tumors spur depression

The Scientist | Tumors can cause classic symptoms of depression in rats, according to a new study published online in PNAS this week.

"What's really cool about this paper is that it shows without a doubt that there are depressive-like behaviors induced in these rats before these rats become [sick]," said Keith Kelley, an immunophysiologist at the University of Illinois at Urbana-Champaign, who was not involved in the research.

Researchers have long known that individuals suffering from chronic illness are at a greater risk of depression, but whether it was a direct cause of the illness or a psychological reaction to being sick was unclear. "By using this animal model of cancer we were able to isolate just the physiological effects of the tumors from the psychological effects that you get in human studies," said Leah Pyter, a behavioral neuroscientist at the University of Chicago, who led the study. "The tumors themselves are sufficient to induce depression."

Pyter and her colleagues induced mammary tumors in rats using a chemical carcinogen known as N-nitroso-N-methylurea (NMU). In a forced swimming test, the rats with chemically induced tumors spent more time floating instead of swimming compared with healthy controls, a classic sign of depression. And while healthy rats prefer weak sugar water to tap water, the rats with tumors showed no such preference.

The rats exhibited these depressive-like behaviors well before they showed any overt signs of illness from the tumors themselves. They showed no difference in eating habits or social behavior, and they did not lose weight, like rats with an induced acute infection often do.