The study of rats found that both males and females lose neurons in the ventral prefrontal cortex between adolescence and adulthood, with females losing about 13 percent more neurons in this brain region than males.
This is the first study to demonstrate that the number of neurons in the prefrontal cortex decreases during adolescence. It is also the first to document sex differences in the number of neurons in the PFC. The study appears in the Feb. 9 issue of the journal Neuroscience.
Earlier studies in humans have found gradual reductions in the volume of the prefrontal cortex from adolescence to adulthood, said psychology professor and principal investigator Janice M. Juraska. "But the finding that neurons are actually dying is completely new. This indicates that the brain reorganizes in a very fundamental way in adolescence."
Juraska, graduate student Julie Markham and undergraduate student John Morris found that the number of neurons decreased in the ventral, but not dorsal, prefrontal cortex during adolescence. The number of glial cells, which surround and support the neurons, remained stable in the ventral PFC and increased in the dorsal PFC.
These findings challenge current models of brain development by showing that some parts of the brain are still being organized well after puberty.
This could have implications for understanding human psychopathologies, such as schizophrenia, which often arise in late adolescence, Juraska said.
Other psychological conditions, such as depression, often first occur in adolescence. And alcohol and nicotine addictions that start in adolescence are harder to overcome than those that begin in adulthood, Juraska said.
"We know that experiences are very potent in younger children because their brains are developing," Juraska said. "So if there is another time that the brain is changing, then everything that happens can be written in and magnified more than during stable times."
The finding that females lose more neurons in the ventral PFC than males during adolescence also is new. Juraska had found earlier that adult female rats had fewer neurons than males in the visual cortex, a brain region associated with perception. But no other studies have looked for sex differences in the number of neurons in the prefrontal cortex.
It is unclear whether sex differences seen in the rat PFC also occur in humans, Juraska said. One contributing factor may be that female rats in the wild are almost always pregnant or nursing.
"The metabolic demands on female rats are so heavy that it might be worthwhile to do away with some very costly cortical cells," she said. "So this may just be a rodent phenomenon."
The loss of neurons is also a necessary part of brain development, she said.
"We always think that having more neurons is better, and it might not be," Juraska said. In some stages of early child development up to half of the neurons in some brain regions are lost. The pruning away of unneeded or disruptive neural circuits appears to be as important to development as the growing of new neural connections, Juraska said.
Although other researchers had seen reductions in the size of the cortex, "no one thought neurons were lost, unless some terrible thing were happening," Juraska said. "Now we are seeing that some major changes are occurring in adolescence that no one has suspected."