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

Abstract

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 24 hours, 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 24 hours 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.

Effects of neonatal handling on play and anxiety in F344 and Lewis rats

“…As such, we also assessed behavior in the elevated plus maze in the present study. When tested as juveniles, F344 rats spent more time on the open arms of the elevated plus maze than LEW rats and this is consistent with what we have observed previously (Siviy et al, 2017). However, handled rats spent as much time on the open arms as the non-handled rats suggesting that handling did not affect baseline levels of anxiety in either strain.…”

“…Increasing isolation increased play solicitation to a comparable extent in both control and handled F344 rats. This pattern of results for LEW rats is remarkably similar to what we observed in a previous study looking at the effects of cross-fostering on play in LEW and F344 rats (Siviy et al, 2017). In particular, in-fostered LEW rats in that study had the expected increase in play solicitation as pre-testing isolation increased from 4 hours to 24 hours whereas cross-fostered LEW rats failed to show any motivational modulation of play solicitation.…”

“…In order to further examine the dynamic interplay likely to be occurring between early postnatal experiences and genetic background, the present study assessed the effects of neonatal handling on play in the inbred F344 and LEW strains. Rats of the F344 strain have been shown to be consistently less playful than rats of the LEW and other strains to which they have been compared (Siviy et al, 1997; Siviy et al, 2011; Siviy et al, 2017; Siviy et al, 2003) so comparing the impact of a well-defined manipulation such as neonatal handling in two inbred strains with well-characterized phenotypic differences may allow for a better understanding of how these types of early postnatal experiences can operate on an existing genotype to influence the behavioral phenotype.…”

“…In a recent cross-fostering study play was assessed in both cross-fostered and in-fostered F344 and LEW rats (Siviy et al, 2017) and it was found that the overall strain difference in playfulness was relatively unaffected by fostering status. In other words, F344 rats that were reared by LEW mothers were no more playful than those reared by F344 mothers.…”

“…Using a protocol originally developed by Myers and colleagues (Myers, Brunelli, Squire, Shindeldecker, & Hofer, 1989) and used previously in our lab (Siviy et al, 2017) a discrete observation was made every 4 minutes for each litter over the course of each 1 hour period. With 16 observations in a 1 hour observation period and 28 observation periods over 10 days, this resulted in a total of 448 discrete observations.…”