Tonic convulsive thresholds and responses during the postnatal development of rats administered 6-hydroxydopamine or 5,7-dihydroxytryptamine within three days following birth.

The maturation of the electroshock tonic convulsive pattern and threshold was investigated in rats between the ages of 4 and 30 days following intracisternal injections of 6-hydroxydopamine (6-OHDA) on postnatal days 1 and 2; or 5,7-dihydroxytryptamine (5,7-DHT) after desipramine on postnatal day 3. In 6-OHDA treated rats decreases in brain norepinephrine (mean values of 55% of control) and dopamine (mean values of 17% of control) were associated with a large reduction in the convulsive threshold and intensification of the pattern on postnatal day 4. Whereas the reduction in catecholamine concentrations and the intensification of the pattern were still evident on postnatal day 30, the last day of testing, the threshold effect was not evident by postnatal day 15. Although 5,7-DHT reduced brain serotonin concentrations (mean values of 59% of control) as early as postnatal day 4, the pattern was not intensified until postnatal day 8, and the threshold was not reduced until postnatal day 21. These effects were still evident on postnatal day 30. The results demonstrate a sequential maturation of monoaminergic regulation in seizure susceptibility and severity, with an apparent transition from catecholaminergic to serotonergic regulation of the tonic threshold during the third postnatal week.