When I look back on my teenage years, I don’t remember the easiest of times. I remember a period of physical and emotional change that was often accompanied by uncertainty, angst, and turmoil. I also remember times of excitement – many firsts including boyfriends, going to the mall without parents, cruising around in the car with friends, and drinking. Teenagehood was definitely a roller coaster ride of emotions.
When we think about the brain, adolescence is a time of significant change. During this time, the brain continues to grow, developing into its fully functional adult form. One of the hallmarks of adolescents is puberty, the time period of becoming sexually mature. Hormones released from the gonads signal the brain to undergo a variety of physical and emotional changes. One of these changes is our response to the opposite sex – boys or girls somehow and all of a sudden seem appealing to us (thus those first kisses).
One session at the conference, Adolescence and Reward: Making sense of neural and behavioral changes amid the chaos, explored this very exciting topic. During adolescence, social, emotional, and cognitive skills develop at an expedited rate. This occurs while exploring and risk-taking also significantly increase. In the adolescent brain, the reward circuitry undergoes distinct changes, making stimuli more rewarding. This occurs while the inhibitory control circuits remain underdeveloped – creating a state that causes a “hypersensitivity to reward.” Several researchers spoke about their work in this area including Dr. Deena Walker who studies sex differences in the amygdala and reward circuitry during adolescence and Dr. Joshua Gulley who studies maturation of the cortico-accumbens circuit.
Dr. Margaret Bell from Depaul University uses the male Syrian hamster to study the underlying neural circuitry in the gains in social reward that occur during adolescence. Hamsters are very sensitive to olfactory cues, and male hamsters are especially sensitive to how their female counterparts smell. She uses a behavioral task called the conditioned place preference model to examine the olfactory cues that males like. Animals will spend more time in a place where they find rewarding stimuli such as sugar, drugs, or pups (if you’re a mom). In this case, Dr. Bell used vaginal secretions from the female hamster to identify whether males found the smell rewarding. She found that the smell of the female only becomes rewarding during the adolescent period. Further, she revealed that this social response is dependent on both the maturation of both the testosterone system and the dopamine system. That is, in the male hamster, the gonadal system interacts with the reward system to produce an appropriate adult social response to the female hamster.
In another session, Dr. Cecilia Flores at McGill University discussed the role of netrin in the involvement of brain development in the adolescent brain. Netrins are a class of proteins that are involved in axon growth, target recognition, axon arborization, and synaptogenesis, leading neurons to their final destination in the brain. There are two netrin receptors – DCC and UNC-5. Normal expression of netrin as well as its receptors are needed for proper brain development to occur. DCC receptor signaling in the dopamine neurons, specifically during adolescence, determines the extent of their innervation to the medical prefrontal cortex in adulthood. Proper connections in this region of the reward circuitry are needed for our ability to stay motivated as well as our response to rewarding stimuli. Additionally, proper dopaminergic innervation of the prefrontal cortex translates into proper behavioral responses in areas such as cognitive flexibility, response inhibition, and sensitivity to drugs in adulthood. Dr. Flores importantly highlighted that experimenting with drugs during adolescence alters the expression of both netrin and its receptors – thus altering how dopamine neurons innervate their final targets. For example, amphetamine exposure during adolescence downregulates DCC expression, causing an expanse of dopamine terminals in the medial prefrontal cortex. Therefore, experimentation with drugs during adolescence may have severe consequences for the structure of the prefrontal cortex as well as a variety of behavioral outcomes.
Collectively, this work highlights that the adolescent period is a time of extensive brain development. Proper brain development during this time is needed for proper behavior to emerge in adulthood. Therefore, it is an extremely important time to engage in healthy behaviors and to stay away from stimuli like drugs and alcohol, which may have severe consequences for structural and functional outcomes in adulthood.
Julia C. Basso, PhD