M. Mizutani

Behavioral Characteristics of Rats Selectively Bred for High and Low Avoidance Shuttlebox Response

Abstract Selective breeding of rat strains showing high and low avoidance behavior in the shuttlebox test has been performed since 1985. To elucidate the postnatal behavior of the high and low avoidance strains, animals from both lines were subjected to an open‐field test, wheel cage activity test and Biel maze test in the 13th, 14th and 16th generations, respectively. No significant differences in any parameter in the open‐field test were observed between high and low avoidance lines. However, animals from the high avoidance line showed higher motor activity in the wheel cage and fewer errors in the Biel maze than the low avoidance line.

Comparative study of behavioral development between high and low shuttlebox avoidance rats

Previously, we reported that high- and low-avoidance animals (HAA and LAA, respectively), selectively bred for different avoidance response rates in a shuttlebox avoidance test, showed additional behavioral differences in wheel cage activity and in water maze performance after weaning. In the present study, physical and behavioral development were examined in HAA and LAA during the preweaning period. As compared to HAA, LAA offspring showed lower body weight, delayed eye opening, poorer performance in pivoting and negative geotaxis, and increased open-field activity. A fostering study indicated that these differences observed in eye opening, pivoting, negative geotaxis, open-field activity, swimming speed, shuttlebox avoidance and wheel cage activity were independent of maternal care. Only the pup weight was strongly dependent on the maternal line. These results indicate that behavioral differences between HAA and LAA observed in the pre- and postweaning periods may be linked to the avoidance genotype, but the difference in pup weight may be caused by maternal care.

Effects of Mitomycin C Treatment before Implantation on Development of Mouse Embryo

Abstract ICR mice were treated intraperitoneally with mitomycin C (MMC) at a single dose of l‐5mg/kg on day 0, 1, 2 or 3 of pregnancy (vaginal plug = day G). Embryotoxicity and teratogenicity were examined at term.

Behavioral characteristics of micrencephalic rats in high and low shuttlebox avoidance lines

Rats of two lines, selectively bred for high and low shuttlebox avoidance response, and unselected line of the Sprague-Dawley (SD) rats were given an IP injection of methylnitrosourea (MNU) at 5 mg/kg on day 13 of gestation. Their offspring, all with micrencephaly, were subjected to the Biel maze, shuttlebox avoidance, and wheel cage activity tests during 6 to 9 weeks of age. Results from micrencephalic rats were compared with those obtained from untreated control offspring of the respective lines. The MNU-treated offspring in each line made more errors than controls in the Biel maze, and showed similar avoidance responses to controls in the shuttlebox test. Line differences in effects of MNU treatment were observed in elapsed time in the straight channel, extinction of shuttlebox avoidance responses, and activity in the wheel cage. These line differences in effects of MNU treatment may be related to control baseline levels of behavior in which the three lines differed widely, and suggest that the behavioral effects of prenatal MNU treatment vary in rats depending on the lines used.

Palatal Slit and Cleft Palate in Rats Treated with a Glucocorticoid 1. Teratogenicity of Dexamethasone

The sensitive period and the dose‐response characteristics for dexametha‐sone‐induced cleft palate and palatal slit in rats were examined. Pregnant rats were injected subcutaneously with dexamethasone in a single dose of 2–6 mg/kg on one day between day 8 and 16 of the gestational period or once each day for two successive days (days 14 and 15 of gestation). All animals were euthanized on day 20 of gestation and the palatal condition of their fetuses was inspected after fixation under a dissecting microscope. The sensitive stage of palatal slit and cleft palate induction was determined and the dose‐response relationship was analyzed by the log‐probit transformation method.