A new type of specific interneuron in the monkey hippocampus forming synapses exclusively with the axon initial segments of pyramidal cells.

By means of Golgi staining and gold-toning, we have found an interneuron in the pyramidal cell layer of the hippocampus which forms synapses exclusively on the axon initial segments of pyramidal neurons. An individual initial segment receives up to 30 symmetrical synapses from one axo-axonic cell. Each axo-axonic cell is in synaptic contact with the axon initial segments of several hundred pyramidal neurons. The interneuron is thus ideally situated to synchronize the output of a large population of pyramidal cells and so might be involved in the generation of rhythmic activity and in epileptogenesis.