When they have pups to protect, normally timid female mice will ferociously attack male mice. This radical change in behaviour is because nursing mothers have low levels of a brain hormone associated with fear and anxiety.
Corticotropin-releasing hormone (CRH) is released in stressful situations. Lactating animals, including humans, have low levels of CRH in the brain, which is thought to make them calmer. Stephen Gammie and colleagues at the University of Wisconsin in Madison wondered if the low levels of CRH in nursing animals might also be responsible for their famed bravery.
The team injected three different concentrations of CRH or saline into the brains of nursing mice and then introduced males into their cages. Mice that had not received CRH immediately assaulted the intruders. But mice injected with the two higher CRH doses were slow to act, and made shorter and less frequent attacks (Behavioral Neuroscience, vol 118, p 805). 鈥淟ow levels of CRH seem to be necessary for maternal protective behaviour,鈥� says Gammie.
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CRH blockers are already being developed to treat anxiety and depression. The finding raises the possibility of another use: a drug for fearlessness. 鈥淚n war, soldiers are under high stress. In those cases, a CRH blocker might help,鈥� says Tracy Bale at the University of Pennsylvania in Philadelphia.