The generation of theta rhythm in hippocampal formation maintained in vitro

The most spectacular example of oscillations and synchrony which appear in the brain is the rhythmic slow activity (theta) of the limbic cortex. Theta rhythm is the best synchronized electroencephalographic activity that can be recorded from the mammalian brain. Hippocampal formation is considered to be the main structure involved in the generation of this activity. Although detailed studies of the physiology and pharmacology of theta-band oscillations have been carried out since the early 1950s, the first demonstration of atropine-sensitive theta rhythm, recorded in completely deafferented hippocampal slices of a rat, was performed in the second half of the 1980s. Since the discovery of cholinergically induced in vitro theta rhythm recorded from hippocampal formation slices, the central mechanisms underlying theta generation have been successfully studied in in vitro conditions. Most of these experiments were focused on the basic question regarding the similarities between the cholinergically induced theta activity and theta rhythm examined in vivo. The results of numerous in vitro experiments strongly suggest that cholinergically induced theta rhythm recorded in hippocampal slices is a useful analogue of theta observed in intact animals, and could be helpful in searching for the mechanisms of oscillations and synchrony in the central nervous system neuronal networks. The objective of the present review is to discuss the main results of experiments concerning theta oscillations recorded in in vitro conditions. It is our intent to provide, on the basis of these results, the characteristics of essential mechanisms underlying the generation of atropine-sensitive in vitro theta.