Impaired Biel and radial arm maze learning in rats with methylnitrosourea-induced microcephaly.

Jc1:SD rats were given methylnitrosourea (MNU; 5 mg/kg, IP) on day 13 of gestation. Male offspring with MNU-induced microcephaly were examined on the Biel water maze and its mirror-image maze at 6 weeks of age and on a radial eight-arm maze at 14 weeks of age. The MNU rats showed postnatal depression in body weight. Their brain weight was about 60% of the control value, and they were thus microcephalic. The MNU animals made significantly more errors on the Biel maze and its mirror-image maze than the controls. In the radial arm maze test, they required more trials to acquire the learning criterion than the controls, and the animals with the acquired learning criterion were fewer. In the retest, however, no significant difference appeared in number of trials required for reacquisition of the criterion between the MNU and control animals. The autopsy of the MNU animals revealed the thinned cerebral cortex and hypoplastic hippocampus. The present results with the MNU rats confirmed the learning impairment and suggested no effect of microcephaly on retention of the acquired memory.