Chronic morphine treatment exaggerates the suppressive effects of sucrose and cocaine, but not lithium chloride, on saccharin intake in Sprague-Dawley rats

Three experiments examined the effect of chronic morphine treatment on coca ine-, sucrose-, and lithium chloride (LiCl)-induced suppression of sacchari n intake in Sprague-Dawley rats. All rats were either water- or food-depriv ed and then implanted subcutaneously with 1 morphine (75 mg) or vehicle pel let for 5 days. They were then given brief access to 0.15% saccharin and so on thereafter injected with either cocaine (10 mg/kg sc), LiCl (0.009 M, 1. 33 ml/100 g body weight ip), or saline, or, in Experiment 2, given a 2nd ac cess period to either a preferred 1.0 M sucrose solution or the same 0.15% saccharin solution. There was 1 taste-drug or taste-taste pairing per day f or a number of days. The results showed that a history of chronic morphine treatment exaggerated the suppressive effects of a rewarding sucrose soluti on and cocaine but not those of the aversive agent, LiCl. These data provid e further support for the reward comparison hypothesis.