The Importance of Mothers

Mind Matters, David Dobbs' research blog over at Scientific American, is indispensable weekly reading. This week is no different. The topic is a paper documenting the importance of maternal presence in rat-pups. Apparently, the absence of a rat mother during a critical period of pup development permanently alters the behavior of the pup*:

In their recent Nature Neuroscience article, researchers Stephanie Moriceau and Regina Sullivan explore learned olfactory preference and aversion as mediated by maternal presence. In this study rat pups were exposed to a peppermint odor that was paired with a light (0.5 milliamp) shock -- a classic conditioning exercise that taught the pups to associate the odor with the shock. It has long been established that a mature rat exposed to such conditioning will learn to avoid the associated odor. It is also well established -- but less well known -- that an extremely young rat pup (less than 8 day old) exposed so such conditioning will actually become attracted to the odor. This study by Moriceau and Sullivan, however, discovered that there is a period during rat youth during which the reaction to the odor-shock conditioning -- that is, whether the rat learns to prefer or avoid the odor -- depends on whether the pup's mother is present during the conditioning. It's a fascinating finding that raises intriguing questions about human behavior.

While this paper focused on the role of the amygdala, previous investigations have demonstrated that the stress of an absent rat mother causes widespread effects in the infant rat brain. I've previously written about some of the work done by Elizabeth Gould, who studies the relationship between stress and reduced neurogenesis.

For the last several years, she and her post-doc, Mirescu, have been depriving newborn rats of their mother for either 15 minutes or three hours a day. For an infant rat, there is nothing more stressful. Earlier studies had shown that even after these rats become adults, the effects of their developmental deprivation linger: They never learn how to deal with stress. "Normal rats can turn off their glucocorticoid system relatively quickly," Mirescu says. "They can recover from the stress response. But these deprived rats can't do that. It's as if they are missing the 'off' switch."

Gould and Mirescu's disruption led to a dramatic decrease in neurogenesis in their rats' adult brains. The temporary trauma of childhood lingered on as a permanent reduction in the number of new cells in the hippocampus. The rat might have forgotten its pain, but its brain never did.

*It's worth noting that this research is not applicable to questions about whether human children are better off if their mother stays home with them. This research is about extremes: a vulnerable rat pup is deprived of the one presence it has evolved to expect.