[Effects of low level toluene exposure during the developing stage of the brain on learning in high avoider rats]

THA (Tokai High Avoiders) rats were exposed in the exposure chambers to 100 and 500 ppm of toluene and air for 7 h every day from 13th day of gestation to 48 d of age. After the termination of toluene exposure, Sidman avoidance test was conducted on the rats for 10 d from 49 d of age (first test series), 100 d of age (second test series) and 150 d of age (third test series). The results obtained are as follows. No difference could be observed between the toluene exposed groups and controls in the initiation of pinna detachment, primary coat of downy hair, incisor eruption and eye opening. The body weight of the 100 ppm exposed group was slightly greater than that of the control and the 500 ppm exposed group. No difference was indicated between these three groups in reflex and locomotor activity. Avoidance learning: Avoidance rate in the two exposed groups of male rats was significantly lower than those in the control during the 60 min of the test in the first test series. The magnitude of individual variations of avoidance rate in the two exposed groups of male rats was also remarkably greater than that of the control. No difference was seen between the two exposed groups in both avoidance rate and magnitude of avoidance variation. In the second and third test series, the avoidance rate of the two exposed groups still did not attain the control levels during the period of the test series. In female rats, the avoidance rate of the two exposed groups during the first 30 min of the test in the first test series was slightly lower than the control. This difference in avoidance rate observed between the exposed groups and the control disappeared during the latter 30 min of the test. In the second and third test series, the avoidance rate of both exposed groups reached the control level during the test period. It may be concluded from the foregoing findings that 100 ppm and 500 ppm toluene exposure during the fetal and suckling periods affects learning in the rats after termination of the growth period, and that the learning disorder still remained as long as 100 days after the end of toluene exposure, although the effect decreased gradually. It was also observed that this learning disorder appeared more distinctly in male rats than in female rats.