Propagation of neuronal activity along the neocortical-perirhinal-entorhinal pathway in the guinea pig.

The study of synaptic interactions within the parahippocampal region is crucial to understand the integrative functions performed by this region during memory information processing. Despite the extensive anatomical studies, the intrinsic physiology of the parahippocampal area has been poorly investigated. We describe here the organization pattern of the synaptic network formed by the temporal neocortex, areas 36 and 35 of the perirhinal cortex (PRC) and the entorhinal cortex (EC), in the in vitro isolated guinea-pig brain. Current source density analysis of laminar field potential profiles was performed with multichannel silicon probes positioned in different parahippocampal subfields. Stimulation of the temporal neocortex induced monosynaptic and polysynaptic potentials in areas 35 and 36, respectively. Area 36 stimulation evoked monosynaptic responses within areas 36 and 35. Stimuli in area 35 induced responses that propagated longitudinally along area 35 itself. No local field responses were observed in the EC after stimulation of both neocortex and areas 35/36. Despite the absence of a local extracellular response, intracellular recordings demonstrated that subpopulations of superficial layer neurons in medial and lateral EC showed polysynaptic EPSPs after stimulation of area 35 and area 36. The results demonstrate that the propagation of neuronal activity across the rhinal sulcus in the direction from the PRC to the EC is finely and diffusely distributed. In agreement with previous reports, these findings suggest that the PRC-EC pathway is highly regulated by inhibitory network interactions.