Sex, Stress, and Neurogenesis

There’s an article in the latest issue of Wired by Jonah Lehrer explaining just how dangerous stress can be to our health.  It’s a fascinating read -- and instead of relying on my poor attempt to paraphrase -- I suggest checking out the article in its entirety.

The part of the story that struck a particular chord with me was Lehrer’s explanation of the experiments done by Elizabeth Gould, who studies how stress hormones affect the growth of new brain cells in adult brain, a process called neurogenesis.  Gould’s previous work, as noted by Lehrer, showed that when animals get stressed out, levels of glucocorticoids -- one type of stress hormone -- skyrocket in their brains.  With brain cells wading in a constant bath of these stress hormones, neurogenesis comes to a screeching halt.

The take-home message from Lehrer’s article: glucocorticoids are bad.  And indeed, they do make bad things happen in the brain.  Aside from the fact that stressed-out animals have less neurogenesis, if you take an animal and inject glucocorticoids directly, new brain cells also stop forming.  Lehrer’s suggests that if we find ways to prevent or otherwise interfere with stress hormones (through a vaccine or otherwise), we could mitigate the effect stress has on our emotional well-being and, ultimately, its complex interaction with disease.

I’ve been putting off (for several weeks now) writing a post on the most recent experiment to come out of Gould’s lab, published in mid-July in PLoS One.  Lehrer’s story finally lit the fire under me.

The term “stress” has a very deliberate negative connotation.  We need this term to bucket somewhat-hard-to-explain feelings, like experiencing “pressure” at work.  But the term is far more encompassing than that.  Stress, by definition, is a measure of how the body responds to a challenge.  Sometimes the challenge can be a threat -- a deadline at work or a difficult family situation -- and triggers the all-too-familiar anxiety we’ve come to expect.  This is the bad type of stress Lehrer discussed.  But the challenge can also be a something, well, good, that temporarily takes our body out of balance.  Consider what happens when you exercise.  Going out for a run will create a physiological burden as the heart beats faster and faster, trying to match blood flow to the demands of the muscles and lungs.  This physical exertion is also a type of stress, a good kind of stress, if you will.

The effects of the two types of stress on the brain are completely different: While the bad stress decreases neurogenesis, the good type of stress, on the other hand,  actively stimulates extra brain cell growth.  Although the brain responds in different ways, both good and bad stress increase the levels of glucocorticoids in the body.  Knowing glucocorticoids are dangerous to the brain, researchers still scratch their heads over how exercise could battle these stress hormones, and win.

But Elizabeth Gould has a new theory.

Exercise makes us feel good about ourselves.  We like the sense of accomplishment.  We celebrate the weight we’ve lost and our increased fitness.  Gould believes that this hedonistic value of exercise could somehow trump the nasty effects seen when glucocorticoid levels rise.  But exercise is such a complex action.  Sure, there’s a hedonistic component, but there’s also a hefty physiologic one.

To give her theory some teeth, Gould would have to prove that another stressor with hedonistic value also boosts neurogenesis.  So this time around, instead of exercise, Gould’s lab used a simpler, less physically-demanding, but equally powerful positive stressor: sex.

While not typically considered a stress by popular definition, sex fits the bill, as it’s been shown to increase glucocorticoid levels in the brain.

Gould’s results show that a single sexual encounter is enough to raise glucocorticoids and increase neurogenesis in the hippocampus of male mice.  After repeated sexual experiences, the glucocorticoid levels stabilize, but the brain continues to grow new neurons and the number of synapses increases.

While this study doesn’t answer all of the questions surrounding glucocorticoids, stress, and the brain, it shows the story is far more complicated than initially thought.  Chronic good stress continually increases neurogenesis, but it also seems to level off the stress hormones themselves.  Gould’s results support the notion that the hedonistic aspect of good stress may in fact be the active ingredient that keeps the dangerous effects of glucocorticoids at bay.

Leuner, B., Glasper, E., & Gould, E. (2010). Sexual Experience Promotes Adult Neurogenesis in the Hippocampus Despite an Initial Elevation in Stress Hormones PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011597