Synchronized bursting of CA1 hippocampal pyramidal cells in the absence of synaptic transmission

Abstract

The synchronization of neuronal firing, seen at its most dramatic in the epilepsies, has generally been attributed to synaptic interactions^1–5. We have now discovered a rhythmic spontaneous bursting activity produced by non-synaptic mechanisms. It develops in rat hippocampal slices after chemical synaptic transmission has been blocked by incubation in low Ca²⁺, increased Mg²⁺ solutions, and persists with almost clockwork regularity for several hours. We report here that spontaneous firing of all the neurones recorded in the slice increased, consistent with the known effects of Ca²⁺ on membrane properties and synaptic transmission^6–10, but the synchronous ‘field bursts’, and presumably their underlying mechanisms, were restricted to the population of pyramidal neurones in the hippocampal CA1 region. Thus, these low Ca²⁺ field bursts are different from the Ca²⁺-dependent synchronous bursts induced in slices by penicillin which originate in the population of pyramidal cells of the CA3 region^1–3.