The bullied mice emerged drastically cowed. Four weeks later, they still fearfully withdrew from even presumably friendly little mice.Is it possible that introverts are formed through such a simple mechanism as genetically-determined greater BDNF levels?
What was happening in their brains?
BDNF is a chemical important for the growth and maturation of nerve cells. Some antidepressants are thought to increase BDNF levels in the hippocampus, helpfully boosting neurons.
But in this different brain region, encompassing the so-called mesolimbic dopamine pathway, Dr. Eric Nestler, UT Southwestern's psychiatry chairman, found too much BDNF was bad: The bullied mice experienced marked BDNF increases, which in turn switched on several hundred genes located deep in the front part of the brain. That unusual gene activation paralleled the animals' social withdrawal.
Then Nestler's team injected mice with a virus that switched off BDNF production only in this one brain region, and repeated the bullying experiment. Mice lacking the BDNF didn't become cowed — they essentially couldn't learn how to respond to this emotional threat, evidence of BDNF's role in social stress.
"The ability of stress to induce BDNF in this reward circuitry is probably a good thing" from an evolutionary standpoint, Nestler said. "If you're constantly subjected to something bad like being beaten up, it makes sense to avoid what's beating you up."
Also, lack of an easy reset switch on things that have been learned can be pretty annoying.