Ontogeny of hyperphagia in the Zucker (fa/fa) rat.

The ontogeny of hyperphagic behavior in the Zucker fatty (fa/fa) rat was examined. Wild-type, +/fa, and fa/fa pups aged postnatal day 5 (P5), P9, P12, P15, and P18 were evaluated using a test that measured ingestive behavior independent of the dam. The independent ingestive test consisted of giving pups access to a test solution [half-and-half (cream and milk)] on a tissue on the floor of a test chamber for 20 min. The latency to ingest and the intake (weight gain and percent weight gain) were measured and normalized to +/fa littermates. Pups were tested once to eliminate any effects of test experience. fa/fa Pups ingested significantly more than lean pups (+/+ and +/fa) on P12, P15, and P18, but not on P5 or P9. The latencies of fa/fa pups did not differ significantly from the latencies of +/+ pups except on P18, when the latencies of fa/fa pups were significantly shorter. The latencies of +/fa pups were significantly longer than the latencies of fa/fa or +/+ pups on P5 and P12. These results demonstrate that hyperphagia in fa/fa rats emerges between P9 and P12 under the test conditions used.