Significance of the deep layers of entorhinal cortex for transfer of both perirhinal and amygdala inputs to the hippocampus

In the rat, a number of sensory modalities converge in the perirhinal cortex (PC). The neural pathway from the perirhinal cortex to the entorhinal cortex (EC) is considered one of the main routes into the entorhinal–hippocampal network. Evidence accumulated recently suggests that EC and PC, far from being passive relay stations, actively gate impulse traffic between neocortex and hippocampus. Using slice preparation maintaining the neurocircuit connecting PC, EC, hippocampal formation and amygdala, we investigated the associative function of PC and EC with respect to sensory and motivational stimuli and the influence of the association on the neurocircuit. In horizontal slices located ventrally to the rhinal sulcus, where we stimulated area 35 and the lateral amygdala, both inputs can be independently conveyed to the dentate gyrus. In slightly more dorsal slices where we stimulated area 36 and the lateral amygdala, the coincidence of the two inputs was needed to activate the hippocampus. This need for association of the two inputs was apparently mediated by the deep layer of EC. In all instances activation of the deep layers of EC was sufficient to activate the dentate gyrus, suggesting the relevance of the deep layers in cortico–hippocampal interactions.