Ontogenetic development of the locomotor response to levodopa in the rat

Administration of exogenous levedopa triggers locomotion in young rats prior to the onset of quadripedal movement. The same substance decreases locomotion in adult animals. The ontogenetic development of the response to levodopa was investigated in rats. Intraperitoneal injection of levodopa (150 micrograms/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, including running, jumping, and wall climbing. This effect can be attributed to the activation of postsynaptic dopaminergic receptors that are already present during the early stages of life. At 25-30 days of age, levodopa-induced motor activity was decreased in comparison with that of the 18- to 20-day-old rats, possibly due to changing patterns of D1/D2-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 autoreceptors at this age may account for the change.