Ontogenetic transitions in the psychopharmacological response to serotonergic manipulations.

The effects of various doses of the serotonergic agonist quipazine and antagonist methysergide on mouthing behavior under both low and high baseline mouthing conditions (i.e., in the absence and presence of milk, respectively) was examined in deprived and non-deprived rat pups at 3-4, 10-11 and 16-17 days postnatally. Quipazine was observed to induce an increase in mouthing in neonatal rat pups, both increases and decreases in mouthing at 10-11 days of age, while exerting a depressant effect on mouthing at 16-17 days postnatally. Methysergide conversely depressed mouthing at the two earlier testing periods, while having little effect on this behavior at 16-17 days postnatally. These ontogenetic transitions in the influence of serotonergic manipulations on mouthing are reminiscent of the ontogenetic reversal previously reported to occur in the influence of serotonergic manipulations on suckling (Ristine and Spear 1984). These developmental discontinuities in the serotonergic system may provide an excellent model system for the investigation of neural mechanisms mediating age-specific behaviors and their ontogenetic decline.