Fundamental Differences in Spontaneous Synaptic Inhibition Between Deep and Superficial Layers of the Rat Entorhinal Cortex Gavin L. Woodhall, Sarah J. Bailey, Sarah E. Thompson, D. Ieuan P. Evans, and Roland S.G. Jones * ABSTRACT: We have previously shown that there are clear differences between spontaneous excitatory synaptic currents recorded in layers V and II of the rat entorhinal cortex (EC) in vitro, and have suggested that these might contribute to a more pronounced susceptibility of the deeper layer to epileptogenesis. In the present study, we have made a detailed comparison of spontaneous synaptic inhibition between the two layers by recording spontaneous inhibitory synaptic currents (sIPSCs) using whole- cell patch-clamp techniques in EC slices. Pharmacological studies indi- cated that sIPSCs were mediated exclusively by -aminobutyric acid (GABA) A receptors. There was little difference in average amplitudes, rise or decay times of sIPSCs in layer II compared with layer V. However, in the former, events occurred at 4 –5 times the frequency seen in the latter, and frequencies of <40 Hz were not uncommon. When activity-indepen- dent, miniature IPSCs were isolated in tetrodotoxin (TTX), the frequency in layer V was more than halved, but in layer II only a small reduction was seen, and the frequency remained very high. In terms of kinetics, while averaged sIPSCs in each layer were very similar, detailed comparison of individual sIPSCs within layers revealed distinct differences, possibly re- flecting inputs from different subtypes of interneurons or inputs at differ- ent somatodendritic locations. In layer V, sIPSCs could be divided into three groups, one with slow rise and decay kinetics and a second with fast rise kinetics, further distinguished into two groups by either fast or slow decay kinetics. The distinction between events in layer II was simpler, one group having both fast rise and decay times and the second with both parameters much slower. Finally, IPSCs could occur in high-frequency bursts in both layers, although these were much more prevalent in layer II. The results are discussed in terms of the overall level of background inhibition in the two layers, as well as how this might relate to their susceptibilities to epileptogenesis. © 2004 Wiley-Liss, Inc. KEY WORDS: spontaneous inhibition; GABA; laminar differences INTRODUCTION Increasing attention is being paid to the role of the entorhinal cortex (EC) as a dynamic processor of infor- mation both entering and leaving the hippocampus. It receives convergent inputs from primary sensory and as- sociation cortices, as well as many subcortical regions. In turn, the EC provides the major source of afferent infor- mation to the hippocampus, primarily via projections from layer II to the dentate gyrus and from layer III to CA1 and CA3. Most hippocampal output is directed back to neocortical and other areas through the EC, pri- marily via CA1 and subicular projections to the pyrami- dal neurons of layer V (Witter, 1993; Witter et al., 2000a; van Groen et al., 2003). The dynamic balance of inhibitory and excitatory synaptic interactions, and the level of excitability of neurons within the different layers of the EC, are likely to be important in determining the processing of and destination of information entering and exiting the hippocampus (Jones, 1993; Witter et al., 2000b). Consideration of dysfunction also highlights the im- portance of the EC, especially with respect to epilepsies involving the limbic system and temporal lobe (TLE). Although the focus of attention has been overwhelmingly on the hippocampus, there is increasing evidence from both clinical and basic studies that the EC may be a major site of importance in TLE (Ben-Ari et al., 1981; Rutecki et al., 1989; Kim et al., 1990; Lothman et al., 1990; Siegel et al., 1990; Du and Schwarz, 1992; Du et al., 1993, 1995; Bertram and Cornett, 1994; Spencer and Spencer, 1994; Sperling et al., 1996; Bernasconi et al., 1999; Wennberg et al., 2002). In vitro experiments in rat brain slices have demonstrated a pronounced susceptibil- ity of the EC to acutely provoked epileptogenesis (Walther et al., 1986; Jones, 1988; Wilson et al., 1988; Jones and Lambert, 1990a,b; Bear and Lothman, 1993; Rafiq et al., 1993; Avoli et al., 1995, 1996). Pharmaco- logically induced seizures arise predominantly in the EC and propagate to adjacent cortical and hippocampal areas (Jones and Lambert, 1990a; Iijima et al., 1996; Avoli et al., 1996; Buchheim et al., 2002; Weissinger et al., 2000; D’Arcangelo et al., 2001). Such epileptiform activity ap- pears to be initiated in the deep layers of the EC and to propagate to the deeper and more superficial layers and to the hippocampus (Jones and Lambert, 1990a,b; Avoli et Department of Physiology and MRC Centre for Synaptic Plasticity, School of Medical Sciences, University of Bristol, Bristol, United Kingdom Gavin L. Woodhall is currently at the Molecular Biosciences Research Group, School of Life and Health Sciences, Aston University, Birmingham, UK. Sarah J. Bailey is currently at the Department of Biochemistry, School of Medical Sciences, University of Bristol, Bristol, UK. Sarah E. Thompson is currently at the Department of Pharmacology, McGill University, Faculty of Medicine, Montreal, Quebec, Canada. D. Ieuan Evans is currently at the Division of Neuroscience, University of Edinburgh, Edinburgh, UK. Grant sponsor: The Wellcome Trust; Grant number: 037329; Grant num- ber: 050680; Grant number: 067472; Grant sponsor: Epilepsy Research Foundation. *Correspondence to: Roland S.G. Jones, Department of Pharmacy and Pharmacology, University of Bath, Claverton Down, Bath, BA2 7AY, UK. E-mail: roland.jones@bath.ac.uk Accepted for publication 9 August 2004 DOI 10.1002/hipo.20047 Published online 22 September 2004 in Wiley InterScience (www. interscience.wiley.com). HIPPOCAMPUS 15:232–245 (2005) © 2004 WILEY-LISS, INC.