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Intake of a saccharin-conditioned stimulus (CS) can be suppressed following pairing with an aversive agent such as lithium chloride (LiCl) or x-rays (referred to as a conditioned taste aversion or CTA), a highly rewarding sucrose solution (referred to as an anticipatory contrast effect), or a drug of abuse such as morphine or cocaine. Although the suppressive effects of LiCl and sucrose are clear examples of aversive and appetitive conditioning, respectively, it is not certain which properties (aversive or appetitive) mediate the suppressive effects of drugs of abuse. It is known, however, that the suppressive effects of a rewarding sucrose US are attenuated when using a caloric sucrose CS in food deprived rats, while LiCl induced CTAs are much less effected. Standard CTA testing typically is conducted in water-deprived rather than food-deprived rats and, although LiCl is known to suppress intake of a sucrose CS in water-deprived rats, the suppressive effects of drugs of abuse have not been evaluated under these conditions. The present experiment, then, compared the suppressive effects of a standard dose of morphine (15 mg/kg) and a matched dose of LiCl (0.009 M) on intake of a sucrose CS in water-deprived and free-feeding rats. The results showed that both drugs suppressed intake in free-feeding subjects, but only the aversive agent, LiCl, reduced CS intake in the water-deprived rats. This finding dissociates the suppressive effects of morphine and LiCl and, in so doing, aligns the suppressive effects of morphine with those of an appetitive sucrose US.
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