New research conducted in the Department of Psychology at the University of Haifa that examined the behavior patterns of laboratory rats revealed that exposure to stress at a young age increases the likelihood of developing mood and anxiety disorders in adulthood. Research has found that the behavior patterns of pre-adolescent rats are very similar to those of young children and adolescents. In both humans and rats, the brain functions that respond to stress develop relatively early in childhood whereas the areas of the brain that regulate emotional responses and learning processes develop later.

This recently completed research revealed that lab rats that were exposed to stress in early childhood (up to age 28 days) developed symptoms related to anxiety and depression later on in life. Thirty-seven percent of the rats that were exposed to stress developed symptoms of depression whereas none of the rats in the control group did.

Most of the research done in this field has been on rats that were exposed to stress in infancy, while this research, which was conducted by Michael Tsoory, PhD, under the direction of Gal Richter-Levin, PhD, exposed the rats to stress in pre-adolescence. The research examined adult behavior patterns, capacity to learn and biological changes in the brain of rats that were exposed to stress in pre-adolescence.

The research revealed that rats that were exposed to stress in pre-adolescence were less inclined to investigate their surroundings and less apt to learn "avoidance responses" - in which one develops advance responses to negative stimuli. In analyzing the rats' behavior patterns, the researchers found that 37 percent of the rats that were exposed to stress exhibited behaviors associated with depression and 37 percent exhibited behaviors associated with heightened anxiety while in the control group 20 percent exhibited behaviors associated with anxiety and none exhibited symptoms of depression.

The researchers hypothesized that during the maturation phase (after weaning until the beginning of sexual maturation) the pre-adolescence is an especially sensitive period during which exposure to stress will have the greatest effect. To prove their hypothesis they exposed both pre-adolescent and young adult rats (34 days old) to the same stress. Both groups failed to learn specific tasks and neither were inclined to explore their surroundings. However, the adult behavior patterns of both groups were not similar. The pre-adolescent rats in this study exhibited behavior similar to those in the first study: 42 percent of the rats exhibited anxiety and 29 percent symptoms of depression. Among the rats that were exposed to stress later in adolescence, 50 percent were anxious and not one was depressed.

This study has further evaluated the developmental dynamics of the limbic system, the brain area which regulates emotional responses and learning processes, and found that the exposure to stress during the pre-adolescence disrupts certain development related alterations within the limbic system.

"The results show that exposing rats to stress in pre-adolescence harms their ability to cope with stress in adulthood and results in responsive behavior patterns similar to mood disturbances and anxiety. This can be attributed to abnormal brain development," says Tsoory.

Source: University of Haifa