Morphine enhances taste preference and aversion: Evidence for opiate mediation in reinforcement or memory

In pursuance of the proposition that opiates are involved in the mediation of reinforcement/retention, the influence of morphine on acquisition of taste preference was examined. Water-deprived rats were permitted access to novel sucrose (1.5%) or quinine (0.0024%) solutions for 10 minutes and were then immediately injected (IP) with saline, 2.7 or 4.2 mg/kg morphine HCl. This conditioning procedure was repeated seven times at 2-3 day intervals, each group (n = 9) receiving only one treatment combination. Subsequently, preference ratios for water or solution obtained in a two-bottle choice test were compared between tastes. Morphine (4.2 mg/kg) reduced quinine, and increased sucrose preference relative to water (F (1,48) = 3.31, p < 0.05), suggesting a reinforcing effect on conditioning, rather than reflecting morphine's known aversive and rewarding properties.