Ketamine, a drug used to induce anaesthesia could treat depression in patients suffering from treatment-resistant bipolar disorder, finds a new study from the U.S. National Institute of Health.

The researchers studied 18 patients with bipolar depression who showed resistance to the commonly used medications such as lithium or valproate. Two-thirds of the patients were on psychiatric disability and nearly all of them were unemployed.

They gave the patients an intravenous infusion of either ketamine or a placebo on two days in two- week intervals. Then the NIH researchers then assessed their depression symptoms after 40, 80, 120 and 230 minutes. They repeated these checks after one, two, three, seven, 10 and 14 days later. The depressive symptoms were assessed before each injection.

The investigators reported significant improvements in depressive symptoms within 40 minutes of receiving ketamine compared to those who received the placebo. The improvements were most noticeable at day two and remained significant through day three.

As many as 71 percent of the patients responded to ketamine injections while the response rate was six percent to the placebo. Ketamine injections were also found safe and reported no serious side effects during the trial.

Depression is a major aspect of bipolar disorder. Many patients do not respond to most of the currently available treatments for bipolar disorder.

Ketamine acts on the brain's glutamatergic system, which plays a role in information processing and memory formation. Recent research suggests that dysfunction in this brain system may contribute to bipolar disorder, says Dr. Nancy Diazgranados and colleagues at the U.S. National Institute of Health.

The brain's glutamatergic system may play a role in bipolar disorder. This could provide more clues to develop treatments that target glutamatergic system, suggests the study, published in the August issue of the journal Archives of General Psychiatry.

The research team, however, suggested that larger studies will be required to explore how ketamine's rapid effect on depression can be maintained over the long-term.