Neurophysiology of the caudally directed hippocampal efferent system in the rat: Projections to the subicular complex

We studied the responses of rat subicular neurons to electrical stimulation of the hippocampus. Four main results were obtained: (1) orthodromic excitatory responses were recorded in neurons of the subicular complex following electrical stimulation of the ipsilateral hippocampus. Responses were usually characterized by a single action potential immediately followed by an inhibitory period. Intracellular recordings showed that spikes were triggered by EPSPs. Preliminary data indicated that the excitatory effects extend to the entorhinal cortex as well; (2) neither fornical nor commissural afferents were necessary for the responses; they were present even in animals that had received surgical sections of the fornix and hippocampal commissures prior to the neurophysiological experiments; (3) some neurons showed evidence of frequency potentiation when stimulated at 5/sec or 10/sec; and (4) neurons in hippocampal fields CA1 and CA3 with the physiological characteristics of pyramids could be antidromically activated by electrical stimulation of subicular white matter. The results indicate an excitatory, caudally directed hippocampal efferent system that originates in fields CA1 and CA3, and that projects to all ventrocaudal regions of the subicular complex.