Exploration in immature rats: effects of drugs.

The effects of d-amphetamine (4 mg/kg), scopolamine (1 mg/kg), methylscopolamine (1 mg/kg), and parachlorophenylalanine (400 mg/kg) on exploration were studied in male rats at 16, 21, and 28 days of age. Amphetamine elicited stereotypy at all ages tested, reduced exploration (measured by the time spent head-dipping during a 10-min trial in a holeboard) from Day 21, but did not significantly increase rearing at the ages tested. (Reduced head-dipping, accompanied by stereotypy, is the pattern of results previously seen in adult rats with this dose of amphetamine.) An interesting difference emerged with scopolamine, which reduced head-dipping at all the ages tested, whereas in adults it produced an increase. The age-related difference in drug effects suggests that muscarinic pathways are functioning from Day 16, but that the system concerned with exploration functions differently in mature and immature rats. Both scopolamine and methylscopolamine reduced rearing which suggests that the change is due to peripheral actions of the drugs. Parachlorophenylalanine, which decreases serotonin levels, increased head-dipping, as it has been found to do in adult rats.