[Theta rhythm recorded in the hippocampal formation in vitro]

Theta rhythm is the best synchronized EEG activity that can be recorded in the mammalian brain. Hippocampal formation (HPC) is considered to be the main structure involved in the generation of this activity. Numerous data indicate that theta rhythm is involved in long-term potentiation, spatial learning, spatial navigation, verbal and spatial working memory, REM sleep, locomotor activities, and sensori-motor integration. Since the discovery of cholinergically-induced theta rhythm recorded from the hippocampal formation slices, central mechanisms underlying theta generation have been successfully studied in the in vitro conditions. Most of in vitro studies have been focused on the basic question regarding the similarities between cholinergically-induced theta oscillations and theta rhythm examined in the in vivo conditions. The results of these experiments have clearly demonstrated that the main properties of theta rhythm in both, in vitro and in vivo preparations are similar. The present review has one main objective: to characterize the basic mechanisms underlying the generation of theta rhythm in the hippocampal formation maintained in vitro.