Development of mouse activity, stimulus reactivity, habituation, and response to amphetamine and scopolamine

Twenty-four litters of non-inbred Swiss-derived mice were used to study the development of locomotion and tendency to approach a novel object in an open field, as well as the effects of dl-amphetamine sulfate and scopolamine hydrochloride (1 or 2 mg/kg IP). Brief (7 min) tests repeated for three consecutive days were preferred in order to obtain information on between-session habituation. Animals tested on days 14-16 showed low levels of activity without changes in successive sessions. In contrast, an adult-like pattern with a high initial activity and marked between-session decrements prevailed on days 21-23 and 28-30. Dl-amphetamine elevated activity only on days 14-16, while scopolamine produced hyperactivity and impaired habituation only on days 21-23 and 28-30. Latency to approach a novel object by untreated animals showed a substantial reduction between the end of the second week and subsequent developmental stages. This went hand in hand with an appearance of latency increases after dl-amphetamine treatments, while an opposite trend in the scopolamine data failed to reach statistical significance. Activity tests in a photocell apparatus at 61-72 days (without prior treatment) showed a reduction of locomotion relative to the level measured in animals from other litters raised in parallel and not subjected to early testing. Overall, the present data and those of the literature indicate that some developmental phenomena in small rodents are relatively insensitive to a variety of organismic, environmental, and test factors, while others (e.g., inverted U-shaped activity trends and successive modifications of the amphetamine profile) depend on complex interactions between several variables.