LONG-RANGE HORIZONTAL CONNECTIONS BETWEEN SUPRAGRANULAR PYRAMIDAL CELLS IN THE EXTRASTRIATE VISUAL-CORTEX OF THE RAT

In this study, we examined the morphological structure and synaptic ph ysiology of long-range axon projections among supragranular pyramidal cells in the extrastriate visual cortex of the rat. Intra- and extrace llular recordings from layer II/III pyramidal cells were performed in brain slices of area 18a following extracellular stimulation of either the underlying white matter or within layer II/III. Neurons were inje cted with biocytin for two-dimensional reconstruction of their axon ar borizations. The conduction velocity of afferent fibers (0.58 mis) was twice as high as that of intracortical tangential fibers (0.28 mis). Layer II/III cells were mainly di- or polysynaptically driven by affer ent activation, but predominantly monosynaptically driven from intraco rtical stimulation sites. The afferent as well as intracortically evok ed postsynaptic potentials showed a very similar time course and shape . From both stimulation sites, suprathreshold action potentials could be elicited. The current threshold for a postsynaptic response and the slope and width of excitatory postsynaptic potentials (EPSPs) increas ed with the distance of lateral stimulation. The morphological propert ies of layer II/III pyramidal cell axon collaterals closely correspond ed to the electrophysiological results. Long-range intraareal axon col laterals could be followed up to 1 mm within the supragranular layers. Their length-distance distribution showed an inverse relationship to the threshold currents of EPSPs. Pyramidal cells exhibited regularly s paced patches of horizontal axon collaterals with an interpatch distan ce of about 250 mu m. We concluded that the supragranular horizontal n etwork in the extrastriate visual cortex of the rat is qualitatively v ery similar to that of cats and monkeys. However, quantitative differe nces exist in its spatial extent and physiological characteristics. (C ) 1994 Wiley-Liss, Inc.