Synchronized burst discharge in disinhibited hippocampal slice. I. Initiation in CA2-CA3 region

Abstract

A type of stimulus-evoked synchronized burst recorded in the [guinea-pig] CA2-CA3 region has been studied by using the hippocampal slice preparation in the presence of penicillin or bicuculline. These synchronized bursts had long and variable latencies (30-150 ms). Since these responses occurred after a long delay following the stimulus, they were termed delayed synchronized bursts. The delayed synchronized burst occurred in an all-or-none fashion. For any region of the slice, the waveform and amplitude of the multipeaked field potential (associated with the delayed synchronized burst) are independent of the source of the afferent stimuli applied to evoke the event. Field potentials were larger and had shorter onset latency in the CA2 region than in the various CA3 regions. Intracellular recordings showed that CA2 pyramidal cells had lower bursting thresholds than neurons in the various CA3 regions. Delayed synchronized bursts could be elicited in a surgically isolated CA2 region by a stimulus applied to the local region. Hyperpolarizing a CA2 pyramidal cell during the delayed synchronized burst revealed that in a single cell, the burst event was triggered by a slow membrane depolarization. Extracellular Ca2+ (10 mM) application increased the threshold of the CA2 pyramidal cells and blocked the above-mentioned slow depolarization in a graded fashion. With sufficient Ca2+, the entire event was blocked. Direct activation of a small number of CA2 cells by pressure ejection of K+ (20 mM) produced a delayed synchronized burst. The latter event involved a significantly larger aggregate of neurons than those initially activated by K+. A K+-induced delayed synchronized discharge can reset the rhythm of the spontaneous periodic synchronized bursts recorded in the CA2 region. Recordings in the CA1 region showed that delayed synchronized burst in CA2-CA3 region produced a synchronized afferent volley eliciting large-amplitude excitatory post-synaptic potentials (EPSP) in the CA1 pyramidal cells. The delayed synchronized burst recorded in the CA1, CA2 and CA3 regions of the hippocampal slice was evidently initiated in the CA2 region. The generation process involved an initial activation of a small number of cells, the output of these cells then activated a local synaptic integration process, which eventually brought about the synchronized burst. The long delay required for the event to occur may be due to the time required for the local integration process. It also appeared that spontaneous and delayed synchronized bursts share a similar generation mechanism and involve the same neuronal aggregates.