Commentary: Is maternal stimulation the mediator of the handling effect ininfancy?

In a recent report. a series of studies is described showing that individua l differences in rat maternal licking and grooming are correlated with thei r offsprings' later adrenal response to a stressor (Liu et al., 1997). Pups that received more maternal stimulation in infancy had lower ACTH, cortico sterone, and CRH mRNA, while they had greater amounts of GR mRNA. Liu et al. also compared maternal behavior in litters where rat pups were h andled daily to maternal behavior of non handled litters. They found that m others of handled pups licked nad groomed their young significantly more of ten than did mother of control litters. In their discussion, Liu et al. pro posed that their data support Levine's (1975) thesis that handling effects in infancy are mediated via the mother. Levine had proposed that handling o f pups will modify the mother-pup interaction, thereby causing a change in maternal behavior. The purpose of this commentary is to summarize data obtained with mice and rabbits which, in part, support and, in part, support and, in part, challen ge the conclusions of Liu et al. (C) 1999 John Wiley & Sons, Inc.