<p class="first" id="P2">After receiving 2.0 mg/kg amphetamine, rats show two phases
of reduced food intake,
short-term hypophagia, during the first several hours after treatment, and longer-term
hypophagia, approximately 19 to 26 hours after treatment. The longer-term hypophagia
may be an indicator of an acute withdrawal. This study assessed whether D1 and D2
receptor activation were important early events in the elicitation of longer-term
hypophagia. Throughout a series of five-day tests, rats could lever press for food
pellets for one-hour periods beginning every three hours. On test day 1, rats were
given a saline pretreatment, and fifteen minutes later they were given a saline treatment.
On test day 3, they were given a pretreatment of either saline or a selective dopamine
receptor antagonist, and fifteen minutes later they were given a treatment of either
saline or amphetamine (2.0 mg/kg). In Experiment 1, pretreatments included 3, 12,
31, and 50 μg/kg of the selective D1 receptor antagonist SCH 23390. In Experiment
2, pretreatments included 25, 50, and 100 μg/kg of the selective D2 receptor antagonist
eticlopride. Distance moved was monitored for the first six hours following pretreatment-treatment
combinations to obtain an indirect behavioral measure of receptor blockade (antagonist
attenuation of amphetamine hyperactivity). Food intake at each meal opportunity was
monitored throughout each five day test. Patterns of food intake following day 1 saline-saline
and day 3 pretreatment-treatment were compared. The combination saline-amphetamine
produced short-term and longer-term hypophagia. Combinations involving antagonist-saline
did not produce longer-term changes in food intake. Pretreatment with 12 to 50 μg/kg
of SCH 23390 produced substantial blockade of amphetamine hyperactivity and prevented
amphetamine-induced acute-withdrawal-related longer-term hypophagia. Eticlopride produced
a partial blockade of longer-term hypophagia. Both D1 and D2 receptor activation are
required for full expression of longer-term hypophagia following amphetamine administration.
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