High-frequency oscillations and seizure-like discharges in the entorhinal cortex of the in vitro isolated guinea pig brain

We analyzed the patterns of seizure-like activity and associated high-frequency oscillations (HFOs) induced by the K+ channel blocker 4-aminopyridine (4AP, 50 M) or the GABAA receptor antagonist bicu- culline methiodide (BMI, 50 M) in the in vitro isolated guinea pig brain preparation. Extracellular field recordings were obtained from the medial entorhinal cortex (EC) using glass pipettes or silicon probes; 4AP or BMI were applied through the basilar artery. Ripples (80–200 Hz) or fast ripples (250–500 Hz) occurred at higher rates shortly before ictal events induced by 4AP or BMI, respectively. In addition, dur- ing the ictal period, ripples were mostly associated with 4AP-induced ictal events whereas fast ripples predominated during ictal discharges induced by BMI. Finally, ripples occurred at higher rates during the clonic phase of 4AP-induced ictal events compared to the tonic phase, while higher rates of fast ripples characterized the clonic phase in both 4AP- and BMI-induced ictal discharges. These differences in HFO occurrence presumably reflect the diverse action of these two convulsants on GABAA receptor signaling