A. Dhillon et Rsg. Jones, Laminar differences in recurrent excitatory transmission in the rat entorhinal cortex in vitro, NEUROSCIENC, 99(3), 2000, pp. 413-422
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.