071P Brighton
Winter Meeting December 2007

LAYER-SPECIFIC EFFECT OF MK801 ON EVOKED FIELD POTENTIALS RECORDED IN RAT PIRIFORM CORTICAL SLICES USING A MULTI-ELECTRODE ARRAY

Naghmeh Fouladi2, Erik Arstad2, Andrew Constanti3, Benjamin J. Whalley1
1University of Reading, Reading, Berkshire, United Kingdom, 2Hammersmith Imanet, Hammersmith Hospital, London, United Kingdom, 3London School of Pharmacy, University of London, London, United Kingdom

N-methyl-D-aspartate-type glutamate receptors (NMDARs) are implicated in wide range of biological processes, including neuroplasticity, learning and neurodegeneration (Kew & Kemp, 2005). Moreover, overactivation of NMDARs is linked to a variety of acute and chronic diseases including stroke, naturopathic pain, schizophrenia and epilepsy (Waxman & Lynch, 2005). The piriform cortex (PC) is a particularly epileptogenic area of the brain (Loscher & Ebert, 1996), although NMDAR contributions to PC synaptic transmission under normal and epileptic conditions have not been investigated in detail. In this study, we examined whether a significant NMDAR-mediated component was present in synaptically-evoked extracellular field potentials (EFPs) recorded from adult rat PC slices using multi-electrode array (MEA) recording methods; layer specificity of NMDAR contributions was also assessed. The non-competitive NMDAR antagonist, MK801, was bath-applied and effects upon EFPs evoked in various PC cell layers (I-III) by local electrical stimulation were assessed. Transverse slices of rat PC (450 μm-thick; adult P>40; female; Hampshire; outbred) were prepared as previously described (Whalley et al., 2005), adhered to the MEA surface and perfused with carboxygenated (95% O2/5% CO2) artificial cerebrospinal fluid (aCSF) at 23°C. EFPs were evoked by 3V bipolar stimuli (stimulating electrodes located in PC layer I, II and III; 30 μm diameter; individual, not concurrent stimuli applied) and recorded from electrodes 200 μm from each stimulation site in each layer. EFPs were recorded in control aCSF and in the presence of bath-applied MK801 (20 μM). Effects of MK801 were measured following 0.2 Hz stimuli for 5 minutes as changes in negative (N)-wave amplitude, expressed as normalised percentages vs control (n=3 for all values; Table 1). Significance was calculated by Wilcoxon Signed Rank test and significance accepted at P<0.05.

Table 1. Layer-specific effects of MK801 in rat PC slices; *P<0.05 vs control; ΨP<0.05 vs layers I/II
PC Layer Distance from pial surface (m) Control N-wave amplitude (absolute % values and V) N-wave amplitude in 20 M MK801 (% of control)
I 216 ± 3 100 (65 ± 11) 50 ± 8*
II 566 ± 2 100 (85 ± 8) 57 ± 5*
III 1000 ± 2 100 (115 ± 8) 82 ± 7*Ψ

These data suggest that MK801 exerts a significant layer-specific blocking effect on evoked EFPs recorded in rat PC, indicative of a greater contribution of NMDARs to normal synaptic transmission in layers I and II vs layer III. Future work will investigate possible MK801 use-dependence in the PC as has been previously shown in hippocampal brain slice recordings (Grover, 1998).

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