Cortical and thalamic participation in the generation of seizures after blockage of inhibition.
Diego Contreras, Alain Destexhe and Mircea Steriade

Society for Neuroscience Abstracts 22: 2099, 1996.

We explored the relative contributions of cortical and thalamic networks in the generation of seizures induced by decrease of GABAergic inhibition. Seizures were induced by local injections of bicuculline (30 mM) in cortex and thalamus of barbiturate-anesthetized cats. Seizures were recorded as field potentials by means of an array of 8 tungsten electrodes (interelectrode distance of 1 mm) placed over the suprasylvian gyrus or in the thalamus spanning from the rostral pole of the reticular nucleus to the lateral geniculate nucleus. (1) injection of bicuculline in the cortex induced the appearance of epileptic-like single spikes in the cortex occurring during spontaneous spindle sequences, that developed into full-blown seizures characterized by highly synchronized spike-and-wave (SW) complexes at 3 Hz, combined with runs of 10-20 Hz spikes. During cortical-induced seizures, the thalamus could reflect cortical activity, but in other epochs it was still able to generate spindle sequences. (2) Injection of bicuculline in the thalamus decreased spindling frequency in a dose-dependent manner and increased the synchrony of spike bursts from thalamocortical and thalamic reticular cells. No SW patterns appeared in the cortex following thalamic injections of bicuculline. Decortication did not modify the effect of bicuculline in the thalamus. (3) Injections of bicuculline in the cortex of athalamic cats showed similar components as those with intact thalamus. (4) Computational models of thalamocortical circuits showed 3 Hz SW-like patterns following the decrease of intracortical GABA_A-mediated inhibition, but not by alteration of intrathalamic inhibition alone. Taken together, these results suggest that the cortex is capable of generating 3 Hz SW patterns as well as 10-20 Hz runs of spikes that may be dissociated from thalamic spindles. Supported by MRC of Canada (MT-3689) and the Savoy Foundation.