Activation of pyramidal neurons by intrinsic longitudinal connections in the hippocampus.

In the guinea pig, EPSPs and population spikes were found to be generated in the apical dendrites of pyramidal neurons of middle and ventral hippocampus, in response to dorsal hippocampal commissure (PSD) stimulation, without any involvement of dentate gyrus granule cells of corresponding segments. These long-latency synaptic effects were evoked only by repetitive (0.2-2.0 c/sec) PSD stimulation and showed increasing latency in ventral direction. A cross section between dorsal and middle hippocampus was followed by the disappearance of the responses ventrally to the section. The results show that the postsynaptic discharge of dorsal pyramidal neurons is transferred to more ventral hippocampal segments by an intrahippocampal longitudinal association system.