Conditioned taste preference produced by pairing a taste with a low dose of morphine or sufentanil

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Abstract
Taste conditioning produced by pairing a taste with low doses of morphine or sufentanil was studied in rats in five experiments. Conditioned taste preferences were obtained with a trace conditioning procedure in which ingestion of a flavored solution was followed by an injection of sufentanil, either 0.25 mcg/kg in experiment 1 or 0.50 mcg/kg in experiment 2. Morphine produced less consistent results than sufentanil. When a similar trace conditioning procedure was used with morphine, a dose of 0.25 mg/kg produced no observable taste conditioning in experiment 3 while 0.42 mg/kg was marginally effective in producing a conditioned taste aversion in experiment 4. In experiment 5, however, conditioning of a taste preference was produced by 0.42 mg/kg morphine with a simultaneous conditioning procedure in which the morphine injection preceded ingestion of the flavored solution. The simultaneous procedure was presumed to facilitate the conditioning of taste preference by minimizing the conditioning of taste aversion.