Morphine-induced conditioned taste aversions in LEW/N and F344/N rat strains

The present experiment assessed the ability of morphine to condition taste aversions in the LEW/N and F344/N rats. Specifically, on four conditioning trials 35 LEW/N and 33 F344/N rats were allowed access to a novel saccharin solution and then injected with varying doses of morphine (0, 10, 32 and 56 mg/kg). On intervening recovery days, subjects were allowed 20-min access to water. Following the fourth trial, a final aversion test was administered. The F344/N rats, but not the LEW/N rats, rapidly acquired morphine-induced taste aversions at all doses of morphine. Pharmacokinetic differences between the strains were also assessed. Specifically, 10 mg/kg morphine (or vehicle) was administered to subjects of both strains and plasma morphine levels were analyzed at 0.5, 2 or 4 h post injection. No differences in plasma levels between the strains were observed. Implications for the differential sensitivity to morphine-induced taste aversions between the strains are discussed.