Single-fibre EPSPs in layer 5 of rat visual cortex in vitro

SIMULTANEOUS intracellular recordings were made from pairs of sy naptically connected layer 5 pyramidal neurones less than 150 μm apart, in slices of rat visual cortex. Action potentials were evoked in one cell of a pair by depolarizing current pulses, while spike-triggered averaging was employed to reveal connections in the other cell and vice versa. Only 4 excitatory postsynaptic potentials (EPSPs) (0.7-1.2 mV) were recorded out of 270 cell pairs tested, equivalent to a connection probability of less than 0.02. Therefore, single-fibre EPSPs between pyramidal cells in layer 5 are less frequent, but individually larger in amplitude, than those found previously between pyramidal cells in layer 2/3 of rat visual cortex. © Rapid Communications of Oxford Ltd.