The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings may not have been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. For more information, see the Explanation of Levels of Evidence for Developmental Toxicity. The findings and conclusions for this study should not be construed to represent the views of NTP or the U.S. Government.
Exposure of Sprague-Dawley-derived (CD ) rats to boric acid at 0.1. 0.2. or 0.4% in the diet (w/w) on gd 0-20, or 0.8% on gd> 6-15 resulted in fetal body weight deficits (greater than or equal to O.1% BORA), increased incidence of malformations (greater than or equal to O.2% BORA), and increased prenatal mortality (0.8% BORA) (Heindel et al., 1992). Among the anatomical anomalies observed. enlarged lateral ventricle(s) of the brain occurred with an increased incidence in the 0.4% and 0.8% BORA groups. These findings were accompanied by severe reductions in fetal body weights (63% and 46% of control weights, respectively). The present study was conducted to determine whether induction of ELV could be separated from fetal body weight deficits by focusing the exposure around a known sensitive period for induction of hydrocephaly in rat fetuses. The potential contribution of decreased maternal food intake to the developmental toxicity at 2.4% boric acid was also examined.
In the present study, BORA (0, 0.8, 1.6 or 2.4% in the diet) was provided to timed-mated female rats from gd 14 to 17. During the treatment period, the vehicle control group was fed undosed diet ad libitum, and a pair-fed control group was provided with the median amount of food consumed (g of food/kg of body wt/day) by the 2.4% BORA group. During the remainder of the study, dams and pups had ad libitum access to undosed diet, except during the second week of lactation (Phase I) when food consumption exceeded the available supply of food during one measurement period. Thus, dam and pup data collected on pnd 14, 21 and 26 may have been compromised due to this temporary shortage of food.
Mated females (n=16-17/group/study phase) were assigned to either Phase I (postnatal study) or Phase II (teratology study). In Phase I, dams were allowed to deliver and the offspring were evaluated for body weight, viability and major morphological defects (specifically, external craniofacial defects, palatal defects and CNS defects observed in free-hand sections) through postnatal day (pnd) 21. Pups were terminated on pnd 21, except that the median weight male in each litter was terminated on pnd 26 for evaluation of regional and total brain weights. In Phase II, dams were necropsied on gestational day (gd) 20 and the uterine contents were evaluated to determine prenatal viability. Live fetuses were weighed and examined for morphological defects as in Phase 1.
During the treatment period (gd 14 to 17), maternal body weight gain and food consumption were depressed in all BORA groups. Actual weight loss was experienced by the 2.4% BORA and pair-fed control groups, with pair-fed animals experiencing the most severe weight loss. Rebound increases in maternal weight gain and food consumption were observed following cessation of BORA exposure or pair feeding. Maternal water consumption was depressed during the first 24 hours of exposure (all BORA groups and pair-fed controls), but returned to control levels by the last day of exposure or pair feeding; thus, increased water intake after the treatment period was probably secondary to rebound increases in food consumption. Maternal body weight on gd 20 (both phases), maternal weight gain for the gestational period as a whole (gd 0 to 20, both phases), gravid uterine weight (Phase II) and corrected maternal weight gain on gd 20 (Phase II) were not affected by BORA exposure or by pair feeding. Maternal relative liver weight was increased on gd 20 (Phase II) by 1.6 and 2.4% BORA, but not by pair feeding. This effect on relative liver weight was not present on pnd 21. No changes were noted for maternal relative kidney weight on gd 20 or pnd 21 in either the BORA or pair-fed groups relative to the vehicle (ad libitum) control group.
The pair-fed control group did not differ significantly from the ad libitum vehicle control group for any parameter related to offspring development in either phase of this study. In contrast, adverse effects were observed in all BORA-exposed groups. Fetal body weight was significantly reduced in the low, mid, and high dose BORA groups on gd 20 (82%, 68% and 69% of control weight, respectively). By pnd 21, recovery from body weight deficits was complete for the low dose group (104% of control weight). Persistent body weight deficits were observed at the mid dose (83% of control weight; biologically relevant but not statistically significant) and at the high dose (75% of control weight: statistically significantly).
Prenatal mortality was not affected on gd 20 (Phase II), but cumulative post-implantation mortality through pnd 21 was increased at the high dose (34%, 44%, 33% and 72% for the vehicle control through high-dose BORA groups, respectively). The incidence of treatment-related mortality was most pronounced during the early postnatal period (pnd 0 to 4) as follows: 2%, 8%, 27% and 44% of pups per litter in the vehicle control through high-dose BORA groups, respectively.
No treatment-related effects were noted for the incidence of craniofacial, palatal or CNS structural defects. Changes in absolute brain weights (regional or total) observed in the mid- and high-dose groups were proportional to alterations in body weight on pnd-26, except that the relative weight of the medulla/pons was increased at the high dose. Analysis of brain regions (telencephalon, diencephalon, medulla oblongata/pons and cerebellum) as a percentage of total brain weight indicated that the relative weight of the telencephalon was decreased, while the relative weight of the medulla/pons was increased. No effects on regional or total brain weights were observed at 0.8% BORA.
In summary, gd 14 to 17 was not a sensitive period for BORA-induced ventricular enlargement. However, fetal body weight was reduced at all doses. Low-dose offspring recovered completely from growth deficits by pnd 21, but body weight effects persisted in the mid- and high-dose groups. These results indicate that BORA can reduce fetal body weight to 69% of control weight on gd 20 without a concomitant increase in the incidence of ventricular enlargement. However, this study did not answer the question of whether BORA could induce more severe weight reductions without concomitant ventricular enlargement, or whether there is a sensitive period earlier in gestation for induction of ventricular enlargement in the absence of fetal body weight effects. Preliminary evidence is provided that exposure to BORA (1.6% or 2.4% on gd 14 to 17) may alter brain weights (total and/or regional), but such effects were not found at 0.8% BORA. BORA-induced developmental toxicity in this study occurred independently of maternal food intake and body weight deficits, since the pair-fed control group did not differ from the ad libitum control group for any developmental measure in this study.