Frequency bands and spatiotemporal dynamics of β burst stimulation induced afterdischarges in hippocampus in vivo

Temporal and spatial characteristics of hippocampal neuronal network activation are modified during epileptiform afterdischarges. We developed a β burst stimulation protocol to investigate subregional variations and substrates of rhythmic population spike discharges in vivo in urethane anesthetized Wistar rat hippocampus with a 14-electrode recording array and extracellular single electrode recordings. Our 64 pulse β burst stimulation protocol was constructed from electrical pulses delivered at intervals corresponding to β (14-25 Hz), Δ (2 Hz), and slow (0.5 Hz) frequencies. In each experiment these interleaved pulses were all repeated four times with unchanged intervals. Stimulation of either perforant path or fimbria fornix induced a prolonged afterdischarge pattern peaking at 200 Hz fast, 20 Hz β, and 2 Hz Δ frequencies. Analysis of variance confirmed that the response pattern of the discharges remained constant regardless of the stimulation β frequency. Within the afterdischarge the fast frequencies were restricted to independent hippocampal subfields whereas β and slow frequencies correlated across the subfields. Current source density (CSD) analysis revealed that the original signal propagation through subfields of the hippocampus was compromised during the β burst stimulation induced afterdischarge. In addition, the CSD profile of the epileptiform afterdischarge was consistently similar across the different experiments. Time-frequency analysis revealed that the β frequency afterdischarge was initiated and terminated at higher γ (30-80 Hz) frequencies. However, the alterations in the CSD profile of the hippocampus coincided with the β frequency dominated discharges. We propose that hippocampal epileptiform activity at fast, β and Δ frequencies represents coupled oscillators at respectively increasing spatial scales in the hippocampal neuronal network in vivo.