Pup odor and ultrasonic vocalizations synergistically stimulate maternal attention in mice

Abstract

Pup ultrasonic vocalizations (USVs), which are emitted by hypothermic pups, and pup odor are thought to be triggers of maternal behavior in mice. We investigated whether pup odor stimulated maternal responses to pup USVs in mother C57BL/6 mice. Two-choice tests were conducted by introducing mothers into a test cage in which a tube was attached on each long wall, and the duration spent in each tube was compared. Pup USVs were reproduced by an ultrasonic speaker at the tube end. In some cases, cotton with pup odor was also presented at the end of the tube. Compared to no stimuli, mothers did not specifically approach the sole presentation of either reproduced pup USVs or pup odor. However, compared to the sole presentation of pup odor, the simultaneous presentation of pup USVs and odor induced a specific approach response. These results suggested that pup USVs and odor synergistically stimulated maternal behavior. In addition, it was confirmed that mothers approached hypothermic pups emitting pup USVs for longer than anesthetized silent pups. To investigate the underlying neural mechanisms, we observed neural responses to various stimuli with the immunohistochemistry of c-fos expression. In the bed nucleus of the stria terminalis, the medial preoptic area, the central nucleus of the amygdala, and the basolateral amygdala, the numbers of c-fos-positive cells were significantly increased following the simultaneous presentation of pup USVs and odor compared to the presentation of each alone, suggesting that these nuclei were involved in multimodal processing related to maternal behavior.