Abstracts

RATIONALE: Previous experiments from our lab have shown definite involvement of the anterior thalamic nucleus (AN) in pentylenetetrazol (PTZ) seizure expression. AN may be activated even in periods lacking high correlation with cortex (CTX). Distinctive EEG waveforms at each site represent specific neuronal activation. Strong EEG phase-locking or synchrony accompanies rhythmic ictal activity and is created by coupled neuronal firing. We show that the thalamus and cortex express different patterns of phase synchrony throughout the clonic seizure phase. The conventional spectrum cannot capture these changes faithfully. METHODS: Awake rats (S-D, 250-350 gm, n=6) with jugular catheters and surface/depth EEG were infused with PTZ at 5.5 mg/kg/min occurs until the first ictal event. Bispectral (BIS) and cross-bispectral (XBIS) analyses were done to examine modulations by harmonics of the spike-wave frequency of 2.5 Hz and capture frequency couplings. Overlapping 4 sec segments were input to the BIS. Using the Hinich BIS test we summed significant (p?.05) peaks along slices of the BIS. We compared differences up to 60 Hz in freq. couplings from CTX and AN during clonic seizure episodes. RESULTS: The strength of inter-harmonic coupling varies by time and electrode. CTX generates the largest number of couplings overall with an average of 16.2 vs 10 (AN) at 2.5 Hz. At 2.5 Hz both CTX and AN frequency couplings decrease at the beginning of an ictal event and then rise near the end. For 5 Hz AN couplings decrease from an average of 9 to 3. CTX rises from 3 at the beginning, peaks at 9.33 and drops to 8 at the end. At 7.5 Hz the both AN and CTX decrease in coupling from the onset of seizures. XBIS analysis shows 30-40 Hz gamma band freq. generation from AN and CTX. CONCLUSIONS: AN and CTX elements display definite alterations in phase synchrony across the spectrum during seizures. The bispectrum indicates a stronger, more developed synchrony in CTX as a seizure progresses whereas AN shows a decline. Ictal events are never completely static though they might appear to be spectrally. Increases in synchrony may represent stronger couplings and recruitment by neuronal generators. Supported by NIH NS35528.