A GABAergic inhibitory component within the hippocampal commissural pathway.

Previous results from immunocytochemical and retrograde transport studies indicated that some GABAergic neurons in the hilus of the dentate gyrus may have axonal projections through the hippocampal commissure. This study has utilized a combined immunocytochemical and retrograde transport method as well as 2 anterograde electron-microscopic methods to determine the existence of this projection in rats. Combined tracer and immunofluorescence studies showed several double-labeled GABAergic neurons in the hilus contralateral to the injection site. The electron-microscopic studies revealed both degenerating and HRP-labeled commissural axons that formed symmetric synapses, the type shown to be formed by GABAergic terminals in the hippocampus. These data demonstrate a GABAergic component within the hippocampal commissural pathway and add further evidence that cortical GABAergic terminals are not derived exclusively from intrinsic neurons.