Fostering and environmental enrichment ameliorate anxious behavior induced by early weaning in Balb/c mice.

Postnatal stimuli affect many aspects of physiological and behavioral development. In mice, earlier weaning augments anxiety, putatively as a result of removing mother-pup interactions during the weaning period. Here, we examined the ameliorating effects of social and environmental enrichment on anxiety related to early weaning. Mice weaned at postpartum day 14 were fostered by virgin females, who displayed some nursing behavior during the 1-week fostering period. In elevated plus-maze tests, 10-week-old pups reared with a foster mother spent more time in the open arms than early-weaned mice, and entered into the open arms at a rate between that of normally- and early-weaned mice. Subsequently, the mice from each rearing group were transferred into either standard housing or housing enriched with toys that were changed periodically. Elevated plus-maze tests were conducted again when the mice were 18 and 26 weeks old. The enriched environment increased the duration of time spent in the open arms, but the magnitude of the effect varied with the rearing condition. Furthermore, mice that lived in the enriched environment showed lower activity than those kept in standard housing. These results suggest that fostering after early weaning attenuates increases in anxiety levels, and maternal care during this period may be important in the development of an offspring's emotionality. Environmental stimuli in adulthood may act to blunt the effects deprivation in early life.