A tiny molecule found only in humans and in other primates could help battle depression, according to a recent study.

Depression is a common cause of disability, and while viable medications exist to treat it, finding the right medication for individual patients often amounts to trial and error for the physician.

Researchers from McGill University in Canada found that levels of the molecule miR-1202 may provide a marker for depression and help detect individuals who are likely to respond to antidepressant treatment. Levels of the molecule, which are lower in the brains of depressed individuals, may hold a key to improving treatment options for those who suffer from depression.

""We identified this molecule, a microRNA known as miR-1202, only found in humans and primates and discovered that it regulates an important receptor of the neurotransmitter glutamate," Dr. Gustavo Turecki, a psychiatrist at the Douglas and professor in the Faculty of Medicine, Department of Psychiatry at McGill, said in a statement.

For the study, researchers used samples from the Douglas Bell-Canada Brain Bank to examine brain tissues from individuals who were depressed and compared them with brain tissues from psychiatrically healthy individuals. The team conducted a number of experiments that showed that antidepressants change the levels of this microRNA.

"In our clinical trials with living depressed individuals treated with citalopram, a commonly prescribed antidepressant, we found lower levels in depressed individuals compared to the non-depressed individuals before treatment," Turecki said. "Clearly, microRNA miR-1202 increased as the treatment worked and individuals no longer felt depressed."

Antidepressant drugs are the most common treatment for depressive episodes, and are among the most prescribed medications in North America.

"Although antidepressants are clearly effective, there is variability in how individuals respond to antidepressant treatment," Turecki said. "We found that miR-1202 is different in individuals with depression and particularly, among those patients who eventually will respond to antidepressant treatment."

Researchers said the discovery may provide "a potential target for the development of new and more effective antidepressant treatments."

The findings were recently published in the journal Nature Medicine.