Associative interactions within the superficial layers of the entorhinal cortex of the guinea pig.

Associative fiber systems in the entorhinal cortex (EC) have been extensively studied in different mammals with tracing techniques. The largest contingent of intra-EC cortico-cortical fibers runs in the superficial layers and is distributed predominantly within longitudinal cortical bands. We studied the patterns of intrinsic EC connectivity in the in vitro isolated guinea pig brain preparation by performing current-source density analysis of field potential laminar profiles recorded with multi-channel silicon probes. The response pattern evoked by stimulation of the lateral olfactory tract was utilized to identify the lateral (l-EC) and medial (m-EC) entorhinal cortex. Stimulation of the deep layers did not evoke consistent responses. Local stimulation of the superficial layers in different portions of the EC induced an early, possibly direct response restricted to layer II-III in the close proximity to the stimulating electrode, followed by a late potential in the superficial layer I, that propagated at distance with a progressively increasing latency. The monosynaptic nature of the delayed response was verified by applying a pairing test. The results demonstrated that stimulation in the rostral-medial part of the EC generated activity restricted to the rostral pole of the l-EC, stimulation of the m-EC induced an associative activation that propagated rostrocaudally within the m-EC, stimulation of the caudal pole of the m-EC induced an additional response directed laterally, and stimulation of the lateral band of the EC determined a prominent longitudinal propagation of neuronal activity, but also induced associative potentials that propagated medially. The results are in partial agreement with the general picture derived from the anatomical studies performed in different species. Even though the largest associative interactions between superficial layers are restricted within either the m-EC or the l-EC, both rostral and caudal stimuli in the EC region close to the rhinal sulcus induced activity that propagated across the border between l- and m-EC.