An Animal Model of Poor Judgment in Adolescence: Previous Learning Suppressed

March 1, 2013 · Posted in Neurobiology 


As young mice transition into adolescence, they experience a “sensitive” period in which their context-based fear memories are temporarily suppressed. In a recent study, young animals learned to avoid an environment associated with a mild shock. Later, when they entered adolescence, this learning was temporarily forgotten or suppressed. However, when the same mice aged into adulthood, they reacquired this learned fear memory and began to again avoid the environment associated with the earlier shock. This temporary loss of fear memory differs in mice depending on their genes.

At the 2012 meeting of the Society of Biological Psychiatry, researcher Francis S. Lee reported that mice with a certain genetic variation display an impairment of this fear memory process. There are several common variants of the gene responsible for the production of brain-derived neurotrophic factor (BDNF), which protects neurons and is necessary for long-term memory. Mice with the poorer functioning variant known as Val66Met (as opposed to the better functioning Val66Val) fail to recall the earlier fear-related events not only in adolescence, but also in adulthood when the fear memory is usually retrievable again.

Editor’s Note: In mice and humans, Val66Val is the most frequently occurring allele in the population, but Val66Met is also a fairly common variation of the BDNF gene. It is this Val66Met allele that is associated with not retaining earlier learned experience about a “dangerous” environment that should be avoided.

These data suggest an intriguing explanation for some of the “wild” behavior and poor judgment to which even the smartest adolescents are prone. This kind of behavior may be based in part on the temporary forgetting in adolescence of earlier learning about which situations or environments are safe versus which ones are dangerous.

Other data suggest that adolescent animals and humans are particularly prone to developing substance abuse. Are they temporarily forgetting or suppressing what they may have already learned about the dangerousness of drugs of abuse because they are in the critical adolescent period?

Other explanations for the chaotic behavior of adolescents are possible. According to the data of the famous neuroanatomist Pasko Rakic, adolescent primates prune back 100,000 synapses per second. (This editor’s wild speculation: it is even possible that the two phenomena are related. It is possible that adolescents can’t remember previously learned avoidance behaviors because their synapses are being pruned back so fast that their brain is scrambled. When the pruning reorganization finally slows down and excitatory synapses are largely replaced with inhibitory synapses, more effective brain processes return and the post-adolescent individual is again able to recall previous learning.)

In any event, adolescents should be viewed with some sympathy because of the unique physiological processes occurring in them—light speed brain reorganization and the inability to remember previous learning about safe environments and actions.


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