Asymmetrical distribution of the Schaffer projections within the apical dendrites of hippocampal field CA1

Continuous current source densities were calculated in two dimensions (proximo-distal vs. medio-lateral) from slices of hippocampal field CA1 placed on a 64-electrode array in the presence of GABA blockers. The synaptic sink generated by stimulation of the Schaffer-commissural fibers spread across the extent of field CA1 within the same sublamina of the stratum radiatum as the stimulation electrode. The size and shape of the current sink varied according to the proximo-distal position of the stimulation site. Sinks generated by stimulation close to the cell body layer were more compact when compared to those produced by stimulation near the top of stratum radiatum which were broad and skewed in the proximal direction. These distributions were obtained with stimulation at either the CA3 or the subicular border of CA1. Induction of LTP increased the intensity of the current field but did not notably affect its distribution. It is concluded that (1) axons remain at the same proximo-distal level as they traverse stratum radiatum of CA1 and (2) generate proximally directed collaterals. Because of this, fibers that enter CA1 stratum radiatum immediately above the pyramidal cell bodies form compact synaptic fields while those entering CA1 at the top of the lamina form much broader and asymmetrical distributed fields.