Dissociation between neonatal novelty-induced preferential maternal care and enhancement in cognitive, social, and emotional functions

Early life stimulation is known to produce long-lasting changes in the brain and behavior. One such early stimulation method is the neonatal novelty exposure procedure which allows the isolation of the novelty effect from several prominent confounding factors inherent to the neonatal handling procedure. In two previous studies, we found long-lasting novelty effects on different sets of functional measures without accompanying preferential maternal care, even when the observation was made immediately after the novelty manipulation, a time when such preferential care is most likely to be expressed. Here, within a single cohort of Long-Evans male rats, we demonstrate that novelty exposure leads to enhancements across several functional domains, including increased disinhibition to novelty, enhanced spatial and social memory, and reduced aggression, again without the accompaniment of preferential maternal care. These findings extend novelty exposure effects to aggression and replicate previously known novelty exposure effects on spatial and social memory with extension to new developmental stages. Most importantly, these findings do not support the hypothesis that preferential maternal care towards novelty-exposed pups mediates the observed novelty effects. We discuss the possibility that the effects of neonatal novelty exposure are mediated via repeated activation of the hypothalamic–pituitary–adrenal (HPA) axis that serves to inoculate pups for future exposures to novelty and novelty-induced HPA activation and that maternal influence is likely to be expressed via its modulatory role—the mother sets the individual-family specific behavioral and hormonal context to allow the same early life experience to have a family-specific effect.

• Early experience of novelty reduces aggression during dyadic social interaction in a neutral environment. • Previously demonstrated novelty effects are replicated with extension to a broader age range. • The dissociation between preferential maternal care and these multiple novelty effects is replicated with variations. • The results are better explained by the stress-inoculation hypothesis than the maternal mediation hypothesis. • The role of the mother in these novelty effects is better viewed as modulatory as opposed to mediatory.