Role of the thalamus in generalized penicillin epilepsy: Observations on decorticated cats

Abstract

Experiments in bilaterally and unilaterally decorticated cats determined whether or not the thalamus plays a primary role in the production of spike and wave (SW) discharge in feline generalized penicillin epilepsy (FGPE). In bilaterally decorticated cats an i.m. dose of penicillin sufficient to induce FGPE in intact cats did not change thalamic spindles appreciably; it failed to induce thalamic SWs or any other form of epileptic activity in the decorticated thalamus. Low-frequency spindles bearing some resemblance to SWs were often seen in the nucleus lateralis posterior (LP) before penicillin administration and showed some slight increase thereafter. This may have some bearing on earlier evidence suggesting that the thalamocortical sector comprising the LP and suprasylvian gyrus plays a leading role in the elaboration of SWs in FGPE. In unilaterally decorticated cats the SW discharges in the intact hemisphere were associated with lower voltage synchronous discharges in the decorticated thalamus which persisted after transection of the massa intermedia or of the mesencephalic reticular formation, but disappeared after section of the anterior commissure. This pathway thus seemed to be responsible for transmitting rhythmic activity at the SW frequency from the intact hemisphere to the decorticated thalamus. These findings support earlier conclusions that the development of generalized SW discharge in FGPE primarily depends on a change in cortical excitability, even though a thalamic trigger is essential. Thalamic SW activity appearing in intact animals after penicillin thus seems to be imposed upon the thalamus by the cortex.