The functional role of kainate receptors in layers II and V of the rat entorhinal cortex

E Robson, RSG Jones. University of Bath, Bath, UK

Temporal lobe epilepsy (TLE) is the most prevalent type of partial epilepsy, and the entorhinal cortex (EC) is a major site of dysfunction in TLE. Glutamatergic kainate receptors (KARs), particularly those containing the GluK1 subunit, have multiple roles in synaptic transmission in layer III of the EC and are important in the synchronisation of neuronal networks in this area [1, 2, 3]. However, the the physiological roles of KARs in other layers of the EC is largely unknown.

In the current experiments we have begun to investigate the roles of KARs, specifically GluR5-containing KAR in layers II and V of the EC. Spontaneous excitatory postsynaptic currents (sEPSCs) used to monitor glutamteric excitation and were recorded and compared in principal neurones in layers II and V using the whole cell voltage-clamp technique in brain slices from juvenile male Wistar rats (P28-38).

Control sEPSCs in the two layers showed clear differences. In layer II (mean (±SEM) IEI (inversely proportional to frequency) was 0.31±0.02 s, which was considerable less than in layer V (1.16±0.06 s). Average amplitude (15.33±0.38 pA v 10.00±0.24 pA) was greater showing that the baseline characteristics of glutamate release of the two layers are fundamentally different, and there were also differences in the effects of KAR agonists.

The non-specific KAR agonist, Kainic acid (KA; 400 nM), decreased the inter-event interval (IEI) in layer V in the first 5 minutes after application from 2.2±0.2 s to 1.2± 0.1 s (mean±SEM; P<0.001, n=4; Kolmogorov-Smirnov (K-S) test), reflecting a substantial increase in frequency. The amplitude of sEPSCs was unchanged (paired t-test). The IEI returns to the control level after 15 mins in the continued presence of KA, however, suggesting that KARs slowly desensitise, or that the increase in glutamate release depletes releasable stores.

Specific activation of GluK1-containing KARs, using the selective agonist (RS)-2-Amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), in contrast to KA, increased the IEI of sEPSCs in layer V from 1.2±0.1 s to 1.6±0.1 s reflecting a decrease in frequency (P=0.004, n=7; K-S test). In layer II, ATPA also increased the IEI from 0.31±0.02 s to 0.44±0.01 s after 15 mins (P<0.001, n=3; K-S test). No changes in amplitude were observed in either layer (paired t-test).

Thus, generalised KAR activation increased the frequency of glutmatergic sEPSCs in layer V, whilst selective activation of GluK1-containing KARs decreased sEPSC frequency in both layer V and layer II. The studies point to substantial differences between these layers and layer III [1] And form the basis of studies to delineate the integrated role of KAR normal and pathologically oscillating neuronal networks, such as in TLE.

(1) Chamberlain et al., (2010) Hippocampus 22(3):555-76

(2) Stranger, K.L.A., et al., (2008) Neural Plasticity, Article ID: 401645

(3) Woodhall, G.L., et al., (2005) Hippocampus 15(2): p. 232-245