Neonatal handling shapes the adult development of social behavior in rats: enduring effects after prolonged social deprivation.

Neonatal handling is an early-life intervention known to produce long-lasting reductions in emotional reactivity and physiological responses to stress in rodents. Although its effects on social behavior remain less well characterized, previous studies have shown that neonatal handling enhances juvenile play behavior, suggesting broader effects on the social behavioral repertoire. The present study examined whether the social effects of neonatal handling observed in younger animals persist into adulthood by assessing social interaction in 10-month-old rats. Social interaction was quantified as time spent in physical contact during two consecutive test sessions following either prolonged social deprivation (seven months of individual housing) or four days of resocialization after the same isolation period. At 2 months of age, we replicated the well-established finding that handled rats showed lower emotional reactivity than non-handled controls in both the open field and elevated plus maze. More importantly, at 10 months of age, handled rats generally spent more time in physical contact than non-handled controls. Although prolonged social deprivation increased social contact in all animals, handled rats generally showed higher levels of affiliative social interaction. Together with previous findings on juvenile play, these results indicate that neonatal handling produces enduring changes in affiliative social behavior that remain detectable into adulthood and may reflect long-lasting alterations in social motivational processes.