Potency of SCH 23390 for decreasing sucrose intake in rat pups depends on mode of ingestion.

To investigate the role of dopaminergic mediation of the positive reinforcing effect of sucrose during development, we tested the effects of the D-1 antagonist, SCH 23390, on the intake of 10% sucrose in 7-, 14-, and 21-day-old rats during independent ingestion (pups lick sucrose from the floor of a beaker) or during continuous intraoral infusion of sucrose. SCH 23390 inhibited intake more in independent ingestion tests than in intraoral catheter tests. At 7 and 14 days this difference was qualitative--SCH 23390 was efficacious in independent ingestion tests, but not in intraoral catheter tests. At 21 days the difference was quantitative--SCH 23390 decreased intake in both tests, but was more potent in independent ingestion tests. SCH 23390 decreased intake during independent ingestion tests without changing latency to eat or time-sampled activity scores at 7 and 14 days, but not at 21 days; thus the inhibition of intake is not accounted for by a generalized motor deficit at 7 and 14 days. Possible explanations for the differential potency include density of reinforcement, pattern of central dopamine metabolism, and interference with the appetitive movements required to maintain contact with the sucrose in the independent ingestion test.