Effects of serotonergic agonists and antagonists on the locomotor activity of neonatal rats.

The locomotor activity of neonatal rats was measured after treatment with serotonin agonists or antagonists. Treatment with the serotonin agonists 5-hydroxytryptophan or quipazine resulted in the elimination of the peak in activity which normally results from increases in activity from days 10 to 15 of life followed by decreases from days 15 to 20 of life. The drug-induced decreases in activity occurred at doses that did not alter locomotor activity after day 17, when most of the peak in activity had passed. The dose of 5 mg/kg of the serotonergic antagonist methysergide eliminated the peak in activity without changing locomotor activity after the peak had passed. The antagonists methiothepin and cinanserin only produced decreases in locomotor activity which did not appear to be related to the peak in activity. The serotonergic agonist data are compatible with the hypothesis that the development of the serotonin system contributed to the inhibition of locomotor activity. The methiothepin and cinanserin data neither confirm nor dispute the hypothesis, as their effects may have been either nonserotonergic or on serotonin receptors that were different than those acted on by the agonists.