The effects of light cycle phase on morphine-induced conditioned taste aversions in the Lewis, Fischer and Sprague-Dawley rat strains.

The present experiment investigated the effect of light cycle phase on morphine-induced conditioned taste aversions in the Lewis (LEW), Fischer (F344) and Sprague-Dawley (SD) rat strains. Separate groups of rats from each strain were trained during either the light phase or the dark phase on a procedure in which saccharin was paired with one of two doses of morphine (or vehicle). With 3.2mg/kg morphine, strain differences were observed during the light phase, with F344 rats displaying a significantly stronger taste aversion than the LEW rats, who displayed a significantly stronger aversion than the SD rats. In contrast, during the dark phase, 3.2mg/kg morphine produced comparable, moderately strong aversions in all strains. With 10.0mg/kg morphine, F344 rats developed stronger aversions than either the LEW or SD rats in both phases of the light cycle. The effect of light cycle was most clearly seen in the SD rats, where stronger aversions were produced in the dark phase for both morphine doses. For the LEW rats, stronger aversions were produced in the dark as compared to the light only with the low dose of morphine. For the F344 rats, aversions of comparable strength were observed in both phases of the light cycle for both morphine doses. The finding that light cycle differentially affects morphine-induced taste aversions in these strains is consistent with what is known about strain differences in circadian patterns of corticosterone activity and with previous results relating corticosterone to morphine-induced taste aversions.