Motherly love seems to be hardwired in most females, but sometimes there's a glitch and moms abandon their childcare responsibilities. Among mice, the moms even sometimes eat their young when this apparent switch goes haywire.

A new study on mice reveals a possible clue for such anomalous behaviors. Turns out, the hormone serotonin, known to boost moods in humans and mice, could be linked with maternal care.

Mice lacking the so-called "happy hormone" neglected their pups, stepping on them and even eating them in some cases, new lab research finds. While mice aren't perfect models for human behavior, the researchers think such results could be transferrable to us.

"We think that data explaining how serotonin influences maternal care will also tell us something about the human situation," said study team member Michael Bader of the Max Delbrück Center for Molecular Medicine Berlin-Buch, Germany.

The finding, published in the June 23 issue of the journal Proceedings of the National Academy of Sciences, also has implications for the treatment of depression in humans with drugs that increase actual serotonin levels in the brain.

Serotonin switch

In the study, researchers "switched off" a gene called Tph2 in a group of mice. The gene is responsible for producing an enzyme linked with serotonin in the brain. So one group of mice showed nearly no serotonin production in the brain and the "control" group produced brain serotonin.

(While we are most familiar with serotonin produced in the brain, the hormone is also produced in the gut and gets released into the bloodstream. The so-called blood-brain barrier keeps that serotonin from entering the brain.)

The serotonin-deficient mice were just as fertile as their normal counterparts, with half of them surviving to adulthood to produce offspring. But the female moms lacking serotonin neglected their pups and ate many of them.

The neglect meant that by day two or three after birth, most of the pups born to such moms were dead.

Lousy moms

To figure out whether the offspring's demise was related to the pups' own health or lack of maternal care, the researchers placed each of the serotonin-deficient moms into a cage with pups born to the "normal" mice, and vice versa (control moms were plopped into cages with mice born to moms without serotonin).

After a half-hour acclimation, the nest tended by the serotonin-deficient mothers was intentionally destroyed by researchers and the pups were scattered throughout the cage. The experimenters gave the mouse moms 30 minutes to see if they would collect pups and rebuild the nest.

That task was a walk in the park for the normal moms. In an average of about four seconds, such females had collected all litter pups, placing each in her mouth and carrying it to a hill-like nest. (Average litter size was about 10 pups.)

"The serotonin-deficient mother, she runs and even steps on her babies and doesn't care," Bader told LiveScience. "She just runs around the cage. She has nothing to do with these babies."

The lousy caregivers also were about five times more likely to eat their offspring compared with the normal female mice.

"It's really a maternal problem and not that the offspring is already somehow damaged," Bader said.

Human depression

Bader and his colleagues suspect the brain serotonin is somehow linked to two other hormones, oxytocin and vasopressin, which are known to play a role in both motherly and romantic love. This could explain the bad-mom finding.

And they say the gene Tph2 could be a new drug target for humans, though any such drug is a ways off in the future, Bader said.

"So our idea is to develop drugs that would activate the enzyme to make more serotonin," Bader said, adding that it could have similar or even better effects than current drugs on the market to treat depression, called selective serotonin reuptake inhibitors (SSRIs). (These drugs, rather than resulting in more serotonin, block its reabsorption, leaving more of the hormone available for use in the brain.)

Plus, he said, the new drug target could be more precise, affecting only serotonin levels in the brain. Many SSRIs can impact other tissues in the body, he said.