Mouse mothers will attack any intruder to protect their offspring.© Alamy.comA mother on the warpath can be a fearsome foe. Now researchers studying mice believe they have identified the hormone responsible for females' fiercely protective attitudes to their offspring.
A maternal mouse will attack any intruder to her nest if she thinks her pups are in danger. This apparently selfless response may be due to low levels of a protein called corticotropin-releasing hormone (CRH), report Stephen Gammie and his colleagues at the University of Wisconsin-Madison.
High levels of CRH in the brain are linked to fear and anxiety, Gammie explains. So without it, a mouse is more likely to leap towards an intruder without a thought for herself. "We've known for a while that CRH promotes fear, so we thought it might be important," he adds.
To test the idea, the researchers took nursing mothers and put a male mouse in their cage after injecting some of them with CRH. Untreated mice launched a vicious attack on the intruder, attempting to bite vulnerable areas such as the head, neck and back. But, as the researchers report in the journal Behavioral Neuroscience1, mice injected with extra CRH were far less valiant and cowered in fear.
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Although the link between low levels of CRH and increased maternal aggression seems straightforward, Gammie admits that researchers do not know exactly how the process works. Perhaps lactation stimulates the brain to produce less CRH, he suggests. Or maybe the brain's cells simply become less responsive to the hormone.
Whatever the mechanism, the discovery will be relevant to studies of human postnatal depression, Gammie argues. Depression is often linked to high levels of stress hormones, he says. The fact that many depressed new mothers seem not to care about their baby's wellbeing could be because their raised levels of stress hormones prevent the low levels of CRH that seem to be required for the care of offspring.
"Our study suggests that CRH is an important factor in the procedure of protecting offspring," Gammie adds. "This study will make a lot of sense to people studying CRH in humans."
If the same process does occur in humans, then drug companies might be able to focus on CRH in the search for treatments for postnatal depression. Some 10-15% of new mothers experience depression lasting more than two weeks after giving birth, and around half of these require treatment with existing antidepressant medications.
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