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The effect of a dopamine antagonist and an agonist on rats’ perception of reward quantity : an examination of the anhedonia hypothesis Martin-Iverson, Mathew Thomas


A procedure was developed to determine the effect of a dopamine (DA) antagonist (haloperidol) and a DA agonist (d-amphetamine) on rats' perceptions of the hedonic value of food. Eighteen rats were trained to discriminate between two quantities of sweet food pellets (1 and 4), in a forced-choice two-lever successive discrimination procedure. To control for non-specific perceptual effects of the treatments, the rats were also trained to discriminate between 1 and 4 tones. It was established that rats attended to the value of food, as well as the proportional differences in quantity, when discriminating food quantities. This was accomplished by altering the value of the food in two ways. Firstly, "hunger" was altered by changing the degree of food deprivation during testing. Secondly, unsweetened food pellets were introduced as probe cues. These two methods of altering the value of food pellets were utilized while quantity generalization gradients were determined, by presenting animals with 1,2, 3 and 4 numbers of stimuli as probe cues. Two measures were derived from these generalization gradients: the point of subjective equality (PSE), which is the calculated number of stimuli that would maintain responses equally distributed between the two levers, and the slope of the gradient. The PSE primarily reflects perceptual processes, while the slopes of the gradients provide an index of performance impairment. It was observed that decreasing the value of food by either decreasing food deprivation or reducing the sweetness of the food pellets resulted in the rats perceiving a given quantity of food as larger than before these treatments (decreased the food PSE). Neither altering food deprivation nor introducing novel tone probes had an effect on the numerical attributes of tones, as reflected by the tone PSE. Haloperidol (0.030, 0.50 and 0.083 mg/kg, i.p.) produced a statistically significant, but slight dose-dependent performance deficit, as reflected by the slope of the generalization gradients. It did not affect the perception of food pellet quantities at any dose, as reflected by the food PSE. Haloperidol decreased the number of tones a given quantity was perceived as by rats (increased the tone PSE). Amphetamine (0.25, 0.50 and 1.0 mg/kg, i.p.) decreased the perception of a given quantity of food (increased the food PSE) in a dose-dependent manner, without a significant effect on performance. Thus, amphetamine enhanced the hedonic value of food. Amphetamine also increased rats' perceptions of a given number of tones (decreased the tone PSE). It therefore appears that while d-amphetamine can enhance the perceived hedonic value of food, haloperidol has no effect on rats' perceptions of the hedonic value of food. Furthermore, evidence that DA systems are involved in the mechanism of an "internal clock" or "counter" was obtained.

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