Differential participation of some 'specific' and 'non-specific' thalamic nuclei in generalized spike and wave discharges of feline generalized penicillin epilepsy.

Extracellular single unit and electroencephalographic (EEG) activity during generalized spike and wave discharges (SW) induced by i.m. penicillin was recorded simultaneously in the cortex, in a 'specific' thalamic nucleus (n. lateralis posterior, LP) and in some 'non-specific' thalamic nuclei (n. centralis medialis, NCM; n. centrum medianum, CM; n. centralis lateralis, CL) Computer-generated EEG averages and histograms of single unit activity were triggered by either peaks of EEG transients or action potentials. The time at which cortical neurons (66/66) were most likely to fire was during the 'spike' of the SW complex while absence of firing was the rule during the 'wave'. Most LP neurons (23/26) showed a similar pattern, 3 cells firing preferentially during the 'wave'. In NCM only 17 of 39 neurons fired during the 'spike', 8 of 39 neurons during the 'wave' while the others showed no change in their firing pattern during SWs. Twenty-six of 30 CM and 20 of 24 CL neurons fired during the 'spike' of SW; the other cells in these nuclei did not change their firing pattern during SWs. When present, rhythmic fluctuations in firing linked to SW discharge were less prominent in these 'non-specific' thalamic nuclei than in cortex and LP. Furthermore, participation of NCM, CM and CL neurons in the SW rhythm occurred only after neurons in cortex and LP had become involved in it. Thus, as is the case for cortical neurons, the main firing pattern of thalamic cells during SWs consists of an oscillation between 'excitation' during the 'spike' and 'inhibition' during the 'wave' of the SW complex. However, the coupling between cortical and thalamic neuronal firing is less intimate for cells of the 'non-specific' thalamic nuclei than for a 'specific' nucleus such as LP. Thus, at least some 'specific' thalamic nuclei are more intimately involved in the mechanism of SW discharge than the midline intralaminar nuclei.