Ontogeny of hypertonic preabsorptive inhibitory control of intake in neonatal rats

The ontogenetic development of postingestive inhibitory control of ingestion by the osmotic load of a preload was examined in rats. On postnatal days 6 (P6) and 12 (P12), pups were deprived for either 6 or 24 h. Gastric preloads (5% body wt) of water, mannitol (a sugar alcohol that is not absorbed) in six concentrations [from 0.125 M (hypotonic) to 1.0 M (hypertonic)], or sham preloads were administered 5 min before a 30-min intake test. Compared with sham treatment, isotonic mannitol (0.25 M), a probe of volumetric control, significantly reduced intake on P12, but not on P6. Compared with isotonic mannitol, the three highest hypertonic concentrations (0.5, 0.66, and 1.0 M) significantly decreased intake on P12, at both levels of deprivation. On P6, 0.66 and 1.0 M mannitol reduced intake after 24 h, but not after 6 h, of deprivation. Thus, on P6, the hypertonic control was detectable only after prolonged deprivation and the volumetric control was not present. On P12, both controls were observed and the hypertonic control was more potent than on P6.