[A temporary reduction of emotional reactivity in postweaning, prejuvenile rats: examination by the Runway Test]

Developmental change of emotional reactivity in Wistar rats was investigated by two experiments using the Runway Test. Rats were tested at 20, 30, 40, 60, and 90 days of age for three successive days. First, a cross-sectional comparison of behavior in the Runway Test showed that 30-day-old rats were faster to enter the runway region, faster to reach the end section, and traversed more sections than other age groups. Rats at 60 and 90 days of age defecated more bolles and urinated more. Second, longitudinal comparisons showed that the ambulation of 30- and 40-day-old rats increased gradually over test days, suggesting lowered emotional reactivity. But 60- and 90-day-old rats showed asymptotic level of ambulation and no increase between days and less defecation. These results suggested that postweaning prejuvenile rats showed a temporary reduction of emotional reactivity in novel environments. Also, their emotional reactivity elevated gradually, and remained stable in each individual level.