The effects of cross -fostering on morphine-induced conditioned taste aversion in Fischer and Lewis rats

<p>Inbred Fischer (F344) and Lewis (LEW) rats differ on a number of physiological and behavioral indices, some of which have been reported to be affected by maternal experience. To assess whether epigenetic factors contribute to the differential behavioral responses to drugs in these strains, the effect of cross-fostering on morphine-induced conditioned taste aversion in males and females was examined in the present experiment. Specifically, at birth 67 LEW pups were cross-fostered to F344 dams while 60 LEW pups were raised by LEW dams (in-fostered). Similarly, 60 F344 pups were cross-fostered to LEW dams while 50 F344 pups were raised by F344 dams (in-fostered). None of the pups was raised by their own dam. On the first day of testing (Post natal Day 60), saccharin was paired with one of two doses of morphine (10 and 32 mg/kg) or saline (control group). This procedure was repeated three more times (Trial 1--4) followed by a Final Aversion Test. In-fostered F344 rats, both males and females, acquired stronger aversions than in-fostered LEW at all doses of morphine. The cross-fostering manipulation affected the LEW strain more than the F344 for both males and females. Specifically, cross-fostering attenuated these strain differences for females at both doses. Cross-fostered F344 and LEW males showed similar effects but only at 10 mg/kg. From these results, it is clear that epigenetic factors such as maternal rearing can affect the acquisition of conditioned taste aversions in F344 and LEW strains. It remains unknown to what extent (if any) other differences between F344 and LEW rats are mediated by early life experiences. The fact that epigenetic factors are important for the acquisition of taste aversions and that drug use is a function of both the rewarding and aversive effects of drugs, these findings may have implications for the use of these strains in assessing the genetic mediation of drug acceptability.</p>