Harauma, A., Murai, M., Nakamura, I. et al. 2023. Anxiety-like behaviors in artificially reared mice is reduced by contact with foster mothers. Physiology & Behavior 260, 114052.

Experimental systems using animal models are important for nutritional and pharmacological research on newborns and infants. Accordingly, artificial nursing systems for rodents based exclusively on formula are critical to studying nutrient requirements, chemical safety, and immune system development in newborns and infants while eliminating the influence of mother’s milk. Such systems can also be used to study the influence of rearing environment-related factors, including physical contact between newborns and mothers or caregivers. However, artificially reared (AR) mice exhibit higher anxiety levels than dam-reared (DR) mice. Therefore, in addition to AR and DR groups, we produced a third group of mice cared for by ovariectomized foster mothers except during nursing time (AR+OVX) and investigated the impact of infant rearing environment on emotional behaviour in adult male C57BL/6 J mice. In the behavioural evaluation with mild stress such as fasting, AR+OVX mice exhibited intermediate anxiety levels between those of DR and AR mice. AR+OVX mice reached anxiety levels similar to those of DR mice in a behavioural evaluation under less stressful conditions, although AR mice remained at high anxiety levels. This suggests that care with physical contact and warmth from foster mothers leads to emotional development similar to that of DR mice, even when reared on artificial milk. This experimental system also makes it possible to investigate the importance of nutrients during the neonatal period while suppressing the influence of rearing environment-related factors.