In the study, alterations in the rats' brains were comparable to changes in human brains when exposed to antidepressants.
Their findings, published in the Proceedings of the National Academy of Sciences, may explain why smells or objects that trigger associations with early life abuse can sometimes reduce stress in adults.
"Children form a strong attachment to their caregiver - even when that caretaker is abusive," write the authors. "Paradoxically, despite the trauma experienced within this relationship, the child develops a preference for trauma-linked cues - a phenomenon known as trauma bonding."
The team found that trauma and pain experienced during infancy led to increased rates of depression-like behavior in adult rats. However, upon being presented with odor cues linked to their earlier trauma, the adult rats experienced a lowering of their depression symptoms.
The authors state that they were surprised by these findings, as cues associated with trauma and pain normally provoke fear.
The impact of serotonin pathways
Previously, senior study investigator Dr. Regina Sullivan and her team have demonstrated that infant brains are limited in their capacity to connect trauma to the area of the brain associated with processing fear. Instead, areas of the brain crucial to approach and attachment are activated.
A connection is made between the cue and the attachment circuit within the brain, which can allow the cue to continue to activate the attachment circuit from that point onward.
Dr. Sullivan believes the findings of the new study illustrate an important aspect of how mammals are biologically affected by infant trauma and that human brains may be affected in a similar way.
The role of serotonin - a neurotransmitter that many scientists regard as a mood regulator - in relation to trauma is a crucial aspect of the study. The neurotransmitter appeared to have opposite effects in infant rats and adult rats.
Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed antidepressants that prevent serotonin from being reabsorbed in the brain.
Trauma experienced by infant rats increased their levels of serotonin. Giving the rats SSRI medication had the same effect on the brain as trauma and led to depression for the rats in adulthood.
Different results for infant and adult rats
Further tests were conducted on the rats when they reached adulthood. The researchers found that odor cues associated with the infant trauma continued to increase their levels of serotonin. This time, however, the increased levels of serotonin helped to alleviate depression-like symptoms in the same way that SSRIs normally work.
"We believe that our research offers the first evidence for the impact of serotonin pathways," explains Dr. Sullivan. "The infant trauma increases serotonin to produce brain programming of later life depression and the infant trauma cue increases serotonin to alleviate the adult depressive-like symptoms."
This seemingly paradoxical finding could have implications for the treatment of human children. "It is possible that giving SSRI medications to children could be detrimental to mental health in adulthood," she suggests.
The team demonstrated that the odor cues altered the activity of genes in the area of the brain that processes fear and pleasure, leading to an uplift in mood. The uplift was the same as that experienced by adult rats whose serotonin levels were increased after having their stress pathways blocked by the researchers.
Dr. Sullivan details how the findings can be used in further research:
"Starting with our results in this study, we can use the same rat model system to better understand other aberrant behaviors and investigate whether changes in serotonin, or other neurotransmitters, can similarly influence adult behavior."
"Our findings suggest that trauma-linked cues have an unexpected positive value in adulthood (i.e., antidepressant properties) and may provide insight as to why victims of childhood abuse are attracted to abuse-related cues," the authors conclude.
Last year, Medical News Today reported on a study that found SSRI use during pregnancy was linked to a higher risk of autism in male children.