Transient and long-lasting electrical responses to direct hippocampal stimulation

¹ Laboratory of Neurophysiology, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland

Widely accepted use of the direct cortical response (DCR) for the study of neocortical apical dendrites prompted this study of the response of the surface of hippocampal pallium to direct electrical stimuli in rabbits anesthetized with Dial or Evipal. The hippocampus was directly exposed by radical decortication. The most typical response to direct hippocampal stimulation (DHR) is a monophasic 20–25 msec. negative wave. The DHR is linearly graded throughout the early part of its input-output curve, shows no refractoriness, exhibits long lasting (400 msec.) potentiation of a previously conditioned test response, is rapidly (3–5 sec.) inverted by GABA and is associated with two types of d.c. shifts: a) d.c. shift without concomitant loss of the DHR and b) a variant of spreading hippocampal depression. From these properties the DHR would appear to be quite similar to the DCR. However, different bioelectric generators must be postulated since the hippocampal neural geometry is different from neocortex with respect to the orientation of its predominant neurons.