Inhibition in subicular and entorhinal principal neurons in response to electrical stimulation of the fornix and hippocampus.

We performed experiments studying the responses of rat subicular and entorhinal neurons to electrical stimulation of the fornix and hippocampus. Four main results were obtained: (1) extracellular recordings from principal neurons showed prolonged inhibition in response to stimulation, and intracellular recordings showed prominent IPSPs; (2) neither fornical nor commissural afferents were necessary for the inhibitory responses; they were present even in animals that had received prior surgical sections of the fornix and hippocampal commissures; (3) antidromic responses to fornix or hippocampal stimulation were recorded in neurons of the subicular complex; and (4) 3 cells in the subicular and entorhinal cortex were encountered that showed some of the properties associated with interneurons. The results suggest that principal neurons of the subicular complex share a number of properties with hippocampal pyramidal cells, including intrinsic recurrent inhibitory circuitry. Further study is required to determine the pathway for entorhinal inhibitory responses.