ONTOGENIC DEVELOPMENT OF THE LOCOMOTOR RESPONSE TO LEVODOPA IN THE RAT

Administration of exogenous levodopa triggers locomotion in young rats prior to the onset of quadripedal movement. The same substance decrea ses locomotion in adult animals. The ontogenetic development of the re sponse to levodopa was investigated in rats, Intraperitoneal injection of levodopa (150 mg/kg body weight) caused characteristic ''crawling' ' or ''swimming-like'' locomotion patterns in 5- to 6-day-old animals. Noradrenergic mechanisms may be involved in this behavior. In 18- to 20-day-old rats, levodopa caused excessive locomotor activity, includi ng running, jumping, and wall climbing. This effect can be attributed to the activation of postsynaptic dopaminergic receptors that are alre ady present during the early stages of life. At 25-30 days of age, lev odopa-induced motor activity was decreased in comparison with that of the 18- to 20-day-old rats, possibly due to changing patterns of D-1/D -2-dopamine receptor subtype interactions. In contrast to observations in younger rats, the same dose of levodopa suppressed motor activity in 60- to 75-day-old rats. The presence of functional dopamine autorec eptors at this age may account for the change.