A chemical messenger in the brain that dramatically decreases aggressive behavior in male mice appears, on the contrary, 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 The Johns Hopkins University.
The new results, reported in the September 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 versus 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 at Johns Hopkins 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 Johns 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.