A chemical messenger in the brain that dramatically decreases aggressive behavior in male mice appears to be essential to a mother mouse's ability to lash out at strange mice in defense of her pups, according to a new report by scientists at Johns Hopkins.
The new results, reported in the Sept. 15 issue of the Journal of Neuroscience, establish a potential link between a single brain chemical--the neurotransmitter nitric oxide--and opposing effects in male vs. female brains.
"This result was a big surprise, given what we saw in the male mice," says Stephen Gammie, a postdoctoral fellow in the Department of Psychology in the Krieger School of Arts and Sciences and lead author of the paper. "This may reflect the fact that male and female rodents need to be aggressive in different situations and as a result have different mechanisms for controlling aggression. It's one more piece of evidence of how complex brain chemistry is, even at the level of the mouse."
The new findings could also help scientists begin to understand at a very basic level some of the brain and behavioral changes brought about by pregnancy.
The experimental mice, originally created by scientists to study brain damage from stroke, have been given a defective copy of the gene for nitric oxide synthase, a protein that creates nitric oxide for use as a signal in brain cells. Theoretically, this produces a near-complete absence of nitric oxide in brain cells.
Four years ago, researchers at Hopkins announced that this modified mouse line had suffered an unexpected side effect: The males were unusually aggressive, relentlessly attacking other males and ignoring female rejection of attempts to mate. The latest research sought to expand that finding by determining the effect of a lack of nitric oxide on females' aggressiveness.
"Male mice are aggressive in a variety of contexts, but the female mouse normally only attacks other mice when she's caring for pups after a pregnancy and is approached by a strange male mouse," says Randy Nelson, professor of psychology in the School of Arts and Sciences and a co-author of the new paper. "The female's aggression reduces the danger that the male will attack her pups."
For the experiment, funded by a grant from the National Institute of Mental Health, researchers used nine genetically modified pregnant mice and 13 normal pregnant mice. Four days after the mouse pups were born, they began to expose the new mothers to strange male mice for 10 minutes daily. To protect them from harm, the pups were removed from the cage before each exposure, a step previously shown to have no effect on maternal aggression. The results were videotaped.
The experimental mice made significantly fewer efforts to drive off the stranger, and often failed to make any discernible attack.
"Compared to the normal mice, the experimental mice spent one-fifteenth of the time attacking the strange males," says Gammie. "On the rare occasions when the experimental mice did appear to make some kind of attack, it was so mild that it produced little reaction in the male mice." When exposed to the attacks and bites of normal females, males either fought back or fled.
To see if the genetic alteration had affected other aspects of maternal behavior, Gammie and Nelson gave mice from both groups a "pup retrieval" test immediately after exposures to the male mice. They returned the pups to the cage at random spots, and noted the time it took the mothers to gather the first and fourth pups back to their nests.
"There were no significant differences in this and other aspects of maternal behavior," Gammie says. "The experimental mice seemed perfectly good at nest building, and the weight and survival rate among their pups was actually higher."
Researchers also analyzed the brains of the mice for citrulline, a chemical by-product created when brain cells make nitric oxide. The number of brain cells making the compound significantly correlated with the aggressiveness of the female mouse. In in-stances where males were aggressive, though, no significant citrulline production occurred.
"In males, it's useful to be aggressive throughout the breeding season because this can increase access to females and also can lead to attacks on the pups of another male," says Gammie, speculating on the reasons for the differences in brain chemistry. "For females, it is dangerous to challenge a larger male, so it makes sense to limit expression of aggression to times when the pups are in the nest and need protection. Females, then, need a way of quickly turning on aggression but then quickly turning it off."
To further explore the link, scientists plan to test whether they can replicate the effects of the altered gene using nitric-oxide suppressing drugs. Scientists hope the new experiment will solidify the connection between nitric oxide and maternal aggression, and help them more closely understand how brain changes create aggression.