Science Watch

Chronic stress has long been shown to fuel depression, but how?

Answers are coming from scientists like those at Dalhousie University in Nova Scotia, who've found that rats repeatedly exposed to the stress hormone corticosterone show more depression-like behavior and greater signs of anxiety. Their study, which offers a rare look at sex differences, also indicates that the hormone affects males more than females. The findings appear in the December issue of Behavioral Neuroscience (Vol. 118, No. 6).

In the study, a team of four led by Lisa Kalynchuk, PhD, now at the University of Saskatchewan, studied 30 male and 30 female rats. By studying links among repeated stress, depression-like behavior and the function of the hippocampus in rats, the investigators were hunting for clues about the biological cause of depression in humans.

Kalynchuk and her co-authors cite medical evidence that depression can stem from chronic overstimulation of the body's hypothalamic-pituitary-adrenal axis, which produces stress hormones such as cortisol in humans and cortico-sterone in rats. For example, patients with Cushing's disease have high levels of cortisol and are often depressed, and depressed people often have hippocampus-linked cognitive problems, due perhaps to smaller hippocampi.

Thus, after the researchers injected rats with high levels of stress hormone for three weeks, they found that compared with controls, the animals showed significantly more behaviors that could be considered anxious and depressed. Voila: The biological agent was not only unmasked, but unmasked with meticulous care.

"This is the most systematic study I've seen to date to demonstrate the stress-depression link," notes Thomas Minor, PhD, associate professor of behavioral neuroscience at the University of California, Los Angeles (UCLA).

Just keep swimming

It was important to be systematic given that, in studying the stress-hormone system and depression, it can be hard to control between-subject variation in hormone levels, the researchers say. Kalynchuk and her colleagues tried a new approach: They bypassed that system and directly injected CORT for 21 days to control its levels. Each treated rat got the same supplemental dose of stress hormone to simulate in a controlled fashion what prolonged stress might do to the body. The researchers injected the control groups--half of each sex group--with salt water alone.

After three weeks of treatment, Kalynchuk and her colleagues--graduate student Andrea Gregus, undergraduate Daniel Boudreau, now a medical student, and Tara Perrot-Sinal, PhD--monitored the rats for signs of anxiety and depression. The tests included:

  • Open field. Do rats venture out into an unfamiliar open field to explore or do they hold back? Holding back is a sign of depression.

  • Resistance to capture. How much do the rats fight being picked up? Not fighting is a sign of depression.

  • Forced swim. When placed in a tank of water for 10 minutes, how much time do rats spend struggling, swimming or being immobile? After this, they get toweled off and dried under a heat lamp. Giving up is a sign of depression.

  • Predator odor. Do they seem upset or run away when a collar that's been worn by a cat gets put in their cage? Showing such defensive behaviors is a sign of depression.

Sure enough, the repeated cortisone injections--which simulated three weeks of chronic stress response--increased depression-like behavior in the rats. In the forced-swim test, both male and female hormone-injected rats spent more time immobile, and they became immobile faster. In the predator-odor test, the artificially stressed rats showed more of one subset of defensive behaviors when in the presence of a cat collar.

The research team drew three main conclusions. First, exposure to repeated CORT injections produced depression-like behavior. Second, the hormonally stressed rats showed more anxiety in specific situations, such as the predator odor test, which the researchers think might be especially sensitive to anxiety in rats. Third, although the effects were generally similar in male and female rats, the hormone appeared to affect males more strongly on these tasks.

Stress hormones and sex hormones

Given the greater vulnerability of male rats to stress in their study, the authors wonder whether the females' higher levels of circulating sex hormones, such as estrogen and progesterone, are "neuroprotective," supporting healthier nerve-cell life from the get-go. Prior research has linked higher sex hormones to less depression-like behavior in female rats; the inclusion of stress hormone makes this study different and an advance over previous studies, says Bruce McEwen, PhD, the Alfred E. Mirsky Professor and head of the Harold and Margaret Milliken Hatch Laboratory of Neuroendocrinology at Rockefeller University.

It's a welcome advance, says Tracey Shors, PhD, of the Center for Collaborative Neuroscience at Rutgers University. "It is my hope that these types of studies will encourage others to consider testing females when using animal models of depression," says Shors. "After all, women are much more likely to experience depression and other stress-related mental illness than men."

Although noting the limits of generalizing rat findings, author Kalynchuk says new experiments could show why female rats seem more resistant to stress-induced depression than do female humans. One obvious explanation could be that rats and humans are just plain different, and that, as she puts it, "The increased depression rate in female humans is due to an interaction between biological and psychosocial factors, which we simply cannot model in rats." The rate could also be higher, in part, because women are more likely to consult a doctor when they feel depressed, whereas men are more likely to self-medicate with alcohol or other drugs.

However, Kalynchuk also points to a possible hormonal explanation for which there's some support. It could be that, in humans, normal levels of female sex hormones could help protect against depression, but hormone fluctuations could put women at risk for problems such as postpartum depression and premenstrual dysphoric disorder. In the Dalhousie experiment, the female rats had normal levels of gonadal hormones. But, says Kalynchuk, other researchers have shown that female rats without ovaries--who have low levels of these hormones--are much more susceptible to stress.

Further experimentation, says Kalynchuk, could show whether male rats are actually more susceptible to the effects of stress than female rats with normal levels of these hormones.

Connecting the dots

Having confirmed that the hormones released by chronic stress cause depression, researchers want to pin down the mechanism: Exactly how does CORT make us feel blue? Minor of UCLA says the study has bearing on the debate over whether CORT facilitates neurodegeneration or results in neuroprotection to produce behavioral depression.

On the one hand, Kalynchuk cites evidence that repeated stress in lab animals reduces neurogenesis--the birth of new brain cells--in the hippocampus, leading to depressive symptoms. She notes, though: "It's not clear whether a decrease in neurogenesis can cause depression or is a by-product of depression. A direct link has not yet been made."

On the other hand, Minor postulates that chronic stress hormone trips a neurological circuit breaker by causing receptors in the hippocampus and amygdala to block glucose intake, sparing these regions from neurotoxic overexcitement. This long-term coping response, he says, would drag down other responses and behaviors, causing what people experience as depression.

"Stress is when you're anxious and fatigued," says Minor. "Anxiety causes energy production, but in fatigue your energy is down, a compensatory shift." He cites accumulating evidence of depression as a metabolic problem in the brain--a view that, if confirmed, would promote regimens that help the brain build new neurons, including the traditional change of scenery, along with fresh air, exercise and regular sleep.

"In some depressed patients, the hypothalamic-pituitary-adrenal axis looks like it's being dysregulated," Minor says, noting that restoring balance to a stressed-out system--and re-regulating glucose transport to the brain--could help. Minor and his colleagues are preparing to test this hypothesis in depressed people.

Ronald Duman, PhD, a professor of psychiatry and pharmacology at the Yale University School of Medicine, also hopes to broaden the findings to improve treatment. The forced-swim test is, he notes, typically used to test antidepressants on rats, but their recovery is paced differently from humans. If someone could replicate the new findings with other indicators of depression, says Duman, it could lead to a better model for testing new drugs and other therapies.

For now, the Dalhousie research helps answer the "nagging question of how chronic stress gets inside the brain," says Gregory Miller, PhD, a psychology professor at the University of British Columbia. "To the extent that these findings generalize to the kinds of stressors and symptoms that humans experience, they'll provide valuable mechanistic insights and potentially important clinical implications."

Rachel Adelson is a science writer in Raleigh, N.C.