The University of Michigan study finds the number of chemical receptors in the brain affects not only the severity of symptoms but how well a depressed patient responds to antidepressants.

"There's a substantial amount of biological difference even among people who have major depression, which is just as important as the biological differences between people with depression and people without," lead researcher Dr. Jon-Kar Zubieta says in a statement. "The more we can understand about these differences, the better we can address treatment."

Preliminary findings were presented at the American Psychiatric Association's annual meeting in Washington.

The researchers compared positron emission tomography, or PET scans, of the brains of non-depressed volunteers to those of patients meeting the criteria for major depression but not yet receiving treatment. They found the more depressed the patient, the lower the concentration of 5HT1a receptors -- linked to the chemical neutrotransmitter serotonin.

In another group, researchers not only found the more depressed patients had fewer receptors -- the gateway for signals sent by chemicals involved in stress response including response to pain, but these receptors were more active in depressed people than in non-depressed people