Exercise stimulates the growth of new brain cells, a new study on rats finds. The new cells could be the key to why working out relieves depression.

Previous research showed physical exercise can have antidepressant effects, but until now scientists didn't fully understand how it worked.

Astrid Bjornebekk of the Karolinska Institute in Sweden and her colleagues studied rats that had been genetically tweaked to show depressive behaviors, plus a second group of control rats. For 30 days, some of the rats had free access to running wheels and others did not.

Then, to figure out if running turned the down-and-out rats into happy campers, the scientists used a standard "swim test." They measured the amount of time the rats spent immobile in the water and the time they spent swimming around in active mode. When depressed, rats spend most of the time not moving.

"In the depressed rats, running had an antidepressant-like effect after running for 30 days," Bjornebekk told LiveScience. The once-slothful rodents spent much more time in active swimming compared with the non-running depressed rats.

The researchers also examined the hippocampus region of the brain, involved in learning and memory. Neurons there increased dramatically in the depressed rats after wheel-running.

Past studies have found that the human brain's hippocampus shrinks in depressed individuals, a phenomenon thought to cause some of the mental problems often linked with depression.

"The hippocampus formation is one of the regions they have actually seen structural changes in depressed patients," Bjornebekk said.

Running had a similar effect as common antidepressants called selective serotonin reuptake inhibitors (SSRIs) on lifting depression.