Propagation pattern of entorhinal cortex subfields to the dentate gyrus in the guinea-pig: an electrophysiological study.

Anatomical studies demonstrated that neurons located in the superficial layers of the medial and lateral aspects of the rat entorhinal cortex (EC) project to temporal and septal portions of both the dentate gyrus (DG) and the CA1 region of the hippocampus, respectively. In the present study we investigated with electrophysiological techniques the propagation pattern of different EC subfields to the DG of the in vitro isolated brain of the guinea-pig. Laminar field potential profiles from different portions of the DG were recorded with multi-channel silicon probes following direct stimulation of the ipsilateral EC surface performed in different positions under direct visual control. Current source density analysis of laminar profiles demonstrated that i) stimulation of the rostral-medial EC induced monosynaptic responses exclusively in the temporal pole of the DG, ii) stimulation of both the lateral and the caudal portions of the EC determined a diffuse monosynaptic activation of both the intermediate and septal DG. The regions of the EC that project to different sectors of the DG in the guinea-pig do not correlate to the EC subfields identified on the basis of cytoarchitectonic criteria. The EC-evoked monosynaptic DG potentials were followed by disynaptic responses coupled with sinks located in the inner molecular layer, proximal to the EC-induced sink, where intra-DG associative synapses were demonstrated by anatomical studies. The present detailed topographical study of the EC connections with the DG in the guinea-pig demonstrates with an electrophysiological approach a projection pattern similar, even if not identical, to that described with tracer techniques in the rat. This report is essential for future studies of the dynamic parahippocampal-hippocampal interactions in the guinea-pig, and in particular in the isolated guinea-pig brain preparation.