UC Davis School of Medicine Study of Teen Sleep Shows Link Between Timing of Brain and Sexual Maturation

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Outcome provides important benchmark for evaluating adolescent-onset mental illnesses

In the largest longitudinal study to date of electrical activity in the brains of sleeping adolescents, UC Davis scientists have discovered that the timing of brain maturation is associated with the timing of puberty.

Published online in the Proceedings of the National Academy of Sciences, the study used electroencephalogram (EEG) measures of the brain together with physical indicators of puberty to show a link in the development of the systems that control behavior and reproduction. The authors believe the outcome could lead to studies that reveal markers of brain abnormalities that contribute to mental illness -- such as schizophrenia and bipolar disorder -- that first become apparent during adolescence.

"A connection between brain and body maturity has long been suspected, but demonstrating its existence has been difficult," said Ian Campbell, project scientist in psychiatry and behavioral sciences at UC Davis and senior author of the study. "Because we were able to track both processes within a large group of teenagers over the course of the crucial years of adolescence, we could demonstrate this association. Exploring the mechanisms that underlie this relationship could ultimately help us better understand the mental-health issues of teenagers."

In previous research, Campbell and colleague Irwin Feinberg, professor-in-residence in psychiatry and behavioral sciences at UC Davis and lead author of the current study, showed that EEG activity declined sharply -- by more than 60 percent -- during adolescence. They attributed this decline to a reduction of neuronal activity due to synaptic pruning as the brain advances toward adulthood. Their new study shows that this process is time-linked to the progression of puberty. Boys and girls in the study who went through puberty earlier also showed an earlier decline in EEG activity.

"The central nervous system is very clever because it produces many more neurons than are necessary early in life, giving the younger brain the backup neurons it needs to reorganize and compensate for brain functions lost following an injury," said Feinberg, author of an earlier study on adolescent-brain reorganization. "But while the adult brain loses that reconstructive power, it instead has the more efficient number of neurons necessary to streamline executive brain functions, making adult cognitive functions possible."

For the current study, the team evaluated 67 healthy adolescents -- 34 girls and 33 boys -- with twice-yearly nighttime EEGs, which record fluctuations in electrical activity within the brain's neurons. One group of participants began the study at age 9 and was evaluated through age 16, and another group entered the study at age 12 and was evaluated through age 18. Within a month of each child's EEG, his or her stage of puberty was determined by a physician using the Tanner scale of breast development in girls and genital development in both boys and girls.

The results showed that EEG fluctuations during nonrapid eye movement sleep -- when the brain is most recuperative -- declined for all participants, with the most rapid decline between the ages of 12 and 13 for girls and between the ages of 13 and 14 for boys. When compared to the Tanner scale results, there was a robust relationship between the timing of this brain change and pubertal development.

Campbell and Feinberg hope their finding inspires research to determine if neuroendocrine events of puberty drive adolescent brain maturation or if these are independent but parallel processes.

"Now that we know the processes are linked in time, we need to determine how," said Campbell. "There could be one physiologic mechanism of control, or they could just simply have common timing with separate triggers. There is still much more that we need to know about the brain during this critical stage of human development."

In addition to Campbell and Feinberg, study co-authors were Kevin Grimm and Evan de Bie of UC Davis. Their study -- "Sex, Puberty and the Timing of Sleep EEG Measured Adolescent Brain Maturation" -- was funded by the U.S. Public Health Service. A copy can be requested by e-mailing pnasnews(at)nas(dot)edu.

The UC Davis School of Medicine is among the nation's leading medical schools, recognized for its research and primary-care programs. The school offers fully accredited master's degree programs in public health and in informatics, and its combined M.D.-Ph.D. program is training the next generation of physician-scientists to conduct high-impact research and translate discoveries into better clinical care. Along with being a recognized leader in medical research, the school is committed to serving underserved communities and advancing rural health. For more information, visit medschool.ucdavis.edu.

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