On August 6th, 2015 we learned about

Manipulating memories of mistreated mice mends their moods

When you’re feeling down, you might think to do one of your favorite activities to cheer up. Have a favorite meal to make yourself feel better. Revisiting a pleasant activity might not be enough to pull you out of a funk though, especially if you’re struggling with real depression. Thanks to some optogenetically stimulated mice, we now know that remembering your happy place may somehow be more effective than actually going there.

Spotlighting brain cells

Optogenetics involves implanting fiber optics into a subject’s brain at selected clusters of neurons. Light can then be shined on those brain cells, which activates them, letting researchers manipulate brain activity with amazing specificity. While the technique was developed with fruit flies, studies with mice have been able to go as far as reencoding memories to be recalled as more positive than the actual event had actually been.

Setting them up then knocking them down

In this study, the mice were given some time with a female companion so that they’d form some positive memories. That brain activity was observed and labeled in the mouse’s hippocampus before the mice were taken away for ten “stressful” days. Those days included confinement and isolation, and seemed to instill a sense of hopelessness and depression in the mice, as they no longer reacted to negative stimuli, like being held from their tails, or positive stimuli, like sweetened water.

The emotionally broken mice then had their happy memories of their time with the female triggered with an optogenetic implant. Within minutes their mood and behavior improved. After five days of this stimulation in their hippocampus, they seemed to be cured, with no return of depressive symptoms.

Why would memory succeed where experience fails?

Seeing positive feedback in science can be exciting, but sometimes the points of failure can be more informative. In this case, while the stimulation of the positive memories was wildly successful in perking up the mice, this was in contrast to what happened when stressed mice were allowed to spend time with the female again. Having a chance to relive the initial, positive experience made no difference in their behavior. The fact that triggering the memory led to such different outcomes may help reveal some important questions about how depression actually works. Is depression blocking happy memories normally, but the optogenetics can overpower it? Does the depression block benefits that memories would normally provide? Is a memory somehow more powerful than the real thing?

We’re still a long way away from anyone plugging a fiber optic in your brain, but these mice are helping give us a more granular look at what’s actually getting us down on a physiological level.

Source: Switching on happy memories 'perks up' stressed mice by Jonathan Webb, BBC Science and Environment

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