Columnar activity supports propagation of population bursts in slices of rat entorhinal cortex

Population bursts including epileptiform spikes and sharp waves can be generated in and propagate through the retrohippocampal cortices. The propagation of these events within the entorhinal cortex was studied with field potential recordings in horizontal slices from rat brain. Population bursts were elicited by repetitive extracellular stimuli in normal media. Transections of particular laminae and microknife cuts parallel to the pial surface were used to interrupt propagation or to isolate potential pathways. Population spikes were not found to propagate more than 0.5 mm in either superficial (layers I-III) or deep (layers V-VI) strips of tissue formed by cuts along layer IV. Events propagated over 2 mm when 'columnar' connectivity was intact. Population spikes propagated past microknife cuts which started at the pial surface and sectioned layers I-IV. Population spikes also propagated past microknife cuts which sectioned the angular bundle and layers V-VI. When deep layer cuts included layer IV, horizontal propagation of population spikes was blocked. Cuts running approximately in layer IV diminished the amplitude and duration of population events recorded in layers II and V (above and below the cut, respectively) and eliminated a high frequency oscillation which occurred during the burst event. It is concluded that superficial and deep layer neurons interact during propagation of population events in entorhinal cortex and that this 'columnar' activity: (a) intensifies the excitatory activity of superficial and deep layer neurons; and (b) provides multiple paths for the spread of activity from column to column within the entorhinal cortex.