Unequal development of thresholds for various phenomena induced by cortical stimulation in rats

Electrical stimulation of sensorimotor cortical area was performed in 9-, 12-, 18-, 25-, 35- and 90-day-old rats with implanted electrodes to establish threshold intensities of currents necessary to elicit four different motor or EEG phenomena. Two different stimulation frequencies (8 and 50 Hz) were used. Development of thresholds for stimulation-bound movements, spike-and-wave afterdischarges and clonic seizures accompanying these afterdischarges was similar: the lowest threshold was found in 18-, respectively, in 18- and 25-day-old rats with the 8 and 50 Hz frequencies. Younger as well as older animals exhibited higher threshold intensities. The fourth phenomenon, transition into another, 'limbic' type of afterdischarges appeared only exceptionally in the youngest rats and its incidence increased whereas the threshold decreased with age. Higher frequency was more efficient in elicitation of limbic afterdischarges than the 8 Hz stimulation in rats aged 18 and more days. Our data represent a background for pharmacological studies and indicate the development of cortical excitability and of connections between the thalamocortical system and limbic structures.