Network activity evoked by neocortical stimulation in area 36 of the guinea pig perirhinal cortex

The perirhinal cortex is a key structure involved in memory consolidation a nd retrieval. In spite of the extensive anatomical studies that describe th e intrinsic and extrinsic associative connections of the perirhinal cortex, the activity generated within such a network has been poorly investigated. We describe here the pattern of synaptic interactions that subtend the res ponses evoked in area 36 of the perirhinal cortex by neocortical and local stimulation. The experiments were carried out in the in vitro isolated guin ea pig brain. The synaptic perirhinal circuit was reconstructed by integrat ing results obtained during intracellular recordings from layer II-III neur ons with simultaneous current source density analysis of laminar profiles p erformed with 16-channel silicon probes. Both neocortical and local stimula tion of area 36 determined a brief monosynaptic excitatory potential in lay er II-III neurons, followed by a biphasic synaptic inhibitory potential pos sibly mediated by a feed-forward inhibitory circuit at sites close to the s timulation electrode and a late excitatory postsynaptic potential (EPSP) th at propagated at distance within area 36 along the rhinal sulcus. During a paired-pulse stimulation test, the inhibitory postsynaptic potential (IPSP) and the late EPSP were abolished in the second conditioned response, sugge sting that they are generated by poli-synaptic circuits. Current source den sity analysis of the field responses demonstrated that 1) the monosynaptic activity was generated in layers II-III and 2) the sink associated to the d isynaptic responses was localized within the superficial layer of area 36. We conclude that the neocortical input induces a brief monosynaptic excitat ion in area 36 of the perirhinal cortex, that is curtailed by a prominent i nhibition and generates a recurrent excitatory associative response that tr avels at distance within area 36 itself. The results suggest that the perir hinal cortex network has the potentials to integrate multimodal incoming ne ocortical information on its way to the hippocampus.