Reduced excitatory effect of kainic acid on rat CA3 hippocampal pyramidal neurons following destruction of the mossy projection with colchicine

Summary Rats were injected unilaterally with colchicine in the dentate gyrus of the dorsal hippocampus. Two weeks later, under urethane anesthesia, extracellular recordings were obtained on both sides from pyramidal neurons of the CA₁ and of the CA₃ regions of the dorsal hippocampus. Microiontophoresis was used to assess the responsiveness of these neurons to kainate, glutamate and ibotenate. The colchicine injection produced a nearly complete destruction of the granule cells of the ipsilateral dentate gyrus and of their mossy fiber projections to CA₃ without apparently affecting other hippocampal neurons. On the lesioned side, the potency of kainate in activating CA₃ pyramidal neurons was reduced by 94% compared to the same neurons on the intact side. However, the excitatory effect of glutamate was unchanged and that of ibotenate only slightly reduced. Kainate was 80 times more potent in activating CA₃ than CA₁ pyramidal neurons on the intact side, whereas this ratio had dropped to 2.6 on the lesioned side. The selective decrease of the effectiveness of kainate in activating CA₃ pyramidal neurons following the colchicine lesion suggests that this amino acid, but not glutamate and ibotenate, produces most of its excitatory effect in the intact CA₃ region by releasing (an) excitatory neurotransmitter(s) from mossy fibers terminals, the nature of which remains to be identified.