Several features make the septohippocampal system useful, especially for the study of physiological mechanisms of graft-host interactions, such as the hippocampus possesses electrical rhythms, which vary with ongoing behavior in a specific manner, and the excitability changes within the hippocampus are easy to monitor. This chapter discusses the possible mechanisms of electrophysiological recovery in the damaged septohippocampal system following transplantation of solid pieces or suspensions of fetal brain tissue into the lesion cavity or into the hippocampus, and the epileptogenic properties of hippocampal grafts. The experiments, using the septohippocampal model to study the mechanisms of action of brain grafts, suggest that a likely mechanism of restoration of physiological activity of the deafferented hippocampus is a passive bridging action of the graft between the host septum and hippocampus. The chapter demonstrates the reciprocal physiological and anatomical connections between the graft and host, and shows that hippocampal grafts can serve as an implanted epileptic focus, which may further worsen the function of the already damaged brain. However, experiments are required that determine why under certain conditions the grafts produce epileptic activity, while under seemingly similar conditions they lead to the restoration of physiological function of damaged brain circuitries.
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