The role of dopaminergic and serotonergic mechanisms in the development of swimming ability of young rats.

Neonatal swimming behavior was studied after a single subcutaneous injection of L-dopa methyl ester (50 mg/kg; 200 mg/kg) apomorphine (0.1 mg/kg; 1.0 mg/kg), DL-amphetamine (0.5 mg/kg; 10 mg/kg), haloperidol (0.5 mg/kg; 1.0 mg/kg), L-tryptophan (50 mg/kg; 100 mg/kg), methysergide (1.0 mg/kg; 5.0 mg/kg) as well as intraventricular injection of 100 microgram 6-OHDA. 1-, 3-, 5- and 7-day-old rats were placed into a temperature-controlled aquarium (37 degrees C) and the pattern of motor coordination, latency time to swimming (LTS) and the number of foreleg strokes for 10 s (FS) were measured. When compared to the physiological saline-injected controls, rats that received L-dopa showed a striking increase of FS at all ages but the most striking improvement of motor coordination was found in newborn rats. On day 1 both doses of DL-amphetamine induced increases in FS and improvement of motor coordination, whereas apomorphine failed to show any effect at this age. On days 3, 5 and 7 low doses of DL-amphetamine and apomorphine increased the FS. However, high doses resulted in a decrement in swimming performance. Haloperidol impaired swimming on day 1 but produced a significant increase of FS on days 5 and 7. Neonatal injection of 6-OHDA delayed development of motor coordination, reduced FS and increased LTS. On days 3, 5 and 7 high doses of L-tryptophan elicited an increase of FS, while high doses of methysergide caused significant impairment of performance. It is suggested that the brain rapidly converts the administered L-dopa to dopamine during the first week of life and there appears to be a strong dependent relationship between the pattern of motor coordination and the amount of available dopamine in the developing brain.