The developmental progress from daredevil teen to risk-averse senior is more complex than we thought, according to a new study that identified changing relationships between neural structures related to risk avoidance at different life stages.
Neuroscientists from the University of California, Los Angeles, led an investigation into a critical part of the brain that help us determine whether we ought ‘take the leap’ or avoid life-threatening danger.
Ours isn’t the only species where adolescents engage in markedly risky behaviors – a pattern that’s at odds with protecting ones’ survival at all costs. Other animals, such as mice for example, share this trait.
“These behaviors may compete with the drive to avoid threatening situations, leading to a reduction in avoidance behaviors in PMA (platform-mediated avoidance assays),” the authors report in their new paper.
“Here, we uncover a circuit mechanism that causally contributes to lower levels of threat avoidance in adolescence.”
By studying the brains of mice, they found the dorso-medial prefrontal cortex (dmPFC) ‘referees’ neural pathways that take on distinct structures at certain points throughout life.
It’s as if the prefrontal cortex – the part of the brain credited for our ability to steer our emotional meat ship on a more deliberate course – is negotiating with the structures that advocate for what we might call ‘instinct’ (the basolateral amygdala, or BA, being the locus of fear and pain memory; the nucleus accumbens, NA, being crucial to reward, reinforcement and aversion).
These negotiations, the experiments showed, depend greatly on age.
In an experiment reminiscent of James Dean’s game of ‘chicken run’ in Rebel without a cause, mice were trained to step on a platform to dodge a threat; a decision made more difficult with a smorgasbord laid out in front of them just out of reach of the platform.
Despite knowing very well how to escape the beep they’d come to associate with an electric shock, juvenile and adolescent mice chose to take their chances and keep on eating for longer, while older mice generally stepped dutifully onto the platform, waiting until the threat had passed.
“Although mice of all ages had similar levels of conditioned fear and some exploratory behaviors during the retrieval test, juveniles and adolescents explored the threatening part of the environment more than adults,” the authors report.
Fluorescent molecules injected into the test subjects’ brains allowed the researchers to track the physiology underpinning these behaviors. Higher levels of glowing molecules generally indicate greater amounts of neural activity.
Activating genes using light through a process of optogenetics revealed further details on how activity in these brain structures related to threat avoidance strategies in juvenile, adolescent and adult mice.
The dmPFC, it turns out, becomes more sensitive to threats with age. Much like aging in the rest of the body, however, changes in the structure’s configuration occur in staggered stages characterized by maturation of synapses and re-arrangement of the circuits connecting the BA and NA.
The brain’s risk-avoidance system may be wired to best suit age-specific challenges as they arise, prioritizing risk when the nest is getting too crowded, and safety when it comes time to settle down.
This is a mouse study, so it’s not clear if these same patterns hold true for humans. But, as mammals, we aren’t too far-removed from mice, providing us with a proxy understanding of how our own brains might navigate the tug-of-war between rewarding risk and safety.
“The lack of studies on the causal functions of the mPFC, BLA and NAc circuits in the developing brain has left a major gap in our understanding of how interactions between these regions produce developmental transitions in threat-induced behaviors,” the authors write.
“In revealing the processes by which top-down circuit maturation guides changes in threat-induced behaviors, we establish a foundation to understand how they can become disrupted.“
This research is published in Nature Neuroscience.
