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 inducedresponses 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