Physiological studies of the reciprocal connections between the hippocampus and entorhinal cortex

Abstract

Neurophysiological recordings were obtained from the hippocampus, entorhinal cortex, and dentate gyrus under conditions of controlled electrostimulation at interconnecting pathways in order to confirm their bidirectional nature as suggested by recent anatomical findings. The existence of a hippocampal to entorhinal pathway was confirmed physiologically by the presence of evoked field potentials and unitary driving of the entorhinal cortex following stimulation of the CA3 subfields of the ipsilateral and contralateral hippocampus. Activation of the entorhinal cortex by such procedures led to a subsequent excitation of the granule cells of the dentate gyrus through the axonal projections of the perforant pathway. The findings are discussed in the context of known anatomical circuitry which might provide the basis for such bidirectional interactions. The functional significance of the demonstrated physiological connections is indicated by the fact that the entorhinal cortex responds to hippocampal activation in a consistent manner and transmits that information back to the dentate gyrus; thereby completing an important three chain loop between three major components of limbic system circuitry.