Background synaptic activity in rat entorhinal cortex shows a progressively greater dominance of inhibition over excitation from deep to superficial layers.
The entorhinal cortex (EC) controls hippocampal input and output, playing major roles in memory and spatial navigation. Different layers of the EC subserve different functions and a number of studies have compared properties of neurones across layers. We have studied synaptic inhibition and excitation in EC neurones, and we have previously compared spontaneous synaptic release of glutamate and GABA using patch clamp recordings of synaptic currents in principal neurones of layers II (L2) and V (L5). Here, we add comparative studies in layer III (L3). Such studies essentially look at neuronal activity from a presynaptic viewpoint. To correlate this with the postsynaptic consequences of spontaneous transmitter release, we have determined global postsynaptic conductances mediated by the two transmitters, using a method to estimate conductances from membrane potential fluctuations. We have previously presented some of this data for L3 and now extend to L2 and L5. Inhibition dominates excitation in all layers but the ratio follows a clear rank order (highest to lowest) of L2>L3>L5. The variance of the background conductances was markedly higher for excitation and inhibition in L2 compared to L3 or L5. We also show that induction of synchronized network epileptiform activity by blockade of GABA inhibition reveals a relative reluctance of L2 to participate in such activity. This was associated with maintenance of a dominant background inhibition in L2, whereas in L3 and L5 the absolute level of inhibition fell below that of excitation, coincident with the appearance of synchronized discharges. Further experiments identified potential roles for competition for bicuculline by ambient GABA at the GABAA receptor, and strychnine-sensitive glycine receptors in residual inhibition in L2. We discuss our results in terms of control of excitability in neuronal subpopulations of EC neurones and what these may suggest for their functional roles.
Department of Pharmacy and Pharmacology, University of Bath, Claverton Down, Bath, United Kingdom.
Stuart David
SD
Greenhill
Department of Pharmacy and Pharmacology, University of Bath, Claverton Down, Bath, United Kingdom.
Sophie Elizabeth Lyn
SE
Chamberlain
Department of Pharmacy and Pharmacology, University of Bath, Claverton Down, Bath, United Kingdom.
Alex
A
Lench
Department of Pharmacy and Pharmacology, University of Bath, Claverton Down, Bath, United Kingdom.
Peter Vernon
PV
Massey
School of Biomedical & Healthcare Sciences, Plymouth University Peninsula Schools of Medicine and Dentistry, Plymouth, United Kingdom.
Kathryn Heather
KH
Yuill
Aston Brain Centre, School of Life and Health Sciences, Aston University, Birmingham, United Kingdom.
Gavin Lawrence
GL
Woodhall
Department of Pharmacy and Pharmacology, University of Bath, Claverton Down, Bath, United Kingdom.
Roland Spencer Gwynne
RS
Jones
D000818:Animals; D018728:Entorhinal Cortex / Q000378:metabolism / Q000502:physiology*; D018698:Glutamic Acid / Q000378:metabolism; D008297:Male; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D013569:Synapses / Q000502:physiology*; D009435:Synaptic Transmission; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid