Cortical synchronized activity evoked by thalamocortical stimulation in vitro.

Epileptiform activity was studied in the thalamocortical (THC) slice preparation. Inhibition was gradually attenuated with increasing doses of bicuculline methiodide (BMI). We compared the ability of thalamic and intracortical (INC) stimulation to evoke epileptiform activity. Synchronized population activity was identified by its all-or-none appearance at a threshold stimulus intensity, by a variable latency and by horizontal propagation to large distances. For most slices (20 out of 24) we could establish a threshold dose of BMI (0.4-0.7 microM) under which THC-evoked population events had epileptiform properties, while INC-evoked ones did not. Increasing the BMI dose by 0.2 microM resulted in the appearance of epileptiform field potentials when stimulating intracortically. It is concluded that the tendency of the neocortex to generate synchronized population activity is higher when it is activated through the THC pathway.