The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings were not evaluated in accordance with the levels of evidence for reproductive or developmental criteria established by NTP in March 2009. The findings and conclusions for this study should not be construed to represent the views of NTP or the U.S. Government.
The "Reproductive Assessment by Continuous Breeding" (RACB protocol) consists of four related tasks. These tasks include: Task 1, dose-finding; Task 2, continuous breeding phase; Task 3, identification of the affected sex, and Task 4, offspring assessment. At Task 3, the rats are 32 to 33 weeks old. Because fertility declines with age, Task 3 can be insensitive in determining the affected sex. This particular study was conducted to test an alternate Task 3 design. In this alternative, younger animals (approximately 12 weeks of age; referred to as "surrogates") were used for cross-mating with the F0 animals to test if younger Sprague-Dawley rats could successfully mate with older F0 Sprague-Dawley rats, and to determine mating and fertility indices in the alternate Task 3 design.
Four groups of animals were tested:
- Group 1-- Twenty (20) F0 control males were mated with F0 control females.
- Group 2-- Twenty (20) F0 control males were mated with twenty (20) surrogate females.
- Group 3-- Twenty (20) F0 control females were mated with twenty (20) surrogate males.
- Group 4-- Twenty (20) surrogate males were mated with twenty (20) surrogate females.
For the number of fertile pairs/number of females with a vaginal sperm/plug, the age of both partners appeared important:
Group 4 (100%)> Group 2 (94%)> Group 3 (88%)> Group 1 (76%)
There were no differences between groups in the mating indices, in the number of live, dead, or total pups per litter, or in live pup weight. The new design provides higher fertility indices than the present Task 3 design and is also more cost-effective.