning); the amygdala, (the emotional center); the hippocampus (a brain center involved in learning and emotions); and the striatum (a brain zone related to learned emotions), all of which possess receptors for estrogen and progesterone.
To solve the mystery of how hormones act in the reward circuit, the team scanned - using functional magnetic resonance imaging (fMRI) - the brain activity of 13 women and 13 men while they accomplished a task requiring simulated slot machines.
The women were investigated before and after ovulation. The scanned images revealed that when women were forecasting a reward, the amygdala and prefrontal cortex, linked to emotion and reward-related planning behavior, were turned on for longer periods during the pre-ovulation phase (four to eight days after their period began) than in the post-ovulatory phase. When women really won a reward, those in the pre-ovulatory stage turned on the striatum and pleasure-reward related area for longer periods than in the post-ovulatory stage.
The amygdala and hippocampus appeared equally sensitive for estrogen no matter the level in which the cycle period was; these two zones were also activated by progesterone during the post-ovulatory phase.
The researchers believe that the estrogen's action during the post-ovulatory period is impaired by progesterone.
Male's pattern for anticipation and delivery of rewards was very different: the striatum was more activated in men during anticipation compared to women, while their frontal cortex was less excited at reward delivery also by comparison to that of women.