CHEMICAL MODULATION OF EPHAPTIC ACTIVATION OF CA3 HIPPOCAMPAL PYRAMIDS

In rats under rethane anaesthesia, antidromic population spikes were evoked in CA3 pyramidal layer by fimbrial/commissural stimulation at a very low frequency (.apprx. 0.5 Hz). Submaximal population spikes-between 20 and 90% of maximum-were enhanced by 8-38% by applications of acetylcholine and bicculline, or by medial sepatal stimulation. Noradrenaline had a less pronounced and regular facilitatory action, whereas .gamma.-aminobutyrate and glutamate only depressed population spikes. Maximal enhancement by acetylcholine or bicuculline was observed when the antidromic population spike was initially at 38-53% of maximum amplitude. A simple explanation of these results is that acetylcholine and bicuculline, by raising their excitability, facilitate the excitation of non-invaded pyramidal cells by antidromic field potentials. They are fully in keeping which previous intracellular observations on ephaptic interactions between CA3 neurons, and provide a further illustration, in situ, of the importance of increased excitability and disinhibition-whether caused by drugs or synaptic action-in promoting synchronized excitation by ephaptic currents.