Measurement of solid feed consumption patterns in neonatal rats by 141Ce-radiolabeled microspheres.

Dams nursing litters of Fischer 344 rats were allowed access to feed containing nonabsorbable 141Ce-labeled NEN-TRAC microspheres for 24-hr intervals during the later half of a 28-day nursing period. Neonatal solid feed consumption began during the third week of nursing and rose to peak amounts as high as 2.1 times normal adult levels on a gram consumed per kilogram body weight per day basis at 28 days of age, with an average daily feed consumption (g kg-1 day-1) of approximately 1.5 times that normally eaten by adults. Feed consumption in lactating females peaked at concentrations as high as 3.2 times and averaged approximately 2.5 times the amount normally consumed by nonpregnant adult females of comparable age. These results should be considered when interpreting the results observed in reproduction studies when exposure is via feed. Effects which could be attributed to "enhanced neonatal sensitivity" may, in fact, merely reflect the toxicity resulting from increased chemical exposure. It is thus clear that, if reproduction studies are to be conducted at or near the MTD, adjustment of dietary concentration of test agents during lactation is necessary to maintain target dose levels and to prevent overexposure.