 A French-American team from Universit� Lyon 1 and National Institute of Mental Health (Bethesda, Maryland) has detected for the first time in humans - employing functional Magnetic Resonance Imaging (fMRI) - the neural circuits from the brain involved in the moods experimented by the women due to variation in the female sex hormones during the menstrual cycle. The human brain is conditioned by reward (food, money, drugs�) to be motivated for cognitive actions like learning. The brain possesses a reward system, based on dopaminergic neurons1 located in the mesencephalon and their projection sites2: the ventral striatum, the anterior cingulate cortex, and the orbitofrontal cortex. The natural neurotransmitter/neuromodulator is dopamine. Impairments in the reward system result in addictions and various psychiatric and neurological pathologies, like schizophrenic disorders. Researches made on animals proved that the dopaminergic3 system is sensitive to female sex hormones (estrogen, progesterone). Female rats self-administered higher doses of cocaine (which stimulates the dopamine system) after injections with estrogens. Women's response to cocaine is higher in the follicular phase of the menstrual cycle (the first part of the menstrual cycle starting from the first day of the period) than in the luteal phase (the second pat of the menstrual cycle that begins after ovulation and ends on the last day of the period), but schizophrenia seems to install later in women than in men. An increase in brain activity with anticipation of uncertain monetary rewards is observed during the follicular phase in the amygdala and the orbitofrontal cortex. To see how hormones affect the reward system neuron network, the research team employed fMRI, investigating brain activity in a group of volunteers twice during their menstrual cycle. The women played at virtual slot machines showing different probabilities of winning when they passed through fMRI. The reward games activated specific brain regions, particularly the amygdala and the orbitofrontal cortex, to a greater extent during the follicular phase (4 to 8 days after the start of the period) than during the luteal phase (6 to 10 after the ovulation). These brain recompense regions were more activated during the follicular phase, when the estrogens do not oppose the progesterone. When the same experiment was made on men, the reward-related region proved to be the ventral striatum (which in women is more linked to emotions) and the amygdalo-hippocampal region. The activation of this brain region goes beyond the craving for money; it implies higher receptiveness and desire, supposed to facilitate procreation during the ovulation. These results explain the role played by sex hormones on reward, motivation and learning behaviors but also impairments observed in schizophrenia, normal aging and drug and gambling addictions. |
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