Neural circuits that suppress parental behavior in adult laboratory rats (Rattus norvegicus) and mice (Mus musculus)

This review examines the nature of a defensive neural system that suppresses parental behavior in sexually inexperienced (virgin) female laboratory rats and virgin male laboratory mice. Virgin female mice are not initially examined because many virgin female laboratory mice are 'spontaneously' maternal. While virgin female rats typically avoid pups, many virgin male mice initially attack pups. Despite these different responses, there are strong commonalities in the nature of their respective defensive neural systems. A comparison of the rat and mouse literature indicates that input from the vomeronasal organ activates the medial amygdala (MeA). The MeA and its connections to the bed nucleus of the stria terminalis, in turn, inhibit the medial preoptic area (MPOA), a hypothalamic region that promotes parental behavior, while also activating a medial hypothalamus-periaqueductal gray defensive region that causes virgin rodents to avoid or attack pups. While the foundational research on this circuit was performed on female rats, more recent research on mice, using advanced neuroscientific techniques, has added more detail to the defensive neural system by uncovering the involvement of urocortin 3-expressing neurons in the perifornical area of the hypothalamus and corticotropin releasing factor receptor 2-expressing MeA neurons. When female rats and male mice become parents, MPOA efferents inhibit the defensive system and activate parental motivation. Although many virgin female mice are 'spontaneously' maternal, a latent defensive system exists which can be activated by environmental stressors. The postpartum mouse is less vulnerable to such stressors, and the neural basis of such resilience is examined.