Feed-forward inhibition in the hippocampal formation.

An overview of the current literature reveals a richness and complexity of anatomical, pharmacological and physiological features of the input systems to the archicortex. Evidence is cited to demonstrate that several afferent paths terminate on and directly excite hippocampal formation interneurons ("non-principal" cells) besides their contacts with pyramidal and granule cells (principal cells). Since all interneurons are thought to be inhibitory, afferent excitation results in a dual effect: direct excitation of principal cells is coupled with concurrent disynaptic feed-forward inhibition. Interneuron activation generally precedes principal cell activation when both are driven by a given afferent path. At least some interneurons take a part in both feed-back and feed-forward inhibition. It is suggested that most of the major inputs to the hippocampal formation dually innervate both interneurons and principal cells and that the excitability of the principal cells depends upon the relative strengths of the inputs to these two cell types. The hypothesis of dual innervation appears powerful in resolving existing anatomical and physiological controversies.