Recurrent Excitation In The Ca3 Region of Cat Hippocampus

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Abstract
The deafferented fornix preparation was utilized to detect recurrent excitatory connections of hippocampal pyramidal cells. The intensity and axosomatic locus of the well-known recurrent inhibitory action posed special difficulties for detecting the more subtle recurrent excitation. Therefore, initially negative extra-cellular unitary recordings, in which cellular damage should be minimal, were employed to study the recurrent excitatory actions. Such recordings from identified pyramidal cells of the CA3 region of cat hippocampus revealed synaptic types of activation as well as antidromic invasion upon stimulation of the deafferented fornix. These data thus demonstrate the existence of a recurrent excitatory pathway in this region. This is a previously undisclosed circuit required by recent hypotheses concerning the origin of experimentally induced interictal spike discharges in the hippocampus.