"Chronic pain and depression are common and often overlapping syndromes," the authors said.
"Understanding the neurobiological basis of this relationship is important because the presence of comorbid pain contributes significantly to poorer outcomes and increased cost of treatment in major depressive disorder," the authors added.
For the study, Irina A. Strigo, Ph.D., of the University of California San Diego, La Jolla, and colleagues studied 15 young adults with major depressive disorder (average age 24.5) who were not taking medication and 15 individuals who were the same age (average 24.3 years) and had the same education level but did not have depression.
Patients with depression completed a questionnaire that evaluated their tendencies to magnify, ruminate over or feel helpless in the face of pain.
All participants underwent functional magnetic resonance imaging (fMRI) while their arms were exposed to a thermal device heated to painful levels and also to non-painful temperatures.
Visual cues (a green shape for non-painful warmth and a red shape for painful warmth) were presented before the heat was applied.
The researchers found that patients with depression showed increased activation in certain areas of their brain, including the right amygdala, during the anticipation of painful stimuli as compared to the controls.
They also displayed increased activation in the right amygdala and decreased activation in other areas, including those responsible for pain modulation, during the painful experience.
In order to examine whether the activation of the amygdala was associated with passive coping styles, the researchers compared the percentage change in the activations of the amygdala with the helplessness, rumination and ramification reported by the participants with depression.
"Significant positive correlations were observed in the major depressive disorder group between greater helplessness scores and greater activity in the right amygdala during the anticipation of pain," the authors said.
"The anticipatory brain response may indicate hypervigilance to impending threat, which may lead to increased helplessness and maladaptative modulation during the experience of heat pain. This mechanism could in part explain the high comorbidity of pain and depression when these conditions become chronic," they added.
The study is published in the November issue of Archives of General Psychiatry, one of the JAMA/Archives journals.