Maternal aggression: new insights from Egr-1.

Lactating mice display fierce aggression towards novel, male mice. This study compares neuronal activity in the brains of aggression-tested (T) and -untested (U) mice using early growth response factor 1 (Egr-1; also known as Krox 24, NGFI-A, Zif268, Tis8, and ZENK) as a measure of neuronal activity. Animals were sampled 90 min after either a sham or real 7-min test with a male intruder, after which their brains were examined for immunoreactivity to Egr-1 (Egr-IR). Significant increases in Egr-IR in T mice were identified in 11 of 40 brain regions, including paraventricular nucleus of the hypothalamus; anterior and lateral hypothalamus (both posterior portion); ventromedial hypothalamus; lateral periaqueductal gray; and medial, central, and basolateral amygdala. Posterodorsal (MePD) and posteroventral medial amygdala were examined for the first time in association with maternal aggression. MePD, a region associated with both sexual and aggressive behaviors in rats, hamsters, and mice, showed increased Egr-IR in association with testing. Taken together, the results from this study provide new insights into the neural circuits regulating maternal behaviors.