Laminar differences in recurrent excitatory transmission in the rat entorhinal cortex in vitro

Paired intracellular recordings were used to investigate recurrent excitato ry transmission in layers II, III and V of the rat entorhinal cortex in vit ro. There was a relatively high probability of Ending a recurrent connectio n between pairs of pyramidal neurons in both layer V (around 12%) and layer III (around 9%). In complete contrast, we have failed to find any recurren t synaptic connections between principal neurons in layer II, and this may be an important factor in the relative resistance of this layer in generati ng synchronized epileptiform activity. In general, recurrent excitatory pos tsynaptic potentials in layers III and V of the entorhinal cortex had simil ar properties to those recorded in other cortical areas, although the proba bilities of connection are among the highest reported. Recurrent excitatory postsynaptic potentials recorded in layer V were smaller with faster rise times than those recorded in layer III. In both layers, the recurrent poten tials were mediated by glutamate primarily acting at alpha-amino-3-hydroxy- 5-methyl-3-isoxazole receptors, although there appeared to be a slow compon ent mediated by N-methyl-D-aspartate receptors. In layer III, recurrent tra nsmission failed on about 30% of presynaptic action potentials evoked at 0. 2 Hz. This failure rate increased markedly with increasing (2, 3 Hz) freque ncy of activation. In layer V the failure rate at low frequency was less (1 9%), and although it increased at higher frequencies this effect was less p ronounced than in layer III. Finally, in layer III, there was evidence for a relatively high probability of electrical coupling between pyramidal neur ons. We have previously suggested that layers IV/V of the entorhinal cortex read ily generate synchronized epileptiform discharges, whereas layer II is rela tively resistant to seizure generation. The present demonstration that recu rrent excitatory connections are widespread in layer V but not layer II cou ld support this proposal. The relatively high degree of recurrent connectio ns and electrical coupling between layer III cells may he a factor in it's susceptibility to neurodegeneration during chronic epileptic conditions. (C ) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.