ACUTE, EARLY THERMAL EXPERIENCE ALTERS WEANING ONSET IN RATS

We hypothesized that first ingestion of solid food (weaning onset) wou ld be accelerated in young rats with advanced thermoregulatory develop ment. To manipulate the pups' thermoregulatory development, we exposed rat pups, but not their dams, to a Cold (10 degrees C), Moderate (21 degrees C), or Warm (31 degrees C) ambience for 2 h/day from postnatal Day 2-14, expecting that early exposure to cooler temperatures would accelerate development of thermoregulatory capabilities and thus accel erate nest egression as well as onset of feeding. Contrary to expectat ion, cold exposure was associated with a profile of developmental dela ys in both growth and maturation. Pups in the Cold condition began fee ding later than pups with Moderate or Warm thermal experiences. We the n evaluated thermoregulatory status (mechanisms for heat production an d temperature conservation) on Day 15-16 (just prior to weaning onset) . Thermogenesis, measured by oxygen consumption, was unaltered by the thermal manipulation. In contrast, pelage development (insulation) was altered. Pups in the Warm condition had greater fur density and an in creased frequency of longer hairs relative to pups in the Cold conditi on. Although the developmental response to early cold exposure was in the direction opposite to our predictions, the hypothesized relation o f thermoregulatory development to weaning onset was supported: Thermor egulatory status correlated with weaning onset. (C) 1998 Elsevier Scie nce Inc.