Electrophysiological analysis of the hippocampal projections to the entorhinal area

Abstract

Responses evoked in the entorhinal area by impulse volleys originating in the ipsilateral hippocampus were analysed in the guinea-pig by means of field potential analysis. Perforant path volleys, synaptically elicited by stimulation of the dorsal psalterium of one side, were used to activate the hippocampal lamellar circuit of the same side and, through interhippocampal impulses, the hippocampal pyramidal neurons of the contralateral side. Discharge of the hippocampal pyramidal neurons was followed by a response, a fast negative deflection preceded and followed by slow waves, in the dorsal third of the ipsilateral entorhinal area. Laminar distribution of the fast negative deflection and of the time-locked unit activity suggested that excitatory synaptic effects followed by neuron discharge were generated in neurons of layers VI-II of the entorhinal area. The increasing latency of the fast negative deflection and of unit firing over the cortical depth suggested that these synaptic effects were generated in temporal sequence, going from layer VI to layer II. The entorhinal response disappeared after a lesion at the caudal border of the hippocampus interrupting the caudally-directed hippocampal efferents. The anatomy of the hippocampal and subicular projections to the entorhinal area in the guinea-pig, together with electrophysiological data obtained in recordings from the ipsilateral subiculum, suggested that the hippocampal impulses were relayed to layers VI-V of the entorhinal area by the subiculum. The delayed activation of layers IV-II was possibly mediated by intracortical connections. Double-shock experiments showed that impulses of hippocampal origin inhibited the response to dorsal psalterium volleys of entorhinal neurons giving origin to perforant path fibers. The data show that the hippocampal output activates the deep layers of the entorhinal area from which it is possibly relayed to numerous cortical and subcortical regions. Moreover, the inhibitory effects exerted on neurons originating perforant path fibers give evidence of a negative feedback control system operating in the hippocampal region.