Effects of piribedil and of d-amphetamine on locomotor activity in immature rats

The effects of piribedil, a direct dopamine (DA) agonist, and of d-amphetamine, an indirect DA agonist, on locomotor activity in 28-day old rats have been investigated. d-Amphetamine (1,2 and 4 mg/kg) increased activity in a dose dependent manner whereas only a 40 mg/kg dose of piribedil significantly increased activity; low doses of piribedil (1.25-10.0 mg/kg) had no effect on activity. Since hyperactivity induced by a direct DA agonist, such as piribedil, is thought to reflect post-synaptic DA receptor activation, the results appear to indicate that post-synaptic DA receptor mechanisms are fully developed in 28-day old rats. In contrast, as low doses of piribedil which cause sedation in adult animals failed to decrease locomotor activity under the present experimental conditions, it seems that DA autoreceptors may not be well developed in 28-day old rats.