Effects of cross-fostering on play and anxiety in juvenile Fischer 344 and Lewis rats.

A cross-fostering design was used to assess the relative involvement of genetic variability and early postnatal experiences on differential levels of playfulness in juvenile Fischer 344 (F344) and Lewis (LEW) rats and the extent to which strain differences in anxiety may influence play in these two strains. F344 dams were found to lick and groom their pups less than LEW dams and this was not dependent upon the strain of the pups in the litter. As expected, F344 rats were less playful than LEW rats as demonstrated by fewer playful contacts directed to the nape of a Sprague-Dawley (SD) partner and a decreased likelihood of rotating completely to a supine position when their nape was contacted by the SD partner. These differences could not be readily explained by parallel strain differences in anxiety. The pattern of effects on play as a function of cross-fostering depended on both the genetic background of the pup and the motivational state of the pup prior to testing. Whereas in-fostered LEW pups solicited more play as isolation prior to testing increased from 4 to 24h, cross-fostered pups of both strains as well as in-fostered F344 pups were relatively insensitive to the motivational modulation of play solicitation. Responsiveness to play solicitations in pups of both strains reared by F344 dams was insensitive to prior isolation whereas pups reared by LEW dams were less likely to respond with a complete rotation to a supine position when solicited as isolation increased from 4 to 24h prior to testing. These data suggest that the overall level of playfulness in a particular strain of rat is fairly resistant to cross-fostering and may be particularly sensitive to genetic variation whereas how this urge is titrated and modified by motivational factors may be influenced more by epigenetic factors.