Effects of chronic naltrexone on amphetamine locomotor activity.

Animals from two different populations of Wistar rats were chronically pretreated with naltrexone for 8 days. After a 2-day rest period, animals were tested with amphetamine for locomotor activity in the open-field with or without white noise (Day 2). The rats were similarly retested on Day 7 and Day 14. New colony animals showed a significant attenuation in amphetamine locomotor activity in the absence of noise only. In contrast, chronic naltrexone significantly decreased amphetamine activity in old colony animals only under noise conditions. The possibility of an inhibitory role opiate receptors on the dopamine neuro-transmission is discussed.