Puberty is often accompanied by making dumb decisions. Yet, despite its reputation, Michael Brecht and colleagues from Humboldt University of Berlin, Germany, recently discovered that the awkward period of adolescence coincided with parts of the brain getting bigger in rats (doi:10.1016/j.cub.2015.11.041). Appropriately enough, the region that expands during puberty is called the ‘genital cortex’, a part of the brain that's active when its namesake region is aroused. The scientists made this discovery by comparing snapshots of the brain taken from different rats before, during and after puberty. However, comparing the same brain cells in the same region of the same animal across development would be a more reliable way to compare how puberty remodels the brain.
Following up on this study, Johanna Sigl-Glöckner, also from Brecht's laboratory, turned to another lab animal, the house mouse (Mus musculus), for which there is a wide range of fancy molecular tools that allow scientists to flag specific brain cells. Using this technology, she and her colleagues spied on brain cells in the genital cortex of individual mice several times during their development, as recently reported in Current Biology.
First, the team determined if, like rats, male and female mice grew more touch-sensitive brain cells in the genital cortex during puberty. While mice have other cell types in their genital cortex, since sex is all about touch, the team focused on tallying touch-sensing cells as they play a more obvious role. To accomplish this, Sigl-Glöckner brought in specially designed mice that produce glow-in-the-dark tags only within brain cells receiving touch information from the thalamus, the brain's sensory relay station. This allowed the team to peek in on the mice periodically and count how many new touch-sensitive cells were born in the genital cortex as the mice grew up. Following puberty, both male and female mice doubled the size of their pre-pubertal genital cortex and added nearly two new touch-sensitive cells a day across adolescence. Therefore, mice, like rats, expand the size of their genital cortex during sexual maturation. However, while (brain) size matters, it wasn't clear how responsive to touch these new cells were.
In order to measure how genital cortex cells respond to stimulation, Sigl-Glöckner tweaked her designer mice so that the touch-sensitive cells would glow even brighter when they were activated: the brighter the cell, the more responsive to touch. Then, the team turned on a tiny vibrating device, the same one that makes your cell phone buzz, to sexually stimulate mice while measuring how bright their brain cells glowed. Despite having a similar number of cells in the genital cortex, the male mouse cells shone brighter than the female cells – both pre- and post-puberty – suggesting that male mice have a more sensitive erogenous zone across development. However, if female mice mated with a male before puberty or if females were abstinent and just reaching adulthood, their genital cortex cells became more responsive to stimulation compared with pre-puberty. Taken together, the changing response to sexual stimulation depends not only on mouse age, but also their sex and sexual experience.
These findings by Sigl-Glöckner and her colleagues expand our understanding of how different factors drive brain remodelling across development and reveal that just as hormonal pre-teens increase in size and sensitivity during puberty, so too does the mouse genital cortex, thus stimulating more questions to be answered about how puberty changes the brain.