Maternal Separation

He also says that studies published in research journals report that those who are abused or neglected as children tend to abuse drugs more often as adults. So in a study published in the journal Psychoneuroendocrinology, Wayne and his colleagues speculated that a trauma early in childhood, such as being separated from one’s mother, would affect the dopamine system in the brain.

The researchers decided to test the idea on rats. They separated rat babies from their mothers for several hours a day during the first two weeks of their lives. This evokes stress in the mother who eventually, “spends less time licking and grooming and nursing and caring for her pups”, according to Wayne. Then they waited until the rats were 3 months old, at which time they are considered adults. They tested the rats’ sensitivity to different doses of drugs like cocaine and amphetamine. Wayne says that although all the rats responded to high doses of the drugs, the rats separated from their mothers “were much more responsive or hyper in response to the drugs at lower doses.”

Next, Wayne and his team wanted to know if their theory about dopamine was correct. When they analyzed the brain tissue, they found that the rats separated from their mothers released a lot more dopamine than those that grew up next to mom. According to Wayne, this suggests that the dopamine system in the brain is functioning at a higher level in the rats that were separated from their mothers.

The researchers dug further to find out what was causing this change in dopamine levels and found that the separated animals showed a decrease in a protein in the brain called dopamine transporters.

The system works like this. Dopamine is released into the space between two nerve cells, stimulating the neighboring nerve cell. In normal animals, once the signal has been sent, dopamine is taken back up by the dopamine transporters into the cell from which it was released. This function is called dopamine reuptake. “This,” says Wayne, “is what regulates the amount of dopamine released in the brain and how much motivational drive we have to take drugs”.

When there is a decrease in the amount of dopamine transporters in the brain, the dopamine reuptake does not happen as it normally would. This means the dopamine signals last longer and are stronger than those in normal individuals.

Brake’s team also studied a number of genes in the front part or prefrontal cortex of the rat’s brain. They found that several genes that are involved in the development of the dopamine system were either turned on or off or changed in their expression.

Wayne says all this appears to be regulated by the care a mother gives early in the rat’s life. He explains, “while we don’t know a lot about their (genes) exact nature, we do know that it may be important for generating how the adult brain will be wired later on”.

The team plans to examine the genes individually to confirm the reliability of the changes in the genes and to study their role in brain development.

Brake points out that although we cannot automatically translate the findings in rats to humans, the study does show that in the brains of mammals, a mother’s care is extremely important for brain development.

This study was funded by grants from the National Institute on Drug Abuse, and the Canadian Institutes of Health Research.