Ultrasonic vocalizations as diagnostic tools in studies of developmental toxicity: an investigation of the effects of prenatal treatment with methylmercuric chloride.

Ultrasonic vocalizations were recorded during two tasks from four groups of neonatal CD rat pups. Groups 0, 2, 4 and 6 were the offspring from pregnant dams treated with 0, 2, 4 or 6 mg/kg methylmercuric chloride by gavage on gestation day 7. On the day of birth, Day 1, litters were randomly culled to 8 pups (4 males, 4 females). The pups were weighed on Days 1, 7, 14, 21 and 30, and no weight differences due to treatment were observed. At 5, 7, 9 and 11 days of age, ultrasonic vocalizations were recorded from the animals for 1 minute. Individual animals were placed in a small test chamber containing either soiled home cage bedding or clean bedding material. Half of the pups in each litter were tested in each "odor" condition, and the rate and duration of the vocalizations were measured for 1 minute. On days 8 and 9, pups were tested on a negative geotaxis incline during which time vocalizations were recorded. In both the "odor" and negative geotaxis tests, methylmercuric chloride affected vocalization rates in a nonlinear dose-response fashion. Regardless of treatment group, the pups vocalized at a higher rate and for a longer duration in the clean than in the soiled bedding test condition. These data showed the variability of the ultrasonic vocalization responses to be smallest for the animals tested at 11 days of age in the clean bedding condition. The results of this study suggest that the value of ultrasonic calls as dependent measures of toxicity may be strengthened by the use of multiple stimulus conditions in order to elicit a graded response pattern. This would facilitate the interpretation of potential nonlinear dose-response effects.