Isolation during the play period in infancy decreases adult social interactions in rats

Abstract

The effects of 1 or 2 weeks of social isolation immediately after weaning on social activity in adulthood were investigated in rats. In addition, it was studied whether these effects were influenced by social experiences of the cagemate when rehoused after the isolation period. Isolation during weeks 4 and 5 of age caused a reduction of social activity as compared to non-isolated controls. Previous social experiences of the cagemate (isolated or non-isolated) did not affect this decreased social activity. Isolation during week 4 of age resulted in similar effects, but the reduced social activity was not present when the rats were rehoused with non-isolated rats. Isolation during week 5 of age did not influence social activity patterns in adulthood.These findings support the idea of a sensitive period in infancy for subsequent social behavior in rats. It is suggested that especially deprivation of acquiring play behavior underlies the social disturbances in adulthood.

Introduction

For animals that live in social groups, like rats, adequate social functioning requires a complex and well-organized repertoire of social behaviors. Adult social behavior is characterized by the ability to respond with appropiate behaviors within the correct context at the right moment 18, 20. Social skills depend not only on the genetic constitution, but is also influenced by factors as prenatal conditions, sex, social rank of the animal and early social experiences 20, 34. Several detailed and descriptive studies on the development of social behavior in the rat 1, 20, 24have shown that during this development certain specific periods can be distinguished 31, 32. The critical period hypothesis [31]postulates that the young of most social species pass through chronological periods during which specific social stimulation is necessary for adequate subsequent social development. Rats are unusually susceptible to environmental influences during the period in which primary social bonds are formed [32]. During this period, young rats spent more and more time away from their mother and their nests and the first non-maternal directed social interactions are observed, i.e social play behavior. Parts of adult aggressive, sexual and social behavioral patterns can be recognized in social play, although most of the time in modified and exaggerated forms 25, 26, 35. The occurrence of social play follows an inverted U-shape during development, it emerges at about 18 days postnatally, peaks during weeks 4 and 5 of age, and decreases with age till sexual maturity 1, 21, 36, 38.

Several deprivation studies have been performed in order to investigate the importance of early social experiences for subsequent social development. When tested in adulthood, rats reared without any social experiences early in life exhibit altered patterns of mating behavior 8, 9, 10, 11, agonistic 18, 37and social behaviors 20, 24, 37. Since all these disturbances have in common that social interactions are involved, it is concluded that early social experiences are essential for the development of normal social responsiveness in the rat.

In the present study, the consequences of 1 or 2 weeks of social isolation immediately after weaning followed by different rehousing procedures on adult social activity were investigated. Social activity was measured by using an objective investigation method, i.e automated social interaction test as described and applied earlier 15, 29, 33. The different rehousing procedures were conducted to investigate whether the previous social experiences (isolated or non-isolated) of the cagemate could influence the isolation-induced social behavioral effects. It is known that resocialization can reverse changes caused by previous isolation 3, 7, 27. However, isolation introduced at weaning often causes irreversible disturbances 3, 13, 39.