The hippocampus is a ridge on the bottom of each lateral ventricle of the brain that plays a critical function in memory processes. It is also one of many areas affecting human behavior. However, the underlying causes in its effect on behavior are not well understood.

Investigating these uncertainities, researchers, lead by bioengineering professor Karl Deisseroth of the Department of Bioengineering at Stanford University, California, treated a group of laboratory rats for five to seven weeks with stress related conditions (such as lack of sleep, change of eating times, loud noises, and adverse lighting conditions). Another group of rats was given normal conditions—those conducive to a non-stress environment. Of the stressed-applied rats, some were given antidepressants while others were not given such medicines.

They then subjected all of the rats to a scary situation, at least scary for such animals: being submerged in water and then placed in a box of sand. The depressed rats did not swim as vigorously as the normal rats, indicating a hopeless attitude.

When the researchers examined the rats’ hippocampus--specifically, the portion called the dentate gyrus--they found that electrical circuits sent across the hippocampus in the depressed rats (those given stressful conditions and not given antidepressents) were unable to transit the brain properly, and the circuits eventually died out. When electrical circuits were sent across the hippocampus of normal rats and depressed rats given antidepressants, they traveled across successfully—in a normal manner.

The Deisseroth study suggests that depression (what they called hopelessness) is caused by a “short-circuit” of a once-normal hippocampus within the brain. Any negative event in a human’s life, such as family death, tragic event, or other negative situation, can cause a short circuit in the brain. Antidepressants seem to help new brain cells grow in the hippocampus, which allows information to successfully transmit across the hippocampal circuit.

The Deisseroth team is not yet certain about the origin of depression and how the hippocampus exactly is associated with the condition. However, they suggest that, in the future, researchers look for ways to fix the hippocampal circuit in people with depressed states. They, in addition, caution that their study is an early experiment using high-speed medical imaging of the hippocampus so their results need to be verified in further studies.

The study was published online on July 5, 2007, in the journal Science under the title “High-Speed Imaging Reveals Neurophysiological Links to Behavior in an Animal Model of Depression. Other collaborators include Raag D. Airan, Leslie A. Meltzer, Madhuri Roy, Madhuri Roy, Yuqing Gong, and Han Chen.

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