Age-dependent effects of scopolamine on avoidance, locomotor activity, and rearing

In Experiment 1, 15-, 17-, 21-, 36- and 90-day-old rats were injected with either physiological saline or 0.5, 1.0, 4.0, 8.0, 16.0 or 32.0 mg/kg of scopolamine (an anticholinergic). Immediately after the injection, shuttle crossings during adaptation were recorded for 8 min, and then the rats were given a single session of 100 two-way avoidance trials. In all ages, scopolamine increased two-way avoidance and intertrial responses throughout training and avoidance and intertrial responses were positively correlated, suggesting that increased avoidance was due, in part, to increased locomotor activity. In addition, scopolamine increased locomotor activity at an earlier age in a stressful situation, i.e. during the intertrial interval, than in a less stressful environment, i.e. during adaptation. In Experiment 2, photocell crossings and rearing were examined in 15-, 17-, 21-, 36-, 90- and 275-day-old rats injected with saline or 0.5, 1.0, 4.0, 8.0 or 16.0 mg/kg of scopolamine hydrobromide. Scopolamine increased photocell crossings in rats 21 days of age and older but did not increase rearing until 36 days of age. Scopolamine also had different behavioral effects in the three oldest ages. Thus, cholinergic involvement in these behaviors changes from at least 15 to 275 days of age. Methylscopolamine, which does not cross the blood-brain barrier, did not alter the behaviors examined in Experiments 1 and 2, suggesting that development of cholinergic neurons in the CNS is responsible for the age-dependent behavioral effects of scopolamine.