between the first study (0.5–4 mg/kg) and the second study (0.2,
0.5 mg/kg) showed no differences in control values or the effects at
0.5 mg/kg. The only exception was the PND17 RBC control data, which
differed significantly (t-tests); however, the effects at 0.5 mg/kg, when
expressed as percent of concurrent control,were not different. Thus, the
data were subsequently combined for statistical analyses and for
graphical purposes. There was a significant dose-by-age interaction
for brain ChE (Fig. 3) and motor activity (Fig. 5), but not for RBC ChE. At
both ages, ChE inhibition in even the lowest dose groupwas significantly
different from control. The apparent leftward shift (1.4-fold) of the
PND17 brain ChE curve is supported by the lower ED50 (Table 1). In
contrast, the ED50 for RBC ChE was significantly lower in the adults. The
motor activity dose–response curve for the PND17 pups was shifted to
the right of the adults, indicating less effect.
3.4. Dicrotophos
The range finding study for dicrotophos suggested the same time
course as seen with monocrotophos, and therefore the same test
times, and molar-equivalent doses, were used. Brain ChE data (Fig. 4)
showed a significant dose-by-age interaction, and suggests that both
the PND11 and 17 pups were more sensitive compared to adults. On
the other hand, there was not a significant factor of age or an
interaction of dose and age for the RBC ChE data. These findings are
reinforced by the significant differences in ED50 estimates for brain
but not RBC ChE (Table 1). The lowest dose of 0.107 mg/kg
significantly decreased brain ChE in PND11 pups, and for all ages
and both tissues, doses of 0.213 mg/kg and above were significantly
different from control. PND11 pups were not tested in the motor
activity chambers, thus age differences could only be examined
between the PND17 and adult rats (Fig. 5). For motor activity counts,