Interaction of cortex and thalamus in spike and wave discharges of feline generalized penicillin epilepsy

Abstract The transition from spindles to spike and wave (SW) discharges of feline generalized penicillin epilepsy was studied using simultaneous EEG recordings from mutually related cortical and thalamic sites after i.m. injection (350,000 IU/kg) or diffuse cortical application of a weak solution (100–300 IU/hemisphere) of penicillin. Both procedures induced similar changes at cortical and thalamic levels, those in the thalamus developing at the same time or slightly later but nerver earlier than in the cortex. These changes consisted of: (i) amplitude increase of spindles, development of positive phases, and decrease in amplitude, followed by disappearance of every second spindle wave as SW discharges developed; (ii) facilitation, progressive amplitude increase, and increase or development of positive phases of recruiting responses to midline thalamic stimulation. Once SW had developed, a decrease in cortical excitability by cortical application of 15% KCl caused the cortical and thalamic SW discharges to disappear and to be replaced by spindles. These results demonstrate that important changes in thalamic activity occur during the development of cortical SW discharge whether induced by i.m. penicillin or by diffuse cortical application of a weak penicillin solution. Changes in thalamic activity appear to be secondary to changes in cortical activity. Thus, although cortical SWs are triggered by thalamocortical inputs which originally were spindle-inducing, these inputs change after penicillin, and reflect an alteration in thalamic activity imposed by the cortex through corticothalamic volleys. In their turn, they modify the cortical response.

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