Embryonic and postnatal injections of bromodeoxyuridine produce age-dependent morphological and behavioral abnormalities

Abstract

The mitotic marker 5-bromodeoxyuridine (BrdU) was injected twice daily (60 mg/kg) into pregnant hooded rats on one of embryonic days (E) 11, 12, 13, 15, 17, or 21, or into rat pups on postnatal day (P) 10. The principal findings were the following: (1) BrdU exposure on E11 produces profound effects on body morphology, and animals must be fed a special diet because of chronic tooth abnormalities; (2) BrdU exposure at E17 or earlier produces a change in coat spotting pattern, the precise pattern varying with age; (3) BrdU exposure on E15 or earlier produces a reduction in both brain and body weight; (4) BrdU exposure on E17 or earlier reduces cortical thickness; (5) BrdU exposure on E11-E13 and at P10 reduces cerebellar size relative to cerebral size; (6) spatial learning is significantly affected after injections of BrdU at E11-E17, but the largest effect is on E17; (7) the deficit in spatial learning may be related in part to a reduction in visual acuity; and (8) skilled forelimb ability is most disrupted after BrdU exposure at E15 but is also impaired after injections on E13 or earlier. BrdU thus has teratological effects on body, brain, and behavior that vary with the developmental age of the fetus or infant.

Fig. 1.

Photographs of the ventrum of male rats exposed to vehicle (CON) or BrdU at different embryonic ages. The head is to the right. Injections at E11–E13 produced large spots; injections at E15–E17 produced progressively smaller spots. Injections at E21 or P10 had no effect on coat pattern.

Fig. 2.

Photograph of physical abnormalities in teeth, digits, and tails of rats with E11 BrdU injections. These abnormalities were not present in any animals with later injections.

Fig. 3.

Summary of skilled forelimb reaching. Rats with E15 BrdU injections were severely impaired at the task. The top panel shows reaching accuracy. The bottom shows the percentage of rats in each group that performed worse than 1 SD below control mean.

Fig. 4.

Summary of latency performance on the Morris water task. The top panel shows the mean total latency summed across all 20 trials. The bottom shows performance on each day of training.

Fig. 5.

Summary of error scores on the Morris task. Thetop panel shows the mean total errors summed across all 20 trials. The bottom shows performance on each day of training. The maximum number of errors per day is four.

Fig. 6.

Summary of performance in the Morris task in Experiment 2. There was only one trial per day. The top panel summarizes time to find the hidden platform, themiddle summarizes the distance swum to find the platform, and the bottom summarizes the swim speed. Rats with E13 BrdU injections took longer to find the platform and swam further but swam at the same speed as control rats.

Fig. 7.

Performance on the probe trial in the Morris task in Experiment 2. The platform had been located in the southwest quadrant. Control animals preferentially searched in this quadrant for the platform. The BrdU-injected rats did not.

Fig. 8.

Summary of performance in the visual detection task. The BrdU-treated rats were slower to locate the platform, which was indicated by a target of different sizes, than the control animals.

Fig. 9.

Summary of the performance of the rats on the visual acuity task. The performance of the BrdU rats dropped off to chance levels around 0.7 cycles/degree, whereas the performance of the control rats was considerably better.

Fig. 10.

Photographs of a control brain and an E12 BrdU-treated brain. The cerebral and cerebellar hemispheres of the BrdU brain are visibly smaller.

Fig. 11.

Summary of the ratio of the cerebellar area (from dorsal photographs) to the total cerebral + cerebellar area. The rats with injections at E13 or earlier or at P10 have relatively smaller cerebellums.

Fig. 12.

Summary of cerebral cortical thickness. Thetop panel shows the mean overall thickness, and thebottom panel shows the mean thickness at each of five planes. Plane 1 is the most anterior. Rats with BrdU injections between E11 and E17 have thinner cerebral cortex than the control and >E21 rats.